Giant piezoelectricity in oxide thin films with nanopillar structure

Abstract

High-performance piezoelectric materials are critical components for electromechanical sensors and actuators. For more than 60 years, the main strategy for obtaining large piezoelectric response has been to construct multiphase boundaries, where nanoscale domains with local structural and polar heterogeneity are formed, by tuning complex chemical compositions. We used a different strategy to emulate such local heterogeneity by forming nanopillar regions in perovskite oxide thin films. We obtained a giant effective piezoelectric coefficient d33,f* of ~1098 picometers per volt with a high Curie temperature of ~450°C. Our lead-free composition of sodium-deficient sodium niobate contains only three elements (Na, Nb, and O). The formation of local heterogeneity with nanopillars in the perovskite structure could be the basis for a general approach to designing and optimizing various functional materials.

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

“…It is clear that the nanopillars are exactly the same as the matrix, but with a shift of a half unit cell along both in-plane directions. Consistent with our previous report 22 , the relative unit-cell shift δ C between nanopillars and the matrix along the c direction, observed along the cross-sectional views, can be described by < a /2, b /2, δ C > , where a and b are unit vectors along in-plane directions and δ C is the displacement along the out-of-plane direction. The relative misfit (δ C ) along the out-of-plane direction clearly differentiates the present boundary from the conventional antiphase boundary.…”

“…However, in the present case an AO (A being A-site atom) layer is missing between two perfect perovskite regions with one region shifted by (a/2, b/2, δ C ) instead of (a/2, b/2, c/2). This is different from an antiphase boundary but still belongs to an OOP boundary, displacing a lattice mismatch of a fraction of a unit cell dimension in neighboring regions of the crystal 22 , 25 . Thus, we still refer to it as an OOP boundary.…”

“…We simulated the ferroelectric domain structure of NP-NNO films with different conditions at the STO substrate. As discussed in the Supplemental Information, the misfit strain in the nanopillar region is set to be −0.8%, whereas the misfit strain in the matrix is set to be 0.1% along a axis and 0.4% along b axis as the overall structure of the film displays different a and b 22 . As shown in Fig.…”

“…Very recently, we have demonstrated a giant electromechanical response in sodium niobate thin films with extensive alkali vacancies that are likely playing a key role in determining the electromechanical behavior 22 . Herein, we investigate the mechanism in which 0D vacancies drive the formation of 3D nanopillars with 2D strained and charged out-of-phase (OOP) boundaries in lead-free alkali niobate thin films.…”

“…The charged OOP boundaries then act as metastable states for polarization rotation between the nanopillars and the surrounding matrix. Following this strategy, we had previously demonstrated a giant effective piezoelectric coefficient d 33 ~1100 pm/V with a high Curie temperature of ~450 °C in NNO films of ~200 nm in thickness 22 , 23 . Here we demonstrate how a large electromechanical response with good temperature stability can be obtained.…”