The Physical Properties And Preparation Of Ferroelectric Polymer Films Nanostructure Devices

Ferroelectric films have attracted much attention due to their outstanding properties, such as ferroelectricity, dielectricity, pyroelectricity, piezoelectricity etc. PVDF based ferroelectric copolymer P(VDF-TrFE) became one kind of the most studied polymers because of their good chemical stability, easy fabrication and flexibility. Recently, high quality copolymer P(VDF-TrFE) ultrathin films have been prepared by the Langmuir-Blodgett(LB) technique. In this dissertation, ferroelectric polymer nanostructure devices were fabricated by LB method. The origin of self-polarization in the ferroelectric polymer films was discussed. The leakage current in the ferroelectric polymer films was investigated. The coupling mechanism in the ferroelectric multilayer, electroresistance in the ferroelectric tunnel junction, and magnetoelecctric coupling in the ferroelectric polymer with a magnetic electrode were studied. The main results are listed as following:1, Self-polarization of the ultrathin ferroelectric polymer: Ultrathin copolymer films of vinylidene fluoride and trifluoroethylene were deposited on Al-coated polyimide substrates, by the Langmuir-Blodgett method. A top Al electrode was evaporated onto the polymer film, to form an Al/polymer/Al structured infrared detector. The pyroelectric voltage response of the detector under various polarizing processes was characterized. The detector with only one transferred polymer layer exhibited a preferential polarization direction. This was considered to result from the self-polarization of the ultrathin polymer film. It was due to the preferred alignment of the dipoles on the Al substrates. The leakage current mechanism of ferroelectric copolymer was investigated in the temperature range from 100 K to 350 K. The electron as the dominant injected carrier was observed in the ferroelectric copolymer films. The transport mechanisms in copolymer strongly depend on the temperature and applied voltage. From 100 K to 200 K, Schottky emission dominates the conduction. With temperature increasing, the Frenkel-Poole emission instead of the Schottky emission controls the carrier transport. When the temperature gets to 260 K, the leakage current becomes independent of temperature, and the space charge limited current conduction was observed.2, Electric properties enhancement in ferroelectric multilayers: Multilayers consisting of alternative ferroelectric poly(vinylidene fluoride-trifluoroethylene)(P(VDF-TrFE)) copolymer and relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene)(P(VDF-TrFE-CFE)) terpolymer with different periodicities in thickness were prepared. Both X-ray diffraction and Raman spectroscopic measurements indicate that the structure of the multilayer with thin alternating layer is similar to that of the ferroelectric copolymer. Compare with that of the copolymer, it is found that the piezoelectric coefficient of the multilayer could be improved. The dielectric constant of the multilayer with a small periodic thickness is two times higher than that of the P(VDF-TrFE) copolymer over a temperature range between 300 K and 350 K. The multilayer also shows a good ferroelectricity in the same temperature range. The enhanced electrical properties of the multilayers are due to the long-range ferroelectric coupling and electrostatic coupling.3, High temperature coefficient of resistance in ferroelectric tunnel junction: An infrared detector was proposed that is based on a ferroelectric tunnel junction(FTJ) working under bolometer-like principles. Electron tunneling, either direct or indirect, through the ferroelectric barrier depends on the temperature of the devices. During tunneling, infrared radiation alters the polarization of the ferroelectric film via pyroelectricity, resulting in a change in the barrier height of the tunnel junction. A high temperature coefficient of resistance of up to-3.86% was observed at room temperature. These results show that the FTJ structure has potential to be adapted for use in uncooled infrared detectors.4, Magnetoelectric coupling mechanism in P(VDF-TrFE)/Co heterostructure: The magnetoelectric effect at the interface of ferroelectric copolymer/ferromagnetic Co multiferroic film was studied. The ferroelectric copolymer poly(vinylidene fluoride- trifluoroethylene) was fabricated by Langmuir-Blodgett deposition technique. Large magnetization changes of the Co films emerge in response to ferroelectric switching of poly(vinylidene fluoride- trifluoroethylene) controlled by applied electric field. The magnetization of the Co films was detected by the Magneto-optical Kerr effect system. Although ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene) have the lower piezoelectric coefficient about 30 pC/N than inorganic ferroelectrics, interface strain coupling is also the primary mechanism altering the induced magnetic anisotropy in the poly(vinylidene fluoride- trifluoroethylene)/Co heterostructure.