Growth of ultrathin Ru oxide films on perovskite and corundum substrates

Abstract

The ability to control the dispersion of RuO2 monolayers on a range of perovskite- and corundum-structured oxide substrates was investigated using complementary experiment and theory. Controlled deposition under UHV conditions and subsequent in situ characterization by x-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), reflection high energy electron diffraction (RHEED) and low energy electron diffraction (LEED) was used to follow the chemical state, dispersion and structure of the RuO2 layers as a function of growth and post-growth processing conditions. The experimental results reveal that monolayer RuO2 on ferroelectric PbxZr1-xTiO3(001) and non-polar SrTiO3(001) α-Al2O3(11¯02) tends to dewet the surface to form three-dimensional clusters, though some kinetic stability of dispersed RuO2 monolayers was achieved on the SrTiO3 surface. Although an RuO2 termination is not favored for SrRuO3(001), deposition onto SrRuO3(001) yields a stable RuO2 surface which ISS suggests is capped by chemisorbed oxygen. The complementary density functional theory modeling and ab initio thermodynamics indicate that Ru oxide monolayers on the perovskite substrates tend to be unstable under the low oxygen pressure experimental conditions; however, either lower temperatures or higher oxygen pressures can lead to a kinetic and even thermodynamic stability. The theoretical findings also provide guidance on how the instability of dispersed RuO2 monolayers can be overcome by inserting buffer layers and increasing the partial pressure of molecules that selectively, strongly adsorb at coordinatively unsaturated Ru sites on the oxide surface.