12950 | Chem. Commun., 2015, 51, 12950--12953 This journal is©The Royal Society of Chemistry 2015

Cite this:Chem. Commun., 2015,

51, 12950

Exfoliated semiconducting pure 2H-MoS2 and2H-WS2 assisted by chlorosulfonic acid†

Georgia Pagona,*a Carla Bittencourt,b Raul Arenalcd and Nikos Tagmatarchis*a

Chlorosulfonic acid assisted the exfoliation of MoS2 and WS2

resulting in retaining their semiconducting 2H-phase, sharply con-

trasting the semiconducting-to-metallic phase-transition observed

with the currently available exfoliation techniques.

Two-dimensional (2D) layered materials have attracted tremen-dous research enthusiasm due to their intriguing physics andpotential applications for electronic devices.1 Unlike graphene,inorganic 2D transition metal dichalcogenides (TMDs) exhibit athickness dependent band-gap,2 while in field-effect-transistorsthey show high on-off ratios,3 with mobility reaching 100 cm2 V�1

s�1.4 Furthermore, TMDs hold strong promise in technologicalapplications, particularly in the electrochemically assisted produc-tion of hydrogen,5 energy storage,6 and sensing.7 Such layeredTMDs are composed of 2D sheets, stacked on top of one anotherby weak interlayered interactions, facilitating the shearing ofthe TMD layers. Notably, those TMDs with analogous structuresto that of graphene, represented by the general formula MX2

(M = Mo, W, Ti, etc.; X = S, Se, Te), are realized in two crystalstructures, depending on the metal coordination with sixchalcogens, specified as 2H-MX2 and 1T-MX2. Particularly, the2H-MX2 is a trigonal prismatic structure, with D6h symmetry,formed by the stacking of weakly interacting 2D MX2 layers,while the 1T-MX2 is an octahedral structure, with D3d sym-metry.8 Markedly, the electronic properties of each phase aredissimilar, namely, the 2H-phase is semiconducting and emis-sive, while the 1T-phase is metallic without photoluminescenceproperties.9

Favorite routes toward preparing exfoliated 2D TMDs fromcommercially available layered bulk crystals mostly rely ondiverse top-down approaches mainly comprising mechanicalcleavage10 and liquid phase exfoliation. A significant amountof the latter, encompassing wet-chemistry, is related to thechemical and electrochemical intercalation with BuLi11 andsonication in common solvents that possess a surface energycomparable to that of the TMDs,12 while avoiding hazardousorganometallic intercalants. However, regardless of the progressachieved so far, each methodology suffers from certain limitationsand drawbacks. For example, the mechanical exfoliation is a lowthroughput method with inherent inconvenience, hence, yieldinguninterrupted films of TMD flakes, while BuLi intercalation, as themain exfoliation route, so far, results in the conversion of thesemiconducting 2H-phase of bulk TMDs to the metallic 1T-phaseupon exfoliation. The latter semiconducting-to-metallic transforma-tion is stabilized by intercalated Li due to the electron-transferphenomena,9,13 hence, restricting TMDs from practical applica-tions in which novel electronic properties are required. Therefore, itis both timely and of paramount importance to develop alternativeroutes and processes to overcome such constraints.

Herein, build up on the knowledge that chlorosulfonic acid hasbeen successfully applied not only to debundle single-walledcarbon nanotubes14 but also to exfoliate graphite;15 the issue ofexfoliating TMDs while simultaneously retaining their originalsemiconducting properties, thereby contrasting all existing exfolia-tion methodologies, is addressed. Considering the lone electrons ofsulphur, both MoS2 and WS2 act as a weak base, which can beprotonated by chlorosulfonic acid, however, without being oxi-dized. This fractional removal of electrons in protonated TMDs isthe initial step toward exfoliation due to repulsive electrostaticforces developing between the layers of TMDs. Notably, deprotona-tion can take place in the presence of a competing base such aswater, similar to what observed in carbon nanotubes.16 This is,retaining their electronic properties, the major advantage foremploying chlorosulfonic acid toward exfoliating TMDs, as com-pared with e.g. BuLi, which in sharp contrast results in irreversibledoping of the material. This unprecedented outcome will facilitate

a Theoretical and Physical Chemistry Institute, National Hellenic Research

Foundation, 48 Vassileos Constantinou Avenue, 11635 Athens, Greece.

E-mail: tagmatar@eie.gr, gpagona@eie.grb Chimie des Interactions Plasma-Surface, University of Mons, 20 Place du Parc,

7000 Mons, Belgiumc Laboratorio di Microscopias Avanzadas, Instituto de Nanosciencia de Aragon,

Universidad de Zaragoza, 50018 Zaragoza, Spaind ARAID Foundation, 50018 Zaragoza, Spain

† Electronic supplementary information (ESI) available: Experimental sectionand additional characterization data. See DOI: 10.1039/c5cc04689k

Received 8th June 2015,Accepted 6th July 2015

DOI: 10.1039/c5cc04689k

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and enable charge-transfer phenomena within exfoliated TMD-basedhybrid materials and electrodes.

Briefly, chlorosulfonic acid was added to bulk MoS2 and WS2

and the mixture was bath sonicated, allowing the insertion ofchlorosulfonic acid molecules between the layers of TMDs,keeping them apart due to developed electrostatic repulsiveforces. Then, under vigorous stirring, water was added, resultingin thermal decomposition of the superacid, simultaneously gen-erating a large amount of local heat facilitating exfoliation, while,pressure built up by the gaseous HCl produced eventually causesadditional expansion of the layers and further exfoliation of theTMD sheets. The exfoliated MoS2 and WS2 were soluble in NMPand DMF as well as in water, while precipitated within 24 h inMeOH, CH2Cl2 and toluene. Nevertheless, those stable solutionsin NMP allowed initiating spectroscopic studies under liquidconditions.

Undisputable evidence for exfoliated MoS2 retaining the semi-conducting 2H-phase initially arose from electronic absorptionspectroscopic studies. In the UV-Vis spectrum of exfoliated MoS2

(Fig. 1a), four characteristic absorption bands—excitonic A and Bbands at 670 and 610 nm, respectively, and direct transitions fromthe valence band to the conduction band, C and D at 450 and400 nm, respectively—due to the trigonal prismatic semiconducting2H-phase were observed,9,17 emphasizing the presence of a directband-gap. Notably, the highly resolved A and B excitonic bands,18

resembling those derived from mechanically exfoliated mono-layered MoS2, guarantee the presence of a vastly ordered structure.Furthermore, the high absorption coefficients of A and B bands andthe overall smooth absorbance depression of the electronic absorp-tion spectrum in the NIR region, below the band-gap of semicon-ducting 2H-MoS2, confirm the absence of metallic components.Similarly, as far as exfoliated WS2 concerns, the 2H-phase wasestablished by registering the corresponding UV-Vis spectrum, inwhich the characteristic excitonic A band at 628 nm was prominent,while a shoulder associated with the indirect B excitonic transitionat 525 nm, however, not as prominent as for the isomorphous MoS2

analogue, was evident (Fig. 1b). The C band at 454 nm, developedfrom optical transitions between the DOS peaks in the valence and

conduction bands, is characteristic of the semiconducting characterwith a direct band-gap. Collectively, these results significantlydiverge from the state-of-the-art (i.e. exfoliated TMDs by BuLi, inwhich mixed-phase structures, semiconducting 2H and metallic 1T,co-exist), where the excitonic A and B bands are basically notfeatured,9 while in the current case they are fully developed andresolved.

The molar absorptivity for exfoliated MoS2 and WS2 wascalculated to be 1.887 L g�1 m�1 at 448 nm and 1.249 L g�1 m�1

at 525 nm, respectively. Solubility values in NMP for exfoliatedMoS2 and WS2 were estimated to be 8.9 and 6.1 mg L�1,respectively (for solubility values of exfoliated MoS2 and WS2

in other solvents, see the ESI,† Table S1), by applying the Beer–Lambert law.

The photoluminescence spectrum of exfoliated 2H-MoS2 in NMPupon excitation at 450 nm showed, beyond direct excitonic transi-tions at 627 and 685 nm, a strong emissive band at 535 nm(Fig. 1c).9,18 Similar fluorescence emission spectra were recordednot only when the excitation was varied in the range of 400–600 nm,though red-shifted with increasing excitation wavelength (ESI,†Fig. S1), but also when registered as thin-films (ESI,† Fig. S2). Thestrong emission at 535 nm verifies the presence of semiconducting2H-MoS2, with a direct band-gap, notably contrasting the situationof bulk MoS2, which as an indirect-gap material only shows weakphonon-assisted processes with negligible quantum yield. It isneedless to mention that photoluminescence in metallic 1T-MoS2

is absent. Similarly, the photoluminescence properties of exfoliatedWS2, in NMP upon excitation at 450 nm, were revealed. The mostnotable feature was a strong emissive band in the range of 500–700 nm, which was deconvoluted into two discrete bands withpeaks at 528 nm and 630 nm (Fig. 1d). The latter is attributed tothe energy of the A excitonic absorption band, while the formercorresponds to the direct band-gap emission.18 A collection ofphotoluminescence spectra in NMP, with varied excitations in therange of 450–590 nm, showing characteristic emissions due to theband-gap and excitonic transitions are presented in the ESI,†Fig. S3, while photoluminescence spectra based on thin-films ofexfoliated WS2 are presented in Fig. S4 (ESI†). Cooperatively, thephotoluminescence assays validate the finding that chlorosulfonicacid only acts as an intercalant without modifying or disturbing theelectronic properties of exfoliated MoS2 or WS2, thereby, preservingtheir semiconducting character.

A solid support for the manifestation of the 2H-MoS2 semi-conducting phase was conveyed by X-ray photoelectron spectro-scopy (XPS) studies. From the high-resolution XPS deconvolutedspectra of the Mo 3d orbitals, peaks at 232.3 and 229.1 eV,corresponding to Mo4+ 3d3/2 and 3d5/2, respectively, were observed(Fig. 2a). The latter peaks are directly related to the 2H-phase.9

Likewise, in the S 2p region, peaks at 163.1 and 161.9 eV weredeconvoluted (Fig. 2b) and attributed to the S 2p1/2 and 2p3/2

orbitals, respectively, due to the semiconducting 2H-phase. The S2s peak was evident at 226.4 eV. At this stage, it is also of primaryimportance to note the almost complete absence of a signal ataround 236 eV, corresponding to oxidized Mo6+ 3d5/2, hence,verifying that chlorosulfonic acid only promotes the exfoliationof MoS2, due to intercalation, without damaging by oxidation the

Fig. 1 (a and b) UV-Vis spectra of exfoliated 2H-MoS2 and 2H-WS2. (c and d)Photoluminescence (red) of exfoliated 2H-MoS2 and 2H-WS2 (NMP,lexc. 450 nm) and their deconvoluted (blue) spectra.

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12952 | Chem. Commun., 2015, 51, 12950--12953 This journal is©The Royal Society of Chemistry 2015

lattice, while also without participating in electron-transfer pro-cesses that would result in the semiconducting-to-metallic phase-transition. Along the same lines, the same holds true whenmonitoring the S 2p orbitals, where no signals due to oxidizedsulfur emerged (i.e. expected at around 168 eV). Interestingly,according to the literature, the XPS analysis of BuLi exfoliatedMoS2 revealed a different picture. In fact, comparing when Li wasintercalated within galleries of MoS2, the high-resolution deconvo-luted spectra of Mo 3d, owed to Mo4+, revealed not only peaks dueto the trigonal prismatic and semiconducting 2H-phase but alsodue to the octahedral and metallic 1T-phase at lower energies.9

Similarly, additional signals to those ascribed to the 2H-phase(cf. Fig. 2b) were observed in the deconvoluted spectrum of S 2p1/2

and 2p3/2, at lower energies, owing to metallic 1T-MoS2.9 Collec-tively, the outcome of our XPS results strongly supports the notionthat the semiconducting 2H-phase of MoS2 is preserved whenexfoliation is assisted by chlorosulfonic acid, contrasting thetransition to metallic 1T-MoS2, evidently ensued upon Li intercala-tion. As far as the XPS of exfoliated WS2 concerns, peaks due to thepresence of W 5p3/2, 4f5/2 and 4f7/2 orbitals were identified at 38.9,35.3 and 33.1 eV (Fig. 2c), respectively, while also peaks for Scorresponding to the 2p1/2 and 2p3/2 orbitals at 163.2 and 162.1 eV,respectively, were also observed (Fig. 2d). The energy positions ofthose signals are supportive of W4+ as well as the formation of pure2H-WS2.19

Aberration-corrected STEM was employed to investigate thelocal (atomic) structure of the exfoliated 2H-MoS2 and 2H-WS2

materials. Indeed, high-angle annular dark field (HAADF)STEM is a technique offering chemical Z-contrast via themeasurement of the intensity of the image.20 In Fig. 3a a low-magnification HAADF-STEM image of one MoS2 flake having alateral size of around 1 mm is shown. The atomic resolutionHAADF-STEM imaging (Fig. 3b) confirms the 2H-MoS2 crystal-line structure of the particular sheet, which is more evident inthe zoomed image (Fig. 3c). In the latter micrograph, thedifference in intensity between the spots forming the hexagonalnetwork indicates their different chemical nature, Mo (Z = 42)and S (Z = 16), respectively. An intensity profile, obtained fromthe white line marked in this figure, is plotted in Fig. 3d.

Notably, from the latter intensity profile and micrograph, aswell as based on recent studies, it is deduced that this area iscomposed of three layers of MoS2 possessing semiconductingproperties.21 Atomically-resolved HAADF-STEM analyses werealso confirmed the 2H crystalline structure of exfoliated WS2.Particularly, in Fig. 3e and f, representative low and highHAADF-STEM images of a WS2 flake are depicted and thedifferent layers are clearly observed. Comparing to the previouscase, i.e. MoS2, here the Z difference between W (Z = 74) andS (Z = 16) is higher, having an evident impact in the differencesin contrast in the atomic-resolved Z-contrast STEM images(Fig. 3h and j).

Finally, Raman spectroscopy was employed to confirm theatomic structural arrangement in exfoliated MoS2 and WS2. Ingeneral, Raman measurements, upon in-resonance 633 nmexcitation, revealed more bands as compared with assaysperformed under off-resonance scattering conditions, with514 nm excitation, due to strong electron–phonon couplinginteractions. For 2H-MoS2, two strong Raman bands, deconvo-luted by a single Lorentzian centred at 383 cm�1 and 407 cm�1

(ESI,† Fig. S5), were assigned to in-plane E12g and out-of-plane

Fig. 2 Deconvoluted XPS of selected core level peak regions: (a) Mo 3dand S 2s, and (b) S 2p, for exfoliated MoS2; (c) W 5p and 4f, and (d) S 2p, forexfoliated WS2.

Fig. 3 (a) Low magnification HAADF-STEM image of a MoS2 flake.(b) HAADF-STEM micrograph showing one of the areas at the edge ofthe MoS2 flake examined in (3a). Scale bar is 1 nm. (c) Zoom of the red-marked area shown in (b). (d) Intensity profile extracted from the white lineplotted in (c). (e and f) Low and high magnification HAADF-STEM images ofa WS2 flake, in which the stacking of the layers is clearly observed.(g) HAADF-STEM micrograph showing one of the areas at the edge ofthe WS2 flake examined in (f). Scale bar is 1 nm. (h and j) Zoom of the redand green marked areas shown in (g). Scale bars are 0.5 nm. (i and k)Intensity profiles extracted from the dotted red lines plotted in (i) and (k).

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A1g vibrational modes, with no evidence of structural distortion,10,22

inferring the absence of structure damage and/or covalent bondformation upon treatment with chlorosulfonic acid. Careful inspec-tion of the spectra further revealed that the A1g and E1

2g modes forexfoliated MoS2 appeared with equal intensities, contrasting thecase of the bulk material, indicating weaker coupling between theelectronic transition at the K point with the A1g phonon existing inultra-thin MoS2 flakes.22 The frequency difference between A1g andE1

2g found to be 24 cm�1, reduced when compared to the valuederived from the bulk material (ca. 27 cm�1), signifying the successof exfoliation and the existence of 2H-MoS2 in few layers.22

Acquisition of numerous Raman spectra at different positions ofthe sample showed that the peak positions and FWHM valuesfor E1

2g and A1g remain identical in all spectra, revealing highhomogeneity for exfoliated MoS2. Similarly, intense Raman bandscentred at 350 cm�1 and 419 cm�1 assigned to the E1

2g and A1g

modes,23 respectively, for exfoliated 2H-WS2 were evident (ESI,†Fig. S6). The observed bands were clearly different from those ofbulk WS2, both in terms of Raman frequency and signal intensity.

Concluding, the exfoliation of MoS2 and WS2, by treating thecorresponding bulk materials with chlorosulfonic acid wassucceeded. This achievement led not only to solubility enhance-ment, but most importantly also to the preservation of thesemiconducting 2H-phase for exfoliated MoS2 and WS2, whichwas confirmed by optical studies and XPS spectroscopy.Furthermore, the exfoliated materials were characterized usingRaman spectroscopy as well as HAADF-HR-STEM imaging. Theparticular facile approach, together with the easy manipulationof the exfoliated layered materials in solution, will allow theirfull exploitation for diverse technological applications.

CB is a research associate at the National Fund for ScientificResearch (FRS-FNRS, Belgium). Financial support from FRS-FNRS under contract 2.4577.11 (Chemographene) is acknowl-edged. The HR-STEM studies were conducted at the Laboratoriode Microscopias Avanzadas, Instituto de Nanociencia de Aragon,Universidad de Zaragoza, Spain and supported by EU FP7 underGrant Agreements 312483 – ESTEEM2 (Integrated InfrastructureInitiative – I3) and 604391 Graphene Flagship. RA acknowledgesfunding from the Spanish Ministerio de Economia y Competitividad(FIS2013-46159-C3-3-P). RA and NT acknowledge financial supportfrom the EU H2020 ETN project ‘‘Enabling Excellence’’ under GrantAgreement 642742.

Notes and references1 M. Xu, T. Liang, M. Shi and H. Chen, Chem. Rev., 2013, 113, 3766.2 T. Cheiwchanchamnangij and W. R. L. Lambrecht, Phys. Rev. B:

Condens. Matter Mater. Phys., 2012, 85, 205302.3 B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis,

Nat. Nanotechnol., 2011, 6, 147.4 Q. H. Wang, K. Kalantar-zadeh, A. Kis, J. N. Coleman and

M. S. Strano, Nat. Nanotechnol., 2012, 7, 699.

5 A. B. Laursen, S. Kegnase, S. Dahl and I. Chorkendorff, EnergyEnviron. Sci., 2012, 5, 5577; D. Voiry, H. Yamaguchi, J. Li, R. Silva,D. C. B. Alves, T. Fujita, M. Chen, T. Asefa, V. B. Shenoy, G. Eda andM. Chhowalla, Nat. Mater., 2013, 12, 850; D. Voiry, M. Salehi,R. Silva, T. Fujita, M. Chen, T. Asefa, V. B. Shenoy, G. Eda andM. Chhowalla, Nano Lett., 2013, 13, 6222; J. R. McKone,S. C. Marinescu, B. S. Brunschwig, J. R. Winkler and H. B. Gray,Chem. Sci., 2014, 5, 865.

6 H. Hwang, H. Kim and J. Cho, Nano Lett., 2011, 11, 4826; T. Stephenson,Z. Li, B. Olsen and D. Mitlin, Energy Environ. Sci., 2014, 7, 209.

7 J. Z. Ou, A. F. Chrimes, Y. Wang, S.-Y. Tang, M. S. Strano andK. Kalantar-zadeh, Nano Lett., 2014, 14, 857.

8 M. Chhowalla, H. S. Shin, G. Eda, L.-J. Li, K. P. Loh and H. Zhang,Nat. Chem., 2013, 5, 263.

9 G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen andM. Chhowalla, Nano Lett., 2011, 11, 5111.

10 C. Lee, Q. Y. Li, W. Kalb, X. Z. Liu, H. Berger, R. W. Carpick andJ. Hone, Science, 2010, 328, 76; S. Ghatak, A. N. Pal and A. Ghosh,ACS Nano, 2011, 5, 7707.

11 H. S. S. R. Matte, A. Gomathi, A. K. Manna, D. J. Late, R. Datta,S. K. Pati and C. N. R. Rao, Angew. Chem., Int. Ed., 2010, 49, 4059;Z. Y. Zeng, Z. Y. Yin, X. Huang, H. Li, Q. Y. He, G. Lu, F. Boey andH. Zhang, Angew. Chem., Int. Ed., 2011, 50, 11093.

12 J. N. Coleman, M. Lotya, A. O’Neill and S. D. Bergin, et al., Science,2011, 331, 568; G. Cunningham, M. Lotya, C. S. Cucinotta,S. Sanvito, S. D. Bergin, R. Menzel, M. S. P. Shaffer and J. N.Coleman, ACS Nano, 2012, 6, 3468.

13 G. Eda, T. Fujita, H. Yamaguchi, D. Voiry, M. Chen and M. Chhowalla,ACS Nano, 2012, 6, 7311.

14 V. A. Davis, A. Nicholas, G. Parra-Vasquez and M. J. Green, et al., Nat.Nanotechnol., 2009, 4, 830.

15 N. Behabtu, J. R. Lomeda, M. J. Green, A. L. Higginbotham,A. Sinitskii, D. V. Kosynkin, D. Tsentalovich, A. Nicholas, G. Parra-Vasquez, J. Schmidt, E. Kesselman, Y. Cohen, Y. Talmon, J. M. Tourand M. Pasquali, Nat. Nanotechnol., 2010, 5, 406; N. I. Kovtyukhova,Y. Wang, A. Berkdemir, R. Cruz-Silva, M. Terrones, V. H. Crespi andT. E. Mallouk, Nat. Chem., 2014, 6, 957; W. Lu, S. Liu, X. Qin,L. Wang, J. Tian, Y. Luo, A. M. Asiri, A. O. Al-Youbi and X. Sun,J. Mater. Chem., 2012, 22, 8775.

16 S. Ramesh, L. M. Ericson, V. A. Davis, R. K. Saini, C. Kittrell,M. Pasquali, W. E. Billups, W. W. Adams, R. H. Hauge andR. E. Smalley, J. Phys. Chem. B, 2004, 108, 8794.

17 E. P. Nguyen, B. J. Carey, T. Daeneke, J. Z. Ou, K. Latham, S. Zhuiykovand K. Kalantar-zadeh, Chem. Mater., 2015, 27, 53; B. J. Carey,T. Daeneke, E. P. Nguyen, Y. Wang, J. Z. Ou, S. Zhuiykov andK. Kalantar-zadeh, Chem. Commun., 2015, 51, 3770.

18 A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galliand F. Wang, Nano Lett., 2010, 10, 1271; K. F. Mak, C. Lee, J. Hone,J. Shan and T. F. Heinz, Phys. Rev. Lett., 2010, 105, 136805; Y. Yu,C. Li, Y. Liu, L. Su, Y. Zhang and L. Cao, Sci. Rep., 2013, 3, 1866;W. Zhao, Z. Ghorannevis, L. Chu, M. Toh, C. Kloc, P.-H. Tan andG. Eda, ACS Nano, 2013, 7, 791.

19 X. Mao, Y. Xu, Q. Xue, W. Wang and D. Gao, Nanoscale Res. Lett.,2013, 8, 430.

20 S. J. Pennycook and P. D. Nellist, Scanning Transmission ElectronMicroscopy: Imaging and Analysis, Springer, 2011; F. L. Deepak,A. Mayoral and R. Arenal, Advanced Transmission Electron Micro-scopy: Applications to Nanomaterials, Springer, 2015.

21 R. J. Wu, M. L. Odlyzko and K. A. Mkhoyan, Ultramicroscopy, 2014,147, 8.

22 H. Li, Q. Zhang, C. C. R. Yap, B. K. Tay, T. H. T. Edwin, A. Olivier andD. Baillargeat, Adv. Funct. Mater., 2012, 22, 1385; S.-L. Li,H. Miyazaki, H. Song, H. Kuramochi, S. Nakaharai andK. Tsukagoshi, ACS Nano, 2012, 6, 7381.

23 H. Terrones, E. Del Corro, S. Feng, J. M. Poumirol, D. Rhodes, D. Smirnov,N. R. Pradhan, Z. Lin, M. A. T. Nguyen, A. L. Elias, T. E. Mallouk,L. Balicas, M. A. Pimenta and M. Terrones, Sci. Rep., 2014, 4, 4215.

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