Research ArticleCONDENSED MATTER PHYSICS

Optical identification of sulfur vacancies: Bound excitons at the edges of monolayer tungsten disulfide

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Science Advances  28 Apr 2017:
Vol. 3, no. 4, e1602813
DOI: 10.1126/sciadv.1602813
  • Fig. 1 Bound excitons at the edges.

    (A) Atomic structure of monolayer 1H-WS2. Scale bar, 1 nm. (B) Optical image of triangular WS2 islands. (C) PL spectra obtained from the marked regions in (B). PL intensity image at 77 K of (D) X0 peak centered at ~1970 meV and (E) Embedded Image peak centered at ~1690 meV. Scale bars, 10 μm. (F) X0 and Embedded Image intensity profiles acquired along the dashed lines in (D) and (E), respectively. arb. units, arbitrary units.

  • Fig. 2 Identification of sulfur vacancies.

    (A) Optical image of transferred WS2 triangle onto QUANTIFOIL (left), which is within 1 μm from the edge of the WS2 triangle, and high-magnification ADF images from center part (middle) and edge part (right). Monosulfur vacancies (VS) and one tungsten + three sulfur vacancies (VWS3) were marked by yellow circles and orange triangles, respectively. (B) Comparison of experimental and simulation ADF image of VS and VWS3 vacancies. The line profile was acquired along the line in ADF images. (C) Calculated monosulfur vacancy (VS) density from the center and edge regions. The error bar means SD of monosulfur vacancy density.

  • Fig. 3 Evolution of PL spectra with laser power density.

    (A) Interior area and (B) at the edge. The inset shows the low-energy region marked by a dashed rectangle around the bound-exciton emission at 1750 meV. (C) Logarithmic plot of the Embedded Image (blue) and Embedded Image (pink) intensity of bound excitons, as a function of the neutral-exciton emission intensity Embedded Image. Lines are power-law fits, and the solid line α = 1 is included for comparison.

  • Fig. 4 Dependence of PL spectra on temperature.

    (A) PL intensity map of the neutral and bound energy emission changing with temperature between 77 and 113 K; the white dashed line is a guide to the position of Embedded Image. (B) PL spectra extracted from (A) for five different temperatures. (C) Normalized Embedded Image peak as a function of temperature and the corresponding fit to the data. The thermal activation energy of bound exciton is 36 ± 6 meV.

  • Fig. 5 First-principles calculations of monosulfur vacancy.

    (A) Band structure of 5 × 5 WS2 supercell containing a monosulfur vacancy, where the red component of the colored bands represents the projection of the total wave function onto the atomic orbitals of the three W atoms nearest to a sulfur vacancy. (B) The same band structure superimposed with colored circles representing the transition energies and magnitudes of the optical transition matrix elements. For each transition, a pair of identical circles is added to the initial and final state, with colors and sizes indicating the transition energy and the magnitude of the matrix element, respectively. (C) Unfilled curves are the imaginary part of the RPA dielectric function for the 5 × 5 supercell with (red) and without (black) sulfur vacancies. Red (gray) filled curves are the joint density of states (JDOS) from the three highest valence bands to the two defect (six lowest conduction) bands. (D) Defect formation energy for a sulfur vacancy with q = 0 and −1, as a function of the Fermi energy [referenced to the conduction band (CB) minimum]. Solid lines are energies of q = 0 (q = −1) obtained at their respective equilibrium configurations; dashed lines are obtained at the equilibrium configuration of the alternative q = −1 (q = 0) state. The thermodynamic charge transition level can be found at the crossover between the two solid lines. (E) Schematic for the defect energies of neutral and charged defects, as a function of the collective coordinates of a system. Optical emission energies can take values between EPL1 and EPL1.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/4/e1602813/DC1

    Characterization of the WS2 triangles

    PL spectroscopy at low temperature

    Density functional calculations of gas adsorption and WS3 vacancy

    fig. S1. Details on the characterization of the WS2 monolayer.

    fig. S2. The decomposition of the PL spectra.

    fig. S3. PL images at low temperature.

    fig. S4. Temperature-dependent PL for the interior and edge regions.

    fig. S5. Band structure of a 5 × 5 WS2 supercell with an N2 or O2 molecule adsorbed on the surface at a sulfur vacancy site.

    fig. S6. Band structure of a WS3 vacancy within a 5 × 5 supercell.

  • Supplementary Materials

    This PDF file includes:

    • Characterization of the WS2 triangles
    • PL spectroscopy at low temperature
    • Density functional calculations of gas adsorption and WS3 vacancy
    • fig. S1. Details on the characterization of the WS2 monolayer.
    • fig. S2. The decomposition of the PL spectra.
    • fig. S3. PL images at low temperature.
    • fig. S4. Temperature-dependent PL for the interior and edge regions.
    • fig. S5. Band structure of a 5 × 5 WS2 supercell with an N2 or O2 molecule adsorbed on the surface at a sulfur vacancy site.
    • fig. S6. Band structure of a WS3 vacancy within a 5 × 5 supercell.

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