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Magnetic brightening and control of dark excitons in monolayer WSe2

Nature Nanotechnology volume 12, pages 883–888 (2017)Cite this article

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Abstract

Monolayer transition metal dichalcogenide crystals, as direct-gap materials with strong light–matter interactions, have attracted much recent attention. Because of their spin-polarized valence bands and a predicted spin splitting at the conduction band edges, the lowest-lying excitons in WX2 (X = S, Se) are expected to be spin-forbidden and optically dark. To date, however, there has been no direct experimental probe of these dark excitons. Here, we show how an in-plane magnetic field can brighten the dark excitons in monolayer WSe2 and permit their properties to be observed experimentally. Precise energy levels for both the neutral and charged dark excitons are obtained and compared with ab initio calculations using the GW-BSE approach. As a result of their spin configuration, the brightened dark excitons exhibit much-increased emission and valley lifetimes. These studies directly probe the excitonic spin manifold and reveal the fine spin-splitting at the conduction band edges.

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Figure 1: Conduction band structure of monolayer WSe2 and magnetic brightening of dark excitons.
Figure 2: Magnetic field-dependence of emission from dark exciton states.
Figure 3: Schematic illustration of bright and dark neutral and charged excitonic states.
Figure 4: Valley properties of dark excitons based on polarized PL.

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Acknowledgements

Support at SLAC/Stanford was provided by the AMOS program, Chemical Sciences, Geosciences, and Biosciences Division, Basic Energy Sciences, US Department of Energy under contract no. DE-AC02-76-SFO0515 and the Betty and Gordon Moore Foundation's EPiQS Initiative through grant no. GBMF4545 (T.F.H.). for data analysis, and by Air Force Office of Scientific Research through the MURI Center for dynamic magneto-optics under grant. no. FA9550-14-1-0040 for optical measurements. The theoretical studies were supported by the Theory of Materials Program funded by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, US Department of Energy under contract no. DE-AC02-05CH11231, which supported the GW and GW-BSE calculations, and by the National Science Foundation (NSF) grant no. DMR-1508412, which supported the magnetic-field-induced exciton mixing analyses. Computational resources were provided by the DOE at Lawrence Berkeley National Laboratory's NERSC facility and by the NSF through XSEDE resources at NICS. Y.-C.L. and J.A.R. acknowledge the support from the Center for Low Energy Systems Technology (LEAST), one of six centres supported by the STARnet phase of the Focus Center Research Program (FCRP), a Semiconductor Research Corporation program sponsored by MARCO and DARPA. Sample preparation at Columbia University was supported by the NSF MRSEC for Precision Assembly of Superstratic and Superatomic Solids through grant no. DMR-1420634. Z.L. and D.S. acknowledge the support from the US Department of Energy (grant no. DE-FG02-07ER46451) for high-field CW PL measurements that were performed at the National High Magnetic Field Laboratory, which is supported by the NSF Cooperative Agreement no. DMR-1157490 and the State of Florida.

Author information

Authors and Affiliations

  1. Department of Physics, Columbia University, New York, 10027, New York, USA

    Xiao-Xiao Zhang

  2. Department of Applied Physics, Stanford University, Stanford, 94305, California, USA

    Xiao-Xiao Zhang & Tony F. Heinz

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

    Xiao-Xiao Zhang & Tony F. Heinz

  4. Department of Physics, University of California, Berkeley, 94720, California, USA

    Ting Cao & Steven G. Louie

  5. Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, 94720, California, USA

    Ting Cao & Steven G. Louie

  6. National High Magnetic Field Laboratory, Tallahassee, 32312, Florida, USA

    Zhengguang Lu, Ying Wang, Zhiqiang Li & Dmitry Smirnov

  7. Department of Physics, Florida State University, Tallahassee, 32310, Florida, USA

    Zhengguang Lu & Ying Wang

  8. Department of Materials Science and Engineering and Center for 2-Dimensional and Layered materials, The Pennsylvania State University, University Park, 16802, Pennsylvania, USA

    Yu-Chuan Lin & Joshua A. Robinson

  9. Department of Mechanical Engineering, Columbia University, New York, 10027, New York, USA

    Fan Zhang & James C. Hone

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Contributions

X.-X.Z. and T.C. conceived the experiment. X.-X.Z. and Z.G.L. performed the experiment, with assistance from Z.Q.L., Y.W. and D.S. at the National High Magnetic Field Laboratory. T.C. performed the theoretical calculation and analysis under the guidance of S.G.L. The exfoliated samples were prepared by F.Z. under the guidance of J.C.H., and Y.-C.L. and J.A.R. prepared the chemically grown samples. Analysis and interpretation of the data were performed by X.-X.Z. and T.F.H. All authors discussed the results and contributed to the writing of the manuscript.

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Correspondence to Tony F. Heinz.

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Zhang, XX., Cao, T., Lu, Z. et al. Magnetic brightening and control of dark excitons in monolayer WSe2. Nature Nanotech 12, 883–888 (2017). https://doi.org/10.1038/nnano.2017.105

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