Abstract
In this work, we have examined the ultrafast dynamics of shape- and composition-controlled bimetallic Au/Ag core/shell nanostructures with transient absorption spectroscopy (TAS) as a function of Ag layer thickness (0 – 15 nm) and pump excitation fluence (0.25 – 1 μJ/pulse). Our synthesis approach generated both bimetallic nanocubes and nanopyramids with distinct dipolar plasmon resonances and plasmon dephasing behavior at the resonance. TAS revealed a rise in electron-phonon coupling lifetime (τ1) with increasing Ag shell thickness for both nanocubes and nanopyramids attributable to the increasing size of the nanostructures. Whereas the phonon-phonon scattering lifetime (τ2) increased for the nanopyramids with increasing Ag shell thickness, bimetallic nanocubes demonstrated an unanticipated decrease in τ2 for the thickest, 15 nm, Ag shell. We attributed this to the lightning rod-effect, in which intense photoemission is localized at the corners of the nanocubes. Since cubes have more corners and edges than nanopyramids, a higher photoemission results in decreased phonon dissipation. Further we observe at the thin, 2 nm, Ag shell regime a strong coupling between Au and Ag strongly impacts both τ1 and τ2 despite minimal change in overall size. Electron dynamics the 0 nm and 2 nm Ag shell revealed a linear dependence of τ1 with increasing power. But τ1 decreased at the highest power (1 μJ/pulse) for the thicker, 8 nm and 15 nm, Ag shells, which we attributed to the saturation of transient signal and stronger light scattering that result in weaker plasmon dephasing giving rise to shorter lifetime.
