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

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.

Figure 1

(a) STEM image of a 20 atom % Sm-doped-CeO₂–STO VAN film, showing the two dominant types of interface, reproduced in part with permission from ref (30), Copyright 2019 Zhu et al., AIP publishing (www.creativecommons.org/licenses/by/4.0/). (b, c) Schematics of the orientations of the STO and CeO₂ structures at the two types of interface. The elements are colored as follows: O: red, Ti: blue, Sr: green, and Ce: yellow.

Figure 2

(a) ¹⁷O NMR spectrum of CeO₂–STO nanopillar lift-off films enriched at 450 °C, recorded at 9.40 T and 50 kHz MAS using a Hahn echo pulse sequence with a single rotor period delay, a 0.1 s recycle delay, and 800 000 scans (22 h). The spectrum has been deconvoluted using Voigt functions. (b, c) DFT-calculated isotropic shifts as a function of distance from the interface for different predicted interfacial structures: (b) 0° interface, low-energy structures (A–I); (c) 45° interface, three structures—STO- and CeO₂-like interfaces of the simple model and a low-energy structure with an intermediate stoichiometry found from random structure searching (RSS).

Figure 3

Model structures for the STO–CeO₂ interfaces. (a) Model A for the 0° interface found from RSS. (b, c) STO- and CeO₂-like interfaces from the simple model for the 45° interface. (d) 45° interface found from RSS. The elements are colored as follows: O: red, Ti: blue, Sr: green, and Ce: yellow.

Figure 4

(a) ¹⁷O NMR spectra of CeO₂–STO lift-off films as a function of the ¹⁷O₂ enrichment temperature, recorded at 9.40 T and 50 kHz MAS using a Hahn echo sequence, a 1 s recycle delay and between 80 000 and 160 000 scans (1–2 days). The spectra in (b) have been rescaled to give the same intensity of the interface signal. The inset shows the ratio of the integrated CeO₂ intensity to the intensity of the interface and STO signals; the errors were estimated by fitting a peak to a region of noise.

Figure 5

¹⁷O T₂ decay curves for the CeO₂ and interface signals measured at two different temperatures, for CeO₂–STO lift-off films ¹⁷O-enriched at 350 °C. The integrated intensity was measured as a function of the total echo length, t, in a Hahn echo experiment performed at 9.40 T and 50 kHz MAS with a 0.1 s recycle delay.

Figure 6

¹⁷O NMR spectra of 1% Gd-doped CeO₂–STO lift-off films, recorded at 9.40 T and 50 kHz MAS using a Hahn echo sequence, a 0.1 s recycle delay and between 800,000 and 1,660,000 scans (1–2 days). (a) Deconvolution of the sample enriched at 450 °C; the signal marked with a † is discussed in the text. (b) Comparison of the same sample enriched at different temperatures. (c) The ratios of the deconvoluted integrated intensities for the CeO₂, STO and interface signals as a function of enrichment temperature. The errors are estimated by fitting a peak to a region of noise. (d) The spectrum of the sample enriched at 450 °C after unenriching in air at 250 °C, as well as a deconvolution of the difference spectrum.