Molecular Dynamics Simulation of Laser Melting of Nanocrystalline Au

Abstract

The mechanisms and kinetics of short pulse laser melting of single crystal and nanocrystalline Au films are investigated on the basis of the results of simulations performed with a model combining the molecular dynamics method with a continuum-level description of the laser excitation and subsequent relaxation of the conduction band electrons. A description of the thermophysical properties of Au that accounts for the contribution of the thermal excitation of d band electrons is incorporated into the model and is found to play a major role in defining the kinetics of the melting process. The effect of nanocrystalline structure on the melting process is investigated for a broad range of laser fluences. At high fluences, the grain boundary melting in nanocrystalline films results in a moderate decrease of the size of the crystalline grains at the initial stage of the laser heating and is followed by a rapid (within several picoseconds) collapse of the crystal structure in the remaining crystalline parts of the film as soon as the lattice temperature exceeds the limit of the crystal stability against the onset of rapid homogeneous melting (the limit of superheating). At low laser fluences, close to the threshold for the complete melting of the film, the initiation of melting at grain boundaries can steer the melting process along the path where the melting continues below the equilibrium melting temperature and the crystalline regions shrink and disappear under conditions of substantial undercooling. The unusual melting behavior of nanocrystalline films is explained on the basis of thermodynamic analysis of the stability of small crystalline clusters surrounded by undercooled liquid.