Three-fold diffraction symmetry in epitaxial graphene and the SiC substrate Page: 3 of 4
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3-fold symmetry. The symmetry relationship between
adjacent terraces separated by a step can be understood
by considering the stacking sequence of layers perpendic-
ular to the surface in 6H SiC ABCACB. Consider two
terraces separated by a 3-layer step. If one terrace has
AB termination, the other has AC termination. Since
the AB-stacking is 60-degrees rotated from AC-stacking,
the diffraction patterns from the two terraces are also 60-
degrees rotated. This is why anywhere in panel (a) that
has buffer layer is either black or white in panel (d). In
contrast, the intensity of diffraction from the single-layer
graphene into the SiC LEED spots is significantly lower,
which limits our ability to determine the symmetry; this
is likely why monolayer graphene in panel (d) is grey. A
summary of these results is given in table 1.
Figure 3 presents the diffraction asymmetry from the
graphite spots for monolayer and multilayer graphene
from two samples. The data show that the apparent 6-
fold symmetry is preserved only for the buffer layer and
monolayer graphene, while it is clearly broken for mul-
tilayer graphene (>2 graphene layers plus buffer layer).
The local film thickness is determined from the bright
field images of panels (a) and (b). In the case of buffer
layer (panel c) and monolayer graphene (panel c and d)
we do not observe contrast in the asymmetry image for
any electron energy studied, despite the presence of a
strong signal. We note that, in comparison to panel (d),
there are some regions of faint contrast in panel (c), but
these artifacts are at the boundaries of domains and re-
sult from imperfections in the image subtraction process.
This lack of contrast indicates that monolayer graphene
maintains a 6-fold diffraction pattern. The lack of 3-
fold symmetry is surprising since one might expect the
monolayer graphene to be Bernal (or rhombohedrally)
stacked above the buffer layer, as predicted by theoret-
ical calculations10. There is more than one possible ex-
planation for this result. First, graphene may still be
Bernal stacked above the buffer layer if the buffer layer
domain size (lateral extent) is smaller than the resolution
of the LEEM (approximately 10nm). If the domains are
small, the average stacking (e.g., AB plus AC) could ap-
pear to be 6-fold symmetric. Second, graphene may not
be Bernal stacked above the buffer layer. For example,
it would be 6-fold symmetric if the carbon atoms of the
first graphene monolayer were positioned directly above
the carbon atoms of the buffer layer (AA stacking).
On the contrary, within each 2- and 3-layer region in
panel 3(d), the asymmetry contrast image reveals regions
of black and white contrast suggesting the presence of 3-
fold symmetry in these regions. Bilayer films that are
nearly AA-stacked, such as proposed in Ref.25, are not
apparent here since they would possess a 6-fold diffrac-
tion pattern and appear grey in panel (d). They would
also be distinguishable from monolayer graphene by the
presence of two minima in the bright field reflectivity21'22.
The black and white regions observed in panel (d) cor-
respond to stacking domains with sizes on the order of
100nm for our samples, and are smaller than the domains
Sample 1 Sample 2
(a) son mrm
C/ (- ,- -
FIG. 4: (Color online) 2pm x 2pm LEEM images. (a,b)
Bright field images of two samples (the same images as panels
(a,b) of figure 3). (a) was taken at 3.7eV, where the buffer
layer is light grey, and single layer graphene is medium-grey.
In (b), taken at 5.4eV, single-layer graphene is light grey or
white (rippled-looking), bilayer graphene is medium-grey, and
3-layer graphene is black. (c, d) Dark field contrast of two 6 x 6
LEED spots (labeled "E" and "F" in figure 1), taken at 39.6eV
and 14.5eV, respectively. Positive and negative contrast are
given by the black and white regions of the image, and regions
of zero contrast are grey. Outlines (red (darker) for monolayer
graphene, green (lighter) for bilayer) are drawn to help the
of uniform thickness seen in the bright-field images (fig-
ure 3a and b). Where the stacking domains impinge, a
linear defect (domain boundary) occurs as in panel (e).
Since the boundary between a black and white region in
panel (d) corresponds to a disruption in one or more of
the graphene planes, these domains likely play an impor-
tant role in the transport properties of bilayer and mul-
tilayer graphene films. Thus, the dark-field imaging re-
veals that stacking domains and their associated domain
boundaries occur within regions of otherwise uniformly
thick graphene. These defects are in addition to those
defects that result from changes in graphene thickness,
and illustrate the complexity that can occur in graphene
synthesized from thermally decomposing SiC. Our find-
ing of 3-fold domains in bilayer graphene agree with the
recent results of Hibino et al.26
Figure 4 shows the diffraction asymmetry from the 6 x 6
satellite spots, which result from electrons that inter-
act with the SiC lattice and overlayers through multi-
ple diffraction27. These satellite spots also possess 3-fold
symmetry (a summary of diffraction symmetries is given
in table 1). In the asymmetry contrast image of panel
(c), the buffer layer has shades of light and dark grey,
and the single layer graphene has regions of black and
white. Comparing panel 4(c) to figure 2(d) (the same
region of the sample), the regions of asymmetry in the
buffer layer are the same for both the satellite spots and
the SiC spots. LEED from one-layer graphene on a single
Ru(0001) terrace28 is also 3-fold symmetric, as are some
patterns from graphene on SiC29, likely because the sub-
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Siegel, D A; Zhou, S Y; El Gabaly, F; Schmid, A K; McCarty, K F & Lanzara, A. Three-fold diffraction symmetry in epitaxial graphene and the SiC substrate, article, December 10, 2009; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc1014852/m1/3/: accessed November 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.