Nanoscale Engineering and Characterization of Synthetic and Biological Ferroelectrics

Abstract

The works presented in this dissertation focus on the characterization, manipulation, and engineering of synthetic and biological ferroelectrics using piezoresponse force microscopy (PFM) and nanoimprint lithography (NIL). Various PFM techniques are introduced first, and are then applied to probe and manipulate inorganic perovskite ferroelectric films and crystals, organic polyvinylidene fluoride trifluoroethylene [P(VDF-TrFE)] films and nanostructures, as well as biological aortic walls and elastins. Using these techniques, sophisticated domain structures are imaged, domain switching characteristics are revealed, and biological ferroelectricity is discovered. The principle of scanning probe microscopy (SPM) is introduced briefly first, followed by detailed discussions on PFM. Damped harmonic oscillator model was also used to enable quantitative analysis of PFM signal. The switching of polarization by conductive SPM tip is then demonstrated, PFM hysteresis and butterfly loops characteristics are also obtained. These PFM techniques are applied to probe inorganic perovskite ferroelectric films and crystals. These studies revealed characteristic domain structures in ferroelectric films and crystals. It also confirmed excellent piezoelectric and ferroelectric properties in PZT nanostructures and stretchable PZT ribbons that are comparable to the flat PZT films on rigid silicon substrates, making it possible to use these PZT structures in a wide range of applications. We also use PFM to study P(VDF-TrFE) films. The effects of processing parameters on film properties are investigated first, and then the thermal stability of P(VDF-TrFE) polar structures are probed at a series of temperatures across Curie point. It is observed that the piezoresponse remains relatively stable up to 110 oC, and then drops rapidly to zero. Furthermore, a rapid nanoimprinting technique is also developed to pattern P(VDF-TrFE) copolymers in just 3 minutes without any post-imprinting annealing. Using PFM, we discovered that the porcine aortic walls are not only piezoelectric, but also ferroelectric, confirmed by tip induced hysteresis and butterfly loops characteristic of polarization reversal. In addition, we also discovered that elastin is switchable by an electric field. Furthermore, it is observed that the ferroelectricity in elastin is largely suppressed by glucose, and such loss of ferroelectricity may have important physiological and pathological implications to elastin's functionalities.