Chalcogenide-Ligand Passivated CdTe Quantum Dots Can Be Treated as Core/Shell Semiconductor Nanostructures

Abstract

Chalcogenide ligands (S^2–, Se^2–, Te^2–) are attractive candidates for passivation of surfaces of colloidal quantum dots (QDs) because they can enhance interparticle or particle–adsorbate electronic coupling. Devices made with QDs in which insulating long-chain aliphatic ligands were replaced with chalcogenide ligands have exhibited improved charge transfer and transport characteristics. While these ligands enable promising device performance, their impact on the electronic structure of the QDs that they passivate is not understood. In this work, we describe significant (up to 250 meV) changes in band gap energies of CdTe QDs that occur when native aliphatic ligands are replaced with chalcogenides. These changes are dependent on the ligand and the particle size. To explain the observed changes in band gap energies, we used the single band effective mass approximation to model the ligand layer as a thin shell of Cd-chalcogenide formed by the bonding of chalcogenide ligands to partially coordinated Cd surface atoms. The model correctly predicted the observed trends in CdTe QD band gap energies. The model also predicts that electrons and holes in chalcogenide-capped QDs can be significantly delocalized outside the core/shell structure, enhancing electronic coupling between QDs and adjacent species. Our work provides a simple description of the electronic structure of chalcogenide-capped QDs and may prove useful for the design of QD-based devices