Recently, researchers at UCD, ORNL, UCSF and Asylum Research investigated the feasibility of intermittent contact mode piezoresponse force microscopy (PFM) in a liquid environment based on simultaneous mechanical and electrical probe modulation to image electromechanical coupling in a mode consistent with reduced surface damage of soft materials.

The measured signal in a PFM experiment is generally a combination of electromechanical and electric forces and, at first glance, the signal in an intermittent contact mode version of PFM will be dominated by long-range electric or electrostatic forces. The electromechanical contribution to the displacement signal is expected to dominate only if (1) the electrical modulation frequency corresponds to the contact resonance, (2) the electrostatic interactions are effectively screened and (3) imaging is performed at small amplitude set-points, maximizing the residence time of the probe in contact with the surface.

The researchers reported in Nanotechnology that these conditions can be met by imaging at frequencies corresponding to the first contact resonance in liquid. The ions in the liquid are found to effectively screen the electrostatic interactions, allowing contrast consistent with the electromechanical signal to be obtained on model ferroelectric materials and piezoelectric tooth dentin.

The authors noted that further control of the screening can be implemented through the choice of solvent and that additional improvements may be attainable with the use of shielded probes, which allow precise control over the application and measurement of local response in solution. The ability to measure contrast consistent with electromechanical response in an intermittent contact mode provides a pathway to measure electromechanical coupling in a wide range of soft biosystems in liquid environments.