Electronic Relaxation Dynamics in Coupled Metal Nanoparticles

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.

Plasmonic Modulation of the Upconversion Fluorescence in NaYF4:Yb/Tm Hexaplate Nanocrystals Using Gold Nanoparticles or Nanoshells

“…[20,21] Slight red-shift is observed with increasing density of gold NPs on NC surface, which is also consistent with previous observations that the SPR peak would red shift with the aggregation of gold NPs. [22][23][24] This plasmonic resonance frequency of gold NPs overlaps well with the two major emission peaks in the upconversion NCs (452 nm and 478 nm). Therefore, the gold NP SPR can effectively couple with the upconversion emission and can thus increase the radiative decay rate, emission efficiency and intensity.…”

Optical Properties and Ultrafast Dynamics of Metallic Nanocrystals

“…Studies were performed regarding the collective optical and conductive properties as a function of the number of deposited layers as well as the nature of the crosslinker. Feldstein et al (99) measured the femtosecond electron dynamics (as described in detail above) for such a film and found that the electron-phonon relaxation time increases with increasing film thickness.…”

Ultrafast resonant optical scattering from single gold nanorods: Large nonlinearities and plasmon saturation

“…8,9,10,11,12,13,14 Exciting and probing nanoparticles on resonance with their plasmon frequencies can reveal nonlinearities associated with the coherent oscillation of the plasmons themselves. Unfortunately, the optical response of the ensemble is broadened by the inhomogeneous distribution of particle sizes and shapes.…”