Ilka Hermes & Romain Stomp explore dual-frequency resonance tracking for piezoresponse force microscopy
Due to their internal structure, ferroelectric materials feature a spontaneous electrical polarisation stable in two or more orientation states, which arrange in domains of coinciding orientation. Sufficiently high electrical fields can switch the polarisation, which allows customising domain patterns[.1] Applications of ferroelectric materials in industry are numerous, including communication and IT or data storage. In academia, researchers study the potential of ferroelectrics for novel applications such as energy harvesting.[3,4]
What is Piezoresponse force microscopy?
The customisation of ferroelectric domain patterns for each application requires an electromechanical imaging method with high spatial resolution. Piezoresponse force microscopy (PFM) is an atomic force microscopy (AFM) method, in which a conductive tip scans samples in contact mode, while applying an AC voltage. Due to the inverse piezoelectric effect, inherent in all ferroelectrics, the AC voltage induces a periodic deformation that is detected by the cantilever. The phase signal of the piezoresponse maps the local domain orientation, while the amplitude images the position of domain walls.
In its basic configuration, PFM applies the AC excitation at frequencies far below the clamped contact resonance of the cantilever. However, materials with low intrinsic piezoresponse, such as ferroelectric thin films or samples sensitive to high excitation fields, require an amplification of the piezoresponse for sufficient resolution. Here, resonance-enhanced PFM makes use of a natural amplification via the cantilever by driving with an AC voltage close to the contact resonance. Since the contact resonance strongly depends on the stability of the tip-sample contact, a high surface roughness, e.g. in polycrystalline thin films, can introduce jumps in the PFM amplitude by shifting the contact resonance. In the worst case, a resonance shift can lead to a complete inversion of the phase signal.[2,5]
Dual-frequency resonance tracking (DFRT) stabilises the resonance-enhanced piezoresponse by tracking the contact resonance of the cantilever. Two sidebands are generated on each side of the resonance (Fig. 2). A second feedback, in addition to the topography feedback, compares the amplitudes ratio of both sidebands and regulates the AC excitation frequency, to keep the sideband amplitude ratio constant.
DFRT Piezoresponse and ferroelectric materials
Here, we demonstrate the DFRT PFM capabilities on the ferroelectric material bismuth ferrite (BFO) using a Park Systems NX20 AFM coupled with a Zurich Instruments HF2 lock-in amplifier (LIA).
Due to the surface roughness of the BFO, single frequency resonance-enhanced PFM suffered severe instabilities, most prominent in the PFM amplitude. Measuring the sample in DFRT PFM(Fig. 3) not only stabilised the PFM amplitude, but also lowered the overall topographic cross-talk from step edges or dents in the sample surface. The PFM amplitude delivered a particularly sharp domain wall contrast, which can be identified by the local signal minima. The PFM phase gave a distinct 180° phase difference between oppositely oriented domains. Neither the amplitude nor the phase correlated to any topographic feature, excluding a topographic origin of the PFM signals.
Moreover, collecting of all available signals, including the amplitudes, phases as well as frequency shifts from both sidebands and the carrier signal, is available via the data acquisition module on the HF2 LIA ̶ facilitated by the straightforward synchronisation of Park Systems AFMs and the HF2 LIA.
To conclude, the capability of DFRT PFM to stabilise the piezoresponse imaging on ferroelectric materials promises a more reliable characterisation of domain patterns especially on polycrystalline samples.
Stabilising the piezoresponse on ferroelectric materials References
1. Tagantsev, A. K., Cross, L. E. & Fousek, J. Domains in ferroic crystals and thin films. 13, (Springer, 2010).
2. Soergel, E. Piezoresponse force microscopy (PFM). J. Phys. D. Appl. Phys. 44, 464003 (2011).
3. Butler, K. T., Frost, J. M. & Walsh, A. Ferroelectric materials for solar energy conversion: photoferroics revisited. Energy Environ. Sci. 8, 838–848 (2015).
4. Fridkin, V. M. Photoferroelectrics. 9, (Springer Science & Business Media, 2012).
5. Rodriguez, B. J., Callahan, C., Kalinin, S. V & Proksch, R. DFRT AFM. Nanotechnology 18, 475504 (2007).
Ilka Hermes is with Park Systems, Romain Stomp is with Zurish Instruments