Melting of Pd clusters and nanowires: A comparison study using molecular dynamics simulation

We present results from a molecular dynamics simulation study of a Pd cluster and a nanowire of the same diameter using the Sutton-Chen many body potential function. Changes in thermodynamic and structural properties of these two systems during heating were studied. We found that the melting temperature of the Pd nanowire of $1200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is lower than the simulated bulk value $(1760\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ but higher than that of the cluster at $1090\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Melting behaviors were characterized by a number of thermodynamic, structural, and dynamical parameters. Surface premelting at much lower temperatures than the near first-order transition temperatures noted above was observed in both Pd systems. The surface premelting temperature range was higher for the nanowire than for the cluster. Surface melting in nanowires manifests itself as large amplitude vibrations followed by free movement of atoms in the plane perpendicular to the nanowire axis, with axial movement arising at temperatures closer to the transition temperature. Increase in nanowire diameter as well as shape change is seen to result from this axial mixing. Bond-orientational order parameters indicated that the nanocluster retained the initial fcc structure at low temperatures. The nanowires, however, were seen to be stable at a solid structure that was close to hcp as established by bond-orientational order parameter calculations. Melting point depressions in both systems agree better with a liquid-drop model than with Pawlow's thermodynamic model.