Electron transfer driven by tip-induced flexoelectricity in contact electrification

Contact electrification (CE) has been known for over 25 centuries, but the origin of the CE remains mysterious. Recent theoretical studies suggest that flexoelectricity may drive the CE, but experimental evidence is lacking. Here, the CE between a nanoscale tip and flat polymers is studied by using atomic force microscopy. The contributions of flexoelectricity to the CE are analyzed. We focus on the effect of the load, which is coupled to the strain gradient at the contact region. It is revealed that the flexoelectric polarization in general polymers can drive electron transfer, and even reverse the intrinsic polarity of electron transfer in the CE. It implies that the flexoelectricity is one of the driving forces for the CE. The flexoelectricity induced electric field is measured by applying a voltage between the tip and the sample, which counteracts the flexocoupling voltage. Further, a band structure model is proposed, in which the surface states of the solid are suggested to be shifted by the flexoelectric polarization.