Research ArticlePHYSICAL SCIENCE

Interlayer couplings, Moiré patterns, and 2D electronic superlattices in MoS2/WSe2 hetero-bilayers

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Science Advances  06 Jan 2017:
Vol. 3, no. 1, e1601459
DOI: 10.1126/sciadv.1601459
  • Fig. 1 Atomic structure of as-grown MoS2/WSe2 vdW heterostructures revealed by STM and STEM.

    (A) STM image for a MoS2/WSe2 vdW heterostructure on the highly oriented pyrolytic-graphite (HOPG) substrate. (B) Close-up STM image showing the hexagonal Moiré pattern with a periodicity of 8.7 nm. (C) Schematic model of the Moiré pattern on an R-stacked MoS2/WSe2 hetero-bilayer. By using the lattice constants of 3.16 Å for MoS2 and 3.28 Å for WSe2, the simulated supercell marked by black solid lines shows a periodicity of 8.64 nm. (D) Atomically resolved STEM image. Typical regions in an R-stacked heterostructure—AA, ABSe, Br, and ABW—are labeled in both (C) and (D). The close-up STEM images for each region are shown in the right column of (E). The simulated images (based on an R-type stacking) and their corresponding atomic models are displayed in the middle and left columns of (E), respectively. (A and B) −3.0 V, 10 pA. Exp., experimental.

  • Fig. 2 First-principles calculations for the interlayer separations and electronic structures of representative sites in an R-stacked MoS2/WSe2 heterostructure.

    (A) Side views of the atomic models for AA, ABSe, Br, and ABW regions with an average lattice constant (Supplementary Materials). The calculated interlayer separations for four atomic alignments are labeled in (A). (B) A perspective view of an STM image zoomed in on a unit cell of the Moiré pattern. A height profile along the diagonal line from AA to AA [gray dashed line in (B)] is shown in (C). Energy band structure of the AA registry is displayed in (D), whereas its corresponding density of states (DOS) diagrams are shown in (E). The size of the green (red) circles represents the projected weight on the d orbitals of Mo (W), and the states are labeled in the subscript based on this project. The corrections for the strain resulting from the average lattice constant used in the calculation are labeled for the typical critical points in the DOS diagram. Results for other sites can be found in the Supplementary Materials. (B) −3.0 V, 10 pA.

  • Fig. 3 Scanning tunneling spectra of AA, ABSe, Br, and ABW regions.

    (A) dI/dV spectra. (B and C) (∂Z/∂V)I and decay constant κ spectra of valence bands, respectively. (D) Calculated energy values at key critical points for AA, ABSe, Br, and ABW sites, respectively. The energies are with respect to the vacuum level. The shaded regions in (B) and (D) represent the valence band edges and show consistent movements of the energy locations of ΓW (black) and KW (cyan). In a deeper lying energy range, the spectral features marked by red and green arrows in (A) to (C) correspond to the energy window where the QW↑,↓, KMo↑,↓, and ΓMo states and a lower ΓW (labeled as ΓW2) state are located. The complicated movements in their relative energy locations result in a complex behavior in κ spectra [red arrows in (C)], making the direct identification of individual states nontrivial.

  • Fig. 4 Summary of the site-dependent electronic structures in MoS2/WSe2 hetero-bilayers.

    (A) Energy differences between KW and ΓWK−Γ) for the four different local lateral alignments. The experimental values are labeled as blue triangles, whereas the calculated DFT results are presented as brown circles. (B) Local bandgap Eg formed between the CBM of MoS2 and the VBM of WSe2. Experimental and calculated DFT results are displayed in purple and brown, respectively.

  • Fig. 5 Bias-dependent STM images of the Moiré pattern.

    The corresponding sample bias voltage is labeled for each image as shown. A dashed red rhombus in (E) and (F) represents a unit cell of the superlattice. A color bar is shown at the bottom to represent the relative height differences (that is, low and high) in (A) to (L). Scale bars, 5 nm in all images.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/1/e1601459/DC1

    Determining the R stacking and local atomic registries by intensity profiles of STEM images

    First-principles calculation of the site-dependent band structures

    Additional discussions for STM and STS results

    fig. S1. ADF-STEM results of the MoS2/WSe2 heterostructures.

    fig. S2. Intensity profile along AA-to-ABW in STEM images.

    fig. S3. First-principles calculations of the electronic structures of ABSe, Br, and ABW.

    fig. S4. Atomic model and the electronic structures of the Br2 site.

    fig. S5. Statistical distributions of the ΓW energy locations.

    table S1. Calculation results of the energy differences between key critical points.

  • Supplementary Materials

    This PDF file includes:

    • Determining the R stacking and local atomic registries by intensity profiles of STEM images
    • First-principles calculation of the site-dependent band structures
    • Additional discussions for STM and STS results
    • fig. S1. ADF-STEM results of the MoS2/WSe2 heterostructures.
    • fig. S2. Intensity profile along AA-to-ABW in STEM images.
    • fig. S3. First-principles calculations of the electronic structures of ABSe, Br, and ABW.
    • fig. S4. Atomic model and the electronic structures of the Br2 site.
    • fig. S5. Statistical distributions of the ΓW energy locations.
    • table S1. Calculation results of the energy differences between key critical points.

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