Subsurface particulate film formation in softenable substrates: Present status and possible new applications

The status of our knowledge of subsurface particulate film formation in softenable substrates is reviewed. Particular emphasis is given to recent developments in our understanding of the formation mechanism, which has progressed via both experiment and theory. Subsurface particulate monolayer formation is now appreciated as a very general phenomenon for most inorganic materials in combination with organic polymer substrates; partially embedded structures are generally formed by organic particulate materials. These configurations are explained by thermodynamic formulations involving the surface and interfacial tensions. With the convenient technique of vacuum deposition, subsurface formation is generally limited to low melting point inorganic materials, and even with these materials is further limited to certain ranges of substrate temperature and deposition rate. These limitations can be ascribed to either sinking rate or growth mode limitations, and calculations are in excellent quantitative agreement with experimental data. When vacuum deposition onto soft substrates does not form a subsurface structure, the resulting above-surface film generally grows in the rare Stranski-Krastanov mode. Based on our present understanding, several possible new applications of these unique subsurface structures (in addition to their important photographic uses) are proposed. These include techniques for improving the substrate adhesion of many inorganic and organic evaporated films, as well as uses as one-step optical recording media and as solar absorbers. It seems likely that other applications will arise as the existence of these structures becomes more widely known.