CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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97 nonadiabatic case the electron tunneling occurs, injection times and efficiency of injection depend on the electron coupling between the dye and the semiconductor or in other words the density of states has major effect on the injection time and yield, whereas in the case of an adiabatic electron transfer process it is independent on the density of states. As mentioned earlier, studies [3.4] on alizarin/ TiO 2 system revealed an ultrafast injection with a time of 6 fs. Theoretical studies on Alizarin/ TiO 2 [3.31] revealed that the electron injection event in this system could be explained on the basis of adiabatic process. While the molecular dynamics by the same group on the electron injection from alizarin to hydrated Ti 4+ ion revealed that unlike bulk TiO 2 the injection takes place via nonadiabatic process [3.31]. Duncan et al [3.32] based on their molecular dynamics study concluded that in case of the hydrated Ti 4+ the non availability of the acceptor levels leads to weaker coupling between the dye and semiconductor. The experimental results by Huber et al [3.4] and molecular dynamics calculations [3.31] suggested that electron injection process in alizarin/ TiO 2 system can be an adiabatic process. Again alizarin molecule strongly coupled with TiO 2 through catecholate moiety which makes a 5 member ring like catechol/TiO 2 complex. Both experimental observation [3.33] and theoretical calculations [3.34-3.35] suggested that on interaction with TiO 2 nanoparticles, catechol molecule a new band originated and a direct charge transfer from catechol to semiconductor surface take place during photoexcitation justifying the process an all adiabatic process. Now by observing the type of binding in alizarin/TiO 2 system and from ultrafast measurements [3.4], it is obvious to think that electron injection process can also be an all adiabatic process. However, recent consolidated theoretical review by Duncan and Prezhdo [3.36] suggests that the story is completely different as compared to catechol/TiO 2 system. Being a larger molecule than catechol,
98 alizarin has extended -electron system and therefore, smaller excitation energy. Due to the presence of electronegative quinine oxygens lowers the absolute value of the ground and excited states. As a result the optical activity for the charge-transfer excitations in the TiO 2 - bound alizarin is significantly smaller than in catechol system, even though the chromophore-semiconductor coupling is exactly the same. As suggested by Duncan and Prezhdo [3.36] that the π* orbital of alizarin, spread over the entire molecule and is much more extended the π * orbital of catechol and therefore mixed less with Ti orbitals and as a result it offers weakly optically active charge-transfer transition bound to Ti through catecholate moiety. So it clearly realized that electron injection process in alizarin/TiO 2 cannot be all adiabatic process. It has been realized that to demonstrate the theoretical prediction it is very important to create a perfect platform where one can observe electron injection process in alirarin/TiO2 system can be mixture of both adiabatic and nonadiabatic process. In the present investigation we have observed that the electron transfer process is multi-exponential and there are longer injection components corresponding to times of 17 ps and 50ps in addition to the ultrafast component (Table 3.1). Therefore the electron transfer event in the present case purely suggests that electron injection in alizarin/TiO 2 cannot be all adiabatic as suggested by Duncan and Prezhdo in their recent review. According to the optical absorption in UV-vis measurements and the size of the particles as measured in high resolution TEM suggest that the present system is a finite one which could in fact lead to a decrease in the density of the acceptor states on the TiO 2 surface and this could give rise to multi-exponential injection. This fact itself is significant in the sense that if one can control the injection into different acceptor levels within the conduction band. The loss of energy via the emission of phonons once the electron is injected into the conduction band gap be
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Charge Transfer Dynamics in Quantum
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DECLARATION I, hereby declare that
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ACKNOWLEDGEMENTS It is my privilege
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1.3.5. Defect Mediated Relaxation 2
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2. 8. 4. White Light Generation- 45
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6.2. Experimental 6.2.1.Synthesis o
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devices based on QDs it has been sh
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Furthermore generation of pump (~40
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shell). This clearly indicated that
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12. V. I. Klimov, J. Phys. Chem. B,
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1.13 Reactant and Product Potential
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light. Inset: Kinetic traces monito
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6.5 Transient decay kinetics of gra
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at 670 nm after exciting at 400 nm
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ABBREVIATIONS BET BQ CB CCD CdS CdT
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1 Chapter 1
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3 size dependent optical properties
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5 1.2. Physics of Semiconductors 1.
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7 As seen from the schematic, poten
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9 Substituting this in Schrödinger
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11 ( r, r ) ( r ) ( r ) (1.13) e
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13 E E E 2 2 2 EX ne, le nh,
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15 spherical symmetry of field. The
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17 gE ( ) 2Em 2 3 3 (1.25) For a
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19 1.3.2. Electron-Hole energy tran
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21 Impact Ionization Figure 1.7. Sc
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23 understanding on mechanistic asp
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25 carriers are unable to sample th
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27 CB VB QD Metal Figure. 1. 10. Sc
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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
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
Page 112 and 113: 71 Figure 2.6. Optical layout of Ti
Page 114 and 115: 73 Grating Convex Mirror Concave Mi
Page 116 and 117: 75 changes the polarization from ho
Page 118 and 119: 77 Grating Output Input Mirror Grat
Page 120 and 121: 79 μJ has a very high peak power.
Page 122 and 123: 81 2.6. Dynamical Theory of X-Ray D
Page 125: 83 Chapter 3
Page 128 and 129: 85 Therefore the study of interfaci
Page 130 and 131: 87 3. 2. Experimental Section 3.2.1
Page 132 and 133: 89 and intensity show absence of ot
Page 134 and 135: 91 which is also neutral; therefore
Page 136 and 137: 93 electron transfer times. This re
Page 138 and 139: 95 r B a * 0 3.1 me / me Now the
Page 142 and 143: 99 drastically reduced leading to a
Page 144 and 145: 101 While the study of injection dy
Page 146 and 147: 103 study. Based on the injection d
Page 148 and 149: 105 17. L. E. Brus, J. Phys. Chem.
Page 154 and 155: 109 of the QDs. As a result surface
Page 156 and 157: 111 4.2.2. Synthesis: The CdTe QDs
Page 158 and 159: 113 lower hole state. In CdSe the l
Page 160 and 161: 115 4.2 inset. The kinetic traces a
Page 162 and 163: 117 growth of the bleach kinetics (
Page 164 and 165: 119 previous section we have discus
Page 166 and 167: 121 lived charge transfer complex w
Page 168 and 169: 123 100 times concentration of the
Page 170 and 171: 125 4.5. References 4.1. Efros, Al.
Page 176 and 177: 129 been made in the synthesis of t
Page 178 and 179: 131 metallic synthesis by arrested
Page 180 and 181: 133 5.3. Result and discussion: 5.3
Page 182 and 183: 135 indicating a confinement induce
Page 184 and 185: 137 absorption studies that on form
Page 186 and 187: 139 1000 c b PL Intensity 100 10 a
Page 188 and 189: 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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