Slow Charge Carrier Relaxation in Gold Nanoparticles

Abstract

We report on ultrafast transient absorption (TA) studies of surface plasmon-excited ∼25 nm-diameter Au nanoparticles in solution, monitoring the response of the surface plasmon resonance in the visible region (1.7–3.0 eV) and that of the inter- and intraband transitions in the deep UV region (3.4–4.5 eV). The former reflects lattice heating by electron–phonon energy transfer, while the latter monitors charge carriers, that is, excess holes and electrons, respectively, below and above the Fermi level. Although the steady-state UV absorption spectrum of Au is featureless, the TA spectra reveal three bands that appear promptly upon plasmon excitation and decay over hundreds of picoseconds (ps). Based on the band structure diagram of bulk Au, we attribute these to inter- and intraband transitions around the X and L symmetry points. The decay of the hot electrons monitored via the interband transitions takes 150–180 ps. On the other hand, probing the intraband transitions at the L symmetry points below the Fermi level reveals both electron and hole relaxation, with the latter requiring tens of ps. These results show that after the initial ultrafast electron–electron and electron–phonon scattering processes, hot charge carriers above and below the Fermi level remain in equilibrium with the hot lattice for several tens to hundreds of ps.