Semimetal or Semiconductor: The Nature of High Intrinsic Electrical Conductivity in TiS2.

As an intensively studied electrode material for secondary batteries, TiS2 is known to exhibit high electrical conductivity without extrinsic doping. However, the origin of this high conductivity, either being a semimetal or a heavily self-doped semiconductor, has been debated for several decades. Here, combining quasi-particle GW calculations, density functional theory (DFT) study on intrinsic defects, and scanning tunneling microscopy/spectroscopy (STM/STS) measurements, we conclude that stoichiometric TiS2 is a semiconductor with an indirect band gap of about 0.5 eV. The high conductivity of TiS2 is therefore caused by heavy self-doping. Our DFT results suggest that the dominant donor defect that is responsible for the self-doping under thermal equilibrium is Ti interstitial, which is corroborated by our STM/STS measurements.