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 Au-13 the relaxation time is significantly slower, indicating that the molecular behavior starts to emerge.