Surface and quantum-size effects in Pt and Au nanoparticles probed by 197Au Mossbauer spectroscopy -: art. no. 205418

Molecular metal nanoclusters and metal colloids are attractive systems to study surface and quantum-size effects on conduction electrons and phonons. Mossbauer spectroscopy is an excellent tool to study such phenomena. We present a detailed analysis of Au-197 Mossbauer spectra taken from a series of Au and Pt nanoparticles of sizes 1-17 nm. A central force model with only nearest-neighbor interactions gives a satisfactory description of the recoil-free fractions. A site-dependent local density-of-states approach is used to describe the surface and size effects on the electronic properties of the nanoparticles, as reflected in the Mossbauer isomer shifts and quadrupole splittings. A correlation of these parameters with the location of the site on the cluster surface is found. We conclude that. even for ligated metal clusters, the surface-atomic layer has predominantly "metallic" character, in the sense that it is a part of the total volume over which the electrons are delocalized, so that free charge exchange is possible between surface atoms and the inner core of the particle. For the smaller particles, the electron density is found to vary throughout the metal core, which is interpreted in terms of surface screening and quantum-size effects.