Layer-dependent optical conductivity in atomic thin WS₂ by reflection contrast spectroscopy.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Physics

Background:

• Optical conductivity is crucial for understanding electronic states in transition metal dichalcogenides (TMDCs).
• Layered TMDCs, including tungsten disulfide (WS2), are emerging materials with unique electronic properties.
• Electron-phonon interactions drive interband transitions, influencing optical conductivity.

Purpose of the Study:

• To report the optical conductivity of WS2 layers for the first time.
• To investigate the frequency-dependent optical conductivity of WS2.
• To correlate optical conductivity features with the transition from direct to indirect band gaps in WS2.

Main Methods:

• Reflection contrast spectroscopy was used to measure optical conductivity.
• WS2 layers were prepared directly on a SiO2/Si substrate.
• Calculations were performed to predict electronic band structure changes with layer number.

Main Results:

• Monolayer WS2 exhibits a direct excitonic transition at point K with an optical conductivity of 0.37 e²/πℏ in the visible spectrum.
• Optical conductivity of WS2 layers is frequency-dependent.
• Additional spectral features emerge for bilayer to bulk WS2, indicating a band gap transition.

Conclusions:

• The study provides the first measurement of WS2 optical conductivity.
• Observed spectral features confirm the transition from direct to indirect band gaps with increasing WS2 layer number.
• Optical conductivity serves as a sensitive probe for electronic structure evolution in layered TMDCs.