NANO
LETTERS
Growth of Semiconducting Graphene on
Palladium
2009
Vol. 9, No. 12
3985-3990
Soon-Yong Kwon,†,‡ Cristian V. Ciobanu,*,§ Vania Petrova,| Vivek B. Shenoy,⊥
Javier Bareño,|,# Vincent Gambin,¶ Ivan Petrov,| and Suneel Kodambaka*,†
Department of Materials Science and Engineering, UniVersity of California
Los Angeles, Los Angeles, California 90095, DiVision of Engineering, Colorado School
of Mines, Golden, Colorado 80401, Frederick Seitz Materials Research Laboratory,
UniVersity of Illinois, Urbana, Illinois 61801, DiVision of Engineering, Brown
UniVersity, ProVidence, Rhode Island 02912, and Northrop Grumman Space and
Technology, Redondo Beach, California 90278
Received July 5, 2009; Revised Manuscript Received August 31, 2009
ABSTRACT
We report in situ scanning tunneling microscopy studies of graphene growth on Pd(111) during ethylene deposition at temperatures between
723 and 1023 K. We observe the formation of monolayer graphene islands, 200-2000 Å in size, bounded by Pd surface steps. Surprisingly,
the topographic image contrast from graphene islands reverses with tunneling bias, suggesting a semiconducting behavior. Scanning tunneling
spectroscopy measurements confirm that the graphene islands are semiconducting, with a band gap of 0.3 ( 0.1 eV. On the basis of density
functional theory calculations, we suggest that the opening of a band gap is due to the strong interaction between graphene and the Pd
substrate. Our findings point to the possibility of preparing semiconducting graphene layers for future carbon-based nanoelectronic devices
via direct deposition onto strongly interacting substrates.
Graphene1,2sa two-dimensional crystalline sheet of carbon
atoms arranged in a honeycomb latticesgenerated enormous
interest in the research community owing to its ultrathin
geometry and properties such as high carrier mobility,3
excellent thermal conductivity,4 and high mechanical
strength.5 One of the attractive features of free-standing
graphene, a semimetal, is its semiconducting behavior6,7 at
length scales below 500 Å with a size-dependent band
gap.8-10 Previous reports11-13 have shown that a band gap
can also be opened in graphene grown on insulating
SiC(0001) and BN(0001) via interactions with the substrate.
Here, we report the formation of semiconducting graphene
layers with a band gap of 0.3 ( 0.1 eV on Pd(111), a metallic
substrate. Using in situ scanning tunneling microscopy
(STM) and spectroscopy (STS), we determine the electronic
structure of graphene islands grown in situ via chemical
* Towhomcorrespondencemaybeaddressed.E-mail:kodambaka@ucla.edu,
cciobanu@mines.edu.
†
Department of Materials Science and Engineering, University of
California Los Angeles.
‡
Present address: School of Mechanical and Advanced Materials
Engineering, Ulsan National Institute of Science and Technology, Ulsan
689-798, Korea.
§
Division of Engineering, Colorado School of Mines.
|
Frederick Seitz Materials Research Laboratory, University of Illinois.
⊥
Division of Engineering, Brown University.
#
Present address: Department of Physics, Chemistry and Biology (IFM)
Linköping University, SE-581 83 Linköping, Sweden.
¶
Northrop Grumman Space and Technology.
10.1021/nl902140j CCC: $40.75
Published on Web 09/28/2009
 2009 American Chemical Society
vapor deposition on Pd(111). In contrast to recent reports
on nanoribbons, where the band gap originates from size/
edge effects,8-10 the band gap in epitaxial graphene on
palladium is caused by a strong interaction with the Pd
substrate and the ensuing breaking of translational symmetry
between the two hexagonal close-packed sublattices of
graphene. Our experiments illustrate the control over the
electronic properties of graphene through interactions with
substrates in well-defined epitaxial configurations. This approach opens up the possibility of preparing metal-semiconducting graphene structures and metal-doped graphenebased devices with potentially new applications.
Using STM, we followed the formation and growth of
graphene on Pd(111) during ethylene deposition over a range
of pressures, substrate temperatures, and times. Panels A and
B of Figure 1 are representative STM images of graphene
islands acquired from a Pd(111) surface in situ during
ethylene deposition at 968 K. In our experiments, island sizes
vary between 200 and 2000 Å and are commonly observed
at or near the Pd step edges as in Figure 1A, or spanning
across multiple terraces as in Figure 1B. During ethylene
deposition, we observe graphene islands on the surface,
which likely form via precipitation of the carbon atoms
dissolved in the substrate. This is plausible because C
dissolves readily in Pd with a temperature-dependent solubility14 that increases from 0.2 atom % at 723 K to 1.4 atom %
Figure 1. STM images of graphene on Pd(111) acquired in situ
during growth. (A and B) Derivative filled-state STM images (VT
) -1.3 V) acquired from a Pd(111) sample at 968 K during
exposure to ethylene. Images show graphene islands near or
spanning across substrate steps. IT ) 0.13 nA in (A) and 0.22 nA
in (B). Fourier transforms (insets) of the images in (A) and (B)
show 6-fold symmetry. (C) High-resolution filled-state STM image
(VT ) -1.3 V, IT ) 0.13 nA) of a graphene island at 968 K. (D)
Surface height profile along the white line shown in panel C.
Graphene forms a hexagonal Moiré pattern with a spatial periodicity
of 21 ( 1 Å. (E) Atomic model showing the [21j1j0] orientation of
graphene (honeycomb lattice) aligned with the Pd [11j0] atoms (large
spheres). This specific arrangement of the two lattices is consistent
with the most frequently observed Moiré periodicity of 21 Å.
at 1023 K. The decrease of the substrate temperature
following the high-temperature deposition and/or extended
deposition times lead to an increase of the C supersaturation
in the bulk, which eventually results in precipitation of
crystalline carbon at the surface.15 A similar process has been
previously reported for graphene growth on other metals.16-19
In all our growth experiments, graphene islands appear
instantaneously, presumably due to faster nucleation and
growth rates compared to the STM scan rates. After graphene
islands are formed, their shapes and sizes do not change
significantly with time and appear to be independent of C2H4
pressure at a given temperature. More interestingly, we
observe considerable rearrangement of the substrate surface (refer to Figure S1 in Supporting Information for more
details). Similar morphological changes of the substrate have
been recently reported for graphene growth on ruthenium
and attributed to the displacement of substrate atoms by
carbon.20
We now focus on the structure of graphene islands. Fourier
transforms (FT) of the STM images (shown as insets in
panels A and B of Figure 1) indicate that the islands exhibit
an ordered superstructure with a 6-fold symmetry. Figure
1C is a higher resolution image of the ordered superstructure within a graphene island. These superstructures are
Moiré patterns formed by the superposition of the honeycomb
lattice of graphene and the hexagonal lattice of Pd(111) and
have also been observed on other metal surfaces.17-20 From
the FTs, we measure spatial periodicities of ∼19 and ∼20
Å for the islands in panels A and B of Figure 1, respectively.
3986
Figure 2. Surface morphologies of graphene islands on Pd substrate.
(A and B) Higher magnification STM images showing portions of
the graphene island in Figure 1B. (C and D) Surface height profiles
obtained along the lines highlighted in panels A and B, respectively.
The observed step height difference between the substrate and the
graphene island is <0.4 Å for the configuration shown in A and is
∼2.3 Å for the island geometry in B. The bottom panel shows
schematics of the suggested growth morphologies for graphene on
Pd(111).
From the surface height profile (Figure 1D) measured along
the white line shown in Figure 1C, we obtain a periodicity
of ∼20 Å. In our experiments, most islands exhibit Moiré
patterns with periodicities of 21 ( 1 Å; however, we have
also observed periodicities as small as 15 ( 1 Å (see Figure
S2 in Supporting Information), but they are relatively few.
Using the in-plane lattice constants of graphene (2.46 Å)
and Pd(111) (2.75 Å), we calculated the relative orientation
between the two lattices that is required to explain the
observed structures. We find that a Moiré pattern with 21 (
1 Å periodicity is obtained when the [21j 1j 0] direction of
graphene is aligned along the [11j0] direction of Pd lattice;
such alignment can be realized by superposition of 9-10
graphene unit cells onto 8-9 unit cells of Pd(111). An atomic
model of such a commensurate Moiré superstructure is shown
in Figure 1E, along with the determined epitaxial relationship
between monolayer graphene and Pd(111).
Interestingly, STM images indicate that the graphene
islands lie in the substrate surface. Panels A and B of
Figure 2 are higher magnification STM images of the
graphene island in Figure 1B. They show the most commonly
observed graphene/substrate configurations in our experiments: graphene bounded by monatomic steps (Figure 2A)
and graphene blanketing substrate steps and terraces in
between (Figure 2B). The surface height profiles in panels
C and D of Figure 2 taken along the lines highlighted in the
images confirm these observations. That is, the height
difference between graphene and the Pd surface is <0.4 Å,
which is significantly lower than the Pd-Pd or graphenegraphene interlayer spacings. This result is typical of several
tens of STM images of graphene islands acquired from
different regions of the substrate during growth at any
temperature between 723 and 1023 K and ethylene pressures
between 10-8 and 10-7 Torr. Moreover, the observed
topologies are independent of tunneling biases between -1.9
Nano Lett., Vol. 9, No. 12, 2009
Figure 3. Tunneling bias-dependent STM images of graphene
islands. (A) Empty- and (B) filled-state STM images (310 × 310
Å2) of the ordered superstructure acquired at room temperature.
Notice the contrast reversal with the change in tunneling voltage
bias. Tunneling bias VT is changed from +1.1 V in panel A to
-1.1 V in panel B while current IT ) 0.24 nA is held constant.
V and +1.9 V, suggesting that this lack of difference in
surface heights is not a tunneling artifact.
In order to establish the location of carbon atoms on
Pd(111), we used density functional theory (DFT) calculations to test the stability of two of the most likely C/Pd
configurations: (1) a honeycomb lattice of C atoms
interspersed among Pd atoms in the uppermost Pd(111)
layer and (2) single six-atom rings made of C atoms
located at interstitial surface sites in the top (111) layer.
DFT calculations indicated that both these configurations
are unstable and showed that the C atoms relax upward
reaching an equilibrium distance of ∼2.3 Å aboVe the first
Pd(111) layer. This distance is consistent with previous
theoretical results,21 and is nearly the same as the Pd (111)
step height of 2.245 Å.
To reconcile the results of these DFT calculations with
the STM images that show graphene islands in the surface,
we suggest that the topologies observed in our experiments
are due to monolayer graphene surrounded by Pd steps. Our
reasoning is as follows: (1) Graphene islands are surrounded
by Pd substrate steps during growth. This is based upon our
in situ high-temperature STM observations, which indicate
that the Pd steps are relatively mobile during ethylene
deposition and that they can move without affecting the
graphene layer (see Figure S3 in Supporting Information).
(2) Presence of multiple (g2) layers of graphene would yield
graphene-graphene or graphene-Pd step heights that are
incompatible geometrically with the height of the monatomic
steps on Pd(111), and would also yield fainter contrast of
the Moiré superstructures in the STM images. However, we
have never observed such graphene island step heights in
our experiments. Therefore, we suggest that the observed
Moiré superstructure is due to monolayer height graphene
islands on Pd(111).
We now discuss the electronic characteristics of the
graphene layers. Surprisingly, we observe bias-dependent
contrast in the STM images of the graphene superstructures;
i.e., the image contrast reverses with the polarity of the bias
voltage on the tip. Figure 3 shows typical empty (3A) and
filled (3B) state STM images of graphene on Pd surface.
This contrast reversal, characteristic of semiconducting
surfaces,22 suggests that the observed graphene islands may
also be semiconducting. To understand the origin of biasNano Lett., Vol. 9, No. 12, 2009
dependent contrast, we used DFT to simulate the STM
images. For comparison, we have simulated STM images
for flat free-standing graphene in vacuum and for graphene
on Pd. In the case of free-standing graphene the simulated
filled and empty state STM images are very similar; i.e., the
image contrast did not change with bias. In the case of
graphene on Pd with large surface unit cells (which is
relevant to our experiments), we have found that tunneling
contrast in the simulated STM images always has a strong
bias dependence; in several cases, the contrast of the
simulated STM images reversed upon changing the sign of
the voltage bias (Figure S4 in Supporting Information). This
effect may be understood as follows. Carbon atoms in the
graphene lattice that are farther away from the Pd surface
(i.e., at the top of the hills in the graphene layer) behave
much like the atoms of free-standing graphene and thus yield
similar contrast in both filled and empty states. The carbon
atoms at the bottom of the valleys interact more strongly
with Pd, which results in substantial distortion of their π
orbitals and in charge transfer with the substrate. This charge
transfer can increase the density of filled states in the valleys,
which in turn would change the contrast of the (simulated)
STM images. Furthermore, the electronic band structure of
flat free-standing monolayer graphene does not exhibit an
energy band gap (Figure S4 in Supporting Information),
while there is a small band gap for the graphene islands on
Pd as shown in the following sections. This reinforces our
suggestion that the experimentally observed contrast reversal
is indicative of the semiconducting nature of graphene on
Pd.
In order to draw firm conclusions about the electronic
structure of graphene on Pd(111), we used point-mode
scanning tunneling spectroscopy (STS) and collected I vs V
data from the graphene islands (Figure 4A). From the STS
data, we extracted normalized conductance values [(dI/dV)/
(I/V)]. This normalized conductance is expected to be nearly
independent of the tip-sample separation23,24 and is a
measure of the local surface density of states (LDOS). Figure
4B shows plots of [(dI/dV)/(I/V)] vs V obtained using three
different tunneling currents. The data are an arithmetic
average of values measured at over 70 different points on
the graphene island. The absence of electronic states, as
indicated by the zero conductance over a range of voltages
centered around 0 V, confirms the existence of a band gap.
This result is typical of all of our STS measurements acquired
using a range of tunneling currents from several locations
up to 300 Å apart within the same as well as different
graphene islands. From the data, we measure band gap values
between 0.2 and 0.4 eV for different graphene islands. Within
our experimental uncertainties of (0.1 eV, we did not
observe any significant variation of the band gap with island
size or shape. The uncertainty value of 0.1 eV originates
from limited spectroscopic resolution at room temperature
due to the thermal broadening of both the sample and tip
electron energy distribution. We note that for a given
graphene island, band gap values do not vary significantly
(e10%) with the change in tunneling currents IT between
0.1 and 1.1 nA, a measure of sample-tip tunneling height.
3987
Figure 4. Scanning tunneling spectroscopy (STS) of graphene on Pd(111). (A) High-resolution STM image from a graphene island on
Pd(111). (B) Typical plots of (dI/dV)/(I/V) vs V data obtained using point-mode STS at three different tunneling currents IT ) 0.3, 0.5, and
0.7 nA with VT ) +1.075 V. Inset in B shows the (dI/dV)/(I/V) vs V data over a smaller range of V around 0 V. We note that (dI/dV)/(I/V)
) 0 over a range of 0.25 V (band gap) centered around 0 V. Similar band gap values are obtained in all our STS measurements. (C)
Average local density of states (LDOS) spectrum from a 20 × 20 Å2 region within a graphene island, extracted from current imaging
tunneling spectroscopy (CITS) measurements (VT ) +1.1 V, IT ) 0.2 nA). (D) DFT calculated projected density of states (PDOS)
corresponding to the carbon atoms of single-layer graphene on Pd(111).
This rules out any tip-induced artifacts in STS data. As a
consistency check, we also used current imaging tunneling
spectroscopy (CITS) to measure LDOS on graphene islands.
Figure 4C shows the average LDOS calculated from five or
more I-V spectra collected from within a 20 × 20 Å2 area
on a graphene island using VT ) +1.1 V and IT ) 0.2 nA.
Band gap values extracted from the CITS data are in good
agreement with the STS measurements and thus provide a
validation for our results.
The observed semiconducting nature of the graphene layer
on Pd(111) is intriguing, because free-standing graphene is
a semimetal with zero band gap and in our experiments
graphene is grown on a metallic substrate. The possible
causes for the presence of a band gap are (i) the finite-size
effects as in refs 9, 10 and (ii) the interaction of graphene
with the substrate as in refs 13, 21. The range of sizes for
the quantum confinement regime in graphene-based structures was determined recently by several groups.9,10 Using
DFT, Son et al. showed that the band gap induced by size
effects in nanoribbons becomes smaller than 0.05 eV at
dimensions larger than ∼60-80 Å.9 Han and co-workers
studied lithographically patterned graphene structures10 and
showed that the band gap drops below 0.01 eV for ribbons
with widths greater than 320 Å. For dimensions of 200 Å
(the smallest island sizes observed in our experiments), Han
et al. find a band gap of ∼0.03 eV (last figure in ref 10);
this value is an order of magnitude smaller than that
measured in our experiments, ∼0.3 eV. Furthermore, from
the STS measurements obtained from graphene islands
located in different regions of the substrate, we always find
a band gap; within our experimental errors, this band gap is
nearly the same for smaller (200 Å) or larger (2000 Å)
islands. On the basis of our measurements and on previous
reports on the size/edge effects in graphene ribbons,9,10 we
assess that the effects of island size on the electronic behavior
of graphene are not significant and that the origin of the band
gap in graphene lies in its interactions with the substrate.
The opening of a band gap in graphene on Pd(111) can
be understood by analyzing the interaction of graphene and
the substrate at the level of density functional theory (DFT,
methods section). We constructed Moiré superstructures with
periodicities ranging from ∼8 to 22 Å and calculated the
strengths of interaction between carbon and Pd atoms. For
all the superstructures simulated, the calculations reveal
3988
strong interaction energies (∼0.13 eV/carbon atom) between
graphene and the Pd substrate, consistent with experimental
observations16 other recent reports of DFT calculations of
graphene on metals.21 This interaction is due to the large
deformation (partial sp3 hybridization) of the π orbitals of
the carbon atoms in the vicinity of Pd atoms. In free-standing
graphene, the gapless energy spectrum is caused by the
perfect symmetry of the two identical hexagonal sublattices.
However, on strongly interacting substrates, this symmetry
can be broken.12,13 As seen in Figure 1E, some of the carbon
atoms are directly above Pd atoms, while some other carbon
atoms lie above the midpoint of a surface Pd-Pd bond. Such
local variations in C-Pd coordination for C atoms within
the graphene lattice cause different distortions of the carbon
orbitals, thus breaking the translational symmetry of the two
hexagonal sublattices. Once the symmetry is broken, a
nonzero band gap is expected to appear,25 which could be
seen both in the energy dispersion relations and in the density
of states corresponding to graphene. In analyzing the DFT
results, we found that the total density of states (i.e., with
contributions from all atoms in the supercell and all angular
momentum components) has no band gap because of the
dominant contribution from the substrate Pd atoms. However,
the projected density of states (PDOS) corresponding to the
carbon atoms alone (Figure 4D) shows a band gap of ∼0.25
eV for a simulated Moiré pattern with a periodicity of 10 Å
(refer to Figure S4 in Supporting Information). This result
is consistent with the experimental LDOS (Figure 4C), in
that they both show a band gap of similar magnitude. We
emphasize that the agreement can only be qualitative because
of the possible influence of the top layer Pd atoms on the
LDOS spectrum.
In conclusion, we used in situ high-temperature STM to
follow the chemical vapor deposition of graphene on Pd(111)
and room-temperature STS to determine the electronic
structure of as-grown layers. Interestingly, these graphene
islands are semiconducting as indicated by the STM image
contrast reversal with change in tunneling bias polarity. We
confirm this behavior using point-mode STS and CITS
measurements, which yield band gap values between 0.2 and
0.4 eV. Our DFT calculations agree with the experimental
results. We attribute this opening of the band gap to strong
interactions between the carbon atoms and the Pd substrate,
which break the symmetry of the two hexagonal sublattices
Nano Lett., Vol. 9, No. 12, 2009
of the graphene lattice. We expect that a similar behavior
can be realized on other strongly interacting metallic as well
as insulating substrates. This opens up the exciting possibility
of tailoring electronic properties of graphene layers by
appropriate metal doping and for fabricating metalsemiconducting graphene-metal sandwich structures26 with
potentially new applications.
Methods. All our graphene growth experiments were
carried out on epitaxial Pd(111) layers, 1600 Å thick, sputterdeposited on Al2O3(0001) using the procedure described in
ref 27. The Pd(111)/Al2O3(0001) samples were transferred
to an ultrahigh vacuum (UHV) multichamber (base pressure
<2 × 10-10 Torr) STM system equipped with facilities for
electron-beam evaporation, ion etching, and low-energy
electron diffraction (LEED). The Pd(111) sample was
degassed in UHV at 873 K for approximately 1200 s and
then at 1073 K for 60 s. This procedure resulted in sharp 1
× 1 LEED patterns corresponding to an in-plane atomic
spacing of 2.75 Å. STM images showed atomically smooth
terraces wider than 1000 Å and separated by monolayerheight steps with a measured step height of ∼2.3 Å. Both
these results are consistent with the values expected for clean
Pd(111).28 Carbon was deposited using ethylene (C2H4) at
pressures between 1 × 10-9 and 5 × 10-7 Torr and temperatures T between 300 and 1100 K. Substrate temperatures
were measured using optical pyrometry and are accurate to
within 50 K. At each temperature, the sample and the tip
were allowed to stabilize for up to 2 h prior to STM data
acquisition. STM images were acquired as a function of
deposition time t and C2H4 pressure in the constant current
mode using commercially available Pt-Ir tips. Typical
tunneling currents IT of 0.1-0.8 nA and bias voltages VT of
( 0.8-2.0 V were used. Scan rates varied between 45 and
200 s/frame. Pixel resolution in the images varied from 0.5
× 0.5 Å2 to 5.0 × 5.0 Å2. Scan sizes, scan rates, and
tunneling parameters were varied to check for tip induced
effects. We observed no such effects in the results presented
here. STM images were processed using Image SXM
software.29
The as-deposited graphene layers were characterized using
point-mode STS and CITS at room temperature. In the STS
mode, I vs V data was acquired over a range of bias voltages
VT between -2 V and +2 V. For CITS, I vs V data were
obtained at each pixel (resolution ) 1.0 × 1.0 Å2) within a
scanned area. The acquisition time at each pixel was 10 ms.
During the measurements, tip-sample separation was held
constant by interrupting the feedback loop.
DFT calculations were performed in the generalized
gradient approximation, using the projector-augmented
pseudopotentials30 and Perdew-Burke-Ernzerhof exchangecorrelation functional.31 We used the VASP package30 to
perform structural relaxations of a single graphene layer
on a palladium slab with up to six (111) layers of Pd
(bottom two layers fixed) and a vacuum spacing of 15 Å
along Pd[111]; for the in-plane directions (i.e., which are
perpendicular to the Pd[111] axis) there is no vacuum so
as to simulate a 2D periodic system with no edges. The
energy cutoff for plane waves was 400 eV during the geoNano Lett., Vol. 9, No. 12, 2009
metry relaxations and was increased to 500 eV for the
calculation of electronic properties and the simulation of
STM images. The Brillouin zone was sampled using a 2
× 2 × 1 Γ-centered grid for structural optimizations, and
a 24 × 24 × 1 grid for computing the density of states.
The projection of the density of states onto atoms and
angular momentum components were calculated using the
Wigner-Seitz radii of 1.43 and 1.03 Å for the Pd and
the C atoms, respectively.
Acknowledgment. We gratefully acknowledge support
from the Department of Materials Science and Engineering
and the Henry Samueli School of Engineering and Applied
Sciences at the University of California Los Angeles,
University of California Discovery grant, from the National
Science Foundation through Grant No. CMMI 0825592/
0825771 and the MRSEC programs at Colorado School of
Mines (DMR-0820518) and Brown University (DMR0520651). This work benefited from computing resources
from the National Center for Supercomputing Applications
(Grant. No. DMR-090121), and was carried out in part at
the Frederick Seitz Materials Research Laboratory Central
Facilities, University of Illinois, which are partially supported
by the U. S. Department of Energy under grants DE-FG0207ER46453 and DE-FG02-07ER46471.
Supporting Information Available: Figures showing
STM images of graphene growth on Pd(111), graphene island
superstructures with different periodicities, Pd step motion
under the graphene island, and density of states of monolayer
graphene. This material is available free of charge via the
Internet at http://pubs.acs.org.
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