COMPARATIVE STUDY OF GRAPHENE FLAKE ON SiO2 SUBSTRATE
USING ATOMIC FORCE MICROSCOPE AND RAMAN SPECTROSCOPY
Felicia Dhivya Thiagaraj
A comprehensive study of graphene flakes was conducted using atomic force microscope
(AFM) and Raman spectroscopy. The sample preparation was done by micromechanical
cleaving of bulk graphite on ~285 nm SiO2 layer. The graphene surface was not found to
be flat but contained folds, wrinkles and impurities which contributed to the intrinsic
stability of the graphene flake. The respective Raman spectra related to the electronic
band structure of graphene were obtained at different regions of the flake with D band
(~1360 cm-1) G band (~1576 cm-1), 2D band (~2724 cm-1) and 2G band (~3250 cm-1).
The asymmetric nature of the 2D band indicated that the selected graphene flake was a
few-layered. Combining Atomic force microscope and Raman spectroscopy detailed
information regarding the morphology, thickness and electronic structure of graphene is
obtained.
1. Introduction
Graphene, the building block of graphite has been of great scientific interest in recent
years due to its unique electronic and structural characteristics. In essence, graphene is
a flat single layer of sp2 hybridized carbon atoms packed into a two dimensional
honeycomb lattice. For a long time it was assumed that 2D crystals were
thermodynamically unstable and could not exist under ambient conditions. However, in
2004 Novoselov et al. successfully demonstrated that it was possible to produce
graphene and other 2D atomic crystals by extracting them from 3D materials [1]. These
extracted 2D crystals were found to exhibit long range crystalline order. Also, the
microscopic roughening on the surface of suspended graphene sheets contributed to the
stability of the crystals [2]. The fact that the roughness is reproducible for different
positions on the flake, becomes notably smaller for bi-layer graphene and disappears
for thicker flakes prove that these corrugations are intrinsic to graphene membranes.
Further, theoretical investigations of 2D crystallite graphene had predicted their
thermodynamic stability through static microscopic crumpling. This discovery has led
to investigations into the electronic and mechanical properties of graphene. Presently,
there are two methods of producing graphene. In the first method, graphene layers are
mechanically exfoliated from bulk graphite [3]. This method obviously cannot be
employed for large-scale applications since only a single sheet of graphene is removed
at a time from highly ordered pyrolytic graphite. Hence, the second method which
involves the full graphitization of silicon carbide under vacuum conditions is employed
[4].
The Atomic Force Microscope is a widely used tool used to characterize morphology
and to measure the thickness of the graphene flakes on the silicon oxide substrate but it
has low throughput. Due to the chemical contrast-caused by variation in the tip-sample
interaction between two samples-between the graphene and substrate, which results in
an apparent chemical thickness of 0.5-1 nm, much greater than that expected from the
interlayer graphite spacing, in practice it is only possible to distinguish between one or
two layers by AFM when graphene films contain folds and wrinkles [5]. Though the
measurements obtained from AFM operating in the contact mode are more reliable, this
mode could nevertheless damage the sample surface. Hence, the AFM is commonly
operated in the non-contact or tapping mode.
Raman Spectroscopy is a very reliable, non-destructive and high throughput tool used
to identify single-layer and multi-layer graphene [6]. The Raman spectra for single- and
few-layer graphene reflect changes in the electronic structure and electron-phonon
interactions. Variations have been observed in the spectral regions of the graphite G
(~1580 cm-1) and 2D (~2700 cm-1) modes as a function of the number of graphene
layers. In addition, the disorder induced D mode (~1350cm-1) is related to the high
energy optical phonons in the vicinity of the reciprocal space vector κ point in the
graphite. It essentially gives an indication of the defect, damage and impurity level in
the graphene. The G mode is the first-order optical phonon mode and is related to the
Raman active tangential E2g phonon, where the two atoms in the graphene unit cell are
vibrating tangentially one against the other. The 2D mode is the second-order optical
phonon mode near the κ point in the graphene Brillouin zone and is due to the double
inelastic phonon scattering. For single-layer graphene the 2D peak can be fitted to a
single Lorentzian, whereas in few-layer graphene it requires fitting to two or more
Lorentzians. The 2D band changes from a narrow and symmetric feature in a singlelayer graphene to one that shows an asymmetric shape on the high-energy side in a fewlayered graphene [6]. Also, the increase in the G-band intensity going from single- to
few-layer graphene further supports the claim that the electronic band structures are
unique to the number of graphene layers.
In this paper, the morphology of the graphene and its number of layers have been
investigated and also a comparative study of the graphene flake using AFM and Raman
Spectroscopy is performed.
2. Materials and Experimental method
Graphene films for this study were prepared by micromechanical cleavage of highly
oriented
pyrolytic graphite as described by Novoselov et.al in 2004 [2]. Using a
scotch tape, flakes of graphite were repeatedly peeled and then deposited on the surface
of a silicon wafer covered by a 285 nm thick silicon dioxide film. Graphene layers
thinner than 50 nm are transparent to light. However, they become visible on the silicon
dioxide surface as even a monolayer adds to the optical path of reflected light to change
the interference color with respect to the substrate. Typically, the color of the silicon
dioxide wafer is violet-blue but the deposition of graphene layers shifts the color to
blue. Graphene flakes were then identified under an optical microscope.
Veeco Dimension 3100 Atomic Force Microscope was used in the tapping mode to
characterize and identify the selected graphene flake. The images were collected and
analyzed using “section or profile” analysis to measure the height of the graphene flake
relative to silicon dioxide surface.
Raman spectra for the selected graphene flake were recorded using Renishaw Raman
Microscopy version 5 for which the excitation source was the 514 nm Ar ion laser with
incident power in the mW range to avoid heating of the sample. The laser spot size is
sub-micron. Also, the Raman measurements were performed with a microscope set-up
in the backscattering geometry at room temperature. The spectra measurements were
collected in the range 1000-4000 cm-1.
3. Results and Discussion
An AFM image of the graphene flake on silicon dioxide wafer selected for this
experiment is shown in Fig. 1. Various features of the flake were investigated under
the AFM. It was confirmed that the surface of the graphene was not flat but rather
consists of folds or wrinkles (Fig. 1) [3]. This means that the stability of the graphene
flake is due to the presence of this 3D roughening on its surface. Therefore, in order to
determine the true height at which the graphene flake is relative to the substrate, the
heights at which the different features are from the SiO2 surface should be taken into
account. Impurities (Fig.1) on the surface of the graphene film were also observed.
These impurities also cause roughening of the graphene surface and their presence can
be attributed to the preparation of graphene on the SiO2 surface by repeated peeling
using a scotch tape. It can be clearly seen that different features of the flake are at
different heights from the SiO2 surface. This could be due to the pleating of graphene or
anomalies induced by the preparation of graphene. Though like other scanning probe
techniques the AFM is not free of measurement artifacts, it still gives reasonably
accurate information regarding the structure and thickness of the graphene layers.
(Insert Figure. 1)
By using Raman spectroscopy with 514 nm excitation laser the same graphene flake
features were analyzed (Fig. 2). As per literature the G band occurs at approximately
1576 cm-1 and the 2D band occurs at approximately 2724 cm-1. The asymmetrical
nature of the 2D band for features A, B and C within the same graphene flake (see
Fig.3) tells us that these regions belong to a few layered graphene [6]. Also, the 2D
peak for a few-layered graphene is divided into 2D1 and 2D2 [8, 7]. This splitting of the
2D band can be explained in the following way: an electron is excited from point A in
the valence л band to point B in the conduction л* band by absorbing a photon. The
excited electron is inelastically scattered to a point C by emission of a phonon. Inelastic
backscattering to the vicinity of point A by emission of another phonon and electronhole recombination leads to emission of photon of energy less than that of the incident
photon [9]. Also, the presence of the disorder induced D peak at about 1360 cm-1
indicates that small defects are present in the flake. Another peak can be seen at about
3250 cm-1 which corresponds to the double-resonant disorder induced G band or the 2G
band. Its presence is again related to the defects in the graphene. The variation in the
intensities and the FWHM of the G peaks and 2D peaks within the same graphene flake
is because the spectra were obtained at different regions of the flake where folds,
wrinkles or impurities would have been present (see Fig.2). Therefore, there is a
variation in the electronic band structure within the same graphene flake while
considering different regions. One set back of using the Raman Spectroscopy is that
sample edges are always seen as defects giving rise to disorder induced peaks [10].
(Insert Figure. 2, Figure. 3)
The darker flake shaped features found on the SiO2 substrate were initially assumed to
be thick graphene layers under the AFM. While, in reality these were
damage/impurities on the surface of the SiO2 substrate, as confirmed by Raman
spectroscopy
Though the AFM provides information regarding the surface morphology and
thickness of graphene layers, yet it cannot indicate whether the chosen graphene layer
is single-layered or a few-layered. In contrast, excellent information can be obtained
regarding electronic structure and number of layers by using the Raman Spectroscopy.
However, this technique cannot provide information regarding the morphology of the
graphene flakes. Also, it is better to use the Raman Spectroscopy first if studies on only
the nature of single- or a few-layered graphene flake has to be performed. In
conclusion, a comprehensive study of the nature of the graphene flake can be
performed by using a combination of the AFM and Raman Spectroscopy.
4. Conclusion
The different features of the graphene flake were studied using both the AFM and
Raman spectroscopy. The surface of the flake was found to contain folds and impurities
which provide intrinsic stability. The intensities of the G bands and the asymmetry of
the 2D bands of the Raman spectra indicate that the graphene flake was few-layered.
Also, the 2D band was split into 2D1 and 2D2 in a few-layered graphene due to double
resonant phonon scattering. By combining the AFM and the Raman Spectroscopy one
can estimate the morphology, number of layers and electronic band structures of
graphene.
Acknowledgments
I would like to thank Dr. Christina Giuzca and Chris Buxey at the Advanced Technology
Institute, University of Surrey, UK for their technical support while using AFM and
Raman Spectroscopy. Also, I would like to thank Dr. Vlad Stolojan for making this
experiment a valuable learning experience.
B
A
C
Folds
A
Impurity
Folds at
the edge
Figure 1
G
2D
2G
D
Figure 2
2D2
2D1
Figure 3
CAPTIONS
Fig.1 shows the AFM image of a few layered graphene. In this figure the dark brown
color corresponds to the SiO2 surface, the light brown is the graphene flake and the white
spots and pleating on its surface corresponds to the impurities and folds. The height of the
pleated layer is 8.25nm relative to the graphene surface and that of the folded edge of the
graphene flake relative to substrate is 11.68nm. This implies that the graphene surface is
not flat but rather consists of folds and impurities which are intrinsic to its structure and
provides stability.
Fig. 2 shows the Raman Spectra using 514 nm laser excitation corresponding to regions
A (dashed line), B (solid line) and C (dotted line) from Fig. 1. The D peak appears at
~1360 cm and gives an indication of the defects in the graphene layer. The G peak
appears at ~ 1576.4cm-1, 2D peak appear at ~2724cm-1 and 2G peak at ~3250cm-1. The
variation in the intensities and FWHM of the G bands and 2D bands is due to the
presence of impurities and folds in those regions.
Fig. 3 shows 2D bands of the Raman Spectra corresponding to regions A (dashed line), B
(solid line) and C (dotted line) from Fig.1. The 2D band peaks are finger prints of the
graphene layers and occur at ~2724 cm-1. Its asymmetrical nature indicates that the
graphene flake is few-layered. The 2D band is split into 2D1 and 2D2 due to double
resonant phonon scattering.
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