Date of this Version
Keith Foreman, The Organic Ferroelectric Vinylidene Fluoride Oligomer: Vacuum Deposition, Properties, and Interfaces (Dissertation), University of Nebraska-Lincoln (2017)
Organic ferroelectric materials combine the versatility and customizability afforded by organic synthesis with the functionality of ferroelectric materials. The model ferroelectric polymer, poly(vinylidene fluoride) (PVDF), is used in a wide variety of applications and is still the subject of fundamental research nearly 80 years after it was first polymerized. Unfortunately, PVDF suffers from thermal decomposition during thin film evaporation in vacuum. Since PVDF thin films cannot be deposited in the ferroelectric phase in vacuum conditions, its use in new, 21st century technologies may be limited since the interface between the organic and adjacent metallic thin films is less than pristine.
Thin films of the VDF oligomer, which is comprised of short, well-defined chains of the –CH2CF2– monomer, can be deposited in the ferroelectric phase in vacuum conditions. The work in this dissertation establishes the VDF oligomer as a viable organic ferroelectric material suitable for use in modern organic-based electronics.
This dissertation describes the design, construction, and operation of a novel thermal evaporator capable meeting the demanding set of vacuum deposition parameters of the VDF oligomer. Also described is the optimization of those deposition parameters to maximize the quality and yield of the VDF oligomer thin films. A wide battery of experimental techniques are used to definitively establish that vacuum deposition preserves the interface between the VDF oligomer and thin films of Co, an important high Curie temperature ferromagnetic metal. Several important physical properties and characteristics of VDF oligomer thin films are reported with particular emphasis on the ferroelectric properties of the films, including: the temperature and thickness dependence of the coercive field, spontaneous polarization, and the stability of the remanent polarization. Compelling evidence for the existence of the ferroelectric-to-paraelectric phase transition is also presented. Lastly, the remaining challenges and possible future experiments using the VDF oligomer are discussed.