Dynamics of Energy Transfer in Large Plasmonic Aluminum Nanoparticles

We report the first photophysical characterization of energy-transfer dynamics in large (100 nm diameter) plasmonic aluminum nanoparticles suspended in liquid 2-propanol. The spectral response of the particles to ultrafast excitation is characterized by a decrease in light transmission broadly across the visible and near-infrared on a 700 fs time scale that is consistent with predictions for electron–lattice relaxation processes. Time-dependent bleaching of the interband transitions is largely isolated from spectral changes to the intraband transition associated with light scattering and provides a window into electron–electron thermalization dynamics that complete on a ∼350 fs time scale. Subsequent relaxation in these particles is characterized by a 250 ps energy transfer to the surrounding medium, comparable to energy-transfer rates expected for much smaller particle sizes (<10 nm in diameter). Using a two-interface model, we find that the rapid thermal energy transfer is accounted for by the presence of a compact ∼4 nm native oxide layer on the aluminum nanoparticles. We propose that using surface modifications, including controlled oxidation, could be an effective tool to engineer heat-transfer rates from large particles to the surrounding medium and could be a handle for controlling thermal decay processes in a broad range of applications involving metal nanoparticles.