Size-dependent melting of spherical copper nanoparticles embedded in a silica matrix

Abstract

We report size-dependent melting of spherical copper nanoparticles embedded in a silica matrix. Based on the temperature dependence of the surface-plasmon resonance energy and its width, we observe two distinct melting regimes. For particles smaller than $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the absorption spectrum changes monotonically with the temperature, and this allows us to assume the gradual solid-liquid phase transition (melting) of the nanoparticles or the existence of superheated solid nanoparticles. In contrast, for nanoparticles larger than $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, we observe a jumplike increase of the bandwidth and a nonmonotonic shift of surface-plasmon band with the increase of temperature below the bulk melting point. This indicates that the melting of large nanoparticles is a first-order phase transition similar to the melting of bulk copper.