Exciton States in Spherical Non-Concentric Core–Shell Quantum Dots

Abstract

The electronic and excitonic structure of non-concentric CdSe/CdS core-shell spherical quantum dots is investigated theoretically, with the core rigidly displaced from the shell center. Hole states are computed within the four-band Luttinger–Kohn model expanded in a plane-wave basis, with the displaced-core confining potential treated exactly, and exciton states are obtained by configuration interaction including the direct electron-hole Coulomb attraction and the short-range exchange interaction. The displacement reduces the confining symmetry from spherical to axial and lifts the degeneracy of the band-edge hole multiplet, yielding a fine-structure splitting that increases monotonically with the core offset. The same mechanism reshapes the exciton spectrum: the ground-state transition undergoes a blueshift as the core approaches the shell boundary, while the exchange-induced dark–bright splitting grows steadily with displacement. The calculated absorption edge is in quantitative agreement with the experimentally measured value. This agreement is obtained only when excitonic effects are included, with the electron-hole Coulomb attraction supplying the redshift required to reconcile theory and experiment. The results identify the core displacement as an effective structural degree of freedom for tuning both the emission energy and the exciton fine structure of colloidal core-shell nanocrystals.