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

Despite being successfully synthesized [Zhang, Nat. Mater. 20, 1073 (2021)1476-112210.1038/s41563-020-00899-9], 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.