3D mapping of compositional gradients of core-shell structures in AgIn(x)Ga(1-x)S(2) quantum dots by atom probe tomography.

Colloidal quantum dots (QDs), which exhibit tunable band gaps depending on their size and composition, are widely studied for light-emitting and optoelectronic applications. AgInGaS-based QDs are particularly promising due to their pure green emission, high blue absorption, and environmental friendliness. However, a comprehensive understanding of these quaternary QDs remains challenging because of the difficulty in examining their complex compositional structure. Here, we three-dimensionally characterize quaternary QDs (AgInGaS-based heterostructured QDs with core/shell and core/shell/shell structures) on the atomic scale using atom probe tomography. We reveal that both the AgInGaS/AgGaS QDs with and without an outer ZnS shell have compositional gradients at their interfaces and elemental inhomogeneity among their cores. Furthermore, an Ag-deficient AgInGaS layer is identified on the outer surface of the AgGaS shell, where the stoichiometric fractions satisfy x » y, arising from differences in the precursor reactivity. Meanwhile, in the AgInGaS/AgGaS/ZnS QDs, the outer ZnS shell evolves into ZnGaS through a cation exchange process, ensuring structural and chemical compatibility with the inner shell. Our findings uncover the internal architecture and nanoscale elemental distributions of quaternary QDs, providing guidance for the future development of QDs.