Recent progress in the exfoliation of layered materials and the nanofabrication of functional structures has revived the interest in two-dimensional materials with properties complementary to graphene, in particular transition metal dichalcogenides such as MoS2.The absorption spectra of 2D semiconductors are dominated by excitons with binding energy of up to several hundreds of meV. Nevertheless, even single layers show an appreciable photovoltaic effect and can work as the active material in high sensitivity photodetectors, thus indicating some degree of charge carrier photogeneration. Using femtosecond optical pump-probe spectroscopy we show that in few-layer MoS2, the hot electron-hole pairs created by photoexcitation relax and form free carriers with a time scale of 700 fs. In the monolayer, on the other hand, during this relaxation there is a branching into excitons and free carrier pairs. Already a moderate in-plane electric field of a few kVcm-1, applied via the source-drain voltage in a field-effect transistor, can significantly change the branching ratio in favour of free carrier pairs.
The fate of photogenerated carriers is trapping at defects or, at higher fluencies, once the defects are saturated, via non-geminate recombination. From the fluence dependent pump-probe signal on different flake thicknesses we find that thicker flakes have a much higher defect content.
Due to the strong electron-electron interaction in 2d materials, many body effects become important already at relatively low excitation densities. We find that the fs optical pump probe signal originates mostly from a combination of band gap renormalization and screening of the exciton and trion binding energy during the first few picoseconds, while at longer delays Pauli blocking is the dominate signal origin.