Binding versus Trapping: Band Structure Guided Ultrafast Dynamics in Monolayer WS2 and Few-Layer ReS2 under Identical Above-Gap Excitation

Abstract

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, 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.