Research ArticleMATERIALS SCIENCE

Room temperature in-plane ferroelectricity in van der Waals In2Se3

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Science Advances  13 Jul 2018:
Vol. 4, no. 7, eaar7720
DOI: 10.1126/sciadv.aar7720
  • Fig. 1 Linear dichroism of In2Se3.

    (A) Crystal structure of layered β-In2Se3. (B) Schematic of the linear polarization optical microscopy measurement. (C) Optical image sequence of β′-In2Se3 imaged by light of different polarization angles. Scale bar, 25 μm. (D) Polarization angle dependence of the light transmission of the regions A and B as shown in (C). a.u., arbitrary units. (E) Schematic of the optical axes of regions A and B.

  • Fig. 2 Low LEEM measurement.

    (A) Bright-field LEEM image of β′-In2Se3 surface taken by a tilted electron beam at 9.9 eV. Scale bar, 1.5 μm. (B) LEED patterns of the three domains. (C) Intensity profile of the subdiffraction spots between the (−1,0) and (0,−1) spots.

  • Fig. 3 Curie temperature of β′-In2Se3.

    (A) LEEM image of a long-stripe domain in β′-In2Se3 at RT. (B) Shrinking and disappearance of domain as temperature increases and recovery of domain during cooling. (C) LEED pattern of β′-In2Se3 at 190°C. (D) LEED pattern of β-In2Se3 at 204°C. (E) Width of the domain as a function of temperature during heating and cooling. Scale bars, 1 μm (A and B).

  • Fig. 4 PFM measurements.

    (A) AFM topography image of β′-In2Se3. Inset: Optical image of the cantilever and the exfoliated crystal. The horizontal scanning direction is indicated by the black double-headed arrow. (B and C) PFM amplitude and phase of vertical signal. (D and E) PFM amplitude and phase of lateral signal. (F) Schematic of the example optical axis (white double-headed arrow) and ferroelectric polarization directions (white/black arrows) of the domains. Scale bars, 5 and 30 μm (inset).

  • Fig. 5 STM measurements.

    (A) Large-area STM image of β′-In2Se3 at 77 K. Scale bar, 10 nm. The bottom graph presents the height profile of 1D superstructures, and the inset indicates the fast Fourier transform pattern showing spots of the superstructures. (B) Zoomed-in STM image showing the atomic structure of unit cell and the 1D superstructures due to height modulation. Scale bar, 1 nm. (C) Atomic structure of 1D superstructures taken from another region. The bottom graph shows the height profile taken from the blue line. Scale bar, 1 nm.

  • Fig. 6 DFT calculation.

    The top view (top) and the side view (bottom) of the β phase before and after relaxation, respectively. Ferroelectricity exists in a crystal structure relaxed from the β phase. The Se atoms in the middle of the five-atom layer shift along one of the threefold symmetry directions.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/7/eaar7720/DC1

    Fig. S1. X-ray diffraction spectrum of In2Se3 flakes.

    Fig. S2. Air stability of In2Se3 and domains.

    Fig. S3. Image contrast of domains under a tilted electron beam in LEEM.

    Fig. S4. The control of electron beam tilt in LEEM.

    Fig. S5. Tilt angle–dependent domain contrast.

    Fig. S6. Proposed ferroelectric polarizations.

    Fig. S7. TEM measurements.

    Fig. S8. Ferroelectric polarization calculated using Berry phase method.

    References (4047)

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. X-ray diffraction spectrum of In2Se3 flakes.
    • Fig. S2. Air stability of In2Se3 and domains.
    • Fig. S3. Image contrast of domains under a tilted electron beam in LEEM.
    • Fig. S4. The control of electron beam tilt in LEEM.
    • Fig. S5. Tilt angle–dependent domain contrast.
    • Fig. S6. Proposed ferroelectric polarizations.
    • Fig. S7. TEM measurements.
    • Fig. S8. Ferroelectric polarization calculated using Berry phase method.
    • References (4047)

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