Color selective manipulation in Li2ZnGe3O8:Mn2+ by multiple-cation substitution on different crystal-sites.

Controlling the occupation sites of Mn2+ emitters by multiple-cation substitution, the Li2ZnGe3O8:Mn2+ phosphor could be optionally tuned in the green to NIR region. For the Mn2+ single doped Li2ZnGe3O8 phosphor, the tetrahedral coordinated Mn2+ (ZnO4 site) affords a green emission, and the octahedrally coordinated Mn2+ (ZnO6 site) shows an NIR (832 nm) emission. Interestingly, the Li2ZnGe3O8 host has three cation crystallographic sites, in which the octahedrally coordinated Mn2+ could exhibit a red emission by occupying GeO6 sites. The different luminescence centers for Mn2+ have been demonstrated using time-resolved emission spectra (TRES), excitation spectra and the decay curves. However, how to selectively regulate these fluorescence emissions corresponding to the different occupation sites is critical. In this paper, a common effect between the group's transition and energy transfer makes it possible to enhance the green emission (ZnO4 site) continuously and to restrain the NIR emission (ZnO6 site) through increasing the Zn/Li ratio in the Li2ZnGe3O8:Mn2+ phosphor. When Zn2+ is substituted by bigger ions from Ca to Sr and Ba, the local environments of Mn2+ around ZnO6 are influenced which leads to part of the Mn2+ emitters occupying the Ge4+ site, and it has the most enhanced effect on the Mn red-emission (GeO6 site). In general, we show that the spectral property of Mn2+ in different occupation sites could be efficiently regulated within the Li2ZnGe3O8 host, and provide a method for photoluminescence tuning.