First-principles prediction of 2D non-centrosymmetric Al ₂ O ₃ with tunable piezoelectric and electric-field-responsive properties

We predict a novel 2D non-centrosymmetric Al₂O₃ (NCS-Al₂O₃) structure using first-principles calculations combined with the MAGUS structure search package. This puckered hexagonal structure, featuring an "O-Al-O-Al-O" stacking sequence, exhibits exceptional stability with a formation energy of -2.88 eV per atom, surpassing that of a previously reported planar monolayer phase (-2.39 eV per atom) and closer to the bulk α-Al₂O₃ value (-3.16 eV per atom). It is an indirect bandgap semiconductor (3.98 eV) with intermediate mechanical stiffness and relatively strong anisotropy (A^U ∼ 7.99). Crucially, the broken inversion symmetry enables strong in-plane piezoelectricity (∼0.6 C m^-2). Most significantly, an out-of-plane electric field can reversibly switch the NCS-Al₂O₃ structure between centrosymmetric and non-centrosymmetric states by modulating a low energy barrier (∼0.11 eV). This unique electric-field-responsive behavior, coupled with its high stability and piezoelectricity, positions NCS-Al₂O₃ as a promising candidate for applications in 2D electronics, in-plane piezoelectric sensors, and electric-field-gated non-volatile memory devices.