Is Charge-Transfer Doping Possible at the Interfaces of Monolayer VSe₂ with MoO₃ and K?

Being a metallic transition-metal dichalcogenide, monolayer vanadium diselenide (VSe₂) exhibits many novel properties, such as charge density waves and magnetism. Its interfaces with other materials can potentially be used in device applications as well as for manipulating its intrinsic properties. Here, we present a scanning tunneling microscopy and synchrotron-based X-ray photoemission spectroscopy study of the surface charge-transfer doping using efficient electron-withdrawing and electron-donating materials, that is, molybdenum trioxide (MoO₃) and potassium (K), on the molecular beam epitaxy-grown monolayer VSe₂ on highly oriented pyrolytic graphite (HOPG). We demonstrate that monolayer VSe₂ is immune to MoO₃- and K-doping effects. However, at the monolayer edges where the local chemical reactivity is higher because of Se deficiency, MoO₃ is seen to react with VSe₂ to form molybdenum dioxide (MoO₂) and vanadium dioxide (VO₂). Compared to the obvious charge-transfer doping effects of MoO₃ and K on HOPG, the electronic structure of monolayer VSe₂ is barely perturbed. This is attributed to the large density of states at the Fermi level of monolayer VSe₂ carrying the metallic character. This work provides new insights into the chemical and electronic properties of monolayer VSe₂, important for future VSe₂-based electronic device design.