Haisheng Xu

Domain stabilization effect of interlayer on ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin film

A charge injection material, poly(3,4-ethylene dioxythioohene)-poly(styrene sulfonic) acid, is introduced as a buffer layer between metal electrodes and a ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer ultrathin film. The buffer layer not only prevents the reactions between P(VDF-TrFE) layer and the top electrode during metallization process, which leads to the loss of polarization, but also supplies the amount of charges needed for compensation of the ferroelectric dipoles to stabilize the domain during switching process so that the fatigue property was improved tremendously. The sandwiched structure shows prominent ferroelectric properties in a 50 nm P(VDF/TrFE) film. A coercive electric field of 52 MV/m and a remnant polarization of 88 mC/m2 are recorded. With two additional organic interlayers, 33 nm thick in total, the coercive field decreases. After more than 1×107 cycles of switching, the polarization remains as high as 68 mC/m2. Moreover, the cell still has good ...

High electric tunability of relaxor ferroelectric Langmuir–Blodgett terpolymer films

Poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) terpolymer films have been fabricated on Al-coated polyimide substrates using the Langmuir–Blodgett (LB) technology. The electrical properties of the films have been investigated from 150 to 350 K. The temperature of the dielectric maximum and the measured frequency obeys the Vogel–Fulcher [J. Am. Ceram. Soc. 8, 339 (1925); Phys. Z. 22, 645 (1921)] law. The electric tunabilities of the films at room temperature are 42.6% and 80% under 50 and 240 MV/m, respectively. The reasons for the high tunability are discussed based on the special microstructure of the LB films.

Fast switching protocol for ferroelectric random access memory based on poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin films

The polarization switching behaviors with different pulse processes for ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer ultrathin films have been studied. The faster switching speed with imprint time is observed for the films with the certain switching and imprint directions. The internal electric fields for these processes are well analyzed, and it is found that the effective fields as well as charge trap states in ferroelectric layers are strongly responsible for the switching behaviors. This result provides an effective route to design the optimum protocol for ferroelectric random access memory based on P(VDF-TrFE) copolymer ultrathin film.

The preparation and ferroelectric properties of defect-free ultrathin films of vinylidene fluoride oligomer

The ultrathin, defect-free ferroelectric films of vinylidene fluoride oligomer were fabricated by vapor deposition. A narrow substrate temperature window was found to get high quality ferroelectric films. Ferroelectric phase was obtained when the substrate temperature was −90 °C. The ferroelectric phase is stable at room temperature, even after annealing at 120 °C for 30 min. A full memory stack based on the ferroelectric VDF oligomer may work as nonvolatile random access memory with superhigh density due to the smaller crystal grain comparing with that of poly(vinylidene fluoride-trifluoethylene) [P(VDF-TrFE)] copolymers. The cell shows prominent ferroelectric properties even as the thickness of VDF oligomer film is down to 60 nm with a coercive field of 95 MV/m and a remnant polarization of 125 mC/m2. After 1×105 cycles of switching, no ferroelectric degradation was observed, the ratio of polarization before and after fatigue is close to 1, which is in contrast to P(VDF-TrFE) thin film where the ferroel...

The Role of Proton for the Effect of Interlayer on Ferroelectric Behavior of Poly(vinylidene fluoride-trifluoroethylene) Copolymer Ultrathin Films

A functional interlayer, poly(3,4-ethylene dioxythioohene)-poly(styrene sulfonic) acid, is introduced as a buffer layer between the metal electrodes of a 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 to 50 nm and the fatigue property is also improved tremendously. We demonstrate that the proton in the PSSH instead of the electron in the PEDOT plays a significant role to improve the ferroelectric properties of P(VDF-TrFE), furthermore, polyvinyl phosphoric acid as well as other polyacid also supplies the proton.

High-temperature ferroelectric behaviors of poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin films with electroactive interlayers

The high-temperature ferroelectric behaviors for poly(vinylidene fluoride-trifluoroethylene) (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 appropri...

The effect of electroactive interlayer on the ferroelectric properties in poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin films

A series of ferroelectric polarization reversal measurements have been conducted on spin-coated ultrathin poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer films sandwiched with different electroactive materials between P(VDF-TrFE) and metal electrodes. Three types of electroactive interlayers, including highly polarizable material copper-phthalocyanine (CuPc), electron conductive polyethylene dioxythiophene/poly(styrene sulfonate acid), and ion (proton) conductive poly (vinyl phosphonic) acid, have been employed in our experiment. In addition, the ferroelectric film with CuPc exhibits larger remnant polarization and the enhancement of fatigue resistance although higher drive voltage and more switching time can be observed, which differs from the samples with electron and proton conductors as interlayers. The further analysis shows that this may be attributed to higher dielectric constant of CuPc than that of P(VDF-TrFE) layer. These results suggest that three types of interlayers can le...

The improved imprint effect in poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin films

The imprint behaviors for poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer ultrathin films with electroactive interlayers have been studied. The different electroactive polymers are used as the interlayer between P(VDF-TrFE) and metal electrodes. It is found that the smaller voltage shift is observed for the samples with electroactive interlayers and the imprint properties are improved substantially after the electroactive interlayers are introduced. The charge trap accumulation in P(VDF-TrFE) films without and with interlayers is analyzed and it is thought that the traps would be partially compensated by the free charges from the electroactive interlayers, illustrating the importance of the appropriate interlayer materials.

Modeling of ferroelectric switching process in poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin films with electroactive interlayer

A universal model and experiments on the polarization switching for ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer ultra-thin films with an interface layer have been studied. It is found that 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.

The effect of ac field amplitude on the relaxor behaviors in Langmuir–Blodgett terpolymer films

The ac field (Eac) amplitude dependence of dielectric response has been investigated for poly (vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) relaxor terpolymer films. The frequency dependence of the temperature of the permittivity maximum Tm can be well described by the Vogel–Fulcher relation for Eac in the range from 1.33 to 80 MV/m. Tm remains nearly constant at an Eac level lower than 13.3 MV/m, and starts decreasing with Eac increases further. The two parameters fitting to the Vogel–Fulcher relation, freezing temperature Tf and activation energy Ea, vary little for Eac below 26.7 MV/m and then Tf decreases, while Ea increases for higher Eac level.