Absorption spectra of CdSe–ZnS core–shell quantum dots at high photon energies: Experiment and modeling

•CdSe–ZnS core–shell Quantum Dots show relatively high absorption at high energies.•QD potential and shape asymmetry lead to normally disallowed (ND) transitions.•Such ND transitions can account for the high absorption at high photon energies.

Absorption spectra of CdSe–ZnS core–shell quantum dot (QD) ensembles, with average core diameters ranging from 2.6 nm to 7.2 nm have been obtained using both transmission and photoluminescence excitation measurements. In agreement with previous reports, the absorption coefficient at energies ≃1eV above the effective bandgap increases monotonically as in bulk solids. A simple effective-mass spherical core–shell potential model cannot explain the relatively high absorption at higher energies. The calculated electron and hole radial envelope wavefunctions show asymmetry due to the core–shell structure. It leads to normally symmetry-disallowed transitions acquiring a weak oscillator strength, with their number and strength increasing with energy. A phenomenological model that invokes normally disallowed transitions in general is shown to reproduce the absorption spectrum at higher energies quite well. The oscillator strength scaling factor for such transitions increases with decrease in QD size, consistent with expectations.

Graphical abstractWe show that to explain the measured high absorption coefficient of CdSe–ZnS core–shell quantum dots at energies ~1eV above their effective bandgap, it is necessary to consider the role of normally disallowed transitions. We provide a simple model under effective mass approximation to account for such transitions.Figure optionsDownload full-size imageDownload as PowerPoint slide