  Two-dimensional (2D) van der Waals (vdW) materials and their bilayers have
stimulated enormous interests in fundamental researches and technological
applications. Recently, a group of 2D vdW III2-VI3 materials with out-of-plane
ferroelectricity have attracted substantial attentions. In this work, the
structural, electronic and optical properties of 2D ferroelectric Ga2O3 bilayer
system are systematically studied using ab-initio computational method.
Intrinsic dipoles of the two freestanding 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 polarization reversal transition of the monolayers
without 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 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
atomic-thin layered systems.
