Doping-Mediated Size and Structure Tailoring of CaS Nanocrystals

Lanthanide-doped CaS crystals are extensively studied as light-emitting materials, but the investigation of CaS nanocrystals is largely lagging. This work reports a doping-induced modification of surface charge density and the local coordination environment in CaS:Ce nanocrystals, which offers precise control over the nanocrystal size (39-14 nm) and emission color (green to yellow). Our mechanistic investigations, corroborated by theoretical calculations, reveal a S-enriched nanocrystal surface that accommodates excess charges of the aliovalent lanthanide dopants. Owing to the altered surface structure, we identify a dark Ce state that selectively extinguishes light emission of the CaS:Ce nanocrystals. We demonstrate that the doping-induced surface effects can be offset by coating the CaS:Ce nanocrystals with an undoped CaS shell, giving rise to a shell-thickness-dependent emission. Our findings suggest that nanocrystals may respond differently to impurity dopants than the corresponding bulk crystals, thereby giving rise to extended control over materials properties for applications such as optical encryption.