Ellipsometric Characterization of Monolayer MoSe₂ from 31 to 300K

The layered transition metal dichalcogenides (TMDCs) has recently gained significant interest due to their distinctive physical properties. Molybdenum diselenide (MoSe₂) is well known as one of transition metal dichalcogenides, which suggests promise as a potential substitute for silicon in state-of-the-art transistors, sensors, and photodetectors. A systematic study on temperature dependence of the dielectric function and critical point energies of MoSe₂ is therefore strongly needed.
In this work, we investigate the dielectric function of 2D MoSe₂ in 0.74 to 6.42 eV energy range at temperatures from 31 to 300 K by spectroscopic ellipsometry. The CP transitions are observed and their energies are obtained by fitting standard analytic expressions to second energy derivatives of the data. We found six new CPs at low temperature beside another six CPs realized at room temperature. Blue shift and enhancement of most CP energies at low temperatures were observed and understood by the reduced lattice constant and electron-phonon interaction. The temperature dependences of these CPs were determined by fitting the data to the coefficients in a phenomenological expression that contains the Bose-Einstein statistical factor.