Two-Dimensional Ferroelectric Ga2O3 Bilayers with Unusual Strain-Engineered Interlayer Interactions

Two-dimensional (2D) van der Waals (vdW) materials and their bilayers have stimulated enormous interests in fundamental research and technological applications. Recently, a group of 2D vdW III2–VI3 materials with out-of-plane ferroelectricity have attracted substantial attention. In this work, the structural, electronic, and optical properties of a 2D ferroelectric Ga2O3 bilayer system are systematically studied using an ab initio computational method. Intrinsic dipoles of the two free-standing monolayers lead to three distinct dipole models (one ferroelectric and two antiferroelectric models). The stable stacking configurations of ferroelectric and antiferroelectric dipole models can be transferred with a polarization reversal transition of the monolayers without an additional operation. Interlayer perturbation effects combined with biaxial-strain engineering lead to high tunablility of the electronic and optical properties of the bilayer systems. Surprisingly, the results reveal a phase transition from a vdW to ionic interlayer interaction induced by in-plane biaxial tensile strain. Detailed analyses suggest a transition mechanism based on the ionic bonding nature of the Ga2O3 system, involving interlayer rearrangement of anions to compensate the symmetry breaking of the heavily distorted ionic folding configurations. These insights can open new prospects for future experimental synthesis, characterization, and application of 2D Ga2O3 atomically thin layered systems.