Ferroelectric phase transition in individual single-crystalline BaTiO ₃ nanowires

We report scanned probe characterizations of the ferroelectric phase transition in individual barium titanate (BaTiO ₃) nanowires. Variable-temperature electrostatic force microscopy is used to manipulate, image, and evaluate the diameter-dependent stability of ferroelectric polarizations. These measurements show that the ferroelectric phase transition temperature (T _c) is depressed as the nanowire diameter (d _nw) decreases, following a 1/d _nw scaling. The diameter at which T _c falls below room temperature is determined to be ∼3 nm, and extrapolation of the data indicates that nanowires with d _nw as small as 0.8 nm can support ferroelectricity at lower temperatures. We also present density functional theory (DFT) calculations of bare and molecule-covered BaTiO ₃ surfaces. These calculations indicate that ferroelectricity in nanowires is stabilized by molecular adsorbates such as OH and carboxylates. These adsorbates are found to passivate polarization charge more effectively than metallic electrodes, explaining the observed stability of ferroelectricity in small-diameter BaTiO ₃ nanowires.