The determination of the atomic-scale structure
of a solid–solid interface is a major outstanding problem in the physical
sciences, the structure controlling many properties including stability, ionic
and electronic transport, magnetism, multiferroicity and superconductivity. NMR
spectroscopy is sensitive to local structure but is not typically sufficiently
sensitive or selective to observe solid–solid interfaces. In this work, CeO<sub>2</sub>–SrTiO<sub>3</sub>
vertically aligned nanocomposite (VAN) thin films are studied and, by combining
selective isotopic enrichment with a lift-off technique to remove the substrate,
the <sup>17</sup>O NMR signal from single atomic layer interfaces can clearly
be seen. The interfacial structure is solved by calculating the NMR parameters
using density functional theory combined with random structure searching. 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 in part from enhanced oxygen mobility at this interface; oxygen motion
at the interface is further identified from <sup>17</sup>O relaxometry experiments.
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.
17O solid-state NMR spectroscopy of A2B2O7 oxides: quantitative isotopic enrichment and spectral acquisition?