Stable phases of freestanding monolayer TiO2 : Emergence of out-of-plane ferroelectricity

Despite being successfully synthesized [Zhang et al., Nat. Mater. 20, 1073 (2021)], the monolayer structure of stable hexagonal TiO₂ 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-TiO₂ and √3×√3 1T-TiO₂, both of which are energetically more favorable 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 are wide-band-gap semiconductors with band gaps larger than their bulk counterparts. In this paper, we not only deepen the understanding of structural instability in wide-gap semiconductors but also add a member to the rare family of two-dimensional out-of-plane ferroelectrics.