**4. Conclusions**

In summary, manganese-activated gallium germanate phosphors were successfully synthesized in two approaches, including a high-temperature solid-state reaction approach and a hydrothermal method. We found, for the first time, that these Mn-activated Zn3Ga2GeO8 phosphors prepared by a hydrothermal method and solid-state reaction method have different crystal phase structures and spectral properties. The phosphors prepared by the hydrothermal method exhibited a double-peak emission, including green PL with a PersL feature and red PL without a PersL effect, due to simultaneously having two kinds of occupancy sites. While the Mn-activated Zn3Ga2GeO8 phosphors prepared by a solid-state reaction method only exhibited red PL. In addition, Li+ ions were selectively substituted for the tetrahedral Zn site or the octahedral Ga site, leading to green light emission at 540 nm (4T1(G) →6A1(S) transition of Mn2+) with an afterglow and red light centered at 701 nm (2E→4A2 transition of Mn4+) in the absence of afterglow. Particularly, Mn-activated Zn3Ga2GeO8 phosphors synthesized by a hydrothermal approach via Li+ replacing the Zn2+ or Ga3+ sites exhibit dynamic and multicolor emissions as luminescence labels for multiple anti-counterfeiting, thus revealing the grea<sup>t</sup> potential of these phosphors in multicolor display, anti-forgery, and other potential applications.

**Author Contributions:** Conceptualization, D.G. and W.C.; methodology, D.G. and P.W.; validation, P.W., F.G. and W.N.; formal analysis, D.G. and W.C.; investigation, P.W., F.G. and W.N.; resources, D.G. and W.C.; data curation, P.W.; writing—original draft preparation, D.G.; writing—review and editing, W.N., D.G. and W.C.; visualization, D.G. and P.W.; supervision, D.G.; project administration, D.G. and F.G.; funding acquisition, G.D and W.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Natural Science Foundation of China, gran<sup>t</sup> number 11604253 and Natural Science Foundation of Shaanxi Province of China, gran<sup>t</sup> number 2018JM1036.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We would like to acknowledge the support from Guangxi Jialouyuan Medical Inc., Solgro, and the distinguished award from UT Arlington.

**Conflicts of Interest:** The authors declare no conflict of interest.
