ZnGeN2 and ZnGeN2:Mn2+ phosphors: hydrothermal-ammonolysis synthesis, structure and luminescence properties

Nitride phosphors have drawn much interest because of their outstanding thermal and chemical stability and interesting photoluminescence properties. Currently, it remains a challenge to synthesize these phosphors through a convenient chemical route. Herein we propose a general and convenient strategy based on hydrothermal-ammonolysis reaction to successfully prepare zinc germanium nitride (ZnGeN2) and Mn2+ doped ZnGeN2 phosphors. The crystal structure, composition, morphology, luminescence and reflectance spectra, quantum efficiency, and the temperature-dependent photoluminescence behavior were studied respectively. The phase formation and crystal structure of ZnGeN2 were confirmed from powder X-ray diffraction and Rietveld refinement. EDX analysis confirmed the actual atomic ratios of Zn/Ge and N/Ge and suggested the presence of Ge vacancy defects in the ZnGeN2 host, which is associated with its yellow emission at 595 nm with a FWHM of 143 nm under UV light excitation. For Mn2+ doped ZnGeN2 phosphor, it exhibits an intense red emission due to the T-4(1g) -> (6)A(1g) transition of Mn2+ ions. The unusual red emission of Mn2+ at the tetrahedral Zn2+ sites is attributed to the strong nephelauxetic effect and crystal field between Mn2+ and the tetrahedrally coordinated N3-. Moreover, the PL intensity of ZnGeN2:Mn2+ phosphors can be enhanced by Mg2+ ions partially substituting for Zn2+ ions in a certain concentration range. The optimal Mn2+ doping concentration in the ZnGeN2 host is 0.4 mol%. The critical energy transfer distance of this phosphor is calculated to be about 27.99 angstrom and the concentration quenching mechanism is proved to be the dipole-dipole interaction. With increasing temperature, the luminescence of ZnGeN2:Mn2+ phosphors gradually decreases and the FWHM of the emission band broadens from 54 nm to 75 nm. The corresponding activation energy Ea was reckoned to be 0.395 eV. And the nonradiative transition probability increases with the increasing temperature, finally leading to the lifetime decrease with the increase of the temperature.