CVD formation of graphene on SiC surface in argon atmosphere.
ABSTRACT We investigate the microscopic processes leading to graphene growth by the chemical vapor deposition of propane in an argon atmosphere at the SiC surface. Experimentally, it is known that the presence of argon fastens the dehydrogenation processes at the surface, at high temperatures of about 2000 K. We perform ab initio calculations, at zero temperature, to check whether chemical reactions can explain this phenomenon. Density functional theory and supporting quantum chemistry methods qualitatively describe formation of the graphene wafers. We find that the 4H-SiC(0001) surface exhibits a large catalytic effect in the adsorption process of hydrocarbon molecules, this is also supported by preliminary molecular dynamics results. The existence of the ArH(+) molecule, and an observation from the Raman spectra that the negative charge transfers into the SiC surface, would suggest that presence of argon atoms leads to a deprotonization on the surface, which is necessary to obtain a pure carbon adlayer. But the zero-temperature description shows that the cold environment is insufficient to promote argon-assisted surface cleaning.
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ABSTRACT: In the present paper, we study the effects of functionalization of graphene with simple organic molecules OH, and NH2, focusing on the stability and band gaps of the structures. We have performed DFT calculations for graphene supercells with various numbers of the attached molecules. We have determined adsorption energies of the functionalized graphene mono- and bilayers, the changes in the geometry, and the band structure. We observe the characteristic effects such as rehybridization of the bonds induced by fragments attached to graphene and opening of the graphene band gap by functionalization. We have also studied the dependence of the adsorption energies of the functionalized graphene on the density of the adsorbed molecules. Our calculations reveal that the –OH and –NH2 groups exhibit the strong cohesion to graphene layers. Further, we determine the critical density of the OH fragments which lead to the opening of the band gap. We also show how to engineer the magnitude of the band gap by functionalizing graphene with NH2 groups of various concentrations.Acta Physica Polonica Series a 11/2011; 120:842-844. · 0.53 Impact Factor
Article: A stable argon compound[show abstract] [hide abstract]
ABSTRACT: The noble gases have a particularly stable electronic configuration, comprising fully filled s and p valence orbitals. This makes these elements relatively non-reactive, and they exist at room temperature as monatomic gases. Pauling predicted in 1933 that the heavier noble gases, whose valence electrons are screened by core electrons and thus less strongly bound, could form stable molecules. This prediction was verified in 1962 by the preparation of xenon hexafluoroplatinate, XePtF6, the first compound to contain a noble-gas atom. Since then, a range of different compounds containing radon, xenon and krypton have been theoretically anticipated and prepared. Although the lighter noble gases neon, helium and argon are also expected to be reactive under suitable conditions, they remain the last three long-lived elements of the periodic table for which no stable compound is known. Here we report that the photolysis of hydrogen fluoride in a solid argon matrix leads to the formation of argon fluorohydride (HArF), which we have identified by probing the shift in the position of vibrational bands on isotopic substitution using infrared spectroscopy. Extensive ab initio calculations indicate that HArF is intrinsically stable, owing to significant ionic and covalent contributions to its bonding, thus confirming computational predictions that argon should form a stable hydride species with properties similar to those of the analogous xenon and krypton compounds reported before.Nature 09/2000; 406(6798):874-6. · 38.60 Impact Factor
This journal is c the Owner Societies 2013
Phys. Chem. Chem. Phys., 2013, 15, 8805--8810
CVD formation of graphene on SiC surface in argon
Małgorzata Wierzbowska,*aAdam Dominiakband Kamil Tokara
We investigate the microscopic processes leading to graphene growth by the chemical vapor deposition
of propane in an argon atmosphere at the SiC surface. Experimentally, it is known that the presence of
argon fastens the dehydrogenation processes at the surface, at high temperatures of about 2000 K. We
perform ab initio calculations, at zero temperature, to check whether chemical reactions can explain this
phenomenon. Density functional theory and supporting quantum chemistry methods qualitatively
describe formation of the graphene wafers. We find that the 4H-SiC(0001) surface exhibits a large
catalytic effect in the adsorption process of hydrocarbon molecules, this is also supported by preliminary
molecular dynamics results. The existence of the ArH+molecule, and an observation from the Raman
spectra that the negative charge transfers into the SiC surface, would suggest that presence of argon
atoms leads to a deprotonization on the surface, which is necessary to obtain a pure carbon adlayer.
But the zero-temperature description shows that the cold environment is insufficient to promote argon-
assisted surface cleaning.
Recent progress in nanotechnology has attracted much atten-
tion to graphene.1–3Due to its elastic and electronic properties,
this material is a very good candidate for novel devices with
extraordinary features.4–8The preparation of pure, good quality,
and large graphene wafers is of major technological interest. For
many years, SiC surfaces have been used for graphene sheet growth
in an epitaxy process by Si sublimation.9This method, however,
introduces many defects and results in graphene does not possess
satisfactory electronic transport properties. The structure of the
epitaxial graphene and its interactions with the SiC surface have
been studied by Raman spectroscopy.10
A new method of epitaxy, chemical vapor deposition11,12
(CVD), is much less sensitive to surface defects and enables high
electron mobilities in the graphene layers (up to 1800 cm2Vs?1),
and the grown wafers are large; even up to 150 mm in
diameter.13Additionally, the graphene multilayers may be
oriented in many stacking sequences.14A difference between
the graphene growth on SiC by the sublimation and CVD
processes is pronounced.15Very recent analysis of the experi-
mental parameters in the CVD growth of graphene and graphite
sheets has been reported.16The CVD method has also been
applied to silicon dioxide substrates (SiO2),17copper,18–20
nickel21and iron.22It enables the transfer of graphene onto
In this technology, the gas mixture of Ar and propane
(C3H8), in the role of a carbon precursor, is used as an
ingredient in graphene epitaxy by the CVD process.13,24,25
Propane is used in role of carbon precursor in the graphene
layer creation process. It is desirable to understand how these
compounds participate in the formation of the carbon layers,
and especially, what is the mechanism of the removal of
the hydrogen atoms from the Si-terminated SiC surface. The
substrate surface must be very clean in order to obtain a good
quality graphene. A possible functionalisation of graphene with
the adsorbed hydrogen is a different issue.26,27
In this work, the chemical reactions behind the CVD process
are described, and the mechanisms of the surface dehydro-
genation are checked. These mechanisms are closely related to
the noble gases’ tendency to form diatomic molecules with
protons or, in specific conditions, with the neutral hydrogen
atom. The propane molecule, obviously, chemisorbs neither on
the Si- nor the C-terminated 4H- or 6H-SiC surface (4H and
6H means the hexagonal crystal structure with a stacking
period in the z-axis of 4 or 6, respectively). This is because
C3H8is a molecule with all chemical bonds saturated. It will be
aInstitute of Theoretical Physics, Faculty of Physics, University of Warsaw, ul. Ho:za 69,
00-681 Warszawa, Poland. E-mail: firstname.lastname@example.org
bInstitute of Heat Engineering, Faculty of Power and Aeronautical Engineering,
Warsaw University of Technology, ul. Nowowiejska 21/25, 00-665 Warszawa,
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
Received 5th December 2012,
Accepted 29th March 2013
Phys. Chem. Chem. Phys., 2013, 15, 8805--8810 This journal is c the Owner Societies 2013
shown that it is absolutely sufficient to remove any hydrogen
atom from the propane molecule in order to adsorb such a
created species at the SiC surface. Further dehydrogenation of
the molecule makes the adsorption stronger. The problem that
arises is: what is the propane dehydrogenation initialization
event, since there are only the saturated propane molecules
present in the gas phase? Therefore, we start from investiga-
tions of the reactions with isolated propane in the gas phase,
and later we model the following chemical reactions at the
surface. The possible role of argon in deprotonization reactions
will be discussed. If the deprotonization scenario was true, then
it would explain the Raman measurements, showing that the
charge transfers from the adsorbates to the SiC surface.28
2 Calculation details
All calculations in this work were performed with density
functional theory (DFT),29using the plane-wave package Quantum
ESPRESSO.30In order to verify the correctness of the results
obtained by the DFT tool, used in further studies, the solutions
for the specific reactions were validated by all-electron calculations
with the quantum chemistry package GAMESS,31which employs
the localized basis sets and treats the Coulomb interactions by
means of perturbative and/or multiconfiguration methods.
To gain insight into the mechanisms of hydrocarbon dehydro-
genation on the surface, some preliminary molecular dynamics
(MD) simulations at thermostat temperatures E1500 K were
performed with the SIESTA code.32
3.1Molecular reactions in the gas phase
Initially, we investigated a scenario with the C3H8- C3H7+ H and
C3H8- C3H6+ H2reactions in vacuum. The reaction energies
presented in Table 1 were obtained with the following schemes:
restricted (open shell) Hartree–Fock, R(O)HF, without and with
the second order perturbation corrections for the dynamical
correlations at a level of the Mo ¨ller–Plesset, MP2, method33
(both by GAMESS) and DFT (by Quantum ESPRESSO). Addition-
ally, the dissociation energies of H2were calculated to complete
a description of the reaction energetics. Details of a set-up used
in the calculations are given in the ESI.†
Independently of the approximation level, the removal of
one hydrogen from propane needs a considerable amount of
energy to be provided into the system (ca. 4 eV). In a case of the
propene molecule (C3H6), a part of the energetic cost has been
consumed by the formation of the H2diatomic bond. Because
of the high Ar concentration in the gas mixture, it is quite
plausible that argon atoms could assist in the above reactions
leading to freeing of a hydrogen atom or a proton. This state-
ment is supported by the results of the quantum chemistry work
on the dissociation of the HeH+molecule, led by Wolniewicz,34
where the separation of a proton is an exothermic reaction of
about 2.04 eV. Thus, a possibility of argon binding with a proton
in our system was calculated. The results are presented in
Table 1. The energy gained from ArH+formation is smaller than
the energy needed to remove one of the hydrogen atoms from
the propane molecule. However, it is still necessary to take the
hydrogen ionization energy into account. Some energy might
be obtained from any of the kinetic processes, which occur at
high temperatures, or from the catalytic reaction with the SiC
surface. Indeed, our preliminary results obtained with MD
support that fact. At an average simulation temperature of the
Nose ´ thermostat of around 1500 K, C3H8releases one hydrogen
with a kinetic energy around 5 eV and the remaining C3H7
moiety with a kinetic energy of 1.5 eV hits the surface zone and
binds at the Si-site.
Fragmentation of the propane molecules might be also
caused by the electron transfer from the neutral propane into
the positively charged noble gases (with unpaired electrons in the
valenceshell),as ithas beendemonstrated experimentally.35,36On
the other hand, at high temperatures in the range of 1300–1700 K,
a similar decomposition of propane could be obtained without
noble gases. This process was studied with IR laser absorption
kinetic spectroscopy and discussed without any role of argon.37
However, in the aforementioned experiment, the gas mixture of
C3H8and Ar (as a major compound) has been used.
To complete this overview of the argon role in the investi-
gated microscopic mechanisms, it is necessary to consider the
possibility of dehydrogenation assisted by the formation of the
neutral ArH molecule. Such a process seems to be forbidden,
since the noble gases have closed valence shells and are not
expected to form molecules with other atoms. We have checked,
using the DFT and the ROHF methods, that indeed the neutral
system ArH does not bind. However, the van der Waals complexes
of Ar with propane have been studied,38and also the HeH+and
ArH+charged molecules can be formed due to this type of
interaction. Moreover, there are also the diatomic molecules of
NeH+, KrH+and XeH+with corresponding dissociation energies
of 2.08, 4.35 and 4.32 eV,39respectively. Even more interesting are
the molecules containing noble gases and some other atoms,
where one or more ingredients are in the excited state. It is known
from experiment that the molecule HArF is stable40and the
existence of the HArCl and HHeF molecules have been predicted
theoretically41,42to be stable too. Recently, the next two new
molecules FArCCH and FArSiF3have also been proposed.43
The crucial information for our investigation into the role of
argon comes from the multiconfigurational calculations for a
minus the total energies of the substrates, for the removal of hydrogen from
propane. The parameter reindicates the bond lengths (in Å). C3H7is obtained
from C3H8by a dissociation of H from the middle C and C3H6is the propene
molecule (hydrogens are dissociated from the middle and terminal C of propane)
Reaction energies (in eV), defined as the total energies of the products
Reaction R(O)HF MP2DFT Exp.33
C3H8- C3H7+ H
C3H7- C3H6+ H
C3H8- C3H6+ H2
H2- H + H
ArH+- Ar + H+
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Phys. Chem. Chem. Phys., 2013, 15, 8805--8810
dissociation of the ArH* molecule in the excited state, performed
by Vance and Gallup.44The main results of the work mentioned
above are summarized in the ESI.† Focusing on those data, we
suppose that it is impossible that argon could build a diatomic
molecule with neutral hydrogen in our system. This is because
the curve minima in the dissociation channels of the excited
argon are shallow, with an energy of 1–1.5 eV. This energy is
much less than the hydrogen binding energy to the surface
or hydrocarbon, and the argon excitations are about 11.5 and
11.7 eV. Such energy excitations of the system cannot be
accessible on this scale without a strong laser beam.
3.2 Adsorption at the surface
Assuming that, in a high temperature process, one hydrogen is
removed from propane, the C3H7system can be adsorbed at the
surface. Two possibilities of creating such species were defined
by (1) symmetric or (2) nonsymmetric removal of the hydrogen
atom from the original hydrocarbon molecule. Since an adsorp-
tion at the 4H-SiC(0001) surface occurs for both cases, the
symmetric CH3–CH–CH3 molecule and the nonsymmetric
CH2–CH2–CH3molecule, further removal of hydrogen atoms
was considered and the adsorption energies were calculated.
Following this procedure, the adsorption of a series of the
species C3H8?n, with n = 1,...,7, was calculated. Finally, the
hydrogen-free system, C3, was adsorbed at the 4H-SiC(0001)
surface. This type of hydrocarbon molecular residue might
serve as a precursor for the graphene layer or a graphitic buffer
layer.45The studied adsorbent species built one, two or three
valence bonds with the Si-terminated SiC surface. For any
studied molecule, the bond order formed with the surface
atoms is strongly dependent on the species–surface geometry
and on the number of hydrogens. Some of the adsorbent
created C–C bonds have a double bond character. The relaxed
geometries of the adsorbed species are presented in Fig. 1.
All calculated adsorption energies, except of the C3H8mole-
cule, are negative, which means the binding state. The modeled
surface was considered to be metallic due to a saturation of the
surface with hydrogens.46,47
Adsorption energies were obtained from a formula valid for
the neutral and charged systems:
Eads.= Eslab+mol.? Eslab? Emol.? Nme, (1)
where N is the number of additional electrons in the charged
systems (N a 0 only in the cases presented in Fig. 2). In the
adsorption of charged molecules the total energies Eslab+mol.
and Emol were calculated with additional electrons, and the
energy Eslabcorresponds to the neutral surface. For the chemical
potential of the electrons, i.e. me, we assumed the Fermi level of
the pure slab (without the adsorbent) could be obtained from
the quadrature of the electronic density to the proper number of
valence electrons in the system with the used pseudopotentials.
Modeling interactions in crystals, using the periodic supercells,
introduces spurious interactions between periodic images
starting and final configurations, C3H8and C3, are in the first row. The second and third rows present the symmetric and the nonsymmetric cases, respectively, for the
descending number of hydrogen atoms from the left- to the right-hand side.
Adsorption geometries of propane and all transition C3H8?nspecies, where n = 1,2,...,8 (up to the ‘‘naked’’ carbons) at the Si-terminated SiC surface. The
surface, obtained from a removal of the hydrogens from propane.
Adsorption energies of the first three neutral and charged species at the
Phys. Chem. Chem. Phys., 2013, 15, 8805--8810This journal is c the Owner Societies 2013
especially in the case of charged cells with the compensating
charge uniform background. In order to take account of these
effects, we use the Makov and Payne method48implemented in
the Quantum ESPRESSO code. All geometries of the systems
taking a part in the adsorption process were optimized sepa-
rately and none of the configurations was fixed.
The resulting values of the energies for the first three
species: C3H7, C3H6and C3H5, are depicted in Fig. 2. Since it
has been assumed that the dehydrogenation could be assisted
by ArH+molecule formation, the calculations for charged
systems were also performed. It follows that negatively charged
species bind more weakly to the surface. The binding energy
depends on the number of bonds, but also on the local surface
strain induced by the adsorbed molecules. For example, the
symmetric configuration of the C3H6group binds much more
strongly than the nonsymmetric one, due to a match of the
Si-terminated SiC surface lattice with the molecular C–C–C
chain. On the other hand, the C3H7nonsymmetric molecule
binds much more strongly than the symmetric one, because the
CH3group in this species is more distant from the surface
when the terminal hydrogen is removed from propane.
There exists a proposal of the charge transfer scenario from
the deprotonized site to the SiC surface states (which have
extended delocalized character) assisted by formation of the
ArH+molecule. The experimental data showed,28that the
charge distribution near the SiC surface is enhanced after
graphene layer adsorption. Also the binding energy of ArH+,
of the order of 4.15 eV, is slightly larger than the adsorption
energy of the hydrogen atom at the Si-site of the 4H-SiC(0001)
surface, which amounts to 3.92 eV (from the DFT results). On
the other hand, the energy of removal of a proton from the
surface is higher than the H ionization potential, about 13.6 eV,
minus the work-function of the SiC surface, circa 3.87 eV. Thus,
the dissociation energy of a proton amounts to around 13.65 eV.
This fact indicates that the zero temperature scenario with argon-
assisted surface chemical reactions does not take place.
Further, the adsorption energies of the species with four or
less hydrogens were compared with the adsorption energies of
rich hydrogenated molecules. In this comparison, the hydrogens
dissociated from a molecule were adsorbed at the surface Si-sites
near the molecule (somewhere in the middle of the primitive cell
used in the calculations). The adsorption sites were distant
enough that the adsorbed species do not interact chemically,
although in an indirect way the surface deformations around the
adsorbed molecules affect the adsorption energies. Thus, the
final reaction was not just a sum of two separate reactions with
the surface. Such picture corresponds to the experimental situa-
tion much better than a separated adsorption scenario, with
hydrogens at an infinite distance from the molecule. The results
of calculations for the aforementioned processes are included in
the ESI,† since the barriers were calculated via the reactant in
vacuum, and they do not include the catalytic role of the surface.
3.3Energy barriers for the surface catalyzed dehydrogenation
Since the dehydrogenation processes which occur via the geo-
metric configurations in vacuum show very high transition
energies (see the ESI†), we also calculated the minimum-energy
paths for chosen reactions which take place at the surface. In
order to obtain the barriers for the reactions close to the
surface, we applied the climbing-image nudged-elastic-band
method (NEB), implemented in the Quantum ESPRESSO code.30
The results for the chosen reactions are presented in Table 2.
Barrier energies are collected in columns corresponding to the
symmetric and nonsymmetric geometries and to the forward
and backward reaction directions. The difference between the
highest energy on the reaction path and the energy of the
starting (or the final) geometric configuration gives the barrier
for the reaction forward - (or backward ’). The energy
differences between the starting and the final configurations
can be obtained from the differences (’) ? (-). The barriers
obtained on the minimum-energy path are not high. This
implies, that the surface acts as a strong catalyzer in the
dehydrogenation process of the hydrocarbon molecules.
The preliminary MD simulations of processes after the
adsorption of C3H7also show a cascade of dissociations. First,
the released hydrogen from C3H8, or some other H from the
atmosphere, collides with the remaining middle H of C3H7,
dissociating it and effectively creating H2going back to the
atmosphere. In the following dynamical evolution (time scale of
90–280 fs), one H atom from the tail CH3-group of C3H6species
remaining at the surface is released, and immediately attracted
to the surface Si-site neighbouring the adsorption site of the
The role of argon and the SiC surface as catalysts in the
dehydrogenation processes has been investigated. We started
with a removal of one hydrogen atom from the C3H8molecule
and found it to be sufficient to initiate the adsorption reactions,
which may continue with further dehydrogenation of molecules
and stronger binding, up to the C3moiety at the 4H-SiC(0001)
surface. Barriers for the dehydrogenation of molecules at the
surface, with one of the reactants in vacuum and other at
the surface, are very high; except for the first dehydrogenation
of propane (see the ESI†). On the other hand, the barriers
for the symmetric and nonsymmetric adsorbates. The reaction directions are
denoted by arrows (-) and (’) and defined by the differences between the
highest energy configuration on the way from the left- to the right-hand side of
given reaction and the energy of the starting (for -) or the final (for ’)
configuration, respectively, calculated within the NEB approach
Barriers (in eV) for the reactions below, which occur at the SiC surface,
C3H8- C3H7+ H
C3H7- C3H6+ H
C3H6- C3H5+ H
C3H5- C3H4+ H
C3H4- C3H3+ H
C3H3- C3H2+ H
C3H2- C3H1+ H
C3H1- C3+ H
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Phys. Chem. Chem. Phys., 2013, 15, 8805--8810
obtained on the minimum-energy paths for the hydrogen
transfer from the adsorbed hydrocarbons onto the nearest
Si-site at the SiC surface are rather low. We conclude, that the
SiC surface should act as a strong catalyzer in graphene epitaxy
by the chemical vapor deposition process.
For the first time, we studied the chemical character of the
dehydrogenation of molecules at the SiC slab, and not just
the mechanical removal of H atoms by the floating gas. We
check a microscopic mechanism for the dehydrogenation of the
SiC surface, assisted by the binding reaction of a proton to
argon forming the ArH+molecule. After this process, the
electronic charge could remain on the surface.28The zero-
temperature description, however, indicates that all proposed
chemical reactions cannot occur without additional processes
caused by the high temperature kinetics or by a strong laser
Preliminary MD simulations without Ar in the atmosphere
above the surface, performed at a high temperature of about
1500 K, confirm the scenario with a cascade of dehydrogena-
tion of the adsorbed hydrocarbons, and the fact that some of
the dissociated hydrogens remain at the surface.
We would like to thank Jacek Majewski for many useful
discussions. This work has been supported by the European
Funds for Regional Development within the SICMAT Project
(Contract No. UDA-POIG.01.03.01-14-155/09) and by the European
Union in the framework of European Social Fund through The
Didactic Development Program of The Faculty of Power and
Aeronautical Engineering of The Warsaw University of Technology.
Calculations have been performed in the Interdisciplinary
Centre of Mathematical and Computer Modeling (ICM) of the
University of Warsaw within the grant G47-7 and in Polish
Infrastructure of Informatic Support for Science in European
Scientific Space (PL-Grid) within the projects no POIG.02.03.00-
00-028/08-00 and MRPO.01.02.00-12-479/02.
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