Piezoresponse force microscopy (PFM) is a technique used to characterize the electromechanical coupling of piezo- and ferroelectric materials. Electromechanical coupling is an intrinsic property that underlies the functionalities of many material systems. Piezoelectricity, for example, refers to the accumulation of an electric charge in a material that is subjected to pressure or another form of mechanical stress. This effect results from the linear interaction between electrical and mechanical states in piezoelectric materials. It may also result in the converse, generating motion in response to an applied electrical field.
This blog post will explore the working principles and potential applications of PFM in more detail.
PFM operates on the principles of atomic force microscopy (AFM). The conductive AFM tip applies a local voltage to a sample surface which subsequently causes it to deform. This localized expansion or contraction deflects the cantilever and the mechanical response can be measured, on the order of approximately 1 – 100 pm/V, to determine the sample’s piezoelectric properties. The measurement is made at a higher frequency where the voltage is applied as AC bias and the deflection is measured using a lock-in amplifier at the same frequency. This greatly improves the signal-to-noise of the measurement. Furthermore, Asylum uses the proprietary DART technique (Dual AC Resonance Tracking) to increase the sensitivity of the measurement by making it at the resonance frequency of the tip-sample contact, which acts as an amplifier of the response.
An additional benefit of the lock-in technique is that the phase of the response can be measured and related to the orientation of the piezoelectric response. In-plane surface displacement is subsequently measured as a function of the lateral components of AFM tip vibrations. A third and final displacement vector can be acquired by measuring the same sample region after it has been rotated by 90°. Calibrating both the lateral and vertical PFM signals enables the determination of the total electromechanical response vector of a sample domain.
The high functionalities of PFM research boast a broad range of established areas of application. It is broadly used for characterization of piezoelectric materials such as sensors, actuators, capacitors, energy storage and harvesting devices, and microelectromechanical systems (MEMS). PFM is also used for ferroelectric material studies for data storage devices and domain energetics and dynamics.
Beyond piezoelectricity and ferroelectricity, PFM is used for fundamental materials science and bio-electromechanical research. Characterizing the electromechanical responses of varying materials is becoming more crucial in understanding and optimizing products and tools in an extensive range of complex areas of study. This requires enhanced PFM tools capable of eliminating artifacts from measurements and enhancing the reproducibility of results.
Asylum Research offers the Cypher AFM with an Interferometric Displacement Sensor (IDS) option. Interferometric direction allows for direct measurement of cantilever deflection with optical beam detection. This mitigates artifacts that may arise from the electrostatic coupling while allowing the sample’s electromechanical responses to be measured at a fixed frequency, or by tracking resonance.
Our PFM capabilities have demonstrated unique capabilities for measuring piezoresponses and electromechanical characteristics of existing and novel materials. If you would like to learn more, please read our white paper on Piezoresponse Force Microscopy with Asylum Research AFMs. Or, contact us directly if you have any questions.