Piezoelectric response in the contact deformation of piezoelectric materials

Abstract

Micro-and nanoelectromechanical systems (MEMS and NEMS) using lowdimensional piezoelectric structures (piezoelectric thin films, nanowires, and nanobelts) [1-10] have potential applications in many areas, including biosensors, actuators, and motion-controllers due to intrinsic electromechanical coupling. The electromechanical coupling provides a unique route for sensing mechanical stimuli from the change in electric potential/field, and for controlling structural deformation via electrical loading [11], which determines the performance and lifetime of micro-and nanodevices during the device operation. It becomes of primary interest to experimentally and theoretically investigate the piezoelectric behavior of materials on both the micro-and nanoscales under electrical and mechanical loading for the development, design, and process control of MEMS and NEMS devices. Piezoelectricity is an interaction between mechanical deformation and electric field. Several techniques have been used to characterize the piezoelectric behavior of piezoelectric structures and materials, which are based on either the direct piezoelectric effect or the inverse piezoelectric effect. The direct piezoelectric effect is that mechanical deformation produces electric polarization, and the inverse piezoelectric effect represents electric field-induced mechanical strain [12]. The techniques using the direct piezoelectric effect include stress-induced charge (Berlincourt method) and indentation, and the techniques using the inverse piezoelectric effect consist of laser interferometers, laser scanning vibrometers, and piezoresponse force microscope. The Berlincourt method, the laser interferometers, and the laser scanning vibrometers can be readily used in determining the piezoelectric behavior of bulk piezoelectric materials, while it is very difficult if not impossible to apply these techniques to evaluate the electromechanical functionality on the nanoscale in lowdimensional piezoelectric structures and materials. This is due to the size constraint and the sensitivity/resolution of the techniques, which limit their applications in characterizing low-dimensional structures and materials. Advances in the micro-and nanofabrication of nanoelectronics and micro-and nanodevices have resulted in the development of surface force microscopy, including nanoindentation and scanning force microscopy (SFM), which has been used to evaluate the mechanical behavior of low-dimensional structures. The principle of the nanoindentation and SFM techniques is based on the surface-contact interaction through direct contact and/or intermediate contact between the probe-tip and the surface of specimen. The sensitivity and resolution are determined by the stiffness of the plate for the nanoindentation and the cantilever beam for the SFM. The advantages of using the surface force techniques involve the use of a small amount of materials such as low-dimensional structures and possibly the evaluation of the local behavior of heterogeneous materials. The use of contact mechanics was proposed by Lefki and Dormans [13] in measuring the piezoelectric coefficients of lead zirconate titanate (PZT) thin films through continuous charge integration. A similar technique was later used by Fu et al. [14] to measure the piezoelectric effect of PbZr0.53Ti0.47O 3 thin films. Suresh and his co-workers [15-17] extended the indentation technique to evaluate the electric response of 1-3 piezoelectric ceramic-polymer composite, lead zirconate titanate, and barium titanate by monitoring the electric current passing through the conductive indenter and the counter electrode. They did not evaluate the piezoelectric response. Recently, Rar et al. [18] modified nanoindentation to assess the piezoresponse of polycrystalline lead zirconate titanate and BaTiO3 piezoceramics by applying an oscillating voltage between the indenter-tip and the backelectrode, similar to the technique of the piezoresponse force microscopy (PFM), as demonstrated by Birk et al. [19]. In parallel, SFM modulated by electric field, such as electric force microscopy [20] and PFM [19, 21], has been developed to map surface charges and/or examine local electromechanical behavior. The concept of PFM was demonstrated first by Birk et al. [19], using scanning tunneling microscopy to measure local piezoelectric activity of piezoelectric thin films made of vinylidene fluoridetrifluoroethylene copolymer. The principle of the PFM technique is based on monitoring the deflection of a cantilever beam responding to local surface oscillation of a piezoelectric material, which is modulated by an AC electric voltage between the conductive tip of the SFM and the counter electrode underneath the piezoelectric material [19, 21-24]. The development of MEMS and NEMS devices in the last decade has made the PFM as an important technique for evaluating nanopiezoelectric activities of low-dimensional structures and materials. This has imposed a tremendous challenge in understanding the local piezoelectric activities for quantifying the piezoelectric behavior of materials. This chapter is devoted to the common theme of the contact deformation in piezoelectric materials and its applications in the indentation testing and the PFM for the characterization of the piezoelectric behavior in piezoelectric materials. It summarizes the dependence of the contact deformation on the electromechanical interaction and the interconnection that correlate local piezoelectric activities with piezoelectric properties and the deformation behavior. It reviews the achievements in evaluating the fracture behavior of piezoelectric materials using the indentation technique. In particular, it emphasizes the need for much-deeper studies of the electromechanical interaction present in the contact deformation of piezoelectric materials. © Springer Science+Business Media, LLC, 2008.