  The optoelectronic properties of metal chalcogenide colloidal nanoplatelets
are often interpreted in terms of excitonic states. However, recent
spectroscopic experiments evidence the presence of trion states, enabled by the
slow Auger recombination in these structures. We analyze how the presence of an
additional charge in trions modifies the emission energy and oscillator
strength as compared to neutral excitons. These properties are very sensitive
to dielectric confinement and electronic correlations, which we describe
accurately using image-charge and variational Quantum Monte Carlo methods in
effective mass Hamiltonians. We observe that the giant oscillator strength of
neutral excitons is largely suppressedin trions. Both negative and positive
trions are redshifted with respect to the exciton, and their emission energy
increases with increasing dielectric mismatch between the platelet and its
surroundings, which is a consequence of the self-energy potential. Our results
are consistent with experiments in the literature, and assess on the validity
of previous theoretical approximations.
