# Optical Studies in Red/NIR Persistent Luminescent Cr-Doped Zinc Gallogermanate (ZGGO:Cr)
## 4 Conclusions  
Zn\({}_{1+x}\)Ga\({}_{2-2x}\)Ge\({}_{x}\)O\({}_{4}\):Cr doped with 0.5% molar of chromium and with x = 0.1 synthesised by a solid-state reaction during 36 h at 1350 \({}^{\circ}\)C exhibits a single ZGO crystalline cubic spinel phase, as demonstrated by XRD. According to the group theory, five vibrational modes at the zone centre were identified by Raman spectroscopy for the mentioned ideal structure. An additional mode was observed at 270 cm\({}^{-1}\) that was assigned to local lattice disorder effects, including inverse spinel. When excited with above and below bandgap excitation, the main recombination processes observed at RT in ZGGO:Cr corresponds to the red/NIR Cr\({}^{3+}\) luminescence with the ions placed at substitutional sites and to the N2 Cr-related complex defects. An additional broad emission band peaked in the ultraviolet region was also observed upon band-to-band excitation. PLE monitored at the red emission indicates that the main population paths of the Cr\({}^{3+}\) emission are achieved by photon energies corresponding to the ion excited levels, as well as by a defect absorption band and the band edge absorption. The width of the excitation bands suggests a spectral overlap of the excited energy levels involved in populating the emitting ones for the different defects. From the comparison of the diffuse reflectance and PLE spectra, a RT bandgap energy of 4.82 eV was estimated for ZGGO. Site-selective excitation and temperature-dependent PL (15 K to RT) reveal the spectral complexity of the presence of several optically active Cr\({}^{3+}\) centres in the ZGGO host that emit almost in the same spectral region and are excited by close energies. Additionally, thermal populating processes between the different optical centres were identified by increasing the temperature, supporting the widespread of energy levels in the doped host that may be able to trap carriers as measured by the RT persistent luminescence decay. The peak position of the R-lines was seen to be dependent on the temperature, the behaviour of which can be further explored for temperature sensing applications. Furthermore, the obtained data are consistent with carriers released preferentially to the N2 Cr\({}^{3+}\)-related optical centre in the ZGGO:Cr host, from which a persistent luminescence can be measured for about 8 h after switching off the excitation light. Figure 14: RT wavelength dependence of the red/NIR persistent luminescence decay for the ZGGO:Cr sample and correspondent fittings according to Equation (4). **Author Contributions:** Conceptualisation, M.S.B., J.R., F.M.C., S.O.P., and T.M. ; methodology, M.S.B., M.S.R., J.R., and S.O.P. ; formal analysis, M.S.B., J.R., S.O.P., and T.M. ; investigation, M.S.B., J.R., M.S.R., J.Z., A.V.G., and S.O.P. ; writing--original draft preparation, M.S.B. and T.M. ; writing--review and editing, M.S.B., J.R., M.S.R., J.Z., A.V.G., A.P., F.M.C., S.O.P., and T.M. ; supervision, J.R., S.O.P., and T.M. All authors have read and agreed to the published version of the manuscript. **Funding:** This work was developed within the scope of the project 3N UID-B/50025/2020 & UID-P/50025/2020, and CICECO (UID-B/50011/2020+UID-P/50011/2020), financed by national funds through the FCT/MEC. This work was also developed in the scope of the project PTDC/CTM-CTM/3553/2020 (DEOFET). **Institutional Review Board Statement:** Not applicable. **Infrared Consent Statement:** Not applicable. **Data Availability Statement:** Data is available upon request. **Acknowledgments:** [PERSON] thanks i3N and FCT for the PhD grant (SFRH/50025/2021). [PERSON] thanks the DEOFET project for the PhD grant (Ref BI 23 16124 2021). The research contract of [PERSON] thanks the received funding by national funds, through FCT, in the framework contract foreseen in article 23 of the Decree-Law 57/2016, changed by Law 57/2017. [PERSON] thanks i3N for the BPD grant (BPD/UI96/5808/2017). The authors also acknowledge [PERSON] and [PERSON] from CICECO for the DR and XRD measurements, respectively.