Research ArticleMATERIALS SCIENCE

Dislocation-driven growth of two-dimensional lateral quantum-well superlattices

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Science Advances  23 Mar 2018:
Vol. 4, no. 3, eaap9096
DOI: 10.1126/sciadv.aap9096
  • Fig. 1 Structure and strain analysis of a WS2 quantum well embedded in a WSe2 matrix.

    (A) Atomic-resolution STEM-ADF image of the embedded WS2 quantum well with a width of 1.2 nm. The coherent interfaces between the WS2 quantum well and the WSe2 matrix are highlighted by the yellow dashed line. The hexagon highlights the orientation of the lattice. (B and C) Chemical mapping from the same region as in (A) showing the spatial distribution of WS2 and WSe2, respectively. (D and E) High-resolution STEM-ADF image of a WS2 quantum well and the corresponding atomic structural model. The dashed lines highlight the coherent lateral interface along the armchair direction. (F to H) STEM-ADF of the entire 65-nm-long WS2 quantum well and the corresponding strain distribution (see Materials and Methods) around the quantum well. (I and J) STEM-ADF image showing the atomic arrangement of the dislocation core at the tip of the WS2 quantum well in (F) and the corresponding atomic model.

  • Fig. 2 Formation of periodic dislocation arrays and dislocation-driven growth of WS2 quantum wells at the WSe2/WS2 lateral interface.

    (A) Schematic showing (I) the formation of periodic dislocation array, (II) dislocation-driven growth of WS2 quantum wells, and (III) formation of 2D quantum-well superlattice in the WSe2/WS2 lateral heterostructure. (B) STEM-ADF image of a WSe2/WS2 lateral interface without the formation of WS2 quantum wells. The epitaxial interface is highlighted by the yellow dashed line. (C) Corresponding strain distribution, overlaid onto the ADF image, showing the formation of periodic dislocation array at the heterointerface. (D) STEM-ADF image of a WSe2/WS2 lateral interface with the formation of WS2 quantum wells. The WS2 quantum wells appear as darker stripes with the same width. (E) Corresponding strain map, overlaid onto the ADF image, showing the presence of a dislocation core at the tip of each WS2 quantum well. Insets in (C) and (E) are magnified views of the strain maps at the highlighted dislocation cores.

  • Fig. 3 Growth mechanism of the WS2 quantum well in WSe2.

    Atomic models of (A) 5|7 dislocation at the WS2/WSe2 interface due to lattice mismatch, (B) dislocation climbs one unit cell into WSe2 by inserting a W atom and an S2 pair, (C) substitution of Se by S in pentagon of the 5|7 dislocation, and (D) subsequent substitution of Se atoms next to the 5|7 dislocation by S resulting in a four–unit cell–wide WS2 nanoseed. (E) Energy barrier for SSe substitution under different levels of compressive strain. (F) Strain mapping based on bond length analysis for the atomic model in (B). See Materials and Methods for details. The green dashed line indicates the WS2/WSe2 interface. (G) Structural model showing a short WS2 strip four unit cells wide and six unit cells long after repeating the insertion-substitution process six times.

  • Fig. 4 Toward 2D quantum-well superlattice with atomically sharp lateral interfaces.

    (A) Atomic structural model and band alignment for a WSe2/WS2 superlattice calculated with the HSE06 functional. Valence band maximum (VBM) and conduction band minimum (CBM) are plotted by black and red lines, respectively. Yellow, S; purple, Se; gray, W. (B) Low-magnification STEM-ADF image showing the formation of parallel MoS2 quantum wells of a few hundred nanometers long in a MoSe2 monolayer toward the formation of quantum-well superlattice.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/3/eaap9096/DC1

    fig. S1. Reference EELS spectra from pure WSe2 and WS2 monolayers.

    fig. S2. STEM-ADF of the entire 65-nm-long WS2 quantum well and the corresponding strain distribution around the quantum well.

    fig. S3. Optical images and spectroscopy measurements of the WSe2/WS2 lateral heterostructure.

    fig. S4. Atomic model of WSe2/WS2 heterostructure.

    fig. S5. Additional structural characterization data from the lateral WSe2/WS2 heterointerface.

    fig. S6. Additional low-magnification STEM-ADF images showing the formation of arrays of WS2 quantum wells at the WSe2/WS2 lateral interface, driven by dislocations.

    fig. S7. Comparison between dislocation climb and extension of a WS2 edge during the sample growth.

    fig. S8. Atomic models for the SSe substitution barrier calculations.

    fig. S9. Band structure of lateral WSe2/WS2 superlattice.

    fig. S10. Optical images and spectroscopy measurements of the MoSe2/MoS2 lateral heterostructure.

    fig. S11. Dislocation climb and formation of nanosize kinks during the growth of quantum well.

    movie S1. Scheme showing the dislocation climb and the corresponding quantum well growth process.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Reference EELS spectra from pure WSe2 and WS2 monolayers.
    • fig. S2. STEM-ADF of the entire 65-nm-long WS2 quantum well and the corresponding strain distribution around the quantum well.
    • fig. S3. Optical images and spectroscopy measurements of the WSe2/WS2 lateral heterostructure.
    • fig. S4. Atomic model of WSe2/WS2 heterostructure.
    • fig. S5. Additional structural characterization data from the lateral WSe2/WS2 heterointerface.
    • fig. S6. Additional low-magnification STEM-ADF images showing the formation of arrays of WS2 quantum wells at the WSe2/WS2 lateral interface, driven by dislocations.
    • fig. S7. Comparison between dislocation climb and extension of a WS2 edge during the sample growth.
    • fig. S8. Atomic models for the SSe substitution barrier calculations.
    • fig. S9. Band structure of lateral WSe2/WS2 superlattice.
    • fig. S10. Optical images and spectroscopy measurements of the MoSe22/MoS2 lateral heterostructure.
    • fig. S11. Dislocation climb and formation of nanosize kinks during the growth of quantum well.

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    Other Supplementary Material for this manuscript includes the following:

    • movie S1 (.mp4 format). Scheme showing the dislocation climb and the corresponding quantum well growth process.

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