Abstract
Lead-free ferroelectrics aiming at replacing market dominant lead-based ones have been extensively searched due to increased environmental awareness and regulations. Among these lead-free candidates, potassium niobate (KNO) ceramics reported in 2004 had electromechanical properties that are close to those of lead-based ceramics and the goal appeared to be achievable. Although various strategies were explored, the advent of techniques to both synthesize and characterize nanoscale ferroelectric materials led to a burst of research in this area, which has continued to date. Despite some success in demonstrating the newly developed nanoscale KNO, the outcome didn’t seem so exciting as these lead-free materials are not yet the choice for commercial products due to issues such as a) cost-effective processing of high quality KNO nanostructures and b) lack of understanding on the ferroelectric behavior at the nanoscale. Motivated by these issues, this thesis work strived to take a novel approach to provide solutions and contribute incremental knowledge in enabling real-world applications of nanoscale lead-free KNO. Firstly, a novel synthesis process to obtain high-quality perovskite-orthorhombic KNO nanofibers using sol-gel assisted far-field electrospinning process is presented. The existing methods used to produce KNO nanostructures were bogged down by issues such as aspect ratio, poor densification and stoichiometry. The aspect ratio and densification have a direct influence on the electromechanical properties of the KNO and consequently their potential applications. The use of excessive solvents triggered hygroscopic secondary phases creating an imbalance in chemical composition which affects the dielectric property and weaken the stability of the KNO nanostructures. In this work, electrospinning, a facile and cost-effective approach offers a pathway to produce KNO nanofibers with control over morphology (solid & porous) and stoichiometry (no secondary phases or impurities). A sequential analysis was developed to systematically determine the appropriate process parameters. Under optimized conditions, the KNO nanofibers were centimetres long with an average diameter of 100 nm leading to high aspect ratio. Low temperature (550 °C for 5 hour) annealing allowed the fibers to grow uniform grains that are densely stacked along the direction of the nanofiber axis. A near perfect chemical composition with no significant trace of impurities was achieved by avoiding water as a solvent in sol-gel preparation to circumvent the hygroscopic issues. Secondly, a systematic investigation on the ferroelectric property of the electrospun KNO nanofibers through probing and manipulation using a family of piezoresponse force microscopy (PFM) techniques was carried out. Given the complexity in probing ferroelectricity on the nanoscale, PFM observations and hysteresis loop measurements have often become de-facto proof of ferroelectric behavior on the nanoscale material, without considering the existence of non-ferroelectric signal originating from tip-sample interactions. In this work, the limitations of conventional single frequency PFM (SF-PFM) such as a) the dependence of piezoresponse amplitude on excitation frequency, b) unable to study the non-ferroelectric signal origins from tip-sample interactions and c) inaccurate quantification due to changes in tip-contact conditions affecting the quality factor are addressed by utilizing Band Excitation method (BE) and contact Kelvin probe force microscopy (cKPFM). Unlike SF-PFM, wherein the piezoresponse is measured at the resonance, BE simultaneously excites and detects within a band of frequencies close to resonance. The BE-PFM mapping of a single KNO nanofiber showed that results from conventional single frequency PFM are erroneous due to large variations of contact stiffness stemming from topographic crosstalk. The cKPFM technique was utilized to decouple the electrostatic contributions in the piezoresponse amplitude of KNO nanofibers. The systematic analysis of cKPFM data demonstrated that tip-surface mechanisms can cause ferroelectric-like characteristics through charge injection and electrostatic forces on the tip. In addition, the similarities in surface potential curves and hysteresis loops revealed that the PFM switching in KNO nanofibers is a pure artefact due to trapping/de-trapping of charges and arguing the ferroelectric functionality in nanofibers may be attributed to electret-like or relaxor-like behavior. By differentiating the non-ferroelectric signal contributions and decoupling the topographic noise from the piezoresponse, piezoelectric coefficient of KNO nanofibers was accurately quantified. The peak piezoelectric constant was around ~3.689 pmV-1. Furthermore, the influence of humidity and substrate in piezo amplitude of nanofibers was observed. Finally, based on the observations from the synthesis and characterization, two different applications utilizing KNO nanofibers were demonstrated. A fast and high sensitive humidity sensor based on KNO nanofibers displaying a logarithmic-linear dependence behavior of the conductance with the relative humidity (RH) was fabricated. The fibers were ultra-long that enhanced the surface area which led to dramatic change in conductance (4 orders of magnitude) while RH varied from 15% to 95%. The sensor exhibited ultrafast response and recovery times, mainly attributed to the greatly reduced interfacial area between the sensing active region and the underlying substrate. The influence of RH on the ferroelectric coercive field of KNO nanofibers was observed. This finding implies that ferroelectric devices developed using KNO nanostructures might require proper encapsulation or packaging to avoid changes in the non-linear dielectric property at higher humidity levels for desired performance. Besides, a flexible high output nanogenerator was fabricated using a nanocomposite comprising porous KNO nanofibers and polydimethylsiloxane. When a compressive force was applied to KNO based piezoelectric nanogenerator, a peak-to-peak output voltage of ~16 V and a maximum closed-circuit current of 230 nA was obtained, which are high enough to realize self-powered nanodevices. This report is the first to explore the possibility of using porous nanofibers and demonstrate that the porosity in piezoelectric nanofibers could enhance the output in energy harvesting applications. The introduction of porosity in the KNO nanofibers improved the generated output by ~20% compared to the solid, non-porous nanofiber based nanogenerators. Therefore, this study provides a comprehensive understanding from the cost-effective synthesis of high quality lead-free ferroelectric KNO nanofibers to demonstrations of their practical applications utilizing their inherent properties. The PFM characterization methodologies from this work can be used as a guideline in reliably studying newly developed lead-free ferroelectrics at the nanoscale. The novel approach and scientific insights could enable broad real-world applications in the fields of environment sensing, nano-electro-mechanical systems and energy harvesting devices and help the technology move forward.