Relationships between Compositional Heterogeneity and Electronic Spectra of (Ga1-XZnX)(N1-XOX) Nanocrystals Revealed by Valence Electron Energy Loss Spectroscopy

Many ternary and quaternary semiconductors have been made in nanocrystalline forms for a variety of applications, but we have little understanding of how well their ensemble properties reflect the properties of individual nanocrystals. We examine electronic structure heterogeneities in nanocrystals of (Ga1-xZnx)(N1-xOx), a semiconductor that splits water under visible illumination. We use valence electron energy loss spectroscopy (VEELS) in a scanning transmission electron microscope to map out electronic spectra of (Ga1-xZnx)(N1-xOx) nanocrystals with a spatial resolution of 8 nm. We examine three samples with varying degrees of intraparticle and interparticle compositional heterogeneity and ensemble optical spectra that range from a single band gap in the visible to two band gaps, one in the visible and one in the UV. The VEELS spectra resemble the ensemble absorption spectra for a sample with a homogeneous elemental distribution and a single band gap and, more interestingly, one with intraparticle compositional heterogeneity and two band gaps. We observe spatial variation in VEELS spectra only with significant interparticle compositional heterogeneity. Hence, we reveal the conditions under which the ensemble spectra reveal the optical properties of individual (Ga1-xZnx)(N1-xOx) particles. More broadly, we illustrate how VEELS can be used to probe electronic heterogeneities in compositionally complex nanoscale semiconductors.