Dynamics of exciton formation, recombination, and trapping in monolayers of 2D-TMD materials

The optical and electrical properties of two-dimensional transition metal dichalcogenide materials (2D-TMDs) are determined by their exciton dynamics. When the thickness of these materials is thinned down to a single layer, the spatial confinement makes the impact of exciton dynamics on the functionality of 2D-TMD-based devices even more important. Here, we present an investigation of the dynamics of formation and decay of the lowest excitons 2D-TMD monolayers. Excitation energy-dependent transient absorption studies indicated that the exciton formation time increases linearly from similar to 150 fs upon resonant excitation to similar to 500 fs following excitation that is similar to 1.1 eV above the band-gap. This dependence is attributed to the time it takes highly excited electrons in the conduction band (CB) to relax to the CB minimum (CBM) and contribute to the formation of the X-A exciton. Additionally, excitation energy dependent studies suggested that the exciton average lifetime increases from similar to 10 ps in the case of resonant excitation to similar to 50 ps following excitation well above the band-gap. Furthermore, studies of the dependence of exciton dynamics on the excitation density suggested that exciton decay in these materials is dominated by defect-assisted recombination (DAR).