Charge dependence of surface plasma resonance on 2 nm octanethiol-protected Au nanoparticles: Evidence of a free-electron system

Charge dependence of surface plasma resonance (SPR) was investigated on 2 nm octanethiol-protected An nanoparticles. Remarkably, if assuming that each Au-thiol bond localizes one free electron, the observed variations in peak position of the SPR bands as a function of charge can be precisely calculated by simple Mie theory without any adjustable parameters but by using the number of the remaining free electrons in the particle and the experimentally determined dielectric constant for the protecting layer of octanethiol. Additionally, the variations in line-width of the SPR bands can also be quantitatively described by the phenomenological damping law DeltaGamma = A(vF)/R with A = 1.2, a value that suggests a strong chemical-interface damping effect that is consistent with a strong metal-thiol bonding. In the calculations, Perdew's classic spherical jellium model is used to account for the electron spill-out effect. Altogether, these results not only strongly support recent findings by Dickson and co-workers (Phys. Rev. Lett. 2004, 93, 077402) who have shown that the nascent electronic properties of subnanometer Au nanoclusters are of free-electron characteristics but also provide much needed evidence of a continuation of a free-electron system into a particle size range where the collective plasmonic behavior has just emerged.