Chemical Migration and Dipole Formation at van der Waals Interfaces between Magnetic Transition Metal Chalcogenides and Topological Insulators

Metal and magnetic overlayers alter the surface of the topological insulator (TI) bismuth selenide (Bi$_2$Se$_3$) through proximity effects but also by changing the composition and chemical structure of the Bi$_2$Se$_3$ sub-surface. The interface between Bi$_2$Se$_3$ and Mn metal or manganese selenide was explored using x-ray photoelectron spectroscopy (XPS) revealing chemical and electronic changes at the interface. Depositing Mn metal on Bi$_2$Se$_3$ without an external source of Se shows unexpected bonding within the Mn layer due to Mn-Se bonding as Se diffuses out of the Bi$_2$Se$_3$ layer into the growing Mn film. The Se out-diffusion is further evidenced by changes in Bi core levels within the Bi$_2$Se$_3$ layers indicating primarily Bi-Bi bonding over Bi-Se bonding. No out-diffusion of Se occurred when excess Se is supplied with Mn, indicating the importance of supplying enough chalcogen atoms with deposited metals. However, Bi$_2$Se$_3$ core level photoelectrons exhibited a rigid chemical shift toward higher binding energy after depositing a monolayer of MnSe$_{2-x}$, indicating a dipole within the overlayer. Stoichiometry calculations indicated that the monolayer forms MnSe preferentially over the transition metal dichalcogenide (TMD) phase MnSe$_2$, providing a consistent picture of the dipole formation in which a plane of Se anions sits above Mn cations. This study shows that chemical diffusion and dipole formation are important for Mn-Bi$_2$Se$_3$ and MnSe$_{2-x}$-Bi$_2$Se$_3$ and should be considered carefully for TMD/TI interfaces more generally.