Dalton Transactions COMMUNICATION Cite this: Dalton Trans., 2014, 43, 10388 Received 27th August 2013, Accepted 24th September 2013 DOI: 10.1039/c3dt52353e www.rsc.org/dalton Electrical and optical characterization of atomically thin WS 2 Thanasis Georgiou, a Huafeng Yang, b Rashid Jalil, c James Chapman, a Kostya S. Novoselov a 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, atomically thin tungsten disulphide (WS 2 ), a layered semiconduc- tor. We provide the distinctive Raman and photoluminescence sig- natures for single layers, and prepare field-effect transistors where atomically thin WS 2 serves as the conductive channel. The transis- tors present mobilities μ = 10 cm 2 V -1 s -1 and exhibit ON/OFF ratios exceeding 100 000. Our results show that WS 2 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 of unusual properties that sparked interest in two-dimensional materials. Its unusual electronic properties 1,2 and mechanical stability 3,4 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 cm 2 V -1 s -1 make it a prime candidate for such applications. However, the gapless Dirac-cone nature of graphene’s electronic spectrum does not make this a straightforward task, since a band gap is required for applications in digital electronics. 5 To this end, efforts con- centrated on inducing a band gap in graphene, either by using nanoribbons, 6 quantum dots or chemical derivatives. 7 Such efforts however have detrimental effects on graphene’s elec- tronic quality. Beyond graphene, there exist many other materials that are layered and can be exfoliated. 8 A prime example of this is hexa- gonal boron nitride (hBN), often seen as graphene’s insulating counterpart. Thick flakes of boron nitride have proven to be particularly useful for serving as a substrate for improving gra- phene’s electronic quality, 9 while the availability of single and few-layer hBN 10 is particularly useful for investigating double- layer graphene heterostructures, 11–13 with hBN serving as a nanometre-thick insulating spacer. Layered transition metal dichalcogenides (TMDs) consist of a large family of materials with the general form TX 2 , where T is a transition-metal from the 4 th –6 th group of the periodic table and X is a chalcogen – sulphur, selenium, or tellurium. Generally, TMDs formed by metals from the 4 th and 6 th groups are semiconductors or insulators, e.g. MoS 2 , whereas those formed by metals from the 5 th group exhibit metallic behav- iour, e.g. TaS 2 and NbSe 2 . 14 Layered semiconductor TMDs have proven to be important candidates for use as an absorber layer in low cost thin film solar cells. 15 This is due to their relatively small band gap (∼1–2 eV) and the large absorption coeffi- cient. 16 Among TMDs, MoS 2 has been steadily attracting more attention. While the bulk of the material has an indirect band gap, single layer MoS 2 is a direct-gap semiconductor. Recently, top-gated MoS 2 transistors have been demonstrated with high ON/OFF ratios of 10, 8 while both MoS 2 and WS 2 were used as a spacer layer for vertical field effect tunnelling transistors, showing very promising characteristics. 17,18 Here we study tungsten disulphide (WS 2 ), yet another member of the TMD family structurally and electronically similar to MoS 2 , as both W and Mo reside in the same column of the periodic table. However, WS 2 has superior thermal and oxidative stability than that of MoS 2 . 19,20 Fig. 1a shows the arrangement of atoms within a trilayer of WS 2 : a single layer of W atoms sandwiched by two sheets of S, in a trigonal prismatic coordination. While the bonds within the trilayers are covalent, adjacent layers are held together by weak van der Waals forces, enabling the well-known method of mechanical exfoliation. Thus, one can cleave the material down to single- layer thickness. Bulk WS 2 is an indirect gap semiconductor, with a gap of 1.3 eV, while as the material transitions to a monolayer the gap becomes direct with size ∼2 eV. Atomically thin flakes of WS 2 were prepared by micro- mechanical exfoliation of bulk WS 2 crystals obtained from TX materials and deposited on a degenerately doped silicon sub- strate covered with 290 nm of silicon oxide. The silicon oxide a School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK. E-mail: artem.mishchenko@gmail.com b School of Chemistry, University of Manchester, Manchester M13 9PL, UK c Manchester Centre for Mesoscience and Nanotechnology, University of Manchester, Manchester M13 9PL, UK 10388 | Dalton Trans. , 2014, 43, 10388–10391 This journal is © The Royal Society of Chemistry 2014 Published on 27 September 2013. Downloaded by The University of Manchester Library on 13/05/2015 11:43:46. View Article Online View Journal | View Issue