  For high-speed optoelectronic applications relying on fast relaxation or
energy transfer mechanisms, understanding of carrier relaxation and
recombination dynamics is critical. Here, we compare the differences in
photoexcited carrier dynamics in 2D and quasi-3D colloidal methylammonium lead
iodide perovskite nanoplatelets via differential transmission spectroscopy. We
find that the cooling of excited electron-hole pairs by phonon emission
progresses much faster and is intensity-independent in the 2D-case. This is due
to the low dielectric surrounding of the thin perovskite layers, for which the
Fr\"ohlich interaction is screened less efficiently leading to higher and less
density dependent carrier-phonon scattering rates. In addition, rapid
dissipation of heat into the surrounding occurs due to the high
surface-to-volume ratio. Furthermore, we observe a sub-picosecond dissociation
of resonantly excited 1s excitons in the quasi-3D case, an effect which is
suppressed in the 2D nanoplatelets due to their large exciton binding energies.
The results highlight the importance of the surrounding environment of the
inorganic nanoplatelets on their relaxation dynamics. Moreover, this 2D
material with relaxation times in the sub-picosecond regime shows great
potential for realizing devices such as photodetectors or all-optical switches
operating at THz frequencies.
