Direct Observation of Density of States in the Energy Gap of Polyethylene Films using High-sensitivity Ultraviolet Photoemission Spectroscopy: Implications for Electron Transfer in Contact Charging

Electrostatic charging of insulating polymers is a fundamental phenomenon, yet its microscopic mechanism remains controversial due to the lack of direct experimental evidence of electronic states within the bandgap. This paper reports the first direct observation of the density of states (DOS) in micrometer-scale self-standing polyethylene (PE) films using high-sensitivity ultraviolet photoemission spectroscopy and constant final state yield spectroscopy. The observed spectra clearly demonstrate the existence of in-gap states distributed throughout the entire bandgap, showing that the DOS follows an exponential function of the binding energy. We negatively charge the film via electron beam irradiation and observe that the photoemission threshold shifts to lower energy, which indicates that injected electrons fill these specific in-gap trap states. Furthermore, exoelectron emission is detected without photo-irradiation, which is attributed to the negative electron affinity of PE. Electrostatic simulations using the measured DOS successfully reproduce contact charging characteristics, confirming that the in-gap states have sufficient density to quantitatively account for the electrification. These simulations further allow for the determination of the charge penetration depth. These findings provide compelling evidence that in-gap states govern electron transfer during contact electrification, offering new insights into space charge accumulation, voltage endurance, and dielectric breakdown in polymeric insulators.