Physics

Infrared

Nanotechnology

Analytical Chemistry (journal)

Optics

Band gap

Ellipsometry

Chemistry

Thin film

Materials science

Photonics

Condensed matter physics

Molecular physics

Monolayer

Atmospheric temperature range

Refractive index

Chromatography

Optoelectronics

Wavelength

Thermodynamics

Scientific Reports

The temperature-dependent ( $$T = 4.5 \, \hbox {-} \, 500 \, \hbox {K}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>T</mml:mi> <mml:mo>=</mml:mo> <mml:mn>4.5</mml:mn> <mml:mspace /> <mml:mtext>-</mml:mtext> <mml:mspace /> <mml:mn>500</mml:mn> <mml:mspace /> <mml:mtext>K</mml:mtext> </mml:mrow> </mml:math> ) optical constants of monolayer $${\text {MoS}}_2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mtext>MoS</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> </mml:math> , $${\text {MoSe}}_2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mtext>MoSe</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> </mml:math> , $${\text {WS}}_2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mtext>WS</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> </mml:math> , and $${\text {WSe}}_2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mtext>WSe</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> </mml:math> 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 $$6.1 \times 10^{-5}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>6.1</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>5</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> to $$2.6 \times 10^{-4} \, \hbox {K}^{-1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>2.6</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> <mml:mspace /> <mml:msup> <mml:mtext>K</mml:mtext> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> 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