Probing the dynamics of plasmon-excited hexanethiol-capped gold nanoparticles by picosecond X-ray absorption spectroscopy.

Flavio Zamponi,Andrea Lübcke,Maarten Nachtegaal,Thomas J Penfold,Jeroen A Van Bokhoven,Jochen Rittmann,Majed Chergui,Chris J Milne

doi:10.1039/c4cp03301a

Abstract

Picosecond X-ray absorption spectroscopy (XAS) is used to investigate the electronic and structural dynamics initiated by plasmon excitation of 1.8 nm diameter Au nanoparticles (NPs) functionalised with 1-hexanethiol. We show that 100 ps after photoexcitation the transient XAS spectrum is consistent with an 8% expansion of the Au-Au bond length and a large increase in disorder associated with melting of the NPs. Recovery of the ground state occurs with a time constant of ∼1.8 ns, arising from thermalisation with the environment. Simulations reveal that the transient spectrum exhibits no signature of charge separation at 100 ps and allows us to estimate an upper limit for the quantum yield (QY) of this process to be <0.1.

Highlights

The surface of a catalyst is required to promote the formation of intermediate species that exhibits moderate stability
A number of recent works have highlighted the effectiveness of picosecond X-ray absorption spectroscopy (XAS) applied to study photoinduced dynamics in NPs.[41,42,43,44]. In this contribution, we report a time-resolved XAS study of the photoexcited dynamics of Au nanoparticles functionalised with hexanethiol chains
By monitoring the temporal evolution of the X-ray absorption near-edge structure (XANES) and using theoretical simulations to analyse the data around the Au-L3 edge, we demonstrate that an 8% lattice expansion is accompanied by a melting of the nanoparticles

Summary

Introduction

The surface of a catalyst is required to promote the formation of intermediate species that exhibits moderate stability. This thermalization process occurs in a few hundreds of femtoseconds.[29,30] The second relaxation step is energy transfer to the lattice, via the electron–phonon coupling, resulting in a large temperature jump of the lattice.[31,32] These temporal evolutions of the electronic and lattice temperatures are generally described using the two-temperature model.[33] The final step is energy transfer to the surrounding matrix This process corresponds to the heat transfer from the metal particles to the environment, whose rate exhibits a strong dependence on the size of the nanoparticle[34,35] and the thermal conductivity of the surrounding medium.[36]. The experiment was performed at the SuperXAS beamline of the Swiss Light Source at PSI, Switzerland

Computational details

Results

Analysis of the spectrum

Conclusions