Impact of Cation Distribution on Photoluminescence of Ag-In-Se/ZnSe Core/Shell Nanocrystals

Ag-In-Se/ZnSe core/shell nanocrystals exhibit good photoluminescence quantum yield (PLQY), yet intriguingly, the maximum PLQY is first reached after several days of storage. We hypothesize that this may be due to cationic rearrangement in the nanocrystal post-synthesis. To test this hypothesis, we computationally generated ternary Ag-In-Se and quaternary Ag-In-Zn-Se nanocrystals with varying degrees of cationic disorder, as quantified by the distribution of the metal cation valence electrons in the tetrahedra around Se anions. We then used density functional theory-parametrized tight-binding simulations to study the electronic structure and optical properties of these systems as a function of the homogeneity of the valence electron distribution in a tetrahedron. We found that homogeneous distribution of cations leads to a larger band gap and optical coupling, and that, in the presence of Ag_In or In_Ag antisite defects, the introduction of intermediate valence Zn cations decreases the variance in valence electrons and improves the optical properties. We further simulated the impact of a Zn-gradient shell and rearrangement of cations in the outer layers of the nanocrystals and find that diffusion of Zn into the nanocrystal and cationic rearrangement can explain the post-synthetic increase of PLQY. This work highlights the importance of developing syntheses for multinary nanocrystals that result not only in size and composition uniformity but also in nanocrystals with a uniform distribution of charge.