An unprecedented azobenzene-based organic single-component ferroelectric

Abstract

Organic single-component ferroelectrics, as an important class of metal-free ferroelectrics, are highly desirable because of their easy processing, mechanical flexibility, and biocompatibility. However, although nearly 50 years have passed since the discovery of photochromism in azobenzene-doped cholesteric liquid crystals, ferroelectricity has never been found in azobenzene-based crystals. Here, we use an amino group to substitute a fluorine atom of 2,2',4,4',6,6'-hexafluoroazobenzene, which successfully introduces ferroelectricity into 2-amino-2',4,4',6,6'-pentafluoroazobenzene (APFA). APFA shows an extremely high Curie temperature (T _c) of 443 K, which is outstanding among single-component ferroelectrics. It also exhibits an indirect optical band gap of 2.27 eV as well as photoisomerization behavior between the trans-form and the cis-form triggered by pedal motion. To our knowledge, APFA is the first azobenzene-based ferroelectric crystal. This work opens an avenue to design excellent single-component ferroelectrics and will inspire the exploration of azobenzene-based ferroelectrics for promising applications in biofriendly ferroelectric devices.

Fig. 1. (a) Design strategy of ferroelectric APFA by replacing the F atom with an amino group. (b) Crystal structure of APFA. The dashed line denotes the intramolecular N–H–N hydrogen bonding interaction. (c) Electron density distribution on the selected plane built with the three N atoms of APFA at 298 K. (d) Packing views of the structure of APFA at 298 K. (e) Packing views of the structure of APFA along the a axis represented by the dot-surface model at 298 K. The direction of spontaneous polarization in the ferroelectric phase is shown with a red arrow.

Fig. 2. Phase transition of APFA. (a) DSC curve. (b) Temperature-dependent ε′ at 1 MHz. (c) Temperature-dependent SHG response. Inset: comparison of the SHG intensity of APFA and KDP. (d) Variable-temperature PXRD patterns.

Fig. 3. Domain switching for the APFA thin film. Topography images (a), PFM amplitude images (b), and PFM phase images (c) of an 8 × 8 μm² region. Vertical PFM amplitude (d) and (f) and phase (e) and (g) images superimposed on the topographic image, which were recorded after writing a square area with a voltage of −140 V (d) and (e) followed by a smaller central square with a voltage of +110 V (f) and (g) using a biased conductive tip. The blue and yellow regions in the phase images indicate the regions with out-of-plane polarization oriented upward and downward, respectively.

Fig. 4. (a) P–E hysteresis loops of the APFA thin film via a double-wave method at room temperature. (b) UV-vis absorption spectra of APFA. Inset: the Tauc plot for APFA. (c) HOMO and (d) LUMO of the APFA molecule. (e) The calculated band structure of APFA and (f) the corresponding PDOS.