CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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170 as compared to that of GO. It is clearly seen that transient signal intensity increases dramatically in RGO-2 as compared that of GO. Transient spectrum broad absorption in the visible region (500 nm to 700 nm) shows higher absorption in the blue region of the spectrum. Furthermore the broad feature at 670nm is significantly reduced indicating that this feature might come from oxygen related trap state absorptions. We have monitored the transient decay kinetics at 670 nm and shown in inset of Figure 6.7. A (m O.D.) 40 30 20 10 0.1ps 0.4ps 1ps 4ps 10ps 100ps A (m O.D.) 12 8 670 nm 4 0 0 50 100 Time Delay (ps) 0 500 550 600 650 700 Wavelength (nm) Figure 6.7: Transient absorption spectra of reduced (24 hours) graphene oxide (RGO-2) at different time delay after exciting 400 nm laser pulse. Inset: Transient decay kinetics at 670 nm. The transient data can be fitted multiexponentially with time constants of τ 1 = 0.26 ps (65%), τ 2 = 2.5 ps (24.5%), τ 3 = 30 ps (6.5%), τ 4 = >400 ps (4%) (Table 6.2).
171 Sample τ 1 (ps) τ 2 (ps) τ 3 (ps) + τ 4 (ps) GO 0.9 ± 0.2(33.3%) 9 ± 2 (38.5%) >300 (28.2%) RGO-1 0.7 ± 0.2 (37%) 10 ± 2 (36.5%) >300(26.5%) RGO-2 0.26 ± 0.1 (65%) 2.5 ± 0.5 (24.5%) 30 ±5 (6.5%) + >300 (4%) Table 6.2: Life times of the transient absorption signal of graphene oxide (GO), reduced (1 hour) graphene oxide (RGO-1) and reduced (for 24 hours) graphene oxide (RGO-2) and at 670 nm after exciting at 400 nm laser light. It is clear that the transient signal decay significantly faster as compared to that of GO. The faster two lifetimes components (0.26ps and 2.5ps) contribute almost 90% to the relaxation process, which indicates a significant percolation in GO from sp 3 to sp 2 carbon. The first two components matches with excited dynamics of photo-excited graphene as measured in earlier reports. These fast components have been assigned to the relaxation by electron-phonon interactions. It is reported in the literature [6.31] that carrier relaxation process in graphite are hindered by accumulation of hot phonons at higher carrier density which is called hot phonon bottleneck effect as a result one can see slower component in the relaxation dynamics. Kamfrath et al. [6.32] measured 2.5 ps component of carrier relaxation dynamics in photo-excited graphite phonon mediated relaxation. In the present studies the slower 0.26ps and 2.5 ps can be attributed interactions with optical and acoustic phonons respectively. However in the decay kinetics of RGO-2, we can still observe ~ 4% contribution >400ps component. In case of transient decay analysis of GO we have explained that >400 ps component arises due to deep trap states. We can see clearly see that on reduction of GO to RGO-2 the contribution of 30ps and >400 ps component. These
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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
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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
Page 168 and 169: 123 100 times concentration of the
Page 170 and 171: 125 4.5. References 4.1. Efros, Al.
Page 173: 127 Chapter 5
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
Page 190 and 191: 143 dynamical spectrum is negligibl
Page 192 and 193: 145 Earlier authors [5.4] reported
Page 194 and 195: 147 Sample t r(ps) t 1(ps) t 2(ps)
Page 196 and 197: 149 In addition to the cooling dyna
Page 198 and 199: 151 pulse-width time scale followed
Page 200 and 201: 153 5.20. Sreejith Kaniyankandy, Sa
Page 203 and 204: 155 CHAPTER 6 Charge Separation in
Page 205 and 206: 157 Studies by Wang et. al. [6.15]
Page 207 and 208: 159 graphene in a 1M NaOH solution.
Page 209 and 210: 161 Intensity (a.u.) 5 4 3 2 1 CdTe
Page 211 and 212: 163 dynamics of graphene oxide and
Page 213 and 214: 165 GO as shown in the Figure 3 rev
Page 215 and 216: 167 electron acceptor for Graphene.
Page 217: 169 governs the recombination dynam
Page 221 and 222: 173 A (mO.D) 10.0 0.0 -10.0 -20.0 0
Page 223 and 224: 175 involved in the relaxation may
Page 225 and 226: 177 Electron quencher (benzoquinone
Page 227 and 228: 179 Scheme 6.1. Schematic of Electr
Page 229 and 230: 181 6.6. Reina, A.; Jia, X.; Ho, J.
Page 231: 183 6.26. Kaiser, A. B.; Navarro, C
Page 234 and 235: 185 assigned to injection into diff
Page 236 and 237: 187 transfer from CdSe core to ZnTe
Page 238 and 239: 189 However the position of bands i
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CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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