Electronic structure of polytetrafluoroethylene with surface states and its role on contact electrification

First-principles calculations on polytetrafluoroethylene (one-dimensional PTFE, 21, 103, and 41) with surface states (SS) are performed. The SS are modelled by defluorination along the chain, and each case is spin optimized, where all possible spin configurations are investigated to determine the lowest energy structure with various charge states. Based on this, the calculated band structures are compared with each other where SS are marked, particularly for the lowest unoccupied level (LUL). The locations of these energy levels are used to explain how PTFE shows strong negative triboelectric material for contact electrification. The lower the level of LUL, the more negative the triboelectric PTFE, due to the increase of probability of electron trap into that level during contact electrification. The SS formation energy was calculated and its relation to the LUL is clearly shown to be inter-correlated. The lower the stability of PTFE, the higher the electron accepting property of PTFE. Also, the hybrid exchange-correlation functional, HSE06 is used to compare vertical electron affinity more quantitatively, to explain the relation to LUL and the negative triboelectric property of PTFE.