Temperature dependence of optical properties of monolayer WS₂ by spectroscopic ellipsometry

The complex dielectric function ɛ = ɛ₁ + iɛ₂ of monolayer tungsten disulfide (WS₂) is investigated for the energy range from 1.5 to 6.0 eV and temperatures from 41 to 300 K. Measurements were performed under ultra-high vacuum conditions to avoid degradation and overlayer contamination. Fourteen critical-point (CP) energies were observed and their origins in Brillouin-zone were identified by band structure calculations. At low temperature the A and B excitonic peaks split into four CPs, which is understood as the neutral and charged exciton states of monolayer WS₂. At low temperatures the CP energies show blue shifts with enhanced structure due to the reduction of electron-phonon interaction. The temperature dependence of these data was obtained by a phenomenological expression with Bose-Einstein statistical factor.