Field-Modulation Imaging of Ferroelectric Domains in Molecular Single-Crystal Films

Yohei Uemura, Shunto Arai, Jun’ya Tsutsumi, Satoshi Matsuoka, Hiroyuki Yamada, Reiji Kumai, Sachio Horiuchi, Akihito Sawa, and Tatsuo Hasegawa

Phys. Rev. Applied 11, 014046 – Published 23 January 2019

Abstract

Many hydrogen-bonded organic ferroelectrics exhibit low-field switching of large spontaneous polarizations. Although the switchable electric dipoles of π-conjugated organic molecules account for the large spontaneous polarizations, their relevant optoelectronic processes have not been used to probe the ferroelectricity. We show that the variation in electro-optic response enables visualization of the ferroelectric domains and domain walls in single-crystal films of a hydrogen-bonded molecular cocrystal. Highly sensitive and rapid visualization is realized by difference optical image sensing between the forward and reverse field applications. We call this technique “ferroelectrics field modulation imaging (FFMI).” The unique optical-probe nature reveals the existence of two types of domain walls showing different three-dimensional orientations within the films; one is roughly perpendicular to the film plane, whereas the other is considerably tilted from the normal to the plane. We explain that both of the domain walls are stabilized to generate substantial neutrality by being directed parallel to the direction of polarization. This study opens a route for exploring the three-dimensional topological nature of domain walls in ferroelectric materials.

Received 16 September 2018
Revised 11 December 2018

DOI:https://doi.org/10.1103/PhysRevApplied.11.014046

Physics Subject Headings (PhySH)

Domain walls Ferroelectric domains

Devices Molecular solids Organic compounds Single crystal materials Thin films

Optical microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Yohei Uemura^1,*, Shunto Arai¹, Jun’ya Tsutsumi², Satoshi Matsuoka¹, Hiroyuki Yamada³, Reiji Kumai⁴, Sachio Horiuchi², Akihito Sawa³, and Tatsuo Hasegawa^1,2

¹Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan
²Flexible Electronics Research Center (FLEC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan
³Electronics and Photonics Research Institute (ESPRIT), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan
⁴Condensed Matter Research Center (CMRC) and Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan

^*uemura@hsgw.t.u-tokyo.ac.jp

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Vol. 11, Iss. 1 — January 2019

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Images

Figure 1
Hydrogen-bonded organic ferroelectric, Hdppz-Hca, and its optical response. (a) Crystal structure of Hdppz-Hca. Hydrogen-bonded chains are along the c axis. (b) Molar absorption spectra of chloranilic acid in acetonitrile solution at different protonation states; neutral H₂ca, deprotonated Hca⁻ monoanion, and ca²⁻ dianion. Reproduced with permission (R. Kumai, et al., J. Chem. Phys. 125, 084715 (2006).) [26]. (c),(d) Schematic of proton displacement under external electric fields. When the field is applied along the direction of the spontaneous polarization, the protons move closer to ca²⁻, which is expected to increase the absorbance (c). When the field is applied opposite to the spontaneous polarization, the protons move further away from ca²⁻, which is expected to decrease the absorbance (d). (e) Electro-absorption spectrum of Hdppz-Hca film. The blue and red curves are the field modulation spectra obtained for a ferroelectric domain at the initial ferroelectric state and after the polarization reversal, respectively. The dashed curve is the normal absorbance spectrum. Black open circles are the average FFMI signal intensity obtained at each photon energy (see Supplementary Fig. S4 [28]).
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Figure 2
FFMI and typical obtained images. (a) Schematic representation of the FFMI measurement. (b) Typical FFMI image of Hdppz-Hca single-crystal film. (c) Lateral PFM phase image of the same area as (b). The same ferroelectric domains and domain walls are clearly identified in (b) and (c). (d) FFMI images of Hdppz-Hca single-crystal film measured at the initial ferroelectric state (left) and at 20 min (center) and 96 min (right) after the application of an external electric field of 20 kV/cm. Domain wall motions are clearly observed during the polarization reversal. White arrows depict the horizontal component of spontaneous polarization. Scale bar is 20 µm.
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Figure 3
Comparison of FFMI image and PFM phase image around the domain walls. (a) Typical FFMI image of Hdppz-Hca single-crystal film around the domain wall, which is moved along the direction shown by the white dotted arrow. White curve in the image indicates the domain wall as observed by the lateral PFM phase image (d). (b),(c) Variation of FFMI signal intensity as a function of the film position across DW1 (x||b) (b) and across DW2 (x||c) (c), along the black dashed arrows in (a). (d) Lateral PFM phase image of the same area as (a). (e),(f) Variation of PFM phase as a function of the same film position as in (b),(c), respectively. Scale bar is 10 µm.
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Figure 4
Spatial width of domain walls as observed in the FFMI. (a) Film thickness dependence of spatial width (w) for the FFMI signal variation around DW1 (blue circles) and DW2 (red squares). Dashed line is a linear fit for the data of DW2. (b) Schematics for the orientations of DW1 (top) and DW2 (bottom) within the film. White arrows depict the horizontal component of spontaneous polarization.
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Figure 5
Observed domain wall orientation and its relationship with crystallographic axes. (a) Crystallographic axes of Hdppz-Hca single-crystal film on a substrate. b and c axes are parallel to the substrate. (b) FFMI image and (c) Lateral PFM phase image of the same area of an Hdppz-Hca single-crystal film. White curves in (b) indicate the domain walls observed in (c). These domain walls stay at the locations after they are moved along the direction shown by the white dotted arrows in (b), respectively. Black arrows depict the horizontal component of spontaneous polarization. Scale bar is 10 µm.
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