Electronic properties of grains and grain boundaries in graphene grown by chemical vapor deposition

Article abstract of DOI:10.1016/j.ssc.2011.05.023

We synthesize hexagonal shaped single-crystal graphene, with edges parallel to the zig-zag orientations, by ambient pressure CVD on polycrystalline Cu foils. We measure the electronic properties of such grains as well as of individual graphene grain bound

Full text of DOI:10.1016/j.ssc.2011.05.023

Solid State Communications 151 (2011) 1100–1104
Contents lists available at ScienceDirect
Solid State Communications
journal homepage: www.elsevier.com/locate/ssc
Electronic properties of grains and grain boundaries in graphene grown by
chemical vapor deposition
Luis A. Jaureguia,b, Helin Caoa,c, Wei Wud,e, Qingkai Yud,e,f, Yong P. Chenc,a,b,∗
a Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907, USA
b School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907, USA
c Department of Physics, Purdue University, West Lafayette, IN 47907, USA
d Center for Advanced Materials, University of Houston, Houston, TX 77204, USA
e Department of Electrical and Computer Engineering, University of Houston, Houston, TX 77204, USA
f Ingram School of Engineering and Materials Science, Engineering and Commercialization Program, Texas State University, San Marcos, TX 78666, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Accepted 15 May 2011
by V. Fal’ko
We synthesize hexagonal shaped single-crystal graphene, with edges parallel to the zig-zag orientations,
by ambient pressure CVD on polycrystalline Cu foils. We measure the electronic properties of such grains
as well as of individual graphene grain boundaries, formed when two grains merged during the growth.
The grain boundaries are visualized using Raman mapping of the D band intensity, and we show that
individual boundaries between coalesced grains impede electrical transport in graphene and induce
prominent weak localization, indicative of intervalley scattering in graphene.
Available online 25 May 2011
Keywords:
A. Graphene
B. Chemical vapor deposition
C. Grain boundary
D. Electronic transport
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
over the nucleation of individual graphene grains and to avoid the
grain boundaries in fabricated graphene devices [16].
Since its discovery, graphene has attracted a lot of attention
in many fields. Graphene’s potential applications have motivated
the development of large-scale synthesis of graphene grown by
several methods, including graphitization of SiC surfaces [1,2] and
chemical vapor deposition (CVD) on transition metals such as
Ni [3–5] and Cu [6]. In particular, it has been shown that large
and predominantly monolayer graphene of excellent quality can
be synthesized by CVD on polycrystalline Cu foils [6–8]. This
relatively simple and low-cost method has been used to produce
graphene that can reach impressive sizes [7] and can be easily
transferred to other substrates [7,9]. So far, all of the large-scale
synthetic graphene films produced are polycrystalline [10–14],
consisting of many single-crystalline grains separated by grain
boundaries formed when grains coalesced. The grain boundaries
(which, by definition, are defective) degrade the electrical
[12,14–16] properties of the resulting films. In order for graphene
to realize its promise in ‘‘carbon-based’’ electronics, progresses
have been made to synthesize either large-scale, high-quality
single-crystalline graphene films [17], or to achieve better control
Recently, we have studied the growth of hexagonal shaped
graphene grains by ambient pressure CVD. We have shown
by transmission electron microscopy and scanning tunneling
microscopy studies that the edges of graphene grains are
preferably oriented along the zig-zag direction. Additionally, we
studied grains and grain boundaries by Raman and electrical
measurements [16]. Here we present further results from electrical
transport measurements of graphene grains and grain boundaries,
primarily how individual grain boundaries impede electronic
transport and scatter charge carriers.
2. Experimental details
The graphene studied in this work is synthesized on polycrys-
talline Cu foils by ambient pressure CVD. First, a Cu foil (25-µm-
thick, 99.8%, Alfa Aesar) is loaded into a CVD furnace and heated
up to 1050 °C under 300 sccm Ar and 10 sccm H2. After reach-
ing 1050 °C, the sample is annealed for 30 min or longer with-
out changing the gas flow rates. The growth is then carried out at
1050 °C for ∼10 min under a gas mixture of 300 sccm diluted (in
Ar) CH4 (concentration 8 ppm) and 10 sccm of H2. We halt growth
before the graphene grains merge with each other to form a glob-
ally continuous (but polycrystalline) graphene film [8,18]. Finally,
∗
Corresponding author at: Department of Physics, Purdue University, West
Lafayette, IN 47907, USA. Tel.: +1 765 494 0947; fax: +1 765 494 0706.
E-mail address: yongchen@purdue.edu (Y.P. Chen).
0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2011.05.023

L.A. Jauregui et al. / Solid State Communications 151 (2011) 1100–1104
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Fig. 1. (a) Optical image of multi-terminal device on two merged graphene grains. (b) Raman mapping of the D band intensity (ID) for the sample depicted in (a) prior to
the device fabrication.
Fig. 2. (a, b) I–V and field effect in conductance curves measured within each graphene grain (left and right grains) and across the grain boundary for a device similar to
the one shown in Fig. 1(a). (c, d) I–V and field effect in conductivity (σ = G(L/W)) curves measured within a graphene grain (left grain) and across the grain boundary for a
Hall bar (width = 2.5 µm) patterned device depicted in the inset of Fig. 2(d). All the measurements are performed at zero magnetic field using 4-probe configurations. The
sub-indexes LG, RG, and CG refer to left, right, and cross grain respectively. The field effect curves are measured at low temperature to suppress gate leakage.
the sample is rapidly cooled to room temperature under the pro-
tection of Ar and H2, and then taken out of the furnace. Graphene
samples are transferred by a PMMA (polymethyl methacrylate) as-
sisted process in a Cu etchant (iron nitrate) onto SiO2/Si wafers
(doped Si covered by 300 nm SiO2) for Raman and electrical trans-
port studies. The graphene grains are typically hexagonally shaped,
with ∼120° corners, and their edges are shown to be parallel to the
zig-zag directions [16]. The hexagonal shape is notably different
from the flower-like shape of previously reported graphene grains
grown by low pressure CVD [6,12,19].
in a variable temperature probe station (Lakeshore CPX-VF). The
low temperature high field magnetotransport data in Fig. 6(a) and
(b) are measured in a He-3 superconducting magnet system at the
National High Magnetic Field Laboratory (NHMFL).
3. Results and discussion
Fig. 1(a) shows a typical device with multi-terminal electrodes
on merged graphene grains that allow us to measure intra-grain
(within grain) and inter-grain (across graphene grain boundary)
electrical properties. Fig. 1(b) shows the Raman mapping of the
D band intensity of the sample represented in Fig. 1(a) before
device fabrication. An intense Raman D-peak can be detected
between graphene grains corresponding to the graphene grain
boundary [16].
Fig. 2(a) shows the I–V curves measured at room temperature
for a device similar to the one shown in Fig. 1(a). From the
I–V curves, the conductance across the grain boundary is found
to be less than the conductance of each individual grain (left
or right). Conductance across the grain boundary is lower than
the conductance of each grain at different carrier densities (n0)
tuned by the backgate voltage (Vg ), as shown in Fig. 2(b). Even
After transferring graphene grains from the Cu foil to SiO2
by the PMMA method, we protect the chosen grains or pair of
merged grains by negative e-beam resist (MA-N 2403). Then, we
etch everywhere else by O2 plasma etching, to avoid shortening
of the electrical contacts by other graphene grains. The electrical
contacts (Cr/Au, 5 nm/35 nm, e-beam evaporated) to graphene
grains are patterned by e-beam lithography. The resistances
are measured using low frequency lock-in detection (Stanford
Research 830) with a driving current of 1 µA. DC I–V curves are
measured with a DC source meter (Keithley 2400). The highly
doped silicon substrate is used as a backgate. All the electrical
−5
transport data in the main text are taken in vacuum (∼10 Torr)

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L.A. Jauregui et al. / Solid State Communications 151 (2011) 1100–1104
Fig. 3. (a) ρCG/ρG ratio between inter-grain resistivity (ρCG) and intra-grain resistivity (ρG) for various samples. (b) Ratio (RCG/r) between measured inter-grain resistance
(RCG) and calculated (neglecting the grain boundary) inter-grain series resistance (r). (c) Effective grain boundary resistance (RCG −r). (d) Effective grain boundary resistance
per unit length of grain boundary (WGB). WGB is the length of the grain boundary (e.g. W3 in Fig. 1(a)).
Fig. 4. (a) Low field magnetoresistances measured at 4.3 K and backgate voltage Vg = 0 V. (b) Magnetoconductivity and its respective weak localization fittings (solid lines)
for T = 4.3 K and at Vg = 0 V and Vg = 70 V. (c, d) Characteristic lengths extracted from such fittings for data measured at various temperatures and two different carrier
densities ∼5.0×1012 cm−2 (Vg = 0 V) and near the Dirac point (Vg ∼ 70 V) respectively. All measurements where performed on the device depicted in the inset of Fig. 2(d).
after considering the device geometry, the effective cross-grain
conductivity is still lower than within the grain. Similar results are
also found on one pair of merged graphene grains patterned into a
Hall bar structure with uniform width, and an optical image of the
device is shown in the inset of Fig. 2(d). So far, all the devices we
have measured are p-type doped (likely due to adsorbates from the
environment and residues from the transfer and device fabrication
processes), and the Dirac point is not reached within the range of
backgate voltage (up to 60 V) used.
grain and ρR for the right grain) for all the devices, as depicted in
Fig. 3(a), indicating that the grain boundary impedes the electric
transport. We can also calculate an inter-grain series resistance




L
L
1
dx
dx
r = ρL
+ ρR
that neglects the grain boundary,
0
W(x)
L
W(x)
1
by simply integrating the intra-grain resistivities [16]. This gives
an r significantly smaller than the measured RCG, indicating that
the grain boundary provides an ‘‘extra’’ resistance (Fig. 3(b)). We
have found qualitatively similar results in all of the coalesced
grain devices we have measured (Fig. 3(a) and (b)): inter-grain
resistivities (ρCG) are always higher than corresponding intra-
grain resistivities (ρG, on each side of the grain boundary), and
RCG is always higher than the calculated r. Additionally, we have
extracted the effective grain boundary resistance (RCG − r) and its
value per unit length of grain boundary as depicted in Fig. 3(c) and
(d) respectively.
We have measured the intra-grain (RG) and inter-grain (RCG
)
resistances for several different devices. Taking into account the
device geometry, we can extract an effective resistivity, ρCG
=

L
1
1
dx
R
CG/(L/Wa), where Wa and L are defined as
=
and
W
a
L
0
W(x)
L = L1 + L2 (Fig. 1(a)), respectively [16]. The inter-grain resistivity
is higher than both of the intra-grain resistivities (ρL for the left

L.A. Jauregui et al. / Solid State Communications 151 (2011) 1100–1104
1103
earlier study in Ref. [22]. It has been suggested that puddles can
produce fluctuating electromagnetic fields, that enhances the in-
elastic scattering [21]. Furthermore, we extract the phase-breaking
rate (τφ) by τ−1
=
D/L2 , where D is the diffusion coefficient
φ
φ
√
(D = vF l/2, here vF is the Fermi velocity, l = h/(2ρCGe2 πn0)
is the mean free path, e is the elementary electric charge, and h is
the Plank’s constant). We obtain for Vg = 70 V (near Dirac point)
−1
−1
and Vg = 0 V phase-breaking rates of 3.91 ps and 0.36 ps re-
spectively at 4.3 K.
The intra-grain mobility (µG) extracted from low temperature
Hall measurements in all of the devices we studied (fabricated on
isolated as well as coalesced grains) ranged from <103 cm2/V s
to ∼104 cm2/V s as shown in Fig. 5. Inter-grain mobility is not
extracted as the two coalescing grains often had different carrier
densities (in the cases where their densities are comparable,
the inter-grain mobility is lower than µG by similar factors as
the resistivity ratios shown in Fig. 3(a)). Our work has clearly
indicated the detrimental effect of grain boundaries on electronic
transport, and that avoiding grain boundaries is beneficial for
improving the mobility. However, the wide variation of µG in
different samples (sometimes even between neighboring grains)
and the occasional low µG observed suggest that other sources
of disorder could also strongly affect the mobilities. Such ‘‘extra’’
disorder may also be partially responsible for the small intravalley
Fig. 5. Hall mobility histogram for
a series of graphene grain devices, the
measurement is performed at 4.3 K and Vg = 0 V. Some of the grains are grown
by a seeded process [16].
Magnetotransport measurements are performed under a per-
pendicular magnetic field (B). Fig. 4(a) presents low tempera-
ture (4.3 K) magnetoresistance (Rxx(B)) measured across the grain
boundary compared to Rxx(B)measured within one of the graphene
grains. The inter-grain Rxx(B) displays a prominent peak at B =
0 T, associated with weak localization (WL). Such a WL peak is
much weaker or even unobservable for intra-grain Rxx(B). WL re-
sults from an interplay between impurity scattering and quantum
coherent transport of carriers (such interplay also leads to re-
producible ‘‘universal conductance fluctuations’’ (UCF) in the re-
sistance, which are also observable in our devices). Raising the
temperature (T) destroys the phase coherence, and therefore di-
minish the WL feature, as was indeed observed in our samples [16].
In graphene, due to the chiral nature of carriers, WL requires the
presence of sharp defects that cause intervalley scattering with
large momentum (q) transfer. Our observation of prominent inter-
grain WL but much weaker or negligible intra-grain WL indicates
that grain boundaries are major sources of intervalley scattering in
our graphene devices (where inter-grain current has to cross the
grain boundary), while such scattering is less significant within the
single-crystal graphene grains.
scattering length (L
<
30 nm, Fig. 4(c) and (d)) observed
∗
(indicating a significant amount of small-q scattering defects such
as charged impurities, line defects and ripples etc. that may
arise from, e.g. graphene transfer or device fabrication processes).
Improving related fabrication processes to reduce such defects will
be required to achieve consistent high mobilities in graphene-
based devices.
We have studied the quantum Hall effect (QHE) in the intra-
grain and inter-grain Hall resistances, measured under high
magnetic fields and low temperatures (450 mK), as shown in
Fig. 6(a) and (b) respectively. For the intra-grain Hall resistance
(Fig. 6(a)), we observe quantized or developing Rxy plateaus at
e2/10h, e2/14h, e2/18h, e2/22h, corresponding to the Landau level
(LL) filling factors ν = 10, 14, 18, 22 respectively. For the inter-
grain Hall resistance (Fig. 6(b)) we observe a quantized Rxy plateau
at e2/6h, corresponding to ν = 6. Such half integer quantum
Hall effect, where ν = 4(n + 1/2) (n is a non-negative integer),
first observed in exfoliated graphene [23,24] and also later in
CVD graphene [3,7,8], reflects the electronic properties of chiral
massless Dirac fermions in graphene. The intra-grain and average
inter-grain carrier densities for the samples measured in Fig. 6(a)
and (b) are ∼4.4 × 1012 cm−2 and ∼1.9 × 1012 cm−2 respectively.
The inter-grain WL feature we observe at low temperature
can be well fitted to the WL theory developed for graphene [20].
Fig. 4(b), shows the cross-grain boundary (CG) magnetoconduc-
tivity (1σCG(B) = σCG(B) − σCG(B = 0)), normalized by e2/h
and antisymmetrized to eliminate the small Hall component. We
present corresponding weak localization fittings for two different
carrier densities. From the fitting at different temperatures, we ex-
tract various inelastic (L , phase-breaking) and elastic (Li, interval-
ϕ
ley; and L , intravalley) scattering lengths for carrier densities of
∗
∼5.0×1012 cm−2 (Vg = 0 V) and near the Dirac point (Vg ∼ 70 V),
as shown in Fig. 4(c) and (d) respectively. There is no significant
4. Conclusions
variation of elastic scattering lengths (Li and L ) for different car-
∗
rier densities, however the L measured at 4.3 K decreases from
ϕ
Grain boundaries can reduce the electrical conductivity of
polycrystalline graphene. The presence of a graphene grain
boundary can be visualized by the mapping the intensity of the
∼250 nm for Vg = 0 V to ∼100 nm for Vg = 70 V. A similar varia-
tion was reported in Ref. [21]. Electron–hole puddles, formed near
Dirac point, can be one cause of this reduction, as suggested in an
Fig. 6. Intra-grain (a) and inter-grain (b) Hall resistance (Rxy) for 2 different devices shows Quantum Hall effect of monolayer graphene. Temperature in both cases is 450 mK.

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L.A. Jauregui et al. / Solid State Communications 151 (2011) 1100–1104
Raman D-band. This D-band comes from an intervalley scattering
process, which involves the presence of sharp defects, such as grain
boundaries. Weak localization is found in the measurement of the
inter-grain magnetoresistance, suggesting that grain boundaries
causes intervalley scattering. For our sample, carrier mobility of
the grains shows a wide variation, maybe due to the uncontrollable
amount of disorder introduced during the transferring and device
fabrication process. Electrical measurements under high magnetic
fields show the characteristic quantum Hall effect of single layer
graphene, within graphene grains and across the grain boundary.
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Acknowledgments
QY acknowledges support from NSF and CAM Special Funding.
YPC acknowledges support from NSF, DTRA, DHS, IBM, Miller Fam-
ily Endowment, Midwest Institute for Nanoelectronics Discovery
(MIND). Parts of the measurements were performed at the National
High Magnetic Field Laboratory (supported by NSF, DOE and the
State of Florida) and the authors thank E. Palm and G. Jones for ex-
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