CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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Furthermore generation of pump (~400nm) by second harmonic generation, probe (450- 1000nm) by white light continuum generation is delineated. Layout of the set up used in the present thesis is also described in detail CHAPTER 3: CHARGE CARRIER DYNAMICS IN ULTRASMALL TiO 2 - ALIZARIN SYSTEM Present chapter deals with effect of size quantization on carrier dynamics in a dye sensitized ultrasmall TiO 2 system. A QD presents a discrete set of acceptor states as opposed to a continuum of states in a bulk semiconductor. This is bound to influence ET rates from (injection) and back (BET) to dye. In such a system owing to discreteness it is might also be possible to extract hot electrons in presence of reduced relexation within individual bands in a QD. The system investigated in the present thesis is TiO 2 sensitized with alizarin dye. TiO 2 nanoparticles were synthesized by colloidal methods. Nanoparticles were characterized by XRD, TEM, Raman spectroscopy. Femtosecond dynamics were monitored by transient absorption spectroscopy. Conclusions on dynamics of injection and BET were drawn by monitoring the electron absorption signal after injection into TiO 2 . These studies clearly indicate an efficient charge separation in the system investigated apart from observation of multiple injection components. CHAPTER 4: EFFECT OF SIZE QUANTISATION ON CHARGE CARRIER DYNAMICS IN CdTe QDs Effect of size quantization on carrier relaxation (cooling and recombination) dynamics in size series of CdTe QD is investigated in this chapter. Series of CdTe QDs of different size was synthesized by colloidal methods in water. These samples were characterized by structural and photophysical techniques. Apart from discreteness in individual bands, surface to volume xvi
atio varied with size of samples. These factors affect not only recombination dynamics but also cooling dynamics. The bleach formation is an index of electron cooling times and recovery corresponds to recombination times. In a QD cooling of electrons is governed by several mechanisms. Therefore we have attempted to draw conclusions on cooling processes involved by studying size dependent dynamics in CdTe QDs. The dynamics were monitored throughout visible region to arrive at mechanism of cooling and recombination in QDs. These studies help in understand factors influencing relaxation dynamics in QDs. CHAPTER 5: CHARGE CARRIER DYNAMICS IN TAILORED CdSe/ZnTe CORE SHELL NANOSTRUCTURE Control over relaxation behavior of QDs is of immense interest as such a control will provide us a way to manipulate a nanostructure according to different application for e.g. a straddled line up in a core shell helps in improving luminescence behavior or a staggered line up helps in charge separation therefore in photovoltaics. In such nanostructures charge relaxation is modified compared to core due modification ban alignment but also due to formation of an interface. In this chapter we explore how a staggered interface alignment in a QD core shell influences carrier dynamics. Core shell samples were synthesized by colloidal methods by growing a shell over core under conditions where homogeneous nucleation is negligible. The system chosen for core shell is CdSe/ZnTe as this system has a staggered band alignment. In this case charge transfer either an electron or hole takes place depending on the band alignment. This is particularly useful in applications where charge separation is required for e.g. photovoltaics. In the present work we have attempted to answer how fast this transfer takes place by monitoring bleach formation kinetics of CdSe. The cooling dynamics with respect to thickness of shell revealed ultrafast hole transfer from CdSe to ZnTe (core to xvii
Page 1: Charge Transfer Dynamics in Quantum
Page 7: DECLARATION I, hereby declare that
Page 11: ACKNOWLEDGEMENTS It is my privilege
Page 14 and 15: 1.3.5. Defect Mediated Relaxation 2
Page 16 and 17: 2. 8. 4. White Light Generation- 45
Page 18 and 19: 6.2. Experimental 6.2.1.Synthesis o
Page 20 and 21: devices based on QDs it has been sh
Page 24 and 25: shell). This clearly indicated that
Page 26 and 27: 12. V. I. Klimov, J. Phys. Chem. B,
Page 28 and 29: 1.13 Reactant and Product Potential
Page 30 and 31: light. Inset: Kinetic traces monito
Page 32 and 33: 6.5 Transient decay kinetics of gra
Page 34 and 35: at 670 nm after exciting at 400 nm
Page 37 and 38: ABBREVIATIONS BET BQ CB CCD CdS CdT
Page 39: 1 Chapter 1
Page 42 and 43: 3 size dependent optical properties
Page 44 and 45: 5 1.2. Physics of Semiconductors 1.
Page 46 and 47: 7 As seen from the schematic, poten
Page 48 and 49: 9 Substituting this in Schrödinger
Page 50 and 51: 11 ( r, r ) ( r ) ( r ) (1.13) e
Page 52 and 53: 13 E E E 2 2 2 EX ne, le nh,
Page 54 and 55: 15 spherical symmetry of field. The
Page 56 and 57: 17 gE ( ) 2Em 2 3 3 (1.25) For a
Page 58 and 59: 19 1.3.2. Electron-Hole energy tran
Page 60 and 61: 21 Impact Ionization Figure 1.7. Sc
Page 62 and 63: 23 understanding on mechanistic asp
Page 64 and 65: 25 carriers are unable to sample th
Page 66 and 67: 27 CB VB QD Metal Figure. 1. 10. Sc
Page 68 and 69: 29 energy barrier. Therefore it is
Page 70 and 71: 31 1 f q 2 A qB (1.30) 2 In equa
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33 Vibrational contribution can be
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35 1. 7. 1. Electron Injection ET i
Page 76 and 77:
37 Under assumption of invariance o
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39 distribution. To achieve good si
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41 initially achieved monodispersit
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43 and rate of charge transfer acro
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45 1.32 P. V. Kamat, J. Phys. Chem.
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47 Chapter 2
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49 chemical species. Since a partic
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51 fluorescence forms an important
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53 eliminated by use of standard wh
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55 sample. Raman spectroscopy is ba
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57 will appear bright and region wi
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59 volatile solvent is drop casted
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61 spots arise from diffraction fro
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63 The electrical signal is then ch
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65 delayed and inverted. The two si
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67 Pump-probe technique is one of t
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69 Amplifier Jade Stretcher fs Osci
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71 Figure 2.6. Optical layout of Ti
Page 114 and 115:
73 Grating Convex Mirror Concave Mi
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75 changes the polarization from ho
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77 Grating Output Input Mirror Grat
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79 μJ has a very high peak power.
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81 2.6. Dynamical Theory of X-Ray D
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83 Chapter 3
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85 Therefore the study of interfaci
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87 3. 2. Experimental Section 3.2.1
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89 and intensity show absence of ot
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91 which is also neutral; therefore
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93 electron transfer times. This re
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95 r B a * 0 3.1 me / me Now the
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97 nonadiabatic case the electron t
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99 drastically reduced leading to a
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101 While the study of injection dy
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103 study. Based on the injection d
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105 17. L. E. Brus, J. Phys. Chem.
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107 Chapter 4
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109 of the QDs. As a result surface
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111 4.2.2. Synthesis: The CdTe QDs
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113 lower hole state. In CdSe the l
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115 4.2 inset. The kinetic traces a
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117 growth of the bleach kinetics (
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119 previous section we have discus
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121 lived charge transfer complex w
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123 100 times concentration of the
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125 4.5. References 4.1. Efros, Al.
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127 Chapter 5
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129 been made in the synthesis of t
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131 metallic synthesis by arrested
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133 5.3. Result and discussion: 5.3
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135 indicating a confinement induce
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137 absorption studies that on form
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139 1000 c b PL Intensity 100 10 a
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141 samples due to a very weak exci
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143 dynamical spectrum is negligibl
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145 Earlier authors [5.4] reported
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147 Sample t r(ps) t 1(ps) t 2(ps)
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149 In addition to the cooling dyna
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151 pulse-width time scale followed
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153 5.20. Sreejith Kaniyankandy, Sa
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155 CHAPTER 6 Charge Separation in
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157 Studies by Wang et. al. [6.15]
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159 graphene in a 1M NaOH solution.
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161 Intensity (a.u.) 5 4 3 2 1 CdTe
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163 dynamics of graphene oxide and
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165 GO as shown in the Figure 3 rev
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167 electron acceptor for Graphene.
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169 governs the recombination dynam
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171 Sample τ 1 (ps) τ 2 (ps) τ 3
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173 A (mO.D) 10.0 0.0 -10.0 -20.0 0
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175 involved in the relaxation may
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177 Electron quencher (benzoquinone
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179 Scheme 6.1. Schematic of Electr
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181 6.6. Reina, A.; Jia, X.; Ho, J.
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183 6.26. Kaiser, A. B.; Navarro, C
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185 assigned to injection into diff
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187 transfer from CdSe core to ZnTe
Page 238 and 239:
189 However the position of bands i
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