Intrinsic flexoelectricity of van der Waals epitaxial thin films

Abstract

The direct measurement of flexoelectric coefficients in epitaxial thin films is an unresolved problem, due to the clamping effect of substrates which induces a net strain (and hence parasitic piezoelectricity) in addition to strain gradients and flexoelectricity. Herein, we propose and demonstrate the use of van der Waals epitaxy as a successful strategy for measuring the intrinsic (clamping-free = flexoelectric coefficients of epitaxial thin films. We have made, measured, and compared BaTiO 3 and SrTiO 3 thin film capacitor heterostructures grown both by conventional oxide-on-oxide epitaxy and by van der Waals oxide-on-mica epitaxy, and found that, whereas the former is dominated by parasitic piezoelectricity, the response of the latter is truly flexoelectric. The results are backed by theoretical calculations of the film-substrate mechanical interaction, as well as by direct measurements that confirm the strain-free state of the films. van der Waals epitaxy thus emerges as powerful new tool in the study of flexoelectricity and, in particular, they finally allow exploring flexoelectric phenomena at the nanoscale (where strain gradients are highest) with direct experimental knowledge of the actual flexoelectric coefficients of thin films.

Domain Matching Strategy for Orientation Control of van der Waals Epitaxial Perovskite Thin Films

“…In conventional bond-to-bond domain matching epitaxy, the misfit dislocation present in each domain represents the difference between the m and n parameters. However, in vdW epitaxy, the epilayer grows with its bulk lattice constant despite the large lattice mismatch and exhibits a fully relaxed strain state. − Therefore, vdW epitaxy is an incommensurate epitaxy that presents incoherent interfaces without any misfit dislocation. Nevertheless, the crystal axes of the grown film and the substrate align well to each other, indicating that the film and the substrate are rotationally commensurate in vdW epitaxy .…”

“…This alternative low-energy material–substrate connection approach does not involve any dangling bond at the substrate surface and therefore does not rely on chemical processing or electron transfer/sharing between ions or molecules participating in the bond. The initial vdW technique has quickly been extended to the heteroepitaxial growth of 3D materials onto 2D layered substrates or vice versa, which is called quasi-vdW epitaxy. − Thin films (3D) grown by quasi-vdW epitaxy are also expected to be completely strain-free due to the weak interaction regardless of the lattice mismatch.…”

Advancements and Prospects of Flexoelectricity

“…Recently, Shu et al proposed a direct measurement method for the flexoelectric coefficient of nanoscale dielectric thin films that can eliminate the influence of piezoelectric effects. 41 By preparing a BaTiO 3 (BTO)/mica van der Waals epitaxial heterostructure, the use of the van der Waals epitaxy technique on a two-dimensional layered substrate, mica, eliminates the substrate clamping effect on the thin film. This method avoids the generation of net strain during the bending process, thus excluding the influence of the piezoelectric effect on the measurement of the flexoelectric coefficient.…”

“…However, for nanoscale dimensions with significant flexoelectric effects, measuring the flexoelectric coefficient poses challenges, as obtaining induced currents generated during the measurement process is difficult. Recently, Shu et al proposed a direct measurement method for the flexoelectric coefficient of nanoscale dielectric thin films that can eliminate the influence of piezoelectric effects . By preparing a BaTiO 3 (BTO)/mica van der Waals epitaxial heterostructure, the use of the van der Waals epitaxy technique on a two-dimensional layered substrate, mica, eliminates the substrate clamping effect on the thin film.…”