Structure of monolayer $2H\text{−}{\mathrm{TaS}}_2$ on Au(111)

We determined the structure of epitaxial $2H\text{−}\mathrm{Ta}{\mathrm{S}}_2$ on Au(111) using the method of x-ray standing waves (XSW), supported by density functional theory (DFT) calculations and scanning tunneling microscopy (STM). The lattice mismatch between substrate and overlayer gives rise to a moiré superstructure, which modulates the structural and electronic properties. For a specific registry (S atoms directly above Au substrate atoms), local covalentlike bonds form, whereas globally weak van der Waals bonding prevails. Still, the $\mathrm{Ta}{\mathrm{S}}_2$ layer remains rather flat. Significant charge transfer from Au(111) into the conduction band of the two-dimensional material is found.

Figure 1

High-resolution STM image of the moiré lattice of $\mathrm{Ta}{\mathrm{S}}_2$/Au(111). The unit cell is indicated by a white rhombus. Tunneling conditions are as follows: $U_{\mathrm{sample}}=-0.2$ V, $I_{\mathrm{tunnel}}=3.3$ nA. (Inset) Corresponding LEED pattern (inverted contrast, electron energy 80.5 eV) obtained at DLS. Characteristic spots of Au and $\mathrm{Ta}{\mathrm{S}}_2$ are marked.

Figure 2

(a) X-ray photoemission spectrum of Ta $4f$ measured with $h\nu =150$ eV. (b) X-ray photoemission spectrum of S $2p$ measured with $h\nu =360$ eV. (Circles) Data points after background subtraction. (Colored areas) Fits of the indicated components (see Table 1). (Solid black line) Sum of fits.

Figure 3

XSW analysis for (a) XPS components of Ta $4f$ [Figs. 2 and 2] XPS components of S $2p$ [Fig. 2]. The XSW profiles (blue, green, red) show the variation in total photoelectron yield as a function of the photon energy scan around the Bragg energy of $E_{\mathrm{Bragg}}=2.635$ keV for Au(111). The colors denote the components as defined in Fig. 2; see also Table 1. The x-ray reflectivity is shown in black. The experimental data is shown as open circles and the best fit from dynamical diffraction theory [33] is shown as solid lines.

Figure 4

Relaxed structure of $\mathrm{Ta}{\mathrm{S}}_2$/Au(111) with $7\times 7\mathrm{Ta}{\mathrm{S}}_2$ unit cells on $8\times 8$ Au(111) unit cells in the ${\mathrm{Ta}}_{\mathrm{fcc}}{\mathrm{S}}_{\mathrm{top}}$-stacking. S atoms are colored green, Ta atoms blue, and Au atoms in different layers gold, red, and black. Dashed lines indicate the unit cell. Regions of high symmetry are labeled. (a) Top view. (b) Side view along a cut following the main diagonal. (c) 2D color gradient plot of the charge density difference upon adsorption along the same plane as shown in (b). (Blue) Minimum ($-16.7\times {10}^{-3}{\mathrm{e}}^{-}/{\AA }^3$) (electron depletion); (red) maximum ($6.6\times {10}^{-3}{\mathrm{e}}^{-}/{\AA }^3$) (electron accumulation).