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Colossal Ionic Conductivity at Interfaces of Epitaxial ZrO2:Y2O3/SrTiO3 Heterostructures

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Science  01 Aug 2008:
Vol. 321, Issue 5889, pp. 676-680
DOI: 10.1126/science.1156393

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  1. Fig. 1.

    (A) Z-contrast scanning transmission electron microscopy (STEM) image of the STO/YSZ interface of the [YSZ1nm/STO10nm]9 superlattice (with nine repeats), obtained in the VG Microscopes HB603U microscope. A yellow arrow marks the position of the YSZ layer. (Inset) Low-magnification image obtained in the VG Microscopes HB501UX column. In both cases a white arrow indicates the growth direction. (B) EEL spectra showing the O K edge obtained from the STO unit cell at the interface plane (red circles) and 4.5 nm into the STO layer (black squares). (Inset) Ti L2,3 edges for the same positions, same color code. All spectra are the result of averaging four individual spectra at these positions, with an acquisition time of 3 s each.

  2. Fig. 2.

    Real part of the lateral electrical conductivity versus frequency of the trilayer with 1-nm-thick YSZ in a double log plot. Isotherms were measured in the range of 357 to 531 K. The solid line represents a NCL contribution (σ′ ∼ Aω, where A is a temperature-dependent proportionality factor and ω is the angular frequency), as explained in the text. Stars identify the value of σdc. The uncertainty of conductance measurements is 1 nS (10–2 S/cm in conductivity for the sample shown, see error bar). (Inset) Imaginary versus real part of the impedance (Nyquist) plots at 492, 511, and 531 K. Whereas the high-frequency contribution is a Debye-like process characterized by a conductivity exponent n = 0, the “grain boundary” term observed in the Nyquist plots shows a clear deviation from a Debye behavior, as reflected by the distorted impedance arcs.

  3. Fig. 3.

    Dependence of the logarithm of the long-range ionic conductivity of the trilayers STO/YSZ/STO versus inverse temperature. The thickness range of the YSZ layer is 1 to 62 nm. Also included are the data of a single crystal (sc) of YSZ and a thin film (tf) 700 nm thick [taken from (7)] with the same nominal composition. (Top inset) 400 K conductance of [YSZ1nm/STO10nm](ni/2) superlattices as a function of the number of interfaces, ni. (Bottom inset) Dependence of the conductance of [STO10nm/YSZXnm/STO10nm] trilayers at 500 K on YSZ layer thickness. Error bars are according to a 1 nS uncertainty of the conductance measurement.

  4. Fig. 4.

    (A) EELS chemical maps. The ADF image in the upper panel shows the area used for EELS mapping (spectrum image, marked with a green rectangle) in the [YSZ1nm/STO10nm]9 superlattice. The middle panel shows the averaged ADF signal acquired simultaneously with the EEL spectrum image, showing the STO (low-intensity regions) and YSZ (higher-intensity) layers. The lower panel shows the Ti (red) and Sr (dark yellow) EELS line traces across several consecutive interfaces. These line traces are averaged from the elemental 2D images shown in the insets, each framed with the same color code (red for Ti, dark yellow for Sr). Data was obtained in the VG Microscopes HB501UX. Because the STEM specimen was relatively thick (several tens of nanometers), the wide chemical interface profiles are most likely attributable to beam broadening. (B) Solid spheres model of the YSZ/STO interface showing: (1) The compatibility of the perovskite and fluorite (rotated) structures. (2) A side view of the interface between STO (at the bottom) and YSZ (on top) with realistic ionic radius. The shaded oxygen positions in the interface plane are presumed absent or displaced because of volume constraints, enabling the high ionic conductivity. (3) A 3D view of the interface, with the ionic radius reduced by half to better visualize the plane of oxygen vacancies introduced in the interface. The square symbol in the legend indicates the empty positions available for oxygen ions at the interface.