Persistent luminescence of zinc gallogermanates

Zinc gallogermanate samples have been optimized for maximum performance by zinc depletion and three overlapping photoluminescence and persistent luminescence bands at 3.0, 2.6 and 2.4 eV are assigned to emission from F+ (2T1 → 2A1) and F0 (3T1, 1T1 → 1A1) centres, respectively. The emission exhibits a strong temperature dependence so that only the 2.4 eV emission is observed at room temperature. In the spectral range from 3.4-2.2 eV the measured persistent luminescence average lifetime from biexponential fits at different temperatures can be explained from the extreme values measured at these energies together with the analysis of the deconvoluted persistent luminescence spectra. The systematic study on the influence of the irradiation time, irradiation power and delay time between irradiation and measurement of the persistent luminescence decay curve has been employed at 10 K to investigate the occurrence of tunneling from the trap to the emission centre. It is found that tunneling may occur only at a very low dose rates for the trap leading to emission at 3.0 eV since the number of traps is much smaller than the number of emission centres. The trap depths for the emission at 2.6 eV and 2.4 eV are 0.003 eV, and in the range from 0.1 eV to at least 0.6 eV, respectively, where there is a continuous distribution of traps with increasing temperature, attributed to the different geometry and distances of traps feeding oxygen vacancies. The persistent luminescence decays are well-fitted by hyperbolic functions and it follows that power-law fits give variable power indices at short times. The slopes of power law fits to the persistent luminescence decay at 10 K become closer to zero, for decays over the range up to 100 s, with increasing delay time. The room temperature green persistent luminescence may find application as a photosensitizer.