  Recently, the fabrication of CdSe nanoplatelets became an important research
topic. Nanoplatelets are often described as having a similar electronic
structure as 2D dimensional quantum wells and are promoted as colloidal quantum
wells with monolayer precision width. In this paper, we show, that
nanoplatelets are not ideal quantum wells, but cover depending on the size: the
strong confinement regime, an intermediate regime and a Coulomb dominated
regime. Thus, nanoplatelets are an ideal platform to study the physics in these
regimes. Therefore, the exciton states of the nanoplatelets are numerically
calculated by solving the full four dimensional Schr\"odinger equation. We
compare the results with approximate solutions from semiconductor quantum well
and quantum dot theory. The paper can also act as review of these concepts for
the colloidal nanoparticle community.
