GHz adhesion dynamics of gold nanoparticles down to 40 nm in diameter

We investigate the acoustic vibrations of single gold nanoparticles on a glass substrate using time-resolved optical spectroscopy. Using a two-color pump-probe setup, we measure the transient change of a particle's reflectivity induced by an optical pump pulse. Our measurements identify both the axial mode, which corresponds to an oscillation of the distance between nanoparticle and substrate, and the particle's fundamental breathing mode, an oscillation of particle diameter. We investigate the scaling of the axial mode frequency with the radial mode frequency. Our analysis reveals that the scaling established for larger particles extends down to diameters of 40 nm. This finding strongly suggests that continuum models of contact mechanics hold for contact areas comprising only a few thousand atoms.