Ultrafast time-resolved investigations of excitons and biexcitons at room temperature in layered WS2

Strong light-matter interactions in layered transition metal dichalcogenides (TMDs) open up vivid possibilities for novel excitonic quasiparticle-based devices. The optical properties of TMDs are dominated mostly by the tightly bound excitons and more complex quasiparticles, the biexcitons. Instead of physically exfoliated monolayers, the solvent-mediated chemical exfoliation of these 2D crystals is a cost-effective, large-scale production method suitable for substantial practical implications. Here, we explore the ultrafast excitonic phenomena in layered WS2 (mono-to-quad) dispersion using broadband (350-750 nm) femtosecond pump-probe spectroscopy at room temperature (300 K) which are inaccessible to the steady-state absorption or emission spectroscopy. The transient absorption spectra (TAS) suggest that the mono-to-quad layered dispersion of WS2 has similar spectral features as monolayer WS2 in terms of saturation absorptions (SA) and excited state absorptions (ESA). Similar to monolayer TMDs, we are able to identify excitons and biexcitons in multi-layered 2D stratum of WS2 as well as calculate the biexciton binding energies ( 69 meV and 66 meV), which are in excellent agreement with earlier theoretical predictions. Furthermore, using many-body physics, we demonstrate that the excitons in layered WS2 behave like Wannier-Mott excitons and explain their origins via first-principles calculations. Our detailed time-resolved investigation provides ultrafast radiative and non-radiative lifetimes of the excitons and biexcitons in layered WS2. Indeed, our results unravel the complex optical response of layered TMDs, which should lead to numerous technological applications for developing excitonic quasiparticle-based valleytronic devices and ultrafast biexciton lasers at room temperature.