Kazuki Asami

Evidence of valence state change of Ce 3+ and Cr 3+ during UV charging process in Y 3 Al 2 Ga 3 O 12 persistent phosphors

To elucidate the persistent luminescence mechanism of the Ce3+-Cr3+-codoped Y3Al2Ga3O12 garnet (YAGG), the valence states of Ce and Cr ions before and during UV charging were investigated by X-ray absorption near edge structure (XANES) spectroscopy. In the XANES spectra for Ce LIII and Cr K edges of YAGG:Ce3+-Cr3+ under UV illumination, the valence states of Ce4+ and Cr2+ were detected, but not in the XANES spectra of YAGG:Ce3+ and YAGG:Cr3+. We conclude that the combination of Ce3+ and Cr3+ causes the valence state change into Ce4+ and Cr2+ by UV illumination, which is the mechanism of persistent luminescence.

1.2 μm persistent luminescence of Ho3+ in LaAlO3 and LaGaO3 perovskites

Persistent luminescence (PersL) in the short-wave infrared (SWIR: ∼900–1700 nm) region has attracted much attention for potential biomedical imaging and night-vision surveillance applications. Although trivalent lanthanide (Ln3+) ions are considered to be one of the most promising luminescent centers working for this region, the lack of suitable persistent phosphor hosts is still the bottleneck of this field. Herein, by using the persistent energy transfer from Cr3+ to Ho3+, Ho3+ PersL at 1.2 μm has been realized in both Ho3+–Cr3+–Sm3+ tri-doped LaAlO3 (LAO:Ho–Cr–Sm) and Ho3+–Cr3+ co-doped LaGaO3 (LGO:Ho–Cr) perovskites. The Ho3+ photoluminescence (PL) intensity ratio between the 5I6 → 5I8 transition at ∼1.2 μm and the 5I7 → 5I8 transition at ∼2.0 μm in LAO is lower than that in LGO due to the higher phonon energy of the LAO host, resulting in its preferred population of the 5I7 excited level through multi-phonon relaxation (MPR) processes. However, the LAO:Ho–Cr–Sm phosphor still exhibits superior Cr3+/Ho3+ PersL intensity than that of LGO:Ho–Cr, and it is even comparable with that of the most widely used deep-red ZnGa2O4:Cr3+ persistent phosphor. This can be explained by the efficient electron traps of Sm3+ ions in LAO:Ho–Cr–Sm while this kind of sensitization by lanthanide ions seems not to work in LGO:Ho–Cr based on prediction from the vacuum referred binding energy (VRBE) diagram. These results demonstrated by the flexible ABX3 perovskites could give a vivid example to choose suitable material hosts, electron trapping centers and energy donor–acceptor combinations toward long PersL in the SWIR region.