Revealing the Structure and Oxygen Transport at Interfaces in Complex Oxide Heterostructures via ¹⁷O NMR Spectroscopy

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 structures of the interfaces and their influence on conductivity remain unclear, since buried solid–solid interfaces are extremely challenging to characterise.<br/>In this work, ¹⁷O NMR spectroscopy was used to study CeO₂−SrTiO₃ VAN thin films [1]: selective isotopic enrichment was 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 was shown to arise from enhanced oxygen mobility at this interface. This was confirmed by ¹⁷O relaxometry experiments, which demonstrated enhanced oxygen motion at the interface. The structure of this interface was 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 Gd-doped sample was prepared, for which the ¹⁷O NMR signal was selectively enhanced by harnessing the spin polarisation of the paramagnetic ion via dynamic nuclear polarisation [2]; this enabled a two-dimensional correlation experiment to be performed, showing spin-diffusion between Gd-CeO₂ and the solid–solid interface. 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.<br/>1. Hope, M. A.; Zhang, B.; Zhu, B.; Halat, D. M.; MacManus-Driscoll, J. L.; Grey, C. P. Chem. Mater. 2020. 32 (18), 7921–7931.<br/>2. Hope, M. A.; Björgvinsdóttir, S.; Halat, D. M.; Menzildjian, G.; Wang, Z.; Zhang, B.; MacManus-Driscoll, J. L.; Lesage, A.; Lelli, M.; Emsley, L.; Grey, C. P. Endogenous 17O Dynamic Nuclear Polarization of Gd-Doped CeO2 from 100 to 370 K. J. Phys. Chem. C 2021, 125 (34), 18799–18809.