Abstract
Vertically aligned nanocomposite (VAN) films, comprising nanopillars of one phase embedded in a matrix of another, have shown great promise for a range of applications due to their high interfacial areas oriented perpendicular to the substrate. In particular, oxide VANs show enhanced oxide-ion conductivity in directions that are orthogonal to those found in more conventional thin-film heterostructures; however, the structure of the interfaces and its influence on conductivity remain unclear. In this work, 17O NMR spectroscopy is used to study CeO2–SrTiO3 VAN thin films: selective isotopic enrichment is combined with a lift-off technique to remove the substrate, facilitating detection of the 17O NMR signal from single atomic layer interfaces. By performing the isotopic enrichment at variable temperatures, the superior oxide-ion conductivity of the VAN films compared to the bulk materials is shown to arise from enhanced oxygen mobility at this interface; oxygen motion at the interface is further identified from 17O relaxometry experiments. The structure of this interface is solved by calculating the NMR parameters using density functional theory combined with random structure searching, allowing the chemistry underpinning the enhanced oxide-ion transport to be proposed. Finally, a comparison is made with 1% Gd-doped CeO2–SrTiO3 VAN films, for which greater NMR signal can be obtained due to paramagnetic relaxation enhancement, while the relative oxide-ion conductivities of the phases remain similar. These results highlight the information that can be obtained on interfacial structure and dynamics with solid-state NMR spectroscopy, in this and other nanostructured systems, our methodology being generally applicable to overcome sensitivity limitations in thin-film studies.
Highlights
Aligned nanocomposite (VAN) films, comprising nanopillars of one phase embedded in a matrix of another, have shown great promise for a range of applications due to their high interfacial areas oriented perpendicular to the substrate
Oxide heterostructures combine two or more different phases, the interfaces of which induce novel or enhanced functional properties due to their unique local environments.[1−4] In particular, vertically aligned nanocomposite (VAN) films, comprising nanopillars of one phase embedded in a matrix of another, have shown great promise for applications as high-temperature superconductors,[5−8] ferroelectrics,[9−12] multiferroics,[13] data storage media,[14,15] and electronic/ionic conductors.[16−18] Unlike conventional planar multilayered heterostructures, the interfaces in VAN films are perpendicular to the substrate, resulting in significantly higher interface-to-volume ratios, more uniform strain, and control over the orthogonal transport properties,[4,19] leading to their potential use in, for example, micron-sized fuel cells
The inclusion of these minor signals in the deconvolution is based on the environments predicted by density functional theory (DFT) calculations, vide infra, but was found to improve the fits
Summary
Aligned nanocomposite (VAN) films, comprising nanopillars of one phase embedded in a matrix of another, have shown great promise for a range of applications due to their high interfacial areas oriented perpendicular to the substrate. A comparison is made with 1% Gd-doped CeO2−SrTiO3 VAN films, for which greater NMR signal can be obtained due to paramagnetic relaxation enhancement, while the relative oxide-ion conductivities of the phases remain similar These results highlight the information that can be obtained on interfacial structure and dynamics with solid-state NMR spectroscopy, in this and other nanostructured systems, our methodology being generally applicable to overcome sensitivity limitations in thin-film studies. Solid-state NMR is a powerful technique to study local structure and dynamics, and for oxide materials, 17O NMR in particular is extremely sensitive to local distortions caused by, e.g., surfaces,[31−34] substituents,[35−37] and other defects.[38−40]
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