Piezoelectric application and domain visualization of ferroelectric materials

Abstract

Ferroelectric materials have many essential properties, such as spontaneous polarization, switchable polarization sates, pyroelectric effects, and piezoelectric effects. These features enable ferroelectrics to be used in a wide range of applications, e.g. sensors, energy harvesters, actuators, transducers, and memory devices. In particular, ferroelectric polymers, e.g. polyvinylidene fluoride (PVDF) and its copolymers, have unique properties of high flexibility, chemical stability, and feasibility of simple fabrication processes compared with ferroelectric oxides. Their properties enable them to be widely applied to wearable applications, such as fabric-based wearable electronics. Ferroelectric domain structures, internal defects, and doping states are very important for a great performance in those applications. The ferroelectric domains can be visualized via scanning probe microscopy (SPM) techniques, such as piezoresponse force microscopy (PFM), electrostatic force microscopy (EFM), and Kelvin probe force microscopy (KPFM). In addition, the local domains and properties of ferroelectrics can be modified electrically or mechanically using atomic force microscopy (AFM) tips. In this research, we design and fabricate a fabric-based wearable electronic device and investigate local ferroelectric domain structures of ferroelectric thin films using SPM. First, we report a cost-effective, high throughput, and strongly integrated fabric-based wearable piezoelectric energy harvester (fabric-WPEH) with a heterostructure of a ferroelectric polymer, poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)] and two conductive fabrics via simple fabrication of tape casting and hot pressing. Our fabrication process would enable the direct application of the unit device to general garments using hot pressing as graphic patches can be attached to the garments by heat press. Simulation and experimental analysis demonstrate fully bendable, compact and concave interfaces and a high piezoelectric d$_{33}$ coefficient (-32.0 pC N$^{-1}$) of the P(VDF-TrFE) layer. The fabric-WPEH generates piezoelectric output signals from human motions (pressing, bending) and from quantitative force test machine pressing. Furthermore, a record high interfacial adhesion strength (22 N cm$^{-1}$) between the P(VDF-TrFE) layer and fabric layers has been measured by surface and interfacial cutting analysis system (SAICAS) for the first time in the field of fabric-based wearable piezoelectric electronics. Second, we demonstrate a new issue about significant discrepancies of lateral PFM (LPFM) signals of the trace scan and those of the retrace scan, which increases as the scan angle between the long axis of the cantilever and the fast scan direction in ferroelectric thin films. Based on the controlled experiments regarding PFM modes, lateral friction, cantilever tilting, and sample positioning, we correlate the LPFM discrepancy issue with the capacitive rocking and the lateral lag. In particular, we show the LPFM phase is significantly governed by the capacitive rocking. Third, we report two mechanical modification techniques of lamellar roughening via high eigenmode tapping and lamellar alignment via large force low pressure scanning using AFM tips of P(VDF-TrFE) thin films. Through PFM, we show that the mechanical modifications can induce the alignment of in-plane (IP) polarization. In addition, the aligned lamellar structure had anisotropic piezoelectric and tribological properties visualized by all angle-resolved PFM (AR-PFM), lateral force microscopy (LFM), and transverse shear microscopy (TSM). Analysis using AR-PFM, AR-LFM, and AR-TSM revealed that the aligned lamellar region was transformed into both the IP ferroelectric domain and the friction domain with the collinearity and the linear relation in intensity due to the aligned lamellar corrugations. Last, we demonstrate a new noncontact SPM technique, potential gradient microscopy (PGM), using surface potential gradients at domain boundaries. PGM is conducted in the second scan of KPFM while measuring the surface potential with slow potential feedback, which means the simultaneous surface potential measurement and domain boundary visualization. The entire ferroelectric domain structure of interest can be visualized by conducting PGM twice with a scan angle difference of 90°. Our research will provide advanced technologies and insights into piezoelectric applications, domain visualization, modification, analysis of ferroelectric materials. We believe that our study will contribute to the advancement of the basic science about SPM and ferroelectrics and the improvement of the properties of electronic devices using ferroelectrics.