Observations on Valence‐Band Electronic Structure and Surface States of Bulk Insulators Based on Fast Stabilization Process of Sample Charging in UPS

The valence-band electronic structure is one of the most basic properties of materials and ultraviolet photoelectron spectroscopy (UPS) is the most commonly used technique for its measurement. To obtain credible UPS spectra on insulator surfaces, eliminating the charging effects at the surfaces is essential but is very difficult. Here, a reliable measurement for the valence-band structure of bulk insulators is shown. It has been observed that, during UPS measurements without any charge compensation, the charging on irradiated insulator surfaces reaches its steady state very quickly and the fluctuation of the charging-induced surface potential is very small. Therefore, the whole UPS spectrum undergoes just a parallel shift and can be calibrated by comparison with the corresponding X-ray photoelectron spectroscopy (XPS) spectrum even though XPS often has a poorer resolution than UPS. From the calibrated valence-band spectra, it is found that noticeable electronic energy levels (i.e., the "surface states") exist in the bandgap, and based on the density of states (DOS) calculations, the surface states are attributed to the truncation of the bulk periodic structure at the surfaces. This work is anticipated to provide a way to obtain the valence band structure and the surface states of bulk insulators which play crucial roles in many physical processes. Fast stabilization of surface charging is observed in UPS measurements on bulk insulators, indicating the UPS spectra undergo a parallel shift that is derived by comparisons between the spectra of UPS and XPS. The calibrated spectra show noticeable surface states in the bandgap that is attributed to the truncation of the bulk periodic structure at the surfaces. image