Electromechanical coupling is one of the fundamental mechanisms underlying the functionality of many materials. These include inorganic macro-molecular materials, such as piezo- and ferroelectrics, as well as many biological systems. This application note discusses emerging applications for and applications of piezoresponse force microscopy (PFM).
Emerging Applications for PFM
High Frequency PFM
High-frequency imaging allows for an improved SNR by avoiding 1/f noise. Furthermore, inertial stiffening of the cantilever improves contact conditions. By probing the PFM signal with higher resonances, topographic imaging is performed with a soft cantilever, while PFM is performed with a higher mode where the dynamic stiffness is much greater. This both reduces the electrostatic contribution to the signal and improves the tip-surface electrical contact through effective penetration of the contamination layer. Finally, resonance enhancement using the higher mode amplifies weak PFM signals. It should be noted that in this regime, the response is strongly dependent on the local mechanical contact conditions, and hence, an appropriate frequency tracking method is required to avoid PFM/topography cross-talk, e.g. using DART or Band Excitation (BE).
The limiting factors for high-frequency PFM include inertial cantilever stiffening, laser spot effects, and the photodiode bandwidth. Inertial stiffening is expected to become a problem for resonances n>4-5, independent of cantilever parameters. This consideration suggests that the use of high-frequency detector electronics, shorter levers with high resonance frequencies, and improved laser focusing will allow the extension of high-frequency PFM imaging to the 10-100MHz range. Asylum's microscopes allow cut-off at ~2-8 MHz and potentially higher, opening a pathway for high frequency studies of polarization dynamics. Figure 1 illustrates the different information that is revealed by imaging a ceramic PZT material at various frequencies.
Figure 1. High Frequency PFM using Asylum's fast photodiode on a ceramic PZT sample at different frequencies (phase left, amplitude right) - below first resonance (top row) and at cantilever resonances (all others) using a MikroMasch NSC 35B cantilever. 1µm scans. Image courtesy of K. Seal, S. Kalinin, S. Jesse, and B. Rodriguez, Center for Nanophase Materials Science, ORNL.
High-Speed PFM (HSPFM)
HSPFM utilizes high speed data acquisition and sample actuation to significantly enhance imaging speeds by increasing line rates from roughly 1 Hz to well above 100Hz. The strong amplitude and phase contrast achievable in PFM, as well as the resolution enhancement provided by this contact-mode based method, have allowed 10nm spatial resolution even at image rates of up to 10 frames per second.
In addition to higher throughput, the primary benefit of this advance is dynamic measurements, for example tracking the evolution of ferroelectric domains during switching, exposure to light, changing temperature, and other effects Figures 2 and 3
Figure 2. This image sequence (left to right, top to bottom) is excerpted from a movie of 244 consecutive High Speed PFM images (4µm scans) depicting in situ ferroelectric memory switching. For the first half of the movie, the tip is biased with a positive DC offset throughout the measurements. By monitoring the phase of the piezoresponse, this allows direct nanoscale observation of ferroelectric poling, in this case from white to black contrast (a 180 degree polarization reversal). The second half of the movie is then obtained with a continuous negative DC bias, causing a black to white contrast shift. The switching mechanism is clearly nucleation dominated for this sample and experimental conditions. Each image is acquired in just 6 seconds. The PZT film is courtesy of R. Ramesh, UC Berkeley, and the HSPFM measurements were performed by N. Polomoff, HueyAFMLabs, UConn. Click here to watch the movie.
Figure 3. (001) domains in a PZT thin film, 3.8um scan. Image courtesy N. Polomoff and B. D. Huey, University of Connecticut Institute of Materials Science. Sample courtesy R. Ramesh, UC Berkeley.
The more general High Speed Scanning Property Mapping (HSSPM) allows rapid measurements of mechanical compliance, electric fields, magnetic fields, friction, etc, with similar benefits for novel dynamic measurements of surfaces.
For quantitative materials properties measurements, AFMs have a few well-known shortcomings. One is that the shape of the tip is usually ill-defined. Forces between the tip and sample have a strong dependence on this tip shape and, therefore, extracting materials properties such as the Young's modulus are at best problematic. Another issue is that the cantilever geometry means that the motion of the cantilever tip is not well defined. Specifically, when the cantilever deflects, there is motion along the vertical axis (z-axis) that is well defined, but there is also motion parallel to the sample surface. This motion is not well characterized and in most cases is not even measured.
The ability to probe forces and directly image the piezo response of a sample with the Asylum Research MFP NanoIndenter is an emerging application area. The NanoIndenter consists of a flexure with a calibrated spring constant to which diamond tips are mounted. This flexure is attached to the NanoIndenter AFM head and replaces the standard cantilever holder. Displacement of the indenting flexure is performed with a piezo actuator (head) and measured with a patented nanopositioning sensor (NPS™). The force is computed as the product of the spring constant and the measured indenter flexure displacement. This measurement is done by converting the vertical flexure displacement into an optical signal measured at the standard MFP-3D photodetector. Because the quantities of indentation, depth and force are computed based on displacements measured with AFM sensors, the indenter has much better spatial and force resolution than previous systems.
Figure 4 shows an example image of PPLN acquired with the NanoIndenter. Note that the topographic resolution is not as high as it would be with an AFM cantilever tip, as expected given the larger indenter tip. The amplitude and phase channels show clear, high SNR domain structure, similar to the results one would expect with cantilever-based PFM.
Figure 4. PPLN amplitude (top) and phase image (bottom) acquired with the MFP NanoIndenter, 50µm scan.
Another example of the experiments that can be performed with the combination of the NanoIndenter and PFM imaging is to study the effects of surface stresses on ferroelectric domain structures with quantitative scratch testing as shown in Figure 5. The top image shows the surface topography of PPLN after it has been purposefully scratched with different loading forces using the NanoIndenter tip.
Figure 5. Surface topography of PPLN after it has been purposefully scratched with different loading forces using the NanoIndenter, 10µm scan (top images). 1µm scan (bottom).
The next image shows the associated phase signal indicative of the domain structure. The domain boundaries have been distorted by the scratches which implies a lattice change which, in turn, has affected the local polarizability. The final figure in this sequence shows a higher resolution scan where the phase has been overlaid onto the rendered topography, showing a close-up of the distortion in the domain structure.
PFM allows organic and mineral components of biological systems to be differentiated and provides information on materials microstructure and local properties. The use of vector PFM may also enable protein orientation to be determined in real space, for example, the internal structure and orientation of protein microfibrils with a spatial resolution of several nanometers in human tooth enamel.
Additional progress will bring understanding of electromechanical coupling at the nanometer level, establish the role of surface defects on polarization switching (Landauer paradox), and probe nanoscale polarization dynamics in phase-ordered materials and unusual polarization states. In biosystems, PFM can also potentially open pathways for studies of electrophysiology at the cellular and molecular levels, for example, signal propagation in neurons.
Ultimately, on the molecular level, PFM may allow reactions and energy transformation pathways to be understood, and become an enabling component to understanding molecular electromechanical machines. Recently, PFM performed on biomolecules has demonstrated electromechanical behavior in lysozyme polymers, bacteriorhodopsin, and connective tissue. Figure 6 shows an example of vertical PFM height and phase images of collagen fibers. PFM has also recently been performed on biological systems such as cells as shown in Figure 7. This image shows a zoom of a red blood cell with the PFM phase channel painted on top to show piezo response.
Figure 6. Topographic (top) and PFM phase (bottom) images of collagen fibers, 1.4µm scan. Image courtesy D. Wu and A. Gruverman, UNL. Sample courtesy G. Fantner.
Figure 7. Zoom of the top surface of a red blood cell. The surface shape was rendered to show the topography while the phase channel is overlaid on top to show piezo response. A small sub-micron region on top (white) of the cell exhibited a much different piezo response than the surronding cell surface. 2µm scan. Image courtesy of B. Rodriguez and S. Kalinin, ORNL.
Applications of Piezoresponse Force Microscopy
Fundamental Materials Science
Applications of piezoresponse force microscopy (PFM) in Fundamental Materials Science include:
- Phase Transitions and Critical Phenomena
- Size Effects
- Nucleation Dynamics
- Ferroelectric Polymers
- Liquid Crystals
- Relaxor Ferroelectrics
Applications of piezoresponse force microscopy (PFM) in Piezoelectric Materials include:
- Micro ElectroMechanical Systems (MEMS)
- Sensors and Actuators
- Energy Storage and Harvesting
- RF Filters and Switches
- Balance and Frequency Standards
- Giant k Dielectrics
Applications of piezoresponse force microscopy (PFM) in Ferroelectric Materials include:
- Domain Engineering
- Non-volatile Memory
- Data Storage Devices
- Domain Energetics and Dynamics
Applications of piezoresponse force microscopy (PFM) in Bio-Electromechanics include:
- Cell Signaling
- Structural Electromechanics
This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.
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