Impact of stoichiometry and size on the persistent luminescence in Cs2(Na1-xAgx)InCl6:Mn2+ double perovskites.

Persistent Luminescence (PeL) is a peculiar type of luminescence where a material, after a proper activation (by X-rays, electrons, or ultraviolet/visible light), continues to emit radiation even for several hours once the excitation has ceased. PeL occurs due to the existence of metastable trap levels that can temporarily store the excitation energy. Most of the PeL materials (> 75%) are oxides; recently, PeL has been discovered in bulk halide double perovskites (HDPs) too. In particular, after UV activation, Cs2(Na1-xAgx)InCl6:Mn2+, in form of single crystals (SCs) have a red emission (~ 620 nm) that lasts for more than 1 hour. Here, we explored the influence of stoichiometry and size on the PeL behaviour in HDPs. We synthesized different compositions of SCs pertaining to the Cs2(Na1-xAgx)InCl6:Mn2+ system. Then, we prepared the homologues in forms of micrometric and nanometric crystals (nanocubes - NCs and nanoplates - NPs). To gain inside this behaviour we carried different optical analyses: photoluminescence excitation, photoluminescence, PeL and thermoluminescence (10K to 300K; 300K to 450K). Our results showed how the PeL is strongly influenced by the materials size, i.e. PeL disappears from bulk to nano-structured samples. We hypothesise that this phenomenon is due to a different trap depth of bulk and nano samples.