Stable phases of freestanding monolayer TiO_2: The emergence of out-of-plane ferroelectricity

Despite being successfully synthesized [Zhang et al., Nat. Mater. 20, 1073 (2021)], the monolayer structure of stable hexagonal TiO_2 is unknown, and it is not even clear whether it can exist in a freestanding form. Through first-principles calculations, we have identified two previously uncharted stable structures, namely, distorted 1×√(3) 1T-TiO_2 and √(3)×√(3) 1T-TiO_2, both of which are energetically more favourable than commonly adopted 1H and 1T phases. Here structural distortions are characterized by the out-of-plane shifts of Ti atoms due to the pseudo-Jahn-Teller interactions, which break one and all two inversion symmetries of 1T configuration. As a consequence, the 1×√(3) 1T remains centrosymmetric while the √(3)×√(3) 1T exhibits out-of-plane ferroelectricity. Electronic structure calculations show that both two are wide-bandgap semiconductors with bandgaps larger than their bulk counterparts. Our study not only deepens the understanding of structural instability in wide-gap semiconductors but also adds a new member to the rare family of two-dimensional out-of-plane ferroelectrics.