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Abstract

CVD-graphene on silicon was irradiated by accelerated heavy ions (Xe, 160 MeV, fluence of 1011 cm-2) and characterized by Raman spectroscopy. The defectiveness of pristine graphene was found to be dominated by grain boundaries while after irradiation it was determined by both grain boundaries and vacancies. Respectively, average inter-defect distance decreased from ~ 24 to ~ 13 nm. Calculations showed that the ion irradiation resulted in a decrease in charge carrier mobility from ~ 4.0 × 103 to ~ 1.3·103 cm2/V s. The results of the present study can be used to control graphene structure, especially vacancies concentration, and charge carrier mobility.
JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ
Vol. 9 No 3, 03020(4pp) (2017) Том 9 3, 03020(4cc) (2017)
2077-6772/2017/9(3)03020(4) 03020-1 2017 Sumy State University
Raman Study of CVD Graphene Irradiated by Swift Heavy Ions
E.A. Kolesov1, M.S. Tivanov1,
*
, O.V. Korolik1, P. Yu. Apel2,3, V.A. Skuratov2,3,4, A.M. Saad5,
I.V. Komissarov6, A. Swic7, P.V. Żukowski8, T.N. Koltunowicz8,
1 Belarusian State University, 4, Nezavisimosti Ave., 220030 Minsk, Belarus
2 Joint Institute for Nuclear Research, 6, Joliot-Curie, 141980 Dubna, Russia
3 Dubna State University, 141980 Dubna, Russia
4 National Research Nuclear University MEPhI, Moscow, Russia
5 Al-Balqa Applied University, PO Box 4545, 11953 Amman, Jordan
6 Belarusian State University of Informatics and Radioelectronics, 6, P. Brovka Str., 220013 Minsk, Belarus
7 Institute of Technological Systems of Information, Lublin University of Technology, 20-618 Lublin, Poland
8 Department of Electrical Devices and High Voltage Technology, Lublin University of Technology, 20-618 Lublin,
Poland
(Received 28 Febriary 2017; revised manuscript received 19 March 2017; published online 30 June 2017)
CVD-graphene on silicon was irradiated by accelerated heavy ions (Xe, 160 MeV, fluence of 1011 cm-2)
and characterized by Raman spectroscopy. The defectiveness of pristine graphene was found to be domi-
nated by grain boundaries while after irradiation it was determined by both grain boundaries and vacan-
cies. Respectively, average inter-defect distance decreased from ~ 24 to ~ 13 nm. Calculations showed that
the ion irradiation resulted in a decrease in charge carrier mobility from ~ 4.0 × 103 to ~ 1.3·103 cm2/V s.
The results of the present study can be used to control graphene structure, especially vacancies concen-
tration, and charge carrier mobility.
Keywords: Irradiated graphene, Defects, Raman spectroscopy, CVD, Swift heavy ions.
DOI: 10.21272/jnep.9(3).03020
PACS numbers: 81.05.ue, 61.48.Gh,
78.30.Ly, 61.80. x
*
tivanov@bsu.by
t.koltunowicz@pollub.pl
1. INTRODUCTION
Graphene is a promising material for a variety of
applications. It is a two-dimensional sp2-carbon
allotrope [1]. The importance of graphene studies is
driven by its unique physical properties: optical
transparency, high values of rigidity, thermal and
electrical conductivity [2]. At present, graphene is used
in transparent electrodes, field effect transistors,
biosensors, etc.
For several applications, graphene structure
modification using controlled defect introduction is
needed [3]. Swift heavy ions (SHI) irradiation method
is a versatile and convenient tool for this purpose [4]. It
allows one to create defects using a wide ion energy
range, different fluences for obtaining different defect
densities, and various ion types. However, the
mechanism of defect formation in SHI-irradiated
graphene is still unclear.
Raman spectroscopy is a versatile tool for obtaining
information on structural properties of various
materials. This method is particularly useful in the
context of graphene studies, due to monoatomic
thickness of the material [1, 2]. As shown in [5], it is
possible to determine the average inter-defect distance
in laser spot area for graphene from D and G peaks
maximum intensity ratio. Moreover, the correlation
presented in [6] allows one to estimate graphene charge
carrier mobility from the full width at half-maximum
(FWHM) of 2D peak.
Purpose of the present work is to study structural
properties of SHI-irradiated graphene using Raman
spectroscopy in order to understand how defect-caused
structural changes influence graphene physical
properties.
2. EXPERIMENTAL
Graphene studied in this paper was obtained by
atmospheric pressure chemical vapor deposition (CVD).
Prior to the synthesis, copper 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. Copper foil was pre-
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 4 μl/min, H2 flow rate 60 cc/min, N2 carrier flow
rate 100 cc/min, synthesis time 30 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 Si/SiO2 substrate by
wet-chemical room-temperature etching without
polymer support in two steps. First, one side of copper
foil was treated for 3 min in a solution of H2NO3 and
H2O mixed in a volume ratio of 1:3. Then 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 the substrate.
Graphene was irradiated by 160 MeV xenon ions
with total fluence of 1011 cm 2 at the IC100 cyclotron
at the FLNR JINR in Dubna [7]. Ion beam homogeneity
E.A. KOLESOV, M.S. TIVANOV, O.V. KOROLIK ET AL. J. NANO- ELECTRON. PHYS. 9, 03020 (2017)
03020-2
over the irradiated specimen surface was controlled
using beam scanning in the horizontal and vertical
directions and was better than 5 %.
Raman spectra were obtained with a spectral
resolution not less than 3 cm 1 using a confocal Raman
spectrometer Nanofinder HE (LOTIS TII). For
excitation of Raman radiation, a continuous wave solid-
state laser with a wavelength of 473 nm was used.
Room-temperature Raman measurements were carried
out using laser power of 800 µW, the diameter of laser
spot on the sample surface being about 0.6 µm.
3. RESULTS AND DISCUSSION
Typical Raman spectra for graphene before and
after SHI irradiation are presented in Fig. 1. The
presence of single-layer graphene in both cases is
indicated by I2D/IG peak intensity ratios together with
2D peak FWHMs and single Lorentzian
approximations (insets in Fig. 1) [2, 8].
1200 1500 1800 2100 2400 2700
D'
2500 2600 2700 2800 2900
Intensity, arb. un.
Raman Shift, cm-1
FWHM = 44 cm-1
2500 2600 2700 2800 2900
Intensity, arb. un.
Raman Shift, cm-1
FWHM = 38 cm-1
Intensity, arb. un.
Raman Shift, cm-1
D
G2D
Fig. 1 Typical Raman spectra for graphene before (top) and after (bottom) ion irradiation. Insets: 2D peak approximations with
single Lorentz functions
As seen from the figure, D peak in pristine
graphene spectrum has relatively low intensity
(ID/IG ~ 0.3), which corresponds to a low defectiveness
of the sample structure [1, 2]. A different situation is
observed for irradiated graphene: typical Raman
spectra in this case demonstrate ID/IG intensity ratios
not less than ~ 0.5. However, in several areas ID/IG
values up to of ~ 1.0 are observed. Based on this fact,
we can conclude that graphene defectiveness increased
after irradiation.
According to [5], the average inter-defect distance in
the laser spot area LD can be calculated from the ratio
of D and G peak maximum intensities using the
following expression:
1
3
22
44
4.3 10 ( )
() ()
()
D
L
ID
L nm
IG
E eV



, (3.1)
where EL is an excitation energy.
It is important to note that since the ID/IG ratio does
not depend on a defect geometry [9], LD values
calculated from the formula represent the average
inter-defect distance for all Raman active defects that
are involved in elastic scattering (since the process
leading to D peak arising includes inelastic electron-
phonon scattering and elastic electron-defect scattering
events [2]).
The calculation results were averaged over all
obtained spectra (400 spectra for each sample). For
pristine graphene, the obtained inter-defect distance
value was ~ 24 nm. The calculation has shown that
after the irradiation the LD value decreased to ~ 13 nm,
signifying the increased number of defects. At the same
time, calculated average distance between the ions
fallen onto graphene surface is ~ 16 nm.
In order to obtain detailed information on defect
distribution over graphene surface before and after the
irradiation, Raman mappings across the surface were
performed. Fig. 2 presents LD maps for graphene both
before and after the irradiation.
As seen from Fig. 2, the distribution of LD is regular
over all scanned area both for pristine and irradiated
graphene. The inter-defect distance uniformly
decreased (and therefore the defect density increased)
after the irradiation.
In [9] it was shown that graphene defect type can be
obtained from ID/ID intensity ratio. D peak for
graphene is usually partially overlapped with G peak
leading to combined asymmetric shape, as can be
observed in Fig. 1, but its intensity can be easily
obtained by simple deconvolution. Typical spectra of
pristine graphene demonstrate ID/ID values of 1.0 - 3.5
corresponding to grain boundaries. At the same time,
irradiated graphene spectroscopy data shows
ID/ID ~ 3.9 - 5.7. According to presented in [9]
dependencies, these values indicate presence of both
grain boundaries and vacancies.
Fig. 3 shows Raman maps of the ID/ID intensity
ratio giving the distribution of defect types over the
scanned area.
RAMAN STUDY OF CVD GRAPHENE IRRADIATED BY SWIFT HEAVY IONS J. NANO- ELECTRON. PHYS. 9, 03020 (2017)
03020-3
Fig. 2 Raman maps (20 × 20 m) of the average inter-defect
distance for pristine (top) and irradiated (bottom) graphene;
scanning step of 1 m
It is seen from the figure that scanned pristine
graphene area is dominated by grain boundaries. In
turn, irradiated graphene map contains a large number
of vacancies besides the boundaries. Moreover, Raman
maps of LD and ID/ID for irradiated graphene show
a very strong correlation, giving almost identical values
distribution. This fact indicates that nearly all defects
in this case are represented by vacancies induced
during the irradiation.
According to [6], charge carrier mobility μ in
graphene can be determined from 2D peak full width at
half-maximum (FWHM). The authors of [6] used
combined Hall effect measurements and Raman
spectroscopy. This method resulted in finding strong
correlation between Hall mobility and 2D peak FWHM.
Determination performed according to dependencies
described in [6] gave the value of charge carrier
mobility in pristine graphene averaged over all 400
scanned spectra
pr ~ 4.0·103 cm2/V s. For irradiated
graphene, the average μirr was about 1.3·103 cm2/V s.
The charge carrier mobility decrease after the
irradiation can be explained by intensifying of charge
carriers scattering by defects induced.
4. CONCLUSION
Raman studies of CVD-graphene on silicon substrate
Fig. 3 Raman maps (20 × 20 m) of the ID/ID intensity ratio
for pristine (top) and irradiated (bottom) graphene; scanning
step of 1 m. Types of defects corresponding to specific ID/ID
values are indicated under the scale
substrate irradiated by accelerated heavy ions (Xe,
160 Me V, fluence 1011 cm 2) have shown the uniform
distribution of defect types and inter-defect distances
over graphene surface. The defectiveness of pristine
graphene was dominated by grain boundaries while
after irradiation it became determined by both grain
boundaries and vacancies. The average inter-defect
distance was found to decrease from ~ 24 to ~ 13 nm.
Charge carrier mobilities in graphene before and after
the ion irradiation were calculated. It was found that
the irradiation resulted in a decrease of charge carrier
mobility from ~ 4.0·103 to ~ 1.3·103 cm2/V s. The results
of the present study can be used to control graphene
structure, especially vacancies concentration, and
charge carrier mobility.
AKNOWLEDGEMENTS
This work was partly supported by the statute tasks
of the Lublin University of Technology, at the Faculty
of Electrical Engineering and Computer Science,
(S-28/E/2017), entitled “Researches of electrical,
magnetic, thermal and mechanical properties of
modern electrotechnical and electronic materials,
E.A. KOLESOV, M.S. TIVANOV, O.V. KOROLIK ET AL. J. NANO- ELECTRON. PHYS. 9, 03020 (2017)
03020-4
including nanomaterials and electrical devices and
their components, in order to determination of
suitability for use in electrical engineering and to
increase the efficiency of energy management”.
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