The Growth and Structure of Ultrathin Metal Films on TiO₂(110)

Abstract

We have studied the growth kinetics and structure of 3d transition metals (Cr, Fe, Cu) evaporated onto TiO₂(110), a model oxide substrate. Using Low Energy Ion Scattering (LEIS) and X-ray Photoelectron Spectroscopy (XPS), we find that Cu grows in the Volmer-Weber (3-D island) mode. The mobility of Cu atoms is very high, and 3-dimensional clusters are formed even at 160 K for Cu thicknesses < 30 Å. In contrast, the Cr and Fe films completely cover the TiO₂ substrate for thicknesses of a few ML, indicating that Cr and Fe “wet” the surface better than Cu. The ability of the metal film to cover (wet) the oxide substrate correlates directly with the reactivity of the metal toward oxygen. There is no XPS evidence for a strong chemical interaction between the Cu film and the oxide substrate. However, for the Cr and Fe films, we observe a strong interaction between the films and the oxide substrate, i.e., oxidation of the metal overlayers and reduction of the substrate, at submonolayer thickness. A bcc(100) structure is observed for Cr and Fe films with thickness ≥ 10 Å, on both stoichiometric and sputtered substrates using Angle-Resolved XPS (ARXPS), Medium Energy backscattered Electron Diffraction (MEED) and LEED. The overlayer structure orientation is related to the substrate local structure.