Search for: All records

Understanding excitonic dynamics in two-dimensional semiconducting transition metal dichalcogenides is important for developing their optoelectronic applications. Recently, transient absorption techniques based on resonant excitonic absorption have been used to study various aspects of excitonic dynamics in these materials. The transient absorption in such measurements originates from phase-space state filling, bandgap renormalization, or screening effects. Here we report a new method to probe excitonic dynamics based on exciton intraband absorption. In this Drude-like process, probe photons are absorbed by excitons in their intraband excitation to higher energy states, causing a transient absorption signal. Although the magnitude of the transient absorption is lower than that of the resonant techniques, the new method is less restrictive on the selection of probe wavelength, has a larger linear range, and can provide complementary information on photocarrier dynamics. Using the WS 2 monolayer and bulk samples as examples, we show that the new method can probe exciton–exciton annihilation at high densities and reveal exciton formation processes. We also found that the exciton intraband absorption cross section of the WS 2 monolayer is on the order of 10 −18 cm 2 . 

The electron dynamics in heterostructures formed by multilayer graphite and monolayer or bulk MoS 2 were studied by femtosecond transient absorption measurements. Samples of monolayer MoS 2 -multilayer graphite and bulk MoS 2 -multilayer graphite were fabricated by exfoliation and dry transfer techniques. Ultrafast laser pulses were used to inject electron–hole pairs into monolayer or bulk MoS 2 . The transfer of these photocarriers to the adjacent multilayer graphite was time resolved by measuring the differential reflection of a probe pulse. We found that photocarriers injected into monolayer MoS 2 transfer to graphite on an ultrafast time scale shorter than 400 fs. Such an efficient charge transfer is key to the development of high performance optoelectronic devices with MoS 2 as the light absorbing layer and graphite as electrodes. The absorption coefficient of monolayer MoS 2 can be controlled by the carriers in graphite. This process can be used for interlayer coupling and control. In a bulk MoS 2 -graphite heterostructure, the photocarrier transfer time is about 220 ps, due to the inefficient interlayer charge transport in bulk MoS 2 . These results provide useful information for developing optoelectronic devices based on MoS 2 -graphite heterostructures.