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Photoluminescence from MEH-PPV and DP-PPV in bulk and encapsulated in porous alumina 2001

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P H O T O L U M I N E S C E N C E F R O M M E H - P P V A N D D P - P P V IN B U L K A N D E N C A P S U L A T E D IN P O R O U S A L U M I N A by Dong Feng Qi B.Sc. Zhongshan University, Guangzhou, China, 1990 M.Sc. Jinan University, Guangzhou, China, 1993 A THESIS S U B M I T T E D IN P A R T I A L F U L F I L 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 OF A P P L I E D S C I E N C E in 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 PHYSICS A N D A S T R O N O M Y We accept this thesis as conforming to the required standard T H E U N I V E R S I T Y O F BRITISH C O L U M B I A September, 2001 © D o n g Feng Qi, 2001 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 of Pty'C* & Afr»**>ntj The University of British Columbia Vancouver, Canada Abstract Two P P V derivatives, M E H - P P V and DP-PPV, are encapsulated in the nanopores of anodized alumina with different pore sizes (50/200 nm) and depth (10/60 urn). Time-integrated and time resolved optical emission properties of these structures have been studied and compared with those of thin films of the polymers. Significant blue shifts of the P L spectra (0.034 eV-0.183 eV) are found in the porous materials as compared to the bulk. The relative intensity of the one-phonon assisted transition increases in comparison with the intensity of the no-phonon transition in the nanostructured samples. The time constants of the optical emission from the bulk M E H - P P V film are very similar in their energy dependence to results obtained previously on bulk P P V films when considered with respect to the excess energy above the peak of the emission spectrum. The decay times are, however, two to three times shorter in the parent. The energy dependence of the time constants measured in DP-PPV films and DP-PPV encapsulated in nanopores (pore diameter 200 nm, pore depth 60 um) are also very similar to each other, and to the results obtained on the M E H - P P V . These results suggest that the intrachain diffusion of electron-hole excitations in all of these polymers, either in bulk or encapsulated form, is very similar. Ill Table of Contents A B S T R A C T . . . , ii T A B L E OF C O N T E N T S iii L I S T O F F I G U R E S v L I S T O F T A B L E S , viii A C K N O W L E D G E M E N T S ix CHAPTER 1 I N T R O D U C T I O N I C H A P T E R 2 T H E O R Y 5 2 1 S U M F R E Q U E N C Y G E N E R A T I O N A N D U P C O N V E R S I O N 5 2.2 O P T I C A L P R O P E R T I E S O F CONJUGATED P O L Y M E R S 11 C H A P T E R 3 E X P E R I M E N T A N D S A M P L E P R E P A R A T I O N 14 3.1 O P T I C A L S Y S T E M O V E R V I E W 14 3.2 M A J O R C H A L L E N G E S A N D S O L U T I O N S . . . 1 9 3.2.1 G E N E R A L A L I G N M E N T 19 3.2.2 G A T E A N D P L S P O T W A L K - O F F O N U P C O N V E R S I O N C R Y S T A L 1 9 3.2.3 S H A P E O F E X C I T A T I O N S P O T 20 3.2.4 N O N L I N E A R C R Y S T A L F O R U P C O N V E R S I O N 20 3.2.5 G A T E A N D P L B E A M G E O M E T R Y 21 3.2.6 P M T D A R K C U R R E N T 23 3.2.7 L N / C C D A S A D E T E C T O R F O R E X T R E M E L Y W E A K S I G N A L S 23 3.3 U P C O N V E R S I O N D A T A A C Q U I S I T I O N 25 3.3.1 T H E Z E R O D E L A Y POINT: L A S E R U P C O N V E R S I O N 25 3.3.2 A L I G N M E N T O F C O L L E C T I O N O P T I C S 27 3.3.3 M A X I M I Z I N G T H E I N T E N S I T Y O F P L 28 3.3.4 T H E S U M F R E Q U E N C Y S I G N A L 28 3.3.5 D A T A C O L L E C T I O N 29 3.3.6 W H I T E L I G H T U P C O N V E R S I O N 30 3.4 S A M P L E P R E P A R A T I O N 32 3.4.1 B U L K S A M P L E S : F I L M O N M I C R O S C O P E - S L I D E S 32 3.4.2 H O S T M A T E R I A L P R E P A R A T I O N 34 3.4.3 E N C A P S U L A T I N G M E H - P P V INTO A N O D I Z E D A L U M I N A O R A N O D I S C 39 3.4.4 E N C A P S U L A T I N G D P - P P V INTO A N O D I Z E D A L U M I N A O R A N O D I S C 39 IV C H A P T E R 4 E X P E R I M E N T A L R E S U L T S 40 4.1 T I M E - I N T E G R A T E D S P E C T R A 41 4 .1 .1 . EFFECTS OF N A N O P O R O U S HOSTS 41 4.1.2. T E M P E R A T U R E D E P E N D E N C E 47 4.1.3 E X C I T A T I O N INTENSITY D E P E N D E N C E 48 4.1.4 C O M P A R I S O N OF P L INTENSITY F R O M DIFFERENT SAMPLES 49 4.1.5 PHOTODEGRADATION 50 4.2 T I M E - R E S O L V E D S P E C T R A 54 4.2.1 P H A S E - M A T C H I N G CALIBRATION 54 4.2.2 B U L K M E H - P P V 57 4.2.3 B U L K D P - P P V 64 4.2.4 ANODISC D P - P P V 68 4.2.5 B U L K VS. ANODISC D P - P P V : LIFETIME D E P E N D E N C E O N E N E R G Y 72 4.2.6 B U L K M E H - P P V vs. B U L K P P V : L IFETIME D E P E N D E N C E O N E N E R G Y 73 4.2.7 L I F E T I M E DEPENDENCE O N E N E R G Y FOR P P V A N D DERIVATIVES 74 C H A P T E R 5 S U M M A R Y A N D C O N C L U S I O N S 75 5.1 S U M M A R Y 75 5.1.1 E X P E R I M E N T A L A P P A R A T U S 75 5.1.2 EFFECTS OF E N C A P S U L A T I O N O N THE TIME-INTEGRATED L U M I N E S C E N C E S P E C T R U M OF M E H - P P V A N D D P - P P V A T 77 K 76 5.1.3 EFFECTS OF E N C A P S U L A T I O N O N THE T I M E - R E S O L V E D L U M I N E S C E N C E S P E C T R U M OF M E H - P P V A N D D P - P P V A T 77 K 77 5.2 C O N C L U S I O N S 78 R E F E R E N C E 80 A P P E N D I X A 82 A P P E N D I X B 84 A P P E N D I X C 86 V List of Figures 1.1: Chemical structure of PPV, M E H - P P V and D P - P P V 2 2.1: Schematic optical setup of sum frequency experiment using a nonlinear crystal 5 2.2: Optics of an uniaxial crystal 7 2.3: Negative uniaxial crystal 8 2.4: Geometry of the gate and P L beams for determination of phase matching 10 2.5: Photoluminescence processes of organic molecules 11 2.6: Schematic diagram showing the segments of polymer chain 13 3.1: Schematic set-up for femto-second time-resolved photoluminescence spectroscopy 18 3.2: Polar plot of the phase matching angle as a function of the P L wavelength 21 3.3: The angular difference between the sum frequency propagation and the gate propagation as a function of P L wavelength under phase matched conditions 22 3.4: Schematic Setup for White Light Upconversion 31 3.5: Synthesis of M E H - P P V 33 3.6: Synthesis of D P - P P V 33 3.7: Set-up for Anodization of Aluminum 34 3.8: Schematic graph of sample before and after anodizing 35 3.9: S E M pictures of anodized alumina on aluminum foil 36 3.10: Pores of Anodisc 37 3.11: Chemical reaction route of silanization of Anodisc 38 V I 4.1: Photoluminescence Spectra of bulk M E H - P P V and bulk D P - P P V at 77 K 41 4.2: Photoluminescence Spectra of Anodisc M E H - P P V and Anodisc D P - P P V at 77 K 42 4.3: Two-Gaussian fits for bulk M E H - P P V and bulk D P - P P V P L at 77 K and three-Gaussian fits for bulk M E H - P P V and bulk D P - P P V P L at 77 K 44 4.4: Two-Gaussian fits for Anodisc M E H - P P V and Anodisc D P - P P V P L at 77 K and three-Gaussian fits for Anodisc M E H - P P V and Anodisc D P - P P V P L at 77 K 45 4.5: Normalized 77 K P L spectra of M E H - P P V and D P - P P V 46 4.6: Effects of Temperature on the time-integrated photoluminescence of bulk DP-PPV, Anodized D P - P P V and bulk M E H - P P V 47 4.7: Intensity of bulk D P - P P V P L at 77 K versus excitation intensity 48 4.8: 77 K Photoluminescence spectra of M E H - P P V and D P - P P V in bulk and nanostructured forms 49 4.9: Evolution of Spectra of Anodisc M E H - P P V and Anodisc D P - P P V with excitation time 52 4.10: Photo-Degradation of Anodisc M E H - P P V and Anodisc D P - P P V 53 4.11: White Light Upconversion spectra at different crystal orientation angles 55 4.12: Wavelength at which the mixing efficiency is maximum for a given angular position of the nonlinear crystal 56 4.13: Short time regime of time-resolved bulk M E H - P P V photoluminescence at 77 K 58 4.14: Bi-exponential fits of time-resolved bulk M E H - P P V photoluminescence at 77 K 59 4.15: Time constants extracted from the time-resolved decay profiles using the Bi-exponential fits 60 4.16: Population lifetime versus estimated electronic energy 61 4.17: Population lifetime versus E C L energy for bulk P P V 63 4.19: Extracted time constants versus probe energy for bulk D P - P P V P L at 77 K 65 4.18: Time-resolved profiles for bulk D P - P P V photoluminescence at 77 K 66 4.20: Population Lifetime versus estimated E C L energy 67 4.22: Extracted time constants versus probe energy for Anodisc D P - P P V P L at 77 K 69 vii 4.21: Decay profiles for Anodisc D P - P P V photoluminescence at 77 K 70 4.23: Population Lifetime versus estimated E C L energy for Anodisc D P - P P V 71 4.24: Comparison of population lifetimes of bulk and Anodisc D P - P P V 72 4.25: Comparison of population lifetimes of bulk P P V and bulk M E H - P P V 73 4.26: Comparison of population lifetimes of PPV and its derivatives 74 / viii List of Tables Table 1.1 Full names of the polymers studied 2 Table 3.1 Specifications of optical components in Figure 3.1 17 Table 4.1 Sample Properties and Sample Labels 40 Table 4.2 Comparison of Spectra of Bulk and Nano structured M E H - P P V and D P - P P V 43 ix Acknowledgements I would like to thank my supervisor Dr. Jeff F . Young for his continuous guidance and encouragement in the past two years. I am so fortunate to have worked with him, through whom I could expose myself to the academic and industrial atmosphere of Canada and North America. I am also in debt to Dr. Katja Rademacher, M r . Andras Pattantyus and Ms. Keri Kwong in Dr . Michael O. W o l f s group in Chemistry Department of U B C for preparing various conjugated polymer samples used in this thesis. I would also like to thank all my colleagues in Photonics Nanostructures Lab, especially M r . Alex Busch, Dr. Xiaonong Shen and M s . Lilian Fan, for their constructive discussion and resourceful help. In addition, I would like to sincerely thank my dear wife Qiaomin, for her full support through out my graduate study in U B C , my mother Shaowen, who flow from China to Vancouver, helping me take care of my new born son, so that I could concentrate on the research work. dedicated to Qiaomin, Hao, Ming-Hau and Wing-Hau Chapter 1 Introduction 1 Chapter 1 Introduction Conjugated polymers have been the subject of intensive research in the last twenty years, mainly due to their semiconductor-like optical and electronic properties. These organic materials offer promise for use in opto-electronic devices such as organic light emitting diodes (OLEDs) [1], light emitting electrochemical cells [2], photodiodes [3], sensors [18], waveguides [5] and lasers [6], etc. As compared to the traditional inorganic semiconductors, such as GaAs, conjugated polymer based devices are easier to fabricate. Currently, full color O L E D based products are commercially available, such as cell phone displays [7]. However, the lifetime of such conjugated-polymer-based products, e.g. the O L E D display, is much shorter as compared to the L C D / C R T based display and yields for manufacturing are inconsistent. For example, L C D s tend to operate for 10,000 to 15,000 hours, but O L E D s last only 5,000 hours [8]. Research is needed to find new materials and structures to improve the device performance, to increase the device lifetime and make the device more robust. To do that, conjugated polymers have been mixed with nanoparticles such as C 6 0 [9,32], carbon nanotubes [10], CdS [11] and T i 0 2 [12], etc. to form polymer - nanoparticle-composites. Conjugated polymers are also encapsulated in nanoporous materials, such as porous silica [13,14,23,31] or porous GaP [21], to modify their optical properties In this thesis, the optical properties of PPV derivatives, M E H - P P V and DP-PPV are compared in bulk form and when encapsulated in the nanopores of anodized alumina. The samples were synthesized by members of Dr. M . O . W o l f s research group in the Chemistry Chapter 1 Introduction Department of U B C . Figure 1.1 shows the chemical structures of P P V , M E H - P P V and DP- P P V . The full names for them are given in Table 1.1. The polymers are prepared in two forms, bulk form, by spin-coating from the polymer solutions onto microscopic glass and nanostructured form, by encapsulating the polymer into porous alumina. Table 1.1 Full names of the polymers studied Abbreviation Full Name PPV po/y(p-phenylenevinylence) MEH-PPV poly( 1 -methoxy-4-(2-ethylhexyloxy-2, 5-phenylenevinylene) DP-PPV /?o/y(2,3-diphenyl-/?-phenylenyvinylene) Figure 1.1: Chemical structures of (a) PPV, (b) MEH-PPV, and (c) DP-PPV. Chapter 1 Introduction 3 This thesis reports a study of the time-integrated and time-resolved optical emission properties of these samples, predominantly obtained at 77 K . Owing to the relatively weak emission from these, as compared to bulk P P V samples, modifications had to be made to the time-resolved optical detection system originally developed by McCutcheon [18] for studying bulk P P V . Chapter 2 describes some of the basic optical properties of the materials used in the nonlinear optical sum-frequency system and the polymers themselves. Chapter 3 describes how the time-resolved photoluminescence system was modified in order to increase its sensitivity by more than a factor of three times. Chapter 4 presents the time-integrated and time-resolved results obtained in bulk and nanostructured M E H - P P V and DP-PPV, and compares them with those previously obtained from bulk PPV. The bulk spectra of the three types of polymer are all similarly shaped, but are centered at different wavelengths in the visible part of the spectrum. The energy dependence of the decay time constants obtained from the time-resolved experiments are remarkably similar for all samples when the energy scales are shifted so the peaks of the emission spectra overlap. These time constants decrease almost exponentially with excess energy above the peak in the density of states associated with the inhomogeneously broadened distribution of coherent segment lengdis in the polymer chains. While these dynamics are similar in bulk and nanostructured forms of the polymers, the time-integrated spectra are changed significantly when the polymers are encapsulated in the nanoporous material. Chapter 4 summarizes the blue shifts, broadening, and shape changes that occur in these time- integrated spectra from the nanostructured samples. Chapter 1 Introduction 4 In Chapter 5, the conclusions drawn from the experimental results are summarized and some future work is suggested. Together with the previous work on bulk PPV, the present results motivate further, more detailed studies of PPV-based polymers encapsulated in nanoporous alumina. The aim of these studies should be to understand better the interaction of the polymers with the pore walls, and how they influence their optical properties. Appendix A and B present the Maple code to numerically calculate the phase matching conditions for the sum frequency generation. Appendix C presents the chemical process to silanizing the anodized alumina. Chapter 2 Theory 5 Chapter 2 Theory 2.1 Sum Frequency Generation and Upconversion The sum frequency generation technique based on non-linear optical crystals was first used by Mahr and Hirsch [16] in 1975. J. Shah has given a detailed review in 1988 on this P L of sample excited by Ultra-short Pump Pulse Figure 2.1: Schematic optical setup of sum frequency experiment using a nonlinear crystal. technique [17], which is shown schematically in Figure 2.1. The photoluminescence (PL) excited by an ultra-short pump pulse* is mixed with an ultra-short gate pulse in a properly oriented non- linear crystal, such as beta-barium borate, (3-BaB 204 (BBO), to generate sum frequency radiation. There are two conditions to be satisfied for efficient sum frequency generation. * The ultra-short pump pulse originates from the ultra-short laser pulse by frequency doubling. Delay i Ultrashort Laser Pulse (100 fs) (gate) Chapter 2 Theory 6 The mixing process only takes place when both the P L and gate pulses are present and interact in the crystal at exactly the same time. This can be achieved by varying and monitoring the light-path lengths of the ultra-short laser pulse (gate) and the ultra-short pump pulse (excitation/PL) beams, as described in Chapter 3. This is the condition that allows the technique to be used to achieve ultrafast time resolution. However, even if the P L and gate beams temporally and spatially overlap in the crystal, a sum frequency signal will only be generated if the two are "phase matched". The phase matching condition is given by equations (2.1) and (2.2). COg+0)pl=COs (2.1) K + Ki = K (2.2) where cog , copi and cos are the frequencies of the gate, P L and sum frequency beams respectively, and kg,kpl and ks are the corresponding wave vectors of the gate, P L and sum frequency beams in the nonlinear crystal. To simplify the analysis, we consider a collinear phase matching, which requires, kg+kpl=ks (2.3) In terms of the refractive indices «(co), (2.3) can be written as [15] ^8[n(o)J-n(cog)] + o)pl[n(coJ-n(a)pl)] = 0 (2.4) In transparent isotropic materials with normal dispersion, the index of refraction n{co) increases with co, so the above relation can never be satisfied. By using a negative uniaxial crystal, such as B B O , and choosing the cos beam to be polarized in the extraordinary direction, it is possible to arrange for n(cos) - n(cog ) and n(cos) - n(copt) to have opposite signs. Two types Chapter 2 Theory 7 of phase matching are commonly used. In type I, both n(a>g) and n(copl) are ordinary. In type II, either n(cog) or n{copJ) is ordinary [15]. To further understand the mechanism of sum frequency generation using negative uniaxial crystals, such as B B O , one should know the optics of the uniaxial crystals, which was Principal plane Beam propagation wavevector 9. Figure 2.2: Optics of uniaxial crystals. described in Reference [33]. As shown in Figure 2.2, in uniaxial crystals, there exists a specific direction called the optic axis (the Z axis). The plane containing the Z axis and the wave vector of the light wave is termed the principal plane. The light beam whose polarization is normal to the principal plane is called an ordinary beam or an o-beam. The beam polarized in the principal plan is called an extraordinary beam or e-beam. The refractive index of the o-beam, which is denoted by n0, does not depend on the propagation direction, while the refractive index of the e- beam depends on the propagation direction. The difference between the refractive indices of the ordinary and extraordinary beams is known as the birefringence An. The value of An is zero along the optic axis Z and reaches a maximum, An= n0- ne, in the direction normal to the Z axis. Chapter 2 Theory 8 In an uniaxial crystal, the index of refraction of the e-beam, ne.beam(Q) depends on the angle between the propagation beam and the Z axis, denoted as 0, as shown in Figure 2.2-2.3, and is determined by the ellipse defined by na and ne, the principal values of the refractive indices [33] . In this work, non-collinear type I phase matching was used, with both the gate and P L beams polarized as o-beams. Upconversion was accomplished in a negative (na>ne) uniaxial B B O crystal cut at an angle (9Cut=380 with respect to the optic axis (the Z axis) as shown in Figure 2.3. Figure 2.3: Negative uniaxial crystal: Principal values of refractive indices. Index ellipse. Cut angle with respect to the optic axis. Chapter 2 Theory 9 As shown in Figure 2.4, the gate and P L beams are not collinear. To achieve phase matching for such a geometry, the nonlinear crystal must be oriented to the right phase matching angle, which is given by [17] [18] s m ( 0 J - (»:r-«r (2-5) where 0m is the phase matching angle (the internal angle between sum frequency wave vector and the optical Z axis of the nonlinear crystal), nHe and n"0 refer to the principal values of refractive index at the sum frequency cos, respectively, and ns(0m) is determined by ns =kscla>, (2.6) where cos is determined by (2.1), ks is given by equation (2.2), and kg and kpl are magnitudes of the gate and the P L wave vectors inside the nonlinear crystal, determined by (for Type I phase matching) kg=n°scoglc (2.7) kpl=n0pl(Dpllc (2.8) where n° and npl° are the ordinary indices of the gate and P L inside the nonlinear crystal. The indices of refraction, ng° and np°, are wavelength dependent but are simply determined by using the empirical dispersion relations given in Reference [33] for B B O . Referring to Figure 2.4, the external angle between the gate and P L beams (9 u +6 2 i ) can be easily obtained by using the method described in Chapter 3. The internal angle between the gate and P L beams (B u +9 2 t ) is determined by refraction at the interface according to Snell's law. Based on the equations (2.1), (2.2) and (2.5)-(2.8), McCutcheon [18] developed a useful Maple code to find Chapter 2 Theory \Q numerically the incident angle of the P L beam, 9 l b required to phase match a specific frequency component of the P L . This code was used throughout this thesis work and it was modified to extract the angular difference between the sum frequency propagation and the gate propagation under phase matching conditions. The Maple codes are attached in Appendix A and B. A more detailed description of the code can be found in the Appendix of Reference [18]. Incident angle of PL . corresponding to the I phasematching condition given J by the code (Appendix A) I Entrance surface of Exit surface of Nonlinear Crystal Nonlinear Crystal Figure 2.4: Geometry of the gate and PL beams for determination of phase matching. Chapter 2 Theory \ \ 2.2 Optical Properties of Conjugated Polymers Conjugated polymers exhibit many semiconductor-like properties. However, the optical Energy Photo Excitation Sj Anti-bonding State (TI* orbital) Photon Emission hv So Bonding State (71 orbital) Atomic coordinates Figure 2.5: Photoluminescence processes between the S0 and Si electronic states of organic molecule, properties are more conveniently described by a molecular picture, which was outlined in Reference [26] by Greenham and Friend. First consider a single isolated benzene ring (C 6 H 6 ) , which ultimately forms part of the backbone of the polymer chain. The electronic structure can be conveniently described in terms of o bonds formed by overlap of sp2 hybrid orbitals and n bonding formed by overlap of pz orbitals on adjacent carbon atoms. The cr orbitals are relatively deep bonding orbitals. The semiconductor-like properties of conjugated polymer arise from the derealization of the relatively shallow re orbital electrons. The estates in the benzene molecule act very much like Chapter 2 Theory • 12 the valence (bonding TC orbital) and conduction (anti-bonding TC orbital) states in inorganic semiconductors, such as GaAs. The promotion of electrons from occupied TC to unoccupied TC states typically results in a change in the quasi equilibrium geometry of the molecule. The transitions associated with photoluminescence from an organic molecule can be represented as in Figure 2.5. The energies of the ground (estate) and excited (TC state) states are shown as a function of the configuration coordinates of the molecule system, which represents the positions of the atomic nuclei. The parabolic shape of the energy bands indicates an energy increase due to displacements from the equilibrium configuration for a specific electronic state. Each electronic state is associated with a set of vibrational sublevels that are spaced much closer than the electronic energy levels. When the molecule is excited (absorption of high energy photons), electrons are promoted from the bottom of the S0 state to high lying Sj sublevels, from which they non-radiatively decay very quickly (< 10"13 s) down to the bottom of Sj. Radiative emission from S,° can result from a transition to any one of the vibrational sublevels in S0 . The transition from the bottom of S, to the bottom of S0 is a purely electronic transition, and is denoted as Sj-^S0 /0—>0 or SQ.0. The transition from the bottom of S; to the other vibrational sublevels in S0 are denoted as 5 i->5'0/0->i, S1^S0/0->2, etc. Polymer chains are formed by bonding identical monomer molecules in a linear chain. When defect free, the 7r electrons can be delocalized over several monomers. The number of monomer units over which the orbitals are delocalized is determined by defects on the polymer chain, such as twists, bends or impurities, as shown schematically in Figure 2.6. The effective conjugation length (ECL) will determine the energy spacing of the S0 and Sj levels, and thus the emission energies from the segments [30]. The transition energy is smaller for longer Chapter 2 Theory 13 effective conjugation lengths and is larger for short effective conjugation lengths. The distribution of conjugation lengths, typically between 10 and 30 monomers [30], produces an inhomogeneously broadened density of states (DOS) distribution. Defects Coherent Segments Figure 2.6: Schematic diagram showing the segments of a polymer chain jointed by defects such as twists, bends or impurities. Chapter 3 Experiment and Sample Preparation 14 Reference [18] gives a detailed description of the basic upconversion system used in this work. This chapter will give a brief review and discuss the improvements made to optimize the pre-existing system to achieve a better sensitivity and to reduce the noise floor. 3.1 Optical System Overview Figure 3.1 shows a schematic representation of the setup for the time-resolving experiment. A Spectra-Physics Mode-locked Titanium Sapphire (Ti:Sapph) laser is pumped by an A r + laser with a constant pumping power of 7 W . The Titanium Sapphire laser is tunable over a range of 735-820 nm with a maximum output power of 800 m W at 800 nm. When the laser system is well mode-locked, it emits "100 femtosecond (full width at half maximum intensity, sech2(f) shaped) ultra-short laser pulses with a repetition rate of 82 M H z . The TkSapph laser beam is split by BS1 into an excitation beam and a gate beam. The gate beam first hits a retro-reflector mounted on a translation stage, then is directed by mirrors M 3 , M 4 , M 5 and M8, and finally focused by L3 onto the surface of the non-linear crystal, C2, for upconversion. The excitation beam first hits M 6 and another retro-reflector mounted on a mini-shaker whose frequency can be adjusted from 5 Hz to 30 Hz. This mini-shaker is useful in finding the zero delay point described later in section 3.3. Directed by M 7 , the excitation beam is focused by L I onto a non-linear crystal C I where it is frequency doubled to generate Chapter 3 Experiment and Sample Preparation \ 5 U V pulses with the same repetition rate as the gate beam. The U V beam is directed by M 9 and M10, and focused by L2 onto the sample in the cryostat. The photoluminescence excited by the U V pulse is collimated by a 9 0 ° parabolic mirror (PM), directed by M11/M12 and focused by L4 onto the same spot as the gate beam on crystal C2. The sum frequency signal generated in C2 is collimated by L 5 , directed by M13/M14 and focused by L 6 onto the entrance slit of the monochromator. Filter F l is used to filter out the second harmonic background from the strong gate pulses (even though the crystal CI is oriented to phase match cog and (opl, there is significant, non-phasematched second harmonic generation at 2cog due to the strength of the gate beam). Photon counting is done by either a photo-multiplier tube (PMT) connected to a computer-monitored discriminator or a liquid nitrogen cooled C C D detector. Table 3.1 gives the major specifications of all die optical components used in the time-resolved photoluminescence experiments illustrated in Figure 3.1. The whole experimental system is controlled by Labview programs via computer interfaces, including rotation of the non-linear crystal C2, pulse counting from the P M T discriminators and positioning the monochromator for a given detection wavelength. A C C D detector cooled by liquid nitrogen is also used for detecting very weak sum frequency signals. The C C D offers lower noise, but it is not as convenient as the P M T for measuring full spectra at a fixed delay. The P M T is made by Thorn E M I Electron Tubes Ltd. (Type: 9784A). The C C D is from Princeton Instruments Inc. (Model: Specl0:100BR). The upconversion crystal C2 is mounted on a three-dimensional translation stage on top of a motor-driven rotation stage. The surface of the crystal is parallel to the x direction of the translation stage. Z and y directions are used to align the crystal so that the focused spots of the Chapter 3 Experiment and Sample Preparation \ 6 gate and P L do not walk off when the crystal is rotated. A small piece of Teflon tape (~2 x 4 mm) is placed on the surface of the crystal to image P L and gate pulse spots. A P a n a s o n i c ® C C D camera is used to view the surface of the crystal on a monitor. The sample is mounted using Teflon tape at the end of a one-meter-long probe inserted down the center of the cryostat. The cryostat was designed to cool the samples to 77 K with liquid nitrogen or to 4 K with liquid Helium. Polymer samples studied in this thesis were cooled to 77 K to avoid photo-degradation. Chapter 3 Experiment and Sample Preparation Table 3.1 Specifications of optical components in Figure 3.1 Symbols in Specifications Figure 3.1 M 1 - M 8 Newport Round Pyrex Mirror M12 25.4 mm Diameter, 1/5 wave, 0.5-18 pm M i l 50 x 50 mm Square Flat Mirror, 1/5 wave, 0.5-18 jum M9-M10 Newport Round Pyrex Mirror 25.4 mm Diameter, 1/5 wave, 250-600 nm M13-M14 Newport Round Pyrex Mirror 50.8 mm Diameter, 1/5 wave, 250-600 nm C I 5x5x0.5 mm Beta-Barium Borate (BBO) Crystal, cut f at 9 = 2 9 . 2 ° C2 5x5x0.5 mm Beta-Barium Borate (BBO) Crystal, cut* at 9 = 38° RF1-RF2 Newport Gold-coated retro-reflector L l Focal length 5 cm L2-L3 Focal length 10 cm L4 Focal length 15 cm L5 Focal length 7.5 cm, 250-430 nm, A R coated L 6 Focal length 7.5 cm, 250-430 nm, A R coated GSI-Lumonics Inc. Gold Coated Aluminum, 9 0 ° off Axis Parabolic P M Mirror 2.875 cm Focal length BS1 50/50 beam splitter F I Newport UG11 filter or Schott D U G 1 1 X filter Refer to Figure 2.3 for definition of cut angle. Refer to Figure 2.3 for definition of cut angle. Chapter 3 Experiment and Sample Preparation 18 Figure 3.1: Schematic set-up for femto-second time resolved photoluminescence spectroscopy. Chapter 3 Experiment and Sample Preparation \ g 3.2 Major Challenges and Solutions The previous setup used in Reference [18] had a sufficient signal to noise ratio to time- resolve the transient P L emitted by bulk PPV. The P L strength of the signal from samples studied in this work was considerably less than that of bulk PPV. Several changes were made to increase the system sensitivity by a factor of approximately three times. Details of these improvements are described in this section. 3.2.1 General Alignment Proper alignment of all components is critical to achieving high sensitivity in this complex optical setup. The first phase of the procedure involves ensuring that all beam paths lie in a common plane parallel to the optical table, and that all focusing optics are aligned precisely on the optical axis of the beam passing through them. 3.2.2 Gate and PL Spot walk-off on Upconversion Crystal Usually, during an upconversion experiment, the nonlinear crystal C2 needs to be rotated to maximize the signal at different wavelengths. The focused gate and P L spots on the crystal must not "walk off" each other as the crystal is rotated. Gate and P L beams must therefore be focused right on the rotation axis of the rotation stage. T o achieve this, first rotate crystal C2 so that the gate beam is perpendicular to the surface of the crystal. Second, translate crystal C2 in the x direction, so that the spot is focused on the Teflon and imaged by the C C D camera. Mark the position of spot on the screen. Now rotate crystal C2 and adjust the x or y positions of the crystal until the gate spot does not walk when crystal C2 is rotated. Finally, Chapter 3 Experiment and Sample Preparation 2 0 adjust the position and focus of the P L beam so that it overlaps with the gate spot on the rotation stage axis. 3.2.3 Shape of Excitation Spot The U V excitation beam is generated by focusing the IR laser beam onto the non-linear crystal C I . In order to make sure the shape of the excitation spot on the sample is a circle for optimum overlap with the gate beam, the Beta-Barium Borate (BBO) crystal is cut at an angle of 2 9 . 2 ° to the optic axis. Such a cut angle allows normal incidence second harmonic generation with "800 nm laser pulses, which minimizes the distortion in the second harmonic generation beam. 3.2.4 Nonlinear crystal for upconversion A negative uniaxial crystal (BBO, ne < na) was used for upconversion. For optimum results, the crystal needs to be cut properly so that it can upconvert a wide frequency range of the P L spectrum. In our upconversion experiment, the B B O crystal is cut at 38° . With such a cut angle, this B B O crystal can be used to upconvert P L wavelengths ranging from the visible (500 nm) to the IR (1200 nm), when used in combination with gate wavelengths from 745 nm- 800 nm. The polar plot in Figure 3.2 gives the phase matching angle as a function of the P L wavelength for a 798 nm gate pulse (thick solid curve) as obtained using the code in Appendix A . The angle between the gate and P L beams is taken to be 2 0 ° . Less than 30° of rotation of the crystal can cover the P L wavelength range of 500 nm-1200 nm. Chapter 3 Experiment and Sample Preparation 21 Normal of Upconversion Crystal Gate Beam Incident Angle ofPLGnOfor Phase Matching Figure 3.2: Polar plot of the phase matching angle as a function of the PL wavelength (thick solid curve). Wavelength of the gate was 798 nm. The external angle between the gate and the PL beams was 20°. 3.2.5 Gate and P L beam geometry T o measure the angle between the gate and the P L , the most accurate method is to determine the angles at which the crystal surface reflects the gate and the P L beams back on themselves, using the Labview program rotate.vi to rotate the crystal and using MM2000TellPosition.vi to indicate the position of the crystal. This method is more straight- forward and quicker than that described in Reference [18] which was based on second harmonic generation from the gate and the reflected laser beam from the sample surface. The P L spectra of the organic semiconductors studied in this work are 150 nm -200 nm wide (taking M E H - P P V as an example, the range is 550 nm - 700 nm). For an angle of 18° between the gate and the P L beams, the angular difference between the sum frequency Chapter 3 Experiment and Sample Preparation 22 propagation and the gate propagation under phase matching condition is plotted as a function of the P L wavelengths in Figure 3.3. The direction of sum frequency propagation only varies by ~1° for P L wavelengths between 550 nm to 700 nm. Thus, once the collection optics (M13/M14/L6) are well aligned for upconversion of one P L wavelength, even though the crystal C2 is rotated for upconverting different parts (wavelengths) of the photoluminescence spectra, the adjustment of the collection optics is minor. We just need to rotate the crystal C2 to the right angle (calculated by the Maple code listed in Appendix A) , position the monochromator to the right wavelength, and set the delay line translation stage to ~3 pico- second delay. A very minor adjustment of M13/M14 then results in an optimized upconverted signal for the next P L wavelength. The same principal applies to other polymer samples which have a similar P L bandwidth. Figure 3.3: The angular difference between the sum frequency propagation and the gate propagation as a function of PL wavelength under phase matched conditions. The wavelength of the gate was 798 nm, and the external angle between the gate and the PL was 18°. Chapter 3 Experiment and Sample Preparation 23 3.2.6 PMT Dark Current The dark count rate of the detector directly impacts the achievable signal-to-noise-ratio (SNR). The P M T dark count rate is sensitive to the ambient temperature and internal heating effects. Practice has shown that if the room temperature is kept stable below 18° and the P M T is turned on immediately before the time-resolved experiment, the dark count rate starts at only 2-5 counts per second, and increases slowly to around 10 counts per second within three hours. A typical run involves acquiring signal counts in 20 second integrations for each of ~70 different delays, where the background dark counts over the same 20 seconds are monitored every five delay points. Between each run at fixed wavelength, the P M T is turned off and a fan is used to cool the P M T before the next run. 3.2.7 LN/CCD as a Detector for Extremely Weak Signals Although much effort has been implemented to reduce the background count rate of the P M T , it is still not low enough to obtain a good signal to noise ratio from some of the weaker sources studied here, which yield peak signals of <3 counts per second. To overcome this problem, a liquid nitrogen cooled C C D detector from Princeton Instruments Inc. (Model: Specl0:100BR) was used in place of the P M T as a detector when the upconverted signal was very weak. The C C D detector is M g F l coated for broad-band operation in the 200 nm - 1100 nm range. Liquid nitrogen was used to cool the detector, so that it effectively has zero dark counts. The detector consists of a 1340 x 100 array of pixels and is specifically designed for spectroscopy measurements. The size of each pixel is 20 x 20 jum. Chapter 3 Experiment and Sample Preparation 24 T o use the detector in upconversion experiments, we want to integrate every photon that reaches the detector. 35,000 (350 x 100) pixels are therefore combined to form a "super pixel" by setting an appropriate region of interest (ROD in the C C D control software [27]. To correctly set the ROI, the monochromator is positioned to the peak wavelength of the S H G of gate, and the monochromator output is imaged on the full C C D chip. M13/M14 are adjusted so that the image on the C C D chip is sharp and strong (typically over 1000 counts per second). As the upconverted signal and the S H G originate from the same spot on the nonlinear crystal and pass through the same optics, they should be imaged at the same position on the C C D chip. Considering the wavelength difference between S H G and upconverted signal and the chromatic aberration of the optics, a ROI which is larger than the S H G image is set to make sure the region can detect every photon of the signal. In our experiment, a 350 x 100 rectangular ROI was centered on the S H G image. A complication arises due to the relatively large IR response of the C C D detector as compared to the P M T . To isolate the detector from any unwanted light in the lab, three layers of black electrical tape were used to seal the gap between the C C D window and the wall of the monochromator. A l l the edges of the monochromator were also sealed with black electrical tape. The height of the entrance slit of the monochromator is narrowed down to ~2 mm by black electrical tape. Several pieces of black cardboard were used to shield the P L , scattered gate beam and the light from the computer monitor from entering the entrance slit of the monochromator. Most importantly, a D U G 1 1 X filter from Schott Glass Technology Inc. was used in place of the UG11 filter to filter out the strong IR background of the gate. The Chapter 3 Experiment and Sample Preparation 2 5 D U G 1 1 X is a UG11 filter with a special coating, which only has one transmission window in the 250 nm - 400 nm range. The C C D detector is equipped with a shutter in front of the chip to control photon integration time. For the above mentioned 350 x 100 ROI, the pure dark count rate with the shutter totally closed is 385+10/30 sec. With the D U G 1 1 X filter inserted in place of the UG11 filter and with the pure dark count rate (385+10/30 sec) subtracted by the control software [27], the gate beam background is below 100/30 sec while the net upconverted signal is in the range of 50-250/30 sec, depending on the wavelength being upconverted and the delay. Data acquisition with the C C D detector is performed in Mode I described in Section 3.3.5. A typical run consisting of 30-40 delays and a 30 second integration time per delay takes about one hour. Occasionally, the integration is corrupted by high intensity spikes due to cosmic rays. 3.3 Upconversion Data Acquisition 3.3.1 The zero delay point: Laser upconversion After all the optical components preceding crystal C2 are properly aligned, the next step is to determine the position of the delay translation stage (DLTS) that corresponds to zero time delay. The laser beam is split at BS1 into gate and excitation beams. The excitation beam hits the sample before being focused onto the upconversion crystal, C2. It is not practical to measure and set the two path lengths to be closer than ~3 cm. Therefore, laser upconversion is used to determine the exact zero delay point. Chapter 3 Experiment and Sample Preparation 26 • Referring back to Figure 3.1, the frequency doubling crystal C I and focusing lens L I are removed from the excitation beam path. The sample in the cryostat is lifted slightly so that the laser is focused on the Teflon below the sample. The scattered laser beam is then focused on C2 in the same manner as the P L . After the scattered spot and gate spot are overlapped on crystal C2 and the crystal is rotated to the right angle for laser upconversion, a lens with a focal length of "200 mm focuses the non-phase-matched second harmonic generated from the gate onto a Si-detector. As the laser upconversion signal is generated at the same point in the crystal, and is the same wavelength as the S H G of the gate pulses, the scattered laser upconversion signal will also be focused on the Si-detector when it is available. The Si-detector is A C coupled to an oscilloscope, and the mini-shaker M S is turned on. The delay line translation stage D L T S is slowly moved, and as the translation stage passes through the zero delay point, a number of spikes appear on the oscilloscope screen. The spikes appear in pairs, one of each pair represents the forward and backward pass of the shaker through the zero delay point. To find exactly the zero delay point, the delay translation stage is moved so all the spikes are evenly spaced on the oscilloscope screen. If the mini-shaker is then turned off and the Si-detector is D C coupled into the oscilloscope, the upconverted signal appears as a D C level. A combination of fine tweaking of the delay line and M12, and fine rotations (in 0 . 1 ° increments) of crystal C2 can result in a scattered laser upconverted signal close to 1 volt. This scattered laser upconverted signal is visible to the eye on florescent paper and is a valuable tool for aligning the collection optics, the monochromator, and for positioning the P M T . The zero delay point determined from the scattered laser upconversion is not exactly the same as that for the actual P L upconversion, after the frequency doubling crystal C I and Chapter 3 Experiment and Sample Preparation 27 L l are re-inserted. Experience has shown that they are within 5 mm apart on the delay line translation stage. The real zero delay point of the actual P L upconversion is determined after the P L upconversion signal is observed. Details of this are discussed in section 3.3.4. 3.3.2 Alignment of Collection Optics The collection optics consists of M13, M14 and L6 . M13 and M14 are 2 inch diameter U V mirrors. L 6 is a U V lens mounted on a 3-D translation stage. Positioning L6 is particularly important. The laser upconversion signal (LUS) is used to align the collection optics, including M13/M14 and the P M T . This is preferred over using the second harmonic generation (SHG) of the gate alone, as the L U S is a dot while the S H G of the gate is ring shaped. Firstly, the entrance and middle slits of the monochromator are opened to 2000 pm, and the lid of the monochromator and the focus lens L6 are removed. The L U S is collimated by L 5 , reflected by M13/M14 to the entrance slit of the monochromator. By tweaking M13/M14, the L U S is aligned to be parallel to the optical stage and perpendicular to the entrance slit of the monochromator. After positioning the monochromator to the peak wavelength of the L U S , adjust the U V mirror and the U V lens inside the monochromator for maximum signal on P M T § . Finally, replace the lid, and adjust L6 for maximum signal on the P M T . Normally, 3-5 million counts per second can be achieved from the L U S . § The second grating of the monochromator was bypassed by placing a U V mirror and lens after the middle slit, for the reasons described in Reference [18]. The signal is directed to the detector through a hole in the side of the lid. Chapter 3 Experiment and Sample Preparation 28 3.3.3 Maximizing the Intensity of PL The nonlinear crystal C I and lens L l are re-inserted for the P L upconversion experiment. The intensity of the P L should be maximized before being upconverted. The P L excited by U V pulses is collimated by the parabolic mirror (PM), reflected by M11/M12 and focused by L4 onto crystal C2. C2 is translated so that the P L focused on the Teflon tape is imaged by the C C D camera. A combination of tweaking L2 and L4 and moving the sample up and down results in a tightly focused, bright and round spot on the screen. 3.3.4 The Sum Frequency Signal To obtain good overlap of the P L and the gate beams on crystal C2, neutral density filters should be inserted into the gate path to reduce the intensity of the gate spot being imaged. When the attenuated gate and maximized P L spots are overlapped on the surface of the Teflon tape using the C C D camera, their respective diameters on the monitor should be ~1 cm and ~0.5 cm. C2 is then translated back and the neutral density filters are removed so that the P L and gate beams are focused on the surface of the nonlinear crystal. The geometry of the gate, the P L and the crystal C2 was described in section 3.2.5. C2 is rotated to the phase matching angle for the wavelength 6f interest, as calculated by the Maple code (Appendix A) . The delay line translation stage (DLTS) is set to approximately +5 ps delay from the zero delay point determined by laser upconversion. Next, the monochromator is positioned to the peak wavelength of the S H G from the gate alone and the P M T signal is maximized by adjusting M13/M14. Then the monochromator is set to the wavelength corresponding to the desired upconverted signal. The wavelength of the Chapter 3 Experiment and Sample Preparation 29 upconverted signal for M E H - P P V P L (550 nm - 700 nm) with an 800 nm gate is in the range of 325-370 nm. As the upconverted signal is much weaker than both the S H G and the P L background, a Newport UG11 filter that has a transmission window between 250 nm and 400 nm is inserted between M13 and L5 to filter both of them. Most likely, a small signal should be observed. If not, adjust the delay line. Once a small signal is available, a combination of adjusting M13/M14, rotating C2, and adjusting M12 for better gate/PL overlap results in an optimized upconverted signal. The zero delay point of the upconverted signal of the P L is approximately determined by moving the D L T S towards the "negative delay" direction until the upconverted signal suddenly becomes equal to the background. The precise zero delay point position is not determined, and is not crucial for interpreting the data obtained to this point. 3.3.5 Data Collection Two modes of data collection were used. Mode I consists of fixing the monochromator and varying the delay. Mode II involves scanning the monochromator at a given delay. For Mode I, with a high signal-to-noise ratio, the P M T counts were monitored in 20 second bins for each delay. The dark count rate of the P M T is controlled to be as low as possible using the method described in section 3.2.5. The P M T dark count background was recorded every five data points by blocking the excitation beam. The temperature of the sample is controlled within 0.1K. The best temperature stability was achieved when the liquid nitrogen level was above the bottom of the sample holder but well below the sample itself. A typical run consisting of 60-70 data points takes about one hour. Chapter 3 Experiment and Sample Preparation 30 For Mode II, the upconverted spectrum is scanned at given delays with a step-size of 0.5 nm. At the shorter delays (<30 ps), the more intensive spectra were scanned at 1 second/point. At longer delays (50-200 ps), the weaker spectra was scanned at 2 second/point. Each spectrum consists of 80 data points. 3.3.6 White light upconversion The spectra taken at fixed delay (Mode II) require calibration if they are to be quantitatively analyzed. The reason that more than a single frequency can be upconverted for a given crystal orientation has partly to do with the large cone angle (~10°) with which the P L signal is focused onto the upconversion crystal. The P L in fact is not incident at a single angle, but over a range of angles, and so different frequency components of the P L spectrum are simultaneously phase-matched. There is also a finite extent of ~ ± 1 . 5 ° to the phase-matching range for a given frequency at a fixed angle of incidence. In principal, the system could be calibrated by replacing the P L source with a diffuse, broadband white light source. This proved to be difficult to achieve without removing the entire cryostat. Instead, to perform white light upconversion, as shown in Figure 3.4, the white light of a 100W tungsten quartz halogen bulb (equivalent to a 3200 K blackbody, Model Oriel 77501) was coupled into an Ocean Optics Inc. fiber (model: P600-2-VIS/NIR) with a facet diameter of 100 pm. The light output from the fiber was collimated by a lens L w (focal length 25 mm) and then focused by L4 onto the nonlinear crystal, in the same manner as the P L was focused. The main difference between this and the P L source was the f-number of L4 imaged on the crystal. In the P L setup, it was ~f4, while in the white light setup it was ~f8. A l l other Chapter 3 Experiment and Sample Preparation 3 \ conditions such as the wavelength of the gate beam, angle between the gate beam and the P L beam (now the white light beam), collection optics and insertion of the UG11 filter were unchanged from the P L upconversion setup. The spectra of upconverted photons are measured in Mode II. As the spectrum of polymer P L is in the visible range, a Newport GG.455 filter was inserted in the tungsten lamp light path to cut off wavelengths shorter than 455 nm, in order to eliminate strong white light background in the sum frequency beam. The spectral response curves were taken at different phase-matching angles, as presented in Chapter 4. Monochromator Figure 3.4: Schematic Setup for White Light Upconversion. Chapter 3 Experiment and Sample Preparation 3.4 Sample preparation 32 In this thesis, two types of P P V derivatives, M E H - P P V and D P - P P V are studied. They were prepared in three forms. 1. Bulk film on microscope slide glass; 2. Encapsulated in anodized alumina on aluminum foil; 3. Encapsulated in commercially anodized disks (Anodisc 13) from Whatman Inc.;[28] 4. Encapsulated in silanized Anodisc material. A l l the samples described below were prepared by members of Dr. M . O . W o l f s group in the Department of Chemistry, University of British Columbia, Canada. 3.4.1 Bulk Samples: Film on microscope-slides 1. M E H - P P V M E H - P P V films were synthesized using the method described in Reference [19]. A 0.6% by weight solution of M E H - P P V in T H F (Tetrahydrofuran) was spin-coated onto microscope slides. The chemical reaction route used to prepare the M E H - P P V is illustrated in Figure 3.5. 2. D P - P P V D P - P P V films were synthesized following a chlorine precursor process described in the literature [22]. First, a soluble chlorine precursor polymer was synthesized and dissolved in toluene. 0.03% by weight prepolymer in toluene was spin-coated onto microscope slides. After drying in vacuum, the sample was heated to 2 8 0 ° C for 2 hours. The chemical reaction route used to prepare the polymer is illustrated in Figure 3.6. Chapter 3 Experiment and Sample Preparation 33 Figure3.6: Synthesis of DP-PPV. Chapter 3 Experiment and Sample Preparation 3.4.2 Host Material Preparation 34 layer (anode) Figure3.7: Set-up for Anodization of Aluminum. 1. Anodized Alumina on Aluminum Foil Anodization is a process in which an aluminum layer is oxidized electrochemically. Under certain conditions, self-organized nanopore structures are formed in the aluminumoxide (alumina) layer [20]. The aluminum foil, which is 0.13 mm thick with a purity of 99.99%, is first degreased with acetone, then it is anodized with 0.3 M oxalic acid ( C 2 H 2 0 4 ) as electrolyte, with an applied voltage of 40-60 V . The anodization is complete once the current starts dropping from its steady value. This process normally takes about one hour. The sample is rinsed with distilled water. The pores are then widened by dipping the oxidized aluminum layer into 5 % phosphoric acid (H 3 P0 4 ) for 45 minutes. A thin aluminum layer sometimes remains at the bottom of the alumina matrix. Figure 3.7 gives a schematic setup for the anodizing process. Chapter 3 Experiment and Sample Preparation 3 5 Figure 3.8 gives a schematic diagram of the aluminum sample before and after the anodization process. Figure 3.9 gives S E M pictures of anodized alumina on aluminum foil with both top and cross sectional views. The diameter of the pores varies between 50-90 nm across the sample. The pores are self-organized with a roughly hexagonal order but they are not uniform across the whole sample. The depth of the pores of the anodized alumina is approximately 10 pm. A l foil Alumina Air Hole Figure 3.8: Schematic graph of sample before (a) and after (b) anodizing. Chapter 3 Experiment and Sample Preparation 36 Figure 3.9: SEM pictures of anodized alumina on aluminum foil. (a) Top view and (b) cross sectional view. (Images obtained by Keri Kwong) Chapter 3 Experiment and Sample Preparation 3 7 2. Anodisc 13 membranes from Whatman Inc. Anodisc 13 membranes, manufactured by Whatman Inc., consist of a thin wafer of Figure3.10: Pores of Anodisc [28]. porous alumina, which can be used as a host to light emitting materials. These porous membranes have an asymmetric structure [35]. The majority of the membrane is comprised of pores with 200 nm pore diameters with a "60 pm depth. The other side of the membrane contains ~20 nm pore diameters with a 2 pm depth. Figure 3.10 shows the pores in the anodisc membrane [28]. More information about this material can be found in References [28] [35]. 2. Silanization of Anodisc Silanization is a chemical process used to modify the internal walls of porous materials, such as alumina or silica. This process allows the interim of the pores to be modified with alkyl groups, as shown in Figure 3.11, before the polymer chains are encapsulated. The purpose of silanization of porous materials is to help encapsulate more polymers into the pores by modifying the interaction of the polymer chain with the silanized pore walls. In principal, the silanization process also offers better protection to the encapsulated polymer chains because Chapter 3 Experiment and Sample Preparation 3 8 the oxygen in the alumina, which is believed to be involved in the degradation of the polymer, is screened from the polymer chain by silanization [34] [23]. Some of the Anodisc membranes were silanized with M e 3 S i C l (trimethylchlorosilane). Full details are described in Appendix C . Figure 3.11 shows the chemical reaction route used for the silanization. Trithylammine (N(CH 3 ) 3 ) is used as a catalyst which reacts first with the silane and then reacts with the alumina. In this thesis, the Anodisc used to host D P - P P V was silanized, while the Anodisc used to host M E H - P P V was not silanized. R R-Si—R + N(CH 3) 3 »• R CI R - S i - R +N(CH 3) 3 R 1 D _ q i _ R OH OH OH R . .N(CH 3 ) 3 K 01 r\ I I I r^ g j |_j , \ + — A l — O - A I — O — A l — »~ 1 1 + |\|(OH3)3 O O OH Porous alumina wall !, _ !, _ !, — A l — O - A I — O — A l — Note: R= Methyl (TMS) Figure3.11: Chemical reaction route for silanization of Anodisc. Chapter 3 Experiment and Sample Preparation 39 3.4.3 Encapsulating MEH-PPV into anodized Alumina or Anodisc To encapsulate M E H - P P V into the host materials described in section 3.4.2, the host material is immersed in a 0.025wt% M E H - P P V T H F solution for two days in the dark, under nitrogen atmosphere to avoid photodegradation. The sample is then rinsed with distilled water, dried and stored in a sealed bottle filled with nitrogen. 3.4.4 Encapsulating DP-PPV into anodized Alumina or Anodisc As D P - P P V is not soluble in T H F , a soluble D P - P P V prepolymer was synthesized as described in section 3.4.1. The host material is immersed in 0.03% by weight DP-PPV prepolymer toluene solution for two days in the dark, nitrogen atmosphere to avoid photo degradation. The sample was then rinsed with distilled water, dried in vacuum and heated to 2 8 0 ° C for 2 hours to convert the prepolymer to D P - P P V . Chapter 4 Experimental Results 40 Table 4.1 summarizes the properties of the M E H - P P V and D P - P P V samples and their labels used in this work. Methods of preparing these samples were described in Section 3.4. In the rest of this thesis, bulk M E H - P P V or bulk DP-PPV refers to the polymer on microscopic slides, Anodisc M E H - P P V or Anodisc DP-PPV refers to the polymer encapsulated in the Anodisc 13 membranes from Whatman Inc., and Anodized M E H - P P V or Anodized D P - P P V refers to the polymer encapsulated in the anodized alumina made by members of Dr. M.O.Wol f ' s group in U B C . Table 4.1 Sample Properties and Sample Labels ^ - ^ ^ ^ Sample 1 .ibeh I'lopcMlc-i Hulk M L H - P P V Anudi/cd MEH-PPV \nodisc M E H - P P Y Bulk D P - P P Y -\iiodi/vd D P - P P Y A iiud isc D P - P P \ Pol j met Type MEH-PPV M E H - P P V M E H - P P V DP-PPV DP-PPV DP-PPV HoslTspc Glass Slide Anodized Alumina on Aluminum Foil Anodisc 13 (Whatman Inc.) Glass Slide Anodized Alumina on Aluminum Foil Anodisc 13 (Whatman Inc.) H U M Flat 50-90 nm diameter pores, 10 (im thick Side l:~200nm diameter pores, ~60 Mm thick Side 2:-20 nm diameter pores, ~2 (mi thick Flat 50-90 nm diameter pores, 10 nm thick Side 1: -200 nm diameter pores, —60 ^m thick Side 2: -20 nm diameter pores, —2 |xm thick Sil.-nii/ed'' - No No - No Yes Note: Anodized samples and anodisc samples are both referred to as "nanostructured samples". Chapter 4 Experimental Results 41 4.1 Time-integrated Spectra 4.1.1. Effects of Nanoporous Hosts The time-integrated photoluminescence spectra for bulk M E H - P P V and bulk D P - P P V at 77 K are plotted in Figure 4.1. Their shape is quite similar to the published [16] spectra from other conjugated polymers such as PPV. Referring to Section 2.2, in each of the spectra, the large peaks at the highest energies (2.05 eV for M E H - P P V and 2.45 eV for DP-PPV) result from a purely electronic transition from the bottom of the first excited state S2 to the bottom of the ground state So (the S0_0 or the 0-phonon transition). The less intensive peaks at lower energies (1.90 eV for M E H - P P V and 2.30 eV for DP-PPV) are attributed to a one- phonon assisted transition from the bottom of 57 to the first vibrational state in S0, and are 0-phonon 2.05 eV 0-phonon 2.45 eV § 0.0 o Z 1.8 2.0 2.2 Energy (eV) 2.4 1.8 2.0 2.2 2.4 2.6 2.8 3.0 Energy (eV) Figure 4.1: Photoluminescence Spectra of (a) bulk MEH-PPV and (b) bulk DP-PPV at 77 K. Excitation wavelength was 398 nm. referred to as the S0.} or the 1-phonon transitions. The less obvious little "bump" near 2.11 eV for D P - P P V is attributed to a two-phonon assisted transition and is referred to as the S0_2 or the 2-phonon transition. For bulk M E H - P P V , the S0_2 or 2-phonon transition was not observed. The broad peaks reflect the inhomogeneously broadened density of states (DOS) due to the distribution of effective conjugation lengths (ECL) in the polymer chains. Chapter 4 Experimental Results 42 Energy (eV) Energy (eV) Figure 4.2: Photoluminescence Spectra of (a)Anodisc MEH-PPV and (b) Anodisc DP-PPV at 77 K. Excitation wavelength was 398 nm. The time-integrated photoluminescence spectra of Anodisc M E H - P P V and Anodisc DP- PPV at 77 K are plotted in Figure 4.2. The encapsulation clearly results in a significant effect of the on the shape and energies of the spectra. In order to quantitatively analyze these spectra, multiple Gaussian functions are used to fit the time-integrated spectra of bulk and Anodisc samples for MEH-PPV and DP-PPV. In all cases considered, two-Gaussian functions do not yield as good fits as those obtained with three- Gaussians. With 3-Gaussians, two of the three (labeled 1 and 2) always have similar line widths and are separated by the expected phonon energy, while the third has a different line width and energy. For direct comparison, the normalized PL spectra of MEH-PPV and DP-PPV at 77 K for both bulk and nanostructured samples are plotted in Figure 4.5. One can observe that there is a significant blue-shift for the P L of the nanostructured samples, ranging from 0.034 - 0.183 eV. A similar blue-shift (approximately 0.15 eV) was also observed by Zojer et al. on PPV chains isolated in a self-assembled lyotropic liquid crystal host [24,25]. In the nanostructured Chapter 4 Experimental Results 43 samples, the amount of blue-shift of M E H - P P V is greater than that of D P - P P V . For the same guest polymer, the Anodisc samples result in a greater blue shift than the Anodized samples. The relative (with respect to the 0-0 transition peak) intensity of the 0-1 transition peak increases in the nanostructured samples compared to the bulk samples. Similar results were also observed by Zojor on isolated P P V polymer chain [25]. In addition, it is observed that the spectra of the nanostructured samples become wider, indicating a greater degree of inhomogeneous broadening of the E C L energy density of states. Similar results were also observed by Zojor on isolated P P V polymer chain [25]. Table 4.2 summarizes the observed blue shifts and the broadening of the 0-0 transition peaks, the relative amplitude of the 0-1 peak with respect to the 0-0 peak, and the blue shift of the third Gaussian for both M E H - P P V and D P - P P V in different host materials. Table 4.2 Comparison of Spectra of Bulk and Nanostructured M E H - P P V and D P - P P V Sample Label Blucsliift of 0-0 peak Broadening of 0-0 peak \mnliuulc 1-0 Amplitude 0-0 Blucsliift of third Gaussian Broadening of third Gaussian Anodized MEH-PPV +0.058 eV Anodisc MEH-PPV +0.183 eV +100% 81% 0.163 eV +115% Anodized DP-PPV +0.034 eV Anodisc DP-PPV +0.068 eV +35% 58% 0.101 eV +10% Bulk MEH-PPV 25% Hulk DP-PPV 35% Chapter 4 Experimental Results 44 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.0 2.2 2.4 2.6 2.8 1.9 2.0 2.1 Energy (eV) 2.3 2.0 2.2 2.4 2.6 2.8 Energy (eV) Figure 4.3: Two-Gaussian tits for (a) bulk MEH-PPV and (b) bulk DP-PPV PL at 77 K and three-Gaussian fits for (c) bulk MEH-PPV and (d) bulk DP-PPV PL at 77 K. In all cases, the dashed lines show the individual Gaussians, and the solid lines show the sum of the Gaussians. Figure 4.4: Two-Gaussian fits for (a) Anodisc MEH-PPV and (b) Anodisc DP-PPV PL at 77 K and three-Gaussian fits for (c) Anodisc MEH-PPV and (d) Anodisc DP-PPV PL at 77 K. In all cases, the dashed lines show the individual Gaussians, and the solid lines show the sum of the Gaussians. Chapter 4 Experimental Results T 1 T 1.8 2.0 2.2 2.4 2.6 2.8 3.0 Energy(eV) Figure 4.5: Normalized 77 K PL spectra of MEH-PPV and DP-PPV for bulk (solid lines), Anodized (dashed lines) and Anodisc (dash-dotted lines) samples. Excitation wavelength was 398 nm. Chapter 4 Experimental Results 47 4.1.2. Temperature Dependence The effects of temperature on the time-integrated photoluminescence of bulk M E H - P P V , bulk D P - P P V and anodized D P - P P V are plotted in Figure 4.6. The data shows that both bulk and nanostructured samples exhibit similar temperature dependences. The S0_0 and S0_j peaks red-shift with decreasing temperature, which is attributed to the freezing out of torsional modes of the phenylene rings [29], resulting in a longer effective conjugation length and a lower emission energy. tc to c 2.5 2.0 1.5 1.0 0.5 0.0 - I I I l I 4 77K - • • 110K S \\ 180K J v 210K ' ••' \ / / \ — — — — — — s - ' : A \ - / / ! \ \ (a) — " i i i i i 1 1.8 "T 3 CD c 0.0 h 2.0 2.2 2.4 2.6 2.8 2.0 2.2 2.4 2.6 2.8 Energy(eV) 1.8 2.0 2.2 2.4 Energy (eV) — i 77K - • • 120K 150K 210K (C) 2.6 2.8 Figure 4.6 : Effects of Temperature on the time-integrated photoluminescence of (a) bulk DP-PPV, (b) anodized DP-PPV and (c) bulk MEH-PPV. Excitation wavelength was 398 nm. Chapter 4 Experimental Results 48 4.1.3 Excitation Intensity Dependence With an excitation wavelength of 397 nm, the 77 K P L spectra of bulk D P - P P V have been measured at different excitation powers, as shown in Figure 4.7 (a). The intensity of the P L (at 510 nm) as a function of the excitation intensity is plotted in Figure 4.7 (b). The intensity of excitation Iex was monitored by inserting neutral density slides into the excitation beam path. The power of the excitation without any neutral density filter in place was 0.1 mW. Figure 4.7 (a) shows that the shape of the P L spectrum does not depend on the excitation intensity. In Figure 4.7 (b), Ipl is found to proportional to l)£ , where the power 1.2 has an error of ± 0.2. The response of the system is therefore linear within the experimental uncertainty. Figure 4.7: Intensity of bulk DP-PPV PL at 77 K versus excitation intensity. Wavelength of excitation was 397 nm. Chapter 4 Experimental Results 49 4.1.4 Comparison of PL intensity from different samples With the same excitation power of ~0.1 m W and same excitation wavelength of 400 nm, the intensity of the P L for different polymers in different forms varies significantly. In Figure 4.8, the P L spectra of bulk and nanostructured M E H - P P V and D P - P P V at 77 K are plotted. Among the six spectra, the P L intensity of bulk M E H - P P V is the strongest, almost ten times stronger than bulk DP-PPV. The P L intensities of the two nanostructured M E H - P P V 400 450 500 550 600 650 700 750 Wavelength (nm) Figure 4.8: 77 K Photoluminescence spectra of MEH-PPV and DP-PPV in bulk and nanostructured forms. Solid line: bulk MEH-PPV, Short dashed line: Anodisc MEH-PPV, Dotted line: Anodized MEH-PPV, Long dashed line: Anodized DP-PPV, Solid line single dot: bulk DP-PPV, Solid line double dots: Anodisc DP- PPV. Excitation wavelength was 400 nm. Chapter 4 Experimental Results 50 samples reduced by factors of 100 and 1000 from the bulk for the anodized and anodisc hosts, respectively. On the other hand, the intensities of the bulk and both nanostructured D P - P P V samples are similar, and considerably stronger that nanostructured M E H - P P V . It is tempting to interpret these results as implying that encapsulation has a strong quenching effect on M E H - P P V , but not on DP-PPV. Unfortunately, there is currently too little knowledge about the relative or absolute polymer incorporation efficiencies in the nanoporous materials to draw any firm conclusions from this data. Its main significance here is to explain why it was impossible to obtain time-resolved data on nanostructured M E H - P P V . 4.1.5 Photodegradation While both bulk and nanostructured samples exhibit photodegradation, the effect is stronger in the nanostructured samples. Figure 4.9 shows a series of 77 K P L spectra obtained at different delays under constant excitation conditions for Anodisc M E H - P P V and Anodisc D P - P P V . It took about one minute to obtain each spectrum. It is noticed that the shape of the spectra does not change appreciably with photo-degradation. The intensity of the P L at the S0.0 peaks of Anodisc M E H - P P V and Anodisc D P - P P V are plotted versus exposure time in Figure 4.10. The intensity of the Anodisc M E H - P P V P L dropped by a factor of 2.3 in 60 minutes after which it continues to decay, but at a slower rate. On the other hand, the intensity of Anodisc D P - P P V P L dropped by a factor of 2 in 20 minutes, after which it remains quite stable. When the time-resolved experiment was carried out on Anodisc D P - P P V , for each detection wavelength, a fresh spot on the sample was exposed to the excitation beam for 20 minutes. Data in Mode-I was then acquired after the P L intensity stabilized. Chapter 4 Experimental Results 51 In summary, based on the experimental results in Section 4.1.4 and 4.1.5, bulk M E H - P P V is the most intense emitter. Time-resolved experiments were performed on bulk M E H - P P V using the P M T as the sum frequency signal detector, bulk and Anodisc D P - P P V are the less intense emitters, so time-resolved experiments on them were performed using the liquid nitrogen cooled C C D . nanostructured M E H - P P V s are the least intense emitters and their upconverted signals could not be detected even using the C C D . Chapter 4 Experimental Results 52 450 Wave' 1.2e+6 1.0e+6 3 (/> -»-» c O O 8.0e+5 650 Wave' Figure 4.9: Evolution of Spectra of (a) Anodisc MEH-PPV and (b) Anodisc DP-PPV with excitation time. Wavelength of excitation was 400 nm. Chapter 4 Experimental Results 140 120 5 100 to c CD 80 60 40 Anodisc MEH-PPV • • • 0 20 40 60 80 100 120 1200 1000 Z3 800 -t—> 600 400 200 0 Anodisc DP-PPV 0 20 40 60 80 100 120 Excitation Time (min) Figure 4.10: Photodegradation of Anodisc MEH-PPV and Anodisc DP-PPV as obtained by the intensity of their respective S0.o peaks. Chapter 4 Experimental Results 54 4.2 Time-resolved Spectra 4.2.1 Phase-matching calibration As described in section 3.3.6, a tungsten lamp, which is equivalent to a 3200K- blackbody radiation source, was used to evaluate the effectiveness of the Maple code used to estimate the phase-matching angle. The angle between the gate beam and the white light was set to 1 6 . 5 ° ± 0 . 1 ° , the same angle used in time-resolving experiments performed on bulk M E H - P P V film. The measured upconverted spectra at different crystal orientation angles are plotted in Figure 4.11. Considering the asymmetric nature of the spectra, a function of form I = I0+ Al exp ' ( 1 - 1 0 ) 2 A ^ J + aX was used to fit them. Figure 4.12 plots the wavelength (k0 in the equation) at which the upconversion mixing efficiency is the maximum for a given angular position of the nonlinear crystal, as obtained from the fits, compared to the predictions from the Maple code in Appendix A . The above results show that the Maple code can give a good prediction for the phase matching angle with a small offset. The actual wavelengths of peak conversion efficiency are all ~ 0 . 2 ° greater than that predicted by the Maple Code for a given gate/PL geometry. In Figure 4.12, the error bars originate from the accuracy of the gate/PL angle (± 0 . 1 ° ) and the curve fitting to the white-light upconversion spectra have been taken into consideration. Chapter 4 Experimental Results Wavelength (nm) Figure 4.11 : White light upconversion spectra at different crystal orientation angles (incident angle of the gate, 0 2 i in Figure 2.4). Solid line is the fit obtained as described section 4.2.1. Wavelength of the gate: 797.5 nm. Chapter 4 Experimental Results 56 380 370 « 360 350 h- 340 h 330 h 320 h- H 700 H 650 600 550 6 7 8 Crystal Position (Degrees) Figure 4.12: Solid dot: Wavelength at which the mixing efficiency is maximum for a given angular position of the nonlinear crystal, obtained from Figure 4.7. Solid line: results predicted by the Maple code in Appendix A (angle between the gate and the PL beams is 16.5°). Dotted lines: results predicted by the Maple code when the angles between the gate and PL beams are 16.4° and 16.6°. Chapter 4 Experimental Results 57 4.2.2 Bulk MEH-PPV The photoluminescence of bulk M E H - P P V at 77 K was time-resolved using the upconversion technique described in Chapter 3. We first consider the decay profiles that are measured using data collection Mode I. The decay at short time delays (0-20 ps) is plotted in Figure 4.13. The full decay curves (0-500 ps) are plotted in Figure 4.14. From these plots one can see two distinct regimes ih the data: a fast decay which occurs on timescales less than 20 ps, followed by a slower tail which extends to hundreds of picoseconds. Following the approach of McCutcheon's on bulk PPV decay profiles [18], the data that decrease monotonically with delay were fit to a bi-exponential function of the form I =I0+wle-t/Ti +w2e~t/T2, (4.1) where I0 is a constant background, z} and r2 represent the short and long time constants, respectively, and w ; and w 2 are the respective weighting factors. The fits are superimposed on the data points in Figure 4.14, and match the experimental data well. In Figure 4.15(b), the extracted fast and slow time constants are plotted versus the corresponding detection energies. These time constant data points can be categorized into three groups; each group is represented by a circle. Based on the model described in Reference [18], the time constants in the 0-0 group are associated with the pure electronic transition S0.0, while the time constants in the 0-1 group are associated with one-phonon assisted transition , and the time constants in the 0-2 group are associated with two-phonon assisted transition S2.0. Chapter 4 Experimental Results 58 400 o <u </> O CO r 200 o o 0 580nm *•»*••• . . . ^ • • • • 10 15 20 800 600 400 200 \ 594nm • • . . 10 15 20 800 800 600 400 200 • ••• 611nm 0 * 10 15 20 800 600 \ o 400 -I O 200 624nm 5 10 Delay (ps) 15 20 1200 1000 800 600 400 200 0 640nm 5 10 Delay (ps) 15 20 Figure 4.13: Short time regime of time-resolved bulk MEH-PPV photoluminescence at 77 K. Chapter 4 Experimental Results 1200 „ 1000 I 800 §. 600 | 400 o 200 ° 0 1.94 ev 800 600 400 200 0 0 100 200 300 400 500 Delay (ps) 1.99 ev • 0 100 200 300 400 500 Delay (ps) 800 800 600 400 200 0 2.05 ev 0 100 200 300 400 500 Delay (ps) 0 100 200 300 400 500 Delay (ps) Figure 4.14: Bi-exponential fits of time-resolved bulk MEH-PPV photoluminescence at 77 K. Chapter 4 Experimental Results 60 3 ro c 0) cu N £ 1 . 2 r 0.8 0.4 0.0 0-phonon 2.05(eV) 2-phonon 1.71(eV). 1-phonon 1.90(eV) 1.6 1.8 2,0 2.2 Energy(eV) to CL -I—' c ro -»—< to c o O CD E 1000 r 100 10 - 1 - 0.1 L 1.6 0-0 Group 0-2 Group • 1.8 2.0 2.2 2.4 Energy(eV) Figure 4.15: Time constants extracted from the time-resolved decay profiles using the Bi-exponential fits. Chapter 4 Experimental Results 1 3 CO c CD T 3 CD N E o z 1.7 1.8 1.9 2.0 co CD E .CD c o ' CL O CL 1000 100 10 h 1 0.1 It I L 2.1 2.2 2.3 2.4 Enerjgy (eV) (b) .1 J I 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 Energy(eV) Figure 4.16 : Population lifetime versus estimated electronic energy. Time-integrated spectrum (a) includes the 3-Gaussians used to obtain an excellent fit to the data, (see Section 4.1). Chapter 4 Experimental Results 62 We consider the time constants to be associated with a single D O S of the E C L of the coherent segments of the polymer chains, but that DOS is manifest via 0-0, 0-1 and 0-2 transitions. Thus to obtain a consistent energy dependence for the time constants, the points in 0-1 and 0-2 groups are translated one phonon energy 0.153 eV and two phonon energies 0.337 eV to higher energy, respectively. The one phonon energy was determined by measuring the energy separation between first and second Gaussians in the 3-Gaussian fit discussed in Section 4.1.1. As the 2-phonon transition peak is not in the range of our measurement, the two phonon energy of 0.337 eV was derived from the photoluminescence spectra in Reference [37]. In Figure 4.16, the translated time constants are plotted versus the estimated energy of the associated coherent segments. For energies above the 0-0 peak in the time-integrated spectrum (actually above the intermediate energy Gaussian shown in Figure 4.15(a)), the population lifetime decreases with increasing energy. This behavior is very similar to that obtained on PPV, the parent conjugation polymer of M E H - P P V , by McCutcheon [18], as shown in Figure 4.17. The explanation of our results reinforces the hypothesis raised in Reference [18]. Chapter 4 Experimental Results 63 Energy (eV) Figure 4.17: Population lifetime versus E C L energy for bulk PPV, the parent polymer of MEH-PPV, at 77 K, from Reference [18]. (a) Bulk PPV PL at 77 K with three Gaussian fit in dashed line, (b) The dashed line is the estimated E C L density of states (DOS) from the 0-0 transition peak of the bulk PPV PL at 77 K. Chapter 4 Experimental Results 64 4.2.3 Bulk DP-PPV Time-resolved experiments on bulk DP-PPV at 77 K were performed with probing energies at 2.23 eV, 2.27 eV, 2.30 eV, 2.36 eV, 2.43 eV and 2.50 eV, as shown in Figure 4.18. The intensity of the P L from bulk DP-PPV is less than one tenth that of bulk M E H - P P V . The peak intensity of the upconverted signal is ~3 counts per second, which was almost the same as the dark counts from the P M T . A L N / C C D detector was therefore used in place of the P M T as the photo detector (see Section 3.3.7). As in Section 4.2.2, a bi-exponential function was used to fit the decay profiles, and the results are plotted in Figure 4.18, along with the data. Within the latitude allowed by the signal to noise ratio, the bi-exponential fits reproduce the data points reasonably well. However, for the 495 nm (2.50 eV) profile, a tri-exponential function was used to give a better fit, resulting in a 20% reduction of the chi-squared value. As the error of the longest time constant is very large (1500+13500 ps), the longest life constant is omitted. In Figure 4.19 the extracted time constants are plotted versus probe energy. Again, the time constants can be categorized into three groups. After translating them by the one- phonon energy (0.15 eV) for the 0-1 group and the two-phonon energy (0.34 eV) for the 0-2 group, respectively, we obtain the time constants as a function of coherent segment energy, as shown in Figure 4.20. The one-phonon energy (0.15 eV) was obtained by measuring the separation of the first and second Gaussians of the three-Gaussian fits to the time-integrated P L spectrum of bulk DP-PPV, as discussed in Section 4.1.1. The 2-phonon peak is actually a small "bump". The position of the "bump" is at 2.11 eV, the two-phonon energy is therefore Chapter 4 Experimental Results 65 estimated to be 0.34 eV, very close to the results quoted in Reference [36]. The symmetric inhomogeneously broadened E C L density of states obtained from the three-Gaussian fit in Section 4.1.1 is also plotted. Again, the population lifetime decreases with the increasing energy, or the decreasing E C L of the coherent segments in the polymer. 1.8 2.0 2.2 2.4 2.6 2.8 3.0 Energy (eV) Figure 4.19 : Extracted time constants versus probe energy for bulk DP-PPV PL at 77 K. Chapter 4 Experimental Results 66 -100 100 200 300 300 300 -100 100 200 300 100 Delay (ps) 200 300 140 I I I • 2.50 e V " •s. — i i + -100 100 Delay (ps) 200 300 Figure 4.18: Time-resolved profiles for bulk DP-PPV photoluminescence at 77 K. Chapter 4 Experimental Results 67 0.1 t A ^ m ^ 1.0 g. 3 o.8 & 0 Q. 0.6 m O o 0.2 CD 0.0 c 1.8 2.0 2.2 2.4 2.6 2.8 3.0 Energy (eV) Figure 4.20: Population Lifetime versus estimated energy of the corresponding coherent segments for bulk DP-PPV at 77 K. Chapter 4 Experimental Results 4.2.4 Anodisc DP-PPV 68 Time-resolved experiments were performed on Anodisc D P - P P V at 77 K at probe energies of 2.30 eV, 2.36 eV, 2.43 eV, 2.54 eV and 2.58 eV. At 77 K the intensity of the Anodisc D P - P P V P L is similar to that of the bulk DP-PPV. The L N / C C D detector was therefore used to detect the upconverted signal. The bi-exponential function was again used to fit the decay profiles, and the results are shown with the data in Figure 4.21. Similar to bulk D P - P P V and bulk M E H - P P V , the bi-exponential fits reproduce the data points reasonably well. However, here for the 480 nm (2.58 eV) profile, a tri-exponential function was used to give a better fit resulting in a 20% reduction of the chi-squared value. But as the longest lifetime has a very large error (940+6000 ps), it is omitted. For the 540 nm (2.30 eV) profile, a single exponential was used. In Figure 4.22, the extracted time constants are plotted versus probe energy. Again, the time constants are categorized into three groups and translated by the one-phonon energy (0.15 eV) for the 0-1 group and the two-phonon energy (0.34 eV) for the 0-2 group, respectively. Again, the one-phonon energy (0.15 eV) was obtained by measuring the separation of the first and second Gaussians of the three-Gaussian fits to the time-integrated P L spectrum of Anodisc DP-PPV, as per discussed in Section 4.1.1. As expected the one- phonon energy is the same as that obtained from bulk DP-PPV. The 2-phonon transition peak in Anodisc D P - P P V time-integrated P L spectrum is not obvious. As the encapsulation of polymer chains into the nanopores should not change the structures of the material, it is assumed that both the bulk and Anodisc DP-PPV have the same energy separation between the first and second vibrational levels in the S0 state. Thus the two-phonon energy was taken to be the same as that obtained from bulk D P - P P V . Chapter 4 Experimental Results 69 The results after shifting are shown in Figure 4.23. Again, the symmetric inhomogeneously broadened E C L density of states obtained from the three-Gaussian fit in Section 4.1.1 is also plotted, and the population lifetime decreases with the increasing energy, or the decreasing E C L of the coherent segments in the polymer. It is noticed that the dependence on the E C L energy below the DOS peak becomes more flat as compared to that above the D O S peak. C O Q _ CO -«—• CD -i— i CO c o O CD E 1000 100 10 1 - o.i L 0-0 group 0-1 group 0-2 group 1.8 2.0 2.2 2.4 2.6 2.8 3.0 Energy (eV) Figure 4.22: Extracted time constants versus probe energy for Anodisc DP-PPV PL at 77 K. Chapter 4 Experimental Results 70 o <u co o CO c o O o CD (0 O CO c o o 200 150 h 100 200 300 100 h 100 200 300 FF o Q> CO O CO c O O o a> co o co CO c o O 70 60 50 40 30 20 - 10 -100 • 2.43 e V cu co o CO 13 o O 200 300 0 100 Delay (ps) 300 -100 0 100 Delay (ps) 200 300 Figure 4.21 : Decay profiles for Anodisc DP-PPV photoluminescence at 77 K. Chapter 4 Experimental Results -*—* w £Z CD CD "ro E CO C L E -4—• CD 1000 F 100 10 0.1 Energy (ejV) I I I 1 1 4 - i i i •f- i t" 4 ' l \ ~ \ \ 2.8 3.0 7j 1.0 0.8 0.6 0.4 0.2 0.0 1.8 2.0 2.2 2.4 2.6 2.8 3.0 Energy (eV) m 0) i—»- CD Q. m O o O co b Figure 4.23 : Population lifetime versus estimated energy of the corresponding coherent segment for Anodisc DP-PPV. Chapter 4 Experimental Results 72 4.2.5 Bulk vs. Anodisc DP-PPV: Lifetime Dependence on Energy The population lifetime dependence on energy for both bulk D P - P P V and Anodisc DP- P P V shown in Figure 4.20 and Figure 4.23 look similar. Figure 4.24 shows both sets of data on the same graph for easy comparison. The time constants for Anodisc D P - P P V have been shifted -0.068 eV, the blue-shift energy extracted from the time-integrated spectra of bulk and Anodisc D P - P P V as discussed in section 4.1.1, so that the peaks of DOSs associated with the 0-0 peaks are aligned. The dependence of population lifetime on the estimated E C L energy for the coherent segments, near and above the respective E C L energy DOS peaks, are remarkably similar for the bulk and Anodisc DP-PPV. CO Q_ 1000 100 o 10 0.1 i I < I f O Bulk DP-PPV • Anodisc DP-PPV shifted -0.068ev from raw data \ 1 1 *L v \ l _ 02 k 1.0 0.5 H 0.0 2.2 2.4 2.6 Energy (eV) 2.8 3.0 o —i 3 Q3_ N CD Q. m CO 0) r-t-CD CL m O D O (J) Figure 4.24:Comparison of population lifetimes of bulk and Anodisc DP-PPV. Time constants of Anodisc DP-PPV have been shifted -0.068 eV, the blue-shift energy of the 0-0 transition extracted from the time-integrated spectra of bulk and Anodisc DP-PPV, to align the DOS peaks, shown in dashed line for bulk DP-PPV and dot-dashed line for Anodisc DP-PPV. Chapter 4 Experimental Results 73 4.2.6 Bulk MEH-PPV vs. Bulk PPV: Lifetime Dependence on Energy The population lifetime dependence on energy for both bulk M E H - P P V and bulk P P V shown in Figure 4.16 and Figure 4.17 also look similar. To compare them, Figure 4.25 shows their population lifetime constants on the same graph for easy comparison. The time constants of bulk M E H - P P V have been shifted +0.289 eV, the energy difference of the respective 0-0 transition peaks, so that the estimated E C L DOS peaks are aligned. The population lifetimes of bulk M E H - P P V are significant longer that those of bulk PPV, but their dependence on the energy with respect to the peak in the respective DOS is very similar. 1000 100 y- 10 Mi •I ui. I * £ ii • l / l \ \ \ 0.1 • Bulk P PV O Bulk MEH-PPV shifted +0.289ev from raw data 1.0 H 0.5 0.0 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 Energy (eV) Figure 4.25:Comparison of population lifetimes of bulk PPV and bulk MEH-PPV. Time constants of bulk MEH-PPV have been shifted +0.289 eV, the energy difference of the 0-0 transition extracted from the time- integrated spectra of bulk PPV and bulk MEH-PPV, to align the DOS peaks, shown in dashed line for bulk PPV and dot-dot-dashed line for bulk MEH-PPV. Chapter 4 Experimental Results 74 4.2.7 Lifetime dependence on Energy for PPV and derivatives In Figure 4.26, the population lifetime constants of bulk PPV, bulk M E H - P P V , bulk D P - P P V and Anodisc D P - P P V are shown on the same graph with the respective 0-0 transition peaks aligned for easy comparison. For clarity, all the error bars are omitted. The energy dependences of the lifetimes in P P V and these two P P V derivatives in bulk and different encapsulated forms are quite similar. Q. £ .CD 1000 100 b- • 10 0.1 A O • A BULK DP-PPV Anodisc DP-PPV Bulk PPV Bulk MEH-PPV DOS Bulk DP-PPV DOS Anodisc DP-PPV DOS Bulk PPV DOS MEH-PPV O 1.5 1.0 0.5 0.0 2.2 2.4 2.6 2.8 3.0 3.2 3 £D_ N ' CD Q . m a) .—»- 3 SD CD Q. m o r~ D O cn •QT c Energy(eV) Figure 4.26:Comparison of population lifetimes of PPV and its derivatives MEH-PPV and DP-PPV, in bulk or in Anodisc forms. Time constants of bulk PPV, bulk MEH-PPV and Anodisc DP-PPV have been shifted +0.128 eV, +0.416 eV and -0.068 eV, respectively, to align their respective 0-0 transition peaks. 75 Chapter 5 Summary and Conclusions 5.1 Summary 5.1.1 Experimental Apparatus A pre-existing photoluminescence upconversion system [18] used to time-resolve the optical emission from bulk P P V polymers was enhanced to increase the achievable signal to noise ratio by a factor of more than three. Using a liquid nitrogen cooled C C D detector, a new nonlinear upconversion crystal, and new alignment procedures, upconverted signals less than "0.3 counts per second have been detected. The tuning curve for the new nonlinear upconversion crystal (BBO cut at 38°) was calibrated by recording the upconverted spectrum obtained using a white-light source at various crystal angles. These enhancements made it possible to time-resolve the emission from M E H - P P V and D P - P P V light emitting polymers, even when they are encapsulated in pores formed in two different types of electrochemically etched alumina. Chapter 5 Summary and Conclusions 76 5.1.2 Effects of Encapsulation on the Time-Integrated Luminescence Spectrum of MEH-PPV and DP-PPV at 77 K Members of Dr . M . O . Wolf's group in the Chemistry Department of U B C successfully encapsulated two types of light emitting polymers ( M E H - P P V and DP-PPV) in nanoporous. alumina. Porous alumina structures made at U B C had pore sizes of 50-90 nm, while porous alumina obtained commercially predominantly consisted of pores with diameters on the order of 200 nm. The time-integrated photoluminescence spectra of both types of polymers, in bulk form and when they are encapsulated in the two different alumina hosts, were obtained under identical excitation conditions at 77 K . For both polymers, there is a significant blue shift of the 0-0 transition peaks in both nanostructured samples as compared to the bulk samples. Referring to Table 4.2, for the same guest polymer, the Anodisc 13 alumina membrane from Whatman Inc. results in a larger blue shift than the "home-made" anodized alumina. The relative intensity of the 0-1 transition peak (with respective to the 0-0 transition peak) is increased for both nanostructured samples as compared to bulk samples. The relative intensity of the 0-1 transition increases from 25% (bulk) to 81% (Anodisc) for M E H - P P V and from 35% (bulk) to 58% (Anodisc) for DP-PPV. In addition, the spectra of the nanostructured samples are broader than their bulk counterparts. The F W H M s of the 0-0 peaks of Anodisc M E H - P P V and Anodisc DP-PPV P L increased by 100% and 35%, as compared to those of bulk M E H - P P V and bulk DP-PPV, respectively. A relatively strong Gaussian shaped feature distinct from the usual 0-n electronic/vibrational transitions is found to help in quantitatively Chapter 5 Summary and Conclusions 77 fitting the data from the nanostructured samples. There is some evidence of a weaker contribution from this additional feature in the bulk M E H - P P V and D P - P P V spectra. By comparing the intensities of time-integrated P L for different polymers in different forms, it is tempting to imply that encapsulation has a strong quenching effect on M E H - P P V , but not on D P - P P V . Unfortunately, there is currently too little knowledge about the relative or absolute polymer incorporation efficiencies in the nanoporous materials to draw any conclusions from this data. 5.1.3 Effects of Encapsulation on the Time-Resolved Luminescence Spectrum of MEH-PPV and DP-PPV at 77 K Time-resolved upconversion experiments were performed on bulk M E H - P P V , bulk D P - P P V and Anodisc DP-PPV. It was found that the population lifetime increases as the E C L of the coherent segments increases above the average segment length, for both bulk and nanostructured polymer samples. When the energy dependence of the population lifetime data for both bulk DP-PPV and Anodisc DP-PPV are shifted in energy by the difference in the respective 0-0 transitions, they exhibit very similar behavior. Applying the same shift to the population lifetime dependence on segment energy for bulk PPV and bulk M E H - P P V , it is found that the shape of the curves are almost identical, but the lifetimes of M E H - P P V are two to three times longer than those of the pure PPV. The similarity of the energy dependence of the lifetimes in pure P P V and these two P P V derivatives in bulk and different encapsulated forms is quite striking. Chapter 5 Summary and Conclusions 78 5.2 Conclusions The time-integrated photoluminescence spectra from both M E H - P P V and D P - P P V polymers are substantially altered from their bulk form when they are encapsulated in nanoporous alumina. The changes, including blue shifts, broadening, and enhancements in phonon assisted transitions, are all greater for both PPV derivatives when encapsulated in commercial Anodisc material with randomly distributed pores, "200 nm in diameter, as compared to alumina films with 50-90 nm diameter pores arranged in a regular hexagonal array. As the pores in both alumina hosts are considerably larger than the diameter of the polymer molecules, the changes are most likely due to interactions between the polymers and the inside walls of the alumina pores. These results are very encouraging, and suggest that there is significant potential for modifying the optical emission properties of PPV-based polymers by controlling their interaction with alumina surfaces. The significance of the distinct new feature in the time-integrated data from the nanostructured samples is not understood, but it is a relatively large effect that should be pursued. It is unlikely that this new feature plays a significant role in the emission at short times (less than a few hundred picoseconds), as the energy dependence of the time-resolved data is remarkably similar for all bulk and nanostructured samples when considered with respect to the peak in the respective D O S . A l l four samples studied with the upconversion system exhibit lifetimes that decrease monotonically, and nearly exponentially with excess energy above the peak in the D O S . This is qualitatively consistent with the interpretation of Kersting et al. [38,39] who attribute the decay dynamics in terms of an intrachain diffusion mechanism of the Chapter 5 Summary and Conclusions 79 photoexcited electron-hole pairs. These results suggest that the interactions with the alumina pore walls does not affect these intrachain dynamics on sub-nanosecond timescales. These results motivate future studies that should include sample characterization using complementary interface sensitive techniques. The substantial blue shifts observed here in the nanoporous materials, up to 0.183 eV, may prove useful in light emitting device applications if methods are developed to improve the emission efficiency and to reduce photodegradation. Further optical experiments, including analysis of polarization dependences and the influence of magnetic fields at temperatures from 4 K to room temperature are also warranted. 80 Reference [I] D . Braun, et al. Appl. Phys. Lett., 58(1991) 1982 [2] Q. Pei, etal. Science, 269(1995) 1086 [3] G. Y u , etal. Science, 270 (1995) 1789 [4] G . Y u , et al. Synthetic Metals, 111-112 (2000) 133 [5] F. Hide, etal. Science, 273 (1996) 1833 [6] T. Gabler, etal. Opt. Comm., 137(1997) 31 [7] G. Y u , et al. Appl. Phys. Lett., 64(1994) 3422 [8] Industrial analysis web site, e.g. http://news.cnet.com/news/0-1006-201-4745832-0.html [9] C. Lee, et al. Molecular Crystal and Liquid Crystal, 256(1994) 85 [10] H . Ago, et al. Phys. Rev. B, 61(2000) 2286 [II] M . Gao, et al. J. of Phys. Chem. B, 102(1998) 4096 [12] M . Baraton, et al. Nanotechnology, 9(1998) 356 [13] T. Nguyen, et al. Appl. Phys. Lett., 76(2000)2454 [14] B . Schwartz, et al. Synthetic Metals, 116(2001) 35 [15] Y . R. Shen, "The Principles of Non-linear Optics", New York: Wiley 1984 [16] H . Mahr et al. Hirch, Opt. Comm., 13 (1975) 96 [17] J. Shah, I E E E J. of Quantum Electronics, 24 (1998) 276 [18] Murray McCutcheon, M.Sc. thesis, University of British Columbia, 1999 [19] US Patent # 5189136, N . N . Barashkov etc. Synthetic Metals 75 (1995) 153 [20] A . P. L i , et al, J. of Appl. Phys., 84 (1998) 6023 [21] L . Rendu, etal. Opt. Materials, 17(2001) 175 Reference 81 [22] Bing R. Hsieh, et. al. Macromolecules 31 (1998), 631 [23] Junjun Wu, et al. , J. Phys. Chem. B 103(1999), 2374 [24] D. Gin etal, Synthetic Metals 101(1999) 52 [25] E . Zojer etal, Synthetic Metals 102(1999)1270 [26] N . C . Greenham, R H . Friend, Solid State Physics, 29(1995) 32 [27] User's Manual, WinSpec, Princeton Instruments Spectroscopic Software, Version 2.4.M, September 20, 1999 Chapter 11 [28] http ://www. whatman. com [29] K. Pichler et al. J. of Phys.: Condensed Matter, 5(1993) 7155 [30] G. R. Hayes et al. Synthetic Metals 84(1997) 889 [31] T. Nguyen et al. Science, 288(2000) 652 [32] C. Y . Yanga, et al. Synthetic Metals 83(1996) 85 [33] V . G. Dmitriev et al. "Handbook of Nonlinear Optical Crystals", Berlin: Springer, 1997 [34] Personal communication with Dr . Katja Rademacher and Keri Kwong in M . O . W o l f s group [35] A . W. Ott et al. , Chem. Materials, 9(1997), 707 [36] B. R. Hsieh, et al., Advanced Materials, 7( 1995), 3 6 [37] A . K. Sheridan, et al, Synthetic Metals, 111-112(2000), 531 [38] R. Kersting, et al, J. of Chem. Phys., 106(1997), 2850 [39] R. Kersting, et al, Phys. Rev. Lett., 70(1993), 3820 82 Appendix A Maple code for calculating phase matching angle for BBO crystal cut at 38° . Incident angle of PL corresponding to the right phasematching condition given by the code (Appendix A) Entrance surface of Exit surface of Nonlinear Crystal Nonlinear Crystal r e s t a r t ; n02:=proc(x); # ordinary index squared for BBO 2.7359+0.01878/(xA2-0.01822)-0.01354*xA2; end: ne2:=proc(x); # extraordinary index squared f o r BBO 2.3753+0.01224/(xA2-0.01667)-0.01516*xA2: end: lambdaG:=0.798: lambdaPL:=0.610: anglediff:=20.5: phasematch:=proc(thetar) Appendix 83 l o c a l kPL, kG, kS, nS, thetaM,lambdas,theta2i,thetali,alpha,beta, thetalt,theta2t,gamma,thetarn; lambdas:=(1/lambdaG + 1/lambdaPL) A(-1); # sum frequency beam thetali:=thetar*Pi/180; theta2i:=thetali - anglediff*Pi/180; theta2t:=arcsin(-sin(theta2i)/sqrt(n02(lambdaG))); thetalt:= arcsin(-sin(thetali)/sqrt(n02(lambdaPL))); alpha:=theta2t - t h e t a l t ; kPL:=sqrt(n02(lambdaPL))/lambdaPL: # PL beam wavevector kG:=sqrt(n02(lambdaG))/lambdaG: # Gate beam wavevector kS:=sqrt(kPL A2 + kGA2 - 2*kPL*kG*cos(Pi-alpha)); # Sum beam nS:=kS*lambdas; # Index of sum ray beta:=arccos((—kPL A2 + kGA2 + kS A2)/(2*kG*kS)); thetam:= -38*Pi/180 + beta - theta2t; (sin(thetarn)) A2 - (l/nS A2-l/n02(lambdas))/(l/ne2(lambdas)-l/n02(lambdas)) ; end: angle:=fsolve(phasematch(thetar)=0,thetar); #Solve numerically f o r thetar rotatefromgate:=anglediff-angle; lambdas:=(1/lambdaG + 1/lambdaPL) A(—1)*1000; angle := 8.833027055 rotatefromgate := 11.66697295 lambdaS := 345.7244319 Appendix 84 Appendix B Maple code for calculating angular difference between the sum frequency propagation and the gate propagation, as a function of the P L wavelength. Wavelength of the gate is 798 nm. Angle between the gate and the P L is 18°. r e s t a r t ; n02:=proc(x); # ordinary index squared f o r BBO 2.7359+0.01878/(xA2-0.01822)-0.01354*xA2; end: ne2:=proc(x); # extraordinary index squared for BBO 2.3753+0.01224/(xA2-0.01667)-0.01516*xA2: end: lambdaG:=0.798 : anglediff:=20: phasematch:=proc(thetar) l o c a l kPL, kG, kS, nS, thetaM,lambdas,theta2i,thetali, alpha,beta,thetalt,theta2t,gamma,thetarn; lambdas:=(1/lambdaG + 1/lambdaPL) A(-1); # sum frequency beam thetali:=thetar*Pi/180; theta2i:=thetali - anglediff*Pi/180; theta2t:=arcsin(-sin(theta2i)/sqrt(n02(lambdaG))); thetalt:= arcsin(-sin(thetali)/sqrt(n02(lambdaPL))); alpha:=theta2t - t h e t a l t ; kPL:=sqrt(n02(lambdaPL))/lambdaPL: # PL beam wavevector kG:=sqrt(n02(lambdaG))/lambdaG: # Gate beam wavevector kS:=sqrt(kPL A2 + kGA2 - 2*kPL*kG*cos(Pi-alpha)); # Sum beam nS:=kS*lambdaS; # Index of sum ray beta:=arccos((-kPL A2 + kGA2 + kS A2)/(2*kG*kS)); thetam:= -cutangle*Pi/180 + beta - theta2t; (sin(thetam)) A2 - (l/nS A2-l/n02(lambdaS))/(l/ne2(lambdaS)-l/n02(lambdas)); end: Angle:=[seq(x[i],i=l..31)]: for i from 1 by 1 to 31 do lambdaPL:=0.550+(i-1)/200; PhaseMatchingl:=fsolve(phasematch(thetar)=0,thetar); lambdaS:=(1/lambdaG + 1/lambdaPL) A(-1); thetalil:=PhaseMatchingl*Pi/180: t h e t a 2 i l : = t h e t a l i l - anglediff*Pi/180: Appendix 85 theta2tl:=arcsin(-sin(theta2il)/sqrt(n02(lambdaG))); thetaltl:=arcsin(-sin(thetalil)/sqrt(n02(lambdaPL))): alphal:=theta2tl - t h e t a l t l : kPLl:=sqrt(n02(lambdaPL))/lambdaPL: kGl:=sqrt(n02(lambdaG))/lambdaG: kSl:=sqrt(kPLl A2 + kGl A2 - 2*kPLl*kGl*cos(Pi-alphal)): nSl:=simplify(kSl*lambdaS); betal:=simplify(arccos((-kPLl A2 + kGl A2 + kSl A 2 ) / ( 2 * k G l * k S l ) ) ) ; thetaSl:=simplify(betal-theta2tl); thetaOutl:=simplify(arcsin(-sin(thetaSl)*nSl)); Angle[i]:=simplify(anglediff-PhaseMatchingl-thetaOutl*180/Pi); od: print(Angle); 1 0 . 6 8 9 5 9 5 5 6 , 1 0 . 6 4 6 3 5 3 0 0 , 1 0 . 6 0 3 4 5 8 6 9 , 1 0 . 5 6 0 9 0 7 3 7 , 1 0 . 5 1 8 6 9 4 8 4 , 1 0 . 4 7 6 8 1 6 2 9 , 1 0 . 4 3 5 2 6 7 0 6 , 1 0 . 3 9 4 0 4 3 2 3 , 1 0 . 3 5 3 1 4 0 1 7 , 1 0 . 3 1 2 5 5 3 8 1 , 1 0 . 2 7 2 2 8 0 9 7 , 1 0 . 2 3 2 3 1 6 0 0 , 1 0 . 1 9 2 6 5 7 2 6 , 1 0 . 1 5 3 2 9 9 4 3 , 1 0 . 1 1 4 2 3 9 5 0 , 1 0 . 0 7 5 4 7 3 9 9 , 1 0 . 0 3 6 9 9 8 9 2 , 9 . 9 9 8 8 1 0 8 1 7 , 9 . 9 6 0 9 0 6 5 1 0 , 9 . 9 2 3 2 8 2 8 0 6 , 9 . 8 8 5 9 3 6 3 2 6 , 9 . 8 4 8 8 6 3 8 8 0 , 9 . 8 1 2 0 6 2 3 4 4 , 9 . 7 7 5 5 2 7 8 8 8 , 9 . 7 3 9 2 5 9 0 1 6 , 9 . 7 0 3 2 5 1 9 9 3 , 9 . 6 6 7 5 0 2 9 6 4 , 9 . 6 3 2 0 1 0 9 7 7 , 9 . 5 9 6 7 7 1 5 4 4 , 9 . 5 6 1 7 8 3 2 1 6 , 9 . 5 2 7 0 4 1 7 4 0 Appendix 86 Appendix C Procedures for Silanization of Anodisc 13 purchased from Whatman Inc. 1. Place Anodisc under vacuum and heat to 100° C for 24 hours to remove residual moisture. 2. Under N 2 environment, add 0.1 ml of dry trithylammine to catalyze the reaction between alumina surface groups and silane. Then 1 ml trimethylchlorosilane was added to the Anodisc. The Anodisc was allowed to dark under N 2 for 2 hours and the excess was removed by vacuum. 3. Wash the Anodisc two times with dry hexane. 4. Dry the Anodisc and heat to 100°C for 1 hour under N 2 . 5. Wash the Anodisc with ethanol and distilled water and dry under vacuum.

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