Journal Name COMMUNICATION This journal is © The Royal Society of Chemistry 20xx J. Name ., 2013, 00, 1-3 | 1 Please do not adjust margins Please do not adjust margins a School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore. b Centre for Disruptive Photonic Technologies, School of Physics and Mathematics Sciences, Nanyang Technological University, Singapore 637371, Singapore. c National Graphene Institute, University of Manchester, Booth St E, Manchester M13 9PL, United Kingdom d Physics Department, Lancaster University, Lancaster LA1 4YB, United Kingdom. e NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798. † Authors contributed equally to this work *Address correspondence to z.liu@ntu.edu.sg (Z.L.), xxhe@ntu.edu.sg (X.H.) Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/ Optoelectronic properties of atomically thin ReSSe with weak interlayer coupling Fucai Liu, a,† Shoujun Zheng, b, † Apoorva Chaturvedi, a Viktor Zólyomi, c Jiadong Zhou, a Qundong Fu, a Chao Zhu, a Peng Yu, a Qingsheng Zeng, a Neil D. Drummond, d Hong Jin Fan, b Christian Kloc, a Vladimir I. Fal’ko, c Xuexia He * , a Zheng Liu *a,e Rhenium dichalcogenides, such as ReS 2 and ReSe 2 , have attracted a lot of interests due to the weak interlayered coupling in these materials. Studies of rhenium based dichalcogenide alloys will help us understand the differences between each binary rhenium dichalcogenides. They will also extend the applications of two-dimensional (2D) materials through alloying. In this work, we studied the optoelectronic properties of ReSSe with S and Se ratio of 1:1. The band gap of ReSSe alloy is investigated by optical absorption spectra as well as theoretical calculations. The alloy shows weak interlayered coupling, as evidenced by the Raman spectrum. A field-effect transistor based on ReSSe shows typical n-type behavior with a mobility of about 3 cm 2 V -1 s -1 and an on/off ratio of 10 5 , together with the in-plane anisotropic conductivity. The device also shows good photoresponse properties, with a photoresponsivity of 8 A/W. The results demonstrated here will provide new avenues for the study of 2D materials with weak interlayer interactions and in-plane anisotropy. Owing to their interesting physical properties and promising applications in nanoelectronics, optoelectronics, and valleytronics 1-10 , two-dimensional (2D) materials have attracted a lot of interests since the discovery of graphene. Graphene, a single layer of carbon atoms arranged in a honeycomb lattice, has shown extremely high mobilities 11, 12 , a high Young’s modulus 13 , and an excellent thermal conductivity 14 . However, the absence of a band gap in graphene reduces its applicability in semiconducting and optoelectronic devices. This has triggered research interests in other semiconducting 2D materials. Among the 2D semiconductors that have been studied, transition-metal dichalcogenides (TMDs) are some of the most promising candidates for optoelectronic applications due to their direct band gaps and strong absorption 15-19 . The recently discovered black phosphorus (BP) has a direct band gap, increasing monotonically from ∼0.3 eV in bulk to ∼1.7 eV in a monolayer 20, 21 . It also shows high mobilities of up to 1000 cm 2 V -1 s -1 at room temperature 22, 23 . These properties make it suitable for applications such as broadband photodetectors, solar cells, and digital electronics 24, 25 . In addition, the bulk structure in BP opens up a new field for studying the in-plane anisotropy of 2D materials and designing conceptually new optoelectronic devices 26-28 . However, the poor environmental stability of BP restricts its applicability 29 . Discovering new, air- stable 2D materials with in-plane anisotropy and other novel functionality would be very rewarding. Re dichalcogenides are promising candidates for the study of anisotropy. ReS 2 forms a distorted 1T structure with triclinic symmetry (Figure 1a) 30 . The Peierls distortion of the 1T structure results in buckled S layers and zigzag Re chains along one of the lattice vectors (b-axis) in the plane. As a consequence, the optical and electric properties in the layer plane are strongly anisotropic 31 . Exploiting the anisotropy of the electronic conductivity of ReS 2 , an integrated inverter based on ReS 2 has been demonstrated 32 . Moreover, the interlayered coupling in ReS 2 is much weaker than other TMDs, with the adjacent monolayers being largely decoupled. Unlike other TMDs, from bulk to monolayers, the Raman spectrum shows almost no dependence on the thickness of ReS 2 33 . The unique properties of ReS 2 make Re compounds attractive for applications in novel optoelectronic devices. To realize high-efficiency solar cells or other optoelectronic devices based on Re compounds, it is crucially important to develop a strategy to tune their optical band gap. Band-gap engineering of TMDs has become urgent. Strain engineering and stacking of various 2D materials have been proposed as