Nanoscale influence on photoluminescence and third order nonlinear susceptibility exhibited by ion-implanted Pt nanoparticles in silica.

A systematic study has been carried out to investigate photoluminescence and third order nonlinear ultraviolet properties exhibited by platinum nanoparticles nucleated in a high-purity silica matrix. The modification in the characteristic photoluminescence spectra of the nanocomposites, ranging between 400 and 600 nm, was obtained with the assistance of a thermal annealing process that changed the average size of the platinum nanoparticles. The influence of temperature, between 200 °C-1100 °C, during the thermal treatment of the nanostructures was analyzed. UV-vis spectroscopy studies corroborated changes in the optical absorption resonances of the ion-implanted samples after annealing, which could then be correlated with the average size of the nanoparticles. The estimated average size was also corroborated by transmision electron microscopy. For temperatures below 600 °C the system is mainly composed of ultra-small photoluminescent platinum nanoparticles. Larger platinum nanoparticles were formed at higher annealing temperatures but photoluminescence quenching was observed as the typical plasmonics response of larger metal nanoparticles started to emerge. The photoluminescence emission for samples with a particle size of less than 2 nm is enhanced approximately 12 fold with respect to the samples with a particle size in the range of 3-7 nm. Differences in the resulting photoluminescence spectra were revealed by substituting the participation of argon, hydrogen or nitrogen, as environmental gases for thermal annealing. A weak PL emission, featuring 1.5 nW at a laser excitation power of 800 μW, related to larger platinum nanoparticles was observed. New emission peaks emerging from the larger platinum nanoparticles were associated with possible hydrogen adsorption on the nanoparticles' surface. Third order nonlinear ultraviolet measurements were conducted using a time-resolved two-wave mixing method with self-diffraction at 355 nm wavelength. The observed self-diffraction decay time is less than 25 ps, regardless of the average size of the nanoparticles studied. The evolution of the self-diffracted intensities derived from temperature was also linked to the mean size of the nanoparticles in the samples. Comparative two-wave mixing evaluations also validated a modification in third order nonlinear susceptibility exhibited by annealed samples. An important role of the localized surface plasmon resonance phenomena associated with the platinum nanoparticles for photoluminescence and optical nonlinearities was identified. A proposed hypothetical electronic mechanism that may explain the exceptional optical transitions related to low-dimensional platinum systems is discussed.