Surface-Induced Melting of Metal Nanoclusters

We investigate the size effect on melting of metal nanoclusters by molecular dynamics simulation and thermodynamic theory based on Kofman’s melt model. By the minimization of the free energy of metal nanoclusters with respect to the thickness of the surface liquid layer, it has been found that the nanoclusters of the same metal have the same premelting temperature T_pre=T₀-T₀(γ_sv-γ_lv-γ_sl)/(ρLξ) (T₀ is the melting point of bulk metal, γ_sv the solid--vapour interfacial free energy, γ_lv the liquid--vapour interfacial free energy, γ_sl the solid--liquid interfacial free energy, ρ the density of metal, L the latent heat of bulk metal, and ξ the characteristic length of surface-interface interaction) to be independent of the size of nanoclusters, so that the characteristic length ξ of a metal can be obtained easily by T_pre, which can be obtained by experiments or molecular dynamics (MD) simulations. The premelting temperature T_pre of Cu is obtained by MD simulations, then ξ is obtained. The melting point T_cm is further predicted by free energy analysis and is in good agreement with the result of our MD simulations. We also predict the maximum premelting-liquid width of Cu nanoclusters with various sizes and the critical size, below which there is no premelting.