Properties Of Small Metal Clusters Research Articles

The size-dependent influence of palladium doping on the structures of cationic gold clusters.

The physicochemical properties of small metal clusters strongly depend on their precise geometry. Determining such geometries, however, is challenging, particularly for clusters formed by multiple elements. In this work, we combine infrared multiple photon dissociation spectroscopy and density functional theory calculations to investigate the lowest-energy structures of Pd doped gold clusters, PdAun−1+ (n ≤ 10). The high-quality experimental spectra allow for an unambiguous determination of the structures adopted by the clusters. Our results show that the Pd–Au interaction is so large that the structures of PdAun−1+ and Aun+ are very different. Pd doping induces a 2D to 3D transition at much smaller cluster sizes than for pure Aun+ clusters. PdAun−1+ clusters are three-dimensional from n = 4, whereas for Aun+ this transition only takes place at n = 7. Despite the strong Au–Pd interaction, the Aun−1+ cluster geometries remain recognizable in PdAun−1+ up to n = 7. This is particularly clear for PdAu6+. In PdAu8+ and PdAu9+, Pd triggers major rearrangements of the Au clusters, which adopt pyramidal shapes. For PdAu4+ we find a geometry that was not considered in previous studies, and the geometry found for PdAu8+ does not correspond to the lowest-energy structure predicted by DFT, suggesting kinetic trapping during formation. This work demonstrates that even with the continuous improvement of computational methods, unambiguous assignment of cluster geometries still requires a synergistic approach, combining experiment and computational modelling.

First principles study of Ag absorption mechanism in amorphous large silica clusters

Abstract Ag nanoparticles cause interesting modifications to the electronic and optical properties of amorphous silica scaffold that is used in optical fibers. The present study is first of its kind to implement first principles based calculation of large amorphous dielectric host with metal inclusions in it. Ground state structural parameters and equilibrium geometries of amorphous ( SiO 2 ) n clusters (n = 8,16,24,32,40) with Ag atoms doped inside the interstitial space of the scaffold have been calculated using DMol3 DFT program. We have shown that the configuration of small Ag clusters depend strongly on the structural characteristics of the scaffold. This creates a unique opportunity to control the microscopic properties of small metal clusters via Ag impregnation. Our study reveals that small sized silica clusters have lower level of absorption due to self-purification effect and Ag absorption efficiency depends on the nature of amorphicity of clusters having different geometries but same number of atoms. The nature of amorphicity of two different geometries of equal sized silica clusters has been verified via two independent approaches viz. (a) pair correlation and bond length/angle distributions plot of Si atoms and (b) the quasi-entropy calculated using USPEX code. More amorphous network of host leads to incorporation of more number of Ag atoms. The effect of host silica matrix on the structures of Ag nanoclusters are evaluated and the results show that bond distances in Ag clusters increase when encapsulated in silica matrix. The effect of doped Ag atoms on the structural characteristics of silica clusters are also evaluated and correlated with the shift in frequencies and change in Raman spectral intensities.

Photoelectron imaging of small aluminum clusters: quantifying s–p hybridization

Photoelectron imaging experiments and detailed calculations are conducted on Al(n)(-) clusters (n = 3-6) and a calibration method is developed for connecting experimental observations of photoelectron angular distributions to theoretical predictions. It is shown that this method can be used to quantify the degree to which the molecular orbitals are built from s- or p-like atomic orbitals. The highest occupied molecular orbitals of these small aluminum clusters are found to contain varying degrees of s-p mixing, with Al(3)(-) containing the "most hybridized" orbital and Al(4)(-) containing the "least hybridized" orbital. It is shown experimentally that s-p hybridization is already present for the trimer species and, similar to other properties of small metal clusters, oscillates with cluster size.

Aluminum Foils: The Contrasting Characters of Hyperconjugation and Steric Repulsion in Aluminum Dimetallocenes

The novel sandwich complex Cp2*Al2I2, which was recently synthesized by Minasian and Arnold, has been characterized using ab initio and density functional methods. A large family of related compounds was also investigated. Although a few Al(II)–Al(II) bonds are known, this is the first such bond to be supported by Cp-type ligands. In addition, in the remarkable Cp4*Al4 synthesis by Roesky, Cp2*Al2I2 is the Al(II) intermediate; Cp4*Al4 is important as a precursor to novel organoaluminum species. Halogen and ligand effects on the Al–Al bond in Cp2*Al2I2 were systematically explored by studying a series of 20 Cp2*Al2I2 derivatives using density functional theory with relativistic basis sets for the halogens. Comparison was made with the focal point treatment, which uses extrapolation to estimate the full configuration interaction and complete basis set limit energy. Torsional potential energy curves, natural population analyses, and enthalpies of hydrogenation were computed. Using the focal point approach, torsional barriers were computed with 0.05 kcal mol(–1) uncertainty. The interplay of steric and electronic effects on the torsional potential energy curves, enthalpies of dehydrogenation reactions, and geometries is discussed. In species with small ligands (R = H, Me), hyperconjugative effects determine the torsional landscape, whereas steric repulsions dominate in species with Cp* alkyl ligands. Species with Cp ligands represent an intermediate case, thus providing insight into how ligands modulate the structures and properties of small metal clusters.

A Density Functional Study of Ni<SUB>2</SUB> and Ni<SUB>13</SUB> Nanoclusters

Monodisperse metal nanoclusters with diameters smallerthan 10 nm in general exhibit unique physical and chem-ical properties as compared to their counterparts that arecomposed of particles.Studies of size-dependent proper-ties of small metal clusters are important for developingfundamental understanding of the transition from atomicto bulk properties.Nanoparticles have been used for yearsin the field of catalysis.The basic reason for producingnanoparticle-based catalysts was to improve the active sur-face to total metal volume ratio.It also appeared that thecatalytic properties changed qualitatively with change ofthe metal particle size.This change was not surprising inretrospect since electronic properties are strongly depen-dent on particle size as they approach the atomic scale(Schmid et al.

Structure and properties of small sodium clusters

We have investigated structure and properties of small metal clusters using all-electron ab initio theoretical methods based on the Hartree-Fock approximation and density functional theory, perturbation theory and compared results of our calculations with the available experimental data and the results of other theoretical works. We have systematically calculated the optimized geometries of neutral and singly charged sodium clusters having up to 20 atoms, their multipole moments (dipole and quadrupole), static polarizabilities, binding energies per atom, ionization potentials and frequencies of normal vibration modes. Our calculations demonstrate the great role of many-electron correlations in the formation of electronic and ionic structure of small metal clusters and form a good basis for further detailed study of their dynamic properties, as well as structure and properties of other atomic cluster systems.

Properties of isomers of the cluster Ni8 from density functional studies

Properties of small transition metal clusters are of considerable importance for a variety of applications, yet information on how they vary with structure is scarce. As an example, we discuss nine structural isomers of Ni8. Bond lengths, average binding energies, ionization potentials, and cluster magnetism have been calculated using a gradient-corrected density functional approach as implemented in the parallel program PARAGAUSS. At variance with the expectation that properties of small metal clusters vary significantly with the specific electronic structure, quite moderate variations were observed for the structural isomers. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 567–574, 2000

CLUSTERS WITH NOVEL PROPERTIES

The size specific nature of the electronic and structural properties of small metal clusters is reviewed. Three specific examples are illustrated: the nature of hydrogen interaction with neutral and charged metal clusters, the stability and electronic properties of metal-carbon complexes and the role of electron shell filling and close atomic packing on synthesizing very stable and chemically inert clusters.

Ion-induced crystal-to-glass transition in alloys

Abstract Bombardment of a thin film with energetic particles provides a typical case of a system internally driven to a state far from equilibrium, which is suitable for the occurrence of the crystal-to-glass transition. In this paper, critical experiments, on both ion implantation and ion mixing, are re-examined and correlated to interpretative models with the aim of identifying important features in such glass formation processes. Attention is devoted to materials presenting intermediate quasi-crystalline phases, whose characteristic structural and compositional properties constitute a bridge between crystalline and amorphous states. Recent research on glass-forming materials, their chemical stability and local properties is discussed with reference to a recent interpretative model of glass nucleation. Correlations between the structural properties of small metal clusters and those of elementary structural units in amorphous alloys are presented.

Measurements of magnetic moments of metal atom clusters (abstract)

We report the first measurements of the magnetic moments of isolated metal atom clusters. In particular we have measured the magnetic moments of iron atom clusters ranging in size from 2 to 15 atoms and for aluminum atom clusters Al2 and Al3 and their corresponding mono-oxides Al2O, Al3O. We produce a supersonic beam of metal atom clusters by pulsed laser vaporization of a metal substrate within the narrow throat of a pulsed nozzle. The clusters are passed through a Stern–Gerlach magnet and the spatial profile of the magnetic deflection pattern is measured using UV laser photoionization combined with time-of-flight mass analysis of the resulting photoions. The magnetic moments of the iron clusters are found to increase linearly with the number of atoms in the cluster consistent with a net spin per atom of about 3.0 μB compared to the bulk value of 2.2 μB. These observations are consistent with theoretical predictions of the magnetic moments of iron clusters of 4, 9, and 15 atoms. For Al2 we observe Stern–Gerlach behavior and for Al3 we measure a maximum moment of approximately 1/3 μB. The implication of these findings regarding the electronic and structural properties of small metal clusters will be discussed.

CNDO calculations of nickel atom clusters

CNDO molecular orbital calculations for nickel atom clusters containing from one to thirteen atoms in various geometric arrangements are presented. The parameters were selected so that an octahedral Ni6 cluster had an equilibrium inter-nuclear distance, d band occupancy, binding energy, Fermi level, and d band width approximating those of bulk nickel. For clusters with a given number of atoms the stability always increased in the order linear < planar < three-dimensional cluster. The general assumption that the binding energy per atom in metal clusters is proportional to the number of nearest neighbours is supported by these CNDO calculations although this relation is certainly not exact for small clusters. Examination of the calculated orbitals does not indicate a separation of the d band into one part made up from atomic t2g orbitals and another from eg orbitals. Overall the CNDO method appears to present a reasonable approach to calculating properties of small metal clusters.