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 inter-particle or particleadsorbate 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 (EMA) 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.

Efficient photocatalytic hydrogen evolution with ligand engineered all-inorganic InP and InP/ZnS colloidal quantum dots

“…Alternatively, InP/ZnS-S QDs may display more active interfaces compared to InP/ZnS-MPA QDs. Compared to organic ligands, such as OLA and MPA, the single anion S 2− can far more easily circumvent the physical and electrical barriers arising from the large volume and insulator carbon chains of organic ligands 58 , 60 , 61 . This may facilitate the migration of charge carriers from the QDs towards other components of the reaction systems, in particular H 2 A or Ni 2+ .…”

Susceptible Surface Sulfide Regulates Catalytic Activity of CdSe Quantum Dots for Hydrogen Photogeneration

“…Here the possibility of anion exchange occurring in the crystal structure can be excluded, as a blueshift should be observed if Se 2− is replaced by S 2− . The initial larger redshift for CdSe‐ n S QDs ( n ≤ 25) is resulted from the growth of CdS, and the following slight redshift ( n > 25) may be caused by the delocalization of exciton to S 2− ligands in QDs . Such a redshift also appears in the emission spectra of CdSe‐ n S QDs, but accompanied with the decrease of emission intensity (Figure d), which indicates the existence of S 2− adsorbing on the surface of QDs.…”

“…This diameter change is much less than that caused by formation of a whole CdS shell layer (0.7 nm). [4a] In combination with XRD and Raman results, we propose that a part of introduced S 2− can transform into lattice S, but a complete CdS layer is not formed for CdSe‐ n S QDs as the absence of Cd 2+ precursors,[10,11b,14] which is in contrast to previous system with large amounts of Cd 2+ and MPA presented in solution. [4a]…”

“…[4a] Recently, S 2− ions have also been used to directly replace the original organic ligands to offer stable and soluble QDs in water for photocatalysis. [7b,c] These studies suggest that S 2− on the surface of QDs can probably either form a shell or work as ligands . Considering the diverse adsorption states of S 2− on the surface of QDs with different bonding form and bonding energy, we initiated to gradually introduce S 2− ions into MPA capped CdSe (MPA‐CdSe) QDs to understand the real role of surface S 2− in photocatalytic hydrogen evolution.…”

“…Thus, the electron transfer from QDs to Ni(OH) 2 is facilitated. Subsequently, the S 2− as ligands (CdSe‐ n S n > 25) can promote the hole transfer from QDs to IPA molecules in solution, as such inorganic ligands could effectively avoid the physical and electrical barriers originated from the large volume and insulated carbon chains of organic ligands . Although the hole transfer also occurs quickly from QDs to MPA ligands, the residual carbon chain would still be a barrier for further hole transfer to IPA.…”