Coherent Vibrational Oscillations of Hollow Gold Nanospheres

Ultrafast pump−probe spectroscopy was used to characterize coherent vibrational oscillations of hollow gold nanospheres (HGNs) composed of a polycrystalline Au shell and a hollow, solvent-filled interior. Different HGN samples show heavily damped radial breathing mode oscillations with a period ranging from 28 ± 2 to 33 ± 3 ps. We theoretically modeled the oscillation period of HGNs while varying both the shell thicknesses and particle radii. Creation of a hollow cavity was predicted to increase the oscillation period relative to solid gold nanoparticles, and this result was verified experimentally. Our theoretical predictions of oscillation period are significantly lower than the experimental measurements. We propose that this difference is due to the polycrystalline nature of HGNs that softens the vibration of the lattice compared with a single-crystalline shell. We compare our system to solid Au nanoparticles and Au nanoparticle aggregates and find a general trend of longer oscillation period with increasing particle polycrystallinity.