Evidence for a narrow band gap phase in 1T′ WS2 nanosheet

While 1T′ phase-pure MX2 (M = Mo, W; X = Se, Te) have recently been reported to be superconductors, Weyl semimetals, or quantum spin Hall insulators, the electronic properties of phase-pure 1T′-WS2 samples are still lacking thorough investigation. Here, we report the study of single-layer 1T′-WS2 nanosheets prepared from lithium exfoliation of WS2. We confirmed the composition and structure of single layer 1T′-WS2 flakes using X-ray photoelectron spectroscopy, Raman spectroscopy, and aberration corrected transmission electron microscopy (STEM). The distorted octahedral structure related to the 1T′ phase with a 2a × 2a superstructure is evidenced using STEM. Photoemission and electronic measurements uncover the presence of a narrow bandgap (>120 meV) in the 1T′-WS2 nanosheets, which is completely different from semiconducting bulk or single-layer 1H-WS2. The material is found to be ambipolar with a p-type nature. At low temperatures, a slow photoresponse is also observed.