Direct Observation of Electron Trapping States in SiO$_2$ by Constant-Final-State Photoelectron Yield Spectroscopy: Insights into the Electron-Transfer Model of Contact Electrification
Understanding electrostatic charging of silicon dioxide (SiO$_2$) is important for electrostatic control in a wide range of applications. Contact electrification (CE) is a major source of such charging, and electron transfer is considered one of its leading mechanisms. Several electron-transfer models have been proposed to explain CE; however, the density of states (DOS) involved in electron transfer has not been experimentally characterized. Determining this DOS is therefore essential for understanding the charging mechanism. However, measuring the DOS of insulating materials such as SiO$_2$ remains challenging: photoemission spectroscopy, the standard technique for probing the DOS of materials, is unsuitable for highly insulating materials because sample charging during measurement can cause spectral distortion or even complete signal loss. Here, we directly and quantitatively determined the in-gap DOS of amorphous SiO$_2$ using constant-final-state photoelectron yield spectroscopy (CFS-YS). Weak occupied states were observed within the band gap, and their marked increase after electron injection revealed that electron-trapping states are distributed over a broad energy range. Vacuum annealing markedly altered the in-gap DOS and electron-trapping behavior. The observed changes suggest a role for surface adsorbates, including water, in the charging behavior of SiO$_2$. Charging simulations based on the experimentally measured in-gap DOS approximately reproduced the previously reported charging behavior and showed that the accumulated charge was concentrated near the surface. These findings indicate that in-gap states distributed over a broad energy range mediate electron transfer during CE and highlight the importance of considering experimentally determined DOS when evaluating proposed electron-transfer models.