Research ArticleAPPLIED SCIENCES AND ENGINEERING

CH3NH3PbI3 perovskites: Ferroelasticity revealed

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Science Advances  14 Apr 2017:
Vol. 3, no. 4, e1602165
DOI: 10.1126/sciadv.1602165
  • Fig. 1 Polarized light optical images and domain configurations in a MAPbI3 single crystal (sample C).

    (A to C) Reflection-mode polarized optical micrographs (crossed Nicols) showing contrast reversal of domain groups as a function of sample rotation relative to the polarizer-analyzer (P-A) pair. At 0° relative angle, the domain groups are indistinguishable, and, at 45° and −45°, the domain contrast is maximum; that is, θe = 90°. The red and black arrows indicate the analyzer orientation and polarizer orientation, respectively. The light intensity of a bright dust particle is insensitive to the sample rotation. Scale bars, 5 μm. (D) Permissible domains of the MAPbI3 tetrahedral phase. At the cubic-to-tetrahedral transition, the tetrahedral unit cell can form with the c axis aligned along any of the three axes (x, y, and z) of the parental cubic phase, leading to three possible configurations (X, Y, and Z). To preserve crystal integrity, the a axes of different domains must coincide (32), leading to six possible domain orientations that are characterized by domain walls intercepting at 0°, 90°, or 45° in the principal planes of the parental phase and at ≈70° and ≈110° in the (110) plane.

  • Fig. 2 Evolution of the domain structure in sample C under external stress.

    Domain area fraction (F) versus applied uniaxial stress graph and polarized light micrograph images corresponding to letter labels. Arrows indicate the sequence of the applied stress throughout the experiment. (A) to (H) Correspond to selected points on the graph. Application of tensile stress (positive) leads to shifts of domain boundaries and formation of new types of domains (tilted 70° and 109° to the old domains). A prominent nonlinearity is observed. Compressive stress eventually erases all domains and, after point G (where only one bright domain exists in the field of view), leads to fracturing (cracking) of the crystal and to a drastic change in the domain structure. Data points following cracking are marked in green. Uncertainty in the F values was estimated to be ≈0.03, taken as the largest of the uncertainties calculated from selected points in the graph (see Materials and Methods for details). Scale bars, 10 μm. The domain area fraction was calculated for a field of view larger than shown (530 μm × 710 μm). Micrographs were grayscaled. The graph insets show schematics of sample bending relative to the incident light (yellow cone). Stress in the graph is shown in arbitrary units (a.u.).

  • Fig. 3 Observation of ferroelastic domain pattern by PFM.

    (A and B) Topography and PFM amplitude images of sample B1 (fabricated by doctor blade–coating) showing different oriented ferroelastic domains. (C and D) High-resolution topography and PFM amplitude images of sample B1 showing that the application of a localized load by the AFM tip does not change the long-range characteristics of stripes.

  • Fig. 4 PFM observation of ferroelastic domain pattern modulated by external stress.

    Topography and PFM amplitude images of sample B2 (fabricated by doctor blade–coating) in pristine state (A and B), under tensile stress (C and D), and after relieving the stress (E and F). The blue arrow in (C) indicates the direction of the applied stress.

  • Fig. 5 Observation of ferroelastic domains by PTIR and their insensitivity to the applied electric field.

    (A) Schematic illustration of the PTIR measurement. An AFM cantilever measures the thermal expansion resulting from light absorption. (B) AFM topography image of the sample A2 area between electrodes and corresponding PTIR images of (C) CH3 asymmetric deformation of the methylammonium ion (1468 cm−1) and (D) electronic transition above the bandgap (13,250 cm−1 and 1.64 eV) of the as-prepared sample. (E) Representative electronic (left) and vibrational (right) absorption spectra obtained from contiguous bright (red dot) and dark (blue dot) striations visible in PTIR images. (F) Sample A2 AFM topography image and corresponding PTIR images at 1468 cm−1 (G) and 13,250 cm−1 (H) obtained after applying a bias of 0.86 V·μm−1 for 1 min (in plane electric field). Scale bars, 2 μm.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/4/e1602165/DC1

    fig. S1. PFM measurements on sample A1 do not show evidence of ferroelectric behavior.

    fig. S2. Representative PFM images showing strong amplitude contrast and very weak phase contrast.

    fig. S3. Observation of ferroelastic domains by PTIR.

    fig. S4. Observation of ferroelastic domains by PTIR with s- and p-polarization.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. PFM measurements on sample A1 do not show evidence of ferroelectric behavior.
    • fig. S2. Representative PFM images showing strong amplitude contrast and very weak phase contrast.
    • fig. S3. Observation of ferroelastic domains by PTIR.
    • fig. S4. Observation of ferroelastic domains by PTIR with s- and p-polarization.

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