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AIST has been promoting the research and development of organic ferroelectric small molecules composed of light elements, which contain no rare metal nor toxic lead and could be suitable for print production technologies. It has developed many organic ferroelectrics, including a binary component molecular compound with excellent ferroelectric properties (AIST press release on January 24, 2005) and a single component material that exhibits the best ferroelectric properties at room temperature (AIST press release on February 12, 2010). To build them into devices, it is necessary to fabricate pinhole-free, uniform thin films with oriented molecules. These demands motivated the researchers to search an appropriate compound and to adopt an advanced printing technique.
This study is supported by the Japan Science and Technology Agency through CREST, as “Creation of Materials Science for Advanced Ferroelectrics of Organic Compounds.” (Research Period： FY2011 - FY2015)
Details of Research
The researchers selected 2-methylbenzimidazole (MBI) as a promising candidate for an organic ferroelectric material (Fig. 1a). MBI is one of the hydrogen-bonded organic ferroelectric materials, is soluble in organic solvents, shows polarization reversal at a low coercive electric field (few tens kV/cm), and exhibits excellent ferroelectric properties at room temperature. Within a single crystal, remnant polarization P would appear in two orthogonal directions. In devices to which voltage is applied in a direction normal to the thin film, P should have a component normal to the thin film. MBI is expected to grow in plate-like crystals having a desired polarization direction.
Figure 1b shows the schematics of the developed thin film print fabrication process under ambient pressure at room temperature. First, the surface of a 1 cm square SiO2/Si substrate was treated with hydrophilic/hydrophobic patterning consisting of a 100 µm line and space (L&S) structure. An array of crystalline thin films can be formed on the hydrophilic regions by shearing the solution of MBI with a flat blade and successive drying. Synchronized light extinction by rotating cross-polarizers in the crossed-Nicols optical micrographs indicated a high degree of crystallographic alignment of these thin plate-like crystals (Fig. 1c).
Figure 1: (a) Molecular structure of MBI. (b) Schematics of fabrication method for single-crystalline thin film. (c) Crossed-Nicols polarized micrographs of MBI thin films. Rotation of the sample (or rotation of the polarizers) changed the brightness. Crossed arrows indicate the directions of the polarizers. Scale bars: 400 µm.
The lattice parameters, crystal orientations, and directions of spontaneous polarization of the MBI film were determined by synchrotron X-ray diffraction measurements at the Photon Factory of KEK. A single diffraction spot (in the dashed red circle) was observed for each diffraction plane (Fig. 2a) suggesting the formation of a single crystal. Figures 2b and 2c show schematics of the molecular packing structure and crystal orientation on the substrate. It was found that one of the hydrogen-bonded chains is directed perpendicular to and another is parallel to the substrate surface, respectively. It means that the principal polarization axes are tilted by 45 degrees relative to the substrate surface. As the spontaneous polarization has a component perpendicular to the substrate, it may be possible to reverse the polarization in electrode/ferroelectric/electrode layered structure by applying voltage between upper and lower electrodes.
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