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

Abstract

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.