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Janus monolayers of transition metal dichalcogenides

Nature Nanotechnology volume 12, pages 744–749 (2017)Cite this article

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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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Figure 1: Synthesis of the Janus MoSSe monolayer.
Figure 2: Energy-dependent X-ray photoelectron spectroscopy.
Figure 3: Out-of-plane dipole probed by angle-resolved SHG.
Figure 4: Characterization of out-of-plane piezoelectricity in the Janus MoSSe monolayer.

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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).

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Author notes

  1. Ang-Yu Lu, Hanyu Zhu and Jun Xiao: These authors contributed equally to this work.

Authors and 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, 94720, California, 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, 14850, New York, 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, 94720, California, USA

    Yiming Yang & Peidong Yang

  8. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, 94720, California, USA

    Yiming Yang, Yuan Wang, Peidong Yang & Xiang Zhang

  9. SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, 94025, California, USA

    Dimosthenis Sokaras & Dennis Nordlund

  10. Kavli Institute at Cornell for Nanoscale Science, Ithaca, 14853, New York, 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, 30332, Georgia, USA

    Mei-Yin Chou

Authors
  1. Ang-Yu Lu
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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.

Corresponding authors

Correspondence to Xiang Zhang or Lain-Jong Li.

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The authors declare no competing financial interests.

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Lu, AY., Zhu, H., Xiao, J. et al. Janus monolayers of transition metal dichalcogenides. Nature Nanotech 12, 744–749 (2017). https://doi.org/10.1038/nnano.2017.100

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