Substrate Suppression of Charge-Density-Wave Phase in Monolayer 1H-TaS₂ on Au

Recent STM/LEED measurements of monolayer 1H-TaS₂ grown on Au(111) found no charge-density wave (CDW) phase, even well below the bulk CDW transition temperature. Additionally, ARPES data suggested that the monolayer becomes n-doped by the substrate. Density-functional calculations (DFT) have found that the freestanding monolayer has a 3x3 CDW instability similar to that of the bulk, but the instability is progressively removed with electron doping. We present DFT calculations of monolayer 1H-TaS₂ on Au(111) which indicate that the CDW instability is suppressed primarily by S-Au interactions beyond simple doping. Calculations find lateral and out-of-plane distortions of the TaS₂ monolayer with a 7x7 periodicity dictated by the commensurate interface with Au rather than by intrinsic instabilities of the monolayer. Simulated STM images of this 7x7 distortion are consistent with experimental STM Moiré patterns. In contrast, we find the intrinsic 3x3 CDW instability to be robust in monolayer 1H-TaS₂ on a graphene substrate.