  Motivated by the experimental synthesis of two-dimensional (2D) perovskite
materials, we study the stability of 2D SrTiO$_3$ from first principles. We
find that the TiO$_6$ octahedral rotations emerge in 2D SrTiO$_3$ with a
rotation angle twice that in the 3D bulk. The rotation angle decreases
significantly with the film thickness, reflecting the strong interlayer
coupling that is absent in the conventional 2D materials. Using the molecular
dynamics simulations, the cubic-like phase is found to appear above 1000 K that
is much higher than the transition temperature of 3D SrTiO$_3$.
