Electronic relaxation dynamics in isolated and aggregated hollow gold nanospheres

Electronic relaxation and interparticle electromagnetic coupling processes in hollow gold nanospheres (HGNs) and HGN aggregates are described. These plasmon-tunable HGNs exhibit an unexpected, but systematic, blue shift of the surface plasmon resonance spectral position when the particles are aggregated. Femtosecond transient absorption measurements and finite-difference time-domain (FDTD) calculations are used to demonstrate that this blue shift is the result of delocalization of the Fermi-gas over multiple particles, an effect not observed with solid spherical particles. The ultrafast electron-phonon coupling lifetimes for the thin-shelled HGNs increase upon aggregation, indicating significant enhancement in interparticle electromagnetic coupling. For instance, a 48-nm HGN with a shell thickness of 7 nm shows ultrafast electron-phonon coupling with a lifetime of 300 +/- 100 fs, and upon aggregation, this lifetime increases to 730 +/- 140 fs. The experimental data strongly suggest that confinement effects in HGNs allow for enhanced energy transport over nanometer distances and this effect can be applied to developing more efficient devices, including photovoltaics.