The electronic and optical properties of tetrahedral CdSe magic clusters (average diameter ∼1.5 nm) protected by carboxyl and amine ligands, which correspond to previously reported experimental structures, are studied using density functional theory. We find extreme ligand packing densities, capping every single dangling bond of the inorganic core, strong dependence of the Z-type metal carboxylate binding on the amount of excess amine, and potential for improved photoluminescence upon replacing phenyl ligands with alkanes. The computed absorption spectra of the Cd 35 Se 20 cluster agree well with experiments, resolving the 0.2 eV splitting of the first exciton peak due to spin−orbit coupling. We discuss the origin of the significant broadening of the optical spectra as due to phonons and structural variations in the ligand configurations and inorganic core apexes.
The structural, electronic and magnetic properties of small Fe m Rh n clusters having N = m+n ≤ 8 atoms are studied in the framework of a generalized-gradient approximation to density-functional theory. For N = m + n ≤ 6 a thorough sampling of all cluster topologies has been performed, while for N = 7 and 8 only a few representative topologies are considered. In all cases the entire concentration range is systematically investigated. All the clusters show ferromagnetic-like order in the optimized structures. As a result, the average magnetic moment per atom µ N increases monotonously, which is almost linear over a wide range of concentration with Fe content.A remarkable enhancement of the local Fe moments beyond 3 µ B is observed as result of Rh doping. The composition dependence of the binding energy, average magnetic moment and electronic structure are discussed.
The electronic structure and adsorption properties of 1.5 nm sized Pt, Au, and PtAu nanoclusters are studied by density functional theory. We explain the recent experimental finding that 20% Au content in PtAu nanoparticles is optimal to induce a dramatically different catalytic behavior. Our results show that the d-band center together with the density of states at the Fermi energy can be used as an indicator of the chemical activity of PtAu nanoclusters. The most favorable adsorption sites on the cluster surfaces as a function of the Pt/Au ratio are identified using atomic H as a probe.
The structure, chemical order, and magnetic behavior
in small FeRh
clusters having N ≤ 19 atoms have been investigated
theoretically. For N ≤ 6 atoms, a thorough
global geometry optimization is performed by considering all possible
cluster topologies, while for 7 ≤ N ≤
19 only a few representative structures are considered. In all cases,
the starting structures are fully relaxed in the entire range of concentrations
and spin polarizations. The calculations are based on a generalized-gradient
approximation to density-functional theory. The results are analyzed
systematically as a function of size and composition. The optimized
cluster structures are compact with a clear tendency to maximize the
number of nearest-neighbor FeRh pairs. For very small sizes, the low-lying
isomers present usually a topology different from that of the optimal
structure, while for larger clusters the lowest-energy isomerizations
imply mainly changes in the chemical order. The most stable structures
are in general ferromagnetic. Antiparallel spin arrangements are found
in some low-lying isomers. An important enhancement of the local Fe
moments is observed as result of Rh doping. This is shown to be a
consequence of an increase in the number of Fe d holes due to Fe–Rh
charge transfer. The local moments at the Rh atoms, which are significant
already in small pure Rh clusters, are not strongly enhanced by Fe
doping. Nevertheless, the overall stability of magnetism, as measured
by the energy gained upon spin polarization, increases with Fe content.
The influence of spin–orbit interactions on the cluster stability
and spin order is discussed.
The
structure and chemical ordering of PtAu nanoclusters of 79, 135, and
201 atoms are studied via a combination of a basin hopping atom-exchange
technique (to locate the lowest energy homotops at fixed composition),
a symmetry orbit technique (to find the high symmetry isomers), and
density functional theory local reoptimization (for determining the
most stable homotop). The interatomic interactions between Pt and
Au are derived from the empirical Gupta potential. The lowest energy
structures show a marked tendency toward PtcoreAushell chemical ordering by enrichment of the more cohesive Pt in the core
region and of Au in the shell region. We observe a preferential segregation
of Pt atoms to (111) facets and Au atoms to (100) facets of the truncated
octahedron cluster motif. Exotic surface patterns are obtained particularly
for Pt-rich compositions, where Pt atoms are being surrounded by Au
atoms. These surface arrangements boost the catalytic activity by
creating a large number of active sites.
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