Thermodynamic and Electronic Properties of Two-Dimensional SrTiO3

Perovskite oxides are star materials due to their particularly rich properties such as superconductivity, ferroelectricity, magnetic property, and photocatalytic activity. A recent study [Ji, D. Freestanding crystalline oxide perovskites down to the monolayer limit. Nature 2019, 570, 87 90] showed that freestanding two-dimensional (2D) perovskite oxides down to the monolayer limit can prepared. The intrinsic nanoscale of 2D perovskite oxides may impart them more intriguing properties and more possibility for their application in nanoscale electronic and spintronic devices. Taking SrTiO3 as an example, we investigate the impact of dimension reduction from three-dimension (3D) to 2D on the thermodynamic and electronic properties of perovskite oxides. We find that the thermodynamic stability of 2D structures is termination-dependent. Moreover, 2D SrTiO3, regardless of terminations (SrO- or TiO2-, or both), exhibit abnormal band-gap trends, with a gap value lower than the bulk limit, suggesting an unusual quantum size effect in 2D SrTiO3. The origin of abnormal trends can be attributed to the band splitting of conduction bands and surface states of valence bands. Our results provide a clear picture about the evolution of the electronic properties of materials with the change of dimension in perovskite oxides.