On surface pre-melting of metallic nanoparticles: molecular dynamics study

Employing the isothermal molecular dynamics and the embedded atom method, we simulated melting of metallic nanoparticles (Au, Ag, Cu, Ni, and Pb ones). In more detail, the results for Au and Ag nanoparticles are presented and discussed. At first, we analyzed the behavior of the temperature dependences for the potential (cohesive) term into the specific (per atom) internal energy and for the degree of crystallinity in the course of heating nanoparticles. We have found that the results obtained for nanoparticles of about 4 and 8 nm in size (containing 2093 and 20,113 atoms, respectively) demonstrate the continuous melting. Employing the dependence of the specific potential energy on the distance to the nanoparticle center of mass and the common neighbor analysis, we showed that the continuous melting occurs via the surface pre-melting mechanism. Then, we evaluated the self-diffusion coefficient in the surface disordered layers of Au and Ag nanoparticles and found that our results agree in order of magnitude (10−9 m2/s) with the values of the self-diffusion coefficient for the bulk Au and Ag melts at the corresponding bulk melting temperatures. Finally, combining in our molecular dynamics experiments continuous heating Au nanoparticles with annealing them at some constant selected temperatures, we have shown that the liquid nucleation and growth mechanism should be most adequate to the melting behavior of metallic nanoparticles.