22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Hydrogen plasmas interaction with graphene surfaces
E. Despiau-Pujo1, A. Davydova1, G. Cunge1 and D.B. Graves2
1
2
University of Grenoble Alpes, CNRS, CEA-Leti Minatec, LTM, 38054 Grenoble Cedex, France
University of California Berkeley, Dept Chem. & Biomol. Engng., 94720 Berkeley, CA, U.S.A.
Abstract: Molecular dynamics simulations, coupled with experiments, are developed to
assist the development of plasma processes to clean, etch and pattern graphene layers in a
controlled way. Interactions between hydrogen plasmas and various types of graphene
surfaces (monolayer, multilayer, nanoribbons) are more specifically investigated.
Keywords: plasma-surface interactions, hydrogen, graphene, molecular dynamics
1. Introduction
Due to its 2D structure and unique physico-chemical
properties, graphene is a promising candidate for novel
applications in microelectronics, transparent conducting
electrodes, sensors or energy storage devices. The
successful development of graphene-based technologies
relies on our capability to grow and integrate this new
material into sophisticated devices, but the nm-scale
control of graphene processing overwhelms current
plasma-based technology.
Since the presence of
multilayers or defects/contaminants on the graphene
surface can significantly degrade its intrinsic properties,
the development of new techniques to clean graphene
surfaces from polymer residues, etch graphene films
layer-by-layer or pattern graphene nanoribbons (GNR)
with minimal edge disorder, are major challenges.
Hydrogen plasmas are shown to be promising to
specifically treat graphene films but little is known about
the fundamental mechanisms of plasma-graphene
interaction. We therefore develop Molecular Dynamics
(MD) simulations, coupled with experiments, to assist the
development of plasma processes to clean, dope and
pattern graphene layers in a controlled way. We more
specifically investigate the interactions between hydrogen
plasmas and various types of graphene surfaces
(monolayers, multilayers, nanoribbons).
2. Molecular Dynamics (MD) method
Classical Molecular Dynamics (MD) refers to a class of
simulations that solve Newton’s equations of motion for a
system of interacting particles, treating each atom as a
classical point and modelling the quantum effects from
electrons by an interatomic potential energy function. In
this work, we use the 2nd generation C-H REBO potential
developed by Brenner and co-workers [1] to study
interactions between H species and various types of
graphene
surfaces
(monolayers,
multilayers,
nanoribbons). Long-range Van der Waals interactions,
which can be important beyond the cut-off of the REBO
potential, are included for multilayer graphene
simulations.
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Fig. 1. Multilayer graphene simulation cell. Top surface
is exposed to ion and neutral flux (random location).
Plasma-surface interactions are simulated by randomly
impacting the graphene surface with low energy
(5 - 50 eV) directed H x + (x = 1,2) ions and isotropic H
radicals. Cell dimensions vary depending on the type of
simulation, with L x = L y = 3 - 10 nm and L z = 1 - 10 nm.
Except for graphene nanoribbons (GNR), periodic
boundary conditions are imposed in the lateral dimensions
to mimic a semi-infinite plane. For all trajectories,
simulations are performed in the microcanonical (NVE)
ensemble. The numerical integration scheme used to
compute positions and velocities is the velocity-Verlet
algorithm with a timestep varying between 5x10-3 fs and
0.1 fs. The graphene surface can be quenched at various
temperatures T S (300 K, 600 K, 800 K, 1000 K) using a
Berendsen thermostat.
3. Results
Elementary C-H interactions and associated energy
barriers are investigated on surface-clean monolayer
graphene as function of the H impact location (top,
bridge, hollow or edge sites of GNR). The influence of
graphene temperature and incident species energy on
1
adsorption, reflection and penetration probabilities is
presented in Fig. 2. Except for impacts at GNR edges or
defects location, H species are shown to experience a
repulsive force which prevents any species with less than
~0.6 eV to adsorb on the graphene surface.
H+ bombardment in the 1 - 10 eV range leads to a
functionalization of the graphene basal plane
(H chemisorption) while irreversible damages are
expected for Ei > 12 eV (penetration of atomic H through
the layers or C-C bond breaking) [2].
radical without damaging the graphene basal plane.
Recent experiments and XPS/AFM/Raman measurements
confirm that H 2 plasmas are promising to clean PMMA
residues from graphene with almost no damage after
annealing (Fig. 4).
Fig. 3. Hydrogenation and etching of a zigzag-GNR as
function of the fluence of H radicals (E R = T S = 800 K).
Fig. 2. Influence of H incident energy and graphene
surface temperature on adsorption (black), reflection
(red), and penetration (blue) probabilities on surface-clean
monolayer graphene [2].
Although graphene exhibits great electrical properties,
nanoelectronic applications are handicapped by the
absence of a semiconducting gap in pristine graphene.
One approach to open a bandgap is to pattern graphene
nanoribbons (GNR) with sub-5 nm width and well
controlled edges.
Simulations of suspended GNRs
trimming in downstream H 2 plasmas show that lateral
etching is maximum for surface temperatures ~600 K and
occurs via a specific mechanism occurring in three
phases: edge hydrogenation, unzipping and chemical
sputtering of edge C atoms (Fig. 3). No formation of
volatile hydrocarbon etching products is observed in this
3-phase mechanism, which explains why the ribbon edges
can be sharp-cut without generation of line-edge
roughness, as also observed experimentally.
The synthesis or successive steps used to produce
graphene devices (PMMA stamps used to transfer CVD
graphene onto specific substrates or photolithographic
masks used to pattern graphene nano-features) often lead
to graphene contamination (polymeric residues remain on
surface). As a first step to model graphene cleaning, the
mechanisms of CH3 groups (a crude approximation for
resist residues) removal from graphene by atomic
hydrogen are investigated numerically. Depending on the
incident energy range and the surface temperature, MD
shows the possibility for chemical etching of the methyl
2
Fig. 4. AFM images of monolayer graphene transferred
on SiO 2 before and after 30 s of H 2 plasma treatment.
PMMA residues are eliminated from the graphene
surface.
Finally, energetic H+ or H 2 + bombardment of stacked
multilayer graphene (s-MLG) is investigated and the
possibility to store hydrogen (trapped as H 2 molecules)
between adjacent layers or etch a full single graphene
sheet is discussed.
4. Acknowledgments
We are grateful to the Nanosciences Foundation of
Grenoble, for the financial support of this work in the
frame of the 2010 Chairs of Excellence Program. DBG
acknowledges partial support from the US Department of
Energy Office of Fusion Science.
5. References
[1] W. Brenner, et al. J. Phys. Cond. Mat., 14, 783
(2002)
[2] E. Despiau-Pujo, et al. J. Appl. Phys., 113, 114302
(2013)
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