CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

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luminescence from the 5 D J states of Eu 3+ . The intensity of these transitions depends strongly on the site symmetry in a host crystal. Magnetic dipole allowed f-f transitions are not affected much by the site symmetry. The J selection rule in this case is ΔJ = 0, ±1 (except for 0→0). For electric dipole transition, the difference in the J values (ΔJ) is ±2. Oscillator strengths are of the order of 10 –5 to 10 –8 for electric dipole transitions, and 10 –8 for magnetic dipole transitions [31]. 1.5.3 Luminescence quenching: Luminescence can be quenched due to the non-radiative decay of the excited state. This non-radiative decay is brought about by different mechanisms involving energy transfer from the excited state of the donor to different types of acceptors like host lattice, organic molecules on the surface, defect levels or nearby ions which may or may not act as activator. In the following section different types of non-radiative path ways of the excited state is given. Multiphonon emission: As discussed in the previous section (equation 5) it is clear that the extent of quenching due to phonons depends on the energy gap between the emitting states and value of phonon energy. General observation is that if the energy difference is more than 5 times the phonon energy, then luminescence is the main de-excitation process than the multi-phonon quenching [42]. This also explains the quenching of lanthanide ions excited state by organic molecules containing functional groups with high phonon energy. Hence, efforts for the development of new efficient luminescent materials based on Ln 3+ ions are directed towards the search for low effective phonon energy host materials. The general order of phonon energy of hosts are fluorides < sulfides < oxides < phosphates. Energy transfer: An excited state can also relax to the ground state by non-radiative energy transfer to a second nearby state/ center. For energy transfer to take place the energy levels of the donor and acceptor should match. However, when there is a mismatch between the energy levels/ transitions of the donor and acceptor ions, the energy transfer process needs to be 18
assisted by lattice phonons of appropriate energy, ħΩ, and this is usually known as phononassisted energy transfer process. The interaction Hamiltonian for the energy transfer process involves different types of interactions; namely, multipolar (electric and/ or magnetic) interactions and/ or quantum mechanical exchange interaction. The dominant interaction is strongly dependent on the separation between the donor and acceptor ions and on the nature of their wave functions. Exchange interactions only occur if the donor and acceptor ions are close enough for direct overlap of their electronic wave functions. Consequently, energy transfer due to quantum mechanical exchange interactions between the D and A ions is only important at very short distances (nearest neighbor positions). Irrespective of the mechanism of energy transfer, the fluorescence lifetime of the donor center, τ D , is affected as a result of any energy transfer process to an acceptor [42]. Therefore, the lifetime of the donor ions (τ D ) can be expressed by equation 6. 1 1 = + Anr + Pt ……………………… (6) τ ( τ ) D D 0 where (τ D ) 0 is the radiative lifetime of the donor ion, A nr is the non-radiative rate due to multi-phonon relaxation, and P t is the transfer rate due to energy transfer. Hence the observed lifetime of the donor is a measure of extent of energy transfer. In addition to this, variation of the emission intensity as a function of time can also give information regarding the distribution of donor and acceptor centres in the system. Concentration Quenching: In principle, an increase in the concentration of a luminescent center in a given material should be accompanied by an increase in the emitted light intensity, this being due to the corresponding increase in the absorption efficiency. However, such behavior only occurs up to a certain critical concentration of the luminescent centres. Above this concentration, the luminescence intensity starts to decrease. This process is known as concentration quenching of luminescence. In general, the origin of luminescence 19
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SYNTHESIS AND CHARACTERIZATION OF L
Page 3 and 4:
STATEMENT BY AUTHOR This dissertati
Page 5 and 6: Dedicated to ……… My beloved b
Page 7 and 8: also thankful to all the members of
Page 9 and 10: 2.3 Synthesis of binary oxide nanom
Page 11 and 12: CHAPTER 5: Zinc Gallate ...........
Page 13 and 14: imaging, scintillators, lasers, etc
Page 15 and 16: morphology by heating at 500 and 90
Page 17 and 18: nanoparticles having hexagonal stru
Page 19 and 20: 100°C and 185°C, respectively in
Page 21 and 22: into poly methyl methacryllate (PMM
Page 23 and 24: 3. N. M. Dimitrijevic, Z. V. Saponj
Page 25 and 26: Fig.13: Fig.14: (a) Simplified ray
Page 27 and 28: Eu 3+ ions after heat treatment at
Page 29 and 30: 615 nm Fig.50: FT-IR patterns (a) a
Page 31 and 32: area electron diffraction pattern a
Page 33 and 34: different amounts of Eu 3+ . Fig.86
Page 35 and 36: and 545 nm, respectively. Fig.106:
Page 37 and 38: List of Tables: Table 1: Variation
Page 39 and 40: CHAPTER 1: Introduction 1.1 Histori
Page 41 and 42: Rods, cylinders, wires and tubes ar
Page 43 and 44: must be supersaturated either by di
Page 45 and 46: - + - + charge stabilized nanoparti
Page 47 and 48: exothermic reaction between the met
Page 49 and 50: constant, ħ is h/2π, e is the ele
Page 51 and 52: these platinum complexes have been
Page 53 and 54: (IR), visible and ultra-violet (UV)
Page 55: in Ce 3+ , Pr 3+ , Tb 3+ and CT abs
Page 59 and 60: decay-time reduction is much easier
Page 61 and 62: nanocrystals is shown in Fig.10 [58
Page 63 and 64: depends strongly on the nature of t
Page 65 and 66: corresponding emission and excitati
Page 67 and 68: doped nanomaterials of the above me
Page 69 and 70: will be formed and subsequently it
Page 71 and 72: Eu 3+ ions were also subjected to s
Page 73 and 74: transferred into a two-necked RB fl
Page 75 and 76: Where λ is the wavelength of X-ray
Page 77 and 78: Micro-structural characterization b
Page 79 and 80: electrons (i.e. diffraction mode).
Page 81 and 82: diffraction spots. By tilting a cry
Page 83 and 84: surface. The contact mode can obtai
Page 85 and 86: three types lines in the scattered
Page 87 and 88: solids tend to be broadened because
Page 89 and 90: sample fluoresce. The fluorescent l
Page 91 and 92: pulse frequency) as excitation sour
Page 93 and 94: Lanthanide ions doped Ga 2 O 3 nano
Page 95 and 96: atoms at three corners and the lone
Page 97 and 98: Figure 19 (a-d) shows SEM images of
Page 99 and 100: Based on these results, it can be i
Page 101 and 102: almost normal to the equatorial pla
Page 103 and 104: Table 2. Summary of important modes
Page 105 and 106: systematic disappearance of these t
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comparison, Eu 3+ ions alone in wat
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the samples, suggesting that Eu 3+
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These inferences are further substa
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Similar DTA peaks have been reporte
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GaOOH samples doped with different
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The cell parameters increases with
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strong emission compared to corresp
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curves are shown in Fig.39. Lifetim
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Similar results are also observed f
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intensity of XRD peaks correspond t
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compared to the bulk material and a
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is almost 1½ times that of peak B1
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3.8 Interaction of Eu 3+ ions with
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The spectrum shows strong emission
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The decay curves are found to be bi
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As prepared undoped Sb 2 O 3 sample
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Intensity(arb.units) 1.0 0.8 0.6 0.
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3+ having surface lanthanide ions.
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diffraction patterns from the GaPO
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5 D 0 7 F 2 level to 7 F 7 7 1 , F
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The second possibility is the excha
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chemical shift anisotropy is much h
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not have strong interaction with th
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(001) (110) (011) (101) (020) (101)
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growth centre compared to higher vi
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observed after excitation at 250 an
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3+ 3+ concentration quenching. Tb l
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Compared to the selected area elect
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lattice. Lanthanide ions occupying
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obtained after 275 nm excitation is
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The luminescence dynamics associate
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that the broad peak can be resolved
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espectively. Value of this integral
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ethylene glycol moiety (stabilizing
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Room temperature and 100°C synthes
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heavier metal ion as compared to La
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Similar studies were also carried o
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3+ 3+ faster decay component is ass
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the other hand the Eu 3+ lifetime h
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3+ 4.4.8 Luminescence studies on Sm
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CHAPTER 5: Zinc gallate (ZnGa 2 O 4
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In pure EG an amorphous product is
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water content in the reaction mediu
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nucleation, thereby leading to incr
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particles. Thus the TEM studies als
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value. The lattice parameters calcu
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Quantum yield of the blue emission
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5.5.2. Eu 3+ doped ZnGa 1.5 In 0.5
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CHAPTER 6: Tungstates [MWO 4 (M = C
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ionic radius of the metal cation ca
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lattice. Asymmetric ratio is ~ 12 f
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700 Intensity (arb.units) 600 λ ex
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Intensity (arb.units) 25000 20000 1
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Strong green emission has been obse
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consists of strong band at 255 nm a
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peak can be attributed to the cryst
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nm) is observed from Er 3+ doped Ga
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and Dy 3+ doped CaWO 4 nanoparticle
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REFERENCES 1. D. J. Barber, I. C. F
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32. K. A. Gschneidner, Jr., L. Eyri
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65. Z. Deng, F. Tang, D. Chen, X. M
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100. A. Rouanel, J. J. Serra, K. Al
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130. F. Li, W. Jianhuai, L. Jiongti
Page 233 and 234:
166. L. Fu, Z. Liu, Y. Liu, B. Han,
Page 235 and 236:
205. R. Sasikala, V. Sudarsan, C. S
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238. E. Oldfield, R. A. Kinsey, K.
Page 239 and 240:
271. B. G. Hyde, S. Andersson, ”I
Page 241 and 242:
B. S. Naidu, B. Vishwanadh, V. Suda
Page 243:
9. Room temperature synthesis of mu
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