Computer simulations of HREM images of metal clusters

Multislice calculations have been performed for Ag, Pd and Au clusters in the size range of 5.0 nm diameter of cuboctahedral, icosahedral and decahedral structures. It could be shown that tilt series are necessary for the classification of the structures. Particularly for arbitrary orientations, i.e. deviations from main directions such as 2-, 3- and 5-fold axes, the performance of computer simulations is mandatory. The influence of absorption is also studied for the case of a 100 kV microscope by introducing a complex potential.     

Structural and dynamical properties of transition metal clusters

Results of molecular dynamics simulation studies of structural and dynamical properties of 12-, 13-, and 14-atom transition metal clusters are presented. The calculations are carried out using a Gupta-like potential expressed in reduced units. The transformation to absolute units involves two size-dependent parameters which effectively convert the potential into a size-dependent one. The minimum energy geometries of the clusters are obtained through the technique of simulated thermal quenching. A melting-like transition is observed as the energy of the clusters is increased. A novel element of the transition is that it may involve a premelting state.     

Structural and electronic properties of compound metal clusters

The equilibrium geometries, relative stabilities, and vertical ionization potentials of compound clusters involving Li _n , Na, Mg, and Al atoms have been calculated using ab initio self-consistent field linear combination of atomic orbitals — molecular orbital (SCF-LCAO-MO) method. The exchange energies are calculated exactly using the unrestricted Hartree-Fock (UHF) method whereas the correlation correction is included within the framework of configuration interaction involving pair excitations of valence electrons. While the later correction has no significant effect on the equilibrium geometries of clusters, it is essential for the understanding of relative stabilities. Clusters with even numbers of electrons are found to be more stable than those with odd numbers of electrons regardless of their charge state and atomic composition. The equilibrium geometries of homo-nuclear clusters can be significantly altered by replacing one of its constituent atoms with a hetero-nuclear atom. The role of electronic structure on the geometries and stabilities of compound clusters is discussed.     

Structural characterization of catalytically active metal clusters in polymer membranes

Metal clusters were generated and stabilized in pore free, mechanically stable poly(amide imide) (PAI) polymer membranes in high dispersion and high concentration of typically 15wt-%. These membranes have been successfully tested for catalytic applications. Pure Pd-loaded and bimetallic Pd/Ag, Pd/Cu, Pd/Co, Pd/Pb PAI films were investigated by means of x-ray absorption spectroscopy, small and wide angle x-ray scattering and transmission electron microscopy to characterize the structure of the metal clusters in the protective polymer. The measurements consistently show a homogeneous distribution of metallic nanoclusters of 10–30 Å size within the membranes. Also, a smaller amount of larger aggregates up to 300 Å is observed in most of the films. The precise cluster size distribution critically depends on the solvents used as well as on other preparation parameters such as the stirring time of the metal precursor/polymer solution. In case of Pd/Ag and Pd/Pb bimetallic films no clear evidence for the formation of bimetallic clusters in the membrane, i.e. alloying of both metal components, is found. In Pd/Cu and Pd/Co membranes, chlorine from CuCl₂ and CoCl₂ precursors reacts with Pd which may influence the Pd catalytic behavior. Reduction and oxidation of the metal nanoclusters is quantitatively studied by means of x-ray absorption spectroscopy. Membrane properties are discussed with respect to catalytic applications.     

Structural and dynamic investigations of aqueous clusters of Na+ and K+

Using computer modeling, we have studied Na⁺–24H₂O and K⁺–24H₂O clusters. We propose ion-water interaction potentials. We obtain structural, energy, and dynamic characteristics of the studied clusters. We show different mechanisms for exchange of water molecules surrounding the Na⁺ and K⁺ ions: single-particle in the case of Na⁺, and close to K⁺, along with single-particle exchange, a large percentage of multiparticle cooperative exchange of water molecules. This difference is explained by the different degrees of orientation of the water molecules surrounding these ions, by the presence of a unified deformed network of H bonds in the K⁺ cluster and its absence in the Na⁺ cluster. We propose a negative hydration mechanism for the K⁺ ion.Institute of General and Inorganic Chemistry, Russian Academy of Sciences. Institute of Physical Chemistry, Russian Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 2, pp. 96–104, March–April, 1993.     

Excitation of metal clusters by intense lasers

We discuss recent theoretical investigations of the non-linear response of metal clusters subject to short and intense laser pulses. After reviewing major results, as obtained in a series of papers over the last few years, we present and discuss detailed examples from ongoing research concerning the spectra of emitted electrons.     

HREM studies of the structure and the oxidation process of copper clusters created by inert gas aggregation

Structure and reactivity with oxigen of Cu clusters in the size range of 4.5±2.5 nm created by the inert gas aggregation technique were studied by HREM. The pure Cu clusters investigated under clean conditions show the structures of MTP's with a small lattice dilatation of the (111) plane of 1.25%. For icosahedral and decahedral particles this dilatation corresponds to a splitting of the nearest neighbour distance showing two different values, i.e. dilatation of 2.2% and contraction of 2.8% for the two edges of the deformed tetrahedral subunits, respectively. Oxidation at room temperature and air pressure of 1 bar only begins after a few minutes of exposure to air, after having undergone creation of probably non-stoichiometric intermediate states in the cuprite Cu₂O structure with the bulk values of the bond lengths.     

Simple metal clusters

The response of alkali cluster ions to an optical excitation is investigated for two different photon energy domains. Below the ionization potential giant resonances in the photoabsorption cross-section are observed for closed shell species. Above the ionization potential, the ionization process competes with the photofragmentation process. The number of valence electrons determines both the behavior of the photoabsorption spectrum and the evolution of the ionization cross-section with the cluster size. The stability of the clusters against an excess of charge is examined through the observation of an asymmetric fission of Na _n ⁺⁺ . Experimental results are discussed in term of an electrostatic model giving an estimate of the critical size of stability and of the height of the coulombic barrier.     

Characterization of large supported metal clusters by optical spectroscopy

Small sodium and silver particles were generated on dielectric substrates like LiF, quartz and sapphire under ultrahigh vacuum conditions. The optical transmission spectra of the clusters were measured as a function of cluster size and shape, for low and high substrate temperatures as well as for s- and p- polarization of the incident light. Excitation of dipolar surface plasmon oscillations in the directions normal and parallel to the substrate surface could be identified. Furthermore, optical spectra for Na and Ag clusters were calculated with the classical Mie theory. The measured spectra vary strongly if the experimental conditions are changed and can be exploited, for example, to characterize the particles with regard to their size and shape. In particular, the axial ratio of the spheroidal clusters could be determined. Its value is considerably different for the two investigated metals and depends on the substrate material. Furthermore, the temperature of the substrate has a pronounced influence on the shape of the particles. At low temperature of T=100 K two-dimensional island growth is predominant. The particles extend only little in the direction perpendicular to the surface and coalesce readily at small coverage of metal atoms. In contrast, the clusters are truly three-dimensional at T=300 K. At this stage, sodium particles still exhibit a rather small axial ratio whereas silver clusters appear almost spherical. Thus, measurements of the optical spectra permit direct in situ monitoring of cluster growth during the nucleation of adsorbed atoms and of temperature induced shape variations. In addition to investigations of the shape of the particles, the quadrupolar surface plasmon mode was observed for Ag clusters.     

Calorimetric investigations of urease inhibition by heavy metal ions

The subject of this publication is calorimetric investigations on the inhibitor effect of heavy metal ions on the enzyme urease. The obtained results allow quantification of the inhibitors (Cd²⁺-, As³⁺-, Zn²⁺-, Pb²⁺-ions) via the initial reaction rate of enzymatically catalysed urea hydrolysis. The interpretation potential of the calorimetric measurements is underlined by the determination of the inhibiting mechanisms for the example of Cd²⁺- and As³⁺-ions, the findings on the time regime of the inhibition process, and the detection of heavy metal ions in the ppm range. The use of several different buffer substance revealed the influence of the latter on the intensity of heavy metal inhibition. This opens the path to both the selective analysis of heavy metals via pattern recognition and to the improvement of detection sensitivity.     

Binary metal alloy clusters

Binary alloy clustersA _n B _m (A,B: Bi, Sb, Pb, Ag, in different compositions) have been generated by simultaneous inert gas condensation of the two vapours. Under suitable condensation conditions the adsorption of single atomsA to clustersB _m can be studied. The adsorption probabilities are found to vary with cluster size. The cluster reactivities depend on the number of absorbed foreign atoms. Lead and bismuth clusters are found to be highly reactive in cracking the strongly bound Sb₄-tetrahedron. One single interaction channel is found for Pb _n /Sb₄ (symmetrical dissociation) and three interaction channels are found for Bi _n /Sb₄ (symmetrical and nonsymmetrical dissociation). Pb- and Bi-clusters have different onset thresholds for dissociative reactivity.     

Electronic structure of metal clusters

Photoemission spectra of valence electrons in metal clusters, together with threshold ionization potential measurements, provide a coherent picture of the development of the electronic structure from the isolated atom to the large metallic cluster. An insulator-metal transition occurs at an intermediate cluster size, which serves to define the boundary between small and large clusters. Although the outer electrons may be delocalized over the entire cluster, a small cluster remains insulating until the density of states near the Fermi level exceeds 1/kT. In large clusters, with increasing cluster size, the band structure approaches that of the bulk metal. However, the bands remain significantly narrowed even in a 1000-atom cluster, giving an indication of the importance of long-range order. The core-electron binding-energy shifts of supported metal clusters depend on changes in the band structure in the initial state, as well as on various final-state effects, including changes in core hole screening and the coulomb energy of the final-state charge. For cluster supported on amorphous carbon, this macroscopic coulomb shift is often dominant, as evidenced by the parallel shifts of the core-electron binding energy and the Fermi edge. Auger data confirm that final-state effects dominate in cluster of Sn and some other metals. Surface atom core-level shifts provide a valuable guide to the contributions of initial-state changes in band structure to cluster core-electron binding energy shifts, especially for Au and Pt. The available data indicate that the shift observed in supported, metallic clusters arise largely from the charge left on the cluster by photoemission. As the metal-insulator transition is approached from above, metallic screening is suppressed and the shift is determined by the local environment.     

Photoionization of alkali metal clusters

The photoionization cross section for spherical alkali metal clusters is predicted to oscillate as a function of the photon wavenumber with a frequency determined by the diameter of the cluster. The oscillations and other principal features of the photo cross section can be worked out analytically using semiclassical techniques. An accurate numerical calculation with different cluster potentials confirms these results qualitatively. Quantitative details depend sensitively on the actual potential. Hence, properties of the true cluster potential can be inferred from the experimental cross section. This might turn out to be useful for improving theoretical cluster potentials.     

Structural investigations of alumino-metalloaminocarbene complexes

[Al(NMe₂)₃]₂ combines with Fe(CO)₅ and Mn₂(CO)₁₀ forming bimetallic compounds Fe₂(CO)₈[C(NMe₂)OAl(NMe₂)₂]₂ and Mn₂(CO)₉[C(NMe₂)OAl₂-(NMe₂)₅]. X-ray diffraction analyses reveal the formation of aluminometallocarbene compounds with central connectivities, MC[NMe₂)][OAl(NMe₂)(μ-NMe₂)₂].     

Structural investigations of silicon-rich CVD-SiC

Silicon Carbide (SiC) and Silicon Carbide with free Silicon [SiC(Si)] thin films were prepared by chemical vapor deposition (CVD) using an CH₃SiCl₃-H₂-Ar gas mixture at temperatures of 1173 to 1323 K. The structure of these films were investigated by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The effects of crystallinity, morphology and Si-concentration of the deposits were examinated in some detail.     

Structural investigations of lead-strontium fluoroapatites

Solid solutions in the system Pb_(10−x)Sr_x(PO₄)₆F₂, 0?x?10, were obtained as apatitic phases from aqueous medium. They were investigated by X-ray diffraction, chemical analysis and infrared (IR) spectroscopy. The results of the structural refinements indicated that the substitution of lead by strontium induces a regular decrease of the lattice constant “a” and a preferential strontium distribution in site M(1). A progressive shift of the F⁻ ion position along the apatitic channel was detected and confirmed by IR evidence. The different character of the M-F and M-O interactions was invoked to justify the structural differences observed as a function of composition.     

Structural study of gold clusters

Density functional theory (DFT) calculations were carried out to study gold clusters of up to 55 atoms. Between the linear and zigzag monoatomic Au nanowires, the zigzag nanowires were found to be more stable. Furthermore, the linear Au nanowires of up to 2 nm are formed by slightly stretched Au dimers. These suggest that a substantial Peierls distortion exists in those structures. Planar geometries of Au clusters were found to be the global minima till the cluster size of 13. A quantitative correlation is provided between various properties of Au clusters and the structure and size. The relative stability of selected clusters was also estimated by the Sutton-Chen potential, and the result disagrees with that obtained from the DFT calculations. This suggests that a modification of the Sutton-Chen potential has to be made, such as obtaining new parameters, in order to use it to search the global minima for bigger Au clusters.     

Quasi-bound resonances in electron scattering by small metal clusters

A quantal calculation to evaluate theoretically the elastic cross section of electrons scattered by metal clusters is presented. The method is applied to the elastic dispersion of low energy electrons (up to 5 eV) from the spherical sodium clusters Na₈ and Na₂₀. Strong resonances in the total cross sections are found for some energies which are closely related to the existence of quasi-bound states, i.e. states completely bound in the classical limit.     

Structural investigations on polymer latices by small-angle x-ray scattering

An overview is given on recent studies of latices by small-angle x-ray scattering (SAXS). It is demonstrated that SAXS-studies performed at different contrast (contrast variation) allows the full elucidation of the fine structure of the particles down to a size-scale of a few nanometers. Examples are given including studies on core-shell latices composed of polystyrene (PS) and poly(methyl methacrylate) (PMMA) as well as of carboxylated latices.