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Two-dimensional ferroelectric films

Nature volume 391, pages 874–877 (1998)Cite this article

Abstract

Ultrathin crystalline films offer the possibility of exploring phase transitions in the crossover region between two and three dimensions. Second-order ferromagnetic phase transitions have been observed in monolayer magnetic films1,2, where surface anisotropy energy stabilizes the two-dimensional ferromagnetic state at finite temperature3. Similarly, a number of magnetic materials have magnetic surface layers that show a second-order ferromagnetic–paramagnetic phase transition with an increased Curie temperature4. Ferroelectricity is in many ways analogous to ferromagnetism, and bulk-like ferroelectricity and finite-size modifications of it have been seen in nanocrystals as small as 250 Å in diameter5, in perovskite films 100 Å thick6 and in crystalline ferroelectric polymers as thin as 25 Å (7-10). But these results can be interpreted as bulk ferroelectricity suppressed by surface depolarization energies, and imply that the bulk transition has a minimum critical size11,12,13. Here we report measurements of the ferroelectric transition in crystalline films of a random copolymer of vinylidene fluoride and trifluoroethylene just 10 Å (two monolayers) thick. We see a first-order ferroelectric phase transition with a transition temperature nearly equal to the bulk value, even in these almost two-dimensional films. In addition, we see a second first-order transition at a lower temperature, which seems to be associated with the surface layers only. The near-absence of finite-size effects on the bulk transition implies that these films must be considered as two-dimensional ferroelectrics.

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Figure 1: Structure of the polymer chains and films.
Figure 2: Dielectric properties of the FLP films.
Figure 3: Pyroelectric response and spontaneous polarization Ps, obtained by integration over temperature, of P(VDF-TrFE 70:30) films.
Figure 4: Polarization hysteresis loops at 25 °C, measured by the pyroelectric technique, of the P(VDF-TrFE 70:30) films with different thicknesses.

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Acknowledgements

We thank P. A. Dowben and J. F. Scott for suggestions for improving the manuscript. Work at the University of Nebraska was supported by the USA NSF Division of Electronic and Communications Systems and by the Nebraska Research Initiative through the Center for Materials Research and Analysis. Work at the Institute of Crystallography was supported by INTAS.

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Authors and Affiliations

  1. the Department of Physics and Astronomy and the Center for Materials Research and Analysis, University of Nebraska, Lincoln, 68588-0111, Nebraska, USA

    A. V. Bune, V. M. Fridkin & Stephen Ducharme

  2. Institute of Crystallography of the Russian Academy of Sciences, 117 333, Moscow, Russia

    A. V. Bune, V. M. Fridkin, L. M. Blinov, S. P. Palto, A. V. Sorokin, S. G. Yudin & A. Zlatkin

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  1. A. V. Bune
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  2. V. M. Fridkin
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  3. Stephen Ducharme
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  4. L. M. Blinov
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  5. S. P. Palto
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  6. A. V. Sorokin
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Correspondence to Stephen Ducharme.

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Bune, A., Fridkin, V., Ducharme, S. et al. Two-dimensional ferroelectric films. Nature 391, 874–877 (1998). https://doi.org/10.1038/36069

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