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.