  Solution-processable two-dimensional (2D) semiconductors with chemically
tunable thickness and associated tunable band gaps are highly promising
materials for ultrathin optoelectronics. Here, the properties of free charge
carriers and excitons in 2D PbS nanosheets of different thickness are
investigated by means of optical pump-terahertz probe spectroscopy. By
analyzing the frequency-dependent THz response, a large quantum yield of
excitons is found. The scattering time of free charge carriers increases with
nanosheet thickness, which is ascribed to reduced effects of surface defects
and ligands in thicker nanosheets. The data discussed provide values for the DC
mobility in the range 550 - 1000 cm2/Vs for PbS nanosheets with thicknesses
ranging from 4 to 16 nm. Results underpin the suitability of colloidal 2D PbS
nanosheets for optoelectronic applications.
