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.
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