Interfacial Charge Rearrangement-Driven Sliding Ferroelectricity with Ultralow Switching Barriers and Strain-Reversible Polarization in HfXY (X, Y = S, Se) Janus and Homogeneous Bilayers

Abstract

Two-dimensional sliding ferroelectrics have emerged as a research frontier for post-Moore information devices, owing to their low energy barriers, reversible polarization switching, and potential compatibility with flexible electronics. Based on first-principles calculations, we systematically investigate the sliding ferroelectric properties and strain-tunable behavior of HfXY (X, Y = S, Se) homobilayer and Janus bilayer systems. Through structural optimization, elastic constant analysis, and ab initio molecular dynamics simulations, we confirm the equivalent thermodynamic and mechanical stability of the AB and BA stacking configurations. Using Berry-phase polarization theory, we find that these bilayer systems exhibit degenerate bistability during the AB ↔ BA stacking transition, with out-of-plane polarization magnitudes ranging from 0.19 to 0.82 pC/m, and the polarization reversal predominantly originates from charge redistribution driven by interfacial chalcogen atoms. This charge redistribution mechanism endows the systems with extremely low polarization switching barriers (approximately 12–15 meV per unit cell). Further biaxial strain engineering reveals that compressive strain enhances interlayer coupling and interfacial charge transfer, thereby increasing polarization magnitude, whereas tensile strain weakens the interfacial interaction, leading to reduced polarization and, in some cases, even reversal of the polarization direction. This work reveals the sliding ferroelectric properties of Hf-based bilayer systems, provides a theoretical basis for their microscopic origin, and demonstrates the potential of strain engineering in the polarization manipulation of two-dimensional van der Waals materials.