Enhanced Ferroelectric Polarizations and Complex Switching Pathways with Small Barriers in In₂Se₃ and MXene Bilayers

Ferroelectric (FE)-based devices have long been investigated for their non-volatility, high speed, and low power consumption, but the need to scale down electronics has led to the search for new materials. 2D ferroelectrics (2DFEs), which exhibit spontaneous electric polarizations in only a monolayer or bilayer, represent the ultimate scaling limit. However, the magnitude of the spontaneous polarizations in 2DFEs is often small, especially in the technologically useful out-of-plane direction, and the polarization switching mechanisms can be complex. In this work, we used density functional theory calculations to investigate 2DFE bilayers consisting of monolayers of the non-centrosymmetric 2DFE In₂Se₃ and the centrosymmetric non-FE MXene Ti₂CO₂. First, we find that heterostructures of Ti₂CO₂/Ti₂CO₂ can be either polar or non-polar depending on the twisting angle between the two monolayers, leading to the presence of a small out-of-plane polarization. We then show that coupling two 2DFEs to form In₂Se₃/In₂Se₃ enhances the magnitude of the out-of-plane polarization from that of the monolayers, which can be further increased by changing the twisting angle. Finally, we interestingly find that the polarization reaches a maximum in In₂Se₃/Ti₂CO₂ bilayer heterostructures, despite being built from a centrosymmetric building unit. We show that these changes in the polarization are linked to an increase in the magnitude and asymmetry in charge transfer across the interface. Finally, we investigate the switching of the polarizations in these heterostructures. We find a complex interplay between sliding to switch the between-layer polarization, and atomic displacements to switch the in-layer polarization, with both displaying a bimodal switching pathway with ultralow barriers (< 0.1 eV). We also find that these two mechanisms can be coupled, leading to complex dynamics. Finally, we show that replacing the Ti₂CO₂ with the magnetic MXene V₂CO₂ to form In₂Se₃/V₂CO₂ leads to a polar metal. This work demonstrates that judicious selection of monolayers to form novel 2DFE bilayer heterostructures, including those which are centrosymmetric, can result in materials with unique properties and enhanced polarizations greater than the sum of their parts.