High Performance Poly(Vinylidene Fluoride-trifluoroethylene) Ferroelectric Nano-film And Mechanism Of The Electroactive Interlayer

More and more attention has been paid to ferroelectric thin films such as poly(vinylidene flouridetrifluoroethylene) copolymers, because of their advantages compared with inorganic materials like lower switching voltage, lower annealing temperature, easier fabrication process, higher dielectric constant, and cheaper cost. However, the researches show that the polarization level is decreased and switching field is increased as the thickness is reduced to below100nm, which is due to boundary conditions and processes at electrode-copolymer interfaces. Thus, in order to search for the proper electrodes or interface layer to improve switching characteristics, we propose a universal and easy-to-implement model for ferroelectric capacitors with electroactive polymeric interface layer. Concentrating on charge switching in ferroelectric capacitors, this model describes both the basic capacitive and ferroelectric switching behavior. Based on this model, electroactive interlayer is introduced as a buffer layer between electrode and P(VDF-TrFE) film and switching properties are investigated. A functional interlayer is introduced as a buffer layer between the metal electrodes of a sandwiched capacitor composed of ferroelectric poly(vinylidene fluoridetrifluoroethylene) copolymer ultrathin film. The sandwiched structure shows prominent ferroelectric properties even as the thickness of P(VDF-TrFE) film is reduced to50nm and the fatigue property is also improved tremendously, even at high temperature60℃. The first chapter is an overview, and the second through seventh chapters exhibit the authors’s achievement:1) The experimental intrinsic coercive field of ferroelectric poly(vinylidene fluoride-trifluoethylene) copolymer films, with both bottom and top gold electrodes is measured at a wide temperature range. In the lower temperature region from-20to25℃, the temperature dependence of coercive field shows good agreement with the prediction by the Landau-Ginzburg (LG) mean-field theory. In the higher temperature region from25to80℃, the coercive field shows a slow decrease with the increased temperature, where the LG theory is suggested not applicable any more. The temperature-dependent changes in the polymer chains have been analyzed.2) A reversible "inherent fatigue" is observed from the partially recovered remnant polarization after re-annealing a fatigued P(VDF-TrFE) film. FTIR spectra indicate that the interchain spacing does not change from10to10’ switching cycles while the degree of all-trans ferroelectric phase decreases gradually with applied switching cycles. After a re-annealing treatment, ferroelectric phase recovers and dipoles at the boundary of crystallites acquire much higher energy.3) A universal model and experiments on the polarization switching for ferroelectric P(VDF-TrFE) copolymer ultra-thin films with interface layer have been studied. It is found that the polarization switching could complete if the capacitance of the interface layer is large enough compared with that of the ferroelectric film. Based on this model, a conducting polymer, poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonic) acid, is introduced as an interface layer between metal electrodes and ferroelectric polymer film and the switching property measurements well agree with the simulation results of the model. These findings provide an effective route to enhance the polarization switching for the ferroelectric films, which is promising for potential applications.4) Three types of electroactive interlayers, including highly polarized material copper-phthalocyanine (CuPc), electron conductor polyethylene dioxythiophene/poly (styrene sulfonate acid)(PEDOT-PSSH) and ion (proton) conductor poly (vinyl phosphonic) acid (PVPA) have been employed in our experiment. It is demonstrated that three types of interlayers can significantly improve the ferroelectric properties. Besides, the CuPc sample exhibits special properties, such as higher remnant polarization and fatigue endurance, which may be attributed to its dielectric response of the in-series capacitor system. Unlike electron and proton conductor, the effect of highly polarized layer on the ferroelectric properties can be further enhanced when the permittivity of the interlayer is much higher than that of the ferroelectric. These results suggest that three types of interlayers can lead to a large improvement of ferroelectric switching, which is promising for potential applications.5) The high-temperature ferroelectric behaviors for P(VDF-TrFE)copolymer ultrathin films with electroactive interlayers have been studied. The different electroactive polymers, commercial poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonic) acid (PEDOT-PSSH), in-situ synthesized PEDOT-PSSH with high PEDOT ratio and poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonic) ammonia (PEDOT-PSSNH4), are used as the interlayer between P(VDF-TrFE) and metal electrodes. It is found that the ferroelectric properties at high temperature are strongly dependent on the interlayer and the faster degradation occurs on the sample with the interlayer which has enough protons as compensating charges. Further analysis on the polarization response behaviors and capacitance shows that the high-temperature ferroelectric properties are more closely associated with the compensating charges in interlayer than the ferroelectric film itself, illustrating the importance of the appropriate interlayer materials.6) The structure and ferroelectric properties of ultrathin blend film fabricated by spin coating process at room temperature from the mixed solution of vinylidene fluoride (VDF) oligomers (n=18) and poly(vinylidene fluoride-co-trifluoroethylene) are investigated. The thin film ferroelectric capacitor with poly(styrenesulfonate)(PSSH) interlayer shows prominent performance with a remnant polarization of120mC/m2even as the thickness of the blend film is down to80nm. No ferroelectric degradation was observed after4×105cycles of switching. When oligomer content in mixture is40%, the blend film shows the highest polarization. With the oligomer content increasing, the molecular chain orientation of the P(VDF-TrFE)/OVDF blend films is no significant change and always parallel to the substrate.In summary, we have studied the properties for P(VDF-TrFE) ferroelectric nano-film and mechanism of the effect of electroactive interlayer. From the model and corresponding experimental results, an effective route to enhance the ferroelectric switching is provided. Our work has laid a solid foundation for carring our research of printed electronics in our team next step.