Structural symmetry-breaking plays a crucial role in determining the electronic band structures of two-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with electric fields on AB-stacked bilayers1,2 or stacked van der Waals heterostructures3,4. In contrast, transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics5,6. Apart from their in-plane inversion asymmetry, an additional degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external electric fields7,8 or, as theoretically proposed, with an asymmetric out-of-plane structural configuration9. Here, we report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS2 monolayer, we fully replace the top-layer S with Se atoms. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements.
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L.-J.L. acknowledges support from the King Abdullah University of Science and Technology (Saudi Arabia), the Ministry of Science and Technology (MOST), the Taiwan Consortium of Emergent Crystalline Materials (TCECM), Academia Sinica (Taiwan) and Asian Office of Aerospace Research & Development (AOARD) under contract no. FA2386-15-1-0001 (USA). C.-P.C. and M.Y.C. acknowledge support from the Thematic Project of Academia Sinica. M.Y.C. acknowledges support from the National Science Foundation (NSF, grant no. 1542747). X.Z. acknowledges support from the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division of the US Department of Energy under contract no. DE-AC02-05-CH11231 (van der Waals heterostructures programme, KCWF16) for PFM imaging and analysis; and Samsung Electronics for nonlinear optical characterization. Y.H. and D.A.M. were supported by the Cornell Center for Materials Research, NSF MRSEC (DMR-1120296) and NSF grant no. MRI-1429155. P.Y. acknowledges support from the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division under contract no. DE-AC02-05CH11231 (PChem KC3103).