Impact\nof Cation Distribution on Photoluminescence\nof Ag–In–Se/ZnSe Core/Shell Nanocrystals

Ag–In–Se/ZnSe core/shell nanocrystals exhibit\ngood\nphotoluminescence quantum yield (PLQY), yet intriguingly, the maximum\nPLQY is first reached after several days of storage. We hypothesize\nthat this may be due to cationic rearrangement in the nanocrystal\npost-synthesis. To test this hypothesis, we computationally generated\nternary Ag–In–Se and quaternary Ag–In–Zn–Se\nnanocrystals with varying degrees of cationic disorder, as quantified\nby the distribution of the metal cation valence electrons in the tetrahedra\naround Se anions. We then used density functional theory-parametrized\ntight-binding simulations to study the electronic structure and optical\nproperties of these systems as a function of the homogeneity of the\nvalence electron distribution in a tetrahedron. We found that homogeneous\ndistribution of cations leads to a larger band gap and optical coupling,\nand that, in the presence of Ag<sub>In</sub> or In<sub>Ag</sub> antisite\ndefects, the introduction of intermediate valence Zn cations decreases\nthe variance in valence electrons and improves the optical properties.\nWe further simulated the impact of a Zn-gradient shell and rearrangement\nof cations in the outer layers of the nanocrystals and find that diffusion\nof Zn into the nanocrystal and cationic rearrangement can explain\nthe post-synthetic increase of PLQY. This work highlights the importance\nof developing syntheses for multinary nanocrystals that result not\nonly in size and composition uniformity but also in nanocrystals with\na uniform distribution of charge.