Selective Fabrication of Mott-Insulating and Metallic Monolayer TaSe2

We report the selective fabrication of monolayer TaSe2 with trigonal-prismatic (1H) or octahedral (1T) crystal structure on bilayer graphene by the molecular-beam-epitaxy method. By angle-resolved photoemission spectroscopy, we found that monolayer 1H-TaSe2 shows a metallic electronic structure with a dispersive band across the Fermi level (E F) similar to the bulk crystal, while monolayer 1T-TaSe2 has a sizable energy gap at E F indicative of the insulating nature in contrast to the metallic nature of bulk 1T-TaSe2. We discuss the origin and implications of differences in the electronic structure among monolayer and bulk TaSe2 by comparing the experimental results with first-principles band-structure calculations as well as previous studies on bulk TaSe2.