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Conjugated polymers in mesoporous hosts Pattantyus-Abraham, Andras Geza 2003

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CONJUGATED  P O L Y M E R S IN MESOPOROUS HOSTS by  ANDRAS GEZA PATTANTYUS-ABRAHAM B . S c . , Queen's U n i v e r s i t y , 1996 B . S c . , Queen's U n i v e r s i t y , 1997  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS  F O R T H ED E G R E E OF  DOCTOR OF PHILOSOPHY in THE F A C U L T Y OF G R A D U A T E STUDIES (Department o f C h e m i s t r y )  W e accept this thesis as c o n f o r m i n g to the r e q u i r e d standard  THE UNIVERSITY OF BRITISH C O L U M B I A September 2 0 0 3 © A n d r a s G e z a Pattantyus-Abraham, 2003  Abstract T h e subject o f this thesis is the synthesis and characterization o f c o m p o s i t e materials based o n electroluminescent conjugated p o l y m e r s i n m e s o p o r o u s hosts. These materials w e r e studied w i t h the g o a l o f p r o d u c i n g a structure i n w h i c h the electrical properties o n encapsulated conjugated p o l y m e r chains c o u l d be measured. T o w a r d s this g o a l , both the creation o f t h i n film hosts w i t h oriented and ordered mesopores a n d n e w methods for the i n c o r p o r a t i o n o f p o l y m e r s into m e s o p o r o u s hosts are described, a l o n g w i t h characterization techniques for s h o w i n g the p o l y m e r d i s t r i b u t i o n o n the nanometre scale. T h e preparation o f the conjugated p o l y m e r p o l y ( 1,4-phenylene v i n y l e n e ) ( P P V ) inside the  3.1  nm  channels  o f a h e x a g o n a l l y ordered  described. The M C M - 4 1  surface  was  first  mesoporous  silica material, M C M - 4 1 ,  d e r i v a t i z e d w i t h an organic base.  is  Subsequent  i n t r o d u c t i o n o f m o n o m e r d i s s o l v e d i n ethanol resulted i n base-initiated p o l y m e r i z a t i o n i n the pores o f M C M - 4 1 . A pore size r e d u c t i o n o f 0.3 n m w a s seen i n the c o m p o s i t e material b y n i t r o g e n p h y s i s o r p t i o n . Electron-energy loss spectroscopy ( E E L S ) s h o w e d that the c o m p o s i t e h a d a distinct loss signal related to the Tt-electron system o n the p o l y m e r .  Energy-filtered  t r a n s m i s s i o n electron m i c r o s c o p y ( E F T E M ) w i t h 2 0 0 k e V electrons s h o w e d that the p o l y m e r w a s e v e n l y distributed throughout the c o m p o s i t e m a t e r i a l through m a p p i n g o f the  7i-electron  losses near 6 e V . A p o l y m e r mass content o f ~8 % i n d i c a t e d the presence o f approx. 6 p o l y m e r chains i n each pore. T h e p h o t o p h y s i c a l properties o f P P V i n s i d e the c o m p o s i t e were f o u n d to be s i m i l a r to b u l k P P V . F o r the preparation o f m e s o p o r o u s t h i n f i l m s w i t h channels oriented n o r m a l l y to the surface, three literature approaches were investigated: the self-assembly o f mesoporous films  w i t h the S B A - 2  structure, the t h e r m a l o x i d a t i o n o f FeO/Si02  f i l m s , and the  silica anodic  o x i d a t i o n o f a l u m i n u m substrates. T h e latter approach, carried out at l o w temperature, is s h o w n to y i e l d a l u m i n a f i l m s w i t h the desired pore structure. F i l m s w i t h a pore size o f 4 ± 1 n m are created w i t h an a p p l i e d potential o f 15 V i n 1.2 M sulfuric a c i d ( i n 1:1 water:methanol) at -39 °C. E E L S and E F T E M were a p p l i e d to the analysis o f c o m p o s i t e materials created b y the adsorption  o f a t h i n layer o f p o l y [ 2 - m e t h o x y - 5 - ( 2 ' - e t h y l h e x y l o x y ) - l , 4 - p h e n y l e n e v i n y l e n e ]  ( M E H - P P V ) o n the surface o f porous a l u m i n a m e m b r a n e w i t h 6 0 n m pore diameter.  The  measurements w e r e c a r r i e d out w i t h a 2 0 0 k e V electron b e a m t r a v e l l i n g p a r a l l e l to the pores o f the  host m e m b r a n e .  T h e TC-electron losses o f the p o l y m e r c o u l d not be d i s c e r n e d i n this ii  geometry. S t r o n g surface losses w e r e present at 8, 13 a n d 18 e V . A long-range loss m o d e not associated w i t h b u l k or surface losses appeared at 7.0 e V , u p to 3 0 n m from the pore surface. B o t h these loss m o d e s interfered w i t h the detection o f the 7t-electron losses a n d the p o l y m e r d i s t r i b u t i o n c o u l d not be c o n f i r m e d . T h e o r i g i n o f the long-range loss m o d e w a s i d e n t i f i e d as the C h e r e n k o v effect.  EELS  w i t h 120 k e V electrons shifted the peak energy o f this loss m o d e to 8.3 e V , w h i c h i n d i c a t e d a dependence o n the electron speed. S a m p l e s w i t h different pore diameters but a f i x e d interpore s p a c i n g also s h o w e d shifts i n the p e a k p o s i t i o n . T h e o r e t i c a l m o d e l l i n g o f the loss spectrum o f a c y l i n d r i c a l pore suggested that these observations arise from the interaction o f the generated C h e r e n k o v radi at ion w i t h the nearby pores i n the m e m b r a n e . T h i s introduces the p o s s i b i l i t y o f s t u d y i n g p h o t o n i c nanostructures b y E E L S . Different methods for i n t r o d u c i n g the conjugated p o l y m e r into an oriented p o r o u s host are e x p l o r e d . T h e i d e a o f creating surface-grafted  conjugated p o l y m e r s o n s i l i c o n substrates  t h r o u g h step p o l y m e r i z a t i o n is investigated; as p r o o f o f concept, a surface-grafted  d i m e r is  synthesized t h r o u g h the W a d s w o r t h - H o r n e r - E m m o n s reaction. It is further s h o w n that s i m p l e centrifugation o f a p o l y m e r s o l u t i o n , w h i l e a l l o w i n g solvent evaporation, p r o v i d e s a sufficient d r i v i n g force for p o l y m e r i n s e r t i o n into the host. T h i s c o m p o s i t e is investigated b y electron m i c r o s c o p y . E E L S a n d E F T E M analysis w e r e also a p p l i e d to u l t r a m i c r o t o m e d t h i n sections o f this material, w i t h the electron b e a m p e r p e n d i c u l a r to the pore axis. T h e results s h o w e d that relativistic effects m a y also be important i n this geometry, effectively m a s k i n g the d i s t r i b u t i o n o f the 7t-electron losses associated w i t h the p o l y m e r . P o s s i b l e routes to the preparation o f a l i g h t - e m i t t i n g d e v i c e ( L E D ) based o n p o r o u s a l u m i n a films are d e s c r i b e d . T h e use o f the u n d e r l y i n g a l u m i n u m substrate as electron-injecting electrode  was  investigated  but  devices  prepared  in  this  manner  did  not  show  electr olu m i nescence. T h e f o r m a t i o n o f p o r o u s a l u m i n a films o n c o n d u c t i v e substrates s u c h as s i l i c o n , i n d i u m t i n o x i d e a n d g o l d w a s also investigated a n d the encountered  experimental  difficulties are reported.  m  Table of Contents Abstract  ii  T a b l e o f Contents  iv  List o f Tables  vii  List o f Figures  vii  List o f Symbols and Abbreviations Acknowledgements  CHAPTER 1 Introduction 1.1  xi xiv  Conjugated Polymers: Organic Conductors and Semiconductors  1 2  1.1.1  O r g a n i c M o l e c u l e s w i t h C o n j u g a t e d Tt-Electron S y s t e m s  3  1.1.2  C o n j u g a t e d M o l e c u l e s i n the S o l i d State  4  1.1.3  Organic Conjugated Polymers  4  1.2  Luminescence i n Conjugated M o l e c u l e s  7  1.3  Electroluminescence i n Conjugated Polymers  8  1.3.1  E l e c t r o l u m i n e s c e n c e Processes i n C o n j u g a t e d P o l y m e r s  1.3.2  Electroluminescence Efficiency  1.3.3  A n Ideal D e v i c e Structure  11  Encapsulated Conjugated Polymers  13  1.4  8 10  1.4.1  A p p r o a c h e s to E n c a p s u l a t i o n  13  1.4.2  O r d e r e d P o r o u s M a t e r i a l s as E n c a p s u l a n t s  14  1.4.3  Oriented Mesoporous Silica Encapsulant  15  1.4.4  L i q u i d Crystal Encapsulant  17  1.4.5  C l a y Encapsulant  20  1.4.6  Cyclodextrin Encapsulant  21  1.4.7  Literature S u m m a r y  23  1.5  Thesis Summary  References  23 25  CHAPTER 2 Characterization of Nanocomposite Materials  31  2.1  Scanning Electron Microscopy  32  2.2  Transmission Electron Microscopy ( T E M )  33  2.2.1 2.3 2.3.1  T E M Sample Preparation H i g h Resolution Chemical Analysis X - R a y Photoelectron Spectroscopy  34 37 38  2.3.2  Energy-Dispersive X - R a y Spectroscopy and Scanning A u g e r M i c r o s c o p y . 39  2.3.3  Electron Energy-Loss Spectroscopy ( E E L S )  40  2.4  General Principles o f E E L S  2.5  E E L S Instrumentation  40 43  2.6  A p p r o a c h e s to E E L S D a t a A c q u i s i t i o n  45  2.7  Quantitative A n a l y s i s o f E E L S S p e c t r a  46  2.8  A p p l i c a t i o n o f E E L S to O r g a n i c M a t e r i a l s  47  2.9  Conclusion  48  CHAPTER 3 A PPV/MCM-41 Composite Material... 3.1  Ordered Porous Host Materials  51 52 iv  3.1.1  Zeolites  52  3.1.2  Mesoporous Materials  53  3.2  Characterization o f M C M - 4 1 Materials  56  3.2.1  Diffraction Techniques  3.2.2  Physisorption  58  3.2.3  Other Techniques  59  3.3  57  Polymerization within M C M - 4 1  3.3.1  60  P P V in MCM-41  60  3.4  Experimental Results  62  3.5  Discussion  3.5.1  71  Thermogravimetric Analysis  71  3.5.2  Physisorption Data..!  72  3.5.3  X - r a y and Neutron Diffraction  73  3.5.4  E E L S and E F T E M  74  3.5.5  U V / V i s A b s o r b a n c e and P h o t o l u m i n e s c e n c e  76  3.6  Conclusion  76  Experimental Details  78  References  80  CHAPTER 4 Preparation of Mesoporous Thin Film Host 4.1  84  Alignment o f Porous T h i n Films  85  4.1.1  Interface-Induced A l i g n m e n t  86  4.1.2  Field-Induced Alignment  88  4.1.3  Oriented Porous T h i n F i l m s by Other Approaches  90  4.2  Further Investigation o f S B A - 2 M e s o p o r o u s S i l i c a F i l m s  4.3  Further Investigation o f the F e O / S i 0  4.4  Porous A l u m i n a Films ( A n o d i c A l u m i n u m Oxide)  4.4.1  2  92  System  94 99  Pore W a l l Structure  101  4.4.2  P o r e G r o w t h Processes  102  4.4.3  Pore Lattice Formation  103  4.4.4  Preparation o f O p t i m a l H o s t from P o r o u s A l u m i n a  105  4.4.5  Barrier Layer Thinning  108  4.5  Conclusion  109  Experimental Details  110  References  113  CHAPTER 5 Characterization of a Porous Alumina/MEH-PPV Composite Material  118  5.1  E E L S Samples  120  5.2  M E H - P P V L o w - L o s s Spectra and Z e r o - L o s s P e a k R e m o v a l  121  5.3  P o r o u s A l u m i n a L o w - L o s s Spectra  5.4  P o r o u s A l u m i n a / M E H - P P V C o m p o s i t e Spectra  5.5  Conclusion  Experimental Details  124 ,  129 132  .  (  References  CHAPTER 6 Aloof Cherenkov Effect in Porous Alumina  132 134  135  6.1  The Cherenkov Effect  135  6.2  Further M e a s u r e m e n t s  139  6.3  Effect o f P r i m a r y B e a m E n e r g y  139 v  6.4  M o d e l l i n g o f the A l o o f C h e r e n k o v Effect  6.4.1  144  M o d e l l i n g o f 197 k e V D a t a  145  6.4.2  M o d e l l i n g o f 117 k e V D a t a  149  6.4.3  Comparison with Experiment  149  6.5  E f f e c t o f P o r e D i a m e t e r o n the C h e r e n k o v P e a k  151  6.6  Conclusion  156  Experimental Details  157  References  158  CHAPTER 7 Polymer Guest Incorporation 7.1  160  Internal P o l y m e r S y n t h e s i s  160  7.1.1  Surface-Graft P o l y m e r i z a t i o n o f C o n j u g a t e d P o l y m e r s  161  7.1.2  D e r i v a t i z a t i o n o f P o r o u s S i l i c o n Surfaces  165  7.1.3  W a d s w o r t h - H o r n e r - E m m o n s R e a c t i o n o n S i l i c o n Surface  167  7.2  E x t e r n a l P o l y m e r Synthesis  169  7.2.1  Polymer Adsorption Loading  170  7.2.2  V a c u u m (Filtration) L o a d i n g  170  7.2.3  Centrifugal L o a d i n g  171  7.3  Preparation o f C e n t r i f u g e d S a m p l e s  172  7.4  Characterization o f Centrifuged Samples  174  7.4.1  Scanning Electron Microscopy  174  7.4.2  Scanning Electron M i c r o s c o p y  174  7.4.3  Transmission Electron Microscopy  178  7.4.4  Energy-filtered Transmission Electron M i c r o s c o p y  180  7.4.5  STEM/EELS  183  7.5  Conclusion  :  184  Experimental Details  185  References  189  CHAPTER 8 Comments on Device Fabrication  193  8.1  D e v i c e Structure  193  8.2  D e v i c e s from P o r o u s A l u m i n a F i l m s o n A l u m i n u m F o i l  195  8.3  D e v i c e s from T h i n F i l m s o n C o n d u c t i n g Substrates  8.3.1  198  A l u m i n u m F i l m Deposition  199  8.3.2  Porous A l u m i n a / S i l i c o n  200  8.3.3  Porous A l u m i n a / I T O  201  8.3.4  Porous A l u m i n a / G o l d  203  8.4  Conclusion  203  Experimental Details  204  References  205  vi  List of Tables Table 1.1 E l e c t r i c a l c o n d u c t i v i t y o f p o l y m e r s and some c o m m o n materials Table 1.2 Properties o f the i d e a l host m a t e r i a l for conjugated p o l y m e r s Table 2.1 C u r r e n t spatial r e s o l u t i o n l i m i t s o f c h e m i c a l analysis techniques Table 3.1 I U P A C c l a s s i f i c a t i o n o f porous materials b y pore size Table 3.2 Parameters for c a l c u l a t i o n o f P P V mass fraction i n M C M - 4 1 Table 3.3 P o l y m e r mass fraction F for Npolymer chains per pore i n M C M - 4 1 Table 4.1 P u b l i s h e d parameters for self-ordered porous a l u m i n a g r o w t h Table 6.1 C h e r e n k o v c o n d i t i o n for c o m m o n T E M b e a m energies Table 6.2 C h e r e n k o v loss peak parameters for ordered samples  6 13 38 52 72 72 103 138 152  List of Figures Figure Figure Figure Figure Figure Figure Figure  1.1 B u t a d i e n e , a s i m p l e conjugated m o l e c u l e  3  1.2 C o n j u g a t e d p o l y m e r s 1.3 Structure o f a s i m p l e conjugated p o l y m e r - b a s e d electroluminescent d e v i c e  5  1.4 E l e c t r o l u m i n e s c e n c e processes i n a s i m p l e p o l y m e r d e v i c e  9  8  1.5 Ideal d e v i c e structure c o n s i s t i n g o f oriented and isolated p o l y m e r chains 12 1.6 Incorporation o f M E H - P P V into oriented m e s o p o r o u s s i l i c a 16 1.7 (a) Structure o f l i q u i d crystal m e s o g e n , (b) structure o f l y o t r o p i c l i q u i d crystal w i t h  p o l y m e r guest  18  Figure 1.8 High-temperature c o n v e r s i o n o f a water-soluble precursor p o l y m e r to P P V  18  Figure 1.9 W a t e r - s o l u b l e P P V derivative  20  Figure 1.10 Preparation o f M E H - P P V / C l a y c o m p o s i t e m a t e r i a l  20  Figure 1.11 E n c a p s u l a t i o n o f conjugated p o l y m e r c h a i n w i t h (3-cyclodextrin  22  Figure 2.1 T r u n c a t e d p y r a m i d geometry o f e p o x y - e m b e d d e d sample for sectioning b y ultramicrotomy  36  Figure 2.2 T h e s m a l l angle cleavage ( S A C ) technique a p p l i e d to a t h i n f i l m  36  Figure 2.3 E l e c t r o n i c e x c i t a t i o n and d e - e x c i t a t i o n m e c h a n i s m s i n a s o l i d  38  Figure 2.4 G e o m e t r y o f (a) elastic, (b) inelastic, inner-shell a n d (c) inelastic, outer-shell scattering events i n v o l v i n g a n electron a n d a c a r b o n atom  41  Figure 2.5 P r i n c i p a l features o f an E E L s p e c t r u m 42 Figure 2.6 S c h e m a t i c o f p o s t - c o l u m n G a t a n I m a g i n g F i l t e r o n a T E M 44 Figure 2.7 Illustration o f c h e m i c a l analysis o f a t w o - c o m p o n e n t sample b y S T E M / E E L S over a set o f points and E F T E M over the w h o l e i m a g e  46  Figure 3.1 C h e m i c a l structure o f p o l y ( l , 4 - p h e n y l e n e v i n y l e n e ) ( P P V ) Figure 3.2 E x a m p l e s o f pore topologies w i t h (a) 1-D, (b) 2 - D and (c) 3 - D c o n n e c t i v i t y Figure 3.3 Stages i n the f o r m a t i o n o f M C M - 4 1 Figure 3.4 T r a n s m i s s i o n electron m i c r o g r a p h o f M C M - 4 1 material obtained u s i n g  52  Ci6H33(CH3)3NCl surfactant, s h o w i n g h e x a g o n a l lattice s p a c i n g and w a l l thickness  55  53 54  Figure 3.5 N i t r o g e n ( o ) adsorption and ( • ) desorption isotherms for M C M - 4 1 58 Figure 3.6 T h e G i l c h route to P P V starting from d i c h l o r o - p - x y l e n e . 61 Figure 3.7 Synthetic scheme for the preparation o f P P V / M C M - 4 1 h y b r i d m a t e r i a l . . 62 Figure 3.8 (a) T h e r m o g r a v i m e t r i c analysis o f T B A O H - t r e a t e d M C M - 4 1 , (b) nitrogen adsorption i s o t h e r m a n d B J H pore d i s t r i b u t i o n for e m p t y a n d T B A O H - t r e a t e d M C M - 4 1  64  Figure 3.9 (a) P o w d e r X - r a y diffraction pattern and (b) neutron scattering data for M C M - 4 1 , P P V / M C M - 4 1 sample 1 a n d sample 2 65 Figure 3.10 (a) T h e r m o g r a v i m e t r i c analysis o f P P V / M C M - 4 1 , a n d (b) B J H pore d i s t r i b u t i o n o f e m p t y M C M - 4 1 , P P V / M C M - 4 1 sample 1 a n d sample 2 Figure 3.11 Spectral signature o f P P V / M C M - 4 1 and M C M - 4 1 b y (a) F T - I R a n d (b) E E L S  67 68  Figure 3.12 E F T E M images for (a) M C M - 4 1 and (b) P P V / M C M - 4 1 69 Figure 3.13 (a) R o o m temperature U V / V i s absorbance a n d p h o t o l u m i n e s c e n c e o f P P V / M C M - 4 1 composite, (b) temperature-dependent p h o t o l u m i n e s c e n c e spectra o f P P V / M C M - 4 1 70  Figure 4.1 2 - D h e x a g o n a l p a c k i n g o f surfactant m i c e l l e s i n aqueous solution onto a graphite surface  86 Figure 4.2 Surfactant templates for (a) M C M - 4 1 a n d M C M - 4 8 structures (Ci6H 3N(CH )3X) a n d 3  3  (b) S B A - 2 structure ( C i e H s s N C C H s M C ^ N C C H s ^ X : ) , w h e r e X = B r or C l 87 Figure 4.3 T h e t w o extreme p o s s i b i l i t i e s for surfactant m i c e l l e alignment i n a p o r o u s support: (a) i n the a x i a l d i r e c t i o n a n d (b) i n the c i r c u m f e r e n t i a l d i r e c t i o n 88 Figure 4.4 O r i e n t a t i o n o f m e s o p o r o u s channels b y electro-osmotic f l o w 90 Figure 4.5 X - r a y diffraction pattern o f S B A - 2 f i l m g r o w n o n m i c a before a n d after c a l c i n a t i o n . 93  Figure 4.6 Preparation o f t h i n f i l m w i t h oriented channels from a FeO:Si02 film 95 Figure 4.7 T E M i m a g e o f cross-section o f FeO:Si02 film o n glass (a) after o x i d a t i o n , before etching, (b) after etching a n d P b - s t a i n i n g . T h e cross-section w a s prepared b y the S A C technique 96 Figure 4.8 E D X spectrum obtained from cross-sections o f F e O : S i 0 2 f i l m s : (a) o x i d i z e d (some A u c o n t a m i n a t i o n is apparent), (b) etched to r e m o v e Fe203, (c) Pb-stained 98 Figure 4.9 S E M images o f porous a l u m i n a f i l m p r o d u c e d at 4 0 V i n 0.3 M o x a l i c a c i d , u s i n g M a s u d a ' s two-step approach: (a) top v i e w , (b) cross-section 99 Figure 4.10 A s i m p l e e l e c t r o c h e m i c a l c e l l for a n o d i z a t i o n o f a l u m i n u m substrates 100 Figure 4.11 Structure o f porous a l u m i n a films g r o w n o n a l u m i n u m : (a) geometry o f pore p a c k i n g , (b) cross-section s h o w i n g barrier layer o f thickness ~ L / 2 at b o t t o m o f pores Figure 4.12 Preparation o f f u l l y - o r d e r e d porous a l u m i n a f i l m b y two-step a n o d i z a t i o n  101 104  Figure 4.13 P o r o u s a l u m i n a samples a n o d i z e d at (a) 2 0 ° C , (b) -8 ° C a n d (c) -40 ° C at 15.0 V i n 1.2 M H S 0  (1:1 F f 0 : M e O H ) , w i t h resulting pore size distributions 106 Figure 4.14 Effect o f temperature o n pore diameter for samples a n o d i z e d at 15.0 V i n 1.2 M sulfuric a c i d 107 2  4  2  Figure 5.1 A d s o r p t i o n o f t h i n layer o f M E H - P P V onto porous a l u m i n a host, s h o w i n g (a) e m p t y host, a n d p o l y m e r - c o a t e d host i n (b) p l a n v i e w a n d (c) as t h i n section for T E M  119  Figure 5.2 P o r o u s a l u m i n a film after s o a k i n g i n M E H - P P V solution, seen i n cross-section, as s h o w n b y (a) light m i c r o s c o p y (b) fluorescence m i c r o s c o p y  119  Figure 5.3 S E M i m a g e o f cross-section o f sample for E E L S experiments  121  Figure 5.4 T E M i m a g e o f M E H - P P V film supported b y a lacey carbon g r i d Figure 5.5 L o w - l o s s spectrum o f M E H - P P V t h i n film, also s h o w i n g v a r i o u s approaches to  122  r e m o v i n g the zero-loss peak: r a w data, m a t r i x d e c o n v o l u t i o n , F o u r i e r - l o g d e c o n v o l u t i o n , p o w e r l a w fit over 1.3 - 2 . 0 e V  123  Figure 5.6 L o w - l o s s spectrum o f pore i n porous a l u m i n a film, s h o w i n g collected data a n d data w i t h zero-loss peak r e m o v e d b y a p o w e r l a w fit over 1.3 to 2.0 e V  125  Figure 5.7 S T E M d a r k - f i e l d i m a g e o f 1.4 u m t h i c k porous a l u m i n a film, s h o w i n g the l o c a t i o n o f line a l o n g w h i c h loss spectra w e r e a c q u i r e d at 2 n m intervals  127  viii  (  Figure 5.8 Representative l o w - l o s s spectra for p o r o u s a l u m i n a f i l m : near pore centre, ~7 n m from pore w a l l , a n d at w a l l Figure 5.9 E n e r g y - f i l t e r e d images o f 0.2 u\m p o r o u s a l u m i n a  128 film  128  Figure 5.10 (a) S T E M d a r k - f i e l d i m a g e o f p o r o u s a l u m i n a / M E H - P P V c o m p o s i t e , s h o w i n g the l i n e a l o n g w h i c h E E L spectra w e r e a c q u i r e d  130  Figure 5.11 C o m p a r i s o n o f l o w - l o s s spectra near pore axis a n d at ~ 7 n m from the pore w a l l , for porous a l u m i n a and porous a l u m i n a / M E H - P P V composite, respectively  131  Figure 5.12 C o m p a r i s o n o f l o w - l o s s spectra o f e m p t y a n d M E H - P P V - t r e a t e d p o r o u s a l u m i n a , nearest to pore w a l l  131  Figure 6.1 G e o m e t r y o f C h e r e n k o v radiation due to an electron t r a v e l l i n g (a) t h r o u g h a m e d i u m and (b) near a m e d i u m w i t h d i e l e c t r i c function e(co)  137  Figure 6.2 D i e l e c t r i c f u n c t i o n o f a l u m i n a 137 Figure 6.3 (a) S T E M d a r k - f i e l d i m a g e , s h o w i n g pore u s e d for E E L S analysis at 197 k e V (b) E E L S spectra a c q u i r e d over the pore diameter w i t h (3 = 0.34 m r a d  141  Figure 6.4 (a) S T E M d a r k - f i e l d i m a g e , s h o w i n g pore u s e d for E E L S analysis at 117 k e V (b) E E L S spectra a c q u i r e d over the pore diameter w i t h (3 = 1.5 m r a d  142  Figure 6.5 C o m p a r i s o n o f l o w - l o s s spectra at 197 a n d 117 k e V p r i m a r y b e a m energies: (a) a x i a l (5 = 0 n m ) a n d intermediate (s = 22 n m ) spectra, a n d (b) w a l l - g r a z i n g (s = 2 8 n m ) spectra  143  Figure 6.6 C o m p a r i s o n o f (a) e x p e r i m e n t a l electron b e a m w i t h convergence angle a a n d (b) theoretical m o d e l  145  Figure 6.7 T h e o r e t i c a l loss d i s t r i b u t i o n for 2 0 0 k e V electrons t r a v e l l i n g ( M o d e l A ) d o w n a single pore i n a l u m i n a , a n d ( M o d e l B ) d o w n a c y l i n d r i c a l pore o f outer r a d i i 6 1 , 9 4 a n d 127 n m . T h e inner pore r a d i u s is 2 9 n m  147  Figure 6.8 T h e o r e t i c a l loss d i s t r i b u t i o n for 2 0 0 k e V electrons t r a v e l l i n g d o w n a c y l i n d r i c a l hole w i t h 0, 6 a n d 12 n e i g h b o u r i n g pores ( M o d e l C ) , s h o w i n g the loss p r o b a b i l i t y for (a) a x i a l (5 = 0 n m ) a n d (b) n e a r - w a l l (s = 27 n m ) trajectories. T h e pore r a d i u s is 2 9 n m a n d the s p a c i n g i s 9 0 nm  148  Figure 6.9 Effect o f electron v e l o c i t y o n the loss p r o b a b i l i t y f u n c t i o n illustrated u s i n g M o d e l B .150  Figure 6.10 C o m p a r i s o n b e t w e e n theoretical a n d e x p e r i m e n t a l spectra at different i m p a c t parameters s: (a) 0 n m , (b) 2 2 n m , (c) 2 8 n m , u s i n g the 1+6 c y l i n d e r m o d e l (radius 2 9 n m , spacing 9 0 nm)  150  Figure 6.11 S T E M d a r k field images s h o w i n g geometry o f ordered p o r o u s a l u m i n a m e m b r a n e s p r o d u c e d b y a two-step a n o d i z a t i o n at 4 0 V i n 0.3 M o x a l i c a c i d : (a) 6 2 n m , (b) 7 4 n m , (c) 84 n m diameters; (d) l o w e r m a g n i f i c a t i o n i m a g e s h o w i n g size o f ordered d o m a i n s  152  Figure 6.12 C r o s s - s e c t i o n o f ordered p o r o u s a l u m i n a m e m b r a n e Figure 6.13 (a) C h e r e n k o v peak shift for a fixed lattice s p a c i n g (105 n m ) w i t h different  153  diameters: 6 2 n m , 74 n m , a n d 82 n m at 197 k e V a n d (o) 62 n m at 117 k e V ; (b) losses d o w n to 2 e V r e v e a l e d b y subtraction o f the z e r o - l o s s p e a k (diameter 62 n m , 197 k e V )  155  Figure 7.1 Surface-initiated a n i o n i c p o l y m e r i z a t i o n o f M E H - P P V  163  Figure Figure Figure Figure Figure  164  7.2 7.3 7.4 7.5  Step p o l y m e r i z a t i o n o f surface-grafted P P V b y the W H E r e a c t i o n P o r o u s s i l i c o n d e r i v a t i z a t i o n b y cathodic electrografting Protection o f aldehyde i n 4-ethynylbenzaldehyde. „ Synthetic route to conjugated d i m e r o n p o r o u s s i l i c o n surface  164 .'. 165 166  7.6 F T - I R spectra after (a) cathodic electrografting, (b) deprotection o f aldehyde a n d (c)  W H E r e a c t i o n o n porous s i l i c o n substrate  168  ix  Figure 7.7 P h o t o l u m i n e s c e n c e o f d e r i v a t i z e d porous s i l i c o n : excitation spectrum a n d e m i s s i o n s p e c t r u m o f W H E reaction product; e m i s s i o n o f deprotected aldehyde  168  Figure 7.8 C h e m i c a l structure o f p o l y [ 2 - m e t h o x y , 5 - ( 2 ' - e t h y l h e x y l o x y ) - l , 4 - p h e n y l e n e v i n y l e n e ) (MEH-PPV)  169  Figure 7.9 (a) V a c u u m - d r i v e n p o l y m e r infiltration into a p o r o u s m e m b r a n e , (b) S E M i m a g e o f p o l y m e r i n 2 0 0 n m pores o f A n o p o r e m e m b r a n e  171  Figure 7.10 C e n t r i f u g a l p o l y m e r l o a d i n g into a porous a l u m i n a f i l m  172  Figure 7.11 (a) Centrifuge rotor a s s e m b l y w i t h t w o substrate holders, (b) detail o f substrate h o l d e r f r o m above and (c) f r o m inside, s h o w i n g O - r i n g seal  173  Figure 7.12 S E M i m a g e o f cross-sections o f centrifuged samples, s h o w i n g (a) p o l y m e r over layer a n d (b) s o m e p o l y m e r penetration into pores  175  Figure 7.13 A n a l y s i s o f p o l y m e r penetration into porous a l u m i n a b y X P S a n d S E M 175 Figure 7.14 S E M images o f centrifuged samples, observed from b o t t o m w i t h the host etched away  :  Figure 7.15 X P S results from b o t t o m o f e m p t y porous a l u m i n a a n d centrifuged sample  176 177  Figure 7.16 T E M images o f t h i n sections o f (a) e m p t y porous a l u m i n a host a n d (b) centrifuged sample  ;  179  Figure 7.17 (a) T E M i m a g e a n d (b) E E L S o f t h i n section o f e m p t y porous a l u m i n a host: zeroloss peak, porous a l u m i n a before a n d after zero-loss p e a k subtraction, a l u m i n u m  180  Figure 7.18 U n f i l t e r e d ( T E M ) a n d energy-filtered (5, 2 5 e V ) images o f e m p t y p o r o u s a l u m i n a a n d centrifuged M E H - P P V / p o r o u s a l u m i n a c o m p o s i t e  182  Figure 7.19 (a) S T E M i m a g e o f t h i n section o f centrifuged sample. T h e contrast has b e e n increased i n the inset to s h o w the p o l y m e r tubules, (b) E E L S associated w i t h d r i l l i n g i n a l u m i n a a n d a p o l y m e r tubule  184  Figure 8.1 Ideal conjugated p o l y m e r d e v i c e c o m p o n e n t s and their assembly: (a) p o l y m e r insertion into host, (b) cathode evaporation a n d (c) anode d e p o s i t i o n  194  Figure 8.2 D e v i c e fabrication sequence from porous a l u m i n a f i l m o n a l u m i n u m f o i l : (a) a n o d i z a t i o n o f a l u m i n u m f o i l , (b) barrier layer t h i n n i n g b y potential r e d u c t i o n , (c) p o l y m e r i n t r o d u c t i o n b y centrifugation, (d) I T O d e p o s i t i o n b y R F sputtering, (e) contact lead b o n d i n g w i t h silver e p o x y a n d e p o x y e m b e d d i n g o f upper surface, (f) a l u m i n u m f o i l r e m o v a l b y c h e m i c a l etching, (g) a l u m i n u m cathode d e p o s i t i o n b y t h e r m a l evaporation. 196 Figure 8.3 S E M images o f porous a l u m i n a f i l m o n a n-type s i l i c o n wafer w i t h the barrier layer e l i m i n a t e d b y the potential r e d u c t i o n m e t h o d  197  Figure 8.4 S E M images o f porous a l u m i n a f i l m , s h o w i n g e l i m i n a t i o n o f barrier layer b y r a p i d potential r e d u c t i o n  197  Figure 8.5 D e v i c e fabrication sequence from an a l u m i n u m f i l m o n a c o n d u c t i v e substrate: (a) i n i t i a l a l u m i n u m f i l m o n substrate, (b) porous a l u m i n a film g r o w t h , (c) final porous a l u m i n a f i l m w i t h barrier layer r e m o v e d , (d) complete d e v i c e w i t h conjugated p o l y m e r s a n d w i c h e d between cathode a n d anode  198  Figure 8.6 S E M images o f h i l l o c k s o n porous a l u m i n a f i l m s prepared from R F sputtered a l u m i n u m f i l m s , s h o w i n g (a) top surface a n d (b) cross-section  199  Figure 8.7 Defects i n porous a l u m i n a films a n o d i z e d from electron-beam evaporated a l u m i n u m f i l m , s h o w i n g (a) S E M i m a g e o f cross-section o f a film a n o d i z e d at 20 ° C i n 1.2 M sulfuric a c i d a n d (b) T E M i m a g e i n p l a n v i e w o f a film a n o d i z e d at -39 ° C  201  Figure 8.8 S E M i m a g e o f porous a l u m i n a f i l m o n I T O - c o a t e d glass substrate, s h o w i n g an area w h e r e most o f the I T O w a s c o n s u m e d  202  List of Abbreviations and Symbols Abbreviation/Symbol Description 0  degree, unit of angle  a  pore radius, n m  a  b e a m convergence semi-angle  A  Angstrom, unit o f length, 1 0 " m  AMPEL  A d v a n c e d M a t e r i a l s and Process E n g i n e e r i n g Laboratories  P  spectrometer c o l l e c t i o n semi-angle  p  ratio o f particle speed to the speed o f light i n v a c u u m  BET  Brunauer-Emmett-Teller  BJH  Barret-Joyner-Halenda  c  speed o f light i n v a c u u m , 3.00 x 1 0 m s"  C  C o u l o m b , unit o f charge  °C  degrees C e l s i u s , unit o f temperature  CCD  charge-coupled d e v i c e  cm  centimetre, unit o f length, 10" m  d  pore s p a c i n g  dhki  interplanar s p a c i n g  D  pore diameter  DC  direct current  e  dielectric function  Ci  real part o f the dielectric function  £  i m a g i n a r y part o f the dielectric function  1 0  8  2  1  2  EDX  energy dispersive x-ray spectroscopy  EELS  electron energy  EFTEM  energy-filtered t r a n s m i s s i o n electron m i c r o s c o p y  e.g.  ( e x e m p l i gratia, L a t i n ) for e x a m p l e  et al.  (et a l i i , L a t i n ) and c o - w o r k e r s  etc.  (et cetera, L a t i n ) a n d others  eV  e l e c t r o n - V o l t , unit o f energy, 1.602x 10"  ex situ  ( L a t i n ) outside, external  loss spectroscopy  19  J  FIB  focused i o n b e a m  FT-IR  F o u r i e r transform infrared  FWHM  full w i d t h at h a l f - m a x i m u m  g  g r a m , unit o f m a s s  GIF  G a t a n i m a g i n g filter  h  hour, unit o f time, 3 6 0 0 s  i.e.  ( i d est, L a t i n ) as i n  in situ  ( L a t i n ) inside, internal  IR  infrared  ITO  Indium T i n Oxide  IUPAC  International U n i o n o f P u r e a n d A p p l i e d C h e m i s t r y  J  Joule, unit o f energy, k g m  K  degrees K e l v i n , unit o f temperature  keV  k i l o - e l e c t r o n - V o l t , 1.602x10"  kg  k i l o g r a m , unit o f mass, 1 0 g  A,  wavelength  L  pore s p a c i n g  LED  light-emitting device  m  metre, unit o f length  M  m o l a r , unit o f concentration, m o l L "  MCM  M o b i l C o m p o s i t i o n o f Matter  MEH-PPV  poly[2-methoxy,5-(2'-ethylhexyloxy)-1,4-phenylene vinylene]  min  m i n u t e , unit o f time, 60 s  mmol  millimol, 10" mol  mol  m o l e , constant o f v a l u e 6.02 x 1 0  mrad  m i l l i r a d i a n , 10" r a d  ms  m i l l i s e c o n d , unit o f t i m e , 1 0  N  n e w t o n , unit o f force, k g m s"  n-D  n-dimensional  nm  nanometre, unit o f length, 10~ metre  p  pressure  p  0  2  s"  2  J  16  3  1  3  2 3  3  s  3  2  9  standard pressure  xii  pH  h y d r o g e n i o n concentration o n l o g a r i t h m i c scale, - l o g i o [ H ]  PL  photoluminescence  NMR  nuclear m a g n e t i c resonance  PPV  p o l y ( 1,4-phenylene v i n y l e n e )  d  C h e r e n k o v angle  Q  m a g n i t u d e o f scattering vector  rad  radian, unit o f angle, % rad = 1 8 0 °  ref.  reference  RF  radio-frequency  RIE  reactive i o n e t c h i n g  RPM  revolutions per m i n u t e  s  second, base unit o f time  s  i m p a c t parameter, n m  SACT  small-angle cleavage technique  sat  saturated s o l u t i o n  SBA  Santa B a r b a r a  seem  surface c u b i c centimetre per m i n u t e , u n i t o f f l o w rate  SEM  s c a n n i n g electon m i c r o s c o p y  STEM  s c a n n i n g t r a n s m i s s i o n electron m i c r o s c o p y  STM  scanning tunnelling microscopy  t  thickness o f adsorbed layer  T  T e s l a , u n i t o f magnetic flux density  TBA  tetrabutyl a m m o n i u m  TEM  t r a n s m i s s i o n electron m i c r o s c o p y  TGA  t h e r m o g r a v i m e t r i c analysis  c  THF  +  tetrahydrofuran  Torr  unit o f pressure, 1 T o r r = 1 m m H g = 133 k g m " s"  UV/Vis  ultraviolet/visible  V  Volts, J C"  XPS  x-ray photoelectron spectroscopy  co  particle energy i n e l e c t r o n - V o l t s  wt.  weight  1  xiii  Acknowledgements I w o u l d l i k e to a c k n o w l e d g e foremost m y supervisor, Prof. M i k e W o l f , for a l l the help and support he p r o v i d e d d u r i n g the course o f this thesis. M y f e l l o w C a n a d i a n s have also supported m e generously through the N a t u r a l S c i e n c e s and E n g i n e e r i n g R e s e a r c h C o u n c i l . T h i s w o r k d r e w o n the expertise o f m a n y talented staff m e m b e r s , students and professors at U B C . a n d elsewhere.  A n u m b e r o f graduate students h e l p e d m y w o r k i n important w a y s . T h e  m e m b e r s o f the W o l f group w e r e generous and tolerant o f m y w a y s , and I w o u l d l i k e to thank D r . O l i v i e r C l o t a n d D r . C e r r i e R o g e r s especially for tutoring m e o n the finer points o f organic synthesis. D r . K a t j a R a d e m a c h e r , K e r i K w o n g , G l e n K u r o k a w a a n d J o s h E d e l also w o r k e d o n s i m i l a r projects and their c o l l a b o r a t i o n w a s v e r y useful. D a r r e n B r o u w e r and J i m S a w a d a from the Fyfe group h e l p e d w i t h the analysis o f m y porous materials. A m o n g the departmental staff, I w o u l d l i k e to thank e s p e c i a l l y the m e c h shop staff B r i a n S n a p k a u s k a s , C e d r i c N e a l , D e s L o v r i t y , R o n M a r w i c k , K e n L o v e a n d R a z v a n N e a g u - for their w o r k o n m y project a n d their  friendship.  B r i a n D i t c h b u r n p r o v i d e d c u s t o m glassware w i t h  superior helpfulness. A n d r e W o n g , from the F a c u l t y o f D e n t i s t r y , taught m e u l t r a m i c r o t o m y o f h a r d materials a n d h e l p e d w i t h the v e r y early electron m i c r o s c o p y w o r k . M a r y M a g e r , from M e t a l s a n d M a t e r i a l s E n g i n e e r i n g , p r o v i d e d s i m i l a r help w i t h s a m p l e preparation a n d electron m i c r o s c o p y . E l a i n e H u m p h r e y and Garnett H u y g e n s , from the B i o i m a g i n g facility, ran a great facility w i t h fabulous m i c r o s c o p e s and p r o v i d e d lots o f advice as w e l l . J e f f Y o u n g and M u r r a y W . M c C u t c h e o n , i n the P h y s i c s Department, c a r r i e d out the variable-temperature l u m i n e s c e n c e studies reported i n chapter 3. T h e most important results o f this thesis can a l l be traced b a c k to the expert help o f Prof. G i a n l u i g i B o t t o n , w i t h w h o m I started the w o r k on electron energy-loss spectroscopy. T h e i n i t i a l  xiv  help he p r o v i d e d p r o v e d i n v a l u a b l e for c o m p l e t i n g this w o r k . Further o n , R i c h a r d H u m p h r e y at the U n i v e r s i t y o f C a l g a r y h e l p e d w i t h measurements, a n d Prof. K a r e n K a v a n a g h at S i m o n Fraser U n i v e r s i t y gave access to her n e w m i c r o s c o p e for the most important measurements. T h e investigation into the C h e r e n k o v effect w a s h e l p e d i m m e n s e l y b y a collaboration w i t h P r o f s . A l b e r t o R i v a c o b a , F . Javier G a r c i a de A b a j o a n d N e r e a Z a b a l a at E u s k a l H e r r i k o U n i b e r s i t a t e a i n S p a i n . T h e i r theoretical insight into the p r o b l e m g u i d e d the investigation and established the s i g n i f i c a n c e o f the results. O n the fabrication side, A l i n a K u l p a , D o u g W o n g a n d J i m M a c k e n z i e assisted i n v a r i o u s w a y s . I p a r t i c u l a r l y appreciated the great help A l S c h m a l z p r o v i d e d , a n d his friendship j u s t as m u c h . D r . K e n W o n g a n d D r . P h i l i p W o n g c a r r i e d out a l l the X - r a y photoelectron  spectroscopy  o n m y samples. Lastly, I w o u l d l i k e to a c k n o w l e d g e the G r e e n C o l l e g e c o m m u n i t y for three f u l f i l l i n g (and thesis-delaying!) years, w h i c h have m a r k e d m e as m u c h as any experience i n m y life.  xv  CHAPTER 1  The  three  Introduction  challenges  of  materials  chemistry  are  synthesis,  characterization  and  understanding. A l l o f these have seen r a p i d progress i n the past decades, largely d u e to advances i n e x p e r i m e n t a l methods a n d i n s t r u m e n t i o n for investigating materials o n the nanometre scale. T h i s f o l l o w s f r o m the i m p o r t a n c e o f interactions o n this scale: the properties o f a l l materials, from atom clusters to the b u l k , whether h o m o g e n e o u s or not, are g o v e r n e d b y interactions on the nanometre  level. U n d e r s t a n d i n g and e x p l o i t i n g these interactions has thus b e c o m e centrally  important i n materials c h e m i s t r y . C o n s e q u e n t l y , the d e v e l o p m e n t o f n e w tools for structural characterization o n this scale is o f e q u a l importance. F o r crystalline materials, the tools for c o m p l e t e characterization o n the a t o m i c l e v e l have been available for a l o n g t i m e through X - r a y diffraction. Y e t this technique o n l y p r o v i d e s an average picture o f the material and m a y o v e r l o o k structural features w h i c h m a y material  function, such  as  interfaces  a n d defects.  Furthermore, amorphous  dominate  materials  and  materials w i t h structure o n a different scale are also e x c l u d e d from investigation b y diffraction. Structural analysis o f s u c h materials c a n o n l y be done by h i g h resolution m i c r o s c o p y , and this area r e m a i n s i n d e v e l o p m e n t for o r g a n i c materials. T h i s thesis describes the preparation o f c o m p o s i t e materials c o n s i s t i n g o f conjugated p o l y m e r s w i t h i n m e s o p o r o u s hosts. In this case, the material properties o f interest -  charge  transport  further  and l u m i n e s c e n c e i n the conjugated p o l y m e r -  c o u l d not be investigated  w i t h o u t first establishing the detailed structure o f the c o m p o s i t e . A s this structure v a r i e d o n the 1  nanometre scale, the characterization w a s not t r i v i a l a n d the d e v e l o p m e n t o f the appropriate techniques b e c a m e the focus o f this w o r k . In a larger context, this reflects the need for p r o p e r material characterization before property studies c a n be undertaken. T h e methods f o l l o w e d i n this w o r k s h o u l d be readily a p p l i c a b l e to other nanostructured systems w i t h conjugated p o l y m e r components. In this chapter, the properties o f conjugated  p o l y m e r s are i n t r o d u c e d a l o n g w i t h  a  d e s c r i p t i o n o f their m a i n a p p l i c a t i o n i n l i g h t - e m i t t i n g devices. T h e m o t i v a t i o n for i n t r o d u c i n g s u c h p o l y m e r s into porous host materials is d i s c u s s e d , a n d the literature o n this d e v e l o p i n g field is r e v i e w e d . F i n a l l y , the scope o f this thesis is presented a l o n g w i t h a short s u m m a r y o f each chapter.  1.1  Conjugated Polymers: Organic Conductors and Semiconductors T h e optical a n d electrical properties o f m o l e c u l a r crystals o f conjugated m o l e c u l e s have  been the subject o f study since the 1 9 4 0 ' s , w h i l e the study o f conjugated p o l y m e r s began i n the 1  1 9 7 0 ' s . O r g a n i c p o l y m e r s w i t h an electron system d e l o c a l i z e d o v e r the c o m p l e t e c h a i n are o f interest  for a n u m b e r o f reasons,  i n c l u d i n g the novelty o f charge transport  in polymers,  theoretical interest i n 1-D conductors, the useful m e c h a n i c a l properties o f p o l y m e r i c materials, and their s i m p l e a n d i n e x p e n s i v e p r o c e s s i n g . V e r y significant breakthroughs have o c c u r r e d i n the study o f conjugated p o l y m e r s o v e r the past 3 0 years. These p o l y m e r s have been used as the active m a t e r i a l i n a n u m b e r o f applications, including lasers. ' 1 1  sensors, ' 2  1 2  3  transistors, " 4  6  light-emitting devices, "  E l e c t r o a c t i v e p o l y m e r s are also p l a y i n g a r o l e  electronics. ' 1 4  1 5  7  1 3  9  photovoltaic d e v i c e s ,  1 0  and  i n the d e v e l o p m e n t o f m o l e c u l a r  Organic Molecules with Conjugated 7t-Electron Systems  1.1.1  In m a n y o r g a n i c m o l e c u l e s , interesting and useful electrical properties  arise  from  the  presence o f d e l o c a l i z e d 7t-electron systems. These systems are referred to as conjugated %electron systems. F r o m the organic c h e m i s t ' s point o f v i e w , s u c h a system is r e c o g n i z e d i n a c h e m i c a l structure as a sequence o f alternating single and double bonds between carbon atoms (Figure  1.1(a)). E l e c t r o n s o n other atoms,  s u c h as o x y g e n , sulfur and nitrogen, can  participate i n a conjugated system. F r o m the p h y s i c a l c h e m i s t ' s point o f v i e w , a conjugated  also n-  electron system can be defined as a set o f adjacent, parallel, half-filled p electron orbitals o n a m o l e c u l e ( F i g u r e 1.1(b)). In the  framework  o f m o l e c u l a r orbital theory, these p orbitals c o m b i n e  to f o r m m o l e c u l a r orbitals that are spread over the c h a i n o f interacting atoms ( F i g u r e 1.1(c)). A r o m a t i c m o l e c u l e s are a special case w h e r e 4 N + 2 conjugated 7i-electrons f o r m a r i n g , w h i c h imparts a d d i t i o n a l stability to the m o l e c u l e . T h e highest o c c u p i e d m o l e c u l a r orbital ( H O M O ) is a n b o n d i n g orbital, and the lowest u n o c c u p i e d m o l e c u l a r orbital ( L U M O ) is a n a n t i b o n d i n g (or 7t*) orbital.  In large  conjugated  electron systems, the b o n d i n g orbitals have a quasi-continuous set o f energy levels, w h i c h is referred to as the valence b a n d . A n a l o g o u s l y , the a n t i b o n d i n g orbitals f o r m the c o n d u c t i o n b a n d . M o s t importantly, the electrons i n the conjugated system are d e l o c a l i z e d over the extent o f the  Figure 1.1  B u t a d i e n e , a s i m p l e conjugated  m o l e c u l e : (a) c h e m i c a l structure,  (b) p - o r b i t a l s  f o r m i n g conjugated system, (c) lowest energy ( o f four) 7t m o l e c u l a r orbital. 3  conjugated bonds. A conjugated m o l e c u l e m a y therefore act as a p a t h w a y for charge transport i f a charge carrier is i n t r o d u c e d into the H O M O or L U M O . T h i s can be a c c o m p l i s h e d by either oxidizing  or  reducing  the  molecule,  either  chemically  (also  known  as  doping)  or  electrochemically.  1.1.2  Conjugated Molecules in the Solid State T h e electrical properties  energy  o f a material depend  largely o n the d i s t r i b u t i o n o f available  states a b o v e the highest o c c u p i e d state. T h i s is the energy difference between  the  c o n d u c t i o n b a n d a n d the valence b a n d , w h i c h determines the a m o u n t o f energy r e q u i r e d to p r o m o t e an electron f r o m the H O M O to the L U M O . T h e size o f the b a n d gap is used to classify a material as a conductor, s e m i c o n d u c t o r or insulator. In conjugated m o l e c u l e s , the b a n d gap is d e t e r m i n e d b y the size o f the conjugated  system, a n d i n most cases it falls i n the range o f  semiconductors. T h e electronic properties o f conjugated m o l e c u l e s i n the s o l i d state (i.e., as a material) are altered b y the effects o f interactions between n e i g h b o u r i n g m o l e c u l e s . E l e c t r o n i c processes, s u c h as charge transport, are then a c o m b i n a t i o n o f intra- and i n t e r m o l e c u l a r processes. F o r s m a l l m o l e c u l e s , i n t e r m o l e c u l a r processes are necessarily important. F o r larger m o l e c u l e s , i n particular p o l y m e r s , the relative importance o f these processes varies from m a t e r i a l to material.  1.1.3  Organic Conjugated Polymers S i n c e the 1 9 7 0 ' s , m o l e c u l e s w i t h l o n g extensions o f conjugated 71-electrons have been the  subject o f scientific pursuit, as a result o f interest i n their electrical a n d o p t i c a l properties. T h e concept o f o r g a n i c m o l e c u l e s as m o l e c u l a r w i r e s has seen significant d e v e l o p m e n t . ' 1 6  1 7  In  4  H  A  B  C  OR  F Figure  1.2  polyaniline,  G  C o n j u g a t e d p o l y m e r s : (a) polyacetylene, (b) p o l y p y r r o l e , (c) polythiophene, (e)  poly(l,4-phenylene),  (f)  poly(l,4-phenylene  vinylene) ( P P V ) ,  (g)  (d)  poly[2-  m e t h o x y , 5 - ( 2 ' - e t h y l h e x y l o x y ) - l , 4 - p h e n y l e n e v i n y l e n e ] ( R = M e , R ' = 2-ethylhexyl) ( M E H PPV). particular, p o l y m e r s w i t h conjugated u-electron systems e x t e n d i n g a l o n g the w h o l e length have been investigated. These are s e m i c o n d u c t o r s i n the pristine state. T h e simplest s u c h p o l y m e r , polyacetylene ( F i g u r e 1.2(a)), w a s reported to be a conductor w h e n d o p e d b y S h i r a k a w a , H e e g e r and M a c D i a r m i d in 1 9 7 7 .  1 8  T h e d o p e d p o l y m e r exhibits an electrical c o n d u c t i v i t y that c a n reach  m e t a l l i c levels (Table 1.1). T h e c o n d u c t i v i t y o f polyacetylene w a s an e n o r m o u s l y  important  scientific d i s c o v e r y that earned the N o b e l P r i z e for c h e m i s t r y i n 2 0 0 0 . T h e p h y s i c s associated w i t h the excited states o n the p o l y m e r c h a i n also p r o v e d to be very r i c h .  1 9  T h e i n c o r p o r a t i o n o f aromatic subunits (benzene, thiophene, p y r r o l e , etc. - see F i g u r e 1.2) a l l o w s c o n t r o l over the structure and properties o f the p o l y m e r . These aromatic subunits m a y be c h e m i c a l l y m o d i f i e d to adjust their electrical a n d o p t i c a l p r o p e r t i e s ,  20  and to impart  other 5  T a b l e 1.1 E l e c t r i c a l c o n d u c t i v i t y o f p o l y m e r s and s o m e c o m m o n materials. Conductivity  Polymer  Other Materials  d o p e d polyacetylene, or  copper, i r o n  doped polypyrrole,  graphite,  (S c m ' ) 1  10  5  1  doped silicon  p o l y t h i o p h e n e , etc. 10"  indium,  fr-arcs-polyacetylene  5  tin, s i l i c o n water  ew-polyacetylene 1 0  -io  diamond polythiophene, polypyrrole  lO" 1 0  -  nylon  1 5  quartz  Teflon  2 0  desirable properties s u c h as s o l u b i l i t y . T h e i n c o r p o r a t i o n o f metal centres into the p o l y m e r c h a i n is also a n area o f active research but falls outside the scope o f this thesis. In general,  the  i n c o r p o r a t i o n o f functional units that interact w i t h the conjugated 7r-system c a n be used to m a k e the c o n d u c t i v i t y sensitive to the presence o f external s t i m u l i . ' ' 2  3  6  T h e structures o f conjugated p o l y m e r s i n s o l u t i o n and i n the b u l k c a n v a r y significantly. O n short length scales, the c h a i n structure is usually planar for o p t i m a l conjugation. O n a longer scale, the p o l y m e r m a y deviate significantly from the i d e a l o f a m o l e c u l a r w i r e and exist i n a c o i l e d - u p c o n f i g u r a t i o n . T h i s structure is difficult to determine i n the s o l i d state but m a y be investigated i n solution b y d y n a m i c light-scattering m e a s u r e m e n t s . The  conductivity  of  environmental e x p o s u r e . ' 2 3  2 4  doped  polymers  was  found  to  22  degrade  significantly  through  A s u n d o p e d s e m i c o n d u c t o r s , they are s o m e w h a t m o r e stable a n d  this area has b e c o m e the m a i n focus o f w o r k i n the past decade.  6  1.2  Luminescence in Conjugated Molecules T h e process o f light e m i s s i o n b y a m o l e c u l e i n an excited state is termed l u m i n e s c e n c e ,  and m a n y conjugated m o l e c u l e s are h i g h l y luminescent. T h e excited state m a y be a s p i n singlet or a s p i n triplet, w h i l e the g r o u n d state is n o r m a l l y a singlet i n organic m o l e c u l e s . T h e transition from  the e x c i t e d state to the g r o u n d state c a n o c c u r t h r o u g h both radiative and non-radiative  processes. A radiative decay process i n w h i c h spin angular m o m e n t u m is c o n s e r v e d (also c a l l e d a spin-allowed  transition,  e.g.  between  two  singlet  states or t w o  triplet states) is  termed  fluorescence.  T h i s is a r a p i d process, w i t h a t y p i c a l lifetime o n the n a n o s e c o n d scale. A s p i n -  f o r b i d d e n radiative transition (e.g. from a triplet to a singlet state) is m u c h s l o w e r ( o n the m i c r o s e c o n d scale) i n organic m o l e c u l e s and is called phosphorescence. T h e singlet g r o u n d state causes fluorescence to be the n o r m a l e m i s s i o n process i n organic m o l e c u l e s . A  t y p i c a l non-radiative  decay process  takes the  m o l e c u l e to a v e r y h i g h l y excited  v i b r a t i o n a l level o f the g r o u n d state, and the excess v i b r a t i o n a l energy is dissipated eventually as heat. T h i s m e c h a n i s m  may  dominate  i f the  radiative p a t h w a y  is s l o w ; for this  reason,  phosphorescence is not usually observed i n organic conjugated m o l e c u l e s . T h e initial excited state can be generated i n a n u m b e r o f different w a y s , and this is used to distinguish  different  types  of  luminescence:  photoluminescence  from  optical  excitation,  electroluminescence from electrical excitation, and c h e m i l u m i n e s c e n c e from c h e m i c a l reaction. These m o d e s o f excitation m a y each be important for different applications; they m a y also differ significantly i n the n u m b e r o f singlet and triplet states that they initially create. F o r conjugated p o l y m e r s , the most important o f these processes is electroluminescence.  7  1.3  Electroluminescence in Conjugated Polymers Electroluminescent  materials  are  of  special  importance  in  display  technology.  E l e c t r o l u m i n e s c e n c e is d e s c r i b e d as the e m i s s i o n o f light f r o m condensed matter under action o f an electric  field.  molecules  It w a s  in  1987.  2 6  25  This phenomenon first  reported  w a s reported  for a conjugated  v i n y l e n e ) ( P P V ) , i n 1990 by F r i e n d and c o - w o r k e r s .  2 7  for s m a l l conjugated polymer,  the  organic  poly(l,4-phenylene  A s i m p l e p o l y m e r - b a s e d d e v i c e is s h o w n  i n F i g u r e 1.3: the active p o l y m e r layer is s a n d w i c h e d between  a m e t a l l i c cathode and  a  transparent anode. W h i l e the performance o f these devices has been c o n t i n o u s l y i m p r o v e d , the m a i n barrier to c o m m e r c i a l i z a t i o n has been d e v i c e d e g r a d a t i o n .  6  C o n j u g a t e d p o l y m e r s are v e r y susceptible to  p h o t o - o x i d a t i o n ; the o x i d i z e d m o l e c u l e s then serve as low-energy traps for electrons, w h i c h reduces the l u m i n e s c e n c e e f f i c i e n c y . ' 2 8  1.3.1  2 9  O t h e r routes to d e v i c e degradation also e x i s t . ' 3 0  3 1  Electroluminescence Processes in Conjugated Polymers T h e t e c h n o l o g i c a l importance o f electroluminescence has m a d e understanding the v a r i o u s  electronic  processes  in  these  devices  an  important  objective.  The  mechanism  electroluminescence i n p o l y m e r s has been w i d e l y investigated and r e v i e w e d i n d e t a i l . ' ' 9  1 9  2 5  of The  Cathode: aluminum or calcium Polymer: - 1 0 0 nm thick Anode: indium tin oxide on glass  Light emission  Figure 1.3 Structure o f a s i m p l e conjugated p o l y m e r - b a s e d electroluminescent d e v i c e .  8  Electric  field •  C o n d u c t i o n band  ^  >,  OA  u» O  Light  C  ^^^^ ,  m  \;\:  •  •  Valence band Anode  Polymer  Cathode  F i g u r e 1.4 E l e c t r o l u m i n e s c e n c e processes i n a s i m p l e p o l y m e r device. k e y processes, illustrated i n F i g u r e 1.4,  are charge injection, transport, r e c o m b i n a t i o n and de-  excitation. U n d e r the action o f the a p p l i e d electric field, the p o l y m e r is o x i d i z e d at the anode ( i n d i u m tin o x i d e ( I T O ) or g o l d ) , w h i c h introduces a positive charge by r e m o v i n g an electron from the H O M O (valence band). T h i s species is referred to as a hole. A t the cathode, the p o l y m e r is reduced, w h i c h places an electron into the L U M O ( c o n d u c t i o n band). T h e w o r k function o f the cathode material is chosen to m a t c h the p o l y m e r L U M O level as closely as possible; a l u m i n u m and c a l c i u m are used c o m m o n l y . I f there is substantial m i s m a t c h , the injection process does not p r o c e e d efficiently. It f o l l o w s that the devices d o not conduct u n d e r reverse bias. These charges migrate towards each other through the material due to the a p p l i e d  field  through a c o m b i n a t i o n o f intrachain and interchain transport. W h e n a hole and an electron meet w i t h i n the p o l y m e r , they m a y r e c o m b i n e to create either a singlet or triplet excited state. A s discussed  above,  electroluminescence  the  singlet  process.  The  state  will  triplet  usually  state  usually  decay decays  radiatively,  completing  non-radiatively;  it m a y  the be  9  converted to a singlet state t h r o u g h the a d d i t i o n o f a s e n s i t i z e r ; ' 3 2  also be converted to light through  phosphorescence. ' 3 4  3 3  the triplet state energy m a y  3 5  T h e e x c i t e d state m a y be l o c a l i z e d o n one p o l y m e r c h a i n or spread over adjacent chains. In the latter case, it has been f o u n d that the radiative lifetime is substantially l o n g e r , ' 2 2  3 6  w h i c h can  then favour non-radiative processes.  1.3.2  Electroluminescence Efficiency The power  efficiency o f an  electroluminescent  d e v i c e is v e r y important  for  many  applications, as it determines the brightness a n d p o w e r c o n s u m p t i o n o f the d e v i c e . Important progress  has  been  made  in  improving  this  efficiency  following  the  discovery  of  electroluminescence i n conjugated p o l y m e r s . M a n y factors affect the p o w e r efficiency, and here 9  o n l y the internal q u a n t u m efficiency is discussed. T h i s is defined as the n u m b e r o f photons created per injected e l e c t r o n . T h e o v e r a l l internal q u a n t u m efficiency is d e t e r m i n e d p r i n c i p a l l y 9  b y the p h o t o l u m i n e s c e n c e y i e l d and the singlet y i e l d . T h e p h o t o l u m i n e s c e n c e efficiency, w h i c h c a n be measured separately, places an upper l i m i t on electroluminescence efficiency. O p t i c a l excitation o f the p o l y m e r creates an excited state i d e n t i c a l to the o n e p r o d u c e d b y the charge r e c o m b i n a t i o n process. S i n c e o p t i c a l excitation creates o n l y singlet states, this a l l o w s the relative i m p o r t a n c e o f radiative and non-radiative processes to be investigated. T h e presence o f interchain interactions is important i n this respect, as an excited state w i t h a s l o w radiative decay c a n be m o r e susceptible to  non-radiative  q u e n c h i n g . In the d e v i c e , the h i g h e r m o b i l i t y o f holes relative to electrons i n the p o l y m e r causes r e c o m b i n a t i o n to o c c u r close to the cathode interface. T h i s interface is k n o w n to have m a n y chemical  defects  at  which  non-radiative  decay  can  occur, ' 3 7  3 8  which  reduces  the  p h o t o l u m i n e s c e n c e efficiency relative to the pristine material.  10  In electroluminescence,  the singlet y i e l d corresponds  to the fraction o f excited states  generated as singlets. T h e creation o f the non-radiative triplet states d u r i n g r e c o m b i n a t i o n reduces this y i e l d . B a s e d o n s i m p l e s p i n statistics, the singlet s p i n fraction is expected to be one quarter, and this w a s l o n g b e l i e v e d to be the l i m i t i n g factor o f organic devices. H o w e v e r , some recent e x p e r i m e n t a l " 3 9  4 2  and t h e o r e t i c a l ' 4 3  4 4  electroluminescent  results have indicated that  the probabilities o f singlet and triplet formation m a y not be the same, and that singlet creation is favoured i n p o l y m e r i c materials (as o p p o s e d to s m a l l m o l e c u l e s ) . H e n c e the singlet fraction m a y approach unity i n conjugated p o l y m e r s . M o r e sophisticated d e v i c e structures p r o v i d e higher efficiency: m u l t i - l a y e r devices used to eliminate the p r o b l e m o f higher hole m o b i l i t y i m p r o v e d b y o p t i m i z i n g the cathode m a t e r i a l ultrathin p o l y m e r l a y e r s .  4 7  4 6  a n d the injection processes can be  or by m o d i f i c a t i o n o f the anode interface b y  T h e c o m p l e x c h e m i s t r y o f evaporated metal contacts has l i m i t e d the  u n d e r s t a n d i n g o f processes at the cathode i n t e r f a c e .  1.3.3  4 5  are  46  An Ideal Device Structure T h e electronic processes i n conjugated p o l y m e r devices are difficult to study due to the  a m o r p h o u s nature o f the p o l y m e r film. T h e disorder i n these films gives rise to c o m p l i c a t e d 2a n d 3 - D p h e n o m e n a despite the 1-D nature o f the p o l y m e r c h a i n . It is then o f scientific interest to develop systems where interchain processes are e l i m i n a t e d , thereby a l l o w i n g the intrachain properties to dominate. T h i s amounts to electrical isolation o f i n d i v i d u a l p o l y m e r chains. It is clear that complete isolation o f each c h a i n w o u l d e l i m i n a t e a l l charge transport, and therefore a functional d e v i c e c a n o n l y be a c h i e v e d i f each p o l y m e r c h a i n is i n electrical contact w i t h both electrodes ( F i g u r e 1.5). A n ordered and oriented encapsulant is necessary to achieve s u c h a structure. A n ideal d e v i c e d e s i g n w o u l d also i n c l u d e c h e m i c a l l y w e l l - d e f i n e d interfaces to 11  < 5 nm  F i g u r e 1.5 Ideal d e v i c e structure c o n s i s t i n g o f oriented a n d isolated p o l y m e r chains.  the  cathode  and  anode,  such that the  charge  injection processes  could  be studied  more  r i g o u r o u s l y . T h i s area has already seen important progress through the study o f conjugated m o l e c u l e s self-assembled on metal i n t e r f a c e s . ' 1 7  4 8  T h e goal o f m e a s u r i n g single p o l y m e r c h a i n electrical properties thus serves to introduce the theme o f encapsulation. A list o f desirable properties m a y be generated based o n the p r e m i s e that the host material s h o u l d be present o n l y to induce the desired o r d e r i n g o f the conjugated p o l y m e r guest and otherwise not interfere w i t h measurements b e i n g made o n the guest ( T a b l e 1.2).  In practice, it must be r e c o g n i z e d that host-guest interactions cannot be entirely a v o i d e d ,  and m a y be difficult to account for.  *  12  1.4  Encapsulated Conjugated Polymers E n c a p s u l a t i o n can be generally defined as the preparation o f materials i n w h i c h there is a  reduced degree o f interaction between the guest m o l e c u l e a n d its surroundings. T h e r e are t w o fundamental m o t i v a t i o n s b e h i n d the g o a l o f p r o d u c i n g encapsulated conjugated p o l y m e r s . First, as d i s c u s s e d above, the study o f single p o l y m e r chains isolated i n the s o l i d state is expected to r e v e a l the fundamental p h o t o p h y s i c a l and e l e c t r i c a l b e h a v i o u r o f the conjugated p o l y m e r c h a i n , without  any effects  due  to aggregation  with  other  polymer chains.  4 9  Second, a  suitable  encapsulant w o u l d p r o v i d e protection against e n v i r o n m e n t a l agents, m o s t importantly o x y g e n , w h i c h w o u l d a l l o w d e v i c e s to operate for longer periods o f t i m e under a m b i e n t c o n d i t i o n s . In this context, encapsulation i m p l i e s the r e d u c t i o n o f c h e m i c a l a n d electrical interactions o f the p o l y m e r chains.  1.4.1  Approaches to Encapsulation T h e p h o t o p h y s i c s o f isolated chains can be studied b y d i s p e r s i n g the p o l y m e r i n an inert  Table 1.2 Properties o f the ideal host m a t e r i a l for conjugated p o l y m e r s . Basis  Property 1. A l i g n e d channels n o r m a l to the substrate  A l l o w g o o d electrical transport  2. N a r r o w pores (< 5 n m )  M i n i m i z e n u m b e r o f p o l y m e r chains per pore, ideally o n l y one per pore (guaranteed i f diameter is < 2 n m )  3. O p t i c a l transparency  A l l o w o p t i c a l characterization o f p o l y m e r guest M e a s u r e o n l y guest electrical properties,  4. E l e c t r i c a l insulator  M i n i m i z e interchain processes 5. C h e m i c a l l y inert  M i n i m i z e interactions w i t h the p o l y m e r guest  6. E a s i l y prepared as t h i n film  A l l o w l i g h t - e m i t t i n g d e v i c e fabrication  (controlled thickness, defect-free) 7. G o o d "analytical contrast" polymer m a t r i x . ' 5 0  5 1  Ease o f characterization  H o w e v e r , this a p p r o a c h does not p r e c l u d e effects due to interactions a m o n g 13  different segments o f the same c h a i n , i f the p o l y m e r is i n a c o i l e d configuration. P o s s i b l e phase segregation o f the p o l y m e r is also an issue i n these materials, and cannot be e x c l u d e d w i t h o u t detailed m i c r o s t r u c t u r a l analysis o f the dispersed p o l y m e r . S i m i l a r l y , dilute p o l y m e r solutions c a n be studied. In this w a y , the intrachain hole m o b i l i t y has been measured for isolated p o l y m e r chains  5 2  and  determined.  the  effect  o f interchain  interactions  on  photophysical behaviour  has  been  22  These approaches to encapsulation are useful but afford a degree o f encapsulation w h i c h prevents direct electrical measurements on the isolated chains. F u r t h e r m o r e , a degree o f disorder r e m a i n s o n the level o f the p o l y m e r c h a i n c o n f o r m a t i o n . F o r this reason, n o v e l approaches to encapsulation  are  desirable  and  one  c a n l o o k to ordered  porous  materials. M a n y  o f the  requirements o f T a b l e 1.2 c a n be satisfied b y porous materials based o n s i l i c a a n d a l u m i n a .  1.4.2  Ordered Porous Materials as Encapsulants T h e synthesis o f conjugated p o l y m e r s w i t h i n ordered host materials has been r e v i e w e d  extensively i n recent y e a r s , ' ' 2 1  5 3  5 4  and a b r i e f o v e r v i e w is g i v e n here. Further details o n v a r i o u s  host materials are g i v e n i n chapters 3 and 4 . In the late 1 9 8 0 ' s , the initial w o r k o n the synthesis o f polyacetylene w i t h i n the channels o f zeolites (crystalline, m i c r o p o r o u s a l u m i n o s i l i c a t e materials) w a s carried out b y B e i n and E n z e l . T h e y incorporated p o l y p y r r o l e ,  4 9  polythiophene,  55  and p o l y a n i l i n e  5 6  into different zeolites. T h i s  w o r k w a s then extended to p r o d u c e conjugated p o l y m e r s w i t h i n the channels o f a m e s o p o r o u s silica  material. "  conjugated  5 7  These  5 9  polymers  indicated that the  in  a  results  p r o v i d e d the  confined  first  environment:  i n d i c a t i o n s about  microwave  the  conductivity  c o n f i n e d p o l y m e r s c o u l d be m o r e c o n d u c t i v e than  i n the  behaviour  of  measurements bulk.  6 0  The  14  composite  materials w e r e  in powder form,  which  p r e c l u d e d a c h i e v i n g any m a c r o s c o p i c  orientation o f the samples. M a r t i n e t al. d e v e l o p e d oriented, encapsulated materials based o n p o r o u s a l u m i n a a n d track-etch m e m b r a n e s . " 6 1  6 3  T h e pore diameters i n these m e m b r a n e s were i n a l l cases larger than  10 n m , w h i c h a l l o w e d for c o n v e n i e n t preparation o f p o l y m e r m i c r o t u b u l e s but d i d not a l l o w any p o l y m e r confinement effects to be o b s e r v e d . Substantial progress towards the goal o f encapsulating conjugated p o l y m e r chains into oriented hosts w a s reported i n the literature d u r i n g the course o f this thesis. T h e w o r k b y the groups o f T o l b e r t a n d S c h w a r z at the U n i v e r s i t y o f C a l i f o r n i a , Santa B a r b a r a " 6 4  b y the group o f G i n at the U n i v e r s i t y o f C a l i f o r n i a , B e r k e l e y " 6 9  d e v e l o p m e n t s i n this  field.  7 5  6 8  and the w o r k  represent the m o s t significant  T h e y prepared host-guest systems w h e r e the conjugated p o l y m e r  guest w a s encapsulated i n v e r y n a r r o w channels, a n d their evidence suggested that interactions between different p o l y m e r chains were e l i m i n a t e d . T h i s w a s a c c o m p l i s h e d t h r o u g h investigation o f the  t i m e - r e s o l v e d o p t i c a l properties  o f the  c o m p o s i t e material. H o w e v e r , the  goal o f  m e a s u r i n g the electrical properties o f single conjugated p o l y m e r c h a i n s d i r e c t l y has not been a c h i e v e d to date; the nearest result describes the hole m o b i l i t y on single conjugated p o l y m e r chains i n s o l u t i o n .  5 2  These a n d other relevant results are r e v i e w e d here to p r o v i d e a perspective o n the field o f encapsulated conjugated p o l y m e r s .  1.4.3 Oriented Mesoporous Silica Encapsulant  15  T h e use o f a large external magnetic field to a l i g n the channels o f a mesoporous  s i l i c a m a t e r i a l w a s pioneered by F i r o u z i e t a l .  1  6  surfactant-templated  T h i s m e t h o d m a y be used to  p r o d u c e s o l i d samples w i t h an o v e r a l l c h a n n e l orientation parallel to the a p p l i e d magnetic w i t h a lattice s p a c i n g o f 3.5 n m and a diameter o f 2.2 n m . PPV  derivative,  7 7  field,  It w a s then s h o w n that a soluble  poly[2-methoxy,5-(2'-ethylhexyloxy)-l,4-phenylene  vinylene]  (MEH-PPV,  F i g u r e 1.2(g)), c o u l d be introduced into the channels o f s u c h a host f o l l o w i n g proper c h e m i c a l functionalization o f the c h a n n e l surfaces (diameter reduced to ~ 1.7 n m ) .  6 4  The polymer loading  was s i m p l y a c c o m p l i s h e d by p l a c i n g the host i n a heated p o l y m e r solution for some t i m e ( F i g u r e 1.6), then f o l l o w i n g an o p t i m i z e d solvent w a s h i n g sequence to m a x i m i z e r e m o v a l o f external p o l y m e r . T h e initial evidence for p o l y m e r i n c o r p o r a t i o n w a s based on measurements o f the fluorescence p o l a r i z a t i o n . It w a s r e c o g n i z e d that at least some unencapsulated  polymer was  present i n larger cavities i n the host material, but its effect c o u l d be m i n i m i z e d through selective o x i d a t i o n . It was further argued o n geometrical considerations (the p o l y m e r p a c k i n g radius is 0.8 - 0.9 n m i n the solid state) that o n l y one p o l y m e r c h a i n c o u l d be present i n each pore. Investigation  o f the  photoluminescence  polarization  dynamics  indicated  that  initial  excitation energy l o c a l i z e d o n the unencapsulated p o l y m e r m i g r a t e d q u i c k l y to the lower-energy  interacting polymer chains  isolated polymer chains F i g u r e 1.6 Incorporation o f M E H - P P V into oriented mesoporous silica.  16  encapsulated c h a i n s . 6 5  6 6  Further energy m i g r a t i o n a l o n g the encapsulated c h a i n w a s a s l o w e r  process. T h e measurement o f transient absorption d y n a m i c s i n the femtosecond r e g i m e a l l o w e d the encapsulated p o l y m e r to be d i s t i n g u i s h e d v e r y clearly, based o n c o m p a r i s o n s w i t h  MEH-  P P V i n dilute s o l u t i o n and as a t h i n f i l m . O v e r a l l , these results p r o v i d e d v e r y c o m p e l l i n g arguments for the presence o f isolated M E H - P P V chains i n the pores o f the s i l i c a host. T h e single chains were not observed directly i n the host channels, but the p h o t o p h y s i c a l b e h a v i o u r o f the c o m p o s i t e indicated that c h a i n isolation had been a c h i e v e d . T h e determination o f the m i c r o w a v e c o n d u c t i v i t y o f the sample, w h i c h w o u l d g i v e a value for the c o n d u c t i v i t y o f a single conjugated p o l y m e r c h a i n , was reportedly under w a y  1.4.4  6 4  but has not yet been p u b l i s h e d .  Liquid Crystal Encapsulant T h e use o f l y o t r o p i c l i q u i d crystals as a host m a t e r i a l was p u r s u e d b y S m i t h e t a l , u s i n g a  p o l y m e r i z a b l e m e s o g e n to f o r m an inverse h e x a g o n a l m a t r i x ( F i g u r e 1 . 7 ) .  69  T h e y initially used a  water-soluble P P V precursor ( F i g u r e 1.8), w h i c h w a s expected to segregate into the aqueous phase o f the l i q u i d crystal. C r o s s - l i n k i n g o f the m a t r i x s t a b i l i z e d the l i q u i d crystal host, after w h i c h the P P V precursor c o u l d be converted to the final f o r m b y heating under v a c u u m . T h e interpore distance w a s 4.0 n m and the pore diameter ~1.5 n m . T h e resulting c o m p o s i t e s h o w e d a blue-shifted p h o t o l u m i n e s c e n c e spectrum relative to b u l k P P V . T h e r e w a s a large increase i n the p h o t o l u m i n e s c e n c e intensity, and the absolute p h o t o l u m i n e s c e n c e efficiency w a s reported 3 0 % , substantially larger than for other samples i n the same study ( 5 - 2 0 % ) other values reported for b u l k P P V ( 2 7 % ) .  7 3  as  but c o m p a r a b l e to  7 8  17  F i g u r e 1.8 High-temperature c o n v e r s i o n o f a water-soluble precursor p o l y m e r to P P V . T h e p h o t o i n d u c e d absorption spectrum was also shifted to higher energy and s h o w e d an excitation  intensity  dependence  consistent  Photoluminescence-detected  magnetic  material  to  behaved  femtosecond  similarly  photoluminescence  with  resonance  conjugated  interchain  measurements  p o l y m e r dispersed  d y n a m i c s were  not  energy  transfer  inhibition.  s h o w e d that the i n an  inert  investigated, though  7 4  composite  matrix.  7 2  this w o u l d  p r o v i d e d the best evidence for p o l y m e r c h a i n i s o l a t i o n . T h e difficulty w i t h a l l the  The have above  c o m p a r i s o n s is that it is not k n o w n whether the p o l y m e r precursor c o n v e r s i o n step proceeds in the c o m p o s i t e i n the same m a n n e r as i n the b u l k .  F i g u r e 1.7 (a) Structure o f l i q u i d crystal m e s o g e n , (b) structure o f lyotropic l i q u i d crystal w i t h p o l y m e r guest.  18  Subsequent  work with  a water-soluble P P V d e r i v a t i v e  7 9  ( F i g u r e 1.9)  allowed more  definitive experiments: the p o l y m e r c o u l d be extracted from the c o m p o s i t e after synthesis and the p h o t o p h y s i c a l properties o f the encapsulated and free p o l y m e r chains c o u l d be c o m p a r e d directly.  7 5  T h i s s h o w e d that the local dielectric e n v i r o n m e n t o f the m a t r i x , w h i c h is h i g h l y polar  due to the carboxylate groups o f the m e s o g e n , had little effect on the observed differences. It w a s also argued (without direct evidence, due to l o w p o l y m e r content i n the c o m p o s i t e ) that the effective conjugation length o f the p o l y m e r w a s unaffected b y the encapsulation process. T h i s left the isolation o f the p o l y m e r chains as the o n l y r e m a i n i n g factor responsible for the observed differences. H o w e v e r , it is not clear whether the b e h a v i o u r o f this P P V derivative c a n be g e n e r a l i z e d to P P V itself. T h e c o m p o s i t e material c o u l d also be prepared as an oriented reportedly r u n n i n g p e r p e n d i c u l a r to the s u b s t r a t e ,  71  film,  w i t h the  channels  by p r e s s i n g the l i q u i d crystal between t w o  glass plates. T h e alignment w a s i n d u c e d by the interactions at the g l a s s / l i q u i d crystal interface. H o w e v e r , n o direct evidence o f this alignment w a s p r o v i d e d . W i t h such an oriented film, it w a s c l a i m e d that a nearly ideal d e v i c e structure was p r o d u c e d . T h e electrical properties o f this d e v i c e structure w e r e investigated, and it w a s f o u n d that both the p o l y m e r and the stacked benzene rings o f the m a t r i x conducted electricity. E l e c t r o l u m i n e s c e n c e w a s detected  a n d it w a s found to  originate from both the m a t r i x and the p o l y m e r . T h u s the electroluminescence b e h a v i o u r o f the isolated chains c o u l d not be investigated by this approach.  19  COO"Na COO"Na +  F i g u r e 1.9 W a t e r - s o l u b l e P P V d e r i v a t i v e .  1.4.5  +  79  Clay Encapsulant Recently, O . O . P a r k and c o - w o r k e r s reported the successful intercalation o f M E H - P P V  between the layers o f an organoclay material ( F i g u r e 1 . 1 0 ) . ' 8 0  8 1  The incorporation o f some  p o l y m e r w a s e v i d e n c e d by an increase i n the s p a c i n g o f the clay layers, as determined b y X - r a y diffraction. N o further details o f the structure o f the c o m p o s i t e w e r e reported. W h i l e the layered structure o f the organoclay does not a l l o w full isolation o f the p o l y m e r chains, the e x p e r i m e n t a l results seemed to indicate that there w a s a large enhancement o f p h o t o l u m i n e s c e n c e intensity (18x;  absolute efficiencies were not reported). T h e absolute p h o t o l u m i n e s c e n c e efficiency o f  MEH-PPV  is k n o w n to be  10-15%;  7 8  therefore  the  c l a i m e d increase m a y be  considered  s o m e w h a t suspect. E l e c t r o l u m i n e s c e n t devices c o u l d be fabricated u s i n g this c o m p o s i t e material as the active  F i g u r e 1.10 Preparation o f M E H - P P V / C l a y c o m p o s i t e material.  20  layer between I T O and a l u m i n u m electrodes. T h e electroluminesence efficiency o f these devices w a s enhanced. T h i s increase w a s attributed to the confinement o f the charged species. T h e hole m o b i l i t y i n the c o m p o s i t e w a s also r e d u c e d relative to b u l k M E H - P P V , w h i c h m a y have led to a better b a l a n c i n g o f the hole and electron d i s t r i b u t i o n . S u r p r i s i n g l y , the d e v i c e operated equally w e l l i n reverse bias, i n d i c a t i n g that the c h o i c e o f electrode material w a s not affecting d e v i c e operation. H o w e v e r , this fact w a s not d i s c u s s e d i n any detail. C l e a r l y , this material e x h i b i t e d s o m e v e r y n o v e l behaviour, but the l i m i t e d characterization d i d not a l l o w the o r i g i n o f this b e h a v i o u r to be identified. S o m e further w o r k has been carried out o n the i n c o r p o r a t i o n o f P P V into layered host m a t e r i a l s . ' 8 2  1.4.6  8 3  Cyclodextrin Encapsulant A n d e r s o n et al.  have  s h o w n that l u m i n e s c e n t conjugated  c y c l o d e x t r i n have substantially enhanced p h o t o s t a b i l i t y . ' 8 4  8 5  m o l e c u l e s encapsulated  in  T h e y have extended this approach  to p o l y m e r s b y s y n t h e s i z i n g water-soluble conjugated p o l y m e r chains threaded inside stacked Pc y c l o d e x t r i n rings ( F i g u r e 1 . 1 1 ) .  84  Sufficiently large c a p p i n g groups at the end o f the p o l y m e r  c h a i n prevent dethreading. T h i s produces p o l y m e r chains w h i c h are very tightly encapsulated. Direct  investigation  o f thin  film  morphology  by  atomic  force  microscopy allowed  the  encapsulated p o l y m e r strands to be r e s o l v e d , w h i c h indicated that there w a s substantial r e d u c t i o n in interactions between chains. T h e absolute p h o t o l u m i n e s c e n c e efficiency o f the p o l y m e r c h a i n was s h o w n to i n c r e a s e  86  b y 3 - 4 x but the photostability o f these p o l y m e r s w a s not discussed.  T h e encapsulation p r o v i d e d b y these m a c r o c y c l e s is not complete, w h i c h still a l l o w s the i n t e r m o l e c u l a r charge transport necessary for d e v i c e o p e r a t i o n .  86  E n c a p s u l a t i o n w a s s h o w n to  increase the electroluminescence efficiency by 2 - 5 x . A g a i n d e v i c e stability w a s not discussed.  21  HO  F i g u r e 1.11  E n c a p s u l a t i o n o f conjugated p o l y m e r c h a i n w i t h p - c y c l o d e x t r i n .  8 6  T h i s approach to encapsulation appears v e r y p r o m i s i n g and w i l l h o p e f u l l y be subject to further study. T h e current results clearly indicate that this a p p r o a c h is useful but have not y i e l d e d any further insight into the electroluminescence processes themselves. A s noted i n the literature, one l i m i t a t i o n o f this a p p r o a c h is the m o b i l i t y o f the m a c r o c y c l e a l o n g the p o l y m e r c h a i n , w h i c h c a n cause a n o n - u n i f o r m d i s t r i b u t i o n o f the e n c a p s u l a n t . Short,  encapsulated  oligomers  prepared  86  i n this  fashion  c o u l d also be  grafted  to  a  c o n d u c t i n g surface, a n d s u c h a configuration w o u l d be i d e a l for electrical measurements on single p o l y m e r chains. T h i s has not b e e n reported to date.  22  1.4.7  Literature Summary The work of Nguyen  et al.  65  p r o v i d e d the clearest results on the properties o n encapsulated  conjugated p o l y m e r s . T h e results o n l i q u i d crystal e n c a p s u l a t i o n  75  w e r e important but further  investigation o f the encapsulated p o l y m e r w o u l d be needed to s h o w the effect o f c h a i n isolation o n electronic excitations. T h e w o r k o n clay e n c a p s u l a t i o n ' 8 0  w a s interesting but d i d not p r o v i d e m u c h  8 1  more  understanding o f the electronic processes i n conjugated p o l y m e r s . T h e w o r k o n P-cyclodextrin encapsulation  86  is important because a functional d e v i c e w a s demonstrated, and further insight  m i g h t be gained f r o m p h o t o p h y s i c a l studies. These w o r k s illustrate c o l l e c t i v e l y that encapsulated conjugated p o l y m e r s e x h i b i t n o v e l behaviour,  i n particular  enhanced  luminescence  efficiencies. H o w e v e r , the  cause o f this  b e h a v i o u r is difficult to identify w i t h o u t a detailed analysis o f the nanostructure o f the material — a n d incorrect c o n c l u s i o n s m i g h t be d r a w n i f s u c h analysis is neglected. It is also clear that the ideal d e v i c e structure, c o n s i s t i n g o f a l i g n e d and isolated conjugated p o l y m e r chains, remains to be reported.  1.5  Thesis Summary T h i s thesis describes the synthesis and characterization o f conjugated p o l y m e r guests i n  m e s o p o r o u s host materials. T h e p r i n c i p a l m o t i v a t i o n o f the w o r k w a s to create materials a n d device  structures  conjugated  w h i c h w o u l d further  the  understanding  o f electronic  processes  p o l y m e r s . H o w e v e r , it was r e c o g n i z e d early o n that little understanding  within c a n be  gained from materials w h i c h are not fully characterized. A change i n material properties cannot be rationalized i f the structure o f the material is not k n o w n i n detail, w h i c h is w e l l illustrated b y the w o r k o n M E H - P P V / c l a y composites discussed above. T h i s realization g u i d e d this w o r k 23  towards the a p p l i c a t i o n o f h i g h resolution characterization techniques to conjugated p o l y m e r c o m p o s i t e materials. T h e u n i f y i n g theme o f the final w o r k is then characterization o f s u c h c o m p o s i t e materials o n the nanometre scale. In chapter 2 , the m a i n characterization techniques used for analysis o f c o m p o s i t e materials are r e v i e w e d , w i t h an e m p h a s i s o n electron m i c r o s c o p y techniques. T h e most important o f these is  electron  energy-loss  spectroscopy  (EELS)  and  energy-filtered  transmission  electron  m i c r o s c o p y ( E F T E M ) . T h e a p p l i c a t i o n o f E E L S to the analysis o f o r g a n i c materials is r e v i e w e d . A general i n t r o d u c t i o n to ordered porous host materials is presented i n chapter 3, and the literature o n introduction o f p o l y m e r s into such hosts is r e v i e w e d . T h i s is f o l l o w e d by a study o f the i n c o r p o r a t i o n o f P P V into a m e s o p o r o u s s i l i c a c o m p o s i t e material. C h e m i c a l analysis o f the c o m p o s i t e material w i t h nanometre resolution b y E E L S and E F T E M is used to s h o w directly that the conjugated p o l y m e r is present i n s i d e the pores o f the host. C h a p t e r 4 is devoted to the preparation o f oriented porous thin films towards the g o a l o f p r e p a r i n g an ideal electroluminescent d e v i c e structure. P o r o u s a l u m i n a m e m b r a n e s prepared at l o w temperatures are presented as v e r y g o o d candidates for p r e p a r i n g s u c h a structure. The  initial  analysis  o f conjugated  polymer/porous  alumina  composite  materials  is  described i n chapter 5. T h e E E L S and E F T E M measurements w e r e carried out parallel to the pores o f the host. T h e analysis w a s h i n d e r e d b y spectral features due to surface effects i n E E L S . F u r t h e r m o r e , the samples s h o w e d a further loss at a large distance from the sample surface that c o u l d not be associated w i t h a surface p l a s m o n . C h a p t e r 6 deals w i t h this unexpected loss at large distance from the sample surface. T h e C h e r e n k o v effect  is identified as the source o f this spectral feature.  T h e e x p e r i m e n t a l and  theoretical results s h o w that the spectroscopy o f the samples o n the l o c a l scale w a s affected b y the large scale structure o f the sample, due to the radiative nature o f the C h e r e n k o v effect.  24  C h a p t e r 7 discusses further w a y s o f p r e p a r i n g p o l y m e r / p o r o u s a l u m i n a f i l m  composites  and their characterization. Initial w o r k i n the d i r e c t i o n o f p r e p a r i n g conjugated o l i g o m e r s grafted to a s i l i c o n surface is d e s c r i b e d . T h e use o f different d r i v i n g forces for c o m p o s i t e preparation is d i s c u s s e d , and the centrifugal force is investigated i n detail. T h i n sections o f the samples are investigated b y E E L S a n d E F T E M . L o w - e n e r g y losses i n the a l u m i n a host at 2 0 0 k V are found to interfere w i t h the p o l y m e r - s p e c i f i c losses. 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L . ; Yonezawa, K .  Synth. Met. 2 0 0 1 , 1 2 1 ,  1291.  7 6 . F i r o u z i , A . ; Schaefer, D . J . ; Tolbert, S. TL; S t u c k y , G . D . ; C h m e l k a , B . F .  1997,119,  J. Am. Chem. Soc.  9466-9477.  7 7 . Tolbert, S. H . ; F i r o u z i , A . ; S t u c k y , G . D . ; C h m e l k a , B . F .  Science 1997, 278,  264-268.  7 8 . G r e e n h a m , N . C ; S a m u e l , I. D . W . ; H a y e s , G . R . ; P h i l l i p s , R . T . ; Kessener, Y . A . R . R . ; M o r a t t i , S. C ; H o l m e s , A . B . ; F r i e n d , R . H . 79. Wagaman, M . W . ; Grubbs, R . H . 80. Lee, T. W . ; Park, O . O . ; K i m , 81.  1995,  Macromolecules 1997, 30,  J. J.; H o n g , J. M . ; K i m ,  Lee, T. W . ; Park, O . O . ; Y o o n , J.; K i m ,  82. Lee, H . C ; Lee, T. W . ; Park, O . O . 83.  Chem. Phys. Lett.  J. J.  Y . C.  Synth. Met.  Opt. Mater.  L e e , H . C ; L e e , T . W . ; L i m , Y . T . ; Park, O . O .  89-96.  3978.  Chem. Mater. 2001,13,  2001,  2003, 21,  241,  121,  2217.  1737.  187.  Appl. Clay Sci.  2002, 21,  287.  84. T a y l o r , P . N . ; O ' C o n n e l l , M . J . ; M c N e i l l , L . A . ; H a l l , M . J . ; A p l i n , R . T . ; A n d e r s o n , H . L .  Angew. 85.  Chem. Int. Ed. Engl.  2000, 39,  608.  Craig, M . R.; Hutchings, M . G . ; Claridge, T. D . W . ; Anderson, H . L .  Engl.  2002, 41,  Angew. Chem. Int. Ed.  1769-1772.  86. C a c i a l l i , F . ; W i l s o n , J . S.; M i c h e l s , J . J . ; D a n i e l , C ; S i l v a , C ; F r i e n d , R . H . ; S e v e r i n , N . ; Samori, P.; Rabe, J. P.; O'Connell, M . J.; Taylor, P . N . ; Anderson, H . L .  Nature Mater. 2002,1,  160-164.  30  CHAPTER 2 Characterization of Nanocomposite Materials  T h e c h e m i c a l analysis o f a p e r i o d i c structures at h i g h spatial resolution is p r o v i n g to be a central challenge o f nanoscience.  A s n e w methods are d e v i s e d to assemble  materials  into  structures o n the nanometre scale, it is b e c o m i n g m o r e important to s h o w b y means as direct as possible that the desired p h y s i c a l and c h e m i c a l structures are obtained. W i t h the advent o f field-emission tunnelling  electron m i c r o s c o p e s , the a t o m i c force m i c r o s c o p e ( A F M ) a n d the microscope  (STM),  topographical  characterization  at  nanometre  scanning-  resolution  has  b e c o m e a relatively routine experiment. H o w e v e r , c h e m i c a l analysis at the nanometre scale is still far from r e a c h i n g the same level o f s i m p l i c i t y , and there are instrumental l i m i t s on the achievable r e s o l u t i o n .  1  T h e development o f characterization techniques for c o m p o s i t e materials based o n p o r o u s i n o r g a n i c hosts was an o n g o i n g challenge w i t h i n our research group. T h e central g o a l w a s to establish the p o l y m e r d i s t r i b u t i o n w i t h i n the c o m p o s i t e material o n the nanometre scale. Optical  techniques  for  characterization  are  pushed  to  their  present-day  limit  by  c o n f o c a l fluorescence m i c r o s c o p y , w h i c h is a w i d e l y used t o o l for establishing the d i s t r i b u t i o n o f fluorescent  materials w i t h 0.1  u.m resolution i n 3 - D . T h i s c a n be readily used for  many  conjugated p o l y m e r s i n o p t i c a l l y transparent hosts. A recently d e v e l o p e d technique, near-field scanning optical m i c r o s c o p y ,  2  c a n be used to m a p  resolution. B u t for w o r k at resolution better than  fluorescence  i n 2 - D w i t h even  higher  10 n m , electron m i c r o s c o p y is used  for  31  t o p o g r a p h i c a l a n d c h e m i c a l analysis. T h i s is i n t r o d u c e d b e l o w a n d is f o l l o w e d b y a r e v i e w o f h i g h resolution c h e m i c a l analysis techniques. T w o forms o f electron m i c r o s c o p y are most relevant to current research i n materials science: s c a n n i n g electron m i c r o s c o p y ( S E M ) a n d t r a n s m i s s i o n electron m i c r o s c o p y ( T E M ) . S E M is a m o r e w i d e l y used technique due to its e v e r - i m p r o v i n g s i m p l i c i t y , w h i l e T E M is often necessary w h e n details i n the nanometre range are important.  2.1  Scanning Electron Microscopy T h e S E M is a v e r y versatile instrument for the characterization o f surface t o p o g r a p h y and  material c o m p o s i t i o n . A n energetic b e a m o f electrons (typically 1 to 3 0 k V ) is rastered across 3  the surface o f the s p e c i m e n , a n d an i m a g e is constructed b y detecting electrons emitted from the surface. I f an electron interacts elastically w i t h an a t o m i c nucleus i n the sample a n d is returned out, it is s a i d to be backscattered. These electrons p r o v i d e s o m e sensitivity to the a t o m i c n u m b e r o f the sample but are emitted i n relatively l o w y i e l d . A backscattered electron detector m a y be used to obtain s o m e elemental contrast i n a s a m p l e but w i t h l i m i t e d resolution. I f an electron undergoes a n u m b e r o f inelastic processes i n the sample before b e i n g reemitted out, it is called a secondary electron. T h e s e are emitted i n a y i e l d a p p r o a c h i n g u n i t y and are largely independent o f m a t e r i a l c o m p o s i t i o n . T h e intensity o f secondary electron e m i s s i o n o b s e r v e d at each p o i n t is m o d u l a t e d b y the l o c a l sample structure, as m o r e electrons c a n escape from protrusions than from depressions. T h i s a l l o w s a t o p o g r a p h i c a l i m a g e to be f o r m e d . T h e secondary electron detector n o r m a l l y used i n S E M presents a surface i m a g e w i t h a point resolution o f - 1 . 5 n m at accelerating voltages f r o m 10 to 2 0 k V ( H i t a c h i S - 4 7 0 0 f i e l d - e m i s s i o n S E M , U B C E l e c t r o n M i c r o s c o p y L a b ) . L o w e r accelerating voltages m a y be used to reduce c h a r g i n g effects and to increase surface detail, at the expense o f resolution. 32  F o r i n s u l a t i n g samples, a c o n d u c t i v e c o a t i n g m u s t be a p p l i e d to prevent charge b u i l d - u p o n the surface, as l o w - v o l t a g e i m a g i n g does not p r o v i d e sufficient resolution. T h i s is usually a c c o m p l i s h e d by sputtering a t h i n c o a t i n g o f g o l d - p a l l a d i u m onto the sample. T h i s coating is adequate for most S E M w o r k but begins to s h o w s o m e structure at h i g h e r m a g n i f i c a t i o n s (above 100,000x), which  was the  usual operating r e g i m e for this w o r k . O t h e r c o a t i n g materials  ( p l a t i n u m , c h r o m i u m ) are smoother and are m o r e suitable for h i g h resolution w o r k but w e r e not readily available for use at U B C . In the S E M , the contrast between an i n o r g a n i c host m a t e r i a l and an o r g a n i c guest is not v e r y large, especially w i t h a c o n d u c t i v e c o a t i n g on the surface. S o m e inferences m a y be m a d e f r o m changes i n the geometry (e.g., complete filling o f pores).  2.2  Transmission Electron Microscopy In the T E M , electrons are transmitted t h r o u g h a very t h i n s a m p l e .  4  T h e contrast is  p r o v i d e d b y the ability o f the sample to scatter electrons, w h i c h is largely a p r o d u c t o f t h i c k n e s s and a t o m i c n u m b e r . T h e accelerating voltage ( b e a m energy) is t y p i c a l l y set at 2 0 0 k V for materials science, i n order to observe t h i c k e r specimens, w i t h s o m e loss o f contrast  over  operating at 80 k V (as used for b i o l o g i c a l samples). In general, a T E M is r e q u i r e d to investigate structures w i t h detail b e l o w 5 - 1 0 n m . S u b nanometre  resolution is r e a d i l y achievable o n standard  information limit on a modern  field-emission  T E M instruments.  T h e achievable  T E M at 2 0 0 k V is 0.12 n m ( T e c n a i F 2 0 T E M ,  N a n o - I m a g i n g F a c i l i t y , S F U ) . A T E M c a n also be operated i n s c a n n i n g m o d e ( S T E M ) , w h e r e it functions transmitted  s i m i l a r l y to an S E M , but w i t h a h i g h e r resolution and the c h o i c e o f detecting electrons (bright-field i m a g i n g for unscattered  electrons, dark-field  i m a g i n g for  strongly scattered electrons) o r secondary electrons. 33  T h e large electron-beam energy causes damage to the sample over t i m e . P o r o u s s i l i c a and a l u m i n a films tend to be fairly sensitive to b e a m damage i n the T E M , as has been observed for the m e s o p o r o u s material M C M - 4 1 . > O r g a n i c materials are s i m i l a r l y sensitive. 5  6  O r g a n i c materials scatter electrons i n the T E M o n l y w e a k l y , due to the l o w a t o m i c n u m b e r o f c a r b o n . T h e simplest approach to i m p r o v i n g the contrast o f o r g a n i c materials is h e a v y - m e t a l staining, w h i c h is w i d e l y used for b i o l o g i c a l s p e c i m e n s . T h e v i n y l i c carbons i n p o l y ( l , 4 - p h e n y l e n e v i n y l e n e ) ( P P V ) and its derivatives are readily stained b y o s m i u m tetroxide. T h e p h e n y l group m a y also be stained by r u t h e n i u m t e t r o x i d e .  7  T h i s s h o u l d p r o v i d e better  contrast i n the T E M because o f the increased electron scattering b y the heavy n u c l e i . H o w e v e r , i n situations w h e r e the amount o f p o l y m e r is l i m i t e d , this m a y still not p r o v i d e sufficient analytical contrast, a n d the use o f m o r e s p e c i a l i z e d techniques for c h e m i c a l analysis is r e q u i r e d .  2.2.1  T E M Sample Preparation T h e central d r a w b a c k o f the T E M is the need for electron-transparent  samples. S m a l l  particles m a y be investigated directly, but b u l k samples must be t h i n n e d or sectioned to be observed. T h e useful thickness range is a function o f the accelerating voltage and c o m p o s i t i o n ; for w o r k at 2 0 0 k V , a thickness b e l o w 2 0 0 n m is usually r e q u i r e d . W h i l e s a m p l e preparation techniques are w e l l established, they are generally t i m e - c o n s u m i n g . T h e observation o f porous films i n the p l a n geometry, that is l o o k i n g d o w n the channels, is straightforward and not sensitive to  film  thickness, p r o v i d e d free-standing  films  c a n be  prepared. T h e film cross-sections are m o r e difficult to image, as they must be prepared i n the f o r m o f t h i n sections less than 2 0 0 n m t h i c k i n order to be  electron-transparent.  T h e preparation o f these t h i n sections is not entirely t r i v i a l for hard materials. T h e c o n v e n t i o n a l technique for cross-section preparation o f s u c h materials is d i m p l i n g and i o n 34  m i l l i n g . M a n y samples are g l u e d to f o r m a s a n d w i c h structure, w h i c h is then s a w e d into 1 m m slices. A 3 m m d i s k is cut from this slice, g r o u n d to 100 urn t h i c k n e s s , a n d then d i m p l e d i n the centre u n t i l the sample is - 2 5 u m t h i c k at the centre. T h e sample is then i o n - m i l l e d u s i n g an argon i o n b e a m u n t i l a s m a l l hole is f o r m e d . T h e cross-sections are finally o b s e r v e d i n the T E M a l o n g the edges o f the m i l l e d hole. T h i s process is e v i d e n t l y tedious a n d suffers from artifacts introduced b y the i o n - m i l l i n g process ( a m o r p h i s a t i o n , preferential r e m o v a l o f elements). T h e state-of-the-art i n thin-section preparation i n v o l v e s the focused i o n - b e a m ( F I B ) technique. It is executed inside an S E M w i t h a b e a m o f g a l l i u m ions: the sample is m i l l e d a w a y at a precisely k n o w n location u n t i l the desired section is obtained. T h i s approach is also p r o n e to p r o d u c i n g damage artifacts. F o r the duration o f most o f this w o r k , there w a s n o ready access to the p r o p e r instrument - o n l y one existed i n C a n a d a outside o f i n d u s t r i a l research laboratories. In late 2 0 0 2 , a n e w F I B instrument w a s installed at S F U . A s a s i m p l e alternative, the s m a l l - a n g l e cleavage ( S A C ) technique w a s d e v e l o p e d b y M c C a f f r e y for routine cross-section preparation o f t h i n f i l m s for T E M . " 8  1 0  T h e r e are t w o major  requirements for successful e x e c u t i o n o f the t e c h n i q u e : a substrate that cleaves r e a d i l y a n d a t h i n f i l m (< 3 0 0 n m ) w i t h g o o d adhesion to the substrate. T h e s a m p l e is cleaved at a s h a l l o w angle (< 3 0 ° ) to f o r m a sharp w e d g e ( F i g u r e 2.2). T h e last m i c r o m e t r e o f the w e d g e near the tip is then sufficiently t h i n for observation i n cross-section b y T E M . W i t h practice, S A C samples c a n be p r o d u c e d w i t h i n h a l f a day, w h i c h is substantially faster than the d i m p l i n g technique. H o w e v e r , the technique is less useful for f i l m s that are not h o m o g e n e o u s i n the plane o f the substrate, as the gradual change i n t h i c k n e s s o f the w e d g e m a k e s it difficult to v i s u a l i z e films w i t h a d d i t i o n a l structure i n the depth o f the cross-section. F o r s u c h films, u l t r a m i c r o t o m y a n d f o c u s e d - i o n b e a m m i l l i n g are m o r e  suitable approaches  to t h i n  section preparation. Nevertheless, the S A C  technique is v e r y useful for v i e w i n g cross-sections w i t h o u t artifacts f r o m the s e c t i o n i n g process.  35  Figure 2.2 The small angle cleavage (SAC) technique applied to a thin film: the tip of the final specimen will often be thin enough over a sufficient length for TEM observation if a < 30°. Ultramicrotomy is commonly used to prepare cross-sections of biological specimens. The specimen is first embedded in epoxy resin, then trimmed with a razor blade or glass knife to form a truncated pyramid around the area of interest (Figure 2.1). After careful cleaning and washing to remove any small particles, the pyramid is sectionedfrombottom to top using a  Thin  film  Substrate  Sectioning direction  Figure 2.1 Truncated pyramid geometry of epoxy-embedded sample for sectioning by ultramicrotomy. The resulting sections float into a water bath where they are collected with TEM grids. 36  d i a m o n d k n i f e ; the resulting t h i n sections float into a water bath, where they m a y be collected u s i n g a T E M g r i d . T h e sections are t y p i c a l l y 2 0 to 60 n m t h i c k . T h i s technique is m o s t r e a d i l y a p p l i c a b l e to soft materials, but i n fact c a n be used to section even the hardest m a t e r i a l s . ' 1 1  1 2  Cross-sections o f self-assembled m e s o p o r o u s s i l i c a  o n m i c a and graphite were p r o d u c e d b y u l t r a m i c r o t o m y . ' 1 3  F u r n e a u x e t a l . successfully used  1 4  u l t r a m i c r o t o m y to obtain t h i n sections o f porous a l u m i n a films for T E M .  2.3  films  1 5  High Resolution Chemical Analysis T h e different  approaches  to spatially resolved c h e m i c a l analysis are r e v i e w e d  here,  f o l l o w e d b y a m o r e detailed d e s c r i p t i o n o f the technique that w a s used i n this w o r k , electron energy-loss  spectroscopy  (EELS)  within  the  T E M . The  S T M , which  can give chemical  i n f o r m a t i o n at very h i g h resolution, is not discussed here as it is not a p p l i c a b l e to i n s u l a t i n g materials. These a n d other e m e r g i n g techniques,  especially s c a n n i n g probe techniques,  d e s c r i b e d i n the p r o c e e d i n g s o f a recent w o r k s h o p o n nanoscale s p e c t r o s c o p y .  are  2  M o s t o f the techniques for c h e m i c a l analysis rely o n initially e x c i t i n g inner-shell (core) electrons b y an energetic b e a m o f electrons or X - r a y s ( F i g u r e 2.3) and then detecting the results o f the de-excitation process. T h e outer-shell electrons also p r o v i d e less direct i n f o r m a t i o n for c h e m i c a l analysis under certain circumstances. T h e spatial resolution is l i m i t e d , i n a l l cases, b y the  i n i t i a l excitation v o l u m e . T h e resolution l i m i t s o f the  techniques  discussed  here  are  summarized in Table 2.1.  37  Light  Auger  Vacuum level Fermi level Valence electrons X-Ray  Core electrons  4—•-  -  Inner-shell E x c i t a t i o n  •  —  •  -  Outer-shell Excitation  De-excitation  Figure 2.3 E l e c t r o n i c excitation and de-excitation m e c h a n i s m s i n a solid, (adapted from ref. 1)  Table 2.1 C u r r e n t spatial r e s o l u t i o n l i m i t s o f c h e m i c a l analysis techniques. Technique  Sampling Depth  Lateral Resolution (nm)  (55») X - r a y photoelectron spectroscopy  -1.0  10  6  (30 ) r  Scanning Auger microscopy  -1.0  > 10  E n e r g y - d i s p e r s i v e X - r a y spectroscopy i n S E M  1000  1000  E n e r g y - d i s p e r s i v e X - r a y spectroscopy i n T E M  100  10  E l e c t r o n energy-loss spectroscopy in T E M  100  1  +  for a synchrotron X-ray source  2.3.1  X-Ray Photoelectron Spectroscopy Monochromatic  X-rays  are  used  as  the  excitation source  for  X-ray  photoelectron  spectroscopy ( X P S ) ; these result i n the e m i s s i o n o f photoelectrons and A u g e r electrons from the sample. T h e k i n e t i c energy o f the ejected electrons is measured, y i e l d i n g the core electron energies.  The  relative abundance  o f any  element  may  be  readily calculated, a n d  some  i n f o r m a t i o n on the c h e m i c a l state o f the element m a y also be obtained. A l t h o u g h the lateral resolution is l i m i t e d b y the b e a m diameter (-1  m m for c o n v e n t i o n a l sources; - 3 0 n m for 38  synchrotron s o u r c e s ) , this technique is v e r y surface-sensitive as a result o f the l i m i t e d m e a n 16  free path o f the generated photoelectron, w h i c h is o n the order o f 1 n m . H o w e v e r , the a d s o r p t i o n o f atmospheric  hydrocarbons  o n the  surface  c a n affect  the  analysis o f c a r b o n - c o n t a i n i n g  samples. W i t h the d e v e l o p m e n t o f m o r e refined X - r a y optics, this technique m a y c o m e to be v e r y important for h i g h resolution analysis, as it induces less s a m p l e damage than electron microscopy.  2.3.2  1 6  Energy-Dispersive X-Ray Spectroscopy and Scanning Auger Microscopy T h e electron b e a m w i t h i n an S E M can be used as a h i g h l y focused source o f excitation  ( d o w n to ~1 n m diameter for a field e m i s s i o n source) for the generation o f X - r a y s and A u g e r electrons. T h e s e are c o l l e c t e d and analysed to f o r m the basis for energy-dispersive X - r a y ( E D X ) analysis and s c a n n i n g A u g e r m i c r o s c o p y ( S A M ) , respectively. In the case o f b u l k samples, the X - r a y signal used for E D X originates f r o m a relatively large d o m a i n s u r r o u n d i n g the p o i n t excited b y the electron b e a m , due to the penetration depth o f the energetic electrons and the l o w absorption o f X - r a y s . T h i s large excitation v o l u m e effectively reduces the spatial resolution to 1 um  but also a l l o w s for a t h i n c o n d u c t i v e f i l m to be a p p l i e d to the surface i f the sample is  insulating. O n the other h a n d , o n l y A u g e r electrons released near the surface can escape and be detected (as w i t h photoelectrons), w i t h the result o f higher lateral resolution ( - 1 0 n m ) than E D X but w i t h the requirement that the sample be c o n d u c t i v e (or very t h i n w i t h a c o n d u c t i v e b a c k i n g ) to a v o i d c h a r g i n g effects. W h e n c o u p l e d to a S T E M , E D X m a y also be u s e d to quantify the c o m p o s i t i o n o f t h i n film  samples. S i n c e the sample must be sufficiently t h i n to transmit electrons, the excitation  v o l u m e for X - r a y s is greatly r e d u c e d and the lateral resolution is l i m i t e d b y scattering o f the electron b e a m w i t h i n the t h i n section, effectively r e a c h i n g 10 n m . In practice, sufficient material 39  m u s t also be present to obtain reasonable c o u n t i n g statistics, w h i c h l i m i t s the m i n i m u m sample t h i c k n e s s that m a y be used. B e a m d a m a g e to the s a m p l e m a y also affect the results i f a particular c o m p o n e n t is b e i n g r e m o v e d at a h i g h e r rate.  2.3.3  Electron Energy-Loss Spectroscopy F i n a l l y , the d i s t r i b u t i o n o f energy losses i n c u r r e d b y the transmitted electron beam i n a  T E M m a y also be m e a s u r e d to y i e l d elemental c o m p o s i t i o n . T h i s a p p r o a c h belongs to a f a m i l y o f techniques described generally as electron energy-loss spectroscopy. A l t h o u g h it is l i m i t e d to t h i n samples, it p r o v i d e s h i g h e r r e s o l u t i o n (~1 n m ) a n d sensitivity (as f e w as 1 to 10 atoms c a n be detected ) than a l l the techniques listed above (see T a b l e 2.1). It is ideally suited for the 1  analysis o f n a n o c o m p o s i t e materials a n d is d e s c r i b e d i n m o r e detail b e l o w .  2.4  General Principles of EELS T h e standard reference for E E L S i n the T E M is the b o o k b y E g e r t o n . A m o r e recent 1  p u b l i c a t i o n by B r y d s o n  1 7  covers some n e w developments and focuses o n the e x p e r i m e n t a l  aspects o f E E L S . In general, the interactions between a t r a v e l l i n g electron a n d the c o m p o n e n t s o f a s o l i d are t e r m e d scattering events ( F i g u r e 2.4). I f the electron is scattered f r o m an a t o m i c nucleus, the process occurs w i t h o u t the electron l o s i n g any significant a m o u n t o f energy. T h i s elastic scattering can cause large deviations i n the trajectory o f the electron, to the extent w h e r e it m a y be backscattered out o f the s o l i d , or s m a l l e r deviations (e.g., diffraction for crystalline materials). S o m e electrons w i l l also excite c o l l e c t i v e o s c i l l a t i o n s o f the atoms i n the s o l i d ( p h o n o n s ) ; these o c c u r at l o w energies i n the m e V range and are not d i s t i n g u i s h a b l e from the unscattered a n d elastically scattered electrons i n the T E M (though they m a y be i n other f o r m s o f  40  B  Figure  2.4 G e o m e t r y o f (a) elastic, (b) inelastic, inner-shell and (c) inelastic, outer-shell  scattering events i n v o l v i n g an electron and a carbon atom, (adapted f r o m ref. 1)  E E L S w i t h higher energy resolution). These electrons are detected b y the spectrometer as the zero-loss peak, the w i d t h o f w h i c h g i v e s the energy resolution o f the instrument. A t y p i c a l loss spectrum is s h o w n i n F i g u r e 2.5. Inelastic scattering occurs w h e n the electron interacts w i t h either the inner-shell or outershell electrons o f an atom ( F i g u r e 2.4). T h e scattering from inner-shell electrons produces an energy loss characteristic for each element, w i t h a v a l u e t y p i c a l l y between 5 0 and 2 0 0 0 e V . In the loss spectrum, these appear as an i o n i z a t i o n edge (e.g., at 2 8 4 e V for carbon Is electrons) and are u s u a l l y e m p l o y e d for elemental analysis. T h e fine structure o f the i o n i z a t i o n edge is also used to derive a d d i t i o n a l i n f o r m a t i o n about the c h e m i c a l state o f the element. T h e r e has been extensive w o r k o n m o d e l l i n g these e d g e s , ' 1  1 8  but since they play n o role i n this w o r k they are  not d i s c u s s e d any further. T h e excited state p r o d u c e d b y inelastic scattering can also manifest itself as a c o l l e c t i v e excitation o f the outer-shell electrons, generally referred to as a p l a s m o n . O n longer t i m e scales, the energy o f the p l a s m o n is distributed over m a n y electrons, but all the energy m a y be carried b y a single electron o n short t i m e scales, w h i c h m a k e s  such excitations possible even i n  41  Zero-loss peak  c Inner-shell ionization edges  100  1 200  1  1 300  r  r  500  400  Energy (eV) F i g u r e 2 . 5 P r i n c i p a l features o f an E E L spectrum, (adapted f r o m ref. 17)  insulators.  1  T y p i c a l l y , p l a s m o n s i n v o l v i n g both a a n d n electrons appear between 5 a n d 5 0 e V  for most materials. A r o m a t i c organic materials also s h o w a distinct  7T.-7T.*  p l a s m o n around 7 e V .  T h e outer-shell electrons that are e x c i t e d i n these b u l k p l a s m o n s are difficult to m o d e l f r o m first p r i n c i p l e s , especially i n non-metals. T h e y m a y be m o r e r e a d i l y treated b y c o n s i d e r i n g the response o f the w h o l e s o l i d to the t r a v e l l i n g electron, w h i c h is v e r y s i m i l a r to the response to a p a s s i n g photon. T h e latter is g i v e n b y the c o m p l e x dielectric function  s(oS).  I f the sample  t h i c k n e s s is k n o w n accurately, the l o w - l o s s spectrum m a y be used to calculate s(co) for the m a t e r i a l a n d c o m p a r e it to data f r o m optical m e a s u r e m e n t s .  19  T h e correlation between the t w o is  sufficient to a l l o w the use o f either o n e to predict the other, a n d as such E E L S c o m p l e m e n t s a n d extends o p t i c a l techniques for d e t e r m i n i n g  £(co). 42  T h e b u l k p l a s m o n s o c c u r w i t h i n the b u l k o f the m a t e r i a l a n d must be d i s t i n g u i s h e d from excitations that appear at interfaces, w h i c h are t e r m e d surface p l a s m o n s . G e o m e t r i e s w h e r e the electron b e a m interacts extensively w i t h the surface o f the material c a n lead to important surface p l a s m o n effects, t y p i c a l l y b e l o w 2 0 e V i n energy. Together, the b u l k and surface d o m i n a t e the  l o w - l o s s spectrum  plasmons  (< 50 e V ) . P l a s m o n s o f both types c a n be excited  from  substantial distances from the sample: up to 8.0 n m for the b u l k p l a s m o n a n d o v e r 12 n m for surface p l a s m o n s .  2 0  T h e f a l l - o f f is h o w e v e r e x p o n e n t i a l a n d reasonable c o u n t i n g statistics are  obtained up to h a l f these distances. T h e intensity o f the loss peaks relative to the total s p e c t r u m area is a function o f sample t h i c k n e s s . F u r t h e r m o r e , both types o f p l a s m o n s c a n lead to m u l t i p l e scattering events per electron ( p l u r a l scattering), l e a d i n g to the appearance o f m u l t i p l e peaks i n the loss spectrum w i t h a P o i s s o n d i s t r i b u t i o n . In this case, the single scattering d i s t r i b u t i o n c a n be r e c o v e r e d by d e c o n v o l u t i n g the spectrum (see chapter 5). T h e b e a m energies t y p i c a l l y used i n T E M lead to electrons w i t h relativistic speeds, w h i c h m a y also lose energy t h r o u g h the C h e r e n k o v effect. T h i s loss m o d e p r o v e d to be v e r y important for the samples that w e r e investigated here a n d is discussed i n m o r e detail i n chapter 6.  2.5  EELS  Instrumentation  T h e schematic o f a m o d e r n T E M w i t h E E L S capabilities is s h o w n i n F i g u r e 2.6. T h e type o f electron source determines the brightness, energy spread a n d S T E M probe size; a f i e l d e m i s s i o n source is usually e m p l o y e d for materials science w o r k . T h e b e a m is accelerated to the desired energy (usually 2 0 0 k V for materials science), a n d m a g n e t i c lenses are used to f o r m the probe. In T E M m o d e , the probe is diffuse a n d i l l u m i n a t e s a large part o f the sample; the convergence semi-angle a is also s m a l l , w h i c h is equivalent to nearly parallel i l l u m i n a t i o n . T h e 43  Electron source  ~^Ss> Probe forming optics  TEM  Sample  GIF  Energy filtering slit  CCD detector  F i g u r e 2.6 S c h e m a t i c o f p o s t - c o l u m n G a t a n I m a g i n g Filter o n a T E M .  transmitted electrons are transferred (GIF).  2 1  by further magnetic lenses to the G a t a n i m a g i n g  filter  T h e G I F consists o f a magnetic p r i s m spectrometer c o u p l e d to a c h a r g e - c o u p l e d d e v i c e  ( C C D ) detector b y m u l t i p o l e m a g n e t i c lenses. In i m a g i n g m o d e , the T E M i m a g e is f o r m e d o n the C C D ; i f energy filtering is required, a slit is inserted after the spectrometer to select electrons w i t h the desired energy loss. In spectroscopy m o d e , the loss spectrum is projected o n the C C D . In S T E M m o d e , the probe is focused to a spot that c a n be as s m a l l as 0.2 n m i n diameter. T h i s strong f o c u s i n g entails a larger a (>10 m r a d ) . T h e image is r e c o r d e d u s i n g a dark-field detector ( w h i c h detects electrons that are strongly scattered b y the sample), thereby a l l o w i n g the m a i n electron b e a m to continue into the G I F w h e r e the loss spectrum is measured. T h u s it is straightforward to p e r f o r m E E L S experiments w i t h the m i c r o s c o p e i n S T E M m o d e : the electron  44  probe is scanned across the s a m p l e to p r o d u c e the i m a g e ; the probe m a y then be r e l i a b l y p o s i t i o n e d i n the location o f interest o n the sample to r e c o r d a loss spectrum.  2.6  A p p r o a c h e s to E E L S D a t a A c q u i s i t i o n T h e o p t i m a l a p p r o a c h to c h e m i c a l analysis b y E E L S depends o n the c o m p l e x i t y o f the  loss spectrum. In T E M m o d e , the energy o f the i m a g e - f o r m i n g electron b e a m m a y be filtered u s i n g a slit o f set w i d t h (energy-filtered T E M , E F T E M ) . E l e m e n t a l distributions m a y be i m a g e d directly b y filtering o n the appropriate spectral feature ( F i g u r e 2.7). T h i s approach is generally useful w h e n a l i m i t e d n u m b e r o f w e l l - u n d e r s t o o d c o m p o n e n t s need to be m a p p e d over a large area. T h e energy resolution is set b y the slit w i d t h and is u s u a l l y 1 e V or greater. T h e spatial resolution is essentially the same as the T E M , but the m a g n i f i c a t i o n is l i m i t e d b y the presence o f theslitto~100,000x. In S T E M m o d e , loss spectra m a y be a c q u i r e d at specific points, lines or areas ( F i g u r e 2.7). T h i s a l l o w s detailed e x a m i n a t i o n o f s p e c i f i c areas o n the sample. T h e spatial resolution is l i m i t e d to ~1 n m b y the electron probe size a n d scattering w i t h i n the sample; the effect o f the latter m a y be l i m i t e d b y restricting the c o l l e c t i o n angle. T h e energy r e s o l u t i o n i n S T E M m o d e is d e t e r m i n e d b y the energy spread o f the electron b e a m . W i t h a  field-emission  electron source, this  spread is about 0.6 to 0.9 e V , d e p e n d i n g o n the current output o f the electron source. T h e process o f a c q u i r i n g a c o m p l e t e loss spectrum for each p o i n t i n an area o f interest is called spectrum i m a g i n g . T h i s a l l o w s detailed p r o c e s s i n g o f the spectra over the w h o l e i m a g e and is useful i n cases w h e r e it is not possible to separate the requisite spectral i n f o r m a t i o n b y E F T E M . H o w e v e r , this process c a n be fairly t i m e - c o n s u m i n g .  45  E,  F i g u r e 2.7 Illustration o f c h e m i c a l analysis o f a t w o - c o m p o n e n t sample b y S T E M / E E L S o v e r a set o f points and E F T E M over the w h o l e image.  2.7  Quantitative Analysis of E E L S Spectra E E L S and E F T E M are most c o m m o n l y used to p r o v i d e qualitative analysis o f material  c o m p o s i t i o n . Q u a n t i f i c a t i o n o f the elemental or phase c o m p o s i t i o n is less straightforward. A n accurate thickness m a p must be first generated, to correct for the thickness dependence o f the losses. P l u r a l scattering effects must be also be e l i m i n a t e d . T h e scattering cross-section o f each c o m p o n e n t , at a specific i o n i z a t i o n edge or p l a s m o n , m u s t be determined t h r o u g h c a l c u l a t i o n or reference samples. A complete d i s c u s s i o n o f the quantification process is p r o v i d e d b y E g e r t o n .  1  T h e focus o f this w o r k w a s o n the qualitative level and n o attempts w e r e m a d e to quantify material c o m p o s i t i o n b y these techniques.  46  2.8  Application of EELS to Organic Materials B o t h E E L S and E F T E M have been used to investigate m a n y o r g a n i c materials. In m a n y  cases, special precautions are taken to l i m i t the radiation damage to the samples, e.g., b y u s i n g a diffuse  beam,  c o l l e c t i n g over m a n y different  areas, and k e e p i n g the s a m p l e at c r y o g e n i c  temperatures. T h e r e is also still debate over whether a diffuse b e a m (i.e., E F T E M ) or a focused probe (i.e., S T E M ) causes the least amount o f damage to o r g a n i c m a t e r i a l s .  22  It has  become  evident that each material must be studied separately to understand its b e a m stability. T h e first important study o f o r g a n i c materials b y E E L S , b y Isaacson, investigated thin f i l m s o f the n u c l e i c a c i d b a s e s .  T h e c o m p l e x dielectric function e(co)  23  was obtained w i t h a  resolution o f ~ 0 . 2 5 e V . T h e d e r i v e d absorption coefficient s h o w e d v e r y g o o d agreement w i t h U V absorption data, p r o v i d e d that the electron dose w a s carefully l i m i t e d to m i n i m i z e damage. M o r e recently, b i o l o g i c a l c e l l - s t a i n i n g c h r o m o p h o r e s have also been m a p p e d at h i g h resolution by  filtering  o n the l o w - l o s s peaks o f the c h r o m o p h o r e s .  24  T h e resolution l i m i t o f this technique  w a s estimated to be 1.6 n m , based o n the edge sharpness o f larger s t r u c t u r e s . C a r b o n nanotubes have been extensively studied by E E L S ,  2 6  "  2 8  25  as they are fairly stable  to the b e a m . T h e l o w - l o s s spectra s h o w the expected % p l a s m o n a r o u n d 4 to 6 e V and the b u l k p l a s m o n near 23 e V . E F T E M has p r o v e d to be a v e r y useful t o o l for the study o f p o l y m e r b l e n d s . 7  2 9  The  i o n i z a t i o n edges o f m i n o r c o m p o n e n t elements (nitrogen for p o l y a m i d e s , o x y g e n for p o l y ( m e t h y l methacrylate),  sulfur for p o l y ( p h e n y l e n e sulfide)) m a y be used w i t h s o m e effectiveness  determine p o l y m e r d i s t r i b u t i o n s . ' 3 0  3 1  to  B u t certain phases can s h o w trace o x y g e n c o n t a m i n a t i o n  (e.g., polybutadiene) that c o m p l i c a t e s the identification process. P o l y m e r s w i t h aromatic groups, s u c h as polystyrene, can be r e a d i l y identified i n a b l e n d due to the distinctive n-%* p l a s m o n at 7 eV.  3 2  T h e p o l y m e r s m a y also be stained i n v a r i o u s w a y s to increase contrast; this c o m e s w i t h the 47  advantage o f phase stabilization b y the c r o s s - l i n k i n g o f chains and the disadvantage o f altered c h e m i c a l states and possible s h r i n k a g e .  32  These reports clearly indicate that conjugated p o l y m e r s c a n be readily d i s t i n g u i s h e d w i t h E E L S and E F T E M by the d i s t i n c t i v e n p l a s m o n o f aromatic rings; analysis u s i n g the carbon K edge is also possible w h e n there is sufficient material present. T h e analysis o f the l o w - l o s s spectrum requires accurate r e m o v a l o f the zero-loss peak; this is discussed i n m o r e detail i n chapter 5.  2.9  Conclusion EELS  and  EFTEM  are  both  h i g h l y suited  for h i g h resolution  characterization  of  nanostructured conjugated p o l y m e r s . T h i s a p p l i c a t i o n is successfully demonstrated i n chapter 3 on a conjugated p o l y m e r / m e s o p o r o u s s i l i c a c o m p o s i t e , and less successfully i n chapters 5 and 7 on a conjugated p o l y m e r / p o r o u s a l u m i n a c o m p o s i t e . These latter results s h o w e d h o w the sample geometry and relativistic effects c a n play a large role i n the l o w - l o s s spectrum, m a k i n g c h e m i c a l analysis less straightforward.  48  References 1. E g e r t o n , R . F .  Electron energy-loss spectroscopy in the electron microscope; 2 n d  ed.; P l e n u m  Press: N e w Y o r k , 1996. 2. 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M . ; A r s e n a u l t , A . L . ; Ottensmeyer, F . P . Micron  1998, 29, 97. 2 6 . K u z u o , R . ; T e r a u c h i , M . ; T a n a k a , M . Jpn. J. Appl. Phys. 1992, 31, L I 4 8 4 . 2 7 . B u r s i l l , L . A . ; Stadelmann, P . A . ; P e n g , J . 1.; S., P . Phys. Rev. B 1994, 49, 2 8 8 2 . 2 8 . Stephan, O . ; K o c i a k , M . ; H e n r a r d , L . ; Suenaga, K . ; G l o t e r , A . ; Tence, M . ; Sandre, E . ; C o l l i e x , C . J. Electron. Spectrosc. Relat. Phenom. 2001, 114, 2 0 9 . 2 9 . D u C h e s n e , A . Macromol. Chem. Phys. 1999, 200, 1813. 3 0 . H o r i u c h i , S.; Y a s e , K . ; K i t a n o , T . ; H i g a s h i d a , N . ; O u g i z a w a , T . Polym. J. 1997, 29, 3 8 0 . 3 1 . H o r i u c h i , S.; I s h i i , Y . Polym. J. 2000, 32, 3 3 9 . 3 2 . V a r l o t , K . ; M a r t i n , J . M . ; Quet, C . Polymer  2000, 41,  4599.  50  CHAPTER 3 A PPV/MCM-41 Composite Material  M a n y o f the host properties desired for o p t i m a l conjugated p o l y m e r encapsulation c a n be f o u n d i n self-assembled porous i n o r g a n i c materials. These materials have been o f longstanding scientific and i n d u s t r i a l interest, due to their large internal surface area, large sorption capacity, t h e r m a l stability and catalytic activity towards s m a l l m o l e c u l e s .  M o r e recently, as instruments  1  for structural and c h e m i c a l analysis o n the nanometre scale have b e c o m e c o m m o n p l a c e , p o r o u s materials have been  investigated as templates  for n o v e l nanostructured  materials. A  brief  o v e r v i e w o f the properties and applications o f self-assembled i n o r g a n i c porous materials is g i v e n here, w i t h an e m p h a s i s o n the m e s o p o r o u s material M C M - 4 1 . M C M - 4 1 , i n its pure s i l i c a f o r m , possesses m a n y o f the properties o f an ideal host for conjugated  p o l y m e r s : it has  narrow  and a l i g n e d channels, w i t h  o p t i c a l l y transparent  and  electrically i n s u l a t i n g w a l l s . T h i s m a d e it attractive to us for an initial study on p o l y m e r / h o s t c o m p o s i t e material synthesis a n d characterization. T h e literature already contained work  o n the  synthesis  of polymer/MCM-41  composite  materials,  i n particular w i t h  l u m i n e s c e n t conjugated p o l y m e r s . T h e preparation and characterization o f a n e w material i n v o l v i n g M C M - 4 1  as  a host  for the  luminescent  important  conjugated  polymer  non-  composite poly(l,4-  p h e n y l e n e v i n y l e n e ) ( P P V , F i g u r e 3.1) is described i n this chapter.  51  ro4 Figure 3.1 C h e m i c a l structure o f p o l y ( l ,4-phenylene v i n y l e n e ) ( P P V ) .  3.1  Ordered Porous Host Materials Porous  microporous,  materials  are  mesoporous  categorized  and  a c c o r d i n g to  macroporous  (Table  their 3.1).  pore For  size into three the  purpose  of  classes: polymer  encapsulation and orientation, the materials o f interest w o u l d have an ordered pore structure w i t h a pore diameter b e i n g near the b o u n d a r y between the m i c r o p o r o u s and m e s o p o r o u s classes (~2 nm).  Table 3.1 I U P A C classification o f p o r o u s materials b y pore s i z e . Pore Diameter  Designation  <2 nm  microporous  2 - 50 n m  mesoporous  >50 nm  macroporous  3.1.1  1  Zeolites T h e major class o f ordered m i c r o p o r o u s materials is the zeolite f a m i l y .  1  Zeolites are  crystalline a l u m i n o s i l i c a t e s possessing a w i d e variety o f pore structures, i n c l u d i n g one, t w o and three d i m e n s i o n a l pore n e t w o r k s ( F i g u r e 3 . 2 ) .  2  A n u m b e r o f naturally o c c u r r i n g zeolites are  k n o w n but a large n u m b e r o f synthetic zeolites have been d i s c o v e r e d since the 1 9 4 0 ' s . T h e synthesis  usually  temperature  consists  o f hydrothermal  i n a sealed v e s s e l .  3  crystallization o f a  reactive  gel  at  elevated  T h e i n o r g a n i c zeolite f r a m e w o r k condenses into a structure  d e t e r m i n e d b y the t e m p l a t i n g (or structure-directing) agents, w h i c h c a n i n c l u d e water m o l e c u l e s , 52  A  B  C  F i g u r e 3.2 E x a m p l e s o f pore topologies w i t h (a) 1-D, (b) 2 - D and (c) 3-D connectivity.  o r g a n i c cations and salts. F o l l o w i n g the synthesis, the trapped t e m p l a t i n g agent can be r e m o v e d b y c a l c i n a t i o n or ion-exchange to expose the pore structure. T h e crystalline nature o f zeolites a l l o w s very precise structural characterization u s i n g X - r a y diffraction a n d solid-state nuclear magnetic  resonance.  T h e w e l l - d e f i n e d structure  also leads to h i g h selectivity i n catalytic  reactions. T h e largest pore size reported to date i n zeolites is o n the order o f 1.2 n m i n d i a m e t e r .  4  These zeolites can a c c o m m o d a t e s m a l l e r p o l y m e r s (e.g. p o l y s t y r e n e ) but not larger p o l y m e r s 5  (in particular conjugated p o l y m e r s w i t h s o l u b i l i z i n g side-chains). A further c o m p l i c a t i o n arises from the s m a l l crystallite size e x h i b i t e d by zeolites: the preparation o f continuous t h i n films is not readily possible, although recent results i n this area appear p r o m i s i n g . F o r these reasons, 6  m e s o p o r o u s materials are m o r e useful as general hosts for conjugated p o l y m e r s .  3.1.2  Mesoporous Materials M e s o p o r o u s host materials are desirable for m a n y applications, but were not available  w i t h w e l l - d e f i n e d pore structures u n t i l 1992, w h e n B e c k et al. reported the d i s c o v e r y o f the M 4 1 S family o f mesoporous aluminosilicate materials. ' 7  8  W h e r e a s zeolites are templated by  53  s m a l l organic m o l e c u l e s , M 4 1 S materials were s h o w n to be templated b y l i q u i d crystalline phases f o r m e d by straight-chain surfactants ( F i g u r e 3.3). O f particular interest i n this f a m i l y w a s the siliceous material M C M - 4 1 , w h i c h s h o w s u n i f o r m 1-D channels p a c k e d i n a 2 - D hexagonal array ( F i g u r e 3.4). T h i s s i m u l t a n e o u s l y e n d o w s the material w i t h a large surface area ( 1 0 0 0  m  2  g" , almost a l l internal) and a large pore v o l u m e (0.8 c m g" ). T h e straight channels are ideal for 1  3  1  p o l y m e r guest a c c o m m o d a t i o n , t h o u g h 1 8 0 ° defects i n c h a n n e l d i r e c t i o n are possible. T h e pore w a l l s u s u a l l y consist o f s i l i c a but other o x i d e s can be r e a d i l y incorporated into the f r a m e w o r k T h i s general approach to t e m p l a t i n g has since been a p p l i e d to the synthesis o f m a n y n e w p o r o u s materials.  9  T h e synthesis o f M C M - 4 1 proceeds from a m i x t u r e o f water, surfactant, s i l i c a source and an a c i d or base catalyst; this f o r m s a gel that is then heated i n a sealed container. T h e s p a c i n g o f the hexagonal phase can be readily altered b y the c h o i c e o f surfactant c h a i n length and the a d d i t i o n o f organic s w e l l i n g agents. counter-ions ' 8  from  1 3  8  V a r i a t i o n s o n the s i l i c a s o u r c e ,  and reaction t e m p e r a t u r e  14  11  p H o f the m i x t u r e , , 1 2  also affect the final structure. T h u s pore diameters  1.5 to 10 n m m a y be obtained; recent developments w i t h p o l y m e r i c t e m p l a t i n g agents have  A  B  C  D  E  F i g u r e 3.3 Stages i n the f o r m a t i o n o f M C M - 4 1 : (a) surfactant m i c e l l e w i t h the h y d r o p h o b i c chains i n the center and the polar head groups l i n i n g the outside, (b) c y l i n d r i c a l  surfactant  m i c e l l e , (c) hexagonal array formed cooperatively b y surfactant m i c e l l e s and silicate species, (d) condensed material (as-made M C M - 4 1 ) , (e) c a l c i n e d material, (adapted from ref.  1 0  )  54  Figure  3.4  Transmission  electron  micrograph  of  MCM-41  material  obtained  using  C i 6 H 3 3 ( C H ) N C l surfactant, s h o w i n g h e x a g o n a l lattice s p a c i n g a n d w a l l thickness, (courtesy o f 3  3  G . Botton, C A N M E T / N a t u r a l Resources Canada)  extended this range to 50 n m . > 1 5  1 6  A t the c o m p l e t i o n o f the synthesis, the organic t e m p l a t i n g agent is n o r m a l l y r e m o v e d b y c a l c i n a t i o n , a process that also induces further condensation o f the s i l i c a f r a m e w o r k dehydration. Solvent e x t r a c t i o n  1 7  a n d supercritical fluid e x t r a c t i o n  18  through  have also been s h o w n to be  effective i n r e m o v i n g the t e m p l a t i n g agents. T h e final f o r m o f the material is then a p o w d e r w i t h a t y p i c a l particle size o f 1 u m . T h e f o r m a t i o n o f M C M - 4 1 t h i n f i l m s has also been extensively investigated; it is d i s c u s s e d i n chapter 4. T h e h i g h degree o f order i n the M 4 1 S f a m i l y o f materials derives from the cooperative arrangement o f the c y l i n d r i c a l surfactant m i c e l l e s a n d the silicate ions i n the l i q u i d crystalline phase. T h i s long-range  order  i n the  p a c k i n g o f the  channels  can be observed  by  X-ray  diffraction. H o w e v e r , the s i l i c a f r a m e w o r k itself is a m o r p h o u s and m i c r o p o r o u s , and its inner surface  has m a n y slightly different  c h e m i c a l sites. T h u s M C M - 4 1  is referred  as a z e o l i t i c  m a t e r i a l to reflect that is an a l u m i n o s i l i c a t e material w i t h o n l y long-range o r d e r i n g . T h e loss o f  55  crystallinity, w h e n c o m p a r e d to the zeolites, m a k e s characterisation m o r e difficult; this is offset, i n the context o f guest i n c o r p o r a t i o n , b y the benefits o f tunable pore diameters. Mesoporous  silica-based materials  may  be r e a d i l y m o d i f i e d since there are  many  accessible surface h y d r o x y groups: it has been f o u n d that 2 6 - 3 0 % o f all S i atoms i n M C M - 4 1 bear a surface h y d r o x y g r o u p . chemistry. ' 8  framework,  2 0  2 1  1 9  T h e surface d e r i v a t i z a t i o n can proceed b y w e l l - d e v e l o p e d silane  A d d i t i o n a l functionality c a n be a c h i e v e d by substituting other elements into the and u s i n g o r g a n i c a l l y - m o d i f i e d f r a m e w o r k s o u r c e s .  22  T h i s f l e x i b i l i t y a n d the large internal surface area m a k e M C M - 4 1 material, and the recently. ' 2 2  2 3  creation o f c o m p o s i t e materials based  on M C M - 4 1  an attractive  has  In m a n y studies, it has not a l w a y s been evident that M C M - 4 1  been  host  reviewed  p r o v i d e s any  substantial advantage o v e r disordered p o r o u s materials, except p o s s i b l y i n its s o m e w h a t larger specific surface area. H o w e v e r , the w e l l - d e f i n e d channels are evidently necessary for the role o f template for nanometre-scale w i r e s and ordered encapsulation.  3.2  Characterization of MCM-41 Materials T h e characterization techniques d e s c r i b e d i n chapter 2 are d i r e c t l y applicable to M C M -  4 1 , especially h i g h resolution t r a n s m i s s i o n electron m i c r o s c o p y ( T E M ) , electron  energy-loss  spectroscopy ( E E L S ) and energy-filtered t r a n s m i s s i o n electron m i c r o s c o p y ( E F T E M ) , a l l o w local c h e m i c a l characterization. A n u m b e r  o f other techniques  which  are used to measure  material-averaged properties. W h i l e there is general agreement o n the formation m e c h a n i s m for M C M - 4 1 , there is w i d e s p r e a d disagreement  on its exact structure. T h i s seems to be due to the fact that the  m e s o p o r o u s 2 - D hexagonal phase c a n be r e a d i l y m a d e under a variety o f different c o n d i t i o n s u s i n g different starting materials, a n d the exact characterization o f the w a l l structure is difficult. 56  3.2.1  Diffraction Techniques  Diffraction is used to identify the presence o f an ordered phase. The interplanar spacing dhki associated with a plane identified by the Miller indices hkl must satisfy the Bragg condition:  2d s\n0  = nA, « = 1,2,3,...  hkl  (Eq. 3.1)  For a hexagonal system, diffraction cannot be observed between planes when / is odd, or (h + 2k) = 3n, where n is an integer. However, / = 0 for a 2-D system, so peaks are expected for all the planes (100), (110), (200), (210), etc. The 2-D interplanar spacing dm is related to the lattice constant as: A hk0  a  -  a  ~  4  / , 2  - ( h  + h k  , 2 N  + k  )  v  (Eq. 3.2) H  '  Powder X-ray diffraction is routinely used to characterize M C M - 4 1 materials. Neutron scattering has also been used to investigate the pore structure in more d e t a i l . ' 2 4  2 5  While X-ray  powder diffraction data is usually plotted against diffraction angle (26), neutron scattering data is presented as a function of the magnitude o f the scattering vector, Q: ^ 4;rsinf? Q = A  ,^ „ „ (Eq.3.3) N  Diffraction techniques may be used to investigate the presence o f molecular guests within the channels o f M C M - 4 1 i f there is contrast matching between the guest and the host. In the case of X-ray diffraction,  sufficient electron density to match the pore walls is obtained for  halogenated organic compounds.  26  For neutron scattering, contrast matching has been reported  for 59% deuterated benzene in M C M - 4 1  2 7  Therefore, not all guests can be readily detected  using diffraction techniques. 57  3.2.2  Physisorption P h y s i s o r p t i o n is w i d e l y used to characterize the surface a n d pore structure o f mesporous  materials.  28  A d s o r b a t e s o f different  size and c h e m i c a l functionality are used to probe  the  structure and reactivity o f the material surface. M i c r o p o r o s i t y is generally p r o b e d u s i n g argon w h i l e mesoporosity is investigated w i t h nitrogen. T h e nitrogen adsorption isotherm c a n be separated into three regimes: (1) m i c r o p o r e f d l i n g and m o n o l a y e r formation, (2) m u l t i l a y e r f o r m a t i o n , a n d (3) c a p i l l a r y condensation and further superficial adsorption (Figure 3.5). R e g i m e (2) y i e l d s the total surface area o f the sample, w h i c h is u s u a l l y determined u s i n g the B r u n a u e r -  600  500 BET "O s_  o  o o  400  CO TJ  ©  < CD  300  E o  >  G  Analysis  CD  -Q  W  200  CD  B J H Analysis  G  o  100  0.0  0.2  0.6  0.4  0.8  1.0  P/Pn F i g u r e 3.5 N i t r o g e n (o)  adsorption and ( • ) desorption isotherms for M C M - 4 1 . T h e lack o f  hysteresis is characteristic o f M C M - 4 1 materials. T h e points used for total surface area ( B E T ) and pore size d i s t r i b u t i o n ( B J H ) analysis are indicated. 58  Emmett-Teller ( B E T ) approach.  R e g i m e (3) is u s u a l l y analysed to determine the pore size  2 9  distribution, w h i c h is the most  important  a n d also most debated  property o f  mesoporous  materials. T h e classic determination o f pore size distributions from adsorption isotherms is that o f Barrett, J o y n e r a n d H a l e n d a ( B J H a n a l y s i s ) ; shown  to  yield  distributions. ' 3 1  3 2  better  absolute  3 0  recently m o r e sophisticated approaches have been  agreement  with  other  techniques  for  determining  size  In the B J H approach, the thickness o f the adsorbed nitrogen layer for a g i v e n  relative pressure is g i v e n b y the H a r k i n s - J u r a e q u a t i o n calibrated for M C M - 4 1 materials:  3  3 3  w i t h the f o l l o w i n g e m p i r i c a l parameters  4  -10.3968  0.1  60.65 0.03071-log  + 0.3 nm ( E q . 3.4)  2-  H o w e v e r , there is no single u n a m b i g u o u s technique for m e a s u r i n g this d i s t r i b u t i o n for m e s o p o r o u s materials. T h e variations i n the structure o f M C M - 4 1 due to differing synthesis c o n d i t i o n s m a k e it difficult to c o m p a r e results between different studies i n the literature, a n d lead to contradictory c o n c l u s i o n s about the pore s t r u c t u r e . - ' ' 1 6  2 4  3 5  3 6  Nevertheless,  behaves as an ideal adsorbent a n d can be used as a reference m a t e r i a l . analysis is used to quantify the  shift i n the  c a p i l l a r y condensation  3 4  MCM-41  In this w o r k , B J H  p o i n t i n the  nitrogen  adsorption isotherm.  3.2.3  Other Techniques T h e r m o g r a v i m e t r i c analysis ( T G A ) is used to reveal the o r g a n i c content o f c o m p o s i t e  materials based o n M C M - 4 1 . T h e host i t s e l f s h o w s n o mass loss up to 1000 ° C . T h e t h e r m a l degradation processes for an organic material o c c u r at specific temperatures and this m a y be 59  used to determine  the m a s s content o f specific organic c o m p o n e n t s  i n the c o m p o s i t e .  The  derivative o f the T G A data is used to e m p h a s i z e the presence o f different degradation processes, and these can be fitted satisfactorily b y G a u s s i a n functions. T h i s a l l o w s the mass content o f each process to be determined m o r e accurately. Fourier-transform components  infrared spectroscopy  t h r o u g h their characteristic  ( F T - I R ) c a n be used to identify the  organic  m o l e c u l a r vibrations, as the s i l i c a m a t r i x is m o s t l y  transparent to infrared radiation.  3.3  Polymerization within MCM-41 Reports o n p o l y m e r i n c l u s i o n i n M C M - 4 1 appeared i n the m i d 1 9 9 0 ' s , i n v o l v i n g both  In situ p o l y m e r i z a t i o n  conjugated and insulating p o l y m e r s . reported initially b y W u and B e i n .  3 7  w i t h i n the channels o f M C M - 4 1 w a s  O x i d a t i v e p o l y m e r i z a t i o n o f aniline was a c h i e v e d by first  c o n d e n s i n g the m o n o m e r f r o m the v a p o u r phase into the host channels. T h e loaded host w a s then soaked  i n an aqueous solution o f oxidant, w h i c h p r o d u c e d  encapsulated  polymer  A c r y l o n i t r i l e w a s p o l y m e r i z e d s i m i l a r l y , u s i n g instead a solution o f r a d i c a l i n i t i a t o r . al.  prepared  other  free-radical  initiated  methacrylate) a n d p o l y v i n y l a c e t a t e .  39  polymers  in  MCM-41:  polystyrene,  38  chains.  Unger e t  poly(methyl  T h e m o n o m e r s w e r e loaded into M C M - 4 1 through v a p o u r  exchange, and the gas-phase r a d i c a l initiator w a s subsequently diffused into the loaded  MCM-  41.  3.3.1  PPV in MCM-41 T h e introduction o f P P V into M C M - 4 1 m u s t also proceed through  in situ synthesis  because o f the i n s o l u b l e and infusible nature o f the p o l y m e r . T h e p o l y m e r i z a t i o n o f P P V can be  60  carried out b y n u m e r o u s r o u t e s , Gilch route  4 1  40  the simplest b e i n g a base-initiated condensation k n o w n as the  ( F i g u r e 3.6). H o w e v e r , this route w a s not exploitable w i t h i n the M C M - 4 1  host:  w h i l e the m o n o m e r c o u l d be readily loaded into M C M - 4 1 t h r o u g h s u b l i m a t i o n , the subsequent introduction o f a base o f sufficient strength was not possible. A q u e o u s bases attacked and d i s s o l v e d the host, a n d non-aqueous bases s o l u b i l i z e d the m o n o m e r a n d extracted it f r o m the pores before p o l y m e r i z a t i o n c o u l d occur. O t h e r p o l y m e r i z a t i o n routes were investigated and an elegant solution to this p r o b l e m w a s f o u n d through the w o r k o f K u m a r  et al.,  42  w h o had  prepared P P V w i t h i n the pores o f V y c o r , a disordered porous glass. T h i s a p p r o a c h used a m o r e reactive m o n o m e r , x y l y l e n e bis(tetrahydrothiophenium c h l o r i d e ) , w h i c h c o u l d be p o l y m e r i z e d b y deprotonated surface h y d r o x y groups. In s i m i l a r fashion, M C M - 4 1 w a s converted to a basic f o r m b y deprotonating its surface hydroxy  groups  with  a non-aqueous  base  (tetrabutylammonium  hydroxide ( T B A O H )  in  methanol) and isolated. T h i s activated f o r m o f M C M - 4 1 thus contained the initiating base w i t h i n its channels. P o l y m e r confinement o c c u r e d t h r o u g h the r a p i d p o l y m e r i z a t i o n o f the m o n o m e r s w i t h i n the host channels ( F i g u r e 3.7). T h e k e y evidence to each step i n this process is based o n F T - I R spectroscopy, T G A , nitrogen p h y s i s o r p t i o n , E E L S and E F T E M . X - r a y and neutron diffraction experiments p r o v i d e d no a d d i t i o n a l i n f o r m a t i o n o n the c o m p o s i t e materials.  NBuOK/THF  F i g u r e 3.6 T h e G i l c h route to P P V starting f r o m d i c h l o r o - p - x y l e n e . 61  F i g u r e 3.7 Synthetic scheme for the preparation o f P P V / M C M - 4 1 h y b r i d material. O n l y one c h a i n is s h o w n i n the pore for clarity.  3.4  Experimental Results T h e pure  Characterization  silica M C M - 4 1 with  X-ray  host w a s  diffraction  synthesized f o l l o w i n g  and  nitrogen  a literature  physisorption  procedure.  indicated  43  substantial  variations from batch to batch i n the lattice constant (4.3 to 4.5 n m ) and B J H pore diameter (3.1 to 3.6 n m ) . T h u s c o m p a r i s o n s were o n l y m a d e w i t h materials prepared i n the same batch. B E T surface area w a s (1.0 ± 0.1) x l O  3  The  m g" , and the total pore v o l u m e w a s 0.8 c m g" . T h e F T 2  1  3  1  I R s p e c t r u m o f the e m p t y host s h o w e d absorption bands for the surface h y d r o x y groups f r o m 3 7 0 0 to 3 0 0 0 c m " , the s i l i c a f r a m e w o r k from 1100 to 6 0 0 c m " , and adsorbed water at 1700 1  1  cm" . 1  62  F o l l o w i n g T B A O H treatment o f M C M - 4 1 , T G A s h o w e d 1 0 % water content a n d a 3 8 % mass loss between 100 a n d 3 0 0 ° C ; this w a s a s c r i b e d to the d e c o m p o s i t i o n o f the T B A counteri o n ( F i g u r e 3.8(a)). T h e mass content o f T B A suggests that 1 5 % o f a l l S i atoms i n M C M - 4 1 h a d an associated w i t h a T B A counter-ion. T h e presence o f the counter-ion reduced the B J H pore diameter b y 1.2 n m ( F i g u r e 3.8(b)). T h e p o l y m e r i z a t i o n step then y i e l d e d a bright green p o w d e r . E x c e s s m o n o m e r and possible side-products c o u l d be easily separated  from  the p o w d e r b y  w a s h i n g d u r i n g filtration. T h e r m a l c o n v e r s i o n o f the p o l y m e r u n d e r v a c u u m l e d to a  fluorescent  yellow powder. X - r a y diffraction o f the resulting P P V / M C M - 4 1  c o m p o s i t e i n d i c a t e d that the s a m p l e  order r e m a i n e d ( F i g u r e 3.9(b)). T h e result o f neutron scattering experiments c a r r i e d out by L . F a n , Z . T u n and J . Y o u n g o n M C M - 4 1 a n d t w o p o l y m e r - c o n t a i n i n g samples is s h o w n i n F i g u r e 3.9(b). T h e strong scattering peak at Q « 0.16 is i n agreement w i t h the X - r a y data. T h e presence o f p o l y m e r i n the sample d i d not seem to alter this peak substantially.  63  0.30  100-f  ^ 0.25 g CD  -1 0.20 2. 5L  H0.15  I co  0.10  §  - 0.05 Q - 0.00 50  0.05 100 200 300 400 500 600 700 800 900 1000  ->—i— —i— —I— —I—'—i— —I—'—I— —I— —I— 1  0  1  1  1  1  1  r  Temperature ( ° C ) Relative Pressure, p/p 0.0  600  0.2  0.4  0  0.6  0.8  1.0  0.18  B o  500-  o  ooooooo'  TJ  o —i  o> 400-I  CD  CO  E "S  •a  < O  c 3  300  CD  ,o°  l_ O  TBA/MCM-41 D ~ 2.2 nm  CO "CD 200  a  3 CQ  Cd  E  _2  3  > 100  OA  1.5 Pore Diameter (nm) F i g u r e 3 . 8 (a) T h e r m o g r a v i m e t r i c analysis o f T B A O H - t r e a t e d M C M - 4 1 , (b) ( o ) , ( • ) nitrogen adsorption i s o t h e r m and  ( A ) , ( A ) B J H pore size d i s t r i b u t i o n for empty and T B A O H - t r e a t e d  M C M - 4 1 , respectively. 64  F i g u r e 3.9 (a) P o w d e r X - r a y diffraction pattern a n d (b) neutron scattering data for ( • )  MCM-  4 1 , P P V / M C M - 4 1 ( • ) s a m p l e 1 a n d ( • ) sample 2 .  65  T G A o f the c o m p o s i t e material ( F i g u r e 3.10(a)) s h o w e d an initial water desorption at 4 0 ° C , f o l l o w e d b y a 1% w e i g h t loss at 2 5 0 ° C , w h i c h is most likely due to r e s i d u a l T B A i n the material  that  did  not  decompose  during  the  thermal  conversion  process.  The  polymer  degradation began at 3 5 0 ° C and s h o w e d at least t w o different d e c o m p o s i t i o n processes, at 5 2 5 ° C and 7 3 0 ° C . S a m p l e s consistently s h o w e d a p o l y m e r content o f 8 ± 2 wt. % . A n a l y s i s o f the 2  nitrogen adsorption isotherm (not s h o w n ) indicated a r e d u c e d B E T surface area ( 8 . 5 x 1 0 '  2 1  mV)  and pore v o l u m e (0.51 ± 0.04 c m g " ' ) . T h e B J H pore diameter w a s observed to decrease b y 0.3 ± 3  0.1 n m ( F i g u r e 3.10(b)). T h e F T - I R spectrum ( F i g u r e 3.11(a)) s h o w e d n e w bands characteristic o f P P V . T h e b a n d at 3 0 2 5 c m " is associated w i t h the C - H stretch i n fnms'-vinylene, w h i l e the bands at 1517 and 1  1422 c m " are due to 1,4-phenylene r i n g stretching m o d e s . 1  4 4  The distribution o f polymer was  investigated b y E E L S and E F T E M . T h e l o w loss spectra o f the empty and p o l y m e r - f i l l e d M C M 41 are c o m p a r e d i n F i g u r e 3.11(b). T h e zero-loss peak has been r e m o v e d by careful subtraction o f a reference peak, r e v e a l i n g that o n l y P P V / M C M - 4 1 has losses b e l o w 8 e V . E F T E M  images  a c q u i r e d at 0, 6 and 12 e V w i t h a 2 e V w i n d o w were used to i m a g e the d i s t r i b u t i o n o f the losses ( F i g u r e 3.12). T h e U V / V i s absorbance spectrum o f the c o m p o s i t e s h o w e d an onset at 5 0 0 n m and a peak at 4 2 0 n m , w h i l e the p h o t o l u m i n e s c e n c e ( P L ) spectrum s h o w e d peaks at 5 1 7 , 547 and 5 9 0 n m ( F i g u r e 3.13(a)). T h e temperature-dependent P L spectra o f encapsulated P P V w e r e measured in c o l l a b o r a t i o n w i t h M . M c C u t c h e o n and J . Y o u n g , u s i n g an excitation w a v e l e n g t h o f 3 8 6 n m ( F i g u r e 3.13(b)).  66  100 4  4 0.12  98 H  4 0.10  4 0.08  4 0.06  03  -  ^. o CO CO  CD  4 0.04  0.02  o  o  4 0.00  i—•—i— —r 1  100  200  300  400  500  600  700  800  900  1000  Temperature (°C) 0.008  B 0.007 - |  E c 'o>  0.006-  MCM-41 D - 3 . 1 nm  PPV/MCM-41 D~2.9nro D - 2 . 8 nm  0.005 •  E <u E O  >CD i  O  0.0040.0030.002 -  0.  0.001 0.0001.5  - i —  2.5  3.0  3.5  4.0  4.5  5.0  Pore Diameter (nm)  Figure  3.10  (a)  T h e r m o g r a v i m e t r i c analysis  of PPV/MCM-41,  and  (b)  B J H pore  size  d i s t r i b u t i o n o f ( • ) e m p t y M C M - 4 1 , P P V / M C M - 4 1 ( A ) s a m p l e 1 a n d ( o ) sample 2.  67  3000  2500  2000  1500  Wavenumber (cm ) 1  8000  0  10  20  30  40  50  60  Energy (eV) F i g u r e 3.11 Spectral signature o f (—) P P V / M C M - 4 1 a n d (—) M C M - 4 1 , u s i n g (a) F T - I R and (b) EELS. 68  A F i g u r e 3.12 E F T E M images for (a) M C M - 4 1  B and (b) P P V / M C M - 4 1 ,  filtered  with a 2 e V  w i n d o w centered o n the g i v e n energies. T h e 6 e V image reveals the p o l y m e r d i s t r i b u t i o n i n the c o m p o s i t e material; the contrast has been enhanced o n the inset. N o t e the presence o f the lacey carbon support i n (a), w h i c h also s h o w s a strong n-ic p l a s m o n . T h e m a i n scale bar is 5 0 n m a n d the inset scale bar is 2 0 n m . 69  I——I——I——I—'—I—'—I——I——I——I——I 1  250  1  1  1  1  1  1  300 350 400 450 500 550 600 650 700  Wavelength (nm) l—•—i— —i— —i—«—i— —i—•—i—•—r 1  1  1  —i—•—i—•—i— —i— —i—•—i— —i— —i— 1  480  1  1  1  510 540 570 600 630 660  690  Wavelength (nm) F i g u r e 3.13 (a) R o o m temperature (—) U V / V i s absorbance a n d  (  ) photoluminescence o f  P P V / M C M - 4 1 c o m p o s i t e , (b) temperature-dependent p h o t o l u m i n e s c e n c e spectra o f P P V / M C M 41. 70  3.5  Discussion T h e presence o f P P V is clearly i n d i c a t e d by the U V / V i s absorbance, fluorescence a n d  F T - I R spectrum o f the c o m p o s i t e sample. T h e central issue i n the characterization o f this material is whether the p o l y m e r chains actually reside i n the channels o f M C M - 4 1 , or p o s s i b l y f o r m a t h i n coat o n the outside o f the host particles.  3.5.1  Thermogravimetric Analysis T h e T G A data for P P V / M C M - 4 1  ( F i g u r e 3.10(a))  clearly indicate that the t h e r m a l  b e h a v i o u r o f the p o l y m e r is altered from its unencapsulated degradation process at 5 4 0 ° C .  4 5  T h e higher temperature degradation process at 7 3 0 ° C has also  been observed for P P V encapsulated i n m o n t m o r i l l o n i t e . o f some  encapsulated  f o r m , w h i c h s h o w s o n l y one  46  p o l y m e r . T h e l o w e r degradation  T h i s strongly suggests the presence process  m i g h t be associated  with  unencapsulated p o l y m e r . A s i m p l e m o d e l for the c o m p o s i t e material c a n be used to determine the approximate p o l y m e r mass content for a g i v e n n u m b e r o f p o l y m e r chains per channel i n M C M - 4 1 . W i t h the assumptions that (1) the p o l y m e r chains have no c o n f o r m a t i o n a l defects, (2) they are oriented parallel to the channels, (3) the effect o f end-groups c a n be i g n o r e d , and (4) the p o l y m e r o n the external surface is m i n i m a l , the mass fraction F o f p o l y m e r i n M C M - 4 1 for a g i v e n n u m b e r o f chains Nper pore can be calculated, u s i n g parameters g i v e n i n T a b l e 3.2: JV-2.57xlQ~ 1.90 x 1 0 '  2 0  +N-  ( E q . 3.5)  2 2  2.57 x l O "  2 2  71  Table 3.2 Parameters for c a l c u l a t i o n o f P P V mass fraction i n M C M - 4 1 Source  Property  Value  P P V repeat length  0.66 n m  P P V repeat unit mass  102.1 g m o l "  M C M - 4 1 w a l l density  2.2 g c m "  M C M - 4 1 lattice constant  ,  M C M - 4 1 pore diameter V a l u e s o f F for s m a l l values o f N p o l y m e r c h a i n per pore  Ref. 47 1  4.3 n m  a m o r p h o u s Si02 X - r a y diffraction  3.1 n m  N2 p h y s i s o r p t i o n  3  are g i v e n i n T a b l e 3.3. T h i s clearly s h o w s that even one  w o u l d represent a substantial mass  l o a d i n g o f the  sample.  The  e x p e r i m e n t a l mass content o f 8% suggests that the pores o f M C M - 4 1 contain 6 p o l y m e r chains each on average. I f a s s u m p t i o n (4) is i n v a l i d , and some o f the p o l y m e r ( - 5 0 % ) is externally located as suggested by the T G A , each channel contains 3 p o l y m e r chains o n average.  Table 3.3 P o l y m e r mass fraction N F(%)±  3.5.2  1%  F for Npolymer  chains per pore i n M C M - 4 1 , u s i n g E q . 3.5.  1  2  4  6  8  10  1  3  5  8  10  12  Physisorption Data T h e reduction o f the B J H pore diameter o f the c o m p o s i t e material, albeit s m a l l , w a s  observed r e p r o d u c i b l y and is significant. T h e B J H m e t h o d assumes a c y l i n d r i c a l geometry for the pores and does not p r o v i d e any i n f o r m a t i o n about the arrangement o f the p o l y m e r chains w i t h i n the pores; one p o s s i b l e interpretation is that the p o l y m e r is present as a t h i n layer o n the w a l l s o f the pores. A s the P P V b a c k b o n e is planar, a p o l y m e r c h a i n c a n lie flat against the pore w a l l and p r o d u c e a relatively s m a l l change i n the B J H pore diameter. T h e observed differences i n pore diameter r e d u c t i o n c o u l d suggest differences i n p o l y m e r c o n f o r m a t i o n w i t h i n the pores, l e a d i n g to a different effect o n the isotherm. T h e s m a l l amount o f residual T B A ions observed by  72  T G A (1 wt. %) w o u l d be expected to reduced the B J H pore diameter b y 0.03 n m , based o n the effect o f 38 w t . % T B A content. I f p o l y m e r i z a t i o n o c c u r r e d strictly i n s o l u t i o n , due to proton transfer from the solvent or trace water and consequent m i g r a t i o n o f the basic species into the b u l k s o l u t i o n , the p o l y m e r m i g h t be present strictly as a layer c o a t i n g the outside o f the M C M - 4 1 particles. T h e effect on the nitrogen adsorption isotherm w o u l d then be v e r y different: i f the pores w e r e fully b l o c k e d , clearly n o substantial adsorption w o u l d o c c u r , w h i c h is not the case. P a r t i a l b l o c k a g e o f the pores b y a s u p e r f i c i a l layer w o u l d not alter the observed capillary condensation point, but w o u l d introduce hysteresis into the i s o t h e r m , w h i c h w a s also not observed. A n external layer that d i d not i m p e d e the adsorption process w o u l d not reduce the o b s e r v e d pore diameter, but s i m p l y reduce the specific surface area and pore v o l u m e due to the presence o f a d d i t i o n a l , non-porous m a s s i n the sample. T h e relative decrease i n the specific surface area ( 1 5 % ) and pore v o l u m e ( 3 6 % ) is greater than the a d d i t i o n a l mass (8%) and also i n agreement w i t h the relative decrease expected for a c y l i n d r i c a l geometry ( d V / V = 2 d A / A ) .  3.5.3  X-ray and Neutron Diffraction T h e X - r a y diffraction data i n d i c a t e d that the p o l y m e r i z e d s a m p l e retained the o v e r a l l  order o f the host material, w i t h n o substantial shift i n the diffraction peak positions. N o attempt w a s m a d e to interpret the changes i n the peak intensities. The  neutron  scattering  results  showed  only  small  differences  between  the  empty  M C M - 4 1 and the P P V / M C M - 4 1 c o m p o s i t e . M o d e l l i n g o f M C M - 4 1 coated w i t h a 0.1 n m - t h i c k c o a t i n g o f m a t e r i a l contrast-matched to the pore w a l l s has been s h o w n to decrease the a m p l i t u d e o f the m a i n scattering peak and shift the relative p o s i t i o n o f the s m a l l e r p e a k .  4 8  The small  difference o b s e r v e d i n the m a i n peak ( Q « 0.16) between the t w o P P V / M C M - 4 1 samples does 73  not correlate w i t h the B J H pore diameters, a n d the s m a l l e r peak ( Q « 0.28) is not sufficiently resolved o n a l l samples to determine any shift relative to the m a i n peak. T h e difference i n scattering cross-section between P P V a n d M C M - 4 1  cannot be d e t e r m i n e d w i t h o u t  further  m o d e l l i n g a n d it is not p o s s i b l e to state w h a t the effect o f a s m a l l a m o u n t o f P P V i n the pores w o u l d have o n the diffraction pattern. A s they stand, these results are not i n disagreement w i t h the hypothesis o f a s m a l l n u m b e r o f chains present i n the pores.  3.5.4  E E L S and  EFTEM  N o r m a l T E M investigation o f the M C M - 4 1  a n d P P V / M C M - 4 1 particles revealed n o  r e a d i l y v i s i b l e differences. A substantial surface layer o f p o l y m e r w o u l d have been v i s i b l e as a n a m o r p h o u s (structure-free) layer at the particle edges. F o r 1 u m particles, an external p o l y m e r mass content o f 4 % w o u l d g i v e rise to a u n i f o r m external layer o f ~7 n m ( a s s u m i n g a p o l y m e r density o f 1 g c m " ) A l s o , n o b u l k p o l y m e r particles w e r e observed. The  electron energy-loss s p e c t r u m o f P P V / M C M - 4 1 particles clearly s h o w e d the loss  attributable to the 71-71* p l a s m o n at 6-7 e V , w h i c h w a s absent from the e m p t y M C M - 4 1 sample. L o s s e s b e l o w 5 e V w e r e due to the o p t i c a l absorption o f P P V , w h i c h has an onset at 2.5 e V a n d a peak near 3 e V  4  9  A t energies above 8 e V , the spectra are d o m i n a t e d b y the b r o a d b u l k p l a s m o n  o f the s i l i c a m a t r i x centered at 2 2 e V , i n agreement w i t h the value o b s e r v e d for a f o r m o f mesoporous s i l i c a . energy  5 1  5 0  T h e p o l y m e r b u l k p l a s m o n w o u l d be expected to appear at a s i m i l a r  and c o u l d not be d i s t i n g u i s h e d . T h e shoulders v i s i b l e between 10 and 2 0 e V are m o s t  l i k e l y due to w e a k surface p l a s m o n s . T h e stability o f the m a t e r i a l seemed v e r y g o o d under a diffuse b e a m w i t h o u t any special measures to l i m i t b e a m damage. Energy  filtering  o n the 7i-7t* p l a s m o n is e x p e r i m e n t a l l y most p r a c t i c a l as the tail o f the  zero-loss peak is substantial b e l o w 5 e V . T h e 6 e V energy-filtered images o f M C M - 4 1  and 74  P P V / M C M - 4 1 m a y be c o m p a r e d to determine the p o l y m e r d i s t r i b u t i o n i n the c o m p o s i t e . E m p t y M C M - 4 1 does not s h o w any substantial losses at 6 e V , and the filtered image is c o r r e s p o n d i n g l y dark. P P V / M C M - 4 1 , o n the other hand, s h o w s 6 e V losses distributed throughout the particle. Intensity variations are seen, and these are due to thickness variations o f the sample, as seen i n the 0 e V image. T h e r e is n o i n d i c a t i o n o f edge brightness, w h i c h w o u l d have suggested  the  presence o f a t h i n p o l y m e r c o a t i n g o n the outside o f the particles. T h e p e r i o d i c i t y o f the host material is also v i s i b l e i n the 6 e V i m a g e o f P P V / M C M - 4 1 . W h i l e this structure is not fully r e s o l v e d , it strongly suggests that the p o l y m e r is c o n f i n e d i n the channels. These results corroborate the p h y s i s o r p t i o n and T G A data i n suggesting that the p o l y m e r chains are incorporated into the channels o f M C M - 4 1 . W h i l e the presence o f s o m e external p o l y m e r m a y be indicated b y the T G A data, the E F T E M results indicate that this can o n l y be i n the f o r m o f a v e r y t h i n layer (< 2 n m ) , c o r r e s p o n d i n g to < 1 wt. % external p o l y m e r . T h e measurements were carried out on the same samples that w e r e used for neutron further  scattering,  i n d i c a t i n g that neutron scattering w a s not p a r t i c u l a r l y suited for investigating these  samples w i t h o u t resorting to deuteration to m a t c h the scattering cross-section o f the host and guest. T h i s demonstrates the ability o f E F T E M to reveal directly the presence o f conjugated p o l y m e r at v e r y h i g h resolution. T h e p r e v i o u s estimate o f the resolution l i m i t for m a p p i n g s m a l l conjugated m o l e c u l e s w a s based o n the edge sharpness o f a m o l e c u l a r c r y s t a l ,  5 2  whereas these  results s h o w directly that s u c h h i g h resolution is achievable. F u r t h e r m o r e , E F T E M w a s used to image directly the presence o f an o r g a n i c guest i n the channels o f M C M - 4 1  for the first t i m e . T h e l o w n u m b e r o f p o l y m e r chains per pore also  suggests that it m a y be possible to image a single isolated conjugated p o l y m e r c h a i n (perhaps dispersed i n a non-conjugated p o l y m e r i c m a t r i x ) b y this technique.  75  3.5.5  UV/Vis Absorbance and Photoluminescence T h e U V / V i s absorbance  spectrum o f the c o m p o s i t e is v e r y s i m i l a r to that o f P P V ,  i n d i c a t i n g a substantial degree o f p o l y m e r i z a t i o n . H o w e v e r , it d i d not s h o w any o f the structure observed i n the absorbance  spectrum o f stretch-ordered  PPV.  5 3  T h e P L spectrum at r o o m  temperature is i n close agreement w i t h literature reports. T h e structure observed i n the spectrum is s i m i l a r to that o f unencapsulated P P V . T h e peaks are red-shifted w i t h decreasing temperature; this effect has been attributed i n b u l k P P V to the r e d u c t i o n o f t o r s i o n a l m o d e s i n the p o l y m e r c h a i n that affect the effective conjugation l e n g t h . T h e o p t i c a l measurements d i d not  5 3  reveal any  substantially n e w  behaviour  in  the  c o m p o s i t e material. Initial attempts at m e a s u r i n g P L d y n a m i c s were unsuccessful, as the s a m p l e r a p i d l y bleached under laser e x c i t a t i o n at 3 8 6 n m .  3.6  Conclusion T h e e x p e r i m e n t a l evidence s h o w e d that a P P V / M C M - 4 1  c o m p o s i t e w a s successfully  synthesized. C h e m i c a l analysis b y E E L S and E F T E M a l l o w e d the presence o f P P V inside the channels to be established u n a m b i g u o u s l y . T h e p o l y m e r mass content suggested that 3 to 6 p o l y m e r chains resided i n each pore. H o w e v e r , this material is less than ideal for property c o m p a r i s o n s for a n u m b e r o f reasons. T h e in situ synthesis leads to a p o l y m e r that cannot be fully characterized due to its i n s o l u b i l i t y . C o m p a r i s o n o f the P L d y n a m i c s w i t h n o r m a l P P V w o u l d be difficult, due to the simultaneous change i n structure and e n v i r o n m e n t , w h i c h has also been a n issue i n the w o r k o f G i n et al. on P P V i n a l y o t r o p i c l i q u i d crystal h o s t .  5 4  T h e r e is m o s t l i k e l y some p o l y m e r c o a t i n g  the outside o f the M C M - 4 1 particles, w h i c h w o u l d further c o m p l i c a t e the analysis. T h i s w a s also f o u n d i n the w o r k o f T o l b e r t et al. o n M E H - P P V i n an oriented m e s o p o r o u s s i l i c a host, and the 76  energy m i g r a t i o n between p o l y m e r outside and inside the channels was s t u d i e d .  5 5  S u c h studies  are not p o s s i b l e o n the c o m p o s i t e prepared here because o f the lack o f m a c r o s c o p i c orientation. These c o m p l i c a t i o n s c a n be e l i m i n a t e d i f the host is prepared i n the f o r m o f an oriented t h i n film, and i f a soluble f o r m o f P P V is used instead. T h i s a l l o w s a fully characterized p o l y m e r to be used and m o r e m e a n i n g f u l c o m p a r i s o n s to be m a d e . Therefore further efforts w e r e directed t o w a r d s the creation o f an appropriate t h i n film host.  77  Experimental Details MCM-41  was  synthesized  according  to  h e x a d e c y l t r i m e t h y l a m m o n i u m c h l o r i d e as the s u r f a c t a n t .  a 43  literature  procedure  using  T h e synthesis w a s carried out at 80  ° C for 2 days i n a T e f l o n - l i n e d stainless steel b o m b for m o s t samples. O n e large batch w a s synthesized i n a p o l y p r o p y l e n e bottle to p r o v i d e enough s a m p l e for neutron scattering. A f t e r w a s h i n g w i t h m e t h a n o l and water, the collected p o w d e r was c a l c i n e d under air w i t h a heating rate o f 1 ° C m i n " to 5 4 0 ° C and h e l d at that temperature for 6 hours. N i t r o g e n adsorption analysis 1  w a s p e r f o r m e d o n a M i c r o m e r i t i c s A S A P 2 0 1 0 instrument. T h e r m o g r a v i m e t r i c analysis w a s carried out u s i n g a T A Instruments T G A 51 under N2 f l o w a n d a heating rate o f 10 ° C m i n " . Infrared spectra w e r e obtained f r o m K B r pellets u s i n g a 1  BOMEM  M B 1 5 5 S F T - I R spectrometer. P o w d e r X - r a y patterns w e r e collected o n a R i g a k u  R o t a f l e x rotating-anode diffractometer. Chemicals  were  obtained  from  Aldrich  Inc.  Xylylene  bis(tetrahydrothiophenium  c h l o r i d e ) w a s p u r i f i e d b y recrystallization f r o m water. E t h a n o l w a s d r i e d o v e r 4 A m o l e c u l a r sieves. A d a p t i n g the w o r k o f K u m a r et a l . , 100 ° C and then treated  with  4 2  the c a l c i n e d M C M - 4 1 w a s d r i e d under v a c u u m at  1 M t e t r a b u t y l a m m o n i u m h y d r o x i d e i n m e t h a n o l under dry  c o n d i t i o n s . T h i s m i x t u r e was left to stand for 4 h at r o o m temperature, after w h i c h the basic M C M - 4 1 was filtered o f f and d r i e d under v a c u u m . T h e resulting s o l i d w a s placed i n a 1 0 - 2 0 % w/w  s o l u t i o n o f x y l y l e n e bis(tetrahydrothiophenium c h l o r i d e ) i n d r y ethanol at 5 0 ° C for 2 4 h ,  and then w a s h e d w i t h ethanol and water to r e m o v e excess m o n o m e r and base. T h e bright y e l l o w - g r e e n p o w d e r w a s d r i e d under v a c u u m at r o o m temperature; subsequent heating to 2 0 0 ° C u n d e r v a c u u m (10" T o r r ) for 6 h resulted i n the p o w d e r t u r n i n g bright y e l l o w i n c o l o r . 2  78  For T E M analysis, samples w e r e deposited o n lacey earbon-coated C u g r i d s ( T e d P e l l a , Inc.)  from  a suspension i n m e t h a n o l . E l e c t r o n energy-loss spectroscopy a n d energy-filtered  t r a n s m i s s i o n electron m i c r o s c o p y w e r e c a r r i e d out on a T e c n a i F 2 0 T E M e q u i p p e d w i t h a G a t a n I m a g i n g Filter. T h e accelerating voltage w a s 197 k V (200 k V n o m i n a l l y , offset b y 3 k V by the G I F ) . L o s s spectra w e r e recorded i n T E M m o d e b y p l a c i n g the particle o f interest above the G I F entrance aperture (diameter 2.0 m m ) . T h e zero-loss peak w a s r e c o r d e d separately for subtraction by m o v i n g to an e m p t y area o n the g r i d . T h e system energy resolution, g i v e n by the F W H M o f the zero-loss peak, w a s 0.9 e V . T h e energy d i s p e r s i o n o f the spectrometer w a s 0.10 e V / p i x e l , w h i c h w a s calibrated u s i n g a 5 0 e V w o b b l e o n the drift tube. E F T E M images w e r e a c q u i r e d w i t h a 2 e V slit. The U V / V i s absorbance s p e c t r u m w a s measured for P P V / M C M - 4 1 as a nujol m u l l , u s i n g a U n i c a m U V - 2 spectrometer.  L o w - t e m p e r a t u r e P L spectra w e r e obtained b y p r e s s i n g the  samples between quartz plates i n a cryostat a n d u s i n g the  frequency-doubled  output o f a  T i : S a p p h i r e laser ( 3 8 6 n m ) to excite the sample. T h e l u m i n e s c e n c e w a s passed t h r o u g h a Czerny-Turner scanning monochromator ( D i g i k r o m 242  from  C V I Laser Corporation, using  o n l y one 6 0 0 grooves m m " grating) a n d c o l l e c t e d w i t h a H a m a m a t s u R 2 2 5 7 p h o t o m u l t i p l i e r 1  tube. T h e spectra w e r e corrected for the response o f the grating a n d the p h o t o m u l t i p l i e r tube b y m e a s u r i n g the s p e c t r u m o f a quartz tungsten h a l o g e n l a m p ( O r i e l 7 7 5 0 1 I l l u m i n a t o r ) o f k n o w n output.  79  References 1. v a n B e k k u m , H . ; F l a n i g e n , E . M . ; Jacobs, P . A . ; Jansen, J . C , E d s .  Science and Practice;E\sevier: 2. M c C u s k e r , L . B .  A m s t e r d a m , 2001  Structural Aspects of Molecular Sieves;  C h e m i s t r y ; E l s e v i e r S c i e n c e : N e w Y o r k , 1996; V o l . 3.  Kessler, H .  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Microporous  445.  83  CHAPTER 4 Preparation of Mesoporous Thin Film Host  M i c r o p o r o u s and m e s o p o r o u s materials often have anisotropic pore structures. A g o o d e x a m p l e is the m e s o p o r o u s material M C M - 4 1 , d i s c u s s e d i n the p r e v i o u s chapter: it has straight 1-D channels arranged  i n a 2 - D h e x a g o n a l lattice. T h i s anisotropy c a n o n l y be e x p l o i t e d ,  h o w e v e r , i f the m a c r o s c o p i c orientation o f the material c a n be c o n t r o l l e d either d u r i n g or after synthesis. S u c h c o n t r o l over the orientation is necessary for a n u m b e r o f applications, i n c l u d i n g p h o t o n i c materials, m e m b r a n e s a n d sensors. It is also k e y to fabricating the i d e a l d e v i c e w h e r e o n l y intrachain processes c a n o c c u r : proper c h a n n e l orientation w i l l a l l o w g o o d charge transport a l o n g the conjugated p o l y m e r chains i n the d e v i c e , a n d a n a r r o w pore diameter w i l l favour the isolation o f the p o l y m e r chains ( F i g u r e 1.5; T a b l e 1.2). T h e d e v e l o p m e n t o f a porous thin f i l m host w i t h oriented 1-D channels w a s thus o f central i m p o r t a n c e to this research effort. N o t w i t h s t a n d i n g a l l the requirements expressed i n T a b l e 1.2, the central challenge is the fabrication o f p o r o u s t h i n  films  w i t h 1-D channels a l i g n e d i n the d i r e c t i o n o f the substrate  n o r m a l . T h e synthesis o f films a n d m e m b r a n e s w i t h s u c h oriented porous structures has been the subject o f c o n t i n u o u s scientific p u r s u i t . " I m p r e s s i v e progress has been m a d e i n m a n y areas but 1  5  the g o a l o f fabricating u n i f o r m defect-free m e m b r a n e s w i t h v e r y s m a l l pores r e m a i n s elusive. M e m b r a n e s w i t h pores d o w n to 10 n m diameter are r e a d i l y available, but the size r e g i m e b e l o w that is not w e l l d e v e l o p e d . A s the m e m b r a n e s o f interest for this w o r k lie i n this uncharted territory, it w a s clear at the outset that substantial effort w o u l d be r e q u i r e d to d e v e l o p the desired t h i n f i l m host. H o w e v e r , a n u m b e r o f p r o m i s i n g first reports w e r e present i n the literature, w h i c h 84  suggested that the g o a l w a s w i t h i n reach. These reports are r e v i e w e d here a n d the investigations into the m o r e p r o m i s i n g routes d i s c u s s e d . M a n y o f the characterization techniques a p p l i c a b l e to a n a l y s i n g b u l k m e s o p o r o u s samples c a n also be a p p l i e d to t h i n f i l m s . X - r a y diffraction c a n be u s e d to evaluate the orientation o f the film.  6  P h y s i s o r p t i o n measurements are not straightforward, due to the l i m i t e d total surface area  o f the films. T h e most important techniques are based o n electron m i c r o s c o p y : s c a n n i n g electron microscopy  ( S E M ) and  transmission  electron  microscopy  ( T E M ) allow  high  resolution  i n s p e c t i o n o f the film structure o n the l o c a l l e v e l , as r e v i e w e d i n chapter 2.  4.1  Alignment of Porous T h i n Films Self-assembly is a n attractive route for materials synthesis because o f the inherent order o f  the r e s u l t i n g materials. O n c e the appropriate  c o n d i t i o n s for self-assembly are k n o w n , the  synthesis is u s u a l l y straightforward. T h u s a lot o f effort has been devoted to the pursuit o f systems w h i c h self-assemble into porous t h i n films and m e m b r a n e s w i t h v a r i o u s geometries. F o r self-assembled m e s o p o r o u s materials, the methods u s e d to achieve alignment fall into t w o general categories:  interface-induced  alignment a n d  field-induced  alignment ( i n c l u d i n g  electric, m a g n e t i c a n d f l o w fields). These v a r i o u s methods are r e v i e w e d here, and it w i l l b e c o m e o b v i o u s that n o ideal approach to the fabrication o f the desired m e m b r a n e v i a self-assembly exists as o f yet. S o m e v e r y recent w o r k o n solvent e v a p o r a t i o n - i n d u c e d alignment does appear v e r y p r o m i s i n g i n creating m e s o p o r o u s t h i n films w i t h channels n o r m a l to the substrate.  7  85  4.1.1  Interface-Induced Alignment Surfactant-templated materials, i n c l u d i n g the m e m b e r s o f the M 4 1 S f a m i l y  (MCM-41,  M C M - 4 8 ) , have been f o u n d to a l i g n themselves i n specific w a y s o n a n u m b e r o f interfaces. It is thought that the arrangement o f the initial layer o f surfactant m i c e l l e s o n the interface largely determines the a l i g n m e n t o f the g r o w i n g film. M i c a a n d graphite i n d u c e a p a r a l l e l a l i g n m e n t o f c y l i n d r i c a l m i c e l l e s (Figure 4.1) t h r o u g h fairly w e a k electrostatic a n d v a n der W a a l s interactions r e s p e c t i v e l y . ' T h e r e s u l t i n g films still have substantial in-plane disorder ( d i r e c t i o n o f channels, 8  9  1 8 0 ° defects), w h i c h l i m i t s their usefulness. T h e air-water interface has also been s h o w n to favour a s i m i l a r type o f o r d e r i n g .  1 0  F i l m s g r o w n o n S i ( 1 0 0 ) a n d ( 1 1 1 ) s h o w n o preferential  o r d e r i n g , but S i (110) does i n d u c e a l i g n m e n t p a r a l l e l to the [001] d i r e c t i o n , i n d i c a t i n g that 6  proper registry o f the t e m p l a t i n g mesophases w i t h the interface is necessary to achieve o r d e r i n g . H o w e v e r , a straightforward a p p r o a c h to p r e d i c t i n g w h i c h surfaces w i l l p r o d u c e proper registry has not been f o u n d yet. It is also p o s s i b l e to m o d i f y a surface to i n d u c e a l i g n m e n t i n a preferred d i r e c t i o n , as s h o w n b y the r u b b i n g m e t h o d ,  1 1  w h i c h is c o m m o n l y u s e d to a l i g n l i q u i d crystalline materials for  d i s p l a y applications. A t h i n p o l y i m i d e film is coated o n the substrate and then r u b b e d i n a particular d i r e c t i o n , w h i c h aligns the channels o f the m e s o p o r o u s m a t e r i a l parallel to the r u b b i n g  F i g u r e 4.1 2 - D h e x a g o n a l p a c k i n g o f surfactant m i c e l l e s i n aqueous s o l u t i o n onto a graphite surface, after ref. 8. 86  d i r e c t i o n . In-plane X - r a y diffraction  s h o w s that the d i s t r i b u t i o n o f c h a n n e l directions has a  FWHMof29°. T h e orientation o f the i n i t i a l template layer o n the surface c a n also be c o n t r o l l e d b y m o d i f y i n g the c h e m i c a l structure o f the t e m p l a t i n g agent ( F i g u r e 4.2). T h i s has b e e n s h o w n t h r o u g h the use o f a two-headed quarternary a m m o n i u m salt to f o r m a structure designated S B A 2.  1 2  T h e initial reports on the structure o f S B A - 2 f i l m s , based o n T E M studies, suggested that  there w a s a c o n t i n u o u s c h a n n e l n o r m a l to the interface. A later study u s i n g h i g h - r e s o l u t i o n T E M s h o w e d that the i n i t i a l structure assignment w a s erroneous a n d that there w a s a h i g h e r degree o f c o n n e c t i v i t y i n the p o r o u s structure than o r i g i n a l l y i d e n t i f i e d .  13  It is therefore still difficult to self-assemble a f i l m w i t h oriented channels n o r m a l to the interface. O n e p l a u s i b l e a p p r o a c h a r o u n d this p r o b l e m is the use o f a substrate that presents vertically-oriented interfaces, i.e., b y u s i n g a larger p o r o u s support to nucleate the film. P o r o u s alumina " 1 4  1 7  (see b e l o w ) a n d c a p i l l a r i e s  18  h a v e b e e n u s e d as substrates for the g r o w t h o f zeolites  a n d m e s o p o r o u s silicates. These oriented supports present interfaces oriented parallel to the substrate n o r m a l ( F i g u r e 4.3). T h i s has b e e n recently used to prepare m e m b r a n e s o f M C M - 4 8 ,  A  B  F i g u r e 4.2 Surfactant templates for (a) M C M - 4 1 a n d M C M - 4 8 structures ( C i H 3 N ( C H ) 3 X ) a n d 6  3  3  (b) S B A - 2 structure ( C i e H a s N C C H s M C ^ N C C F f e ^ ) , w h e r e X = B r or C l . 87  w h i c h has an isotropic c u b i c structure, and therefore cannot s h o w a preferred orientation. A supported  MCM-41  structure  has not been reported to date, perhaps because the  desired  orientation is not a c h i e v e d . Nevertheless, one w o u l d expect that the surface curvature o f the porous support (i.e., the inverse o f the c h a n n e l radius) w o u l d have a significant effect i n f a v o u r i n g alignment o f the channels i n the a x i a l d i r e c t i o n over the c i r c u m f e r e n t i a l d i r e c t i o n .  A  B  F i g u r e 4.3 T h e t w o extreme p o s s i b i l i t i e s for surfactant m i c e l l e alignment i n a porous support: (a) i n the a x i a l d i r e c t i o n a n d (b) i n the c i r c u m f e r e n t i a l d i r e c t i o n . A smaller radius o f curvature R w o u l d be expected to favour the a x i a l orientation.  4.1.2  Field-Induced Alignment E x t e r n a l l y a p p l i e d fields offer an attractive approach to o b t a i n i n g the desired c h a n n e l  a l i g n m e n t i n porous materials. In m a n y cases, the product m a t e r i a l is o n l y partially a l i g n e d w i t h the  a p p l i e d field,  as  o n l y an average  alignment  is a c h i e v e d , w i t h  substantial  deviations  p r e s u m a b l y due to t h e r m a l disorder o f the templates. It has been s h o w n that the a p p l i c a t i o n o f a large m a g n e t i c field ( - 1 2 T ) c a n be u s e d to a l i g n the channels i n a m e s o p o r o u s m a t e r i a l .  1 9  T h i s results from anisotropy i n the d i a m a g n e t i c  88  susceptibilities o f the t e m p l a t i n g agents. O n c e the template m i c e l l e s reach e q u i l i b r i u m w i t h the m a g n e t i c field ( w i t h 6 0 % o f the d o m a i n s a l i g n e d w i t h the m a g n e t i c field, as seen b y solid-state N M R ) , condensation o f the s i l i c a f r a m e w o r k is a c h i e v e d b y a l l o w i n g gaseous H C 1 to diffuse i n . T h e resulting m a t e r i a l w a s characterized b y X - r a y diffraction, w h i c h suggested that 7 8 % o f the pores were a l i g n e d i n the d e s i r e d d i r e c t i o n .  2 0  H i l l h o u s e et al. have u s e d a f l o w field to i n d u c e preferential alignment o f a m e s o p o r o u s silica film g r o w n i n a capillary t u b e .  21  the  (< 2 0 0 n m ) , based  external f l o w  for t h i n  m o r p h o l o g y o f the  film  films  T h e channels were f o u n d to be oriented i n the d i r e c t i o n o f o n observation o f the  macroscopic  b y S E M . T h i c k e r films tended to lose this preferred alignment and  b e c a m e s i m i l a r to films g r o w n under static c o n d i t i o n s . M o r e w o r k i n this d i r e c t i o n w a s c a r r i e d out b y K i m a n d Y a n g .  2 2  T h e flow field p r o v i d e d b y l a s e r - i n d u c e d ablation has also been u s e d to generate a l i g n e d structures. ' 2 3  guest  2 4  T h e laser w a s u s e d to create a seed film o f material oriented b y the ablation o f a  m o l e c u l e , ferrocene.  Further  treatment w i t h  the  c o n t i n u o u s film w i t h an orientation d e r i v e d from the seed  synthesis  mixture  then  formed  a  film.  T h e use o f b o t h an interface a n d an a p p l i e d electric field to fully c o n t r o l alignment has also been d e m o n s t r a t e d .  25  A n elastomeric m o l d w a s u s e d to create m i c r o n - s c a l e channels o n a  substrate b e t w e e n t w o electrodes (Figure 4.4). T h e electric field creates an electro-osmotic  flow  b e t w e e n the t w o electrodes, w h i c h is further g u i d e d b y the channels. T h u s b o t h the flow and the presence o f the interfaces contribute to a l i g n i n g the template m i c e l l e s , l e a d i n g to a v e r y h i g h degree o f alignment parallel to the condensation o f the s i l i c a  framework.  flow  direction. Localized  Joule heating then  induces  W h i l e this approach created h i g h quality a l i g n m e n t o f the  channels ( X - r a y diffraction i n d i c a t e d that the d i r e c t i o n a l spread w a s 1.7°, substantially better than other methods for a l i g n i n g the m i c e l l e s ) , it w o u l d not be straightforward to fabricate t h i n 89  F i g u r e 4.4 O r i e n t a t i o n o f m e s o p o r o u s channels b y electro-osmotic f l o w , after ref. 2 5 .  f i l m s (i.e., w i t h c h a n n e l lengths from 100 to 2 0 0 n m ) u s i n g this technique. O n e p o s s i b i l i t y m a y be to c o m b i n e it w i t h a porous support o f the type d i s c u s s e d above.  4.13  Oriented Porous T h i n Films by O t h e r Approaches Track-etch m e m b r a n e s  2 6  are prepared b y e x p o s i n g a polycarbonate or polyester f i l m to a  b e a m o f energetic heavy ions i n a linear accelerator. T h e ions leave damage tracks that are r o u g h l y n o r m a l to the f i l m  surface, a n d w h i c h are then etched c h e m i c a l l y to the d e s i r e d  diameter. T h e obtainable diameters range from 10 n m to several m i c r o n s . These are available commercially  from  W h a t m a n , Inc. u n d e r the N u c l e o p o r e trademark. W h i l e the t e c h n o l o g y  b e h i n d these m e m b r a n e s is fairly w e l l d e v e l o p e d , it is unclear i f they c a n be prepared i n the f o r m o f thin f i l m s , or obtained w i t h smaller pore diameters. 90  K o n d o h e t al. reported an almost i d e a l porous f i l m FeO:Si02 t h i n films w i t h a specific s t o i c h i o m e t r y at 6 0 0 ° C . i n d u c e d b y the d i r e c t i o n o f o x y g e n  flow,  2 7  structure,  attained b y  oxidizing  T h e a l i g n m e n t w a s thought to b e  as the o x i d i z e d F e precipitated out (as Fe2C>3) i n  c o l u m n s perpendicular to the film surface. T h e Fe2C>3 c o l u m n s c o u l d be c o n v e n i e n t l y etched out, l e a v i n g b e h i n d a porous s i l i c a m a t r i x . T h e resulting film h a d an average pore size o f 4 n m , as d e t e r m i n e d b y nitrogen p h y s i s o r p t i o n a n d supported b y T E M studies. P o r o u s a l u m i n a (anodic a l u m i n u m o x i d e ) is also c o m m e r c i a l l y important, especially for microfiltration.  28  It is p r o d u c e d b y a n o d i z i n g a l u m i n u m samples i n aqueous electrolytes. U n d e r  the appropriate c o n d i t i o n s , the resulting o x i d e has v e r t i c a l pores r u n n i n g through it, a n d the pore s p a c i n g c a n be controlled b y the a p p l i e d potential. T h e exact cause o f the a l i g n m e n t o f the channels is still unclear but appears to be related to b o t h the a p p l i e d potential a n d the strain created b y the e x p a n s i o n o f the lattice f r o m a l u m i n u m to a l u m i n u m o x i d e . T h e a l i g n m e n t o f the channels  is almost perfect  after  extended g r o w t h a n d p u b l i s h e d reports  i n d i c a t e d that the  diameter m a y be adjusted f r o m 20 to 5 0 0 n m . T h i c k m e m b r a n e s are c o m m e r c i a l l y available from W h a t m a n , Inc. under the A n o p o r e trademark. It m a y be p o s s i b l e to prepare s i m i l a r structures b y electron-beam lithography a n d c h e m i c a l e t c h i n g o f an appropriate substrate. A t the early stages o f this thesis, the feature size for this technique w a s o n the order o f 50 n m , a n d thus too large for the purpose o f this project. D i r e c t electron-beam d r i l l i n g o f a l u m i n a or s i l i c a films u s i n g a  field-emission  T E M i n s c a n n i n g m o d e is  a v e r y p r o m i s i n g approach for creating a s m a l l n u m b e r o f v e r y s m a l l diameter h o l e s .  2 9  P o r o u s t h i n films based o n the self-assembled S B A - 2 structure, the FeO/SiC»2 system and porous a l u m i n a were i n i t i a l l y j u d g e d to be p r o m i s i n g t h i n  film  hosts. T h e i r synthesis  and  characterization w a s undertaken to investigate further their suitability as oriented t h i n film hosts for conjugated p o l y m e r s .  91  4.2  Further Investigation of SBA-2 Mesoporous Silica Films Self-assembled films are attractive candidates for study as they are often h i g h l y ordered  a n d u n i f o r m . A s s u m i n g that the c h e m i s t r y to y i e l d the correct geometry c a n be f o u n d , these films  w o u l d be i d e a l host materials for conjugated p o l y m e r s . A s stated above (section 4.1), the  geometry o f a film nucleated o n an interface is thought to d e p e n d o n the arrangement o f the v e r y first  layer o f t e m p l a t i n g agent o n the interface. A s s u c h , c o n t r o l o f the  template-interface  interactions s h o u l d i n p r i n c i p l e a l l o w the preparation o f films w i t h the desired pore orientation. M o s t o f the w o r k reported i n the literature has focused o n t h i n films related to the M C M - 4 1 structure a n d based o n straight-chain a l k y l a m m o n i u m surfactants.  T h e s e adopt the  well-  established parallel a l i g n m e n t w i t h the interface i n a h e x a g o n a l structure. A different orientation c a n o n l y be expected from films w i t h substantially different structures. T h e structure type is c o n t r o l l e d b y the c h o i c e o f surfactant a n d p H . A s m a l l n u m b e r o f g e o m e t r i c a l parameters have been f o u n d to g o v e r n the type o f structure p r o d u c e d b y a g i v e n surfactant. T h e h y d r o p h o b i c tail length, h y d r o p h i l i c h e a d size a n d charge c a n a l l be adjusted to y i e l d fairly predictable s t r u c t u r e s .  30  T o l b e r t et a l . f o u n d that the use o f a two-headed surfactant  ( F i g u r e 4.2(b)), w h i c h templates the 3 - D h e x a g o n a l S B A - 2 structure, a l l o w e d the synthesis o f t h i n films w i t h a possible c h a n n e l n o r m a l to the interface. In this case the i n i t i a l layer o n the surface w a s f o u n d to consist o f p a c k e d hemispheres, representing the h i g h e r s y m m e t r y o f S B A 2. T h e p r o p o s e d structure consisted o f large cages, the c o n n e c t i v i t y o f w h i c h w o u l d determine the f o r m o f the channels t h r o u g h the material.  92  As  s u c h , the material appeared p r o m i s i n g a n d w a s investigated  reported S B A - 2 structure w a s synthesized o n  freshly-cleaved  i n m o r e depth.  The  m i c a substrates. T h e presence o f  the S B A - 2 phase w a s c o n f i r m e d b y X - r a y diffraction ( F i g u r e 4.5). T h e ( 0 0 2 ) peak appears strongly, i n d i c a t i n g that the film is a l i g n e d w i t h the c axis perpendicular to the film. T h e (112) a n d (004) peaks appear w e a k l y . T h e c a l c i n e d sample s h o w e d a shift i n the (002) peak to higher angle, i n d i c a t i n g that the  framework  size h a d decreased d u r i n g c a l c i n a t i o n .  In order to investigate the microstructure o f S B A - 2 films, the samples g r o w n o n m i c a w e r e e m b e d d e d i n e p o x y resin a n d sectioned u s i n g a d i a m o n d k n i f e . H o w e v e r , the  film  d i d not  s u r v i v e the sectioning process and thus no further i n f o r m a t i o n o n the pore structure was obtained  7  o o  6 co  5  C M  o o  CL O  o 4 £  Peak (002) (002) (112) (004)  29 2.29 (as made) 2.59 (calcined) 4.6 5.1  3  CO  c  CD  1  2  £  2  8  x10  1 1X1  0  8 29 ( ° )  F i g u r e 4.5 X - r a y diffraction pattern o f S B A - 2 film g r o w n o n m i c a (—) before a n d  (—)  after  calcination. 93  i n this fashion. A report appearing at this t i m e indicated that S B A - 2 d i d not have the desired connectivity, based o n h i g h resolution T E M i m a g i n g .  1 3  T h i s study o f b u l k S B A - 2 indicated that  the structure g r o w n as a f d m w o u l d have straight 1-D channels parallel to the interface a n d zigzag  channels  r u n n i n g p e r p e n d i c u l a r l y to the interface,  a geometry  n o t c o n d u c i v e to  conjugated p o l y m e r i n c l u s i o n . In v i e w o f this i n f o r m a t i o n , a n d p r o m i s i n g developments w i t h porous a l u m i n a f i l m s , n o further characterization w a s attempted.  4.3  F u r t h e r Investigation o f the F e O / S i C h System T h e FeO/SiC»2 system i s the subject o f one report i n the l i t e r a t u r e .  27  T h e i n s p i r a t i o n for  this approach reportedly c o m e s from the observation o f elongated Fe203 structures i n natural samples. I n this material, the o r d e r i n g is d e t e r m i n e d b y the d i r e c t i o n o f o x y g e n diffusion as F e O is o x i d i z e d to Fe203 w i t h i n the SiC>2 m a t r i x . T h e m o r p h o l o g y o f the resultant materials depends strongly o n the relative concentration o f F e O a n d Si02. A t c o m p o s i t i o n s between 70:30 a n d 60:40 FeO:Si02, it is reported that c o l u m n s o f Fe203 g r o w p e r p e n d i c u l a r l y to the substrate w i t h i n the Si02 m a t r i x ( F i g u r e 4.6). These c o l u m n s c a n then b e etched out b y aqueous H C 1 , l e a v i n g b e h i n d the Si02 m a t r i x . T h e resulting 1-D channels i n Si02 have a diameter o f 4 n m , as determined b y a B a r r e t t - J o y n e r - H a l e n d a ( B J H ) analysis o f the nitrogen p h y s i s o r p t i o n i s o t h e r m . W h e n the FeO:Si02 ratio is l o w e r than 60:40, the resulting Fe203 particles are trapped i n the Si02 m a t r i x a n d cannot be etched out. H i g h e r F e O levels lead to a n o x i d i z e d f i l m w i t h larger, d i s o r d e r e d Fe203 structures. U s e o f the correct FeO:Si02 ratio s h o u l d p r o d u c e a t h i n f i l m host w i t h nearly ideal characteristics: porous Si02 films w i t h v e r t i c a l 4 n m channels w i t h easily controlled  thickness. T h e slight d r a w b a c k  o f this process  is the rather  high processing  temperature ( 6 0 0 ° C ) .  94  O2 diffusion  F i g u r e 4.6 Preparation o f t h i n film w i t h oriented channels f r o m a FeO:Si02 f i l m .  T h e d u p l i c a t i o n o f these p u b l i s h e d results w a s attempted. U s i n g a 65:35 FeO:Si02 target, t h i n films w e r e sputtered unto glass a n d s i l i c o n substrates u s i n g radio-frequency ( R F ) sputtering. Freshly-sputtered films were initially transparent w i t h a deep green c o l o u r . F o l l o w i n g o x i d a t i o n at 6 0 0 ° C , the films turned r e d i n appearance, i n d i c a t i n g the f o r m a t i o n o f Fe203. T h i s c o l o u r disappeared c o m p l e t e l y u p o n e t c h i n g w i t h 1:1 H C h H b O , suggesting that most o f the Fe203 h a d been r e m o v e d from the t h i n  film.  These observations w e r e i n agreement w i t h the c h e m i s t r y  suggested b y the i n i t i a l reference. T h e m i c r o s t r u c t u r e o f the films at different stages o f p r o c e s s i n g w a s investigated b y T E M . Cross-sections o f the t h i n  films  w e r e r e a d i l y obtainable b y the small-angle cleavage ( S A C )  technique. 95  T h e cross-section o f the o x i d i z e d , unetched film is s h o w n i n F i g u r e 4.7 (a). V e r y little structure is apparent, i n contrast w i t h the p u b l i s h e d m i c r o g r a p h s , w h e r e c o l u m n a r structures w e r e v i s i b l e . T h e energy-dispersive X - r a y ( E D X ) analysis o f the thin film w i t h a S T E M ( F i g u r e 4.8(a)) y i e l d e d a F e : S i a t o m i c ratio o f 2.1 ± 0.2, w h i c h w a s w i t h i n the range d e e m e d o p t i m a l (1.9 - 2.3). X P S analysis, o n the other h a n d , y i e l d e d a F e : S i ratio o f 0.9, suggesting the surface F e O content w a s w e l l b e l o w the desired level. T h e etched film is s h o w n i n F i g u r e 4.7 (b), a n d there is insufficient contrast to observe the pore structure. T h e E D X spectrum s h o w e d that the been fully r e m o v e d ( F i g u r e 4.8(b)), i n d i c a t i n g that there w e r e n o isolated  Fe203 h a d  Fe203 particles. T h i s  suggested that the F e : S i ratio w a s near or above o p t i m a l . T h e X P S results, w h i c h reflect the c o m p o s i t i o n at the surface, m a y indicate that the film h a d a l o w e r concentration o f Fe near the surface. H e a v y - e l e m e n t staining o f the pore surface w a s attempted to i m p r o v e the contrast between  glass substrate  F i g u r e 4.7 T E M i m a g e o f cross-section o f F e O : S i 0 f i l m o n glass (a) after o x i d a t i o n , before etching, (b) after e t c h i n g a n d P b - s t a i n i n g . T h e cross-section w a s prepared b y the S A C technique. T h e scale bars are 5 0 n m l o n g . 2  96  the pores a n d the s u r r o u n d i n g m a t r i x . L e a d acetate is k n o w n to b i n d to surface h y d r o x y groups, but it d i d not i m p r o v e the contrast o f the pores, despite clear i n d i c a t i o n o f the presence o f P b b y the E D X spectrum (Figure 4.8(c)). T h e n i t r o g e n adsorption i s o t h e r m o n a 2 0 0 - n m t h i c k s a m p l e (anticipated total surface area ~ 0 . 5 m ) d i d not s h o w a clear condensation step, suggesting a w i d e d i s t r i b u t i o n o f p o r e sizes. 2  T h e measurements were also l i m i t e d i n a c c u r a c y b y instrumental drift. T h i s stood i n contrast w i t h the reported  adsorption i s o t h e r m , w h i c h  s h o w e d a clear c a p i l l a r y condensation  step  c o r r e s p o n d i n g to a 4 n m B J H diameter. W h i l e the t h i n films w e r e p o r o u s , as e v i d e n c e d b y the c o m p l e t e r e m o v a l o f the Fe203 b y etching, n o w e l l - d e f i n e d channels c o u l d be detected electron m i c r o s c o p y or n i t r o g e n p h y s i s o r p t i o n . A t t e m p t s to contact the authors to  by  discuss  p o s s i b l e p r o c e s s i n g issues were unsuccessful. Further efforts to r e p r o d u c e this w o r k  were  a b a n d o n e d since porous a l u m i n a s h o w e d m o r e p r o m i s e .  97  0.00  2.56  5.12  7.68  10.24  X - R a y E n e r g y (keV)  Figure 4.8 E D X spectrum obtained from cross-sections of F e O : S i 0 2 films: (a) oxidized ( A u contamination is apparent), (b) etched to remove Fe203, (c) Pb-stained.  4.4  Porous Alumina Films (Anodic Aluminum Oxide) A n o d i c films o n a l u m i n u m have a l o n g h i s t o r y o f i n d u s t r i a l a p p l i c a t i o n as cosmetic and  protective layers. T h e c h o i c e o f a n o d i z a t i o n c o n d i t i o n s (electrolyte, temperature, potential) has a dramatic effect o n the structure o f the resulting a l u m i n a film. T h e porous films obtained under the appropriate c o n d i t i o n s were f o u n d to e x h i b i t a structure w h i c h  fitted  the  requirements  expressed i n T a b l e 1.2 v e r y closely. T h e p o r o u s nature o f some o f these films has been r e v i e w e d i n depth b y T h o m p s o n and Wood.  D u e to their i n d u s t r i a l importance, these films have been extensively studied since the  3 1  1930's. M a n y characteristics o f the  films  were w e l l understood b y the  1980's, a n d it w a s  r e c o g n i z e d that v e r y ordered films c o u l d be p r o d u c e d under certain c o n d i t i o n s . T h e self-ordering p h e n o m e n o n was not fully e x p l o i t e d u n t i l the 1990's: i n 1993, M a s u d a e t al. reported a s i m p l e technique  r  for the p r o d u c t i o n o f p o r o u s a n o d i c a l u m i n a w i t h h i g h l y ordered pores and a  ~  m  &  A  ™  dt  m a t  41  m**m  <  '^ • • •  ^ €P  •  in* Figure  ^Km.  A  ^  JMIK-  & ^ JI ^ A ^  g|  w  —  4.9 S E M images o f porous a l u m i n a film p r o d u c e d at 4 0 V i n 0.3 M o x a l i c a c i d , u s i n g  M a s u d a ' s two-step approach: (a) top v i e w , (b) cross-section. T h e scale bars are 100 n m l o n g . Images courtesy o f D r . K . R a d e m a c h e r . 99  corresponding  narrow  pore size d i s t r i b u t i o n  (Figure 4 . 9 ) .  T h i s development  3 2  l e d to  w i d e s p r e a d a p p l i c a t i o n o f porous a l u m i n a m e m b r a n e s b o t h as hosts for other m a t e r i a l s " 3 3  as t e m p l a t e s  37  for the synthesis o f t u b u l e s  38  and w i r e s  3 9  '  4 0  3 6  the and  w i t h u n i f o r m diameters i n the 30 to  100 n m r e g i m e . T h e d e v e l o p m e n t o f w e l l - o r d e r e d large p o r e samples a l l o w e d the fabrication o f p h o t o n i c crystals either directly u s i n g patterned porous a l u m i n a f i l m s ' 4 1  pattern to another substrate b y dry c h e m i c a l e t c h i n g . " 4 3  4 6  or b y transferring the  T h e a n o d i z a t i o n o f other materials,  m o s t notably s i l i c o n , also produces v e r y useful porous s t r u c t u r e s . P o r o u s a l u m i n a is prepared  4 2  47  b y a n o d i z i n g a l u m i n u m i n an a c i d i c electrolyte, t y p i c a l l y  p h o s p h o r i c , sulfuric or o x a l i c a c i d . T h e anode i s attached to the a l u m i n u m substrate, w h i l e the cathode usually consists o f a p l a t i n u m m e s h electrode ( F i g u r e 4 . 1 0 ) . U n d e r the c o n d i t i o n s , the pores are straight and h e x a g o n a l l y p a c k e d , 4 8  4 9  appropriate  as s h o w n i n F i g u r e 4 . 9 . T h e  structural parameters are s h o w n i n F i g u r e 4 . 1 1 : the pore diameter is D and the lattice s p a c i n g is  Glass cell w i t h O - r i n g  Figure 4.10 A s i m p l e e l e c t r o c h e m i c a l c e l l for a n o d i z a t i o n o f a l u m i n u m substrates. 100  L. T h e o x i d e g r o w t h process leaves a barrier layer at the b o t t o m o f the pores w i t h a thickness o n the order o f L/2.  A n i o n - c o n t a m i n a t e d layer  B a r r i e r layer B  Figure 4.11 Structure o f porous a l u m i n a f i l m s g r o w n o n a l u m i n u m : (a) geometry o f p o r e p a c k i n g , (b) cross-section s h o w i n g barrier layer o f thickness ~L/2 at b o t t o m o f pores.  4.4.1  Pore Wall Structure T h e nature o f the pore w a l l s has b e e n studied extensively. T h e results o f m a n y early  studies have been r e v i e w e d b y T h o m s o n and W o o d .  3 1  T h e pore w a l l s consist o f a relatively  thick, a m o r p h o u s , anion-contaminated surface layer over a m o r e dense core o f p u r e a l u m i n a . T h e t w o different material. ' ' 3 1  4 9  zones 5 0  o f a l u m i n a c a n be r e a d i l y d i s t i n g u i s h e d i n T E M m i c r o g r a p h s  There exists considerable v a r i a t i o n i n a n i o n i n c o r p o r a t i o n a m o n g the  f i l m s prepared u s i n g sulfuric, o x a l i c , a n d p h o s p h o r i c a c i d s .  5 1  U s i n g T E M and E D X ,  5 0  of  the  different the a n i o n  d i s t r i b u t i o n has b e e n s h o w n to be u n i f o r m through the less dense layer. A f t e r w a s h i n g , the d i s t r i b u t i o n o f anions i n the c e l l w a l l is lowest i n the pure a l u m i n a core, reaches a m a x i m u m w i t h i n the less dense layer and falls o f f near the pore surface. T h i s d i s t r i b u t i o n c a n be the result o f an initially u n i f o r m a n i o n c o n t a m i n a t i o n , w h i c h is altered d u r i n g the post-synthesis  water 101  rinse b y preferential r e m o v a l o f anions near the pore s u r f a c e . ' 5 2  5 3  T h e nature o f the a n i o n  c o n t a m i n a t i o n m a y be v e r y important for electric d e v i c e applications o f porous a l u m i n a , as the anions  may become  m o b i l e under  h i g h electric fields, c a u s i n g electrical short-circuits  or  affecting the electrical properties o f the p o l y m e r host. These anions m a y be r e m o v e d c o m p l e t e l y b y t h e r m a l treatment o f the  free-standing  membrane. " 5 4  5 9  M e m b r a n e s prepared u s i n g o x a l i c a c i d  as the electrolyte s h o w the r e m o v a l o f oxalate i o n over 2 0 0 - 4 0 0 ° C , w i t h c r y s t a l l i z a t i o n o f the pore w a l l s o c c u r r i n g at 8 2 0 - 8 4 0 ° C . W h e n sulfuric a c i d is u s e d as electrolyte, the c o n t a m i n a t i n g anions are r e m o v e d at 9 7 0 ° C .  4.4.2  Pore Growth Processes  A n u m b e r o f e m p i r i c a l relationships have been established for porous a l u m i n a g r o w t h : (1) the s p a c i n g o f the pores is p r o p o r t i o n a l to the a p p l i e d potential (2.5 to 2.8 n m V " ) ; 1  4 9  (2) the pores f o r m an ordered lattice at certain potentials for each electrolyte; (3) the pore depth ( f i l m thickness) is p r o p o r t i o n a l to the total charge passed ( p r o p o r t i o n a l to time under galvanostatic anodization). W h i l e these e m p i r i c a l relationships have b e e n t h o r o u g h l y investigated, the  processes  w h i c h g o v e r n the g r o w t h o f the pores are not fully understood. A qualitative analysis has been presented b y T h o m p s o n a n d W o o d .  3 1  I n a c i d i c electrolytes, pore f o r m a t i o n m u s t be the result o f  the concurrent o x i d a t i o n o f the a l u m i n u m to a l u m i n a a n d d i s s o l u t i o n o f the a l u m i n a b y the electrolyte, a l o n g w i t h a higher a l u m i n a d i s s o l u t i o n rate i n depressions. T h i s suggests that  field-  assisted d i s s o l u t i o n is a k e y d r i v i n g force b e h i n d pore g r o w t h . A l o c a l i z e d temperature increase at the m e t a l - o x i d e interface due to Joule heating m a y also p l a y a role i n the process. T h e larger effect  o f electrolyte temperature o n the  film  parameters indicates  that l o c a l  temperature  variations d o not have a d o m i n a t i n g role i n the process. 102  T h e pore structure m a y be further altered b y etching w i t h p h o s p h o r i c a c i d , w h i c h attacks the a l u m i n a m u c h faster than sulfuric a c i d or o x a l i c a c i d . T h e anion-contaminated layer is also attacked faster than the the m o r e dense c e l l boundary. T y p i c a l l y , the pore w a l l s are etched i n 5 w t . % p h o s p h o r i c a c i d at a rate o f 8 n m h" at r o o m t e m p e r a t u r e . 1  4.4.3  60  Pore Lattice Formation F o r a g i v e n electrolyte, self-ordering m a y be obtained over a range o f a p p l i e d potentials.  P u b l i s h e d values are s h o w n i n T a b l e 4 . 1 . U n d e r these c o n d i t i o n s , the pore lattice b e c o m e s ordered w i t h a d o m a i n size between  1 a n d 4 p m . B e l o w this range o f potentials, there is  considerable pore b r a n c h i n g d u r i n g the g r o w t h process. T h u s not a l l pore sizes are r e a d i l y accessible i f an ordered pore lattice is desired. T h e m e t h o d for o b t a i n i n g fully ordered structures process. ' 3 2  4 8  6 1  consists o f a two-step  anodization  A n i n i t i a l l o n g a n o d i z a t i o n p e r i o d is c a r r i e d out, u n t i l the pores are g r o w i n g i n  the i d e a l arrangement at the metal/oxide interface. T h e o x i d e is then stripped u s i n g a p h o s p h o r i c a c i d / c h r o m i c a c i d solution, w h e r e the c h r o m i c a c i d protects  the a l u m i n u m substrate f r o m  d i s s o l u t i o n once the o x i d e is r e m o v e d ( F i g u r e 4 . 1 2 ) . T h i s leaves a scalloped surface that is  T a b l e 4.1 P u b l i s h e d parameters for self-ordered porous a l u m i n a g r o w t h . Electrolyte  Concentration  Oxalic acid  0.3 M  Potential  Temperature  Spacing  (V)  (!Q  (nm)  40  1  105  Reference 49,61  Sulfuric acid  20wt%  18.7  1  50  62  Sulfuric acid  0.3 M  25  not reported  66.3  49  Sulfuric acid  20wt%  15-25  1  41 - 6 8  63  Phosphoric acid  1 wt %  195  not reported  50J  49  103  5.8 w t % H P 0 3  1.5 w t % C r 0  •  4  + Anodize  3  12 h , 6 0 ° C  Figure  4.12  Preparation  o f fully-ordered p o r o u s  alumina film  b y a two-step  anodization  process.The i n i t i a l o x i d e layer is stripped, f o l l o w e d b y further a n o d i z a t i o n o f the substrate. a n o d i z e d i n a s e c o n d step for the desired duration, y i e l d i n g a fully  ordered f i l m  o f the  appropriate thickness. M a s u d a has elegantly s h o w n that ordered pore g r o w t h is initiated b y the s c a l l o p e d surface b y p r e p a r i n g s i m i l a r nanoindentations o n a l u m i n u m surfaces w i t h a patterned SiC wafer fabrication  6 1  a n d w i t h an a t o m i c force m i c r o s c o p e . o f defect-free  lattices o n the m m  6 4  T h i s nanoidentation technique a l l o w e d the  scale. M a s u d a also reported  that electrolyte  concentration a n d temperature have n o significant effect o n the f o r m a t i o n o f the lattice. T h e self-ordering o f the pore lattice has also been the subject o f several investigations but still r e m a i n s to be u n d e r s t o o d . - ' ' 4 7  ordered interpore  lattices c a n arise  from  s p a c i n g . H o w e v e r , the  4 9  6 2  6 5  It has been s h o w n through computer s i m u l a t i o n that  i n i t i a l pore f o r m a t i o n o n isolated d e f e c t s , o r i g i n o f the  fixed  51  given a  lattice s p a c i n g r e m a i n s unclear.  fixed The  m e c h a n i c a l stress o f e x p a n s i o n from a l u m i n u m to a l u m i n a has been suggested as a source o f r e p u l s i v e interaction b e t w e e n g r o w i n g p o r e s . alumina  films  6 2  N i e l s c h e t al. p o i n t e d out that a l l ordered p o r o u s  have a 1 0 % porosity (ratio o f pore area to u n i t c e l l area) independent o f the  electrolyte and p o t e n t i a l .  49  These points are d i s c u s s e d further b e l o w .  104  4.4.4  Preparation of Optimal Host from Porous Alumina P o r o u s a l u m i n a is e v i d e n t l y a v e r y versatile material, as the pore size a n d s p a c i n g c a n be  easily adjusted over a w i d e range and the thickness o f the porous film is s i m p l e to c o n t r o l . T h i n f i l m s m a y be prepared on v a r i o u s substrates b y a n o d i z i n g evaporated a l u m i n u m f i l m s o f the correct t h i c k n e s s .  6 0  T h i s also a l l o w s cross-sections for S E M to be prepared i n a convenient  manner. T h e smallest pore size reported i n the literature is 2 0 n m , obtained u s i n g 2 0 w t % sulfuric a c i d at 15 V .  6  3  T h e r e is a recent report o f a m e m b r a n e w i t h 5 n m pore s i z e  6 6  but it w a s s i m p l y  obtained at a l o w potential w i t h o u t any o r d e r i n g , a n d characterized i n d i r e c t l y b y gas diffusion measurements. T h e c o n d i t i o n s for the preparation o f f i l m s w i t h a n ordered lattice o f pores w i t h diameters b e l o w 2 0 n m are not established i n the literature. H o w e v e r , M o s k o v i t s  reported  u n p u b l i s h e d results at a conference s h o w i n g that the pore diameter c o u l d be v e r y effectively r e d u c e d through m a n i p u l a t i o n o f the electrolyte t e m p e r a t u r e .  67  In order to r e a c h temperatures  w e l l b e l o w 0 ° C , a m i x t u r e o f m e t h a n o l and water was u s e d as the s o l v e n t .  68  U s i n g 1.2 M  sulfuric a c i d i n a 3:1 m i x t u r e o f m e t h a n o l a n d water, pore sizes d o w n to 4 n m c o u l d be reached at a n o d i z a t i o n temperatures o f -40 ° C . In our hands, this a p p r o a c h w a s also successful. It w a s f o u n d that a 1:1  methanol:water  m i x t u r e w a s effective as the solvent d o w n to -50 ° C . W h i l e k e e p i n g the a n o d i z a t i o n potential and electrolyte concentration constant, f i l m s were a n o d i z e d at 2 0 ° C , - 8 ° C a n d - 4 0 ° C ( F i g u r e 4 . 1 3 ) . T h e f i l m s a n o d i z e d at 2 0 ° C a n d - 8 ° C w e r e observed b y S E M , whereas the film obtained at -40 ° C r e q u i r e d a T E M to d i s t i n g u i s h the pores. It can be seen that w h i l e the o r d e r i n g is not perfect, the pore s p a c i n g does not v a r y significantly f r o m 4 0 n m - the s p a c i n g is clearly fixed b y the  a p p l i e d potential. T h e p o r e  diameter,  o n the  other  hand,  changes  dramatically with  temperature: f r o m 21 n m at 2 0 ° C to 4 n m at -40 ° C ( F i g u r e 4 . 1 4 ) . 105  Diameter (nm) F i g u r e 4.13 P o r o u s a l u m i n a samples a n o d i z e d at (a) 2 0 ° C , (b) -8 ° C a n d (c) -40 ° C at 15.0 V i n 1.2 M H2SO4 (1:1 H 0 : M e O H ) , w i t h resulting pore size distributions. T h e scale bar is 20 n m . 2  106  -1  1  -40  1  '  -20  1  1  0  r~  20  Temperature ( ° C ) F i g u r e 4.14 Effect o f temperature o n pore diameter for samples a n o d i z e d at 15.0 V i n 1.2 M sulfuric a c i d .  T h e effect o f electrolyte concentration w a s also investigated b y r a i s i n g the concentration o f sulfuric a c i d to 5.0 M . H o w e v e r , there was n o effect o n the pore size at fixed potential and temperature. T h i s stands i n contrast w i t h the results o f P a t e r m a r a k i s et al,  w h o reported that the  square o f the pore base diameter is inversely p r o p o r t i o n a l to the proton activity at the base o f the pore, based o n r o o m temperature data obtained galvanostatically w i t h different concentrations o f sulfuric a c i d .  5 3  T h e samples  a n o d i z e d at -40°C clearly s h o w some  ordering. T h i s fact a l l o w s  some  c o m m e n t to be m a d e o n the p r o p o s a l that the o r d e r i n g o f the pore lattice is d r i v e n b y m e c h a n i c a l stress i n the a l u m i n u m to a l u m i n a transformation. T h e i r coefficients o f t h e r m a l e x p a n s i o n are 23.1X10  -6  K " and 8 . 4 x l 0 " K " , r e s p e c t i v e l y . 1  suggest that any effects  6  1  69  T h i s difference i n e x p a n s i o n coefficients w o u l d  due to the stress o f e x p a n s i o n w o u l d be v e r y different  at l o w e r  temperatures, a n d any process dependent o n this stress w o u l d be altered. T h e fact that the lattice s p a c i n g remains the same despite the change i n temperature w o u l d argue against any d o m i n a t i n g role for m e c h a n i c a l stress i n the o r d e r i n g process. Further experiments w o u l d be r e q u i r e d to validate this hypothesis fully. T h e d r a w b a c k o f a n o d i z i n g at l o w e r temperatures is that the a n o d i z a t i o n rate is r e d u c e d drastically. T h e current density declines to 10 u A c m " at -40 ° C and the o x i d e g r o w t h rate 2  becomes - 2 0 0  nm/24  h. While  unreasonable for o b t a i n i n g a f i l m  this  is m u c h  slower  than  normal  w i t h a p o r e size that is otherwise  anodization, difficult to  it is  not  fabricate.  M o r e o v e r , since the s p a c i n g o f the pores is the same at r o o m temperature, the i n i t i a l a n o d i z a t i o n of  the  two-step process  for p r e p a r i n g  highly-ordered  films can  be  carried  out  at  room  temperature.  4.4.5  Barrier Layer Thinning T h e barrier layer created d u r i n g the a n o d i z a t i o n process ( F i g u r e 4.9) must be r e m o v e d to  a l l o w electrical contact to the conjugated p o l y m e r guest. T h e a l u m i n u m substrate for the porous a l u m i n a film m a y i t s e l f be u s e d as the cathode material. T h e requirement then is for a m e t h o d to r e m o v e , or at least to t h i n significantly, the barrier layer present between the pores a n d the aluminum. P h o s p h o r i c a c i d e t c h i n g is the simplest approach to barrier layer r e m o v a l , p r o v i d e d the a l u m i n u m substrate is r e m o v e d a n d the film etched from the b o t t o m . w i t h an a r g o n - i o n b e a m m a y be e m p l o y e d .  7 0  3 2  Otherwise, ion milling  T h e p r a c t i c a l l i m i t a t i o n o f this m e t h o d is the  diameter o f the b e a m i n the i o n m i l l , w h i c h is o n the order o f 1 m m o n the instrument available at U B C ( V C R i o n m i l l , M e t a l s and M a t e r i a l s E n g i n e e r i n g ) . T h i s restricted the sample area w h i c h c o u l d be processed easily, m a k i n g it i m p r a c t i c a l for d e v i c e fabrication. 108  R e a c t i v e i o n e t c h i n g ( R I E ) m a y be u s e d attack the a l u m i n a selectively a n d d i r e c t i o n a l l y , a n d this approach w a s investigated b y D r . K . R a d e m a c h e r approach,  a b o r o n trichloride/argon p l a s m a  i n our research  is used to etch a w a y the  tetrafluoride/oxygen m i x t u r e c a n also be u s e d to etch a l u m i n a ,  7 1  group. I n this  alumina. A  carbon  but these gases w e r e not r e a d i l y  available o n the etcher at U B C . A p p l i e d from the b o t t o m o f the sample ( w i t h the a l u m i n u m r e m o v e d b y saturated H g C k ) , this m e t h o d w a s f o u n d to o p e n some pores. H o w e v e r ,  the  c o n d i t i o n s for o p e n i n g the pores f r o m the top o f the sample were not f o u n d . B a r r i e r layer t h i n n i n g m a y also be a c h i e v e d b y r e d u c i n g the potential at the e n d o f the a n o d i z a t i o n step. T h i s c a n be executed stepwise,  72  either stepwise  or gradually. I f this is c a r r i e d out  p o r e - b r a n c h i n g occurs as smaller pores nucleate at the b o t t o m o f the o r i g i n a l pores;  the b r a n c h i n g leads to an inverted tree structure at the b o t t o m o f the a n o d i z e d layer. T h i s is u s e d i n the c o m m e r c i a l l y - a v a i l a b l e A n o d i s c m e m b r a n e s to obtain 2 0 n m pore sizes. Better c o n t r o l o f the rate o f potential r e d u c t i o n leads to a s m a l l h o l e at the b o t t o m o f each p o r e .  7 3  It s h o u l d b e  noted that extended a n o d i z a t i o n at v e r y l o w potential (< 1 V ) eventually leads to the separation o f the a l u m i n a m e m b r a n e f r o m the a l u m i n u m substrate.  4.5  Conclusion It w a s s h o w n that a porous a l u m i n a host for conjugated p o l y m e r s c a n be r e a d i l y prepared  w i t h the desired properties. T h e f o r m a t i o n o f s m a l l diameter films w a s a c h i e v e d u s i n g the l o w temperature a n o d i z a t i o n m e t h o d reported b y M o s k o v i t s . T h e resulting films s h o w a l i g n e d 1-D channels w i t h a diameter o f 4 ± 1 n m , and the barrier layer c o u l d be effectively t h i n n e d b y a s i m p l e potential reduction m e t h o d . H o w e v e r , m a n y aspects o f porous a l u m i n a film formation are still p o o r l y understood, a n d it m a y be p o s s i b l e to i m p r o v e the l o w temperature film g r o w t h rate b y further e x p l o r i n g different c o m b i n a t i o n s o f electrolyte, temperature a n d a n o d i z a t i o n potential. 109  Experimental Details  1. SBA-2 Films  8 1 2  T h e 16-3-1 g e m i n i surfactant w a s synthesized a c c o r d i n g to the literature p r o c e d u r e . T h i n 7 4  f i l m s w i t h the S B A - 2 structure were then g r o w n o n m i c a substrates. T h e reaction m i x t u r e w a s o f the  following  molar  composition:  1.0  H 0: 2  0.076  HC1: 9.5x10^  16-3-1:  1.8xl0"  3  tetraethylorthosilicate. A f t e r 5 m i n u t e s o f stirring, this m i x t u r e w a s transferred to a T e f l o n - l i n e d stainless steel b o m b . A  freshly  c l e a v e d m i c a substrate w a s then floated u p s i d e d o w n o n the  solution, and the sealed b o m b w a s p l a c e d i n an o v e n at 8 0 ° C for 16 h . After c o o l i n g to r o o m temperature, the m i c a substrate w a s retrieved a n d r i n s e d w i t h d i s t i l l e d water a n d d r i e d i n air. T h e surfactant w a s r e m o v e d b y c a l c i n a t i o n i n air: the temperature w a s s l o w l y raised to 5 4 0 ° C over 18 h to a v o i d c r a c k i n g the film, and h e l d at 5 4 0 ° C for 6 h . X - r a y diffraction patterns w e r e obtained o n a R i g a k u 6 k W p o w d e r Nitrogen  p h y s i s o r p t i o n measurements  were  carried out  on  a Micromeritics  diffractometer. ASAP  2010  instrument at 77 K . S a m p l e s w e r e e m b e d d e d i n E P O N r e s i n for u l t r a m i c r o t o m y w i t h a d i a m o n d knife (Microstar).  2. FeO:Si0 System 2  A 3 " sputtering target w i t h a c o m p o s i t i o n o f 65:35 F e O : S i 0  2  by powder mixing volume  (Pure T e c h ) w a s u s e d to prepare t h i n films o n glass a n d s i l i c o n substrates b y R F sputtering. T h i s process produces a p l a s m a i n the w o r k i n g gas (argon), w h i c h then strips m a t e r i a l from the target a n d deposits it o n the substrate. It is an efficient w a y to deposit dielectric materials. T h e R F p o w e r was set to literature value, 2 0 0 W , w h i l e the argon pressure was 10 m T o r r , the highest obtainable o n the sputtering system b e i n g used. A sputtering t i m e o f 4 h y i e l d e d 2 0 0 n m - t h i c k 110  films.  T h e films w e r e d a r k green i n c o l o u r . T h e o x i d a t i o n o f the films w a s carried out at 6 0 0 ° C  i n air, resulting i n a change o f c o l o u r to red. E t c h i n g w i t h 1:1 H C L E ^ O overnight p r o d u c e d a clear film. W h e n cross-sections w e r e required, 0.1 m m - t h i c k c o v e r s l i p s (#0, E . M . Sciences) w e r e u s e d as the substrate. T h e cross-sections prepared b y S A C were observed w i t h a H i t a c h i H - 8 0 0 T E M e q u i p p e d w i t h a n X - r a y detector for E D X analysis. T h e analysis area for E D X w a s selected b y u s i n g the m i c r o s c o p e i n S T E M m o d e a n d s c a n n i n g over the area o f interest o n l y . Quantitative analysis  o f the  E D X spectra  was  c a r r i e d out  u s i n g the  ZAPTEM  program,  yielding  a  composition o f 2 5 % Fe, 12% S i and 6 3 % O . X P S analysis w a s p e r f o r m e d o n a L e y b o l d M A X 2 0 0  system u s i n g m o n o c h r o m a t e d A l  K « radiation as the e x c i t a t i o n source. T h e peaks w e r e fitted u s i n g the X P S P E A K  computer  p r o g r a m ( R . W . M . K w o k , C h i n e s e U n i v e r s i t y o f H o n g K o n g ) . T h e F e 2p, S i 2 p and O Is peaks were u s e d to calculate a t o m i c percentages: 12 % F e , 13 % S i , 7 4 % O . N i t r o g e n p h y s i s o r p t i o n measurements were c a r r i e d out o n a M i c r o m e r i t i c s A S A P 2 0 1 0 instrument at 77 K . A 2 0 0 n m film w a s deposited onto c o v e r s l i p s w i t h a total projected area o f 60 c m . T h e a p p r o x i m a t e total surface area w a s 0.5 m , u s i n g an estimated p o r o s i t y o f 2 5 % and 2  2  the reported specific surface area o f 8 0 0 m  2  g" . 1  3. P o r o u s a l u m i n a A l u m i n u m foil ( 9 9 . 9 9 + % , A l d r i c h ) w a s degreased i n acetone, then r i n s e d w i t h distilled water. N o e l e c t r o p o l i s h i n g w a s done. A l u m i n u m t h i n films were prepared o n n-type s i l i c o n substrates b y electron-beam evaporation ( w i t h a base pressure o f 2 x 1 0 " T o r r ) from a l u m i n u m 6  slugs ( 9 9 . 9 9 9 % , A l f a A e s a r ) i n a graphite c r u c i b l e or R F sputtering ( 3 " target, 9 9 . 9 9 % p u r i t y , 3 0 0 W w i t h 6 m T o r r argon).  Ill  A n o d i z a t i o n w a s carried out under the c o n d i t i o n s g i v e n i n the text. M o s t samples were a n o d i z e d w i t h o u t a n y s t i r r i n g o f the electrolyte, although s t i r r i n g w i l l i m p r o v e the o r d e r i n g o f the p o r e lattice. F o r the two-step process, the i n i t i a l l y g r o w n o x i d e layer w a s stripped b y i m m e r s i n g the substrate i n 5.8 wt. % H3PO4 + 1.5 wt. % C r 0  3  at 50 to 60 ° C overnight. A n o d i z a t i o n w a s then  c o n t i n u e d at the initial c o n d i t i o n s . P o r e - w i d e n i n g was effected b y 5 % H3PO4 at 20 ° C for the g i v e n periods. L o w temperature a n o d i z a t i o n was c a r r i e d out i n a F T S S y s t e m s (Stone R i d g e , N . Y . ) M u l t i - C o o l chiller. S a m p l e s w e r e coated w i t h a t h i n layer o f A u / P d for S E M ( H i t a c h i S - 4 7 0 0 ) , t y p i c a l l y w i t h an accelerating voltage between 10 and 20 k V a n d a 6 m m w o r k i n g distance. Free-standing f i l m s a n d u l t r a m i c r o t o m e d cross-sections w e r e observed b y T E M ( H i t a c h i H - 8 0 0 at 2 0 0 k V , H i t a c h i H - 7 6 0 0 at 80 k V ) . P o r e size distributions w e r e d e t e r m i n e d u s i n g the S c i o n Image  computer  p r o g r a m ( S c i o n C o r p . , M D , w w w . s c i o n c o r p . c o m ) or the equivalent I m a g e J p r o g r a m ( N a t i o n a l Institute o f H e a l t h , w w w . n i h . g o v ) . T h e pore diameters were calculated for. a c i r c l e o f area equivalent to the i m a g e d pores. A r i o n m i l l i n g was done u s i n g a V C R I o n m i l l w i t h 5 k V i o n energy, 4 0 p A b e a m current a n d 9 0 ° angle o f i n c i d e n c e . R e a c t i v e i o n etching w a s done w i t h a P l a s m a Q u e s t E C R etcher. T o o p e n a m a j o r i t y o f pores f r o m the b o t t o m o f the sample, the R F p o w e r w a s  150 W , the  m i c r o w a v e p o w e r 3 0 0 W , a n d the substrate temperature w a s 4 8 ° C . T h e gas flow rates for A r / B C l / C l were 20.5/20.1/2.8 seem, respectively. T h e e t c h i n g t i m e was 4 8 0 s. 3  2  112  References  1. 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Langmuir 1991, 7,  1072.  117  CHAPTER 5 Characterization  of  a  Porous Alumina/MEH-PPV  Composite Material  C o m p o s i t e materials  based  on porous  a l u m i n a and conjugated  p o l y m e r s have  been  extensively investigated i n our research group. A full characterization o f these materials w a s important for understanding their properties, and the central g o a l was to establish the p o l y m e r d i s t r i b u t i o n w i t h i n the c o m p o s i t e material o n the nanometre scale. T h e i n i t i a l experiments w e r e aimed  at  investigating  samples  prepared  by  adsorbing  the  conjugated  polymer poly[2-  m e t h o x y , 5 - ( 2 ' - e t h y l h e x y l o x y ) l , 4 - p h e n y l e n e v i n y l e n e ) ( M E H - P P V ) f r o m solution onto  porous  a l u m i n a ( F i g u r e 5.1). T h i s straightforward procedure y i e l d e d samples i n w h i c h the presence o f p o l y m e r w a s e v i d e n c e d by the c o l o u r and fluorescent e m i s s i o n o f the porous a l u m i n a layer ( F i g u r e 5.2). Confocal  fluorescence  microscopy was  used  to establish  that M E H - P P V  was  distributed  throughout the t h i c k n e s s o f the porous a l u m i n a host w i t h 0.1 urn resolution. H o w e v e r , the conjugated  p o l y m e r c o u l d not be located w i t h higher spatial resolution b y either  scanning  electron m i c r o s c o p y ( S E M ) or t r a n s m i s s i o n electron m i c r o s c o p y ( T E M ) , o w i n g to the p o o r b e a m contrast o f the p o l y m e r guest. T h e v i n y l i c carbons i n M E H - P P V were readily stained b y o s m i u m tetroxide, as e v i d e n c e d b y the disappearance o f the red-orange c o l o u r o f the sample, but this d i d not i m p r o v e the contrast i n T E M or S E M , m o s t l i k e l y due to the l i m i t e d amount o f p o l y m e r on the porous a l u m i n a surface.  118  D ~ 60 n m  F i g u r e 5.1 A d s o r p t i o n o f t h i n layer o f M E H - P P V onto porous a l u m i n a host, s h o w i n g (a) empty host, and p o l y m e r - c o a t e d host i n (b) p l a n v i e w and (c) as t h i n section for T E M .  F i g u r e 5.2 P o r o u s a l u m i n a f i l m after s o a k i n g i n M E H - P P V solution, seen i n cross-section, as s h o w n b y (a) light m i c r o s c o p y (b) fluorescence m i c r o s c o p y . T h e scale bar is 10 urn. S a m p l e s and images prepared by D r . K . R a d e m a c h e r .  119  T h e r e w e r e n o p r e v i o u s l y p u b l i s h e d studies o f porous a l u m i n a f i l m s b y E E L S at the outset o f this w o r k ; subsequently, a v e r y relevant study o f e p o x y infiltration into p o r o u s a l u m i n a appeared i n the literature. In that w o r k , the d i s t r i b u t i o n o f c a r b o n , a l u m i n u m a n d o x y g e n w a s 1  m a p p e d b y S T E M / E E L S u s i n g the c o r r e s p o n d i n g i o n i z a t i o n edges. T h e samples w e r e also held at -134 ° C u s i n g a c r y o g e n i c h o l d e r to m i n i m i z e b e a m damage to the sample. In p r i n c i p l e , the porous a l u m i n a / M E H - P P V sample c o u l d have been m a p p e d i n the same m a n n e r as P P V / M C M - 4 1 (chapter 3) t h r o u g h the use o f the 7 e V % p l a s m o n . T h e c a r b o n K - e d g e c o u l d also have been used. N e i t h e r o f these methods w a s successful but the experiments d i d p r o d u c e s o m e interesting results for further investigation.  5.1  E E L S Samples Two  geometries are possible for investigating the p o r o u s a l u m i n a / M E H - P P V c o m p o s i t e  materials b y E E L S : the p l a n geometry ( F i g u r e 5.1(b)) and the cross-section ( F i g u r e 5.1(c)). T h e cross-sections m a y be prepared b y u l t r a m i c r o t o m y , y i e l d i n g t h i n sections ( - 3 0 n m t h i c k ) o f the material. A s the samples w e r e k n o w n to contain v e r y little p o l y m e r - p o s s i b l y as little as a m o n o l a y e r o n the surface o f the pores - this geometry w o u l d present a v e r y s m a l l a m o u n t o f p o l y m e r to the electron b e a m . In chapter 7, this geometry is investigated for samples w i t h larger amounts o f p o l y m e r . It w a s d e e m e d advantageous to investigate these particular samples i n the p l a n geometry initially, s u c h that the electron b e a m w o u l d pass d o w n the pores a n d interact w i t h a l l the p o l y m e r distributed a l o n g the length o f the pores. A n u m b e r o f samples w e r e prepared for E E L S measurements. In order to obtain the clean loss s p e c t r u m o f M E H - P P V , t h i n film samples w e r e prepared by casting f r o m T H F over water 120  and c o l l e c t i n g the resulting f i l m w i t h a lacey carbon g r i d . T h e empty host material consisted o f porous a l u m i n a m e m b r a n e s (0.2 and 1.4 u m t h i c k ) , and the c o m p o s i t e material investigated w a s a 1.4 u m porous a l u m i n a m e m b r a n e w h i c h had been soaked i n M E H - P P V .  F i g u r e 5.3 S E M image o f cross-section o f sample for E E L S experiments. T h e scale bar is 1.0 u m .  5.2  M E H - P P V Low-Loss Spectra and Zero-Loss Peak Removal T h e initial investigation o f M E H - P P V focused on the l o w - l o s s spectrum and the distinctive  7r-7t*  p l a s m o n o f conjugated organic materials. T h e bright field T E M image o f the  MEH-PPV  f i l m is s h o w n i n F i g u r e 5.4, and the collected loss spectrum is i n F i g u r e 5.5. T h e 7t-7i* p l a s m o n w a s seen clearly at 6.4 e V , whereas the b u l k p l a s m o n appeared at 22 e V , as anticipated for an aromatic material. H o w e v e r , there were no initially d i s c e r n i b l e features associated w i t h the onset o f o p t i c a l absorption o f the p o l y m e r c h a i n at -3 e V , largely due to the tail o f the zero-loss peak. For  these experiments,  a clean  zero-loss  peak  was  not  recorded  immediately,  and  later  a c q u i s i t i o n y i e l d e d a peak w i t h a significantly different shape. H e n c e a n u m b e r o f different  121  F i g u r e 5.4 T E M image o f M E H - P P V f i l m supported by a lacey carbon g r i d . T h e scale bar is 100 nm. approaches to r e m o v i n g the zero-loss peak f r o m the spectrum were evaluated, as the tail o f the zero-loss peak affects  most o f the features b e l o w 10 e V . T h e possible approaches  include  F o u r i e r - l o g d e c o n v o l u t i o n , m a t r i x d e c o n v o l u t i o n , extrapolation o f the tail by a p o w e r - l a w fit 2  3  a n d direct subtraction o f the zero-loss p e a k . B a s i c for E x c e l  5  4  These procedures w e r e i m p l e m e n t e d as V i s u a l  scripts. F o u r i e r - l o g and m a t r i x d e c o n v o l u t i o n were based on  p r o g r a m s presented  by Egerton,  2  FORTRAN  w h i l e the p o w e r - l a w fit w a s a s i m p l e linear least-squares  regression procedure. F o u r i e r - l o g d e c o n v o l u t i o n was found to r e m o v e the effect o f p l u r a l scattering very w e l l , but it d i d introduce some artefacts near 4 e V , where the routine chooses the separation point between the zero-loss peak and the r e m a i n d e r o f the spectrum. T h e algorithm was not designed to deal w i t h spectra s h o w i n g substantial losses o n the tail o f the zero-loss peak. T h i s c o u l d have also been a v o i d e d by a c q u i r i n g a clean zero-loss spectrum separately. M a t r i x d e c o n v o l u t i o n d i d not r e m o v e the zero-loss peak v e r y effectively but d i d r e m o v e the p l u r a l scattering s m e a r i n g out the b u l k p l a s m o n peak. A s such it w a s not v e r y useful for e x a m i n a t i o n o f the spectrum o v e r the 2 to 5 e V range.  122  Energy (eV) F i g u r e 5.5 L o w - l o s s spectrum o f M E H - P P V t h i n removing  the  zero-loss  peak:  (•)  raw  data,  (•)  film,  also s h o w i n g v a r i o u s approaches  matrix  to  deconvolution, ( A ) Fourier-log  d e c o n v o l u t i o n , ( o ) p o w e r l a w fit over 1.3 - 2.0 e V . T h e inset s h o w s the detail over 0 - 1 0 e V .  D i r e c t subtraction o f the zero-loss peak can be a p p l i e d i n a very accurate w a y b y the use o f spline interpolation and curve-fitting techniques, as has been reported r e c e n t l y .  4  This method  seemed to be the most reliable for r e v e a l i n g w e a k spectral features i n the 2 to 5 e V range. H o w e v e r , it again r e q u i r e d an accurate spectrum o f the clean zero-loss peak; the lack o f this i n f o r m a t i o n made it i m p o s s i b l e to apply to the data presented here. F i n a l l y , the p o w e r - l a w fit does not address p l u r a l scattering but does r e m o v e the t a i l o f the zero-loss peak fairly s m o o t h l y , w i t h m i n i m a l artefacts i n the 2-5 e V range. T h e p o s s i b i l i t y o f a w e a k shoulder feature b e i n g fitted out b y this procedure cannot be ruled out, h o w e v e r (later 123  experiments s h o w e d that s u c h a shoulder w a s present i n s o m e o f the spectra - see section 6.5). Nevertheless, this approach seemed the most reliable one and as such, the p o w e r - l a w fit w a s used to process all the spectra, u s i n g least-squares linear regression to calculate the  curve  parameters. T h e i m p l e m e n t a t i o n as a script a l l o w e d large n u m b e r s o f spectra to be processed easily.  5.3  Porous A l u m i n a Low-Loss Spectra T h e l o w - l o s s spectrum o f porous a l u m i n a ( F i g u r e 5.6) s h o w e d a b u l k p l a s m o n at 23 e V ,  w h i c h is i n agreement w i t h the literature v a l u e for a m o r p h o u s a l u m i n a .  6  Crystalline alumina  presents a p l a s m o n at a slightly h i g h e r energy (26 e V ) . T h i s c o n f i r m e d the a m o r p h o u s nature o f 2  the pore w a l l s , as reported i n the literature f r o m T E M o b s e r v a t i o n s . ' 7  8  A d d i t i o n a l loss m o d e s  appear b e l o w the b u l k p l a s m o n : one or t w o b e l o w 10 e V , and one at 13 e V . T h e nature o f these a d d i t i o n a l m o d e s has not been fully established i n the literature; it has been suggested that they are due to a n a l u m i n u m ( O ) surface p l a s m o n w h i c h w o u l d arise f r o m a d e v i a t i o n from the i d e a l a l u m i n a s t o i c h i o m e t r y , but this is v e r y u n l i k e l y for porous a l u m i n a . R e d u c t i o n o f a l u m i n a to 6  a l u m i n u m b y the electron b e a m is not observed i n a m o r p h o u s a l u m i n a , as a l u m i n u m atoms are r e m o v e d preferentially over o x y g e n a t o m s .  9  T h e presence o f these a d d i t i o n a l loss m o d e s , especially the ones b e l o w 10 e V , presents a difficulty as they w o u l d p o s s i b l y m a s k the conjugated p o l y m e r  71-71*  p l a s m o n near 6 e V . U n d e r  these circumstances, it w a s necessary to study these a d d i t i o n a l loss m o d e s i n m o r e detail.  124  I  '  0  1  10  '  1  20  '  1 30  '  1-  40  Energy (eV) F i g u r e 5.6 L o w - l o s s s p e c t r u m o f pore i n p o r o u s a l u m i n a film, s h o w i n g ( • ) collected data and ( o ) data w i t h zero-loss peak r e m o v e d b y a p o w e r l a w fit over 1.3 to 2.0 e V .  T h e d i s t r i b u t i o n o f a l l the loss m o d e s i n a pore was d e t e r m i n e d by a c q u i r i n g loss spectra at regular intervals o v e r a line c r o s s i n g a. pore i n S T E M m o d e ( F i g u r e 5.7). T h i s revealed a strong loss m o d e near the pore w a l l , s o m e a d d i t i o n a l loss m o d e s at intermediate distances  from  the  w a l l , and one loss m o d e w h i c h extended at nearly constant level throughout the pore. T y p i c a l spectra for these three r e g i m e s are s h o w n i n F i g u r e 5.8: (a) at the w a l l , the a l u m i n a b u l k p l a s m o n is seen at - 2 2 e V , w i t h a strong tail due to p l u r a l scattering; (b) at - 7 n m from the w a l l , three surface p l a s m o n s at 8, 13 and 18 e V appear, w i t h an additional shoulder at 7 e V ; the shoulder at 3 e V is p r o b a b l y an artifact due to zero-loss peak subtraction; (c) at - 23 n m from the w a l l , a strong peak at 7 e V is seen, and the surface p l a s m o n s are still seen as w e a k e r shoulders. 125  T h e s e peak p o s i t i o n s are most l i k e l y only accurate to about 1 e V , due to the presence o f o v e r l a p p i n g peaks a n d a substantial b a c k g r o u n d due to p l u r a l scattering. T h e d i s t r i b u t i o n o f these l o w - l o s s m o d e s can also be v i s u a l i z e d b y E F T E M , as s h o w n i n F i g u r e 5.9 for a thinner (0.2 urn) p o r o u s a l u m i n a film. T h e filtered images c o n f i r m that the 7 e V m o d e is w e a k e r but e v e n l y distributed throughout the h o l e . T h e losses at 13 e V are seen to be c o n f i n e d to the n e i g h b o u r h o o d o f the surface, and the b u l k p l a s m o n can be seen throughout the film  and j u s t outside the surface. T h e d i s t r i b u t i o n o f the 2 2 e V losses also s h o w s the areas o f  l o w e r a n d h i g h e r w a l l density: the m a t e r i a l m o s t l y consists o f l o w density a l u m i n a w i t h a higher density core f o r m i n g a lattice between the p o r e s .  8  126  F i g u r e 5.7 (a) S T E M d a r k - f i e l d i m a g e o f 1.4 u m thick porous a l u m i n a  film,  s h o w i n g the  location o f line a l o n g w h i c h loss spectra were acquired at 2 n m intervals. T h e scale bar is 100 nm.  x  0  10  20  30  40  50  Energy (eV) F i g u r e 5.7 (b) D e t a i l o f analysis line and spatially-resolved l o w - l o s s spectra o f porous a l u m i n a film.  T h e zero-loss peak w a s r e m o v e d b y a p o w e r - l a w fit. 127  60000  Energy (eV) F i g u r e 5 . 8 Representative l o w - l o s s spectra for porous a l u m i n a f i l m : ( • ) near pore centre, ( o ) ~ 7 n m pore w a l l , a n d ( A ) at w a l l .  F i g u r e 5 . 9 Energy-filtered images o f 0.2 u m porous a l u m i n a f i l m , u s i n g a 2 e V w i n d o w : (a) 0 e V , (b) 6 e V , (c) 13 e V , (d) 22 e V . T h e scale bar is 100 n m .  128  from  5.4  Porous A l u m i n a / M E H - P P V Composite Spectra T h e spatial d i s t r i b u t i o n o f the l o w - l o s s spectra i n a p o r o u s a l u m i n a / M E H - P P V c o m p o s i t e  material w a s obtained i n the same manner, as s h o w n i n F i g u r e 5.10. T h e d i s t r i b u t i o n o f loss m o d e s w a s v e r y s i m i l a r to that o f empty p o r o u s a l u m i n a host. T h e same strong peak at 7 e V w a s o b s e r v e d o v e r the centre o f the pore, but w i t h n o s h o u l d e r p e a k s at h i g h e r energies. A t intermediate distances, the surface p l a s m o n s w e r e identical to those o f the e m p t y p o r o u s a l u m i n a . T h e s m a l l differences observed i n these m o d e s m a y reflect the presence o f p o l y m e r ; detailed m o d e l l i n g o f these effects w o u l d h o w e v e r require k n o w i n g the d i e l e c t r i c function o f M E H - P P V to at least 10 e V , a n d at present it is o n l y k n o w n to 5 e V . d e t e r m i n e d f r o m a carefully measured M E H - P P V  1 0  In p r i n c i p l e , this c o u l d be  loss s p e c t r u m ; h o w e v e r that w a s d e e m e d  b e y o n d the scope o f this thesis. The  n e a r - w a l l spectra are s h o w n i n m o r e detail i n F i g u r e 5.12. T h e l o w signal-to-noise  l e v e l does not a l l o w any significant c o n c l u s i o n s to be m a d e about the presence o f the p o l y m e r t h r o u g h the 7t p l a s m o n . S o m e attempts w e r e m a d e to f i n d the c a r b o n K - e d g e loss near 2 8 0 e V but w i t h o u t any success. T h i s approach has been s h o w n to be successful for m a p p i n g e p o x y penetration into p o r o u s a l u m i n a ; therefore the absence o f the s i g n a l near 2 8 0 e V c a n be a s c r i b e d to the l o w 1  a m o u n t or r a p i d degradation o f the p o l y m e r i n the samples investigated.  129  F i g u r e 5.10 (a) S T E M d a r k - f i e l d image o f porous a l u m i n a / M E H - P P V composite, s h o w i n g the line a l o n g w h i c h E E L spectra were a c q u i r e d . T h e scale bar is 100 n m .  x  0  10  20  30  40  50  Energy (eV)  F i g u r e 5.10 (b) D e t a i l o f analysis line and spatially-resolved l o w - l o s s spectra o f porous a l u m i n a / M E H - P P V c o m p o s i t e film. T h e zero-loss peak w a s r e m o v e d by a p o w e r - l a w fit from 1.3 to 2.0 e V .  130  40000 \-  20  30  Energy (eV) F i g u r e 5.11 C o m p a r i s o n o f l o w - l o s s spectra (o,«)  near pore axis and ( • , • )  at ~ 7 n m from the  pore w a l l , for porous a l u m i n a a n d p o r o u s a l u m i n a / M E H - P P V composite, respectively.  1000  20 30 Energy (eV) F i g u r e 5.12 C o m p a r i s o n o f l o w - l o s s spectra o f empty (•) and M E H - P P V - t r e a t e d ( A ) f o r o u s a l u m i n a , nearest to pore w a l l .  131  5.5  Conclusion A l t h o u g h the use o f the TT-TT* p l a s m o n to identify conjugated o r g a n i c m o l e c u l e s b y E E L S  and E F T E M is w e l l established, its a p p l i c a t i o n to the p o r o u s a l u m i n a / M E H - P P V c o m p o s i t e i n the plan geometry w a s not successful. L o w - e n e r g y surface p l a s m o n s and an unexpected l o n g range  interaction  at  7  e V effectively  masked  the  region  o f interest;  the  difficulty  was  c o m p o u n d e d by the s m a l l a m o u n t o f p o l y m e r present i n the c o m p o s i t e . T h e effect o f surface p l a s m o n s w o u l d have been m i n i m i z e d by e m p l o y i n g a cross-sectional geometry  instead. A larger p o l y m e r content  i n the c o m p o s i t e w o u l d have also p r o v i d e d a  stronger p l a s m o n loss peak. S u c h samples are investigated b y the same methods i n chapter 7. H o w e v e r , the most interesting feature a r i s i n g from these results is indeed the loss peak at 7 e V . T h e nature o f this interaction w a s studied i n m o r e detail and is the subject o f the f o l l o w i n g chapter.  Experimental Details P o r o u s a l u m i n a thiri f i l m s were prepared b y a n o d i z i n g 0.2 and 1.0 u m t h i c k a l u m i n u m films  evaporated onto rc-type s i l i c o n ( l l l ) wafers b y e-beam e v a p o r a t i o n .  11  The anodization was  c a r r i e d out i n 0.3 M o x a l i c a c i d at 2 0 ° C w i t h an a p p l i e d potential o f 4 0 . 0 V , u s i n g a glass c e l l w i t h an O - r i n g seal to the sample. U p o n c o m p l e t i o n o f the a n o d i z a t i o n , the p o r o u s a l u m i n a film (1.4  u m t h i c k ) had detached  aluminium  film.  f r o m the substrate, but was still attached  to the  surrounding  T h e barrier layer w a s r e m o v e d by e t c h i n g i n 5 wt. % p h o s p h o r i c a c i d for 4 0  m i n . T h e film was r i n s e d w i t h distilled water, f o l l o w e d by ethanol a n d then d r i e d . S o m e  films  w e r e soaked i n a s o l u t i o n o f M E H - P P V (0.038 wt. % i n T H F ) for 48 h . A d i a m o n d scribe w a s used to cut the porous a l u m i n a film to fit a 3 m m C u T E M g r i d , to w h i c h it w a s fixed u s i n g a 132  s m a l l a m o u n t o f e p o x y glue. M E H - P P V films were cast over water from a 0.05 w t % solution i n T H F and collected w i t h a holey c a r b o n g r i d ( T e d P e l l a Inc.). O s m i u m tetroxide s t a i n i n g w a s c a r r i e d out before e m b e d d i n g the samples  epoxy  for  u l t r a m i c r o t o m y . S a m p l e s were exposed to o s m i u m tetroxide v a p o u r b y p l a c i n g t h e m next to a drop o f 4 % aqueous solution i n a c o v e r e d d i s h for 3 0 m i n . D i r e c t i m m e r s i o n i n this solution w a s also used. T h i n sections w e r e obtained b y u l t r a m i c r o t o m y . T h e samples were first sputter-coated w i t h A u / P d to prevent e p o x y penetration into the porous a l u m i n a . T h e e m b e d d i n g m e d i u m w a s either E P O N e p o x y (various suppliers) or 3 0 2 - 3 M e p o x y ( E p o t e k , Inc.). S e c t i o n i n g w a s then c a r r i e d out u s i n g a d i a m o n d k n i f e w i t h a 4 5 ° edge set w i t h a 4 to 6 ° clearance angle, u s i n g water as the section c o l l e c t i n g l i q u i d . Electron  energy-loss  spectroscopy  C A N M E T / N a t u r a l Resources  was  carried  out  on  a  Canada equipped with a Schottky  Philips C M 20 field-emission  T E M at  electron  operated n o m i n a l l y at 2 0 0 k V , but l o w e r e d to 197 k V for spectroscopy. M e a s u r e m e n t s m a d e w i t h a G a t a n I m a g i n g F i l t e r u s i n g a d i s p e r s i o n o f 0.05 e V / c h a n n e l on a detector, c o v e r i n g -5 to 4 5 e V losses.  S p a t i a l l y - r e s o l v e d spectra  gun were  1024-channel  w e r e collected at  regular  intervals w i t h an estimated p r o b e size o f < 1 n m and an energy resolution o f 1.0 e V ( F W H M o f zero-loss peak).  133  References 1. A r a y a s a n t i p a r b , D . ; M c K n i g h t , S.; L i b e r a , M . 2.  Egerton, R . F.  J. Adhes. 2001,  76,  353.  Electron energy-loss spectroscopy in the electron microscope;  2 n d ed.; P l e n u m  Press: N e w Y o r k , 1996. 3. S u , D . S.; Schattschneider, P . 4. R e e d , B . W . ; S a r i k a y a , M .  J. Microsc. 1 9 9 2 , 1 6 7 ,  Ultramicroscopy2002,  93,  63. 25.  5. M i c r o s o f t ; R e d m o n d , W A . 6. K a p s a , R . ; Stara, I.; Z e z e , D . ; G r u z z a , B . ; M a t o l i n , V . 7. T h o m p s o n , G . E . ; W o o d , G . C .  Anodic films on aluminum;  T e c h n o l o g y ; A c a d e m i c Press: N e w Y o r k , 1983; Vol. 8. N i e l s c h , K.;  C h o i , J . ; S c h w i r n , K.;  9. B e r g e r , S. D .  Philos. Mag.  317,  77.  Treatise o n M a t e r i a l s S c i e n c e a n d  23, p. 205.  Wehrspohn, R . B . ; Gosele, U .  B 1987,  M o n k m a n , A . P.  Thin Solid Films 1998,  Nano Lett. 2002, 2,  677.  55, 341.  Adv. Mater. 2002, 14,  10.  Tammer, M.;  11.  Crouse, D . ; L o , Y . H . ; Miller, A . E . ; Crouse, M .  210.  Appl. Phys. Lett. 2000, 76,  49.  134  CHAPTER 6 Aloof Cherenkov Effect in Porous Alumina  T h e unexpected l o w - l o s s spectral feature observed b y electron energy-loss  spectroscopy  ( E E L S ) i n the centre o f the pores o f p o r o u s a l u m i n a f i l m s , as described i n chapter 5, warranted further investigation. S e v e r a l aspects o f this feature w e r e u n u s u a l : the lack o f p l u r a l scattering, the rather large distances - up to 30 n m - f r o m the pore w a l l at w h i c h it w a s still present w i t h o u t v e r y m u c h decay i n intensity, and the fact that it appeared  i n the spectral r e g i o n usually  d o m i n a t e d b y the o p t i c a l properties o f the m a t e r i a l u n d e r g o i n g analysis. These characteristics suggested that the observed feature was not a surface p l a s m o n . P r e l i m i n a r y d i s c u s s i o n s w i t h theoreticians experienced w i t h m o d e l l i n g E E L S suggested that the most p l a u s i b l e hypothesis for its o r i g i n w a s rooted i n the C h e r e n k o v effect. A collaborative effort w a s then undertaken to validate this hypothesis. T h i s chapter describes the further e x p e r i m e n t a l and theoretical results that were used to s h o w that the C h e r e n k o v effect is responsible for these losses. In this chapter, the energy o f the electron b e a m i n the t r a n s m i s s i o n electron m i c r o s c o p e ( T E M ) is g i v e n i n units o f k e V , w h i c h is preferred from the theoretical standpoint. T h i s is equivalent to the e x p e r i m e n t a l accelerating voltage i n the T E M .  6.1  T h e C h e r e n k o v Effect T h e C h e r e n k o v effect is described as the e m i s s i o n o f radiation w h e n a charged particle  m o v e s t h r o u g h a m e d i u m at a speed greater than the speed o f light i n that particular m e d i u m .  1  135  T h e effect w a s observed by C h e r e n k o v i n 1 9 3 4 ,  2  i n the f o r m o f a g l o w i n fluids exposed to a  radioactive source. T h e a d d i t i o n o f q u e n c h i n g agents d i d not alter the luminescence,  suggesting  that the radiation w a s extraordinary i n nature, and a full explanation o f the effect w a s p r o v i d e d b y T a m m and F r a n k i n 1 9 3 7 .  3  T h e c o m p l e x dielectric function  e(co) = £i((o) + isiico) defines  the interactions between a  m e d i u m and electromagnetic radiation. T h e real part alters the wavelength o f the  propagating  radiation, thus r e d u c i n g the speed o f propagation o f the radiation at an energy co to c I ^ j s ( c o ) , x  w h e r e c is the speed o f light i n v a c u u m . T h e i m a g i n a r y part o f the dielectric function describes the attenuation at a g i v e n energy co. T h u s the C h e r e n k o v c o n d i t i o n is satisfied for electron velocities v w h e n w h i c h results i n the e m i s s i o n o f radiation w i t h frequency g i v e n by  cost9 = c  clv^s (co) x  s (co)> c x  co i n a h o l l o w cone w i t h  ( F i g u r e 6.1). T h i s radiation  is analogous  to the  2  /v , 2  half-angle Shockwave  p r o d u c e d b y objects t r a v e l l i n g faster than the speed o f sound i n air. F o r 2 0 0 k e V electrons, the C h e r e n k o v c o n d i t i o n is  £\(co) > 2 . 1 ,  w h i c h is a m p l y satisfied for energies up to 12 e V for  a l u m i n a ( F i g u r e 6.2). T h e thresholds for other electron energies c o m m o n l y used i n T E M are s h o w n i n T a b l e 6.1. C h e r e n k o v radiation can be p r o d u c e d by electrons t r a v e l l i n g i n v a c u u m but p a s s i n g near a material, p r o v i d e d the C h e r e n k o v c o n d i t i o n is satisfied w i t h i n the material. T h e radiation is then p r o d u c e d by the i n d u c e d p o l a r i z a t i o n i n the material and it is expected to be important up to electron-surface  separations o f the order o f the range o f the evanescent field o f the electron i n  co,  v a c u u m . F o r a g i v e n frequency c o m p o n e n t  the decay constant is  ~v/co = Xv/2nc,  w h e r e A is  the w a v e l e n g t h o f the C h e r e n k o v r a d i a t i o n . F o r 7 e V radiation, this is a p p r o x i m a t e l y 2 0 n m . It 4  136  w i l l be s h o w n b e l o w that the C h e r e n k o v losses are actually sensitive to the sample structure up to m u c h larger distances, due to the radiative nature o f the C h e r e n k o v effect.  i (a)  B F i g u r e 6.1 G e o m e t r y o f C h e r e n k o v radiation due to an electron t r a v e l l i n g (a) t h r o u g h a m e d i u m and (b) near a m e d i u m w i t h dielectric function s(co): light is emitted i n the f o r w a r d direction i n a h o l l o w cone. —1—1—1—1—1—1—1—1—t—J 1 1 1  1  1  1 1  1  1  1 1  j  1  1  11  1  1  1  11  1  1  1  1  1  1 1 1—  lRe[e((o)] V\ r \ \ \ i  1  •1 \ 1 lm[e(o))]  1  1  -  I  l \  1  I  1  1 1  ~  \  X *  —  *  I  \ 1  1  \  I I \  \ \  \ \  I  _ \ \  \  N  \  N  \  \ \  \  \  ^  9 §  i  0  i  i  i  1  5  i - *•  i  i  1 i i i i 1 i r T i  10  15  1 i i i  20  i—i—i—i—1—i—i—i—i—1—i—i—i—i—  25  30  35  40  co(eV)  F i g u r e 6.2 D i e l e c t r i c function o f a l u m i n a , after data from ref. 5. T h e C h e r e n k o v c o n d i t i o n is satisfied up to 12 e V for 2 0 0 k e V electrons. F i g u r e courtesy o f A . R i v a c o b a .  137  T a b l e 6.1 C h e r e n k o v c o n d i t i o n for c o m m o n T E M b e a m energies.  P  Beam Energy  M i n i m u m s^co)  (keV)  (v/c)  80 100  0.50 0.55  4.0  120  0.59  2.9  200  0.70  2.1  3.3  U s i n g E E L S , losses due to the C h e r e n k o v effect have been observed i n d i e l e c t r i c s and 6  semiconductors ' 6  7  w i t h the electron b e a m p a s s i n g t h r o u g h the material, a n d the a c c o m p a n y i n g  o p t i c a l e m i s s i o n s have been recently m e a s u r e d . f i e l d " C h e r e n k o v effect nanoparticles.  9  has  been  8  It is o n l y recently that the " a l o o f , or "near-  c o n s i d e r e d , i n a report o n the loss spectra o f a l u m i n a  These results s h o w e d loss features b e l o w 10 e V for non-penetrating  trajectories  w h i c h c o u l d not be simulated w i t h a non-relativistic m o d e l , but a relativistic m o d e l was not presented. P o r o u s s i l i c o n also s h o w s u n u s u a l spectral features at large beam-surface w h i c h m a y be due to the C h e r e n k o v e f f e c t .  10  separations  A s s u c h , the a l o o f C h e r e n k o v effect had not been  fully validated i n the literature and merited further e x p e r i m e n t a l and theoretical investigation. A s a p o i n t o f interest, the m i c r o s c o p i c nature o f C h e r e n k o v radiation is still the subject o f debate. T h e long-standing v i e w p o i n t has been that the radiation is not emitted by the charged particle itself, rather b y oscillations i n the m e d i u m . ' > 1  12  H o w e v e r , it has been recently suggested  that the radiation is i n fact better d e s c r i b e d as o r i g i n a t i n g f r o m the t r a v e l l i n g c h a r g e .  13  debate m a y be p u r e l y a c a d e m i c , as certainly both the m e d i u m a n d the t r a v e l l i n g charge required  for the p h e n o m e n o n .  In the  theoretical m o d e l s presented  b e l o w , the  This are  Cherenkov  e m i s s i o n clearly arises f r o m the response o f the m e d i u m to the p a s s i n g external electron.  138  6.2  Further Measurements A n u m b e r o f variables c o u l d affect the 7 e V peak: pore diameter, lattice s p a c i n g , sample  t h i c k n e s s , and p r i m a r y electron b e a m energy. T h e p r i m a r y b e a m energy w a s chosen as b e i n g the m o s t i n d i c a t i v e factor for the C h e r e n k o v effect, as it w o u l d alter the C h e r e n k o v c o n d i t i o n , thus i n d u c i n g a change i n any loss features associated w i t h C h e r e n k o v radiation. It also addresses the c o n c e r n that the observed peak is due to a finite pore length effect i n the s a m p l e or an unexpected surface p l a s m o n . T h e effect o f v a r y i n g the pore diameter w i t h a fixed lattice s p a c i n g w a s also investigated.  6.3  Effect of Primary Beam Energy T h e effect o f p r i m a r y b e a m energy w a s investigated b y r e c o r d i n g loss spectra w i t h a l o w e r  p r i m a r y b e a m energy. M e a s u r e m e n t s at 5 0 k e V w o u l d have been the most r e v e a l i n g , since the C h e r e n k o v c o n d i t i o n w o u l d n o longer have been satisfied and any associated losses w o u l d have disappeared altogether. T h i s w a s not p o s s i b l e due to instrumental l i m i t a t i o n s , as the available G a t a n I m a g i n g Filter ( G I F ) was set up for operation at 117 and 197 k e V . T h e  measurements  w e r e carried out o n the s a m p l e used earlier (chapter 4 ) , a 1.4 u m p o r o u s a l u m i n a m e m b r a n e . T h e m i c r o s c o p e c a m e r a length a n d G I F entrance aperture w e r e v a r i e d to investigate the effect o f the spectrometer c o l l e c t i o n angle ((3) o n the collected spectra. T h e n e w measurements at 197 k e V c o n f i r m e d the r e p r o d u c i b i l i t y o f the peak at 7 e V i n a 6 0 n m pore ( F i g u r e 6.3). T h e results o f the measurements at 117 k e V o n the 1.4 u m film are s h o w n i n F i g u r e 6.4: the general f o r m o f the loss spectra was v e r y s i m i l a r to those taken at 197 k e V , but certain subtle differences were apparent w h e n the spectra w e r e c o m p a r e d ( F i g u r e 6.5). T h e r e is a s m a l l blue-shift o f the loss peaks to h i g h e r energies (from 7 to 8 e V ) , both at the centre o f the pore a n d at intermediate distances. T h e surface p l a s m o n peaks are seen to be at the 139  same energies, i n d i c a t i n g that the shift is not due to c a l i b r a t i o n error or to a change i n geometry. N e a r the pore w a l l , the b u l k p l a s m o n s are also seen to be i n agreement at the t w o b e a m energies, again c o n f i r m i n g that the shift o b s e r v e d i n the a x i a l loss peak is real and caused by the change i n b e a m energy. T h e c o l l e c t i o n angle w a s v a r i e d from 0.34 m r a d to 6.0 m r a d , w h i c h h a d v e r y little effect on the form o f the spectrum. T h i s is i n agreement w i t h the s m a l l scattering angles for electrons interacting w i t h surface p l a s m o n s . T h e scattering angle o f the C h e r e n k o v effect is also very small.  1 4  140  Energy (eV) F i g u r e 6.3 (a) S T E M d a r k - f i e l d image, s h o w i n g pore used for E E L S analysis at 197 k e V (scale bar is 100 n m ) . (b) E E L S spectra a c q u i r e d over the pore diameter w i t h P = 0.34 m r a d . T h e z e r o loss peak has been r e m o v e d u s i n g a p o w e r l a w fit over 1.3 - 2.0 e V . 141  Energy (eV)  F i g u r e 6.4 (a) S T E M d a r k - f i e l d image, s h o w i n g pore used for E E L S analysis at 117 k e V (scale bar is 100 n m ) . (b) E E L S spectra a c q u i r e d over the pore diameter w i t h (3 = 1.5 m r a d . T h e z e r o loss peak has been r e m o v e d u s i n g a p o w e r l a w fit over 1.3 - 2.0 e V . 142  197 k V , 0 = 0.34 mrad  3000  <  1 1 7 k V , p = 1.5 m r a d  s = 22 nm  2000  ^—*  co c 0  -»—'  c  1000  20  30  Energy (eV) • 197 kV, p = 0.34 mrad 117 kV, (3= 1.5 mrad  B  600  <  400  s = 28 nm  CO £= d)  200  Energy (eV) F i g u r e 6.5 C o m p a r i s o n o f l o w - l o s s spectra at 197 and 117 k e V p r i m a r y beam energies: (a) a x i a l (s = 0 n m ) a n d intermediate (s = 22 n m ) spectra, a n d (b) w a l l - g r a z i n g (s = 2 8 n m ) spectra.  6.4  M o d e l l i n g o f the A l o o f C h e r e n k o v Effect The  energy loss spectra w e r e m o d e l l e d b y N . Z a b a l a , A . R i v a c o b a a n d F . J . G a r c i a de  A b a j o at the B a s q u e C o u n t r y U n i v e r s i t y i n S p a i n . T h e details o f the m a t h e m a t i c a l m o d e l s are not presented here but they can be found i n the l i t e r a t u r e . ' 1 5  1 6  A s m e n t i o n e d i n chapter 2, the l o w energy (< 50 e V ) interactions between an electron beam  and a material c a n be d e s c r i b e d by dielectric theory, w h e r e the d i e l e c t r i c function  £($>) defines the response o f the material to the electric field o f the t r a v e l l i n g electrons. T h e electrons experience energy losses due to the potential i n d u c e d o n the m e d i u m : the electric field o f the electron b e a m p o l a r i z e s the m e d i u m , and this i n d u c e d p o l a r i z a t i o n creates an electric field w h i c h acts to s l o w the electron b e a m . T h e losses are first calculated i n the f o r m o f the distribution o f energy loss p r o b a b i l i t i e s per u n i t length, dP(co)/dz. T h i s w o u l d represent the f o r m o f the loss s p e c t r u m w i t h perfect energy resolution a n d w i t h o u t any p l u r a l scattering, a l l o w i n g the expected losses to be seen i n detail. T h e finite energy d i s t r i b u t i o n o f the electron b e a m (i.e., the shape o f the zero-loss peak) is then taken into account t h r o u g h a c o n v o l u t i o n process, a l o n g w i t h the effect o f m u l t i p l e scattering, to y i e l d a direct c o m p a r i s o n w i t h the e x p e r i m e n t a l s p e c t r u m . T h e finite size o f the electron b e a m is not taken into account, and only trajectories parallel to the pore axis are considered. T h e spatial dependence o f the energy losses is d e s c r i b e d i n terms o f the i m p a c t parameter s, w h i c h is the distance between the electron b e a m a n d the centre o f the pore ( F i g u r e 6.6). T h e effect o f finite b e a m convergence angle i n S T E M m o d e leads to a substantial spread o f the b e a m w h e n t h i c k m e m b r a n e s are c o n s i d e r e d - a t y p i c a l v a l u e o f a = 10 m r a d leads to a total spread o f 20 n m o v e r 1 urn.  144  A  B  F i g u r e 6.6 C o m p a r i s o n o f (a) experimental electron b e a m w i t h convergence angle a and (b) theoretical m o d e l .  6.4.1  M o d e l l i n g o f 197 k e V D a t a T h e energy loss p r o b a b i l i t y d i s t r i b u t i o n for relativistic electrons t r a v e l l i n g p a r a x i a l l y to an  infinite c y l i n d r i c a l hole i n a dielectric m e d i u m w a s first described b y Z a b a l a et al}  1  This model  a l l o w s the calculation o f the losses associated w i t h a single hole (radius a = 2 9 n m ) i n b u l k a l u m i n a (Figure 6.7, M o d e l A ) . N e a r the pore w a l l (s = 27 n m ) , surface p l a s m o n s between 10 a n d 25 e V dominate the loss function. T h i s is in reasonable agreement w i t h the experimentally observed s p e c t r u m . N e a r the pore centre (s - 0 n m ) , the loss probability is a m o n o t o n i c a l l y d e c a y i n g c u r v e , falling to zero at 3 0 e V . T h i s represents losses due to the C h e r e n k o v effect, as n o surface p l a s m o n s are excited at this distance. T h i s s i m p l e m o d e l thus fails to describe the e x p e r i m e n t a l spectrum near the pore centre: the a s y m m e t r i c a l peak at 7 e V observed for porous a l u m i n a differs substantially from the m o n o t o n i c a l l y d e c a y i n g curve. T h e observed difference suggests that the C h e r e n k o v losses are cut o f f at 7 e V . T h i s discrepancy was assumed  to arise f r o m the effect o f n e i g h b o u r i n g pores, w h i c h alters the 145  dielectric function o n a l o c a l level. A l t h o u g h these pores lie b e y o n d the range o f the evanescent field  o f the electron b e a m , they alter the m e d i u m t h r o u g h w h i c h the C h e r e n k o v radiation  propagates and directly affect the loss spectrum. T h e effect o f the n e i g h b o u r i n g pores c a n be m o d e l l e d i n a n u m b e r o f w a y s . T h e simplest approach is to consider c y l i n d r i c a l shells o f a l u m i n a w i t h different outer r a d i i , as s h o w n i n F i g u r e 6.7 ( M o d e l B ) . T h i s m o d e l for the porous  a l u m i n a introduces t w o peaks into the  d i s t r i b u t i o n i n the r e g i o n o f interest. C h a n g e s i n the shell t h i c k n e s s shift the p o s i t i o n o f the d o m i n a n t loss peak. T h i s gives an i n d i c a t i o n o f w h y the C h e r e n k o v losses are cut o f f at 7 e V : o n l y the C h e r e n k o v radiation w h i c h can be transmitted b y the m e m b r a n e gives rise to associated electron energy losses. T h e C h e r e n k o v radiation is c o n f i n e d w i t h i n the c y l i n d r i c a l shell, due to total internal reflection at the v a c u u m interfaces (the c o n d i t i o n for total internal reflection is guaranteed by the C h e r e n k o v c o n d i t i o n ) .  1 6  T h e c y l i n d r i c a l shell geometry leads to a quantization  rule for the transmitted r a d i a t i o n , cutting out m u c h o f the low-energy C h e r e n k o v radiation and the losses w h i c h w o u l d have been associated w i t h i t .  1 6  T h e basic p h y s i c s b e h i n d the observed  loss peak is therefore fairly clearly represented b y this s i m p l e shell m o d e l . M o r e refined representations o f the dielectric properties o f porous a l u m i n a were  also  investigated, by m o d e l l i n g the nearest n e i g h b o u r i n g pores directly, either w i t h s i x or t w e l v e c y l i n d r i c a l nearest neighbours (1+6 or 1+12 pores). T h e resulting loss distributions for electrons w i t h a x i a l paths (s = 0 n m ) and near w a l l paths (s = 2 0 n m ) are s h o w n i n F i g u r e 6.8. T h e m o d e l w i t h 1+6 pores clearly p r o v i d e s a peak at 7 e V . T h e 1+12 m o d e l , w h i c h w o u l d be expected to be m o r e accurate than the 1+6 m o d e l , deviates substantially b y presenting a second peak at ~8 e V . H o w e v e r , the subtle differences between these m o d e l s w o u l d not be d i s c e r n i b l e experimentally, due to the l i m i t e d energy resolution (~0.9 e V ) o f E E L S i n a T E M . L o s s e s b e l o w 7 e V are also present i n these m o d e l s and this is discussed further b e l o w .  146  7x10  : " C) CO  5  N T3  '•  \  N  .  )  f  -  a=29 nm  4  3  ;  f\ 1  1  /  /  1  1  1  1  1  1  •  1 —r—i—|—i—i—i—r—j—i—i  i  i  MODEL A :  \  \  -  / \s=27 nm  CL T5  3  /  -Q C C  2  -Q O  1: 0 0  :  X ^ ^ s = 2 3 nm (m=0) s=0  \s=23 nm  nnri\^AA^^~~^----. •  10  4x10  -i  1  r-  15 20 co(eV)  -l—TI  1  r  " ~ l T  25  i  i  r - j T - 1 - I rf- h - i — i —  35  30  | — i  i  i  i  MODEL B  3.5P 3  \ ^  3|_b=94nm  N  §  2.5  t b = 1 2 7 nm:  CL T3  I  1-5 F-  03  -Q §  b=61  1 f. (MODEL A ) \ i  nm  a=29 nm s=0 nm  Q. Hi  0.5  LU  0.  o  Hi 10 o(eV)  15  20  F i g u r e 6.7 T h e o r e t i c a l loss d i s t r i b u t i o n for 2 0 0 k e V electrons t r a v e l l i n g ( M o d e l A ) d o w n a single pore i n a l u m i n a , and ( M o d e l B ) d o w n a c y l i n d r i c a l pore o f outer r a d i i 6 1 , 94 and 127 n m . T h e inner pore radius is 2 9 n m . F i g u r e s courtesy o f N . Z a b a l a , F . J . G a r c i a de A b a j o and A . Rivacoba. 147  F i g u r e 6.8 T h e o r e t i c a l loss d i s t r i b u t i o n for 2 0 0 k e V electrons t r a v e l l i n g d o w n a c y l i n d r i c a l hole w i t h 0, 6 and 12 n e i g h b o u r i n g pores ( M o d e l C ) , s h o w i n g the loss p r o b a b i l i t y for (a) a x i a l (s = 0 n m ) and (b) n e a r - w a l l (s = 27 n m ) trajectories. T h e pore radius is 2 9 n m and the s p a c i n g is 90 n m . F i g u r e s courtesy o f N . Z a b a l a , F . J . G a r c i a de A b a j o and A . R i v a c o b a .  148  6.4.2  M o d e l l i n g o f 117 k e V D a t a A t the l o w e r p r i m a r y b e a m energy, the electron v e l o c i t y has decreased to v i e « 0.6. T h e  C h e r e n k o v c o n d i t i o n is thus  Si(co)  > 2.8,  w h i c h is also satisfied up to ~11 e V for a l u m i n a ( F i g u r e  6.2). T h e energy loss p r o b a b i l i t y does d e p e n d d i r e c t l y on v i e , and the theoretical spectra reflect this ( F i g u r e 6.9). A t s = 0 n m , the loss peak shifts to higher energy b y - 1 . 5 e V , i n qualitative agreement w i t h the e x p e r i m e n t a l result.  6.4.3  Comparison with Experiment T h e loss p r o b a b i l i t y o f the 1+6 c y l i n d e r m o d e l w a s c o n v o l u t e d w i t h the e x p e r i m e n t a l z e r o -  loss peak to y i e l d a direct c o m p a r i s o n w i t h e x p e r i m e n t at three representative i m p a c t parameters. ( F i g u r e 6.10). T h e quantitative agreement at s = 22 n m a n d s = 2 8 n m is quite g o o d . A t s = 0 n m , the m a g n i t u d e o f the C h e r e n k o v peak is overestimated b y a factor o f 2 at 2 0 0 k e V ; at 120 k e V the agreement is i m p r o v e d . T h i s indicates that some effects m a y still not be fully  understood,  either e x p e r i m e n t a l l y or theoretically. O n e p o s s i b i l i t y is the effect o f finite b e a m  convergence  angle i n S T E M m o d e , w h i c h leads to a substantial spread o f the b e a m w h e n t h i c k m e m b r a n e s are c o n s i d e r e d . H o w e v e r , a c o m p a r i s o n o f the spatial d i s t r i b u t i o n o f losses as revealed b y E F T E M ( a s m a l l ) and S T E M / E E L S  indicates that the difference is not v e r y p r o n o u n c e d . O v e r a l l , the  agreement between theory and experiment is satisfactory, and s h o w s that the o r i g i n o f the loss peaks is w e l l represented b y the m o d e l s presented here.  149  0.5  MODEL B  3 CO  N T3  CL  0.4 0.3  C\J  >  0.2  a=29 nm b=94 nm s=0 nm  200 KeV (p=0.7) •;  -Q CD  -Q O  0.1 U 0 0  120 KeV (p-0.6)  10 co(eV)  15  20  F i g u r e 6.9 Effect o f electron v e l o c i t y o n the loss p r o b a b i l i t y function illustrated u s i n g M o d e l B . (|3 = vie i n this context). F i g u r e courtesy o f N . Z a b a l a , F . J . G a r c i a de A b a j o a n d A . R i v a c o b a .  Energy loss (eV) F i g u r e 6.10 C o m p a r i s o n between theoretical  a n d experimental  spectra at different  impact  parameters s: (a) 0 n m , (b) 2 2 n m , (c) 2 8 n m , u s i n g the 1+6 c y l i n d e r m o d e l (radius 2 9 n m , s p a c i n g 9 0 n m ) . F i g u r e courtesy o f F . J . G a r c i a de A b a j o . 150  6.5  Effect of Pore Diameter on the Cherenkov Peak T h e c y l i n d r i c a l shell m o d e l i n d i c a t e d that the C h e r e n k o v loss peak shifts w i t h the t h i c k n e s s  o f the shell, w h i c h is equivalent to c h a n g i n g the pore diameter w h i l e k e e p i n g a f i x e d  pore  spacing. T h i s effect w a s investigated i n m o r e detail b y p r e p a r i n g ordered porous a l u m i n a m e m b r a n e s through a two-step a n o d i z a t i o n process (chapter 4 ) . T h e lattice s p a c i n g w a s fixed b y the a n o d i z a t i o n potential (105 n m at 4 0 V ) , w h i l e the pore diameter was altered b y etching the samples w i t h p h o s p h o r i c a c i d . In this m a n n e r , w e l l - o r d e r e d d o m a i n s o f about 800 n m diameter were obtained, w i t h average pore diameters o f 6 2 , 74 and 82 n m ( F i g u r e 6.11). T h e S E M image o f the cross-section s h o w e d that the pores were straight ( F i g u r e 6.12). T h e c h a n n e l diameters d i d not appear to be u n i f o r m throughout the thickness o f the m e m b r a n e . T h i s m a y have been an artifact due to a slight tilt i n the cleavage plane or it m a y a consequence o f the fabrication process  (see  the  experimental  details),  which  m i g h t have  been  avoided through  further  o p t i m i z a t i o n . T h e sample w o u l d still be expected to s h o w the loss spectrum o f the smaller pore diameter w i t h less intensity. T h e loss peak w a s recorded at the centre o f the pore for each ordered m e m b r a n e ( F i g u r e 6.13(a)). T h e loss peak is shifted to higher energy as the pore diameter is increased, from 6.7 e V at 62 n m to 7.9 e V at 82 n m . T h i s is i n qualitative agreement w i t h the p r e d i c t i o n that a t h i c k e r a l u m i n a shell w i l l lead to a l o w e r energy cutoff, as m o r e o f the C h e r e n k o v radiation is able to propagate i n the m e m b r a n e . C h a n g i n g the p r i m a r y b e a m energy to 117 k e V again shifted the loss peak to h i g h e r energies. T h e intensity o f the loss peak d i m i n i s h e s w i t h pore diameter due to the larger distance to the pore w a l l , resulting i n a smaller interaction between the electron b e a m and the m e m b r a n e (the m e m b r a n e thickness difference also has an effect, but not as substantial, see T a b l e 6.2). T h e loss intensity m a y also decrease due to the r e d u c e d w a l l thickness ( F i g u r e 6.7).  151  T a b l e 6.2 C h e r e n k o v loss peak parameters for ordered samples. Pore D i a m e t e r  Membrane Thickness  Peak Position  (nm)  (um)  62  2.1 ± 0 . 3  6.7  (eV) (197 k e V )  59  2.1 ± 0 . 3  7.8  (117keV)  74  2.1 ± 0 . 3  7.5  82  0.85 + 0.05  7.9  •• • •• •••« • ••• I  •  •  %  4  I  ^^^^^  ^^^^ j ^ ^ ^ ^ ^ ^ ^ * ^ ^ ^ ^  , w  .  „  „  w  -  _  -  „.„  F i g u r e 6.11 S T E M dark field images s h o w i n g geometry o f ordered porous a l u m i n a m e m b r a n e s p r o d u c e d by a two-step a n o d i z a t i o n at 4 0 V i n 0.3 M o x a l i c a c i d : (a) 62 n m , (b) 74 n m , (c) 84 n m diameters; (d) l o w e r m a g n i f i c a t i o n image s h o w i n g size o f ordered d o m a i n s . T h e scale bars are 100 n m . 152  F i g u r e 6.12 C r o s s - s e c t i o n o f ordered porous a l u m i n a m e m b r a n e . N o t e the pore diameters appear larger at the bottom. T h e scale bar is 1 u m .  M o r e careful measurement o f the zero-loss peak also a l l o w e d the e x a m i n a t i o n o f losses d o w n to 2 e V . T h i s revealed that there was n o sharp c u t - o f f at the l o w energy side o f the C h e r e n k o v loss peak ( F i g u r e 6.13(b)). Instead, the losses extend at least to 2 e V at a r o u g h l y constant value. T h e zero-loss peak subtraction w a s not sufficiently g o o d between -2 and 2 e V to extract any further i n f o r m a t i o n . T h e p r e c i s i o n o f the result is affected b y noise i n the spectra. T h e uncertainty i n the calculated spectrum m a y be d e t e r m i n e d from the c o u n t i n g statistics: the C C D detector is susceptible to shot noise, w h i c h is a P o i s s o n process w i t h an uncertainty o f for N c o u n t s .  1 8  ^fN  T h e uncertainty i n the dark current, w h i c h is subtracted from each collected  s p e c t r u m , m a y be estimated from the noise i n the spectrum o f the zero-loss peak, w e l l a w a y from the peak itself. These calculations suggested a noise level o f - 2 0 0 counts at 1 e V and - 1 0 0 0 counts at 0 e V , w h i c h w o u l d not account for the large fluctuations observed i n the calculated spectrum. C h a n g e s i n e x p e r i m e n t a l c o n d i t i o n s (e.g., stray fields) m a y then be the cause o f the  153  imperfect subtraction o f the zero-loss peak, and the range o f v a l i d i t y o f the subtraction c a n o n l y be estimated from the quality o f fit at the left-hand side o f the zero-loss peak. O v e r a l l , the observed b e a m energy a n d p o r e radius dependence agrees w i t h the p r e d i c t i o n s o f the shell m o d e l . It does not clarify the extent o f the l o n g range interaction w i t h the pore lattice, for w h i c h further measurements at different energies and o n different samples w o u l d be necessary. A s this pore lattice effectively forms a 2 - D p h o t o n i c crystal, these results introduce the p o s s i b i l i t y that E E L S c o u l d be used to extract useful i n f o r m a t i o n f r o m 2 - D p h o t o n i c nanostructures  through the C h e r e n k o v e f f e c t .  19  The possibility o f observing novel Cherenkov  radiation effects i n p h o t o n i c crystals has also been the subject o f theoretical d i s c u s s i o n .  2 0  154  6000 h  3  4000 L  CO  • | 2000  0 0  2  4  6  8  10 12 14 16 18 20  Energy (eV) 6000  1  B  — T  i  —  •  —  r  A  4000  - |  0  F i g u r e 6.13 (a) C h e r e n k o v peak  1  1  1  1  1  1  2 4 6 Energy (eV)  shift for a fixed  1  1  8  1 "  10  lattice s p a c i n g (105 n m ) w i t h  different  diameters: ( • ) 6 2 n m , ( • ) 74 n m , a n d ( • ) 82 n m at 197 k e V and ( o ) 6 2 n m at 117 k e V ; (b) ( • ) losses d o w n to 2 e V revealed b y subtraction o f the ( A ) zero-loss peak (diameter 6 2 n m , 197 k e V ) . T h e losses between - 2 and 2 e V are not m e a n i n g f u l . 155  6.6  Conclusion T h e 7 e V loss peak observed b y E E L S i n the centre o f the pores o f porous a l u m i n a  films  w a s found to be m o s t l i k e l y due to the C h e r e n k o v effect. T h e clearest evidence is the shift i n the loss peak to h i g h e r energy w i t h decreasing accelerating voltage. A n u m b e r o f m o d e l s w e r e investigated, and it w a s f o u n d that the local loss spectrum is affected by the sample structure on a m u c h larger scale, due to the radiative nature o f the C h e r e n k o v losses. T h e theoretical m o d e l o f the loss peak at different p r i m a r y b e a m energies m a t c h e d the e x p e r i m e n t a l data reasonably w e l l . C e r t a i n aspects o f the material and geometry w e r e not m o d e l l e d , and c o u l d account for the s m a l l discrepancies observed: the a n i o n c o n t a m i n a t i o n o f the porous a l u m i n a , and the exact geometry o f the pores n e i g h b o u r i n g the pore b e i n g investigated. More  direct evidence for the  generated r a d i a t i o n  8  C h e r e n k o v effect  c o u l d be obtained  b y detecting  the  but the systems i n existence today detect l u m i n e s c e n c e b e l o w 6 e V o n l y .  T h e nature o f these losses c o u l d be further studied b y fabricating a n d s t u d y i n g single pores, pore clusters and pore lattices w i t h exactly defined geometries, s u c h that a closer m a t c h to the theoretical m o d e l s c o u l d be obtained. H o w e v e r , the aspect ratios seen i n porous a l u m i n a are not readily a c h i e v e d u s i n g other fabrication approaches. Further measurements o n an  instrument  e q u i p p e d w i t h an electron m o n o c h r o m a t o r (zero-loss peak F W H M ~0.1 e V ) w o u l d be needed to establish the experimental loss spectrum w i t h sufficient detail to choose the correct theoretical representation o f the pore lattice. F i n a l l y , it s h o u l d be p o i n t e d out that the identification o f this p h e n o m e n o n w a s m o s t l y due to several serendipitous choices o f e x p e r i m e n t a l factors: the measurement geometry, w h i c h i n truth w a s not really useful for c h e m i c a l analysis, a t h i c k m e m b r a n e w h i c h m a x i m i z e d the loss p r o b a b i l i t y , and a pore diameter w h i c h w a s sufficiently large to a v o i d surface p l a s m o n losses near the centre.  156  Experimental Details P o r o u s a l u m i n a t h i n films w e r e prepared b y a n o d i z i n g 0.2 and 1.0 u m t h i c k A l  films  evaporated onto n-type s i l i c o n wafers b y e-beam e v a p o r a t i o n . T h e a n o d i z a t i o n w a s carried out 21  i n 0.3 M o x a l i c a c i d at 2 0 ° C w i t h an a p p l i e d potential o f 4 0 . 0 V , u s i n g a glass cell w i t h an O r i n g seal to the sample. U p o n c o m p l e t i o n o f the a n o d i z a t i o n , the porous a l u m i n a detached  from  the substrate, but w a s still attached to the s u r r o u n d i n g a l u m i n i u m  film  had  film.  The  barrier layer w a s r e m o v e d b y e t c h i n g i n 5 wt. % p h o s p h o r i c a c i d for 4 0 m i n . T h e film w a s r i n s e d w i t h d i s t i l l e d water, f o l l o w e d b y ethanol and then d r i e d . U s i n g the same samples (chapter 5 ) , electron energy-loss spectroscopy w a s carried out o n a Tecnai F 2 0 ( M e d i c a l Imaging Facility, University o f Calgary) T E M with a Schottky  field  e m i s s i o n electron g u n operated at n o m i n a l l y at 120 and 2 0 0 k V . T h e p r i m a r y b e a m energy used for spectroscopy w a s 117 and 197 k e V . M e a s u r e m e n t s w e r e m a d e w i t h a G a t a n I m a g i n g Filter ( G I F ) u s i n g a d i s p e r s i o n o f 0.05 e V / c h a n n e l on a 1024-channel detector, c o v e r i n g -5 to 45 e V losses. T h e c o l l e c t i o n semi-angle w a s between 0.34 and 6.0 m r a d ; the convergence angle w a s not determined. T h e probe size was estimated to be < 1 n m w h i l e the energy resolution was 0.9 e V ( F W H M o f zero-loss peak). M e a s u r e m e n t s o n ordered samples w e r e carried out o n T e c n a i F 2 0 ( N a n o - i m a g i n g F a c i l i t y , S i m o n Fraser U n i v e r s i t y ) e q u i p p e d w i t h a G I F . T h e d i s p e r s i o n w a s set to 0.10 e V / c h a n n e l . T h e F W H M o f the electron probe was m e a s u r e d to be 0.6 n m . T h e energy r e s o l u t i o n w a s 0.90 e V at 197 k e V and 0.87 e V at 117 k e V . O r d e r e d samples were prepared b y a n o d i z i n g 9 9 . 9 9 + % a l u m i n u m f o i l ( A l d r i c h ) for 2-3 hours at 4 0 V i n 0.3 M o x a l i c a c i d for the first step, w i t h stirring. T h e a n o d i z e d layer w a s stripped overnight by i m m e r s i n g i n 5.8 w t . % H 3 P O 4 + 1.5 wt. % Q O 3 at 50 to 6 0 ° C overnight. A  second a n o d i z a t i o n step w a s carried out for 10-20 m i n under the same c o n d i t i o n s . T h e 157  resulting f i l m w a s w a s h e d w i t h water and d r i e d , then g l u e d to a glass slide ( m e m b r a n e d o w n ) w i t h C r y s t a l B o n d , a t h e r m o p o l y m e r adhesive. T h e a l u m i n u m susbtrate w a s r e m o v e d b y e t c h i n g w i t h a saturated H g C ^ s o l u t i o n , l e a v i n g the a l u m i n a m e m b r a n e attached to the glass slide w i t h the barrier layer f a c i n g up. T h i s layer w a s etched for 120 m i n i n 5 % H 3 P O 4 w i t h stirring. A f t e r w a s h i n g w i t h water and d r y i n g , the adhesive w a s r e m o v e d b y s o a k i n g i n acetone, and m e m b r a n e inspected b y T E M . desired. F i l m thicknesses  the  T h e pore w a l l s c o u l d be further etched to increase the diameter as  were  determined  by S E M ; i n some cases there w a s  substantial  v a r i a t i o n over the w h o l e film, l e a d i n g to s o m e uncertainty i n the thickness.  References 1. J a c k s o n , J . D .  Classical Electrodynamics; W i l e y : N e w Y o r k ,  2. C h e r e n k o v , P . A .  Dokl. Akad. NaukSSSR 1934,  3. T a m m , I. E . ; F r a n k , I. M .  5. P a l i k , E . D .  2, 451-453.  Dokl. Akad. NaukSSSR 1937,14,  4. G a r c i a de A b a j o , F . J . ; B l a n c o , L . A .  1999.  107.  Phys. Rev. B 2003, 67, N o .  Handbook of Optical Constants of Solids;  125108.  P a l i k , E . D . , E d . ; A c a d e m i c Press:  O r l a n d o , 1985.  Springer Tracts Mod. Phys.  6. D a n i e l s , J . ; Festenberg, C . v . ; Raether, H . ; Z e p p e n f e l d , K .  1970,  5 4 , 78-135. 7. C h e n , C . H . ; S i l c o x , J . ; V i n c e n t , R .  Phys. Rev. B. 1975,12,  8. Y a m a m o t o , N . ; A r a y a , K . ; T o d a , A . ; S u g i y a m a , Ff. 9. A b e , H . ; K u r a t a , H . ; H o j o u , K . 10.  J. Phys. Soc. Jpn.  64-71.  Surf. Interface Anal.  2000, 69,  2001,  31,  79.  1553.  Williams, P.; Levy-Clement, C ; Albu-Yaron, A . ; Brun, N . ; Colliex, C .  J. Porous Mater.  2000, 7, 159. 11. T y a p k i n , A . A .  Physics of Particles and Nuclei  2001,  32,  506. 158  12. G i n z b u r g , V . L .  Physics-Uspekhi 2002, 45, 3 4 1 .  13. P l a t o n o v , K . Y . ; F l e i s h m a n , G . D . 14. E g e r t o n , R . F .  Physics-Uspekhi 2002, 45, 2 3 5 .  Electron energy-loss  spectroscopy  in the electron microscope;  2nd  ed.;  P l e n u m Press: N e w Y o r k , 1996. 15. Z a b a l a , N . ; R i v a c o b a , A . ; G a r c i a de A b a j o , F . J . ; Pattantyus, A .  Surf. Sci.  2003,  532-535,  461-467. 16. Z a b a l a , N . ; P a t t a n t y u s - A b r a h a m , A . G . ; G a r c i a de A b a j o , F . J . ; R i v a c o b a , A . ; W o l f , M . O .  Phys. Rev. B  , i n press.  17. Z a b a l a , N . ; R i v a c o b a , A . ; E c h e n i q u e , P . M . 18. R e e d , B . W . ; S a r i k a y a , M .  Surf. Sci.  1989,  209, 4 6 5 .  Ultramicroscopy 2002, 93, 2 5 .  19. G a r c i a de A b a j o , F . J . ; P a t t a n t y u s - A b r a h a m , A . G . ; Z a b a l a , N . ; R i v a c o b a , A . ; W o l f , M . O . ; Echenique, P. M .  Phys. Rev. Lett. 2003, 91, N o .  143902.  2 0 . L u o , C ; Ibanescu, M . ; J o h n s o n , S. G . ; J o a n n o p o l o u s , J . D . 21. Crouse, D . ; L o , Y . H . ; Miller, A . E . ; Crouse, M .  Science 2003, 299, 3 6 8 .  Appl. Phys. Lett.  2000,  76, 4 9 - 5 1 .  159  CHAPTER 7 Polymer Guest Incorporation  P o r o u s a l u m i n a films were s h o w n to be a v e r y close a p p r o x i m a t i o n to an ideal host material i n chapter 4: they have 4 n m pores r u n n i n g p e r p e n d i c u l a r to the f i l m n o r m a l , and the f i l m thickness can be easily controlled. T h e i n t r o d u c t i o n o f the conjugated p o l y m e r guest into the porous a l u m i n a host must be considered w i t h i n the constraints i m p o s e d b y the desired lighte m i t t i n g d e v i c e structure and the properties o f the p o l y m e r guest. T h e i n t r o d u c t i o n o f the p o l y m e r guest into the porous t h i n f i l m host c a n be a c c o m p l i s h e d first  through  in situ synthesis.  In this  direction, some  initial w o r k  o n the  surface-graft  p o l y m e r i z a t i o n o f p o l y ( l , 4 - p h e n y l e n e v i n y l e n e ) ( P P V ) on a s i l i c o n surface is presented i n this chapter. S e c o n d l y , b u l k - s y n t h e s i z e d p o l y m e r m a y be introduced into the host t h r o u g h a variety o f methods. A n o v e l approach, centrifugal l o a d i n g , is investigated and used to prepare samples for in-depth characterization, i n particular high-resolution c h e m i c a l analysis b y electron energyloss spectroscopy ( E E L S ) and energy-filtered t r a n s m i s s i o n electron m i c r o s c o p y ( E F T E M ) .  7.1  Internal Polymer Synthesis  In situ  synthesis o f the p o l y m e r guest is attractive due to the relatively r a p i d diffusion o f  m o n o m e r s into the host material. M a n y conjugated p o l y m e r s , s u c h as P P V , polythiophene and polyacetylene, can o n l y be introduced into the host i n this m a n n e r , due to their i n s o l u b l e and infusible nature. T h e groups o f M a r t i n and B e i n separately carried out p i o n e e r i n g w o r k on 160  conjugated  p o l y m e r synthesis i n p o r o u s a l u m i n a  1  and mesoporous  silica,  respectively. T h i s  2  approach w a s also exploited i n the creation o f the P P V / M C M - 4 1 c o m p o s i t e discussed i n chapter 3.  Recent  reports  diethoxyphenylene)  describe  the  formation  of  poly(l,4-phenylene)  3  and  poly(2,5-  b y o x i d a t i v e c o u p l i n g i n porous a l u m i n a m e m b r a n e s . C h e m i c a l v a p o u r  4  deposition o f a p o l y m e r precursor has been s h o w n to introduce P P V into porous a l u m i n a as well.  5  T h e t w o difficulties that face this approach is c o n t r o l over the degree o f p o l y m e r i z a t i o n and confinement o f the p o l y m e r i z a t i o n to the pore v o l u m e . T h e degree o f  in situ p o l y m e r i z a t i o n  m a y differ significantly from the b u l k - s y n t h e s i z e d p o l y m e r . T h i s m a y be especially relevant i n pores w h i c h approach m o l e c u l a r d i m e n s i o n s (< 2 n m ) . T h i s difference c a n m a k e  meaningful  property c o m p a r i s o n s m o r e difficult. Nevertheless, materials prepared i n this m a n n e r m a y have useful applications i n devices. P o l y m e r i z a t i o n confinement m a y be a c h i e v e d b y first l o a d i n g one o f the reactants into the porous host. T h i s is e x e m p l i f i e d i n the synthesis o f c o n f i n e d p o l y a n i l i n e , w h e r e aniline is first diffused into the p o r e s .  2  T h e synthesis o f P P V / M C M - 4 1 discussed i n  chapter 3 relied o n the a b i l i t y o f the M C M - 4 1 surface to act as an i n i t i a t i n g base w h e n m o n o m e r w a s subsequently introduced. H o w e v e r , i f the pre-loaded species can diffuse out o f the pores, bulk  polymerization  cannot  be  prevented  entirely.  p o l y m e r i z a t i o n f r o m a substrate s u p p o r t i n g the porous surface-graft  A  better film  approach  is to  initiate  the  o n one side, w h i c h is k n o w n as  p o l y m e r i z a t i o n . T h i s last approach appears v e r y p r o m i s i n g and s o m e p r e l i m i n a r y  w o r k has been carried out i n this d i r e c t i o n .  7.1.1  Surface-Graft Polymerization of Conjugated Polymers Surface-graft  p o l y m e r i z a t i o n is a route to p o l y m e r confinement  b y virtue o f p o l y m e r  g r o w t h b e i n g l o c a l i z e d to the end o f chains initiated f r o m the surface. T h e use o f a suitable 161  surface-bound initiator is k e y to this route. I f the initiator is b o u n d to a m a t e r i a l suitable for use as an electrode, it c a n be guaranteed that each p o l y m e r c h a i n is attached to the electrode interface i n a c h e m i c a l l y w e l l - d e f i n e d manner. T h e presence o f a p o r o u s host o n top o f this electrode w o u l d then y i e l d a d e v i c e w i t h surface-grafted p o l y m e r c h a i n s separated b y the w a l l s o f host. In this manner, o n l y c o n t i n u o u s p o l y m e r c h a i n s w o u l d exist i n each c h a n n e l o f the host. T h e p r o b l e m o f b u l k p o l y m e r i z a t i o n is e l i m i n a t e d entirely. L i v i n g p o l y m e r i z a t i o n is the m e t h o d o f c h o i c e for p r e p a r i n g surface-grafted p o l y m e r s . T h i s u s u a l l y i n v o l v e s an a n i o n i c o r r a d i c a l p o l y m e r i z a t i o n m e c h a n i s m . T h e r e are t w o established l i v i n g p o l y m e r i z a t i o n routes to conjugated p o l y m e r s : r i n g - o p e n i n g metathesis p o l y m e r i z a t i o n (ROMP) ' 6  7  and e l e c t r o p o l y m e r i z a t i o n . R O M P 8  m a y be used to prepare v e r y w e l l defined  p o l y m e r s , a n d it has been demonstrated to create surface-grafted initiator. ' 9  1 0  polymers from a  surface  H o w e v e r , the preparation o f the r e q u i r e d m o n o m e r , catalyst and surface initiator  w e r e b e y o n d the scope o f the present w o r k . T h e use o f e l e c t r o p o l y m e r i z a t i o n to prepare surfacegrafted conjugated p o l y m e r s has been demonstrated for p o l y a n i l i n e ' 1 1  1 2  and polythiophene.  13  In  practice, e l e c t r o p o l y m e r i z a t i o n has the disadvantage o f p r o d u c i n g chains w i t h m a n y c h e m i c a l defects a n d w a s not e x p l o r e d further for this reason. O n e further p o s s i b i l i t y is the G i l c h route: it has been reported that it proceeds b y an a n i o n i c m e c h a n i s m initiated grafted  from  1 4  a n d c o u l d therefore also be  a surface ( F i g u r e 7.1). P r e l i m i n a r y efforts i n this d i r e c t i o n d i d not y i e l d any  polymer, presumably  because  the  surface  initiation  group  was  not  i n sufficient  concentration to compete w i t h the. b u l k p o l y m e r i z a t i o n process. It is also possible that the surface initiator group reacts w i t h n e i g h b o u r i n g initiators to deactivate the surface.  162  Figure 7.1 Surface-initiated a n i o n i c p o l y m e r i z a t i o n o f M E H - P P V . Step  (or condensation)  polymerization was  e x p l o r e d as  an  alternative route to  the  preparation o f surface-grafted p o l y m e r s . C o n d e n s a t i o n reactions have been used to m a k e w e l l defined  conjugated  oligo(phenylene vinylene)s in high y i e l d .  8  P o l y ( p h e n y l e n e ethynylene)  derivatives o f substantial length have been prepared i n this m a n n e r t h r o u g h repeated c o u p l i n g o f oligomers.  1 5  T h i s a p p r o a c h w a s c o n c e i v e d to be a p p l i c a b l e to the preparation o f P P V c h a i n s b o u n d to a c o n d u c t i v e substrate. A n e x a m p l e o f a synthetic route is s h o w n i n F i g u r e 7.2, based o n the Wadsworth-Horner-Emmons  ( W H E ) synthesis  of  aromatic  olefins  from  aldehydes  and  phosphonate esters. T h i s reaction is k n o w n to p r o d u c e trans conjugated olefins i n h i g h y i e l d and has been a p p l i e d to the synthesis o f P P V d e r i v a t i v e s . " 1 6  solid s u p p o r t . " 2 0  1 9  It has also been demonstrated o n a  2 2  T h e hydrogen-terminated s i l i c o n surface is a convenient substrate for evaluating this route due to its w e l l k n o w n c h e m i s t r y . ' 2 3  2 4  Porous silicon substrates  25  are particularly useful as their  h i g h surface area a l l o w s the use o f t r a n s m i s s i o n F T - I R for characterization. T h e most relevant reactions o n this surface have been d e v e l o p e d b y the groups o f S a i l o r " , B u r i a k " 2 6  Wayner. " 3 3  3 5  2 8  2 9  3 2  and  A m o n g these, cathodic electrografting has been used to graft functionalized  a l k y n e s and alkenes to the s i l i c o n surface ( F i g u r e 7 . 3 ) .  3 0  O f particular interest w a s the grafting 163  PO(OEt)  2  (EtO) OP 2  PO(OEt)  2  j f-BuOK/THF  f-BuOK/THF  Figure 7.2 Step p o l y m e r i z a t i o n o f surface-grafted P P V b y the W H E reaction. o f phenylacetylene and 4 - b r o m o p h e n y l a c e t y l e n e t h r o u g h the acetylene c a r b o n , w h i c h suggested that a fully conjugated  b o n d to the s i l i c o n surface c o u l d be f o r m e d . M o s t importantly, the  presence o f the aromatic halogen a l l o w s the use o f w e l l k n o w n chemistry to further derivatize the surface: the b r o m i n a t e d p h e n y l r i n g c o u l d be used to f o r m grafted oligo(phenylene) chains t h r o u g h the S u z u k i c o u p l i n g . A s P P V derivatives were o f interest i n this w o r k , a grafted 8  aromatic aldehyde w a s pursued to create surface-bound o l i g o ( l , 4 - p h e n y l e n e v i n y l e n e ) chains t h r o u g h the W H E reaction discussed above. T h e actual encapsulation o f p o l y m e r b y s u c h surface c h e m i s t r y w o u l d be a c c o m p l i s h e d b y  R H-  •R H I. '. H -Si—Si—Si—Si-  H H H H -Si—Si—Si—Si-9 raA c m -2  Figure 7.3 P o r o u s s i l i c o n bromophenyl, etc.).  derivatization  by  cathodic  electrografting  (R  =  phenyl,  4-  30  164  the preparation o f p o r o u s a l u m i n a f i l m s o n a s i l i c o n s u b s t r a t e .  36  T h e k e y step i n the process  w o u l d be the successful preparation o f the hydrogen-terminated s i l i c o n surface, w h i c h requires r e m o v a l o f the native s i l i c o n d i o x i d e film. T h i s is usually a c c o m p l i s h e d w i t h aqueous h y d r o g e n fluoride or a m m o n i u m fluoride. Initial tests indicated that p o r o u s a l u m i n a w a s stable to the latter solution. R e a c t i v e i o n etching, c o m m o n l y used i n the s e m i c o n d u c t o r industry, c a n also be used to r e m o v e the native o x i d e layer i n the presence o f p o r o u s a l u m i n a  3 6  but this m a y not w o r k w e l l  w i t h a l l pore sizes.  7.1.2  Derivatization of Porous Silicon Surfaces T h e functionalization o f porous s i l i c o n substrates b y cathodic electrografting w a s studied.  4 - e t h y n y l b e n z a l d e h y d e has the t w o desired functional groups: an a l k y n e for b i n d i n g to s i l i c o n and an aldehyde for further c h e m i s t r y b y the W H E reaction. A s the grafting process is reductive at the s i l i c o n surface, the aldehyde group was protected b y c o n v e r s i o n to the c y c l i c acetal ( F i g u r e 7.4). A  porous  silicon  substrate w a s then  d e r i v a t i z e d b y cathodic electrografting o f this  protected aldehyde (Figure 7.5), u s i n g t e t r a b u t y l a m m o n i u m hexafluorophosphate i n methylene c h l o r i d e as the electrolyte. T h e c h e m i c a l changes w e r e f o l l o w e d by F T - I R spectroscopy. N e w C H stretches and aromatic bands appeared after electrografting ( F i g u r e 7.6(a)); the c y c l i c acetal  r\ ecu reflux  Figure 7.4 P r o t e c t i o n o f aldehyde i n 4-ethynylbenzaldehyde. 165  b a n d at 943 c m " c o u l d also be d i s t i n g u i s h e d . T h e a l k y n e stretch w a s expected to appear near 1  2 1 0 0 c m " but c o u l d not be c o n f i r m e d due to the o v e r l a p p i n g H - S i stretches. T h e strong S i - 0 1  f r a m e w o r k stretch at 1100 c m " indicated the presence o f substantial surface o x i d e . 1  F o l l o w i n g deprotection o f the aldehyde, a strong C = 0 stretch peak appeared at 1700 c m "  1  a l o n g w i t h a w e a k e r H - C = 0 peak at 2 7 3 3 c m " ( F i g u r e 7.6(b)). T h e aromatic bands w e r e also 1  enhanced, w h i l e the c y c l i c acetal b a n d disappeared. T h e aliphatic C - H stretches r e m a i n i n g at 2 9 6 9 and 2 9 3 0 c m " m a y be due to c h e m i - or p h y s i s o r b e d e l e c t r o l y t e . 1  the surface-grafted  30  A l t h o u g h the structure o f  species c o u l d not be fully established, it w a s clear that a surface-bound  aldehyde w a s present.  o.  H H H H Si—Si—Si—Si—  «-Bu NPF /CH2Cl 4  6  4 m A cm"  2  2  .0  H I. H H Si—Si-Si—Si—  O  II  (EtO) P. 2  dilute HCl(aq)  H H H H —Si—Si—Si—Si—  I  I H H ii—Si-Si-Si—  F i g u r e 7.5 Synthetic route to conjugated d i m e r o n p o r o u s s i l i c o n surface.  166  7.1.3  Wadsworth-Horner-Emmons Reaction on Silicon Surface The  terminal  aldehyde  on  the  silicon  surface  was  reacted  with  1,4-  x y l y l e n e b i s ( d i e t h y l p h o s p h o n a t e ) . T h e F T - I R spectrum o f the resulting surface ( F i g u r e 7.6(c)) s h o w e d that the C = 0 stretch at 1700 c m " w a s c o m p l e t e l y r e m o v e d and a n e w shoulder assigned 1  to a P = 0 stretch had appeared at 1260 c m " . T h e aliphatic C - H stretches around 2 9 6 9 and 2933 1  c m " w e r e i n agreement w i t h the C - H stretches observed i n the phosponate ester, although they 1  m a y still be due to the electrolyte. T h i s surface also e x h i b i t e d blue p h o t o l u m i n e s c e n c e , as s h o w n i n F i g u r e 7.7. T h e e m i s s i o n m a x i m u m w a s observed at 4 0 0 n m . T h e excitation spectrum, detected at 4 0 0 n m , had a m a x i m u m at 322 n m . T h i s is i n agreement w i t h the literature v a l u e o f 4-ethynylstilbene  37  and w o u l d suggest that the interaction o f the n electron system w i t h the  s i l i c o n substrate is w e a k . These initial results strongly suggest that the desired surface-grafted species w a s obtained. H o w e v e r , m o r e detailed analysis w i t h m o d e l c o m p o u n d s w o u l d be required to fully c o n f i r m this. W h i l e this c h e m i s t r y w a s carried out o n porous s i l i c o n substrates, it s h o u l d be readily a p p l i c a b l e to hydrogen-terminated w o u l d then  s i l i c o n wafers as w e l l . Short o l i g o m e r s grafted to s i l i c o n  represent a v e r y w e l l defined j u n c t i o n between  an o r g a n i c and an  inorganic  s e m i c o n d u c t o r . T h e rate o f p o l y m e r g r o w t h can be increased b y s y n t h e s i z i n g appropriate d i m e r s or trimers i n b u l k , then u s i n g t h e m to g r o w the surface-grafted p o l y m e r .  1 5  167  I  I  250  300  I  I  350  I  I  400  I  "  ~  l  450  —  I  1  500  1  1—  550  Wavelength (nm) F i g u r e 7.7 P h o t o l u m i n e s c e n c e o f d e r i v a t i z e d p o r o u s s i l i c o n : ( A ) excitation spectrum (detected at 4 0 0 n m ) and ( • )  e m i s s i o n s p e c t r u m (excited at 3 0 0 n m ) o f W H E reaction product;  (•)  e m i s s i o n o f deprotected aldehyde.  168  7.2  E x t e r n a l P o l y m e r Synthesis S o l u b l e conjugated  p o l y m e r s and precursor p o l y m e r s are readily prepared through b u l k  synthesis. S u c h p o l y m e r s m a y then be inserted into the pores o f the host material i n a n u m b e r o f w a y s . T h i s process a l l o w s the use o f k n o w n b u l k synthetic routes to c o n t r o l the properties o f the p o l y m e r , and direct characterization o f the p o l y m e r properties before encapsulation. P u r i f i c a t i o n c o u l d then r e m o v e possible undesirable by-products. T h i s is also the best approach for definite c o m p a r i s o n s between the encapsulated a n d b u l k p o l y m e r , as a m b i g u i t y relating to differences i n the c h e m i c a l structure is e l i m i n a t e d . T h e central challenge lies i n f i n d i n g a suitable d r i v i n g force for p o l y m e r insertion into the host material. O n e key e x a m p l e i n the literature i n the w o r k o f T o l b e r t et al, P P V p o l y m e r ( F i g u r e 7.8) w a s s h o w n to insert itself into a d e r i v a t i z e d material through  diffusion  and  adsorption.  difference ( v a c u u m or filtration l o a d i n g ) . ' 3 9  38  4 0  A  second  example  is the  w h e r e the M E H mesoporous use  silica  o f a pressure  A t h i r d n o v e l e x a m p l e , to date u n e x p l o r e d i n the  literature, is the use o f centrifugal forces to d r i v e the p o l y m e r into the host m e m b r a n e .  These  latter t w o approaches are m o r e useful w h e n the host pore size is larger than a few n m .  O  \ F i g u r e 7.8 C h e m i c a l structure o f p o l y [ 2 - m e t h o x y , 5 - ( 2 ' - e t h y l h e x y l o x y ) - l , 4 - p h e n y l e n e v i n y l e n e ) (MEH-PPV). 169  7.2.1  Polymer Adsorption Loading D i r e c t adsorption is a v e r y s i m p l e m e t h o d for i n c o r p o r a t i n g p o l y m e r into m e s o p o r o u s  hosts. T h e M E H - P P V / p o r o u s a l u m i n a c o m p o s i t e studied i n chapter 5 w a s prepared by this approach. T h e process is d r i v e n b y the favourable interactions between the p o l y m e r and the pore surface. In the case o f a l u m i n a , both the electron-rich p h e n y l rings and the pendant o x y g e n s c a n interact w i t h surface h y d r o x y g r o u p s . ' 4 1  4 2  ether  H o w e v e r , it is not clear i f s u c h an  a p p r o a c h c a n be used to introduce m o r e than a m o n o l a y e r o f p o l y m e r . It is m o s t l i k e l y that this process c a n only achieve h i g h l o a d i n g o f the host w h e n the ratio o f p o r e surface area to pore v o l u m e is h i g h , i.e., w h e n the pore diameter is s m a l l . T h e pore density must be h i g h as w e l l . These c o n d i t i o n s are met b y m e s o p o r o u s s i l i c a materials; o n the other h a n d , porous a l u m i n a can be m a d e w i t h s m a l l pore diameters o n l y w i t h a l o w pore density, due to the fixed pore s p a c i n g at the a n o d i z a t i o n potential for ordered pore g r o w t h (see chapter 4). Since  high  polymer  loading  was  desired  for  evaluation  of  the  high  resolution  characterization techniques, p o l y m e r l o a d i n g by adsorption w a s not investigated further.  7.2.2  Vacuum (Filtration) Loading T h i s a p p r o a c h is very useful for i n t r o d u c i n g m a t e r i a l into p o r o u s m e m b r a n e s , as has been  reported recently for g o l d n a n o p a r t i c l e s . ' 3 9  4 0  T h e p o l y m e r solution is d r a w n t h r o u g h the p o r o u s  m e m b r a n e , and r a p i d solvent evaporation deposits p o l y m e r into the pores. D u e to the large pressure difference exerted o n the m e m b r a n e , it must either be v e r y t h i c k or supported i n s o m e form.  It w a s found that p o l y m e r c o u l d be readily i n t r o d u c e d into t h i c k p o r o u s a l u m i n a  m e m b r a n e s (60 urn t h i c k A n o p o r e c o m m e r c i a l m e m b r a n e s w i t h 2 0 0 n m pore size) b y s i m p l y filtering a p o l y m e r s o l u t i o n ( F i g u r e 7.9(a)). T h e substantial p o l y m e r l o a d i n g afforded b y this 170  F i g u r e 7.9 (a) V a c u u m - d r i v e n p o l y m e r infiltration into a p o r o u s m e m b r a n e , (b) S E M i m a g e o f p o l y m e r i n 2 0 0 n m pores o f A n o p o r e m e m b r a n e . T h e scale bar is 5 0 0 n m .  approach a l l o w e d the p o l y m e r to be seen directly i n the pores o f the m e m b r a n e b y S E M ( F i g u r e 7.9(b)). T h e s e t h i c k e r m e m b r a n e s c o u l d also be used as a support for v a c u u m l o a d i n g p o l y m e r into thinner m e m b r a n e s . Nevertheless, this a p p r o a c h w a s not p u r s u e d further o w i n g to its requirement o f a free-standing m e m b r a n e and the relative fragility o f such m e m b r a n e s .  7.2.3  Centrifugal Loading T h e centrifugal force m a y be used to d r i v e p o l y m e r into the pores o f the a l u m i n a host.  C e n t r i f u g a t i o n is generally used as a separation technique for s m a l l particles i n s o l u t i o n , but can also be used to create density gradients i n p o l y m e r s o l u t i o n s .  43  T w o l o a d i n g processes m a y be  p o s s i b l e , d e p e n d i n g on the speed o f the centrifuge. D i r e c t sedimentation o f the p o l y m e r m a y be a c h i e v e d at v e r y h i g h speeds ( 1 5 0 0 0 0 R P M ) , w h e r e the forces are o n the order o f 9 x l 0  6  N.  4 3  S u c h centrifuges w e r e available for use but c o u l d not be readily m o d i f i e d to incorporate a vertically oriented substrate. P r e l i m i n a r y experiments w e r e c a r r i e d out o n a t y p i c a l c h e m i s t r y 171  Substrate h o l d e r  P o l y m e r solution  S o l v e n t evaporation  Porous alumina C e n t r i f u g a l force  F i g u r e 7.10 C e n t r i f u g a l p o l y m e r l o a d i n g into a porous a l u m i n a f d m . laboratory centrifuge, i n w h i c h the speeds are o n the order o f a few thousand R P M . It w a s realized that this speed m a y be sufficient i f the centrifugal force is a p p l i e d w h i l e a l l o w i n g solvent evaporation, as depicted i n F i g u r e 7.10. T h i s second approach w a s investigated initially, u s i n g M E H - P P V d i s s o l v e d i n tetrahydrofuran ( T H F ) , and w a s f o u n d to be effective.  7.3  Preparation of Centrifuged Samples T h e e x p e r i m e n t a l setup for p o l y m e r l o a d i n g is s h o w n i n F i g u r e 7 . 1 1 : a s i m p l e laboratory  centrifuge has been m o d i f i e d w i t h a holder for flat substrates. T h e holder consisted o f a threaded stainless steel barrel w h i c h c o u l d be sealed to the v e r t i c a l l y oriented substrate. T h e seal between the barrel o f the holder a n d the substrate w a s chosen to be a V i t o n O - r i n g , as rubber O - r i n g s are sensitive to m a n y c o m m o n solvents and T e f l o n O - r i n g s were f o u n d not to p r o v i d e r e p r o d u c i b l e seals. T h e solvent evaporation rate c o u l d be c o n t r o l l e d to s o m e extent b y c a p p i n g the end o f the barrel w i t h a rubber septum. A l t h o u g h porous a l u m i n a m e m b r a n e s w i t h 4 n m pore diameter were available for study, membranes  w i t h larger pore sizes ( t y p i c a l l y - 2 0 n m ) were used. These were m o r e easily  fabricated i n large n u m b e r s for different studies, and the larger a m o u n t o f p o l y m e r w h i c h c o u l d 172  F i g u r e 7.11 (a) Centrifuge rotor assembly w i t h t w o substrate holders, (b) detail o f substrate holder from above and (c) from inside, s h o w i n g O - r i n g seal. be i n t r o d u c e d into these pores w a s  important  for the development  o f the  characterization  techniques for c o m p o s i t e samples i n general. T h e pore v o l u m e i n a porous a l u m i n a film can be calculated from the exposed substrate area (0.28 c m ) and the a p p r o x i m a t e porosity ( 3 0 % ) . F o r a 1 u m - t h i c k film, this amounts to - 8 2  n L . C o m p l e t e pore f i l l i n g w o u l d then require a p o l y m e r m a s s in the tens o f u g . F o r a p o l y m e r s o l u t i o n o f 0.038 w t % M E H - P P V i n T H F , the c o r r e s p o n d i n g v o l u m e is o n the order o f tens o f uL. Consistent results were obtained by u s i n g three consecutive centrifuging steps: (1) M E H - P P V solution, w i t h o u t c a p p i n g the barrel, - 2 m i n (2) a d d i t i o n a l solvent o n l y , w i t h a capped barrel, - 1 5 m i n (3) barrel cap r e m o v e d , - 5 m i n T h e first step deposits a s m a l l amount o f p o l y m e r onto the substrate, and the second step serves to d r i v e it further into the pores and distribute it u n i f o r m l y . T h e last step, w i t h the cap r e m o v e d , ensures that the solvent is fully evaporated.  173  7.4  Characterization of Centrifuged Samples T h e M E H - P P V d i s t r i b u t i o n i n the centrifuged samples w a s investigated u s i n g a n u m b e r o f  different techniques. V i s u a l l y , the samples s h o w e d a t h i c k e r r i n g o f p o l y m e r deposited at the edge o f the e x p o s e d area; i n s o m e cases, s m a l l p o l y m e r particles w e r e v i s i b l e o n the surface. T h e p o l y m e r d i s t r i b u t i o n i n the centre o f the sample w a s usually even.  7.4.1  Scanning Electron Microscopy W i t h larger pore samples, the S E M m a y be used to determine the p o l y m e r d i s t r i b u t i o n .  T h e backscattered electron detector w a s not useful at the m a g n i f i c a t i o n s o f interest (> 50 OOOx) and  the scattering difference between the carbon-based p o l y m e r a n d the a l u m i n a host is not  sufficient for useful contrast. T h e secondary-electron detector, w h i c h p r o v i d e s t o p o g r a p h i c a l i n f o r m a t i o n at h i g h resolution, w a s used for this study. W h e n cross-sections o f the host material are o b s e r v e d b y S E M , a partially filled pore m a y not stand out evidently. A c o m p l e t e l y filled pore w o u l d e l i m i n a t e most o f the v i s i b l e structure i n the porous f i l m . In m a n y cases, a superficial p o l y m e r layer c o u l d be readily d i s t i n g u i s h e d ( F i g u r e 7.12). Part o f this p o l y m e r layer c o u l d also be seen penetrating into the pores, w h i c h indicated s o m e pore f i l l i n g . In the case o f substantial pore f i l l i n g , the p o l y m e r acts as a r e p l i c a o f the pore space. T h i s can be v i s u a l i z e d b y e t c h i n g a w a y the a l u m i n u m substrate a n d the a l u m i n a host, as s h o w n i n F i g u r e 7 . 1 3 . T h e result o f this process is s h o w n i n F i g u r e 7.14: elongated p o l y m e r tubules are v i s i b l e , i n d i c a t i n g substantial penetration o f the p o l y m e r into the porous a l u m i n a host. W h e n t h i c k e r p o l y m e r - l o a d e d samples w e r e investigated i n this fashion, the p o l y m e r tubules w e r e o b s e r v e d to have collapsed into bundles ( F i g u r e 7.14(b)).  174  Figure  7.12  S E M i m a g e o f cross-sections  o f centrifuged  samples,  s h o w i n g (a) p o l y m e r  overlayer and (b) some p o l y m e r penetration into pores. T h e scale bars are 2 0 0 a n d 100 n m .  X P S o n this surface  \ Embed  HgCl2(sat)  Conductive epoxy  S E M o n this surface  \  /  5% H P0 (aq) 3  4  F i g u r e 7.13 A n a l y s i s o f p o l y m e r penetration into porous a l u m i n a by X P S and S E M . 175  F i g u r e 7.14 S E M images o f centrifuged samples, observed from bottom w i t h the host etched a w a y . T h e scale bars are (a) 2 0 0 n m and (b) 1.0 u m ( 2 0 0 n m i n the inset).  176  7.4.2  X - r a y Photoelectron Spectroscopy F o r the purpose o f d e v i c e preparation, it is necessary to create c o m p o s i t e films i n w h i c h  the p o l y m e r penetrates the depth o f the m e m b r a n e . O t h e r w i s e , electrical contact to the p o l y m e r is not possible. T h i s cannot be d e t e r m i n e d reliably from the S E M images, but m a y be obtained b y X - r a y photoelectron spectroscopy ( X P S ) . T h i s is a v e r y surface-sensitive technique, because o f the l i m i t e d m e a n free path o f the generated photoelectrons. T h e s a m p l i n g depth is effectively o n the order o f 1 n m . B y a n a l y s i n g the bottom interface o f the c o m p o s i t e , it m a y be used to establish w h e t h e r p o l y m e r has penetrated to the bottom o f the pores ( F i g u r e 7.13). T h e presence o f a c a r b o n signal o n this surface, after a r g o n - i o n sputtering to r e m o v e any adsorbed atmospheric hydrocarbons, w o u l d indicate c o m p l e t e p o l y m e r penetration to the bottoms o f pores. T h e argon sputtering also served to t h i n the barrier layer at the bottom o f the pores. T h e result o f this e x p e r i m e n t is s h o w n i n F i g u r e 7.15. T h e a l u m i n u m 2 p (74.7 e V ) a n d oxygen  Is (532 e V ) signals are present i n both the e m p t y and centrifuged samples. T h e i r  Oxygen 1 s  A l u m i n u m 2p  " i — • — i — — i — 1 I—i 1  I  I  540  •  I  535  •  I  •  530  Binding Energy (eV)  I  525  I I  I  80  '  1  I  I  75  Carbon 1 s  1—I n — • — i — • — i  1  ,  I  70  Binding Energy (eV)  I  l_l  290  1  1 285  .  r  1  1  .  280  1—1 275  Binding Energy (eV)  F i g u r e 7.15 X P S results from b o t t o m o f ( o ) e m p t y porous a l u m i n a and ( • ) centrifuged sample. 177  positions are very close to the expected energies for a l u m i n u m o x i d e (74.3 and 531.1 e V , respectively) ' 4 4  4 5  H o w e v e r , the c a r b o n Is signal is o n l y present i n the centrifuged sample. T h e  m a i n c a r b o n peak, at 2 8 0 . 4 e V , w a s substantially b e l o w the n o r m a l range (from 2 8 8 e V for o x i d e s to 281 e V for carbides) a n d this w a s assigned to sputtering d a m a g e .  4 6  Quantification o f  the a m o u n t o f p o l y m e r present at the bottom o f the pore w a s not attempted. T h e results clearly indicate that this l o a d i n g a p p r o a c h is effective at p r o d u c i n g fully penetrated c o m p o s i t e materials.  7.4.3  Transmission Electron Microscopy T h i n sections for T E M investigation w e r e prepared b y u l t r a m i c r o t o m y o f e p o x y - e m b e d d e d  samples. P r i o r to e m b e d d i n g , a t h i n layer o f A u / P d w a s deposited b y D C sputtering o n top o f the samples. F o r the e m p t y host, this prevented e p o x y infusion into the pores. F o r the centrifuged samples, this layer prevented d i s s o l u t i o n o f M E H - P P V into the e p o x y . T h e thickness o f the A u / P d layer w a s between 2 0 and 5 0 n m . Sections were cut to thicknesses between 2 0 a n d 80 n m . A s the pore s p a c i n g is 4 0 n m for p o r o u s a l u m i n a prepared at 15 V , section thicknesses o n that same order w i l l c o n t a i n not m o r e than one pore. Substantially thinner sections w e r e m o r e difficult to prepare w i t h o u t i n d u c i n g substantial deformation to the p o r o u s a l u m i n a film. Nevertheless, g o o d quality sections as t h i n as 2 0 n m were obtained for s o m e samples. B e a m damage to the p o r o u s a l u m i n a host was evident i n the appearance o f c i r c u l a r defects i n the a l u m i n a . T h e stability c o u l d be i m p r o v e d substantially b y the use o f a diffuse electron b e a m . U n d e r s u c h c o n d i t i o n s , the s e n s i t i v i t y o f a c h a r g e - c o u p l e d d e v i c e ( C C D ) detector w a s necessary to capture images w i t h i n a reasonable a m o u n t o f t i m e (< 10 s) to a v o i d any sample drift.  178  A  B  Polymer  Porous alumina  Aluminum  F i g u r e 7.16 T E M images o f t h i n sections o f (a) empty porous a l u m i n a host and (b) centrifuged sample (beam damage to the host is apparent). T h e scale bars are 100 n m , and the accelerating voltage was 80 k V . T h e cross-section o f the empty porous a l u m i n a host is s h o w n i n F i g u r e 7.16(a). T h e A u / P d layer is clearly v i s i b l e . T E M images o f a centrifuged samples are s h o w n i n F i g u r e 7.16(b). In this case a p o l y m e r overlayer w a s v i s i b l e , and the A u / P d layer was attached to the e m b e d d i n g e p o x y , w h i c h usually separated a w a y d u r i n g the sectioning process. In order to establish the p o l y m e r d i s t r i b u t i o n , pores that c o u l d be d i s t i n g u i s h e d clearly i n the sections were e x a m i n e d . In the empty film, a t h i n a l u m i n a film was associated w i t h m a n y pores, c o r r e s p o n d i n g s i m p l y to a section w h i c h cut through part o f the pore w a l l . A s s u c h , i n the centrifuged sample, the presence o f p o l y m e r c o u l d not be d i s t i n g u i s h e d from a pore w i t h a partially sectioned w a l l . A l t h o u g h i n m a n y cases the p o l y m e r overlayer seemed to be connected continuously  w i t h the  material  filling  the  pore, the  MEH-PPV  d i s t r i b u t i o n c o u l d not  established u n a m b i g u o u s l y b y s i m p l e e x a m i n a t i o n o f the T E M images. T h e samples  be  were  investigated further by E E L S and E F T E M .  179  7.4.4  Energy-filtered Transmission Electron Microscopy T h e approach demonstrated  i n chapter 3 o n P P V / M C M - 4 1 samples w a s a p p l i e d to the  analysis o f the centrifuged M E H - P P V / p o r o u s a l u m i n a c o m p o s i t e . It was again anticipated that the aromatic n-n  p l a s m o n i n the l o w - l o s s spectrum w o u l d reveal the p o l y m e r distribution i n the  c o m p o s i t e material. D u e to t i m e constraints, o n l y one sample w a s investigated at an accelerating voltage o f 2 0 0 k V . A 2 u m - t h i c k porous a l u m i n a film was used to prepare a centrifuged sample. B o t h the empty host and the centrifuged sample were u l t r a m i c r o t o m e d to p r o d u c e sections ~ 5 0 n m t h i c k . T h e T E M image o f an empty host section is s h o w n i n F i g u r e 7.17(a); l o w - l o s s spectra a c q u i r e d o n the host and o n the a l u m i n u m substrate are s h o w n i n F i g u r e 7.17(b). T h e a l u m i n a b u l k p l a s m o n loss had a m a x i m u m at 23 e V , i n agreement w i t h a p r e v i o u s l y reported value for amorphous a l u m i n a .  4 7  T h e a l u m i n u m substrate s h o w e d a surface p l a s m o n loss at 7 e V and b u l k  Energy (eV) F i g u r e 7.17 (a) T E M image (scale bar is 2 u m ) and (b) E E L S o f t h i n section o f e m p t y porous a l u m i n a host: ( • )  zero-loss peak, porous a l u m i n a ( • )  before  and ( o ) after zero-loss  peak  subtraction, ( A ) a l u m i n u m . 180  p l a s m o n losses at m u l t i p l e s o f 15 e V , i n agreement w i t h the k n o w n a l u m i n u m p l a s m o n s .  4 8  T h e m i n i m u m i n the a l u m i n a spectrum o c c u r r e d near 5 e V ; an attempt w a s therefore m a d e to filter o n this energy w i t h a 2 e V slit. T h e energy-filtered images o f the e m p t y and centrifuged sample are c o m p a r e d i n F i g u r e 7.18: both samples s h o w substantial losses at 5 e V . T h e presence o f s m a l l tubules o f - 2 0 n m diameter i n defect areas s h o w e d that some p o l y m e r w a s present, but the a l u m i n a losses at 5 e V appear to m a s k the p o l y m e r d i s t r i b u t i o n i n the host. O t h e r areas o n the section were s i m i l a r . S u b t r a c t i o n o f the 25 e V i m a g e (due m o s t l y to the a l u m i n a b u l k p l a s m o n ) from the 5 e V i m a g e d i d not p r o v i d e any i m p r o v e m e n t . T h e r e are t w o possible causes. First, it is possible that the s e c t i o n i n g process has altered the centrifuged sample: the p o l y m e r m a y have been d i s p l a c e d from the pores i n the observed sections. T h u s the empty and centrifuged samples appear identical. T h e second possible cause m a y be related to C h e r e n k o v effect i n the loss spectrum, as d i s c u s s e d i n chapter 6. It appears that there are sufficient losses i n a l u m i n a b e l o w 10 e V s u c h that the presence o f p o l y m e r is m a s k e d by thickness variations in the a l u m i n a host. A p r e v i o u s literature report has s h o w n b y E E L S measurements at 100 k V that losses i n a m o r p h o u s a l u m i n a t h i n films are n e g l i g i b l e b e l o w 8 e V , i n agreement w i t h o p t i c a l measurements o f the dielectric function.  4 7  T h e results obtained here differ from this m a r k e d l y since measurable losses c a n be  observed even b e l o w 4 e V ( F i g u r e 7.17(b)). F o r a 100 k V electron t r a v e l l i n g through a l u m i n a , the real part o f the d i e l e c t r i c function (£\(co))  must be above 3.3 (Table 6.1). F o r a l u m i n a , the value o f S\(co) is b e l o w this threshold up  to 5 e V ( F i g u r e 6.2). T h i s suggests that the use o f an accelerating voltage o f 100 k V or l o w e r is m o r e suitable for the investigation o f a l u m i n a - b a s e d c o m p o s i t e materials by l o w - l o s s E E L S and E F T E M , especially for s m a l l amounts o f conjugated material.  181  F i g u r e 7.18 U n f i l t e r e d ( T E M ) and energy-filtered (5, 25 e V ) images o f empty porous a l u m i n a and centrifuged M E H - P P V / p o r o u s a l u m i n a composite. T h e scale bars are 100 a n d 2 0 0 n m , respectively. 182  H o w e v e r , it cannot be e x c l u d e d that s o m e o f the features i n the l o w - l o s s s p e c t r u m o f p o r o u s a l u m i n a are due to a n i o n - c o n t a m i n a t i o n o f the pore w a l l s . Further study at different accelerating voltages w o u l d establish w h i c h effect is p r e d o m i n a n t . F o r silica-based c o m p o s i t e materials (e.g. P P V / M C M - 4 1 ) , there is m o r e flexibility i n the c h o i c e o f accelerating voltage. T h e dielectric f u n c t i o n o f an a l l - s i l i c a zeolite, silicalite, has been reported.  49  These results s h o w that £\(co) is b e l o w 2 up to 5 e V for silicalite and has a m a x i m u m  v a l u e o f 3. T h i s suggests the losses due to the C h e r e n k o v effect w i l l be less p r o m i n e n t for s i l i c a based materials even at 2 0 0 k V , i n agreement w i t h the o b s e r v e d loss s p e c t r u m o f e m p t y M C M 41 ( F i g u r e 3.11). These issues associated w i t h the l o w - l o s s spectrum m i g h t have been a v o i d e d altogether b y filtering o n c a r b o n i o n i z a t i o n edge at 2 8 4 e V instead. T h i s w a s not attempted because o f t i m e constraints. T h i s signal is also expected to be w e a k e r than the p l a s m o n loss, w h i c h w o u l d entail longer collection times and the associated b e a m damage to the s p e c i m e n .  7.4.5  STEM/EELS A t t e m p t s to measure the loss spectrum d i s t r i b u t i o n on t h i n sections i n S T E M  mode  resulted i n substantial damage to the a l u m i n a host. T h e i m a g e i n F i g u r e 7.19(a) s h o w s the d a m a g e associated w i t h earlier s p e c t r u m a c q u i s i t i o n s . T h e loss spectra recorded i n this w a y s h o w e d a n e w peak near 9 e V (Figure 7.19(b)). S u c h a feature has been o b s e r v e d p r e v i o u s l y i n a study o n electron-beam hole d r i l l i n g i n a l u m i n a .  5 0  It w a s assigned to m o l e c u l a r o x y g e n trapped  i n the material, as d r i l l i n g proceeds b y preferential r e m o v a l o f a l u m i n u m atoms. T h e loss spectrum a c q u i r e d d i r e c t l y o n a tubule s h o w e d substantial losses b e l o w 10 e V , suggesting that it w a s c o m p o s e d o f conjugated p o l y m e r .  183  Energy (eV) F i g u r e 7.19 (a) S T E M i m a g e o f t h i n section o f centrifuged sample. T h e contrast has been increased i n the inset to s h o w the p o l y m e r tubules, (b) E E L S associated w i t h ( • ) d r i l l i n g i n a l u m i n a and ( c ) a p o l y m e r tubule. T h e zero-loss peak has been subtracted. Nevertheless, it m a y still be possible to determine the p o l y m e r distribution i n S T E M m o d e . T h i s has certainly been demonstrated i n the core-loss r e g i m e  5 1  and m a y also be possible i n the  l o w - l o s s r e g i m e w i t h careful analysis o f the spectra. S u c h an approach w o u l d require a c r y o g e n i c s a m p l e h o l d e r to m i n i m i z e b e a m damage to the a l u m i n a host, a n d this was not available for this study.  7.5  Conclusion T w o general approaches to p o l y m e r i n t r o d u c t i o n into a porous host were described. T h e  surface-grafted synthesis o f conjugated o l i g o m e r s appears to be a p r o m i s i n g route and s h o u l d be e x p l o r e d i n m o r e detail. T h e synthetic a p p r o a c h described here s h o u l d be readily applicable to the preparation o f larger conjugated m o l e c u l e s o n the s i l i c o n surface. H o w e v e r , its usefulness can o n l y be c o n f i r m e d once the structure o f the surface-grafted species is established i n m o r e 184  detail, p o s s i b l y t h r o u g h s c a n n i n g t u n n e l l i n g m i c r o s c o p y . These structures w o u l d represent a j u n c t i o n between a m o l e c u l a r s e m i c o n d u c t o r a n d a b u l k s e m i c o n d u c t o r ; the electrical b e h a v i o u r o f s u c h a j u n c t i o n is o f fundamental interest. T h e use o f centrifugal force i n c o n j u n c t i o n w i t h solvent evaporation e x p l o r e d as a m e t h o d to introduce a soluble conjugated  has also been  p o l y m e r i n porous a l u m i n a  films.  C h a r a c t e r i z a t i o n o f the resulting c o m p o s i t e clearly s h o w e d p o l y m e r penetration. H o w e v e r , the p o l y m e r c o u l d not be located w i t h h i g h resolution b y E E L S interference  f r o m C h e r e n k o v losses  i n the  a l u m i n a host.  and E F T E M ,  p o s s i b l y due to  Further experiments  at  different  accelerating voltages w o u l d also be r e q u i r e d i n this d i r e c t i o n .  Experimental Details 1. Surface Chemistry Surface d e r i v a t i z a t i o n o f porous s i l i c o n substrates f o l l o w e d a literature p r o c e d u r e .  30  As  s i l i c o n is transparent to I R , F T - I R spectra w e r e a c q u i r e d i n t r a n s m i s s i o n . C h e m i c a l s w e r e purchased  from  S i g m a - A l d r i c h , Inc. and used as-received. C a r b o n tetrachloride and ethylene  g l y c o l were d r i e d over m o l e c u l a r sieves. M e t h y l e n e c h l o r i d e w a s d r i e d b y p a s s i n g through activated a l u m i n a . N M R spectra were a c q u i r e d o n a B r u k e r A C - 2 0 0 at 2 0 0 M H z . F T - I R spectra were recorded o n a B O M E M M B 155S spectrometer. P h o t o l u m i n e s c e n c e spectra w e r e a c q u i r e d on a Cary  spectrophotometer.  2-(4-ethynylphenyl) 1,3-dioxolane:  Starting from 4-[(trimethylsilyl)ethynyl]benzaldehyde,  the e t h y n y l g r o u p w a s first deprotected f o l l o w i n g a literature p r o c e d u r e .  52  T h e aldehyde w a s  then protected b y r e f l u x i n g w i t h excess ethylene g l y c o l i n c a r b o n tetrachloride over a c i d i c a l u m i n a for 2 4 h .  5 3  T L C i n 6:1 hexanes/ethyl acetate indicated a single product. T h e .mixture  w a s w a s h e d t w i c e w i t h water and d r i e d over m a g n e s i u m sulfate. S o l v e n t r e m o v a l under reduced 185  pressure y i e l d e d a light orange s o l i d . A f t e r p u r i f i c a t i o n b y s u b l i m a t i o n , a w h i t e p r o d u c t w a s obtained. F T - I R ( K B r ) : 3 2 7 0 , 3241 ( H - C = C ) , 2 8 9 0 ( H C O O ) , 2 1 0 3 ( O C ) , 1424, 1384, 1222, 1074, 941 ( 1 , 3 - d i o x o l a n e ) , 836 c m " . ' H N M R (acetone-d ): 57.4 (q, 4 H , aromatic), 5.6 (s, 1 H , b e n z y l i c ) , 1  6  3.9 ( m , 4 H , O C H C H 0 ) , 3.5 (s, 1 H , O C H ) . M a s s spectroscopy indicated a parent M / z o f 173 2  2  (M - - H ) . +  l,4-xylylenebis(diethylphosphonate):  a,a'-dichloro-/?-xylene  was  reacted  with  equivalents o f triethylphosphite at 140 ° C for 5 h . E x c e s s triethylphosphite w a s r e m o v e d under r e d u c e d pressure. T h e r e m a i n i n g w h i t e p o w d e r w a s p u r i f i e d b y recrystallization from methylene chloride/hexanes at -20 ° C to f o r m w h i t e needle-like crystals. H N M R (acetone-d ): 8 7 . 2 (s, 4 H , ]  6  aromatic), 4.0 (p, 8 H , M e C H O P ) , 3.1 (d, 4 H , C H P ) , 1.2 (t, 12 H , C H ) ; 2  2  3 I  3  P NMR: 526.2. F T -  I R ( K B r ) : 2 9 6 3 , 2 9 0 7 ( C H ) , 1514, 1480, 1439, 1391 (aromatic), 1261 c m " ( P = 0 ) . 1  Porous silicon:  substrates w e r e prepared from n-type s i l i c o n wafers as reported i n the  literature and stored under nitrogen. F T - I R : 1100 cm" ( S i - 0 f r a m e w o r k ) , 2 2 4 0 and 2 0 9 0 cm" 1  1  ( S i - H ) . These were present through a l l the surface m o d i f i c a t i o n s b e l o w .  Cathodic electrografting on porous silicon:  T h e electrografting c e l l consisted o f a porous  s i l i c o n substrate c l a m p e d d o w n to an a l u m i n u m support w i t h a #9 glass j o i n t , and c a p p e d w i t h a rubber septum. A V i t o n O - r i n g w a s used to f o r m the seal to the substrate. A p l a t i n u m m e s h counter-electrode w a s inserted t h r o u g h the septum. A syringe needle w a s also inserted through the septum to a l l o w evacuation o f the c e l l . T h e electrografting w a s carried out under nitrogen atmosphere. T h e electrolyte, c o n s i s t i n g o f 0.1 M t e t r a b u t y l a m m o n i u m hexafluorophosphate  and  0.2 M protected aldehyde i n methylene c h l o r i d e , w a s prepared separately under nitrogen and i n t r o d u c e d into the c e l l through the septum. A current density o f ~4 m A c m " w a s passed through the c e l l for 3 m i n . T h e substrate w a s then w a s h e d w i t h methylene c h l o r i d e , acetone, water and 186  ethanol, then d r i e d under a stream o f nitrogen. F T - I R (not i n c l u d i n g s i l i c o n surface bands): 2 9 6 8 , 2 9 3 0 , 2 8 8 4 ( C - H ) , 1606, 1388, 1510 (aromatic), 943 c m " ( 1 , 3 - d i o x o l a n e ) . 1  Aldehyde deprotection on surface: T h e aldehyde w a s deprotected b y s o a k i n g i n dilute H C 1 for 30 m i n . T h e substrate w a s w a s h e d w i t h water and d r i e d . F T - I R (not i n c l u d i n g s i l i c o n surface bands): 2 9 6 9 , 2 9 3 3 , 2 8 7 4 ( C - H ) , 2 7 3 2 ( H - C O ) , 1700 ( C = 0 ) , 1603, 1510 (aromatic), 1307 cm"  1  (aromatic).  WHE reaction on surface: T h e substrate, 55 m g b i s ( d i e t h y l phosphonate ester) (0.15 m m o l ) and a stir bar w e r e added to a r o u n d bottom flask and p l a c e d under nitrogen. T h i s w a s f o l l o w e d by the a d d i t i o n o f 2.0 m L T H F and 0.13 m L o f 1.0 M p o t a s s i u m  tert-butoxide/THF  (0.13 m m o l ) . A y e l l o w c o l o u r first d e v e l o p e d then changed to deep orange. T h e m i x t u r e w a s refluxed for 5 h . T h e substrate w a s then w a s h e d w i t h ethanol and acetone, f o l l o w e d by d r y i n g under streaming nitrogen. F T - I R (not i n c l u d i n g s i l i c o n surface bands): 2 9 6 9 , 2 9 3 0 , 2 8 6 9 ( C - H ) , 1510 (aromatic), 1260 cm" ( P = 0 ) . P h o t o l u m i n e s c e n c e : E m i s s i o n m a x i m u m at 4 0 0 n m and a 1  shoulder at 3 7 0 n m w i t h excitation at 3 0 0 n m ; excitation m a x i m u m at 3 2 2 n m w i t h detection at 400 nm.  2. V a c u u m L o a d i n g MEH-PPV  was  prepared  a c c o r d i n g to a literature  procedure.  14  C o m m e r c i a l porous  a l u m i n a m e m b r a n e s ( A n o p o r e b y W h a t m a n , Inc.) w i t h a n o m i n a l pore diameter o f 2 0 0 n m o n one side and 20 n m on the other side were used. T h e m e m b r a n e t h i c k n e s s w a s 60 u m . It w a s p l a c e d o n top o f a stainless steel tube, through w h i c h v a c u u m w a s a p p l i e d . M E H - P P V p o l y m e r solution drops (0.038 to 0.8 w t % i n T H F ) were p l a c e d o n top o f the m e m b r a n e . O n c e the solution w a s d r a w n through, the m e m b r a n e w a s r e m o v e d for analysis. A H i t a c h i S - 4 1 0 0 f i e l d e m i s s i o n S E M was used for observation o f the c o m p o s i t e structure.  187  3. C e n t r i f u g e d S a m p l e s P o r o u s a l u m i n a hosts w e r e prepared b y a n o d i z i n g a l u m i n u m f o i l samples i n 1.2 M sulfuric a c i d at 15.0 V or i n 0.3 M o x a l i c a c i d at 4 0 . 0 V at r o o m temperature, as described i n chapter 4. S o m e f i l m s w e r e also prepared o n s i l i c o n substrates b y a n o d i z i n g electron-beam  evaporated  a l u m i n u m f i l m s . These a l l o w e d convenient cross-section preparation b y cleavage o f the s i l i c o n substrate. T h e a n o d i z a t i o n t i m e w a s generally between 2 m i n and 8 m i n , resulting i n f i l m thicknesses o f 0.5 to 2 u m . T h e pore w a l l s were etched for 5 to 10 m i n i n 5 % H3PO4, then soaked i n distilled H2O. T h e samples were then r i n s e d w i t h ethanol, d r i e d w i t h a heat g u n and further d r i e d under v a c u u m . Centrifugation w a s carried out i n a standard laboratory centrifuge w i t h a speed o f 1700 RPM,  u s i n g a substrate holder described above. T h e a p p r o x i m a t e centrifugal force at  the  substrate was 3 x 1 0 N . 3  M E H - P P V w a s prepared a c c o r d i n g to a literature p r o c e d u r e .  14  A 0.038 w t % s o l u t i o n o f  M E H - P P V i n T H F ( t y p i c a l l y 15 - 2 0 u L ) w a s deposited into the holder b y syringe and spun for 5 m i n . P u r e T H F w a s then added (again ~ 2 0 u L ) and the holder sealed to reduce the evaporation rate. A f t e r 15 m i n o f s p i n n i n g , the seal w a s r e m o v e d a n d the u n c o v e r e d h o l d e r s p u n again for 5 min. A H i t a c h i S-4700 field e m i s s i o n S E M w a s used to observe the samples, t y p i c a l l y w i t h an accelerating voltage o f 20 k V for h i g h resolution w o r k . F o r X P S analysis o f the pore bottoms, a layer o f A u / P d w a s first deposited onto the top surface. T h i s prevented e p o x y penetration into the sample d u r i n g e m b e d d i n g o f the top surface. Due  to the need for c o n d u c t i v i t y , a silver-filled e p o x y w a s used as the e m b e d d i n g m e d i u m  ( E p o t e k , Inc.). T h e r e m a i n i n g a l u m i n u m substrate w a s r e m o v e d b y treatment w i t h HgCl2(sat)188  A n a l y s i s w a s carried out o n a L e y b o l d M A X 2 0 0  u s i n g the A l K « radiation as the excitation  source. T h i n sections w e r e obtained b y u l t r a m i c r o t o m y o f samples e m b e d d e d i n e p o x y ( 3 0 2 - 3 M , Inc.). A d i a m o n d knife ( M i c r o s t a r , Inc.) w i t h a 4 5 ° i n c l u d e d angle w a s used. T h e knife clearance angle w a s set to 4 ° , a n d s e c t i o n i n g speeds as l o w as 0.2 m m s" w e r e used. T h e t h i n sections w e r e 1  floated i n a water bath and collected either w i t h lacey carbon-coated C u g r i d s ( T e d P e l l a , Inc.) or similar  Quantifoil-coated C u grids (Qantifoil  Micro  Tools  GmbH).  These  grids  allowed  investigation o f the sections by E E L S w i t h o u t any interference f r o m a support f i l m , due to the presence o f holes i n the coatings. Initial sample observation w a s c a r r i e d out on a H i t a c h i H - 7 6 0 0 T E M at an accelerating voltage o f 8 0 k V . E E L S a n d E F T E M w e r e c a r r i e d out o n a T e c n a i F 2 0 T E M e q u i p p e d w i t h a G a t a n I m a g i n g Filter. T h e accelerating voltage w a s 197 k V ( 2 0 0 k V n o m i n a l l y , offset b y 3 k V by the G I F ) . L o s s spectra w e r e recorded i n T E M m o d e by p l a c i n g the particle o f interest above the G I F entrance aperture (diameter 2.0 m m ) . T h e zero-loss peak w a s recorded separately b y m o v i n g to an e m p t y  area o n the  grid,  i m m e d i a t e l y before  o r after  spectrum  collection.  A collection t i m e o f 5 s w a s used. S u b t r a c t i o n w a s carried out b y shifting and s c a l i n g the zeroloss peak. E F T E M images w e r e a c q u i r e d w i t h a 2 e V slit. In S T E M m o d e , a c a m e r a length o f 150 m m w a s used, and spectra w e r e collected u s i n g a 5 0 m s d w e l l t i m e . T h e s y s t e m energy resolution, g i v e n b y the F W H M o f the zero-loss peak, w a s 0.96 e V . T h e energy d i s p e r s i o n o f the spectrometer w a s 0.10 e V / p i x e l .  References 1. M a r t i n , C . R . Acc. Chem. Res. 1 9 9 5 , 28, 6 1 . 2. W u , C . - G . ; B e i n , T . Science 1994, 264, 1757. 189  3. Y u , B . Z . ; Li, H . 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Chem.  1999,  Langmuir 2002, 18, 2 9 7 1 .  2002, 23,  179.  3 3 . B o u k h e r r o u b , R . ; M o r i n , S.; W a y n e r , D . D . M . ; L o c k w o o d , D . J .  Applied Research  2000,  182,  2479.  Physica Status Solidi a-  117.  3 4 . B o u k h e r r o u b , R . ; M o r i n , S.; W a y n e r , D . D . M . ; L o c k w o o d , D . J .  Solid State Commun.  2001,  Langmuir  18,  775,319. 35. Wojtyk, J. T. C ; M o r i n , K . A . ; Boukherroub, R.; Wayner, D . D . M .  2002,  6081. 36. Crouse, D . ; L o , Y . H . ; M i l l e r , A . E . ; Crouse, M . 3 7 . G l u s a c , K . D . ; Schanze, K . S.  Appl. Phys. Lett.  2000,  76, 4 9 .  Polymer Preprints 2002, 43, 87.  3 8 . W u , J . J . ; G r o s s , A . F . ; Tolbert, S. H .  J. Phys. Chem. B 1999, 103, 2 3 7 4 .  39. Hornyak, G . ; K r o l l , M . ; Pugin, R . ; Sawitowski, T.; S c h m i d , G . ; B o v i n , J. O.; Karsson, G . ; H o f m e i s t e r , H . ; H o p f e , S.  Chem. Eur. J. 1997, 5,  1951.  191  40.  M a r i n a k o s , S. M . ; B r o u s s e a u , L . C ; Jones, A . ; F e l d h e i m , D . L .  Chem. Mater.  1998,  10,  1214. 4 1 . V a n d e r b e e k , G . P . ; Stuart, M . A . C ; Fleer, G . J . ; H o f r n a n , J . E .  Macromolecules  1991,  24,  6600. 42. M a t h u r , S.; M o u d g i l , B . M. 43. Elias, H . G .  Miner. Metall. Process 1998, 75,  Polymere; H u t h i g &  24.  W e p f V e r l a g : H e i d e l b e r g , 1996.  4 4 . T r e v e r t o n , J . A.; W e s t , R . ; J o h n s o n , D . ; T h o r n t o n , M. 45. R e h i m , S. S. A . ; H a s s a n , H . H.; A m i n , M . A .  Appl. Surf. Sci.  J. Appl. Electrochem.  1993,  72, 3 4 9 .  2002, 32,  1257.  4 6 . P . W o n g , personal c o m m u n i c a t i o n . 4 7 . F r e n c h , R . H . ; M u l l e j a n s , H . ; Jones, D . J . 48. Egerton, R . F.  Electron energy-loss  J. Am. Ceram. Soc. 1998, 81, 2 5 4 9 .  spectroscopy  in the electron microscope;  2 n d ed.;  P l e n u m Press: N e w Y o r k , 1996. 49.  McComb, D . W.; Howie, A .  Nucl. Instrum. Methods Phys. Res., Sect. B  1995,  50. B e r g e r , S. D . ; S a l i s b u r y , I. G . ; M i l n e , R . H . ; Imeson, D . ; H u m p h r e y s , C . J .  96,  569.  Philos. Mag. B  1987,55,341. 51. A r a y a s a n t i p a r b , D . ; M c K n i g h t , S.; L i b e r a , M .  J. Adhes.  52. A u s t i n , W . B . ; B i l o w , N.; K e l l e g h a n , W . J . ; L a u , K . S. Y . 53. K a m i t o r i , Y.; H o j o , M.; M a s u d a , R . ; Y o s h i d a , R .  2001,  76, 3 5 3 .  J. Org. Chem.  Tetrahedron Lett.  1981, 48, 2 2 8 0 .  1983,  26, 4 7 6 7 .  192  CHAPTER 8 Comments on Device Fabrication  A  substantial a m o u n t o f effort w a s directed at fabricating a f u n c t i o n i n g l i g h t - e m i t t i n g  d e v i c e ( L E D ) based o n the c o m p o s i t e materials d e s c r i b e d i n this thesis. T h e preparation o f an L E D based on conjugated p o l y m e r inside a porous a l u m i n a host i n v o l v e s three k e y steps: (1) preparation o f porous t h i n f i l m host, (2) i n t r o d u c t i o n o f the conjugated p o l y m e r guest, a n d (3) a p p l i c a t i o n o f electrodes. T h e first step was the subject o f chapter 4 and w i l l be elaborated u p o n here i n consideration o f the t h i r d step, the need for electrodes to m a k e an electrical d e v i c e . T h e processes  d e s c r i b e d here also assume  that the  conjugated  p o l y m e r insertion proceeds  by  centrifugation (chapter 7), w h i c h requires a host f i l m w i t h a s u p p o r t i n g substrate. T h e devices prepared i n this w o r k either d i d not s h o w any electrical c o n d u c t i v i t y or failed r a p i d l y because o f electrical short c i r c u i t s . T h e r e were n o substantially n e w results i n these endeavours but m a n y p r o c e s s i n g issues were r e c o g n i z e d . T h i s chapter is intended to s u m m a r i z e these results and discuss p o s s i b l e routes to the creation o f d e v i c e s based o n the p o r o u s a l u m i n a host.  8.1  Device Structure T h e structure o f a p o l y m e r - b a s e d ( L E D ) is constrained b y the requirements o f t w o k e y  processes: the field-assisted injection o f charges at the p o l y m e r - e l e c t r o d e interfaces, and the r e c o m b i n a t i o n o f charges w i t h i n the p o l y m e r ( F i g u r e 1.4). T h e h i g h electric fields required for  193  charge i n j e c t i o n ' can be obtained at moderate a p p l i e d potentials (< 50 V ) by m i n i m i z i n g the 1  2  thickness o f the p o l y m e r layer between the electrodes. T h e h i g h e r hole m o b i l i t y causes charge r e c o m b i n a t i o n to o c c u r close to the electron-injecting electrode. N o n - r a d i a t i v e q u e n c h i n g o f the e x c i t e d state has been found to be important near this electrode, w h i c h effectively enforces a m i n i m u m d e v i c e thickness o f ~ 9 0 n m .  3  A b o v e this threshold, the efficiency o f a p o l y m e r L E D  r e m a i n s effectively constant w i t h thickness, up to several h u n d r e d  nanometres.  3  T h e ideal structure is s h o w n in F i g u r e 8.1: the central requirement for a functional d e v i c e is the existence o f the contact surfaces to the anode and cathode materials. I n d i u m tin o x i d e ( I T O ) and g o l d are c o m m o n l y used as anode materials for M E H - P P V - b a s e d devices, whereas a n u m b e r o f different l o w - w o r k f u n c t i o n metals ( c a l c i u m , silver, a l u m i n u m ) m a y be a p p l i e d as the cathode. T h e a n o d i z a t i o n procedure  for porous a l u m i n a hosts leaves one surface  immediately  accessible for the a p p l i c a t i o n o f an electrode. T h e s e c o n d surface is usually capped by the a l u m i n a barrier layer, w h i c h m a y be over 2 0 n m t h i c k ( F i g u r e 4.11). T h i s barrier layer must be  F i g u r e 8.1  Ideal conjugated  polymer device components  and their assembly: (a) p o l y m e r  insertion into host, (b) cathode evaporation a n d (c) anode d e p o s i t i o n . 194  r e m o v e d i n all cases to create functional devices.  8.2  Devices from Porous Alumina Films on Aluminum Foil A d e v i c e fabrication sequence u t i l i z i n g a porous a l u m i n a film g r o w n o n an a l u m i n u m foil  is s h o w n i n F i g u r e 8.2. E q u i v a l e n t structures m a y be p r o d u c e d b y partially a n o d i z i n g t h i c k aluminum  films  o n suitable substrates ( s i l i c o n wafer, glass slide). These substrates c a n  be  cleaved easily, w h i c h p r o v i d e s a convenient m e t h o d for o b t a i n i n g cross-sections for S E M . T h e barrier layer is t h i n n e d b y r e d u c i n g the potential at the end o f the a n o d i z a t i o n process (Figures 8.3 and 8.4). A f t e r p o l y m e r i n t r o d u c t i o n , an I T O anode is deposited on the p o l y m e r b y R F sputtering. A lead is attached to the I T O layer w i t h silver e p o x y , and the top surface is e m b e d d e d i n e p o x y to p r o v i d e support for the device. A t this stage, the a l u m i n u m substrate can c o n c e i v a b l y act as the electron-injecting  contact  for the d e v i c e . T h i s w a s tested on several different devices but no electroluminescence  was  observed. A l l devices eventually failed under the a p p l i e d potential. T h e obstacle to d e v i c e operation i n this f o r m is p r o b a b l y the r e m a i n i n g barrier layer at the bottom o f the pores. Further p r o c e s s i n g to r e m o v e this barrier layer w a s not possible, as the structure p r o v e d too fragile for r e m o v a l o f the a l u m i n u m substrate: the host film r e a d i l y separated from the deposited I T O layer d u r i n g the HgCl2(sat) e t c h i n g step ( F i g u r e 8.2(f)). A solution to this p r o b l e m was not found  195  F i g u r e 8.2  D e v i c e fabrication sequence f r o m porous  alumina  film  on a l u m i n u m f o i l :  (a)  a n o d i z a t i o n o f a l u m i n u m f o i l , (b) barrier layer t h i n n i n g by potential reduction, (c) p o l y m e r i n t r o d u c t i o n b y centrifugation, (d) I T O d e p o s i t i o n b y R F sputtering, (e) contact lead b o n d i n g w i t h silver e p o x y and e p o x y e m b e d d i n g o f upper surface, (f) a l u m i n u m foil r e m o v a l by c h e m i c a l etching, (g) a l u m i n u m cathode d e p o s i t i o n b y t h e r m a l evaporation.  196  F i g u r e 8.3 S E M images o f porous a l u m i n a f i l m o n a n-type s i l i c o n wafer w i t h the barrier layer e l i m i n a t e d b y the potential r e d u c t i o n m e t h o d . T h e f i l m w a s a n o d i z e d at 15 V i n 1.2 M sulfuric a c i d at 2 0 ° C f o l l o w e d b y a potential reduction to 9 V over 30 s, then a reduction to 0 V over 5 s. T h e scale bars are 2 0 0 n m and 100 n m , respectively.  F i g u r e 8.4 S E M images o f porous a l u m i n a f i l m , s h o w i n g e l i m i n a t i o n o f barrier layer by r a p i d potential reduction (from 15 V to 0 V over 8 s). T h e scale bars are 2 0 0 n m and 100 n m , respectively.  197  8.3  Devices f r o m T h i n F i l m s on C o n d u c t i n g Substrates T h e integrity o f the p o l y m e r / p o r o u s a l u m i n a c o m p o s i t e f i l m m a y be better retained i f the  porous a l u m i n a host is f o r m e d directly  from  an a l u m i n u m f i l m o n a c o n d u c t i v e  substrate.  Substrates o f interest w o u l d include any material that c a n serve as anode i n an L E D : s i l i c o n , I T O and g o l d . T h e d e v i c e fabrication sequence is illustrated i n F i g u r e 8.5. T h e initial step i n v o l v e s d e p o s i t i o n o f a l u m i n u m t h i n f i l m s . T h e different p h y s i c a l v a p o u r deposition m e t h o d s are r e v i e w e d b e l o w . T h e a n o d i z a t i o n step proceeds n o r m a l l y u n t i l the substrate interface is reached, at w h i c h point the electrolyte m a y or m a y not react w i t h the substrate material. A l u m i n u m f i l m s o n s i l i c o n , I T O and g o l d w e r e investigated, but n o suitable structures c o u l d be p r o d u c e d . T h e results and difficulties encountered are d e s c r i b e d b e l o w .  Al  -  B  ITO  F i g u r e 8.5 D e v i c e fabrication sequence from an a l u m i n u m f i l m on a c o n d u c t i v e substrate: (a) i n i t i a l a l u m i n u m f i l m on substrate, (b) porous a l u m i n a f i l m g r o w t h , (c) final porous a l u m i n a f i l m w i t h barrier layer r e m o v e d , (d) complete d e v i c e w i t h conjugated p o l y m e r s a n d w i c h e d  between  cathode and anode. 198  8.3.1  A l u m i n u m F i l m Deposition T h e a l u m i n u m d e p o s i t i o n step m u s t also be carefully controlled for the preparation  defect-free  films.  of  P r o p e r c l e a n i n g o f the substrate w a s necessary to obtain g o o d adhesion. A  s i m p l e p r o t o c o l c o n s i s t i n g o f t w o 10 m i n sonication steps (detergent solution, methanol)  was  found to be suitable for most substrates. A n u m b e r o f different sputtering  and  electron-beam  film  d e p o s i t i o n methods available at U B C were investigated. R F  evaporation  were  available  i n the  AMPEL  cleanroom.  DC  sputtering w a s available i n the D e p a r t m e n t o f P h y s i c s , w h i l e t h e r m a l evaporation was available i n the Department o f E l e c t r i c a l E n g i n e e r i n g . D C sputtering o f a l u m i n u m a l l o w s convenient deposition o f t h i c k e r films (> 1 u m , rate > 0.5 n m s" ). H o w e v e r , the setup available at U B C was found to produce contaminated 1  films,  as  e v i d e n c e d by gas e v o l u t i o n d u r i n g the a n o d i z a t i o n process. It w a s not used for any further w o r k . R F sputtering is n o r m a l l y used for the deposition o f dielectric materials. It m a y also be used to deposit a l u m i n u m , h o w e v e r h i l l o c k formation is k n o w n to o c c u r (Figure 8.6).  The  resulting surface roughness l o w e r s the reflectivity o f the a l u m i n u m film. T h e addition o f a s m a l l  F i g u r e 8.6  S E M images o f h i l l o c k s o n porous a l u m i n a  films  prepared  from R F sputtered  a l u m i n u m films, s h o w i n g (a) top surface and (b) cross-section. T h e scale bars are 2 0 0 n m . 199  a m o u n t o f copper (~1%) is used i n the s e m i c o n d u c t o r industry to prevent h i l l o c k f o r m a t i o n . T h i s w a s not attempted  here, as the effect o f copper i m p u r i t i e s o n the a n o d i z a t i o n step w a s not  k n o w n . Nevertheless,  R F sputtering  produced  continuous  films  with  g o o d adhesion.  The  aluminum. The  only  d e p o s i t i o n rate was t y p i c a l l y 0.2 n m s" . 1  Electron-beam  evaporation  is also  c o m m o n l y used  to  deposit  difficulty w a s the presence o f p i n h o l e s i n the resulting film i f the deposition process is too r a p i d . T h e associated surface roughness again causes a l o w e r reflectivity i n the deposited  films.  The  a n o d i z e d films then c o n t a i n defects as s h o w n i n F i g u r e 8.7. Defect-free films w e r e achievable b y k e e p i n g the deposition rate b e l o w 0.5 n m s" . 1  T h e r m a l evaporation w a s found to p r o d u c e films w i t h p i n h o l e s as w e l l . In this case, the effect o f deposition rate w a s not investigated i n m o r e detail. S i n c e the p h y s i c a l process is almost i d e n t i c a l to electron-beam evaporation, it is expected that c o n t r o l o f the d e p o s i t i o n rate w o u l d p r o d u c e s i m i l a r pinhole-free  8.3.2  films.  Porous Alumina/Silicon The  possibility o f  directly  using  silicon  wafers  as  an  electroluminescent device has been e x p l o r e d i n the literature. " 4  both degeneratively d o p e d n -  6  electrode  for  an  organic  P a r k e r and K i m s h o w e d that  and /?-type s i l i c o n wafers can be used as anode or cathode i n  devices m a d e w i t h M E H - P P V .  4  Wunsch e t a l .  injecting material than either g o l d or I T O .  f o u n d that /?-type s i l i c o n w a s a better hole-  6  C r o u s e e t a l . investigated the fabrication o f porous a l u m i n a films o n s i l i c o n wafers for the purpose o f h e x a g o n a l pattern transfer to the w a f e r . T h e y f o u n d that « - t y p e s i l i c o n c o u l d be used 7  r e a d i l y as a substrate, as there w e r e no unfavourable reactions w i t h o x a l i c , sulfuric or p h o s p h o r i c a c i d up to potentials o f 110 V . It w a s observed that the barrier layer c o u l d be r e a d i l y r e m o v e d by 200  F i g u r e 8.7 Defects i n porous a l u m i n a f i l m s a n o d i z e d from electron-beam evaporated a l u m i n u m f i l m , s h o w i n g (a) S E M i m a g e o f cross-section o f a film a n o d i z e d at 20 ° C i n 1.2 M sulfuric acid and (b) T E M i m a g e i n p l a n v i e w o f a film a n o d i z e d at -39 ° C . T h e scale bars are 2 0 0 n m long.  a short etching step w i t h 5 % p h o s p h o r i c a c i d . O n the other hand,  p-type s i l i c o n  w a s not suitable  w i t h o u t an insulating coating o f s i l i c o n d i o x i d e . A s degeneratively d o p e d s i l i c o n wafers were not readily available, this approach was not pursued any further but m a y be a p r o m i s i n g avenue for future w o r k .  8.3.3  Porous A l u m i n a / I T O T h e preparation o f porous a l u m i n a films on I T O - c o a t e d glass substrates has been reported  by C h u e t a/.  8  T h e a n o d i z a t i o n process n o r m a l l y c o n s u m e s the I T O layer i f it is not halted once  the a l u m i n u m layer is fully o x i d i z e d . I f anodization is halted at the right point, the I T O layer is preserved and the desired structure is obtained. These results were obtained w i t h a 10 v o l . % p h o s p h o r i c a c i d electrolyte and 130 V . T h e pore size i n the resulting film w a s 8 0 - 1 0 0 n m . Protection o f the I T O layer b y deposition o f a t h i n layer o f s i l i c o n d i o x i d e or a l u m i n u m o x i d e between the I T O and a l u m i n u m layers is also possible. O n e literature report indicated that a thin s i l i c o n d i o x i d e layer above the I T O layer w a s even beneficial to L E D efficiency, through a 201  better balance o f the hole and electron injection rates. A n o p t i m a l thickness near 2 n m w a s 9  reported. A t t e m p t s were m a d e to reproduce these results w i t h a l u m i n u m deposited on I T O - c o a t e d glass and poly(ethylene terephthalate) substrates. T h e latter is advantageous for o b t a i n i n g crosssections by u l t r a m i c r o t o m y . H o w e v e r , p i n h o l e s were a l w a y s present i n the a l u m i n u m f i l m , as e v i d e n c e d b y i m m e d i a t e gas e v o l u t i o n (i.e., reaction o f the I T O ) at the b e g i n n i n g o f the a n o d i z a t i o n process. It is b e l i e v e d that deformation o f the plastic substrate under the thermal load o f the evaporation process w a s the cause o f the p i n h o l e s . O n I T O - c o a t e d glass substrates, both sulfuric and o x a l i c a c i d electrolytes (for pore sizes o f ~21 and 60 n m respectively; see chapter 4) were used to anodize the deposited a l u m i n u m f i l m . H o w e v e r , the a n o d i z a t i o n process, u s i n g the cell depicted i n F i g u r e 4 . 1 0 , d i d not proceed uniformly  across the  substrate, w i t h the outer  edge b e i n g a n o d i z e d faster.  This produced  structures  i n w h i c h the I T O layer w a s c o n s u m e d at the edges (Figure 8.8), and a l u m i n u m  r e m a i n e d i n the centre. T h e deposition o f t h i n s i l i c o n d i o x i d e and a l u m i n a layers (up to 4 n m i n thickness) w a s not  F i g u r e 8.8 S E M image o f porous a l u m i n a film o n I T O - c o a t e d glass substrate, s h o w i n g an area where most o f the I T O was c o n s u m e d . T h e scale bar is 2 0 0 n m . 202  f o u n d to p r o v i d e any significant protection to the I T O layer d u r i n g a n o d i z a t i o n . T h i c k e r layers m a y have p r o v i d e d m o r e protection but w o u l d then have required r e m o v a l b y a different process (e.g., reactive-ion etching). It is also p o s s i b l e that s t i r r i n g o f the electrolyte w o u l d have created a m o r e u n i f o r m a n o d i z a t i o n o n the substrate. L o w e r i n g the electrolyte temperature w o u l d reduce the a n o d i z a t i o n rate, w h i c h m a y a l l o w better c o n t r o l over the end point. A s these difficulties w e r e not resolved, this approach d i d not p r o v i d e any suitable host films for further p r o c e s s i n g .  8.3.4  Porous Alumina/Gold Gold  is also c o m m o n l y used  as  hole-injecting electrode  in polymer-based  devices.  H o w e v e r , there are t w o difficulties associated w i t h fabricating porous a l u m i n a films over g o l d . First, a l u m i n u m and g o l d interdiffuse  r e a d i l y to create an intermetallic c o m p o u n d .  proceeds r a p i d l y at 100 ° C , w i t h films deteriorating w i t h i n 1 h . R o o m - t e m p e r a t u r e proceeds at a s l o w e r rate, w h i c h necessitates i m m e d i a t e p r o c e s s i n g o f deposited  1 0  This  degradation films.  The  s e c o n d difficulty, as w i t h I T O , is i n the a n o d i z a t i o n process: gas e v o l u t i o n occurs at the g o l d interface once the a l u m i n u m is c o n s u m e d . A literature report indicated that it w a s again possible to obtain useful structures by s t o p p i n g the a n o d i z a t i o n at the correct t i m e .  1 1  Efforts to reproduce  this result were unsuccessful due to i m m e d i a t e reaction e v o l u t i o n o f gas from the substrate, w h i c h suggested that p i n h o l e s w e r e present i n the a l u m i n u m film. N o further attempts w e r e m a d e to prepare porous a l u m i n a hosts i n this manner.  8.4  Conclusion T h e fabrication o f a d e v i c e based o n the conjugated p o l y m e r / p o r o u s a l u m i n a c o m p o s i t e  material revealed a large n u m b e r o f p r o c e s s i n g issues. T h e current results indicate that s o m e o f  203  these m a y be o v e r c o m e w i t h further effort. In the case o f films on a l u m i n u m f o i l , m o r e careful p r o c e s s i n g to r e m o v e the a l u m i n u m substrate is necessary  to preserve the integrity o f the  structure. A s for films o n s i l i c o n , I T O and g o l d , c o n t r o l o f the deposited a l u m i n u m m o r p h o l o g y appears to be k e y for o b t a i n i n g the desired structure w i t h o u t c a u s i n g gas e v o l u t i o n f r o m the u n d e r l y i n g substrate d u r i n g a n o d i z a t i o n . D e v i c e s o n  n-type  s i l i c o n s h o u l d still be investigated i n  m o r e detail, as it is k n o w n that it s h o w s no reaction w i t h the electrolyte. T h e efforts described here w e r e a i m e d at relatively crude devices i n w h i c h conjugated p o l y m e r c h a i n confinement w a s not really possible. H o w e v e r , once the obstacles identified here are resolved, the use o f l o w temperature a n o d i z a t i o n s h o u l d produce the desired structure for s u c h confinement and a l l o w the study o f single c h a i n electrical properties.  Experimental Details I T O - c o a t e d glass and poly(ethylene terephthalate) substrates were obtained  from  Delta  T e c h n o l o g i e s , Inc. S i l i c o n wafers ( « - t y p e ) w e r e obtained f r o m M o n s a n t o , Inc. Substrates w e r e cleaned p r i o r to deposition b y s o n i c a t i n g for 10 m i n i n a detergent solution ( 1 0 % F L - 7 0 , F i s h e r S c i e n t i f i c ) and for 10 m i n i n methanol, w h i c h w a s f o l l o w e d b y d r y i n g i n streaming nitrogen. G o l d substrates were prepared b y thermal evaporation o n s i l i c o n w i t h a t h i n c h r o m i u m adhesion layer (~10 n m ) . A l u m i n u m thin  films  o n s i l i c o n , g o l d and I T O w e r e prepared  d e s c r i b e d i n chapter 4. S E M images w e r e a c q u i r e d o n a H i t a c h i S - 4 7 0 0  field-emission  as  SEM.  I T O films were prepared b y R F sputtering at 100 W w i t h 6 m T o r r argon ( 1 2 0 seem f l o w ) . S o m e film d a r k e n i n g w a s observed due to r e d u c t i o n o f metallic i m p u r i t i e s . T h i s m a y be a v o i d e d b y m i x i n g a s m a l l o x y g e n f l o w (< 0.1 seem) to the c h a m b e r d u r i n g deposition. S i l i c o n d i o x i d e  204  and a l u m i n u m o x i d e protective films w e r e prepared b y electron-beam evaporation i n the same process as a l u m i n u m evaporation. For d e v i c e testing, electrical contacts were attached to the I T O layer u s i n g s i l v e r e p o x y (Epotek, Inc.) T h e I T O layer w a s further e m b e d d e d i n o p t i c a l e p o x y ( 3 0 2 - 3 M , E p o t e k , Inc.). D i r e c t contact  was  m a d e to the  aluminum  substrate b y an alligator c l i p .  characteristics w e r e investigated w i t h a D C p o w e r s u p p l y and a multimeter.  Current-voltage  T h e measurements  w e r e i n general difficult to reproduce, p r e s u m a b l y due to the presence o f electrical short circuits i n the d e v i c e that w o u l d degrade w i t h t i m e . C o n d i t i o n i n g at h i g h voltage (>50 V ) i m p r o v e d the stability somewhat but a l l devices eventually burned out.  References 1. P a r k e r , 1. D .  J. Appl. Phys. 1994, 75,  1656.  2. A r k h i p o v , V . I.; E m e l i a n o v a , E . V.; T a k , Y . H . ; Bassler, H . 3. C a o , Y . ; P a r k e r , I. D . ; Y u , G . ; Z h a n g , C ; Heeger, A . J . 4. Parker, I. D . ; K i m H e l e n , H .  J. Appl. Phys.  1998, 84, 848.  Nature 1999, 397, 4 1 4 .  Appl. Phys. Lett. 1994, 64, MIA.  5. Z h o u , X . ; H e , J . ; L i a o , L . S.; L u , M . ; X i o n g , Z . H . ; D i n g , X . M . ; H o u , X . Y . ; T a o , F . G . ; Z h o u ,  C. E.; Lee, S. T. Appl. Phys. Lett. 1999, 74, 609. 6. W u n s c h , F . ; C h a z a l v i e l , J . N.; O z a n a m , F . ; S i g a u d , P . ; Stephan, O . 7. C r o u s e , D . ; L o , Y . H.; M i l l e r , A . E . ; C r o u s e , M . 8. C h u , S. Z . ; W a d a , K ; Inoue, S.; T o d o r o k i , S.  9. D e n g , Z . B . ; D i n g ,  Appl. Phys. Lett.  Appl. Opt.  2002, 149,  Appl. Phys. Lett.  1972,11,  489,  191.  1999,  B321.  74, 2 2 2 7 .  1594.  11. Y a n g , Y . ; C h e n , H . L . ; M e i , Y . F.; C h e n , J. B . ; W u , X . L . ; B a o , X . M .  2002, 123,  2001,  2000, 76, 4 9 .  J. Electrochem. Soc.  X . M.; L e e , S. T . ; G a m b l i n g , W . A .  10. H u n t e r , W . R . ; M i k e s , T . L.; H a s s , G .  Surf. Sci.  Solid State Commun.  279. 205  

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