Melting and freezing of Lennard-Jones clusters on a surface

The properties of clusters containing 13 Lennard-Jones atoms and adsorbed on a substrate were studied using molecular-dynamics simulations. The clusters were found to undergo a transition from one solidlike structure at low temperatures to sets of liquidlike structures at higher temperatures for a wide range of cluster-surface interactions. Coexistence of the solidlike structure with the liquidlike structures was observed over a finite temperature domain, allowing the identification of the transition temperature $T_m$ from plots of the equilibrium energy as a function of temperature. $T_m$, the analog of the melting temperature of bulk materials, was observed to depend on the cluster-substrate interaction strength ${\epsilon }_s$. As $T_m$ is approached from the liquid phase in the cluster phase diagram drawn in terms of the parameter ${\epsilon }_s$, a wetting transition was evident. For weak substrates the cluster desorbed as a whole and the surface remained dry. For strong substrates layered structures built up such that the surface was wet. For intermediate ${\epsilon }_s$ the cluster froze while being adsorbed on the surface, keeping the icosahedral structure at low temperatures. Partial wetting took place.