Direct observation of the transition from indirect to direct bandgap in atomically thin epitaxial MoSe2

Journal name:
Nature Nanotechnology
Volume:
9,
Pages:
111–115
Year published:
DOI:
doi:10.1038/nnano.2013.277
Received
Accepted
Published online

Abstract

Quantum systems in confined geometries are host to novel physical phenomena. Examples include quantum Hall systems in semiconductors1 and Dirac electrons in graphene2. Interest in such systems has also been intensified by the recent discovery of a large enhancement in photoluminescence quantum efficiency3, 4, 5, 6, 7 and a potential route to valleytronics6, 7, 8 in atomically thin layers of transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se, Te), which are closely related to the indirect-to-direct bandgap transition in monolayers9, 10, 11, 12. Here, we report the first direct observation of the transition from indirect to direct bandgap in monolayer samples by using angle-resolved photoemission spectroscopy on high-quality thin films of MoSe2 with variable thickness, grown by molecular beam epitaxy. The band structure measured experimentally indicates a stronger tendency of monolayer MoSe2 towards a direct bandgap, as well as a larger gap size, than theoretically predicted. Moreover, our finding of a significant spin-splitting of ~180 meV at the valence band maximum of a monolayer MoSe2 film could expand its possible application to spintronic devices.

At a glance

Figures

  1. Crystal structure, RHEED patterns and overall ARPES spectra of MoSe2 thin film.
    Figure 1: Crystal structure, RHEED patterns and overall ARPES spectra of MoSe2 thin film.

    a, Crystal structure of MoSe2. b,c, RHEED patterns of epitaxial bilayer graphene over a 6H-SiC(0001) substrate (b) and a monolayer MoSe2 thin film grown on the substrate (c). d, Theoretical band structures calculated using GGA along the Γ–K direction of the monolayer MoSe2 film. Zero energy represents the VBM. e, f, Direct comparison of theoretical and experimental band structures of the monolayer MoSe2 film (e). The experimental band structure is shown in e as a second derivative of the data in f to enhance visibility (black and white intensity plot), and the overlaid green dotted lines are the calculated band structures with renormalized energy scale. ky refers to the momentum along the Γ–K direction, corresponding to the y axis shown in a.

  2. Band evolution with increasing thickness of MoSe2 thin films.
    Figure 2: Band evolution with increasing thickness of MoSe2 thin films.

    ad, ARPES spectra of monolayer, bilayer, trilayer and 8 ML MoSe2 thin films along the Γ–K direction. White and green dotted lines indicate the energy positions of the apices of valence bands at the Γ and K points, respectively, with energy values written in the same colours. eh, Second-derivative spectra of ad, respectively, to enhance the visibility of some bands. Yellow dashed lines indicate the Fermi level. il, Calculated band structures of monolayer, bilayer, trilayer and 8 ML MoSe2. Insets to k and l: zoom-in splitting of the valence band at the K point. Blue and red circles in k indicate opposite spin directions.

  3. Direct bandgap in monolayer and indirect bandgap in 8 ML MoSe2 thin films.
    Figure 3: Direct bandgap in monolayer and indirect bandgap in 8 ML MoSe2 thin films.

    ad, ARPES data for the monolayer sample. eh, ARPES data for the 8 ML sample. a,e, Second-derivative spectra of undoped monolayer (a) and 8 ML MoSe2 (e) films along the Γ–K direction. b,f, Second-derivative spectra along the Γ–K direction after potassium surface doping to shift the chemical potential and reveal the CBM in monolayer and 8 ML MoSe2 thin films, respectively. Yellow dashed lines are Fermi levels. Blue and red arrows in a indicate the opposite spin directions of the spin-split states near the K point in the monolayer MoSe2 film. Green dotted lines indicate the valence bands in monolayer and 8 ML MoSe2 films, moved by 0.13 eV and 0.46 eV with potassium doping, respectively. c,g, Constant energy maps at the CBM of potassium-doped monolayer and 8 ML MoSe2 films, respectively. d,h, Constant energy maps at the VBM of undoped monolayer and 8 ML MoSe2 films, respectively. Red hexagons indicate the first Brillouin zone of the system. kx and ky refer to the momentum along the Γ–K and Γ–M directions, corresponding to the x and y axes shown in Fig. 1a, respectively.

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Author information

Affiliations

  1. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Yi Zhang,
    • Bo Zhou,
    • Yulin Chen,
    • Sung-Kwan Mo &
    • Zahid Hussain
  2. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA

    • Yi Zhang,
    • Yong-Tao Cui,
    • Hao Yan,
    • Zhongkai Liu,
    • Felix Schmitt,
    • James Lee,
    • Rob Moore &
    • Zhi-Xun Shen
  3. Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan

    • Tay-Rong Chang &
    • Horng-Tay Jeng
  4. Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA

    • Bo Zhou,
    • Yong-Tao Cui,
    • Hao Yan,
    • Zhongkai Liu,
    • Felix Schmitt,
    • James Lee,
    • Rob Moore,
    • Yulin Chen &
    • Zhi-Xun Shen
  5. Department of Physics and Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK

    • Bo Zhou &
    • Yulin Chen
  6. Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA

    • Hsin Lin &
    • Arun Bansil
  7. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan

    • Horng-Tay Jeng

Contributions

Y.Z. led the thin-film growth effort with F.S., J.L., R.M. and S.K.M., performed ARPES measurements with B.Z., Z.L. and S.K.M., and analysed the data. Y.Z., H.L. and S.K.M. wrote the paper with suggestions and comments by A.B. and Z.X.S. Y.T.C. and Y.H. characterized samples with Raman spectroscopy and AFM. T.R.C., H.L., H.T.J. and A.B. provided theoretical support. S.K.M., Y.L.C., Z.H., A.B. and Z.X.S. were responsible for project direction, planning and infrastructure.

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The authors declare no competing financial interests.

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