Nucleation Density and Shape of Submonolayer Two-Dimensional Islands of Diphenyl Dinaphthothienothiophene in Vacuum Deposition

Submonolayer two-dimensional (2D) islands of diphenyl dinaphthothienothiophene with various shapes and densities (N) were formed on a SiO2/Si substrate by controlling substrate temperature and the surface treatment for SiO2 in vacuum deposition to investigate the growth mechanism on the basis of their morphology. The statistical analysis shows that the 2D islands have complex shapes when N is small, and there is a constant relationship between N and the shape complexity of the 2D islands, regardless of the deposition conditions. Because the surface morphology is determined by diffusion coefficients for admolecules on a substrate surface (Ds) and along the edge of a 2D island (Dedg), the relationship between (N, shape complexity) and (Ds, Dedg) is studied. The statistical analysis indicates that Dedg is almost independent of the surface conditions and is instead determined by interactions with molecules constructing the 2D island. Therefore, Dedg is considered as a material-dependent parameter to control the morphology for growing high-quality films in vacuum deposition.