  Semiconductor nanoplatelets (NPLs), with their large exciton binding energy,
narrow photoluminescence (PL), and absence of dielectric screening for photons
emitted normal to the NPL surface, could be expected to become the fastest
luminophores amongst all colloidal nanostructures. However, super-fast emission
is suppressed by a dark (optically passive) exciton ground state, substantially
split from a higher-lying bright (optically active) state. Here, the exciton
fine structure in 2-8 monolayer (ML) thick Cs_{n-1}Pb_nBr_{3n+1} NPLs is
revealed by merging temperature-resolved PL spectra and time-resolved PL decay
with an effective mass modeling taking quantum confinement and dielectric
confinement anisotropy into account. This approach exposes a
thickness-dependent bright-dark exciton splitting reaching 32.3meV for the 2ML
NPLs. The model also reveals a 5-16 meV splitting of the bright exciton states
with transition dipoles polarized parallel and perpendicular to the NPL
surfaces, the order of which is reversed for the thinnest NPLs, as confirmed by
TR-PL measurements. Accordingly, the individual bright states must be taken
into account, while the dark exciton state strongly affects the optical
properties of the thinnest NPLs even at room temperature. Significantly, the
derived model can be generalized for any isotropically or anisotropically
confined nanostructure.
