Local photo-mechanical stiffness revealed in gold nanoparticles supracrystals by ultrafast small-angle electron diffraction

TL;DR

This study reveals that ligand-capped gold nanoparticle supracrystals exhibit a local photo-mechanical stiffness comparable to solids like graphite, demonstrated through ultrafast electron diffraction experiments showing rapid structural responses to laser-induced heating.

Contribution

The paper introduces a novel application of ultrafast small-angle electron diffraction to measure local stiffness in nanoparticle assemblies, showing they can reach stiffness levels similar to covalently bonded solids.

Findings

Nanoparticle supracrystals display stiffness comparable to graphite.

Femtosecond laser pulses induce rapid structural changes.

Van der Waals interactions can produce solid-like stiffness in assemblies.

Abstract

We demonstrate that highly-ordered two-dimensional crystals of ligand-capped gold nanoparticles display a local photo-mechanical stiffness as high as that of solids such as graphite. In out-of equilibrium electron diffraction experiments, a strong temperature jump is induced in a thin film with a femtosecond laser pulse. The initial electronic excitation transfers energy to the underlying structural degrees of freedom, with a rate generally proportional to the stiffness of the material. With femtosecond small-angle electron diffraction, we observe the temporal evolution of the diffraction feature associated to the nearest-neighbor nanoparticle distance. The Debye-Waller decay for the octanethiol-capped nanoparticles supracrystal, in particular, is found to be unexpectedly fast, almost as fast as the stiffest solid known and observed by the same technique, i.e. graphite. Our observations unravel that local stiffness in a dense supramolecular assembly can be created by Van der Waals interactions up to a level comparable to crystalline systems characterized by covalent bonding.