Existence and stability of nanometer-thick disordered films on oxide surfaces

Abstract

Disordered films of 1–2 nm thickness have been observed on the surfaces of binary oxide systems in liquid–solid and solid–state equilibrium. The systems Bi2O3–ZnO, Bi2O3–Fe2O3, WO3–TiO2 and MoO3–Al2O3 have been examined. Sufficient data now exist to interpret the observed films as surface phases of thermodynamically determined thickness and composition, coexisting at equilibrium with one or more bulk phases. Phenomenological comparisons to the formation of equilibrium-thickness intergranular films, multilayer adsorption, prewetting and surface melting are made. A thermodynamic model that allows the prediction of systems likely to form stable films is proposed, in which key variables are the relative surface energies, the sign and strength of the dispersion interaction, the extent of ordering (epitaxy) at the crystal–film interface, and the free energy of amorphization and mixing. It is suggested that the relative interfacial energies and volume thermodynamic terms are always important contributions, while the dispersion interaction is relatively unimportant well below the solidus temperature, but may be critical close to and above the solidus temperature.