Melting Mechanism and Structure Evolution of Au Nanofilms Explored by Molecular Dynamics Simulations

The melting mechanism and structure evolution of two-dimensional Au nanofilms with different thicknesses have been investigated in detail by using classical molecular dynamics simulations. The simulation results demonstrate that all Au nanofilms display a two-stage melting behavior of surface premelting and homogenous melting. Furthermore, the premelting behavior only occurs in the outermost layers but the other inner layers always keep a stable solid state until the corresponding melting point, which is different from the premelting behavior from surface into the interior in zero-dimensional Au nanocluster and one-dimensional Au nanowire. Meanwhile, the increase of nanofilm thickness can lead to an increase of melting point. During the premelting process, the surface reconstruction from the {100} plane to the {111} plane has directly been observed at a atomic level for all Au nanofilms. However even for the thinnest L2 nanofilm, the surface stress can't induce such surface reconstruction until temperature is up to 500 K, while similar surface reconstruction induced by surface stress can be observed at much lower temperature for the Au nanowire due to its higher surface-to-volume ratios compared to the Au nanofilm. In addition, our simulation results show that the thinnest Au nanofilm with two atomic layers can be broken into independent one-dimensional nanowires when the temperature reaches a certain value.