Plasmon-Induced Hot-Carrier Excited-State Dynamics in Plasmonic Semiconductor Nanocrystals

The variance of carrier relaxation pathways for WO3-x plasmonic semiconductor nanocrystals (PSNCs) is monitored by transient absorption spectroscopy following excitation of the localized surface plasmon resonance (LSPR) versus the optical band gap (E-g,E-opt). Excitation of the LSPR leads to efficient hot carrier population above the Fermi level in WO3-x via Landau damping, in analogy to noble metal LSPR relaxation mechanisms. Hot carrier depopulation occurs on the femtosecond timescale, observed as the concomitant recovery of an LSPR bleach with the appearance of discrete interband and intraband photoinduced absorption features. By comparison, the direct excitation of E-g,E-opt results in trion recombination at donor-acceptor sites within the WO3-x NC, consistent with exciton decay dynamics observed for typical wide-band-gap semiconductor NCs. From the analysis of pump power dependency data, a hot-carrier electron-phonon coupling constant of 1.47 x 10(11) J K-1 s(-1) cm(-3) is extracted. The direct comparison of the decay dynamics following E-g,E-opt versus LSPR excitation confirms that the observed plasmon in trioctylphosphine oxide passivated, spherical WO3-x is a resonance state in which hot carriers are generated only from excitation on resonance with the LSPR frequency. This study on WO3-x PSNCs provides a toolset that can be used to evaluate the role of hot carriers following LSPR excitation of n-type, plasmonic transparent conducting oxide NCs, where enhancement of photocatalysis, photovoltaic performance, and optical enhancement has been reported.