Neutral and charged dark excitons in monolayer WS₂

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Optics

Background:

• Transition Metal Dichalcogenides (TMDs) like WS2 are crucial 2D materials with unique optoelectronic properties.
• Understanding excitonic states, particularly dark states, is key to advancing TMD device applications.
• Encapsulation in hexagonal boron nitride (hBN) enhances TMD properties by reducing defects and charge noise.

Purpose of the Study:

• Investigate the properties of dark excitons and dark trions in WS2 monolayers.
• Determine the influence of magnetic fields on these states.
• Characterize the decay dynamics and interactions of dark excitons and trions.

Main Methods:

• Low-temperature, polarization-resolved magneto-photoluminescence spectroscopy.
• Experimental setup utilizing perpendicular and parallel magnetic field configurations.
• Analysis of emission spectra to identify and differentiate excitonic species.

Main Results:

• WS2 monolayer is confirmed as an n-type doped semiconductor.
• Dark trions are found to dominate the emission spectrum.
• Coulomb exchange interaction coupling dark and grey excitons identified via polarization, but not for dark trions.
• Determined g-factor for dark trions to be approximately -8.6.
• Measured decay rate for dark trions is ~0.5 ns, over 100 times longer than bright excitons.

Conclusions:

• Dark trions are the primary emissive species in this WS2 system.
• The long decay rate of dark trions suggests potential for applications in quantum information and low-loss devices.
• Magnetic field studies provide crucial parameters like the g-factor for dark trions.