Surface Adsorption and Phase-Selective Electronic Modulation of Monolayer TaSe2

Abstract Surface adsorption doping is a powerful way to tune two-dimensional electronic structures of surfaces and monolayers in van der Waals materials, but its impact often goes beyond simple charge transfer. Here, we investigate using angle-resolved photoemission spectroscopy how different adsorbates, such as alkali metal (K), transition metal (Fe), and simple diatomic molecules (CO and O2), modify the electronic structures of monolayer 1T- and 1H-TaSe2, prototypical transition-metal-dichalcogenide monolayers featuring distinct electronic phases. While K adsorption induces a rigid band shift consistent with electron doping on both 1T and 1H monolayers, Fe adsorption produces spectral broadening without an apparent rigid shift in the 1T monolayer, indicating distinct local interactions. In contrast, CO and O2 adsorptions lead to unexpectedly strong rigid band shifts in the 1T monolayer but leave the 1H band essentially unchanged. The unusual band shift on the 1T monolayer is attributed to the formation of an interfacial dipole layer, whose details are revealed by density functional calculations. These results highlight the rich diversity of adsorbate interactions on transition-metal-dichalcogenide monolayers, which can be a versatile knob controlling the electronic properties of van der Waals materials.