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 200. 3em{0ex}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 200. 3em{0ex}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.