Exciton Fine Structure in Wurtzite Nanorods: Is the Exciton Ground State Always Dark?

Colloidal semiconductor nanorods have demonstrated potential as bright, stable, and polarized light sources. Emission of light in these and other nanocrystals proceeds through recombination of confined electron-hole pairs or excitons with tunable size-dependent resonant frequencies. Usually, their brightness is reduced by the "dark exciton"─a nonemissive state into which electron-hole pairs relax before recombining radiatively. Here we analyze the fine structure of exciton states in wurtzite CdSe nanorods and demonstrate that, for cylindrical nanorods of radii less than a critical value of R ∼ 30 Å and aspect ratios larger than ∼5, the exciton ground state is the bright emissive state polarized parallel to the axis of the nanorod.