Calculations of Second-Order Raman Scattering in Monolayer Black Phosphorus

Ab initio electronic and phonons band structure calculations were performed on a monolayer of Black Phosphorus to simulate and identify the origin of bulk-forbidden Raman modes in the experimental spectrum. In our previous experimental work, we observed four of these new modes in the A_g¹ and A_g² regions, called the D modes. They are present for the three wavelengths at which they were probed and appear to dominate the Raman spectrum for an excitation wavelength of 633 nm. The laser frequency and layer number dependence of the D modes, that seem linked to the decrease of the peak intensity ratio A_g¹/A_g² with degradation time, suggest that these modes are “defect-induced” second-order Raman scattering. The simulated Raman spectrum adds to this conclusion by identifying them to be due to phonon-defect interactions occurring through intravalley scatterings. This talk will focus on the simulation and theoretical details. Using scattering theory and values obtained from DFT calculations, we identified which electronic states were coupled resonantly by a phonon and a defect. From those states, we extracted the corresponding phonon frequency to build the Raman spectrum. It allowed us to locate the regions of the band structures mostly contributing to the Raman spectrum.