Ferroelectric Nanowires: an Investigation in Synthesis, Characterization, Functionality, and Modeling of Finite Size Effects

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 from 20 nm to hundreds of nm has been measured, the c/a ratio of all the measured nanowires has not shown any suppression compared to the bulk value. A maximum c/a up to 1.14, which is larger than the bulk value of 1.065 was observed in nanowires with thickness around 100 nm. DSC measurement confirmed the first-order ferroelectric phase transition and showed a remarkable decrease of the transition latent heat compared to bulk PT/PZT. Annealing PX-PT/PZT on conductive substrate resulted in perovskite PT/PZT nanowires which are attached to the substrates, providing an almost ideal sample for PFM measurements. Prominent piezoelectric response of individual PbZ0.1T0.9 nanowires relative to a reference film with a d33 about 50 pm/V and squared hysteresis loop with sharp swiTching indicated pronounced piezoelectric and ferroelectric properties. Monocrystalline KN nanowires have been processed by a hydrothermal approach. The morphology of the products has been confirmed to be sensitive to the concentration of Nb sources, and could vary from nanowires/nanorods to cubes. Both monodomain and multidomain structures have been observed by PFM. The integration of KN nanowires on the Si substrate as a device prototype has been demonstrated using electron beam lithography. PFM measurements proved that piezoelectric deformation can be excited up to 25 pm by an electric field induced by the lithographically fabricated electrodes. In the theoretical modeling part, we first studied the shift of Tc due to the surface-effect-controlled size effects in ferroelectric nanowires with an axial equilibrium polarization. An effective-surface-energy framework based on the Landau-Ginzburg-Devonshire (LGD) theory has been proposed in the study. The result indicated that for a second order phase transition in the axially-polarized ferroelectric nanowires, Tc shifts down inversely proportional to square of the wire radius (R2) in the case of a strong suppression (close to 0) of polarization at the surface, while it decreases inversely proportional to the radius (R) in the case of a weak suppression of polarization at the surface. The surface tension plays an enhancing role in nanowires of most perovskites, giving an upshift on Tc inversely proportional to R. The competition between the suppressing effect of the short-range surface relaxation and the enhancing role of the surface tension could result in an enhancement of the ferroelectricity in nanowires with certain size. The size-induced shift of Tc due to the surface effect and the surface tension in the axially-polarized nanowires which undergo a first-order paraelectric-ferroelectric phase transition is simply proportional to 1/R in most cases. With the LGD theory, we have also investigated the situation when the axis of the nanowire is not parallel to the crystallographic direction of the polarization of the bulk material. In this case the depolarization field acts "against" the spontaneous polarization, leading, in general, to a polarization rotation. Modeling of the phase transition in cylindrical PT nanowires with their axis parallel to [111]C direction, with the depolarizing field due to the incomplete screening considered as the dominant source of the finite size effects, shows that the competition between the geometrical symmetry of the ferroelectric nanowires and the crystalline symmetry of the material results in an additional phase transition and the usual first order cubic to ferroelectric phase transition of PT becomes a second order phase transition in the small diameter regime.


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