BaO-La2O3-WO3三元系統化合物螢光粉之光致發光特性研究

Abstract

In this work, BaLa2WO7: Ln3+(Ln3+ = Eu3+, Dy3+, Sm3+, Er3+, Ho3+, Tb3+, Tm3+) phosphors have been synthesized via a solid-state reaction. The absorption, excitation, emission and decay curves were obtained to study the luminescence properties. The effects of substituting Ba2+ with Sr2+ and Ca2+ in BaLa2WO7: Ln3+ to improve the optical properties are also studied. The experimental results indicated that the crystal can be assigned to the structural nature of the BaLa2WO7 single phase as the calcination temperature in the range of 900 ~ 1350°C, and the rare-earth ions were satisfactorily substituted for the La3+ ions in BaLa2WO7 monoclinic structure. BaLa2WO7 doped with rare-earth ions at different doping concentrations do not significantly affect morphology. The broad band located around 338 nm of the excitation spectrum is due to the charge-transfer state (CTS) band caused by electron transfer from oxygen to tungsten (ligand-to-metal charge-transfer transitions, LMCT). However, the CTS band of the oxygen 2p orbital to the empty 4f orbital of Ln3+ is weak and immersed in the CTS band of WO6 due to their week covalency. The sharp peaks in the range of 350 to 600 nm are assigned to the typical intra-4f forbidden transitions of the Ln3+ ions. The emission spectra of Eu3+-doped BaLa2WO7 phosphors excited at 395 nm exhibit a series of sharp peaks, which are attributed to the 5D0 → 7FJ (J=0,1,2,3,4) transitions. Luminescence from higher excited states, such as 5D1, 5D2, and 5D3, were also observed at low Eu3+ concentration. The optimal emission intensity of 5D0 → 7F2 red emission is at x=0.4 (BaLa1.6Eu0.4WO7). The chromaticity coordinates of Eu3+-doped BaLa2WO7 phosphors vary with Eu3+ content from white, orange-red, to red. The (5D0 → 7F2) / (5D0 → 7F1) asymmetry ratio increased with increasing Eu3+ concentration, reveling that the local structural symmetry around Eu3+ significantly changed as Eu3+ became incorporated into BaLa2WO7. Dy3+-doped BaLa2WO7 phosphors emit bright near-white light. Under an excitation wavelength of 351 nm, two dominant emission peaks were seen at 485 and 572 nm, which correspond to the 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions of Dy3+, respectively. The intensity of the emission from the 4F9/2 → 6H15/2 transition was higher than that from the 4F9/2 → 6H13/2 transition, revealing that Dy3+ occupied a relatively symmetrical site in BaLa2WO7: Dy3+. The doping of Dy3+ ions did not change the site symmetry even at a high concentration. By analyzing the emission spectra and decay behaviors, the energy transfer due to cross-relaxation and concentration quench effect over ion-ion interaction between two neighboring rare-earth ions provide an extra decay channel, the luminescence centers have different local environments and the existence of more than one relaxation process. When excitation energy from an ion decaying from a highly excited state promotes a nearby ion from the ground state to the metastable level, cross-relaxation can occur easily between two neighboring rare-earth ions, such as Sm3+(4G5/2), Er3+(4S3/2), Tm3+ (1D2), and Ho3+(5S2). Introducing Sr2+ into Ba2+ site can further enhance the emission intensity but do not change the site symmetry around rare-earth ions. However, the substitution of Ba2+ by Ca2+ leads to an intense crystal distortion, resulting in a degradation of the local site symmetry around Eu3+ and Dy3+. Consequently, the color of Ca-substituted phosphors, Ba1-yCayLa2WO7: Dy3+, can be tuned by increasing Ca2+ content. All synthesized phosphors in the study can emit different colors by doping different kinds of activators and varying doping concentrations, such as white to red (Eu3+), green (Er3+, Ho3+), orange-red (Sm3+), blue (Tm3+), and near-white (Dy3+). The experimental results revel that the excitation wavelength of BaLa2WO7: Ln3+ phosphors and the emission wavelength of blue- and UV-LEDs (350 ~ 450 nm) have closely overlapped, and the sharp emission peaks show that BaLa2WO7 is suitable for rare-earth-doped phosphors, making it an attractive candidate for use in optical applications.