Hot Electron Emission Can Lead to Damping of Optomechanical Modes in Core-Shell Ag@TiO₂ Nanocubes

Interactions between light and metal nanostructures are mediated by collective excitations of free electrons called surface plasmons, which depend primarily on geometry and dielectric environment. Excitation with ultrafast pulses can excite optomechanical modes that modulate the volume and shape of nanostructures at gigahertz frequencies. Plasmons serve as an optical handle to study the ultrafast electronic dynamics of nanoscale systems. We describe a method to synthesize core-shell Ag@TiO₂ nanocubes - while successfully maintaining the size and shape of the nanocube. Transient absorbance spectroscopy (TAS) is used to track photophysical processes on multiple time scales: from the ultrafast creation of hot carriers to their decay into phonons and the formation of optomechanical modes. Surprisingly, the TiO₂ shell surrounding the Ag nanocubes caused no appreciable change in the frequency of the optomechanical mode, indicating that mechanical coupling between the core and shell is weak. However, the optomechanical mode was strongly attenuated by the TiO₂ shell and TAS decay at ultrafast time scales (0-5 ps) was much faster. This observation suggests that up to ∼36% of the energy coupled into the plasmon resonance is being lost to the TiO₂ as hot carriers instead of coupling to the optomechanical mode. Analysis of both ultrafast decay and characterization of optomechanical modes provides a dual accounting method to track energy dissipation in hybrid metal-semiconductor nanosystems for plasmon-enhanced solar energy conversion and chemical fuel generation.