# Optical Studies in Red/NIR Persistent Luminescent Cr-Doped Zinc Gallogermanate (ZGGO:Cr)
## 2 Materials and Methods
### Characterisation  
The structural properties of the synthesised powders were determined by XRD and Raman spectroscopy. The XRD patterns were measured on a Philips X'Pert MPD, employed in a \(\theta\)-2\(\theta\) mode with Cu K\({}_{\alpha}\) radiation. The Raman spectra were measured by exciting the samples with the 442 nm line of a He-Cd laser (Kimmon IK Series) and the scattered light was analysed using a Horiba Jobin Yvon HR800 spectrometer. The measurements were performed with a 50\(\times\) objective in the backscattering mode. Morphological and chemical characterisation was attained by acquiring SEM images using a field-emission-Schottky-gun (FEG-SEM) Hitachi SU-70, operated at 15 kV or 30 kV, and equipped with a Bruker EDS detector.  
Diffuse reflectance (DR) of the non-intentionally and intentionally doped pellets was measured in the wavelength range 200-900 nm using a UV/Visible GBC Cintra-303 equipment in the absorbance mode, with a bandwidth of 5 nm, a speed velocity of 100 nm min\({}^{-1}\), and a data pitch of approximately 0.520.  
The steady-state PL and PLE data were obtained by a Fluorologo(r)-3 from Horiba Scientific. A continuous Xe lamp of 450 W was used as the excitation source, coupled to an excitation monochromator, Gemini 180, with a diffraction grating of 1200 lines mm\({}^{-1}\) and a focal distance of 180 mm, in which it is possible to select excitation wavelengths between 250 and 660 nm. The scanning emission monochromator is an iHR550 with a diffraction grating of 1200 lines mm\({}^{-1}\) and a focal distance of 550 mm. The detector used is a R928P PMT, which is sensitive for wavelengths in a range of 200 to 850 nm.  
For the temperature-dependent PL data acquisition, a dispersive system SPEX 1074 Czerny-Turner monochromator with a diffraction grating of 1200 lines mm\({}^{-1}\) and a focal distance of 1 m was used. This equipment was coupled to a PMT Hamamatsu R928 detector. The excitation source used was a He-Cd laser possessing two different excitation wavelengths, one at 325 and another at 442 nm. Additional excitation lines at 405 nm and 532 nm from solid-state lasers were used. The samples were placed in a cold finger of a closed-cycle He cryostat under vacuum conditions, which allowed the temperature to reach a minimum value of 15 K. The spectra acquired in both systems were corrected to the optics of the system and the detector response.  
For the time-resolved PL (TRPL) measurements, the same Fluorologo(r)-3 system was used, now with the aid of a pulsed Xe lamp (operating at up to 25 Hz) coupled to the same monochromator and with the excitation fixed at 310 nm. The full width at half-maximum (FWHM) of each lamp pulse is 3 \(\upmu\)s, so that lamp interference during the acquisition of decay curves is minimised. The measurement conditions were set to a sample window (duration of signal acquisition) of 10 ms and several time delays after flash were employed (from 0.05 to 50 ms).  
To study the persistent luminescence, the pellets were irradiated with selected wavelengths for 5 min. with the Xe lamp (450 W, continuous). After that, the irradiation source was turned off and spectra were acquired to evaluate the signal profile of persistent luminescence. To study the evolution of the intensity of the persistent luminescence decay time, the same equipment was used in kinetics mode. For such purposes, the pellets were irradiated for 5 min. with different wavelengths, then the light was turned off and the persistent luminescence intensity was measured during 10 h, with a time increment of 10 s.