In the last few years, tremendous research interest has been focused on two-dimensional transition metal dichalcogenides, following progress in processing of layers with atomic-size thickness. Among them, molybdenum disulfide (MoS2) nanoflakes have shown unique optical and electrical properties. They are excellent absorbers, despite being ultrathin, with high exciton-binding energies that make excitonic transitions evident even at room temperature. Bulk and few-layer MoS2 are indirect band gap semiconductors, while its monolayer is a direct band gap semiconductor. Also, bound trions have been reported in monolayer MoS2, due to strong interactions between excitons and charges and spatial confinement of the photoexcited species.
Ultrafast spectroscopy has brought important clarification to the aforementioned properties in MoS2 nanoflakes and has unraveled their carrier dynamics and transport properties. However, the strong dependence of the fundamental properties on the nanoflake size and preparation process clearly shows that additional research is needed to understand the rich and unusual photophysics in this system. Therefore, we have used time-resolved absorption and THz transmission spectroscopy to shed light on the photophysical properties of solution-processed, few-layer MoS2 nanoflakes with subpicosecond temporal resolution. Using different excitation photon energies and fluences, we have resolved the carrier and exciton relaxation, the recombination processes and the corresponding time scales. Also, we have used the spectrum of the complex photoconductivity in the THz region to study the carrier transport properties in the nanoflakes as a function of number of layers.
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