| Compared to the traditional inorganic ferroelectrics, ferroelectric polymer poly(vinylidene fluoride-ran-trifluoroethylene)(P(VDF-Tr FE)) shows huge application potential in flexible transducers, infrared detectors, organic solar cells and ferroelectric memories for its special advantages such as soft, low weight, light transmittance, low cost and ease of process. Rotation of molecular chains are involved in polarization reversal process due to the ferroelectricity of P(VDF-Tr FE) stems from aligned molecular dipoles in lamellar crystals. Therefore, regulation and control of P(VDF-Tr FE) molecular chains and lamellae orientation is not only important for deepening the understanding of its ferroelectric properties, but also helpful for the fabrication of organic devices with excellent performance.In this paper, we first study the P(VDF-Tr FE) lamellae orientation behavior with one and two dimensional nanoconfined conditions. In one dimensional space confined P(VDF-Tr FE) thin and ultrathin films, we find lamellae orientation behavior having special relevance with thermal history and film thickness. On thermal history perspective, when the samples are directly annealed in the paraelectric phase temperature, lamellar crystals take edge-on orientation and randomly distribute on the substrate; however, when annealed from melt, crystals change to flat-on orientation with no fixed geometrical shape. In non-continuous ultrathin films, we find the fast growth direction of needle like crystals is along crystallographic a axis. On film thickness perspective, we find the orientation of molecules and lamellar crystals does not change when film thickness varies in the range of 35-150 nm. In two dimensional nanoconfined P(VDF-Tr FE) nanowires and nanostripes, our results show that the molecular of lamellar crystals orientated in the confined spaces. In nanowires samples crystallized from anodic aluminum oxide(AAO) templates with aperture of 40-100 nm, the selected area electron diffraction(SAED) pattern shows the approximate single-crystal structure with lamellaes parallel to the hole walls and polar b axis perpendicular to the long axis of nanowires. In addition, fabricated by nanoimprint lithography, the lamellar crystals in P(VDF-Tr FE) nanostripes take edge-on orientation with a axis and main chains normal and parallel to the stripes, respectively.Taking piezoresponse force microscopy(PFM) as the main technical means, we analyze the effect of lamellar orientation on the ferroelectric properties in nanometer scale. In P(VDF-Tr FE) thin and ultrathin films, we find ferroelectricity exists in edge-on but not in flat-on oriented lamellaes. Thus, molecular chain direction can directly determine the ferroelectricity of P(VDF-Tr FE) when considering the relationship between lamellae orientation and the alignment of molecular chains. The molecular dipole direction can be switched by vertical external electric field when molecular chains are parallel to the substrate. While when the chains are normal to the substrate, the rotation will not happen. In addition, we find that the coercive field of different thickness films does not change, which is in agreement with thickness independent crystal orientation. From single point piezoresponse hysteresis loops of P(VDF-Tr FE) thin films and nanowires, lower coercive field and higher piezoelectric response can be found in nanowires for the promoted crystalline and oriented crystals in confined space.With PFM as the main technique, combined with studies of condensed structure, we explore the effect of molecular and crystal orientation on polarization reversal process. Different from the isotropic propagation process with nearly circular domains in non-ordered thin films, ferroelectric polarization reversal in nanostripes shows anisotropic domain growth along distinct directions. The dynamic analysis of P(VDF-Tr FE) thin films and nanostripes shows that the domain wall motion is a creep process with dynamic exponent found to be close to 0.5 by our calculation, which indicated a random bond disorder system associated with defects in conformation and molecular packing.In summary, we have systematically studied lamellae orientation behavior of P(VDF-Tr FE) with different nanoconfined conditions, analyzed the relationship between the molecular alignment and ferroelectric properties, and explored the effect of lamellae orientation on polarization reversal process. These studies provide effective experimental approaches to regulate and control P(VDF-Tr FE) molecular and lamellar crystal orientation, deepen the understanding of the ferroelectric properties of polymers, lay the foundation for optimization of material performance, and have important significances for the design of nanoscale ferroelectric devices. |
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