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The triboelectric effect is a ubiquitous phenomenon in which the surfaces of two materials are easily charged during the contact‐separation process. Despite the widespread consequences and applications, the charging mechanisms are not sufficiently understood. Here, the authors report that, in the presence of a strain gradient, the charge transfer is a result of competition between flexoelectricity and triboelectricity, which could enhance charge transfer during triboelectric measurements when the charge transfers of both effects are in the same direction. When they are in the opposite directions, the direction and amount of charge transfer could be modulated by the competition between flexoelectric and triboelectric effects, which leads to a distinctive phenomenon, that is, the charge transfer is reversed with varying forces. The subsequent results on the electrical power output signals from the triboelectrification support the proposed mechanism. Therefore, the present study emphasizes the key role of the flexoelectric effect through experimental approaches, and suggests that both the amount and direction of charge transfer can be modulated by manipulating the mixed triboelectric and flexoelectric effects. This finding may provide important information on the triboelectric effect and can be further extended to serve as a guideline for material selection during a nanopatterned device design., Using experimental approaches, the authors show that, in the presence of strain gradient, the charge transfer during triboelectric measurements is resulted from competition between flexoelectricity and triboelectricity. The charge transfer can be enhanced or inversed depending on the relative charge transfer direction of both effects. This finding could be a guideline for material selection for a nanopatterned device design.

Piezoresponse force microscopy (PFM) is an indispensable tool for investigating local piezoelectric and ferroelectric properties by detecting the electromechanical strain response to an applied voltage on a surface. The PFM technique is highly sensitive to the surface state of a sample, such as surface adsorption and surface potential, which could lead to the misinterpretation of the quantitative value of PFM signal as well as the presence of ferroelectricity. In this study, we observed a frequency-dependent PFM signal generated by surface adsorbates. However, the surface adsorbates can be removed by sample cleaning albeit the cleaning induced a significant change in surface potential. Thus, correct evaluation of the effective piezoelectric coefficient was realized by cleaning the sample and ruling out the electrostatic contribution. This work demonstrates that one should be careful to consider the effect of adsorbates and the electrostatic effect to correctly evaluate the piezoelectric coefficient.

Identification of microstructures in steel has been extensively studied to improve the understanding of corrosion behavior. However, identification by expert eyes could be subjective, and most previous works on identification are solely based on morphological features. Furthermore, it is more difficult to identify local microstructures on a small scale-length. In this study, we developed a method for differentiating local microstructures on low carbon steel based on multiple physical properties at the nanoscale combined with machine learning techniques. Machine learning techniques were applied to the atomic force microscopy images of multiple physical properties, that is, not only of morphological features but also of the surface potential and capacitance gradient. Thereafter, we analyzed the corrosion behavior according to the concentration of the NaCl solution in which the samples were immersed, on the basis of the identified local microstructures as well as obtained physical properties. This study, which is based on these multiple physical properties, potentially provides a powerful tool for identifying and visualizing features of data. It could be further extended to electrochemical systems with more complex microstructures.

Piezoresponse force microscopy (PFM) has gradually becomes indispensable tool to investigate local piezoelectric and ferroelectric properties in diverse material systems. However, numerous reports have shown that the PFM signal can originate from several non-piezoelectric origins. Among them, because the electrostatic interaction between the AFM tip/cantilever and sample surface can be readily involved, it can be the most important factor during PFM measurement. In particular, in materials with relatively low piezoelectricity, the situation can be more significant because the PFM signals from weak piezoelectricity can be hidden or buried by the electrostatic interactions. Herein, we examined the significance of the electrostatic interactions induced by the surface potential in PFM. Using piezoelectric and non-piezoelectric materials, we examined how the surface potential-dependent electrostatic interactions can significantly affect the PFM signal. We observed that the electrostatically induced PFM amplitude have a linear relationship with the magnitude of surface potential when the instrumental noise floor is properly considered. Our results demonstrate that electrostatic interactions can be significant and their recognition and minimization are essential during PFM and other AFM-based measurements.

In piezoresponse force microscopy (PFM), ideally, electromechanical strain is induced only by the converse piezoelectric effect. In reality, however, the obtained experimental PFM signal is a combination of many other factors. In particular, the concurrent electrostatic effect between the tip/cantilever and a sample surface can be significant because the electrostatic effect induced by surface potential is inevitable in some cases. However, most previous reports dealt with the extreme case of the electrostatic effect, such as the on-field state. That is, the contributions to the hysteresis loop of electrostatic effects from different sources have not yet been clearly understood. In the present work, we study the electrostatic effect on the hysteresis loop in relation to various measurement parameters. The results indicate that the PFM response is strongly affected by the electrostatic effect caused by external charge injection. This work can provide a guideline for determining the PFM response in a hysteresis loop.