Electron transfer in the contact-electrification between corrugated 2D materials: A first-principles study

Triboelectricity usually occurs between two different materials and can be understood as electron transfer in the framework of the electron-cloud model. Recently, experiments have shown that charge transfer also can happen between chemically identical materials but different surface curvatures. To understand the driving mechanism for this case, we carry out, for the first time, first-principles density functional theory (DFT) investigations of charge transfer due to flexoelectricity and piezoelectricity in two-dimensional materials. Case studies of piezoelectric molybdenum telluride (2H–MoTe2) and non-piezoelectric graphene are analyzed in detail. It is found that a large corrugation of the contacting materials, whether identical or not, causes a nonlinear increase in the charge transfer, leading to electron depletion near concave surfaces and electron accumulation near convex surfaces. The maximum charge transfer is found to occur around an equilibrium separation distance of the contacting materials. In the case of graphene, and different from 2H–MoTe2, both piezoelectric and flexoelectric coefficients are found to increase as the corrugation increases and is followed by a bandgap opening. We stipulate that the present ab initio findings provide new insight toward understanding the origins of triboelectricity.