Excitons in a reconstructed moiré potential in twisted WSe₂/WSe₂ homobilayers
Moiré superlattices in twisted van der Waals materials have recently emerged as a promising platform for engineering electronic and optical properties. A major obstacle to fully understanding these systems and harnessing their potential is the limited ability to correlate direct imaging of the moiré structure with optical and electronic properties. Here we develop a secondary electron microscope technique to directly image stacking domains in fully functional van der Waals heterostructure devices. After demonstrating the imaging of AB/BA and ABA/ABC domains in multilayer graphene, we employ this technique to investigate reconstructed moiré patterns in twisted WSe₂/WSe₂ bilayers and directly correlate the increasing moiré periodicity with the emergence of two distinct exciton species in photoluminescence measurements. These states can be tuned individually through electrostatic gating and feature different valley coherence properties. We attribute our observations to the formation of an array of two intralayer exciton species that reside in alternating locations in the superlattice, and open up new avenues to realize tunable exciton arrays in twisted van der Waals heterostructures, with applications in quantum optoelectronics and explorations of novel many-body systems.
© 2020 Nature Publishing Group. Received 12 February 2020; Accepted 11 November 2020; Published 04 January 2021. We thank B. Urbaszek, R. Bekenstein and L. Ju for helpful discussions. We acknowledge support from the DoD Vannevar Bush Faculty Fellowship (N00014-16-1-2825 for H.P. and N00014-18-1-2877 for P.K.), NSF (PHY-1506284 for H.P. and M.D.L.), NSF CUA (PHY-1125846 for H.P. and M.D.L.), AFOSR MURI (FA9550-17-1-0002) and ARL (W911NF1520067) for H.P. and M.D.L., the Gordon and Betty Moore Foundation (GBMF4543 for P.K.), ONR MURI (N00014-15-1-2761 for P.K.) and Samsung Electronics (for P.K. and H.P.). All the fabrication was performed at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF award 1541959. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, and the CREST (JPMJCR15F3), JST. A.S. acknowledges support from the Fannie and John Hertz Foundation and the Paul & Daisy Soros Fellowships for New Americans. Data availability: All data needed to evaluate the conclusions are presented in the article and the Supplementary Information. Author Contributions: T.I.A., G.S., A.S., K.D.G., H.P. and M.D.L. conceived the project. T.I.A., G.S., A.S. and K.D.G. designed and performed the experiments, analysed the data and wrote the manuscript with extensive input from the other authors. Y.Z. and J.S. assisted with the optical measurements. T.I.A., G.S., J.S., Y.Z., L.A.J. and A.Y.J. fabricated the samples. A.S. and K.D.G. performed the SEM imaging. T.I.A., G.S., A.S., K.D.G, D.S.W. and D.B. developed the theoretical model. H.H. grew the TMD crystals. T.T. and K.W. grew the hBN crystals. P.K., H.P. and M.D.L. supervised the project. The authors declare no competing interests.
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Supplemental Material - 41563_2020_873_MOESM1_ESM.pdf