Experimental study of the interfacial cobalt oxide in Co3O4/a-Al2O3(0001) epitaxial films

TL;DR

This study investigates the structural and chemical evolution of ultrathin cobalt oxide films grown on a-Al2O3(0001), revealing a transition from Co2O3 to Co3O4 with increasing thickness through spectroscopic and diffraction analyses.

Contribution

It provides detailed spectroscopic and structural insights into the growth and phase transition of cobalt oxide films on a-Al2O3(0001), highlighting the formation of Co2O3 at the interface and its evolution.

Findings

Cobalt oxide films become more disordered with thickness.

A transition from Co2O3 to Co3O4 occurs as thickness increases.

Interfacial Co2O3 layer forms during early growth stages.

Abstract

A detailed spectroscopic and structural characterization of ultrathin cobalt oxide films grown by O-assisted molecular beam epitaxy on a-Al2O3(0001) single crystals is reported. The experimental results show that the cobalt oxide films become progressively more disordered with increasing thickness, starting from the early stages of deposition. Low energy electron diffraction patterns suggest that the unit cell remains similar to that of a-Al2O3(0001) up to a thickness of 17 A, while at larger thicknesses a pattern identified with that of Co3O4(111) becomes visible. X-ray photoelectron spectroscopy reveals sudden changes in the shape of the Co 2p lines from 3.4 to 17 A cobalt oxide thickness, indicating the transition from an interfacial cobalt oxide layer towards [111]-oriented Co3O4. In particular, the absence of characteristic satellite peaks in the Co 2p lines indicates the formation of a trivalent, octahedrally coordinated, interfacial cobalt oxide layer during the early stages of growth, identified as the Co2O3 corundum phase.