Abstract
Ferroelectrics are electro-active materials that can store and switch their polarity (ferroelectricity), sense temperature changes (pyroelectricity), interchange electric and mechanical functions (piezoelectricity), and manipulate light (through optical nonlinearities and the electro-optic effect): all of these functions have practical applications. Topological switching of π-conjugation in organic molecules, such as the keto-enol transformation, has long been anticipated as a means of realizing these phenomena in molecular assemblies and crystals1. Croconic acid, an ingredient of black dyes2, was recently found to have a hydrogen-bonded polar structure in a crystalline state3. Here we demonstrate that application of an electric field can coherently align the molecular polarities in crystalline croconic acid, as indicated by an increase of optical second harmonic generation, and produce a well-defined polarization hysteresis at room temperature. To make this simple pentagonal molecule ferroelectric, we switched the π-bond topology using synchronized proton transfer instead of rigid-body rotation. Of the organic ferroelectrics, this molecular crystal exhibits the highest spontaneous polarization (∼20 μC cm-2) in spite of its small molecular size, which is in accord with first-principles electronic-structure calculations. Such high polarization, which persists up to 400 K, may find application in active capacitor and nonlinear optics elements in future organic electronics.
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Acknowledgements
S.H. is grateful for support by a Grant-in-Aid for Scientific Research (No. 20110003) by the Ministry of Education, Culture, Sports, Science and Technology of Japan. The research leading to the theoretical results received funding from the European Research Council under the European Community, 7th Framework Programme - FP7 (2007-2013)/ERC Grant Agreement n. 203523.
Author Contributions S.H. did the sample preparation and the dielectric measurements, and wrote most of the paper, Y. Tokunaga optimized the P-E hysteresis data and made the pyroelectric and resistivity measurements, G.G. and S.P. did the calculations and wrote a significant part of the discussion, H.I. and R.S. performed the second harmonic generation imaging microscopy and wrote the paper, R.K. contributed to the diffraction studies, and Y. Tokura contributed to the design of the studies and writing of the paper.
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Horiuchi, S., Tokunaga, Y., Giovannetti, G. et al. Above-room-temperature ferroelectricity in a single-component molecular crystal. Nature 463, 789–792 (2010). https://doi.org/10.1038/nature08731
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DOI: https://doi.org/10.1038/nature08731
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