Temperature-dependent indirect gaps for two-dimensional bismuth oxychalcogenides probed by spectroscopic ellipsometry

In-plane optical properties of two-dimensional bismuth oxychalcogenides Bi₂O₂X (X = S, Se, and Te) are reported for a wide spectral range of 0.73-6.42 eV and at temperatures of 4.5-500 K by spectroscopic ellipsometry. At room temperature, Bi₂O₂S, Bi₂O₂Se, and Bi₂O₂Te exhibit an indirect band gap of 1.18 ± 0.02, 0.95 ± 0.01, and 0.60 ± 0.01 eV, respectively. As the temperature decreases, the indirect absorption edge of Bi₂O₂S undergoes a blueshift, while the indirect band gap of Bi₂O₂Se shows a redshift, and Bi₂O₂Te remains independent of temperature. The chalcogenide-dependent behavior as a function of temperature may be relevant to electron-phonon interactions in Bi₂O₂X materials. The observed pseudo-isotropic complex dielectric function and optical absorption coefficient by spectroscopic ellipsometry are directly compared with the first-principles calculations with a hybrid functional approach.