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 angle-resolved 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 near-flatband 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.