Scintillators based on Ce-doped garnets are regularly co-doped with Mg²⁺ or Ca²⁺ to form Ce ions in 4+ state and reduce undesired afterglow. However overly high Ce⁴⁺ concentration leads to poor light yield performance. In order to understand the reason for variation in luminescence efficiency of Ce³⁺- and Ce⁴⁺-doped garnets we investigate the differences in energy conversion processes in complex LuGd₂Ga₃Al₂O₁₂:Ce³⁺/Ce⁴⁺ ceramics by means of VUV synchrotron irradiation. At first we have established via transmission spectroscopy and X-ray absorption spectroscopy that LuGd₂Ga₃Al₂O₁₂:Ce, Mg sample contains cerium in the 4+ state only. Then we show with VUV spectroscopy efficient interaction of excitons with Gd³⁺ and Ce³⁺, and lack of exciton absorption edge in LuGd₂Ga₃Al₂O₁₂:Ce⁴⁺ excitation spectrum. Instead, Ce⁴⁺ exhibits charge-transfer absorption band in the range of exciton emission. We suggest that when Ce⁴⁺ concentration becomes too high, the exciton → Gd³⁺ → Ce³⁺ energy transfer path is hindered. It leads to high intensity of Gd³⁺ luminescence in Lu₁Gd₂Ga₃Al₂O₁₂:Ce, Mg ceramics, but lowered Ce³⁺ X-ray excited luminescence. Fine balance between 3+ and 4+ Ce concentrations is necessary to achieve the best performance of garnet scintillators.

Metal-halide perovskite nanocrystals show promise as the future active material in photovoltaics, lighting, and other optoelectronic applications. The appeal of these materials is largely due to the robustness of the optoelectronic properties to structural defects. The photoluminescence quantum yield (PLQY) of most types of perovskite nanocrystals is nevertheless below unity, evidencing the existence of nonradiative charge-carrier decay channels. In this work, we experimentally elucidate the nonradiative pathways in CsPbBr₃ nanoplatelets, before and after chemical treatment with PbBr₂ that improves the PLQY. A combination of picosecond streak camera and nanosecond time-correlated single-photon counting measurements is used to probe the excited-state dynamics over 6 orders of magnitude in time. We find that up to 40% of the nanoplatelets from a synthesis batch are entirely nonfluorescent and cannot be turned fluorescent through chemical treatment. The other nanoplatelets show fluorescence, but charge-carrier trapping leads to losses that are prevented by chemical treatment. Interestingly, even without chemical treatment, some losses due to trapping are mitigated because trapped carriers spontaneously detrap on nanosecond-to-microsecond timescales. Our analysis shows that multiple nonradiative pathways are active in perovskite nanoplatelets, which are affected differently by chemical treatment with PbBr₂. More generally, our work highlights that in-depth studies using a combination of techniques are necessary to understand nonradiative pathways in fluorescent nanocrystals. Such understanding is essential to optimize synthesis and treatment procedures.