2, Instituto de Física, Universidad Nacional Autónoma de México (UNAM), Mexico City, , Mexico
In recent years, small metal nanoclusters (with sizes below 2 nm) have emerged as quite interesting materials because their optical properties differ from those of larger clusters or plasmonic nanoparticles. One of the new properties exhibited by these systems is their photoluminescence, probably due to the discretization of the electron density of states. Light emission from metal nanoclusters can be relevant in technological applications as primary nanosource of light or as a nanoantenna to enhance light-matter interaction. Herein, we study the photoluminescence (PL) emission from Pt metal nanoclusters embedded in silica and sapphire plates, by using the ion implantation technique. The PL signal ranges from 400 nm to 600 nm, with a peak at 550 nm for all samples irrespective of the matrix type. We have observed a gradual change in the behavior of the PL intensity as increasing the power density of picosecond pulse excitation in the UV range. PL intensity increases linearly at lower pump excitation and reaches a saturation behavior for intermediate power density. However, as the excitation power increases above ~7 kW/m2, the PL response become more intense, and it can be considered as superlinear with respect the excitation laser intensity. This optical behavior is in clear contrast compared with the usual observations in semiconductor quantum dots, where the PL intensity saturates at higher fluence excitation. The observed superlinear PL from Pt nanoclusters is analyzed in terms of multi-photon excitations.