Research ArticleMATERIALS SCIENCE

Giant tuning of ferroelectricity in single crystals by thickness engineering

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Science Advances  14 Oct 2020:
Vol. 6, no. 42, eabc7156
DOI: 10.1126/sciadv.abc7156
  • Fig. 1 In situ TEM setup and domain configurations and the thickness dependence of ferroelectric/ferroelastic domain switching under electric biasing.

    (A) Schematic diagram of the experimental setup. Samples marked “1,” “2,” and “3” were of thicknesses of 170, 50, and 50 nm, respectively. A dark-field TEM image of the three samples represents their initial domain configurations. Head-to-tail tetragonal domains with their polarization directions are marked using colored vectors. (B) A STEM-HAADF image showing a typical 90° ferroelastic domain. Scale bar, 2 nm. (C) STEM-HAADF image overlaid with strain color map (strain tensor εxx, the magnitude ranges from −1 to 1%) determined by geometric phase analysis. The εxx component represents the field strains for in-plane direction. Scale bar, 2 nm. (D) A series of images showing the evolution of ferroelectric/ferroelastic domains in the thick sample (sample 1) with increasing bias and after the bias was withdrawn. Colored arrows represent local polarization directions. The switched domain areas are marked by brown/yellow colors, while the newly formed domain boundaries are marked by dashed lines. Diffraction patterns were taken for sample 1 at 0 and +10 V to confirm the domain orientation (see section S2 for detailed information). Scale bar, 500 nm. (E) Schematic diagrams of the domain evolution behavior in (D). The domain evolution exhibited the crossing mode. (F) A series of images showing the evolution of ferroelectric/ferroelastic domain switching behavior in a thin sample (sample 2) with increasing bias and after the bias was withdrawn. The switched domain areas are marked by yellow/purple colors, while the newly formed domain boundaries are marked by dashed lines. The domain switching directions are marked by dashed arrows in different colors. Scale bar, 500 nm. (G) Schematic drawing of the domain evolution behavior in (F). The domain evolution exhibits the directional mode.

  • Fig. 2 Role of the tip of a domain wall in domain switching in a thin sample.

    (A) A series of images (extracted from movie S3) showing the switching behavior of ferroelectric/ferroelastic domains in a thin sample. The switched domain areas are marked by green/brown colors, while the newly formed domain boundaries are marked by dashed lines. High-resolution HAADF-STEM images were taken for sample 3 before and after biasing to confirm the domain orientation (see section S4 and Materials and Methods for details). Scale bar, 500 nm. (B) Schematic drawing of the domain evolution behavior in (A). Domain evolution exhibits a mixture of the crossing and directional modes due to the existence of domain walls and domain wall tips. (C) A STEM-ABF image showing a domain tip area (scale bar, 20 nm). (D and E) Two STEM-HAADF images overlaid with strain color map (strain tensor εxx) showing the strain areas around the domain tip. Scale bars, 2 nm.

  • Fig. 3 Origin of the thickness-dependent effect.

    (A) A high-resolution STEM-HAADF image showing that a randomly polarized region with a thickness of ~30 atomic layers existed in the sample surface layer. The interfacial region of random/aligned polarization is enlarged on the right. (B) A high-resolution STEM-ABF image showing lattice distortion in the sample surface region. The original tetragonal lattice was distorted into an irregular quadrilateral. (C) A STEM-HAADF image overlaid with a strain color map (strain tensor εxx) showing a huge strain field in the ferroelectric surface layer. (D) Schematic drawing showing the effect of strain field on a thick and thin sample. Strain fields (marked with red color) exist in the random/aligned polarization interface and the corner region where ferroelastic domain walls interact with the sample surface. It is obvious that the thick sample is affected less by the surface strain field.

  • Fig. 4 Phase-field simulation.

    (A) Schematic illustration of the difference between the crossing mode and directional mode. The 90° and 180° domain walls are marked with orange solid ellipse and dashed ellipse, respectively. (B and C) Phase-field simulations of a thick sample (strain free) and a thin sample (strained). Domain merging into a single domain is found in the strain-free (thick) sample, while domain reversal (switching) along the domain wall direction occurs in the strained (thin) sample.

Supplementary Materials

  • Supplementary Materials

    Giant tuning of ferroelectricity in single crystals by thickness engineering

    Zibin Chen, Fei Li, Qianwei Huang, Fei Liu, Feifei Wang, Simon P. Ringer, Haosu Luo, Shujun Zhang, Long-Qing Chen, Xiaozhou Liao

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    • Sections S1 to S10
    • Figs. S1 to S10

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