CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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21 Impact Ionization Figure 1.7. Schematic of Impact Ionization Process 1.3.5. Defect Mediated Relaxation Surface to volume ratio in a nanoparticle is very high, which leads to creation of defects at surface. The defects could arise from dangling bonds, surface reconstruction, capping agents etc. If these defect states arise within mid gap region i.e. between the two bands then they can influence carrier relaxation by trapping the carrier [1.23]. In some cases defects can also have levels within the individual valence and conduction bands. Such defect states are called surface resonances. These states can also involve in aiding cooling of carriers. 1. 4. How Can QDs Help in Solar Cells? One of most important application of QDs is in the field of photovoltaics which has in recent time gained much importance due to energy crises as green approach to solve the energy crises. Photovoltaic effect was first observed by E. Becquerel [1.28]. Photovoltaic technology involves use of a mediator which absorbs solar light and converts that to
22 electricity. This forms the basis for a solar cell or photovoltaic cell and material that acts as a mediator is a semiconductor. The semiconductor on exposure of light creates electrons and holes in the CB and VB which has to be separated before recombination to build and efficient solar cell. The separation of electrons and holes created in photovoltaic cells can be achieved by forming and inbuilt field. Solar cells are usually divided into three generations. The first generation solar cells consisted of mainly silicon. Cell is achieved by creation of a pn junction from Si. Such cells are known to be efficient with efficiency ~24%. However cells are to be of electronic grade which means their purity should be extremely high. Therefore it is very expensive. The second generation solar cells consists of thin film based cells mainly consisting of poly-silicon, amorphous silicon, Copper Indium Selenide and Cadmium Telluride. These cells are comparatively cheaper but have a lower efficiency ~19%. The third generation solar cells are based on polymers, nanocrystals and dye sensitized solar cells. Dye sensitized solar cells use wide band gap semiconductor like TiO 2 and ZnO. The semiconductors do not absorb visible light. The light is absorbed by a dye which transfers charges to semiconductor and HOMO of cell is replenished by a redox electrolyte (see figure 1.8.). One of the first studies on influence of mechanistic aspects electron transfer reactions in a cell was carried out by Gerischer and Memming [29-30]. These studies clearly showed potentials of a dye sensitized solar cell. In 1990s, Michael Graetzel in Ru(dcbpy) 2 (NCS) 2 [dcbpy=(4,4’-dicarboxy-2,2’-bipyridine] or (Ru-N3) dye sensitized mesoporous TiO 2 nanoparticles reported conversion efficiency of ~11% in a iodine/iodide based redox electrolyte [32]. The schematic shown in figure 1.8 describes different processes occurring in a DSSC. The efficiency of a DSSC depends on factors like electron injection time, efficiency of electron injection, free energy for back electron transfer (BET) etc. Though significant
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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 22 and 23: Furthermore generation of pump (~40
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 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
Page 72 and 73: 33 Vibrational contribution can be
Page 74 and 75: 35 1. 7. 1. Electron Injection ET i
Page 76 and 77: 37 Under assumption of invariance o
Page 78 and 79: 39 distribution. To achieve good si
Page 80 and 81: 41 initially achieved monodispersit
Page 82 and 83: 43 and rate of charge transfer acro
Page 84 and 85: 45 1.32 P. V. Kamat, J. Phys. Chem.
Page 87: 47 Chapter 2
Page 90 and 91: 49 chemical species. Since a partic
Page 92 and 93: 51 fluorescence forms an important
Page 94 and 95: 53 eliminated by use of standard wh
Page 96 and 97: 55 sample. Raman spectroscopy is ba
Page 98 and 99: 57 will appear bright and region wi
Page 100 and 101: 59 volatile solvent is drop casted
Page 102 and 103: 61 spots arise from diffraction fro
Page 104 and 105: 63 The electrical signal is then ch
Page 106 and 107: 65 delayed and inverted. The two si
Page 108 and 109: 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
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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
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189 However the position of bands i
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