Monolayer 1 T -NbSe 2 as a Mott insulator

The emergence of exotic quantum phenomena is often triggered by a subtle change in the crystal phase. Transition metal dichalcogenides (TMDs) exhibit a wide variety of novel properties, depending on their crystal phases, which can be trigonal prismatic (2 H ) or octahedral (1 T ). Bulk NbSe 2 crystallizes into the 2 H phase, and the charge density wave and the superconductivity emerge simultaneously and interact with each other, thereby creating various anomalous properties. However, these properties and their interplay in another polymorph, 1 T -NbSe 2 , have remained unclear because of the difficulty of synthesizing it. Here we report the first experimental realization of a monolayer 1 T -NbSe 2 crystal grown epitaxially on bilayer graphene. In contrast with 2 H -NbSe 2 , monolayer 1 T -NbSe 2 was found to be a Mott insulator, with an energy gap of 0.4 eV. We also found that the insulating 1 T and metallic 2 H phases can be selectively fabricated by simply controlling the substrate temperature during epitaxy. The present results open a path to crystal-phase engineering based on TMDs.