Alkali-deficiency driven charged out-of-phase boundaries for giant electromechanical response

Abstract

Traditional strategies for improving piezoelectric properties have focused on phase boundary engineering through complex chemical alloying and phase control. Although they have been successfully employed in bulk materials, they have not been effective in thin films due to the severe deterioration in epitaxy, which is critical to film properties. Contending with the opposing effects of alloying and epitaxy in thin films has been a long-standing issue. Herein we demonstrate a new strategy in alkali niobate epitaxial films, utilizing alkali vacancies without alloying to form nanopillars enclosed with out-of-phase boundaries that can give rise to a giant electromechanical response. Both atomically resolved polarization mapping and phase field simulations show that the boundaries are strained and charged, manifesting as head-head and tail-tail polarization bound charges. Such charged boundaries produce a giant local depolarization field, which facilitates a steady polarization rotation between the matrix and nanopillars. The local elastic strain and charge manipulation at out-of-phase boundaries, demonstrated here, can be used as an effective pathway to obtain large electromechanical response with good temperature stability in similar perovskite oxides.

Evolution from Lead‐Based to Lead‐Free Piezoelectrics: Engineering of Lattices, Domains, Boundaries, and Defects Leading to Giant Response

“…[ 84 ] Domains can be visualized using various imaging methods depending on their sizes such as light microscopy, [ 85,86 ] piezoresponse force microscopy (PFM), [ 87–89 ] scanning electron microscopy (SEM), [ 90,91 ] transmission electron microscope (TEM), [ 35,36,92 ] and scanning transmission electron microscopy (STEM) [ 93–95 ] (Figure 5c–h). Advanced imaging techniques such as high‐angle annular dark‐field (HAADF) imaging in aberration‐corrected STEM can reveal quantitative information regarding domains and domain walls even at lattice scale [ 30,41,94,96 ] (Figure 5h).…”

“…During the film growth, 3D nanopillars are formed (Figure 23c–e) with similar crystal structure as the ferroelectric matrix separated by OOP boundaries which display different valence states. [ 41 ] The OOP boundaries surrounding the nanopillars carry two pairs of head‐to‐head and tail‐to‐tail polarizations which appear on the opposite sides of the nanopillar (Figure 23f), showing that the boundaries surrounding the pillar are oppositely charged. Based on phase field simulations, it has been shown that the pairs of oppositely charged OOP boundaries create a strong depolarization field, E dep .…”

“…This E dep facilitates a steady polarization rotation between the matrix and nanopillars, resulting in giant reversible electric‐field‐induced strain (Figure 23g–k). [ 41 ] Since, A‐site deficiency can be deliberately induced by modifying fabrication parameters, this strategy of piezoelectric can be extended to other oxide perovskite piezoelectrics. One of the main advantages of this strategy is the use of simple composition without need of extensive doping which make it highly suitable to be used especially in thin films.…”

“…(a–k) Reproduced under the terms of the CC‐BY 4.0 license. [ 41 ] Copyright 2021, The Authors, Published by Springer Nature.…”

Seeing Structural Mechanisms of Optimized Piezoelectric and Thermoelectric Bulk Materials through Structural Defect Engineering

“…Piezoelectrics, due to their ability of interconversion between electrical energy and mechanical strain, are widely applied in electro-mechanical devices. Environmental problems make it urgent to develop new lead-free piezoelectric materials with high performance [44,55,[113][114][115][116][117][118][119][120][121][122][123][124]. The lack of basic comprehension of the mechanisms at local level has hindered development.…”