Publication
Ferroelectric perovskites exhibit superior dielectric, piezoelectric and ferroelectric properties. Architecting ferroelectrics in the small scale holds interest in both application and fundamental research. Compared to their 2D counterparts – thin films, ferroelectric nanowires –1D structuresndash; are far less studied. A number of important features of ferroelectric nanowires, such as their piezoelectric activity, domain structure, switching behavior and phase transitions, have not been yet fairly studied, nor understood. In this thesis, we treat central issues in the synthesis, structure, and functionality of monocrystalline nanowires of two of the most important ferroelectric perovskites – Pb(Zr, Ti)O3 (PZT) and KNbO3 (KN). This is complemented by theoretical modeling aiming to understand finite size effects in ferroelectric nanowires. Monocrystalline PZT nanowires have been synthesized by a polymer-assisted hydrothermal approach. The ferroelectricity of the PZT nanowires was attested by local switching under DC field applied from the conductive tip of the piezoresponse force microscope (PFM). The ferroelectric-paraelectric phase transition in an individual PZT nanowire and the formation and disappearance of 90° domains upon thermal cycling through its phase transition temperature Tc (Curie temperature) have been observed. PZT nanowires prepared by this direct hydrothermal method showed Ti-rich composition despite of the equal molarity of Zr and Ti in the well-mixed starting materials. This compositional problem was found to be related to the growth mechanism during the hydrothermal treatment, in which an acicular PX-PZT phase was the first to grow, consequently transforming into perovksite PZT nanowires. The existence of an upper limit of solubility of Zr in this precursor PX phase, determined the Ti-rich composition of the converted monocrystalline perovskite PZT nanowires. The atomic structure of the acicular PX phase has been resolved by combining synchrotron X-ray diffraction, electron diffraction and first-principles calculation, showing a unique open-channel structure with 1D cavities about 5.5 Å diameter throughout the acicular wires. The 1D growth habit of the PX phase along its [0 0 1] axis could be related to the large portion of vacant space leading to a high energy of the (001) plane. The doping of Zr in the PX-phase lattice was confirmed to be limited to 17 atom% at most. Taking the advantage of the uniform acicular morphology of the PX phase, we have developed a two-step route to prepare perovskite PZxT1-x (0≤ x ≤0.17) nanowires in which the pure perovskite nano-wires phase has been obtained by annealing PX-PZT in air. An O2 absorption at about 455 °C has been identified as a necessary step occurring during transformation from the PX to perovskite phase in air. The perovskite PZT nanowires had usually a monocrystalline porous structure with 90° domains. The tetragonality of the single PbTiO3 (PT) nanowires with diameters ranging