Paper 2D+NS-WeA12
Metallic Edges in Atomically Thin WSe₂

Wednesday, November 9, 2016, 6:00 pm, Room 103B

Transition metal dichalcogenides (TMDs) is a unique class of layered two-dimensional (2D) crystals with extensive promising applications. Tuning their electronic properties is vital for engineering new functionalities. Surface oxidation is of particular interest because it is a relatively simple and low-cost method compared with other processes involving complicated steps. By means of scanning tunneling microscopy and spectroscopy (STM and STS), and X-ray photoelectron spectroscopy (XPS), we show here the observation of metallic step edges in atomically thin WSe₂ monolayers grown by chemical vapor deposition (CVD) on epitaxial graphene (Gr). STM images show the shape and the structure of WSe₂ step edges and STS reveals their metallic nature. Photoemission demonstrates that the formation of metallic sub-stoichiometric tungsten oxide (WO_x, x<3) is responsible of high conductivity measured along the WSe₂ step edges. DFT Calculations [1,2] revealed that the W₁₈O₄₉ have a metallic behavior which is in excellent agreement with our photoemission estimated WO_x, with 2.61≤x≤2.72. Our findings are in contrary with reported results of air-exposed WSe₂ edges with large band gap measured at ~3.1 eV. [3] We explain this discrepancy by considering the differences in WO_x stoichiometry. [1]

This work was supported in part by the Center for Low Energy Systems Technology (LEAST), one of the six SRC STARnet Centers, sponsored by MARCO and DARPA, and by the SWAN Center, a SRC center sponsored by the Nanoelectronics Research Initiative and NIST, and the US/Ireland R&D Partnership (UNITE) under the NSF award ECCS-1407765.