Piezoresponse Force Microscopy

Piezoresponse force microscopyThis feature is optional on some microscopes such as the Dimension Icon-PI.1 (PFM) is a Contact AFM technique to measure sample displacement out of the sample plane in response to an applied AC bias. The method is most directly applicable to piezoelectric samples such as lead zirconia titanate, but can also work for electrostrictive samples such as lead magnesium niobate, and has been used on ferroelectric materials such as barium titanium oxide. For more on the theory of PFM, see AFM and Piezoresponsive Materials.

With the SPM in Piezoresponse Mode, the tip is engaged with the sample and an AC voltage is applied between tip and sample during scanning. A responsive sample expands and contracts in synchronization with the applied voltage. By feeding the photodetected cantilever deflection signal into a lock-in amplifier whose reference signal is the applied AC bias frequency, background topography is suppressed from the resulting image which features only the sample surface height changes induced by the applied field.

Piezoresponse imaging does not require additional dedicated hardware and works with both fixed tip/rastered sample and fixed sample/rastered tip SPMs. However, this mode is not supported by software for all Bruker SPMs. PFM mode is not presently available on the Multimode and BioScope Catalyst platforms. The FastScan system, despite lacking tip bias capability, supports PFM mode. This requires Sample Setup for Operation Without Tip Bias.

Certain points must be considered when planning PFM experiments:

• Though a light imaging force best preserves samples, the tip must stay in contact with the sample to maintain the applied tip/sample voltage. A particularly rough sample may require a higher load force, higher gains or slower scan speeds to ensure a continuous electrical connection.

• A conductive cantilever is required (see PFM Probe Selection).
• Because active tip cantilevers are not conductive, PFM is not operable with Actuated Probe or dual scan.
• Piezoresponse imaging may be performed in a nonconducting fluid. In a conductive fluid, a conductive cantilever must be insulated from the fluid, except at the tip; this is difficult to accomplish.

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