Temperature-dependent optical constants of monolayer $${\text {MoS}}_2$$, $${\text {MoSe}}_2$$, $${\text {WS}}_2$$, and $${\text {WSe}}_2$$: spectroscopic ellipsometry and first-principles calculations

Abstract

The temperature-dependent ($$T = 4.5 \, \hbox {-} \, 500 \, \hbox {K}$$ T = 4.5 - 500 K ) optical constants of monolayer $${\text {MoS}}_2$$ MoS 2 , $${\text {MoSe}}_2$$ MoSe 2 , $${\text {WS}}_2$$ WS 2 , and $${\text {WSe}}_2$$ 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 $$6.1 \times 10^{-5}$$ 6.1 × 10 - 5 to $$2.6 \times 10^{-4} \, \hbox {K}^{-1}$$ 2.6 × 10 - 4 K - 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.

Dynamic screening of quasiparticles in WS$_2$ monolayers

“…The temperature-induced shift coefficient β, lies within the values reported in literature [10,34,35] and the maximum temperature increase at the highest fluence is approximately 4°K. The excitation-induced broadening parameter for exc-exc scattering D is also consistent with the value reported in the literature [24], and the one for QFC-exc scattering K, is expected to be higher than D due to higher mobility and lower effective mass of free carriers [40], in agreement with our observation.…”

Low energy switching of phase change materials using a 2D thermal boundary layer

“…However, ideally the TMDC material should have negligible interaction with the optical mode propagating in the waveguide. To demonstrate the compatibility of subnanometer thick TMDCs with photonic devices, we compare the changes in optical mode confinement of photonic waveguides with and without the TMDC layer using Finite Dif- being non-absorbing in the infrared due to its large bandgap(Figure S5) 53,54 . Therefore, the WS 2 layer can be incorporated into PCM-based PIC devices with minimal optical effect yet produce a dramatic reduction in the PCM switching energy, which is a highly desirable trait.…”

“…The dimensions were chosen to ensure single mode operation 51,52 . In the simulation model, refractive indices of the Si waveguide, MoS 2 /WS 2 and GeTe layers were obtained from literature 31,53,54 . The refractive index values can be found in the supporting information Figure S5.…”

Thermal Conductivity of Large-Area Polycrystalline MoSe2 Films Grown by Chemical Vapor Deposition

“…In Figure c, two characteristic peaks are observed at 1.54 and 1.77 eV, corresponding to A and B excitonic transitions, respectively. The A and B excitons are associated with the direct-gap transitions at the K point in the Brillouin zone. ,, The energy difference between A and B exciton peaks is attributed to the valence-band splitting induced by strong spin–orbit coupling at 0.23 eV. , These excitonic transitions are considered to be independent of the thickness due to weak coupling between the layers …”