Graphene, 2013, 2, 1-7
http://dx.doi.org/10.4236/graphene.2013.21001 Published Online January 2013 (http://www.scirp.org/journal/graphene)
Na Induced Changes in the Electronic Band Structure of
Graphene Grown on C-Face SiC
Leif I. Johansson, Chao Xia, Chariya Virojanadara
Department of Physics, Chemistry and Biology, Linköping University, Linköping, Sweden
Received October 2, 2012; revised November 16, 2012; accepted December 28, 2012
ABSTRACT
Studies of the effects induced on the electron band structure after Na deposition, and subsequent heating, on a C-face 2
MLs graphene sample are reported. Na deposition shifts the Dirac point downwards from the Fermi level by about 0.5
eV due to electron doping. After heating at temperatures from around 120˚C to 300˚C, the -band appears considerably
broadened. Collected Si 2p and Na 2p spectra then indicate Na intercalation in between the graphene layers and at the
graphene SiC interface. The broadening is therefore interpreted to arise from the presence of two slightly shifted, but
not clearly resolved, -bands. Constant energy photoelectron distribution patterns, E (kx, ky); s, extracted from the clean
2 MLs graphene C-face sample look very similar to earlier calculated distribution patterns for monolayer, but not
Bernal stacked bi-layer, graphene. After Na deposition the patterns extracted at energies below the Dirac point appear
very similar so the doping had no pronounced effect on the shape or intensity distribution. At energies above the Dirac
point the extracted angular distribution patterns show the flipped, “mirrored”, intensity distribution predicted for
monolayer graphene at these energies. An additional weaker outer band is also discernable at energies above the Dirac
point, which presumably is induced by the deposited Na.
Keywords: Graphene on C-Face SiC; Graphene Band Structure; Na Intercalation; Constant Energy Photoelectron;
Angular Distribution Patterns
1. Introduction
Graphene grown epitaxially on the wide band semicon-
ductor SiC is considered a most promising platform [1]
for future carbon-based electronic devices. Chemical ap-
proaches adapted from the field of graphite intercalation
compounds [2] have been used for tailoring the elec-
tronic properties of graphene. For example, sodium (Na)
deposited on monolayer graphene grown on Si-face SiC
intercalates [3-5] first between the carbon layers but with
time also underneath the carbon buffer layer. Heating at
temperatures around 100˚C promotes intercalation [3-5]
but at higher temperatures Na starts to de-intercalate and
desorb from the sample. The intercalation transforms the
carbon buffer layer to a second graphene layer, as re-
vealed by the appearance of a second -band. The gra-
phene layers become fairly strongly n-doped by the Na,
as revealed by a downward shift of the Dirac point by
about 1.0 eV. Studies [6,7] of the electron band structure
of graphene grown on Si-face SiC show that the number
of -bands corresponds directly to the number of gra-
phene layers. The effects induced by other alkali metals
have also been investigated quite thoroughly and there is
a general consensus about these findings [8-11].
For graphene grown on C-face SiC the situation is
quite different. Only one -band (-cone) with the Dirac
point located close to the Fermi level has in general been
observed [12-14] even for multilayer graphene samples.
For Bernal (AB) stacking of graphene layers a single
-band with linear dispersion at the
 -point results [15]
only for single layer graphene (SLG), while bi- and tri-
layer graphene show [15,16] two and three split parabolic
-bands at the

-point, respectively. Calculations of
how the misorientation of stacked graphene layers affects
the electronic structure have demonstrated that the band
structure of SLG is also found in incommensurate multi-
layered graphene systems. The macro Low Energy Elec-
tron Diffraction (LEED) pattern from C-face graphene is
smeared out into a strongly modulated diffraction ring
[12]. This, together with surface X-ray diffraction results
[17] has been interpreted to indicate that the graphene
layers on the C-face stack in such a way that adjacent
layers are rotated with respect to each other. A recent in-
vestigation [14] of C-face graphene using Low Energy
Electron Microscopy (LEEM), X-ray Photoelectron Elec-
tron Microscopy (XPEEM) and selected area (micro)
LEED showed however unambiguously formation of
fairly large crystallographic grains of multilayer graphene
C
opyright © 2013 SciRes. Graphene

L. I. JOHANSSON ET AL. 2
where there was no rotational disorder between adjacent
layers but that the grains had different azimuthal orienta-
tions. The Angle Resolved Photo Electron Spectroscopy
(ARPES) results [14] showed a single -cone with the
Dirac point located close to the Fermi level. Therefore
further studies of the electron band structure of C-face
graphene are well motivated.
In this study the effects induced in the electron band
structure of C-face graphene are investigated after depos-
iting similar amounts of Na and after heating at similar
temperatures as earlier done [3,5] for Si-face graphene
samples. The idea was to see if similar effects could be
induced on C-face graphene, that the Dirac point would
shift downwards by one eV after deposition and if more
than one -band could be observed after Na deposition
and heating. Since Na do intercalate Si-face graphene we
assumed that it would also intercalate C-face graphene
that is known to form in smaller domains/grains and thus
have a higher density of defects on the surface.
2. Experimental
A graphene sample grown [14] on a n-type 6H-SiC sub-
strate at 1800˚C for 20 min in an Ar pressure of 500 mbar
was utilized. A graphene coverage of predominantly 2
MLs was determined using LEEM, Photo Electron Spec-
troscopy (PES) and LEED at beam line I311 at the MAX
laboratory. This beam line is equipped with a modified
SX-700 monochromator that provides light for two end-
stations. One for PES, built around a large hemispherical
Scienta electron analyzer, providing a total energy reso-
lution of 10 to 100 meV at photon energies from 45 to
450 eV. The other is built up around a LEEM (LEEM III,
Elmitec GmbH) instrument. These stations were utilized
for characterizing the graphene sample. ARPES experi-
ments were performed at beam line I4, which is equipped
with a SGM monochromator and a PHOIBOS 100 2D
CCD Specs energy analyzer. The low angular dispersion
(LAD) lens mode was selected, providing an acceptant
angle of ±7˚. A Ta foil mounted on the sample manipu-
lator was utilized as a reference sample for determining
the Fermi level. Deposition of Na was performed as ear-
lier [3,5] using a SAES getter source and with the sample
at room temperature. Subsequent heating was carried out
for five minutes at different selected temperatures. The
sample temperature was determined using optical py-
rometers in the temperature range 350˚C - 950˚C. Lower
sample temperatures had to be estimated from extrapola-
tion of the input power (current × voltage) and sample
temperature relation determined over the temperature
range covered by the pyrometers. The uncertainty in the
low temperatures range is therefore large, ca. ±25˚C.
Data collection was carried out with the sample at room
temperature.
3. Results and Discussion
A LEEM image collected at a voltage of 3.3 eV and a
field of view of 10 mm is shown in Figure 1(a). The
number of graphene layers present in the different do-
mains is determined from the electron reflectivity versus
electron kinetic energy, i.e. the so called I-V curve. The
reflectivity curves extracted from the areas labeled from
A to P are shown in Figure 1(b). Curves A to I are very
similar and show two pronounced minima that indicate a
coverage of 2 MLs. Curves J to N also show two minima
indicating a coverage of 2 MLs, although these curves
have a slightly different shape around 2 eV. Curve O
show one dip indicating single graphene layer coverage
and curve P represents an area with no graphene cover-
age. Most of the surface of this sample is thus found to
have a graphene coverage of 2 MLs. Selected area LEED
diffraction patterns collected from areas E and D at an
energy of 45 eV are shown in Figure 1(c). A probing
area with a diameter of 0.4 μm was used so both patterns
are collected from a single grain/domain, but the two
domains are seen to have different azimuthal orientations.
No rotational disorder between the two graphene layers
is however discernable in these diffraction patterns.
C 1s spectra collected from this 2 MLs graphene sam-
ple at three photon energies are shown in Figure 2(a).
They are modeled accurately using only two components,
the SiC component from the substrate and the G com-
Figure 1. (a) LEEM image recorded at 0.4 eV and a field of
view of 10 m; (b) Electron reflectivity curves extracted
from the different areas labeled A to P in (a), showing a
coverage of mainly 2 ML of grapheme; (c) Selected area
LEED patterns, collected at 45 eV using a probing area
diameter of 0.4 m, from two different grains/domains in
(a).

L. I. JOHANSSON ET AL. 3
Figure 2. (a) C 1s spectrum recorded using three different
photon energies showing contribution from graphene (G)
and the SiC substrate; (b) The -band recorded around the
 -point at a photon energy of 33 eV.
ponent from the graphene layers. In agreement with ear-
lier results [14,18] no trace of a carbon buffer layer
component can be observed. The energy separation of
1.9 eV and peak intensity ratio of around 6 (at 600 eV),
determined between the graphene and the SiC compo-
nents (G/SiC) are consistent with previously determined
values [18] for a graphene thickness of about 2 MLs. The
π-band recorded from this 2 MLs graphene sample
around the
 point and along the A-  -A’ direction
using a photon energy of 33 eV is shown in Figure 2(b).
The Dirac point is located within ca. 75 meV below the
Fermi level indicating a slight n-doping instead of the
slight p-doping earlier reported [13]. Since the graphene
grain size was only on the order of a few micrometers, a
few π-cones from domains of different azimuthal orien-
tations were typically detected when moving the light
spot over the sample. However, the Dirac point of the
different cones appeared always located at the same en-
ergy position. After Na deposition and subsequent heat-
ing the sample was always relocated to the same position
where only one π-cone appeared in the spectrum from the
clean surface.
The effects induced in the -band structure close to the
 point after Na deposition and subsequent heating are
shown in Figure 3. The Dirac point is seen to shift down
by about 0.5 eV after Na deposition. On Si-face graphene
the downward shift [3] was twice as large, i.e. ca. 1.0 eV
for a similar amount of deposited Na. Subsequent heating
at 50˚C and 80˚C is seen to only make the band appear
slightly more diffuse. Heating at temperatures from
120˚C to 300˚C results in a considerable broadening of
the spectral features. At 400˚C the Dirac point has moved
back quite much towards EF so most of the Na appears to
have desorbed, and at 950˚C the initial band is restored.
An interesting thing is what causes the broader features
that are observed particularly well at 200˚C and 300˚C.
First we speculate based on earlier findings, and then we
present supporting experimental evidence. On Si-face
graphene intercalation of Na in between the carbon lay-
ers and also at the graphene SiC interface was strongly
promoted [3,5] by heating at around 100˚C while at
higher temperatures the Na started to de-intercalate and
desorb from the surface, Therefore we suggest similar
things [3,5] to occur also on C-face graphene. After de-
position and after heating at temperatures below ca.
100˚C a large part of the Na stays on the surface and in
between the graphene layers. When heating at tempera-
tures from 120˚C to 300˚C intercalation at the interface
and in between the carbon layers is promoted, while the
Na on the surface decreases considerably since Na begins
to desorb. Then the two carbon layers experience a dif-
ferent electron n-type doping level so two -bands, cones,
are present, one with the Dirac point fairly close to EF
and the other with the Dirac point at around 0.4 eV be-
low EF. After heating at 400˚C most of the Na has de-
sorbed but there is still some intercalated Na, giving rise
to a smaller energy difference between the Dirac points
of the two graphene layers. Intercalation at the interface
can be unambiguously identified by the appearance [3,5]
of shifted substrate components in the C 1s and Si 2p
core level spectra. However, on beam line I4 the C 1s
level is not accessible but a limited set of Si 2p and Na
2p spectra were possible to collect, see Figure 4, and
they provide support for the above speculations.
The Si 2p spectra, displayed in the left panel in Figure
4, show that the Si 2p signal is strongly attenuated after
deposition of Na and that some intercalation may even-
tually have occurred. After heating at 120˚C and 200˚C a
Si 2p component shifted about two eV to lower binding
energy is clearly revealed, which indicates intercalation
[3,5] at the graphene SiC interface. At 400˚C this com-
ponent is not discernable so most of the Na then appears
to have left the interface. The Na 2p spectra displayed in
the right panel tell a similar story. After deposition a
main sharp component, corresponding to Na metal on top
of the graphene [3,5], and a broad feature at higher bind-
ing energy are observed. After heating at 120˚C only two
broad components are observed, like earlier [3,5] for Na
deposited on Si face graphene. The new component at
lower binding energy was then identified to correspond
to Na intercalated at the graphene SiC interface and the
one at higher binding energy to Na intercalated in be-
tween the graphene layers. When increasing the heating
temperature to 200˚C and then to 400˚C only the com-
ponent corresponding to intercalation in between the
graphene layers remains, but at 950˚C also this compo-
nent vanished. Thus the evolution of the core level spec-
tra support the speculations concerning the reason for the
changes induced in the electron band structure after heat-
ing at different temperatures.
Since the Dirac point is shifted downwards from the
Fermi level after Na deposition it is then possible to in-

L. I. JOHANSSON ET AL. 4
Figure 3. The -band structure recorded close to the

point before and after Na deposition and after heating at different
temperatures. A photon energy of 33 eV was used.
Figure 4. Normal emission Si 2p and Na 2p spectra re-
corded before and after Na deposition and after heating at
three different temperatures. A photon energy of 140 eV
was used.
vestigate the band structure both above and below the
Dirac point and compare that with the band structure
from the clean surface and with calculated results. For
that purpose E (kx, ky) angular distribution patterns were
extracted around the

-point at certain energies rela-
tive to the Dirac point. For the clean 2 MLs graphene
C-face sample this pattern is a distinct small circular spot
at ED, see upper panel in Figure 5 in which E (kx, ky); s
from the clean surface are displayed. When moving away
from the Dirac point the pattern has at 0.2 eV the shape
of a circular ring which when moving further away be-
comes gradually more triangular. The intensity distribu-
tion and shape of these constant energy maps at and be-
low the Dirac point agree well with the patterns calcu-
lated [19] for monolayer, but not bi-layer, graphene.
Close to the Dirac point the calculated pattern for mono-
layer graphene is fairly circular and when moving further
away it becomes more triangular due to the increased
effect of triangular warping, see Figure 2 in [19]. The
asymmetry in the intensity distribution, the appearance of
the so called dark corridor [19,20], has for monolayer
graphene been well accounted for by considering the two
source interference pattern from the two inequivalent
atomic sites in the graphene honeycomb lattice. After Na
deposition the patterns extracted below ED appear fairly
similar to those for the clean surface, see lower panel

L. I. JOHANSSON ET AL. 5
Figure 5. Photoelectron angular distribution patterns E (kx, ky) around the

-point extracted from recorded ARPES spec-
tra collected from the graphene sample before (upper panel) and after (lower panel) Na deposition. The wavevector kx is
along the Γ-
 -  direction and ky along A-  -A’.
in Figure 5. However, the circular spot at ED is distinctly
larger and the patterns are slightly more diffuse but the
doping has not induced any other visible changes in the
symmetry of the pattern. When looking at the patterns
extracted above ED they are flipped, “mirrored”, com-
pared to the pattern at the same energy below ED. This
was also predicted [19] at energies located above ED for
monolayer graphene, see Figure 2 in [19], but not for
Bernal stacked bi-layer graphene. Thus both for the as
grown 2 MLs graphene C-face sample and for the doped
graphene after Na deposition the E (kx,ky) angular distri-
bution patterns extracted reflect those predicted for mono-
layer graphene, while the patterns predicted for Bernal
stacked bilayer graphene look distinctly different, espe-
cially at energies below the Dirac point, see Figure 4 in
[19]. It also deserves to be noticed that the -band dis-
persion we observe for Na doped 2 MLs graphene close
to the
 point looks distinctly different compared to
that in Figures 3(c) and (d) in [13], where a hatlike shape
of the bi-layer bands was reported. It is thus clear that the
two graphene layers on our C-face sample are not Bernal
stacked. The micro LEED patterns in Figure 1(c) more-
over show that the layers are not misoriented, i.e. rotated
relative to each other, so the stacking is neither turbo-
stratic. Therefore AA stacking seems the only available
option left [15,16] for multilayer graphene showing linear
dispersion at the  point, since Bernal (AB) and ABC
stacking results in “hatlike” parabolic dispersions and
split bands at the  point. However, a splitting of the
-bands from different layers is also predicted for AA
stacking, but this has not yet been possible to reveal or
verify experimentally. Therefore we speculate that the
interaction and charge transfer between graphene layers
grown on C-face SiC is maybe so weak/small that the
band splitting is not resolvable in conventional ARPES.
At energies above ED there is after Na deposition also
an additional weaker “outer band” discernable in the pat-
terns, in Figure 5, which monolayer graphene cannot
explain the presence of. To further elucidate its presence
cuts from the collected data set, are shown in Figure 6.
In cut a), made at the

point, i.e. at kx ≈ 1.7 [1/Å],
and along the A-

-A’ direction, this additional band is
barely visible. In cut b), at the
 point but along the
Γ-

-

direction, it appears as a weak feature on the
right hand side of the main -band. In cut c), however,
made at kx ≈ 1.8 [1/Å] and along the A-
 -A’ direction,
this additional band becomes more visible. We can only
point out the presence of this weak band, induced by the
Na deposited on the 2 MLs graphene sample, but have no
further explanation for it.
4. Summary and Conclusions
Deposition of Na on a 2 MLs graphene C-face sample
shifted the Dirac point downwards from the Fermi level
by about 0.5 eV due to electron n-type doping. Subse-
quent heating at temperatures from ca. 120˚C to 300˚C
broadened the -band considerably. Collected Si 2p and
Na 2p spectra then indicated Na intercalation in between
the graphene layers and at the graphene SiC interface.
The broadening is therefore suggested to arise from
presence of two -bands, slightly shifted to each other by
a difference in electron doping concentration of the gra-
phene layers.
Constant energy photoelectron distribution patterns, E
(kx, ky); s, extracted for the clean 2 MLs graphene C-face
sample look very similar to earlier calculated distribution
patterns for monolayer graphene, but not the patterns for
Bernal stacked bi-layer graphene. After Na deposition
the E (kx, ky) patterns extracted at energies below the

L. I. JOHANSSON ET AL. 6
Figure 6. Cuts from the data set recorded after Na deposi-
tion in Figure 5. In a) the cut is along the A-  -A’ direction
and at the  point, i.e. at kx ≈ 1.7 [1/Å]. In b) the cut is
also at the  point but along the Γ-  -  direction. In
c) the cut is along the A-  -A’, direction but at kx ≈ 1.8
[1/Å], see lower panel in Figure 5.
Dirac point appear very similar to those for the clean
sample so the doping did not visibly affect the shape or
intensity distribution. Then at energies above the Dirac
point the angular distribution patterns show the flipped,
“mirrored”, intensity distribution predicted for mono-
layer, but not bi-layer, graphene. The deposited Na in-
duced an additional weaker outer band at energies above
the Dirac point, which calculated results for monolayer
graphene could not account for.
5. Acknowledgements
The authors gratefully acknowledge support from the
European Science Foundation, within the EuroGRA-
PHENE (EPIGRAT) program, and the Swedish Research
Council (#621-2011-4252 and Linnaeus Grant).
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