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Cite this: DOI: 10.1039/c3dt52353e

Received 27th August 2013,Accepted 24th September 2013

DOI: 10.1039/c3dt52353e

www.rsc.org/dalton

Electrical and optical characterization of atomicallythin WS2

Thanasis Georgiou,a Huafeng Yang,b Rashid Jalil,c James Chapman,a

Kostya S. Novoselova and Artem Mishchenko*a

Atomically thin layers of materials, which are just a few atoms in

thickness, present an attractive option for future electronic

devices. Herein we characterize, optically and electronically, atomi-

cally thin tungsten disulphide (WS2), a layered semiconductor.

We provide the distinctive Raman and photoluminescence signa-

tures for single layers, and prepare field-effect transistors where

atomically thin WS2 serves as the conductive channel. The transis-

tors present mobilities μ = 10 cm2 V−1 s−1 and exhibit ON/OFF

ratios exceeding 100 000. Our results show that WS2 is an attrac-

tive option for applications in electronic and optoelectronic

devices and pave the way for further studies in this two-dimen-

sional material.

Graphene, a single layer of carbon atoms, presents a range ofunusual properties that sparked interest in two-dimensionalmaterials. Its unusual electronic properties1,2 and mechanicalstability3,4 render it a particularly interesting system for appli-cations in electronic devices.5 Indeed, the large carrier mobili-ties in graphene exceeding μ = 100 000 cm2 V−1 s−1 make it aprime candidate for such applications. However, the gaplessDirac-cone nature of graphene’s electronic spectrum does notmake this a straightforward task, since a band gap is requiredfor applications in digital electronics.5 To this end, efforts con-centrated on inducing a band gap in graphene, either by usingnanoribbons,6 quantum dots or chemical derivatives.7 Suchefforts however have detrimental effects on graphene’s elec-tronic quality.

Beyond graphene, there exist many other materials that arelayered and can be exfoliated.8 A prime example of this is hexa-gonal boron nitride (hBN), often seen as graphene’s insulatingcounterpart. Thick flakes of boron nitride have proven to beparticularly useful for serving as a substrate for improving gra-phene’s electronic quality,9 while the availability of single and

few-layer hBN10 is particularly useful for investigating double-layer graphene heterostructures,11–13 with hBN serving as ananometre-thick insulating spacer.

Layered transition metal dichalcogenides (TMDs) consist ofa large family of materials with the general form TX2, where Tis a transition-metal from the 4th–6th group of the periodictable and X is a chalcogen – sulphur, selenium, or tellurium.Generally, TMDs formed by metals from the 4th and 6th groupsare semiconductors or insulators, e.g. MoS2, whereas thoseformed by metals from the 5th group exhibit metallic behav-iour, e.g. TaS2 and NbSe2.

14 Layered semiconductor TMDs haveproven to be important candidates for use as an absorber layerin low cost thin film solar cells.15 This is due to their relativelysmall band gap (∼1–2 eV) and the large absorption coeffi-cient.16 Among TMDs, MoS2 has been steadily attracting moreattention. While the bulk of the material has an indirect bandgap, single layer MoS2 is a direct-gap semiconductor. Recently,top-gated MoS2 transistors have been demonstrated with highON/OFF ratios of 10,8 while both MoS2 and WS2 were used as aspacer layer for vertical field effect tunnelling transistors,showing very promising characteristics.17,18

Here we study tungsten disulphide (WS2), yet anothermember of the TMD family structurally and electronicallysimilar to MoS2, as both W and Mo reside in the same columnof the periodic table. However, WS2 has superior thermal andoxidative stability than that of MoS2.

19,20 Fig. 1a shows thearrangement of atoms within a trilayer of WS2: a single layer ofW atoms sandwiched by two sheets of S, in a trigonal prismaticcoordination. While the bonds within the trilayers arecovalent, adjacent layers are held together by weak van derWaals forces, enabling the well-known method of mechanicalexfoliation. Thus, one can cleave the material down to single-layer thickness. Bulk WS2 is an indirect gap semiconductor,with a gap of 1.3 eV, while as the material transitions to amonolayer the gap becomes direct with size ∼2 eV.

Atomically thin flakes of WS2 were prepared by micro-mechanical exfoliation of bulk WS2 crystals obtained from TXmaterials and deposited on a degenerately doped silicon sub-strate covered with 290 nm of silicon oxide. The silicon oxide

aSchool of Physics and Astronomy, University of Manchester, Manchester M13 9PL,

UK. E-mail: artem.mishchenko@gmail.combSchool of Chemistry, University of Manchester, Manchester M13 9PL, UKcManchester Centre for Mesoscience and Nanotechnology, University of Manchester,

Manchester M13 9PL, UK

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serves as an optical enhancer enabling the optical identifi-cation of atomically thin flakes,21 while also serving as a dielec-tric for electrical characterization. We investigated thin flakesof WS2 with a combination of techniques. First, we characteri-zed atomically thin layers optically by systematically measur-ing its Raman spectrum as a function of flake thickness, whilealso monitoring the photoluminescence. We employed atomicforce microscopy in order to confirm its thickness and finallywe probed WS2 electronically by transport measurements in aback-gating field effect transistor configuration.

Scanning Raman measurements were recorded using aWitec Raman spectrometer with a 1800 lines per mm gratingand a 514 nm laser line. The Raman light was collected in abackscattering configuration through a 100× objective with NA0.95. The resolution of the Witec spectrometer determined bythe width of the silicon peak is 3 cm−1. Photoluminescencespectra were recorded using a Renishaw spectrometerequipped with a 515.5 nm laser line. The PL spectra were nor-malized with respect to the out-of-plane mode A1g of theRaman spectrum in order to allow comparison of spectra

recorded on flakes of different thickness. In both the cases thelaser power was kept well below 1 mW in order to avoid localheating effects. A multimode Veeco atomic force microscopewas utilised in order to obtain topography scans in thetapping mode. For electrical characterization, we employedoptical lithography in order to expose the contacts. Metalli-zation was carried out by electron beam evaporation using a5 nm Cr/50 nm Au. Electrical characterization was carried outin helium using a Keithley 2636A sourcemeter.

Raman spectroscopy is an invaluable tool in grapheneresearch, being able to unambiguously identify monolayer,22

characterize strain,23,24 doping,25–27 edges28 and disorder.29,30

Similar to graphene, Raman spectroscopy is proving useful inthe case of MoS2 research. Recently reported thickness-depen-dent Raman spectroscopy reveals distinct signatures associatedwith monolayers and few layers,31,32 as the vibrational spectrumis sensitive to the number of layers in the sample. All TMDsshare two main common features in their Raman spectrum.The E2g mode where the metal and chalcogen atoms vibrate in-plane in opposite directions, and the A1g mode where the chal-cogen atoms just vibrate out-of-plane. For WS2 the E2g lies ataround 352 cm−1 whereas the A1g mode lies at 420 cm−1.33

Fig. 1b and c present an optical micrograph and an AFMscan of a monolayer of WS2. Fig. 1d and e present the scanningRaman maps of the same flake. The Raman maps capture pic-torially the transition from bulk to monolayer, as the twophonon modes shift with respect to thickness. For monolayers,the E2g mode red shifts to 354 cm−1 and the A1g mode blueshifts to 418 cm−1. The A1g mode stiffens as the restoring forceacting on the atoms decreases due to the decreased number oflayers, whereas the E2g mode softens due to the strongreduction in dielectric screening of a monolayer compared tothe bulk (Fig. 2).31,34

To unveil the transport characteristics we prepared field-effect transistors (FETs) where the conductive channel consists

Fig. 1 (a) Schematic of atomic trigonal prismatic arrangement in a WS2 layer.The W atoms, in grey, are sandwiched by S, blue. (b) Optical image and tappingmode AFM of a WS2 flake. A height profile across the flake reveals ∼1 nm stepheight in agreement with monolayer thickness. (d, e) Scanning Raman spec-troscopy of a monolayer of WS2 reveals the evolution of the E2g and A1g modes(shown as insets) with respect to flake thickness.

Fig. 2 (a) Photoluminescence spectroscopy for WS2, normalised with respectto the E2g Raman mode. The onset of strong photoluminescence when WS2 is inthe monolayer state indicates a transition from an indirect to a direct-gapsemiconductor.

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of a few layers of WS2. Fig. 3a shows a typical image of one ofour devices. To examine its electrical characteristics, we applya drain-source voltage Vds between two Cr/Au patterned electro-des while sweeping a gate voltage, Vg, through the conductiveSi substrate. Fig. 3b presents the logarithmic conductance ofthe channel as a function of Vg at Vb = 0.1 V. Our transistorsshow n-type behaviour and current modulation spanning fiveorders of magnitude, which is sufficient for digital electronicapplications.5

The field-effect mobility of our WS2 transistors was esti-mated from the slope dG/dVgs of the G–Vg curve:

μ ¼ LWC

dGds

dVgs

where L and W are the length and the width of the channel,C = 1.19 × 10−4 Fm−2 is the gate capacitance per unit area givenby C = ε0εr/d where εr is the dielectric constant of SiO2 and d isthe thickness of the gate oxide (290 nm) correspondingly. Weobtain a field effect mobility μ = 10 cm2 V−1 s−1, which is twoorders of magnitude better than the corresponding MoS2back-gated transistors.8,35 We note that this value representsthe lower limit because of the contact resistance. As our devicepresents with linear I–Vds characteristics (Fig. 3, inset), weexclude the possibility that a Schottky barrier is dominatingthe field effect mobility at the contacts. One can possiblyfurther enhance the mobility by using a high dielectric top-gate, e.g. hafnium oxide or zirconium oxide, which has beenshown to act as a mobility booster.35 In order to probe furtherthe performance of WS2 FETs, we investigated the subthres-hold swing (SS) of our devices. The SS is defined as the voltageswing required for a tenfold increase in the drain current andis given by the following expression

SS ¼ dVgs=dðlog IdsÞ

The SS is an important figure of merit in transistor oper-ation and lowering it remains one of the most non-trivial tech-nological challenges.5 Our devices show SS of ∼4 V per decade.

It is expected that a high dielectric top-gate will also aid inboosting the SS, since in our case the SS is limited by the gatecapacitance.

In conclusion, we have studied atomically thin flakes ofWS2, a layered semiconducting material. The material presentsa large mobility of the order of μ = 10 cm2 V−1 s−1 and a largeON/OFF ratio of the order of 105, making the material parti-cularly interesting for applications in low-power electronicdevices. Raman spectroscopy reveals its thickness-dependentmodes while photoluminescence spectroscopy monitors thetransition of the material from an indirect band-gap semicon-ductor to a direct one. WS2 offers yet another exciting two-dimensional material which invites further studies.

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