Abstract

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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Acknowledgements

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).

Author information

Author notes

    • Ang-Yu Lu
    • , Hanyu Zhu
    •  & Jun Xiao

    These authors contributed equally to this work.

Affiliations

  1. Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal, 23955-6900, Saudi Arabia

    • Ang-Yu Lu
    • , Ming-Hui Chiu
    • , Chih-Wen Yang
    •  & Lain-Jong Li
  2. NSF Nanoscale Science and Engineering Center, University of California, Berkeley, California 94720, USA

    • Hanyu Zhu
    • , Jun Xiao
    • , Yuan Wang
    •  & Xiang Zhang
  3. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan

    • Chih-Piao Chuu
    •  & Mei-Yin Chou
  4. School of Applied & Engineering Physics, Cornell University, Ithaca, New York 14850, USA

    • Yimo Han
    •  & David A. Muller
  5. Research Center for Applied Sciences, Academia Sinica, Taipei 10617, Taiwan

    • Chia-Chin Cheng
  6. Department of Material Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan

    • Chia-Chin Cheng
    •  & Kung-Hwa Wei
  7. Department of Chemistry, University of California, Berkeley, California 94720, USA

    • Yiming Yang
    •  & Peidong Yang
  8. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Yiming Yang
    • , Yuan Wang
    • , Peidong Yang
    •  & Xiang Zhang
  9. SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA

    • Dimosthenis Sokaras
    •  & Dennis Nordlund
  10. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA

    • David A. Muller
  11. Department of Physics, National Taiwan University, Taipei 10617, Taiwan

    • Mei-Yin Chou
  12. School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

    • Mei-Yin Chou

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Contributions

A.-Y.L., H.Z. and J.X. contributed equally to this work. L.J.L., A.-Y.L. and X.Z. conceived the concept. C.-P.C. and M.-Y.C. provided theoretical support. A.-Y.L., C.-C.C. and C.-W.Y. performed the synthesis. M.-H.C., A.-Y.L., S.D. and D.N. ran the X-ray photoelectron spectroscopy experiments and analysed the results. H.Z. and Y.Y. measured the piezoresponses, supervised by P.Y. Y.H. performed the transmission electron microscopy measurements and analysis, supervised by D.A.M. J.X. ran the SHG experiments and analysed the results. A.-Y.L., H.Z., J.X., C.-P.C, M.-Y.C., L.-J.L. and X.Z. wrote the manuscript. All co-authors discussed the results and commented on the manuscript at all stages.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Xiang Zhang or Lain-Jong Li.

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DOI

https://doi.org/10.1038/nnano.2017.100