Synthesis and Enhanced Optical Properties of Cd-Free Multinary Quantum Dots

Semiconductor quantum dots（QDs）exhibit size-dependent optical properties originating from quantum confinement in nanometer-sized crystalline materials. While CdSe-based QDs have achieved narrow emission linewidths and high photoluminescence quantum yields through precise size and interface control, environmental regulations on cadmium have accelerated the development of cadmium-free alternatives. Among them, multinary Ⅰ-Ⅲ-Ⅵ semiconductor QDs, such as CuInS2- and AgInS2-based materials, have attracted attention because of their compositional flexibility and wide bandgap tunability; however, they have long been regarded as defect-emitting materials showing very broad emission spectra associated with donor–acceptor pair recombination. From a crystallographic viewpoint, these optical characteristics are closely related to their complex crystal structures, compositional inhomogeneity, and surface/interface disorder. In this article, we review recent progress in cadmium-free multinary QDs with an emphasis on the crystallographic origin of their optical properties, and describe the emergence of band-edge emission in AgInS2 QDs achieved by coating with amorphous Ⅲ-Ⅵ semiconductor shells（InSx and GaSx）. We further discuss an efficient synthetic strategy based on the separation of nucleation and crystal transformation processes, enabling improved yield, narrow emission linewidths, and multicolor emission. These results indicate that the development of cadmium-free QDs requires crystallographic redesign of zero-dimensional materials rather than simple elemental substitution.