Ultrafast electron–phonon coupling in hollow gold nanospheres

Abstract

Electronic energy relaxation in hollow gold nanospheres (HGNs) was studied using femtosecond time-resolved transient absorption spectroscopy. A range of HGNs having outer diameter-to-shell thickness aspect ratios of 3.5 to 9.5 were synthesized by a galvanic replacement method. The HGNs exhibited electron–phonon relaxation times that decreased from 1.18 ± 0.16 to 0.59 ± 0.08 ps as the aspect ratio increased over this range. The corresponding electron–phonon coupling constants, G, ranged from (1.67 ± 0.22) to (3.33 ± 0.45) × 10¹⁶ W m⁻³ K⁻¹. Electron–phonon coupling was also determined for solid gold nanospheres (SGNs) with diameters spanning 20 nm to 83 nm; no size dependence was observed for these structures. The HGNs with high aspect ratios exhibited larger electron–phonon coupling constants than the SGNs, whose average G value was (1.9 ± 0.2) × 10¹⁶ W m⁻³ K⁻¹. By comparison, low-aspect ratio HGNs exhibited values comparable to SGNs. The electron–phonon coupling of high-aspect ratio HGNs was also influenced by the surrounding fluid dielectric; slightly smaller G values were obtained when methanol was the solvent as opposed to water. This coupling enhancement observed for high-aspect ratio HGNs was attributed to the large surface to volume ratio of these structures, which results in non-negligible contributions from the environment.