The size dependence of the photoluminescence spectra from silver nanoparticles embedded in a silica host medium was observed. The quantum yield of the photoluminescence increased when the size of the nanoparticles was decreased. The quantum yield for 8 nm silver nanoparticle was estimated to be on the order of ${10}^{-2}$ which is ${10}^8$ times higher than the one observed for bulk silver. The two photoluminescence bands observed from silver nanoparticles were rationalized as the radiative electron interband transitions and radiative decay of the surface plasmons in silver nanoparticles. The strong local electric field induced by the surface-plasmon resonance in silver nanoparticles enhances the exciting and emitted photons and increases the quantum yield of the photoluminescence.

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 $20\mathrm{nm}$, 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 $20\mathrm{nm}$, 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.