Scanning tunneling microscopy on epitaxial bilayer graphene on ruthenium (0001)

Article abstract of DOI:10.1063/1.3106057

The atomic structure of epitaxial single and bilayer graphene on Ru(0001) was studied by scanning tunneling microscopy (STM). High-resolution imaging of the surface of single layer graphene shows a moir with pronounced buckling and broken A/B carbon sublayer symmetry due to a strong interaction with the metal substrate. The top sheet of bilayer graphene is largely unperturbed by residual interactions with the substrate. Screened from the metal substrate, it shows the hallmarks of freestanding monolayer graphene: a honeycomb structure with equivalent carbon sublattices imaged in STM and a linear dispersion of π -bands near the Dirac point.

Full text of DOI:10.1063/1.3106057

Scanning tunneling microscopy on epitaxial bilayer graphene on ruthenium
(0001)
E. Sutter, D. P. Acharya, J. T. Sadowski, and P. Sutter
Citation: Appl. Phys. Lett. 94, 133101 (2009); doi: 10.1063/1.3106057
View online: http://dx.doi.org/10.1063/1.3106057
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v94/i13
Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS 94, 133101 ͑2009͒
Scanning tunneling microscopy on epitaxial bilayer graphene
on ruthenium „0001…
E. Sutter, D. P. Acharya, J. T. Sadowski, and P. Suttera͒
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA
͑Received 5 January 2009; accepted 4 February 2009; published online 30 March 2009͒
The atomic structure of epitaxial single and bilayer graphene on Ru͑0001͒ was studied by scanning
tunneling microscopy ͑STM͒. High-resolution imaging of the surface of single layer graphene
shows a moiré with pronounced buckling and broken A/B carbon sublayer symmetry due to a strong
interaction with the metal substrate. The top sheet of bilayer graphene is largely unperturbed by
residual interactions with the substrate. Screened from the metal substrate, it shows the hallmarks of
freestanding monolayer graphene: a honeycomb structure with equivalent carbon sublattices imaged
in STM and a linear dispersion of ␲-bands near the Dirac point. © 2009 American Institute of
Physics. ͓DOI: 10.1063/1.3106057͔
Graphene, a two-dimensional honeycomb lattice of sp2
bonded carbon atoms,1 has shown a number of remarkable
properties that make it a promising material for large-scale
applications in microelectronics2,3 and sensing.4 To realize
this potential, reliable methods for fabricating large-area
single crystalline graphene domains are required. Graphene
synthesis by epitaxy on noncarbide-forming transition
metals5,6 has been considered recently as a promising alter-
native to micromechanical cleavage for producing macro-
scopic graphene domains. On the ͑0001͒ surface of ruthe-
nium ͑Ru͒, in particular, it has been shown that one can
produce large ͑close to 500 ␮m͒ single or bilayer epitaxial
graphene nuclei in a controlled layer-by-layer fashion.5
Graphene interacts strongly with Ru. Thus a single graphene
layer has a significantly altered electronic structure compared
to freestanding graphene and an atomic structure reflecting
the strong substrate coupling.5,7 While much effort has been
invested in determining the properties of monolayer
graphene ͑MLG͒ on Ru͑0001͒,7–9 the atomic structure of bi-
layer epitaxial graphene has not been investigated yet.
The possibility of controlled growth of bilayer graphene
͑BLG͒ domains on Ru͑0001͒ raises several important issues,
which can be addressed by high-resolution microscopy.
While Raman spectroscopy suggests that the top sheet in
BLG on Ru͑0001͒ recovers the electronic structure of free-
standing MLG,5 it is unclear if the sheet is homogeneously
decoupled from the first layer and substrate. Further, many of
the exotic electronic properties of graphene derive from the
chiral nature of its Dirac fermions,2,10 originating in the two-
atom basis ͑i.e., A and B sublattices͒ of the honeycomb struc-
ture of graphene. Thus, it needs to be verified if the A and B
carbon sublattices of the top sheet of the bilayer are equiva-
lent ͑i.e., the charge carriers chiral͒ or if the underlying
buffer breaks the sublattice symmetry ͑leading to nonchiral
carriers͒. Local spectroscopy, finally, can be used to establish
the residual charge-transfer doping of the bilayer by deter-
mining the energy of the Dirac point, i.e., the crossing of the
linear ␲-bands at the K-point of the two-dimensional Bril-
louin zone.
Here, we address these issues by scanning tunneling mi-
croscopy ͑STM͒ imaging and spectroscopy of single and bi-
layer epitaxial graphene on Ru͑0001͒. The study is based on
the ability to grow macroscopic epitaxial graphene sheets
with predictable thickness in a controlled layer-by-layer
mode on Ru. The graphene growth was carried out in a
preparation chamber attached to an ultrahigh vacuum ͑UHV͒
STM following the procedure described in Ref. 5. Briefly, a
clean Ru͑0001͒ single crystal was loaded with interstitial car-
bon by exposure to a hydrocarbon precursor ͑ethylene͒ at
high temperature ͑Ͼ1000 °C͒. Graphene was grown in
UHV by slowly lowering the sample temperature, driving the
controlled segregation of C to the Ru surface. The graphene
growth proceeds via the initial formation of monolayer thick
graphene islands that expand to a size corresponding to the
spacing of the initial nuclei and coalesce to a complete
graphene monolayer covering the entire Ru͑0001͒ substrate.
Upon completion of the first layer, a second graphene layer
nucleates and can again grow to domain sizes of over
100 ␮m if the slow temperature ramp is continued. STM
imaging and spectroscopy on samples with full monolayer
coverage and with additional small second layer nuclei was
performed in situ using a Createc low-temperature STM
cooled to 77 K. The STM measurements were complemented
by selected-area angle-resolved photoemission spectroscopy
͑micro-ARPES͒, performed in situ at 300 K on individual
larger BLG domains in a Elmitec LEEM III low-energy elec-
tron microscope with energy analyzer ͑energy resolution
ϳ200 meV͒, using 42 eV synchrotron radiation.
On Ru͑0001͒, single layer graphene adopts a moiré
structure5,7,8 in which the ͗1010͘ directions of layer and sub-
¯
strate align, with moiré repeat vectors am=2.93Ϯ0.08 nm,
equivalent to 10.8Ϯ0.3 times the nearest-neighbor distance
on Ru͑0001͒.5 Figure 1͑a͒ is an overview STM image of this
moiré structure. The graphene monolayer shows large varia-
tions in apparent height ͑about 1 Å͒ within a highly ordered
moiré structure that has no visible defects over very large
sample areas. High-resolution STM images ͓Fig. 1͑b͔͒ show
no atomic-scale defects, indicating a high crystalline quality
of the graphene layer. Superimposed over the large long-
range height variations is a mostly triangular lattice structure
at the atomic scale, similar to the one observed earlier7 and
a͒
Author to whom correspondence should be addressed. Electronic mail:
psutter@bnl.gov.
0003-6951/2009/94͑13͒/133101/3/$25.00
94, 133101-1
© 2009 American Institute of Physics
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133101-2
Sutter et al.
Appl. Phys. Lett. 94, 133101 ͑2009͒
FIG. 1. ͑Color͒ Morphology of first-layer epitaxial graphene on Ru͑0001͒.
͑a͒ UHV STM image, obtained at 77 K, of a large area of monolayer epi-
taxial graphene on Ru͑0001͒. ͑b͒ High-resolution STM image of the moiré
structure of the graphene layer ͑V= +0.2 V, I=0.2 nA͒. ͑c͒ Magnified view
of the highest region of the moiré, showing a local honeycomb structure.
FIG. 2. ͑Color͒ Morphology of the top sheet of bilayer epitaxial graphene
on Ru͑0001͒. ͑a͒ UHV STM image ͑T=77 K͒ of the surface consisting of
two different phases: a BLG island on a completed MLG layer. ͑b͒ Height
profile along the line displayed in a. ͑c͒ High-resolution STM image show-
ing honeycomb contrast, i.e., sublattice equivalency, in rippled and flat re-
gions of the bilayer. ͑d͒ Closeup view of the graphene honeycomb lattice
͑V= +0.7 V, I=0.2 nA͒.
assigned to a moiré pattern whose atomic contrast varies
across the unit cell depending on the local registry with the
surface Ru atoms. The local registry determines which of the
graphene A or B sublattices is imaged. In most parts of the
moiré unit cell the sublattice symmetry is broken and only
one of the two carbon sublattices is imaged due to the strong,
covalent interaction with the substrate. The full graphene
honeycomb lattice is only visible in areas directly adjacent to
the moiré maxima ͓Fig. 1͑c͔͒. In these regions the graphene
sheet is lifted higher above the Ru lattice and couples weakly
to it.9 As a result, the density of states ͑DOS͒ of the two
sublattices is identical in these areas and the full honeycomb
of both A and B carbon atoms is imaged by STM.
While the atomic structure of the graphene monolayer
shown in Fig. 1 is dominated by the strong covalent interac-
tion with the metal substrate, it is important to establish the
surface structure of the graphene bilayer, whose topmost
sheet is expected to be almost completely decoupled and
only interacting with the support via weak van der Waals
forces.5 In order to ensure the unambiguous identification of
the bilayer, the growth was modified to yield only small nu-
clei of the second graphene layer. This was achieved by
growing the monolayer to saturation coverage, and then rap-
idly lowering the sample temperature.
Figure 2͑a͒ shows a typical BLG island and the sur-
rounding completed monolayer. The island is bounded on
one side by a Ru surface step and on the other by a kinked
but atomically sharp edge whose segments align with the
major crystallographic directions of the underlying moiré
structure of the monolayer. The surface of the bilayer island
shows distinct inhomogeneous STM contrast that appears
less ordered than the surrounding monolayer areas. While a
majority of the sheet is flat, it also carries surface ripples of
varying height, the highest of which protrude about 1 Å. In
the sample plane, the positioning of these ripples shows no
long-range order. In some areas the ripples are in a local
hexagonal arrangement with the periodicity and orientation
of the superstructure of the adjacent monolayer but they gen-
erally do not align with the maxima of the monolayer moiré.
Figure 2͑b͒, a profile of apparent height in STM along
the line traced in Fig. 2͑a͒, shows a height difference of
ϳ2.2 Å between two terraces entirely covered by MLG, cor-
responding to the interlayer spacing of the ͑0001͒-oriented
Ru ͑2.14 Å͒. These areas covered by MLG are thus grown on
adjacent substrate terraces and separated by an atomic Ru
step. Although the contrast in STM invariably reflects a com-
bination of topographic and electronic structure and can be
difficult to interpret on inhomogeneous surfaces, the mea-
sured height difference between the two sheets of the bilayer
͑3.2 Å, relative to the monolayer minima͒ is close to the
c-axis spacing of graphite ͑3.35 Å͒, as expected if the top
sheet of the bilayer is coupled to the underlying support by
weak van der Waals forces only.
The equivalence of A and B carbon sublattices in
graphene requires a description of electronic Bloch states in
terms of two component wave functions and gives rise to
exciting phenomena such as chiral quantum Hall effects2,10
and anisotropic group velocity renormalization11 and elec-
tron supercollimation12 in lateral superlattices. It is therefore
important to establish if the decoupling of top sheet of epi-
taxial BLG on Ru is sufficient to recover the A/B sublattice
symmetry. This question can be addressed by high-resolution
STM. Atomically resolved images ͓Figs. 2͑c͒ and 2͑d͔͒ ob-
tained on the bilayer show symmetrical honeycomb contrast
throughout, i.e., in both planar and rippled areas. The honey-
comb structure is characteristic of a single graphene layer for
which all carbon atoms are equivalent and has been observed
by STM on cleaved MLG on SiO2.13,14 The atomic structure
observed here shows that the top sheet in bilayer epitaxial
graphene on Ru͑0001͒ is weakly perturbed by residual sup-
port interactions and has the characteristics of freestanding
MLG. In particular its carbon sublattices are equivalent so
that its charge carriers should behave as massless and chiral
Dirac fermions, showing the rich behavior observed or pre-
dicted for such charge carriers in isolated MLG.
To further explore the electronic decoupling of the top
sheet of epitaxial BLG, we have performed scanning tunnel-
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133101-3
Sutter et al.
Appl. Phys. Lett. 94, 133101 ͑2009͒
ing graphene buffer and metal substrate. Micro-ARPES spec-
tra on larger BLG domains on Ru͑0001͒ indeed show a linear
band dispersion near K, with Dirac point about 500 meV
below EF ͓Fig. 3͑b͔͒, confirming that the top sheet of the
bilayer behaves like freestanding MLG except for a down-
shift of the Dirac point. Our STM and initial micro-ARPES
results suggest that the decoupling of the top sheet of the
bilayer is due to a suppression of ␲-states of the bottom layer
by hybridization with Ru d-states. Such a process would
leave only the ␲-states of an isolated top layer near EF at the
K-point.
In conclusion, we have established the atomic and elec-
tronic structure of bilayer epitaxial graphene on Ru͑0001͒.
The interfacial graphene sheet in the bilayer plays the role of
a buffer, coupling strongly to the metal and screening the top
sheet from the metal d-states. As a result, the top sheet of the
bilayer recovers the structure ͑a symmetric honeycomb lat-
tice͒ and electronic properties ͑linear dispersion in Dirac
cones͒ of freestanding MLG.
FIG. 3. ͑Color online͒ Electronic structure of the top sheet of bilayer epi-
taxial graphene on Ru͑0001͒. ͑a͒ dI/dV tunneling spectra of MLG and BLG
domains. Horizontal lines mark dI/dV=0; vertical lines denote energies
Ϯប␻ of the K2 acoustic phonon ͑Ref. 16͒. ͑b͒ Micro-ARPES spectrum on
0
BLG, obtained near the K-point of the Brillouin zone in a direction perpen-
dicular to ⌫-⌲.
ing spectroscopy on both monolayer and bilayer domains.
Representative local dI/dV spectra are shown in Fig. 3͑a͒.
The spectrum of the monolayer shows pronounced modula-
tions of the tunneling conductance near the Fermi energy
͑EF͒, reflecting the DOS of a graphene sheet strongly inter-
acting with the metal substrate. The state at about Ϫ0.2 eV,
for instance, is also observed in micro-ARPES spectra at
small in-plane momentum on the monolayer.15 From a mini-
mum at EF, dI/dV increases sharply with energy for both
filled and empty states, consistent with a finite DOS at EF
near the zone center. The situation is different for the bilayer.
Here, the tunneling conductance remains close to zero within
an extended energy window around EF. A similar gaplike
feature observed in tunneling spectra on exfoliated
graphene/SiO2 ͑Ref. 14͒ was interpreted as due to a suppres-
sion in elastic tunneling close to EF, followed by a sharp rise
in the tunneling conductance with the onset of inelastic tun-
neling involving the emission of a K6 acoustic phonon with
energy ប␻0Ϸ67 meV.16 For BLG on Ru͑0001͒, however,
the gap is significantly wider. If the tunneling indeed is due
to the opening of an inelastic channel at the energy of a
K-point acoustic phonon, our data suggest that the excitation
of the lowest-energy out-of-plane K6 phonon mode is sup-
pressed, and that the gap is instead defined by the in-plane K2
phonon with ប␻0Ϸ125 meV.16 At higher negative bias, the
tunneling spectrum of BLG on Ru͑0001͒ is similar to
electron-doped MLG SiO2, which showed a pronounced dip
in dI/dV at the Dirac energy, i.e., the energy at which the
linear ␲-bands intersect.14 Adopting the same interpre-
tation for our spectra, we would place the Dirac energy at
EDϷ−500 meV, consistent with a substantial electron dop-
ing of the decoupled top sheet of the bilayer by the underly-
Work performed under the auspices of the U.S. Depart-
ment of Energy under Grant No. DE-AC02-98CH1-886.
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