Comparison between Trion and Exciton Electronic Properties in CdSe and PbS Nanoplatelets

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 the image-charge and variational Quantum Monte Carlo methods in effective mass Hamiltonians. We observe that the giant oscillator strength of neutral excitons is largely suppressed in trions. Both negative and positive trions are red-shifted 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 the validity of previous theoretical approximations