  A new type of two-dimensional layered semiconductor with weak electrostatic
but not van der Waals interlayer interactions, Bi2O2Se, has been recently
synthesized, which shown 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 (Bi2O2Te and
Bi2O2S). The experimental peaks were fully consistent with the calculated
results, and were successfully assigned. Bi2O2S was predicted to have more
Raman-active modes due to its lower symmetry. The shift of 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 of Raman shifts are anisotropic
for degenerate modes although Bi2O2Se and Bi2O2Te 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 2D Bismuth
oxychalcogenides into nanoelectronic devices.
