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UBC Theses and Dissertations

Leed studies on two surfaces of copper Parkin, Sean Richard 1989

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L E E D S T U D I E S O N T W O S U R F A C E S O F C O P P E R b y S E A N R I C H A R D P A R K I N B . S c , T h e U n i v e r s i t y o f K e n t at Canterbury 1987 A T H E S I S S U B M I T T E D I N P A R T I A L F U L P I L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F M A S T E R O F S C I E N C E i n T H E F A C U L T Y O F G R A D U A T E S T U D I E S D E P A R T M E N T 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 required standard T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A J U N E 1989 © Sean R i c h a r d P a r k i n In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia Vancouver, Canada DE-6 (2/88) n Abstract T h e w o r k presented i n this thesis i n c l u d e s invest igat ions u s i n g l o w energy e lectron d i f f r a c t i o n ( L E E D ) o f the c lean stepped C u ( 3 1 1 ) surface a n d its interact ions w i t h s u l p h u r , a n d a l s o f o r a h a l f m o n o l a y e r o x y g e n superstructure o n C u ( l l O ) designated C u ( l 1 0 ) - ( 2 x l ) - O . I n each case, intensity versus energy (1(E)) curves were measured w i t h a v i d e o L E E D ana lyzer f o r sets o f independent d i f f rac ted beams f o r future comparison to the results o f mul t ip le scattering calculations. T h e c l e a n C u ( 3 1 1 ) surface was cut and p o l i s h e d f r o m a s ingle crys ta l copper r o d and c leaned b y sputter-etching w i t h argon ions f o l l o w e d b y anneal ing . Intensity measurements were recorded for 14 symmetry inequivalent di f f ract ion beams at n o r m a l inc idence , f o l l o w e d b y s ix at 10* o f f n o r m a l inc idence . A d s o r p t i o n o f sulphur o n the C u ( 3 1 1 ) surface was c a r r i e d out b y d o s i n g w i t h H2S f o l l o w e d b y its p r e s u m e d d issoc ia t ion a n d loss o f h y d r o g e n to the v a c u u m . T h i s study i n d i c a t e d that S atoms order themselves o n C u ( 3 1 1 ) o n l y i n the [ O i l ] d i rec t ion . In addi t ion to exper imenta l w o r k o n the C u ( 3 1 1 ) surface, a d i s c u s s i o n is made o f the d i f f i cu l t i es associated w i t h the applicat ion o f L E E D to adsorption o n stepped surfaces. T h e C u ( 1 1 0 ) - ( 2 x l ) - O stucture was prepared by the adsorption o f o x y g e n o n the (110) surface o f copper . T h e condi t ions necessary to produce the best L E E D pattern were f o u n d b y analysis o f adsorption spot prof i les , and experimental L E E D 1(E) curves were recorded f o r nine independent beams at n o r m a l inc idence and a further s ix at 10" o f f n o r m a l incidence. i i i Table of contents Abstract i i Table o f Contents i i i L i s t o f Tables v L i s t o f F igures v i Acknowledgements x i Chapter 1 A n Introduction to L o w E n e r g y E lec t ron D i f f r a c t i o n 1 1.1 M o d e r n Surface Science 2 1.2 Class i f i ca t ion o f Surface Structures 4 1.2.1 T h e Step and M i c r o f a c e t Notat ions for Stepped Surfaces 4 1.2.2 T h e U n i t M e s h and Rec iproca l N e t 7 1.2.3 Over layer Structures - W o o d and M a t r i x Notations 9 1.3 L o w E n e r g y E lec t ron D i f f r a c t i o n 10 1.3.1 E lec t ron Scattering 10 1.3.2 Condi t ions for Elast ic L E E D 14 1.3.3 Surface Crysta l lography b y L E E D 18 1.3.4 Calcula t ion o f L E E D Intensities 21 1.3.5 E v a l u a t i o n o f L E E D 1(E) Curves 25 1.4 A u g e r E lec t ron Spectroscopy 27 Chapter 2 Exper imenta l Methods 31 2 .1 V a c u u m C h a m b e r and P u m p d o w n 32 2 . 2 Sample Preparation and Q e a n i n g 35 2 .3 A E S for Surface C o m p o s i t i o n A n a l y s i s 37 iv 2 .4 Experimental L E E D 39 2 .4 .1 L E E D Optics and Elec tron G u n 39 2 . 4 . 2 L E E D Spot Intensity Measurements 41 2 .4 .3 L E E D Spot P r o f i l e Measurements 43 Chapter 3 S r a d i e s o n m e C b p r x T ( 3 n ) S u r f a c e 47 3.1 Introduction 48 3 . 2 Exper imenta l W o r k o n the C l e a n C o p p e r (311) Surface 50 3.3 A d s o r p t i o n o f Su lphur o n Cu (311) 53 3.3.1 Introduction 53 3.3 .2 Exper imental 63 3.3.3 Interpretation o f the L E E D Pattern for Sulphur o n Cu (311) 65 3 .4 S o m e Considerat ions for A d s o r p t i o n o n Stepped Surfaces 70 Chapter 4 Studies o f the C o p p e r ( 110 ) - ( 2 x l ) - O A d s o r p t i o n system 77 4 .1 Introduction and Previous W o r k 78 4 . 2 E x r ^ r i m e n t a l 83 4 .3 Presentation o f D a t a 85 4 .4 F i n a l C o m m e n t s and Suggestions for Further W o r k 94 References 96 V List of tables 1.1 S o m e surface sensitive techniques. 3 1.2 E x a m p l e s o f the step a n d microfacet notations f o r some f .c .c . stepped surfaces 6 3.1 C u r r e n t c r y s t a l l o g r a p h i c data f o r the (311) surfaces o f some f . c . c . metals. 51 3 .2 B e a m i n d i c e s a n d energy ranges ( e V ) f o r the e x p e r i m e n t a l C u ( 3 1 1 ) L E E D 1(E) curves. 54 3 .3 Characterist ics o f the seven simplest f .c .c . stepped surfaces. 73 3 .4a S o m e character is t ics o f the three s imples t b . c . c . h i g h M i l l e r i n d e x surfaces w i t h rectangular unit meshes. 7 4 3 .4b S o m e characteristics o f the f o u r s implest non-basal h .c .p . surfaces w i t h rectangular unit meshes. 75 4 .1 C u r r e n t s t ructural i n f o r m a t i o n f o r the C u ( 1 1 0 ) - ( 2 x l ) - O a d s o r p t i o n sys tem. 80 4 . 2 B e a m indices a n d energy ranges ( e V ) f o r the exper imenta l C u ( 1 1 0 ) -(2x l ) - 0 L E E D 1(E) curves. 86 VI List of figures 1.1 T h e f i v e Brava i s t w o dimensional meshes i n real and rec iprocal spaces. 8 1.2 W o o d a n d m a t r i x notat ions f o r s o m e o v e r l a y e r structures o n the f . c . c . ( H O ) surface. 11 1.3 A t y p i c a l energy dis tr ibut ion o f backscattered electrons as a func t ion o f the p r i m a r y beam energy Ep. 13 1.4 T y p i c a l energy dis t r ibut ion o f e lectron m e a n free paths, L, i n a meta l l i c s o l i d . 13 1.5 T h e geometry o f the L E E D experiment; an electron beam is incident o n a surface w i t h energy E i n a d i r e c t i o n (8,<|>). T h e i n t e n s i t y o f the dif fracted beam is recorded as a funct ion o f E. 15 1.6 T h e E w a l d sphere construct ion for L E E D . T h e E w a l d sphere is s h o w n at t w o energies f o r the same inc ident d i r e c t i o n ( v i e w p a r a l l e l to the surface) . 19 1.7 F i v e types o f adatom structure o n the f . c . c . ( l 10) surface. E a c h w o u l d p r o d u c e a (2x1) L E E D pattern, i l l u s t r a t i n g that s t ructural analyses require more than just k n o w l e d g e o f the di f f ract ion pattern. 2 0 1.8 Three types o f d o m a i n structure f o u n d f o r over layers o n f .c .c . c rys ta l faces. 22 1.9 Schematic d iagram o f the de-excitation processes o f atomic core holes. a) X - r a y e m i s s i o n . b) A u g e r electron emiss ion. 28 1.10 A n A u g e r spectrum o f contaminated c o p p e r s h o w i n g the total energy distr ibution and its first derivative. 30 v i i 2 .1 T h e V a r i a n 240 v a c u u m chamber and its associated hardware. 33 2 . 2 P u m p and gas l ine configurat ion for the V a r i a n 240 V a c u u m chamber. I G = Ion G a u g e , T C G = T h e r m o c o u p l e G a u g e 34 2 .3 Schematic d iagram o f the laser method used to check correspondence o f the opt ical and crystal lographic planes. 36 2 .4 C o n f i g u r a t i o n o f the L E E D optics for use as a retarding f i e l d analyzer ( R F A ) for A u g e r electron spectroscopy ( A E S ) . 38 2 .5 S i m p l i f i e d d i a g r a m o f the L E E D optics used for d i s p l a y i n g d i f f rac t ion patterns. 4 0 2 .6 B l o c k diagram o f the v ideo L E E D analyzer ( V L A ) . 42 2.7 1(E) curves for symmetr i ca l ly related beams f r o m C u ( l 1 0 ) - ( 2 x l ) - O and C u ( 3 1 1 ) surfaces. These equivalent beams are averaged, smoothed, and i f needed, the background is subtracted. 44 2 .8 L E E D spot pro f i l e analysis : a) a user selected (10x10) p i x e l w i n d o w is p l a c e d over the d i f f r a c t i o n spot b) a L E E D spot p r o f i l e - the F W H M gives a measure o f spot sharpness. 46 3.1 a) T h e real space ideal bu lk truncation representation o f f.c.c.(311) b) T h e rec iprocal space f.c.c.(311) net ( L E E D pattern). 49 3 .2 A u g e r spectra o f the Cu(311) surface. a) P r i o r to c leaning i n U H V . b) A f t e r c leaning by argon i o n bombardment/anneal cycles . 52 3 .3 Exper imenta l 1(E) curves recorded for the clean Cu(311) surface. a) (1,-3) and (-1,-2) beams at n o r m a l incidence. b) (1 ,-2) and (-1,-1) beams at n o r m a l inc idence . 5 5 c) (1,-1) and (-1,0) beams at n o r m a l inc idence . 5 6 Vlll 3.4 3.5 3.6 d) (1,0) and (-1,1) beams at normal incidence. 56 e) (1,1) and (-1,2) beams at normal incidence. f) (1,2) and (-1,3) beams at normal incidence. 57 g) (0,-3) beam at normal incidence. h) (0,-2) beam at normal incidence. i) (0,-1) beam at normal incidence. 58 j) (0,2) beam at normal incidence. k) (0,1) beam at normal incidence. 59 1) (0,3) beam at normal incidence. m) (2,-2) and (-2,0) beams at normal incidence. n) (2,-1) and (-2,1) beams at normal incidence. 60 o) (1,-3) and (-1,-2) beams at 10° off normal incidence, p) (1,-2) and (-1,-1) beams at 10° off normal incidence. q) (1,0) and (-1,1) beams at J 0° off normal incidence. 61 r) (0,-3) beam at 10° off normal incidence, s) (0,2) beam at 10° off normal incidence. t) (0,0) beam at 10° off normal incidence. 62 Auger peak height ratio R = s( 1 5 2 e V)/Cu(920eV) plotted as a function of H2S exposure time at 2x10~8 torr. 64 Photographs of the L E E D pattern for sulphur adsorbed on Cu(311) a)62eV b)67eV c)76eV d)142eV 66 Adsorption of Sulphur on the Cu(311) surface. a) The two equivalent adsorption sites between adjacent terraces. b) Possible arrangements of sulphur on Cu(311) giving (2x1) patterns. c) Possible (2x3) arrangements of sulphur on Cu(311). 69 IX d) Possible (2x5) arrangements of sulphur on Cu(311). 69 4.1 a) A real space ideal bulk truncation representation of the f.c.c.(110) surface. b) The reciprocal space f.c.c.(l 10) net (LEED) pattern. c) The LEED pattern of the Cu(l 10)-(2xl)-O adsorption system. 79 4.2 Proposed models for the Cu(l 10)-(2xl)-O surface, a) Unreconstructed. b) Buckled row. c) Missing row d) Sawtooth. 81 4.3 Normalised 1 / F W H M of the (_1/2.1) and (_1/2,-l) fractional beams from the Cu(l 10)-(2xl)-O surface plotted as a function of exposure time to oxygen at 4xl0"8torr. 84 4.4 Experimental 1(E) curves recorded for the Cu(l 10)-(2xl)-O surface. a) (0,1) and (0,-1) beams at normal incidence. b) (1,1),(1,-1),(-1,1) and (-1,-1) beams at normal incidence. 87 c) (2,0) and (-2,0) beams at normal incidence. d) (2,1),(2,-1),(-2,1) and (-2,-1) beams at normal incidence. 88 e) (l»1/2)»(l»-1/2)»(-l»1/2) and (-I.-V2) beams at normal incidence. f) (2,1/2),(2,-1/2),(-2,1/2) and (-2,-1/2) beams at normal incidence. 89 g) (O,1^) and (O,-1^) beams at normal incidence. h) (0,3/2) and (0,-3/2) beams at normal incidence. i) (l»3/2).(-l»3/2).(l»-3/2) and (-l,-3/2) beams at normal incidence. 90 j) (-1,1) and (-1,-1) beams at 10° off normal incidence. k) (0,1) and (0,-1) beams at 10° off normal incidence. 91 1) (-1,0) beam at 10° off normal incidence. m) (-2,0) beam at 10° off normal incidence. 92 X 4.4 n) (0,2) and (0,-2) beams at 10° off normal incidence. o) (0,0) beam at 10° off normal incidence. 93 Acknowledgements Throughout the course o f m y study at U B C , I have appreciated the interest and advice o f m y research supervisor , P r o f . K . A . R . M i t c h e l l , as w e l l as his comments o n and careful reading o f this thesis. I w o u l d l i k e to acknowledge the members o f the surface science group past and present at U B C . S p e c i a l thanks are due to D r . R . A . M c F a r l a n e f o r i n t r o d u c i n g m e to the exper imental aspects o f L E E D , and M r H . C . Z e n g for his m a n y useful suggestions o n parts o f this w o r k . I a lso acknowledge M r J .R . L o u , D r P . C . W o n g , M s Y . K . W u , and latterly M r s T . V . G r i m s b y a n d P r o f . M . Y . Z h o u . I a m indebted to m a n y members o f the mechanica l and electronics w o r k s h o p s , wi thout whose assistance, the r u n n i n g and maintenance o f the v a c u u m system and its a u x i l i a r y hardware w o u l d have been far f r o m pleasant F i n a l l y , I w o u l d l i k e to thank m y parents and m y g i r l f r iend Stephanie, to w h o m I dedicate this thesis. 1 Chapter 1 A n Introduction to Low Energy Electron Diffraction 1.1 Modern Surface Science The study of surface physico-chemical properties has seen a dramatic shift in emphasis over the last few decades from the investigation of 'real' or polycrystalline dirty surfaces to the exaniination of clean single crystal surfaces. This change was due to a realisation that knowledge of fundamental surface properties could best be achieved by an understanding of ordered, clean surfaces. The modern approach to surface science has been facilitated in the last 25 years by the ability to routinely attain ultrahigh vacuum (UHV, pressure <10"9 torr), essential for maintaining surface cleanliness. In addition, the proliferation of surface sensitive techniques developed in the interim have provided access to a wide range of relevant information, which in turn has led to important advances in materials science [1,2,3]. Some of the more common techniques are listed in table 1.1. Structural geometry and elemental composition are of paramount importance in surface science. Most work has concentrated on the low Miller index surfaces of metals and semiconductors, due in part to their close packed natures and high degrees of symmetry. On the other hand, 'real' surfaces used as heterogeneous catalysts for example, include a variety of crystal faces, exhibiting many stepped surfaces. Indeed it is these steps that are thought to be the 'active sites' in such catalysts [4], and so structural analyses of high Miller index surfaces and of adsorption on them are essential. The combination of low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) has been particularly significant in surface characterization. Surface crystallography using LEED is now a mature science, such that the types of surface that can be studied with confidence grows continually. There is already a sizeable database of several hundred surface structures that have been 'solved' by Table 1.1 Some surface sensitive techniques. Technique Acronym Probe Particle Detected Particle Information Low Energy Electron Diffraction LEED [5,6] electron electron Geometrical structure Auger Electron Spectroscopy AES [7,9] electron electron Elemental composition High Resolution Electron Energy Loss Spectroscopy HREELS [8] electron electron Vibrational modes X-ray Photoelectron Spectroscopy XPS [9] photon electron Composition, electronic states Ultra-violet Photoelectron Spectroscopy UPS [10] photon electron Valence electronic states Angle-Resolved U-V Photoelectron Spectroscopy ARUPS [11] photon electron Structure, Valence states Angle Resolved X-ray Photoelectron Spectroscopy ARXPS [12] photon electron Geometrical structure, composition Surface Extended X-ray Absorption Fine Structure SEXAFS [13] photon electron Geometrical structure Near Edge X-ray Absorption Fine Structure NEXAFS [13] photon electron Intramolecular bonding Grazing Incidence X-ray Scattering GKS [14] photon photon Geometrical structure Reflection Absorption Infrared Spectroscopy RAIR [15] photon photon Vibrational modes Surface Enhanced Raman Spectroscopy SERS [16] photon photon Vibrational modes Helium Atom Diffraction HEAD [17] atom atom Geometrical structure Impact-Collision Ion Scattering Spectroscopy ICISS [18] ion ion Geometrical structure, composition High/Low Energy Ion Scattering Spectroscopy HEIS/LEIS [19] ion ion Geometrical structure, composition Secondary Ion Mass Spectrometry SIMS [20] ion ion Composition Scanning Tunneling Microscope STM [21] electric field tunneling current Geometrical, defect and electronic structures L E E D , so m u c h so that i t has been frequently used to assess the capabi l i ty and potential o f other, newer surface structure probes. 1.2 Classification of Surface Structures 1.2.1 The Step and Microfacet Notations for Stepped Surfaces S i n g l e c r y s t a l surfaces c a n be c l a s s i f i e d b y the M i l l e r i n d i c e s o f the corresponding b u l k crys ta l lographic planes . In surface crys ta l lography, reference to ' l o w ' a n d 'h igh ' M i l l e r indices is frequently made. T h e l o w M i l l e r i n d e x faces o f c u b i c crystals are the (111),(100) and (110) faces. T h e term 'h igh M i l l e r index ' is taken to mean surfaces conta ining step and terrace structure where the terraces are not too large. A s one goes to h igher and h igher M i l l e r i n d i c e s , the surface w i l l resemble the l o w M i l l e r index surface o f its terrace planes, both chemica l ly and p h y s i c a l l y . F o r example , a surface whose M i l l e r indices are (400,1,1) c o u l d equal ly w e l l be thought o f as a (100) surface, m i s a l i g n e d by - 0 . 2 ° , w h i c h was the a l l o w e d misa l ignment error i n this w o r k (see sect ion 2.2) . T h i s is adequate f o r the l o w M i l l e r i n d e x surfaces, but f o r h i g h M i l l e r i n d e x surfaces, no i n d i c a t i o n o f step or terrace structure is g i v e n . T h e s implest stepped surfaces have l o w M i l l e r i n d e x terraces o f f i x e d w i d t h a n d l o w M i l l e r i n d e x steps o f single atom height. T h e second type have steps o f h i g h M i l l e r index , such that the steps have steps, and are k n o w n as ' k i n k e d ' surfaces. T h e step notation o f L a n g et al [22] classifies a stepped surface as (hkl) = n(htktlt)x(hsk8ls) 1.1 where n is the number o f r o w s o f atoms per terrace and (htktlt),(hsksls) are the M i l l e r ind ices o f terrace and step planes respect ive ly . I n some cases, f o r e x a m p l e the face centered c u b i c (f.c.c.) (311) surface, there is no unique step notat ion, s ince i t c o u l d be wri t ten as 2 ( 1 0 0 ) x ( 1 1 1 ) o r 2 (111 )x (100) depending o n h o w the surface is v i e w e d . T h e step nota t ion is adequate f o r s i m p l e stepped surfaces , but f o r k i n k e d surfaces, n o p r o v i s i o n is made f o r the further b r e a k d o w n o f the step indices . A m o r e serious d r a w b a c k i n the case o f k i n k e d surfaces is that n m u s t be n o n - i n t e g r a l , otherwise its value depends o n w h i c h r o w o f atoms is counted. In either case, n cannot be easily recognised f r o m the M i l l e r indices . T h e m i c r o f a c e t nota t ion f o r c u b i c c rys ta l s , i n t r o d u c e d b y V a n H o v e and S o m o r j a i [23] rel ies o n the (hkl) plane b e i n g perpendicular to the (hkl) vector . T h i s vector can be split in to three l inear ly independent vectors (hik-|li),(h2k2l2),(h3k3l3). A n y h igh M i l l e r index plane (hkl) m a y then be written as (hkl) = ax(hikil!) + by(h2k2l2) + c z(h 3k3l 3) 1.2 where the coeff icients a,b,C are the relative amounts o f each component vector, and the subscripts x , y , z g ive the n u m b e r o f unit meshes o f (hnknln) per unit mesh o f (hkl). F o r s imple stepped surfaces, o n l y the f irst t w o terms are needed. E x a m p l e s o f these nomencla tures are g i v e n i n table 1.2. L i k e the step nota t ion , there is n o s ingle microfacet notation for a g i v e n surface. It depends o n w h i c h microfacets are chosen to break the surface d o w n i n t o , the best c h o i c e o f w h i c h m a y v a r y d e p e n d i n g o n the applicat ion. 6 Table 1.2 Examples of the step and microfacet notations for some f.c.c. stepped surfaces. surface Miller Step Microfacet M (533) 4(111)x(100) 33(111)+21(100) (911) 5(100)x(111) oVIOOJ+l^m) 8 (775) 7(111 )x(111) 55(1115+2^110) j f (10,87) 7(111)x(310) 714(111)+11(110) +22(100) 7 1.2.2 The Unit Mesh and Reciprocal Net T h e d i p e r i o d i c i t y o f a s ingle crystal surface means that each p a i r o f equivalent points can be related by a translation vector t=mai + na2 1-3 T h e uni t vectors a i a n d S2 def ine a uni t m e s h ( in analogy w i t h the unit c e l l o f X - r a y crys ta l lography) , w h i c h w i t h the set o f integers {m,n} generate a net analogous to the lattice o f t r iper iodic crystal lography. A di f f rac t ion pattern is a direct representation o f * * the surface net i n rec iprocal space. It is def ined b y vectors fii and 32 such that a-Sj* = 2 « 5 j j 1.4 where the K r o n e c k e r s y m b o l , 8jj=0 i f i^j and 8y=1 i f i=j These unit vectors describe the reciprocal net according to fl = ha-i* + kS2* 1 - 5 where h,k are integers. There are f i v e Brava is unit meshes, analogous to the fourteen Brava is unit cel ls o f b u l k crys ta l lography. These are s h o w n i n both rea l a n d rec iproca l space i n figure 1.1. D e t a i l s o f the var ious convent ions i n surface c rys ta l lography are g i v e n i n the International Tables for X - r a y Crysta l lography [24], and i n an article by W o o d [25]. Real Reciprocal 32 31 32 31 3 2 31 Square 32 Centered Rectangular Hexagonal Oblique Rectangular 32* 31 31 32 Figure 1.1 The five Bravais two-dimensional meshes in real and reciprocal space. 9 1.2.3 Overlayer Structures - Wood and Matrix Notations M i l l e r i n d i c e s , o r the step a n d microface t notat ion, m a y not be suf f i c ient to describe the translat ional sy m m e try o f a surface comple te ly . T h e d i p e r i o d i c i t y o f the surface can be al tered b y reconstruct ion o f the topmost a t o m i c layer(s) and/or b y adsorpt ion, resul t ing i n extra d i f f rac t ion spots i f the over layer unit mesh is larger than the substrate unit m e s h . T o relate the uni t meshes, t w o n a m i n g schemes have been developed, W o o d notation and matr ix notation. T h e W o o d notation [25] designates the superlattice structure as '|S1| IS2l J a i l l & l ©x 1.6 where 3.1,32 and £1 ,§2 are the vectors w h i c h describe the unit meshes o f the substrate and surface structures respec t ive ly , 0 gives the rotat ion angle between the meshes (dropped i f 0 = 0 ) and X denotes the adsorbed species. T h e express ion is sometimes p r e f i x e d b y a 'c' o r 'p* to denote centered or p r i m i t i v e meshes respect ive ly . W o o d notation can o n l y be appl ied i f the angle between 3/i and 3 2 is the same as that between £1 and £ 2 . In cases where these angles are different, the more general matr ix notation [26,27] must be used. It g ives the re lat ionship between the surface r e g i o n probed by L E E D and the substrate as V 3.11 3 1 2 V 321 3 2 2 and the determinant o f A g ives the s ize o f the superlattice m e s h w i t h respect to the substrate mesh . S o m e examples o f the W o o d and matr ix notations are g i v e n i n f igure 1.2. 1.3 Low Energy Electron Diffraction T h e or ig ins o f L E E D can be traced back to experiments p e r f o r m e d i n 1927 by D a v i s s o n and G e r m e r [28], w h o first observed the d i f f rac t ion o f l o w energy electrons a n d r e c o g n i z e d their inherent surface sens i t iv i ty . D e v e l o p m e n t o f the technique h o w e v e r was hampered b y e x p e r i m e n t a l and theoret ical d i f f i c u l t i e s , so that surface structure d e t e r m i n a t i o n was not p o s s i b l e u n t i l the ear ly 1970's. T h e e x p e r i m e n t essential ly consists o f d i rec t ing a monoenerget ic , c o l l i m a t e d electron beam i n the 10-3 0 0 e V range at a surface and s tudying the e last ica l ly backscattered electrons. These electrons are u s u a l l y co l l ec ted o n a f luorescent screen, though there are alternative c o l l e c t i o n devices [5]. I n the c o n v e n t i o n a l d i s p l a y systems, i f the surface is w e l l ordered, a pattern o f spots is produced whose arrangement and sharpness is governed by the ordering o f the surface. 1.3.1 Electron Scattering L o w energy electrons have w a v e l e n g t h s w h i c h , a c c o r d i n g to de B r o g l i e ' s hypothesis 150.4 l1/2 ?i = h/, mv or X(A) = 1.8 lE(eV) Wood Matrix fcc(110)(1x1) fcc(110) 1 Lo e.g. clean surface fcc(110) (2x1) fcc(110) e.g. Cu(110) (2x1)0 lr(110) (2x1) 2 0 .0 1. fcc(110) (2x2) fcc(110) e.g. Cu(110) (2x2) Au Cu/Ni(110) (2x2) CO 2 0 .0 2. fcc(110)c(2x2) fcc(110) e.g. Ni(110) c(2x2) CO Pt(110) c(2x2)0 "1 -1" .1 1. 1.2 Wood and matrix notation for some overlayer structures on the f.c.c.(110) surface. are o f the same o r d e r as a t o m i c spacings i n s o l i d s . A s s u c h , c o n s t r u c t i v e a n d destruct ive interference w o u l d be expected to produce a d i f f rac t ion pattern conta in ing structural in format ion u p o n interaction w i t h an ordered crystal surface. A n energet ical ly w e l l de f ined electron beam (energy Ep) inc ident o n a surface w i l l g ive rise to backscattered and secondary electrons. There are three distinct regions i n the energy distr ibution as s h o w n i n f igure 1.3. L E E D depends o n electrons i n region I, that is the e last ical ly and 'quasi-elastical ly ' scattered electrons (those h a v i n g suffered s m a l l (<0.1eV) energy losses v i a phonon interaction) w h i c h constitute a few percent o f the total i n c i d e n t e lectrons. R e g i o n II contains peaks due to A u g e r electrons and p l a s m o n losses super imposed o n a general scattered background [29], and r e g i o n III is made up o f the 'true secondary' electrons, those h a v i n g undergone a series o f inelast ic scattering events [8]. T h e surface sens i t iv i ty o f L E E D is a direct result o f the h i g h p r o b a b i l i t y o f inelast ic scattering f o r l o w energy electrons. T h i s dictates that L , the mean free path o f an electron i n a s o l i d , de f ined by I = l 0 e x p (-l/L) 1.9 is o f the order o f a f e w atomic layer spacings. In equation 1.9, Io, the inc ident b e a m intensity is attenuated to I after passage through a distance / w i t h i n the c rys ta l . T h e m e a n free path o f an electron i n a meta l l i c s o l i d , as a f u n c t i o n o f energy is s h o w n i n f igure 1.4. 13 UJ True Secondary Peak I I AES Peaks Plasmon Losses Energy (E) I Elastic Peak Ep Figure 1.3 A typical energy distribution of backscattered electrons as a function of the primary beam energy E p . 104 103 102 101 101 105 102 103 104 Electron Energy (eV) Figure 1.4 Typical energy dependence of electron mean free path, L, in a metallic solid. 1.3.2 Conditions for Elastic L E E D In a L E E D experiment , the electron beam impinges o n a surface w i t h direct ion (0,<(>) as s h o w n i n f igure 1.5. Outs ide the inf luence o f a c rys ta l , the inc ident and back dif fracted electrons experience a f i e l d free reg ion , where they can be expressed as plane waves Yk(r) = exp(ik-r) 1.10 where the wavevec tor & specif ies the d i rec t ion o f the beam and relates to wavelength according to |k| = 2 w / x 1.11 and energy by E = f]2|k|2 1.12 2 m where m = electron mass and f i = n /2 j t -F o r incident and dif fracted beams w i t h wavevectors ho and fc', the dif ferential scattering cross section is ' m ^ l(k'|T|fco>|' 1.13 15 Surface Normal -2 Diffracted Figure 1.5 The geometry of the LEED experiment; an electron beam is incident on the surface with an energy E and direction (6,<|)). The intensity of the diffracted beam is recorded as a function of E. where T is the transit ion operator. T h i s cross section gives a measure o f the number o f electrons scattered per unit t ime per unit s o l i d angle i n the direct ion &' for a unit incident f l u x i n d i rec t ion KQ . F o r a s y m m e t r y operat ion S a p p l i e d to the surface, the matr ix element above satisfies [30,31,32] <k'|T|l<o> = <k'|S-iTS|JSo> = <Sk'|T|SiSo> 1.14 If S is a translation by a surface net vector 1, then the t w o ends o f equation 1.14 g ive (k'|T|ko> = exp[iOSo-K)-t](h:|T|iSo> 1.15 T h i s equation is sat isf ied i f (k'|T|ko) = 0 , corresponding to zero scattered intensity, or w h e n exp[i (ko-k ' ) - i ] = 1 1.16 Since the surface net vector {has the f o r m t = m£i+ns2 m , n = integers 1.17 where S1.S2. are the real space unit mesh basis vectors, equation (1.16) w i l l be satisfied w h e n e v e r the p a r a l l e l c o m p o n e n t o f the d i f ference o f the i n c i d e n t a n d d i f f r a c t e d wavevectors equals a vector o f the reciprocal net fl=JSo||-k||' = hSi* + hS2* h,k = integers 1.18 and since 1 is parallel to the surface, the dot product of J with the components of jko and K' perpendicular to the surface will be zero, giving the diffraction condition exp[i{ko||-is'll)-tJ = 1 1.19 which is met if ( h s i * + k£2*)-(m£i + ns_2) = 27t(integer). 1.20 Applying equation 1.4 gives 27t(hm + kn) = 27t(integer) 1.21 and this condition holds if the parallel components of the wave vectors satisfy k'n" = kon+ + fl(h,k) h,k = integers 1.22 The superscripts (+/-) denote wavevector direction into/out of the crystal, and fl(h,k) are vectors of the reciprocal net. Equation (1.22) is simply a statement of momentum conservation parallel to the surface. It, along with the energy conservation condition (elastic scattering), namely Ihgl2 = |ko+l2 1 2 3 defines the diper iodic dif fract ion condit ion i n L E E D . The indices (h,k) are commonly used to label the diffracted beams. Equation (1.22) shows that the specular (0,0) beam is formed by electrons that have interacted with the surface without momentum transfer parallel to the surface, therefore its direction is independent of energy. Non-specular beams however, will tend to converge on the specular beam with increasing energy. This can be seen by adapting the Ewald sphere [5 ] from X-ray diffraction to the diperiodic case of LEED as shown in figure 1.6. The intersection of this sphere with the reciprocal lattice rods define the conditions under which both the parallel momentum and energy conservation requirements are simultaneously met 1.3.3 Surface Crystallography by LEED As with X-ray diffraction, a full structural analysis by L E E D cannot be effected simply by knowledge of the diffraction pattern, as evidenced by figure 1.7. To extract structural information, the spot intensities at varying incident electron energies are recorded as intensity versus energy or 1(E) curves. The experimental 1(E) curves are then matched to theoretical 1(E) curves calculated for various 'model' structures. The model which gives the best agreement between theory and experiment is deemed to be the closest to the actual structure. A L E E D experiment is restricted by instrumental limitations associated with energy and angular deviations in the electron beams. Park et al [33] have defined an instrument response function which yields a characteristic dimension, the transfer width W. For conventional systems, this is typically about 100A and it represents the limit over which structural features can be detected [34]. For a surface whose domain dimensions are greater than W, observed L E E D patterns are made up of superimposed diffraction patterns from many regions of approximate area W 2 . When there are 19 Figure 1.6 The Ewald sphere construction for LEED. The Ewald sphere is shown at two energies for the same incident direction (view parallel to the surface). Figure 1.7 Five types of adatom structure on the f.c.c.(110) surface. Each would produce a (2x1) diffraction pattern, illustrating that the structural analysis requires more than just knowledge of the diffraction pattern. to o ordered regions o n the surface o f this magnitude, the L E E D patterns observed w i l l be as sharp as possible for that particular instrument. Imperfections between these ordered regions w i l l contribute to the general background scattering but w i l l not affect the b e a m sharpness [34]. S o m e surfaces can e x h i b i t o r d e r e d reg ions that are re la ted b y a s y m m e t r y operation. W i t h these symmetr ica l ly related 'domains ' , o f the types i l lustrated i n f igure 1.8, the o b s e r v e d L E E D pattern represents a s u m o f d i f f r a c t i o n patterns f r o m the i n d i v i d u a l domains . A consequence is that the d i f f rac t ion pattern then appears w i t h the point group symmetry o f the substrate rather than that o f a l o c a l ordered reg ion o f the surface. 1.3.4 C a l c u l a t i o n o f L E E D Intensities T h e s t rong in terac t ion o f s l o w electrons w i t h s o l i d s , c a u s e d b y the large scattering cross section, precludes the use o f a single scattering theory as used i n X - r a y di f f ract ion. T h e h i g h probabi l i ty that electrons w i l l scatter several times before leav ing the surface requires a mul t ip le scattering, or d y n a m i c a l theory o f electron di f f ract ion to interpret the structure i n experimental 1(E) curves. S ince the interaction is so strong, it is c lear that the scattering potential s h o u l d be w e l l chosen. T h e usua l approach is to f o l l o w band structure theories [35] and approximate a solid's potential w i t h the ' m u f f i n tin' m o d e l . T h i s m o d e l assumes that the potent ia l w i t h i n each a tom is spher i ca l ly symmetr ic , and that the potential between atoms is a constant V 0 , such that V 0 = V o r + iV0j. 1.24 (2x1) domains on f.c.c.(311) related by a mirror reflection Figure 1.8 Three types of domain structure found for overlayers on f.c.c. crystal faces. The real component, Vor, often loosely termed the 'inner potential', accounts for the lowering in potential energy experienced by an electron on entering the solid and is typically between -5 and -15 eV. The imaginary component, V 0j, also a negative quantity, accounts for energy damping due to inelastic scattering, and is usually treated as a constant, though sometimes it is given a weak energy dependence (e.g. as E 1 /3 [36]). The salient features of any multiple scattering formalism are as follows: The incident electron beam, treated as a plane wave of the form exp(ik/l), interacts with the surface and is scattered by individual atoms. These scattered waves interact with each other and with the primary electron beam to produce an effective electron wave which scatters further. This process can be envisaged to continue until the resulting effective electron wave becomes self-consistent, thereby allowing the diffracted beam intensities to be calculated. The treatment of this overall scattering problem is broken into simpler parts. First consideration is for the scattering by the spherical potentials of the individual atoms, atoms are then built up into layers, and the scattering by a layer is calculated, and finally the layers are stacked. Exact solutions to the interlayer scattering involve calculations to infinite order in either K-space using the Bloch wave method [37,38] or in L-space by the T-matrix method [39]. The high cost in computer time and storage of these exact solutions led to the development of perturbative schemes that effectively speed up the calculations. These methods rely on the inelastic energy damping to limit the number of multiple scattering events [5]. A multiple scattering analysis of the Cu(311) 1(E) data presented in this thesis is being carried out using the so called 'layer doubling' method developed by Pendry [40-42]. This method starts with the scattered amplitudes due to multiple scattering in two individual layers. These layers are then stacked together and expressions are derived to calculate the scattering f r o m the t w o layers together. T h i s procedure is then cont inued such that each i teration doubles the number o f layers f r o m the previous i terat ion, and the resul t ing re f lec t ion matrices converge due to the s m a l l e lectron m e a n free path , t y p i c a l l y o n the th i rd o r fourth iteration (8 or 16 atomic layers) . These b u l k reflections c a n be stored and re-used w i t h v a r y i n g structural parameters such as the in ter layer spacing. T h e layer d o u b l i n g method is eff ic ient and f l e x i b l e , though convergence m a y f a i l i f interlayer spacings are less than around 0.6A. Other perturbat ional schemes i n c l u d e r e n o r m a l i s e d f o r w a r d scattering (RFS) [43] and the c o m b i n e d space m e t h o d (CSM) [41 ,44] . I n R F S , backscat ter ing o f e l e c t r o n s , w h i c h i s u s u a l l y w e a k c o m p a r e d to f o r w a r d sca t te r ing , i s treated perturbational ly w h i l e the latter is calculated exact ly . In the f irst-order ca lcu la t ion , a l l scattering paths f r o m single back scattering processes are considered, w h i l e the second a n d t h i r d order ca lcula t ions have three and f i v e back ref lect ions respec t ive ly . T h e iteration continues u n t i l the s u m o f the amplitudes o f the beams f o r w a r d scattered into the crystal has dropped to a smal l predetermined fract ion (-0.3%) o f the incident beam amplitude, w h i c h typica l ly requires twelve to fifteen layers. W h e n interlayer spacings are s m a l l , the number o f plane waves (beams) needed for ca lcu la t ion becomes large, this increases the size o f the d i f f rac t ion matrices . T h e p r o b l e m can be overcome us ing L - s p a c e methods w h i c h are more ef f ic ient f o r s m a l l interlayer spacings. In the C S M , s m a l l interlayer spacings resul t ing f r o m ad-atoms for e x a m p l e are treated i n the L - s p a c e representation, and the K - s p a c e representation is used for the m a i n part o f the crystal where interlayer spacings are suff ic ient ly large. 1.3.5 Evaluation of L E E D 1(E) Curves St ruc ture d e t e r m i n a t i o n s b y L E E D are c a r r i e d out b y c o m p a r i s o n o f exper imental 1(E) curves w i t h those ca lculated f o r a number o f plausible mode ls . T h e evaluat ion is done both v i s u a l l y and mathemat ica l ly by the use o f re l iab i l i ty indices or R-factors . A n R-factor is a dimensionless number that decreases as the correspondence between curves i m p r o v e s , such that R - » 0 as I t(E)-»Ie(E). There are current ly at least ten R-factors used i n L E E D crystal lography, each is sensitive to different features o f the 1(E) curves [5]. T h e analysis o f the Cu(311) 1(E) curves presented i n chapter 3 w i l l u t i l ize three o f these, w h i c h are n o w brief ly described. T h e R- fac tor R 2 [45] is sensitive to relative peak heights i n the 1(E) curves, and is def ined as where l e and It are experimental and theoretical intensities respectively, and the constant C, g iven by 1.25 c= 1.26 serves to normal ise the two curves, and the prefactor A2, g iven by 1 A 2 = 1.27 normalises R2 and renders it dimensionless. The Zanazzi-Jona R-factor [45,46] is sensitive to the general shape of the 1(E) curves and attempts to quantify all features evaluated in a visual comparison. For a single beam, Zanazzi and Jona have proposed | le , , -cl t "| | le , -cl t , | rzj = Azj 1.28 lle'l + maxle'l where and A ZJ= 1.29 0.027jledE c = JledE JltdE 1.30 is a scaling constant; the single and double primes represent first and second derivatives of intensity with respect to E. The many beam Zanazzi-Jona R-factor is defined by Ijrzj'AEi RZJ= 131 Z ;AE' where i runs over a l l the i n d i v i d u a l beams and AE' is the energy range o f the i t n beam. T h e R - f a c t o r p r o p o s e d b y P e n d r y uses the der ivat ive o f the l o g a r i t h m o f the intensities o f 1(E) curves, and is sensitive to relative peak posit ions. T h e P e n d r y m u l t i -beam R-factor [45,47] is def ined as where i f lYe 1 - Y ti)2dE Rpe = — 1 3 2 X j ( Y e i 2 + Ytj2)dE L-1 Y = 1.33 L-2 + V o i 2 and dlnl(E) L = 1.34 dE V 0 j is the imaginary component o f the m u f f i n - t i n constant potential used i n the mul t ip le scattering calculations. 1.4 Auger Electron Spectroscopy R o u t i n e surface c o m p o s i t i o n analysis under U H V was not poss ib le u n t i l the a v a i l a b i l i t y o f A u g e r e lectron spectroscopy [7] i n the late 1960's. T h e p h e n o m e n o n was n a m e d after Pierre A u g e r w h o first observed and correct ly interpreted it i n 1925 [48] . T h e process is s c h e m a t i c a l l y d e p i c t e d i n f i g u r e 1.9; i ts f i rs t step i n v o l v e s e" Figure 1.9 Schematic diagram of the de-excitation processes of atomic core holes. a) X-ray emission. b) Auger electron emission. ionization by either electron, photon or ion impact of an inner electronic energy level to form a core 'hole'. The system will then relax by an outer electron dropping into the core hole. The excess energy may be released either by X-ray emission, or by transference to a third electron (the 'Auger' electron) which leaves the atom with a kinetic energy characteristic of the element concerned. Thus AES can be used to identify all elements, with the exception of hydrogen and helium. The surface sensitivity of AES stems from the short mean free paths of electrons with energy in the 50-1000eV range (figure 1.5). Auger electrons tend to be accompanied by a large, slowly varying background of secondary electrons; as a consequence Auger electron spectra are conventionally viewed as a plot of the first derivative of the number of electrons versus energy. Figure 1.10 is an example of an Auger spectrum of contaminated copper from this work showing the energy distribution spectrum and its first derivative. Identification of peaks in an Auger electron spectrum is accomplished by comparing the peak position (taken by convention to be the position of the minimum in the derivative peak) to either standard spectra [49], or to tables of Auger emission energies [50]. Estimates of the relative amounts of an element on a surface can be provided by the ratio of an adsorbate AES peak height to that for a substrate peak. Absolute quantitative analysis requires a suitable calibration against a standard sample or some other technique, such as radiotracer analysis [9]. 31 Chapter 2 Experimental Methods 2.1 Vacuum Chamber and Pump down The experiments in this work were performed in a Varian 240 vacuum chamber made of demagnetized stainless steel. The various components are connected by knife edge flanges with copper gaskets. Figure 2.1 shows front and side views of the chamber, which is equipped with conventional hemispherical 4-grid LEED optics (Varian 981-0127) which also act as a retarding field analyzer for AES. Other facilities include an argon ion bombardment gun (Varian 981-1045) for sample cleaning, a nude ion gauge (Varian 971-0003) for pressure measurement and a molybdenum sample holder with a resistive heater (Varian 981-2058) and 0.005 inch chromel-alumel thermocouple wires mounted on a high precision manipulator (Varian 980-0523). The manipulator and sample holder keep the surface at the focus of the LEED screen and allow sample translation in three dimensions and rotation about two perpendicular axes. Stray magnetic fields are neutralized by mutually orthogonal Helmholtz coils [6,51]. The argon used in the cleaning procedures, and the gases for chemisorption experiments are admitted into the chamber through a variable leak valve (Varian 951-5106) via a nozzle from the gas line. The gas line is bakeable separately from the main chamber and has its own small 20 Is-1 ion pump (Varian 981-0200). Pressure in the gas line is monitored with two thermocouple gauges (Varian 972-0006) and the gauge on the small ion pump control unit Ultrahigh vacuum is achieved by a combination of pumps as shown in figure 2.2. Rough pumping from atmospheric pressure down to the IO - 2 torr range, is achieved by the liquid nitrogen cooled sorption pump. The liquid nitrogen trapped, water cooled oil diffusion pump (Varian M2) then takes the chamber into the 10"7 ton-range. At this stage, the system is baked at ~180°C for 15-20 hours with the main ion pump (200 Is-1) on to remove gases adsorbed on the inside of the chamber. After Titanium Sublimation Pump Manipulator Ion Gauge r 1—1 / N 7^ Optics Thermocouple Ion G a u g e ^ x ^ ^ Sample I Ar+ Ion Gun Window Variable Leak Valve ^7 Figure 2.1 The Varian 240 vacuum chamber and its associated hardware. - f e ^ f Electron — L P Gun Titanium Sublimation Pump Diffusion Pump Mechanical Pump Sorption Pump Experimental Chamber ^ G 200 1/s Ion Pump TCG J Gas Line TCG Leak Valve <$> <$> <g> 6 0 6 Glass Bulbs Figure 2.2 Pump and gas line configuration for the Varian 240 vacuum chamber. IG = Ion Gauge, TCG = Thermocouple Gauge bakeout, a l l the f i laments are degassed w h i l e the chamber is s t i l l hot. O n c o o l i n g , the pressure i n the chamber is around 2 x l 0 - 1 0 torr, this degree o f U H V can be mainta ined b y per iodic use o f the t i tanium subl imat ion p u m p . 2.2 S a m p l e P r e p a r a t i o n a n d C l e a n i n g T h e copper (311) and (110) samples used i n this study were prepared f r o m a 99 .999% pure s ingle crystal r o d (grown b y A . A k h t a r , D e p t . o f M e t a l l u r g y , U n i v e r s i t y o f B r i t i s h C o l u m b i a ) . T h e crystal was mounted o n a goniometer and oriented us ing the L a u e back ref lect ion method [52]. A f t e r orientation a sl ice o f about 1 m m thickness was cut by the spark eros ion technique ( 'Agie t ron ' , A g i e , S w i t z e r l a n d ) p a r a l l e l to the des ired crysta l lographic plane. T h e sample was set i n an acry l i c resin ( 'Quickmount ' , F u l t o n M e t a l l u r g i c a l Products C o r p . , U S A ) and g l u e d to a p o l i s h i n g j i g equipped w i t h a l ignment micrometers . T h i s j i g was p l a c e d o n a planetary l a p p i n g po l i sher ( D U 172, C a n a d i a n T h i n F i l m L t d . ) , and p o l i s h e d w i t h 9[im and 6u.m d i a m o n d pastes and the orientat ion was re -checked. T h e sample was then h a n d p o l i s h e d w i t h 3|im a n d l | i m d i a m o n d pastes o n ar t i f i c ia l deerskin (Buehler m i c r o c l o t h 40-7218). A f t e r p o l i s h i n g to opt i ca l f latness, the j i g was f i x e d o n an o p t i c a l bench and a H e / N e laser was used to ensure that the opt ica l face and the desired plane were w i t h i n 0 . 2 ° , as s h o w n i n f igure 2.3. U p o n satisfactory or ientat ion, the res in was d i s s o l v e d i n acetone and the sample was degreased i n tr ichloroethylene and washed consecut ively i n acetone, methanol and d i s t i l l e d water . W h e n d r y , the c r y s t a l was m o u n t e d o n the m a n i p u l a t o r a n d the thermocouple wires were spot w e l d e d to the sample c u p . T h e manipulator was p laced ins ide the v a c u u m chamber , a n d the sys tem was p u m p e d d o w n as descr ibed i n the preceding section. Figure 2.3 Schematic diagram of the laser method used to check correspondence of the optical and crystallographic planes. ON The main impurities on both surfaces were carbon, chlorine and sulphur as determined by AES. Surface cleaning within UHV was achieved by sputter-etching with argon ions. Initially ~5xl0" 5 torr of argon was admitted into the chamber. These argon atoms were then ionized by electrons from a hot filament and accelerated toward the earthed sample by a potential of 400V with a current density of 2.0|iA/cm2. Atoms thus sputtered from the surface were removed using the TSP which has a negligible pumping speed for the rare gases. After 2 hours, the bombardment was stopped and the argon was pumped away with the diffusion pump and the main ion pump. This initial gentle bombardment was followed by an anneal for 15 minutes at 550°C and was sufficient to remove most of the surface contaminants left over from the preparation and cleaning procedures outside of UHV. Subsequent cleaning was done with a higher argon ion gun potential of 590V for 2-4 hours, with anneals up to 650-700°C for 10-15 minutes. AES showed that carbon and chlorine were removed fairly easily, but the high temperature anneal caused sulphur to segregate from the bulk. Many hours of bombard-anneal cycles were needed before the bulk region close to the surface was depleted of sulphur to the extent that the surface sulphur contamination was below the detection limit of the RFA, and hence of the order of 1-2% of a monolayer [53]. As the surfaces became cleaner and more ordered, the annealing temperature was lowered somewhat to around 550-600°C. The cleaned surfaces then gave sharp (lxl) LEED patterns, with low background, free of fractional beams and streaks. 2 .3 A E S f o r S u r f a c e C o m p o s i t i o n A n a l y s i s In this work, AES was used to qualitatively monitor the surface composition with the LEED optics acting as a RFA [54] as shown in figure 2.4. A primary electron beam of 2keV and lOuA with a 1mm2 spot size was directed at the sample. A linear, G I Grids w G2 Sample Vp+ksincot \:VA Electron Gun w ••V.! I . , V P Gun •:::.;* / Control Figure 2.4 Configuration of the LEED optics as a retarding field analyser for AES. negative potential (Vr) difference ramp was applied between the earthed sample and grids G2 and G3 so that only electrons with sufficient energy were collected by the screen. The resulting spectrum of total number of electrons reaching the screen versus energy was then electronically doubly differentiated (3). This was achieved by modulating the applied potential with a small (-1-10V) sinusoidal signal of frequency CO (1250 Hz) and recording the collector current signal with the lock-in amplifier (PAR-128) at the second harmonic frequency, 2co [7]. This gives the derivative of the electron energy distribution, and has the effect of flattening out the slowly varying background and enhancing the small peaks caused by Auger electron emission. 2.4 Experimental L E E D 2.4.1 L E E D Optics and Electron Gun Figure 2.5 is a schematic diagram of the 4-grid LEED optics and electron gun (Varian 951-2125) used in this work. Electrons emitted from a heated filament are collimated by the electrostatic lens and they leave the drift tube with an energy determined by the potential difference between the filament and the earthed sample. The drift tube is also earthed so as to provide a field free space for the electron beam. Typical beam energies used were 50-250eV with a current which varied linearly between -O.lnA at 40eV and - l^tA at lOOeV, and was constant at higher energies. This beam current as a function of energy was recorded following each measurement in order to normalise the LEED spot intensities. The diffraction pattern is formed by the electron beams on the fluorescent screen after passage through the four concentric hemispherical grids labelled G1 to G4 in figure 2.5. Grid G1 is grounded so that backscattered electrons traverse a field free Grids ure 2.5 Simplified diagram of the LEED optics used for displaying diffraction patterns. space f r o m the sample to G 1 . G r i d s G 2 a n d G 3 are at a negative potential ,a f e w e V or so less than the inc ident e lectron energy to suppress inelast ic electrons. G r i d G 4 is earthed i n order to sh ie ld the suppressor grids f r o m the h i g h vol tage o f the co l lec tor screen ( ~ 5 k V ) . T h i s h i g h potent ial accelerates the e last ical ly scattered electrons f r o m G 4 onto the screen. 2 .4 .2 L E E D S p o t I n t e n s i t y M e a s u r e m e n t s T h e L E E D spot intensities were measured as a f u n c t i o n o f inc ident e lectron beam energy us ing a v i d i c o n T V camera ( C o h u 4410 IS IT) and a v ideo L E E D analyzer ( V L A ) f r o m D a t a - Q u i r e [55]. T h e set up is s h o w n schemat ica l ly i n f igure 2.6. T h e v i d i c o n camera sends an i m a g e o f the w h o l e L E E D screen to a m o n i t o r a n d to the m i c r o p r o c e s s o r ( M C 6 8 0 0 ) v i a an a n a l o g u e to d i g i t a l ( A / D ) c o n v e n o r . T h e microprocessor also controls the L E E D g u n v i a a d i g i t a l to analogue ( D / A ) converter . T h e image o f the L E E D pattern o n the m o n i t o r defines a v i d e o f rame cons i s t ing o f 2 5 6 x 2 5 6 p i x e l s w h o s e intensities can vary between 0 and 255. A d i f f rac t ion spot is covered b y a 10x10 p i x e l ' w i n d o w ' under computer contro l and its intensity is obtained b y a d d i n g the intensit ies o f the p i x e l s i n the w i n d o w . T h e computer contro ls the m o v e m e n t o f the w i n d o w d u r i n g data a c q u i s i t i o n so that i t tracks the path o f the d i f f rac t ion spot as the energy i s increased. T h e V L A is capable o f recording up to 49 spots s imultaneously, but since some beams, especial ly the fractional beams, have very m u c h smaller m a x i m u m intensities than other beams, w h i c h c o m b i n e d w i t h the problem o f obtaining adequate t racking f o r each spot, make i t advisable to record different beam types separately, w i t h different camera ga in l eve ls , i n order to m i n i m i z e b a c k g r o u n d effects. T h e 1(E) curves are then normal ized according to the expression T.V. Monitor A/D Convenor Video Signal ISIT T.V. Camera Lens I = Sample / Gun X-Y Recorder Scope Motorola 6800 Terminal D/A Convertor LEED Control Unit Floppy Disks Modem Mainframe Computer Figure 2.6 Block diagram of the video LEED analyser (VLA). where I' and l 0 are measured dif fracted and incident beam currents respect ively. W h e n a surface under inves t igat ion has po in t s y m m e t r y elements , and the d i r e c t i o n o f the incident beam is appropriately chosen, the diffract ion pattern m a y contain symmetr ica l ly re lated spots. T h e procedure u s e d is to average the 1(E) curves m e a s u r e d f o r the s y m m e t r i c a l l y related beams; this helps to i m p r o v e the spectra by averaging out s l ight inhomogenei t ies [56] and r e d u c i n g the noise , as s h o w n i n f igure 2.7. S u c h curves are then smoothed t w i c e u s i n g a c u b i c sp l ine rout ine , a n d i f r e q u i r e d , a b a c k g r o u n d subtraction is made . T h e V L A used i n this w o r k had the o p t i o n o f insert ing a delay between each measurement. T h e purpose o f this delay is to ensure that the d i f f rac t ion spot has stopped m o v i n g w h e n its intensity is recorded. F o r a one second de lay , this causes the surface to be under the electron beam for about ten times as l o n g as w h e n no delay is u s e d T h i s increase i n length o f exposure to the electron beam is not a p r o b l e m w h e n the surface is stable under it , but for C u ( l 1 0 ) - ( 2 x l ) - O , the di f f ract ion pattern was degraded quite q u i c k l y w h e n the electron beam energy was above 2 0 0 V . 2.4.3 L E E D Spot Profile Measurements A d d i t i o n a l i n f o r m a t i o n regarding surface order can be obta ined b y measur ing the intensity d is t r ibut ion across a g i v e n d i f f rac t ion spot. T h e g r o w t h o f chemisorpt ion layers o n a surface, f o r e x a m p l e , w i l l i n e v i t a b l y be i n i t i a t e d at m a n y places o n a surface, w h i c h m a y lead to the f o r m a t i o n o f antiphase domains [27,57]. I f the size o f these domains is less than the transfer w i d t h o f the instrument [33], then the observed di f f rac t ion pattern w i l l have spots whose intensity distr ibut ion contains in format ion o n the d o m a i n structure, as w e l l as step d is t r ibut ion , chemisorbed i s l a n d sizes etc. Spot 40 60 80 100 120 140 160 180 200 220 240 260 Electron Energy (eV) Figure 2.7 1(E) curves for three symmetrically related beams from Cu(110)-(2x1 )-0 and two from Cu(311). These equivalent beams are averaged, smoothed, and if needed, the background is subtracted. p r o f i l e measurements are ach ieved w i t h the V L A by p l a c i n g a 10x10 p i x e l w i n d o w complete ly over the spot to be analysed as s h o w n i n f igure 2.8a. T e n scans are needed to dig i t ize the spot (figure 2.8b). T h e intensity pro f i l e a long the X w i n direct ion is then obta ined b y j o i n i n g the m a x i m a o f the ten prof i l es . T h e f u l l w i d t h at h a l f m a x i m u m ( F W H M ) o f this spot p r o f i l e gives a quantitat ive measure o f spot sharpness [55]. In this w o r k , the R F A was not sensitive enough to record A u g e r spectra o f o x y g e n o n the C u ( l 10) surface. Spot pro f i l e analysis o f f rac t ional beams f r o m the C u ( l 1 0 ) - ( 2 x l ) - O system was thus essential to determine the o p t i m u m o x y g e n exposure required to g ive the best di f fract ion pattern. Figure 2.8 LEED spot profile analysis: a) a user selected 10x10 pixel window is placed over the diffraction spot b) a LEED spot profile; the FWHM gives a measure of spot sharpness. Chapter 3 Studies on the Copper (311) Surface 3.1 Introduction T h e conceptual s i m p l i c i t y o f l o w M i l l e r i n d e x surfaces has resulted i n a great d e a l o f research c o n d u c t e d o n t h e m . T h e r e a c t i v i t y o f ' real ' surfaces h o w e v e r i s thought to arise large ly f r o m the defect structures o f the w i d e var iety o f c rys ta l faces they exhibi t . O n h i g h M i l l e r i n d e x surfaces, the steps and k i n k s p r o v i d e reproducib le defects w h o s e character is t ics are f i x e d f o r a g i v e n surface . T h e y have b e c o m e important theoret ica l ly and exper imenta l ly as models to br idge the gap between the r e l a t i v e l y w e l l unders tood b e h a v i o u r o f l o w M i l l e r i n d e x surfaces a n d the p o o r l y understood surfaces o f real polycrysta l l ine materials. T h e face centered cubic (311) surface is one o f the s implest h i g h M i l l e r index surfaces, w i t h a w e l l de f ined step and terrace structure. F i g u r e 3.1 gives the ideal bu lk truncation, together w i t h its corresponding reciprocal net (i.e. L E E D pattern). It has the c m two dimensional space group, and can thus be represented b y a centered rectangular net [24]. T h i s surface is c o m p o s e d o f steps w i t h (100) and (111) components , as can be deduced f r o m its alternative designations w i t h the step and microface t notat ions: 2 ( l l l )x(100) or 2 (100)x( l l l ) and 2 i ( 1 0 0 ) + l i ( l l l ) respect ively . T h e d iper iod ic i ty o f this surface m a y also be represented b y an ob l ique p r i m i t i v e net, whose unit mesh is sufficiently smal l to make it convenient for a mult iple scattering L E E D analysis. T h e c lean copper (311) surface has been studied p r e v i o u s l y i n this laboratory [58,59,60]. I n that s tudy, a r e laxa t ion o f the f i rs t in ter layer s p a c i n g was the o n l y structural parameter a l l o w e d to v a r y i n m a t c h i n g e x p e r i m e n t a l a n d ca lcu la ted 1(E) curves . T h e resul t ing contract ion o f -5.0±1.5% f r o m the b u l k interlayer spac ing was w e l l b e l o w that predic ted by theory [61] a n d the result was a lso a n o m a l o u s w h e n c o m p a r e d to other f.c.c.(311) surfaces [62]. T h e r e l a t i v e l y o p e n (311) and (110) surfaces o f A l and N i as w e l l as the (110) faces o f other f .c .c . metals , have been shown - [233] Figure 3.1 a) The real space ideal bulk truncation representation of f.c.c.(311) • • • • • • • (2,-4) (2,-3) (2,-2) (2,-1) (2,0) (2,1) (2,2) (1*3) (1*2) (1*1) (1*0) (1*1) (1*2) (1*3) (0*3) (0*2) (0*1) (0*0) (0*) (0*2) (0*3) (-1*2) (-1*1) (-1*0) (-1*1) (-1*2) (-1*3) (-1*4) (-2*2) (-2*1) (-2*0) (-2,1) (-*2) (-2*3) (-2,4) b) The reciprocal space f.c.c.(311) net (LEED pattern). by L E E D [63-66] and other techniques [67] to e x h i b i t m u l t i l a y e r re laxat ions o f the topmost in ter layer spac ings . These effects are m o r e p r o n o u n c e d f o r m o r e o p e n surfaces. R e c e n t l y a re-analys is o f the o r i g i n a l C u ( 3 1 1 ) data, w h i c h cons is ted o f 8 s y m m e t r i c a l l y inequivalent beams, u s i n g m o r e sophist icated software w h i c h a l l o w e d considerat ion o f these m u l t i l a y e r relaxations was p e r f o r m e d [68]. T h e magnitudes o f these d a m p e d osc i l l a tory compress ions a n d expans ions , as g i v e n i n table 3 .1 , were again considerably less than the values expected f r o m theory [69,70] and i n compar ison to other f .c.c.(311) surfaces. T h e o r i g i n a l C u ( 3 1 1 ) study was the f i rs t one p e r f o r m e d i n the V a r i a n 240 v a c u u m cha m b e r i n this laboratory w h e n the surface science group was s t i l l i n its in fancy , and was the first analysis o f a h i g h M i l l e r index surface made anywhere us ing the d y n a m i c a l theory o f L E E D . A s w e l l as testing theories for c lean surfaces, another reason f o r u n d e r t a k i n g n e w w o r k o n the C u ( 3 1 1 ) surface is to attempt structural invest igat ions w h e n it contains adsorbates. N o quanti tat ive i n f o r m a t i o n exists f o r structural arrangements f o r m e d b y chemisorpt ion o n stepped surfaces. These factors p r o v i d e d the impetus for a thorough re-investigation o f the s y s t e m 3.2 E x p e r i m e n t a l W o r k o n t h e C l e a n C o p p e r (311) S u r f a c e T h e c o p p e r (311) sample u s e d i n this w o r k was p r e p a r e d i n the m a n n e r descr ibed i n sections 2.1 and 2.2 . I n i t i a l A u g e r spectra ( f igure 3.2a) i n d i c a t e d the presence o f su lphur , c h l o r i n e and c a r b o n . T h e r e c o u l d also have been substantial amounts o f o x y g e n present, but the i n s e n s i t i v i t y to adsorbed o x y g e n o f the L E E D opt ics used as the A u g e r e lec tron analyser , i n c o m m o n w i t h other R F A ' s [53,71] , meant that it c o u l d not be detected o n a semi-quantitative l eve l . A s the Cu(311) surface became p r o g r e s s i v e l y c leaner, L E E D patterns w i t h streaks corresponding to par t ia l Table 3.1 Current crystallographic data for the (311) surfaces o f var ious f .c.c. metals. Surface d B ( A ) d i % d 2 % d 3 % R-factor R e f . Al(311) 1.22 -13.3 +8.8 0 .0 R 2 , R Z J 6 6 Cu(311) 1.09 -5 R Z J 58 Cu(311) 1.09 -7.3 +3.7 0 .0 R 2 , R P E , R Z J 68 Ni(311) 1.06 -15.9 +4.1 -1.6 R 2 65 Theory -12.2 +4.4 -1.7 6 9 , 7 0 11 chlorine 50 100 150 200 250 300 Energy (eV) Figure 3.2 Auger spectra of the Cu(311) surface. a) Prior to cleaning in UHV. b) After cleaning in by argon ion bombardment/anneal cycles. ordering along the steps were observed, as had been reported previously [60]. This observation led to the sulphur adsorption experiments of section 3.3. After repeated bombard/anneal cycles, the concentrations of all surface contaminants were reduced to below the detection limit of the RFA as shown in figure 3.2b. Intensity versus energy curves were recorded with a one second delay between each measurement for 14 beams including eight equivalent pairs at normal incidence for two separate experiments and a set of six 1(E) curves, including three equivalent pairs, were taken at 10° off normal along the [233] azimuth so as to preserve the mirror plane in the LEED pattern. Table 3.2 gives the energy ranges over which LEED spot intensities were recorded for the Cu(311) surface, and the 1(E) curves are shown in figures 3.3a-3.3t. 3.3 Adsorption of Sulphur on Cu(311) 3.3.1 Introduction The adsorption of sulphur on the low Miller index faces of copper has attracted a great deal of attention from LEED crystallographers, and surface scientists in general [72,73]. In contrast, there have been few such studies of adsorption on the high Miller index surfaces of any metal [74]. The importance of such studies however, is evident from the role that sulphur is known to play in heterogeneous catalysis as a poison and as a catalyst pre-treatment agent [1]. A discussion of the experimental and theoretical difficulties of adsorption on stepped surfaces will be left until section 3.4. Table 3.2 Beam indices and energy ranges (eV) for the experimental Cu(311) LEED 1(E) curves. Beam Indices Normal 1 Normal 2 Off-normal (l,-3),(-l,-2) 120 - 248 120 - 248 74- 248 (l,-2),(-l,-D 66 - 248 62 - 248 50- 218 (1,-D,(-1,0) 50 - 248 50 - 248 (1,0),(-1,1) 50 - 248 80 - 248 50- 248 (1,1),(-1,2) 50 - 248 80 - 248 (1,2),(-1,3) 120-248 120 - 248 (0,-3) 140 - 246 180-246 60- 246 (0,-2) 110-246 104 - 226 (0,-1) 50 - 246 50 - 246 (0,2) 50 - 246 50 - 246 (0,1) 50-116 50-116 50- 246 (0,3) 100 - 246 100 - 246 (2,-2),(-2,0) 150-248 192 - 248 (2,-l),(-2,l) 140-212 140 - 248 (0,0) 90- 246 a) (1,-3) and (-1,-2) beams at normal incidence. 1 st expt. 2nd expt. ave. 40 6C 80 100 120 140 160 180 200 220 240 260 Energy (eV) 1 1 1 1 1 1 — • 1 b) (1,-2) and (-1,-1) beams at normal incidence. 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) Figure 3.3 Experimental 1(E) curves recorded for the clean Cu(311) surface. 1 1 1 1 1 1 r— c) (1,-1) and (-1,0) beams at normal incidence. 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) d) (1,0) and (-1,1) beams at normal incidence. ' I I 1 st expt. / \ — 2nd expt. ' ' ^ ^ ^ ^ ^ ave. V _ ^ _ ^ ^ / \ — I I I I I I l l I i 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) Figure 3.3 cont. Cu(311) experimental l(E) curves. 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) 40 50 80 100 120 140 160 180 200 220 240 260 Energy (eV) Figure 3.3 cont. Cu(311) experimental l(E) curves. g) (0,-2) beam at normal incidence. 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) 40 60 80 100 120 120 140 160 180 200 220 240 260 Energy (eV) Energy (eV) Figure 3.3 cont. Cu(311) experimental l(E) curves. k ) ( 0 , 2 ) b e a m a t n o r m a l i n c i d e n c e . 1 1 1 1 1 V i s t e x p t . A 2nd expt. / A \ /^ -. a v e . — / — 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) Figure 3.3 cont. Cu(311) experimental l(E) curves. m) (2,-2) and (-2,0) beams at normal incidence. ave CE >-t— 01 n) (2,-1) and (-2,1) beams at normal incidence. 1st expt 2nd expt ave. CD cr CL >-t— cn 140 160 180 200 220 240 260 20 140 Energy (eV) 160 180 200 220 240 260 Energy (eV) Figure 3.3 cont. Cu(311) experimental l(E) curves. 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 2 2 0 Energy (eV) 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) Figure 3.3 cont. Cu(311) experimental l(E) curves. 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) CD 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) in 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) Figure 3.3 cont. Cu(311) experimental l(E) curves. 3.3.2 Experimental T h e streaked di f f ract ion patterns caused b y segregation o f sulphur f r o m the b u l k copper c o u l d not be i m p r o v e d appreciably b y longer o r h igher temperature anneal ing. T h i s was ref lected i n the sulphur AES peak at 1 5 2 e V , w h i c h d i d not increase i n height f o r anneals o f m o r e than 10-15 rninutes. T o investigate whether adsorpt ion o f s u l p h u r o n the Cu(311) surface w o u l d produce a L E E D pattern that was analyzable , a series o f experiments w h e r e b y su lphur was adsorbed onto the C u ( 3 1 1 ) surface b y d o s i n g w i t h H2S, f o l l o w e d b y heat ing to g i v e the p r e s u m e d desorpt ion o f H2 were conducted . In this manner, the exposure condit ions were o p t i m i z e d such that a L E E D pattern w h i c h appeared to consist o f (2x3)S and (2x5)S components was observed. T h e c lean Cu(311) surface was exposed at r o o m temperature to h i g h pur i ty H2S (Matheson Research Grade) at 2 x l 0 * 8 torr. D u r i n g d o s i n g , the p r e v i o u s l y sharp ( l x l ) L E E D pattern was observed to degrade considerably . T h e integral spots became more d i f f u s e w i t h streaks j o i n i n g t h e m i n the [233] d i r e c t i o n a n d h a l f w a y between the integral beams, also i n the [233] direct ion. A n n e a l i n g o f this surface for a f e w rninutes at 3 0 0 ° C considerably reduced the background and part ia l ly ordered the streaks to f o r m an i l l -def ined di f f ract ion pattern. T o f i n d the o p t i m u m H2S dose, the L E E D pattern was observed after anneal ing for a series o f exposures, and sulphur uptake curves , measured b y R = A 1 5 2 / A 9 2 0 i .e . the ratio o f the A E S peak-to-peak height f o r S at 152eV to that o f C u at 9 2 0 e V , as a func t ion o f exposure were plot ted (figure 3.4). T h e C u peak at 9 2 0 e V was chosen as it corresponds to a re la t ive ly deep core hole and so i t is less sensit ive to the presence o f s t Figure 3.4 Auger peak height ratio R = s( 1 5 2 e V)/Cu(920eV) plotted as a T i m e (minutes) function of H2S exposure time at 2x10"8 torr. ON adsorbed o v e r l a y e r s . T h e best d i f f r a c t i o n patterns w e r e o b s e r v e d f o r va lues o f R between about 2.5 a n d 3 , w h i c h corresponded to an exposure o f 3-4 minutes at 2 x l O s torr H2S. W i t h h igher exposures the pattern became v e r y d i f fuse and n o n e w L E E D patterns were observed after annea l ing . In a l l cases, the patterns p r o d u c e d b y d o s i n g w i t h H2S were superior to those caused b y sulphur segregation f r o m the b u l k . T h i s w a s p r e s u m a b l y due to the h i g h e r surface concentra t ion , as r e v e a l e d b y A E S , o f sulphur obta ined o n d o s i n g . 3.3.3 Interpretation of the L E E D pattern for sulphur on Cu(311) P h o t o g r a p h s o f the L E E D patterns o b t a i n e d after d o s i n g w i t h H2S a n d anneal ing at 3 0 0 ° C are s h o w n i n f igure 3 .5a-3 .5d. N e w spots i n the L E E D pattern were observed i n the [233] d i r e c t i o n , bo th a l o n g the r o w s o f integra l spots a n d also h a l f w a y between these r o w s . T h e sharpest o f these n e w spots were at V3 and 2/3 o f the distance between integral spots, and so w o u l d be l a b e l l e d (h,k) where h i s an integer a n d k = {..."5/3,*4/3,"2/3r1/3,1/3,2/3...}. T h i s i m p l i e d the presence o n the surface o f regions o f (2x3) translational symmetry . A t certain energies h o w e v e r , s t i l l m o r e spots were seen i n the r o w s o f [233] d i r e c t i o n , f o r e x a m p l e , at 6 2 e V there were spots o n either side o f the (1 ,"5/3) spot w h i c h can not be interpreted as b e i n g (2x3) , but w h o s e separation a n d p o s i t i o n were consistent w i t h a (2x5) pattern. B e t w e e n r o w s o f integral spots, the n e w di f f rac t ion spots were d i f fuse a n d tended to f o r m streaks. A t 6 2 e V a n d 6 7 e V h o w e v e r , pairs o f spots c o u l d be seen w h o s e separation was about a f i f t h o f the integral b e a m separation, again consistent w i t h a (2x5) d i f f rac t ion pattern. F r o m these observations i t is reasonable to suggest that the observed pattern contained components f r o m (2x3) , (2x5) a n d a lso (2x1) ordered regions o f the surface (any f rac t iona l order spots f r o m (2x1) regions w o u l d be i n the same posi t ions as those caused b y di f f rac t ion 66 b) 67eV Figure 3.5 Photographs of the LEED pattern for sulphur adsorbed on Cu(311). d) 142eV Figure 3.5 cont. Photographs of the Cu(311) - sulphur system. o f f (2x3) and(2x5) regions) . M o o r e [60] reported a s i m i l a r c o m b i n a t i o n o f d i f f rac t ion patterns o n N i ( 3 1 1 ) f o r su lphur that h a d d i f f u s e d f r o m the b u l k to the surface d u r i n g anneal ing . T h e occurence o f these compos i te patterns is perhaps not surpr is ing w h e n one examines the f .c.c.(311) surface a n d speculates o n where adsorpt ion c o u l d occur . It is clear that sulphur adsorbs a long the terraces at every second site (as is observed for sulphur o n the Cu(100) surface where i t occupies the f o u r f o l d cc<miinate site to g ive a (2x2)S L E E D pattern [73]), since the reported patterns a l l h a d a s ingle r o w o f fract ional spots h a l f w a y between r o w s o f integral spots i n the [ O i l ] d i rec t ion , though the actual site c o u l d not be ascertained from the d i f f r a c t i o n pattern a lone . T h e source o f the streaks a n d the apparent superposi t ion o f d i f f rac t ion patterns p r o b a b l y arises f r o m the fact that f o r adsorbates o n di f ferent r o w s , there are t w o s y m m e t r i c a l l y equiva lent adsorpt ion sites, as s h o w n i n f i g u r e 3 .6a , w h i c h w i l l l e a d to some randomness o f adsorbate p o s i t i o n between terraces. T h e ac tua l s i tuat ion t h o u g h i s rather m o r e c o m p l e x , as r e a l f .c .c . (311) surfaces have m a n y s u c h terraces. T h i s i n turn w o u l d create L E E D spots w h i c h are s t reaked a l o n g the [233] d i r e c t i o n . T h e genera l dif fuseness o f the d i f f r a c t i o n spots is p r o b a b l y due to the s ize o f the ordered regions . S i n c e sites a a n d b i n f igure 3 .6a are equivalent , l o n g range order is u n l i k e l y , a n d therefore the d i m e n s i o n o f order ing i n the [233] a z i m u t h w i l l be less than the transfer w i d t h W . F i g u r e s 3 .6b-3 .6d s h o w some plaus ib le arrangements f o r sulphur adsorbed o n Cu(311) w h i c h w o u l d g ive r ise to ( 2 x l ) , ( 2 x 3 ) and (2x5) d i f f r a c t i o n patterns. 1(E) curves were not recorded f o r this C u ( 3 1 1 ) - s u l p h u r sys tem as the pattern was not o f h i g h enough qual i ty f o r re l iable data to be obta ined, and f o r reasons to be discussed i n the next section. 69 a) The two equivalent adsorption sites between adjacent terraces. b) Possible arrangements of sulphur on Cu(311) which give (2x1) LEED patterns. Figure 3.6 The adsorption of sulphur of the Cu(311) surface. 3.4 Some Considerations for Adsorption on Stepped Surfaces I n the layer d o u b l i n g a n d RFS methods , d i f f r a c t i o n matr ices o f the substrate a n d adsorbate layers c a n be c a l c u l a t e d independent ly p r o v i d e d that the in ter layer s p a c i n g between t h e m is s u f f i c i e n t l y large (>0.6A a n d 1.0A r e s p e c t i v e l y ) . These matr ices , represent ing d i f f r a c t i o n f r o m the layers m a y then be s tacked to generate ca lcula ted d i f f r a c t i o n beams f r o m the w h o l e surface r e g i o n . T h e general openness o f stepped surfaces h o w e v e r , leads to s m a l l in ter layer spacings both f o r the substrate layers and the substrate-adsorbate layer. T h e former is a result o f the angle at w h i c h the surface i s cut w i t h respect to the b u l k uni t c e l l vectors , a n d can be v i s u a l i z e d f r o m k n o w l e d g e o f the M i l l e r indices , s ince these are propor t iona l to the rec iprocals o f the points at w h i c h the crys ta l lographic p lane intercepts the unit c e l l vectors. Substrate-adatom layer spacings w i l l tend to be s m a l l as the most favourable places for adatoms to s t ick w i l l be i n h i g h coordinat ion number sites adjacent to the steps (and Idnks) o n the surface. T h e i m p l i c a t i o n for m u l t i p l e scattering analyses o n stepped surfaces is that the topmost b u l k layer and the adsorbate layer must often be treated as a single entity w i t h i n the c o m b i n e d space approach . O f t e n large uni t meshes m a y be i n v o l v e d , but this w o u l d severely h i n d e r the a p p l i c a b i l i t y o f d y n a m i c a l scattering ca lcula t ions to such systems, g i v e n the c o m p u t i n g resources at hand . T h e c o m p l e x i t y o f the calculat ions i n a l l m u l t i p l e scattering methods current ly i n use increases w i t h the number o f atoms i n the unit m e s h [5]. T h i s means that study o f adsorpt ion o n stepped surfaces w i l l p lace great constraints u p o n researchers as regards computer time, cost and storage. Severa l schemes have been p r o p o s e d that c a n reduce these requirements . T h e ' b e a m set neglect ' ( B S N ) [75] a n d 'unit c e l l r e d u c t i o n ' methods [76] c o n s i d e r o n l y a s m a l l , care fu l ly chosen subset o f the di f fracted beams d u r i n g analysis . T h e B S N f o r example i m p r o v e s the computa t ion t ime b y a factor o f N o v e r the convent iona l generation a n d stacking o f layer d i f f rac t ion matrices, where N is the number o f atoms i n the unit mesh. In Tensor L E E D , b y break ing d o w n the scattering p r o b l e m into a structure factor and a f o r m factor , such that i t becomes analogous to X - r a y scattering [77] , advantages i n computer t ime over convent ional methods have been s h o w n to approach three orders o f magni tude . I n a d d i t i o n , e f f i c ient use o f s y m m e t r y i n both real a n d r e c i p r o c a l spaces have recently enabled analysis b y d y n a m i c a l L E E D o f the S i ( l 1 l ) - ( 7x7 ) reconstructed surface [78]. A s w e l l as m a k i n g an analysis m o r e d i f f i c u l t , the actual acquis i t ion o f data becomes progress ive ly harder as the unit mesh size increases. T h i s is because the spacing between dif fract ion spots is inversely proport ional to the geometrical size o f the surface uni t c e l l , and therefore emphasises the need f o r h igher reso lu t ion instruments [79]. I n v i e w o f the d i f f i c u l t i e s c o n c e r n i n g q u a n t i t a t i v e L E E D a n a l y s i s f o r c h e m i s o r p t i o n o n stepped surfaces, and the fact that i t is an area as yet u n e x p l o r e d b y d y n a m i c a l L E E D , any sys tem to be s tudied at present s h o u l d satisfy certain cr i ter ia . F i r s t l y , preparat ion o f the surface i tsel f and adsorption o f gases needs to be carr ied out v e r y care fu l ly so as to be sure that the observed d i f f rac t ion pattern is genuine ly sharp and not a composi te o f patterns f r o m different arrangements o f the same adsorbate, as was o b s e r v e d f o r su lphur o n the C u ( 3 1 1 ) surface. S e c o n d l y , the u n i t m e s h o f the substrate needs to be as s m a l l as p o s s i b l e ( c o r r e s p o n d i n g l y the in ter layer s p a c i n g s h o u l d be as large as possible) i n order that a perturbative d y n a m i c a l m e t h o d m a y be ava i lab le f o r the L E E D intensity ca lcula t ions . T h i r d , the uni t m e s h o f the substrate-adsorbate system s h o u l d be as s m a l l as poss ible since the number o f beams required i n a c a l c u l a t i o n (and hence the d i m e n s i o n o f square matr ices i n v o l v e d ) increases i n p r o p o r t i o n to the uni t m e s h area [44]. Therefore the i d e a l systems f o r inves t igat ing adsorpt ion o n stepped surfaces w o u l d s h o w ( l x l ) o r (2x1) d i f f rac t ion patterns. T a b l e 3.3 g ives some characterist ics o f the seven s imples t f . c . c . s tepped surfaces that are pertinent to L E E D crystal lography. W i t h specif ic regard to stepped surfaces o f copper , the C u ( 2 1 0 ) - ( 2 x l ) - O partem reported b y M c K e e et. a l . [84] seems erninently analyzable b y d y n a m i c a l L E E D . T h i s surface has l o n g br idge sites that are not d i s s i m i l a r to those f o u n d o n the C u ( l l O ) surface (see chapter 4) . T h e f .c .c . (210) surface t h o u g h , i n c o m m o n w i t h other surfaces w i t h o b l i q u e uni t meshes , m a y suf fer f r o m the stat ist ical randomness o f a d s o r p t i o n site w h i c h was seen f o r C u ( 3 1 1 ) - s u l p h u r , l e a d i n g to o n e - d i m e n s i o n a l order ing . In that respect, stable and unique adsorbate arrangements are u n l i k e l y unless some f o r m o f surface reconstruct ion occurs o r unless ( l x l ) structures are poss ible w i t h the adsorbed species. W i t h this i n r n i n d , perhaps the best f .c .c . stepped surfaces f o r L E E D crys ta l lographic analyses w o u l d be the (211), (221) a n d (310) faces, as these have rectangular unit meshes. T h e C u ( 4 1 0 ) - ( l x l ) - O a n d C u ( 4 1 0 ) - ( l x l ) - 2 O patterns reported b y M i l n e [89], A l g r a e t a l . [90] and T h o m p s o n et. a l . [91], though apparently w e l l - d e f i n e d , w o u l d not be amenable to d y n a m i c a l L E E D analys is due to the s m a l l interlayer spacings. O n h i g h M i l l e r i n d e x surfaces o f b o d y centered c u b i c (b.c.c.) and hexagonal c lose p a c k e d (h.c.p.) lattices, the same pr inc ip les and suggestions made f o r f . c . c . surfaces s h o u l d a lso a p p l y . T a b l e s 3 .4a a n d 3.4b g i v e some characterist ics h e l p f u l to L E E D crystal lographers f o r the s implest b .c .c . and h .c .p . h i g h M i l l e r index surfaces respect ively w h i c h possess rectangular unit meshes. It should be noted i n the case o f b .c .c . and h .c .p . crysta l lattices however , that not a l l h i g h M i l l e r i n d e x surfaces are i n fact stepped. A l s o some h . c .p . surfaces are not u n i q u e , such that f o r a g i v e n M i l l e r index , there m a y be t w o possible surface structures as a result o f the 'non-unique plane ' p r o b l e m . T h i s i n r u m w o u l d further compl i ca te a ca l cu la t iona l analysis so the best h . c . p . surface to s tudy w o u l d p r o b a b l y be the (110) surface . A l t h o u g h this T a b l e 3.3 Characterist ics o f the seven simplest f .c .c . stepped surfaces. Uni t mesh Notation 0 Dimensions K n o w n simple adsorption systems Refs. 3 ^ 311 2<100)x(lll) ll(100)+ll(lll) a = r b = V3r 6 = 106.78* A =1.6612 d = 0.426r Au: (5x3)Pb, (3x3)Pb Cu: (4x2)Pb Ni: (2xl)S, (2x3)S,(2x5)S P ± (2xl)CO [ 8 0 ] [ 8 1 ] [ 6 0 ] [ 8 2 ] a t f YJ J 210 2(100)x(110) ll(100)+li(110) a=V2r b = \3r 9=114.09* A = V5r2 d = 0.316r Au: (lxl)Pb Cu: (2x1)0, (3x1)0, (2x3)N c(lW2xv2)R45*N Ni* c(l W2XY2)R45*N, (2x3)N Pd: (lxl)CO, (lx2)CO [ 8 3 ] [ 8 4 ] [ 8 5 ] [ 8 5 ] [ 8 6 ] 331 2(110)x(lll) 2i(110)+li(lll) a = r b = \5r 0 = 102.92* A = 2r2 d = 0.324r Ag: (6xl)Cl Rh: ( l x l ) H 2 [ 8 7 ] [ 8 8 ] 211 3(lll)x(100) l 2(lll)+li(100) a = r b = V6r 9 = 90* A = V6r2 d = 0.289r Cu: (4xl)Pb [ 8 1 ] 221 4(l l l)x(lll) ( ^ ( l 11)^2)1(111) a = r b = 3r 9 = 90* A = 3r 2 d = 0.236r 310 3(100)x(110) 22(100)+li(110) a = \2r b = V5r 9 = 90* A = Vl0r 2 d = 0.224r 410 4(100)x(110) 33(100)+li(110) a = V2r b = 0 / l7/V2)r 9 = 104.04* A = 4r2 d = 0.171r Cu: (1x1)0, (1x1)20 [ 8 9 ] [ 9 0 ] [ 9 1 ] Table 3.4a Some characteristics of the three simplest b.c.c. high Miller index surfaces with rectangular unit meshes. Unit mesh Notation Dimensions 211 2(lll)x(100) ll(lll)+li(100) a = r b = d*N3)T A = C/8/V3)r2 d = (2/^ 18^  ^ ^ ^ ^ 210 2(110)x(100) l2(H0)+li(100) a = (2/V3)r b = 2.581r A = 2.981r2 d = 0.258r b j \ > < M 332 3(lll)x(110) 22(lll)+l2(H0) a = C/8/V3)r b = 0/l32/6)r A = 3.127r2 d = 0.246r Table 3.4b Some characteristics of the four simplest non-basal h.c.p. surfaces with rectangular unit meshes. Unit mesh Notation Dimensions 100 1010 a = r b = c/8/V3)r A = C/8/V3)r2 5 = 0-1/3 8 = V3-1 d=1/3C / 3/2)ror2/3C / 3/2) 102 10l2 a = r b = c/5i/3)r A = c/51/3)r2 8 = 0-1/3 8 = 1/3-1 d = 0.198ror0.396r no 1120 a = C/8/V3)r b = V3r A = V8r2 d = 0.5r 112 1122 a = V3r b = O^/Ve* A = c/44/2)r2 d = 0.426r In this table 5 is a number such that 0<8<1. This 8 specifies the location of the dividing plane (i.e. bulk terrnination), leading to non-unique surfaces for a given (hkl) plane when (2h+4k+31) is not a multiple of six. The notations given here are Miller indices and Miller-Bravais indices, a four index system given by 0ik-(h+k)l). The step notation has not been given here since for h.c.p. surfaces it does not give a convenient description of the surface. Also, the microfacet notation is only applicable to cubic lattices. surface is not s tr ic t ly stepped o r e v e n o f h i g h M i l l e r i n d e x i n the n o r m a l sense o f the te rm, i t does conta in regular 'defects' w h i c h stem f r o m its re la t ive ly o p e n nature, and therefore study o f its surface properties w o u l d be h e l p f u l as a precursor to the study o f h .c .p . h i g h M i l l e r i n d e x surfaces proper . It remains to be seen whether the study o f adsorpt ion systems o n h i g h M i l l e r index surfaces u s i n g L E E D w i l l have a comparable degree o f success to studies o n the l o w M i l l e r index surfaces. 77 Chapter 4 Studies on the Copper (110)-(2xl)-O Adsorption System 4.1 Introduction and Previous Work T h e adsorpt ion o f o x y g e n o n m e t a l l i c surfaces i s o f cons iderable interest i n heterogeneous catalysis research [1], F o r m o r e than t w o decades, the interact ion o f o x y g e n w i t h the l o w M i l l e r i n d e x surfaces o f c o p p e r has o c c u p i e d m a n y surface scientists, resul t ing i n a great dea l o f w o r k us ing various surface sensitive techniques. F r o m the earliest L E E D studies [92], i t has been k n o w n that o x y g e n adsorption at r o o m temperature leads to the format ion o f a stable (2x1) superstructure o n C u ( l 10) w h i c h corresponds to a h a l f m o n o l a y e r coverage. U p o n further exposure to O2, this is replaced b y a c (6x2) structure, and then a surface o x i d e l a y e r o f O12O is observed to f o r m [92]. F i g u r e 4.1 shows the C u ( l 10) surface a long w i t h its L E E D patterns before a n d after o x y g e n exposure up to the h a l f m o n o l a y e r coverage . T h e a forement ioned studies have p r o v i d e d m u c h i n f o r m a t i o n r e g a r d i n g adsorpt ion site, w h i c h is n o w universa l ly accepted as the l o n g bridge site i n the [001] d irect ion, as w e l l as c o n f l i c t i n g e v i d e n c e r e g a r d i n g the o c c u r e n c e a n d nature o f r e c o n s t r u c t i o n i n the substrate, interlayer spacings and b o n d lengths. F o r c o n f i r m a t i o n o f a proposed surface structure m o d e l , m o d e m surface science requires that there be agreement between results f r o m dif ferent surface a n a l y t i c a l tools . T h e controversy s u r r o u n d i n g the structure o f the C u ( 1 1 0 ) - ( 2 x l ) - O adsorpt ion system stems f r o m discrepancies that exist i n analyses u s i n g a v a r i e t y o f surface structure probes . T a b l e 4.1 summar ises the s tructural i n f o r m a t i o n obta ined f o r this system to date. Techniques used thus far i n c l u d e three types o f i o n scattering spectroscopies namely I C I S S [93], L E I S [94] a n d H E I S [95] as w e l l as a tom di f f rac t ion [96], S E X A F S [97,99], N E X A F S [98], angular r e s o l v e d U P S [100] and G L X S [101]. T h e surface structure models proposed so far are presented i n f igure 4 .2a-d . T h e p r o b l e m o f attempting to compare the results f r o m these dif ferent techniques is due to t w o m a i n factors. F i r s t l y , each i s sensitive to different features o f [001] 79 (-3,2) (-2,2) (-1,2) (0,2) (1,2) (2.2) (3.2) (-3,1) (-2.1) (-1,1) (0,1) (1.1) (2.1) (3,1) (-3,0) (-2,0) (-1.0) (0.0) (1,0) (2,0) (3,0) (-3,-1) (-2,-1) (-1,-1) (0,-1) (l.-l) (2.-1) (3,-1) (-3,-2) (-2,-2) (-1,-2) (0.-2) (1.-2) (2.-2) (3.-2) b) The reciprocal space f.c.c.(110) net (LEED pattern). • • • • • • • (-3.2) (-2,2) (-U) (0 .2 ) (1 .2 ) (2 .2 ) (3 .2 ) • • • • • • • (-3,3/2) (-2,3/2) (-1,3/2) (0.3/2) (1.3/2) (2,3/2) (3.3/2) # • • • • • • (-3.1) (-2,1) (-1,1) (0 ,1) (1.1) (2 ,1) (3 ,1 ) • • • • • • • (-3.V2) (-2.V2) (-I.I/2) (O.V2) (I.V2) (2.V2) ( 3 , L / 2 ) • • • • • • • (-3.0) (-2.0) (-1.0) (0,0) (1.0) (2.0) (3.0) • • • • • • • (-3,-1/2) ( - V W l . - V z ) (0,-1/2) (1.-1/2) (2.-V2) (3,-1/2) • • • • • • • (-3.-1) (-2,-1) (-1,-1) (0,-D (l.-l) (2 . -1) (3 . -1) • • • • • • • (-3.-3/2)(-2.-3/2)(-l1-3/2) (0.-3/2) (1,-3/2) (2,-3/2) (3,-3/2) • • • • • • • (-3,-2) (-2,-2) (-1.-2) (0.-2) (1.-2) (2 . -2) (3 . -2 ) c) LEED pattern of the Cu(110)-(2x1 )-0 adsorption system. 8 0 T a b l e 4.1 Current structural in format ion for the C u ( l 1 0 ) - ( 2 x l ) - O surface. Technique A d s o r p t i o n site M o d e l d c u - 0 d l d 2 6 R e f . I C I S S l o n g bridge m i s s i n g r o w +25% ± 1 0 % - 1 0 % ± 5 % 0 . 5 M L [93] L E I S l o n g bridge m i s s i n g r o w -0.6±0 .2A 0 . 5 M L [94] H E I S l o n g bridge buck led surface 0 . 5 M L [95] H E A D l o n g bridge m i s s i n g r o w < 0 . 5 M L [96] S E X A F S l o n g bridge m i s s i n g r o w +0.35A 0 . 5 M L [97,99] N E X A F S l o n g bridge +0.35A 0 . 5 M L [98] A R U P S l o n g bridge buckled surface 0 . 5 M L [100] G I X S l o n g bridge buck led surface 0 . 5 M L [101] a) Unreconstructed. b) Buckled row. c) Missing row. d) Sawtooth. Figure 4.2 Proposed models for the Cu(110)-(2x1 )-0 surface. the surface such as l o n g and short range order , a n d also, the condi t ions under w h i c h the surfaces were prepared and analysed were not a l w a y s the same. F o r example , the H E I S s tudy, f a v o u r e d a b u c k l e d r o w m o d e l whereas L E I S a n d I C I S S suggested a m i s s i n g r o w m o d e l . D u e to the h i g h sputtering y i e l d s f o r C u a n d O u s i n g 4 k e V N e + i o n s , the L E I S w o r k w a s c a r r i e d out under condi t ions o f d y n a m i c e q u i l i b r i u m , a n d therefore one cannot rule out the existence o f a different surface structure to that studied b y other techniques a n d H E I S , where the sputtering y i e l d s are less as a result o f the smal ler cross sect ion for sputtering b y 5 0 0 k e V H e + ions . T h e L E I S study c o n c l u d e d that o x y g e n sits 0.6±0 .2A b e l o w the top C u l a y e r and I C I S S , u s i n g 5 k e V L i + i o n s , gave values f o r the top t w o C u inter layer spacings o f + 2 5 ± 1 0 % and - 1 0 ± 5 % f r o m the b u l k C u ( l l O ) in ter layer s p a c i n g . A t o m d i f f r a c t i o n does not suf fer f r o m the same p r o b l e m o f exper imenta l ly i n d u c e d surface damage. I n part icular , h e l i u m atoms w i t h <0 .1eV energy d o not penetrate the topmost copper layer , and so structural in format ion f r o m H E A D s h o u l d be m o r e r e l i a b l e . F o r c l e a n C u ( l 10) i t was f o u n d that h e l i u m atoms are re la t ive ly insens i t ive to the corrugat ion o f the surface, y i e l d i n g a va lue o f 0.09A f o r the depth o f the troughs. T h i s is due to a ' s m o o t h i n g out' o f the C u i o n cores b y valence electrons so that the H e atoms c o u l d not 'see' m u c h o f the corrugat ion o f the surface. A f t e r adsorption o f h a l f a m o n o l a y e r o f o x y g e n however , a corrugation depth o f 0.75A was ob ta ined . T h i s a l o n g w i t h analys is o f the d i f f r a c t i o n peaks , p r o v i d e d e v i d e n c e f o r a ' m i s s i n g r o w ' s tructure, but n o quant i ta t ive va lues f o r inter layer spacings o r C u - 0 b o n d lengths were g i v e n . I n the angle r e s o l v e d U P S a n d G L X S w o r k , the b u c k l e d surface m o d e l expla ined the experimental f indings better than other m o d e l s , but again, no values were g i v e n f o r interlayer spacings o r f o r the heights o f the p r o p o s e d 'buckles ' . Techniques such as S E X A P S a n d N E X A F S are sensit ive to short range order, but g ive less in format ion regarding interlayer spacings or relaxat ion phenomena. In the S E X A F S w o r k , the b u c k l e d r o w m o d e l was rejected pure ly o n nearest ne ighbour b o n d length determinations since a peak corresponding to the distance between o x y g e n a n d copper atoms i n the b u c k l e d r o w were m i s s i n g . N E X A F S also f a v o u r e d the m i s s i n g r o w m o d e l u s i n g a s i m i l a r premiss a n d gave g o o d agreement w i t h S E X A F S f o r the height o f o x y g e n above the copper atoms that they br idge o f +0.35A, a rather different value to that obtained b y L E I S o f -0.6+0.2A. T o date, n o quantitative surface structure analysis b y L E E D has been p u b l i s h e d f o r the system u s i n g f u l l d y n a m i c a l scattering calculat ions o f 1(E) curves . T h i s chapter presents exper imental data obta ined for n ine symmetr ica l ly inequivalent beam types recorded at n o r m a l incidence and s ix at 10° of f -n o r m a l for the C u ( 1 1 0 ) - ( 2 x l ) - O s y s t e m 4.2 E x p e r i m e n t a l T h e Cu(110) sample was prepared i n the same w a y as the Cu(311) sample , as descr ibed i n sections 2.1 and 2.2. P r i o r to c leaning i n U H V , the surface was f o u n d to be contaminated w i t h s u l p h u r , c a r b o n a n d c h l o r i n e . S i n c e both the C u ( 3 1 1 ) a n d C u ( 1 1 0 ) samples w e r e cut f r o m the same s ing le c r y s t a l r o d , a n d the preparat ion procedures were essential ly ident i ca l , i t is not surpr is ing that both conta ined the same amount and type o f contaminants . A f t e r c leaning i n U H V b y argon i o n bombardment , i t was necessary to f i n d the o x y g e n exposure w h i c h produced the best (2x1) d i f f rac t ion pattern. T h i s was ach ieved b y d o s i n g w i t h v a r y i n g amounts o f o x y g e n , f o l l o w e d b y anneal ing at 1 0 0 ° C f o r five minutes . Spot prof i l es were then recorded for the (1,"V2) a n d (-l,'l/2) f r a c t i o n a l beams a n d p l o t t e d (see f igure 4 .3) . T h i s a l l o w e d accurate deterrnination o f the o p t i m u m exposure condit ions . D u r i n g data acquis i t ion, the L E E D 1.0-<D 2 0.9-CL a o.8-(A Q Si 0.7-° 0.6' | g 0.5" "O CD CO "CO E o 0.4^  0.3-0.2-0.H o • (Vfc) beam o (-1 ,-1/2) beam 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Time (min.) of exposure to 4x10"8 torr O2 at room temperature. Figure 4.3 Normalised VFWHM of the (1,-1/2) and (-1,-1/2) fractional beams from the Cu(110)-(2x1)-O surface plotted as a function of exposure time to oxygen at 4x10'8 torr. 00 pattern was observed to degrade slowly under the electron beam, resulting in increased background and diffuse diffraction spots. For this reason, the sample was re-annealed after every third set of beam intensity measurements. Intensity versus energy curves were recorded for five fractional and four integral order beams at normal incidence in two separate experiments, and two sets of data for six integral order beams were recorded at 10° off normal incidence along the [110] azimuth. The surface was then re-positioned normal to the electron beam, and the original nine beams were re-recorded twice to check their correspondence to the first two sets of normal data. 4.3 Presentation of Data The energy ranges over which each beam was recorded are given in table 4.2, and their 1(E) curves are shown in figures 4.4a to 4.4o. In the first two sets of normal data and the off-normal data, a one second delay was inserted between each intensity measurement. This resulted in a visible degradation of the diffraction pattern after recording of three different beam types due to the length of exposure to the electron beam. The third and fourth sets of normal data were recorded without a delay, so as to rriinimize the time that the surface was under the electron beam in each scan. This also allowed multiple scans to be added, with corresponding increase in the signal to noise ratio. After recording three inequivalent beam types, the total time that the electron beam was on, was about the same as that for data collected with a delay. Comparison of peak positions in 1(E) curves recorded by these two methods reveals a systematic shift to lower energy by about 4eV for curves recorded without a delay. The shift can be attributed to the spot intensities being recorded a fraction of a second before the diffracted beams had stopped moving. In a multiple scattering calculation, this shift would be taken into account by variation of the inner potential V 0 r . Since this is a non-86 Table 4.2 Beam indices and energy ranges (eV) for the experimental Cu(l 10)-(2xl)-O L E E D 1(E) curves. Beam Indices Normal 1 Normal 2 Off normal 1 Off normal 2 (0,1),(0,-1) (1,1),(-1,1),(1,-1),(-1,-1) (2.0) ,(-2,0) (2.1) ,(-2,l),(2,-l),(-2,-l) (l , 1y2),(-l , 1 /2),(l ,- 1 /2),(-l ,- 1/ 2) (2,i/2),(-2,i/2),(2,-i/2),(-2,-i/2) (OS/iUO,-1/!) (0,3/2),(0,-3/2) (l , 3 /2) .(-l . 3 /2) ,(l ,- 3 /2) ,(-l .- 3 /2) (0,1),(0,-1) (-1,0) (-2,0) (0,2),(0,-2) (0,0) 50-180 50-248 50-248 100-248 100-230 50-248 104-248 100-248 50-248 50-248 104-248 80-248 50-130 50-148 80-206 80-248 100-206 100-248 50-218 50-218 50-134 50-134 80-218 100-198 50-218 50-134 50-134 80-218 100-198 1 1 1 1 1 1 I I I I I 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) 1 1 1 I 1 1 1 1 1 r b) (1,1 ),(1 ,-1 ),(-1,1) and (-1 ,-1) beams at normal incidence ave i i i i i i i i i i 10 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) Figure 4.4 Experimental 1(E) curves recorded for the Cu(110)-(2x1)-O surface. oo -I 1 1 r -i 1 r c) (2,0) and (-2,0) beams at normal incidence. — i 1 1 1 1 1 1 1 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) 1 r d) (2,1),(2,-1),(-2,1) and (-2,-1) beams at normal incidence. ave. 1&2 ave. 3&4 — i 1 1 1 1 1 1 1 1 1— 10 60 80 100 120 HO 160 180 200 220 240 260 Energy (eV) Figure 4.4 cont. Cu(110)-(2x1)-O experimental l(E) curves. OO OO r — - i 1 1 1 1 1 1 • e) (1,1/2),(1.-1/2),(-1,1/2) and (-1,-1/2) beams at normal incidence. I 1 1 1 1 1 1 1 1 i ' i 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) 1 1 1—-—r 1 1 ' i 1 1 f) (2,1/2),(2,-1/2),(-2,1/2) and (-2,-1/2) beams at normal 40 60 80 100 120 140 160 180 200 220 240 260 Energy (eV) Figure 4.4 cont. Cu(110)-(2x1)-O experimental l(E) curves. OO VO Energy (eV) Energy (eV) Energy (eV) Figure 4.4 cont. Cu(110)-(2x1 )-0 experimental l(E) curves. 40 60 80 100 120 140 160 180 200 220 Energy (eV) k) (0,1) and (0,-1) beams at 10' off normal incidence. 40 60 80 100 120 140 160 180 200 220 Energy (eV) Figure 4.4 cont. Cu(110)-(2x1)-O experimental l(E) curves. I 1 • I) (-1,0) beam at 10° off normal 40 60 8 0 100 120 140 Energy (eV) m) (-2,0) beam at 10° off normal 4 0 6 0 8 0 1 0 0 1 2 0 1 Energy (eV) Figure 4.4 cont. Cu(110)-(2x1)-O experimental l(E) curves. o) (0,0) beam at 10* off normal incidence. 60 80 100 120 140 160 180 200 220 80 100 120 140 160 180 200 Energy (eV) Energy (eV) Figure 4.4 cont. Cu(110)-(2x1)-O experimental l(E) curves structural parameter, it would not alter the shape of the calculated 1(E) curves, only their position on the energy scale during rratching of experimental and theoretical 1(E) curves would be affected. 4.4 Final Comments and Suggestions for Further Work This thesis aims to add to our knowledge of copper surfaces, in particular the stepped Cu(311) surface and its interaction with sulphur and also oxygen adsorption on the Cu(l 10) surface. L E E D and AES together are particularly suited to this task, the former providing detailed structural information, while the latter gives the elemental composition of the surface. It had been hoped to include a multiple scattering analysis for the Cu(311) surface using at least the normal incidence data. These calculations, as well as forthcoming calculations for the off-normal data, have been performed by Dr P.R. Watson at Oregon State University. Due to unforseen circumstances however, the matching of experiment and theory using the R-factors R2, RZJ and RPe has not been completed as yet, though a full analysis using all the experimental data is irrratinent. The work on sulphur adsorption on Cu(311), though inconclusive in that it was not able to unambiguously determine the nature of sulphur atom ordering on the surface, did highlight the many problems associated with chemisorption on stepped surfaces. The conclusions drawn and suggestions made will benefit those attempting L E E D crystallography on such systems in the future. In addition, a full multiple scattering analysis of the Cu(110)-(2xl)-O chemisorption system is currently being initiated in the surface science laboratory at U B C . 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