Functionalization of two-dimensional PbTiO3 film by surface modification: A first-principles study

Highlights: • Strong spontaneous polarization can be induced by surface modification. • Polarization arises from an electric field caused by layer-doping. • VBM and CBM electronic states of ${\mathrm{PTO}}_{\mathrm{Al}{O}_2}^{\mathrm{Nb}{O}_2}$ distribute on different surfaces. • Band structures of ${\mathrm{PTO}}_{\mathrm{Al}{O}_2}^{\mathrm{Nb}{O}_2}$ can be effectively tuned by changing the polarization. Spontaneous polarization in ultrathin ferroelectric films is usually severely suppressed by a depolarization field, which hinders practical applications. Here, using first-principle calculations, we explored a way of introducing spontaneous polarization in two-dimensional (2D) PbTiO₃ films, analyzed the underlying mechanisms, and then discussed its possible applications. Our results demonstrate that strong spontaneous polarization, metallicity, and magnetism can be achieved by making appropriate surface modifications. Polarization arises from an electric field that is caused by layer-doping, and the bandgap can be continuously tuned by varying the polarization. In particular, in double layer-doped systems, electronic states at the valence band maximum (VBM) and conduction band minimum (CBM) tend to be distributed on the p-doped and n-doped surfaces, respectively, and this may contribute to the separation of electron-hole pairs. Our findings should be beneficial to achieve 2D perovskite-based materials that have various functionalities and improve the understanding of their properties.