Superheating and Solid-Liquid Phase Coexistence in Nanoparticles with Nonmelting Surfaces

We present a phenomenological model of melting in nanoparticles with facets that are only partially wet by their liquid phase. We show that in this model, as the solid nanoparticle seeks to avoid coexistence with the liquid, the microcanonical melting temperature can exceed the bulk melting point and that the onset of coexistence is a first-order transition. We show that these results are consistent with molecular dynamics simulations of aluminum nanoparticles which remain solid above the bulk melting temperature.

Figure 1

Model geometries for coexisting clusters. For clusters with SM facets ($\Delta \gamma >0$), the spherically symmetric model on the left can be used. For clusters with NM facets ($\Delta \gamma <0$), we use the spherical cap model on the right.

Figure 2

Caloric curve for a 3.5 nm Al cluster constructed using MCD theory.

Figure 3

Caloric curve for a 4033-atom Al cluster. The curve was constructed by both heating from the solid state (squares) and cooling from a coexisting structure (circles) that emerges as the cluster approaches the liquid state.

Figure 4

Snapshots showing the state of the 4033-atom cluster at $E=-2.966\mathrm{eV}/\mathrm{\text{atom}}$ (left) and $E=-2.960\mathrm{eV}/\mathrm{\text{atom}}$ (right) during the cooling phase of the caloric curve in Fig. 3. Light gray atoms are of low mobility (solid) and the dark gray atoms are of high mobility (liquid).

Figure 5

The dependence of the melting point on size as calculated using MCD theory for an Al (111)-faceted particle (solid lines) and several data points calculated from MD simulations.