Description
Robust excitons dominate the optical properties of atomically thin semiconductors based on transition-metal dichalcogenides. However, a crucial question persists: What is the exciton formation mechanism ?
This study addresses this fundamental problem through polarization-dependent micro-PL studies on WSe2 and MoS2 monolayers close to the neutrality point. The results of our experiments clarify the role played by the two potential formation mechanisms: a) geminate and b) bimolecular. The geminate formation process corresponds to the monomolecular annihilation of the photogenerated correlated electron-hole pair. In contrast the non-geminate formation results from the random bimolecular binding of two free charges, losing all correlation between the excitation photon and the electron-hole pair of the exciton.
For a laser excitation energy below the band gap, we show that the geminate mechanism prevails as expected, whereas above the band gap, both geminate and bimolecular process coexist1. This means that we must go beyond the simple description used until now of either a totally geminate or totally bimolecular formation process.
1 Mourzidis et al, PRX 15 , 031078 (2025)
