Tunable lattice dynamics and dielectric functions of two-dimensional Bi ₂ O ₂ Se: striking layer and temperature dependent effects

Two-dimensional (2D) Bi₂O₂Se semiconductors with a narrow band gap and ultrahigh mobility have been regarded as an emerging candidate for optoelectronic devices, whereas the ambiguous phonon characteristics and optical properties still limit their future applications. Herein, high-quality centimeter-scale 2D Bi₂O₂Se films are successfully synthesized to disclose the lattice dynamics and dielectric functions under the control of thickness and temperature. It has been demonstrated that the stronger electrostatic Bi-Se interactions result in a stiffened phonon vibration of thicker Bi₂O₂Se layers. Three excitons (E_a, E_b, and E_c) exhibit significant red shifts with layer stacking. Interestingly, the dielectric properties in the visible-near infrared region (E_a and E_b) are dominated by the combined effect of the joint density of states and mass density, whereas the dielectric properties in the ultraviolet region (E_c) are dominated by the exciton effect. Furthermore, the temperature-sensitivity of the phonon frequency and exciton transition energies is revealed to be layer-dependent. In particular, the optical response of E_b excitons exhibits a prominent dependence on temperature, which indicates a promising optical modulation by temperature in the visible spectrum. This study enriches the knowledge about phonon dynamics and dielectric properties for 2D Bi₂O₂Se, which provides an essential reference for high-performance related optoelectronic devices.