Binding versus trapping: band structure guided ultrafast dynamics in monolayer WS₂ and few-layer ReS₂ under identical...

Femtosecond transient absorption measurements under identical 400 nm above-gap excitation directly contrast the photoexcited-state pathways of monolayer WS₂ and few-layer ReS₂, providing a unified benchmark for two prototypical yet fundamentally dissimilar TMD semiconductors. Monolayer WS₂ exhibits a strongly excitonic regime, showing pronounced A/B/C exciton bleaches accompanied by discrete biexciton related induced absorption bands (AA and BB). The biexciton binding energies were extracted, and a 55-65 meV transient blue shift of the biexciton resonances was observed, with the largest shift rate occurring at ∼1.5 ps, consistent with ultrafast excitonic cooling-dominated relaxation. The exciton population then depopulates on tens-of-picoseconds timescales through many-body interactions and recombination/trapping. In contrast, ReS₂ shows only weak, sub-ps-lived excitonic bleaches and is dominated by broadband photoinduced absorption that persists for hundreds of picoseconds to ∼1 ns, indicative of unbound and localized carriers consistent with carrier-/trap-associated relaxation. This quantitative dichotomy, Coulomb-bound exciton physics versus trap-limited carrier dynamics, offers a parent-material benchmark relevant to interpreting and designing WS₂/ReS₂ heterostructures that couple ultrafast light harvesting with prolonged charge storage for photodetection and optoelectronic memory concepts.