Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide

Journal name:
Nature Materials
Volume:
12,
Pages:
554–561
Year published:
DOI:
doi:10.1038/nmat3633
Received
Accepted
Published online

Abstract

Recent progress in large-area synthesis of monolayer molybdenum disulphide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined chemical vapour deposition synthesis to grow highly crystalline islands of monolayer molybdenum disulphide up to 120 μm in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror twin boundaries that are stitched together by lines of 8- and 4-membered rings. Density functional theory reveals localized mid-gap states arising from these 8–4 defects. We find that mirror twin boundaries cause strong photoluminescence quenching whereas tilt boundaries cause strong enhancement. Meanwhile, mirror twin boundaries slightly increase the measured in-plane electrical conductivity, whereas tilt boundaries slightly decrease the conductivity.

At a glance

Figures

  1. Large-grain MoS2 growth.
    Figure 1: Large-grain MoS2 growth.

    a, Optical reflection image of a CVD growth of a typical large-grain MoS2 on a SiO2 (285 nm)/Si substrate. The image contrast has been increased for visibility; magenta is the bare substrate, and violet represents monolayer MoS2. b, Optical image of a monolayer MoS2 triangle. The triangle is 123 μm from tip to tip. c, Photoluminescence spectra from monolayer (red) and bilayer (blue) MoS2. Peak height is normalized to the silicon Raman peak. The narrow spikes at high energy are the Raman transitions (see Supplementary Fig. S3a). d, High-resolution ADF-STEM image of freely suspended monolayer MoS2 on a TEM grid. The bright spots are molybdenum atoms; the grey spots are two stacked sulphur atoms. The lattice is composed of hexagonal rings alternating molybdenum and sulphur sites; top view and side views of the structure are overlaid. e, DF-TEM image of a large triangle with the diffraction pattern inset. Together, the diffraction pattern and the DF-TEM image show that the triangle is a continuous single crystal. The ~2–4 μm brighter and darker areas are rotationally aligned bilayers of MoS2. Similar variations in contrast have been observed in bilayer graphene, where they reflect differences in stacking order29.

  2. Diffraction imaging of crystal orientation and edge terminations.
    Figure 2: Diffraction imaging of crystal orientation and edge terminations.

    a, Bright-field image of a single-crystal triangle with a Mo-zigzag edge orientation. b, Diffraction pattern from a. The asymmetry of the Mo and S sublattices separates the diffraction spots into two families: and kb  =  −ka. c, A line profile through experimentally measured diffraction spots (black) and Bloch-wave simulations (red). The higher intensity ka spots point towards the Mo sublattice, as indicated by the arrows in a,b. Both curves were normalized to the height of the peaks, and the red curve was offset horizontally for visibility. d, Bright-field TEM image of two triangles with S-zigzag edge orientations. The curved appearance of the crystal edges contrasts with the sharper crystal edges of the Mo-zigzag edges in a. The inset diffraction pattern shows the location of the aperture used to form the image in e. e, Dark-field image of the region in d. As the triangles are rotated 180° from one another, the aperture simultaneously collects the brighter ka spot for the left triangle and the darker kb spot for the right triangle. This produces a corresponding contrast difference between the two islands that allows us to uniquely infer crystallographic orientation from dark-field images. To improve visibility, we have clipped the bottom of the intensity range in this image.

  3. Tilt and mirror twin grain structures.
    Figure 3: Tilt and mirror twin grain structures.

    a, Bright-field TEM image of two triangles that have grown together. Inset diffraction pattern shows the two crystal orientations are 40°±0.5° apart (measured between the red and cyan lines, which are at equivalent spots in the diffraction pattern). b, Colour-coded overlay of DF-TEM images corresponding with the two red- and cyan-circled spots in a shows a tilt grain boundary as a faceted line connecting the two triangles. c, Bright-field image of a region containing irregularly shaped MoS2 islands, with diffraction pattern inset. Red arrows indicate regions where adjacent grains overlap, forming rotationally misaligned bilayers. d, Coloured DF-TEM overlay shows that the irregular shapes are polycrystalline aggregates. The crystals are connected both by faceted, abrupt grain boundaries, and ~1 μm overlapped bilayers. e, Bright-field image of a mirror twin composed of 180°-rotated triangles, with diffraction pattern inset. f, Dark-field image corresponding with the orange circle in e shows the two triangles have different diffraction intensity, similar to the 180°-rotated triangles in Fig. 2d,e. This intensity change indicates the presence of a mirror twin boundary between the darker and lighter regions. The small triangle in the centre is multilayer MoS2. g, Bright-field image of a 6-pointed star, with inset diffraction pattern. h, DF-TEM image corresponding with the orange circle in g shows that the star contains several rotationally symmetric mirror twins, forming a cyclic twin.

  4. Grain boundary atomic structure.
    Figure 4: Grain boundary atomic structure.

    a, High-resolution ADF-STEM image of a mirror twin boundary. The boundary is visible just below the annotated line. The annotation indicates the nanoscale faceting of the boundary at ±20° off of the zigzag direction. b, Zoomed-in image of the grain boundary shows a periodic line of 8-4-4 ring defects. c, An atomistic model of the experimental structure shown in b. Energy minimization with DFT confirms that this boundary is locally stable. d, The total DOS of defect-free, that is pristine, MoS2 (black), the total DOS of MoS2 with the grain boundary (red dashed) and the projected DOS of the atoms along the grain boundary (blue filled). The dashed grey line denotes the Fermi energy of pristine MoS2, and the light green shaded area indicates the pristine bandgap. All states have been given a Gaussian broadening of 0.07 eV. e, A 2D spatial plot of the local mid-gap DOS (integrated in the plane of the Mo over a 1.7 eV range about the Fermi energy of the pristine MoS2). The colour scale of the density is 0–0.05 states per bohr3.

  5. Optical and electronic properties of mirror and tilt boundaries.
    Figure 5: Optical and electronic properties of mirror and tilt boundaries.

    ad, Optical measurements of an island containing a mirror twin boundary. eh, Corresponding measurements for an island containing a tilt boundary. a,e are optical images; bd and fh are colour plots of photoluminescence. In b,f, red is the relative quantum yield, with colour scale 0–1100 a.u. We see 50% quenching at the mirror twin boundary and a 100% enhancement at the tilt boundary. In c,g, green is the peak position, with colour scale 1.82–1.87 eV. There is an upshift of 8 meV at the mirror twin boundary, and a much stronger 26 meV upshift in the tilt boundary. In d,h, cyan is the peak width with colour scale of 53–65 meV. The peak broadens from 55 to 62 meV at the boundary in both samples. i,j, Linear and logarithmic electrical transport transfer curves of 4 FETs fabricated from the mirror twin MoS2 island shown in the inset of i. The curves correspond with pristine regions (magenta and black), and regions containing a grain boundary running perpendicular (cyan) and parallel (orange) to the flow of electrons. All data were measured at room temperature, using the Si growth substrate as a back-gate and a source–drain bias of 500 mV.

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Author information

  1. These authors contributed equally to this work

    • Arend M. van der Zande,
    • Pinshane Y. Huang &
    • Daniel A. Chenet

Affiliations

  1. Energy Frontier Research Center, Columbia University, New York, New York 10027, USA

    • Arend M. van der Zande,
    • Tony F. Heinz,
    • David R. Reichman &
    • James C. Hone
  2. Department of Mechanical Engineering, Columbia University, New York, New York 10027, USA

    • Arend M. van der Zande,
    • Daniel A. Chenet,
    • Gwan-Hyoung Lee &
    • James C. Hone
  3. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA

    • Pinshane Y. Huang &
    • David A. Muller
  4. Department of Chemistry, Columbia University, New York, New York 10027, USA

    • Timothy C. Berkelbach &
    • David R. Reichman
  5. Departments of Physics and Electrical Engineering, Columbia University, New York, New York 10027, USA

    • YuMeng You &
    • Tony F. Heinz
  6. Samsung-SKKU Graphene Center (SSGC), Suwon, 440-746, Korea

    • Gwan-Hyoung Lee
  7. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA

    • David A. Muller

Contributions

A.M.v.d.Z. supervised and coordinated all aspects of the project. MoS2 growth was carried out by D.A.C. and A.M.v.d.Z. Electrical characterization and analysis was carried out by A.M.v.d.Z., D.A.C. and G-H.L. under the supervision of J.C.H. Electron microscopy and data analysis were carried out by P.Y.H. with D.A.M.’s supervision. Optical spectroscopy and data analysis were carried out by A.M.v.d.Z. and Y.Y. under T.F.H.’s supervision. DFT simulations were carried out by T.C.B. under D.R.R.’s supervision. A.M.v.d.Z., P.Y.H., D.A.C., T.C.B., Y.Y., T.F.H., D.A.M. and J.C.H. wrote the paper.

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The authors declare no competing financial interests.

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