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Low-temperature Raman studies of supported graphene are presented. A linear temperature dependence of 2D peak linewidths was observed with the coefficients of 0.036 and 0.033 cm\(^{-1}\)/K for graphene on copper and glass substrates, respectively, while G peak linewidths remained unchanged throughout the whole temperature range. The different values observed for graphene on glass and copper substrates were explained in terms of the substrate effect on phonon–phonon and electron–phonon interaction properties of the material. The results of the present study can be used to consider substrate effects on phonon transport in graphene for nanoelectronic device engineering.
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This is a pre-print of an article published in
Journal of Low Temperature Physics.
The final authenticated version is available online at:
https://doi.org/10.1007/s10909-017-1807-x
Effect of the Substrate on Phonon Properties of Graphene
Estimated by Raman Spectroscopy
M. S. Tivanov*1, E. A. Kolesov1, O. V. Korolik1, A. M. Saad2, I. V. Komissarov3
1 Belarusian State University, 4 Nezavisimosti Av., 220030 Minsk, Belarus
2 Al-Balqa Applied University, PO Box 4545, Amman 11953, Jordan
3 Belarusian State University of Informatics and Radioelectronics, 6 P. Brovka, 220013
Minsk, Belarus
* Corresponding author: E-mail tivanov@bsu.by, Phone: +375172095451, Fax:
+375172095445
Abstract
Low-temperature Raman studies of supported graphene are presented. Linear
temperature dependence of 2D peak linewidths was observed with the coefficients of 0.036
and 0.033 cm-1/K for graphene on copper and glass substrates, respectively, while G peak
linewidths remained unchanged throughout the whole temperature range. The different values
observed for graphene on glass and copper substrates were explained in terms of the substrate
effect on phonon-phonon and electron-phonon interaction properties of the material. The
results of the present study can be used to consider substrate effects on phonon transport in
graphene for nanoelectronic device engineering.
Keywords: graphene; temperature; phonon-phonon interaction; electron-phonon
interaction; substrate; Raman spectroscopy.
Introduction
Graphene is a promising material for a variety of applications due to its unique physical
properties [1]. Among them one can distinguish an unusually high thermal conductivity [2]. In
the context of present and future applications in nanoelectronic devices, reported graphene
thermal conductivity values of about 5300 W/mK [2] are of great interest to be achieved,
since the use of material with such properties to a large extent reduces the problem of heat
removal from functional elements of a nanoelectronic device.
As reported in the literature, thermal transport in graphene may be affected by
anharmonic phonon processes, as well as electron-phonon coupling (EPC) effects [3-5]. This
points to relevance of studying features of such processes, since they are directly connected to
possible undesirable suppression of graphene thermal conductivity.
At the same time, graphene layers on the substrates are needed for nanoelectronic
applications. The substrates, in turn, may affect graphene anharmonic phonon and EPC
properties, which leads to explicit necessity to achieve genuine understanding of such effects,
for them to be taken into account while a nanoelectronic device is designed.
Raman spectroscopy is a universal tool for nanostructure studies, and in terms of
graphene this method is quite powerful, providing the information on numerous features of
material properties [6, 7]. Particularly, the full width at half-maximum values for two most
typical graphene Raman peaks G which corresponds to a first-order Brillouin zone center
process [7] and 2D which includes second-order intervalley scattering [7] include phonon-
phonon and electron-phonon process contributions [8, 9]. Studying the behavior of the
mentioned values, especially in the low-temperature environment where several complex
contributions are either trivial or constant and others are linearized, gives the possibility to
shed the light on how these processes are related to each other.
The purpose of the present study is to conduct low-temperature Raman studies of
supported graphene on typical dielectric (glass) and metallic (copper) substrates in order to
investigate the substrate effect on the features of electron-phonon and phonon-phonon
interactions in graphene.
Methods
Experimental graphene was synthesized by atmospheric-pressure chemical vapor
deposition on copper foil. Prior to the synthesis, the substrate was electrochemically polished
in 1 M phosphoric acid solution for 5 min with operating voltage of 2.3 V. Synthesis was
performed in a tubular quartz reactor with a diameter of 14 mm. During the preliminary
treatment, copper foil was annealed at 1050 °C for 60 min under the following gas flow rates:
hydrogen 150 cc/min, nitrogen 100 cc/min. Synthesis was performed under the following
conditions: reactor temperature 1050 °C, C10H22 flow rate of 30 µL/min, N2 carrier flow rate
of 100 cc/min, synthesis time 10 min. After the hydrocarbon flow termination, the sample
was cooled down to room temperature at a rate of ~ 50 °C/min.
Graphene was transferred to glass by wet-chemical room-temperature etching without
polymer support in two steps. First, one side (the one that was by reactor wall) of copper foil
was treated for 3 min in a solution of H2NO3 and H2O mixed in a volume ratio of 1:3. Second,
the copper foil was totally dissolved in a water solution of FeCl3. Graphene film was washed
several times in a bath with distilled water prior to being placed onto glass.
Raman spectra were obtained with a confocal Raman spectrometer Nanofinder HE
(LOTIS TII) with a spectral resolution better than 3 cm-1 using a continuous solid-state laser
with a wavelength of 473 nm (power of 800 µW and laser spot diameter of 0.6 µm for room-
temperature measurements) [10]. During low-temperature Raman measurements, the sample
was studied in a vacuum (less than 5×10-4 Pa) temperature-controlled box using laser power
of 5.8 mW (laser spot diameter being of about 1.5 µm) [10]. The measurements were
performed in temperature range from 20 to 294 K.
Results and Discussion
Room-temperature Raman spectra for graphene on glass and copper substrates are
presented in Fig. 1. As seen, typical for graphene G, 2D and D peaks [6] are observed. The
presence of single-layer graphene was confirmed by 2D peak single Lorentz approximations,
as well as 2D peak linewidth values typical for monolayer [6, 7]. The ratio of peak intensities
I2D/IG was of about 1.4 and 1.6 in case of glass and copper substrates, respectively.
1200 1500 1800 2100 2400 2700 3000
FWHM = 18 cm-1
FWHM = 16 cm-1 FWHM = 33 cm-1
2D
Intensity, arb. un.
Raman Shift, cm-1
G
D
FWHM = 36 cm-1
Figure 1. Typical room-temperature Raman spectra for graphene on copper (bottom) and
glass (top) substrates.
Room-temperature G peak full widths at half-maximum (FWHMs) show the values of
about 16 and 18 cm-1 for graphene on copper and glass substrates, respectively. For 2D peak
FWHM, the corresponding values are of 33 and 36 cm-1. The differences may be attributed to
electron and phonon lifetime reduction due to scattering on defects created during graphene
transfer to a glass substrate [11].
As it was shown in [12], average defect density in the laser spot area nD can be
calculated using ratio of peak intensities ID/IG from the following expression:
94D
DL
G
I
n 7.3 10 E I
, (1)
where EL is the laser excitation energy.
Calculation gives nD values of 3.2·1010 and 4.4·1010 cm-2 for graphene on copper and
glass, respectively. Greater defect density value in case of a glass substrate supports the
consideration of defect-induced greater G and 2D peak FWHM values.
Figure 2 presents experimental dependencies of G and 2D peak FWHMs on
temperature. As seen, G peak FWHMs remain unchanged throughout the whole temperature
range for graphene on both copper and glass substrates with the values of 16±1 and 18±1 cm-
1, respectively. For 2D peak, linear temperature dependencies with coefficients of
(3.6±0.2)×10-2 and (3.3±0.2)×10-2 cm-1/K are observed for graphene on copper and glass
substrates, respectively. These coefficients are in a very good agreement with the reported
value of 3.1×10-2 cm-1/K for unsupported vertical graphene sheet [9], as well as with the
theoretical dependencies calculated in [8].
0 50 100 150 200 250 300
15
18
21
24
27
30
33
36
FWHM(G), cm-1 FWHM(2D), cm-1
Temperature (K)
Cu = 0.036 cm-1/K
Copper
0 50 100 150 200 250 300
15
18
21
24
27
30
33
36
gl = 0.033 cm-1/K
FWHM(G), cm-1 FWHM(2D), cm-1
Temperature, K
Glass
Figure 2. Experimental temperature dependencies of G (open symbols) and 2D (solid
symbols) Raman peak linewidths for graphene on glass and copper substrates, as well as
linear approximations (solid lines).
According to calculations presented in [8], electron-phonon coupling contribution to the
FWHM of G and 2D peaks can be considered constant in the low-temperature range (below
~ 300 K), with the values of
-1
(0) 9.1 cm
G
el ph
and
2 -1
(0) 22.0 cm
D
el ph
. The same is valid
for G peak linewidth variations due to anharmonic phonon-phonon interactions (
-1
(0) 7.2 cm
G
ph ph
). However, a linear change in 2D peak linewidth driven by the latter is
expected [8, 9]. Thus, temperature behavior of G and 2D peak FWHMs at temperatures below
300 K can be described by the following expressions:
, (2)
2 2 2 2
4
( ) C (0) (0)
D D D D
ph ph ph el ph
TT
, (3)
where
2
4(0)
Dph
is anharmonic 4-phonon process contribution to 2D peak FWHM at
temperatures close to absolute zero (the initial constant contribution of 3-phonon processes
can be considered negligible) [8].
As seen in Fig. 2, both G and 2D peak linewidth temperature dependencies fit the
formalized description (2) and (3) for graphene on both glass and copper substrates. However,
the constant intercept for 2D peak case which represents sum of 4-phonon and electron-
phonon terms
2
4(0)
Dph
and
2(0)
D
el ph
in (3) obtained from linear approximation takes
different values of 23±1 and 26±1 cm-1 for copper and glass substrates, respectively. This fact
can be explained by screening effect which takes place as metallic substrate electronic sub-
system applies electrostatic field [13, 14] that leads to renormalization of graphene density of
states [15], consequently resulting in suppression of
2D
el ph
term value.
For copper, greater 2D peak linewidth shift coefficient of 3.6×10-2 cm-1/K represented
by
2
CD
ph ph
in (3) indicates stronger influence of the substrate on anharmonic phonon
interactions in this case, as in [10]. The substrate possibly affects the phase space of
anharmonic phonon-phonon scattering [10], leading to change of its contribution to 2D peak
linewidth temperature behavior and its effect on graphene thermal conductivity, which is
known to be dominated by phonon-phonon interactions [16].
The estimated screening-induced suppression of electron-phonon term should also take
place in case of G peak FWHM temperature behavior for graphene on metallic substrate
second term in (2). At the same time, considering the substrate effect on phonon-phonon
interaction the corresponding
G
ph ph
term can simultaneously be increased, counterbalancing
the broadening and leading to G peak linewidth fluctuating around the theoretical value of
about 16 cm-1 given in (2). In case of glass substrate, both effects are expected to be less
pronounced [10, 15]; however, scattering on defects created during graphene transfer most
likely leads to greater overall FWHM(G) values in this case as it was mentioned earlier in the
text.
Conclusion
Low-temperature studies of supported graphene are presented. Linear temperature
dependence of 2D peak linewidths was observed with the coefficients of 0.036 and 0.033
cm-1/K for graphene on copper and glass substrates, respectively, while G peak linewidths
remained unchanged throughout the whole temperature range with the values of 16 and 18
cm-1, the values being in agreement with theoretical studies. In order to analyze the observed
behavior, anharmonic phonon-phonon scattering and electron-phonon interaction effects were
considered. The difference of 2D peak linewidth temperature dependence coefficients, as well
as G peak FWHM values observed for graphene on glass and copper substrates were
explained in terms of metallic substrate electronic sub-system screening effects and the
substrate effect on the phase space of anharmonic phonon-phonon scattering in graphene. The
results of the present study can be used to consider substrate effects on phonon transport in
graphene for nanoelectronic device engineering.
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