  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.
