  We present results on photoexcited carrier lifetimes in few-layer transition
metal dichalcogenide MoS2 using nondegenerate ultrafast optical pump-probe
technique. Our results show a sharp increase of the carrier lifetimes with the
number of layers in the sample. Carrier lifetimes increase from few tens of
picoseconds in monolayer samples to more than a nanosecond in 10-layer samples.
The inverse carrier lifetime was found to scale according to the probability of
the carriers being present at the surface layers, as given by the carrier
wavefunction in few layer samples, which can be treated as quantum wells. The
carrier lifetimes were found to be largely independent of the temperature and
the inverse carrier lifetimes scaled linearly with the photoexcited carrier
density. These observations are consistent with defect-assisted carrier
recombination, in which the capture of electrons and holes by defects occurs
via Auger scatterings. Our results suggest that carrier lifetimes in few-layer
samples are surface recombination limited due to the much larger defect
densities at the surface layers compared to the inner layers.
