Strain-driven autonomous control of cation distribution for artificial ferroelectrics

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Science Advances  28 Apr 2021:
Vol. 7, no. 18, eabd7394
DOI: 10.1126/sciadv.abd7394
  • Fig. 1 A schematic diagram of the developed strain synthesis of composite oxide heterostructures.

    During the epitaxial growth of host materials (BiT) with a large c lattice constant, another material (BFO) with a smaller unit cell is incorporated in situ, resulting in a BiTF composite system. There are four octahedral layers with Ti (blue) and Fe (red) ions between two BiO2 layers. In bulk, there is no way to control the local distribution of Ti and Fe ions among four octahedral layers. However, strain in thin film can work as nanorobot arms in that Fe ions preferentially locate at inner (outer) octahedral layers under tensile (compressive) strain to reduce the total energy of the system.

  • Fig. 2 Structural characterization of BiTF thin films grown on various substrates.

    (A) X-ray diffraction θ-2θ scans of BiTF composite films with the different fraction of BFO blocks. The θ-2θ scans show the structural evolution from BiT with three octahedral layers to BiTF with four octahedral layers as BFO blocks are inserted. The asterisk indicates the 001 peak from the STO substrate. arb. units, arbitrary units. (B) HAADF images of BiT (left) and BiTF (right) composite films. While gray dashed lines are three octahedral layers already existing in the BiT film, the red dashed line shows an additional octahedral layer in the BiTF film. It indicates complete insertion of a BFO perovskite block into BiT structures. (C) Reciprocal space maps of strained BiTF films grown on four different substrates. Black dashed lines highlight the substrate (103) qx.

  • Fig. 3 Strain-dependent Fe distribution in BiTF films.

    Atomically resolved STEM-EDX mapping of BiTF grown on (A) LAO (−0.9%), (B) STO (1.3%), and (C) DSO (1.8%) substrates. The leftmost column exhibits schematic diagrams of local Fe distribution in BiTF. The middle column exhibits HAADF, element-selective EDX, and overlaid EDX images. The Fe K-edge mapping shows that Fe ions are preferentially located at outer (inner) octahedral layer in BiTF/LAO (DSO) and randomly distributed in BiTF/STO. The rightmost column is line profiles of each element along the white arrows in EDX maps.

  • Fig. 4 Bandgap reduction and unexpected out-of-plane ferroelectric polarization in BiTF films.

    (A to D) σ1(ω) of BiT (black) and BiTF (red) films on each substrate. The observed reduction of the bandgap by inserting BFO blocks implies that charge transfer energy between Fe 3d and O 2p orbitals is smaller than that between Ti 3d and O 2p orbitals.

  • Fig. 5 Strain-dependent in-plane and out-of-plane ferroelectric polarizations in BiTF films.

    (A to D) Lateral cKPFM measured along the orthorhombic [100] direction after application of different voltage pulses, as a function of read voltage. Clear hysteresis behaviors are observed in the films on LSAT and STO substrates, while ferroelectricity is unclear and strongly suppressed in the films on LAO and DSO. This result implies that the randomness of Fe ion position plays a role in stabilizing the ferroelectricity. (E to H) Vertical cKPFM curves of BiTF films on each substrate. Only the film on STO shows clear out-of-plane ferroelectric hysteresis behaviors, which are forbidden by symmetry in bulk. We attribute this unexpected polarization to extrinsic asymmetry of cationic distribution signified with intrinsic random preference by moderate tensile strain.

Supplementary Materials

  • Supplementary Materials

    Strain-driven autonomous control of cation distribution for artificial ferroelectrics

    Changhee Sohn, Xiang Gao, Rama K. Vasudevan, Sabine M. Neumayer, Nina Balke, Jong Mok Ok, Dongkyu Lee, Elizabeth Skoropata, Hu Young Jeong, Young-Min Kim, Ho Nyung Lee

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