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

Doping quasi-two-dimensionalsemiconductor nanoplatelets(NPLs)which possess atomically exact thicknesses has attracted intense researchinterests, because it determines their unique optoelectronic properties.Dopants in ultrathin NPLs tend to diffuse to the surface due to theself-purification effect, which can result in reduced optical performancesuch as shortened photoluminescence (PL) lifetimes, low PL quantumyields (PLQYs), and broadening of spectral linewidth. To address theseissues, an effective way is to overgrow the NPLs with a semiconductorshell to locate dopants away from the surface. In contrast to Cd-basedcore/shell NPLs, heavy-metal-free core/shell NPLs made of Zn-chalcogenideshave hardly been explored. Here, we synthesized colloidal ZnSe:Mn/ZnScore/shell NPLs for the first time via a heat-up method. A combinationof zinc diethyldithiocarbamate and ZnCl2 yielded smoothand homogeneous shells on pregrown ZnSe:Mn NPLs. The resulting ZnSe:Mn/ZnScore/shell NPLs exhibit noticeably improved optical properties comparedto the ZnSe:Mn core-only NPLs. In particular, the Mn2+ PLQYis enhanced by more than one order of magnitude upon deposition ofZnS shells, which can be attributed to an increase in the Mn2+ internal quantum efficiency. We systematically investigated boththe matrix- and dopant-related PL kinetics as well as the PLQYs. Further,using a descriptive mathematical model, we recognized the dominantrole of the Mn2+ nonradiative relaxation channels in theenergy-transfer route from ZnSe absorption to the luminescence ofMn(2+) ions. Our findings can contribute to a better understandingand to applications of heavy-metal-free core/shell NPLs with superiorfluorescence, photostability, and low toxicity, for example, in UV-light-convertingdevices, light-emitting diodes, imaging, and bio-labeling.