Growth of Multinary Copper-Based Sulfide Shells on CuInSe₂ Nanocrystals for Significant Improvement of Their Near-Infrared Emission

A range of different copper-based sulfide shells, such as CuInS₂, CuInZnS₂, CuInS_2-xSe_x, and CuInZnS_2-xSe_x, have been grown on CuInSe₂ (CISe) nanocrystals (NCs) in order to improve their photoluminescence quantum yield (PLQY). Starting from initial CISe core NCs with PLQY of 0.4%, the deposition of the CuInS₂ shell has increased the PLQY to 4.5%, while introducing Zn or Se elements into the CuInS₂ shell to form cation-alloyed CuInZnS₂ or anion-alloyed CuInS_2-xSe_x shells improved their PLQYs to 8% and 11%, respectively. Limitations on PLQY for the mentioned shell materials were related to the lattice mismatch with the CISe core material, and the band gap alignment in the core/shell NCs for quasi type-II nanostructures. This could be mitigated by growing a multinary CuInZnS_2-xSe_x shell on the CISe core NCs, which resulted in further improvement of their PLQY to 20% for the emission centered at 920 nm. This study offers guidance for the rational selection of copper-based sulfide shell materials that are able to significantly improve the PLQY of heavy-metal free CISe NCs in the near-infrared spectral range of 834-1028 nm, which is relevant for biological labeling, optoelectronics, and sensing.