NT-MDT Spectrum Instruments launches a new approach for simultaneous study of topography and electromechanical properties of fragile and soft samples. The new AFM mode called HybriD Piezoresponse Force Microscopy (HD PFM)  allows simultaneous nondestructive investigation of surface morphology, mapping of quantitative nanomechanical properties, dielectric properties and piezoelectric domain morphology. Furthermore, they can be studied right under variable temperature thanks to the principle of HD PFM operation. For the very first time, this mode permitted nondestructive electromechanical study of diphenylalanine peptide nanotubes and triglycine sulfate crystal measurements right while phase transformation of the second type. This article presents the principle of HD PFM operation and demonstrates the obtained data.
Atomic force microscopy (AFM) is considered to be a powerful tool for surface imaging and examination of a material’s local properties with nanometer-level spatial resolution. Since the working principle of an AFM is based on the direct interaction between sharp tip and sample, a wide range of unique AFM measurement techniques have been created: conductivity mapping, quantitative nanomechanical measurements, electromagnetic studies etc. One of these AFM techniques is Piezoresponse Force Microscopy (PFM), which helps exploring the electromechanical performance of piezoelectric and ferroelectric materials based on their domain morphology with nanometer spatial resolution in various temperatures and different environments.
The working principle of PFM is based on contact mode AFM – a sample scanning technique in which the tip is in constant contact with the sample surface with a feedback-controlled positive force value. During the scanning process, an AC voltage is applied between the conductive tip and sample causing in-plane and out-of-plane oscillations of the sample according to the domain structure. This allows the study of domain geometry, local piezoelectric coefficients, dynamics and polarization direction along with a spatial resolution of tens of nanometer that is limited by the AFM tip radius. Since the initial development, this technique by Guthner and Dransfeld in 1991, PFM has become an extensively used technique for ferro- and piezoelectric crystals research. A wide range of crystalline materials, whose structure lacks a central symmetry, were explored based on piezoelectric domain structure and dynamics: lead zirconate titanate, triglycine sulfate, BiFeO3 etc.
More commonly, any material with noncentrosymmetric structure has the potential to demonstrate piezoelectric properties. From this point of view, the most interesting area for investigation of electromechanical coupling in life sciences would indeed refer to the case where majority of proteins, organelles and polysaccharides have noncentrosymmetric nature. This effect was called biopiezoelectricity and has consequently been observed in specific types of muscular movement, ion transportation, amino acids, the nervous system etc. and, as a result, its detection has become vital for biomedical and nanomedicine applications.
However, this needs a new method for investigating the electromechanical coupling in life systems. PFM is considered to be a great candidate for this purpose as it permits piezoresponse measurement with nanometer-level resolution. However, PFM being a contact mode technique, it is not suitable for studying biological samples. The lateral tip-sample interaction arising from the constant contact of AFM tip with the surface can be critical enough for destroying or deforming fragile and softer material. NT-MDT Spectrum Instruments launches a new approach for PFM investigation of such fragile and soft objects by utilizing the reduced lateral tip-sample interaction in HybriD Piezoresponse Force Microscopy mode (HD PFM).
HD PFM is an extension of the recently introduced HybriD mode (HD mode) – scanning technique based on fast force-distance curves measurements with real-time processing of tip response. From the hardware point of view, HD PFM is realized employing NT-MDT S.I. new control electronics named HybriD 2.0. In HD mode, the sample or the tip is driven into a vertical oscillation by a Z piezo-element at the frequency well below the resonances of the piezo-element and the probe. As the sample and the probe interact with each other in every cycle, the tip goes from non-contact to contact regime and the cantilever deflects in response to the tip-sample interactions to the specified level.
Figures 1a-b illustrate this sequence of the events and with an idealized deflection profile of the probe in the single cycle of HD mode. The temporal deflection plot comprises of a wealth of useful information that can be detected, also mapped during lateral scanning: quantitative local Young’s modulus, topography, adhesion, long-distance electrostatic and magnetic forces that are sensed by probes with conducting or ferromagnetic coatings, respectively.
Additionally, since tip goes out of the surface in each scanning point, the lateral tip-sample interaction is noticeably decreased in comparison to contact mode AFM and any parasitic drifts of the probe caused by sample heating can be compensated. This permits nondestructive studies of fragile and soft sample including measurements right under variable temperature.
To use simultaneous piezoresponse measurement in HD mode, users are suggested to apply AC voltage between conductive coating of the tip and investigated object in “time window” known as the tip-sample contact (Figure 1c). AC voltage leads to mechanical oscillations of the sample depending on its local polarization. Corresponding lateral (LF signal) and vertical (DFL signal) motion of AFM tip is recorded in defined “time window” and processed to get amplitude and phase signals. Amplitude of LF and DFL signals characterize local piezoelectric coefficient of the material whereas the phase signals provide information about local polarization direction. The full DFL(t) curve of each HybriD mode circle is also processed in order to calculate adhesion, feedback input signals and E modulus. Thus, HD PFM provides multiple information about sample’s properties via a single measurement cycle, and at the same time, makes HD PFM nondestructive by retracting the tip from the sample at each HybriD oscillating cycle.
Figure 1. (a) A model illustrating a performance of HD mode (b), an idealized temporal deflection curve during an oscillatory cycle, (c) principle of piezoresponse measurement in “time window” corresponding to the tip-sample mechanical contact during an oscillatory cycle.
Nondestructive Study of Diphenylalanine Peptide Nanotubes
Peptide nanotubes (PNTs) self-assembled from diphenylalanine monomers were recently discovered to display strong piezoelectric properties. Kholkin et al. demonstrated in-plane PFM contrast and high effective d15 piezoelectric coefficient of at least 60 pm/V (for tubes 200 nm in diameter)  considered to be the greatest value for known biopiezoelectrics. Together with intrinsic biocompatibility and very high elastic modulus for molecular crystals, this makes diphenylalanine PNTs to be potential materials for producing piezo-nanodevices that are considered to be potentially compatible with human tissue.
PNTs are challenging samples for standard contact PFM investigation because of their fragility and weak contact with a substrate. Hence, no PFM images of nondestroyed nanotubes can be found in the literature. Young’s modulus study of these structures is also of major interest since earlier measured values by varied techniques differ from 9 to 32 GPa . Furthermore, there is no data with maps of mechanical properties. Therefore, it was logical to apply HD PFM to diphenylalanine PNTs to measure piezoresponse and map quantitative nanomechanical properties.
Figures 2 and 3 demonstrate the obtained data: lateral piezoresponse, deformation map, topography, adhesion and electrostatic properties maps of less than 100 nm tubes were simultaneously obtained. Measurements were carried out on NT-MDT Spectrum Instruments AFM VEGA with use of NSG30/TiN probe. Lateral PFM phase establishes PNTs with opposite polarization direction corresponding to d15 piezoelectric constant (vertical electric field and polarization parallel to the tube axis). Nonuniform distribution of nanotubes’ stiffness was demonstrated by the deformation map. That was referred to variation of tube’s inner diameter. As none of the standard contact mechanics models (Hertz, DMT, JKR etc.) describe tip-nanotube interaction, FEA simulation was applied in order to quantify transversal Young’s modulus. Obtained value of 29±1 GPa coincides with earlier measured by the nanoindentation and AFM force spectroscopy techniques. Obtained HD PFM data was recently published in the Ultramicroscopy journal.
Figure 2. Nondestructive imaging of diphenylalanine nanotubes electromechanical properties by the HD PFM. Scan size: 7×7 µm. Sample courtesy: Dr. A. Kholkin, University of Aveiro, Portugal.
Figure 3. Nondestructive imaging of diphenylalanine nanotubes electromechanical properties by the HD PFM. Scan size: 8×8 µm. Sample courtesy: Dr. A. Kholkin, University of Aveiro, Portugal.
Piezoresponse Study with Real-Time Temperature Variation
Study of temperature dynamics of ferro- and piezoelectric domains is presently of great interest. Since AFM working principle allows measurements under varied temperatures of a sample, PFM is now extensively used for this type of study. The biggest drawback of standard PFM is that topography measurement is based on feedback control of cantilever deflection. Thus, any change of sample temperature causes parasitic drift of cantilever, thus distorting the obtained image. In contrast, HD PFM working principle permits drift compensation in each scanning point: feedback loop input signal equals not cantilever deflection but difference between maximum cantilever deflection per oscillatory circle and the baseline level.
One of the model samples to study temperature dynamics of ferroelectric properties is triglycine sulfate crystal (TGS). Even though TGS primitive cell comprises of more than 100 atoms, the nature of spontaneous polarization is extremely simple.
This sample was used to demonstrate the potential of continuous piezoresponse measurement under variable temperature. TGS chip was initially measured with a very high AFM temperature gradient: >0.1 °C/sec. Results in Figure 4 shows domain morphology dynamic when temperature value goes through Curie point. Even though parasitic temperature drift of the cantilever was more than 100 nm (Figure 4c), topography and PFM measurements were correct.
Figure 4. In situ HD PFM study of temperature dynamic of TGS crystal: (a) topography, (b) out-of-plane PFM amplitude, (c) out-of-plane PFM phase, (d) parasitic temperature drift (baseline level), nm, (e) temperature of the sample. Sample courtesy: Dr. R. Gainutdinov, Institute of Crystallography of RAS, Russia.
Newly cleaved TGS crystal was also measured near Curie point for in situ observation of domain structure formation. It was established that near Curie point, a quasi-periodic domain structure appears and this is followed by well-known oval domain structure.
The data was attained by using NT-MDT Spectrum Instruments AFM VEGA and NSG30/TiN probes.
Figure 5. In situ HD PFM study of temperature dynamic of TGS crystal near Curie point. Topography is overlayed by out-of-plane PFM phase. Scan size is 15×15 µm. Sample courtesy: Dr. R. Gainutdinov, Institute of Crystallography of RAS, Russia.
This article presented a new approach for nondestructive piezoresponse measurements along with simultaneous electrostatic and nanomechanical studies. New HD PFM mode was employed for nondestructive study of electromechanical properties of diphenylalanine peptide nanotubes and temperature dependence of ferroelectric domain structure of triglycine sulfate crystal.
 A. Kalinin, V. Atepalikhin, O. Pakhomov, A.L. Kholkin, A. Tselev, An atomic force microscopy mode for nondestructive electromechanical studies and its application to diphenylalanine peptide nanotubes, Ultramicroscopy. 185 (2018) 49–54. doi:10.1016/j.ultramic.2017.11.009.
 A. Kholkin, N. Amdursky, I. Bdikin, E. Gazit, G. Rosenman, Strong piezoelectricity in bioinspired peptide nanotubes, ACS Nano. 4 (2010) 610–614. doi:10.1021/nn901327v.
 P. Zelenovskiy, I. Kornev, S. Vasilev, A. Kholkin, On the origin of the great rigidity of self-assembled diphenylalanine nanotubes, Phys. Chem. Chem. Phys. 18 (2016) 29681–29685. doi:10.1039/C6CP04337B.
This information has been sourced, reviewed and adapted from materials provided by NT-MDT Spectrum Instruments.
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