Nanoscale

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Controlling sulph

aDepartment of Materials, University of Ox

E-mail: Jamie.warner@materials.ox.ac.ukbStanford Nanocharacterization Laboratory,

USAcDepartment of Materials Science and Engin

94305-4034, USA

† Electronic supplementary informa10.1039/c4nr04091k

Cite this: Nanoscale, 2014, 6, 12096

Received 19th July 2014Accepted 14th August 2014

DOI: 10.1039/c4nr04091k

www.rsc.org/nanoscale

12096 | Nanoscale, 2014, 6, 12096–121

ur precursor addition for largesingle crystal domains of WS2†

Youmin Rong,a Ye Fan,a Ai Leen Koh,b Alex W. Robertson,a Kuang He,a

Shanshan Wang,a Haijie Tan,a Robert Sinclairc and Jamie H. Warner*a

We show that controlling the introduction time and the amount of sulphur (S) vapour relative to the WO3

precursor during the chemical vapour deposition (CVD) growth of WS2 is critical to achieving large

crystal domains on the surface of silicon wafers with a 300 nm oxide layer. We use a two furnace system

that enables the S precursor to be separately heated from the WO3 precursor and growth substrate.

Accurate control of the S introduction time enabled the formation of triangular WS2 domains with edges

up to 370 mm which are visible to the naked eye. The WS2 domains exhibit room-temperature

photoluminescence with a peak value around �635 nm and a full-width at half-maximum (FWHM) of

�12 nm. Selected area electron diffraction from different regions of the triangular WS2 domains showed

that they are single crystal structures.

Introduction

Atomically thin transition metal dichalcogenides (TMDCs) haveexpanded two-dimensional (2D) materials beyond grapheneand invigorated the promise of 2D materials in electronics.1–7

While the zero-band-gap nature has limited graphene imple-mentation in electronics,8 single layered atomic TMDC mate-rials are direct band-gap semiconductors with applications inelectronics, optoelectronics9–12 and valleytronics.13,14 In recentyears, single-layer MoS2 and WS2 have been fabricated viaseveral methods such as mechanical exfoliation,3,15–17 liquidexfoliation,4,5,18 and sulfurization of transition metal oxides.19–23

However, producing synthetic large area high-quality mono-layers of TMDCs is possibly more challenging than graphenedue to the propensity for the formation of nanoparticles24,25 ormultilayers.26 Chemical vapour deposition (CVD) growth ofmonolayer MoS2 crystal domains and lms has beenrealised,19,27–29 which has facilitated extensive studies regardingtheir unique photoluminescence and electrical transportproperties.28,30,31

The CVD synthesis of WS2monolayer crystals is slightly morechallenging than MoS2 because of the higher melting points ofthe W-based precursors, such as WO3, compared to theMo-based precursor counterparts, and the resulting higher

ford, Parks Road, Oxford, OX1 3PH, UK.

Stanford University, Stanford, CA 94305,

eering, Stanford University, Stanford, CA

tion (ESI) available. See DOI:

03

growth temperatures needed. In the past decade, the CVDapproach has proved to be extremely useful for production oflarge-area single crystalline graphene domains which candiminish grain boundary defects for transport devices.32,33

Although the creation of a single crystalline structure ofmonolayer WS2 has been recently realized,34,35 the controlledCVD growth of large-area monolayers with homogenouscrystallinity still remains an experimental challenge andrequires continual investigation.

The approach for most reports of CVD growth of WS2 relieson the reaction of S vapour with a W-precursor at elevatedtemperatures, with both precursors placed in a single furnace.Not having individual control over each precursor's tempera-ture limits the parameter space of the chemical reaction. Toovercome this limitation, we utilize a CVD approach thatseparates the two precursors into different furnaces, onecontaining the elemental S and the other containing both theW-precursor and the growth substrates. Both the S vapour andW-precursor vapour are carried downstream to the growthsubstrate by pure Ar gas. We examine how the timing of whenthe S precursor is heated relative to theWO3 precursor to releaseS vapour inuences the domain size, crystal thickness andhomogeneity.

Results and discussion

Fig. 1a presents a schematic illustration of the double-furnace 1inch quartz tube atmospheric pressure CVD system designedfor allowing the abundant supply of S vapour both before andduring the formation of WS2 thin crystals on SiO2/Si substrates.The S powder was heated at �180 �C in a low temperature (LT)furnace which generates vapour carried by pure Ar gas to the

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Fig. 1 Atmospheric pressure CVD synthesis of large-area WS2 crystal domains on the SiO2/Si substrate. (a) Schematic presentation of thecorresponding CVD experimental set-up with a highlighted WS2 reaction chamber (black dashed line). (b) Temperature profiles employed forformation of WS2 on the SiO2/Si substrate with sulphur introduced at 0, �10 and �20 min due to the target reaction temperature of 1070 �C/860 �C. (c) Photo of large triangular WS2 crystals deposited on a 2 cm SiO2/Si substrate. (d and e) Zoom-in images chosen from the selected area(red dashed line) in (c) and (blue dashed line) in (d), respectively, showing triangular WS2 domains.

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reaction furnace with high temperature (HT). The HT furnacecontains both WO3 and the growth substrates. The WO3

precursor is placed in the center of the HT furnace and heatedto �1070 �C for the crystal growth on the substrate. The SiO2/Sisubstrate was placed downstream from WO3 where WS2nucleation and growth were enabled at a lower temperature(�860 �C).

Fig. 1b shows the temperature proles employed for ramp-ing or cooling of S and WO3 precursors to the reactiontemperatures (i.e. temperature prole for the SiO2/Si substratesimultaneously ramps and cools along with that of the WO3

precursor). With regard to the WS2 growth stage, all WS2formation reactions were equally given 60 min growth timestarting at 0 min when the temperature of the HT furnacereached 1070 �C (WO3)/860 �C (substrate). The solid S powder inthe LT zone was heated and carried downstream using Ar to thereaction chamber at times of 0, 10 or 20 min before the HTfurnace (and WO3 precursor) reached 1070 �C. These differentintroduction timings of S vapour prior to the WS2 growth stagewere important in the controlled growth of thin crystal domainson as-prepared substrates.27 By optimization of the reactionvariables, such as the amount of precursors used, the reactiontemperature and the positioning of the growth substrate, wehave improved synthesis of WS2 domains so that they are largeenough to be visible to the naked eye, shown in Fig. 1c–e.

Fig. 2a presents an overview of large WS2 crystal domains(200–300 mm) deposited on the SiO2 surface where sulphur wasintroduced 10 min prior to growth, the black line in Fig. 1b. The

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WS2 domains are scattered across the SiO2/Si substrate with lownucleation density which can permit the large crystal domainsize. However, the distribution of domain sizes and crystallinityacross the substrate is due to the dynamics of the local supply ofvapourized precursors. As a close kin to WS2, previously repor-ted vapour phase growth of monolayer MoS2 triangular crystalshas also inevitably encountered similar phenomena.23,28 InFig. 2b and c, we particularly demonstrate an as-deposited areawhere a triangular WS2 domain as large as�370 mmwas formedunder the aforementioned conditions. Fig. 2d shows anexample of the optical contrast from one of these large WS2triangles (�300 mm) on the SiO2/Si substrate. The majority ofas-obtained WS2 single domains have sizes varying from �200to�350 mm excluding relatively small domains that are not fullycrystallized. Characterization using AFM was performed in aselected region of a WS2 triangular domain (Fig. 2e). Fig. 2fshows an AFM image that correlates with the selected region inFig. 2e and a line prole which determines the thickness of theas-grown WS2 crystal to be �1 nm which is in agreement withthe reported thickness of monolayer WS2 triangular crystals.22

The WS2 domains were primarily monolayer, but there weresome bilayer and few layer domains as well. The monolayerdomains were easily identied from the multi-layered domainsby their substantially more intense photoluminescence emis-sion (as illustrated in ESI Fig. S1b†). In this approach to thesynthesis of WS2 atomically thin crystals, we were able toproduce one of the largest triangular single domains(�370 mm) in the recent development of large-area 2D

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Fig. 2 SEM, AFM, optical, Raman, and PL characterization of WS2 single crystals obtained from growth achieved with �10 min sulphur intro-duction. (a) SEM image of overall coverage of monolayer WS2 crystals on SiO2/Si. (b and c) SEM images of the as-deposited area chosen to showextraordinary largeWS2 single crystal domains. (d) Optical image of a monolayer WS2 single crystal on the SiO2/Si substrate. (e and f) AFM result ofthe selected region of the as-grown large WS2 crystal domain in (e). (g and h) Characteristic Raman and PL spectra of as-produced monolayerWS2 domains exhibited in (d).

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dichalcogenides.28,34,36 Some small particulate materials areoccasionally observed on the substrates post-growth and it islikely that these are formed during the cooling stages of theCVD, since they did not cause nucleation of WS2 domains as itwould be expected that they were present before or duringgrowth.

Raman spectroscopy was used to conrm that the triangleswere indeed WS2. The Raman spectrum of a WS2 single domainusing an excitation laser with a wavelength (lex) of 532 nm isshown in Fig. 2g, which demonstrates the characteristic in-plane vibrational (E12g) mode and the out-of-plane vibrational(A1g) mode at �350 and �416 cm�1, respectively, for WS2. Thefrequency difference between the E12g mode and A1g mode isshown to be�66 cm�1 which matches the Raman spectroscopiccharacteristics of single-layer-thick WS2 lms on SiO2/Sisubstrates.17 Photoluminescence spectroscopy (PL) using a532 nm laser for excitation was undertaken, Fig. 2h, whichrevealed a strong PL peak between 550 and 750 nm, typical for amonolayer WS2 crystal. The peak has maxima at the wavelengthof 636 nm (direct band gap� 1.95 eV) which falls in the range ofreported PL peak positions for monolayer WS2 thinlms.16,17,22,37

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In the interest of studying uniformity and crystallinity of theas-formed crystals, PL and Raman mapping were executed onthree major types of the large monolayer WS2 domains pre-sented in Fig. 3a–c. As provided in the ESI Fig. S1c,† the mostprominent difference between single- and bi-layer WS2regarding their Raman spectrum is the ratio of E12g peak inten-sity and A1g intensity. For a bilayer, the E1

2g/A1g ratio is�2 whereas the ratio for a single-layer can be of three timesgreater. Therefore, we have plotted the ratio of these two char-acteristic Raman peaks against XY coordinates where multi-point Raman analysis was conducted on WS2 crystal domainsshown in Fig. 3a–c. As a result, the high consistency of theE12g/A1g peak intensity ratio of �6 across the single domains is

shown respectively in Fig. 3d–f. This has ascertained that a highuniformity of the monolayer WS2 single domain over an area ofup to �0.05 mm2 is plausible.22,28,34,35 Fig. 3g–i elaborate theintegrated PL peak intensity of monolayer WS2 plotted withregard to the XY coordinates of PL multi-point analysis.Accordingly, it is conrmed that the lateral PL yield of theseas-formed WS2 monolayer domains is of good efficacy. It isnoteworthy that the PL efficiency varies due to different WS2monolayer domain congurations. Furthermore, the PLintensity showed no signs of increased PL yield at the edge of

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Fig. 3 Optical microscopy, Raman and PL mapping of as-grown monolayer WS2 crystals. (a)–(c) Optical images of different WS2 domainconfigurations. (d–f) Raman mapping of WS2 crystals shown in (a–c) plotted with regard to the ratio of integrated E2g peaks (330–370 cm�1)/integrated A1g peaks (400–430 cm�1) at a 532 nm excitation laser. (g–i) PL mapping of WS2 crystals in (a–c) plotted according to integratedcharacteristic photon emission intensities from wavelength 600 to 660 nm. (j)–(l) PL spectra of the colour-indicated characterization pointsselected from the edge towards the crystal center in (g)–(i), respectively.

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the crystal domains.22,34 In contrast, an increase in PL intensityfrom the edge towards the central body was discovered throughanalysis of the single-point PL spectrum which is demonstratedin Fig. 4j–l. Moreover, the relatively deteriorated PL as well asthe slightly red-shied peak position due to the edge of WS2monolayer domains has indicated that there might be an edgepassivation incurred by impurities from the reaction chamberor localized oxidation in air.35

The large WS2 monolayer domains characterized in Fig. 1–3were grown for the S introduction time of �10 min, the blackline in Fig. 1b. In order to demonstrate that the introductiontime of S is important, we present results where the time ischanged to 0 min and also �20 min, the dashed green line anddashed blue line in Fig. 1b, respectively. Fig. 4a–c show SEMimages of the samples produced when sulphur is introduced attime ¼ 0 min, the dashed green line in Fig. 1b. We observeddefected or incomplete triangular WS2 crystals formed close tothe regions where possible deposition of WS2 nanowires wasfacilitated. The regions covered with WS2 nanowires are verysusceptible to further deposition of WS2 crystals as the nano-wires are energetically favourable to act as WS2 nucleation

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seeds. Fig. 4c shows a large incomplete WS2 triangle with a sizeof �200 mm being deposited. Fig. 4d–f show SEM images of thesamples when the sulphur introduction time was changed to�20 min, the blue dashed line in Fig. 1b. This case has the Svapour introduced at almost the very beginning of the WO3

temperature ramping stage. In contrast to Fig. 4a, the WS2nanowires were no longer fabricated. Fig. 4e and f show twodifferent examples of WS2 triangular crystals produced by thisapproach. One, shown in Fig. 4e, has uniform regions within alarger triangular crystal that is �200 mm. The other domain,Fig. 4f, shows relatively thick edges towards the crystal centre,which is likely to be due to the early stage of formation of amonolayer WS2 single crystal that normally provides WS2+x inthe thick regions.34

The results in Fig. 1–4 demonstrate that the timing of thesulphur introduction is very important for the growth of largemonolayer crystals of WS2. If S is introduced too early, thenthere is not sufficient WO3 precursor on the growth substrate toobtain large uniform WS2 domains and if S is introduced toolate, then there is too muchWO3 precursor on the substrate andWS2 ends up to a thickmulti-layer and nanowires also form. The

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Fig. 4 Influence of S introduction timing as well as face-up/face-down substrate positioning on WS2 crystallization. (a–c) SEM images of WS2triangular crystals formed using 0 min sulphur introduction. (d–f) SEM images of WS2 triangular crystal domains obtained from �20 min sulphurintroduction. (g) Schematic illustration of the substrate face-down configuration in the reaction chamber with respect to the substrate face-up.(h) A SEM image taken at the edge of the SiO2/Si substrate produced by the substrate face-down approach.

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key step is the balance of the WO3 precursor concentration onthe growth substrate. The WS2 formation on the growthsubstrate occurs within a sulphur rich atmosphere and there-fore growth is controlled by theWO3 precursor. If the S vapour isintroduced into the system before the bulk WO3 precursorreaches the elevated temperature, then it can react to form bulkWS2 which does not sublimate under our conditions easily andthe result is no material on the growth substrate, and a largeamount of bulk WS2 powder where the WO3 powder originated.

The position of the growth substrate within the HT furnace isalso important. In Fig. 4g, we have carried out WS2 depositionwith the substrate face-down in comparison with the previousface-up approach. Face-down synthesis was achieved by placingthe SiO2/Si substrate above the WO3 precursor with the

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SiO2-side facing downward. The precursor and the face-downsubstrate were both heated at 1070 �C in the reaction chamberunder equivalent conditions to that of the black line in Fig. 1b(�10 min). As a result, it can be seen in Fig. 4h that only very fewWS2 domains had deposited at the very edge of the SiO2/Sisubstrate with almost none in the main central region of thesubstrate.

High-resolution transmission electron microscopy (HRTEM)combined with selected area electron diffraction (SAED) wasused to investigate the crystallinity of the CVD-synthesized WS2domains. Fig. 5a shows an optical microscopy image of aselected region of a WS2 crystal transferred onto a holey SiNTEM grid. Fig. 5b provides a zoomed-in view of the WS2 thinlm region on the TEM grid. A comprehensive study of the

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Fig. 5 (a) Optical microscopy image of a region of the WS2 triangular domain transferred onto a holey SiN TEM grid. (b) Higher magnificationoptical image of a covered and uncovered region of the WS2 domain on the TEM grid. (c–h) SAED patterns from the WS2 domain taken fromregionsmarked in (a). (i) Energy-dispersive X-ray spectroscopy (EDX) from aWS2 domain. (j) Aberration corrected HRTEM image of a WS2 domainshowing a hexagonal lattice structure. The scale bar is 1 nm.

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SAED pattern was conducted along the array of 20 consecutiveholes of the TEM grid highlighted in Fig. 5a (red dashed line).Fig. 5c–h show a selected group of the SAED patterns thatcorrespond to the numbered holes in Fig. 5a. All SAED patternsdemonstrate the single crystalline structure except the last hole,number 20, as shown in ESI Fig. S2.† It showed a polycrystallinepattern due to the back-folded edge of the WS2 triangle. Thecontinuity of the single-crystal WS2 diffraction pattern andlattice orientation occurs across dimensions of �86 mm.Furthermore, the energy dispersive X-ray (EDX) spectrum inFig. 5i shows that the domain consists of tungsten (W) andsulphur (S) with minor contamination (Ca) from the solventused in the transfer process. Fig. 5j is an aberration-correctedHRTEM image from a WS2 domain showing a typical hexagonallattice structure.

Conclusions

We have demonstrated that the heating prole of the Sprecursor inuences the growth of WS2 domains by CVD. Usingtwo separate furnaces was crucial in optimizing the growthconditions to achieve large triangular monolayer domains ofWS2 directly grown on silicon wafers with an oxide surface. TheWS2 crystals were photoluminescent and are therefore suitablefor use in opto-electronic applications. These results provideimportant insights into the delicate factors that inuence the

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growth of high quality TMDC semiconducting 2D materials.Further work will involve extending these growth methods toachieve full uniform monolayer coverage of WS2 directly ondevice compatible substrates. One major challenge for growingcontinuous sheets of monolayer WS2 using the currentapproach is that the S vapour reacts with the bulk WO3

precursor and turns it into WS2 bulk powder. This results in thequenching of the WO3 precursor and the CVD growth of WS2domains stop, limiting their size. The key to moving thisforward will be the ability to introduce S and WO3 precursorsinto the growth chamber separately and therefore the WO3 bulkpowder will not quench and this should lead to the continuousgrowth.

MethodsWS2 CVD synthesis and transfer

The synthesis was executed in two individually controlledquartz tube furnaces with only Ar owing at atmospheric pres-sure. Sulphur powder (1 g of purum grade >99.5%) was placedin the low-temperature furnace sublimating at�180 �C whereasWO3 powder (1 g of reagent grade > 99.5%) was heated at1070 �C in the high-temperature furnace. SiO2/Si (300 nm)substrates were positioned 10 cm away at the downstream ofWO3 (i.e. substrates are placed in the region of the high-temperature furnace at �860 �C while WO3 placed in the region

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�1070 �C). Only argon (ow rate 100 sccm) was used fortransporting WO3 and sulphur vapours to the substrates. Thegrowth was initiated by ramping sulphur at �30 �C min�1 andWO3 at �40 �C min�1, and giving 60 min reaction time. Thesulphur pre-treatment of the WO3 precursor was facilitated byheating the sulphur powder at 15 min (i.e. sulphur vapourformation time at 10 min) before the high-temperature furnacewas ramped up to �1070 �C.

The as-grown sample was rst spin-coated with a PMMAscaffold (8 wt% in anisole, 495k molecular weight) at 4700 rpmfor 60 s and then cured at 150 �C for 15 min. The edge of thewafer was ground with a diamond le to expose the SiO2/Siedges to the etchant solution. The underlying SiO2/Si substratewas subsequently detached by oating the sample on a 15 MKOH (Sigma-Aldrich) solution in a water bath for 2 h at 50 �C.The suspended PMMA/WS2 lm was thoroughly cleaned bytransferring and oating onto fresh DI water several times. Forthe TEM characterization, the PMMA/WS2 lm was transferredonto a holey silicon nitride (SiN) TEM grid (Agar ScienticY5385) and le to dry overnight in air. PMMA/WS2/SiN wassubsequently baked at 150 �C for 15 min to improve sampleadhesion. The WS2 TEM sample was eventually prepared byremoving PMMA using a 2 h acetone solution bath at 40 �C.

Characterization of monolayer WS2

The crystal structure of as-produced WS2 triangles was imagedusing a scanning electron microscope (Hitachi-4300 SEM)under an accelerating voltage of 3.0 kV. The thickness andsurface topology were examined by using an atomic forcemicroscope (Veeco Park CP AutoProbe SPM). Raman and pho-toluminescence (PL) spectroscopy were carried out using a JYHoriba Labram Aramis imaging confocal Raman microscope atan excitation wavelength of 532 nm. Selected area electrondiffraction of WS2 was performed using a JEOL 2100 TEMoperating at 80 kV. Aberration corrected TEM (and energydispersive X-ray spectroscopy) was conducted using an FEITitan 80-300 environmental TEM at 80 kV accelerating voltage.

Acknowledgements

JHW thanks the Royal Society for support.

References

1 S. Z. Butler, S. M. Hollen, L. Cao, Y. Cui, J. A. Gupta,H. R. Gutierrez, T. F. Heinz, S. S. Hong, J. Huang,A. F. Ismach, E. Johnston-Halperin, M. Kuno,V. V. Plashnitsa, R. D. Robinson, R. S. Ruoff,S. Salahuddin, J. Shan, L. Shi, M. G. Spencer, M. Terrones,W. Windl and J. E. Goldberger, Progress, Challenges, andOpportunities in Two-Dimensional Materials BeyondGraphene, ACS Nano, 2013, 7, 2898–2926.

2 K. Novoselov, D. Jiang, F. Schedin, T. Booth, V. Khotkevich,S. Morozov and A. Geim, Two-dimensional atomic crystals,Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 10451–10453.

12102 | Nanoscale, 2014, 6, 12096–12103

3 K. F. Mak, C. Lee, J. Hone, J. Shan and T. F. Heinz, AtomicallyThin MoS2: A New Direct-Gap Semiconductor, Phys. Rev.Lett., 2010, 105, 136805.

4 J. N. Coleman, M. Lotya, A. O'Neill, S. D. Bergin, P. J. King,U. Khan, K. Young, A. Gaucher, S. De, R. J. Smith,I. V. Shvets, S. K. Arora, G. Stanton, H. Kim, K. Lee,G. T. Kim, G. S. Duesberg, T. Hallam, J. J. Boland, J. J. Wang,J. F. Donegan, J. C. Grunlan, G. Moriarty, A. Shmeliov,R. J. Nicholls, J. M. Perkins, E. M. Grieveson, K. Theuwissen,D. W. McComb, P. D. Nellist and V. Nicolosi, Two-Dimensional Nanosheets Produced by Liquid Exfoliation ofLayered Materials, Science, 2011, 331, 568–571.

5 H. S. S. R. Matte, A. Gomathi, A. K. Manna, D. J. Late,R. Datta, S. K. Pati and C. N. R. Rao, MoS2 and WS2Analogues of Graphene, Angew. Chem., Int. Ed., 2010, 49,4059–4062.

6 M. Chhowalla, H. S. Shin, G. Eda, L. Li, K. P. Loh andH. Zhang, The chemistry of two-dimensional layeredtransition metal dichalcogenide nanosheets, Nat. Chem.,2013, 5, 263–275.

7 A. K. Geim and K. S. Novoselov, The rise of graphene, Nat.Mater., 2007, 6, 183–191.

8 T. Palacios, Graphene electronics thinking outside thesilicon box, Nat. Nanotechnol., 2011, 6, 464–465.

9 Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman andM. S. Strano, Electronics and optoelectronics of two-dimensional transition metal dichalcogenides, Nat.Nanotechnol., 2012, 7, 699–712.

10 G. Eda and S. A. Maier, Two-Dimensional Crystals: ManagingLight for Optoelectronics, ACS Nano, 2013, 7, 5660–5665.

11 R. S. Sundaram, M. Engel, A. Lombardo, R. Krupke,A. C. Ferrari, P. Avouris and M. Steiner,Electroluminescence in Single Layer MoS2, Nano Lett.,2013, 13, 1416–1421.

12 M. Bernardi, M. Palummo and J. C. Grossman, ExtraordinarySunlight Absorption and One Nanometer ThickPhotovoltaics Using Two-Dimensional MonolayerMaterials, Nano Lett., 2013, 13, 3664–3670.

13 K. F. Mak, K. He, J. Shan and T. F. Heinz, Control of valleypolarization in monolayer MoS2 by optical helicity, Nat.Nanotechnol., 2012, 7, 494–498.

14 H. Zeng, J. Dai, W. Yao, D. Xiao and X. Cui, Valleypolarization in MoS2 monolayers by optical pumping, Nat.Nanotechnol., 2012, 7, 490–493.

15 A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C. Chim,G. Galli and F. Wang, Emerging Photoluminescence inMonolayer MoS2, Nano Lett., 2010, 10, 1271–1275.

16 W. Zhao, Z. Ghorannevis, L. Chu, M. Toh, C. Kloc, P. Tan andG. Eda, Evolution of Electronic Structure in Atomically ThinSheets of WS2 and WSe2, ACS Nano, 2013, 7, 791–797.

17 H. Zeng, G. Liu, J. Dai, Y. Yan, B. Zhu, R. He, L. Xie, S. Xu,X. Chen, W. Yao and X. Cui, Optical signature of symmetryvariations and spin-valley coupling in atomically thintungsten dichalcogenides, Sci. Rep., 2013, 3, 1608.

18 G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen andM. Chhowalla, Photoluminescence from ChemicallyExfoliated MoS2, Nano Lett., 2011, 11, 5111–5116.

This journal is © The Royal Society of Chemistry 2014

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View Article Online

19 Y. Zhan, Z. Liu, S. Najmaei, P. M. Ajayan and J. Lou, Large-Area Vapor-Phase Growth and Characterization of MoS2Atomic Layers on a SiO2 Substrate, Small, 2012, 8, 966–971.

20 J. Song, J. Park, W. Lee, T. Choi, H. Jung, C. W. Lee, S. Hwang,J. M. Myoung, J. Jung, S. Kim, C. Lansalot-Matras and H. Kim,Layer-Controlled, Wafer-Scale, and Conformal Synthesis ofTungsten Disulde Nanosheets Using Atomic LayerDeposition, ACS Nano, 2013, 7, 11333–11340.

21 A. L. Elias, N. Perea-Lopez, A. Castro-Beltran, A. Berkdemir,R. Lv, S. Feng, A. D. Long, T. Hayashi, Y. A. Kim, M. Endo,H. R. Gutierrez, N. R. Pradhan, L. Balicas, T. E. M. Houk,F. Lopez-Urias, H. Terrones and M. Terrones, ControlledSynthesis and Transfer of Large-Area WS2 Sheets: FromSingle Layer to Few Layers, ACS Nano, 2013, 7, 5235–5242.

22 H. R. Gutierrez, N. Perea-Lopez, A. L. Elias, A. Berkdemir,B. Wang, R. Lv, F. Lopez-Urias, V. H. Crespi, H. Terronesand M. Terrones, Extraordinary Room-TemperaturePhotoluminescence in Triangular WS2 Monolayers, NanoLett., 2013, 13, 3447–3454.

23 S. Najmaei, Z. Liu, W. Zhou, X. Zou, G. Shi, S. Lei,B. I. Yakobson, J. Idrobo, P. M. Ajayan and J. Lou, Vapourphase growth and grain boundary structure of molybdenumdisulphide atomic layers, Nat. Mater., 2013, 12, 754–759.

24 I. Wiesel, H. Arbel, A. Albu-Yaron, R. Popovitz-Biro,J. M. Gordon, D. Feuermann and R. Tenne, Synthesis ofWS2 and MoS2 Fullerene-Like Nanoparticles from SolidPrecursors, Nano Res., 2009, 2, 416–424.

25 L. Rapoport, Y. Bilik, Y. Feldman, M. Homyonfer, S. Cohenand R. Tenne, Hollow nanoparticles of WS2 as potentialsolid-state lubricants, Nature, 1997, 387, 791–793.

26 S. Kim, A. Konar, W. Hwang, J. H. Lee, J. Lee, J. Yang, C. Jung,H. Kim, J. Yoo, J. Choi, Y. W. Jin, S. Y. Lee, D. Jena, W. Choiand K. Kim, High-mobility and low-power thin-lmtransistors based on multilayer MoS2 crystals, Nat.Commun., 2012, 3, 1011.

27 Y. Lee, X. Zhang, W. Zhang, M. Chang, C. Lin, K. Chang,Y. Yu, J. T. Wang, C. Chang, L. Li and T. Lin, Synthesis ofLarge-Area MoS2 Atomic Layers with Chemical VaporDeposition, Adv. Mater., 2012, 24, 2320–2325.

This journal is © The Royal Society of Chemistry 2014

28 A. M. van der Zande, P. Y. Huang, D. A. Chenet,T. C. Berkelbach, Y. You, G. Lee, T. F. Heinz,D. R. Reichman, D. A. Muller and J. C. Hone, Grains andgrain boundaries in highly crystalline monolayermolybdenum disulphide, Nat. Mater., 2013, 12, 554–561.

29 X. Wang, H. Feng, Y. Wu and L. Jiao, Controlled Synthesis ofHighly Crystalline MoS2 Flakes by Chemical VaporDeposition, J. Am. Chem. Soc., 2013, 135, 5304–5307.

30 W. Zhang, J. Huang, C. Chen, Y. Chang, Y. Cheng and L. Li,High-Gain Phototransistors Based on a CVD MoS2Monolayer, Adv. Mater., 2013, 25, 3456–3461.

31 Y. Lee, L. Yu, H. Wang, W. Fang, X. Ling, Y. Shi, C. Lin,J. Huang, M. Chang, C. Chang, M. Dresselhaus,T. Palacios, L. Li and J. Kong, Synthesis and Transfer ofSingle-Layer Transition Metal Disuldes on DiverseSurfaces, Nano Lett., 2013, 13, 1852–1857.

32 H. Zhou, W. J. Yu, L. Liu, R. Cheng, Y. Chen, X. Huang,Y. Liu, Y. Wang, Y. Huang and X. Duan, Chemical vapourdeposition growth of large single crystals of monolayer andbilayer graphene, Nat. Commun., 2013, 4, 2096.

33 O. V. Yazyev and S. G. Louie, Electronic transport inpolycrystalline graphene, Nat. Mater., 2010, 9, 806–809.

34 C. Cong, J. Shang, X. Wu, B. Cao, N. Peimyoo, C. Qiu, L. Sunand T. Yu, Synthesis and Optical Properties of Large-AreaSingle-Crystalline 2D Semiconductor WS2 Monolayer fromChemical Vapor Deposition, Adv. Opt. Mater., 2014, 2, 131–136.

35 Y. Zhang, Y. Zhang, Q. Ji, J. Ju, H. Yuan, J. Shi, T. Gao, D. Ma,M. Liu, Y. Chen, X. Song, H. Y. Hwang, Y. Cui and Z. Liu,Controlled Growth of High-Quality Monolayer WS2 Layerson Sapphire and Imaging Its Grain Boundary, ACS Nano,2013, 7, 8963–8971.

36 J. Huang, J. Pu, C. Hsu, M. Chiu, Z. Juang, Y. Chang,W. Chang, Y. Iwasa, T. Takenobu and L. Li, Large-AreaSynthesis of Highly Crystalline WSe2 Mono layers andDevice Applications, ACS Nano, 2014, 8, 923–930.

37 N. Peimyoo, J. Shang, C. Cong, X. Shen, X. Wu, E. K. L. Yeowand T. Yu, Nonblinking, Intense Two-Dimensional LightEmitter: Mono layer WS2 Triangles, ACS Nano, 2013, 7,10985–10994.

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