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Raman spectroscopy and Monte-Carlo simulation studies for supported graphene irradiated by 160 MeV Xe ions are presented. Changes in the density and dominating types of defects with increasing fluence were observed. In order to analyze contribution of defect formation mechanisms, in which the substrate is involved, a comparative study was performed for graphene on SiO2/Si, copper and glass substrates. The major defining mechanisms were found to be atomic recoils and formation of defects induced by hot electrons. For graphene on copper, the impact of substrate recoil atoms was found to be greater comparing to graphene on silicon oxide and glass, where the recoils participated approximately equally. Moreover, a possibility of defect formation in graphene due to hot electrons generated in the substrate near the interface was noted. Finally, a linear dependence of air-induced doping on D and G peak intensity ratio that represents defect density in graphene was found. The study is useful for solving the long-standing controversy on major mechanisms of defect formation in irradiated graphene, as well as for graphene-based nanoelectronic device engineering.
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Defect Formation in Supported Graphene Irradiated by Accelerated Xenon
Egor A. Kolesov1, Mikhail S. Tivanov1,*, Olga V. Korolik1, Pavel Yu. Apel2, 3, Vladimir A.
Skuratov2, 3, 4, Anis Saad5, Ivan V. Komissarov6
1 Belarusian State University, 4 Nezavisimosti Av., 220030 Minsk, Belarus
2 Joint Institute for Nuclear Research, Joliot-Curie 6, 141980 Dubna, Russia
3 Dubna State University, Dubna, Russia
4 National Research Nuclear University MEPhI, Moscow, Russia
5 Al-Balqa Applied University, PO Box 4545, Amman 11953, Jordan
6 Belarusian State University of Informatics and Radioelectronics, 6 P. Brovka, 220013 Minsk,
* Corresponding author: E-mail; Phone +375172095451; Fax +375172095445;
Address: 4 Nezavisimosti Av., 220030 Minsk, Belarus.
Raman spectroscopy and Monte-Carlo simulation studies for supported graphene irradiated
by 160 MeV Xe ions are presented. Changes in the density and dominating types of defects with
increasing fluence were observed. In order to analyze contribution of defect formation
mechanisms, in which the substrate is involved, a comparative study was performed for graphene
on SiO2/Si, copper and glass substrates. The major defining mechanisms were found to be
atomic recoils and formation of defects induced by hot electrons. For graphene on copper, the
impact of substrate recoil atoms was found to be greater comparing to graphene on silicon oxide
and glass, where the recoils participated approximately equally. Moreover, a possibility of defect
formation in graphene due to hot electrons generated in the substrate near the interface was
noted. Finally, a linear dependence of air-induced doping on D and G peak intensity ratio that
represents defect density in graphene was found. The study is useful for solving the long-
standing controversy on major mechanisms of defect formation in irradiated graphene, as well as
for graphene-based nanoelectronic device engineering.
Keywords: graphene; irradiation; swift heavy ions; interface; TRIM; adsorption.
Graphene is a promising material for a variety of applications due to its unique physical
properties [1]. At present, graphene is used in transparent electrodes, field effect transistors,
sensors and other applications. Engineering of several graphene-based nanoelectronic devices
such as biosensors, as well as graphene functionalization techniques, requires structural
modification of the material through a controlled defect introduction [2-4]. Swift heavy ions
(SHI) irradiation method is a versatile and convenient tool for this purpose [5]. It allows one to
control defect densities varying type, energy and fluence of ions. However, the mechanism of
defect formation in SHI-irradiated graphene still requires additional clarification.
It is known that the events of energy transfer to both target lattice nuclei (producing
recoils) and electron sub-system (producing hot electrons) participate in the ion-matter
interaction process, the latter giving non-negligible contribution to the defect formation in
graphene [6]. Moreover, the substrate was ambiguously reported to affect graphene stability
under the irradiation, leading either to creation of additional defects [7, 8] or to reduction of the
resultant defect yield [9].
Raman spectroscopy is a versatile tool for obtaining information on the structural
properties of various materials. This method is particularly useful for graphene studies due to
monoatomic thickness of the material [1]. Purpose of the present work is to study defect
formation processes in SHI-irradiated supported graphene using Raman spectroscopy and
Monte-Carlo TRIM simulations in order to understand the contributions of different defect
formation mechanisms.
Graphene was obtained using an 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 an operating voltage of 2.3 V. The CVD process was performed in a
tubular quartz reactor with a diameter of 14 mm using reagent grade chemicals. Copper foil was
pre-annealed at 1050 °C for 60 min under the following gas flow rates: H2 150 cc/min, N2
100 cc/min. The synthesis was performed under the following conditions: reactor temperature of
1050 °C, C10H22 (decane) flow rate of 4 μl/min, H2 flow rate of 60 cc/min, N2 carrier flow rate of
100 cc/min, and synthesis time of 30 min. After the hydrocarbon flow had been terminated, the
sample was cooled down to room temperature at a rate of ~ 50 °C/min.
Graphene was transferred to SiO2/Si (oxide thickness of 600 nm) and glass substrates by a
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 distilled water bath prior to being placed onto the substrate.
Graphene was irradiated by 160 MeV xenon ions with fluences of 108, 109 and 1011 cm-2 at
the IC100 cyclotron at the FLNR JINR in Dubna [10]. Ion beam homogeneity over the irradiated
sample 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 better than 3 cm-1 using a
Nanofinder HE (LOTIS TII) confocal Raman spectrometer. For excitation of Raman radiation, a
continuous 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 of about 0.6 µm.
The simulations were performed using binary collision Monte-Carlo approach
implemented in TRIM (the Transport of Ions in Matter) code [11]. TRIM group of programs
uses quantum-mechanical collision treatment considering screened Coulomb interaction between
an ion with effective charge and an atom, including exchange and correlation interactions for the
overlapping electron shells and creation of electron excitations and plasmons inside the target
[11]. In order to estimate the role of various defect formation mechanisms in the evolution of
graphene-substrate system under ion irradiation, the Monolayer Collision mode (for every
collision to be calculated without any approximations) with 100,000 incident 160 MeV xenon
ions was utilized for graphene monolayer simulated located on a substrate layer with the
thickness of 300 Å.
It is important to note that TRIM code treats target as an amorphous matrix with a
homogenous mass distribution (which is not generally applicable for nanostructures) and
calculates each collision impact regardless of collision density. Thus, TRIM simulation of
graphene irradiation cannot be used in order to obtain specific quantitative results. However,
TRIM code can still be used for statistical qualitative estimations [12-14]. Besides, supported
graphene can be considered a bulk system within the scale of several effects such as substrate
sputtering. Thus, we preferred TRIM code for the simulation since we were interested in a
statistical description of various defect formation mechanisms attributed to the substrate as a
bulk system participating in the process.
Results and discussion
Typical Raman spectra of pristine and SHI-irradiated graphene on SiO2/Si substrate are
presented in Fig. 1. As seen, D peak in pristine graphene spectrum has relatively low intensity
(ID/IG ~ 0.18), which corresponds to a small disorder of the sample structure [1, 15]. At the same
time, Raman spectra of irradiated graphene demonstrate increase in ID/IG intensity ratio of up to
0.5 (in several points up to 1.0) as the irradiation fluence changes from 108 to 1011 cm-2,
indicating an increase of defect concentration. Moreover, position shift of the second-order 2D
peak from 2687 to 2691 cm-1, the decrease of I2D/IG intensity ratio from 1.7 to 1.0, and the
increase of 2D peak width from 33 to 43 cm-1 are observed. These changes together with the
shift of G peak position from 1587 to 1595 cm-1 can be attributed to adsorption doping of air-
exposed defected graphene and are discussed later.
1200 1500 1800 2100 2400 2700
108 cm-2
109 cm-2
FWHM = 38 cm-1
FWHM = 35 cm-1
Intensity, arb. un.
Raman Shift, cm-1
FWHM = 33 cm-1
FWHM = 43 cm-1
1011 cm-2
ion fluence:
Figure 1. Typical Raman spectra of pristine and SHI-irradiated graphene on SiO2/Si substrate.
Irradiation fluence is given to the right of the plots.
In order to obtain information on defect distribution over the sample surfaces, Raman
mappings of 20×20 µm2 areas were obtained (Fig. 2). A uniform increase of ID/IG intensity ratios
(and therefore, the defect densities) is observed with increasing fluence. Raman map of graphene
irradiated with the fluence of 1011 cm-2 also demonstrates presence of high defect density
regions, possibly corresponding to a partial destruction of graphene lattice.
Figure 2. Raman maps (20×20 µm2) of ID/IG intensity ratio representing defect density distribution over
the sample surface for pristine and irradiated graphene on SiO2/Si substrate; scanning step of 1 µm.
Raman process leading to D peak arising in the spectrum implies relaxation of zero-
wavevector selection rule and includes events of elastic scattering on a defect and inelastic
scattering on A1g phonon at the Brillouin zone edge [15]; therefore, ID/IG ratio is directly related
to the defect density. This dependence includes regions of inverse proportionality (high density
of defects) and direct proportionality (low-density region) [15]. However, the former means the
disorder is large enough to affect G and 2D peak profiles [15], and that was not observed in the
experimental spectra. Thus, we assume that the performed irradiation introduced degree of
disorder specific for a low-density region, and therefore ID/IG is directly proportional to the
defect density, which is confirmed by the relationship between the experimental fluence and
ID/IG values. It is also important to note that since the ID/IG ratio does not depend on a defect
geometry [16], its values represent the average interdefect distance as well as corresponding
density for all Raman active defect sites that are involved in elastic scattering events.
As it was shown in [16], information on dominating defect type can be provided by ID/ID’
intensity ratio in Raman spectra of graphene. The average ID/ID’ values obtained from the maps
are presented in Table 1. Raman spectra of pristine graphene demonstrate <ID/ID’> values of
about 1.5, corresponding to grain boundaries as dominating defects [16]. At the same time,
Raman spectroscopy data for the irradiated graphene shows <ID/ID’> ~ 4.1 7.5. According
to dependencies presented in [16], these values indicate presence of both grain boundaries and
vacancies, with the role of vacancies increasing for greater fluences (Table 1).
Table 1. Peak intensity ratios calculated from the Raman spectra of pristine and irradiated graphene on
Irradiation fluence, cm-2
0 (pristine)
Fig. 3 demonstrates Raman maps of ID/ID’ intensity ratio representing the distribution of
different types of defects over the sample surface for pristine and irradiated graphene. As seen,
grain boundaries are mostly typical for pristine graphene, with several regions demonstrating
ID/ID’ values corresponding to sp3-bonding complexes. The map for graphene irradiated with 108
cm-2 fluence clearly shows the uniform increase of ID/ID’ ratio, indicating the formation of
irradiation-induced vacancies. As the fluence increases, further growth of vacancy density, as
well as formation of more sp3-bonding complexes are observed.
Figure 3. Raman maps (20×20 µm2) of ID/ID’ intensity ratio representing defect type distribution over the
sample surface for pristine and irradiated graphene on SiO2/Si substrate; scanning step of 1 µm. The types
of dominating defects corresponding to specific ID/ID’ values are indicated to the left of the color scale.
The observed ID/IG increase is stronger than expected considering literature data for various
ion types and energies, including data for suspended graphene [7, 17-19]. At the same time, it
was reported that defect yields for suspended and supported graphene can differ significantly
since the substrate can play an important role in defect formation process [7, 8, 17, 18, 20].
Particularly, the substrate might be involved through various different events such as energy
transfer from a recoiled substrate atom to graphene lattice. Moreover, one cannot exclude the
possibility of defect formation mechanism dependence on the crystallinity of the substrate
material. Thus, a correct analysis of the obtained results requires paying a close attention to
irradiation-induced substrate effects.
According to [17], the defect formation process in SHI-irradiated graphene includes
defects generated through an indirect process of substrate sputtering. In order to determine the
role of sputtered substrate atoms in graphene irradiation effects, comparison was carried out for
graphene irradiated with a fixed fluence of 108 cm-2 on copper (as-synthesized), SiO2/Si and
glass substrates. We performed TRIM simulations for graphene on each substrate in order to
obtain specific values of substrate sputtering yields, sputtered atom energy distributions, nuclear
and electronic stopping power (it should be noted that TRIM code is quantitatively applicable for
this case, since it is a bulk target being sputtered). Based on these values as well as defect yield /
nuclear stopping power dependencies from [17], we estimated approximate defect yields for
sputtered atoms. As an additional verification of structural modification trends obtained during
the simulation, Raman maps of pristine and irradiated graphene were scanned on each substrate,
whereupon average <ID/IG> and <ID/ID’> parameters were determined. The comparison of the
obtained values for graphene on various substrates is given in Table 2. Naturally, the number of
backscattered ions was negligibly small for all cases.
Table 2. Values calculated from Raman spectra of pristine and irradiated (fluence of 108 cm-2) graphene
on various substrates, as well as parameters obtained using TRIM simulations.
Pristine <ID/IG>
Irradiated <ID/IG>
Pristine <ID/ID’>
Irradiated <ID/ID’>
Nuclear stopping power Sn, keV/nm
Electronic stopping power Se, keV/nm
Sputtering yield Ys
Sputtered atom energies, keV
Estimated defect yields for sputtered atoms
Due to the fact that <ID/IG> value of 0.15 for pristine as-grown graphene is quite close to
those of 0.16 and 0.18 for graphene transferred to SiO2/Si and glass, we can confirm that the
transfer process introduced only a minor amount of defects into graphene lattice. It is seen that
<ID/IG> increase after graphene irradiation on SiO2/Si and glass has close values, while one for
copper is slightly greater. At the same time, the evolution of the defect system in graphene with
the irradiation has similar direction for all three substrates: from grain boundaries domination in
pristine graphene to both grain boundaries and vacancies in the irradiated material. However,
vacancy formation turned out to be the most discernible effect for graphene on silicon oxide,
according to utilized Raman dependencies from [16].
Due to a high energy of the incident ions, the energy transferred to the substrate nuclei for
all three materials is much smaller than that transferred to the electron sub-system, all the
sputtering yields being less than one (for copper, however, the sputtering yield is almost two
orders of magnitude greater). Performing simple estimations considering presented in Table 2
defect yields for sputtered atoms, sputtering yields and fluence, one can obtain the following
approximate total maximum defect concentrations created by sputtered substrate atoms during
the irradiation: 2.4·107 cm-2 for copper, 9.6·105 cm-2 for SiO2/Si and 9.0·105 cm-2 for glass. These
values are by several orders of magnitude smaller than the typical intrinsic defect density for a
pristine material; thus, greater obtained Ys value for copper still implies a weak participation in
the defect generation process. Thus, we do not consider substrate sputtering to play a major role
in the defect formation in graphene irradiated with 160 MeV xenon ions: substrate sputtering
yields have maxima at smaller incident ion energies (less than 1 MeV). Besides, other works
demonstrate that the sputtering damage of graphene itself effectively takes place for incident ion
energies as small as 20 eV [21], while still occurring most actively for irradiation by heavy ions
[18, 22]. Thus, using high-energy ions (more than ~ 30 MeV, according to our TRIM
simulations) for controlled defect induction in supported graphene can minimize the amount of
defects induced by substrate sputtering.
However, according to our simulation results, there is a non-negligible role of substrate
recoil atoms that do not leave the sample but become moved away from their positions at this
irradiation energy. Due to the energy transferred from the incident ions, such recoils can reach
graphene-substrate interface and participate in the defect formation. Fig. 4 demonstrates spatial
recoil distributions statistically obtained for graphene on copper, SiO2/Si and glass substrates as
another important output of TRIM simulation procedures. The greatest amount of recoils
reaching graphene-substrate interface region is observed for graphene on copper. For graphene
supported by SiO2/Si and glass, almost similar situation is observed, with the amount of oxygen
atoms reaching the interface being slightly greater; however, Na atoms also participate in the
second case.
Figure 4. Statistical depth recoil distributions for graphene irradiated by xenon ions on (a) copper, (b)
SiO2/Si and (c) glass substrates. For glass substrate, the distribution of other element atoms present in
glass (Ca, Mg, Al) is not shown due to its negligibly small rate.
According to our estimations, the approximate amount of displacements which can be
created by recoil atoms in graphene layer was 6·109 cm-2 for copper, 4·109 cm-2 for SiO2/Si and
5·109 cm-2 for glass substrate, these values being more realistic than those estimated for substrate
sputtering damage. Thus, we can consider recoil-dominated damage to be more intensive
mechanism of defect formation in SHI-irradiated supported graphene than the substrate
sputtering in our case.
Another reported mechanism to contribute to the defect formation in graphene irradiated
by swift heavy ions is an electronically-stimulated surface desorption [23]. Due to a high energy
of Xe ions used in our experiment, it can dissipate through an electronic excitation event, and
thus the incident ions (or recoil atoms) can be considered as the hot electron source (that mostly
relates to collisions with reduced direct kinetic energy transfer, i.e. non-central collisions with an
impact outside the target atom cross section) [23]. In turn, the hot electrons disrupt graphene
lattice, leading to the in-plane carbon bonds breakage [24]. According to our calculations, the
electronic stopping energy loss in graphene for the case of 160 MeV xenon ions is 17.2 keV/nm,
with this value being quite enough for the surface atoms to desorb. It should be noted that the
keV/nm units are used here for a comparative purpose only, as in [17]; the physical meaning of
nanometers in the denominator is not defined for graphene and requires adaptation to be
correctly used both for three- and two-dimensional targets.
Naturally, the substrate does not affect the amount of energy lost for ionization in graphene
within the utilized approach; however, according to our calculations, electronic stopping energy
loss for 160 MeV xenon ions in copper is 36.3 keV/nm (it can be considered quantitative since
the substrate is a bulk target). A large value of this parameter suggests an idea that the
contribution to the defect formation in graphene is possible from substrate hot electrons
produced near the interface. Due to silicon oxide giving the greatest contribution to electronic
stopping in glass, the ionization energy loss for SiO2/Si and glass has almost similar values: 14.5
and 14.6 keV/nm, respectively. Energy lost for ionization by recoil atoms turned out to be small
(less than 2 eV), as well as the amount of recoil-induced ionization events for all three cases.
It should be noted that the simulation performed did not take into account the fact that
graphene on copper is as-grown, but it nevertheless agreed with the supporting Raman
experiments qualitatively well for both pristine and irradiated graphene, leading to a conclusion
that substrate-induced irradiation effects in case of a relatively strong graphene-metal interaction
manifest themselves much stronger than the residual synthesis effects such as chemical bonds at
the interface.
Finally, considering the possibility of doping for air-exposed graphene, we analyzed
indicative in this case Raman spectra parameters (G and 2D peak positions, I(2D)/I(G) ratio [25])
for graphene on SiO2/Si substrate. As it was seen in Fig. 1, the evolution of these parameters is
observed as the fluence increases, suggesting the increasing hole doping for greater defect
density values [25]. Summarizing corresponding obtained values for each spectrum in a batch
processing sequence, we obtained fluence-dependent average values of hole concentration
presented in Fig. 5. A near-linear dependence on <ID/IG> is observed in this case, demonstrating
that ‘SHI irradiation + functionalization’ sequence can be considered a comparable alternative to
the low-energy ion beam implantation [26] for graphene doping. Peak ratio value corresponding
to the initial defect density of pristine graphene which is not determined by the irradiation effects
still fits into the obtained line well. This fact suggests that for graphene functionalization
methods based on a defect induction through SHI irradiation, a small but present initial amount
of defects represented by grain boundaries or vacancies does not strongly affect dependencies
needed for controllability of the process. Presented observation can be taken into account in
order to simplify graphene-based nanoelectronic device engineering process.
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Hole concentration ( 1013 cm-2)
Fluence (cm-2)
0 108 109 1011
Figure 5. Hole concentration dependence on <ID/IG> ratio representing the average sample defect density
(fluence also indicated) for graphene on SiO2/Si irradiated with 160 MeV Xe ions. Squares represent
experimental data obtained from Raman spectra and solid line shows a linear fit.
A systematic Raman study, as well as corresponding Monte-Carlo simulations for
supported graphene irradiated by swift xenon ions (energy of 160 MeV) were performed. As the
fluence increased, changes in the defect density and dominating defect types were observed. In
order to analyze the contributions of defect formation mechanisms in which the substrate is
involved, a comparative study was performed for graphene on SiO2/Si, copper and glass
substrates. The contribution of sputtered substrate atoms was small for all cases, while the major
defining mechanisms were atomic recoils reaching graphene-substrate interface and
electronically-stimulated surface desorption. A role of substrate recoils reaching the interface
was found to be greater for graphene on copper, while for SiO2/Si and glass substrates atom
recoils participated approximately equally. Moreover, a possibility of defect formation in
graphene due to substrate hot electrons, generated near the interface, was noted. Considering the
possibility of doping for the air-exposed graphene, a linear hole concentration dependence on the
<ID/IG> was found, with the initial defectiveness point (not defined by the irradiation effects) still
fitting into the obtained dependence well. The present study provides further insight into the
contributions of different defect formation mechanisms in irradiated graphene. The obtained
results are useful for graphene-based nanoelectronic device engineering, including devices that
require controlled graphene functionalization as well as controlled introduction of a known
amount of disorder into graphene structure.
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... A great deal of interest for Graphene applications is found in the area of solar cells, LEDs, electronic skin, touchscreens, micro-/nano-electronics, energy storage devices and super-capacitors [2,3]. In graphene application the patterning and controlled creation of defects in its structure play a crucial role and a well-controlled method for defect creation can provide new functionalities to graphene [1,4,5]. Defects in graphene are defined as a breaking of symmetries of the carbon honeycomb lattice and it can be edges, grain boundaries, vacancies, implanted atoms, and defects that are associated to a change of sp 2 to sp 3 carbon-hybridization [6]. ...
... Defects in graphene are defined as a breaking of symmetries of the carbon honeycomb lattice and it can be edges, grain boundaries, vacancies, implanted atoms, and defects that are associated to a change of sp 2 to sp 3 carbon-hybridization [6]. One form of sp 3 defect creation is fluorination or oxidation of the graphene structure; the vacancy-like defects can be provided using electron or ion energetic beams [1,4,6,7]. ...
... It has been referred [4,20,21], that the substrate have significant influence on the defect formation during supported graphene irradiation that is connected with indirect processes caused by sputtering and recoiling of substrate atoms. Moreover, the displacement energy (minimum energy, which atom must obtain during collision in order to be displaced from the lattice position) in graphene on substrate depends on the direction of atom knock-outing; in direction towards the SiO 2 substrate is much higher (> 60 eV) compare to carbon atoms knocked out into the free space (~22 eV) [15,[21][22][23]. ...
In this work, a graphene monolayer deposited on the SiO2/Si substrate by means of Chemical Vapour Deposition (CVD) was irradiated using He and Au ions. The energy of used ions was 1.8 MeV and ion fluences varied in the range from 1.0 × 10¹³ cm⁻² to 1.0 × 10¹⁵ cm⁻². The different mass of Au and He ions leads to the significantly different electronic and nuclear stopping ratios and thus a difference in ion beam induced defects, of which the nature, size and density were estimated from the evolution of relative intensities of Raman lines. The chemical composition with possible oxidation of graphene layer and structural changes of the irradiated graphene were characterized by Attenuated Total Reflection - Fourier Transform Infrared (ATR-FTIR). The surface conductivity was measured by using standard two point method. The used ion irradiation leads to defect production in graphene structures whose density increases with increasing ion fluence and is significantly more pronounced for heavier gold ions. The graphene modification causes the slight conductivity decrease in the case of He irradiation in accordance with the observed structural degradation and significant decrease of graphene conductivity after Au irradiation.
... It should be noted that within the framework of this study, the TRIM code was used exclusively to consider the irradiation effects associated with the substrate, in accordance with [3,5,8,23]. Calculation of defect formation directly in two-dimensional materials, including sputtering, is not possible using this software [4] due to the fact that the target is modeled as an amorphous matrix with a uniform mass distribution, and the effect of collisions is taken into account regardless of their density. ...
In this paper, we study structural and adsorption properties of graphene irradiated with 46 MeV Ar ions and 240 keV H ions on SiO2/Si and copper substrates by micro-Raman spectroscopy. Graphene irradiated with H ions demonstrated evidence of both high and low defect density regions on a sub-micron scale. TRIM calculations showed that substrate was the dominant defect source with a contribution from about 55% for H ions in graphene on SiO2/Si to 90% for Ar in graphene on SiO2/Si. Charge carrier density analysis showed p-type adsorption doping saturating at (0.48 ± 0.08) × 10¹³ cm⁻² or (0.45 ± 0.09) × 10¹³ cm⁻² with a defect density of 1.5 × 10¹¹ cm⁻² or 1.2 × 10¹¹ cm⁻² for graphene on SiO2/Si or copper, respectively; this was analyzed in the framework of physisorption and dissociative chemisorption. This study is useful towards the development of functionalization methods, molecular sensor design, and any graphene application requiring modification of this material by controlled defect introduction.
Understanding and facilitating defects in two-dimensional transition metal dichalcogenides (TMDCs) are of fundamental importance for their application in optoelectronic devices and valleytronic devices. In this study, swift heavy ion (SHI) irradiation was applied to introduce defects in monolayer WSe 2 in a controlled manner. Temperature-dependent photoluminescence and transient absorption spectroscopy are employed to investigate the excitonic performances in defective WSe 2 . It is observed that the trion emission rises up alongside exciton emission for WSe 2 irradiated with elevated ion fluences. Defects introduced by SHI irradiation can strongly localize carriers and weaken the exciton–phonon coupling and further affect the optical signatures of the excitons. Photoexcited electron–hole pairs were suppressed to form excitons due to the weaken phonon scattering, and the population of exciton was reduced for the irradiated WSe 2 . These results reveal that SHI irradiation is an effective technique to explore defect dependence of exciton formation and evolution dynamics in TMDCs, which have important implications for various optoelectronic applications.
Transition metal dichalcogenides (TMDCs) have attracted immense interest of the scientistific community due to their unique physico-chemical characteristics and applications in optoelectronic devices. Interface between TMDCs and the substrate plays a leading role in the performance and reliability of the devices. Swift heavy ion (SHI) irradiation is used as a most valuable tool for altering material's physical, chemical, structural, surface and interface properties in a controlled manner. In this work, characteristics of SHI irradiation caused defects in MoSe2 sheets on different substrates were presented to study the role of the substrate on defect engineering of MoSe2. Morphologies of the SHI induced latent tracks on MoSe2 were detected by atomic force microscopy (AFM), which suggest that substrate plays a dominate role on determining the shape of the latent tracks on 2D TMDCs. Cross-section morphology of SHI irradiated MoSe2 on different substrates was examined by high resolution transmission electron microscopy (TEM). The lattice mismatch between MoSe2 sheets and the substrate leads to different depths of the atomic mixing at their interface. Dependence of vibrational modes of MoSe2 nanosheets on the substrates was studied by Raman spectroscopy and a counterbalance effect among the electron doping, charge localization and decoupling effect is expected to determining the shift of A1g mode in SHI irradiated MoSe2 sheets. Electrical properties degradation of the MoSe2 field effect transistor (FET) confirms the SHI irradiation that cause charge localization. This study provides fundamental insights in understanding the influence of the substrate on defect engineering in TMDC materials and a practical guide on choosing the conditions to obtain certain parameters of irradiated TMDC materials.
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This study is dedicated to the common problem of how to choose a suitable substrate for ion irradiation of two-dimensional materials in order to achieve specific roles of certain defect formation mechanisms. The estimations include Monte Carlo simulations for He, Ar, Xe, C, N and Si ions, performed in the incident ion energy range from 100 eV to 250 MeV. Cu, SiO2, SiC and Al2O3 substrates were analyzed. The considered substrate-related defect formation mechanisms are sputtering, recoil atoms reaching the interface with a non-zero energy, and generation of hot electrons in close proximity of the interface. Additionally, the implantation of sputtered substrate atoms into the 2D material lattice is analyzed. This work is useful both for fundamental studies of irradiation of two-dimensional materials and as a practical guide on choosing the conditions necessary to obtain certain parameters of irradiated materials.
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This letter reports on a systematic investigation of sputter induced damage in graphene caused by low energy Ar+ ion bombardment. The integral numbers of ions per area (dose) as well as their energies are varied in the range of a few eV’s up to 200 eV. The defects in the graphene are correlated to the dose/energy and different mechanisms for the defect formation are presented. The energetic bombardment associated with the conventional sputter deposition process is typically in the investigated energy range. However, during sputter deposition on graphene, the energetic particle bombardment potentially disrupts the crystallinity and consequently deteriorates its properties. One purpose with the present study is therefore to demonstrate the limits and possibilities with sputter deposition of thin films on graphene and to identify energy levels necessary to obtain defect free graphene during the sputter deposition process. Another purpose is to disclose the fundamental mechanisms responsible for defect formation in graphene for the studied energy range.
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This paper presents a theoretical approach for the quantitative depth evaluation of linear opened surface cracks by using lock-in infrared thermography. In order to simulate heat flow near a crack, a three-dimensional simulation model has been developed by using finite element simulation. We show that, under a periodic local thermal excitation in the vicinity of the crack, the second spatial derivative of the amplitude image can provide information on this depth. The influence of the simulation parameters are discussed for the optimal characterization of defects. Keywords: Infrared lock-in thermography, finite element method, image processing, crack assessment (Some figures may appear in colour only in the online journal)
The ferroelectric phase transition in a small particle depends on its size. Analytical results concerning the size dependence of the transition temperature, and the polarization profile in the ferroelectric phase have been obtained within a phenomenological model, which has been previously studied only numerically. The model does not take into consideration the depolarization field, assuming full compensation of surface charges. The dynamic susceptibility deviates from Debye-like behaviour in exhibiting a broadening at higher frequencies. The static susceptibility obeys the Curie-Weiss law, and exhibits a similar divergency at the point of the size-driven transition.
Despite the frequent use of noble gas ion irradiation of graphene, the atomistic scale details, including the effects of dose, energy, and ion bombardment species on defect formation, and the associated dynamic processes involved in the irradiations and subsequent relaxation, have not yet been thoroughly studied. Here, we simulated the irradiation of graphene with noble gas ions, and the subsequent effects of annealing. Lattice defects, including nanopores, were generated after the annealing of the irradiated graphene, which was the result of structural relaxation that allowed the vacancy-type defects to coalesce into a larger defect. Larger nanopores were generated by irradiation with a series of heavier noble gas ions, due to a larger collision cross-section that led to more detrimental effects in the graphene, and by a higher ion dose that increased the chance of displacing the carbon atoms from graphene. Overall trends in the evolution of defects with respect to a dose, as well as the defect characteristics, were in good agreement with experimental results. Additionally, the statistics in the defect types generated by different irradiating ions suggested that the most frequently observed defect types were Stone-Thrower-Wales (STW) defects for He+ irradiation and mono-vacancy (MV) defects for all other ion irradiations.
Highly oriented pyrolytic graphite (HOPG) and monolayer graphene were irradiated by swift heavy ions (SHI, 479 MeV 86Kr and 250 MeV 112Sn) and highly charged ions (HCI, 4 MeV 86Kr19+). The irradiation effects caused by different types of irradiation were investigated by Raman spectroscopy. It was found that the intensity ratio of D peak to G peak (ID/IG) in the case of HCI was higher than that of SHI for the same ion fluence in HOPG. The larger ID/IG indicates that synergistic effects of kinetic and potential energies of medium energy HCI has to be considered during the energy deposition process. A turning point was detected during the evolution process of ID/IG with fluence obtained from SHI and HCI impacted graphene, while such turning point was absent in the case of HOPG. The Lucchese's phenomenological model was modified and the experimental data of ID/IG vs. fluence for HOPG and graphene was completely following the modified model. According to this model, energetic ions induced both structurally disordered and activated regions in graphene. The competing mechanism of these two regions resulted in three different trends of the ID/IG variation in the case of graphene whereas in HOPG, such mechanism was not observed.
We investigate the structural, electronic and transport properties of substitutional defects in SiC-graphene by means of scanning tunneling microscopy and magnetotransport experiments. Using ion incorporation via ultra-low energy ion implantation the influence of different ion species (boron, nitrogen and carbon) can directly be compared. While boron and nitrogen atoms lead to an effective doping of the graphene sheet and allow to reduce or raise the position of the Fermi level respectively, (12)C(+) carbon ions are used to study possible defect creation by the bombardment. For low temperature transport the implantation leads to an increase in resistance and a decrease in mobility in contrast to undoped samples. For undoped samples we observe in high magnetic fields a positive magnetoresistance that changes to negative for the doped samples, especially for (11)B(+)- and (12)C(+)-ions. We conclude that the conductivity of the graphene sheet is lowered by impurity atoms and especially by lattice defects, since they result in weak localization effects at low temperatures.
Although ion beam technology has frequently been used for introducing defects in graphene, the associated key mechanism of the defect formation under ion irradiation is still largely unclear. We report a systematic study of the ion irradiation experiments on SiO2-supported graphene, and quantitatively compare the experimental results with molecular dynamic simulations. We find that the substrate is, in fact, of great importance in the defect formation process, as the defects in graphene are mostly generated through an indirect process by the sputtered atoms from the substrate.
In this article we review Raman studies of defects and dopants in graphene as well as the importance of both for device applications. First a brief overview of Raman spectroscopy of graphene is presented. In the following section we discuss the Raman characterization of three defect types: point defects, edges, and grain boundaries. The next section reviews the dependence of the Raman spectrum on dopants and highlights several common doping techniques. In the final section, several device applications are discussed which exploit doping and defects in graphene. Generally defects degrade the figures of merit for devices, such as carrier mobility and conductivity, whereas doping provides a means to tune the carrier concentration in graphene thereby enabling the engineering of novel material systems. Accurately measuring both the defect density and doping is critical and Raman spectroscopy provides a powerful tool to accomplish this task.
Monolayer graphene and highly oriented pyrolytic graphite (HOPG) were irradiated by swift heavy ions (209Bi and 112Sn) with the fluence between 1011 and 1014 ions/cm2. Both pristine and irradiated samples were investigated by Raman spectroscopy. It was found that D and D′ peaks appear after irradiation, which indicated the ion irradiation introduced damage both in the graphene and graphite lattice. Due to the special single atomic layer structure of graphene, the irradiation fluence threshold Φth of the D band of graphene is significantly lower (<1 × 1011 ions/cm2) than that (2.5 × 1012 ions/cm2) of HOPG. The larger defect density in graphene than in HOPG indicates that the monolayer graphene is much easier to be damaged than bulk graphite by swift heavy ions. Moreover, different defect types in graphene and HOPG were detected by the different values of ID/ID′. For the irradiation with the same electronic energy loss, the velocity effect was found in HOPG. However, in this experiment, the velocity effect was not observed in graphene samples irradiated by swift heavy ions.
We construct a model to obtain the density of point defects in N-layer graphene by combining Raman spectroscopy and the TRIM (Transport Range of Ions in Matter) simulation package. The model relates the intensity (or area) ratio of graphene’s D and G bands to the defect density on each layer due to Ar+ bombardment. Our method is effective for ion fluences ranging from 1011 to ∼1014 Ar+/cm−2 and it should be in principle extendable to any kind of ion and energy.