Unveiling and quantifying the topology-dependent pre-melting of nanoparticles

TL;DR

This study reveals that the surface pre-melting of hexagonal close-packed Co nanoparticles depends on facet orientation, with stepped facets melting significantly earlier than flat ones, challenging the classical isotropic melting model.

Contribution

It demonstrates facet-dependent pre-melting behavior in nanoparticles, extending understanding of melting phenomena to anisotropic surfaces and providing a size-dependent melting model.

Findings

Stepped facets melt nearly 200K below flat facets.

Surface mobility begins at 0.2×T_M,∞, driven by facet disorder.

Size-dependent critical liquid layer thickness triggers complete melting.

Abstract

The melting of metallic nanoparticles is governed by surface pre-melting, a phenomenon traditionally modeled as the isotropic growth of a uniform liquid shell. Challenging this classical view, we report facet-dependent surface pre-melting in hexagonal close-packed Co nanoparticles, arising from the structural heterogeneity of the nanoparticle surface. Characterizing melting in molecular dynamics simulations (500 to 6000 atoms), we observe the onset of surface mobility, starting as low as $0.2\times T_{M,\infty}$ (the bulk melting point), driven by the early disordering of stepped $\{01\bar{1}1\}$ facets. We found that these facets consistently melt at temperatures nearly 200 Kelvin lower than flat $\{0001\}$ facets, regardless of particle size, and relate facets melting temperatures to the nanoparticle size via a 2D extension of the Gibbs-Thomson relation. We determine a critical liquid layer thickness that triggers the melting of the entire nanoparticle, which is found to be size-dependent. Our results confirm the recent experimental observation of the surface pre-melting effect, and extend it to anisotropic particles with different facet orientations.