Observation of an abnormally large bandgap in monolayer 1T'-WS₂ grown on SrTiO₃(001)

The 1T' structural phase of monolayer transition metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te) has attracted broad interest because of an exotic quantum spin Hall insulator state. Among them, the investigation on the electronic structures of the 1T'-WS2 monolayer is still lacking due to the difficulty in obtaining the 1T'-WS2 as a metastable phase. Here, we report the growth of 1T' phase WS2 monolayer on the SrTiO3 (001) substrate using molecular beam epitaxy. Surprisingly, a large bandgap of 0.65 eV is revealed by angleresolved photoemission spectroscopy and scanning tunneling spectroscopy, which is abnormally larger than the theoretical expectance. Moreover, an additional near-flatband emerges in the grown monolayer 1T'-WS2. The abnormally large bandgap and the emerging nearflatband are suggested to be the effect of interfacial interactions. Our findings provide important information about electronic structures and the interfacial effect of the epitaxial 1T'-WS2 monolayer on the SrTiO3(001) substrate and would stimulate future theoretical and experimental research on the 1T'-MX2 family.