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Tunable electroresistance and electro-optic effects of transparent molecular ferroelectrics

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Science Advances  30 Aug 2017:
Vol. 3, no. 8, e1701008
DOI: 10.1126/sciadv.1701008
  • Fig. 1 Schematic illustration of the formation process of a MOFE ImClO4 thin film and its corresponding morphology.

    (A) IP-LP growth by controlled supersaturation at the liquid-air interface with controlled temperature gradient and external water partial pressure. (B) Typical growth process of an ImClO4 thin film with an average thickness of 390 nm, recorded by an optical microscope on the gas/water surface at a low external water partial pressure. (C) Schematic figure of the “twin-like” grown MOFE ImClO4 films with n intervariant interfaces. The sum of the intervariant interface and depolarization energies, nγA and εrPs2V, admits to a minimum at the wavelength Embedded Image, where V/A = t is the film thickness. (D) Scanning electron microscopy image of the ImClO4 thin film with a bamboo-like surface morphology with an average thickness of 390 nm. The inset displays the as-grown transparent ImClO4 film. (E) External water partial pressure–dependent thickness control of the ImClO4 thin film. The partial water pressure above the supersaturated ImClO4 solution was used to control the growth rate and the uniformity of the ImClO4 crystallized film.

  • Fig. 2 Electromechanical coupling in ImClO4 thin films.

    (A) Ferroelectric hysteresis loop of an ImClO4 film. (B to D) Atomic force microscopy (AFM) height images and corresponding PFM phase and amplitude patterns of ImClO4 film. (E) Local PFM hysteresis loops of ImClO4 film. Top: Phase signal. Bottom: Amplitude signal. a.u., arbitrary units. (F and G) Piezoelectric current and voltage responses on an ImClO4 thin film at an external force of 9.8 N. The polarity reverses upon applying and releasing the force. The inset displays a schematic illustration of the piezoelectric effect of ferroelectric ImClO4 thin films. (H and I) Stress (σ), induced output current (I), and voltage (V) in the range of 2.4 N ≤ σ ≤ 9.8 N, leading to the enhanced current 0.73 nA ≤ I ≤ 3.2 nA and voltage 0.013 V ≤ V ≤ 0.068 V. The output current exhibits excellent reversibility and repeatability for the cyclic loading of 7.8 N for 500 cycles, as illustrated by the inset of (I).

  • Fig. 3 Electro-optic effect of ImClO4 thin films.

    (A) Diagram of the apparatus used to determine the angle-dependent polarization. The light-matter interaction is strongest when the polarization vectors of the ImClO4 thin film are directed parallel to the [2,Embedded Image,0] direction. (B) Angle-dependent transmitted light intensity of ImClO4 thin film. (C) Optical images of negatively and positively poled ImClO4 thin films. Green rectangles represent the electrode area. (D) Comparison of the transmission spectra of the original and negatively and positively poled states. The inset shows the transmittance of the ImClO4 thin film on ITO substrate at three different states.

  • Fig. 4 Polarization-induced resistance switching of MOFE ImClO4 thin films.

    (A) Conducting AFM and related PFM images reveal a distinct resistance contrast between the upward and downward polarized domains. (B) Schematic illustration of the polarization switching–induced resistance of MOFE ImClO4 thin films. When the film polarization is directed parallel to the electric field, the potential barrier of the electrons is reduced (ON state), whereas the average height of the potential barrier increases when the direction of the polarization is reversed (OFF state), resulting in an increase and/or decrease of the resistance. (C) I-V curves in the ON and OFF states for a small reading voltage between −0.5 and 0.5 V. (D) Current ratio between the ON and OFF states of ImClO4 thin-film devices. (E) Device retention performance at Vread = 0.2 V for the ON and OFF states; no significant deterioration was found in both the ON and OFF resistance levels after cycling 103 times. The inset shows the I-V curves before and those of both the ON and OFF states after 103 cycles. (F) Cyclic polarization switching tests the ON and OFF resistance stability of ImClO4 thin-film devices.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/8/e1701008/DC1

    fig. S1. Optical images of ImClO4 molecular crystals synthesized by spin coating an ImClO4 solution (700 mg/ml) on an ITO-prepatterned glass substrate at 3000 rpm, at different magnifications.

    fig. S2. Optical images of ImClO4 molecular crystals synthesized by drop-casting ImClO4 solution on ITO glass substrate at a concentration of 700 mg/ml, at different magnifications.

    fig. S3. Optical images of ImClO4 molecular films synthesized by IP-LP growth on ITO glass substrate at an intermediate evaporation rate, with a water partial pressure of 13.5%; the film grows on the surface of the solution, inducing unidirectional growth.

    fig. S4. Optical images of ImClO4 molecular films synthesized by IP-LP growth on ITO glass substrate at a slow evaporation rate, with a water partial pressure of 45.1%, at different magnifications.

    fig. S5. Morphology and elements distribution analysis of ImClO4 molecular films at a water partial pressure of 45.1%.

    fig. S6. Morphology and elements distribution analysis of ImClO4 molecular films at a water partial pressure of 35.5%.

    fig. S7. Experimental and simulated x-ray diffraction patterns of ImClO4 film on ITO substrate.

    fig. S8. Piezoelectric voltage response of the Ag/ImClO4/Ag sandwiches to external stresses from 2.4, 4.1, and 6.7 to 9.8N, which generate the proportional voltages shown in the right-hand side.

    fig. S9. Evolution of the piezoelectric current outputs at different applied stresses from 2.4, 4.1, and 6.7 to 9.8 N on the ImClO4 film.

    fig. S10. Evolution of the piezoelectric voltage outputs with decreasing magnitude of the applied stress on the ImClO4 film from 2.4, 4.1, and 6.7 to 9.8 N.

    fig. S11. Piezoelectric voltage as a function of the frequency resulting from applied forces, F0sin(νt) (F0 = 4.5 N at 0.40, 0.20, and 0.10 Hz).

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Optical images of ImClO4 molecular crystals synthesized by spin coating an ImClO4 solution (700 mg/ml) on an ITO-prepatterned glass substrate at 3000 rpm, at different magnifications.
    • fig. S2. Optical images of ImClO4 molecular crystals synthesized by drop-casting ImClO4 solution on ITO glass substrate at a concentration of 700 mg/ml, at different magnifications.
    • fig. S3. Optical images of ImClO4 molecular films synthesized by IP-LP growth on ITO glass substrate at an intermediate evaporation rate, with a water partial pressure of 13.5%; the film grows on the surface of the solution, inducing unidirectional growth.
    • fig. S4. Optical images of ImClO4 molecular films synthesized by IP-LP growth on ITO glass substrate at a slow evaporation rate, with a water partial pressure of 45.1%, at different magnifications.
    • fig. S5. Morphology and elements distribution analysis of ImClO4 molecular films at a water partial pressure of 45.1%.
    • fig. S6. Morphology and elements distribution analysis of ImClO4 molecular films at a water partial pressure of 35.5%.
    • fig. S7. Experimental and simulated x-ray diffraction patterns of ImClO4 film on ITO substrate.
    • fig. S8. Piezoelectric voltage response of the Ag/ImClO4/Ag sandwiches to external stresses from 2.4, 4.1, and 6.7 to 9.8N, which generate the proportional voltages shown in the right-hand side.
    • fig. S9. Evolution of the piezoelectric current outputs at different applied stresses from 2.4, 4.1, and 6.7 to 9.8 N on the ImClO4 film.
    • fig. S10. Evolution of the piezoelectric voltage outputs with decreasing magnitude of the applied stress on the ImClO4 film from 2.4, 4.1, and 6.7 to 9.8 N.
    • fig. S11. Piezoelectric voltage as a function of the frequency resulting from applied forces, F0sin(νt) (F0 = 4.5 N at 0.40, 0.20, and 0.10 Hz).

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