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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.

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.

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