CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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devices based on QDs it has been shown that Size of QDs also affects photovoltaic efficiency, maximum current and V oc [7, 8]. This is due to size dependent variation in electronic properties leading to change in free energy and surface properties. Work on QDs has shown that surface plays major roles in determining photovoltaics properties due to the fact that surface to volume ratio of QDs are very high because of their small dimensions. Apart from tunability of electronic properties, in QDs there are several reports on production of multiple excitons induced by excitation of QD with photons of energy many times the band gap energy [9, 10]. This novel effect also has been exploited in photovoltaics to improve the efficiency of energy conversion [11]. Additionally one can use a QD in place of TiO 2 as the electron acceptor which would help in hot carrier based DSSC. As mentioned earlier the surface, interface properties have a clear bearing on photovoltaic efficiency. Therefore it is imperative to understand photo induced dynamics which would further help in improving the properties of quantum dot. In that context transient absorption spectroscopy has been a valuable tool in understanding the influence of surfaces and interfaces in relaxation dynamics in time scale of few hundred fs to ms [12-14]. The main thrust in the present thesis is on synthesis of QDs and QD based nanostructures and study of its dynamics with change in size, surface and nanostructure formation. The QDs synthesized in the present thesis are mostly II-VI semiconductors. Nanostructures were formed with a core shell geometry and QDs decorated metal. The study not only involves influence of size but also charge separation behavior in novel nanostructures like core shell. The QD based nanostructure have been synthesized and structurally characterized by TEM, Raman spectroscopy, XRD. The dynamics was measured by femtosecond spectroscopy transient absorption. xiv
CHAPTER 1: GENERAL INTRODUCTION This chapter gives a brief introduction to nanotechnology and basic semiconductor physics and electronic structure of a bulk crystal with simple quantum mechanical models. A brief description of theoretical framework based on effective mass approximation to describe changes in electronic structure with size. Typical band structures in real systems like CdSe, CdTe etc is also described. A brief account of processes involved in relaxation of carriers on photoexcitation is described. Methods to improve the relaxation behavior are also discussed with particular emphasis on core shell and metal semiconductor nanostructure. A concise description of Marcus theory of electron transfer which gives the theoretical framework for electron transfer is also outlined. Additionally brief discussion on types of electron transfer processes, adiabatic and non adiabatic is included. Importance of synthesis conditions to obtain monodisperse particles is delineated along with synthesis technique used in present thesis for II-VI QDs. CHAPTER 2: EXPERIMENTAL TECHNIQUE A summary of experimental techniques used in this thesis for characterization of materials to dynamics study is described in this chapter. A brief outline of transmission electron microscope and additional information like electron diffraction and elemental analysis is outlined. Additionally description of steady state UV-Vis absorption, photoluminescence spectroscopy is also delineated. A brief description Time correlated single photon counting technique and instrumentations involved is explained. Hot exciton relaxation in QDs takes place in the femtosecond time scale, to study ultrafast phenomena we have used Femtosecond Transient absorption spectroscopy. Basic techniques involved in generation of a femtosecond pulse, amplification and characterization is described. xv
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 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 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
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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
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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
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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
Page 238 and 239:
189 However the position of bands i
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