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Atomic force microscopy (AFM) is a well-known technique for determining the topography of a surface. However, slight variations in the instrument have yielded new methods that can image an entirely different set of properties. One of these is piezoresponse force microscopy (PFM), which uses most of the basic principles of AFM alongside a conductive tip to measure the piezo/ferroelectric domains of a material. In this article, we look at PFM and how it analyzes piezo/ferroelectric materials
What is Piezoresponse Force Microscopy (PFM)?
Piezoresponse force microscopy (PFM) is a variation of atomic force microscopy (AFM) that uses a conductive probe tip to measure the piezoelectric, and more commonly the ferroelectric, domains of a material. Unlike many other variations of AFM, PFM does not take any principles from other techniques.
Instead, it applies the principles of the piezoelectric effect to obtain measurements, alongside the conventional AFM location techniques to determine which domains are being measured. This enables the simultaneous creation of a topographic map, alongside a ferroelectric domain map which has a high spatial resolution.
It should be noted that some of the components of a standard AFM instrument are made from piezoelectric materials, especially the scanners, as the piezoelectric material can be contracted and expanded depending on if there is a voltage applied within the instrument. PFM differs as it requires the principles of the piezoelectric to perform the measurement itself and back out related properties.
The Piezoelectric Effect
The piezoelectric effect is a well-known phenomenon where electric surface charges (or surface polarization) are generated when a piezoelectric material undergoes applied stress. This can take the form of bending, squeezing or other forms of applied pressure. Additionally, the piezoelectric effect can be reversed, and this is known as the converse piezoelectric effect. In the converse piezoelectric effect, the material will expand under an applied electric field.
There are many different types of piezoelectric materials, and these range from polymers and copolymers, to bones and teeth. However, there is also a special class of piezoelectric materials known as ferroelectrics (these are most commonly analyzed using PFM). Ferroelectric materials possess a permanent dipole moment and spontaneous polarization properties that can be permanently switched in the presence of an electric field.
These materials can then switch between binary states (1 and 0) depending on whether they are switched on or off. The analysis of this switching state can tell the researcher a lot about a ferroelectric material’s optical, mechanical and electrical properties. So, PFM can be a powerful technique for obtaining a lot of information from these materials.
How PFM Works
As with all variations of AFM, the regions with a piezo/ferroelectric domain are located by the movement of the cantilever beam. This movement cause the laser positioned towards the back of the cantilever beam to become deflected onto a position-sensitive photodiode (PSPD), and this identifies the region being analyzed. However, PFM does differ in some respects, because how the piezoelectric material responds (contraction or expansion) can also be deduced by where the laser hits the PSPD. PFM is a contact mode technique that uses a conductive tip made of silicon with a coating of either platinum, gold tungsten or conductive diamond thin films.
Because the piezoelectric material under analysis responds to electrical fields, an alternating current (AC) voltage bias is applied across the tip as it scans the surface. This causes the tip to act as an electrode. As the tip reaches a domain that is piezo/ferroelectric, the principles of the converse piezoelectric effect are applied and the tip couples with the surface through polarization and mechanical displacement mechanisms (deflection, torsion, and buckling). This brings the tip closer to the surface.
The properties are determined by the whether the piezoelectric domains expand or contract, and this is based on the voltage bias which causes the piezoelectric material to undergo either an in-phase (expansion) or out-of-phase response (contraction). The difference in whether the material is expanded or contracted is determined by how far the cantilever moves towards it, which is picked up by the PSPD depending on the degree (and direction) of laser deflection of the cantilever.
For the in-plane mode, there is generally a change in the lateral movement on the PSPD and this results from torsional forces that twist the cantilever. On the other hand, out-of-plane modes usually see a difference in vertical movements on the PSPD, and this is due to vertical deflections of the cantilever beam.
Piezoelectric Sensors. Image Credits: Nowwy Jirawat/shutterstock.com
It is a technique that is limited in the types of materials used, with only piezo and ferroelectric materials being the materials that are analyzed by this method. Although, nowadays, this class of materials extends out to MEMS devices, sensors, sonar systems, radio frequency (RF) filters, and data storage devices, among a lot of other devices. So, there is a wide range of complete systems that can be analyzed with PFM.
Additionally, a lot of information can be gained from these materials using PFM, including mapping the size of ferroelectric/piezoelectric domains, determining the switching behavior kinetics of ferroelectric materials, for identifying phase transitions and nucleation dynamics, and for providing information on the 3D polarisation properties of piezo-responsive materials.
Sources and Further Reading