Piezo Response Imaging

A Crosspoint Switch Example Using the MFP-3D AFM

The characterization of the electrical properties of materials on the nanometer scale with atomic force microscopy (AFM) is a rapidly expanding field. As materials are customized for ever more specialized roles, having an instrument that can be configured to perform these measurements is becoming a necessity. Piezo response (PR) imaging on an atomic force microscope, pioneered by Gruveman et al. a number of years ago, is one such electrical characterization technique.

AFMs configured in this mode are used to measure the mechanical response of a piezoelectric material as a function of lateral position when a time-varying voltage is applied across grains or domains in the material. The MFP-3D AFM is ideally suited for these measurements because of the flexibility of the controller and software. Specifically, the all-digital controller and the easy accessibility to change the controller’s crosspoint switch in the IGOR Pro software interface makes PR imaging an ideal application.

How PR Imaging Works
In the MFP-3D system, the cantilever is scanned over the surface in contact mode. An oscillating voltage is applied to the tip, preferably at a frequency considerably higher than the feedback loop of the AFM (in this example, a frequency of 200kHz).

In general, a frequency near, but not exactly on, the in-contact resonance of the cantilever is used. This concept is illustrated in Figure 1. The blue material is polarized “up”. When the tip applies a local potential, the grain shrinks. The red material, polarized in the opposite direction will instead expand. By taking the oscillating AFM deflection and measuring the phase signal with a lockin amplifier, you can clearly differentiate different domains.

Additional information on this technique can be found in the complete version of the application note.

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Figure 1: Oscillating polarization response microscopy, or piezo response (PR) imaging.

Topography (top), piezo response (middler) and in-plane piezo response images of PZT, 1µm scan, courtesy J. Spanier, Mesomaterials Lab, Drexel University.