Electrostatically Driven Polarization Flop and Strain‐Induced Curvature in Free‐Standing Ferroelectric Superlattices

Abstract

The combination of strain and electrostatic engineering in epitaxial heterostructures of ferroelectric oxides offers many possibilities for inducing new phases, complex polar topologies, and enhanced electrical properties. However, the dominant effect of substrate clamping can also limit the electromechanical response and often leaves electrostatics to play a secondary role. Releasing the mechanical constraint imposed by the substrate can not only dramatically alter the balance between elastic and electrostatic forces, enabling them to compete on par with each other, but also activates new mechanical degrees of freedom, such as the macroscopic curvature of the heterostructure. In this work, an electrostatically driven transition from a predominantly out‐of‐plane polarized to an in‐plane polarized state is observed when a PbTiO3/SrTiO3 superlattice with a SrRuO3 bottom electrode is released from its substrate. In turn, this polarization rotation modifies the lattice parameter mismatch between the superlattice and the thin SrRuO3 layer, causing the heterostructure to curl up into microtubes. Through a combination of synchrotron‐based scanning X‐ray diffraction imaging, Raman scattering, piezoresponse force microscopy, and scanning transmission electron microscopy, the crystalline structure and domain patterns of the curved superlattices are investigated, revealing a strong anisotropy in the domain structure and a complex mechanism for strain accommodation.

Enhanced polarization and abnormal flexural deformation in bent freestanding perovskite oxides

“…The flexoelectric polarization becomes so predominant at atomic scale that strain gradient engineering offers a new path toward manipulating electrical and mechanical behaviors in these strongly correlated two-dimensional materials as future potential building blocks in multifunctional flexible electronics 30 – 32 and nanomachines 2 . For example, the strong strain gradient within self-rolling heterostructures provides a powerful tool of flexoelectric polarization for designing novel energy harvesters and field-effect transistors 33 , 34 . Furthermore, the enhanced flexoelectric polarization opens a door for the application of intrinsic nonpolar materials in polarity-dependent electronics.…”

Freestanding Perovskite Oxide Films: Synthesis, Challenges, and Properties

“…[ 65 ] Additionally, Li et al observed a radical transformation of the polarization direction in PbTiO 3 /STO superlattice with a SRO bottom electrode when released from its substrate. [ 126 ] Interestingly, the variation of the polarization direction in the superlattice drove the membrane to curl up into microtubes, in order to minimize the overall elastic energy. These results show that the suppression of the mechanical interaction with the substrates may influence the polarization dynamics of ferroelectric perovskite oxides.…”

Freestanding complex-oxide membranes

“…Being free from substrate clamping, such mechanical boundary conditions can be affected by electrostatic considerations, such as depolarizing fields. This was observed in PbTiO 3 /SrTiO 3 superlattices, where the strain-induced out-of-plane polarization in the epitaxial films is destabilized once the superlattice is released from the substrate, leading to in-plane polarization and flux-closure domains that eliminate the depolarizing field and causes large lattice mismatch with the bottom SrRuO 3 electrode and the curling of the membranes into microscopic rolls [165]. X-ray diffraction and cross-sectional TEM studies reveal the distribution of ferroelastic domains across the thickness of the superlattice rolls, as a mechanism for accommodating the curvature (figure 5(b)).…”