High yield exfoliation of two-dimensional chalcogenides using sodium naphthalenide

Abstract

Transition-metal dichalcogenides like molybdenum disulphide have attracted great interest as two-dimensional materials beyond graphene due to their unique electronic and optical properties. Solution-phase processes can be a viable method for producing printable single-layer chalcogenides. Molybdenum disulphide can be exfoliated into monolayer flakes using organolithium reduction chemistry; unfortunately, the method is hampered by low yield, submicron flake size and long lithiation time. Here we report a high-yield exfoliation process using lithium, potassium and sodium naphthalenide where an intermediate ternary Li(x)MX(n) crystalline phase (X=selenium, sulphur, and so on) is produced. Using a two-step expansion and intercalation method, we produce high-quality single-layer molybdenum disulphide sheets with unprecedentedly large flake size, that is up to 400 μm(2). Single-layer dichalcogenide inks prepared by this method may be directly inkjet-printed on a wide range of substrates.

ARTICLE

Received 21 Aug 2013 | Accepted 22 Nov 2013 | Published 2 Jan 2014

Jian Zheng1, Han Zhang1, Shaohua Dong1, Yanpeng Liu1, Chang Tai Nai1, Hyeon Suk Shin2, Hu Young Jeong2, Bo Liu1 & Kian Ping Loh1

Transition-metal dichalcogenides like molybdenum disulphide have attracted great interest as two-dimensional materials beyond graphene due to their unique electronic and optical properties. Solution-phase processes can be a viable method for producing printable single-layer chalcogenides. Molybdenum disulphide can be exfoliated into monolayer akes using organolithium reduction chemistry; unfortunately, the method is hampered by low yield, submicron ake size and long lithiation time. Here we report a high-yield exfoliation process using lithium, potassium and sodium naphthalenide where an intermediate ternary LixMXn crystalline phase (X selenium, sulphur, and so on) is produced. Using a two-step expansion

and intercalation method, we produce high-quality single-layer molybdenum disulphide sheets with unprecedentedly large ake size, that is up to 400 mm2. Single-layer dichalcogenide inks prepared by this method may be directly inkjet-printed on a wide range of substrates.

DOI: 10.1038/ncomms3995

High yield exfoliation of two-dimensional chalcogenides using sodium naphthalenide

1 Department of Chemistry and Graphene Research Centre, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore.

2 Interdisciplinary School of Green Energy and Low Dimensional Carbon Materials Center, UNIST Central Research Facilities (UCRF), Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 689-805, Korea. Correspondence and requests for materials should be addressed to K.P.L. (email: mailto:[email protected]

Web End [email protected] ).

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Post graphene discovery, single-layered transition-metal dichalcogenides (LTMDs), such as MoS2 and WS2, have attracted great attention as next generation two-

dimensional (2D) materials owing to their large intrinsic bandgap113, which is particularly attractive in view of the gapless nature of graphene1416. Single-layer MoS2 has attractive attributes such as a direct bandgap (1.9 eV), large in-plane mobilities (200500 cm2 V 1 s 1), high current on/off ratios (exceeding 108), as well as remarkable mechanical and optical properties17,18. Two-dimensional quantum connement of carriers can be exploited in conjunction with chemical composition to tune the optoelectronic properties of the metal chalcogenides at the nanoscale. These properties are of great interest for applications in optoelectronic devices such as thin lm solar cells, photodetectors, exible logic circuits and sensors1923.

Transition-metal chalcogenides (TMC) possessing lamellar structures can serve as hosts for the intercalation of a wide variety of electron-donating species ranging from Lewis base to alkali metals24,25. One well-known class of intercalatants is the organolithium compounds. MoS2 can be intercalated with lithium to give the reduced LixMXn phase (X Se, S, and so

on) with expanded lattice, this can be exfoliated in a second step into single-layer sheets by ultrasound-assisted hydration process2629. However long lithiation time (for example, 3 days at 100 C) is typically needed when n-butyl lithium is used as lithiation agent due its inclination to form dimeric, trimeric and higher aggregates, which diffuse slowly into the interlayers in large-sized TMC crystals. Subsequent exfoliation from the lithiated MoS2 suffers a low yield of single-layer akes, disintegration into submicron-sized ake, formation of metal nanoparticles and precipitation of Li2S5,29. This has limited the development of solution-processed LTMDs in most applications that demand clean and large-sized akes. An electrochemical approach has been developed recently to produce single-layer MoS2 and WS2 akes; however, the upward scaling of this process is limited by volumetric resistance in battery-type cells30. In such electrochemical cells, 10% acetylene black nanoparticles are typically added to the host materials to reduce volumetric resistance but this creates one problem: carbon nanoparticles are mixed with LTMDs and these contaminants are hard to be dislodged from the surface. Similar to the research directions in solution-processed graphene, there is a clear need to explore new chemistry to make high-quality single-layer LTMDs in high yield.

The intercalation of lithium into layered molybdenum disulphide may be described as an ionelectron transfer topotactic reaction. In most reported papers, organolithium reagents were used as intercalating agents because of its solubility in a wide range of solvents and the formation of stoichiometric LiMoS2 ternary products2629. Compared with Li ions, other alkali ions such as Na or K were less commonly used in exfoliation chemistry. The ionic radii of Na and K are several times larger than that of Li ions, which means that in principle these ions can expand the lattice in the c-axis direction to a larger extent. More importantly, Na and K intercalation compounds react more violently with water than Li compounds, implying that single-layer TMDs should be exfoliated more efciently. Intercalation of Na and K can also produce different structural and electronic effect compared with Li due to different coordinative complexation by the host. In LixMXn, Li is always octahedrally coordinated; however, K and Na can occupy octahedral or trigonal prismatic site. This has important implications electronically due to the metallic to semiconductor properties transition in these compounds. Metal electride solution consisting of Na in concentrated liquid ammonia can be a

powerful reducing agent, except that ammonia molecule has a tendency to coordinate with Mo and displace S, this can result in decomposition and segregation of Mo nanoparticles31,32. In spite of numerous reports that discussed the exfoliation of LTMDs, few of these methods can meet the demand of producing high yield, high purity and large-sized akes33,34.

Our search for alkali metal adducts lead us to naphthalene, which forms intensely coloured compounds with alkali metals. In sodium naphthalenide (Na C10H8 ), for example, the metal transfers an electron to the aromatic system to produce a radical anion which has strong reducing properties. Although sodium naphthalenide was rst investigated in 1936 by Scott et al.35, the synthetic utility of this alkali metal adduct has not been fully explored. It is interesting to consider whether single-layer LTMDs can be produced by reacting MoS2 with various alkali metal naphthalenide adduct in a radical anion solution.

Motivated thus, we prepare naphthalenide adducts of Li, K and Na and compare the exfoliation efciency and quality of MoS2 generated. Using a two-step expansion and intercalation method, we report the production of high-quality single-layer MoS2 ake sheets with unprecedentedly large ake size, that is, up to 400 mm2 when the Na adduct was intercalated. Single-layer MoS2 inks prepared by this method could be directly jet-printed on a wide range of substrate.

ResultsProduction of LTMDs. Figure 1 shows the schematic diagram of the processing steps involved in obtaining well-dispersed samples of LTMDs. First, bulk MoS2 crystals (or powders) are expanded by reacting with hydrazine (N2H4) in hydrothermal condition (Fig. 1a). The expansion mechanism can be explained by a redox-rearrangement model in which part of the N2H4 is oxidized to

N2H5 upon intercalation. The intercalated N2H5 is not thermally stable and will be decomposed to N2, NH3 and H2 upon

heating the intercalated MoS2 lms at high temperature. Decomposition and gasication of intercalated N2H4 molecules expands the MoS2 sheets by more than 100 times compared to its original volume. In a second step, the expanded MoS2 crystal is intercalated by alkali naphthalenide solution (Fig. 1b). Finally the intercalated MoS2 is exfoliated by dipping in ultrasonicated water operated at low power to avoid fragmentation of the sheets. A black suspension consisting predominantly of 90% single-layer MoS2 can be obtained after centrifugation and decanting the supernatant (Supplementary Fig. S1). The generic applicability of this method has been tested successfully on a wide range of LTMDs, which includes the high yield exfoliation of monolayer TiS2, TaS2, and NbS2, as well as few layer (24) diselenide TiSe2,

NbSe2, and MoSe2 (Supplementary Fig. S2).

Intercalation and exfoliation. XRD measurements were done on freshly intercalated, hermetically sealed samples after drying in argon and vacuum. The position and intensity of the (002) peak originating from the hexagonal 2H-MoS2 can be used to judge the extent of intercalation and exfoliation. In freshly intercalated samples, the (002) peaks of the pristine MoS2 is shifted completely toward lower angles, indicating expansion of the lattice along the c-axis and formation of stoichiometric ternary compounds (Fig. 2). The spacing between adjacent layers in the expanded lattice of the each intercalated phase are K c/2 7.92 ,

Na c/2 7.05 and Li c/2 6.18 , respectively. The increase in

the interlayer distance as a result of the intercalation is Dc/2

(c /2 -c-c0/2), K Dc/2 1.88 , Na Dc/2 1.01 and Li

Dc/2 0.14 . It is worth noting that these Dc/2 values are much

smaller than that of naphthalene-intercalated MoS2 produced by an exfoliating-restacking process, which indicates that the organic

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N2H4

A + A+

A+

e

e

H2O Disperse

=Li+, Na+, K+

Figure 1 | Schematic of fabrication processes. (a) Bulk MoS2 is pre-exfoliated by the decomposition products of N2H4. (b) Pre-exfoliated MoS2 reacts with AC10H8 to form an intercalation sample, and then exfoliates to single-layer sheets in water. (c) Photograph of bulk single-crystal MoS2,(d) photograph of pre-exfoliated MoS2, (e) photograph of Na-exfoliated single-layer MoS2 dispersion in water.

Intensity (a.u.)

=S =Mo =

e

MoS2

Kx(H2O)yMoS2

NaxMoS2

Nax(H2O)MoS2

(C10H8)xMoS2

6.05

7.05

9.35,11.61

11.36

NaxMoS2 Lix(H2O)yMoS2

LixMoS2 Pristine MoS2

KxMoS2

Nax(H2O)yMoS2

6 8 16 4 6 8

20 10 0 10

10

12

14

18

10

12

14

16

18

20

2 theta ()

20

2 theta ()

002

Nax(H2O)yMoS2

K-exfoliated MoS2103

100 104 110

105

LixMoS2

Lix(H2O)yMoS2 NaxMoS2

Intensity (a.u.)

101 004

Na-exfoliated MoS2

Li-exfoliated MoS2

Pristine MoS2

2 theta ()

60

10

20

30

40

50

10

0 10 20

Chemical shift (p.p.m.)

Chemical shift (p.p.m.)

Figure 2 | Characterization of intercalated and exfoliated MoS2. (a) XRD pattern and schematic of pristine MoS2, Na-intercalated MoS2, after exposure of Na-intercalated MoS2 to the ambient for 3 days, exfoliated-and-restacked naphthalene-intercalated MoS2. (b) Li-, Na- and K-intercalated

MoS2, after exposure of intercalated sample to the ambient for 3 days. (c) Li-, Na- and K-exfoliated MoS2 without any annealing. (d) Solid-state 7Li NMR spectra of LixMoS2 and Lix(H2O)yMoS2, (e) Solid-state 23Na NMR spectra of NaxMoS2 and Nax(H2O)yMoS2.

anion is not intercalated in the MoS2 crystal. Fig. 2b shows that after three days of storage in wet air (humidity around27.5 g m 3), the intensity of the (002) peaks is reduced and they shift toward smaller angles (K c/2 8.99 , Na c/2 9.35 and

11.61 and Li c/2 7.08 ). This additional shift is ascribed to

the continuous hydration of the intercalated ions, leading to further lattice expansion.

The intercalated compounds are exfoliated by hydration, and the exfoliated sheets are dried into powder form and characterized by XRD (Fig. 2c). In the case of K-intercalated MoS2, peaks due to the intercalated host compound as well as restacked (002) peak of MoS2 show signicant intensities, which reects incomplete exfoliation. However the (002) peaks vanish completely in the exfoliated Na-intercalated and Li-intercalated

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110 100

110 100

1.0 nm

1.0 nm 1.1 nm

Mo

MoS2 MoS2

S

W

WS2 WS2 S

1.1 nm

0.9 nm 1.1 nm

TiS2

TaS2

NbS2

Ta

Ta

S

S

S

Nb S

S

Ti

Ti

Nb

S

0 1 2 3 4 5 6 KeV

0 1 2 3 4 KeV

0 1 2 3 4 KeV

Figure 3 | Mophology characterization of LTMDs. (a) SEM images of Na-exfoliated single-layer MoS2. (b) AFM images of Na-exfoliated single-layer MoS2. (c) TEM images of Na-exfoliated single-layer MoS2, insets are the corresponding SAED and aberration-corrected HRTEM images. (d) SEM images of

Na-exfoliated single-layer WS2. (e) AFM images of Na-exfoliated single-layer WS2. (f) TEM images of Na-exfoliated single-layer WS2, insets are the corresponding SAED and aberration-corrected HRTEM images. (g) AFM images TiS2, and corresponding EDS and photograph of dispersion in water.

(h) AFM images of TaS2. (i) AFM images of NbS2. (gi) give average thickness of B1 nm, conrming that single-layer is successfully produced by our method. Scale bars in (a) is 10 mm, in (b,d,g,h) are 5 mm, in (c,e) are 1 mm, in (f) is 500 nm, in (i) is 2 mm, in inner images of (c,f) are 1 nm.

samples, which is a signature of complete exfoliation29,30. After annealing at 150 C to remove water, weak (002) peaks recover in the Na and Li-exfoliated samples due to limited degree of restacking (Supplementary Fig. S3). Solid-state NMR is used to study the local coordination environments of the alkali metal cations before and after hydration (Fig. 2d,e). The central peaks in freshly intercalated MoS2 are sharp and symmetrical indicating a highly uniform chemical environment for most intercalated cations. After hydration, both central peaks of Li7 and Na23 solid NMR are broadened and shifted to lower frequencies, which can be attributed to dynamic processes or complex coordination environment with H2O molecules.

Morphology characterization. The uniformity and size distribution of the single-layer LTMDs sheets are examined using scanning electron microscopy (SEM) (Fig. 3a,d). One remarkable

result is that 80% of the single-layer MoS2 sheets has lateral widths of around 10 mm, this about 10 times larger than solution-exfoliated akes reported using n-butyl lithium methods28,30. As shown in Fig. 3a, a typical single-layer MoS2 ake has a surface area of 400 mm2. When the same intercalation-and-exfoliation process is performed on WS2 crystals, 80% of the exfoliated akes obtained are determined to be single layers with lateral dimensions between 3 and 10 mm, which essentially match the grain size of WS2 powder before exfoliation (Fig. 3d). To assess the effectiveness of this method, the exfoliation yield in terms of micron-sized akes is compared with the commonly used exfoliating agent n-butyl lithium on the same starting materials, the results show that only submicron-sized akes can be generated using the latter (Supplementary Figs S4S6).

The thickness of the exfoliated sheets is characterized by atomic force microscopy (AFM). Fig. 3b,e show typical tapping mode AFM images of exfoliated MoS2 and WS2 deposited on a

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Scotch tape-exfoliated monolayer K exfoliated monolayer

Na exfoliated monolayer Li exfoliated monolayer

Scotch tape-exfoliated monolayer K exfoliated monolayer

Na exfoliated monolayer Li exfoliated monolayer

660

Intensity (a.u.)

Intensity (a.u.)

370 380 390 400 410 420 600 620 640 680 700 Raman shift (cm1) Wavelength (nm)

Figure 4 | Raman and photoluminescence spectrum of MoS2. (a) Raman spectra of scotch-tape-exfoliated single-layer MoS2 and Na-, K- and Li-exfoliated single-layer MoS2 deposited on Si/SiO2 substrate. (b) Photoluminescence spectrum of scotch-tape-exfoliated single-layer MoS2, and Na-,

K- as well as Li-exfoliated single-layer MoS2 nanosheet deposited on Si/SiO2 substrate.

SiO2/Si substrate by spin-coating. The average topographic height is around 1 nm, which agrees with typical height of a single-layer MoS2 (between 0.6 and 1.0 nm)30. Statistical analysis of 100 akes produced by the three different alkali metal adduct reveal 20, 90, and 80% of the akes to be monolayer for KxMoS2, NaxMoS2, and

LixMoS2, respectively (Supplementary Fig. S7). Although KxMoS2 reacts more violently with water than NaxMoS2 and LixMoS2, its large ionic radius precludes full intercalation. Intercalating WS2 with sodium naphthalenide also produces single-layer WS2 akes with a yield ofB90%.

Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were performed on the exfoliated ake suspended on a lacey carbon TEM grid (Fig. 3c,f). The SAED patterns of exfoliated MoS2 and WS2 exhibit high crystallinity (the inset in Fig. 3c,f, Supplementary Fig. S8), as judged from the characteristic honeycomb lattice. From XPS analysis, the Na-exfoliated MoS2 lm shows Mo 3d peaks with peak position and width characteristic of the 2H phase28 (Supplementary Figs S9S11).

Optical characterization. The Raman spectra of the exfoliated MoS2 akes were recorded using a 532-nm excitation line (Fig. 4a and Supplementary Fig. S12). For Na-exfoliated samples, the E12g phonons stiffen with decreasing number of layers and a blue shift of the peak from 380 cm 1 of the thick layers to 383 cm 1 of monolayer MoS2 occurs. On the other hand, A1g phonons soften with decreasing number layers, giving rise to a red shift from 407 cm 1 in the bulk material to 403 cm 1 in the monolayer.

The Raman signature obtained is consistent with that of mechanically exfoliated single-layer MoS2 (ref. 36). The corresponding Raman peaks in Li-exfoliated MoS2 are much broader, which can be due to slight doping and possible presence of defects.

Single-layer MoS2 exhibits a unique signature in its optical spectrum in the form of photoluminescence (PL) due to the transition from an indirect to a direct-bandgap semiconductor (Supplementary Fig. S13). MoS2 appears in two distinct symmetry: 2H (trigonal prismatic D3h) and 1T (octahedral Oh) phases. The 2H phase is semiconducting while 1T is metallic (Supplementary Note 1). In this work, PL can be observed on the exfoliated akes after a brief bake at 200 C to transform it to the 2H phase. As shown in Fig. 4b, the PL spectrum of a Naexfoliated single-layer MoS2 exhibits a peak centred at 668 nm

(1.86 eV) with a shoulder at 623 nm (1.99 eV), which agrees with excitonic peaks arising from the K point of the Brillouin zone. The PL peak position and peak width is consistent with mechanically cleaved monolayer MoS2 (ref. 3). The Liexfoliated monolayer sample shows a weaker PL peak, this reects either slight doping or the presence of defects in it. The chemical purity of the Na-exfoliated MoS2 akes is veried by energy-dispersive X-ray spectroscopy (Supplementary Fig. S14) and its electrical property is evaluated in the form of a eld effect transistor (Supplementary Fig. S15 and Supplementary Note 2). The eld effect mobility of monolayer MoS2 ake is measured to be in the range of 18 cm2/(V s)while that of few layer MoS2

akes are in the range of 2080 cm2/(V s), which are

comparable with those of eld effect transistor made from mechanically exfoliated MoS2 akes37.

Optical fibre

Figure 5 | Inkjet printing of MoS2. (a) Wafer-scale MoS2 pattern jet-printing. Large area, continuous and highly uniform MoS2 thin lm pattern were directly printed on a 4-inch Si/SiO2 wafer by inkjet printing.(b) Schematic showing the printing of MoS2 thin lm on the optical bre pigtail. (c) AFM image of MoS2 thin lm, showing a thickness of 14 layers. (d) SEM image of MoS2 thin lm-coated optical bre pigtail. Scale bar is 5 mm in (c,d).

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Inkjet printing. Inkjet printing is highly promising for the high-throughput deposition of micron-sized patterns by virtue of its speed, low cost, additive and direct writing capability38. The good dispersion and high viscosity of our MoS2 dispersion render it highly suitable for jet-printing. The ink is made from0.02 mg ml 1 MoS2 fully dispersed in ethanol/water (2:1 volume) solution (viscosity 2.64 cP and surface tension34.3 mN m 1). To print high-resolution patterns and uniform lms, 10-mm diameter printer nozzle is selected, and the wafers are heated to 60 C before printing (Supplementary Figs S16, S17 and Supplementary Note 3). Owing to the moderate surface energy of the ink, MoS2 inks can be directly printed on plastic,

SiO2, glass and optical bre pigtail (Fig. 5ad) without any chemical/physical modication. Figure 5a shows the word NUS printed directly using the MoS2 akes. The AFM and SEM characterization of the MoS2 printed thin lm show that the printed MoS2 lm is continuous and uniform with a sheet thickness of 23 layers, as shown in Fig. 5c,d.

DiscussionIn summary, we have explored the use of metal naphthalenide for the intercalationexfoliation of metal chalcogenides (MoS2 and

WS2) and obtained high-efciency exfoliation of micron-sized monolayer sheet (widths in the range of 510 mm). The size distribution of the ake is much better than exfoliation using organolithium salts (n-butyl lithium). This can be related to a reduced chemical reaction of the radical anion (C10H8 ) with the host material, and the fact that we apply a pre-expansion procedure with hydrazine to facilitate the efcient intercalation of the metal cations. Evidence from XRD and NMR shows the existence of highly ordered ternary phase after cation intercalation. In terms of exfoliation efciency, sodium naphthalenide (Na C10H8 ) produces higher quality monolayer ake than its lithium and potassium counterparts. This work contributes a high-yield chemical processing method for producing high-quality 2D chalcogenide monolayers with direct relevance to printable photonics.

Methods

Pre-expansion of MoS2. Bulk MoS2 (1.6 g; SPI, single crystal) and 20 ml hydrazine hydrate (Aldrich, 98%) are sealed in an autoclave and heated at 130 C for 48 h. The expanded MoS2, which has a worm-like appearance, is washed three times by water and dried at 120 C for 10 h.

Intercalation of MoS2. Na (0.69 g; Aldrich) (for K or Li, we used 1.08 g or 0.21 g, respectively), 1.92 g naphthalene (aldrich) and 80 ml anhydrous tetrahydrofuran (THF) (Aldrich, fresh redistilled by Na) are stirred for 2 h in ice-water bathin argon atmosphere. Pre-expanded MoS2 powder (1.6 g) is added to the dark blue solution and the mixture is further stirred for 5 h. After reaction, the product is washed ve times by anhydrous THF. The procedures are similarfor both K and Li.

Intercalation process for WS2 is similar except that 2.48 g of WS2 is used with the same amount of reagent above.

Caution: LixMoS2 will self-heat in air, NaxMoS2 will self-ignite in air and KxMoS2 will self-explode in air.

Exfoliation. Distilled water (100 ml) is added to the intercalated sample. The mixture is sonicated in a low-power sonic bath (60 W) for 30 min to form a homogeneous suspension. The mixture is centrifuged at 8,000 r.p.m. for 15 min for several cycles to remove excess impurity, and then at 1,000 r.p.m. for 15 min in the last cycle.

Equipment. The following equipment were used: Raman (Alpha 300R),PL (equipped on Raman), SEM (Jeol JSM-6701F), AFM (Dimension Fast Scan), XPS (SPECS), UV-VIS-NIR (Shimadzu UV-3600), solid NMR (Bruker 400 MHz), powder XRD (Siemens D5005), Jet-Printer (Dimatix, 2800).

References

1. Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. S. Electronics and opto-electronics of two-dimensional transition metal dichalcogenides. Nat. Nanotech. 7, 699712 (2012).

2. Mak, K. F., Lee, C., Hone, J., Shan, J. & Heinz, T. F. Atomically thin MoS2: a new direct-gap semiconductor. Phys. Rev. Lett. 105, 136805 (2010).

3. Splendiani, A. et al. Emerging photoluminescence in monolayer MoS2. Nano Lett. 10, 12711275 (2010).

4. Cao, T. et al. Valley-selective circular dichroism of monolayer molybdenum disulphide. Nat. Commun. 3, 887 (2012).

5. Matte, H. S. S. et al. MoS2 and WS2 analogues of graphene. Angew. Chem. Int. Ed. 49, 40594062 (2010).

6. Gordon, R. A., Yang, D., Crozier, E. D., Jiang, D. T. & Frindt, R. F. Structures of exfoliated single layers of WS2, MoS2, and MoSe2 in aqueous suspension. Phys. Rev. B 65, 125407 (2002).

7. Shi, Y. et al. Van der waals epitaxy of MoS2 layers using graphene as growth templates. Nano Lett. 12, 27842791 (2012).

8. Lee, Y.-H. et al. Synthesis of large-area MoS2 atomic layers with chemical vapor deposition. Adv. Mater. 24, 23202325 (2012).

9. Coleman, J. N. et al. Two-dimensional nanosheets produced by liquid exfoliation of layered materials. Science 331, 568571 (2011).

10. Chhowalla, M. et al. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nat. Chem. 5, 263275 (2013).

11. Butler, S. Z. et al. Progress, Challenges, and opportunities in two-dimensional materials beyond graphene. ACS Nano 7, 28982926 (2013).

12. Zhan, Y., Liu, Z., Najmaei, S., Ajayan, P. M. & Lou, J. Large-area vapor-phase growth and characterization of MoS2 atomic layers on a SiO2 substrate. Small 8, 966971 (2012).

13. Chou, S. S. et al. Ligand conjugation of chemically exfoliated MoS2. J. Am.

Chem. Soc. 135, 45844587 (2013).14. Novoselov, K. S. et al. Electric eld effect in atomically thin carbon lms. Science 306, 666669 (2004).

15. Novoselov, K. S. et al. Two-dimensional atomic crystals. Proc. Natl Acad. Sci. USA 102, 1045110453 (2005).

16. Park, S. & Ruoff, R. S. Chemical methods for the production of graphenes. Nat. Nanotech. 4, 217224 (2009).

17. Radisavljevic, B., Radenovic, A., Brivio, J., Giacometti, V. & Kis, A. Single-layer MoS2 transistors. Nat. Nanotech. 6, 147150 (2011).

18. Yoon, Y., Ganapathi, K. & Salahuddin, S. How good can monolayer MoS2 transistors be? Nano Lett. 11, 37683773 (2011).

19. Zeng, H., Dai, J., Yao, W., Xiao, D. & Cui, X. Valley polarization in MoS2 monolayers by optical pumping. Nat. Nanotech. 7, 490493 (2012).

20. Mak, K. F., He, K., Shan, J. & Heinz, T. F. Control of valley polarization in monolayer MoS2 by optical helicity. Nat. Nanotech. 7, 494498 (2012).

21. Lee, H. S. et al. MoS2 Nanosheet phototransistors with thickness-modulated optical energy gap. Nano Lett. 12, 36953700 (2012).

22. Radisavljevic, B., Whitwick, M. B. & Kis, A. Integrated circuits and logic operations based on single-Layer MoS2. ACS Nano 5, 99349938 (2011).

23. Lee, K. et al. Electrical characteristics of molybdenum disulphide akes produced by liquid exfoliation. Adv. Mater. 24, 23202325 (2012).

24. Benavente, E., Santa Ana, M. A., Mendizabal, F. & Gonzalez, G. Intercalation chemistry of molybdenum disulde. Coord. Chem. Rev. 224, 87109(2002).

25. Golub, A. S., Zubavichus, Y. V., Slovokhotov, Y. L. & Novikov, Y. N. Single-layer dispersions of transition metal dichalcogenides in the synthesis of intercalation compounds. Russian Chem. Rev. 72, 123141 (2003).

26. Py, M. A. & Haering, R. R. Structural destabilization induced by lithium intercalation in MoS2 and related-compounds. Can. J. Phys. 61, 7684 (1983).

27. Eda, G. et al. Coherent atomic and electronic heterostructures of single-layer MoS2. ACS Nano 6, 73117317 (2012).

28. Eda, G. et al. Photoluminescence from chemically exfoliated MoS2. Nano Lett. 11, 51115116 (2011).

29. Joensen, P., Frindt, R. F. & Morrison, S. R. Single-layer MoS2. Mater. Res. Bull. 21, 457461 (1986).

30. Zeng, Z. Y. et al. Single-layer semiconducting nanosheets: high-yield preparation and device fabrication. Angew. Chem. Int. Ed. 50, 1109311097 (2011).

31. Somoano, R. B., Hadek, V. & Rembaum, A. Alkali metal intercalates of molybdenum disulde. J. Chem. Phys. 58, 697 (1973).

32. Zak, A. et al. Alkali metal intercalated fullerene-like MS2 (M W, Mo)

nanoparticles and their properties. J. Am. Chem. Soc. 124, 47474758 (2002).33. Gordon, R. A., Yang, D., Crozier, E. D., Jiang, D. T. & Frindt, R. F. Structures of exfoliated single layers of WS2, MoS2, and MoSe2 in aqueous suspension. Phys.

Rev. B 65, 125407 (2000).34. Yang, D. & Frindt, R. F. Li-intercalation and exfoliation of WS2. J. Phys. Chem. Solids 57, 11131116 (1996).

35. Scott, N. D., Walker, J. F. & Hansley, V. L. Sodium naphthalene. i. a new method for the preparation of addition compounds of alkali metals and polycyclic aromatic hydrocarbons. J. Am. Chem. Soc. 58, 2442 (1936).

6 NATURE COMMUNICATIONS | 5:2995 | DOI: 10.1038/ncomms3995 | http://www.nature.com/naturecommunications

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3995 ARTICLE

36. Li, H. et al. Optical identication of single- and few-layer MoS2 sheets. Small 8, 682686 (2012).

37. Das, S., Chen, H., Penumatcha, A. V. & Appenzeller, J. High performance multilayer moS2 transistors with scandium contacts. Nano Lett. 13, 100105 (2013).

38. Zhang, L. et al. Inkjet printing high-resolution, large-area graphene patterns by coffee-ring lithography. Adv. Mater. 24, 436440 (2012).

Acknowledgements

K.P.L. is grateful for the MOE Tier 1 grant Two dimensional crystals as platforms for optoelectronics (R-143-000-556-112) and also NRF-CRP project Novel 2D materials with tailored properties beyond graphene (R-144-000-295-281). H.S.S. is grateful for a grant (code no. 2011-0031630) from the Center for Advanced Soft Electronics under the Global Frontier Research Program through the National Research Foundation funded by the Ministry of Science, ICT and Future Planning, Korea.

Author contributions

J.Z. designed the work and performed the experiments. K.P.L. conceptualized the work, analysed the data and wrote the paper. H.Z., S.D., Y.P.L., C.T.N., H.S.S., H.Y.J. and B.L. performed some characterization experiments and analysed data.

Additional information

Supplementary Information accompanies this paper at http://www.nature.com/naturecommunications

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How to cite this article: Zheng, J. et al. High yield exfoliation of two-dimensional chalcogenides using sodium naphthalenide. Nat. Commun. 5:2995 doi: 10.1038/ ncomms3995 (2014).

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