Excitonic effects in the K and L2,3 edges spectra of bulk and monolayer black phosphorus from first-principles

In this study, the K and L2,3 X-ray absorption near edge structure (XANES) of bulk and monolayer black phosphorus (BP) were calculated and compared. Calculations were performed in two different levels of theory, with and without considering the excitonic effects. The present calculations indicated that both bulk and monolayer BP showed strong anisotropy in their core absorption spectra. The results demonstrated that the electron-hole coupling and formation of the bound exciton had a significant effect on the edge spectra of both bulk and monolayer BP in a way that the excitonic effect modified the oscillator strengths and the energy positions of the main spectral structures, intensely. The edge spectra of bulk were in a good agreement with the existing experimental core spectra. As the focus shifted from bulk to monolayer, all the K and L2,3 absorption spectra were red-shifted. When excitonic effects were included in the calculations, the spectral features were red-shifted, as well. Decomposition of L2,3 edge to L2 and L3 edges indicated that the contribution of the L3 edge was dominant. Local densities of states (DOS) calculations showed that p-like and d-like states of phosphorus atoms play the main role in the electron transitions of K and L2,3 edges spectra, respectively.