Non-quantum-confinement optics in (CdS)n clusters

Reduction reactions at the interface between CdS quantum dot and Z-type ligands driven by electron injection in the electroluminescent processes

The efficient and stable electroluminescence of quantum dots (QDs) is of great importance in their applications in new display technologies. The short service life of blue QDs, however, hinders their development and commercialization. Different mechanisms have been proposed for the destabilization of QDs in electroluminescent processes. Based on real-time time-dependent density functional theory studies on the QD models covered by Z-type ligands (XAc2, X = Cd, Zn, Mg), the structural evolution is simulated to reveal the mechanism of the reduction reactions induced by electron injection. Our simulations reproduce the experimental observations that the reduction reactions occur at the QD-ligand interface, and the reduced Cd atom is almost in a zero valence state. However, different sites are predicted for the reactions in which the surface metal atom of the QD instead of the metal atom in the ligands is reduced. As a result, one of the arms of the chelate ligand leaves the QD, which tends to cause damage to its electroluminescent performance. Our findings contribute to a mechanistic understanding of the reduction reactions that occurred at the QD-ligand interface.

Variation of the optical properties with size and composition of small, isolated Cdx Sey + clusters

Global energy minimum structures and optoelectronic properties are presented for isolated Cd_x Se_y ⁺ clusters with x + y ≤ 26. The compositional- and size-dependent variation of optical, electronic and geometric properties is systematically studied within the framework of ground state and time-dependent density functional theory. The applied methods are justified by benchmarks with experimental data. It is shown that the optical gap can be tuned by more than 2 eV by only changing the composition for a fixed number of atoms. The stoichiometric species reveal an unexpected size-dependent behavior in comparison to larger colloidal CdSe quantum dots, that is, a redshift of the optical gap was observed with decreasing cluster size in contrast to predictions by quantum-size effects. This unexpected result is discussed in detail taking the positive charge of the clusters into account.

Influence of back donation effects on the structure of ZnO nanoclusters

The structure and properties of ZnO quantum dots is a very popular and rapidly growing field of research for which accurate quantum calculations are challenging to perform. Since the dependence between system size and wall time scales nonlinearly, certain compromises have to be made. A particularly important limiting factor is the size of the basis used, this is especially the case if accurate large calculations are to be carried out. In our work, we discovered that an important O(2p)->Zn(4p) back donation, which greatly influences the strength of the ZnO bond, can be reproduced only if diffuse functions are added to the basis set. We further tested the basis dependence for the magic-sized wurtzite nanophase ZnO clusters which were previously shown to be able to accurately reproduce the magnetically doped II-IV Q-dots. In this work, we outline the minimal basis sets required to properly describe ZnO bonds in a nanocluster. It was demonstrated that the rock salt nanophase is incorrectly stabilized if a basis set does not contain sufficiently diffuse functions while the correct wurtzite phase is stabilized when diffuse functions are added. This tendency, similar to that in the ZnO dimer case, was shown to stem from the incorrect lack of Zn(4p) electron density in calculations when using the diffuse-free basis set.

Analysis of the Structures and Properties of (GaSb)n (n = 4-9) Clusters through Density Functional Theory

An optimization strategy combining global semiempirical quantum mechanical search with all-electron density functional theory was adopted to determine the lowest energy structure of (GaSb)n clusters up to n = 9. The growth pattern of the clusters differed from those of previously reported group III-V binary clusters. A cagelike configuration was found for cluster sizes n ≤ 7. The structure of (GaSb)6 deviated from that of other III-V clusters. Competition existed between core-shell and hollow cage structures of (GaSb)7. Novel noncagelike structures were energetically preferred over the cages for the (GaSb)8 and (GaSb)9 clusters. Electronic properties, such as vertical ionization potential, adiabatic electron affinities, HOMO-LUMO gaps, and average on-site charges on Ga or Sb atoms, as well as binding energies, were computed.

A comparative study of small 3d-metal oxide (FeO)n, (CoO)n, and (NiO)n clusters

Geometrical and electronic structures of the 3d-metal oxide clusters (FeO)_n, (CoO)_n, and (NiO)_n are computed using density functional theory with the generalized gradient approximation in the range of 1 ≤ n ≤ 10.

A study of optical absorption of cysteine-capped CdSe nanoclusters using first-principles calculations

Optical absorption of cysteine-capped CdSe nanoclusters varies with cluster size, ligands and solvents.