Pressure-balance and diffuse-interface models for surficial amorphous films

Recent observations of nanoscale surficial amorphous films in Bi2O3-doped ZnO are briefly reviewed. The experimental results are modeled with two approaches. A pressure-balance model with the volumetric free energy being the dominant temperature-dependent interaction extended from Clarke's intergranular films model is proposed and numerically evaluated. This quantitative model predicts thicknesses versus temperature behavior for subeutectic films consistent with experimental results. Alternatively, the sequence of adsorption and wetting events as a function of temperature and composition is interpreted as a case of combined surface prewetting and premelting in a two-component system with a bulk eutectic reaction as a generalization of Cahn's critical point wetting model. In this second approach that better represents the through-thickness gradients, diffuse-interface formulation is proposed and analyzed for assessing surficial film stability as well as associated drying and complete wetting transitions. The observation made for Bi2O3 on ZnO can be represented by one of the possible surface prewetting/premelting phase diagrams.