Mn2+-Doped ZnSe/ZnS Core/Shell Nanoplatelets as Low-Toxic UV-to-Vis Light-Converters with Enhanced Optical Properties

Doping quasi-two-dimensional semiconductor nanoplatelets (NPLs) which possess atomically exact thicknesses has attracted intense research interests, because it determines their unique optoelectronic properties. Dopants in ultrathin NPLs tend to diffuse to the surface due to the self-purification effect, which can result in reduced optical performance such as shortened photoluminescence (PL) lifetimes, low PL quantum yields (PLQYs), and broadening of spectral linewidth. To address these issues, an effective way is to overgrow the NPLs with a semiconductor shell to locate dopants away from the surface. In contrast to Cd-based core/shell NPLs, heavy-metal-free core/shell NPLs made of Zn-chalcogenides have hardly been explored. Here, we synthesized colloidal ZnSe:Mn/ZnS core/shell NPLs for the first time via a heat-up method. A combination of zinc diethyldithiocarbamate and ZnCl2 yielded smooth and homogeneous shells on pregrown ZnSe:Mn NPLs. The resulting ZnSe:Mn/ZnS core/shell NPLs exhibit noticeably improved optical properties compared to the ZnSe:Mn core-only NPLs. In particular, the Mn2+ PLQY is enhanced by more than one order of magnitude upon deposition of ZnS shells, which can be attributed to an increase in the Mn2+ internal quantum efficiency. We systematically investigated both the matrix- and dopant-related PL kinetics as well as the PLQYs. Further, using a descriptive mathematical model, we recognized the dominant role of the Mn2+ nonradiative relaxation channels in the energy-transfer route from ZnSe absorption to the luminescence of Mn2+ ions. Our findings can contribute to a better understanding and to applications of heavy-metal-free core/shell NPLs with superior fluorescence, photostability, and low toxicity, for example, in UV-light-converting devices, light-emitting diodes, imaging, and bio-labeling.