Imaging moiré flat bands in three-dimensional reconstructed WSe2/WS2 superlattices

Abstract

Moiré superlattices in transition metal dichalcogenide (TMD) heterostructures can host novel correlated quantum phenomena due to the interplay of narrow moiré flat bands and strong, long-range Coulomb interactions1,2,3,4,5,6,7,8,9. However, microscopic knowledge of the atomically reconstructed moiré superlattice and resulting flat bands is still lacking, which is critical for fundamental understanding and control of the correlated moiré phenomena. Here we quantitatively study the moiré flat bands in three-dimensional (3D) reconstructed WSe2/WS2 moiré superlattices by comparing scanning tunnelling spectroscopy (STS) of high-quality exfoliated TMD heterostructure devices with ab initio simulations of TMD moiré superlattices. A strong 3D buckling reconstruction accompanied by large in-plane strain redistribution is identified in our WSe2/WS2 moiré heterostructures. STS imaging demonstrates that this results in a remarkably narrow and highly localized K-point moiré flat band at the valence band edge of the heterostructure. A series of moiré flat bands are observed at different energies that exhibit varying degrees of localization. Our observations contradict previous simplified theoretical models but agree quantitatively with ab initio simulations that fully capture the 3D structural reconstruction. Our results reveal that the strain redistribution and 3D buckling in TMD heterostructures dominate the effective moiré potential and the corresponding moiré flat bands at the Brillouin zone K points.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Fig. 1: Aligned WSe2/WS2 heterostructure.
Fig. 2: Moiré superlattice reconstruction.
Fig. 3: STS measurement of moiré-induced flat bands.
Fig. 4: Ab initio calculations of the electronic structure in reconstructed moiré superlattice.

Data availability

The data supporting the findings of this study are included in the main text and in the Supplementary Information files, and are also available at https://github.com/HongyuanLiCMP/Moire_STM_source_data.

References

  1. 1.

    Tang, Y. et al. Simulation of Hubbard model physics in WSe2/WS2 moiré superlattices. Nature 579, 353–358 (2020).

    CAS  Article  Google Scholar 

  2. 2.

    Wu, F., Lovorn, T., Tutuc, E. & MacDonald, A. H. Hubbard model physics in transition metal dichalcogenide moiré bands. Phys. Rev. Lett. 121, 026402 (2018).

    CAS  Article  Google Scholar 

  3. 3.

    Shimazaki, Y. et al. Strongly correlated electrons and hybrid excitons in a moiré heterostructure. Nature 580, 472–477 (2020).

    CAS  Article  Google Scholar 

  4. 4.

    Regan, E. C. et al. Mott and generalized Wigner crystal states in WSe2/WS2 moiré superlattices. Nature 579, 359–363 (2020).

    CAS  Article  Google Scholar 

  5. 5.

    Wang, L. et al. Correlated electronic phases in twisted bilayer transition metal dichalcogenides. Nat. Mater. 19, 861–866 (2020).

    CAS  Article  Google Scholar 

  6. 6.

    Yankowitz, M. et al. Tuning superconductivity in twisted bilayer graphene. Science 363, 1059–1064 (2019).

    CAS  Article  Google Scholar 

  7. 7.

    Liu, X. et al. Tunable spin-polarized correlated states in twisted double bilayer graphene. Nature 583, 221–225 (2020).

    CAS  Article  Google Scholar 

  8. 8.

    Kim, K. et al. Tunable moiré bands and strong correlations in small-twist-angle bilayer graphene. Proc. Natl Acad. Sci. USA 114, 3364–3369 (2017).

    CAS  Article  Google Scholar 

  9. 9.

    Lu, X. et al. Superconductors, orbital magnets and correlated states in magic-angle bilayer graphene. Nature 574, 653–657 (2019).

    CAS  Article  Google Scholar 

  10. 10.

    Cao, Y. et al. Correlated insulator behaviour at half-filling in magic-angle graphene superlattices. Nature 556, 80–84 (2018).

    CAS  Article  Google Scholar 

  11. 11.

    Cao, Y. et al. Unconventional superconductivity in magic-angle graphene superlattices. Nature 556, 43–50 (2018).

    CAS  Article  Google Scholar 

  12. 12.

    Zondiner, U. et al. Cascade of phase transitions and Dirac revivals in magic-angle graphene. Nature 582, 203–208 (2020).

    CAS  Article  Google Scholar 

  13. 13.

    Burg, G. W. et al. Correlated insulating states in twisted double bilayer graphene. Phys. Rev. Lett. 123, 197702 (2019).

    CAS  Article  Google Scholar 

  14. 14.

    Cao, Y. et al. Tunable correlated states and spin-polarized phases in twisted bilayer–bilayer graphene. Nature 583, 215–220 (2020).

    CAS  Article  Google Scholar 

  15. 15.

    Chen, G. et al. Evidence of a gate-tunable Mott insulator in a trilayer graphene moiré superlattice. Nat. Phys. 15, 237–241 (2019).

    CAS  Article  Google Scholar 

  16. 16.

    Chen, G. et al. Tunable correlated Chern insulator and ferromagnetism in a moiré superlattice. Nature 579, 56–61 (2020).

    CAS  Article  Google Scholar 

  17. 17.

    Chen, G. et al. Signatures of tunable superconductivity in a trilayer graphene moiré superlattice. Nature 572, 215–219 (2019).

    CAS  Article  Google Scholar 

  18. 18.

    Slagle, K. & Fu, L. Charge transfer excitations, pair density waves, and superconductivity in moiré materials. Phys. Rev. B 102, 235423 (2020).

    Article  Google Scholar 

  19. 19.

    Zhang, Y., Yuan, N. F. & Fu, L. Moiré quantum chemistry: charge transfer in transition metal dichalcogenide superlattices. Phys. Rev. B 102, 201115 (2020).

    CAS  Article  Google Scholar 

  20. 20.

    Kerelsky, A. et al. Maximized electron interactions at the magic angle in twisted bilayer graphene. Nature 572, 95–100 (2019).

    CAS  Article  Google Scholar 

  21. 21.

    Xie, Y. et al. Spectroscopic signatures of many-body correlations in magic-angle twisted bilayer graphene. Nature 572, 101–105 (2019).

    CAS  Article  Google Scholar 

  22. 22.

    Jiang, Y. et al. Charge order and broken rotational symmetry in magic-angle twisted bilayer graphene. Nature 573, 91–95 (2019).

    CAS  Article  Google Scholar 

  23. 23.

    Choi, Y. et al. Electronic correlations in twisted bilayer graphene near the magic angle. Nat. Phys. 15, 1174–1180 (2019).

    CAS  Article  Google Scholar 

  24. 24.

    Pan, Y. et al. Quantum-confined electronic states arising from the moiré pattern of MoS2–WSe2 heterobilayers. Nano Lett. 18, 1849–1855 (2018).

    CAS  Article  Google Scholar 

  25. 25.

    Zhang, C. et al. Interlayer couplings, moiré patterns, and 2D electronic superlattices in MoS2/WSe2 hetero-bilayers. Sci. Adv. 3, e1601459 (2017).

    Article  Google Scholar 

  26. 26.

    Zhang, Z. et al. Flat bands in twisted bilayer transition metal dichalcogenides. Nat. Phys. 16, 1093–1096 (2020).

    CAS  Article  Google Scholar 

  27. 27.

    Raja, A. et al. Coulomb engineering of the bandgap and excitons in two-dimensional materials. Nat. Commun. 8, 15251 (2017).

    Article  Google Scholar 

  28. 28.

    Wang, G. et al. Colloquium: excitons in atomically thin transition metal dichalcogenides. Rev. Mod. Phys. 90, 021001 (2018).

    CAS  Article  Google Scholar 

  29. 29.

    Ugeda, M. M. et al. Giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor. Nat. Mater. 13, 1091–1095 (2014).

    CAS  Article  Google Scholar 

  30. 30.

    Waters, D. et al. Flat bands and mechanical deformation effects in the moiré superlattice of MoS2-WSe2 heterobilayers. ACS Nano 14, 7564–7573 (2020).

    CAS  Article  Google Scholar 

Download references

Acknowledgements

This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division of the US Department of Energy under the van der Waals heterostructure program (KCWF16), contract number DE-AC02-05CH11231 (device electrode preparation, STM spectroscopy, DFT calculations and theoretical analysis). Support was also provided by the US Army Research Office under MURI award W911NF-17-1-0312 (device layer transfer), and by the National Science Foundation Awards DMR-1807233 (surface preparation) and DMR-1926004 (GW calculations). S.T. acknowledges support from DOE-SC0020653, NSF DMR 1552220, DMR 1904716 and NSF CMMI 1933214 for WSe2 and WS2 bulk crystal growth and analysis. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, grant number JPMXP0112101001, JSPS KAKENHI grant number JP20H00354 and the CREST(JPMJCR15F3), JST for bulk hBN crystal growth and analysis. E.C.R. acknowledges support from the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program. S.L. acknowledges support from Kavli ENSI Heising Simons Junior Fellowship. M.H.N. thanks S. Kundu and M. Jain for their implementation of noncollinear wavefunction plotting in Siesta. Computational resources were provided by Cori at National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the US Department of Energy under contract no. DE-AC02-05CH11231, Stampede2 at the Texas Advanced Computing Center (TACC) through Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation under grant no. ACI-1053575 and Frontera at TACC, which is supported by the National Science Foundation under grant no. OAC-1818253.

Author information

Affiliations

Authors

Contributions

M.F.C., F.W. and S.L. conceived the project, and S.G.L. supervised the theoretical calculations. H.L. and S.L. performed the STM measurements, and M.H.N. carried out the DFT and GW calculations. H.L., J.X., X.L., J.W., W.Z., S.Z. and S.K. fabricated the heterostructure device. E.R. and D.W. performed the second harmonic generation measurements. K.Y., M.B. and S.T. grew WSe2 and 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 Shaowei Li or Steven G. Louie or Feng Wang or Michael F. Crommie.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature Materials thanks Andrea Young and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Materials and Methods and Supplementary Figs. 1–13.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Li, H., Li, S., Naik, M.H. et al. Imaging moiré flat bands in three-dimensional reconstructed WSe2/WS2 superlattices. Nat. Mater. (2021). https://doi.org/10.1038/s41563-021-00923-6

Download citation

Further reading

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing