Polarity and anti-distortive polarons in WO3 through epitaxial shear strain

TL;DR

This paper reports the discovery of a new polar phase in epitaxial WO3 thin films induced by shear strain, revealing enhanced conductivity and anti-distortive polarons, advancing the understanding of strain-tunable functionalities in oxide electronics.

Contribution

It demonstrates strain-induced polar phase stabilization in WO3 thin films and provides experimental evidence for anti-distortive polarons, a novel insight into oxide material behavior.

Findings

Epitaxial shear strain stabilizes a polar triclinic phase in WO3.

Stripe domain walls exhibit enhanced electrical conductivity.

Experimental evidence for anti-distortive polarons in WO3.

Abstract

Bestowing CMOS-compatible binary oxides with additional functionalities is a powerful strategy toward the realization of oxide electronics. Ideal candidates are thin films which display a strong sensitivity to strain, chemical doping or nanoscale confinement. Among these, crystalline tungsten trioxide WO3 exhibits exceptional structural flexibility, enabling a wide range of functionalities. Here, we reveal the emergence of a previously unreported polar phase in epitaxial WO3 thin films. We accomplish this by imposing epitaxial shear strain, which stabilizes a low-symmetry triclinic structure that persists up to large film thicknesses and elevated temperatures. At the atomic scale, a change in the oxygen octahedral tilt pattern facilitates this symmetry lowering into a polar phase, which manifests as a periodic in-plane polarized stripe domain configuration with needle-like bifurcations at the microscale. The stripe domain walls further exhibit a strongly enhanced electrical conductivity in conjunction with a pronounced reduction of a distortive structural mode, providing the first experimental evidence for the formation of anti-distortive polarons recently predicted in WO3.