Research ArticleMATERIALS SCIENCE

High-density array of ferroelectric nanodots with robust and reversibly switchable topological domain states

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Science Advances  18 Aug 2017:
Vol. 3, no. 8, e1700919
DOI: 10.1126/sciadv.1700919
  • Fig. 1 Structure and PFM images for a BFO nanodot array.

    (A) SEM image. (B) XRD pattern. STO, (001)-oriented SrTiO3; SRO, SrRuO3; a.u., arbitrary units. (C and D) Vertical PFM amplitude (V-amp.) (C) and phase (V-pha.) (D) images for the as-prepared nanodot sample. (E and F) Lateral PFM amplitude (L-amp.) (E) and phase (L-pha.) (F) images. Some single nanodot PFM images are zoomed-in and shown in the insets in the gap between (C) and (D) and (E) and (F), illustrating some typical PFM contrast variants frequently observed in the nanodots. The inset schematic diagram of the cantilever indicates that the cantilever is parallel to the y axis (<010> direction); thus, the contrasts in the lateral PFM image reflect the x components of polarization vectors (along the <100> direction). All the PFM images (C to F) are from the same region.

  • Fig. 2 3D domain reconstruction using vector PFM analysis for a typical nanodomain inside a single nanodot.

    (A to C) The lateral PFM (L-PFM) amplitude and phase images with sample rotation for 0° (A) and 90° (B), along with the vertical PFM (V-PFM) images (C). (D to G) The piezoresponse images (PFM) for both the x, y, and z components derived from the combination of amplitude and phase images according to Rcos(θ) (R, amplitude; θ, phase angle), in which the x component of PFM (PFMx) (D) is converted from (A), (F) from (B), and the z component of PFM (PFMz) (G) from (C). The y component of PFM (PFMy) (E) is obtained by anticlockwise rotation of the image (F) for 90° so that the local PFM signals in (D), (E), and (G) are from the same locations. (H and I) 2D vector contour maps including the amplitude map (H) and phase angle map (I) converted from the combination of PFMx (D) and PFMy (E), presenting the lateral polarization distributions. (J and K) 3D vector map including the amplitude map (J) and phase angle map (K), derived from the combination of PFMx (D), PFMy (E), and PFMz (G). (L) Schematic diagram for the 3D domain structures. All the PFM images and vector maps share the same scale bar.

  • Fig. 3 Vector PFM images and vector maps, along with the simulated contours for some typical topological domains in the nanodots.

    (A to D) PFM images for the four typical domain structures frequently observed in the nanodots: center-convergent domain (A), center-divergent domain (B), double-center domain (C), and reverse double-center domain (D) that is equivalent to the double-center domain in (C). The micrographs from left to right are piezoresponse images derived from Rcos(θ) (R, amplitude; θ, the phase angle) for vertical PFM (PFMz) and lateral PFM for three sample rotation angles of 0° (PFMx), 45° (L-PFM along the <110> direction), and 90° (PFMy), as well as their corresponding 2D vector angle maps and schematic domain configurations. (E) Cylinder model for phase-field simulation of the center domains in the nanodots. (F to H) Three different polar vector contour maps derived from the simulation of the different charge distribution states: negative charge (F), positive charge (G), and half-positive and half-negative charge (H). The arrows in the simulated vector contours present the microscopic polarization directions, and the color scales show the angular distribution of lateral polarization. (I) STIM amplitude contrast map superimposed in 3D surface topology, indicating the nonuniform accumulation of mobile ionic charges. All the PFM images and vector maps are of the same size. All the PFM images (A to D) share the same scale and color bars.

  • Fig. 4 Electric switching behaviors and retention properties for an array of center domains.

    (A to D) The topology and PFM images for a nanodot array illustrating the reversible switching of center domains triggered by applying pulsed electric fields on individual dots, including the topological image (A) and the PFM vertical (top) and lateral (bottom) phase images at the initial state (B), after the first set of electric pulses (C), and after the second set of electric pulses (D). (E to H) PFM images showing the electric switching of center domains by scanning electric bias and the retention properties for the switched center domains, for the initial state (E), the just switching state (F), after a retention duration of 6000 min (G), and after an elongated retention duration of 24,000 min (H). The circles in (G) and (H) indicate the contrast changes in certain nanodots during the retention test. The insets below the panels illustrate the characteristic contrasts in the L-pha. and V-pha. images for center-convergent (left) and center-divergent (right) domains, respectively. PFM images (A) to (E) share the same scale and color bars, and PFM images (F) to (H) share the same scale and color bars.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/8/e1700919/DC1

    Nanoporous AAO membrane mask fabrication

    Scanning thermo-ionic microscopy

    Phase-field simulation details and parameters

    table S1. The coefficients used in phase-field simulations.

    fig. S1. Schematic diagrams for different 1D topologically polar domain.

    fig. S2. Schematic procedures for the fabrication and PFM characterization of BFO nanodot array sample.

    fig. S3. PFM images for the nanodot array after poling by scanning bias voltages of ±8 V.

    fig. S4. 3D PFM images for a nanodot array.

    fig. S5. Single-dot PFM images for four typical topologic domains.

    fig. S6. A comparison of domain structures between an oxygen-deficient BFO film and a less oxygen-deficient film.

    fig. S7. STIM images of a nanodot array in this work.

    fig. S8. Surface potential maps and charge distribution states for different center domains.

    fig. S9. The domain structures for nanodots with a relatively larger lateral size (~300 nm) derived from three different methods.

  • Supplementary Materials

    This PDF file includes:

    • Nanoporous AAO membrane mask fabrication
    • Scanning thermo-ionic microscopy
    • Phase-field simulation details and parameters
    • table S1. The coefficients used in phase-field simulations.
    • fig. S1. Schematic diagrams for different 1D topologically polar domain.
    • fig. S2. Schematic procedures for the fabrication and PFM characterization of BFO nanodot array sample.
    • fig. S3. PFM images for the nanodot array after poling by scanning bias voltages of ±8 V.
    • fig. S4. 3D PFM images for a nanodot array.
    • fig. S5. Single-dot PFM images for four typical topologic domains.
    • fig. S6. A comparison of domain structures between an oxygen-deficient BFO film and a less oxygen-deficient film.
    • fig. S7. STIM images of a nanodot array in this work.
    • fig. S8. Surface potential maps and charge distribution states for different center domains.
    • fig. S9. The domain structures for nanodots with a relatively larger lateral size (~300 nm) derived from three different methods.

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