Effect of electron doping on lattice instabilities in single-layer 1H-TaS2

Recent angle-resolved photoemission spectroscopy measurements of single-layer 1H-TaS2 grown on Au(111) suggest strong electron doping from the substrate. In addition, scanning tunneling microscopy/spectroscopy measurements on this system show suppression of the charge-density-wave (CDW) instability that occurs in bulk 2H-TaS2. We present results from ab initio density functional theory calculations of free-standing single-layer 1H-TaS2 to explore the effects of doping on the CDW. In the harmonic approximation, we find that a lattice instability along the ΓM line occurs in the undoped monolayer, consistent with the bulk 3×3 CDW ordering vector. Electron doping removes the CDW instability, in agreement with the experimental findings. The doping and momentum dependences of both the electron-phonon coupling and of the bare phonon energy (unscreened by metallic electrons) determine the stability of lattice vibrations. Electron doping also causes an expansion of the lattice, so strain is a secondary but also relevant effect.