Visualizing Line Defects in non-van der Waals Bi₂O₂Se Using Raman Spectroscopy

Atomic-layered materials, such as high-quality bismuth oxychalcogenides, which are composed of oppositely charged alternate layers grown using chemical vapor deposition, have attracted considerable attention. Their physical properties are well-suited for high-speed, low-power-consumption optoelectronic devices, and the rapid determination of their crystallographic characteristics is crucial for scalability and integration. In this study, we introduce how the crystallographic structure and quality of such materials can be projected through Raman spectroscopy analysis. Frequency modes at ∼55, ∼78, ∼360, and ∼434 cm^-1 were detected, bearing out theoretical calculations from the literature. The low-frequency modes positioned at 55 and 78 cm^-1 were activated by structural defects, such as grain boundaries and O-rich edges in the Bi₂O₂Se crystals, accompanied by sensitivity to the excitation energy. Furthermore, the line defects at ∼55 cm^-1 exhibited a strong 2-fold polarization dependence, similar to graphene/graphite edges. Our results can help illuminate the mechanism for activating the Raman-active mode from the infrared active mode by defects, as well as the electronic structures of these two-dimensional layered materials. We also suggest that the nanoscale width line defects in Bi₂O₂Se can be visualized using Raman spectroscopy.