Ferroelectric materials feature a spontaneous electrical polarization stable in two or more orientation states because of their internal structure. These are arranged in domains of coinciding orientation.
If electrical fields are sufficiently high then they can switch the polarization, which enables the customization of domain patterns.1 So, there are many applications of ferroelectric materials in the industry, including information technology, communication and data storage.2
In academia, researchers examine the potential of ferroelectrics for novel applications like energy harvesting.3,4 An electromechanical characterization technique with the high spatial resolution is needed for the customization of ferroelectric domain patterns for each application.
Figure 1. Three-dimensional view of the PFM phase on the topography of a bismuth ferrite sample.
Piezoresponse Force Microscopy (PFM) is an Atomic Force Microscopy (AFM) technique where a conductive tip scans samples in contact mode while applying an AC voltage. The AC voltage induces a periodic deformation that is detected by the cantilever due to the inverse piezoelectric effect which is inherent in all ferroelectrics.
The local domain orientation is mapped by the phase signal of the piezoresponse, while the amplitude images the position of domain walls.2 In its basic configuration, PFM applies the AC excitation at frequencies which are a lot less than the clamped contact resonance of the cantilever.
Yet, for sufficient resolution, materials with low intrinsic piezoresponse such as samples sensitive to high excitation fields or ferroelectric thin films, need an amplification of the piezoresponse. In this instance, resonance-enhanced PFM uses a natural amplification via the cantilever by driving with an AC voltage near the contact resonance.
As the contact resonance is strongly dependent on the stability of the tip-sample contact, a high surface roughness, e.g. in polycrystalline thin films, is able to introduce jumps in the PFM amplitude by shifting the contact resonance. In the worst instance, a resonance shift can result in complete inversion of the phase signal.2,5
By tracking the contact resonance of the cantilever, Dual Frequency Resonance Tracking (DFRT) stabilizes the resonance-enhanced piezoresponse: Two sidebands are produced on each side of the resonance, as seen in Figure 2.
Figure 2. Frequency sweep of PPP-EFM cantilever (k=2.8 N/m) on BFO sample with the contact resonance in blue and the two sidebands in red and green 3 kHz from the resonance.
In addition to the topography feedback, second feedback compares the amplitude ratio of both sidebands and regulates the AC excitation frequency to keep the sideband amplitude ratio constant.5
The DFRT PFM capabilities on the ferroelectric material bismuth ferrite (BFO) are demonstrated here by using a Park Systems NX20 AFM coupled with a Zurich Instruments HF2 Lock-in Amplifier (LIA).
Single-frequency resonance-enhanced PFM suffered severe instabilities because of the surface roughness of the BFO, most notably in the PFM amplitude. By measuring the sample in DFRT PFM (Figure 3), the PFM amplitude was stabilized and the overall topographic cross-talk was lowered from step edges or dents in the sample surface.
Figure 3. DFRT PFM measurement on BFO, showing the sample topography (a), PFM amplitude (b), and PFM phase (c) with line profiles through different regions of the image. The AC excitation was 1 V.
The PFM amplitude supplied an especially sharp domain wall contrast, which can be identified by the local signal minima. The PFM phase supplied a distinct 180° phase difference between oppositely oriented domains. Neither the phase nor the amplitude correlated to any topographic feature, excluding a topographic origin of the PFM signals.
Furthermore, gathering of all signals available, including the amplitudes, phases as well as frequency shifts from both sidebands and the carrier signal, is available via the data acquisition module (DAQ) on the Zurich Instruments HF2 LIA ̶ facilitated by the straightforward synchronization of Park Systems AFMs and the HF2 LIA, as seen in Figure 4.
Figure 4. Synchronized DFRT-PFM data imaging via the Park Systems SmartScan software and the Zurich Instruments LabOne Software.
To summarize, the capability of DFRT PFM to stabilize the piezoresponse imaging on ferroelectric materials promises a more reliable characterization of domain patterns, particularly on rough, polycrystalline samples.
Produced from materials originally authored by Ilka M. Hermes from Park Systems Europe GmbH and Romain Stomp from Zurich Instruments AG.
References and Further Reading
- Tagantsev, A. K., Cross, L. E. & Fousek, J. Domains in ferroic crystals and thin films. 13, (Springer, 2010).
- Soergel, E. Piezoresponse force microscopy (PFM). J. Phys. D. Appl. Phys. 44, 464003 (2011).
- Butler, K. T., Frost, J. M. & Walsh, A. Ferroelectric materials for solar energy conversion: photoferroics revisited. Energy Environ. Sci. 8, 838–848 (2015).
- Fridkin, V. M. Photoferroelectrics. 9, (Springer Science & Business Media, 2012).
- Rodriguez, B. J., Callahan, C., Kalinin, S. V & Proksch, R. Dual-frequency resonance-tracking atomic force microscopy. Nanotechnology 18, 475504 (2007).
This information has been sourced, reviewed and adapted from materials provided by Park Systems Europe.
For more information on this source, please visit Park Systems Europe.