Exciton-Driven and Layer-Independent Linear and Nonlinear Optical Properties in NbOCl.

We investigate the electronic structure and linear and nonlinear [second-harmonic generation (SHG)] spectra of the NbOCl monolayer, bilayer, and bulk by using a real-time first-principles approach based on many-body theory. First, the interlayer couplings between NbOCl layers are very weak, due to the relatively large interlayer distance, saturation of the p orbital of Cl atoms, and high degree of localization of charge density around the Nb atom for both the lowest conduction band and the highest valence band. Second, the quasiparticle gaps and exciton binding energy for the three systems show layer-dependent features and decrease with an increase in layer thickness. Most importantly, the linear and SHG spectra of the NbOCl monolayer, bilayer, and bulk are dominated by strong excitonic resonances and exhibit layer-independent features due to the weak interlayer couplings. Our findings demonstrate that excitonic effects should be included in studying the optical properties of not only two-dimensional materials but also layered bulk materials with weak interlayer couplings.