Quasi-free-standing single-layer ${\mathrm{WS}}_2$ achieved by intercalation

Large-area and high-quality single-layer transition metal dichalcogenides can be synthesized by epitaxial growth on single-crystal substrates. An important advantage of this approach is that the interaction between the single layer and the substrate can be strong enough to enforce a single crystalline orientation of the layer. On the other hand, the same interaction can lead to hybridization effects, resulting in the deterioration of the single layer's native properties. This dilemma can potentially be solved by decoupling the single layer from the substrate surface after the growth via intercalation of atoms or molecules. Here we show that such a decoupling can indeed be achieved for single-layer ${\mathrm{WS}}_2$ epitaxially grown on Ag(111) by intercalation of Bi atoms. This process leads to a suppression of the single-layer ${\mathrm{WS}}_2$-Ag substrate interaction, yielding an electronic band structure reminiscent of free-standing single-layer ${\mathrm{WS}}_2$.

Figure 1

Angle-resolved photoemission spectra measured with 30 eV photons of (a) SL ${\mathrm{WS}}_2$ on Ag(111) and (b) SL ${\mathrm{WS}}_2$ on Ag(111) exposed to Bi. The red dashed lines overlaid on the experimental data show the calculated free-standing SL ${\mathrm{WS}}_2$ band dispersion from Ref. [13]. In each case, the dashed lines are aligned with the experimental global valence band maximum, situated at $\overline{K}$. The yellow solid lines correspond to fits to the experimental data. The dashed magenta lines denote the limits of the projected bulk band gap. The contrast in the upper part of the spectra, demarcated by the blue solid lines, has been increased to make fainter features visible, in particular the intensity observed near the $\overline{Q}$ point in (a). (c) Same as (b) but acquired with a photon energy of 28 eV.

Figure 2

Room-temperature LEED measurements, acquired with an electron kinetic energy of 64 eV, of SL ${\mathrm{WS}}_2$ on Ag(111) before (a) and after (b) exposure to Bi. The center insets serve to highlight the diminished intensity of the moiré satellite spots in (b). (c) Diffracted intensity along radial cuts indicated by the lines of corresponding color in (a) and (b) and fits to the intensity (black lines), with the fit components indicated in grey. (d) LEED pattern from panel (b) with an indication of the diffraction spots's origin: Ag(111) (green), ${\mathrm{WS}}_2$ (blue), and Bi (brown). (e) A structural model of Bi intercalated SL ${\mathrm{WS}}_2$ on Ag(111) from both top and side perspectives. The unit cells and lattice parameters (extracted from the LEED data) are indicated on the appropriate layer in the top view.

Figure 3

(a) STM image showing two different regions of the surface separated by the yellow dotted line. The upper region possesses a hexagonal moiré pattern highlighted by the dashed blue circles, which is consistent with the moiré expected for SL ${\mathrm{WS}}_2$ on Ag(111). In the lower region, the hexagonal moiré is absent. The additional features, i.e., dark depressions and thin irregular-shaped lines, are attributed to defects and domain boundaries and are typical topographic characteristics of the SL ${\mathrm{WS}}_2$ epitaxially grown on Ag(111). Image parameters: ${\mathrm{V}}_B=1214\mathrm{mV},{\mathrm{I}}_t=0.220\mathrm{nA},300\AA \times 140\AA$. (b) STM image and corresponding FFT showing a rectangular lattice structure consistent with previous observations of the $\left(\mathrm{p}\times \sqrt{3}\right)$ Bi adlayer on Ag(111) [42]. The extra spots highlighted by the green dashed circles in the FFT pattern arise from the one-dimensional stripes. Image parameters: ${\mathrm{V}}_B=175\mathrm{mV},{\mathrm{I}}_t=1.140\mathrm{nA},50\AA \times 50\AA$ (STM), and $10{\AA }^{-1}\times 10{\AA }^{-1}$ (FFT). (c) Atomic-resolution STM image and FFT of the upper region of panel (a). Image parameters: ${\mathrm{V}}_B=94\mathrm{mV},{\mathrm{I}}_t=1.570\mathrm{nA},30\AA \times 30\AA$ (STM), and $12{\AA }^{-1}\times 12{\AA }^{-1}$ (FFT). (d) Atomic-resolution STM image and FFT of the lower region of panel (a). Image parameters: ${\mathrm{V}}_B=1214\mathrm{mV},{\mathrm{I}}_t=1.110\mathrm{nA},30\AA \times 30\AA$ (STM), and $12{\AA }^{-1}\times 12{\AA }^{-1}$ (FFT).