Journal of the Korean Physical Society, Vol. 55, No. 3, September 2009, pp. 1299∼1303
Variations in the Raman Spectrum as a Function of the Number
of Graphene Layers
Duhee Yoon, Hyerim Moon and Hyeonsik Cheong∗
Department of Physics, Sogang University, Seoul 121-742
Jin Sik Choi, Jung Ae Choi and Bae Ho Park
Department of Physics, Konkuk University, Seoul 143-701
(Received 25 August 2008)
We studied variations in the Raman spectrum as a function of the number of graphene layers by
using micro-Raman spectroscopy and atomic force microscopy (AFM). The sample preparation was
done by micromechanical cleaving of natural graphite on an ∼300 nm SiO2 layer. The Raman G
band (∼1580 cm−1 ), G∗ band (∼2450 cm−1 ) and 2D band (∼2700 cm−1 ) varied as functions of the
number of graphene layers. The Raman 2D band was especially sensitive to the number of graphene
layers. These features are related to the electronic band structure of graphene. Moreover, areas of
different numbers of graphene layers were clearly identified using spatially-resolved micro-Raman
imaging spectroscopy.
PACS numbers: 73.22.-f, 78.30.-j, 78.66.-w
Keywords: Graphene, Raman, AFM
I. INTRODUCTION
Graphene is a two-dimensional carbon material with
sp2 bonds and has a relativistic energy dispersion [1–
3]. Since Geim et al. successfully isolated single-layer
graphene [1,4], many research groups have investigated
the physical properties of graphene. Since graphene
has high carrier mobility and since the carrier type
and density can be easily controlled using the electric
field, graphene has attracted much attention as a nextgeneration electronic device material [1, 2, 5]. Other
novel phenomena have been found, including a halfinteger quantum Hall effect in single-layer graphene and
a Berry’s phase of 2π in bi-layer graphene [2,6,7]. Confinement effects in graphene nano-ribbons were predicted
and observed, which also point to the possibility of using
graphene in nano-electronics [8–10]. Moreover, graphene
is projected to be a spintronic material due to its peculiar electronic properties [11]. If electronic devices are
to be made of graphene on a commercial scale, extended
graphene sheets are needed, and many research groups
are working on producing large-size graphene sheets [3,
12].
Raman spectroscopy has been a useful tool in the investigation of graphitic materials such as fullerenes, carbon nanotubes (CNTs), and graphite [13–15]. Raman
spectra of samples with a few graphene layers have been
∗ E-mail:
published [16], and a method to indentify the number
of graphene layers by using a Raman imaging technique
was developed [17]. Moreover, the thermal conductivity
of graphene [18, 19], the influence of the substrate [20],
and the doping effect [21,22] were investigated using Raman spectroscopy. Although a few research groups have
reported measurements of variations in the Raman spectrum of graphene as a function of the number of layers,
there are still some inconsistencies [16,17,23–25]. Since
the Raman spectrum is used as a fingerprint of the number of graphene layers for samples with a few layers, it is
important to establish the correlation between the Raman spectrum and the number of layers.
In this work, we investigated variations in the Raman
spectrum as a function of the number of graphene layers
by using micro Raman spectroscopy and atomic force microscopy (AFM). The variations in the Raman G band
(∼1580 cm−1 ), G* band (∼2450 cm−1 ), and 2D band
(∼2700 cm−1 ) were observed as functions of the number
of graphene layers. The Raman intensity and Raman
band shape vary as functions of the number of graphene
layers, and the Raman 2D band is especially sensitive
to the number of graphene layers. The Raman 2D band
shape could be used as a fingerprint of the graphene layers. Because of variations in the Raman bands, areas
of different numbers of graphene layers can be clearly
identified using spatially-resolved micro-Raman imaging
spectroscopy.
[email protected]; Fax: +82-2-717-8434
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Journal of the Korean Physical Society, Vol. 55, No. 3, September 2009
Fig. 1. Optical microscope images of (a) a thick graphite
flake, (b) a thin graphite flake with graded thickness, and
(c) single- and bi-layer graphene (SiO2 layer thickness: ∼310
nm). (d) Comparison of the Raman spectra of graphite and
graphene.
II. EXPERIMENTS AND DISCUSSION
The graphene samples were prepared by micromechanical cleavage of graphite flakes (∼50 mesh) on 280- to
310-nm SiO2 /Si substrates [1, 2, 4]. We first approximately identified the number of the graphene layers with
an optical microscope. In spite of the fact that the
thickness of the graphene layer is as thin as 3.4 Å, we
could estimate the number of graphene layers through
the color and the contrast owing to the interference between the graphene and the SiO2 layers [24–27]. The
number of graphene layers was exactly identified using an
AFM (Seiko SPA-300HV, contact mode). We obtained
the Raman spectrum of the graphene samples with a
homemade micro-Raman spectroscopy system. In microRaman spectroscopy, the 514.5-nm (2.41-eV) line of an
Ar ion laser was used as the excitation source with a
power of ∼1 mW. The heating effect can be neglected
at this power range [16]. The laser beam was focused
onto the graphene sample by a 40× microscope objective
lens (0.6 N.A.), and the scattered light was collected in
the backscattering geometry (Z[Pi , Ps ]Z̄). The collected
scattered light was dispersed by a Jobin-Yvon TRIAX
550 spectrometer (1200 grooves/mm) and was detected
with a liquid-nitrogen-cooled CCD detector. The spatial resolution was less than ∼1 µm, and the spectral
resolution was ∼1 cm−1 .
Figure 1(a) is an optical microscope image of a thick
graphite flake, and Fig. 1(b) is an optical microscope
image that shows the variation of color due to the thickness variation in a thin graphite flake. In Fig. 1(b), the
yellow area is thicker than the blue area. The number of
graphene layers increase as the color changes from blue
to yellow. The purple area is the SiO2 substrate. The
variation of the color originates from the interference between the graphene and the SiO2 layer [24–27]. Fig. 1(c)
shows an optical microscope image of a sample consisting of single-layer graphene, bi-layer graphene, and thick
graphene. The faint purple area in the center of Fig. 1(c)
is single-layer graphene. Fig. 1(d) compares the Raman
spectrum of graphene with that of graphite. The Raman spectrum of graphite is well known [28]: there are
two prominent peaks (G and 2D) and three small peaks
(D, G∗ and G0 ) [16,17,23–25]. However, the D band is
absent in the Raman spectrum of single-layer graphene.
The Raman D band originates from disordered carbon
atoms in A01 symmetry [29]. In well-ordered graphene
and graphite, the D band is absent [28]. The shape of
the 2D band of graphite is clearly different from that of
single-layer graphene.
Figure 2 shows the evolutions of the G, G∗ , and 2D
bands as the number of graphene layers is increased.
The Raman G band is related to E2g symmetry (space
group P63 /mmc) [30], and its shape is a single Lorentzian
curve. The intensity of the Raman G band increases with
the number of layers up to 7 layers and then decreases
for thicker samples (∼23, ∼40 layers and graphite), as
shown in Fig. 2(a). This is plotted in Fig. 3(a). Although we did not find the exact turning point of the
G band intensity variation, there has been a report that
the Raman G band intensity increases monotonically until about ten layers [31]. The G band frequency (ωG ) of
single-layer graphene is 1585.5 ± 1 cm−1 , and that of
bi-layer graphene is 1581.5 ± 0.5 cm−1 . The ωG for
other thicknesses is 1582.5 ± 1 cm−1 (Fig. 3(b)). The
shape of the G band did not vary, but the variation in
the intensity is sensitive up to 7 layers. Therefore, the
variation in the G band Raman intensity can be used to
approximately identify the number of graphene layers.
The Raman G∗ band has a small intensity, which does
not vary much, as shown in Fig. 2(b). This band
originates from a combination of the zone boundary inplane longitudinal acoustic (iLA) phonon and the inplane transverse optical (iTO) phonon modes [32, 33].
The frequency of the G∗ band (ωG∗ ) redshifts with increasing number of graphene layers from 2455 cm−1 to
2445 cm−1 . The shapes of the G∗ bands are similar to
each other, except for single-layer graphene. The G∗
band of single-layer graphene is relatively sharp compared to that for other thicknesses. Although the sharp
shape of the G∗ band could be evidence for single-layer
graphene, the G∗ band has a low signal-to-noise ratio.
The 2D band can be used identify single-layer and bilayer graphene [16,17]. The line shape of the 2D band of
single-layer graphene is unique compared to the others.
It has a single Lorentzian lineshape and a high intensity,
as shown in Fig. 2(c). Its frequency is 2685 ± 1 cm−1 .
From Fig. 2(c), it is evident that the 2D band can be
used an exact fingerprint for the number of graphene
layers for 1 to 4 layers. The 2D band can be explained
with a double resonance Raman process [34,35] and has
a close correlation with the electronic band structure of
the graphitic materials [16,17,34–36]. As the number of
graphene layers increases, the electronic band structure
Variations in the Raman Spectrum as a Function of the Number· · · – Duhee Yoon et al.
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Fig. 2. Evolutions of the (a) G band, (b) G∗ band, and (c) 2D band in the Raman spectrum as functions of the number of
graphene layers.
varies and approaches that of graphite [37, 38]. Since
the lineshape of the 2D band reflects the electronic band
structure, we can infer that the electronic band structure
of 5 or 6 layers of graphene is very similar to that of
graphite. There is also an overall blue shift of the 2D
band with increasing number of layers.
We also performed spatially-resolved micro-Raman
imaging spectroscopy for a sample that had areas with
different thicknesses. Fig. 4(a) is an optical microscope
image of the sample. Areas with different shades are observed. Fig. 4(b) is an AFM topography image obtained
from the area indicated by the dotted square in Fig. 4(a).
We estimate the number of graphene layers by using cross
section profiling of the AFM image. Raman images are
constructed for the intensities of the Raman G and 2D
bands because these bands have a good signal-to-noise
ratio. The Raman scanning was performed in a 30 µm
× 40 µm region. Figs. 4(c), (d), and (e) are images
for the Raman G band intensity, the 2D band intensity,
and the ratio of the intensity of the 2D band to that of
the G band, respectively. The boundaries between areas
with different numbers of layers are well distinguished in
Fig. 4(c) because the G band intensity is sensitive to the
number of layers. On the other hand, the boundaries are
not well distinguished in Fig. 4(d), except for the singlelayer area because the 2D band intensity of single-layer
graphene is bigger than the others. Because the G band
intensity increases and then decreases as the number of
graphene layers increases, there may be a case where the
G band intensities are the same for areas with different
numbers of graphene layers. Since the 2D band intensity tends to decrease as the number of graphene layers
increases, one can better estimate the thickness in such
cases by comparing the intensities of the G and the 2D
Fig. 3. Variations of (a) G band intensity and (b) the G
band frequency as functions of the number of graphene layers.
bands. Fig. 4(e) is the image of the ratio of the intensity
of the 2D band to that of the G band. As expected,
areas of different thicknesses are more clearly resolved.
Therefore, by combining the intensity images of the G
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Journal of the Korean Physical Society, Vol. 55, No. 3, September 2009
Fig. 4. (a) Optical microscope image and (b) atomic force microscope (AFM, contact mode) image of a graphene sample
(SiO2 layer thickness: ∼280 nm). Images of (c) the G band intensity, (d) the 2D band intensity, and (e) the ratio of the intensity
of the 2D band to that of the G band.
and the 2D bands, one can reliably estimate the number
of graphene layers.
III. CONCLUSION
We performed micro-Raman spectroscopy as a function of the number of graphene layers and spatiallyresolved micro-Raman imaging spectroscopy for a
graphene sample with areas of different thicknesses. The
intensity of the G band increases monotonically with the
number of graphene layers up to 7 layers. The lineshape
of the 2D band changes significantly as the thickness increases from 1 to 4 layers. By combining these two features one can reliably estimate the number of graphene
layers.
ACKNOWLEDGMENTS
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (R01-2008-00010685-0 and KRF-2008-314-C00111), through Quantum
Photonic Science Research Center, and by Nano R&D
program through NRF funded by MEST (2008-03717).
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