Growth and structure of singly oriented single-layer tungsten disulfide on Au(111)

A singly oriented, single layer of tungsten disulfide (${\mathrm{WS}}_2$) was epitaxially grown on Au(111) and characterized at the nanoscale by combining photoelectron spectroscopy, photoelectron diffraction, and low-energy electron microscopy. Fast x-ray photoelectron spectroscopy revealed that the growth of a single crystalline orientation is triggered by choosing a low W evaporation rate and performing the process with a high temperature of the substrate. Information about the single orientation of the layer was obtained by acquiring x-ray photoelectron diffraction patterns, revealing a 1H polytype for the ${\mathrm{WS}}_2$ layer and, moreover, determining the structural parameters and registry with the substrate. The distribution, size, and orientation of the ${\mathrm{WS}}_2$ layer were further ascertained by low-energy electron microscopy.

Figure 1

(a),(b) High-resolution core-level spectra for W $4f$ ($h\nu =140$ eV) and S $2p$ ($h\nu =260$ eV), respectively, measured at room temperature (RT). The result of a peak-fitting analysis is displayed as well. (c) LEED pattern (primary electron energy $E_p=56$ eV). The dashed yellow line indicates the alignment of the moiré superstructure with the crystallographic orientation of ${\mathrm{WS}}_2$. The solid gray line is a circular profile taken across the ${\mathrm{WS}}_2$ spots and shown in panel (d).

Figure 2

Top panel: Side view (left) of the trigonal prismatic (1H) structural phase of ${\mathrm{WS}}_2$, and top view (right) of the main (Or1) and mirror (Or2) orientations used for XPD simulations. (a) XPD pattern acquired on the sample for the W $4f_{7/2}$ component ($h\nu =170$ eV) (in color) together with a simulation assuming a single orientation of the layer (in gray scale). (b) W $4f_{7/2}$ XPD pattern obtained with $h\nu =360$ eV. (c) Contour plot reporting the $R$-factor as a function of the layer thickness and lattice constant. (d) and (e) $R$-factor along the dashed lines in panel (c). (f) $R$-factor vs percentage of mirror orientation admixture, obtained for layer thickness and lattice constant values corresponding to the minima of the $R$-factor shown in panels (d) and (e). The inset of (d), (e). and (f) shows a magnification of the graph around the minimum of the $R$-factor, with the vertical dashed arrow indicating the confidence interval $\mathrm{\Delta }{R}_{\text{min}}$ and the horizontal arrow the uncertainty in the determination of the lattice constant, layer thickness, and percentage of mirror orientation, respectively, as explained in Sec. 2. The simulated patterns shown are those obtained employing the parameters deduced from the $R$-factor analysis.

Figure 3

(a) Au $4f_{7/2}$ core level acquired on clean Au(111) at $h\nu =400$ eV. Surface (${\mathrm{Au}}_{\text{S}}$) and bulk (${\mathrm{Au}}_{\text{B}}$) components are indicated. (b) XPD pattern acquired at 400 eV photon energy ($\mathrm{KE}\sim 316$ eV), showing the XPD pattern (color) associated with the bulk component ${\mathrm{Au}}_B$ in the Au $4f_{7/2}$ core level together with the multiple scattering simulation (gray) for the clean Au(111) sample. (b) Orientation of the Au sample derived from the comparison of the XPD experiment with the simulation. (d) and (e) Ball model of the adsorption geometry of ${\mathrm{WS}}_2$ on Au(111) at the corner of the moiré unit cell with orientation Or1 (${\mathrm{W}}_{\text{fcc}}{\mathrm{S}}_{\text{top}}$) and Or2 (${\mathrm{W}}_{\text{hcp}}{\mathrm{S}}_{\text{top}}$), respectively. (f) Moiré unit cell for the configuration Or1 (preferred) with the regions of high local symmetry for ${\mathrm{WS}}_2$ on Au(111).

Figure 4

(a) BF LEEM reveals islands that are identified as ${\mathrm{WS}}_2$ islands by their specific LEED pattern. (b) $\mu$LEED pattern obtained using an illumination aperture of 1 $\mu \mathrm{m}$ [illuminated area marked by a yellow circle in (a)]. (c,d) DF LEEM images of area (a) using the (10) beam of the ${\mathrm{WS}}_2$ LEED pattern [cf. (b)], evidencing contrast changes of different ${\mathrm{WS}}_2$ domains at certain energies. The filled triangle points to a specific reference position in the image. (e) BF $I\left(V\right)$ curves from the regions marked with red and black circles. (f) Corresponding DF $I\left(V\right)$ curves for the two domains observed in panels (c) and (d), respectively. The $I\left(V\right)$ curves are vertically shifted for the sake of clarity.