Raman Spectra and Strain Effects in Bismuth Oxychalcogenides

A new type of two-dimensional layered semiconductor with weak electrostatic but not van der Waals interlayer interactions, Bi2O2Se, has been recently synthesized, which shows excellent air stability and ultrahigh carrier mobility. Herein, we combined theoretical and experimental approaches to study the Raman spectra of Bi2O2Se and related bismuth oxychalcogenides (Bi2O2 Te and Bi2O2S). The experimental peaks lie at 160 cm(-1) in Bi2O2 Se and at 147 and 340 cm(-1) in Bi2O2Te. They were fully consistent with the calculated results (159.89, 147.48, and 340.33 cm(-1)) and assigned to the out-of-plane A(1g), A(1g), and B-1g modes, respectively. Bi2O2S was predicted to have more Raman-active modes due to its lower symmetry. The shift in the predicted frequencies of Raman-active modes was also found to get softened as the interlayer interaction decreases from bulk to monolayer Bi2O2Se and Bi2O2Te. To reveal the strain effects on the Raman shifts, a universal theoretical equation was established based on the symmetry of Bi2O2Se and Bi2O2Te. It was predicted that the doubly degenerate modes split under in-plane uniaxial/shear strains. Under a rotated uniaxial strain, the changes in Raman shifts are anisotropic for degenerate modes, although Bi2O2Se and Bi2O2 Te were usually regarded as isotropic systems similar to graphene. This implies a novel method to identify the crystallographic orientation from Raman spectra under strain. These results have important consequences for the incorporation of two-dimensional bismuth oxychalcogenides into nanoelectronic devices.