1 Nature 1998 Vol: 391(6670):874-877. DOI: 10.1038/36069

Two-dimensional ferroelectric films

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 (refs 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.

Mentions
Figures
Figure 1: Structure of the polymer chains and films.a, Fragment of a P(VDF-TrFE 70:30) copolymer chain. 30% of the VDF (–CH2–CF2–) units have been replaced at random by TrFE (–CHF–CF2–) units. The arrows show the direction of the net dipole moments pointing from the fluorine side to the hydrogen side of the chain. b, Atomic-resolution STM image of a single-monolayer film of P(VDF-TrFE 70:30) deposited on graphite. Figure 2: Dielectric properties of the FLP films.a–c, Dielectric constant of the P(VDF-TrFE 70:30) films with different thicknesses, showing peaks at the phase transitions on heating and, due to thermal hysteresis, at different temperatures on cooling: a, 30 ML; b, 7 ML; c, 2 ML. The arrows show the direction of temperature change. d, The inverse capacitance at 25 °C for FLP films with different thicknesses. Figure 3: Pyroelectric response and spontaneous polarization Ps, obtained by integration over temperature, of P(VDF-TrFE 70:30) films.a, 30 ML; b, 5 ML; c, 2 ML. Figure 4: Polarization hysteresis loops at 25 °C, measured by the pyroelectric technique, of the P(VDF-TrFE 70:30) films with different thicknesses.a, 30 ML; b, 5 ML; c, 2 ML.
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References
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    • . . . The first crystalline Langmuir–Blodgett-deposited polymer films, of P(VDF-TrFE 70:30), achieved a great improvement in quality7,8,10 . . .
    • . . . These ferroelectric polymer films, 30 monolayers (ML) thick (150 Å), also showed double hysteresis and the critical point19, and a new conductance switching controlled by the polarization state8 . . .
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    • . . . Also, the polarization of the 2–7 ML films reverts in zero field to an orientation determined by the substrate, independent of the history of electric-field application8,9, consistent with the failure of the pyroelectric response to completely vanish above the Curie temperature in the thinnest films . . .
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    • . . . Finite-size effects have been demonstrated in these films; the coercive field scaled as the -0.7 power of the thickness in films with 35–135 ML (ref. 9). . . .
    • . . . The 5-ML and 2-ML films show considerable bias, probably due to interactions with the substrate or the top electrode, consistent with our earlier observations on the dynamics of switching FLP films8,9 . . .
    • . . . Also, the polarization of the 2–7 ML films reverts in zero field to an orientation determined by the substrate, independent of the history of electric-field application8,9, consistent with the failure of the pyroelectric response to completely vanish above the Curie temperature in the thinnest films . . .
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    • . . . The Ising model for ultrathin films23,26 is a more appealing approach to modelling ferroelectricity in two-dimensional polymer films because the dipole moments have restricted freedom—they can rotate only about the chain axis and are further inhibited from rotations about the axis by both interchain steric interactions and intrachain dihedral stiffness . . .
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    • . . . The crystalline FLP films were formed by horizontal Langmuir–Blodgett deposition from a water subphase, as reported in greater detail previously7,24 . . .
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    • . . . The STM image (Fig. 1b) was obtained using an MTD (Moscow, Russia) instrument with a 1-ML FLP film deposited on a graphite substrate25 . . .
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    • . . . The Ising model for ultrathin films23,26 is a more appealing approach to modelling ferroelectricity in two-dimensional polymer films because the dipole moments have restricted freedom—they can rotate only about the chain axis and are further inhibited from rotations about the axis by both interchain steric interactions and intrachain dihedral stiffness . . .
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