Effects of O defects on adsorption of small Ag clusterson a MgO(001) surface

The interaction of Ag atoms with a defective MgO(001) surface is systematically studied based on density functional theory. The Ag clusters are deposited on neutral and charged oxygen vacancies of the MgO(001) surface. The structures of Ag clusters take the shape of simple models of two- or three-dimensional (2D and 3D) metal particles deposited on the MgO surface. When the nucleation of the metal clusters occurs in the F_s (missing neutral O) centre, the interaction with the substrate is considerably stronger than that in the F_s⁺ (missing O^- ) centre. The results show that the adsorption of Ag atoms on the MgO surface with oxygen vacancy is stronger than on a clear MgO surface, thereby attracting more Ag atoms to cluster together, and forming atomic islands.     

Emission of charged clusters during metal sputtering by ions

A method for simulating processes of metal sputtering by ion bombardment in the form of large neutral and charged clusters with a number of atoms N≥5 based on simple physical assumptions and in fair agreement with experiment is suggested. As an example, the ionization degrees and ionization coefficients, as well as the relative cluster yields, are calculated as a function of the number of atoms in clusters of different metals (Ag, Nb, and Ta) bombarded by singly charged Ar⁺¹ and Au⁻¹ ions. A fluctuation mechanism of charge state formation for large clusters, which describes the dependence of the charge state distributions on cluster size and target temperature, is developed.     

Effect of Polydispersity on the Phase Behavior of Polystyrene (PS)/Poly (Vinyl Methyl Ether) (PVME)

The thermodynamics and kinetics of phase separation in partially miscible blends of poly (vinyl methyl ether) (PVME) and two kinds of polystyrene (PS) with the same weight average molecular weight but different polydispersity were studied. The miscibility of PS/PVME with the monodisperse PS was better than that of PS/PVME with the polydisperse PS. Different morphology was observed for the two kinds of PS/PVME (10/90) blends during the nonisothermal phase separation process. The blend with monodisperse PS presented a co-continuous structure while the blend with polydisperse PS presented a viscoelastic phase separated network structure at a quench depth of 29°C. With increase of the heating rate, the increase of cloud point of PS/PVME (30/70) with polydisperse PS was smaller than that of PS/PVME (30/70) with monodisperse PS. During the isothermal phase separation of the critical composition (20/80) of PS/PVME with a quench depth of 30°C, it was found that the phase morphology of the two kinds of blends was nearly the same at the early stage of phase separation. However, as the dispersed phase, an approximately spherical droplet structure was observed in the blend with monodisperse PS at the late stage of phase separation, which did not appear in the blend with polydisperse PS.     

Self-confined polymer dynamics in miscible binary blends

The segmental dynamics of PVME within the single-phase state of poly(styrene)/poly(vinyl methyl ether) blends (PS/PVME) was examined by dielectric spectroscopy. A particular attention has been given to the high PS concentration regime. In this latter, rather localized, weakly cooperative motions of the PVME segments are detected at low temperatures, in addition of the secondary relaxation processes. This feature is attributed to confinement effects induced by the PS chains on the PVME.     

F+ laser performance and interaction of Cu,Ag and Au at the reduced oxygen coordination of MgO surface: first principles calculations

F⁺ laser performance and interaction of the title group IB transition metals at the reduced oxygen coordination of MgO surface were investigated using the TD and DFT methods of ab initio molecular electronic structure calculations. The considered ion clusters were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces and the nearest neighbor ions to the F⁺ site were allowed to relax to equilibrium in each case. The F⁺ laser performance fades quickly as the reduced oxygen coordination decreases from 5 to 4 to 3. The relaxed excited states (RESs) of the defect containing surfaces are compact and deep below the conduction bands of the perfect MgO surface. The probability of orientational destruction of the center in laser experiment is expected to follow the order flat>corner>edge. The excited state at the edge has higher energy than that at the flat or at the corner. F⁺ is easily formed at the lower oxygen coordination and the disappearance of anisotropy and 2p splitting observed in absorption of F⁺ at the surface follow the order corner>flat>edge. The Glasner–Tompkins relation is generalized to include the F⁺ bands at the reduced oxygen coordination of a metal oxide surface. As far as the adsorbate–substrate interactions are concerned, the F⁺ center enhances the adsorptivity of Ag, Cu and Au by ca. 1.91–3.33 eV and changes the nature of adsorption from physical adsorption to chemical adsorption. The adsorption energies follow the order Cu>Au>Ag and are explainable in terms of electrostatic potential curves, energy gaps and spin pairing. Cu and Ag act as electron donors while Au acts as electron acceptor and the MgO surface cannot be made semiconducting by F⁺ imperfection.     

Electron energy loss spectroscopy of the vibration modes of water on Ag(100) and Ag(115) surfaces and comparison to Au(100), Au(111) and Au(115)

Motivated by rather similar behavior of the Helmholtz capacitances of stepped Au(11n) and Ag(11n) electrodes we have extended a previous study on the vibration spectrum of water adsorbed at low temperatures on stepped gold surfaces to Ag(100), Ag(115) and Au(111) surfaces. On Ag(100) surfaces, the spectra show the presence of the typical H-bonded network of water molecules. The rather weak intensity, the absence of non-hydrogen bonded hydrogen atoms, the similarity to the infrared spectrum of ice crystallites, and the increase in the angular spread of the elastic peak are indicative of adsorption in form of three-dimensional clusters. This is stark contrast to Au(100) and Au(111) where the spectra match to a model involving stacks of water bilayers. The low coverage spectra on Ag(115) resemble the results on Au(115): A considerable fraction of the H-atoms remains in the non-H-bonded state and spectral features of water adsorbed at step-sites are identified. The first layer of water on Ag(115) surfaces should therefore have a similar structure as recently proposed in a theoretical study concerning water on Au(115). For larger doses, the experimental results on Ag(115) suggests the formation of three-dimensional clusters. This is contrary to Au(115) where the layered structure with a constant fraction of non-hydrogen-bonded H-atoms persists at higher doses.     

Structure of quench condensed Ge-metal alloys

Thin films of Ge with about 30 at % of Al, Au or Ag are condensed at low temperatures. Their structural and electrical properties have been investigated in situ after condensation and during annealing. From electron diffraction measurements the atomic distribution functionG(r) is derived and discussed in relation with the electrical resistivity of the samples. In spite of certain similarities in the phase diagrams the metal atoms contribute quite differently to the short range order, reaching from Al atoms on Ge sites of the random network of the amorphous state to Ag clusters in its voids. Correspondingly the electrical properties extend from semi- to superconductivity.     

Effect of Nanoparticles on the Phase Behavior of Polystyrene/Poly(vinyl methyl ether) Blends with Different Polydispersities

Effects of two kinds of silica nanoparticles on the phase behavior of blends of poly (vinyl methyl ether) (PVME) and two kinds of polystyrene (PS), with the same number average molecular weight but different polydispersities, were studied by using optical microscopy. The addition of both the hydrophilic A200-SiO₂ and hydrophobic R974-SiO₂ nanoparticles made the miscibility become worse. The decrease in the phase separation temperatures of PS/PVME with polydisperse PS (pPS) was larger than that of PS/PVME with monodisperse PS (mPS) in the presence of A200-SiO₂; the shifts of the phase separation temperatures of mPS/PVME and pPS/PVME were comparable for the incorporation of R974-SiO₂. The addition of both kinds of silica accelerated the phase separation during the nonisothermal phase separation. The domain size changes of the phase morphology of mPS/PVME (50/50) and pPS/PVME (50/50) were almost the same during the nonisothermal phase separation in the presence of nanoparticles. During the isothermal phase separation, the introduction of silica slowed down the phase separation dynamics by hindering the phase structure transition. The morphology changes of pPS/PVME (50/50) were more evident than those of mPS/PVME (50/50) with the incorporation of SiO₂.     

Structural and electronic properties of silver-doped gold clusters Au n Ag v (2 ≤ n ≤ 10; v = 0, ±1): comparison with pure gold clusters

The structural and electronic properties of silver-doped gold clusters Au _n Ag ^v (2?≤?n?≤?10; v?=?0,?±1) have been systematically investigated using density functional theory. The results show that the ground state optimal structures of the cationic and neutral clusters are found to be planar up to n?=?3 and 9, respectively. However, for the anionic clusters, no three-dimensional lowest-energy structures are obtained according to DFT calculations. The calculated binding energy and dissociation energy as a function of cluster size exhibit odd–even alternations. The natural population analysis indicates that in Au _n Ag ^v clusters charges transfer from the Ag atom to the Au frames. The trends for the vertical detachment energies, adiabatic electron affinities, adiabatic ionization potentials, and chemical hardness of Au _n Ag ^v clusters, as the cluster size increases, are studied in detail and compared with the available experimental data.     

银原子团簇在纳米碳管中的形态与熔化特性

采用分子动力学方法,对纳米碳管中Agn(n=108,402)团簇的形态及熔化特性进行了模拟,并与自由状态下团簇的形态及熔化特性进行了对比。研究表明,①温度T=150 K时,碳管中银团簇的形态呈现为吸附在碳管内侧的单原子层银管,表现为非晶体,而自由状态下的银团簇呈现为近似球形,且具有一定的晶体特征。②自由状态下银团簇的熔化为“晶体熔化”,而纳米碳管中的银团簇为非晶熔化。     

Formation of argentic clusters and small Ag nanoparticles in soda-lime silicate glass

The formation of argentic clusters and very small Ag nanoparticles of 0.5 to 2 nm size in commercial soda-lime glass silver-doped by Ag/Na ion exchange in a mixed nitrate melt has been studied by electron microscopy and EXAFS. Particles formation was induced already during the ion exchange procedure, or by subsequent ion irradiation with 1.5 MeV He⁺ or 3 MeV Au⁺. The presence of nanoparticles was also macroscopically revealed by their surface plasmon resonance. The structural characterization indicates that specific configurations of silver oxide-like structures, so-called argentic clusters, are involved in the initial stage of nanoparticles formation.     

Surface-enhanced Raman scattering (SERS) activity of Ag, Au and Cu nanoclusters on TiO2-nanotubes/Ti substrate

Tubular arrays of TiO₂ nanotubes (ranging in diameter from 40 to 110 nm) on a Ti substrate were used as a support for Ag, Au or Cu deposits obtained by the sputter deposition technique, where the amount of metal varied from 0.01 to 0.2 mg/cm². Those composite supports were intended for surface-enhanced Raman scattering (SERS) investigations. Composite samples were studied with the aid of scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) to reveal their characteristic morphological and chemical features. Raman spectra of pyridine (as a probe molecule) were measured at different cathodic potentials ranging from −0.2 down to −1.2 V after the pyridine had been adsorbed on the metal-covered TiO₂ nanotube/Ti substrates. In addition, SERS spectra on a bulk standard activated Ag, Au and Cu substrates were also measured. The SERS activity of the composite samples was strongly dependent on the amount of metal deposit, e.g. at and above 0.06 mg Ag/cm², the intensity of SERS signal was even higher than that for the Ag reference substrate. The high activity of these composites is mainly a result of their specific morphology. The high SERS sensitivity on the surface morphology of the substrate made it possible to monitor very small temporal changes in the Ag metal clusters. This rearrangement was not detectable with microscopic (SEM) or microanalytical (AES) methods. The SERS activity of Au or Cu clusters was distinctly lower than those of Ag. The spectral differences exhibited by the three kinds of composites as compared to the reference metal samples are discussed.     

Comparison of electronic structures of mass-selected Ag clusters and thermally grown Ag islands on sputter-damaged graphite surfaces

Ag cluster anions consisting of 3–16 atoms were deposited on sputter-damaged HOPG surfaces using a soft-landing technique (mean deposition energy less than 0.2 eV/atom) at room temperature. For investigations of the structures of deposited clusters, X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were used. In addition, the chemical properties of deposited clusters were studied using atomic oxygen and CO. Comparison of the properties of deposited Ag clusters and Ag islands with similar sizes grown by evaporating Ag atoms on the same substrate shows different results, implying that two different preparation methods give either different shapes of Ag clusters and islands, or dissimilar metal–support interactions. PACS 73.22.-f     

Magnetism of Fe clusters embedded in Cu,Ag, and Au fcc matrices: density functional calculations

We present extensive first principles density functional theory (DFT) calculations dedicated to analyze the magnetic properties of small Fe _n clusters (n = 2,3) embedded in Cu fcc, Ag fcc and Au fcc matrices. We consider several dimers and trimers having different interatomic distances. In all cases the Fe atoms are embedded as substitutional impurities in the metallic network. For the case of the Fe dimers we have considered two magnetic configurations: ferromagnetic (antiferromagnetic), when the atomic magnetic moment of the Fe atoms are parallel (antiparallel) each other. For the case of dimers immersed in Cu and Ag matrices, the ground state corresponds to the ferromagnetic Fe dimer whose interatomic distance is a/√2. For Fe dimer immersed in the Au matrix the ground state corresponds to a ferromagnetic coupling when the interatomic distance is a√(3/2). In the case of the Fe trimers we have considered three or four magnetic configurations, depending on the Fe cluster geometry. For the case of Fe trimer immersed in Cu and Ag matrices we have found that the ground state corresponds to the ferromagnetic trimer forming an equilateral triangle with an interatomic distance equal to a/√2. The ground state for the Fe trimer immersed in the Au matrix corresponds to the ferromagnetic Fe trimer forming a right angle triangle.     

Density functional investigation of structural and electronic properties of small bimetallic silver–gold clusters

Structural and electronic properties of bimetallic silver–gold clusters up to eight atoms are investigated by the density functional theory using Wu and Cohen generalized gradient approximation functional. By substitution of Ag and Au atoms, in the optimized lowest energy structures of pure gold and silver clusters, we determine the ground state conformations of the bimetallic silver–gold ones. We reveal that Ag atoms prefer internal positions whereas Au atoms prefer exposed ones favoring charge transfer from Ag to Au atoms. For each size and composition, binding energy, HOMO–LUMO gap, magnetic moment, vertical ionization potential, electron affinity and chemical hardness were calculated. On increasing the size of the cluster by varying number of Ag atoms with fixed number of Au ones, vertical ionization potential and electron affinity show obvious odd–even oscillations consistent with the pure Ag and Au clusters. Au atoms inclusion in the cluster increases the binding energy and vertical ionization potential, indicating higher stability as the number of Au atoms grows. The variation of chemical hardness with the composition in a cluster with the same size shows peaks when the number of Ag atoms is greater than or equal to Au ones, corresponding to transition from planar to tri-dimensional structures. For clusters with even number of atoms, the peaks indicate that the clusters with the same number of Ag and Au atoms are the most stable ones. Analyzing the density of states, we found that increasing the concentration of Ag atoms affects the energy separation between the HOMO and the low lying occupied states.     

Isotope effects in the metastable decay of Ne2 +

Neon dimer ions undergo spontaneous dissociation (metastable decay) several microseconds after formation by electron impact ionization of neon clusters. In this contribution we compare the kinetic energy release distribution (KERD) of the previously reported isotopomer ²⁰Ne₂ ⁺ with that of ²²Ne₂ ⁺. The heavy isotopomer shows the same two components in the KERD as the lighter ones. However, the high-energy component that is due to electronic pre-dissociation is reduced in intensity. The decrease is attributed to a reduced predissociation rate from the II(1/2u) state into I(3/2u).     

Density functional theory study on the structural and electronic properties of Ag-adsorbed (MgO)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> clusters

Equilibrium geometries, charge distributions, stabilities and electronic properties of the Ag-adsorbed (MgO) _n (n = 1–8) clusters have been investigated by density functional theory (DFT) with generalized gradient approximation (GGA) for exchange-correlation functional. The results show that hollow site is energetically preferred for n⩾4, and the incoming Ag atoms tend to cluster on the existing Ag cluster. The Mulliken populations indicate that the interaction between the Ag atom and Magnesia clusters is mainly induced by a weak atomic polarization. The adsorbed Ag atom only causes charge redistributions of the atoms near itself. The effect of the adsorbed Ag atom on the bonding natures and structural features of Magnesia clusters is minor. Furthermore, the investigations on the first energy difference, fragmentation energies and electron affinities show that the Ag(MgO)₄ and Ag(MgO)₆ are the most stable among studied clusters. Supported by the Startup Fund of High-level Personal for Shihezi University (Grant No. RCZX200747)     

ESR and Optical Absorption of Ag in a N2 Matrix

Ag atoms isolated in N₂ matrices were studied by simultaneous observation of optical absorption and electron spin resonance (ESR). In matrices with low Ag/N₂ ratios two ESR signals were observed, both of which could be correlated to the well known absorption triplet corresponding to the 5s ²S → 5p ²P resonance transition. One of the two ESR signals disappeared irreversably upon annealing up to 20 K. The stable species is attributed to Ag atoms in a nanocrystalline environment whereas the unstable species corresponds to atoms in the non-crystalline regions between the crystallites. In higher doped matrices a new Ag spectrum showed up, both in the optical absorption and in the ESR. It is assigned to Ag atoms with near-by complexes, possibly Ag clusters.     

Variations of Ag bonding distances in the AgBr1−xIx solid-solution with rock-salt type structure by X-ray and EXAFS analyses

The local structure of the AgBr_1−xI_x rock-salt type solid-solution (0≤×≤0.4) was investigated by both EXAFS and single crystal X-ray diffraction. The first nearest neighbor distances from Br to Ag, and those from Ag to Br in the solid-solution can be determined from EXAFS method. Because Ag⁺ surrounded by both Br⁻ and I⁻ ions is locally attracted to Br⁻, which has larger effective negative charge than I⁻, the Ag-Br distances gradually decrease with AgI content. There are observed systematic changes in both Br K- and Ag K-edge XANES spectra with increasing AgI content in connection with the change of the local structure in the solid-solution. The Br and I ions are distributed at random over the 4(a) sites of space group Fm3m without forming clusters. Both cation and anion are displaced from the lattice site because the Ag-Br and Ag-I distances in the solid-solution are close, respectively, to the distances in the pure rock-salt type AgBr and a hypothetical rock-salt type AgI. The particular increase in temperature factors by X-ray diffraction results from the static displacements of both cations and anions from the normal octahedral site. A large enhancement of ionic conductivity in AgI rich region will be ascribed to an increase in the concentration of the Frenkel defect caused by changes in local structures in an equilibrium state.     

Excited-state relaxation of Ag8 clusters embedded in helium droplets

Neutral silver clusters Ag(N) are grown in ultracold helium nanodroplets. By exploiting a strong absorption resonance recently found for Ag8, first photoelectron spectra of this neutral species are recorded. Variation of the laser photon energy reveals that direct vertical two-photon ionization is hindered by rapid relaxation into the lower edge of a long-living excited state manifold. The analysis of the dynamics gives a precise value of (6.89+/-0.09) eV for the vertical ionization potential of Ag8. The influence of the helium matrix on photoemission is discussed.