Electron transfer in nano-scale contact electrification: Atmosphere effect on the surface states of dielectrics

Contact electrification (CE) is a common phenomenon, in which the identity of the charge carriers has been debated for decades. Recently, the temperature and photoexcitation effects in CE reveal that the electron in the surface states of dielectrics is the charge carriers, which are responsible for the creation of electrostatic charges. Therefore, the electron transfer in CE may be predicted and controlled by measuring and changing the surface states of dielectrics. Here, a new method to measure the surface states of dielectric based on bias effect in CE is proposed. The CE between metals and dielectrics is performed in different atmospheres, and the surface state density (SSD) and highest occupied surface state level (HOSL) of the dielectric are measured by the method. The results suggest that both the SSD and HOSL of the dielectric can be changed by the adsorption of gas molecules. Particularly, the O₂ molecule is found to shift the HOSL of the dielectric to a lower energy level and make the dielectric more likely to be negatively charged. The findings provide a method to control charge transfer characteristics in CE by changing the O₂ concentration in the atmosphere, and the surface states measurement method has implications for the characterization of the CE properties of materials.