Supplementary material
Carbon support
Applied Physics Letters
2012
Noncatalytic chemical vapor deposition of
graphene on high-temperature substrates
for transparent electrodes
Jie Sun (孙捷),a), b) Matthew T. Cole,c) Niclas Lindvall,b)
Kenneth B. K. Teo (张谋瑾),d) and August Yurgensb)
b)Department of Microtechnology and Nanoscience,
Quantum Device Physics Laboratory, Chalmers University
of Technology, S-41296 Gothenburg, Sweden
c)Department of Engineering, Electrical Engineering
Division, University of Cambridge, 9 JJ Thomson Avenue,
CB3 0FA Cambridge, United Kingdom
d)AIXTRON
(a)
5 nm
(b)
(c)
(d)
Nanoinstruments Ltd., Swavesey, CB24 4FQ
Cambridge, United Kingdom
FIG. 1. (a) High-resolution TEM image of the graphene
deposited on SiO2/Si and subsequently transferred to Cu
grid. (b) Typical electron diffraction pattern showing
monolayer graphene feature. (c) When the beam spot is
somewhat larger than ~10 nm, signals from two or more
domains are detected. (d) Even larger spot gives the signal
similar to that of a-C.
The “nanoflake” mechanism proposed in this letter
can be used to explain noncatalytic graphene
deposition in chemical vapor deposition (CVD)
systems. It complements the common growth mode
based on single C atoms as in molecular beam
controlled processes. For example, graphene on
dielectrics by molecular beam epitaxy has been
reported.1 However, the deposition rate is expected to
be relatively low in these cases.
Black Magic (AIXTRON), since the deposition
primarily occurs on the surfaces of the samples.
The transmission electron microscopy (TEM) data
represent an evidence for the graphene nature of our
thin films. The image shown in Fig. 1 is obtained on
SiO2-grown graphene (Fig. 1(b) in the main text,
middle sample) wet-transferred to a standard Cu grid
with a-C network. Fig. 1(a) suggests that the
graphene membrane is basically uniform (with no
observable holes). At the rippled/folded free-standing
edges in the lower part of Fig. 1(a), a layered
structure is observed, which is unlikely to be seen in
the case of a-C. More importantly, the convergedbeam electron diffraction shows a typical monolayergraphene pattern (Fig. 1(b)). Even though this pattern
can be observed everywhere in the membrane, mixed
signals from two or more domains are observed when
the electron-beam spot is made somewhat bigger than
Under the growth conditions used here, carbon
deposits on any high-temperature materials, including
the quartz tube and sample susceptor. Therefore, after
many runs (Fig. 1(a) of the main text), the exhaust
(left) end of the tube (carbon black region) becomes
really black. The middle zone (graphene region) is
covered with shiny graphite film, which reflects the
radiation and makes it practically difficult to heat the
samples. Therefore, the reaction tube has to be
exposed to open air and heated up to >750 oC for 0.5
h to burn off the carbon. This cleaning, however, is
not a problem for cold-wall CVD systems such as
a)
Author to whom correspondence should be addressed.
Electronic mail: jiesu@chalmers.se.
1
(a)
Ψ (o)
6 nm
(a)
5 μm × 5 μm 0 nm
20
15
(b)
Wavelength (nm)
S
FIG. 2. (a) Ψ(λ) spectra for different incident angles.
D
20 μm
(b) Reconstructed optical constants.
(c)
10
(b)
Height (nm)
n (real)
k (imaginary)
Wavelength (nm)
5
0
-5
0
1
2
X (m)
3
FIG. 3. (a) AFM image scanned over 5 μm × 5 μm area
showing the average surface roughness of ~1 nm. (b)
Optical micrograph of a Hall-bar device fabricated from the
graphene deposited on SiO2/Si. (c) AFM height profile
across an edge of graphene measured on the device in (b).
~10 nm (Fig. 1(c)). This gives an estimate of the
grain size in our graphene. Even larger beam spot
generates a pattern resembling that of a-C, indicating
the long-distance disorder caused by the numerous
nanocrystallites. The nanocrystallites, however, are
not directly observed by the high-resolution TEM due
to the possible presence of polymer residues on the
graphene. The polymer is used as mechanical support
during the transfer.
We have carried out a numerical inversion of the
data in Fig. 2(a), using the measurements on the bare
SiO2/Si substrate as a reference. Assuming the
graphene effective thickness of 1.6 nm, we obtain
rather reasonable dependences shown in Fig. 2(b).
The real part (n) of the refractive index is shown in
red and the imaginary part (k) in green. Both n and k
are similar to the results on exfoliated graphene and
CVD-grown graphene on Cu (n is slightly lower than
in literature).2,3 The expected absorption peak at 276
nm is seen in the data for k.3 The assumed thickness
(1.6 nm) is slightly higher than expected.
Nevertheless, there can be many explanations for this,
including e.g. non-ideal reference substrate (order of
1 nm height variations are to be expected for thick
thermal SiO2). Also, the effective optical length
might be different because of domain size and
morphology of the nanocrystalline graphene and
uncertainty in choosing the “optimal thickness” for
the numerical inversion.
We have performed variable angle spectroscopic
ellipsometry (VASE) measurements on the SiO2grown graphene (Fig. 1(b) in the main text, middle
sample), using a Woollam M2000 ellipsometer. The
results are compared to recent ellipsometry
measurements of both exfoliated and Cu-grown
graphene.2,3 The anisotropic signal is however very
weak in our case (which is also discussed in Ref. 3).
Here we focus on the isotropic data. In Fig. 2(a), the
ellipsometric parameter Ψ(λ) (for definition of Ψ, see
Ref. 2) is displayed for four different angles of
incidence (45o-60o). In green are the data for directly
deposited graphene on SiO2/Si while in red for the
bare SiO2/Si substrate. By comparison with Ref. 2,
our sample is found to have similar optical properties
as the single-crystalline exfoliated graphene.
2
Fig. 3(a) provides an atomic force microscopy
(AFM) image on the SiO2-grown graphene (Fig. 1(b)
in the main text, middle sample). The image has been
captured over larger area (5 μm × 5 μm) compared
with Fig. 1(c) in the main text. The graphene shows a
surface roughness of ~1 nm. Fig. 3(b) is an optical
micrograph of a Hall-bar device fabricated by
photolithography (using S1813 resist) from the assynthesized graphene. AFM line scan across the
graphene edge indicates a step height of ~2 nm. The
height profile is displayed in Fig. 3(c). Typically,
exfoliated monolayer graphene has an AFM
thickness of ~0.8 nm on SiO2, whereas after
lithography process this value often increases to ~1.52 nm due to the resist residue.4,5 Thus, TEM,
ellipsometry and AFM characterization all suggest
that the middle sample shown in Fig. 1(b) of the main
text is indeed a nanocrystalline graphene principally
composed of monolayer flakes.
(a)
Black edge
(b)
FIG. 4. (a) Photo of the quartz and sapphire based graphene
samples. In each row, the first sample is a bare substrate for
comparison. (b) Enlarged image of (a) showing carbon
black edges.
In each row of Fig. 4(a), the far-left sample is bare
quartz (sapphire) (5 mm × 5 mm, double side
polished), while the others are the same type of
quartz (sapphire) substrates with nominal monolayer
graphene deposited under conditions similar to Ref. 6.
Because the precursor gas can penetrate the small
gaps between the samples and holder during the
deposition, graphene grows also on the bottoms of
substrates. We remove this graphene by O 2 plasma
(with the top side protected). The samples are placed
on a piece of paper with a text on it. The text is
clearly seen through the samples due to the high
uniformity and transparency of the graphene. The
three samples of graphene on quartz (sapphire) are
grown in different runs but are alike, indicating the
reproducibility of the process.
cm2/Vs. Further details on these electrical transport
data will be published elsewhere.
Rough substrate contains numerous pits, kinks and
steps, etc. Smaller or misaligned (such as inclined or
vertical) graphene flakes are easily adsorbed onto
these defective sites. As a result, the deposited
material is rough and appears dull black. Fig. 4(b) is
a magnification of Fig. 4(a) (rotated 90o), where
carbon black is seen on the diced edges of the
graphene/quartz samples (left). On the other hand, the
edges of the sapphire substrates (right) are polished,
and therefore the nanocrystalline graphene thin film
grows conformally. Fig. 4(b) implies the important
role of substrate surface flatness in the noncatalytic
CVD of graphene.
Electrical measurements on the Hall-bar devices
fabricated from the as-synthesized graphene on
SiO2/Si, quartz and sapphire show that the I-V curves
are linear, and the contacts to metals are ohmic,
similar to our previous results on Si3N4.6 At room
temperature, up to 13% variation of Rs is observed by
changing the gate voltage, measured in the middle
sample in Fig. 1(b) of the main text (back-gate
configuration). Both the field-effect- and Hall
measurements indicate a carrier mobility of ~40
We thank Dr. T. J. Booth for the help in obtaining
TEM images.
References
1. S. K. Jerng, D. S. Yu, Y. S. Kim, J. Ryou, S. Hong, C. Kim, S.
Yoon, D. K. Efetov, P. Kim, and S. H. Chun, J. Phys. Chem. C 115,
4491 (2011).
2. V. G. Kravets, A. N. Grigorenko, R. R. Nair, P. Blake, S.
Anissimova, K. S. Novoselov, and A. K. Geim, Phys. Rev. B 81,
155413 (2010).
3
3. F. J. Nelson, V. K. Kamineni, T. Zhang, E. S. Comfort, J. U. Lee,
and A. C. Diebold, Appl. Phys. Lett. 97, 253110 (2010).
4. Z. Cheng, Q. Zhou, C. Wang, Q. Li, C. Wang, Y. Fang, Nano
Lett. 11, 767 (2011).
5. Y. Dan, Y. Lu, N. J. Kybert, Z. Luo, A. T. C. Johnson, Nano
Lett. 9, 1472 (2009).
6. J. Sun, N. Lindvall, M. T. Cole, K. B. K. Teo, and A. Yurgens,
Appl. Phys. Lett. 98, 252107 (2011).
4