Metal Chalcogenide Clusters with Closed Electronic Shells and the Electronic Properties of Alkalis and Halogens

Clusters with filled electronic shells and a large gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are generally energetically and chemically stable. Enabling clusters to become electron donors with low ionization energies or electron acceptors with high electron affinities usually requires changing the valence electron count. Here we demonstrate that a metal cluster may be transformed from an electron donor to an acceptor by exchanging ligands while the neutral form of the clusters has closed electronic shells. Our studies on Co6Te8(PEt3),(CO) (m + n = 6) clusters show that Co6Te8(PEt3)(6) has a closed electronic shell and a low ionization energy of 4.74 eV, and the successive replacement of PEt3 by CO ligands ends with Co6Te8(CO)(6) exhibiting halogen-like behavior. Both the low ionization energy Co6Te8(PEt3)(6) and high electron affinity Co6Te8(CO)(6) have closed electronic shells marked by high HOMO-LUMO gaps of 1.24 and 1.39 eV, respectively. Further, the clusters with an even number of ligands favor a symmetrical placement of ligands around the metal core

Magnetic properties of Co2-xTMxC and Co3-xTMxC nanoparticles

Using synthetic chemical approaches, it is now possible to synthesize transition metal carbides nanoparticles with morphology, where the transition metal layers are embedded with intervening layers of carbon atoms. A composite material consisting of Co2C and Co3C nanoparticles has been found to exhibit unusually large coercivity and energy product. Here, we demonstrate that the magnetic moments and the anisotropy can be further enhanced by using a combination of Co and other transition metals (TM). Our studies are based on mixednanoparticles Co2−x TM xC and Co3−x TM xC, in which selected Co sites are replaced with 3d transition elements Cr, Mn, and Fe. The studies indicate that the replacement of Co by Fe results in an increase of both the magnetic moment and the magnetic anisotropy. In particular, CoFe2C is shown to have an average spin moment of 2.56 μ B and a magnetic anisotropy of 0.353 meV/formula unit compared to 1.67 μ B and 0.206 meV/formula unit for the Co3C. Detailed examination of the electronic structure shows that the limited hybridization of carbon p-states with transition metal d-states drives the larger anisotropy

Magnetism in assembled and supported silicon endohedral cages: First-principles electronic structure calculations

First principles electronic structure calculations on a free CrSi12cluster, a (CrSi12)2 dimer, and CrSi12 clusters supported on Si(111) surfaces have been carried out within a gradient corrected density functional formalism using a supercell approach. The ground state of CrSi12 is a Cr centered hexagonal biprism of Si atoms in which the Cr spin moment is completely quenched. As two CrSi12 motifs are brought together, they form different composite units depending on initial direction of approach and, in most cases, the composite cluster is found to have a net spin moment. Cluster assemblies obtained by depositing CrSi12 motifs on a Si(111) surface exhibit similar finite spin moments for several initial directions of approach. An analysis of the electronic states shows that the origin of the magnetic moment lies in those Cr d-states that do not mix with silicon sp states. The studies suggest the possibility of forming silicon-based magnetic semiconductors through such assemblies

Quantum spin transport through magnetic superatom dimer (Cs8V-Cs8V)

Theoretical studies of the spin transport through a magnetic superatom dimer (Cs8V)-(Cs8V) have been carried out within a density functional theory combined with nonequilibrium Green\u27s-function formalism. It is shown that the electronic transport is sensitive to the binding site as well as the contact distance between the dimer and the electrode, and that the conductance at zero bias exhibits an oscillatory behavior as a function of the contact distance. The conductance in ferromagnetic state shows an unusually high spin polarization that exceeds 80% at large separations. The I–V curve shows negative differential resistance for specific contact distances, whose origin lies in the shift of frontier energy levels as well as the charged state of the superatom, under external bias

Oxidation of Pdn (n=1–7,10) clusters supported on alumina/NiAl(110)

First-principles theoretical investigations on the oxidation of Pdn (n=1–7, 10) clusters deposited on alumina/NiAl(110) have been carried out using a gradient-corrected density-functional approach. Our studies indicate that the free Pdn clusters are compact and maintain their compact structures when deposited on the surface, undergoing only small relaxations of the Pd-Pd distance. The clusters bind to the surface via a pair of Pd atoms and with a binding energy of around 1.0 eV. Studies of oxidation through an O2molecule show that O2 occupies sites closer to the surface for Pd1, Pd4, Pd5, and Pd6 while, in other cases, the binding is highest to Pd atoms farther from the surface. An analysis of the charge gained by the O2 molecule upon absorption shows that, while O2 always gains charge, the amount of charge contributed by the Pdn cluster or the surface can vary significantly. In particular, in the case of Pd4, only a small charge is donated by the cluster, thus accounting for the recently observed lack of shift in the x-ray photoelectron spectroscopy levels

Nearly-free-electron gas in a silicon cage

A systematic study of the ground state geometries, electronic structure, and stability of the metal (M) encapsulated MSi12 (M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni) clusters has been carried out within a gradient-corrected density functional formalism. It is shown that the ground state of most MSi12 clusters has the lowest spin multiplicity as opposed to the high spin multiplicity in free transition metal atoms. Consequently, a proper inclusion of the spin conservation rules is needed to understand the variation of the binding energy of M to Si12 clusters. Using such rules, CrSi12 and FeSi12 are found to exhibit the highest binding energy across the neutral while VSi−12 has the highest binding energy across the anionicMSi−12 series. It is shown that the variations in binding energy, electron affinity, and ionization potential can be rationalized within an 18-electron sum rule commonly used to understand the stability of chemical complexes and shell filling in a confined free-electron gas

Electronic counting rules for the stability of metal-silicon clusters

First principles electronic structure calculations have been carried out to examine the stability of cationic, neutral, and anionic MSi15, MSi16, and MSi17 (M=Sc, Ti, and V) clusters. ScSi16−, TiSi16, and VSi16+ are found to be particularly stable in agreement with recent experiments. It is shown that the enhanced stability can be reconciled within a model where each Si atom coordinated to the metal contributes one electron to the valence pool. Clusters where the total number of valence electrons obtained by summing one electron from each Si site coordinated to metal atom and the valence electrons of the metal attain 20 are found to be particularly stable. Combined with the earlier reported stability at 18 electrons, it is proposed that such valence pools might be looked upon as a nearly free electron gas inside a silicon cage

First-principles study of the onset of noncollinearity in Mnn clusters: Magnetic arrangements in Mn5 and Mn6

First-principles theoretical investigations of the noncollinearity of atomic spin moments in manganese clusters have been carried out within a gradient-corrected density-functional approach. Our studies on Mn5 and Mn6 include investigation of both collinear and noncollinear arrangements. It is shown that while the atomic structure of the ground state of Mn5 is a triangular bipyramid, the collinear and noncollinear arrangements have comparable energies and hence are degenerate. For Mn6, while the ground state has a square bipyramid arrangement, the noncollinear configuration is most stable making it the smallest cluster to feature a noncollinear ground state. The results are discussed in view of the recent experimental Stern-Gerlach profiles

Unusually large spin polarization and magnetoresistance in a FeMg8-FeMg8 superatomic dimer

Electronic transport across a FeMg8 magnetic superatom and its dimer has been investigated using a density functional theory combined with Keldysh nonequilibrium Green\u27s-function formalism. For a single cluster, our studies for the cluster supported in various orientations on a Au(100) surface show that the transport is sensitive to the contact geometry. Investigations covering the cases where the axes of Mg square antiprism are 45°, perpendicular, and parallel to the transport direction, show that the equilibrium conductance, transferred charge, and currentpolarizations can all change significantly with orientation. Our studies on the transport across a magnetic superatom dimer FeMg8–FeMg8 focus on the effect of electrode contact distance and the support. The calculated I-V curves show negative differential resistance behavior at larger electrode-cluster contact distances. Further, the equilibrium conductance in ferromagnetic state shows an unusually high spin polarization that is about 81.48% for specific contact distance, and a large magnetoresistance ratio exceeding 500% is also found. The results show that the superatom assemblies can provide unusual transport characteristics, and that the spinpolarization and magnetoresistance can be controlled via the contact geometry