Electronic and Structural Properties of Single-crystal Jahn-Teller Active CoMnOThin Films.

Recent investigations on spinel CoMnOhave shown its potential for applications in water splitting and fuel cell technologies as it exhibits strong catalytic behavior through oxygen reduction reactivity. To further understand this material, we report for the first time the synthesis of single-crystalline CoMnOthin films using molecular beam epitaxy. By varying sample composition, we establish links between cation stoichiometry and material properties using in-situ x-ray photoelectron spectroscopy, x-ray diffraction, scanning transmission electron microscopy, x-ray absorption spectroscopy, and spectroscopic ellipsometry. Our results indicate that excess Co ions occupy tetrahedral interstitial sites at lower excess Co stoichiometries, and become substitutional for octahedrally-coordinated Mn at higher Co levels. We compare these results with density functional theory models of stoichiometric CoMnOto understand how the Jahn-Teller distortion and hybridization in Mn-O bonds impact the ability to hole dope the material with excess Co. The findings provide important insights into CoMnOand related spinel oxides that are promising candidates for inexpensive oxygen reduction reaction catalysts.