  Recent experiments suggest that the photoluminescence line width of CdSe and
CdSe/CdS nanoplatelets (NPLs) may be broadened by the presence of shakeup (SU)
lines from negatively charged trions. We carry out a theoretical analysis,
based on effective mass and configuration interaction (CI) simulations, to
identify the physical conditions that enable such processes. We confirm that
trions in colloidal NPLs are susceptible of presenting SU lines up to one order
of magnitude stronger than in epitaxial quantum wells, stimulated by dielectric
confinement. For these processes to take place trions must be weakly bound to
off-centered impurities, which relax symmetry selection rules. Charges on the
lateral sidewalls are particularly efficient to this end. We propose that the
broad line width reported for core/shell CdSe/CdS NPLs may relate not only to
SU processes but also to a metastable spin triplet trion state. Understanding
the origin of SU processes opens paths to rational design of NPLs with narrower
line width.
