Probing the interplay of atomic structure and properties in perovskite oxide thin films

Abstract text (incl. figure legends and references)

Perovskite-structured oxide thin films provide a fascinating playground for the study of condensed matter physics, owing to the strong coupling of lattice composition and structural distortions to functional properties. Here, we illustrate how recent developments in electron microscopy are vital for study of their structure–property relationships.

Measurements are made in scanning transmission electron microscopy (STEM) mode using a double aberration-corrected FEI Titan Themis 60-300 at 200/300 kV. For measuring atomic structure, high angle annular dark field (HAADF) and annular dark field (ABF) 90° rotation image series are recorded, and then corrected for linear and non-linear scan distortions. Electron energy-loss spectroscopy (EELS) is carried out using a Gatan GIF Quantum ERS. Samples are prepared via mechanical polishing and broad ion beam milling, or by focused ion beam and lift out, and cleaned with an IBSS MCA.

First, we look at NdNiO₃/SmNiO₃ superlattices, where macroscopic measurements of a metal-to-insulator transition (MIT) show an unexpected transition from two electronic phases to a single one as layer thickness decreases below a critical value. Using monochromated EELS to measure subtle changes in the Ni L₃ and O K ionization edges, we not only confirm this behavior, but also map the electronic phases with a sub-nm spatial resolution [1]. STEM structure measurements show, however, that each layer retains the composition and lattice distortions of its parent lattice; a result that helps motivate a new Landau theory of electronic phase boundary energy [2]. To further study this behavior, we test decoupling the layers by incorporating LaAlO₃ interlayers into the superlattices. Here, atomic resolution STEM is essential for diagnosing deposition problems and for measuring the coupling of structural distortions across interfaces; see Fig. 1.

Lattice distortion connections next come to the fore in the study of LaVO₃ films. STEM helps show how epitaxial strain plays a key role in determining the film's orientation, giving an out-of-plane orthorhombic long axis under the tensile strain set by a (110) DyScO₃ substrate [3]. Since the DyScO₃ instead has an orthorhombic long axis that is in-plane, such a film inherently forces an interface that couples the incommensurate oxygen octahedra rotations of the two orthorhombic orientations. Remarkably, STEM reveals this rotation-coupled interface forms within film, rather than at the substrate/film interface, creating a sharp 90° domain boundary within a single compound. Justified by second principles energetic simulations, this finding promises new avenues for creating novel functional properties [4].

In summary, we illustrate myriad ways that aberration-corrected STEM is critical for uncovering the interplay between atomic structure and physical properties in functional thin films. Looking towards the future, in order to achieve further insights, our aim is to couple these precision measurements with variable sample conditions.

[1] B. Mundet et al., Nano Lett. 21, 2437 (2021).

[2] C. Domínguez et al., Nat. Mater. 19, 1182 (2020).

[3] H. Meley et al., APL Mater. 6, 046102 (2018).

[4] D.T.L. Alexander et al., under review.

Fig. 1. (a) HAADF STEM image of a NdNiO₃/SmNiO₃ superlattice with LaAlO₃ interlayers. (b) depth profile of average Δy displacements of A-site cations from their "ideal" cubic positions (right) shows how each layer recovers the distortions of its parent compound.