2 research outputs found
Elasticity of an Assembly of Disordered Nanoparticles Interacting <i>via</i> Either van der Waals-Bonded or Covalent-Bonded Coating Layers
Tailoring physical and chemical properties at the nanoscale by assembling nanoparticles currently paves the way for new functional materials. Obtaining the desired macroscopic properties is usually determined by a perfect control of the contact between nanoparticles. Therefore, the physics and chemistry of nanocontacts are one of the central issues for the design of the nanocomposites. Since the birth of atomic force microscopy, crucial advances have been achieved in the quantitative evaluation of van der Waals and Casimir forces in nanostructures and of adhesion between the nanoparticles. We present here an investigation, by a noncontact method, of the elasticity of an assembly of nanoparticles interacting <i>via</i> either van der Waals-bonded or covalent-bonded coating layers. We demonstrate indeed that the ultrafast opto-acoustic technique, based on the generation and detection of hypersound by femtosecond laser pulses, is very sensitive to probe the properties of the nanocontacts. In particular, we observe and evaluate how much the subnanometric molecules present at nanocontacts influence the coherent acoustic phonon propagation along the network of the interconnected silica nanoparticles. Finally, we show that this ultrafast opto-acoustic technique provides quantitative estimates of the rigidity/stiffness of the nanocontacts
Graphene-to-Substrate Energy Transfer through Out-of-Plane Longitudinal Acoustic Phonons
Practically, graphene is often deposited
on substrates. Given the
major substrate-induced modification of properties and considerable
energy transfer at the interface, the graphene–substrate interaction
has been widely discussed. However, the proposed mechanisms were restricted
to the two-dimensional (2D) plane and interface, while the energy
conduction in the third dimension is hardly considered. Herein, we
disclose the transfer of energy perpendicular to the interface of
the combined system of the 2D graphene and the 3D base. More precisely,
our observation of the energy dissipation of optically excited graphene
via emitting out-of-plane longitudinal acoustic phonon into the substrate
is presented. By applying nanoultrasonic spectroscopy with a piezoelectric
nanolayer embedded in the substrate, we found that under photoexcitation
by a femtosecond laser pulse graphene can emit longitudinal coherent
acoustic phonons (CAPs) with frequencies over 1 THz into the substrate.
In addition, the waveform of the CAP pulse infers that the photocarriers
and sudden lattice heating in graphene caused modification of graphene–substrate
bond and consequently generated longitudinal acoustic phonons in the
substrate. The direct observation of this unexplored graphene-to-substrate
vertical energy transfer channel can bring new insights into the understanding
of the energy dissipation and limited transport properties of supported
graphene