# Optical Studies in Red/NIR Persistent Luminescent Cr-Doped Zinc Gallogermanate (ZGGO:Cr)
## 3 Results
### RT Persistent Luminescence  
RT TRPL measurements were also conducted in the ZGGO:Cr sample to gain further insight regarding the probed emission. Figure 12 shows a comparison of the steady-state and transient PL spectra. The latter was performed using 310 nm of excitation light and delay times of 0.05 and 50 ms after the Xe pulse lamp. The transient spectra clearly evidence a longer decay time for the N2 centre than the ones that originate the R-lines.  
Persistent luminescence was recorded after excitation of the sample with 310 nm of light for 5 min. Figure 13 shows the persistent luminescence spectra measured upon switching off the excitation light, indicating that the ZGGO:Cr sample emits during almost 8 h, with the N2 defect accounting for the observed red/NIR persistent luminescence.  
The wavelength excitation dependence of the persistent luminescence decays for ZGGO:Cr samples is depicted in Figure 14. During the first few minutes, the persistent luminescence intensity decreases rapidly, while over time the decrease is not so pronounced. The decay curves were fitted to a combination of stretched exponentials with dispersion factors varying between 0.4 and 0.9, implying that a wide distribution of the defect trap levels is available for the carriers capture [36]. The following expression was employed for the fittings [36]  
\[I(T)\ =\sum_{i=1}^{n}I_{i}e^{-\delta_{i}(\frac{t}{t})} \tag{4}\]  
where \(0<\delta<1\) denotes the dispersion factor (\(\delta=1\) is equivalent to the simple exponential decay) and \(n=4\) in the case of the decay acquired after the initial excitation with 310 nm and with 275 nm, while \(n=2\) for the 400 nm case.  
Figure 12: RT-normalised PL and TRPL spectra of ZGGO:Cr. The spectra were vertically shifted for clarity.  
As stated above, persistent luminescence relies upon the capture and release by the defect traps of photoexcited charge carriers. As demonstrated, a ZGGO:Cr sample can absorb photons by band-to-band transitions, as well as by defect levels and via the excited states of Cr\({}^{3+}\). Persistent luminescence is observed under all the excitation conditions, meaning that all excitation/de-excitation processes involve traps of charge carriers that store energy, further depopulated by a thermal activation process that is able to feed the emitting levels of Cr\({}^{3+}\). Therefore, different hypotheses should be considered. First (i), upon band-to-band excitation, an electron-hole pair is generated, resulting in the formation of a self-trapped exciton in the defective ZGGO:Cr host, giving rise to a broad emission band peaked at 348 nm/3.56 eV (see the solid blue line in Figure 4a). The released energy is resonantly transferred to the Cr\({}^{3+}\) optical centres, exciting the electrons from the fundamental to higher excited levels, followed by non-radiative relaxation, resulting in the  
Figure 13: RT red /NIR persistent PL of ZGGO:Cr after illumination with 310 nm of a Xe lamp for 5 min. The grey zone in (**a**) is zoomed in (**b**).  
characteristic \({}^{2}E\rightarrow\) \({}^{4}A_{2}\) Cr\({}^{3+}\) red/NIR intraionic emission. Additionally (ii), persistent PL was also observed using below bandgap excitation, which is likely to be due to the presence of defects and distinct Cr\({}^{3+}\) optical centres. Therefore, under these conditions and from the measured persistent luminescence decay, it is fair to assume that the photogenerated carriers can be trapped by a wide distribution of shallow and deep trap centres, including the Cr\({}^{3+}\) cluster defects, from which the carriers are released by a tunnelling mechanism and/or a thermally activated process, preferentially to the N2 Cr\({}^{3+}\)-related centre.