Excitonic Structure in CsPbBr3 Nanocubes, Nanorods, and Nanoplatelets: The Effect of Dimensionality.

We present a theoretical study comparing the excitonic ground-state properties of CsPbBr3 nanocrystals with different dimensionality: nanorods (quasi-1D), nanoplatelets (quasi-2D), and nanocubes (quasi-3D). All three systems are described on an equal footing, by means of a general variational effective mass model, which captures the influence of quantum confinement, dielectric confinement, electron-hole correlations, and polaronic effects (within a Haken model). The strongly confined directions squeeze the exciton (X) wave function and enhance Coulomb attractions along the weakly confined directions. This stimulates super-radiance, causing radiative recombination rates to speed up from cubes to platelets and to rods, consistent with recent experiments. The anisotropic local field factor is a secondary, yet non-negligible, mechanism that further enhances radiative rates. X binding energies are also determined primarily by the directions of strong confinement, which is also consistent with experiments. Weakly confined directions, however, become influential for small aspect ratios. Dielectric confinement plays a major role in determining the binding energies and less so in the interparticle distances. For all dimensionalities, the biexciton (XX) geometry is that of a distorted tetrahedron, rather than squared or linear distributions that would result in Coulomb-governed 2D and 1D structures.