Experimental Evidence for Dark Excitons in Monolayer WSe_{2}

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Transition metal dichalcogenides (TMDs) like WSe2 are 2D semiconductors with unique electronic properties.
• Monolayer TMDs exhibit direct band gaps, making them promising for optoelectronic applications.
• Understanding exciton behavior is crucial for optimizing light emission in these materials.

Purpose of the Study:

• To investigate the photoluminescence and exciton dynamics in monolayer tungsten diselenide (WSe2).
• To identify and characterize the nature of the band-gap exciton states in WSe2.
• To correlate experimental observations with theoretical predictions regarding spin polarization.

Main Methods:

• Temperature-dependent photoluminescence spectroscopy to study emission intensity variations with temperature.
• Time-resolved photoluminescence spectroscopy to probe exciton lifetimes and dynamics.
• Analysis of experimental data in conjunction with theoretical models of electronic band structure.

Main Results:

• Experimental evidence for an optically forbidden dark exciton state in monolayer WSe2, located below the bright exciton state.
• Observation of significant quenching of photoluminescence intensity at reduced temperatures, attributed to the dark exciton.
• Results align with theoretical predictions of spin-polarized conduction and valence bands at the K point.

Conclusions:

• The presence of a low-lying dark exciton state significantly influences the optical properties of monolayer WSe2.
• This dark state contributes to the temperature-dependent quenching of light emission.
• The findings support theoretical models of spin-polarized band structures in 2D TMDs.