CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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23 understanding on mechanistic aspects has been achieved, the efficiency has not improved from ~11%. Quantum dots have significant potential in photovoltaics due to ability to tune electronic structure by size [1.32-1.34]. Therefore V OC can be tuned by just changing the size. Additionally use of wide band gap QD in a DSSC also can lead to improvements as BET process is expected to be delayed according to theory as free energy for BET increases. FET and BET process are interfacial electron transfer phenomena which can be controlled by proper choice of semiconductor and dyes with anchors to adsorb on to semiconductor, therefore, leading to possibly control performance of devices. CB Dye e - LUMO hν e - Ox Red e - HOMO Electrolyte VB TiO 2 Anode Cathode e - Figure 1. 8. Schematic of Dye Sensitized Solar Cell
24 1.5. “Improving on” QDs As mentioned earlier QDs offer opportunities in several thrust areas of research due to tunability of properties with size. However large number of surface ions on a QD could lead to formation of traps leading to loss of carriers. Additionally since carriers are confined within a small volume the interaction between them is strong which leads to nonradiative relaxation which also can be detrimental to any device application. Some of these problems can be overcome by creation of heterostructures suited for that particular purpose. Some typical heterostructures which are used commonly to “improve on” QDs are described below. 1.5.1. Semiconductor-Semiconductor Core Shell Nanostructures In the previous sections we have described how electronic structure is modified in a QD with size. Here we discuss the implications of how a different semiconductor grown over a quantum dot can influence its properties. It is well known that two different semiconductors have different band alignment and electronic structure. Therefore the band alignment at the interface can significantly alter static and dynamics properties due to several reasons. We list few reasons here (i) growth of another semiconductor can annihilate defects at the interface (ii) since band alignment at interface is different it can produce a charge separation or localization depending on alignment (iii) doping. These effects depend on lattice structure, spacing, band alignment etc. Most commonly used core shell nanostructure is Type I or Type II heterostructure. In the case of type I nanostructure as described in the figure 1.9, band alignment is in such a way that on photoexcitation carriers are localized in the core. This has an added advantage that
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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 60 and 61: 21 Impact Ionization Figure 1.7. Sc
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
Page 110 and 111: 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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