Random-Alloying Induced Signatures in the Absorption Spectra of Colloidal Quantum Dots

The optical spectra of homogeneously alloyed colloidal CdSe x S 1– x quantum dots (QDs) for sulfur-rich compositions show additional transitions between the first and second absorption features of pure CdSe and CdS. Using tight-binding calculations for a large ( N = 50) number of stochastic realizations for each composition of the alloyed QDs, we trace back this effect to transitions from deeper valence-band states to the lowest conduction-band level. These transitions are energetically degenerate with the main absorption lines in the pure systems but are shifted in between the lowest two main transitions in alloyed QDs. Moreover, the configurational disorder upon alloying distorts the symmetry properties of electron and hole wave functions, which results in a broad band of allowed transitions from these deeper valence-band states to the lowest conduction-band level. In agreement with experimental findings, the corresponding absorption feature shows a blueshift relative to the first exciton transition with decreasing sulfur to selenium ratio. Many-body calculations using the configuration interaction scheme reproduce the effect and additionally yield a bowing parameter of the optical gap in very good agreement with experimental findings from UV–vis spectra.