Ferroelectricity in Nonstoichiometric SrTiO₃ at the 2D Limit

Strontium titanate (STO) is a prototypical quantum paraelectric, lacking long-range ferroelectric order at any temperature, which has made the engineering of robust polarization a persistent challenge. Here, we demonstrate room-temperature ferroelectricity in nonstoichiometric STO films down to two unit cells in thickness. Direct nanoscale imaging via piezoresponse force microscopy reveals strong, switchable ferroelectric polarization. Atomic-scale imaging of oxygen columns using integrated differential phase contrast confirms a large, long-range ordered polarization of ∼30 μC·cm^-2. Chemical analysis shows that these films are strontium-deficient (Sr/Ti = 0.93), with electron energy-loss spectroscopy indicating that the reduced Ti valence originates from oxygen vacancies. We conclude that the interplay of Sr deficiency and oxygen vacancies drives the emergent ferroelectric phase transition facilitated by the strain that can enhance the polarization. This work establishes nonstoichiometric ultrathin STO as a platform for strong low-dimensional ferroelectricity, opening pathways for nanoscale electronic devices and engineered quantum materials.