Investigation of spatially localized defects in synthetic ${\mathrm{WS}}_2$ monolayers

While the spatially nonhomogeneous light emission from synthetic ${\mathrm{WS}}_2$ monolayers is frequently reported in the literature, the nature of this phenomenon still requires thoughtful investigation. Here, we combine several characterization techniques (optical imaging, scanning probe and electron microscopy) along with density functional theory to investigate the presence of substitutional doping localized at narrow regions along the S zigzag edge of ${\mathrm{WS}}_2$ monolayers. We verified that photoluminescence quenching along narrow regions is not related to grain boundaries but to substitutional impurities of lighter metals at the W sites, which modify the radiative and nonradiative decay channels. We also found potential candidates for occupying the W site through ADF-STEM analysis and discussed their impact on photoluminescence quenching by performing density functional theory calculations. Our findings shed light on how atomic defects introduced during ${\mathrm{WS}}_2$ monolayer's synthesis impact the crystalline quality and, therefore, the development of high-performance optoelectronic devices based on semiconducting 2D materials.

Figure 1

(a)–(c) Optical images of different synthesized ${\mathrm{WS}}_2$ monolayers. (d)–(f) Respective FI images. The intensity profile collected from the white dashed line in (f) is seen in the yellow inset graph. Scale bar: 20 $\mu \mathrm{m}$. (g) Optical image of a star-shaped ${\mathrm{WS}}_2$ monolayer with the overlaid hyperspectral PL intensity image. Scale bar: 10 $\mu \mathrm{m}$. (h) Top: PL spectrum taken at regions A (blue) and B (green) shown in (g). Bottom: Normalized PL spectrum at positions A (blue) and B (green) in (a) shown in logarithmic scale.

Figure 2

(a)–(d) Polarized-resolved SH imaging on different ${\mathrm{WS}}_2$ monolayers. Labels I and II, black and red arrows, respectively, indicate regions exhibiting the dark and bright regions shown in the inset FI images. (e) SH intensity as function of input laser polarization angle for the three different domains shown in (d). (f) Dark-field SH imaging of the ${\mathrm{WS}}_2$ flake shown in (d). Scale bar: 30 $\mu \mathrm{m}$.

Figure 3

(a) FI image of a triangular-shaped ${\mathrm{WS}}_2$ monolayer grown on ${\mathrm{SiO}}_2$/Si. Scale bar: $15\mu \mathrm{m}$. (b) AFM image of the flake displayed in (a). The black arrow indicates the topography profile from the dark region. (c) SKPM mapping of the surface potential of this same flake. (d) EFM image of the same flake acquired at tip bias $V_{\text{tip}}=-3\text{V}$. Scale bar (a)–(d): 5 $\mu \mathrm{m}$. (e) PL image of a ${\mathrm{WS}}_2$ polycrystal on ${\mathrm{SiO}}_2$/Si. The highlighted rectangular region indicates the region where AFM and LFM were recorded. Scale bar: 12 $\mu \mathrm{m}$. (f) AFM and (g) LFM image of the rectangular highlighted in (e). Scale bar in (f) and (g) corresponds to 10 $\mu \mathrm{m}$. Dashed red lines in ( f) and (g) indicate where line profiles (shown by full red lines) were measured.

Figure 4

(a) FI image of a triangular ${\mathrm{WS}}_2$ monolayer transferred on top of a TEM grid. Scale bar: $20\mu \mathrm{m}$. (b)–(d) Scanning transmission electron microscopy (STEM) images taken at the regions shown by holes 1, 2, and 3 shown in (a). Scale bar: $2\mathrm{nm}$. Insets showing magnified FI image and the pins indicate the location investigated by STEM. (e) Histogram of total ADF intensity of atom. The total ADF intensity was collected from a circular area with a diameter of 0.2 nm around an atom. The obtained total ADF intensities were normalized using W and ${\mathrm{S}}_2$ signals as 1.00 and 0.75, respectively. Profiles from holes 2 and 3 are shifted vertically for visibility. The vertical lines in bold correspond to the normalized total ADF intensity of substitutional lighter transition atoms incorporated into the W-site of the ${\mathrm{WS}}_2$ monolayer, obtained from STEM simulations.

Figure 5

(a) Eigenvalues of the highest occupied (red circles) and the lowest unoccupied (black rectangles) defect states versus the cubic root of the calculated formation energies of selected substitutional impurities in ${\mathrm{WS}}_2$. (b) Imaginary part of the dielectric function as a function of energy for the Sc and Cr impurities. Spin-down (red line) and spin-up (black line) contributions are shown separately, and the response of pristine ${\mathrm{WS}}_2$ (blue line) is also shown for comparison. (c), (e) Calculated electronic structure of the substitutional Cr and Sc impurities, respectively. Left: Calculated band structure, relative to the Fermi level, for the spin-down (red line) and spin-up (black line) states of the large unit cell containing the impurity. Right: Total density of states (TDOS), and projected density of states (PDOS) at selected atoms. (d), (f) Probability density of the lowest unoccupied states induced by the Cr (c) and Sc (e) impurities.