Synthesis of few layer graphene by direct exfoliation of graphite and a Ramanspectroscopic studyS. Gayathri, P. Jayabal, M. Kottaisamy, and V. Ramakrishnan

Citation: AIP Advances 4, 027116 (2014); doi: 10.1063/1.4866595 View online: http://dx.doi.org/10.1063/1.4866595 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/4/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Influence of transfer residue on the optical properties of chemical vapor deposited graphene investigated throughspectroscopic ellipsometry J. Appl. Phys. 114, 093505 (2013); 10.1063/1.4819967 Few layer graphene synthesized by filtered cathodic vacuum arc technique J. Vac. Sci. Technol. B 31, 040602 (2013); 10.1116/1.4813762 Terahertz, optical, and Raman signatures of monolayer graphene behavior in thermally reduced graphene oxidefilms J. Appl. Phys. 113, 183502 (2013); 10.1063/1.4803713 Mega-electron-volt proton irradiation on supported and suspended graphene: A Raman spectroscopic layerdependent study J. Appl. Phys. 110, 084309 (2011); 10.1063/1.3647781 Synthesis of few-layered graphene by H 2 O 2 plasma etching of graphite Appl. Phys. Lett. 98, 183114 (2011); 10.1063/1.3589354

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AIP ADVANCES 4, 027116 (2014)

Synthesis of few layer graphene by direct exfoliationof graphite and a Raman spectroscopic study

S. Gayathri,1 P. Jayabal,1 M. Kottaisamy,2 and V. Ramakrishnan1,a

1Department of Laser Studies, School of Physics, Madurai Kamaraj University,Madurai-625021, India.2Department of Chemistry, Thiagarajar College of Engineering, Madurai-625015, India.

(Received 6 January 2014; accepted 10 February 2014; published online 18 February 2014)

The exfoliation of graphene from pristine graphite in a liquid phase was achievedsuccessfully via sonication followed by centrifugation method. Ultraviolet–visible(UV–vis) spectra of the obtained graphene dispersions at different exfoliation timeindicated that the concentration of graphene dispersion increased markedly withincreasing exfoliation time. The sheet-like morphology of the exfoliated graphenewas revealed by Scanning Electron Microscopy (SEM) image. Further, the mor-phological change in different exfoliation time was investigated by Atomic ForceMicroscopy (AFM). A complete structural and defect characterization was probedusing micro-Raman spectroscopic technique. The shape and position of the 2D bandof Raman spectra revealed the formation of bilayer to few layer graphene. Also,Raman mapping confirmed the presence of uniformly distributed bilayer graphenesheets on the substrate. C© 2014 Author(s). All article content, except where other-wise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.[http://dx.doi.org/10.1063/1.4866595]

I. INTRODUCTION

The discovery of a new material brings the most exciting and fruitful periods of scientificand technological research. The recent discovery of graphene, atomically thin layers of graphitebrought such a period. The discovery of monolayer graphene in 2004 has led to the demonstrationof a host of novel physical properties in this most exciting world of nanomaterials.1 Single layergraphene (SLG) and few layer graphene (FLG) exhibit a two dimensional crystal lattice structurewith excellent properties including high surface area, strong electronic, mechanical, thermal andchemical properties.2, 3 With these fascinating properties, graphene may be used as active componentsin various applications such as field effect transistors, transparent conductors, liquid crystal displays(LCDs), electrochemical resonators, and photovoltaic devices.4–6

Generally, graphene is made by micromechanical cleavage. In this process, monolayers arepeeled from Highly Oriented Pyrolytic Graphite (HOPG) using a scotch tape. Though it is the sim-plest method to achieve pure SLG, this process has major disadvantage in terms of mass production.Later, chemical vapour deposition and epitaxial growth have been used for the synthesis of large areagraphene nanosheets. However, it is usually necessary to transfer the sample on to other substratesin order to make useful devices.7, 8 By considering the application of graphene in various areas, therehas been a significant interest on the development of large-scale production methods.9 At present,graphite exfoliation method is widely obtained by chemical reduction of graphite oxide (GO), whichwas obtained by oxidation of graphite to GO using strong oxidizing agent (Hummer’s method).The major drawbacks of this technique include a large number of structural defects, multi-stepprocess, use of concentrated acids in oxidation and hazardous chemicals are preferred to reduceGO. Hence, a non-covalent, liquid-phase method was proposed to produce large quantities of defect

aE-mail: vr.optics1@gmail.com

2158-3226/2014/4(2)/027116/12 C© Author(s) 20144, 027116-1

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free, unoxidized graphene from graphite in certain organic solvents and surfactant assisted aqueousmedia.

Recently, a top down method viz liquid phase exfoliation has been proposed to produce graphenesheets in mass production. Exfoliation of graphite in liquid phase involves two steps. First step isthrough the direct sonication, by using a solvent that overcome the weak van der Waals interactionand the next step is through centrifugation which removes the larger graphitic particles. Severalliquids were used to create stable dispersions of graphene such as surfactants in aqueous media,ionic liquids and organic solvents.10–15 The organic solvent ortho-dichlorobenzene (o-DCB), be-ing aromatic, is able to interact with graphene via π -π stacking. o-DCB has a surface tension of36.6 mJ/m2, which is very close to the proposed range (40–50 mJ/m2).16 Sahoo et al.17 have demon-strated one pot synthesis of graphene by exfoliation of graphite in o-DCB. The synthesis was onlybased on sonication and the sonication time was varied up to 5 hours. Skaltsas et al.10 have exfoliatedgraphene in o-DCB and N-methyl-1,2-pyrolidone via tip sonication for various exfoliation time upto 1 hour. Hamilton et al.16 have demonstrated the production of graphene dispersions from syntheticgraphite, HOPG and expanded graphite in o-DCB by cup-horn sonication for 30 minutes. Graphenecan be readily identified by Raman spectroscopy.18–20 Raman spectroscopy also allows monitoringof doping, defects and strain19, 21 and it is a very sensitive tool to probe the nature of the grapheneedges.22 The number of layers present in the sample can also be determined.18 Raman mappingof graphene23, 24 has been made for micro-mechanically cleaved samples, graphene from CVD,etc.

In the present work, graphene sheets have been successfully exfoliated from graphite in o-DCB by simple sonication for longer period of 8 hours with an hour interval followed withcentrifugation. UV-vis spectroscopic technique was employed to determine the concentration ofgraphene sheets present in the solution. The structural and defect characterizations of exfoliatedgraphene were carried out by micro-Raman spectroscopic technique. Raman mapping of grapheneexfoliated from graphite in liquid phase has not been attempted so far. So, we have employedRaman mapping to determine the number of layers and overall distribution of the graphenesheets on the substrate. In addition, morphological investigations have been made by AFM andSEM.

II. EXPERIMENTAL

A. Materials

The starting materials graphite powder (Alfa Aesar), o-DCB (Merck) were purchased and usedas received without further purification.

B. Graphene dispersion

In the typical synthesis, 1 g of graphite flakes was added to 100 ml of o-DCB. The mixturewas sonicated in a bath ultrasonicator for 8 hours. The water in the bath sonicator was changedfrequently in order to avoid the water heating. With the aim of optimizing the sonication time, 10mlof dispersion was collected in a vial for every one hour. All the samples S1 (1 hour), S2 (2 hours), S3

(3 hours), S4 (4 hours), S5 (5 hours), S6 (6 hours), S7 (7 hours) and S8 (8 hours) were left undisturbedfor two days. The sample S1 appeared very close as the true solution while the remaining samplesappeared light grey to black colour dispersion as the sonication time increased. The colloidal liquiddispersions were centrifuged at the rate of 3500 rpm for 20 minutes. After centrifugation, the largergraphitic flakes got settled down and the exfoliated graphene sheets dispersed in the liquid. The tophalf of the supernatant i.e homogeneous graphene dispersion was carefully pipetted out and collectedin separate vials. Though the samples S1, S2 and S3 appeared as such the true solvent, the dispersionof the graphene was confirmed by performing Tyndall scattering. The exfoliated graphene sheets inthe solvent did not settle down even after six months, hence the centrifugation rate and time werealso optimized.

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C. Graphene film

The liquid dispersions were made in to film on two substrates (single crystalline Silicon (Si)and Quartz) by drop casting method in order to analyze the substrate effect on Raman features ofgraphene. For the substrate cleaning, methanol and ethanol were mixed well in the ratio of 1:1. Thesubstrates were sonicated in the mixture for ten minutes. Then the substrates were transferred tothe diluted hydrochloric (HCl) acid and sonicated for ten minutes followed by rinsing in deionisedwater. All the substrates were well cleansed in order to get pure graphene film. A small drop of theliquid dispersions was then dropped on the substrates and dried at 350oC on hot plate for two hours.The solvent o-DCB was completely evaporated (decomposed) after the heat treatment.

D. Instrumentation

The bath sonication was performed with a CYBERLAB ultrasonic steri-cleaner (Freq. 40 KHzand Power 100 W). The dispersions were centrifuged in a REMI C-854/4 Medico/Doctor Centrifuge.The UV-vis absorption spectroscopy was carried out using SHIMADZU UV-2450 Spectrophotome-ter. Raman scattering measurements were performed at room temperature in 180o back scatteringgeometry by using HORIBA Jobin Yvon LabRAM HR 800 equipped with a thermo-electricallycooled charge coupled device (CCD) detector and an automated XY motorized stage. The line of632.8 nm from He-Ne laser was used as the source of excitation. With a 50X microscope objec-tive, the spot diameter was about ∼1μm with, typically, 17 mW power focussed on the sample.Before the measurements were performed, the micro-Raman spectrometer was well calibrated using521 cm−1 Raman line of single crystalline Silicon (Si) wafer. The spectrograph consists of holo-graphic grating of 1800 grooves/mm and the confocal hole of the spectrograph was set to 100 μm.The Raman mapping measurements were made using the same Raman spectrometer and excitationsource. Rectangular mapping was performed for all the samples. The XY stage moved (20 × 20)steps, totally 400 Raman spectra were collected and the intensity map was deduced for the area of(81 × 67) μm2. SEM was recorded using Tescan VEGA-3 LMU instrument. AFM (A-100 SGSunit) was used to study surface morphology.

III. RESULTS AND DISCUSSION

The stress on the graphite particles is placed by sono-physical energy through the low powersonication technique. This stress is transmitted all over the sp2 hybridized carbons in the grapheneplanes weakening the attraction between the graphene layers created by the weak van der Waalsforce that clasp the graphene sheets together.17 The dispersion of graphite in o-DCB leads to theformation of sono polymer, which sticks on to the graphite layers. With the increasing sonicationtime, the formation of polymer bounded graphite sheets as well as the distance between the graphitelayers increases. This leads to the formation of SLG to FLG sheets by simple sonication.

The digital images of the graphene dispersions before and after centrifugation are presentedin Fig. S1.25 The graphene dispersions after centrifugation exhibit Tyndall scattering effect whereHe-Ne (633 nm) laser line was used. The Fig. 1 shows the UV-vis absorption spectra of the graphenedispersions in different exfoliation time. The absorbance of graphene dispersions increases notablywith the increasing sonication time. It is evident from the figure that the graphene absorbs the entirevisible region and a peak centered at ∼300 nm corresponds to the C-C of graphene sheet. Theconcentrations of graphene dispersions have been determined by Lambert-Beer law. The absorptioncoefficient of graphene at 660 nm in true organic solvent o-DCB was estimated as 3780 L g m−1

using the reported value.10 The concentration of graphene in solvent against the exfoliation timeplot is shown in the inset Figure 1. The concentration of the synthesized graphene sheets alsoincreased remarkably with the increasing sonication time. The obtained graphene concentration ino-DCB ranged from 4.5 to 6.6 μg/ml. The sonication of base graphite material for 8 hours yieldedthe maximum concentration of 6.6 μg/ml. Further increasing the exfoliation time may increase thegraphene concentration but long time sonication leaves structural defect which could disturb theelectronic properties of graphene.26

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FIG. 1. UV-Visible spectra of graphene dispersion, (Inset) concentration of graphene dispersion as a function of exfoliationtime.

The morphological change in different exfoliation time of graphene was analyzed by AFM. Inorder to estimate the thickness of graphene sheets, the dispersions of S4, S5, S6, S7 and S8 weredropped on Si substrates and dried in air at room temperature. AFM is considered as one of thedirect methods for estimating the thickness of graphene sheets.26 AFM images of dispersions coatedon Si substrates are shown in Fig. 2(a) to 2(e). From the Fig. 2(a) to 2(c) it is clearly visible thatthe graphene sheets appear like FLG to MLG and it is confirmed that there are still some graphiticparticles are needed to be exfoliated in the dispersion. But in the case of samples S7 and S8 shownin Fig. 2(d) & 2(e) it is clear that the graphene sheets are well exfoliated. The thickness of thegraphene sheets of S7 and S8 was estimated as ∼0.8 to 2 nm. Therefore, our exfoliated graphenesheets exhibit BLG to FLG.4 The results revealed the sheet like morphology of exfoliated grapheneand the samples S4, S5, S6 consists of FLG to MLG sheets and the sample S7 and S8 consists ofBLG sheets. The dispersion of S8 was dried in air at room temperature and the obtained powderwas subjected to SEM analysis. The SEM image of S8 is shown in Fig. 2(f) and the sheet likemorphology of the exfoliated graphene is clearly observed. The size of the typical graphene sheetwas estimated as ∼800 nm to 1 μm. Even though the thickness of graphene sheets was estimated byAFM, a detailed Raman analysis has also been made to determine the number of layers by Ramanmapping.

Graphene can be easily identified in terms of orientation and number of layers through inelasticlight scattering.18 Raman spectroscopic technique is one of the powerful and non-destructive toolsto characterize carbon nanostructures.21 The Raman spectra of graphene show general features inthe 1000–3000 cm−1 region: the D, G and 2D peaks, which lie at around ∼1350, ∼1560 and∼2700 cm−1, respectively. The G peak corresponds to the E2g phonon at the Brillouin zone centre.The D peak is due to the breathing modes of sp2 atoms and it requires a defect for its activation.27 Itcomes from TO phonons around the K point of the Brillouin zone and it is active by double resonance

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FIG. 2. AFM images of (a) S4 (b) S5 (c) S6 (d) S7 (e) S8 and (f) SEM image of S8 (scale bar: 1 μm).

(DR).28 The intensity of D peak is attributed to the amount of disorder but not related to the numberof graphene layers.29 The second order of zone boundary phonons gives rise to 2D peak18 and itappears as a strong single peak in monolayer graphene. However, it splits into four bands in bi-layergraphene. Even if D peak is absent, the 2D peak is always observed as defects are not necessary for

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FIG. 3. Raman spectra of the graphene on Si substrate.

the activation of two phonons with the same momentum, one backscattering from the other. DR canalso happen as intravalley process, i.e. connecting two points belonging to the same cone aroundthe K or K’ points.30 This gives rise to the D′ peak, which is generally seen around ∼1610 cm−1 indefect graphite.31 Defects induce a significant increase of the D and D′ intensities and decrease ofthe 2D peak height intensity with line broadening. Further, the appearance of the combination mode(D and D′ phonons) around ∼2950 cm−1 is also expected, it also requires a defect for its activation.In defect-free graphene, the variation of Full Width at Half Maximum of the G peak, FWHM (G),is due to doping.32

The Raman spectrum of pristine graphite is provided in Fig. S2.25 The major Raman featuresD, G and 2D bands of graphite appeared at 1334, 1575 and 2676 cm−1, respectively. The Ramanspectra of graphene sheets deposited from solution onto the single crystalline Si substrate are shownin Fig. 3. For all the samples, the D band appeared at ∼1332 cm−1 whereas the G band appearedat 1573, 1574, 1575, 1579 and 1582 cm−1 for the samples S4, S5, S6, S7 and S8, respectively. Theapparent variation of the Raman features of graphene and graphite is the 2D band. The SLG usuallyhas a single Lorentzian 2D peak with a full width at half maximum of ∼30 cm−1, and the intensity

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ratio of 2D to G band (I2D/IG) is >2.13 Maximum ∼30 cm−1 shifting of the 2D band with respect tographite is usually observed in the SLG.33, 34 So the number of layers in the graphene sheet can bepossibly determined from the intensity, shape and position of the 2D band. As the exfoliation timeis increased, considerable red shift of the 2D band with respect to pristine graphite was observed.The Raman spectra of S7 and S8 have a 2D band shape characteristic of graphene bi-layers. The 2Dband position of S7 and S8 appeared at ∼2658 and ∼2656 cm−1, respectively. The 2D band of S7

was shifted by 18 cm−1 to the lower wavenumber side with respect to pristine graphite whereas 20cm−1 shift was observed for S8. The graphene bi-layers were identified by Lorentzian multi peakfit using Origin 8.0 software and the 2D peak analysis of S7 and S8 are shown in Fig. 3. The 2Dband of the bilayer graphene usually has 4 components.18, 21 The bilayer graphene (BLG) of sampleS7 consists of four bands at 2617, 2652, 2676 and 2693 cm−1 and the four bands appeared at 2622,2657, 2686 and 2709 cm−1 for the sample S8. The deconvolution of 2D band in Raman spectra ofS4, S5 and S6 shown in the Figure 3 revealed that FLG (∼5 layers) was formed. The intensity ratioof G and 2D peaks (IG/I2D) of the samples S4 to S8 varied from 2.38 to 1.48 whereas the ratio IG/I2D

of the parent graphite material is 3.30. Hence, it is evident that the graphene layers are exfoliatedfrom graphite by simple sonication and centrifugation.

The Raman spectra of graphene sheets deposited onto the Quartz substrate is shown in Fig. 4.The D band appeared at ∼1328 cm−1 and G band is observed in the region of 1570–1573 cm−1

for all the samples. By comparing the peak positions of D and G peaks of the graphene sheets onSi substrate, no appreciable shift was observed. The band shape of 2D peak of the samples S7 andS8 suggest the presence of BLG sheets and it was confirmed by the Lorentzian multi peak fit. Thebilayer graphene (BLG) of sample S7 consisted of four bands at 2609, 2640, 2662 and 2682 cm−1

and the four bands appeared at 2621, 2647, 2670 and 2690cm−1 for the sample S8. From 2D bandposition and shape of the Raman spectra shown in Fig. 4, it is evident that the samples S4, S5 and S6

exhibit FLG sheets. The exfoliation of SLG from graphite was not achieved which may be due tothe re-aggregation of graphene layers during the film formation. Fig. 5(a) depicts the plot betweenintensity ratio (IG/I2D) and exfoliation time. The decrease in the intensity ratio with increasingsonication time indicates the exfoliation of graphene from graphite. The peak position of 2D bandas a function of exfoliation time of graphene on Si and graphene on quartz is shown in Fig. 5(b).From the plot, it is clear that the wavenumber shift between the two different substrates is in therange of ∼1 to 5 cm−1. The above results suggest that the interactions between graphene sheets andthe substrates play a negligible role in affecting the Raman features of graphene sheets. The aboveresults revealed that BLG and FLG (∼5) can be noticed and thus confirming that our method cansuccessfully exfoliate bulk graphite.

All the Raman spectra shown in Figures S2,25 3 and 4 have small D bands, indicating thepresence of defects. The presence of D band in all the samples is due to the ring breathing vibrationof sp2 carbon atoms. Indeed, as the spot size of the Raman excitation line is ∼1μm which is largerthan the flakes deposited on the substrate, the large quantities of the edges of the graphene sheetswill be covered by the laser beam. Therefore, we suggest that the D band comes from edges ofthe graphene sheets rather from structural defects inside the graphene plane.9, 35 The width of theG band remained unchanged even when the D band intensity increased. The lack of broadening ofthe G band also supports that the D band originates from the edges.19, 28 Even though the defectpeak is observed in all the samples, the intensity ratio of D to G band I(D)/I(G) is very low (∼0.2to 0.4) and hence the electronic properties are not disturbed in the synthesized graphene samples.The presence of D band is a general characteristic as expected that the sonication energy brings indamages in the graphene sheets and create edge defects. The D band appeared for all the samplesaround 1332cm−1. In addition to the D-band, D′ band exists in the spectra at ∼1609 cm−1. TheD′-band is usually reported for disordered graphitic lattice whereas this band is very tiny or evennegligible in the parent graphite material.36 The presence of the D′ band in graphite was confirmedby the deconvolution of superimposed G band using Lorentzian multi peak fit which is shown inFig. S2 inset.25 The D′ peak was noticeable for the samples S7 and S8, which is attributed to longtime sonication.26

Raman mapping of the BLG to FLG samples was made to obtain more support on the av-erage number of graphene layers and the quality of the samples after exfoliation. The mapping

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FIG. 4. Raman spectra of the graphene on Quartz substrate.

measurements were made for all the samples as mentioned in the experimental part. At the endof the measurements, 400 Raman spectra were collected and the intensity map was generated as aRaman image for the area of (81 × 67) μm2. The collection of sequence of such spectra while scan-ning the sample in the X-direction and Y-direction (rectangular mapping) allows complete samplemapping with full Raman spectra at every point. Raman imaging can be used to explicitly determinethe graphene thickness.23 The advantage of Raman spectroscopy and imaging in determining thenumber of graphene layers is that it does not depend on the substrate used.24 Figures 6 and 7 showRaman intensity images of the sample S7 on Quartz substrate obtained when plotting the peak inten-sity as a function of the spatial location for the three bands (1330 cm−1 in red, 1580 cm−1 in greenand 2660 cm−1 in blue). From the mapping images, it is clear that the graphene sheets are uniformly

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FIG. 5. (a) IG/I2D intensity ratio as a function of exfoliation time (b) 2D band position as a function of exfoliation time.

distributed on the substrate. The intensity map of D band is shown in the Fig. 7. The brighter areasin the map represent higher intensity; the defect is greater in the brighter area which is attributedto the edge/basal defect. Spectra at points (a) to (c) correspond to defect ratio ID/IG of 0.19 to 0.28while the spectra at points (d) and (e) hold the defect ratio of 0.31 to 0.36. As the peak intensity ofthe 400 spectra can be directly obtained from the Raman mapping measurement, the histogram ofID/IG ratio of 400 data points was made. From the histogram it is clear that the ID/IG ratio almostmaintained the same value all over the sample and the low intensity of the D band indicates that

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FIG. 6. Raman map of G and 2D band of graphene on quartz and histogram of IG/I2D intensity ratio.

FIG. 7. Raman intensity map of D band and histogram showing defect ratio (ID/IG).

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high quality BLG to FLG is obtained. Characteristic spectra of BLG and FLG were observed, ascould be determined by the analysis of the second order 2D peaks arising from the zone boundaryphonons.18, 37 In order to determine the number of layers present in the graphene sheets, the intensityratio IG/I2D was estimated from the Raman intensity maps of G and 2D bands which are shown inFig. 6. The IG/I2D ratio of 400 data points was made as a histogram. The area (b) to (e) marked in themaps are correlated to the spectra shown nearby. The intensity ratio was almost constant in ∼70% ofthe sample in which the IG/I2D ratio ranged from 1.40 to 1.66, while the spectrum at points (a) and (f)the ratio was remarkably higher of 2.36 and 2.46, respectively. Those spectra indicate the presenceof MLG in a small domain, indeed the ratio did not coincide with that of graphite which indicatesthat the graphene can be synthesized by our method. The Raman mapping of all the samples showsimilar result (all the data are not presented) revealing the quality of the dispersion.

IV. CONCLUSION

The graphene sheets were successfully exfoliated from graphite in o-DCB by simple sonication.The homogeneous graphene dispersions remain stable even after 6 months. The concentration ofthe graphene sheets increased with increasing sonication time. The yield of graphene dispersioncan be increased by varying the optimizing parameters such as sonication time, power of sonicator,centrifugal rotational speed and time, initial amount of graphite addition to the solvent. The sheetlike morphology of the synthesized graphene sheets was revealed by AFM and SEM. From theSEM result, the size of the graphene sheets was estimated as ∼800 nm. Micro-Raman Spectroscopictechnique was carried out to characterize the synthesized graphene sheets. The deconvolution of 2Dband in the Raman spectra of the samples S7 and S8 splits in to four bands revealing the formationof BLG. The Raman mapping measurements indicated that almost all the graphene sheets are of2-layers to few layers and are distributed uniformly. The average defect ratio (ID/IG) of the graphenesheets is also determined from Raman mapping and the ratio is estimated as ∼0.3. Hence BLG,FLG (∼5 layers) and multi-layers (>5 layers) with minimal defect can be directly exfoliated fromgraphite in o-DCB.

ACKNOWLEDGMENT

One of the authors SG would like to thank DST-SERB, Govt. of India for fellowship. The authorsthank UGC-UPE Programme of Madurai Kamaraj University, India for providing micro-Raman andAtomic Force Microscopy (AFM) facilities and the authors acknowledge DST-SERB for providingRaman mapping accessories and financial assistance.

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