Tuning the thermal quenching properties of Ga3+-modified LiTaO3:Bi3+ phosphor through defect engineering strategy

Abstract

Defects inside fluorescent materials are often an important cause of the anti-thermal quenching of fluorescent materials. Adjusting the thermal compensation effect of defects on luminous centers is often achieved by adjusting the types, quantities and depths of defect traps. In this contribution, we found that doping Ga3+ into LiTaO3 matrix containing intrinsic defects of lithium vacancies (VLi，) and antisite-tantalum (TaLi4+) resulted in Bi3+ from non-luminescence to luminescence. It is speculated that the luminescence is mainly due to the charge compensation of Ga3+ instead of Ta5+, which promotes the conversion from Bi2+ to Bi3+, and then makes phosphor produce obvious luminescence. Interestingly, due to the increase of Ga3+ content, when excited at 304 nm, the integral intensity of sample (0.10 Ga3+) at 420–470 nm is 1.98 times that of sample (0.01 Ga3+). The effect of defect traps on luminescent properties was characterized by thermoluminescence (TL) and X-ray photoelectron spectroscopy (XPS). It is confirmed that the thermal compensation from 303 to 383 K comes from VO··, BiLi2+, TaLi4+ and GaLi2+. And the proportion of these defects will change with the change of the heterovalent substitution of Ga3+/Ta5+. At the same time, the increase of Ga3+ concentration from 1 to 10% in defect modification results in the change of crystal structure, which also increases the Bi3+ band gap from 3.40 eV to 3.50 eV, thus increasing the proportion of the integral intensity of 423 K to original intensity of 303 K at 420–470 nm from 0.45 to 0.67. This defect engineering strategy of Ga3+ modification is helpful for a more complete understanding of the causes for the change of luminescence and thermal quenching characteristics of phosphors in the process of heterovalent substitution, as well as the development of anti-thermal quenching phosphors for high-quality w-LEDs.