Coherent Vibrational Oscillations of Hollow Gold Nanospheres

Abstract

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.

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“…In the case of the HGN with the 7 nm shell, T osc is nearly identical to the experimental T osc of ∼5.8 ps reported for a solid Au NP with an overall diameter of 8.9 nm [157]. This is attributed to the very low aspect ratio of the HGN [136,158]. When the shell thickness was kept constant and the overall diameter increased (6.2-12.2 nm) T osc (5.5-16.0 ps) was found to increase linearly with increasing diameter [136].…”

“…Theoretical calculations were used to determine how shell thickness and particle radius impacted the fundamental breathing mode of HGNs. For an HGN with a constant overall radius of 8.9 nm and a shell thickness that increased from 1-7 nm, T osc decreased from 13.6 to 6.6 ps [136]. In the case of the HGN with the 7 nm shell, T osc is nearly identical to the experimental T osc of ∼5.8 ps reported for a solid Au NP with an overall diameter of 8.9 nm [157].…”

“…In order to fully understand and optimize the plasmonic response of a nanoparticle, electron dynamics must be studied. Femtosecond time resolved transient absorption laser spectroscopy is frequently employed to evaluate how differences in aspect ratio, size, shell thickness, aggregation, and the dielectric constant of the surrounding medium influence energetic conversion and transfer [16,31,35,[130][131][132][133][134][135][136][137][138]. However, due to the complex interplay of parameters, optimizing a structure for SPR absorption in the NIR is not the same as optimizing a nanoparticle for maximal generation and transfer of heat.…”

“…The corresponding g values were calculated for HGN aggregates (6.6 × 10 16 W m −3 k −1 ) and solid Au spheres (2.7 × 10 16 W m −3 k −1 ) using the two-temperature model [134]. The larger electron-phonon coupling constant observed for the HGN aggregates is correlated to rapid electron cooling and greater lattice heating efficiency [136,151]. Faster electronic relaxation is attributed to greater electron-phonon coupling [141].…”

“…Ultrafast pump-probe laser spectroscopy has also been used to evaluate the coherent vibrational oscillations of HGNs with average core diameters between 16.4 and 18.8 nm with shell thicknesses between 3.4 and 4.7 nm [136]. All HGNs in the study were found to have heavily damped radial breathing mode oscillations with periods ranging from 28 to 33 ps.…”