Influence of modifying additives on formation of supported copper nanoparticles

The influence of modifying additives of Ce, Zr, La and Cs oxides on changes of electronic state of supported copper during the catalytic reaction of butane complete oxidation has been studied by the methods of IR-spectroscopy of adsorbed CO, XPS, EXAFS and XRD. The modifying additions of cerium and zirconium oxides stabilize the ionic state of copper, while lanthanum and cesium oxides decrease the effective charge of copper ions. The observed effects are caused by variation in metal dispersivity in the modified samples and by electron donor-acceptor interaction of the surface atoms and ions of copper with the modifier.     

Doubly charged clusters - neutral atom charge transfer: the role of the Coulombic repulsion

We have studied experimentally the collisional charge transfer between a neutral atom and a multicharged metal-atom cluster. The charge transfer cross section measured for Na ₃₁ ^{+ +} + Cs is in the range of 400 ?². The time-of-flight mass analysis of the singly charged collision products demonstrates that an energy of about 0.5 eV is deposited in the cluster fragment during the charge transfer collision. This effect can be interpreted as a charge transfer to an excited state of the metal cluster. The measured cross section for Na ₃₁ ^{+ +} + Cs is larger than the one for Na ₃₁ ⁺ + Cs collisions. This difference between these two systems is due to the existence, for the first one, of a Coulombic repulsion term in the collision output channel. Received 24 October 2000     

Hindered Coulomb explosion of embedded Na clusters — stopping,shape dynamics and energy transport

We investigate the dynamical evolution of a Na₈ cluster embedded in Ar matrices of various sizes from N=30 to 1048. The system is excited by an intense short laser pulse leading to high ionization stages.We analyze the subsequent highly non-linear motion of cluster and Ar environment in terms of trajectories, shapes, and energy flow. The most prominent effects are: temporary stabilization of high charge states for several ps, sudden stopping of the Coulomb explosion of the embedded Na₈ clusters associated with an extremely fast energy transfer to the Ar matrix, fast distribution of energy throughout the Ar layers by a sound wave. Other ionic-atomic transfer and relaxation processes proceed at slower scale of few ps. The electron cloud is almost thermally decoupled from ions and thermalizes far beyond the ps scale.     

Matrix embedded cobalt-carbon nano-cluster magnets: behavior as room temperature single domain magnets

A new type of Co-C nanoparticles is synthesized from CH₂Cl₂ solution of Co₄(CO)_{1 2} by heating up to 210 ^°C in a closed vessel. Transmission electron microscope (TEM) and electron energy loss spectroscopy (EELS) observation show that the particles are embedded in amorphous carbon and their average size is 12 nm. The radial structure function obtained from the extended X-ray absorption fine structure (EXAFS) of the Co K-edge absorption of the Co-C nanoparticles provides a Co-C average distance of 2.08 Å and the Co-Co distances of 3.18 Å and 3.9 (±0.2) Å. The particles exhibit the magnetic hysteresis curve with a coercive force of 200 Oe at 20 K and 260 Oe at 300 K. The temperature dependence of the magnetic susceptibility measured under zero-field cooling and 10 Oe field cooling conditions exhibits the behavior characteristic of a set of single magnetic domain nanomagnets in an amorphous carbon matrix.     

A density-functional study of aluminium,iron, zirconium and cerium microclusters

The geometries and electronic structural properties of AB and ABC (A, B, C = Al, Fe, Zr, Ce) microclusters have been systematically investigated by using a hybrid density-functional method (B3LYP) approach. The spectroscopic constants of ground-state AB and ABC (A, B, C = Al, Fe, Zr, Ce) are obtained, and are found to be in agreement with other available experimental and theoretical results. The calculated gaps between highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO-LUMO) are clearly changed when X is doped into the AB dimers (X = Al, Fe, Zr, Ce). The calculated results indicate that a triangular form with D_3h, C_2v or C_s symmetry is the most stable for the corresponding ABC trimers, and, in addition, the possible isomers (linear structure) with D_∞h or C_∞v symmetry of three-atom clusters were found to be of higher energies. We conclude that AlFe and Al₂Fe have the highest chemical stability of all the AB dimers and ABC trimers, respectively, due to the high HOMO-LUMO gap. We also find that the binding energy of Ce₃ is the largest in magnitude among all ABC (A, B, C = Al, Fe, Zr, Ce) trimers, as is the case with Ce₂ among all AB (A, B, C = Al, Fe, Zr, Ce) dimers. The most stable geometry, charge transfer and possible dissociation channels are also discussed.     

Electronic structure and magnetism in 4d-transition metal clusters

We analyze the stability of magnetic states obtained within the tight-binding model for cubooctahedral (O_h) and icosahedral (I_h) clusters of early 4d (Y, Zr, Nb, Mo, and Tc) transition metals. Several metastable magnetic clusters are identified which suggests the existence of multiple magnetic solutions in realistic systems. A bulk-like parabolic behavior is observed for the binding energy of O_h and I_h clusters as a function of the atomic number along the 4 d-series. The charge transfer on the central atom changes sign, while the average magnetic moments present an oscillatory behavior as a function of the number of d electrons in the cluster. Our results are in agreement with other theoretical calculations. Received: 20 November 1997 / Received in final form: 9 March 1998 / Accepted: 30 March 1998     

Trianionic gold clusters

Using Penning-trap experiments and a shell-correction method incorporating ellipsoidal shape deformations, we investigate the formation and stability patterns of trianionic gold clusters. Theory and experiment are in remarkable agreement concerning appearance sizes and electronic shell effects. In contrast to multiply cationic clusters, decay of the trianionic gold clusters occurs primarily via electron autodetachment and tunneling through a Coulomb barrier, rather than via fission. Received 9 January 2001     

Chirality, defects, and disorder in gold clusters

Theoretical and experimental information on the shape and morphology of bare and passivated gold clusters is fundamental to predict and understand their electronic, optical, and other physical and chemical properties. An effective theoretical approach to determine the lowest-energy configuration (global minimum) and the structures of low energy isomers (local minima) of clusters is to combine genetic algorithms and many-body potentials (to perform global structural optimizations), and first-principles density functional theory (to confirm the stability and energy ordering of the local minima). The main trend emerging from structural optimizations of bare Au clusters in the size range of 12-212 atoms indicates that many topologically interesting low-symmetry, disordered structures exist with energy near or below the lowest-energy ordered isomer. For example, chiral structures have been obtained as the lowest-energy isomers of bare Au₂₈ and Au₅₅ clusters, whereas in the size-range of 75-212 atoms, defective Marks decahedral structures are nearly degenerate in energy with the ordered symmetrical isomers. For methylthiol-passivated gold nanoclusters [Au₂₈(SCH₃)₁₆ and Au₃₈(SCH₃)₂₄], density functional structural relaxations have shown that the ligands are not only playing the role of passivating molecules, but their effect is strong enough to distort the metal cluster structure. In this work, a theoretical approach to characterize and quantify chirality in clusters, based on the Hausdorff chirality measure, is described. After calculating the index of chirality in bare and passivated gold clusters, it is found that the thiol monolayer induces or increases the degree of chirality of the metallic core. We also report simulated high-resolution transmission electron microscopy (HRTEM) images which show that defects in decahedral gold nanoclusters, with size between 1-2 nm, can be detected using currently available experimental HRTEM techniques.     

Enhanced multiferroic properties in La and Ce co-doped BiFeO3 nanoparticles

Room temperature multiferroic properties of BiFeO₃ (BFO), Bi_0.9La_0.1FeO₃ ((La)BFO) and Bi_0.9La_0.075Ce_0.025FeO₃ ((La,Ce)BFO) nanoparticles have been reported in this paper. XRD (X-ray diffraction) analyses of the nanoparticles show a decrease in the lattice constants and cell volume with the substitution of La and Ce. It is evident from the SEM (scanning electron microscope) micrographs that the (La,Ce) co-doped sample possesses dense microstructure made of smaller particles. Raman study accounts for the weakening of the strong hybridization between Bi-O by the substitution of La and Ce ions. This is also accompanied by an increase in the remanent magnetization, dielectric constant, and ferroelectric polarization. BFO nanoparticles show exchange bias effect under an applied magnetic field while the (La)BFO and (La,Ce)BFO samples show no trace of such effect. Ac-conductivity of (La,Ce) co-doped sample is observed to be several orders lesser in magnitude than bulk BFO ceramics. These results are interpreted by means of the subtle change in the structure, suppression of the spin cycloid and reduction of oxygen vacancies in the doped samples.     

Direct observation of the non-supported metal nanoparticle electron density of states by X-ray photoelectron spectroscopy

Synchrotron-based X-ray photoelectron spectroscopy on copper and silver cluster beams created by a magnetron-based gas-aggregation source has allowed mapping the electron density of states (DOS) of free metallic nanoparticles. The cluster DOS profiles obtained in the experiments strongly resemble the infinite solid DOS shapes, but the extracted cluster work-functions are lower than those for the bulk metal. The latter observation is explained by the initial negative charge on most of the clusters, created by the source.     

Structural and electronic properties of Bin (n = 2-14) clusters from density-functional calculations

The structural and electronic properties of Bi_n (n = 2-14) clusters have been systematically studied using gradient-corrected density-functional theory. For each cluster size, a number of structural isomers were constructed and optimized to search for the lowest-energy structure. The competition of several structural patterns such as cages, superclusters, and layered structures leads to the alternating appearance of these configurations as global minima. Although the tendency of Bi to form puckered-layer structures is already well-known, the electronic states of Bi_n clusters are still far from that of the bulk. As well, a remarkable even-odd atom number oscillation is observed in the structural and electronic properties of the clusters, implying that the stability of Bi_n clusters is mainly dominated by the electron shell effect rather than by geometrical packing. The theoretically calculated values for electron affinities agree well with available experimental data.     

Ionization of metal clusters by ions in the Fermi velocity range

We simulate excitation of metal clusters by highly charged, energetic ions, analyzing electron emission in terms of discrete ionization probabilities. Our test case is the collision of on the cluster at velocities around the electronic Fermi velocity of bulk sodium. The calculations are performed with a density-functional approach, using the time-dependent local density approximation. We find that ionization takes place on an extremely short time scale of less than 5 fs. The preferred final charge state depends sensitively on the impact parameter. High ionization can easily be achieved in sufficiently close collisions. Direct trapping through the by-passing ion is found to be of little importance at the velocities considered. Received: 28 July 1997 / Received in final form: 23 December 1997 / Accepted: 8 January 1998     

EICO measurements of inner-core excited mixed rare-gas clusters

The spectra of deep inner-core excited mixed rare-gas clusters were recorded by using electron ion coincidence (EICO) and multi-hit momentum imaging (MHMI) techniques. The EICO spectra for Ar₉₉Kr₁ clusters reveal that singly charged ions are emitted from the inner-core excited clusters in addition to the multiple charged ions. The dependence of the EICO spectra on photon energy and cluster size suggests that the holes created through vacancy cascade on the krypton atoms are transferred to the surrounding atoms, and that the singly charged ions are the primary product of the krypton photoabsorption. Charge localization is suggested for the inner-core excited mixed rare-gas clusters from the analysis of the EICO peak width. The MHMI measurements give us direct evidence for the strong charge migration from X-ray absorbing atoms to surrounding atoms. The photon energy dependence of the PSD image for fragment ions suggests that the momentum of the fragment ions depends on the number of charges generated by the vacancy cascade.     

Dissociation channels of silver bromide cluster Ag<Subscript>2</Subscript>Br,silver cluster Ag<Subscript>3</Subscript> and their ions studied by using alkali metal target

Various dissociation channels of silver bromide cluster ion Ag₂Br⁺ and silver cluster ion Ag₃ ⁺ were observed in high-energy collisionally-activated dissociation (CAD) using a Cs target. The fragment patterns of the high-energy CAD were compared with those of the metastable dissociation and low-energy CAD. The difference in the fragment patterns between the high-energy CAD and the other dissociation methods was explained in terms of the internal energy distributions. The dissociation mechanisms of neutral silver bromide cluster Ag₂Br and silver cluster Ag₃ were also investigated by charge inversion mass spectrometry using the Cs target. While the fragment ions AgBr^- and Ag₂ ^- were dominantly observed in the charge inversion spectrum of Ag₂Br⁺, the undissociated ion Ag₃ ^- was observed as a predominant peak in the case of Ag₃ ⁺. The dissociation behavior of Ag₂Br^* can be explained on the basis of the calculated thermochemical data. Contrary to this, the predominant existence of the undissociated Ag₃ ^- cannot be explained by the reported thermochemical data. The existence of undissociated Ag₃ ^- suggests that the dissociation barrier is higher than the internal energy of Ag₃ ^* (theoretical: 1.03 eV, experimental: 2.31 eV) estimated from the ionization potentials of Ag₃ and Cs.     

Photoionization of argon, krypton and xenon clusters in the inner valence shell region

Photoionization of rare gas clusters in the innervalence shell region has been investigated using threshold photoelectron and photoion spectrometers and synchrotron radiation. Two classes of states are found to play an important role: (A) valence states, correlated to dissociation limits involving an ion with a hole in its innervalence ns shell, (B) Rydberg states correlated to dissociation limits involving an ion with a hole in its outervalence np shell plus an excited neutral atom. In dimers, class A states are “bright”, that is, accessible by photoionization, and serve as an entrance step to form the class B “dark” states; this character fades as the size of the cluster increases. In the dimer, the “Mulliken” valence state is found to present a shallow potential well housing a few vibrational levels; it is predissociated by the class B Rydberg states. During the predissociation a remarkable energy transfer process is observed from the excited ion that loses its innershell electron to its neutral partner. Received: 10 February 1998 / Revised: 17 July 1998 / Accepted: 31 July 1998     

Shell structure of cesium layer covering the C fullerene core

Strutinsky shell corrections for the cesium-coated fullerenes were investigated. The single particle levels of electrons are obtained using the spherical mean-field potential of a shifted Wood-Saxon type. The parameters of the potential are adjusted to reproduce the experimental ionization energies of the Cs(N) clusters and the magic numbers observed in their photo-ionization spectra of the C₆₀Cs(N) aggregates. Received 24 May 2002 / Received in final form 9 July 2002 Published online 15 October 2002 RID="*" ID="*"This work has been partly supported by the Polish Committee for Scientific Research under Contract No. 2P03B 115 19 and by the Program of Scientific Exchange between the IN2P3-France and the Polish Research Institution No. 99-95. RID="b" ID="b"e-mail: Krzysztof.Pomorski@umcs.lublin.pl     

C 1s core photoemission of C60 and C48N12

We make a theoretical study of the shake-up of the 1s photoemission of C₆₀. The method takes into account the N-body reactions of the π and σ electrons which appear during the formation of the photoemission hole on one carbon atom. We analyze the origin of the satellite in the spectra due to transitions between N-body states. Our calculation shows that the satellite spectra is essentially given by N-body transitions which involve the creation of one or two electron-hole pairs. The method has been applied also to C₄₈N₁₂. The situation is more complex. The spectra of the two most stable species have been investigated. Moreover the influence on the spectra of the position of the hole created on the carbon atom in C₄₈N₁₂ has been examined (all the carbon positions are not equivalent for some isomers).     

Interpretive approach to collective effects of electrons in multicenter problems

Interpretive theoretical tools prove valuable in guiding the analysis of experiments in the realm of atomic clusters. Here, we review basic elements of an analytic approach that makes it possible to find and visualize the effective electrostatic potential and Coulomb correlations in multicenter problems. To illustrate the utility of these concepts we apply them to exploring molecular-doped metallic clusters. This study is aiming at a systematic, visual assessment of changes induced in screening, Coulomb correlation and effective potential by varying the charge of the electronegative impurity and its position in the cluster cage.