Abstract
The emergence of a domain wall property that is forbidden by symmetry in bulk can offer unforeseen opportunities for nanoscale low-dimensional functionalities in ferroic materials. Here, we report that the piezoelectric response is greatly enhanced in the ferroelastic domain walls of centrosymmetric tungsten trioxide thin films due to a large strain gradient of 106 m−1, which exists over a rather wide width (~20 nm) of the wall. The interrelationship between the strain gradient, electric polarity, and the electromechanical property is scrutinized by detecting of the lattice distortion using atomic scale strain analysis, and also by detecting the depolarized electric field using differential phase contrast technique. We further demonstrate that the domain walls can be manipulated and aligned in specific directions deterministically using a scanning tip, which produces a surficial strain gradient. Our findings provide the comprehensive observation of a flexopiezoelectric phenomenon that is artificially controlled by externally induced strain gradients.
Highlights
The emergence of a domain wall property that is forbidden by symmetry in bulk can offer unforeseen opportunities for nanoscale low-dimensional functionalities in ferroic materials
The ferroelastic twin structure is apparently seen in a crosssectional weak-beam dark-field transmission electron microscopy (TEM) image taken along the zone axes of [001]YAO and 1⁄2110YAO (Fig. 1b–d)
TEM images of the [001]YAO zone axis (Fig. 1b, c and Supplementary Fig. 1 in Supplementary Note 1) show that the fine-domain walls appear in the regions of the B domains, but not in the A domains due to the orthogonal relation of the finedomain walls
Summary
The emergence of a domain wall property that is forbidden by symmetry in bulk can offer unforeseen opportunities for nanoscale low-dimensional functionalities in ferroic materials. Nanoscale materials and devices can tolerate strain gradients as large as 106 m−1, and this can lead to a substantial amount of flexoelectric polarization which may be comparable to or even larger than conventional ferroelectric polarizations[5,6,7] Under these conditions, the walls or boundaries between two crystallographically different domains[8,9] can undergo a large amount of lattice deformation and/or structural reconstruction. To visualize the piezoresponse at the ferroelastic domain walls with large strain gradients, the so-called flexopiezoelectricity, we choose epitaxially grown tungsten trioxide (WO3)[25,26], an extensively studied ferroelastic material that shows periodically ordered domain walls in a herringbone structure, where the intervals between the domain walls can be scaled by film thickness[25] This binary oxide looks like an A-site vacant perovskite structure, and WO3 is elastically flexible, as supported by experimental and theoretical results. The domain walls in WO3 provide an ideal platform to explore the possibility of flexopiezoelectricity
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