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J. of Supercritical Fluids 63 (2012) 99–104

Contents lists available at SciVerse ScienceDirect

The Journal of Supercritical Fluids

journa l homepage: www.e lsev ier .com/ locate /supf lu

reparation of few-layer and single-layer graphene by exfoliation of expandableraphite in supercritical N,N-dimethylformamide

hangqing Liua, Guoxin Hua,∗, Hanyang Gaob

School of Mechanical & Power Engineering, Shanghai Jiaotong University, Shanghai 200240, ChinaSchool of Environmental Science & Engineering, Shanghai Jiaotong University, Shanghai 200240, China

r t i c l e i n f o

rticle history:eceived 5 July 2011eceived in revised form 7 January 2012ccepted 7 January 2012

eywords:

a b s t r a c t

Few-layer graphene (FG) is produced in supercritical N,N-dimethylformamide (DMF) using expandablegraphite (EG) as starting material in less than 15 min. Monolayer graphene is produced by exfoliationof FG in supercritical DMF. The samples were characterized by atomic force microscopy (AFM), X-raydiffraction (XRD), Transmission electron microscopy (TEM), Scanning electron microscopy (SEM), andRaman spectroscopy. AFM result reveals that the average thickness of FG is about 3 nm and monolayer

upercritical DMFew-layer grapheneingle-layer graphenexpandable graphiteield

graphene is about 1.2 nm. Raman result indicates that the ID/IG of FG is 0.3, which proves a small proportionof defects in it. Moreover, we have studied the effect of process parameters on the yield of FG. Resultsshow that a yield of 7 wt% can be obtained at an optimum condition (with concentration being 2 mg/ml,temperature being 673 K, and volume ratio being 0.67). Besides, the temperature of FG formation is verylow (473 K). Finally, these results show that EG is an excellent precursor for the fabrication of FG and

monolayer graphene.

. Introduction

Monolayer graphene, a new carbon nanomaterial, is composedf carbon atoms forming hexagonal rings. It has become a hotesearch topic for scientists of various fields in the last few years.t possesses many unique properties: high specific surface area [1]ue to the size effect of nanomaterials; strong mechanical strength2] derived from the strong interaction between adjacent carbontoms; high values of electrical conductivity and thermal conduc-ivity [3] derived from ultra-fast transfer of charge carriers [4]. Dueo these features, it has great potential to be applied in nanocom-osites [5], transistor, sensor, supercapacitor, fuel cell, solar cell,tc. Few-layer graphene (FG) consists of several overlapping indi-idual monolayer graphene, and the adjacent layers are connectedy van der Waals force. By increasing stack layers, FG will be trans-ormed into bulk graphite. Electrons in monolayer graphene, FGnd graphite exhibit different behaviors. In monolayer graphene,lectron is following a linear dispersion relation [6]. The electronictructure of FG changes dramatically due to interlayer coupling [7].oreover, its two dimensional electronic band structure plays an

mportant role in many of the intriguing properties of FG. There-

ore, FG may show a lot of attractive properties by controlling theumber of layers. So, it is important to find out a simple method

or obtaining large amounts of FG with high quality.

∗ Corresponding author. Tel.: +86 21 34206569; fax: +86 21 34206569.E-mail address: hugx@sjtu.edu.cn (G. Hu).

896-8446/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2012.01.002

© 2012 Elsevier B.V. All rights reserved.

Recently, a large number of papers have reported many kindof techniques for preparing FG, such as chemical vapor deposition[8], super-short-pulse laser produced plasma deposition [9], andthe reduction of few-layer graphene oxide [10]. However, there aresome disadvantages about these techniques. Although the prod-ucts are pure FG sheets in chemical vapor deposition method, itis a great challenge to transfer the sample onto other insulatingsubstrates for making useful devices, and it cannot achieve massproduction as the operating condition is rigorous. On the otherhand, although the method of the reduction of few-layer grapheneoxide is simple and low-cost, having the potential to be used inlarge-scale production, the amount of oxygen-containing groupbonding to the FG was large and cannot be restored completely;moreover, huge amounts of defects would remain after reduction.These impurities seriously affect the characteristics of FG. In thisstudy, we introduce an approach for fabricating FG by exfoliationof expandable graphite (EG) in supercritical DMF. The obtained FGcan be used for preparing monolayer graphene. In addition, weevaluate the influence of process parameter on the yield of FG.The investigated parameters include concentration, temperature,and volume ratio. The supercritical process has several advantagesover other methods: (1) the required setup is very simple (sealedstainless-steel reactor); (2) it is less toxic than other methods; (3)the method is capable for mass production; (4) the products are

of high quality because the process does not introduce any otherimpurities.

The supercritical fluids have been widely used to synthesisnovel nanomaterials. In the last few years, Gulari et al. [11] have

1 itical Fluids 63 (2012) 99–104

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EG

EG immersing in supercritical DMF

FG

FG immersing in

supercritical DMF

Monolayer grapheneSupercritical DMF

Intercalated content

Fig. 1. The schematic for fabricating few-layer graphene (FG) and single-layergraphene by exfoliation of expandable graphite (EG) in supercritical N,N-

00 C. Liu et al. / J. of Supercr

eported the utilization of supercritical CO2 method for interca-ating and exfoliating layered silicate. Supercritical CO2 mediuman be fully diffused in between the interlayer of silicate with highiffusivity and low viscosity. Upon rapid depressurization, the sil-

cate is expanded and delaminated. Nenwen Pu et al. [12] haveeported a supercritical CO2 processing technique for intercala-ion and delamination of layered graphite. FG is obtained throughapid depressurizing of the supercritical CO2 to expand and exfo-iate graphite. In addition, Rangappa et al. [13] have synthesizedraphene by supercritical solvent exfoliation of natural graphite.hen fluids reach or exceed their critical point, they show vari-

us unique properties, such as outstanding wetting of surfaces, lownterfacial tension, low viscosity, and high diffusion coefficients.hese features facilitate supercritical fluid to accomplish penetra-ion and expansion of the interlayer of graphite.

. Experimental

.1. Materials

Expandable graphite powder (CP, 300–500 �m) was pur-hased from commercial products (Shanghai Fuyouqin Tradingo., Ltd, China). N,N-dimethylformamide was analytical reagentSinopharm Chemical Reagent Co., Ltd, China).

.2. Experimental

All supercritical experiments were performed in a 15 ml sealedtainless-steel reactor (caution: the reactor maximum loading is0% (12 ml), the remaining spaces are air). A simple procedureas illustrated as follows. Some expandable graphite powder was

dded to a container and dispersed in DMF by low-power sonicationultrasonic cleaner, 40 kHz, maximum output 180 W) for 15 min.he mixture was carefully transferred into stainless-steel reactor.hen the sealed reactor was heated up to preset temperature within0 min in a tube furnace and the temperature was maintained for5 min (the critical point of DMF: critical temperature 377 ◦C, crit-

cal pressure 4.4 MPa). After the EG was successfully exfoliatedong enough, the heat treatment was finished by putting the hoteactor into an ice-cold water bath. The obtained FG was filteredpolyvinylidene fluoride film (PVDF)) and washed several timesith fresh solvent. The products were vacuum dried overnight at

00 ◦C. Monolayer graphene was produced from FG by the samerocess.

.3. Characterization

XRD experiments were performed using X-ray diffractionD/max-2200/PC, Rigaku Corporation, Japan) with Cu K� radiation.he patterns were recorded using acceleration voltage of 40 kV andcceleration current of 20 mA.

Raman measurements were carried out using a Dispersiveaman Microscope (Senterra R200-L, Bruker Optics) with an exci-ation wavelength of 532 nm. TEM was performed on a JEOLEM-2100F field emission transmission electron microscope withn accelerating voltage of 200 kV. SEM was obtained on a FEI Sirion00 scanning electron microscope working at 5 kV.

Atomic Force Microscopy images of FG and monolayer graphenen freshly cleaved mica surface were taken with a Nanoscope IIIan tapping mode. A droplet of dispersion was cast onto a freshly

leaved mica surface after ultrasonication for 5 min in DMF. Theample was kept in vacuum dried at 60 ◦C overnight to let the resid-al solvent to evaporate. The AFM measurements were operated inir under ambient temperature and pressure.

dimethylformamide (DMF). It includes two steps: (1) producing few-layer grapheneby supercritical DMF from expandable graphite; (2) producing monolayer grapheneby supercritical DMF from few-layer graphene.

3. Results and discussion

High-yielding and good-quality FG is obtained by exfoliationof EG in supercritical DMF. The production process is deduced asshown in Fig. 1. This method consists of superior solvent as interca-lation molecule and EG with peculiar structure as acceptor. Herein,when fluids reach or exceed their critical point, they show vari-ous unique features, such as outstanding wetting of surfaces, lowinterfacial tension, low viscosity, and high diffusion coefficients[11]. These features make supercritical fluid easy to accomplishpenetration and expansion of the interlayer of EG. Meanwhile, EGis intercalated graphite. Because the intercalation of graphite isincomplete, so the space around the intercalated site is larger thanother position, this unique structure provides chance for fluid topenetrate. As a result, exfoliation firstly occurs at these intercalatedsites (Fig. 1), and the majority of the product is few-layer ratherthan monolayer. Monolayer graphene is obtained from FG owingto the unique characteristics of supercritical DMF. SEM is used todemonstrate the products of the supercritical process. Fig. 2 showsthe SEM images of EG, exfoliated EG, and exfoliated FG. Their sizesare 300–500 �m, 30–50 �m, 2–10 �m, respectively. The result indi-cates that supercritical treatment leads to the exfoliation of EG.

AFM is an efficient method to determine the thickness ofproducts. A large number of images were obtained by severalmeasurements (see supporting information). Fig. 4a and b showsa representative tapping mode AFM image of FG sheets and theheight profile in selected locations. The sizes and thicknesses vary,but as a whole, all the thicknesses of FG are roughly around 3 nm.This result shows that FG is successfully produced by exfoliation ofEG in supercritical DMF. The AFM image of exfoliated FG is shownin Fig. 4c and d. We find that the height is about 1.2 nm, which islarger than the theory thickness of single-layer graphene. Consid-ering the residual solvent molecular on both sides of graphene, theproducts are single-layer graphene. This result is also confirmedby TEM observation. Fig. 3 shows the TEM image of exfoliated FGand demonstrates the existence of monolayer graphene. The sizeof the monolayer graphene is micrometer and the edge tends toscroll. Electron diffraction (ED) patterns are used to confirm thecrystallinity of graphene. The ED patterns show the monolayergraphene is well-crystallized.

It is also crucial to ascertain whether the exfoliation of EG ishigh quality FG flakes. The XRD of EG and FG are shown in Fig. 5.

There is a strong diffraction peak for FG at 2� = 26.4◦ (d = 0.336 nm),which is corresponding to the normal graphite spacing. Besides,the intensity of diffraction peak of FG is as the same as EG, whichshows that after supercritical operation, their original pristine

C. Liu et al. / J. of Supercritical Fluids 63 (2012) 99–104 101

Fig. 2. SEM images of (a) EG, scale bar = 200 �m; (b) exfoliated E

Fig. 3. TEM image of exfoliated FG shows the existence of monolayer graphene, scalebar = 200 nm, and the inset shows the selected area electron diffraction patterns.

G, scale bar = 50 �m; (c) exfoliated FG, scale bar = 20 �m.

structure has been retained and no oxygen containing functionalgroups have been introduced. The XRD result indicates that thismethod can produce high quality FG and avoid introducing anystructure destruction, which is also supported by the followingRaman results. Moreover, EG has a peak at 2� = 26.1◦ (d = 0.341 nm),the average interlayer distance was slightly increased comparedto pure graphite (d = 0.335 nm). That is to say, there is a certaindegree of intercalation occurs in EG (Fig. 1), and these intercala-tion sites provide path for DMF intercalating. This unique structurefacilitates EG to fabricate FG rather than monolayer graphene. TheXRD result combining with AFM image exhibits that the EG is anexcellent starting material for preparing FG by exfoliation of EG insupercritical DMF.

Raman spectroscopy is a powerful tool for examining the qual-ity of obtained few-layer graphene. Generally, the Raman spectraof few-layer graphene exhibits two primary features: a G band at1580 cm−1 due to the E2g phonons at the Brillouin zone center, anda D band at 1350 cm−1 corresponding to breathing mode of k-pointphonons of A1g symmetry [8,14,15]. The Raman spectra of EG, FGand exfoliated FG are displayed in Fig. 6. There are three peaks forFG at 1350 cm−1 (D peak), 1580 cm−1 (G peak) and 2714 cm−1 (2Dpeak). The D peak in FG, is derived from the edge effects and impuri-ties of EG precursor [16], because same D peak also appeared in EG

spectrum. In addition, the intensity ratio (ID/IG) is about 0.3, whichis much less than most chemical reduction reports [17,18]. Thisresult confirms that the method of supercritical DMF exfoliationsuccessfully produce high quality FG. Finally, the Raman spectra

102 C. Liu et al. / J. of Supercritical Fluids 63 (2012) 99–104

Fig. 4. Typical tapping mode AFM images of (a) few-layer graphene sheets 2 �m × 2 �m, (b) few-layer graphene sheets 0.8 �m × 0.8 �m, (c) monolayer graphene sheets3 n thea about

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�m × 3 �m, and (d) monolayer graphene sheets 1.25 �m × 1.25 �m deposited overage thickness of few-layer graphene is about 3 nm and monolayer graphene is

re hardly different between FG and EG at 2D peak, further indi-ating that the obtained products are few-layer graphene. This isecause the Raman spectra of more than five atomic layers are hardo be distinguished from that of bulk graphite [19]. The 2D peak ofxfoliated FG shifts to 2699 cm−1 (Fig. 6), which proves that FG isuccessfully exfoliated in supercritical DMF.

In order to find out the unique characteristics of FG and achievearge-scale production, it is highly important to optimize operatingonditions. We have studied the effect of process parameters on theield of FG by exfoliation of EG in supercritical DMF. We have usedisible absorption spectroscopy (� = 660 nm) to characterize the

ield [20] (other data in supporting information). EG was treatedy supercritical DMF, and the products were precipitate, filtratednd dried. The dried products dispersed in fresh DMF by ultrasonic,aid it over night, centrifuged (FG is present in supernatant, the

mica substrate from dispersion, corresponding height cross-sectional profile. The1.2 nm.

remaining is unchanged EG) and subsequent analyzed by visibleabsorption spectroscopy [21]. For determining the standard con-centration, we filtered the FG dispersion through polyvinylidenefluoride films (PVDF), dried the sample in vacuum oven, weightedthe solid mass, and then got the concentration of dispersion. Theabsorbance of FG dispersion is well in line with the concentration(Fig. 7), obeying Beer’s law, which indicates that the dispersity andstability of FG in DMF are excellent.

The effect of concentration on the yield of FG achieved fromsupercritical DMF process was obtained at three levels in therange of 0.5–2 mg/ml. The operating conditions are fixed as in

Table 1. Fig. 8 shows the influence of concentration on the yield.The yield are various from 8.54% to 7%. These results reveal thatincreasing concentration gives rise to yield decreasing. The yield ishigher under lower concentration due to plenty of DMF facilitate

C. Liu et al. / J. of Supercritical Fluids 63 (2012) 99–104 103

10 20 30 40 50 60

FG

EG

(002)

(002)

2θ (degree)

Fig. 5. XRD patterns of EG and FG.

1000 1500 2000 2500 3000

2600 2700 2800

wavenumbers(cm-1)

a

b

c

Wavenumber(cm-1

)

a

b

c

2717

2714

2699

Fig. 6. Raman spectra of (a) EG; (b) FG; (c) exfoliated FG. (Inset) The enlarged 2Dpeak.

0.00 0.03 0.06 0.09 0.120.0

0.5

1.0

1.5

2.0

Absorb

an

ce

Concentration(mg/ml)

Fig. 7. Optical absorbance (� = 660 nm) as a function of the concentration of FG inDMF. The absorbance of FG dispersion is well in line with the concentration, whichindicates that the dispersity and stability of FG in DMF are excellent.

Table 1Processing conditions of different supercritical treatment.

Effect parameters Processing conditions

Time Temperature Volume ratio Concentration

Concentration 15 min 673 K 0.67 –Temperature 15 min – 0.67 2 mg/mlVolume ratio 15 min 673 K – 2 mg/ml

0.5 1.0 1.5 2.0

7

8

9

Yie

ld(%

)

Concentration(mg/ml)

Fig. 8. The effect of the concentration on the yield of FG by exfoliation of EG insupercritical DMF (operating conditions: temperature at 673 K, volume ratio at 0.67).

0.2 0.3 0.4 0.5 0.6 0.7 0.80

2

4

6

8

Yie

ld(%

)

Volume ratio

Fig. 9. The effect of the volume ratio on the yield of FG by exfoliation of EG in

supercritical DMF (operating conditions: temperature at 673 K, concentration at2 mg/ml).

intercalation (Relatively, there are much DMF molecules, but lessEG). Herein, 2 mg/ml is selected as the process concentration infollowing investigates.

The influence of volume ratio (the volume ratio is defined as thevolume of DMF divided by the volume of reactor) on the yield of FGobtained from supercritical DMF was examined at five levels in therange of 0.27–0.80. The process conditions are fixed as in Table 1.Fig. 9 shows the effect of volume ratio on the yield. These resultsindicate that the yield is going up with increasing volume ratio from0.27 to 0.80. It is well known that adding volume ratio will lead toenhancement of pressure when the temperature is constant. Thismay be attributed to increasing supercritical DMF density at higherpressures, leading to better intercalation. Therefore, 0.67 is selectedas the process volume ratio in following studies.

The effect of temperature on the yield of FG received fromsupercritical DMF was scrutinized at eight levels in the range of423–773 K. The process conditions are fixed as in Table 1. Theresults are shown in Fig. 10, which exhibits that the yield enhanceswith increasing temperature from 423 K to 673 K. This is becauseincreasing temperature leads to decreasing viscosity of the DMFand enhancing its diffusion, which is helpful to intercalation. How-ever, above 723 K, the yield drastically reduces with increasingtemperature, which is likely due to DMF decomposition abovethat temperature [13]. Otherwise, the FG begins to formation from

473 K. The low formation temperature suggests that EG is an excel-lent starting material for fabricating FG. Consequently, 673 K isselected as the process temperature.

104 C. Liu et al. / J. of Supercritical F

400 500 600 700 8000

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Temperature(K)

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Krishnamurthy, R. Goodhue, J. Hutchison, V. Scardaci, A.C. Ferrari, J.N. Cole-

ig. 10. The effect of the temperature on the yield of FG by exfoliation of EG in super-ritical DMF (operating conditions: volume ratio at 0.67, concentration at 2 mg/ml).

. Conclusions

In conclusion, we have supplied a simple and rapid methodor producing FG by exfoliation of EG in supercritical DMF. Single-ayer graphene can be gained by exfoliation of FG in supercriticalMF. The FG and monolayer graphene generation process is low-ost, easy to scale up, and less toxic than chemical reduction ofraphene oxide which includes hazardous reducing agents suchs hydrazine, hydroquinone, dimethylhydrazine. Furthermore, weave investigated the influence of process parameters on the yieldf FG. It is found that concentration, temperature, and volume ratioould highly affect the yield. Finally, its liquid phase facilitates thepplication through spray coating, spin coating, drop casting or vac-um filtration. Consequently, we suppose that this method willave the way for the mass production and practical applicationf FG.

cknowledgments

The authors acknowledge financial support from the Nationalatural Science Foundation of China (no. 51076094) and thank

he Instrumental Analysis Center of Shanghai Jiao Tong UniversitySJTU) for AFM, SEM, Raman measurements.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.supflu.2012.01.002.

[

luids 63 (2012) 99–104

References

[1] R.S. Ruoff, Graphene-based ultracapacitors, Nano Letters 8 (2008) 3498–3502.[2] C.G. Lee, X.D. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties

and intrinsic strength of monolayer graphene, Science 321 (2008) 385–388.[3] A.A. Balandin, S. Ghosh, W.Z. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau,

Superior thermal conductivity of single-layer graphene, Nano Letters 8 (2008)902–907.

[4] K.I. Bolotin, K.J. Sikes, Z. Jiang, M. Klima, G. Fudenberg, J. Hone, P. Kim, H.L.Stormer, Ultrahigh electron mobility in suspended graphene, Solid State Com-munications 146 (2008) 351–355.

[5] Z.X. Xu, H.Y. Gao, G.X. Hu, Solution-based synthesis and characterization of asilver nanoparticle-graphene hybrid film, Carbon 49 (2011) 4731–4738.

[6] 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 ingraphene, Nature 438 (2005) 197–200.

[7] D. Graf, F. Molitor, K. Ensslin, C. Stampfer, A. Jungen, C. Hierold, L. Wirtz, Spa-tially resolved Raman spectroscopy of single- and few-layer graphene, NanoLetters 7 (2007) 238–242.

[8] A. Reina, X.T. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M.S. Dresselhaus, J. Kong,Large area, few-layer graphene films on arbitrary substrates by chemical vapordeposition, Nano Letters 9 (2009) 30–35.

[9] H.X. Zhang, P.X. Feng, Fabrication and characterization of few-layer graphene,Carbon 48 (2010) 359–364.

10] Z.S. Wu, W.C. Ren, L.B. Gao, B.L. Liu, C.B. Jiang, H.M. Cheng, Synthesis of high-quality graphene with a pre-determined number of layers, Carbon 47 (2009)493–499.

11] G.K. Serhatkulu, C. Dilek, E. Gulari, Supercritical CO2 intercalation of layeredsilicates, Journal of Supercritical Fluids 39 (2006) 264–270.

12] N.W. Pu, C.A. Wang, Y. Sung, Y.M. Liu, M.D. Ger, Production of few-layergraphene by supercritical CO2 exfoliation of graphite, Materials Letters 63(2009) 1987–1989.

13] D. Rangappa, K. Sone, M.S. Wang, U.K. Gautam, D. Golberg, H. Itoh, M. Ichihara,I. Honma, Rapid and direct conversion of graphite crystals into high-yielding,good-quality graphene by supercritical fluid exfoliation, Chemistry: A Euro-pean Journal 16 (2010) 6488–6494.

14] B. Tang, G.X. Hu, H.Y. Gao, Raman spectroscopic characterization of graphene,Applied Spectroscopy Reviews 45 (2010) 369–407.

15] A.W. Musumeci, E.R. Waclawik, R.L. Frost, A comparative study of single-walledcarbon nanotube purification techniques using Raman spectroscopy, Spec-trochimica Acta Part A: Molecular and Biomolecular Spectroscopy 71 (2008)140–142.

16] W.M. Zhou, X. Liu, Y.F. Zhang, Simple approach to �-SiC nanowires: synthesis,optical, and electrical properties, Applied Physics Letters 89 (2006) 223124.

17] Y. Zhou, Q.L. Bao, L.A.L. Tang, Y.L. Zhong, K.P. Loh, Hydrothermal dehydrationfor the green reduction of exfoliated graphene oxide to graphene and demon-stration of tunable optical limiting properties, Chemistry of Materials 21 (2009)2950–2956.

18] W. Gao, L.B. Alemany, L.J. Ci, P.M. Ajayan, New insights into the structure andreduction of graphite oxide, Nature Chemistry 1 (2009) 403–408.

19] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec,D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Raman spectrum of graphene andgraphene layers, Physical Review Letters 97 (2006) 187401.

20] Y. Hernandez, V. Nicolosi, M. Lotya, F.M. Blighe, Z.Y. Sun, S. De, I.T. Mcgov-ern, B. Holland, M. Byrne, Y.K. Gun’ko, J.J. Boland, P. Niraj, G. Duesberg, S.

man, High-yield production of graphene by liquid-phase exfoliation of graphite,Nature Nanotechnology 3 (2008) 563–568.

21] U. Khan, A. O’Neill, M. Lotya, S. De, J.N. Coleman, High-concentration solventexfoliation of graphene, Small 6 (2010) 864–871.