22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Chemical functionalization of graphene by plasma processes
G.V. Bianco, M. Losurdo, M. M. Giangregorio, P. Capezzuto, and G. Bruno
Institute of Inorganic Methodologies and of Plasmas, CNR-IMIP, Chemistry Department of University of Bari
“Aldo Moro”, via Orabona 4, 70126 Bari, Italy
Abstract: H 2 , O 2 , N 2 and SF 6 modulated plasma processes were applied for the covalent
binding of functional groups on graphene without introducing structural defects related to
ion radiative damaging. Real time monitoring of graphene optical properties by
spectroscopic ellipsometry was used for allowing an unprecedented control over the degree
of functionalization proven by structural, chemical and electrical characterizations.
Keywords: graphene, functionalization, plasma treatment
The peculiar properties of graphene such as high carrier
mobility, optical transparency, fleixibility and high
chemical resistance have stimulated a vast amount of
research in several technological fields. However, the
diffusion of graphene technologies is still limited by
several challenges. The opening of a gap in the graphene
band structure is fundamental for its exploitation in
electronic applications (especially for the development of
graphene-based transistor devices). Similarly, the control
of the doping state of graphene is needed for applications
as low resistance transparent conductive layer substituting
conductive transparent oxides. Moreover, since graphene
is a relatively inert material, it provides weak interactions
with adsorbates and, hence, this limits its potential for
sensing applications.
The chemical functionalization of graphene has been
reported to be effective in addressing these issues by
tailoring both the chemical and electrical properties.
Functional groups covalently linked to graphene are
effective for doping via charge-transfer processes [1] and
for gap opening via the C-sp2 to C-sp3 conversion [2].
Moreover, the graphene functionalization locally changes
the surface reactivity with chemical species also
providing selective binding with specific target molecules
[3].
Several experimental routes have been explored for
decorating graphene with functional groups. In particular,
the covalent attachment of hydrogen [3] and halogen [2]
atoms has received great attention due to their twofold
potential for (i) tailoring the graphene electrical properties
by doping and band-gap opening, and (ii) activating the
graphene
basal
plane
toward
a
controlled
functionalization with more complex organic groups.
Depending on the degree of functionalization, the opening
of an energy gap up to 3.0 eV has been estimated for
fluorinated graphene [4]. Moreover, the fluorine atoms
can be easily replaced with various types of functional
groups through nucleophilic substitution reactions by
using alcohols, amines, Grignard reagents, or alkyl
lithium compounds. In the same way, a band-gap up to
3.5 eV can be opened in graphene by hydrogenation [5],
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which also locally activates the material towards
diazonium functionalization.
Fig. 1. Chemical functionalization of graphene by plasma
processes.
Typically, the covalent modification of graphene is
carried out by photochemical [6], thermal [4] or plasma
generation [2] of atomic or molecular species which
combine with graphene by free radicals addiction
reaction.
In particular, plasma-functionalization of
graphene is attractive since it involves fast, dry and
scalable processes.
However, the control of the
functionalization degree by plasma treatment is still
challenging. In particular, the minimization of structural
damaging by ion bombardment or etching processes
(especially for oxygen plasma) is difficult to achieve.
In this contribution, we present plasma-chemical routes
for tailoring electrical properties in large area chemical
vapor deposition (CVD) graphene by functionalization
with several chemical groups including oxygen and
nitrogen groups as well as hydrogen and fluorine atoms.
Our functionalization processes have been developed and
optimized with the twofold aim: (i) the fine tuning of
graphene electrical properties, and (ii) the strong
minimization of induced structural damage. To this
purpose, we have explored modulated plasmas of H 2 , O 2 ,
N 2 , and SF 6 for the controlled modification of graphene.
We demonstrate that the use of a pulsed plasma source
allows a good control of the functionalization kinetics
without introducing structural defects.
Specifically,
graphene functionalization by a modulated plasma
process can take advantage from the different life-times of
plasma generated species for minimizing the
1
concentration of ions impacting the graphene surface. In
the same way, the interaction between graphene and
vibrationally excited species can be strongly reduced, and
this limits the graphene chemical heating which favours
carbon etching processes.
Our plasma treatments were performed on monolayer
CVD graphene transferred on corning glass. The reactor
is equipped with an in situ spectroscopic ellipsometer to
monitor in real time the graphene optical properties
during functionalization (see Fig. 2) and to provide direct
measurements of the functionalization degree. The latter
was estimated by the analysis of Raman spectra of
functionalized graphene (see Fig. 3). Specifically, the
relationship proposed by Cancado et al. [7] between the
intensity of D peak and the average distance between sp3
carbon atoms was exploited for estimating the surface
density of functional groups covalently attached to
graphene. This analysis was also supported by XPS
studies to attest the chemical nature of functional groups
attached to graphene and to provide further insights on the
interaction between plasma generated species and
graphene.
Finally, electrical characterizations of
functionalized graphene, including sheet resistance,
carrier mobility, and Hall resistance were performed
across a wide temperature range to investigate doping
effects.
In particular the potential of plasma
functionalization for the work function engineering and
band-gap opening of graphene was demonstrated.
Fig. 3. Evolution of the Raman spectra of monolayer
graphene at different hydrogenation times. The pristine
graphene spectrum shows the G band around 1580 cm-1
and the 2D band around 2700 cm-1. After hydrogen
plasma exposure, the D peak (corresponding to defect
activated bands) appears around 1350cm-1.
Acknowledgement
The authors acknowledge funding from the National
Laboratory Sens&Micro LAB Project (POFESR 2007–
2013, code number 15) funded by Apulia Region, and
from the European Community's 7th Framework
Programme under grant agreement no. 314578
MEM4WIN (www.mem4win.org).
Fig. 2. Evolution of the spectra of the pseudoextinction
coefficient of the pristine graphene on corning at different
plasma-hydrogenation times.
At higher energy
(> 4.6 eV), the spectra are dominated by a broad band
arising from interband transition in graphene. For
increasing plasma-treatment times, these band decreases
in intensity and shifts to higher energy, thus providing a
measure of the hydrogenation level.
2
References
[1] G.V. Bianco, et al. Phys. Chem. Chem. Phys., 16,
3632 (2014)
[2] G. Bruno, G.V. Bianco, et al. Phys. Chem. Chem.
Phys., 16, 13948 (2014)
[3] Z. Sun, et al. Nature Comm., 2, 559, (2011)
[4] J.T. Robinson, et al. Nano Lett., 10, 3001 (2010)
[5] R. Balog, et al. Nature Mat., 9,315 (2010)
[6] B. Li, et al. ACS Nano, 5, 5957 (2011)
[7] L.G. Cancado, et al. Nano Lett., 11, 3190 (2011)
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