Plasmon decay induced hot electron generation in metallic nanoparticles is central to the rapidly expanding field of photocatalysis. Hot electrons can be created by interband, plasmon, or intraband excitations. Following excitation, electrons thermalize through electron-electron scattering. The subsequent relaxation by electron-phonon coupling transfers the energy, effectively described by an increased electron temperature, to the lattice. These steps are widely described by the two-temperature model indicating the independence between initial thermalization of hot electrons, via electron-electron scattering, and the overall lifetime of hot electrons, determined by electron-phonon coupling. However, the excitation wavelength dependence of electron-electron scattering and electron-phonon coupling are still not well understood, and conflicting views have emerged suggesting that plasmon excitation leads to different hot electron lifetimes. We used femtosecond (fs) transient absorption with sub-100 fs time resolution to study 40 nm gold nanospheres and 30×60 nm nanorods. Excitation wavelengths were varied from 400 to 800 nm to cover interband, plasmon, and intraband excitation. Comparing nanospheres to nanorods allowed us to tune the spectral overlap of the plasmon with interband and intraband transitions, respectively. To obtain electron-phonon relaxation times we performed a pump power dependence at each excitation wavelength. By analyzing the slopes of these power dependencies, we were able to establish that there is no excitation wavelength dependence of the electron-phonon relaxation as long as the absorbed power is considered. Our finding that the overall hot electron lifetime in colloidal gold nanospheres and nanorods is independent on how they were created has important impact on the further development of plasmonic photocatalysts.

Ultrafast electron dynamics in gold nanoparticles as a function of excitation

ACS Spring 2022

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