Abstract
Piezoresponse force microscopy (PFM), as a powerful nanoscale characterization technique, has been extensively utilized to elucidate diverse underlying physics of ferroelectricity. However, intensive studies of conventional PFM have revealed a growing number of concerns and limitations which are largely challenging its validity and applications. In this study, an advanced PFM technique is reported, namely heterodyne megasonic piezoresponse force microscopy (HM‐PFM), which uses 106 to 108 Hz high‐frequency excitation and heterodyne method to measure the piezoelectric strain at nanoscale. It is found that HM‐PFM can unambiguously provide standard ferroelectric domain and hysteresis loop measurements, and an effective domain characterization with excitation frequency up to ≈110 MHz is demonstrated. Most importantly, owing to the high‐frequency and heterodyne scheme, the contributions from both electrostatic force and electrochemical strain can be significantly minimized in HM‐PFM. Furthermore, a special measurement of difference‐frequency piezoresponse frequency spectrum (DFPFS) is developed on HM‐PFM and a distinct DFPFS characteristic is observed on the materials with piezoelectricity. By performing DFPFS measurement, a truly existed but very weak electromechanical coupling in CH3NH3PbI3 perovskite is revealed. It is believed that HM‐PFM can be an excellent candidate for the ferroelectric or piezoelectric studies where conventional PFM results are highly controversial.
Highlights
Electrostatic force is one of the most intractable issues which continuously influences the Piezoresponse Force Microscopy (PFM) results since its invention
The special measurement offered by Heterodyne Megasonic Piezoresponse Force Microscopy (HM-PFM), the difference-frequency piezoresponse frequency spectrum (DFPFS),[44] is demonstrated on three types of functional materials including dielectric, Lithium-ion battery and ferroelectric materials
The results clearly show that the oscillation amplitude decays dramatically with increasing frequency, implying that the electrostatic force induced cantilever vibration will be significantly minimized if PFM is operated at high frequency
Summary
Electrostatic force is one of the most intractable issues which continuously influences the PFM results since its invention. High-frequency PFM working at high eigenmodes of the cantilever has been put forward to effectively minimize the electrostatic force contribution.[18, 20, 25] But the associated decrease of detection sensitivity, laser spot size effect, large bandwidth requirement for photodetector and lock-in amplifier make the application of highfrequency PFM very limited.[18, 25] Despite high-frequency PFM has seldom received attentions due to the current technical restrictions, using high-frequency excitation to detect piezoelectric strain does provide a meaningful instruction for the development of advanced PFM methods. By performing an overall assessment of using high-frequency excitation in PFM, it is found that several substantial improvements can be realized simultaneously, including minimizing the electrostatic force-induced cantilever vibration, attenuating electrochemical Vegard strain and electrostriction effects as well as reducing the influence of dynamic electrochemical processes. The results indicate that the electrochemical strain has been considerably attenuated in HM-PFM
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