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Imaging moiré excited states with photocurrent tunnelling microscopy

Nature Materials (2024)Cite this article

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Abstract

Moiré superlattices provide a highly tuneable and versatile platform to explore novel quantum phases and exotic excited states ranging from correlated insulators to moiré excitons. Scanning tunnelling microscopy has played a key role in probing microscopic behaviours of the moiré correlated ground states at the atomic scale. However, imaging of quantum excited states in moiré heterostructures remains an outstanding challenge. Here we develop a photocurrent tunnelling microscopy technique that combines laser excitation and scanning tunnelling spectroscopy to directly visualize the electron and hole distribution within the photoexcited moiré exciton in twisted bilayer WS2. The tunnelling photocurrent alternates between positive and negative polarities at different locations within a single moiré unit cell. This alternating photocurrent originates from the in-plane charge transfer moiré exciton in twisted bilayer WS2, predicted by our GW-Bethe–Salpeter equation calculations, that emerges from the competition between the electron–hole Coulomb interaction and the moiré potential landscape. Our technique enables the exploration of photoexcited non-equilibrium moiré phenomena at the atomic scale.

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Fig. 1: Laser-STM measurements of a twisted bilayer WS2 moiré superlattice.
Fig. 2: Electronic structure of twisted bilayer WS2.
Fig. 3: Photocurrent mapping of ICT excitons.
Fig. 4: Interaction between STM tip and ICT excitons.

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Data availability

The data supporting the findings of this study are included in the main text and in the Supplementary Information, and are also available at https://github.com/HongyuanLiCMP/Imaging-Moir-Excited-States-with-Photocurrent-Tunneling-Microscopy.

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Acknowledgements

This work was primarily funded by the United States Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under contract no. DE-AC02-05CH11231 (van der Waals heterostructure programme KCFW16) for device fabrication, STM spectroscopy and force field calculations for structural reconstructions. The Center for Computational Study of Excited-State Phenomena in Energy Materials (C2SEPEM) at Lawrence Berkeley National Laboratory, supported by the United States Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division under contract no. DE-AC02-05CH11231, as part of the Computational Materials Sciences Program, provided advanced codes and experimental support for optical measurements. The Theory of Materials Program (KC2301) funded by the DOE Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division under contract DE-AC02-05CH11231, provided resources to develop the PUMP approach and analysis of the moiré excitons. Computational resources were provided by the National Energy Research Scientific Computing Center (NERSC), which is supported by the DOE Office of Science under contract DE-AC02-05CH11231, and Frontera at TACC, which is supported by the National Science Foundation under grant OAC-1818253. Support was also provided by National Science Foundation Award DMR-2221750 (surface preparation). S.T. acknowledges support from DOE-SC0020653, NSF DMR 2111812, NSF DMR 1552220, NSF 2052527, DMR 1904716 and NSF CMMI 1933214 for WS2 bulk crystal growth and analysis. K.W. and T.T. acknowledge support from JSPS KAKENHI (grant nos. 19H05790, 20H00354 and 21H05233). We thank Y. W. Choi, S. Kundu and J. Ruan for discussions.

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Author notes

  1. These authors contributed equally: Hongyuan Li, Ziyu Xiang, Mit H. Naik.

Authors and Affiliations

  1. Department of Physics, University of California at Berkeley, Berkeley, CA, USA

    Hongyuan Li, Ziyu Xiang, Mit H. Naik, Woochang Kim, Zhenglu Li, Alex Zettl, Steven G. Louie, Michael F. Crommie & Feng Wang

  2. Graduate Group in Applied Science and Technology, University of California Berkeley, Berkeley, CA, USA

    Hongyuan Li & Ziyu Xiang

  3. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    Hongyuan Li, Ziyu Xiang, Mit H. Naik, Woochang Kim, Zhenglu Li, Alex Zettl, Steven G. Louie, Michael F. Crommie & Feng Wang

  4. School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ, USA

    Renee Sailus, Rounak Banerjee & Sefaattin Tongay

  5. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Japan

    Takashi Taniguchi

  6. Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Japan

    Kenji Watanabe

  7. Kavli Energy Nano Sciences Institute, University of California Berkeley and Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    Alex Zettl, Michael F. Crommie & Feng Wang

  8. Department of Materials Science and Engineering, Stanford University, Palo Alto, CA, USA

    Felipe H. da Jornada

  9. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA, USA

    Felipe H. da Jornada

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  1. Hongyuan Li
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Contributions

S.G.L., M.F.C. and F.W. conceived the project. H.L. and Z.X. performed the STM/STS and PTM measurements, M.H.N. and F.H.d.J. formulated the generalized PUMP method, and M.H.N., W.K. and Z.L. performed the ab initio GW-BSE calculations. H.L. and Z.X. fabricated the heterostructure device. R.S., R.B. and S.T. grew the WS2 crystals. K.W. and T.T. grew the hBN single crystal. All authors discussed the results and wrote the manuscript.

Corresponding authors

Correspondence to Steven G. Louie, Michael F. Crommie or Feng Wang.

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Supplementary Figs. 1–13 and discussion.

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Li, H., Xiang, Z., Naik, M.H. et al. Imaging moiré excited states with photocurrent tunnelling microscopy. Nat. Mater. (2024). https://doi.org/10.1038/s41563-023-01753-4

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