Ferroelectricity in Nonstoichiometric SrTiO 3 at the 2D Limit

. 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.