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

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 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 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.