Substrate-induced suppression of charge density wave phase in monolayer $1H\text{−}{\mathrm{TaS}}_2$ on Au(111)

Recent experiments have found that monolayer $1H\text{−}{\mathrm{TaS}}_2$ grown on Au(111) lacks the charge density wave (CDW) instability exhibited by bulk $2H\text{−}{\mathrm{TaS}}_2$. Additionally, angle-resolved photoemission spectroscopy measurements suggest that the monolayer becomes strongly electron doped by the substrate. While density functional theory (DFT) calculations have shown that electron doping can suppress the CDW instability in monolayer $1H\text{−}{\mathrm{TaS}}_2$, it has been suggested that the actual charge transfer from the substrate may be much smaller than the apparent doping deduced from photoemission data. We present DFT calculations of monolayer $1H\text{−}{\mathrm{TaS}}_2$ on Au(111) to explore substrate effects beyond doping. We find that the CDW instability is suppressed primarily by strong S-Au interactions rather than by doping. The S-Au interaction results in a structural distortion of the ${\mathrm{TaS}}_2$ monolayer characterized by both lateral and out-of-plane atomic displacements and a $7\times 7$ periodicity dictated by the commensurate interface with Au. Simulated STM images of this $7\times 7$ distorted structure are consistent with experimental STM images. In contrast, we find a robust $3\times 3$ CDW phase in monolayer $1H\text{−}{\mathrm{TaS}}_2$ on a graphene substrate with which there is minimal interaction.

Figure 1

Top view of a $7\times 7$ supercell, showing ${\mathrm{TaS}}_2$ monolayer and surface Au layer. (a) In region A, Ta atoms are approximately above Au surface sites; in region B, S atoms are approximately above Au surface sites; and, in region C, both S and Ta atoms are approximately above surface hollow sites. (b) Arrows indicate lateral displacements of interface S atoms (not to scale). Ta atoms are omitted for clarity.

Figure 2

Simulated STM images. (a) Height map calculated from a local density-of-states isosurface for fully relaxed structure. (b) FFT of image in (a). (c) Height map calculated from a local density-of-states isosurface for structure without lateral displacements. (d) FFT of image in (c). The constant term represented by the point at the origin in the FFT images is set to the value of the $1\times 1$ signal for clarity.

Figure 3

Top view of a $3\times 3$ supercell, showing ${\mathrm{TaS}}_2$ monolayer and surface Au layer. Arrows indicate lateral displacements of interface S atoms (not to scale). The optimized structure can also be described by a $\sqrt{3}\times \sqrt{3}{\mathrm{TaS}}_2$ on a $2\times 2$ Au(111) supercell indicated with dashed lines. Ta atoms are omitted for clarity.

Figure 4

Projected density of states calculated for freestanding undistorted SL $1H\text{−}{\mathrm{TaS}}_2$ (top) and a relaxed $7\times 7$ supercell of SL $1H\text{−}{\mathrm{TaS}}_2$ on Au(111) (bottom).

Figure 5

Band structure of a 7 × 7 supercell of SL $1H\text{−}{\mathrm{TaS}}_2$ on Au(111) unfolded into the $1\times 1{\mathrm{TaS}}_2$ Brillouin zone. Bands of freestanding $1H\text{−}{\mathrm{TaS}}_2$ monolayer are plotted with solid lines.

Figure 6

Orbital projected band structure of a $\sqrt{3}\times \sqrt{3}$ supercell of SL $1H{\text{-TaS}}_2$ on Au(111). Bands of freestanding $1H\text{−}{\mathrm{TaS}}_2$ monolayer are plotted with solid lines.

Figure 7

Top view of CDW distortion in $3\times 31H\text{−}{\mathrm{TaS}}_2$ monolayer on a graphene substrate. Arrows indicate lateral displacements of Ta atoms (not to scale).

Figure 8

Orbital projected band structure of a 3 × 3 supercell of SL $1H{\text{-TaS}}_2$ on graphene. Bands of freestanding $1H\text{−}{\mathrm{TaS}}_2$ monolayer (with the CDW distortion expressed) are plotted with solid lines.