Ferroelectric Surface Potentials Enable Band-Alignment Engineering for Tunable Quantum Anomalous Hall States

In van der Waals heterostructures hosting the quantum anomalous Hall (QAH) effect, an appropriate band alignment is often needed to prevent extrinsic electronic bands from obscuring the topological gap. However, band alignment in two-dimensional heterostructures is typically regarded as a passive property determined by the material choice rather than an actively tunable degree of freedom. Here, we show that ferroelectric substrates provide a nonvolatile route to engineer band alignment through the surface electrostatic potential generated by ferroelectric polarization. The resulting surface potential shifts the energy levels of adjacent layers while largely preserving their intrinsic band dispersion, thereby enabling the controllable topological phase transitions. Using first-principles calculations, we demonstrate this mechanism in a van der Waals heterostructure composed of a fluorinated MoSe₂ monolayer on a ferroelectric In₂S₃ substrate. Polarization reversal drives a transition of band alignment from type-III to type-I, inducing a phase transition from metallic states to QAH insulating states with a finite topological gap.