# Optical Studies in Red/NIR Persistent Luminescent Cr-Doped Zinc Gallogermanate (ZGGO:Cr)
## 1 Introduction
For the case of the N1 defect, three likely signatures have been proposed: (i) a Cr\({}^{3+}\)-V\({}_{\rm Zn}\) pair, (ii) a Cr\({}^{3+}\)-Zn\({}_{\rm i}\) pair (where V\({}_{\rm Zn}\) and Zn\({}_{\rm i}\) stand for zinc vacancy and zinc interstitial, respectively), or (iii) a Cr\({}^{3+}\) nearby an antisite defect [10; 22; 24; 25]. By using electron paramagnetic resonance (EPR) measurements, [PERSON] et al. [10] proved that the N2 line is due to the recombination of a specific type of Cr complex, involving a Cr\({}^{3+}\) ion with antisite cationic defects (Zn at the Ga site, Zn\({}_{\rm Ga}^{\prime}\), and Ga at the Zn site, Ga\({}_{\rm Zn}^{\prime}\)) as the first cationic neighbour [10; 25]. Such a model was also corroborated by extended X-ray absorption fine structure (EXAFS) measurements [25], indicating that the antisite defects have a pronounced influence on the observation of the persistent luminescence, which is commonly related to the N2 optical centre [10; 11; 25]. Ab initio atomic-scale simulations  
Figure 1: Structural model of ZGO:Cr and ZGGO:Cr. When x = 0.1 Ge replaces Ga atoms, as represented by the pink portion on Ga atoms. The model was obtained using the free software VESTA and the data from [PERSON] et al. [13]. performed with density functional theory (DFT) on ZGO:Cr also suggest that an antisite defect pair is formed on the first coordination shells of the Cr\({}^{3+}\) ions [26]. The N3-line has been assigned to Cr\({}^{3+}\)-Cr\({}^{3+}\) pairs [22]. In the field of in vivo bioimaging, the red/NIR persistent luminescence within the tissue transparency window (650-1350 nm) [27] is of particular relevance as the light attenuation is largely due to scattering (rather than absorption), penetrating deeply into the tissues [27]. Simultaneously, red/NIR persistent luminescence allows optical imaging in the absence of tissue irradiation, avoiding autofluorescence of the tissues and consequently improving the sensing sensitivity of the measurements with a superior signal-to-noise ratio, at the same time avoiding tissue damage by direct irradiation [27]. Persistent luminescence relies on the capture and release of charge carriers by the defect traps. Besides the defect trap levels, optically active centres are necessary to release light at the wavelength of interest [9; 10; 11; 12; 13; 14; 25; 26; 28; 29]. While the latter are essentially related to the intraionic emission of dopants/impurities intentionally incorporated into a given host (e.g., transition metal ions and/or lanthanides), the former are usually correlated with the presence of intrinsic lattice defects [28; 29]. These traps can store the excitation energy and slowly release it to the emitters. The capture of the carriers by the traps cannot promptly recombine from the trap levels and, usually, thermal energy is required to release the carriers to the emitting centre. In the case of very shallow electron traps, these can be very fastly depopulated at RT via the material's conduction band, while deep traps require higher temperatures. Additionally, detraping can also occur via tunnelling when the trap levels are almost resonantly in energy with those of the emitting optical centre(s) [28; 29]  
Red/NIR-emitting Cr\({}^{3+}\) optically active defects can be successfully achieved by in situ doping of ZGGO during the synthesis procedure by solid-state reaction. Though there are several studies on photoluminescence (PL) spectra at RT of Cr-doped ZGO and ZGGO [8; 9; 10; 11; 12; 13; 14; 25; 26; 28; 29], a detailed study of the intraionic emission at low temperatures and its temperature dependence is scarce [9; 10; 11; 18; 22]. Hence, in this work, we attempt to contribute to the understanding of the luminescent properties of Cr-doped ZGGO at cryogenic temperatures and their temperature dependence.