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We report size-dependent melting of spherical copper nanoparticles embedded into 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 existence of superheated solid nanoparticles. In contrast, for nanoparticles larger than 20 nm, we observe a jump-like increase of the bandwidth and non-monotonic dependence of surface plasmon energy at the temperatures 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.

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The temperature dependence of the surface plasmon energy and bandwidth for silver nanoparticles in the size range 8-30 nm embedded in a silica matrix has been studied using diffuse reflection spectroscopy. The dependence shows a non-monotonic jump-like behaviour indicating a low-temperature size-dependent melting of silver nanoparticles. The melting point decreases with the decrease of the nanoparticle size. The hysteresis in the temperature dependence of the surface plasmon bandwidth has been observed, indicating the first-order phase transition.