**1. Introduction**

The ability to tune the luminescent color of materials is essential for various applications, such as anti-counterfeiting, three-dimensional displays, information coding, bioimaging, optoelectronic devices and luminescent labeling [1–3]. Conventional approaches involve utilizing specially designed organic dyes or quantum dots upon ultraviolet or blue light excitation, where the emission color is modulated by tuning the wavelengths or the power density of excitation light [4,5], thus posing limitations in the resolution of imaging due to auto-fluorescence. Persistent luminescence (PersL) phosphors, which can emit luminescence lasting for hours after the stoppage of the excitation light [6], are particularly suitable for such imaging applications as they emit no background fluorescence [7,8].

The transition metal Mn with the multiple oxidation states, e.g., +2, +3 and +4, provides an opportunity for multi-color emission [9]. In recent years, Mn-doped ZnGa2O4 microcrystals with unique luminescence features have attracted much attention because a ZnGa2O4 host has two kinds of stable chemical coordination structure, including Ga3+ sites with octahedral coordination and Zn2+ sites with tetrahedral coordination [10]. Generally, a Mn2+ activation center occupying the tetrahedral-coordinated sites shows a green emission with long PersL, while a Mn4+ activation center with an octahedral-coordinated

**Citation:** Gao, D.; Wang, P.; Gao, F.; Nguyen, W.; Chen, W. Tuning Multicolor Emission of Manganese-Activated Gallogermanate Nanophosphors by Regulating Mn Ions Occupying Sites for Multiple Anti-Counterfeiting Application. *Nanomaterials* **2022**, *12*, 2029. https://doi.org/10.3390/ nano12122029

Academic Editors: Yann Molard and Eleonore Fröhlich

Received: 10 May 2022 Accepted: 9 June 2022 Published: 13 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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structure demonstrates a luminescence emission from red to deep red. Apparently, the Mn activator shows a green to deep red emission, and the emission color is determined by the coordination environment of Mn ions in the crystal structure.

In our work, Mn-activated Zn3Ga2GeO8 phosphors are successfully prepared using a hydrothermal method and solid-state reaction approach. Interestingly, it was found that changing the preparation route was a more efficient method for the spectral tuning of Mn-activated Zn3Ga2GeO8 phosphors relative to a non-equivalent ion doping strategy. Mn-activated Zn3Ga2GeO8 phosphors show an enhanced red photoluminescence (PL) at 701 nm and a strong green emission at 540 nm with PersL and a green photostimulated luminescence (PSL) by Li+ substituted for Zn2+ or Ga3+ sites under hydrothermal conditions. Zn3Ga2GeO8 phosphors synthesized by a solid-state reaction only exhibit an enhanced pure-red broad-band luminescence. Particularly, Zn3Ga2GeO8 phosphors synthesized by a hydrothermal approach exhibit multicolor and multimode luminescence properties, which are especially suitable for multiple anti-counterfeiting and have a grea<sup>t</sup> potential for multicolor display, anti-forgery, and other potential applications.
