Pure Metal Clusters Research Articles

Electronic shell structure in monoxides and dioxides of sodium

Metal–metal bonding interactions are important for the structure and properties of sodium-rich monoxide and dioxide clusters NanO, NanO2n⩽10. DFT calculations show that such bonding is due to occupied molecular orbitals of mostly Na:3s character and provide evidence that these orbitals resemble the 1s, 1p electronic shells of pure metal clusters. Calculated properties of the sodium oxides (ionisation potential and sodium abstraction energy) follow those of pure sodium clusters in both magnitude and overall trend. Structural anomalies can also be explained by the electronic shell model, as illustrated with clusters possessing four ‘metallic’ electrons (1s21p2). Singlets show Jahn–Teller distortion to prolate shapes; triplets distort to become oblate. In addition, the shell model rationalises the stereoelectronic conditions for the metal-to-peroxide electron transfer which leads to O–O cleavage.

Unusual magnetic behavior in imperfect 4d and 5d clusters on the Ag(0 0 1) surface

Small mixed 4d and 5d clusters on Ag(0 0 1) are investigated by means of the KKR Green's function method. It is shown that the mixture of pure transition metal clusters with substrate atoms or the creation of defects in the cluster leads to an unexpected enhancement of the magnetic moments. The results are discussed in the framework of a tight binding model.

Surface-enhanced Raman scattering (SERS) on copper electrodeposited under nonequilibrium conditions

Rough Cu surfaces, with pattern formation, were obtained by a nonequilibrium electrodeposition from acidic electrolyte. Wavelength dispersive spectrometer (WDS)/energy-dispersive spectrometer (EDS) investigations revealed that such surfaces are highly oxidized. Surface enhanced Raman scattering (SERS) investigations confirmed a complex structure and composition of such a substrate; a distinct difference in spectral characteristics of an adsorbate on Cu2O, pure metallic Cu clusters, and some other, not yet well characterized, metallic Cu clusters, which predominate on the copper surfaces activated by standard electrochemical methods, were identified and distinguished.

EXAFS study on metal cluster doped silica glass obtained by ion implantation procedures

Metal-doped glasses were prepared by double ion implantation (Ag+Cu and Au+Cu) in silica glass. X-ray absorption spectroscopy was used to study the different oxidation states of the dopant metals and the possible formation of metal or alloy clusters. We present an investigation on co-implanted silica glass at the Cu K, Ag K and Au L III edges. The local atomic order around each dopant was determined. In particular, no evidence of metal alloying was found. Cu is mainly bound to O atoms of the glass, Ag mainly forms pure metallic clusters and Au is present as small metallic particles and isolated atoms.

Sol-gel synthesis of metal nanoclusters-silica composite films

In a program on the development of metal nanoclusters in sol-gel derived thin films, attempts were made to synthesize pure and mixed metal clusters, control the cluster size and increase the volume fract f the clusters. Thus, Ag, Cu and Ag-Cu nanoclusters were prepared in silica films using dip- and spin-coating techniques. The annealing of Ag/SiO2 films in different atmospheres (air, argon and 5% H2-95% N2 gas) caused modifications of Ag nanoclusters resulting in changes in their surface plasmon resonance (SPR) peak positions. The Cu and Ag-Cu codoped films were annealed in reducing atmosphere (5% H2-95% N2 gas). In order to prepare Cu nanoclusters of different sizes, the concentrations of Cu in Cu/SiO2 composite films were varied from 8 to 30 mol% and annealed at 800°C for different times for growth. The size of the Cu nanoclusters was measured from the half band width of Cu SPR peak (appearing within 570–557 nm range) and X-ray diffraction. In this way Cu-nanoclusters of size ranges from about 3.5 to 10 nm (average diameters) were prepared . The Ag-Cu nanocluster-containing silica films show the existence of both Ag and Cu SPR peaks with some blue shifting in comparison with to their pure analogues depending on the Ag:Cu ratio.

A new cluster source for the generation of binary metal clusters

A new thermal, supersonic cluster source for the investigation of binary metal cluster formation at thermodynamically well-defined expansion conditions is described. The source consists of two separately heatable cartridges. A first cartridge can be heated up to 1220 K and the second high temperature cartridge reaches maximal temperatures of 1800 K. A temperature difference of 1000 K between the two cartridges can be maintained for at least 3 h. Clustering occurs upon supersonic expansion from a conical nozzle. This cluster source has two main applications: (a) the generation of mixed metal clusters and (b) the investigation of pure metal clusters at various expansion conditions. The performance and applications of this source are illustrated by presenting results of the heterocluster formation of mixed sodium/gold and sodium/silver heteroexpansions. In addition, the influence of the oven parameters on the internal temperatures of the generated clusters is illustrated with the example of Na2.

Emergence of metallic properties in alkali-rich alkali-halide clusters

We have used photoelectron spectroscopy to study alkali-rich sodium iodide and cesium chloride clusters in order to determine how their composition affects their electronic properties. Spectral features of (Na I${)}_{\mathrm{n}}$${\mathrm{Na}}_{\mathrm{m}}^{\mathrm{\ensuremath{-}}}$ (m\ensuremath{\geqslant}2) are remarkably similar to those of isolated metal clusters ${\mathrm{Na}}_{\mathrm{m}}^{\mathrm{\ensuremath{-}}}$ for all sizes of n that we investigated. This result suggests that the clusters are comprised of phase separated metallic and ionic components. In contrast, photoelectron spectra of (CsCl${)}_{\mathrm{n}}$${\mathrm{Cs}}_{\mathrm{m}}^{\mathrm{\ensuremath{-}}}$ clusters are unlike those of pure metal clusters, suggesting that the excess metal atoms are mixed into the ionic portion of the cluster. Such mixing may make it possible for these cesium chloride clusters to exhibit the finite system equivalent of a metal-insulator transition.

Study of partial oxidation of Cu clusters by HRTEM

Cu clusters of diameters less than 10 nm were selected as models to study oxidation as a function of size, chemical composition, and structure. The investigations were performed with high-resolution electron microscopy. It was shown that beginning with multiply twinned particles as stable structures for particles with diameters less than 5 nm and of cuboctahedra for larger particles, in the transitional state between pure metal and oxide both states can coexist within the same particle; this latter can be proved by HRTEM. It was also demonstrated that this is due to incorporating distortions as step dislocations within grain boundaries which can be shown in electron micrographs of high resolution better than 0.18 nm after image processing. The creation of sub-oxides with less reactivity than the pure metal clusters could also be demonstrated leading to morphologies which are different from those of the pure metal.

Enhancement of Metallic Silver Monomer Evaporation by the Adhesion of Polar Molecules to Silver Nanocluster Ions

Abstract : We have compared the metallic evaporation channels from metastable Ag(X=5,7,11)(AgI)(Y=1-4)(+) clusters in the 1st FFR of a double focussing mass spectrometer with that of the corresponding pure metallic clusters, Ag(X=5,7,11) (+). It is found that the presence of the polar AgI molecules increases the rate of silver monomer evaporation relative to that of silver dimer evaporation. Using thermodynamic expressions for the heat of evaporation of the different evaporation processes and assuming the absence of reverse activation energies, an expression for the difference between the activation energy of silver monomer and dimer evaporation is derived. It is shown that dipole/induced-dipole forces resulting from the presence of AgI polar molecules lead to an enhancement of silver monomer evaporation if the polarizability of the pure metallic cluster ions increases with the number of Jellium electrons. Our theoretical calculations of the static polarizabilities of (Ag(x))(+) using time dependent density functional theory within the local density approximation, shows a smooth increase in the polarizabilities with the number of the Jellium electrons in these clusters. Finally, we observe that the enhancement of Ag monomer evaporation per AgI added is smaller for clusters with even number of AgI molecules than with odd numbers. This was proposed to result from the contribution of configurations with dipole 'pairing' of the AgI molecules in clusters with even number of AgI molecules. Dipole pairing would decrease the average dipole/induced-dipole interaction between the AgI molecules and the metallic part of these 'mixed' clusters.

Collective electronic excitations in metal-coated C60.

A two-shell jellium-on-jellium model has been used to study the collective electronic excitations of ${\mathrm{C}}_{60}$ coated by an increasing number N of Na atoms. We predict a transition from the \ensuremath{\pi}-electron polarization of the fullerene to the metallic sodium polarization as a function of increasing N. For low coverages, the Na layer produces only a broadening and fragmentation of the \ensuremath{\pi} plasmon of ${\mathrm{C}}_{60}$. However, if the coverage is large enough, a Na surface plasmon appears. This occurs only after the electron density of the cluster clearly shows two distinct regions, the outer one having an ``average'' density similar to that in pure Na metal or Na clusters. The induced density at the collective-mode frequency shows structure corresponding to two interfaces, ${\mathrm{C}}_{60\mathrm{\ensuremath{-}}}$Na and Na vacuum, the first peak becoming less pronounced as the coverage increases due to a transfer of oscillator strength to the metallic counterpart. The static polarizability per electron also shows this trend and tends slowly to the Na value from below after an initial sharp rise in the region N\ensuremath{\sim}13--21.

Evidence for a phase separation in metal-rich alkali-halide cluster anions.

We report photoelectron spectra of metal-rich alkali-halide cluster anions (NaCl${)}_{3}$${\mathrm{Na}}_{\mathit{m}}^{\mathrm{\ensuremath{-}}}$ and (${\mathrm{KI}}_{3}$${\mathrm{K}}_{\mathit{m}}^{\mathrm{\ensuremath{-}}}$) (m=0 to 5) which show evidence of phase separation within the clusters. The spectra are remarkably similar to those observed in pure alkali metal clusters and include such metal-like features as a highest-occupied\char21{}lowest-unoccupied molecular orbit gap that decreases with increasing m and an even-odd alternation in the electron verticle binding energies. These results suggest that the metal-rich alkali-halide clusters consist of metal anions, ${\mathrm{Na}}_{\mathit{m}}^{\mathrm{\ensuremath{-}}}$ or ${\mathrm{K}}_{\mathit{m}}^{\mathrm{\ensuremath{-}}}$, weakly attached to neutral alkali halides, (NaCl${)}_{3}$ or (KI${)}_{3}$

Atomic structure and collective excitations of medium size NanKm clusters

Density functional theory is used to study the atomic and electronic structure of NanKm clusters with up to seventy atoms. The simplifying approximation has been made of replacing the external potential of the ionic background by its spherical average about the cluster centre in the iterative process of solving the Kohn-Sham equations for each geometry tested. The search for the equilibrium geometry is performed by employing steepest descent and simulated annealing techniques. We have found segregation of K to the surface and when the cluster is large enough, a neat stratification of K and Na shells. Those effects (segregation and stratification) do not perturb the electronic magic numbers well known for pure alkali metal clusters. Our results for the atomic structure are rather similar to those reported earlier for NanCsn clusters. We have also studied in a selected case, Na20Cs20, the dependence of the collective electronic excitation spectrum on the segregation and other geometric characteristics of the cluster.

Production of aluminum clusters by direct laser vaporization of aluminum nitride

We report the production of aluminum clusters (Al + x , x up to 62 Al − x , x up to 42) in a FTMS by direct laser vaporization of aluminum nitride. This is the first observation of pure metal clusters from a metal compound. A pressure burst composed of N and/or N 2 accompanies cluster formation and presumably helps stabilize the nascent clusters. The cluster intensity distributions are similar to those obtained using aluminum targets. The thermalized cation clusters do not react with O 2 unless they are kinetically excited. Deviations in the branching ratios from earlier beam investigations may result from pressure/timescale differences in the two experiments.

The magic numbers of metal and metal alloy clusters

Pure metal and metal alloy clusters including Cun, Agn, CunAgm, CunAlm, CunInm, AgnAlm, AgnInm, and CunPbm are produced by a gas aggregation source and investigated by time-of-flight mass spectrometry following ionization with a KrF excimer laser. In the case of pure metal clusters (Cun,Agn,Inn), as well as alloy clusters composed of these metals, magic numbers are observed in their cluster ions which correspond to jellium shell closings (counting the total valence electrons from the component metals). These findings are in good agreement with their expected free-electron behavior. Interestingly, the abundance of pure Pbn+ corresponds to species which are expected to be especially stable due to their geometric structure. A similar situation also arises for the Pb-rich alloy clusters. By contrast, the metal alloy clusters CunPbm+ show magic numbers at jellium shell closing in the series of Cu-rich clusters.

Metallization of ionic clusters.

The energetics and structure of multiple excess electrons in halogen-deficient Na{sub {ital n}}F{sub {ital m}} clusters are studied starting at the stoichiometric ionic lImit Na{sub {ital n}}F{sub {ital n}} and ending with pure metallic clusters Na{sub {ital n}} (for {ital n}=4,3,2 and partially for {ital n}=14). The results exhibit structural transitions upon metallization, odd-even oscillations in {ital n}-{ital m} and possibly shell-closing effects portrayed in formation and ionization energies, and excess-metal face segregation for the larger system.

Silver-halogen cluster compounds Ag n X m (n?2; 0?m?n;X=F, Br)

Nonstoichiometric silver-halogen cluster compounds Ag n X m (0≤m≤n;X=F, Br) are generated by cocondensation of Ag atoms and AgX species using a slightly modified gas aggregation technique. The AgX molecules are produced by partial decomposition of SF6 and Br2 respectively at the surface of the hot silver containing crucible, followed by the reaction of halogen atoms with silver, giving rise to the formation of AgX molecules. In a heterogeneous nucleation between these molecules and evaporated Ag atoms the afore mentioned cluster compounds are formed. The degree of halogenation can either be controlled by the adjustment of the silver evaporation rate, or even more easily by controlling the partial pressure of the halogenating agent. The mass spectra of singly charged halogenated clusters, which are generated by electron impact ionization, reflect the stability of ions. These mass spectra demonstrate that there is an alternation in the intensity pattern up to a relatively high degree of halogenation (m) for each of the investigated compound series Ag n X m ,n≤8. This behavior is similar to the well-known odd-even effect for pure metal clusters, allowing us to postulate the existence of a “metallic” core which governs the stability of the cluster ion (at least for not too high degree of halogenation).

Electronic and atomic structure of Cs N and Cs N O clusters

The electronic and atomic structure of CsN and CsNO clusters up toN=70 is studied using both the spherical jellium model and a spherical average pseudopotential (SAPS) method. Due to shortcomings in the Aufbauprinciple of one-electron shells the spherical jellium model fails to give a consistent description of CsN and CsNO. These problems do not occur in the SAPS scheme, which allows to take into account structural aspects beyond the electronic shell effects. Due to the existence of ionic bonding these structural effects are more evident in CsNO clusters than in the pure metallic CsN clusters. The embedding energy of oxygen in CsN is negative for allN, showing a strong affinity of CsN clusters for oxygen. The embedding energy exhibits large size oszillations, since for certain values ofN the oxygen atom must displace a central Cs atom to the cluster surface.

Pure metal and metal-doped rare-gas clusters grown in a pulsed ARC cluster ion source

The performance of a new pulsed arc cluster ion source (PACIS) is demonstrated for pure metal cluster ions and anions as well as mixed metal-rare-gas clusters. Pure metal clusters like W n + exhibit intensity distributions similar to those from laser vaporization sources. The ion current of the PACIS is comparable to that of a laser vaporization source. For metal-doped argon cluster ions, a single aluminium atom serves as the central ion which is surrounded by argon atoms arranged in an icosahedron. These measurements therefore strongly support the close packing model for argon clusters.

Electronic structure of Cs N and Cs NO clusters

A spherical average pseudopotential method based on density functional theory is used to study the electronic and geometrical structure of Cs N and Cs NO clusters up to N = 70. The embedding of oxygen in a Cs cluster changes the size-dependent electronic and structural properties of the host cluster quite drastically. Due to the existence of ionic bonding in Cs NO clusters the structural effects beyond the electronic shell-closing effects turn out to be more evident in both theory and experiment when compared to the pure metallic host clusters.

Explanation of the Invar Anomalies from Molecular-Orbital Cluster Calculations

The electronic structures of pure and mixed Fe-Ni metal clusters, as computed with the self-consistent-field $X\ensuremath{\alpha}$ scattered-wave method, are used as models for the bonding situation in fcc Fe-Ni alloys. The calculations indicate that several anomalous properties of the Invar alloy ${\mathrm{Fe}}_{64}$${\mathrm{Ni}}_{36}$ are a consequence of the presence, at the Fermi level, of strongly antibonding majority-spin orbitals and nonbonding minority-spin orbitals in this concentration range.