Giant Piezoelectricity in Oxide Thin Films with Nanopillar Structure

Piezoelectric materials – materials that generate electrical charge in response to an applied mechanical force, and vice versa – is currently widely used both in sensors (i.e. generating electrical charge in response to mechanical force) and actuators. High-performance piezoelectric materials with a larger output per unit applied input can lead to sensors and actuators with greater sensitivity.  Current high-performance piezoelectric materials suffer from two important drawbacks. 

First, such materials typically contain lead, and cannot be used in certain countries due to environmental concerns. In addition, these materials tend to have complex chemical compositions and low Curie temperature, and hence are difficult to include in devices without affecting manufacturing yield and cost; each piezoelectric material has its own Curie temperature, and loses its piezoelectric properties after being heated above the Curie temperature.

Experimentalists from IMRE and NUS fabricated lead-free NaNbO3 thin films with nanopillar regions within a homogeneous perovskite matrix, and demonstrated that such thin films with a simple chemical composition have a larger piezoelectric effect and higher Curie temperature than state-of-the-art, chemically-complex piezoelectric lead zirconate titanate (PZT) and potassium sodium niobate (KNN).  For instance, the NaNbO3 thin films have an effective piezoelectric coefficient more than twice that of the best PZT films, and four times that of the best KNN-based films with extensive doping. Density functional theory calculations carried out by IHPC provided the foundation for a theoretical understanding of the physical mechanism underlying the giant piezoelectric effect. 

Specifically, the density functional theory calculations explained the formation of antisite NaNbO3 within the nanopillars, and how antisite atoms induces the local nanoscale structural heterogeneities that causes the giant piezoelectric effect.  This is the first report of a material with this magnitude of piezoelectric effect, and the first report of leveraging nanopillar microstructural heterogeneity for enhancement of functional material properties.

In addition to providing a novel high-performance piezoelectric material with greater ease of application, the theoretical understanding of the physical mechanism underlying the giant piezoelectric effect suggests forming local heterogeneities using nanopillars could be a general approach to designing high-performance functional material properties beyond piezoelectric behaviour.  This paper also enhances Singapore's international reputation in the design of functional materials with wide-ranging engineering applications.  

The article titled "Giant piezoelectricity in oxide thin films with nanopillar structure" by researchers from IMRE, NUS, IHPC, Pennsylvania State University and University of Missouri has been accepted for publication in Science (impact factor of 41.845); this paper is embargoed prior to publication.  The IHPC researcher involved in this work is Dr Ong Phuong Khuong.