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1 Exploring the effect of varying regime of ion fluence on optical and surface electronic properties of CVD Grown Graphene. Tanmay Mahanta 1 , Sanjeev Kumar 1 , Tanuja Mohanty 1, a) 1 School of Physical Sciences, Jawaharlal Nehru University, New Delhi - 110067, INDIA. a) Corresponding author: [email protected] Abstract The fact that the optoelectronic properties of nanomaterials can be manipulated by defect engineering for specific application has enabled graphene to outperform the conventional electronics. In this work, the influence of ion fluence dependent defect formation pathway on the modification of surface electronic properties of graphene has been investigated. The chemical vapor deposited (CVD) graphene was irradiated with swift heavy ion (SHI) at different fluence to study the defect formation mechanism and their role in modulation of its work function. The change in the morphological, optical and vibrational properties in the graphene are analyzed by atomic force microscopy (AFM), UV-Visible spectroscopy (UV-Vis) and Raman spectroscopy. The effect of different regime of ion dose on modification of surface electronic properties of graphene is investigated through scanning Kelvin microscope (SKPM). Ion fluence controlled defect formation channels are identified and their effects on the work function of graphene sheets are analyzed. It is observed that the lower fluence of SHI favors the doping effects while strain effects are dominant at the higher fluence. This preferential behavior of surface electronic properties of graphene can be explained in terms of variability of the defect production processes determined by the ion-beam fluence. Keywords: CVD Graphene; Ion beam irradiation; Raman spectroscopy; UV-Vis spectroscopy; Scanning Kelvin Probe Microscopy.

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  • 1

    Exploring the effect of varying regime of ion fluence on optical and surface electronic properties of CVD Grown Graphene.

    Tanmay Mahanta1, Sanjeev Kumar1, Tanuja Mohanty1, a)

    1School of Physical Sciences, Jawaharlal Nehru University, New Delhi - 110067, INDIA.

    a)Corresponding author: [email protected]

    Abstract

    The fact that the optoelectronic properties of nanomaterials can be manipulated by defect

    engineering for specific application has enabled graphene to outperform the conventional

    electronics. In this work, the influence of ion fluence dependent defect formation pathway on the

    modification of surface electronic properties of graphene has been investigated. The chemical

    vapor deposited (CVD) graphene was irradiated with swift heavy ion (SHI) at different fluence

    to study the defect formation mechanism and their role in modulation of its work function. The

    change in the morphological, optical and vibrational properties in the graphene are analyzed by

    atomic force microscopy (AFM), UV-Visible spectroscopy (UV-Vis) and Raman spectroscopy.

    The effect of different regime of ion dose on modification of surface electronic properties of

    graphene is investigated through scanning Kelvin microscope (SKPM). Ion fluence controlled

    defect formation channels are identified and their effects on the work function of graphene sheets

    are analyzed. It is observed that the lower fluence of SHI favors the doping effects while strain

    effects are dominant at the higher fluence. This preferential behavior of surface electronic

    properties of graphene can be explained in terms of variability of the defect production processes

    determined by the ion-beam fluence.

    Keywords:

    CVD Graphene; Ion beam irradiation; Raman spectroscopy; UV-Vis spectroscopy; Scanning

    Kelvin Probe Microscopy.

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    1. Introduction

    Graphene a single layer of graphite is a two-dimensional material that conquers the field of

    science and technology since its discovery [1]. It possesses several outstanding properties and

    due to the copiousness in nature, it has taken over the fundamental science research as well as the

    applied one. Several novel properties that are not available in other materials at room

    temperature and ambient conditions have been observed in graphene. The charge carrier in

    graphene, unlike other known materials, is massless Dirac Fermions [2] which makes the

    conductivity in graphene the highest at room temperature. The room temperature quantum Hall

    effect [3], as well as the breaking of Born-Oppenheimer approximation, was observed in

    graphene [4]. It's the thinnest and hardest material known to date with a tensile strength of 125

    GPa, an elastic modulus of 1.1 TPa, and a two-dimensional ultimate plane strength of 42 N-m- 2

    [5]. Graphene's carrier mobility is 2×105 cm2 (V-1 s-1) and is only affected by impurities and

    defects [6]. Graphene is made out of carbon in a hexagonal honeycomb lattice with two atoms in

    a unit cell. Each layer forms sp2 hybridization to make a bond with other carbon atoms and the

    non-bonding electrons form π bond move freely between layers [7]. In the Brillouin zone around

    K point or Dirac point, the energy-momentum relation is linear which is responsible for many

    exceptionally outstanding properties in graphene [2]. Nonetheless, despite these novel properties

    graphene is exempted from being used in nano-electronic devices as the requirement is a finite

    on-off ratio which pristine graphene does not possess [2]. As the inversion symmetry is

    preserved in graphene it cannot be used in making piezoelectric devices either [8]. Several

    efforts have been made to modify graphene’s properties and harness it in a way favorable for

    device applications. Controlled doping and creation of vacancies are one of them [9-10]. Ion

    beam irradiation is a suitable technique by which in a controlled manner one could dope any

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    material with donor or acceptor atoms of choice or create a vacancy in materials by externally

    controlling angle, fluence, thus the effect of the ion beam on material could be controlled.

    Depending upon the energy of the ion beam, processes like adsorption, ion implantation, track

    formation, etc. could be realized in the material on which the irradiation takes place. Therefore, it

    is important to investigate the effect of the ion beam on graphene which modifies its physical

    properties.

    In this work, we've irradiated single/bi layers of graphene deposited on SiO2/Si by Si+5 ion

    of 70 MeV of energy. It was observed that at low fluence the vibrational, as well as the surface

    electronic properties, are modified by the introduction of doping. The Raman spectra, absorption

    spectra, and work function results are supporting the fact. At high fluence, a tensile strain is

    imposed in graphene beyond which the complete transformation to amorphous carbon was

    observed.

    2. Experimental Details

    CVD grown Graphene sheets (single layer/bi layers) were purchased from Graphene

    Supermarket. Our sample is of one or two-layer Graphene sheets on SiO2/Si substrate (1cm×1cm

    dimension). The thin film of Graphene was gently cleaned with acetone to remove any external

    elements on it and then used for further experiments. The 16 MV Pelletron accelerator was

    employed to irradiate the samples by Si+5 beam of energy 70 MeV with fluence 5×1011 ions-cm-2

    ,1×1012 ions-cm-2& 5×1012 & 5×1013 ions-cm-2 at normal incidence. The pressure inside the

    chamber was 10-6 Torr. PANlytical X’pert PRO, (Cu-Kα line) was used for XRD studies. AFM

    images of pristine and irradiated graphene were taken by Park systems XE-Series. Raman

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    analysis was done by Witec Alpha 300, (using 532 nm Laser, 100 μW). The UV-VIS

    spectroscopy was done using Shimadzu UV 2600 UV-VIS Spectrophotometer. The work

    function was measured in terms of CPD using SKPM of KP Technology.

    .

    3. Results

    3.1 XRD Studies

    The samples consist of one or two layers of graphene before ion irradiation of the Si+5 beam of 70

    MeV of energy. Here in figure-1, the XRD plot of the pristine sample is shown. We can see the

    characteristic peak of graphene [11] at ~26o angle. The sharpness of the peak is less, which may

    be due to existence of single/bi layers of graphene as well as discontinuity in the film. No other

    peak corresponding to different planes other than (002) was seen, which implies that the (002)

    plane of graphene is abundant in our sample.

    Fig. 1. XRD plot of pristine graphene sample. The peak at around 26o corresponds to the (002) plane in graphene.

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    3.2 AFM Studies

    Fig. 2. AFM images of pristine and irradiated graphene. (a) pristine, (b) graphene irradiated with 5×1011 ions-cm-2 Si+5 beam, (c) graphene irradiated with 1×1012 ions-cm-2 Si+5 beam, (d) graphene irradiated with 5×1012 ions-cm-2 Si+5 beam.

    The above figure (figure-2) showing AFM images of the graphene sample. The layer like

    structure in the pristine sample is visible which is nearly uniform over the SiO2/Si substrate.

    Upon ion irradiation, the morphological changes are also exposed. With higher fluence defects

    introduced in terms of hillocks and troughs are also clearly observable. To estimate the surface

    roughness of a two-dimensional thin film, we calculate the fractal dimension (D) and from that,

    we quantitatively estimate the value of surface roughness in terms of roughness exponent (α).

    The fractal dimension which is a box-counting dimension of an object expressed as

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    Where N(n) is the number of boxes of side n required to cover the object. The roughness

    exponent is related to the fractal dimension as

    For an extremely smooth surface, the value of roughness exponent is 1, wherein that case the

    fractal dimension will be the dimension of the substrate (d=2). As the roughness exponent

    decreases, the fractal dimension increases. The fractal dimension can be calculated as the slope

    of the line obtained by plotting the log of cell size versus the log of cell count which is calculated

    and plotted in figure-3. It's a measure of surface roughness that has minimum value 2 for a

    highly smooth surface to maximum 3 for an extremely rough surface [12].

    Fig. 3. Fractal dimensions of differently irradiated graphene film on Si/SiO2.

    We can see that from the plot that as the fluence of ion beam increases the fractal dimension also

    increases which is a clear sign of enhancement of surface roughness of graphene film.

    3.3 Raman studies

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    The graphene is identified by the Raman spectrum indirectly through its signature peaks. Being a

    member of the sp2 hybridized carbon group, and hexagonal, it shows a sharp 2D peak, caused by

    the vibration of the hexagonal ring of carbon. It's the strongest band seen in the graphene layer,

    the Kohn anomaly is the main contributor of its strength [13]. For a single layer of graphene, it's

    as much as four times the strength of the G band. The G band is associated with the doubly

    degenerate in-plane transverse optical phonon mode and longitudinal optical phonon mode at Γ

    point (iTO & LO) [13]. The increment of defects or deviation from its hexagonal shape by any

    kind of process like doping, defect generation, etc, changes the intensity of its two main Raman

    peaks. The intensity of the 2D Raman band in pristine graphene is higher than that of its G band,

    but for the defected graphene, it loses intensity, as well as the broadening of the 2D peak, occurs

    [13]. The intensity of G peak over-weighted 2D peak, the appearances of new peaks also occur.

    Of them, D and D’ peaks are most important, which quantitatively gives the degree of disorder in

    the sample [13]. The inter-valley double resonance process involving defects and iTO phonons

    and intra-valley double resonance process involving LO phonons and defects are the contributors

    of the peaks respectively.

    The Raman spectra of pristine and irradiated samples are shown in figure 4(a). The two main

    characteristic peaks of graphene can be seen. G peak as a result of the vibration of the hexagonal

    aromatic structured ring leaves its signature in the Raman spectrum in terms of a sharp peak,

    which is seen in our pristine sample at ~1580 cm-1. The energy dispersive Raman peak popularly

    known as 2D peak, resulting from in-plane vibration of carbon atoms in the opposite sense at

    around 2690 cm-1 is also observed. Here it is to be noted that, for a single layer of graphene the

    intensity ratio of 2D and G peak is nearly 4, here in our case, it’s slightly higher than 1, implies

    the presence of single/bi layers of graphene in our sample. 2D peak broadens with the increment

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    of layer number. As ion beam irradiation takes place, the peaks go through changes in the

    following manner. The peak positions (Figure 5(a)) of both the peaks are blue-shifted for the

    initial fluence. For higher fluence, the 2D peak is redshifted compared to the pristine sample. The

    FWHM (Figure-5(b)) also changed upon irradiated of the beam. After a slight decrement for the

    first fluence, it started increasing significantly. These two effects make us conclude that the first

    fluence showing a doping effect on the graphene sample. It is reported that doping shifts Raman

    peak to a lower wavelength i.e., blueshifts Raman peak [14, 15, 16, 17]. In our experiment, we've

    seen the same effect in the sample irradiated with 5×1011 ions-cm-2. At the same time, the FWHM

    of Raman peaks shows an anomalous behavior, firstly for the fluence 5×1011 ions-cm-2 it

    decreases after that it starts increasing. In the presence of dopants, the effective lifetime of a

    phonon increases as it shifts the Fermi level, correspondingly the width of the Raman spectrum

    decreases [14, 18]. Hence, the FWHM is decreasing for 5×1011 ions-cm-2. Again after fitting the

    Raman spectrum of graphene irradiated with 5×1011 ions-cm-2 by Lorentzian fitting, we’ve

    observed from fig4(b) that splitting of G peak occurs, which is a clear signature of doping in

    even number of graphene layers. [19]. after this particular fluence, FWHM starts increasing

    indicating deviation from pristine graphene structure. With increasing fluence we expect strain

    comes to play in the structure, as ion beam could displace carbon atom, create extended defects,

    etc, all of these effects impose finite strain in graphene. Hence, FWHM increases. The

    appearance of several defect induced peaks in the samples irradiated with higher fluence also

    supports the fact. The defect induced ID peak intensity is increasing upon an increment of ion

    fluence.

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    Fig. 4. (a) Raman spectra of pristine and Si +5 irradiated samples. (b) The splitting of G peak in 5×1011 ions-cm-2 irradiated graphene. The doping effect could blueshift the Raman G peak in an odd number of graphene layers or in an even number of layers it splits the G peak.

    Fig. 5. (a) 2D and G Raman peak position vs fluence in pristine and irradiated graphene. (b) FWHM of 2D and G peaks vs fluence.

    As the intensity of D peak is a parameter of measuring defect inside graphene samples [13], we

    can say higher fluence above 5×1011 ions-cm-2 creates defects in graphene samples. The

    appearance of D' peak adjacent to G peak is also a sign of defect in graphene, which also appears

    in those samples. From the Raman peak position, we also have calculated the value of strain

    inside the sample. (Table-1)

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    Strain (%) 1×1012 ions/cm2 5×1012 ions/cm22D -0.06% (T) -0.08% (T)G 0.14% (C) 0.14 % (C)

    Table 1. Strain calculation of ion irradiated graphene from Raman spectra. (T = Transverse strain, C= Compressive strain.)

    To calculate the induced strain we've used equation-3, which includes position shifting of 2D and

    G peaks and original peak positions of the unstrained sample [20].

    Where Δω is the Raman shift, ω is Raman peak position in pristine graphene, is Gruneisen

    parameter and is strain tensor. It can be seen that the ion beam introduces strain inside graphene,

    hence, the peak shifting in Raman spectra occurs. The samples irradiated with ion beam higher

    than 5×1011 ions-cm-2 fluence get strained, and the Raman peak position of the red-shifted which

    can be seen from Figure-5(a).

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    Fig. 6. Raman spectrum of Si+5 beam irradiated graphene with fluence 5×1013 ions-cm-2 complete transformation of graphene into amorphous carbon is visible.

    We also investigated the effect of Si+5 beam above fluence 5×1012 ions-cm-2, and as we can

    observe in the figure-6 complete transformation of graphene film into amorphous carbon occurs

    [21]. The D and G peak is visible with very high intensity and the 2D peak has been almost

    diminished are the clear proof of the aforementioned statement as per earlier reported results.

    3.4 Absorbance spectra studies

    Fig. 7. (a) Absorbance spectra of pristine and irradiated graphene. (b) Zoomed view of the peak at ~265 nm.

    The graphene consists of carbon atoms in a sp2 hybridized state. The presence of the C=C bond

    in graphene makes the electronic transition from π→π* bonding and anti-bonding state [22]. This

    type of transition lies in the UV range. Hence, graphene shows a sharp absorption peak in the

    UV region around ~265 nm. This absorption peak could be a signature of the presence of the

    graphene sample. In the pristine sample, we observed a peak at around 265 nm, connected to

    π→π* transition of C=C bond. After the introduction of ion irradiation, at fluence 5×1011 ions-

    cm-2 the intensity is highly increased. We know acceptor doping increases the absorption,

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    therefore, increases the absorbance peak [23, 24]. Here in our case as we already have discussed

    that at a fluence 5×1011 ions-cm-2, there occurs doping in the sample with acceptor atom hence the

    absorption enhances. At the same time, the red-shift of peak position is observed, implies the

    enhancement of screening for which the electron-hole interaction is reduced and the resonant

    range of excitonic state is smaller than that of pristine graphene, hence redshift occurs [25, 26].

    It's a clear sign of doping in graphene. With the further increment of fluence, in rest two samples,

    the intensity is significantly higher than the pristine sample, maybe due to the presence of

    vacancy sites, which absorbs light very much [27].

    3.5 WF Measurement

    The work function of graphene on SiO2/Si was also calculated using Scanning Kelvin Probe

    Microscopy (SKPM), which allows us to map the contact potential difference (CPD) between the

    gold tip embedded in it and the sample beneath it. Once the value of CPD is known, the work

    function value can be estimated using the following equation [28].

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    Fig. 8. Work function mapping of ion irradiated graphene. (a) pristine, (b) 5×1011 ions-cm-2, (c) 1×1012 ions-cm-2, (d) 1×1012 ions-cm-2.

    Where, e is the electronic charge, ϕtip and ϕsample are work function of the gold tip and work

    function of the sample respectively. The value of ϕtip is ~5.1 eV, from this information and value

    of CPD, we were able to find out the work function value of pristine and irradiated graphene

    samples. From the WF mapping, we see that the work function of pristine graphene is

    approximately 4.8 eV, which is a good value as reported earlier. Now the sample irradiated with

    5×1011 ions-cm-2, it increases to 6.2 eV. The rest samples have a higher value of WF than the

    pristine one.

  • 14

    Fig. 9. (a) WF evolution of graphene under ion irradiation. (b) Strain vs fluence in ion irradiated graphene

    It’s a well-known fact that, doping and strain modulate the work function of a material. The peak

    at around 380 nm in the UV-VIS spectrum of 5×1011 ions-cm-2 is possibly due to the n→π

    transition of the C=O bond [29]. We already observed a blue shift in Raman spectra and a

    redshift in the absorbance spectrum as well as higher absorbance, all of these are sufficient to

    attribute this enhancement of WF to doping. The doping concentration was also calculated using

    equation (5).

    Where νF is the Fermi velocity, has value 1×106 m/s. ΔEF is shifting in WF value, h is Plank’s

    constant. The value of un-doped graphene’s work function was taken from Ref. 30. It’s observed

    that initially p doped graphene with concentration 9.63×1010 cm-2 increases to 2.8×1012 cm-2. It's to

    be noted that the pristine graphene sample was also doped with p doping as observed from the

    calculation, the dopant concentration was increased by the ion beam irradiation with fluence

    5×1011 ions-cm-2. The higher fluence beyond 5×1011 ions-cm-2 also increases WF than pristine but

    the value is lower than the doped sample. This could be due to the tensile strain that had been

  • 15

    imposed in our sample because of ion irradiation. The tensile strain enhances the WF in graphene

    as it shifts the Fermi level downwards while keeping the vacuum level almost unaltered [31]. We

    already have seen from strain calculation that the ion beam introduces systematic tensile strain

    which could be the reason for the increment of the work function value from the pristine one.

    With higher strain WF value increases. Here it’s to be noted that doping is more effective to

    increase the WF value.

    4. Discussion

    The ion beam irradiation on graphene results in a tremendous change in its electronic,

    morphological and vibrational properties. The initial fluence of beam dopes the sample with

    acceptor atoms and upon increasing ion fluence external strain comes to play. It’s very unlikely

    that a high energetic ion beam with energy as many as 70 MeV will dope directly the target

    material. In our case, it might be possible that the charge transfer between broken carbon bonds

    intimidated by ion beam and ambient O2 molecules results in p doping effects [14, 32]. But if this

    is the case the samples irradiated with higher fluence would be more prone to doping as possible

    broken carbon bonds would be more vulnerable to make bonds with ambient O2. But that was not

    observed in our results. We’ve proposed that it might be the O2 coming from the SiO2 substrate

    via sputtering that dopes graphene. The high energetic beam when irradiated on a sample it loses

    its energy in mainly two ways: energy loss to electrons of target material and energy loss to the

    nucleus of the target material. The later energy loss is dominant in the lower energy regime.

    Since, we’ve irradiated graphene/SiO2/Si with 70 MeV Si+5 ions, the collision between the ions

    and SiO2 (since projected range of Si+5 ion beam is ~163.1 μm, the ions go directly to SiO2 with a

    little energy loss to graphene. Fig. S1) are elastic as the energy value falls in the region far below

    maximum electronic energy loss (Fig. S2). This elastic collision results in recoils which leads to

  • 16

    subsequent recoils hence the high energetic ions could start a collision cascade transferring

    energy to atoms of the target material. If a collision cascade reaches the surface of the target

    material, and if the remaining energy is higher than the surface binding energy, an atom will be

    ejected, this is known as sputtering. We’ve investigated the possibilities of O2 implantation

    directly from the sputtering of SiO2 on Si+5 ion irradiation theoretically using SRIM simulation

    [33]. The integral sputtering was plotted (Fig. 10) and it was observed that a fraction of O2 atoms

    whose energy is less than the surface binding energy of SiO2 (3.4 eV), ejected from it but as it is

    less than graphene surface binding energy (7.4 eV) are implanted in graphene. A few fractions of

    O2 atoms could be able to overcome the graphene surface energy hence able to eject from

    graphene. There's a certain probability that being kept in an ambient condition the vacancy sites

    which are major contributors of external strain as it deforms the unit graphene cell while

    preserving the volume might be occupied by environmental O2. It was nonetheless, observed that

    the dominant factor that modulates the physical properties of graphene is the external strain in it

    caused by the Carbon vacancies probably.

  • 17

    Fig. 10. Integral sputtering of SiO2 on irradiation of Si+5 beam as estimated by SRIM simulation. A significant fraction of O2 atoms leaves the surface of SiO2 owning higher energy than the surface binding energy of SiO2 yet end up doping graphene as graphene has far more surface binding energy.

    5. Conclusions

    In summary, CVD grown graphene sample are irradiated with different ion dose of Si+5 ions and

    their effect on its optoelectronic properties are investigated. At low fluence (5×1011 ions-cm-2), it

    is observed that graphene is doped with acceptor dopants whereas at higher fluence (1×1012 ions-

    cm-2, 5×1012 ions-cm-2) vacancy like defects are created in the samples generating strain in the

    graphene. This differential response of graphene to different ion fluence has its origin in the way

    of creating defects in the graphene. This behavior of CVD graphene can help in tailoring its

    properties in controlled way for the intended purpose in opto-electronic devices. The tuned work

    function of graphene is often desirable in the FET and catalytic applications where work function

    and surface charge distribution play a major role.

    Acknowledgments The authors are thankful to AIRF, JNU for Raman, XRD measurements,

    Dr. Supriya Sabbani, SPS, JNU for UV-VIS spectroscopic measurement, Dr. Ashim Kr.

    Pramanik SPS, JNU for AFM and IUAC, New Delhi for beam time.

    References[1].A. K. Geim, K. S. Novoselov, The rise of graphene, Nature Materials 6 (2007) 183-191.

  • 18

    [2].K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V.

    Grigorieva, S. V. Dubonos, A. A. Firsov, Two-dimensional gas of massless Dirac

    fermions in graphene, Nature letters 438 (2005) 197-200.

    [3].Z. Jiang, Y. Zhang, Y. W. Tan, H. L. Stormer, P. Kim, Quantum Hall effect in

    graphene, Solid State Communications 143 (2007) 14–19.

    [4].S. Pisana, M. Lazzeri, C. Casiraghi, K. S. Novoselov, A. K. Geim, A. C. Ferrari, F.

    Mauri, Breakdown of the adiabatic Born–Oppenheimer approximation in graphene,

    Nature Materials 6 (2007) 198-201.

    [5].K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J. H. Ahn, P. Kim, J.

    Y. Choi, B. H. Hong, Large-scale pattern growth of graphene films for stretchable

    transparent electrodes, Nature 457 (2009) 706–710.

    [6].T. B. Tang, C. S. Lee, Z. H. Chen, G. D. Yuan, Z. H. Kang, L. B. Luo, H. S. Song, Y.

    Liu, Z. B. He, W. B. Zhang, I. Bello, S. T. Lee, High-Quality Graphenes via a Facile

    Quenching Method for Field-Effect Transistors, Nano Lett. 9 (2009) 1374–1377.

    [7].B. Partoens, F. M. Peeters, From graphene to graphite: Electronic structure around

    the K point, Phys. Rev. B: Condens. Matter Mater. Phys. 74 (2006) 07540401-07540411.

    [8].K. A. N. Duerloo, M. T. Ong, E. J. Reed, Intrinsic Piezoelectricity in Two-Dimensional

    Materials, J. Phys. Chem. Lett. 3 (2012) 2871-2876.

    [9].J. H. Chen, W. G. Cullen, C. Jang, M. S. Fuhrer, E. D. Williams, Defect Scattering in

    Graphene, Phy. Rev. Lett. 102 (2009) 236805.

    [10]. J. J. Palacios, J. Fernández-Rossier, L. Brey, Vacancy-induced magnetism in

    graphene and graphene ribbons, Phy. Rev. B. 77 (2008) 195428.

  • 19

    [11]. Z. Q. Li, C. J. Lu, J. P. Xia, Y. Zhou, Z. Lou, X-ray diffraction patterns of

    graphite and turbostratic carbon, Carbon 45 (2007) 1686-1695.

    [12]. J. D. Miller, S. Veeramasuneni, J. Drelich, M. R. Yalama, Effect of roughness as

    determined by atomic force microscopy on the wetting properties of PTFE thin films,

    Polymer engineering and Science 36 (1996) 14.

    [13]. A. C. Ferrari, D. M. Basko, Raman spectroscopy as a versatile tool for studying

    the properties of graphene, Nature nanotechnology 8 (2013) 235-246.

    [14]. J. H. Kim, J. H. Hwang, J. Suh, S. Tongay, S. Kwan, C. C. Hwang, J. Wu, J. Y.

    Park, Work function engineering of single layer graphene by irradiation-induced defects,

    Appl. Phys. Lett. 103 (2013) 171604.

    [15]. J. Yan, Y. Zhang, P. Kim, A. Pinczuk, Electric Field Effect Tuning of Electron-

    Phonon Coupling in Graphene, Phys. Rev. Lett. 98 (2007) 16 166802.

    [16]. C. Casiraghi, S. Pisana, K. S. Novoselov, A. K. Geim, A. C. Ferrari, Raman

    fingerprint of charged impurities in graphene, Appl. Phys. Lett. 91(23) (2007) 233108.

    [17]. M. Bruna, A. K. Ott, M. Ijäs, D. Yoon, U. Sassi, A. C. Ferrari, Doping

    dependence of the Raman spectrum of defected graphene, ACS Nano 8(7) (2014) 7432-

    7441.

    [18]. A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S. K. Saha, U. V. Waghmare, K.

    S. Novoselov, H. R. Krishnamurthy, A. K. Geim, A. C. Ferrari, A. K. Sood, Monitoring

    dopants by Raman scattering in an electrochemically top-gated graphene transistor, Nat.

    Nanotechnol. 3(4) (2008) 210–215.

    [19]. M. Bruna, S. Borini, Observation of Raman G-band splitting in top-doped few-

    layer graphene, Phys. Rev. B 81 (2010) 125421.

  • 20

    [20]. N. Ferralis, R. Maboudian, C. Carraro, Evidence of Structural Strain in Epitaxial

    Graphene Layers on 6H-SiC(0001), Phys. Rev. Lett. 101 (2008) 156801.

    [21]. A. C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and

    amorphous carbon, Phys. Rev. B. 61 (2000) 14095-14107.

    [22]. J. Li, C. Y. Liu, Ag/Graphene Heterostructures: Synthesis, Characterization and

    Optical Properties, Eur. J. Inorg. Chem. 8 (2010) 1244–1248.

    [23]. G. Buchowicz, P. R. Stone, J. T. Robinson, C. D. Cress, J. W. Beeman, O. D.

    Dubon, Correlation between structure and electrical transport in ion-irradiated graphene

    grown on Cu foils, Appl. Phys. Lett. 98(3) (2011) 032102.

    [24]. C. Zhu, S. Guo, Y. Fang, S. Dong, Reducing Sugar: New Functional Molecules

    for the Green Synthesis of Graphene Nanosheets, ACS Nano 4 (2010) 2429–2437.

    [25]. L. Yang, Excitonic Effects on Optical Absorption Spectra of Doped Graphene,

    Nano Lett. 11 (2011) 3844–3847.

    [26]. K. F. Mak, L. Ju, F. Wang, T. F. Heinz, Optical spectroscopy of graphene: From the

    far infrared to the ultraviolet, Solid State Communications 152 (2012) 1341-1349.

    [27]. T. Mohanty, N. C. Mishra, A. Pradhan, D. Kanjilal, Luminescence from Si nanocrystal

    grown in fused silica using keV and MeV beam, Surface & Coatings Technology 196 (2005) 34-

    38.

    [28]. J. Shakya, A. S. Patel, F. Singh, T. Mohanty, Appl. Phys. Lett., Composition

    dependent Fermi level shifting of Au decorated MoS2 nanosheets 108 (2016) 013103.

    [29]. H. Shu, Y. Li, X. Niu, J Wang, Greatly Enhanced Optical Absorption of a

    Defective MoS2 Monolayer through Oxygen Passivation, ACS Appl. Mater. Interfaces 8

    (2016) 13150-13156.

  • 21

    [30]. Y. J. Yu, Y. Zhao, S. Ryu, L. E. Brus, K. S. Kim, P. Kim, Tuning the Graphene

    Work Function by Electric Field Effect, Nano Lett. 9 (2009) 3430.

    [31]. X. Peng, F. Tang, A. Copple, Engineering the work function of armchair graphene

    nanoribbons using strain and functional species: a first principles study, J. Phys.:

    Condens. Matter 24 (2012) 075501 1-10.

    [32]. K. F. Mak, K. He, C. Lee, G. H. Lee, J. Hone, T. F. Heinz, J. Shan, Tightly

    bound trions in monolayer MoS2, Nature materials 12 (2013) 207-211.

    [33]. J. F. Ziegler, SRIM: The Stopping and Range of Ions in Matter, Version SRIM-

    2013, (http://www.srim.org/)

    http://www.srim.org/