Control of Elemental Distribution in the Nanoscale Solid-State Reaction That Produces (Ga_1-xZn_x)(N_1-xO_x) Nanocrystals.

Abstract

Solid-state chemical transformations at the nanoscale can be a powerful tool for achieving compositional complexity in nanomaterials. It is desirable to understand the mechanisms of such reactions and characterize the local-level composition of the resulting materials. Here, we examine how reaction temperature controls the elemental distribution in (Ga_1-xZn_x)(N_1-xO_x) nanocrystals (NCs) synthesized via the solid-state nitridation of a mixture of nanoscale ZnO and ZnGa₂O₄ NCs. (Ga_1-xZn_x)(N_1-xO_x) is a visible-light absorbing semiconductor that is of interest for applications in solar photochemistry. We couple elemental mapping using energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope (STEM-EDS) with colocation analysis to study the elemental distribution and the degree of homogeneity in the (Ga_1-xZn_x)(N_1-xO_x) samples synthesized at temperatures ranging from 650 to 900 °C with varying ensemble compositions (i.e., x values). Over this range of temperatures, the elemental distribution ranges from highly heterogeneous at 650 °C, consisting of a mixture of larger particles with Ga and N enrichment near the surface and very small NCs, to uniform particles with evenly distributed constituent elements for most compositions at 800 °C and above. We propose a mechanism for the formation of the (Ga_1-xZn_x)(N_1-xO_x) NCs in the solid state that involves phase transformation of cubic spinel ZnGa₂O₄ to wurtzite (Ga_1-xZn_x)(N_1-xO_x) and diffusion of the elements along with nitrogen incorporation. The temperature-dependence of nitrogen incorporation, bulk diffusion, and vacancy-assisted diffusion processes determines the elemental distribution at each synthesis temperature. Finally, we discuss how the visible band gap of (Ga_1-xZn_x)(N_1-xO_x) NCs varies with composition and elemental distribution.