Ultrafast electron-electron scattering and energy exchanges in noble-metal nanoparticles

Abstract

The conduction electron energy exchanges are investigated in gold and silver nanoparticles with average size ranging from 2 to 26 nm, embedded in different matrices. The experimental studies were performed by following the internal thermalization dynamics of photoexcited nonequilibrium electrons with a femtosecond pump-probe technique. The probe wavelength dependent measurements are in qualitative agreement with the results of a theoretical model based on bulk metal electron kinetics and band-structure modeling. In both metals, the measured electron thermalization times are close to the bulk ones for nanoparticles larger than 10 nm and sharply decrease for smaller ones. The results are independent of the nanoparticle environment and synthesis technique showing that the observed size behavior reflects an increase of the efficiency of the electron-electron energy exchanges in small nanoparticles. It is in agreement with a simple model based on a bulk metal approach of the electron kinetics modified to introduce surface effects. The observed increase of the electron-electron interaction with size reduction is ascribed to reduction of the screening of the Coulomb interaction by the conduction and core electrons close to the nanoparticle surface.