can be generated by the contact or sliding of a P-type semiconductor against an N-type semiconductor [7][8][9] (or semiconductor against metal [10][11][12] ), and a novel electric generator was proposed. [7] This new type of electric generator is gaining attention for that a direct current can be generated without a rectifier module. [8][9][10][11] However, the mechanism of the current generation in the CE involving semiconductors is still under debate.For the contact between a P-type semiconductor and an N-type semiconductor, the PN junction will be formed at the interface. [13,14] It is highly suspected that the built-in electric field in the PN junction makes the CE between the semiconductors different from that between conventional materials. Considering that, Wang et al. pointed out that the generation of the continuous direct current induced by the sliding of P-type semiconductor against N-type semiconductor is similar to the photovoltaic effect, and can be referred to as the "tribovoltaic effect." [15] The only difference is that the electron-hole pairs in tribovoltaic effect are excited during friction, in which the energy is released due to either the transition of electrons from the surface states of one side to that of another side in the contact or sliding, [16][17][18][19][20][21] or the energy released while forming a new bond at the interface, [22] just as it occurs in the CE between two conventional materials. [23][24][25][26] Then, the electron-hole pairs are separated by the built-in electric field and the external current flowing from the P-side to the N-side will be induced, as it occurs in the photovoltaic effect. [27][28][29] According to the tribovoltaic effect, the excitation efficiency of electron-hole pairs highly depends on the density of surface states of the semiconductors and the bonding interactions. Hence, the tribovoltaic current may be related to the density of surface states of the semiconductors and the sliding load, which can significantly affect the bonding interactions at the sliding interface. [22] In this work, the direct current is generated by sliding an N-type semiconductor tip over a Si sample surface using conductive atomic force microscopy (CAFM). [30,31] Both P-type and N-type Si samples with various doping concentrations are used in the experiments. Further, the Si samples are treated by inductively coupled plasma reactive ion etching (ICP-RIE), which can significantly increase the density of surface states of the semiconductors by doping or introducing defects on the surface. [32,33] It is revealed that the current density in the sliding experiments increases with increasing of the density of surface Contact electrification (CE or triboelectrification) is a common phenomenon, which can occur for almost all types of materials. In previous studies, the CE between insulators and metals has been widely discussed, while CE involving semiconductors is only recently. Here, a tribo-current is generated by sliding an N-type diamond coated tip on a P-type or N-type Si wafer...

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Contact electrification (CE) at interfaces is sensitive to the functional groups on the solid surface, but its mechanism is poorly understood, especially for the liquid–solid cases. A core controversy is the identity of the charge carriers (electrons or/and ions) in the CE between liquids and solids. Here, the CE between SiO2 surfaces with different functional groups and different liquids, including DI water and organic solutions, is systematically studied, and the contribution of electron transfer is distinguished from that of ion transfer according to the charge decay behavior at surfaces at specific temperature, because electron release follows the thermionic emission theory. It is revealed that electron transfer plays an important role in the CE between liquids and functional group modified SiO2. Moreover, the electron transfer between the DI water and the SiO2 is found highly related to the electron affinity of the functional groups on the SiO2 surfaces, while the electron transfer between organic solutions and the SiO2 is independent of the functional groups, due to the limited ability of organic solutions to donate or gain electrons. An energy band model for the electron transfer between liquids and solids is further proposed, in which the effects of functional groups are considered. The discoveries in this work support the “two-step” model about the formation of an electric double-layer (Wang model), in which the electron transfer occurs first when the liquids contact the solids for the very first time.