Ab initio study of ferroelectricity in BaTiO3 nanowires

We present a density-functional theory study of finite-size effects in stoichiometric and nonstoichiometric BaTiO3 nanowires of varying cross-sectional sizes and sidewall terminations. The tendencies for axial, transverse, and toroidal ferroelectric polarization instabilities in these nanowires have been characterized and possible driving forces underlying these behaviors have been identified. The critical size for ferroelectricity via polarization along the nanowire axis is determined to be 12 Å, regardless of the stoichiometry or nanowire sidewall terminations. The sidewall terminations alter the manner in which axial polarization manifests; for instance, in nonstoichiometric BaO-terminated nanowires, a “core-shell-type” polarization results beyond the critical size, while in all other cases, roughly uniform polarization along the same direction was observed across the entire cross section. A tendency for transverse polarization i.e., normal to the nanowire axis occurs in nanowires with a cross-sectional size of 16 Å displaying TiO2-terminated sidewall facets. This tendency is accompanied by a toroidal or vortex polarization state, with the toroidal moment along the nanowire axis, to mitigate the depolarizing fields due to transverse polarization.