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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/m⋅K [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:

-1

( ) (0) (0) 16.3 cm

G G G

ph ph el ph

T const

, (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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