ABSTRACT
The formation of disperse systems on a solid substrate by condensation from a foreign vapour phase takes place in several stages. Sorption processes begin upon contact of the vapour medium with the surface or when applying a molecular beam to the surface. With time a stationary concentration of adsorbate particles forms on the surface and the size of the particles is determined by the parameters of the vapour environment, the properties of the substrate and the external physical conditions. If the vapour environment is supersaturated, the adsorbed phase can also become supersaturated. Due to thermal migration of particles and fluctuations agglomerates of particles of the adsorbed phase form continuously on the surface. The state of the agglomerates varies depending on the size of the particles. The agglomerates which have not reached the critical size of stability in the adsorbed and vapour phases start to decay. The agglomerates that are larger than the critical size are stable and able to grow. The result of this second stage, called nucleation or nucleation, is the formation of a disperse system as the particles in the condensate of submicron dimensions distributed on the substrate surface. In the third stage the growth of nuclei and the birth of additional ones leads to the formation of a continuous coating on the substrate surface by the condensate of the vapour phase. Depending on the combination of the surface properties and the properties of the condensing vapour we can obtain a variety of disperse systems. In the case of vapour condensation by the vapour-liquid mechanism the formation of liquid nuclei over time transforms to the liquiddrop condensation on the surface, and in the case of wetting of the last –to film condensation. In condensation by the vapour-liquid-crystal mechanism the formation of liquid nuclei as they grow leads to crystallization of the solid phase and production of a solid film on the substrate. Condensation by the vapour-crystal mechanism is characterized by the formation of solid nuclei, followed by filling the entire surface by the solid condensate of the vapour phase. Depending on the properties of the substrate surface with respect to the
condensate, greatly varying films of the solid condensate can be produced. On single-crystal substrate with a lattice parameter close to the lattice parameter of the condensate, a low-intensity effect of disturbing factors may produce single-crystal films (molecular epitaxy). Polycrystalline substrates usually show the formation of a polycrystalline solid condensate film with the grain size, determined by the surface properties and condensation conditions. Quasiamorphous solid condensates can be produced as a result of condensation at high supersaturation.
