Liquid PFM for Ferroelectric Materials

Why perform PFM in liquid?

Although most PFM measurements are performed in air, many materials operate in liquids rather than under ambient conditions. Biological materials function in aqueous environments, electrochemical devices contain liquid electrolytes, and ferroelectric surfaces can interact strongly with water, ions and other chemical species.

Liquid PFM allows us to image and manipulate electromechanical behaviour while both tip and the sample are immersed in a controlled liquid environment. In our group, we mainly use the technique to investigate ferroelectric thin films. Although the discussion here focuses on ferroelectric materials, similar approaches can also be applied to a range of piezoelectric, biological and electrochemically active materials.

Advantages of Liquid PFM

Measurements in controlled environments

Liquid PFM allows materials to be studied under conditions closer to those in which they operate. This is particularly relevant for biological materials, electrochemical systems, sensors used in fluids and ferroelectric surfaces exposed to water or other chemical species. The properties of the liquid can be varied: changing its conductivity or ionic concentration provides a way to investigate how the surrounding environment affects the measured response. For example, Balke et al. [1] studied BiFeO₃ thin films in Na₂SO₄ electrolytes with different ionic concentrations.

Reduced capillary forces

In air, a thin water meniscus forms between the AFM tip and the sample. This produces capillary forces that affect the tip–sample contact and may complicate the interpretation of PFM measurements. Immersing the tip and sample removes this ambient water meniscus. The mechanical interaction between the tip and the surface can therefore be controlled more directly.

Screening of long-range electrostatic interactions

PFM contrast does not always originate entirely from the local piezoelectric response. Long-range electrostatic forces between the tip, sample and cantilever can also contribute to the measured signal. In conductive liquids, mobile ions screen these long-range electric fields. This can localise the electrical interaction more strongly around the tip–sample contact and reduce non-local electrostatic contributions, as illustrated in Figure 1.

Figure 1. Schematic mechanism of PFM contrast enhancement in a liquid environment. In (a),(b) ambient, electrostatic forces are present between the tip and the sample. Electromechanical coupling (double arrow) is present only for the contact mode, when tip and surface are in contact. In (c),(d) liquid, the electrostatic forces are minimized. [2]

With the lack of capillary forces and long-range electrostatics, liquid measurements can improve the spatial resolution of the PFM response under suitable conditions. Rodriguez et al. demonstrated this using PZT measured in distilled water [2]. They reported a substantially sharper domain-wall profile in liquid than in air, as shown in Figure 2.

Figure 2. (a) Surface topography and PFM (b) amplitude and (c) phase images of PZT acquired in distilled water at 10 V. PFM mixed signal and phase profile across a typical domain wall in (d) ambient and (e) liquid environments. [2]

Challenges of Liquid PFM

Liquid PFM requires careful experimental design and results interpretation as the liquid changes both the mechanical response of the cantilever and the electrical conditions around the tip.

Cantilever dynamics in liquid

The surrounding liquid adds mass and viscous damping to the cantilever [2]. Resonance frequencies usually shift to lower values, resonance peaks broaden, and the PFM amplitude may decrease. The frequency response can also contain several overlapping peaks. This makes the selection and tracking of the contact resonance more difficult than in air and can increase cross-talk between the in-plane, out-of-plane and topographic signals [1]. These effects are particularly important for materials with an already weak piezoelectric response in ambient.

Conductivity and electrochemical reactions

The conductivity of the liquid affects the potential distribution around the biased tip. Part of the applied voltage may be lost through the liquid rather than being applied across the material. Stray currents and electrochemical reactions may also occur, particularly when DC voltages are applied. These reactions can change the surface, causing irreversible sample damage [3].

Electric double-layer formation

In an electrolyte, mobile ions accumulate near the biased tip and the sample surface, forming electric double layers. At higher ionic concentrations, strong screening and voltage losses due to the growing electric double layer can reduce the measurable PFM signal. Balke et al. [1] observed a loss of domain contrast as the concentration of a Na₂SO₄ electrolyte was increased, as seen in figure 3. This is an important limitation for measurements in concentrated electrolytes, including those used in batteries and other electrochemical devices. Ionic screening can reduce unwanted long-range electrostatic interactions, but excessive screening can also prevent a sufficient AC voltage from reaching the sample.

Figure 3. In-plane PFM images for BiFeO3 thin film in Na2SO4 with a molarity of (a) 10-6, (b) 10-4 M, (c) 10-2 M, and (d) 1 M. [1]

Outlook

Liquid PFM is most useful when the liquid environment is part of the scientific question. At the same time, measurements require suitable controls because mechanical damping, voltage losses and electrochemical reactions can all contribute to the observed signal. When these effects are taken into account, liquid PFM provides a useful approach for studying ferroelectric materials, and biological systems [4] under controlled environmental conditions.

References

Balke, N. et al. Probing Local Electromechanical Effects in Highly Conductive Electrolytes. ACS Nano 6, 10139–10146 (2012).
Rodriguez, B. J., Jesse, S., Baddorf, A. P. & Kalinin, S. V. High Resolution Electromechanical Imaging of Ferroelectric Materials in a Liquid Environment by Piezoresponse Force Microscopy. Phys. Rev. Lett. 96, (2006).
Balke, N., Jesse, S., Chu, Y.-H. & Kalinin, S. V. High-Frequency Electromechanical Imaging of Ferroelectrics in a Liquid Environment. ACS Nano 6, 5559–5565 (2012).
Kalinin, S. V. et al. Towards local electromechanical probing of cellular and biomolecular systems in a liquid environment. Nanotechnology 18, 424020 (2007).