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 TiO_2is unknown, and it is not even clear whether it can exist in a freestandingform. Through first-principles calculations, we have identified two previouslyuncharted stable structures, namely, distorted 1×√(3) 1T-TiO_2and √(3)×√(3) 1T-TiO_2, both of which are energeticallymore favourable than commonly adopted 1H and 1T phases. Here structuraldistortions are characterized by the out-of-plane shifts of Ti atoms due to thepseudo-Jahn-Teller interactions, which break one and all two inversionsymmetries of 1T configuration. As a consequence, the 1×√(3) 1Tremains centrosymmetric while the √(3)×√(3) 1T exhibitsout-of-plane ferroelectricity. Electronic structure calculations show that bothtwo are wide-bandgap semiconductors with bandgaps larger than their bulkcounterparts. Our study not only deepens the understanding of structuralinstability in wide-gap semiconductors but also adds a new member to the rarefamily of two-dimensional out-of-plane ferroelectrics.