Direct probe of size-dependent electronic relaxation in single-sized Au and nearly monodisperse Pt colloidal nano-particles

We report the first direct measurement of electronic energy relaxation in single-sized Au and nearly monodisperse Pt colloidal nano-particles using femtosecond laser spectroscopy. The relaxation time is found to be sensitive to both electron-phonon coupling and surface collision, which have competing effects on the relaxation rate. In large particles weakening of electron-phonon coupling dominates, resulting in slower electronic relaxation relative to bulk. In smaller particles surface collision dominates, which makes the relaxation faster. However, in very small clusters such as Au13 the relaxation time is significantly slower, indicating that the molecular behavior starts to emerge.