Probing excitonic dark states in single-layer tungsten disulphide

Area of Science:

• Condensed Matter Physics
• Materials Science
• Nanoscience

Background:

• Transition metal dichalcogenide (TMDC) monolayers are promising 2D semiconductors for electronic and optoelectronic applications.
• TMDCs exhibit unique optical phenomena in monolayer form, including direct bandgaps and strong light-matter interactions, though underlying mechanisms require further investigation.
• First-principles calculations suggest excitonic effects dominate the optical response, predicting larger quasiparticle bandgaps than measured optical gaps.

Purpose of the Study:

• To experimentally investigate excitonic states in single-layer WS2.
• To elucidate the nature and properties of excitons in TMDC monolayers.
• To understand the fundamental mechanisms behind strong light-matter interactions in these 2D materials.

Main Methods:

• Two-photon excitation spectroscopy was employed to detect excitonic dark states in single-layer WS2.
• GW plus Bethe-Salpeter equation (GW-BSE) calculations, a many-body Green's-function approach, were used to model electron-electron and electron-hole interactions.
• Experimental findings were combined with theoretical predictions to characterize the excitons.

Main Results:

• Experimental evidence for a series of excitonic dark states in single-layer WS2 was obtained.
• Excitons were identified as Wannier-type with exceptionally large binding energies (approximately 0.7 eV), resulting in a quasiparticle bandgap of 2.7 eV.
• These strongly bound excitons were found to be stable at room temperature and exhibited a novel energy dependence on orbital angular momentum, deviating from hydrogenic models.

Conclusions:

• The study confirms the presence of excitonic dark states and large exciton binding energies in single-layer WS2, highlighting the significance of many-electron effects.
• The findings provide crucial insights into the strong light-matter interactions in 2D semiconductors.
• The discovered excitonic properties hold potential for future applications in computing, communication, and bio-sensing devices utilizing TMDC monolayers.