Keywords: metal nanoparticles, electron-phonon coupling, time-resolved spectroscopy, heat dissipation, size dependent properties, silver, gold, nanotechnology
Electron-phonon coupling and heat dissipation in metal nanoparticles
This review discusses electron-phonon (e-ph) coupling and heat dissipation in noble metal nanoparticles, specifically, how the time constants for these events depend on size. This has proven to be a difficult question for the e-ph coupling process. For example, results from our laboratory and others, show that for Au particles in aqueous solution the coupling between the electrons and phonons is independent of size, and essentially the same as that for the bulk metal. In contrast, recent work by Voisin and co-workers ((2003) Phys. Rev. Lett., Vol. 90, 177401) showed that the characteristic e-ph coupling time decreases from ~1 ps for particles larger than 10 nm, to ~0.6 ps for 3 nm particles. These conflicting results will be critically discussed. On the other hand, the heat dissipation process is less controversial, and the size dependence is much clearer. We have found that the characteristic time scale for heat dissipation is proportional to the particle radius, at least for particles larger than ca. 10 nm. This is consistent with heat diffusion in the surrounding liquid controlling relaxation. Experiments with bi-metallic particles of Pt and Au are also described. In this system the effective e-ph coupling constant is an average of that for the two metals, weighted by the fraction of electronic states from each. The density of electronic states for Pt is much greater than that for Au, which means that Pt dominates the dynamics, even for particles that are mostly Au by atomic molefraction.