heterostructures*

The use of work function-mediated charge transfer in two-dimensional (2D) heterostructures has recently garnered significant interest as a means of tailoring 2D electrostatic environments at the nanoscale. For example, α-RuCl₃ is a van der Waals material possessing a large work function (Φ = 6.1 eV) and behaves as a 2D electron acceptor when interfaced with graphene and other 2D materials. In our study, we seek to understand how the presence of a large work function mismatch influences the behavior of the wide band gap insulator hexagonal boronitride (hBN) in heterostructures with α-RuCl₃. Using scattering-type scanning near-field optical microscopy (s-SNOM), we image hBN phonon-polaritons (PhPs) as a means of interrogating charge transfer at the hBN/α-RuCl₃ interface. We observe significant frequency-dependent optical losses that are well in excess of those predicted from the intrinsic optical properties of either material, revealing significant emergent optical conductivity associated with interlayer charge transfer. First-principles density functional theory (DFT) calculations corroborate the existence of interlayer charge transfer and concomitant band structure renormalization in hBN/α-RuCl₃ heterostructures.