Research ArticleAPPLIED PHYSICS

Harnessing the topotactic transition in oxide heterostructures for fast and high-efficiency electrochromic applications

See allHide authors and affiliations

Science Advances  09 Oct 2020:
Vol. 6, no. 41, eabb8553
DOI: 10.1126/sciadv.abb8553
  • Fig. 1 Electrochromic process of a CFO thin film.

    (A) Schematic of an electrochromic CFO device. (B) Real-time optical images of a CFO channel during an electroforming process at a bias voltage of 75 V at selected elapsed times. (C) Real-time current monitoring during electroforming processes. Green marks indicate the forming time of each process. (D) Temperature dependences of the forming time and ionic mobility. Error bars were estimated to be one-fifth of the full-width half-maximum of the time derivative of the monitor current. (E) Oxygen 1s binding energy spectra of both BM-CFO and P-CFO, normalized by the integrated intensity of Ca 2p spectra. Each spectrum is deconvoluted into two (main and shoulder) peaks. (F) Oxygen stoichiometry of the formed state is estimated by comparing the spectral areas of the main peaks, on the assumption that the oxygen stoichiometry of BM-CFO is 2.5.

  • Fig. 2 Electronic conduction change and emergence of charge disproportionation.

    (A) Microscopic image of four-probe measurement electrodes (green) post-deposited on an electroformed P-CFO (dark yellow) between two forming electrodes (pink). (B) Electrical resistivity of BM-CFO and P-CFO. Dashed vertical line indicates a metal-insulator transition. (C) Energy-dependent integrated intensity of the (1/2 1/2 1/2) peak of P-CFO near Fe K-edge. The integrated intensity was obtained through areal integration of ω rocking curve. (D) Fluorescence near Fe K-edge at the (1/2 1/2 1/2) reflection. a.u., arbitrary unit. (E) Temperature-dependent integrated intensity of the (1/2 1/2 1/2) peak at 7.128 keV for P-CFO, rescaled by the value at the lowest temperature. The intensity was normalized by the integrated intensity of (0 1/2 2) at 7.128 keV. The inset graphs represent ω-rocking curves of (1/2 1/2 1/2) at 10, 200, and 300 K. Dashed line indicates the charge disproportionation transition.

  • Fig. 3 Transformation of crystal structure in electroforming process.

    (A) X-ray θ-2θ scans for BM-CFO and P-CFO films on LAO substrate. (B) Orthorhombic unit cell of BM-CFO phase and the relationship with the pseudocubic unit cells of a film on LAO substrate. (C) Reciprocal line scan along an in-plane H direction at the BM-CFO (002) position. The reciprocal lattice unit (r.l.u.) is normalized by that of LAO. (D) Reciprocal space maps for (002) and (E) (103) reflections of a BM-CFO thin film. (F) Reciprocal line scan along the H direction at the P-CFO (002) position. (G) Reciprocal space maps for (002) and (H) (103) reflections of a P-CFO thin film. (I) Schematic of the oxygen vacancy arrangements for mosaic BM-CFO domains with pseudomonoclinic (M) cells. Pseudocubic lattice constants of the BM-CFO are measured to be ap = 3.914 Å, bp = 3.675 Å, cp = 3.883 Å, α = γ = 90°, and β = 88.76°. The transformed tetragonal (T) P-CFO phase has lattice constants of ap = bp = 3.789 Å, cp = 3.768 Å, α = β = γ = 90°.

  • Fig. 4 Electroforming effects on BM-CFO films on various substrates.

    (A) The surface topography of a 100-nm-thick BM-CFO film on LAO shows nanodomains of ~50 nm in lateral size. The surface roughness (SD of surface height) is evaluated to be 1.62 nm. (B) Schematics of hypothetical oxygen vacancy migration steps through the mosaic structure. The yellow dotted boxes represent mosaic domains, and the domain walls appear between the domains. Oxygen vacancies (green balls) rapidly move through the wall regions, and they are also supplied from the domain regions through relatively slow diffusion. (C) Surface topographic images of as-grown BM-CFO films on various substrates [from left to right, STO(001), LSAT(001), and LSAO(001)]. (D) Initial (t = 0 min) and final (t = 120 min) microscopic still images of BM-CFOs during the electroforming processes.

  • Fig. 5 The measured dielectric function and absorption coefficient spectra of CFO thin films on LAO substrate.

    (A) Real and imaginary dielectric function spectra of BM-CFO phase. The two main peaks at 3.2 eV (βE) and 4.2 eV (γE) in the imaginary part are attributed to O 2p to Fe 3d optical transition. The relatively weak peak at 1.8 eV is likely caused by defect states. (B) Real and imaginary dielectric function spectra of P-CFO phase. The broad peak at 2.3 eV (ζE) is due to the interband O 2p to Fe 3d transition. (C) Absorption coefficient spectra of BM-CFO and P-CFO phases, calculated from the dielectric functions. Two vertical dashed lines guide the visible light region. (D) Absorption coefficient change and coloration efficiency for the topotactic transition from BM-CFO to P-CFO.

  • Fig. 6 Calculated electronic structure of BM-CFO and P-CFO.

    (A) Site- and orbital-decomposed DOS and band structure of AFM BM-CFO. (B) Schematic electronic structure of BM-CFO. βE and γE indicate the optical transitions in its imaginary part of dielectric function spectra. (C) Site- and orbital-decomposed DOS and band structure of P-CFO. (D) Schematic electronic structure of P-CFO. ζE represents the optical transition in its imaginary part of dielectric function spectra.

Supplementary Materials

  • Supplementary Materials

    Harnessing the topotactic transition in oxide heterostructures for fast and high-efficiency electrochromic applications

    Ji Soo Lim, Jounghee Lee, Byeoung Ju Lee, Yong-Jin Kim, Heung-Sik Park, Jeonghun Suh, Ho-Hyun Nahm, Sang-Woo Kim, Byeong-Gwan Cho, Tae Yeong Koo, Eunjip Choi, Yong-Hyun Kim, Chan-Ho Yang

    Download Supplement

    This PDF file includes:

    • Sections S1 to S12
    • Figs. S1 to S12
    • References

    Files in this Data Supplement:

Stay Connected to Science Advances

Navigate This Article