Real-time Observation of Ferroelectric Domains and Domain-Wall Motions in Plastic/Ferroelectric Crystal Films

Plastic/Ferroelectric crystals are a class of newly discovored ferroelectric molecular materials that attract considerable attentions as potential alternatives to widely used ferroelectric ceramics [1]. In the compounds, spontaneous polarization arises through the freeze of rotational motion of polar globular cations. Because of the solution processability and unique multiaxial ferroelectricity, the plastic/ferroelectric crystals are expected for device application by facile thin-film preparation. Among a variety of compounds reported so far, 1-azabicyclo[2.2.1] heptanium perrhenate (AH-R) exhibits 4-axis spontaneous polarization at room temperature and outstanding ferroelectric properties (<i>P</i>_s = 4.1 µC/cm², <i>E</i>_c = 2 - 4 kV/cm) even in bulk polycrystalline samples [2].<br/><br/>Herein we report ferroelectric domain structures and the response to applied electric fields in solution-processed AH-R crystalline thin films. We adopted birefringent field-modulation imaging technique, an optical imaging for observing ferroelectric domains via the Pockels effect under in-plane electric fields (&lt;&lt; <i>E</i>_c) [3,4]. We successfully observed various patterns of ferroelectric domains that are formed by three different types of domain boundaries in as-grown films. We then visualized step-by-step polarization orientation process by applying pulsed DC electric fields (&gt; <i>E</i>_c) through the field-modulation imaging measurements. We found that the domain walls which involve lattice deformation and are nearly parallel to the external field begin to move along the direction perpendicular to the applied electric field. We finally observed that the polarization of entire area between electrodes is oriented to the same direction after several seconds.<br/><br/>We also performed a high-speed imaging experiment for observing periodic dynamics of domain wall motions during the polarization switching process under applying alternating (AC) electric fields. With gate bias control of the image intensifier installed in front of the image sensor, delay of the exposure to polarized light microscopy was swept so as to visualize the motion as a sequence of the images. We achieved visualization of smooth and repetitive behavior of the domain wall motions, where the striped domain walls perpendicular to the field are maintained while those parallel to the field move to expand the reversed domains. Based on the observations, we discuss motions of domain walls that drive the polarization reversal under electric fields. Such direct observation of domain behavior are expected to elucidate microscopic mechanisms of switching characteristics in multiaxial plastic/ferroelectric crystals.<br/><br/>[1] J. Harada, APL Mater. <b>9</b>, 020901 (2021).<br/>[2] J. Harada et al. J. Am. Chem. Soc. <b>141, </b>9349 (2019).<br/>[3] Y. Uemura et al. Phys. Rev. Appl. <b>14, </b>024060 (2020).<br/>[4] Y. Uemura et al. to be published.