Anisotropy, band-to-band transitions, phonon modes, and oxidation properties of cobalt-oxide core-shell slanted columnar thin films

TL;DR

This study explores the transformation of cobalt slanted columnar thin films into cobalt oxide core-shell structures through oxidation, revealing their optical, structural, and phononic properties, and demonstrating effective passivation against further oxidation.

Contribution

It provides detailed analysis of the oxidation process and properties of cobalt-oxide core-shell nanostructures, including the development of a passivation method using alumina coating.

Findings

Cobalt nanocolumns transform into transparent cobalt oxide core-shell structures upon annealing.

Atomic layer deposition of alumina effectively prevents further oxidation of cobalt structures.

The study characterizes the optical and vibrational properties of the resulting cobalt oxide structures.

Abstract

Highly-ordered and spatially-coherent cobalt slanted columnar thin films were deposited by glancing angle deposition onto silicon substrates, and subsequently oxidized by annealing at 475 $^{\circ}$C. Scanning electron microscopy, Raman scattering, generalized ellipsometry, and density functional theory investigations reveal shape-invariant transformation of the slanted nanocolumns from metallic to transparent metal-oxide core-shell structures with properties characteristic of spinel cobalt oxide. We find passivation of Co-SCTFs yielding CoAl$_2$O$_3$ core-shell structures produced by conformal deposition of a few nanometers of alumina using atomic layer deposition fully prevents cobalt oxidation in ambient and from annealing up to 475 $^{\circ}$C.