[Southeast University News Network, Feb. 14] (Correspondent: Xiong Rengen) Recently, the International Research Institute of Molecular Ferroelectric Science and Application affiliated to the School of Chemistry and Chemical Engineering of Southeast University and Jiangsu Provincial Key Laboratory of “Molecular Ferroelectric Science and Application” have achieved important progress in molecular ferroelectric research. The team first discovered a peculiar “vortex-antivortex” domain structure in a molecular ferroelectric thin film.
As a special topological form, the spontaneous polarization in ferroelectric vortex domains is localized into a closed flux in a limited space, enabling this region, especially the domain walls, highlight profound physical properties. Such process has been widely applied toferroelectric superconducting devices, field effect transistors, photodetectors and luminous diodes, etc.. In addition, as the vortex center features the lowest energy, the reversal between polarization states becomes easier and this domain structure is very suitable for building low-energy memorizers.
Currently, the majority studies on vortex domain structure are focusing on inorganic ferroelectric materials without any report regarding molecular ferroelectrics. In this paper, an organic-inorganic hybrid perovskite molecular ferroelectric (4,4-difluoropiperidinium) 2PbI4 was successfully synthesized by using the “fluorine effect” modification strategy. As limited by the symmetry and boundary conditions, the octet topological domain structure as merged by 8 secondary domains was widely available in the thin films of this material. Based on the crystal structure and piezoelectric microscope (PFM) imaging analysis, a total of four polarization states were observed in the topological domain. Among them, the polarization directions of two domains in the same quadrant were opposite at an angle of 180 °; and the included angle of the polarization directions of adjacent quadrant domains was 90 °. In this way, the polarization in this region featured a + 2π rotation (vortex) in the clockwise direction and a -2π anti-vortex in the counterclockwise direction, forming a beautiful “vortex-anti-vortex” structure. This unique domain structure, coupled with the mechanical flexibility, easy crystallization and easy film formation of the molecular ferroelectrics, would make (4,4-difluoropiperidinium) 2PbI4 very promising in device applications.
Relevant achievement has been published in the Journal of the American Chemical Society, a top journal in the field of chemistry with the title of “Observation of vortex domains in a two-dimensional lead iodine perovskite ferroelectric”. This work was accomplished by Ph.D. candidates Zhang Hanyue, Song Xianjiang and Chen Xiaogang, etc. together while Southeast University is deemed as the first correspondent. This achievement was sponsored by “Top Ten Science and Technology Issues of Southeast University” foundation.
Paper link: https://pubs.acs.org/doi/10.1021/jacs.0c00371
(Editor-in-charge: Tang Tang, reviewed by: Song Yechun)