Electronic band structure of Two-Dimensional ${\mathrm{WS}}_2$/Graphene van der Waals Heterostructures

Combining single-layer two-dimensional semiconducting transition-metal dichalcogenides (TMDs) with a graphene layer in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these heterostructures. Here, we report the electronic and structural properties of transferred single-layer $\mathrm{W}{\mathrm{S}}_2$ on epitaxial graphene using micro-Raman spectroscopy, angle-resolved photoemission spectroscopy measurements, and density functional theory (DFT) calculations. The results show good electronic properties as well as a well-defined band arising from the strong splitting of the single-layer $\mathrm{W}{\mathrm{S}}_2$ valence band at the $K$ points, with a maximum splitting of 0.44 eV. By comparing our DFT results with local and hybrid functionals, we find the top valence band of the experimental heterostructure is close to the calculations for suspended single-layer $\mathrm{W}{\mathrm{S}}_2$. Our results provide an important reference for future studies of electronic properties of $\mathrm{W}{\mathrm{S}}_2$ and its applications in valleytronic devices.

Figure 1

Structural and electronic properties of a $\mathrm{W}{\mathrm{S}}_2$/graphene heterostructure: (a) crystalline structure of $\mathrm{W}{\mathrm{S}}_2$; (b) typical optical image of the $\mathrm{W}{\mathrm{S}}_2$ transferred onto the graphene layer. The contrast has been adjusted in order to improve the visibility of the flake. The $\mathrm{W}{\mathrm{S}}_2$ profile is traced by white dashed lines. (c) Micro-Raman spectra taken at different positions of the $\mathrm{W}{\mathrm{S}}_2$/graphene heterostructure; in the top spectrum the used multipeak Lorentzian fit is shown as blue lines. (d) Raman map images of the peak intensity and position of the 2LA (M) (top) and $A_{1g}\left(\mathrm{\Gamma }\right)$ (bottom) modes of $\mathrm{W}{\mathrm{S}}_2$ on epitaxial graphene.

Figure 2

High-resolution XPS and ARPES of $\mathrm{W}{\mathrm{S}}_2$/graphene heterostructures: (a) $\mathrm{C}-1s$, (b) $\mathrm{W}\text{−}4f$, and (c) $\mathrm{S}\text{−}2p$ core levels at $h\nu =340\mathrm{eV}$. (d) ARPES map of $\mathrm{W}{\mathrm{S}}_2$/graphene heterostructure around the Γ point in the $\mathrm{\Gamma }K$ high-symmetry direction of the $\mathrm{W}{\mathrm{S}}_2$ Brillouin zone. (e) Band structure of the graphene layer in the heterostructure at the $\mathrm{\Gamma }K$ high-symmetry direction. (f) Corresponding momentum distribution curves (MDCs) of (e).

Figure 3

(a) ARPES map of single-layer $\mathrm{W}{\mathrm{S}}_2$ along the $\mathrm{\Gamma }K$ high-symmetry directions. (b,c) Comparison between ARPES map and DFT calculations (PBE and HSE06, respectively).