Spectroscopic Properties Of Rare-earth Ions In Amorphous Materials

This thesis is focused on the spectroscopic properties of rare-earth ions in amorphous materials. The new results obtained felled into two parts:one part is the study of local-field effect on the optical transition of emitters in dielectric media, which is detected using Eu3+embedded in glass systems. The detailed theory and experimental results are presented in Chapters4and5. The other part is on the mechanism of and the temperature dependence on the greatly enhanced upconverted luminescence in glass ceramics (GC), with details in Chapters6and7.The outline of the thesis chapter by chapter is as follows:Chapter1presents the background, significance and status of the area in general. In Chapter2,1discuss some relevant fundamentals of spectroscopy and general features of glass and GC. Chapter3focuses on the preparation of glass and GC and the techniques of measurement and analysis of optical spectra used in this thesis.Chapter4focuses on the study of local-field effects on the photoluminescent spectra using Eu3+as probes in glass. First, the mechanism of the local field effect is presented. Then, according to the time-dependent perturbation theory in modern quantum mechanics and the theory of macroscopic electrodynamics, the spontaneous emission rate of an isolated electric dipole emitter depends on the local structure of the medium and the appropriate models, and both of the two most accepted models, i.e., the virtual-cavity model and the real-cavity model, for the local-field effect are functions of the refractive index of the medium. Based on the theoretical derivation and the comparison of ratios of the enhancement factors in two models, a practical criterion on which model is applicable for a given system is presented with a critical review of the available experimental and theoretical results. According to the criterion, VC model should be more appropriate for the electric dipole emission rates of Eu3+in glass, in contrary to what has been reported in Ref.[Phys. Rev. Lett.91,203903(2003)] that the decay of5D0of Eu3+in lead borate glass follows the RC model. Recently, a more scrutinized experimental study on Eu3+in the same glass system by my supervisor C. K. Duan, showed that the results actually follow the VC model, in consistency with the discussion presented in Chapter4. To further show the generality of the criterion, we also study the local-field effect on the emission spectra of Eu3+ions in two new glass systems. Since the predictions of the two models vary distinctively only when the refractive index n is big enough and varies in a range wide enough, a series of glass samples with refractive indices cover most of the1.5-2.2range are desired. Another requirement is that the local coordination structure of Eu3+is stable when the refractive index is adjusted by varying glass composition, so that the influence of the variation of the electric dipole moment of the5Doâ†'7FJ optical transitions of Eu3+is negligible. By referring to some available data on the influence of glass composition on the optical refractive index, we eventually chose the following two glass series for the study:1)xB2O3+(79-x) TeO2+5La2O3+1/2Eu2O3+10ZnO and2)(99-x)(2/3SiO2+1/3B2O3)+x PbO+1/2Eu2O3. Detailed quantitative studies using the refractive indices, measured intensity ratios and decay lifetimes confirm that the VC model can better describe the results than the real-cavity model. We concluded that, at least for isolated positive luminescent ions in solids, the VC model should be applied to describe the local-field effect on the spontaneous emission rate.Chapter5is mainly focused on the understanding of Eu3+emission spectra in glass, which is an extension of the study presented in Chapter4. In both of Eu3+-doped lead-borosilicate and boro-tellurite glass system, the same changing pattern is observed, i.e., the peak energies for5D0â†'7F0transition are blue-shift and the widths of7F1are broadened when the excitation energies gradually increases. To justify and interpret this experimental phenomenon, firstly we proposed correction to the calculation of the width of7F1CF splitting initially presented in Ref.[Phys. Chem. Chem. Phys.12,9933(2010)] and then obtained the correlation between the intensity ratio of5D0â†'7F0versus5Doâ†'7F2and the width of7F1. By using the the adapted model, quantitative analyses have been carried out and greatlly improved agreement between theory and experiments than using the original model is obtained. These results indicate that the relationship between the spectral intensity ratios and the CF splitting widths in glass should be quite general. The spectral analysis procedure could be put into test in other non-crystalline systems and expected to be useful for the local structure investigations in further studies.Chapter6reports the enhanced upconverted emission in novel Ho3+-doped GC containing BaYbF5nanocrystals. The origin intention of fabricating GC is to further study the local field effects on the spontaneous emission rate, but we find some new phenomena and applications of the spectroscopic properties of rare-earth ions. GC it themselves are more interesting and so in this and the following chapter I will focus on them. GCs, as a kind of the most attractive photoluminescent materials, have been designed for a wide variety of applications owing to the merging of low cut-off phonon energies of fluoride hosts for luminescent ions and the desirable mechanical and chemical characteristics of silicate based glass to encapsulate these luminescent micro/nano crystals. In this work, Ho3+-doped transparent oxyfluoride SiO2-Al2O3-Na2CO3-CaO-BaF2-YbF3GC containing BaYbF5nanocrystals of8-22nm were fabricated via melt-quenching technique with subsequent heat treatment. The formation of crystalline fluoride phase was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Compared to precursor glasses, the greatly enhanced green emission (40-fold), new emission band at ultraviolet-blue region and stark splittings of emission in GC, indicate that Ho3+enters into BaYbF5nanocrystals with low phonon energy. Besides, the origin of the previously unconfirmed emission band at440-460nm is clearly identified by measuring spectra from thermally coupled luminescent levels at various temperatures. The outstanding upconversion properties of Ho3+in GC may present potential application in all-solid-state upconversion lasers operating in the visible and ultraviolet range.Chapter7focuses on the study of the temperature-dependent upconverted luminescence due to their significantly potential applications in temperature sensors. Although the temperature-dependent upconverted spectra were measured in Chapter6, there exists the common heating effect caused by980nm excitation because of high concentrations of Yb3+in GC containing BaYbF5nanocrystals. In this work, we carefully selected and fabricated transparent oxyfluoride SiO2-Al2O3-Na2CO3-CaO-NaF-YF3GC containing25nm-sized NaYF4:Yb3+/Er3+crystals with high upconverted efficiency and low laser-induced temperature. The intensities of green UC emissions from2H11/2â†'4115/2and4S3/2â†'4115/2transitions enhances for about15times relative to those for the PC sample. The FIR of the two green UC emissions is characterized for optical thermometry in the range from298K to693K. A fitting of the FIR by an exponential function gives an effective energy difference of775cm-1, from which a relative temperature sensitivity of1.24%K-1around300K is obtained. The enhanced UC emissions and good temperature sensitivity indicate that the transparent GC containing nanocrystalline NaYF4:Yb3+/Er3+could be a promising candidate for optical fiber point temperature sensing.Finally, I sum up the results obtain in this thesis and give a perspective for further work.