Optical functions and transitions are essential for a material to reveal the light-matter interactions and promote its applications. Here, we propose a quantitative strategy to systematically identify the critical point (CP) optical transitions of 2D semiconductors by combining the spectroscopic ellipsometry (SE) and DFT calculations. Optical functions and CPs are determined by SE, and connected to DFT band structure and projected density of states via equal-energy and equal-momentum lines. The combination of SE and DFT provides a powerful tool to investigate the CP optical transitions, including the transition energies and positions in Brillouin zone (BZ), and the involved energy bands and carries. As an example, the single-crystal monolayer WS2 is investigated by the proposed method. Results indicate that six excitonic-type CPs can be quantitatively distinguished in optical function of the monolayer WS2 over the spectral range of 245-1000 nm. These CPs are identified as direct optical transitions from three highest valence bands to three lowest conduction bands at high symmetry points in BZ contributed by electrons in S-3p and W-5d orbitals. Results and discussion on the monolayer WS2 demonstrate the effectiveness and advantages of the proposed method, which is general and can be easily extended to other materials.
Keywords: first-principle calculations; monolayer WS2; optical functions; optical transitions; spectroscopic ellipsometry.