Mechanism of melting in submonolayer films of nitrogen molecules adsorbed on the basal planes of graphite

The melting mechanism in submonolayer films of ${\mathrm{N}}_2$ molecules adsorbed on the basal planes of graphite is studied using molecular-dynamics simulations. The melting is strongly correlated with the formation of vacancies in the films. As the temperature increases, the edges of the submonolayer patch become atomically rough and vacancies are first created there. Then there is an onset temperature at which the vacancies penetrate into the patch. At an intermediate region of coverages ∼0.3–0.8 commensurate layers, there is sufficient free volume for the film to melt at that temperature. At higher coverages, ∼0.8–1.0 layers, a solid with defects is formed, and additional free volume must be created by higher energy mechanisms such as layer promotion for melting to occur; thus, the melting temperature rises with coverage. In contrast, for very small patches, the atomically rough zone penetrates the entire patch at a lower temperature where the film melts. The calculated melting temperatures are significantly lower than observed experimentally, indicating a severe fault in the potential model. A possible source of the discrepancy is identified.