란탄족이 도핑된 산화물형광체의 합성과 상방전환형광 특성 연구

Abstract

Recently, upconversion (UC) phosphors have attracted much attention because of its broad range of applications, for example in light emitting diodes (LED), flat displays and solar energy converters. Large variety of luminescence materials have been investigated by doping and co-doping of lanthanide and non lanthanide ions to solid matrices. A variety of excellent low phonon frequency oxide host materials have been reported such as Y3Al5O12, YAlO3, ZrO2 and Y2O3. These host materials matches well with the lanthanide dopants in terms of its valency and ionic radius and thus, resulting in its high melting point and low cut-off phonon energy. The present work deals with a detailed investigation on the UC luminescence from lanthanide doped and co-doped oxide phosphors. Various dopants of activator ions and sensitizer ion (Yb3+) have been used in combination with the oxide host for enhancement of UC luminescence resulting in a green, red (Er3+) and yellow (Ho3+) emissions. Additionally, the UC mechanisms of lanthanide doped oxide materials are analyzed using an energy level diagram and rate equation that includes the energy transfer processes. This thesis work follows a systematic studies and can be summarized as follows:

1. The UC luminescence properties of YAlO3:Er3+ nanophosphors are investigated. Initially, the samples are synthesized by a simple solvothermal reaction technique. The structural elucidation and crystallinity studies of the prepared samples are carried out using powder X-ray diffraction (XRD) while, its emission spectra was recorded at 975 nm diode laser. Their UC processes were explained by measuring the UC luminescence spectra and pump power dependence. 2. ZrO2:Er3+ and ZrO2:Er3+, Yb3+ phosphors were synthesized via a simple solvothermal technique. At low concentrations of Yb3+, the crystalline structure changed from tetragonal to monoclinic phase with the increase of sintering temperature. As the concentration of Yb3+ increased to a value above 0.05M, ZrO2 phosphors displayed a very stable tetragonal phase. 3. The UC luminescence properties of ZrO2:Ho3+ and co-doped ZrO2:Ho3+,Yb3+ nanophosphors with various concentrations of Yb3+ ions were synthesized via solvothermal technique. The prepared samples show a strong green, weak red and near infrared UC. The energy transfer processes between the Ho3+ and Yb3+ ions and the mechanisms involved have been investigated. 4. ABO4(A= Ca, Sr; B=W, Mo)¬:Er3+/Yb3+/Li+ phosphors tri-doped with different concentrations of Li+ ion ranging from 0 to 22.5 mol% were prepared using a solid-state reaction method. Their UC luminescence properties were investigated under a 975 nm laser diode excitation. The four kinds of phosphors namely CaWO4, CaMoO4, SrWO4, and SrMoO4 showed a strong green UC emission peaks at 530, 550 nm and a weak red UC emission. A detailed investigation on the optimum doping concentration of Li+ ions and the effects of Li+ concentration on UC emission intensity were studied in detail.

The experimental results obtained from this thesis can be utilized for the synthesis of UC phosphors and for a variety of applications. To conclude, the lanthanide doped oxide UC phosphors can be strongly established as one of the promising candidates in the field of display and white lighting source for LEDs and for biomedical application.