Paul S. Vincett

Subsurface particle monolayer and film formation in softenable substrates: Techniques and thermodynamic criteria

Abstract We have previously shown that subsurface particulate monolayers form rather generally when inorganic materials are appropriately deposited onto softenable organic polymer substrates. Here we analyse, in terms of the surface free energies, the thermodynamic stability on soft substrates of the various conceivable subsurface structures versus the possible above-surface configurations. For both rigid preformed particles and flexible growing clusterd (as occur in vacuum deposition), the tendency for complete immersion is determined by the same inequality among the various surface and interfacial tensions. Estimates of these parameters predict a completely embedded structure to be thermodynamically favourable for most inorganic materials on organic polymer substrates, with generally a partially embedded structure predicted for organic materials. Although, with vacuum deposition of the particle material, subsurface structure formation is often limited by kinetic factors (associated with substrate temperature and deposition rate)), we describe a variety of other fabrication techniques by which such structures can indeed be fabricated with remarkable generality from preformed particles. Moreover, with organics, completely—rather than partially—embedded structures can be formed, if particles are spread on the surface of the polymer and the system is exposed to a suitable solvent vapour. This can be rationalized in terms of the above- mentioned inequality, provided that appropriate surface and/or interfacial tensions are calculated for this situation. Continuous subsurface films are predicted to become thermodynamically favourable during vacuum deposition, compared with embedded particulate structures, when particle coalescence becomes too slow to keep the projected-area coverage below certain maximum levels. This is expected as the particles exceed some minimum size, and we confirm this prediction experimentally.

Vacuum deposition onto softenable substrates: Formation of novel subsurface structures

Abstract The vacuum deposition of a wide range of evaporant materials onto soft (heated thermoplastic) substrates has been studied for the first time. It was previously known that, in some circumstances, selenium deposited onto heat-softened polymers forms a nearly close-packed monolayer of submicron, almost monodisperse, amorphous spheres embedded a few tens of nanometres under the surface; this novel structure has wide applications as a photographic film. It was not known whether this phenomenon was unique to selenium and its alloys (which are unusual in being amorphous and polymeric), and if not what the criteria were for formation of these or other subsurface structures. Under deposition conditions similar to those used for selenium (substrate temperature T s ∽120°C), tin, indium and silver have now also been observed by electron microscopy to form novel subsurface particulate structures. The tin and indium structures are similar to that of selenium, except that the spheres are nearly single crystal with greater poolydispersity and are arranged in a less pristine monolayer with a fair degree of multiple stacking. The silver structure consists of very small (about 25 nm) crystalline aggregates, very tightly packed to a depth of several particles below the polymer surface. For considerably higher T s , the embedded silver become discrete and well separated, resembling more the selenium, tin and indium structures. Under our typical deposition conditions, all other materials tried form only above-surface deposits, as do selenium, tin, indium and silver when either the substrate temperature is too low or the deposition rate too high. Subsurface particle formation is shown to be an activated process, the activation energies depending on both the evaporant and the polymer. The above-surface deposits generally grow in a Stranski-Krastanov mode, with a thin (1–3nm) continuous initial deposit and superposed island structures; this is extremely unusual for a non-single-crystal substrate. Our results, and a theoretical analysis to be published separately, suggest that the criteria for formation of subsurface structures are certain minimum levels of the evaporant surface diffusion and polymer fluidity at T s . Thus, for typical thermoplastics, this criterion usually requires a fairly low melting point evaporant; most other inorganic materials have been shown to have a thermodynamic tendency to form subsurface monolayers, and (like silver) they will probably overcome kinetic limitations if T s can be made high enough. Thermodynamic considerations suggest partially embedded structures for organics, and this was also observed.

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

Abstract 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.