Javier Reguera | Searcharxiv

arXiv2018

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

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.

cond-mat.mes-hall↗

arXiv2016

Order/disorder dynamics in a dodecanethiol-capped gold nanoparticles supracrystal by small-angle ultrafast electron diffraction

In metal nanoparticles (NPs) supracrystals, the metallic core provides some key properties, e.g. magnetization, plasmonic response or conductivity, with the ligand molecules giving rise to others like solubility, assembly or interaction with biomolecules. The formation of these supracrystals depends on a complex interplay between many forces, some stemming from the core, some from the ligands. At present, there is no known approach to characterize the local order of ligand molecules or their dynamics with atomic spatial resolution. Here, we develop a methodology based on small-angle ultrafast electron diffraction combined with angular cross-correlation analysis to characterize a two-dimensional supracrystal of dodecanethiol-coated gold NPs. We retrieve the static arrangement of the ligands, showing that at equilibrium they order in a preferential orientation on the NPs surface and throughout the two-dimensional supracrystal. Upon light excitation, positional disorder is induced in the supracrystal, while its overall homogeneity is surprisingly found to transiently increase. This suggests that transient annealing of the supracrystal takes place within few picoseconds (ps). This methodology will enable the systematic investigation of the dynamical structural properties of nano-assembled materials containing light elements, relevant for biological applications.

cond-mat.mtrl-sci↗