Electronic Relaxation Dynamics in Coupled Metal Nanoparticles

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Abstract

This paper reports the results of hot-electron lifetime measurements in 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 femtosecond optical spectroscopy and scanning probe microscopy. Atomic force microscope measurements are used to characterize the samples and quantify the film's growth pattern. Time-resolved laser spectroscopy measurements are used to determine hot-electron lifetimes. The dependence of the hot-electron lifetimes on the colloid film's structure is analyzed; the lifetimes range from 1 to 3 ps and decrease with greater aggregation. The lifetime is shown to vary in a predictable manner with the film's growth, and a model is presented to describe this relationship. This model allows for the prediction of hot-electron lifetimes over a broad range of film thicknesses and obtains asymptotic agreement with previous experimental results for Au polycrystalline films. Additionally, physical insight into the processes responsible for the range of lifetimes is obtained through an analysis that takes into account two competing phenomena: electron inelastic surface scattering (ISS), which tends to increase electron−phonon coupling with decreasing domain size, and electron oscillation−phonon resonance detuning (EOPRD), which tends to decrease it. The relative contributions of each of these processes has been estimated and shown to agree with the theoretically-predicted tends. Finally, these results have implications for the nature 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, the data presented herein and their analysis in terms of a size-dependent 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 physically separated by organic insulating groups. The films do, however, maintain physical characteristics and electronic influence based on the colloids from which they are built.

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