Properties and mechanism of strain and piezoelectricity of hybrid molecular ferroelectrics

• Main Author: Hu, Yuzhong
• Other Authors: Fan Hongjin
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Science::Physics
• Online Access: https://hdl.handle.net/10356/145124
• Institution: Nanyang Technological University

Ferroelectrics are materials with spontaneous polarization that can be reversed by external electric field. Because of the polarized structure and electrically driven domain reversibility, ferroelectrics possess numerous electronic applications, including memory devices, sensors, and energy conversion device. Based on their structural and composition differences, ferroelectrics can be divided into different categories including oxide perovskite, inorganic salt, organic ferroelectrics and organic-inorganic hybrid systems. Among all these material families, oxide perovskites are so far the most intensively studied ferroelectrics in both fundamental research and industrial fields. For instance, lead zirconate titanate (PZT) is used as translation actuator in various micropositioning applications due to its fast response speed and high stiffness. However, the future perspectives of electronic devices require materials to be lightweight, thin, and bio-friendly, while most of these traditional oxide ferroelectrics have low mechanical flexibility, high temperature synthesis requirement and contain toxic elements. Organic-inorganic hybrid ferroelectrics (OIHFs) possess the unique organic and inorganic mixed composition. The organic part grants this hybrid system with advantages of organic ferroelectrics such as easy processing, lightweight, and mechanical flexibility, while the inorganic framework brining in high mechanical strength and semiconductor properties including photoluminescence, photovoltaics and other photo-electronic effects. With this hybrid composition and high structure flexibility, OIHFs receive great attention as electromechanical materials and multifunctional materials in recent years. Since 2014, more than forty new OIHFs have been successfully discovered. Their structures vary from 0D to 3D and applications cover optical, ferroelectric and electrostrictive fields. Particularly, with rational molecule design, the piezoelectric responses of some hybrid structures have even outperformed traditional oxide ceramics such as bismuth titanate and PZT. In Chapter 3, we report the discovery of a new OIHF and investigate the influence of the halide part on its ferroelectricity. In previous reports, OIHFs were found to lose their ferroelectricity even with a very low level of halide substitution, making the investigation to the role of halide part in OIHFs ferroelectric mechanism quite challenging. In this chapter, the structure and ferroelectric properties of a new solid solution, C6H5N(CH3)3CdBr3xCl3(1-x) ((PTMA)CdBr3xCl3(1-x)) are systematically investigated. Different with other OIHFs, the ferroelectricity of this hybrid structure is well preserved throughout bromide to chloride substitution. This enables us to disclose the role of halide part in OIHF ferroelectricity. Through systematic characterization, we find Br substitution has obvious softening effect, which leads to faster switching speed, lower switchable temperature and lower coercivity (Ec). The crystallography results indicate that this softening effect should originate from strength weakening on hydrogen and Cd-halide (Cd-X) bonds. In Chapter 4, the design strategy and discovery of the colossal shear strain in this OIHF will be presented. Materials with controllable strain is highly desirable in shape memory devices, sensors and actuators. So far, great efforts have been made to realize and optimize strain output in various material systems, including shape memory alloy, electrostrictive polymer and ferroelectrics. However, few of them can achieve high output in both stress and strain. In this chapter, we discuss the properties and origin of the colossal shear strain (21.5%) in (PTMA) CdBr3xCl3(1-x), which is two orders of magnitude higher than general ferroelectrics and much larger than all shape memory alloys. The giant strain stems from non-180 ͦ ferroelastic switch induced by special structure confinement effect, while its decent stress output should come from its strong inorganic backbone. Chapter 5 discusses the piezoelectric properties and relevant mechanism in this solid solution. The Br substitution in this OIHF effectively softens its lattice and leads to this giant piezoelectric response. In Br rich sample, d35 can reach 4800 pm/V, which is much larger than other hybrid structures (up to 2560 pm/V) and is comparable to top-performance Pb based relaxor (5000 pm/V). In summary, this thesis presents the discovery of a new OIHF (PTMA)CdBr3xCl3(1-x) which gets both colossal shear strain and piezoelectric responses. The mechanism of these giant electromechanical properties and the role of the halide part in the ferroelectricity of this solid solution are systematic investigated. These outstanding strain output and piezoelectric performances make this OIHF a good candidate as flexible devices, microelectromechanical systems and wearable electronics. In addition, our design strategy for high strain output crystal structure and insights to the soften mechanism in hybrid ferroelectric system should shed light on the discovery of new OIHFs with excellent electromechanical and ferroelectric properties.