Recently, "Observations on valence-band electronic structure and surface states of bulk insulators based on fast stabilization process of sample charging in UPS" completed by Professor Xu Xuefeng's research group from the School of Technology at BFU was published in Laser & Photonics Reviews, a top journal in the field of physics and astrophysics (IF=11, District 1, Chinese Academy of Sciences).

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.

Zhulei, a Ph.D student of the School of Technology, is the first author of the paper, with Professor Xu Xuefeng serving as the corresponding author. Ph.D. student Li Wei also contributed to the research. Beijing Forestry University is the signature unit of the first author. This research was supported by the National Natural Science Foundation of China (52375166).

Paper link: https://doi.org/10.1002/lpor.202301069