Temperature-dependent optical constants of monolayer MoS ₂ , MoSe ₂ , WS ₂ , and WSe ₂: spectroscopic ellipsometry and first-principles calculations

The temperature-dependent (T=4.5-500K) optical constants of monolayer MoS ₂, MoSe ₂, WS ₂, and WSe ₂ were investigated through spectroscopic ellipsometry over the spectral range of 0.73–6.42 eV. At room temperature, the spectra of refractive index exhibited several anomalous dispersion features below 800 nm and approached a constant value of 3.5–4.0 in the near-infrared frequency range. With a decrease in temperature, the refractive indices decreased monotonically in the near-infrared region due to the temperature-dependent optical band gap. The thermo-optic coefficients at room temperature had values from 6.1 × 10 ^{- 5} to 2.6×10-4K-1 for monolayer transition metal dichalcogenides at a wavelength of 1200 nm below the optical band gap. The optical band gap increased with a decrease in temperature due to the suppression of electron–phonon interactions. On the basis of first-principles calculations, the observed optical excitations at 4.5 K were appropriately assigned. These results provide basic information for the technological development of monolayer transition metal dichalcogenides-based photonic devices at various temperatures.