Adsorption of small molecules on silver clusters

We report investigations of adsorption of N(2) and O(2) molecules on silver cluster cations. We have first revisited structures of small silver clusters based on first-principles calculations within the framework of density functional theory with hybrid functional. The 2D to 3D transition for the neutral clusters occurs from n = 6 to 7 and for cations, in agreement with experiments, from n = 4 to 5. With the refined structures, adsorption energies of N(2) and O(2) molecules have been calculated. We have identified characteristic drops in the adsorption energies of N(2) that further link our calculations and experiments, and confirm the reported 2D-3D transition for cations. We have found that perturbations caused by physisorbed molecules are small enough that the structures of most Ag clusters remain unchanged, even though physisorption stabilizes the 3D Ag(7)(+) structure slightly more than the 2D counterpart. Results for pure O(2) adsorption indicate that charge transfer from Ag(n)(+) to O(2) occurs when n > 3. Below that size oxygen essentially physisorbes such as nitrogen to the cluster. We interpret the experimentally observed mutually cooperative co-adsorption of oxygen and nitrogen using results from density functional theory with generalized gradient approximations. The key to the enhancement is N(2)-induced increase in charge transfer from Ag(n)(+) cations to O(2).     

Far-infrared spectroscopy of small neutral silver clusters

The vibrational spectra of Ag(3) and Ag(4) are recorded in the far-infrared between 100 and 220 cm(-1) using multiple photon dissociation spectroscopy of their complexes with Ar atoms. For Ag(3)-Ar two IR active bands are found at 113 and 183 cm(-1), for Ag(4)-Ar one band at 163 cm(-1) and very weak IR activity at 193 cm(-1) are observed. This, together with recent theoretical studies, allows for a reassignment of the controversial vibrational data reported earlier for the bare Ag(3) cluster. The influence of the number of Ar atoms in the complexes on the frequency of the IR active modes is found to be minor. However, the low-frequency IR-active band of Ag(3) shifts with increasing Ar coverage from 113 cm(-1) for Ag(3)-Ar to about 120 cm(-1) for Ag(3)-Ar(4), the value known for Ag(3) embedded in rare gas matrices.     

The role of dimers in evaporation of small argon clusters

Evaporation of small Lennard-Jones argon clusters has been studied using molecular dynamic simulations. An extensive library of clusters with 4, 5, 6, 11, and 21 atoms has been obtained from an earlier study. Analysis of the evaporation properties of the clusters indicate, that the fraction of dimer evaporations of all evaporation events increases with the total energy of the cluster. The fraction of evaporated dimers from clusters with a constant lifetime is independent of the cluster size for short-lived clusters and increases with cluster size for long-lived clusters. Only a few percent of the clusters which are long lived enough to participate in vapor-liquid nucleation decay by emitting dimers. The mean cluster lifetime as a function of total energy shows the same exponentially decreasing trend for monomer and dimer evaporation channels. The fraction of trimer evaporations is found to be vanishingly small.     

Electron impact ionization of small silver and copper clusters

Using a quadrupole mass spectrometer, relative cross sections for electron impact ionization of neutral Ag _n and Cu _n clusters withn=1 ... 4 have been measured for electron energies between threshold and 125 eV. From the results, the following ionization energies were obtained: Ag₂: 7.26±0.1 eV, Ag₃: 6.19±0.2 eV, Ag₄: 6.33±0.3 eV, Cu₂: 7.46±0.15 eV, Cu₃: 6.14±1.0 eV, Cu₄: 7.00±0.6 eV. With only two exceptions, these values agree with other data published for Ag₂, Cu₂, Cu₃ and Cu₄.     

Binding of ammonia to small copper and silver clusters

We report equilibrium geometries, harmonic frequencies, and thermochemical data for the metal cluster–ammonia complexes Ag_n(NH₃) and Cu_n(NH₃) (n=1,2,3,4), Ag₄(NH₃)₂, and Cu₄(NH₃)₂ calculated by a density functional method. The calculated shifts in ammonia umbrella mode frequency correlate with the observed shifts and the calculated enthalpies of complexation. The preferred site for NH₃ adsorption and the calculated bond enthalpies can be rationalized by considering atomic charges obtained from a natural population analysis and polarization of the metal electron density.     

Early stages in the growth of small silver clusters in aqueous solution

In this mini-review, the growth of silver nanoclusters following the reduction of silver ions in aqueous solution is studied and some clusters are characterized. A model for the molecular structure of trimer silver clusters is discussed as well as the role of aliphatic alcohol radicalsin the growth of silver nanoclusters.     

Development kinetics of silver clusters on silver halides

Silver nuclei are produced by pulse radiolysis at the surface of AgCl nanocrystallites in the presence of an electron donor, the methyl viologen, which induces the growth of silver nuclei. The experimental results observed on the increase of the silver atom concentration and on the decay of the donor concentration during this process, which is similar to the photographic development by an electron donor, are compared with the kinetics obtained from numerical simulation. The model assumes that the formation of silver clusters with a supercritical nuclearity is required before the start of an electron transfer reaction from the two reduced forms of the donor methyl viologen to the silver clusters. The reaction is controlled by the access of the donor to the surface sites of the AgCl crystallite. The rate constant values of the successive steps of the mechanism are derived from the adjustment of calculated kinetics to experimental signals under various conditions, using a single set of parameters which are fairly suitable under all conditions studied.     

The structure of small carbon clusters

New non linear isomers of C _n forn=4, 5, 6 have been observed using the combination of the laser photodetachment technique and the Coulomb Explosion Imaging method. Electron affinities of these isomers were found to be lower than the corresponding known linear isomers. The structure of low electron affinity C₄ isomer was found to be rhombic in accordance with recent theory. Indications for the non linearity of the low electron affinity isomers of C₅ and C₆ are also presented.     

The isomers of small carbon clusters

Absolute cross-sections for photodetachment of negative carbon clusters are reported for C _n ^? (n=3,...,8). These measurements are made using different types of ion sources, which create different isomers. These new results indicate that various negative and neutral isomers exist, some with electron affinities as low as ～1 eV.     

Photoelectron spectroscopy of silver clusters

A comparison of x-ray photoemission from Ag clusters deposited on polygraphite and highly oriented pyrolitic graphite shows the influence of the support both on the valence band and on the core 3d level of the metal. Positive shifts have been obtained with respect to the bulk for the Fermi edge and the 3d peaks depending on the number of silver atoms deposited on the substrates. When the deposition is very small (cluster regime) the positive shifts of the binding energies are quite different for different substrates and cannot have a common origin. In contrast with recent work, we show that several effects contribute to these shifts: initial state effects like charge redistribution as well as final state effects like the hole-electron interaction.     

The Hubbard model for small clusters

The Hubbard model with nearest neighbor hopping, and one type of orbital is applied to clusters of 4 and 5 sites. Results are reported for the ground state as a function of the number of electrons. A rather complicated dependence of the spin of the ground state on occupation number, geometry, and model parameters is found. Ground state energies are compared. The excitation spectrum is illustrated in a particular example.     

Formation of silver clusters in nozzle expansions

The first successful experiments to generate continuum silver cluster beams from nozzle expansions are described. A mixture Ar/Ag expands out of a conical nozzle (0.35 mm dia., 10° cone angle, length 17 mm). At 2150 K, total pressure 300 kPa, silver partial pressure 8 kPa the silver intensity measured with a rate meter 479mm away from the source is 1.8 nm/s, or 0.02 g m^?2s^?1. The data for the onset of clustering confirm the predictions of the scaling laws developed to compare condensation in nozzle expansions of metal vapors with that of rare gases.     

The structure of small gallium nitride clusters

The geometry, vibrational frequencies and stability of the structural isomers of small gallium nitride clusters (n = 2–4) have been investigated using density functional theory. The lowest energy structures are cyclic. The ground electronic state of the cyclic forms for n > 2 is the singlet state. All of the cyclic structures have D_nh symmetry. The caged structures for Ga₄N₄ lie higher in energy than the planar cumulenic monocyclic ring. The Ga‐N bond dominates the structures for many isomers, so that one dissociation channel is loss of a GaN monomer. However, unlike the corresponding boron and aluminum clusters, dissociation into larger fragments is energetically favored. The structural properties of the gallium nitride clusters are similar to those of the analogous AIN (and BN) clusters. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:281–286, 2000     

The electronic structure of small sodium clusters

Ab initio molecular orbital calculations using the STO3-21G basis set has been carried out for the cluster series Na _n ⁺ , Na _n , and Na _n ^– (wheren=2–7). The basis set is shown to be reliable compared with more extensive basis sets at the Hartree-Fock level. Thirty-one optimized structures are reported and discussed, many of which (especially for the anions) have not been considered. The STO3-21G//STO3-21G calculations suggest that for most of the species the optimum geometries are planar. In particular, the optimized structures for the anionic species should provide a starting point for more sophisticated configuration interaction calculations.     

The electronic structure of small lithium clusters

Ab initio molecular orbital calculations using the STO-6G and STO6-21G basis sets have been performed for the cluster series Li _n ⁺ , Li _n , and Li _n ^– (wheren=2–7). Thirty-two optimized structures are discussed and reported, many of which (especially for the anionic structures) have not yet been considered. The calculations suggest that for all three species the optimum geometries are planar. Of the two levels of theories that were investigated, STO-6G//STO-6G and STO6-21G//STO-6G, the latter hybrid theory was found to be less reliable. In particular, for the anionic structures these calculations should provide a platform from which more sophisticated, i.e., configuration interaction, geometry optimization can be performed.     

The mixed ionocovalent bonding in small insulating clusters

We report a selfconsistent tight binding calculation of the electronic and structural properties of small insulating clusters (up to 12 atoms): these include alkaline-earth oxides (MgO and CaO) and NaCl clusters. Interatomic distance contractions are found in most cases, as in small metallic clusters. In addition, new effects take place in some of the compounds; these are a reduction of the ionic charge and, for the first time, the prediction of a dimerization of the interatomic distances. The microscopic processes responsible for their occurence and size dependence are dicussed with analytical arguments. It is shown that for these small clusters the equilibrium configuration and the electronic structure are fixed by a competition between covalent and electrostatic interactions, especially in the oxides.     

The influence of CO on the surface plasmon absorption band of small silver clusters (D≤20 Å)

The surface plasmon absorption band of small Ag particles (diameter≤100 Å) in the gasphase as well as for clusters embedded in noble gas matrices is known to shift to higher energies and to broaden with decreasing diameter. This size effect depends sensitively on the microscopic surface properties of the clusters, which can be influenced by the interaction with the embedding matrix. Previous experiments with silver clusters in the size range 20 Å≤D≤100 Å proved that for SiO₂ and CO the matrix influence is strong enough to supersede the intrinsic size effect as observed in the noble gas matrices. The investigation of the influence of a CO matrix on smaller clusters with D≤20 Å reveals a novel feature: the position as well as the width of the surface plasmon absorption band become independent of the cluster size.     

Stability of multiply charged silver clusters

Using the gasaggregation technique it is possible to generate metal clusters in narrow size distributions and to vary their mean size by adjusting the cell parameters. The high intensity of this source allows to detect besides singly charged clusters also multiply charged ones. Ag _n ²⁺ and Ag _n ³⁺ are observed forn≧9 andn≧31, respectively; i.e. at values well below the critical sizes reported for spheres.     

Long-lived charge-separated states in ligand-stabilized silver clusters

Recently developed synthesis methods allow for the production of atomically monodisperse clusters of silver atoms stabilized in solution by aromatic thiol ligands, which exhibit intense absorption peaks throughout the visible and near-IR spectral regions. Here we investigated the time-dependent optical properties of these clusters. We observed two kinetic processes following ultrafast laser excitation of any of the absorption peaks: a rapid decay, with a time constant of 1 ps or less, and a slow decay, with a time constant that can be longer than 300 ns. Both time constants decrease as the polarity of the solvent increases, indicating that the two processes correspond to the formation and recombination, respectively, of a charge-separated state. The long lifetime of this state and the broad optical absorption spectrum mean that the ligand-stabilized silver clusters are promising materials for solar energy harvesting.