ELECTRONIC RELAXATION DYNAMICS IN COUPLED METAL NANOPARTICLES

This paper reports the results of hat-electron Lifetime measurements i n a series of thin films prepared by wet chemistry techniques from 12- nm colloidal Au nanoparticles. The films, which vary in thickness and domain (Or aggregate) size, are studied by a combined approach of femt osecond optical spectroscopy and scanning probe microscopy. Atomic for ce microscope measurements are used to characterize the samples and qu antify the film's growth pattern. Time-resolved laser spectroscopy mea surements are used to determine hot-electron lifetimes. The dependence of the hot-electron lifetimes on the colloid Film's structure is anal yzed; the lifetimes range from 1 to 3 ps and decrease with greater agg regation, The lifetime is shown to vary in a predictable manner with t he film's growth, and a model is presented to describe this relationsh ip. This model allows for the prediction of hot-electron Lifetimes ove r a broad range of film thicknesses and obtains asymptotic agreement w ith previous experimental results for Au polycrystalline films. Additi onally, physical insight into the processes responsible for the range of lifetimes is obtained through an analysis that takes into account t wo competing phenomena: electron inelastic surface scattering (ISS), w hich tends to increase electron-phonon coupling with decreasing domain size, and electron oscillation-phonon resonance detuning (EOPRD), whi ch tends to decrease it. The relative contributions of each of these p rocesses has been estimated and shown to agree with the theoretically- predicted tends. Finally, these results have implications for the natu re of interparticle coupling and electron mobility. Specifically, they are consistent with and can be taken as evidence for an intercolloid electron conduction mechanism based on activated hopping, In short, th e data presented herein and their analysis in terms of a size-dependen t ISS and EOPRD shows that in thin films Au colloids aggregate in such a way as to be electronically coupled with one another while being ph ysically separated by organic insulating groups. The films do, however , maintain physical characteristics and electronic influence based on the colloids from which they are built.