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

Abstract

The temperature-dependent (T=4.5-500K) optical constants of monolayer MoS , MoSe , WS , and WSe 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 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. 2 2 2 2 - 5