Two-dimensional (2D) van der Waals (vdW) heterostructures have attracted substantial research interest in recent years, due to their tremendous advantages, such as atomically sharp interfaces, digitally controlled layered components, and unconstraint lattice mismatch, and immense potentials, in electronic and optoelectronic applications. The functionality and performance of such vdW heterostructures critically depend on the band alignment between the constituent layers. In this work, based on systematic first-principles calculations, we demonstrate that by taking advantage of the out-of-plane ferroelectricity of a 2D ferroelectric material α-In 2 Se 3 , the band alignments of a large variety of α-In 2 Se 3 based vdW heterostructures can be controllably switched between different types of semiconductor or metal−semiconductor junctions via the polarization reversal of the ferroelectric α-In 2 Se 3 layer upon the application of an external electric field. This work provides a generic guideline for the application of 2D ferroelectric α-In 2 Se 3 in tuning the electronic and optical properties of vdW heterostructures for device applications.

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Domain boundaries in ferroelectric materials exhibit rich and diverse physical properties distinct from their parent materials and have been proposed for broad applications in nanoelectronics and quantum information technology. Due to their complexity and diversity, the internal atomic and electronic structure of domain boundaries that governs the electronic properties remains far from being elucidated. By using scanning tunneling microscopy and spectroscopy (STM/S) combined with density functional theory (DFT) calculations, we directly visualize the atomic structure of polar domain boundaries in two-dimensional (2D) ferroelectric β′-In2Se3 down to the monolayer limit. We observe a double-barrier energy potential with a width of about 3 nm across the 60° tail-to-tail domain boundaries in monolayer β′-In2Se3. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials.