Production and characterization of carbon-based nanomaterials: single- and double-layer graphene for high-pressure Raman spectroscopy

Abstract: Graphene is a novel carbon nanomaterial and a great promise for
nanotechnology applications in a near future, thanks to its outstanding
mechanical and electronic properties. Less than 4 Angstroms (Å) in
thickness, mono-crystal graphene flakes can reach tens of millimeters in
their biggest dimension, making it a 2-dimensional crystal. This one-atom-
thick sheet of pure sp² carbon displays Young modulus reaching 1TPa, a
theoretical hardness higher than diamond, and is also a close-to-perfect
(ballistic) conductor for heat and charge carriers. This dazzling array of
exceptional properties makes graphene a state-of-the-art candidate for a
list of applications in nanoscale electronic devices, bio-nanotechnologies,
and advanced materials.

Raman spectroscopy constitutes a non-destructive and time-efficient
technique that probes many properties: the samples quality, thickness,
doping. Most importantly, Resonance Raman Spectroscopy allows determination
of the number of layers of graphene flakes (Single-Layer Graphene SLG,
Double-Layer Graphene DLG, N-Layer Graphene NLG). A high-pressure study of
graphene species by in-situ Resonance Raman Spectroscopy will provide a
direct probing of the sp² carbon-carbon bond strength when SLG is studied,
and the inter-planar interaction in the case of DLG. For an optimal
comparative study, SLG and DLG graphene flakes need to be probed at High-
Pressure without a supporting substrate.

For this purpose, this thesis aims at the synthesis of high-quality graphene
flakes in a reliable and reproducible manner. The second goal of the thesis
is to extensively characterize these samples at ambient conditions, and to
realize their transfer into a high-pressure device called Diamond Anvil-Cell
(DAC) to prepare the in-situ experiments. Graphene samples are prepared
using a mechanical exfoliation method, which was optimized to reach the fast
and reproducible production of high-quality flakes, whose size can reach up
to 20µm in their biggest dimension. Graphene samples provided by our
collaborators are also investigated.

The characterization confirms the production of high-quality supported
samples (low or no D-band in the Raman spectra), and though transfer of the
samples into the DAC is so far unsuccessful, a slight change in the
materials used (from Cu-based grids to Au grids) should make the transfer
possible and open the path to High-Pressure measurements for both types of
samples.

We have also proposed an innovative method to produce graphene from
nano-lithographically patterned NanoPillars by mechanical exfoliation.
Albeit unsuccessful as we first expected it, the cleavage, as showed by SEM,
provokes a peeling of the pillars’ superior layers, accordingly to the
expected mechanism. These efforts open the way to a new promising route for
the synthesis of large area graphene-based nanostructures.