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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. 

Both absorption and emission of light in semiconductor quantum dots occur through excitation or recombination of confined electron−hole pairs, or excitons, with tunable size-dependent resonant frequencies that are ideal for applications in various fields. Some of these applications require control over quantum dot shape uniformity, while for others, control over energy splittings among exciton states emitting light in different polarizations and/or between bright and dark exciton states is of key importance. These splittings, known as exciton fine structure, are very sensitive to the nanocrystal shape. Theoretically, nanocrystals of spheroidal shape are often considered, and their nonsphericity is treated perturbatively as stemming from a linear uniaxial deformation of a sphere. Here, we compare this treatment with a nonperturbative model of a cylindrical box, free of any restrictions on the cylinder’s aspect ratio. This comparison allows one to understand the limits of validity of the traditional perturbative model and offers insights into the relative importance of various mechanisms controlling the exciton fine structure. These insights are relevant to both colloidal nanocrystals and epitaxial quantum dots of III−V and II−VI semiconductors. 

We derive an effective spin-Hamiltonian accounting for the shape anisotropy of the zinc blende semiconductor nanocrystals within the k · p formalism explicitly taking into account the spin–orbit split-off valence band. It is shown that, for small InP nanocrystals, neglect of the spin–orbit split-off band can lead to significant underestimation of one of the two parameters determining the exciton fine-structure splittings. This parameter is only important for nanocrystals with shape anisotropy. 

We derive an effective spin-Hamiltonian accounting for the exciton fine structure in quasi-spherical zinc-blende semiconductor nanocrystals within the k · p formalism explicitly taking into account the spin-orbit split-off valence band. It is shown that, for excitons in nanocrystals made of III-V and II-VI semiconductors with fairly small spin-orbit splitting, the scaling of the electron-hole exchange interaction with the nanocrystal size insignificantly differs from the inverse nanocrystal volume law predicted within the model neglecting the spin-orbit split-off band. Numerical calculations are performed for InP nanocrystals. 

We investigate the fundamental optical properties of single zinc-blende InP/ZnSe/ZnS nanocrystals (NCs) using frequency- and time-resolved magneto-photoluminescence spectroscopy. At liquid helium temperature, highly resolved spectral fingerprints are obtained and identified as the recombination lines of the three lowest states of the band-edge exciton fine structure. The evolutions of the photoluminescence spectra and decays under magnetic fields show evidence for a ground dark exciton level 0L with zero angular momentum projection along the NC main elongation axis. It lies 300 to 600 μeV below the ±1L bright exciton doublet, which is finely split by the NC shape anisotropy. These spectroscopic findings are well reproduced with a model of exciton fine structure accounting for shape anisotropy of the InP core. Our spectral fingerprints are extremely sensitive to the NC morphologies and unveil highly uniform shapes with prolate deviations of less than 3% from perfect sphericity. 

Analytic equation for energy dispersion of electronic states in lead chalcogenide nanosheets is derived within an effective mass model. Selection rules for interband optical transitions are analyzed and expressions for interband optical matrix elements are obtained. It is shown that the main effect of the lateral confinement in nanoplatelets can be accounted for in terms of the quantized in-plane wave vector.