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Title: Use of luminescence spectra in analyzing metal ion doped systems
Other Titles: Shan za jin shu li zi ti xi de ying guang guang pu fen xi
Authors: Pan, Zaifa (潘再法)
Department: Department of Biology and Chemistry
Degree: Doctor of Philosophy
Issue Date: 2007
Publisher: City University of Hong Kong
Subjects: Spectrum analysis.
Luminescence spectroscopy.
Notes: xxv, 253 leaves : ill. (some col.) 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2007.
Includes bibliographical references (leaves 235-253)
CityU Call Number: QD555.6.S65 P36 2007
Type: thesis
Abstract: The transition metal ions and lanthanides ions may be employed as optically activated centers, due to their unique electronic configurations,which result in special spectroscopic properties. Thus, these ions have always played a prominent role in lighting and light conversion technologies. However, their optical properties not only depend on the nature of the active ion, but also on the site environment provided by the host lattice. In this dissertation, five series of metal ion doped luminescent materials have been synthesized. Luminescence spectra and other characterization techniques have been employed to give the optical information of the doped ions, the structure properties of various hosts and their effects imposed on the chromophore ions. After an introduction concerning the background on luminescence, host lattice structure and transition selection rules in Chapter 1, Chapter 2 describes the experimental part of the thesis. In Chapter 3, Er3+ and Eu3+ luminescence spectra have been applied to determine the structure of the dopant ion in α-Al2O3 powders. Samples of Er3+-doped α-Al2O3 were prepared by the preformed sol or nitrate-citrate methods at concentrations from 25 to 0.1 at.%. The luminescence spectra showed the formation of erbium garnet (ErAlO3) and/or perovskite (Er3Al5O12) compounds and no luminescence from Er3+ substituted at Al3+ sites was detected. Eu3+-doped α-Al2O3 has been synthesized by a drastic method, combustion synthesis, and then characterized by luminescence spectroscopy. Taking advantage of Eu3+, which can serve as a sensitive probe of its crystalline environment, low-temperature site-selective luminescence spectroscopy demonstrates that combustion synthesis can substitute Eu3+ for Al3+ (C3v) in α-Al2O3, despite the large size discrepancy between these ions. Site-selective spectroscopy also shows that some other impurity phases are present at a 1% dopant ion level. In Chapter 4, absorption, excitation and emission spectra for both SrCl2:Eu2+ and SrCl2:Yb2+ samples have been investigated. Samples were prepared by the Bridgeman method. The 10 K ultraviolet emission spectrum and low-energy ultraviolet absorption spectra were reported for dilute SrCl2:Eu2+. Only one luminescent state was observed under various synchrotron radiation excitation wavelengths. The broad room temperature 4f65d → 4f7 emission spectrum, peaking at 410 nm, sharpens considerably at 10 K and vibrational progressions in the totally symmetric Sr-Cl stretching mode of 210 cm-1 upon the zero phonon line and vibronic structure are predominant. The low energy 4f7 → 4f65d absorption spectrum comprises similar vibronic structure but many transitions overlap. The emission, absorption and excitation spectra are well simulated by calculation. Also, for the SrCl2:Yb2+ sample, there are two bands at 385 and 410 nm, respectively. However, we find at low temperature the second band is due to the emission spectra of SrCl2:Eu2+, and only one transition, (4f135d)Γ4u → (4f14)Γ1g, is expected for the Yb2+ emission spectra. Furthermore, these two bands show obvious temperature dependence. At room temperature, the two bands are broad and of similar intensity. On cooling, band 2 decreases in intensity and band 1 increases. Thus, the assignment of the second band is reconsidered and its temperature dependence is also discussed in this chapter. In Chapter 5, Ba2LaNbO6 has been synthesized by a solid state reaction and an unanticipated emission has been observed. In order to understand this emission, a series of Ba2LaNbO6:M (M=Pr3+, Yb3+, Eu3+, Nd3+, Tm3+, Mn4+ and Cr3+) systems have been synthesized using the same method. The powders obtained were examined by XRD and show the double perovskite structure. The emission and excitation spectra have been recorded and the peaks have been assigned under the site symmetry of the host lattice. After comparison of both the emission and excitation spectra, the origin of the anomalous emission in neat Ba2LaNbO6 is assigned to the Mn4+ impurity ion. The luminescence in this host shows concentration quenching and the mechanism for this phenomenon has been proposed. Phonon coupling transitions were recorded for Mn4+, Cr3+ and Eu3+ samples and assigned to different vibrational modes, by consideration together with the IR and Raman spectra. In Chapter 6, four Eu3+ doped trivalent yttrium complexes with different substituted 1,10-phenanthrolines (Y(R-1,10-phen)2(NO3)3:Eu3+; R = 5-Chloro-, 5-nitro, 5-methyl- and 4-methyl-) have been synthesized. The structures of 2 complexes were determined by X-ray crystallography. The luminescence spectra of the complexes have been recorded and interpreted. Vibrational coupling transitions are observed for most of the free ligands and their corresponding complexes. The energy levels of central lanthanide ions could be deduced from the electronic spectra, and the electro-inductive effect of the functional groups of the ligands on the f-electrons has been investigated. In chapter 7, the effects of different synthesis methods on the structure and luminescence properties of metal ion doped TiO2 are discussed. The materials were prepared by sol-gel synthesis and hydrothermal method, characterized by luminescence spectra and other related techniques. The results show that all the samples were composed of nanosized particles. The sol-gel method prepared Bi-TiO2 and Er-TiO2 both possess the anatase phase, while the hydrothermal method prepared Eu-TiO2 possesses the rutile phase. The photoluminescence (PL) emission spectra at 10K were examined and the main PL peak of the neat TiO2 sample were found to occur at 550 nm, while the band of Er3+ doped TiO2 sample was slightly red shifted with lower intensity and some superimposed sharper lines from 4f11-4f11 transitions of Er3+. The PL of Eu-TiO2 show strong Eu3+ emission and disclose the efficiency of energy transfer between the TiO2 and the doped ion. Furthermore, Bi-TiO2 was applied to the photocatalytic degradation experiments and the results demonstrated that the presence of Bi3+ in TiO2 catalysts substantially enhances the photocatalytic degradation of methylparathion in aqueous suspension. The possible mechanisms of photoluminescence quenching and photodegradation are elucidated in the context of donor-acceptor interactions with Bi-O polyhedra acting as electron trapping centres which hinder electron-hole pair recombination.
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