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

Abstract

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 ⁴T_1g → ⁶A_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 Å 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 E_a 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.