Determining the Structure–Property Relationships of Quasi-Two-Dimensional Semiconductor Nanoplatelets

We
report a theoretical study of CdSe nanoplatelets aimed at identifying
the main factors determining their photophysical properties. Using
atomic configurations optimized with density functional theory calculations,
we computed quasiparticle and exciton binding energies of nanoplatelets
with two to seven monolayers. We employed many body perturbation theory
at the GW level and solved the Bethe-Salpeter equation to obtain absorption
spectra and excitonic properties. Our results, which agree well with
recent experiments, were then used to design a model that allows us
to disentangle the effects of quantum confinement, strain induced
by passivating ligands, and dielectric environment on the electronic
properties of nanoplatelets. We found that, for the model to accurately
reproduce our first principle results, it is critical to account for
surface stress and consider a finite potential barrier and energy-dependent
effective masses when describing quantum confinement. Our findings
call into question previous assumptions on the validity of an infinite
barrier to describe carrier confinement in nanoplatelets, suggesting
that it may be possible to optimize interfacial charge transfer and
extraction by appropriately choosing passivating ligands. The model
developed here is generalizable to core–shell platelets and
enables the description of system sizes not yet directly treatable
by first-principles calculations.