# Optical Studies in Red/NIR Persistent Luminescent Cr-Doped Zinc Gallogermanate (ZGGO:Cr)
## 3 Results
### RT Photoluminescence and Photoluminescence Excitation
Figure 4a also displays the DR spectrum of ZGGO:Cr, showing the intraionic \({}^{4}A_{2}\to{}^{4}T_{2},{}^{4}T_{1}\) absorption bands with similar spectral shape and peak position as the ones measured by PLE. From these data, a strong average crystal field strength was estimated as \(Dq/B\) of 2.74 for ZGO and ZGGO, where \(Dq\) and \(B\) stand for the crystal field and Racah parameters, respectively. The estimated values were calculated considering the mean peak energies of the \({}^{4}A_{2}\to{}^{4}T_{2}\) and \({}^{4}A_{2}\to{}^{4}T_{1}\) transitions, according to [PERSON] et al. [33] and [PERSON] et al. [20, 21], and are in good agreement with the previously reported values [7, 18]. Regardless of whether the emission is monitored in the R- or N- lines, the \({}^{4}A_{2}\to{}^{4}T_{2},{}^{4}T_{1}\) excitation bands exhibit FWHM larger than 450 meV, suggesting a spectral overlap of the excited energy levels involved in populating the emitting ones for the different defects, as will be discussed later. A noticeable difference is observed in the PLE of the ZGO:Cr and ZGGO:Cr samples in the high energies region. In the case of ZGO:Cr, the red/NIR emission is preferentially excited by photons with wavelengths shorter than 350 nm/3.54 eV (with a maximum at 243 nm/5.1 eV, that matches with the ZGO bandgap [15, 16, 17]), whereas in the case of ZGGO:Cr, the emission is preferentially populated by longer wavelength photons, corresponding to the intraionic \({}^{4}A_{2}\to{}^{4}T_{1},{}^{4}T_{2}\) absorption. Additionally, it is worthwhile to perform a comparison between the PLE and DR spectra of the non-intentionally and intentionally doped ZGGO samples. The DR spectra of both samples exhibit two main maxima at 257 nm/4.82 eV and 303 nm/4.09 eV, with the latter absorption band showing higher intensity in the non-intentionally doped sample. Therefore, it is unlikely that this band could correspond itself to the \({}^{4}A_{2}\to{}^{4}T_{1}({}^{4}P)\) Cr\({}^{3+}\) transition. Since the band is observed in both the non-intentionally doped and the doped samples, a more plausible explanation is to consider that it is related to other defects/impurities present in both samples (from the host material). Additionally, and as mentioned, the DR spectra of the ZGGO samples exhibit a maximum at shorter wavelengths, suggesting that the energy gap of this material occurs in the vicinity of 4.82 eV. Upon this band-to-band excitation, the ZGGO:Cr sample exhibits a broad ultraviolet emission peaked at \(\sim\)348 nm/3.56 eV that partially matches the absorption energy of Cr\({}^{3+}\) excited states and defect bands. At RT, excitation of the ZGO:Cr and ZGGO:Cr samples into the band edge, as well as defect levels and higher excited Cr\({}^{3+}\) levels, result in a rapid relaxation to the \({}^{2}E\) emitting state, as seen in the excitation-dependent PL spectra shown in Figure 5. It is noteworthy that the relative intensities of the observed lines, their peak positions, and lines' asymmetry show a dependence on the excitation wavelength, suggesting different origins for the different excitation wavelengths. Figure 4: (**a**) **(top)** DR spectra of non-intentionally doped (green dashed line) and chromium-doped (black dashed line) ZGGO samples; (**bottom**) RT PLE (@ 690 nm) and PL (\(\lambda_{\text{exc}}\) = 325 nm) spectra of ZGGO:Cr (black lines) and ZGO:Cr (red lines) samples, respectively; and a PL spectrum of ZGGO:Cr (blue solid line) at band-to-band excitation. (**b**) Schematic representation of the energy levels of Cr\({}^{3+}\) in octahedral sites and corresponding splittings (not to scale) of the fundamental and first excited states under the interplay of a trigonal crystalline field and spin–orbit interaction. Figure 3: (**a1,a2**) SEM images of ZGGO:Cr pellets, (**a2**) shows in a higher magnification image revealing the polyhedral nature of the grains. (**b**) EDS spectrum of ZGGO:Cr pellets showing the identification of the elements Zn, Ga, Ge, Cr, and O; the * correspond to Si contamination.