Temperature dependence of optical properties of monolayer WS2 by spectroscopic ellipsometry

[Display omitted] •Dielectric function of monolayer WS2 is reported from 1.5 to 6.0 eV.•Temperature dependence of the CPs is reported from 41 to 300 K.•Separation of the neutral and charged excitons is observed.•Origins of the CPs are identified by band structure calculation. 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.