With the support of the National Natural Science Foundation of China (Grant number: 51788104, 51625202), the research group of Profs. Yang Shen and Ce-Wen Nan observed the toroidal polar topology in strained ferroelectric polymer and presented the spatial periodic absorption on tera-hertz (THz) wave in the topology. The research work was published in Science on March 5, 2021, entitled “Toroidal polar topology in strained ferroelectric polymer” (2021, 371: 1050-1056; DOI: 10.1126/science.abc4727).

Toroidal assembly of ferroic order parameter has become an emerging research field for exotic phenomena and potential applications in reconfigurable electronic devices, which has been systematically studied and reported in oxide thin films in recent years. As one of the most classic ferroelectrics, poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] has been appreciated for its ability to be solution-cast, low processing cost and intrinsic flexibility. However, the understanding on the polar assembly within P(VDF-TrFE) is limited, raising the question if toroidal polar topology can ever exist or how it will exist in the ferroelectric polymers.

The research group applied a dimension-limiting method to produce P(VDF-TrFE) thin film consisting of single-layered face-on lamellar crystals with thickness of 100 nm and lateral size of ~40 μm. Given the special chain rotation degree of freedom in ferroelectric polymer, the primary dipole moment perpendicular to the polymer chain will be substantially aligned parallel to the film plane (Figure 1A). By applying in-plane piezo-responsive force microscope, the research group successfully observe polar topology with evident toroidal assembly (Figures 1B&1C). Selected area electron diffraction and x-ray scattering confirmed that the Curie transition of P(VDF-TrFE) introduced a biaxial strain of 7.3% in tensile style within the face-on lamellae. As suggested by first-principle and phase-field calculations, the tensile strain would flatten the energy landscape of polarizations along all the in-plane directions, facilitate the free rotation of polymer chain and enable continuous rotation of polarization in real-space, which is key to the formation of toroidal polar topology. An atomic force microscope equipped with THz laser was applied to examine the THz absorption in the face-on lamellae, and a spatially periodic absorption caused by the toroidal polar topology was manifested (Figure 1D).

The research group also took advantage of the special chain alignment in the thin film to measure the dielectric response along the backbone of P(VDF-TrFE), where relaxor behavior at low frequency was observed (Figure 1E) and attributed to the weakened interchain interaction from the biaxial strain. Combining microscopic and macroscopic characterization, substantial ferroelectric switching and piezoelectric response were triggered along the chain direction, and show strong coupling with the toroidal topology where polarizations were perpendicular to the polymer chain. These results prove the existence of dipole along polymer chains, complement the conventional concept that ferroelectric responses of polymers mostly originate from the dipoles perpendicular to the polymer chains, and provide deep understanding on the responsive mechanism within ferroelectric polymers.

This work applied a simple and effective method to align the single crystal of P(VDF-TrFE), and found an emergent toroidal polar topology in ferroelectric polymers. The research presents a novel paradigm for controlling the topology in ferroelctric polymers, and provides opportunities for multi-stimuli conversions in flexible electronics toward reconfigurable devices.

Figure 1: P(VDF-TrFE) face-on lamellae with toroidal polar topology. (A) The schematic of lattice and polarization in the thin film. (B) large-scale face-on lamellae of P(VDF-TrFE). (C) the toroidal polar topology within a single lamella. (D) the spatially periodic absorption on tera-hertz wave within the lamella. (E) the relaxor behavior at low frequency along the chain backbone of P(VDF-TrFE).