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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: tanujajnu@gmail.com

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.

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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.

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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

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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

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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.

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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.

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