THIN FILMS OF A NEW ORGANIC SINGLE-COMPONENT FERROELECTRIC 2-METHYLBENZIMIDAZOLE

 Subject of Research.We present results of structural and dielectric study of organic ferroelectric 2-methylbenzimidazole (MBI) thin films. Method. The films have been grown on substrates of leuco-sapphire, fused and crystalline silica, neodymium gallate, bismuth germanate, gold, aluminium, platinum. The films have been grown by two different methods: substrate covering by ethanol solution of MBI and subsequent ethanol evaporation; sublimation at the temperature near 375 K under atmospheric pressure. Crystallographic orientation studies have been performed by means of «DRON-3» X-ray diffractometer, block structure of the films has been determined by «LaboPol-3» polarizing microscope. Small-signal dielectric response has been received with the use of «MIT 9216A» digital LCR-meter, while strong-signal dielectric response has been studied by Sawyer-Tower circuit. Main Resuts. We have shown that the films obtained by evaporation are continuous and textured. Obtained film structure depends on the concentration of the solution. Films may consist of blocks that are splitted crystals like spherulite. Spontaneous polarization components in such films may be directed both perpendicularly and in the film plane. We have also obtained structures consisting of single-crystal blocks with spontaneous polarization components being allocated in the film plane. Block sizes vary from a few to hundreds of microns. Films obtained by sublimation are amorphous or dendritic. The dielectric properties of the films obtained by evaporation have been studied. We have shown that the dielectric constant and dielectric loss tangent increase under heating. The dielectric hysteresis loops are observed at the temperature equal to 291-379 K. The remnant polarization increases with temperature for constant amplitude of the external electric field, and achieves 4.5mC/cm², while the coercive field remains constant. We propose that such behavior is explained by increase of the number of crystallites with switchable polarization due to decrease in the coercive field under heating. The remnant polarization decreases with frequency increase. Practical Relevance. Proposed method of ferroelectric films manufacture is characterized by low cost and convenience. Unlike many other ferroelectrics, the films contain no lead and rare metals. MBI films demonstrate the low value of the coercive fields. This paper may be useful for electronic components developers.

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PICTURES

 Fig. 1. The structural formula of 2-methylbenzimidazole molecule

 Fig.2. MBI type crystals of spherulite growing in an alcohol solution. Photos are taken in the crossed polarizers at different rotation angles of the sample relative to the polarizers (a)–(b). The overview area size is 4×3 mm2

 Fig. 5. Dielectric hysteresis loops in the MBI films at different temperatures.

UЕ– the voltage applied to inter-digital structures, UP –the voltage on reference capacitor proportional to Ppolarization. Linear dependence UP(UЕ) due to capacity structure has been deducted: A type film (a), B type film (b)

Fig. 6. Dielectric hysteresis loops in the MBI films at various frequencies. Linear dependence UP(UЕ) due to the structure capacity has been deducted: A type film with the temperature of 294 K (a); B type film with the temperature of 295 K (b)

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