Tunable LLP via Energy Transfer between Na2-y(Zn1-xGax)GeO4 Sosoloid Host and Emission Centers with the Assistance of Zn Vacancies

A series of sodium zinc gallogermanate sosoloid Na2-y(Zn1-xGax)-GeO4: yTb(3+) (x = 0, 0.1, 0.12, 0.15, 0.18, and 0.20; y = 0, 0.01, 0.02, 0.03, and 0.04) samples were successfully synthesized and their luminescence properties were investigated. It was found that Zn vacancies could be the predominant contribution to the enhancement of PL and LLP intensities when Ga3+ ions were introduced as solute atoms substituted the crystal sites of Zn2+ in Na2ZnGeO4 host. Zn vacancies act not only as an exciton energy-level participated in the PL process but also as trapping centers contributed to the LLP process. Furthermore, the energy transfer processes from Na-2(Zn0.8Ga0.2)GeO4 host to Tb3+ ions in PL and LLP were identified. Accordingly, the LLP colors changed from blue to green with increasing concentration of Tb3+ and the mechanism of which was proposed. Besides, after irradiation with 800 nm femtosecond pulsed laser, a blue upconversion LLP phenomenon was clearly observed in Na-2(Zn0.8Ga0.2)GeO4 for the first time. Analysis suggested that multiphoton absorption process was dominant in this upconversion luminescence, and subsequently the carriers excited into conduction band were captured by the trapping centers. Then the realization of LLP was due to the thermal stimulated recombination of holes and electrons at traps. Our results indicated that the blue-green emitting phosphor of Na-2(Zn0.8Ga0.2)GeO4: Tb3+ could be a new member in the family of LLP materials, which could also provide potential applications in the fabrication of optical memory.