Obstacles for one-dimensional migration of interstitial clusters in iron

Interstitial clusters are known to cause frequent one-dimensional (1D) jumps (stepwise positional changes) under electron irradiation around room temperature. The distance of 1D jumps in iron was examined in detail through in-situ observation using high-voltage electron microscopy. The 1D jump distance was found to be longer for smaller clusters in specimens of higher purity, although the distance did not depend on the irradiation beam intensity and electron energy. The distribution of the 1D jump distance was well described by the distribution of the free path of interstitial clusters migrating through randomly distributed impurity atoms. The 1D jump process is considered as fast 1D diffusion of interstitial clusters at low activation energy from the point where the cluster detrapped from an impurity atom to the point where the cluster was trapped again by another impurity. Electron irradiation provides a trigger for causing 1D migration by detrapping of clusters from impurity atoms.     

Theoretical description of ultrafast phenomena in clusters

A theoretical study of different ultrafast nonequilibrium processes taking place during and after ultrashort excitation of clusters is presented. We discuss similarities and differences for several processes involving nonequilibrium ultrafast motion of atoms and electrons. We study ultrashort relaxation of clusters in response to excitations produced by femtosecond laser pulses of different intensities. We show how different relaxation processes, such as bond breaking, melting, fragmentation, emission of atoms, or Coulomb explosion, can be induced, depending on the laser intensity and laser pulse duration. We also discuss processes involving nonequilibrium electron dynamics, such as intraband Auger decay in clusters and ultrafast electronic motion during collisions between clusters and surfaces. We show that this electron dynamics leads to Stückelberg-like oscillations of measurable quantities, such as the electron emission yield. Received: 4 April 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Stability of two-component alkali clusters formed on helium nanodroplets

The stability of two-component clusters consisting of light (Na or K) and heavy (Rb or Cs) alkali atoms formed on helium nanodroplets is studied by femtosecond laser ionization in combination with mass spectrometry. Characteristic stability patterns reflecting electron shell-closures are observed in dependence of the total number of atoms contained in the mixed clusters. Faster decay of the stability of mixed clusters compared to the pure light ones as a function of size indicates a destabilizing effect of heavy alkali atoms on light alkali clusters, presumably due to second order spin-orbit interaction.     

Electronic excitation processes in rare gas clusters studied by electron energy loss spectroscopy

We present the electron energy loss spectra for Ar clusters as a function of incident electron energy and of cluster size. In spectra measured with 100 eV incident electron energy the bulk excitation peak becomes visible for a mean cluster size above 170 atoms per cluster. For 250 eV incident electron energy the bulk excitation peak is clearly observable even for a mean cluster size of 120 atoms per cluster. These experimental results are qualitatively reproduced by a simple calculation that accounts for the mean free path of electrons in Ar clusters; i.e., the penetration depth of incident electrons into the cluster.     

Magnetism of palladium clusters in nanoporous carbon

To determine the electronic properties of metal-carbon clusters, palladium clusters are introduced into nanoporous carbon samples. The structural and magnetic properties of the nanocomposite thus obtained are investigated. A ferromagnetic resonance (FMR) component is observed in the electron spin resonance (ESR) spectra measured in a wide temperature range. The electron density of states (DOS) of clusters consisting of a palladium atom surrounded by carbon atoms is calculated, and it is shown that, due to the interaction between electronic terms of carbon and metal, such a cluster should exhibit magnetism. The investigation of the structures of the porous skeleton of nanoporous carbon and of palladium clusters has allowed us to reveal how carbon and palladium atoms are brought into close contact so that to give rise to the observed ferromagnetism.     

Spontaneous alloying of tin atoms into nanometer-sized gold clusters and phase stability in the resultant alloy clusters

Alloying behavior and phase stability has been studied in situ by transmission electron microscopy using clusters in the Au-Sn system. When tin atoms are vapor-deposited onto nm-sized gold clusters, rapid dissolution of tin atoms into gold clusters takes place and as a result Au-rich solid solution, amorphous-like Au-Sn alloy and AuSn compound clusters are formed depending upon the concentration of tin. The remarkable enhancement of solubility has been observed in Au-rich solid solution and AuSn compound. It becomes more difficult to form two phases in the interior of individual clusters even if the composition of alloy clusters falls in the two-phase region in the phase diagram for the bulk alloy and as a result amorphous-like phase is stabilized in nm-sized Au-Sn alloy clusters. Received 2 August 1999 and Received in final form 8 November 1999     

Study of luminescence and optical resonances in Sb2O3 micro- and nanotriangles

Bimetallic clusters display new characteristics that could not be obtained by varying either the size of pure metallic systems or the composition of bulk bimetals alone. Coating of pre-deposited clusters by vapour deposition is a typical synthesis process of bimetallic clusters. Here, we have demonstrated that hierarchical, gold cluster-decorated copper clusters as well as both heterogeneous and homogeneous Cu?CAu bimetallic clusters (4.6 to 10.7?nm) can be prepared by coating pre-deposited, size-selected Cu₅₀₀₀ (4.6?±?0.2?nm) with Au evaporation at various temperatures. These bimetallic clusters were analyzed by aberration-corrected scanning transmission electron microscopy and associated electron energy loss spectroscopy. The results indicate that the growth of bimetallic clusters is controlled by a competition between nucleation and diffusion of the coating Au atoms.     

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.     

Structure and stability of a silicon cluster on sequential doping with carbon atoms

SiC is a highly stable material in bulk. On the other hand, alloys of silicon and carbon at nanoscale length are interesting from both technological as well fundamental view point and are being currently synthesized by various experimental groups (Truong et. al., 2015 [26]). In the present work, we identify a well-known silicon cluster viz., Si₁₀ and dope it sequentially with carbon atoms. The evolution of electronic structure (spin state and the structural properties) on doping, the charge redistribution and structural properties are analyzed. It is interesting to note that the ground state SiC clusters prefer to be in the lowest spin state. Further, it is seen that carbon atoms are the electron rich centres while silicon atoms are electron deficient in every SiC alloy cluster. The carbon–carbon bond lengths in alloy clusters are equivalent to those seen in fullerene molecules. Interestingly, the carbon atoms tend to aggregate together with silicon atoms surrounding them by donating the charge. As a consequence, very few Si–Si bonds are noted with increasing concentrations of C atoms in a SiC alloy. Physical and chemical stability of doped clusters is studied by carrying out finite temperature behaviour and adsorbing O₂ molecule on Si₉C and Si₈C₂ clusters, respectively.     

Observation of multiple scattering in (e, 2e) experiments on small argon clusters

A kinematically complete experiment for 100 eV electron-impact ionization of small argon clusters was realized. The triple coincidence detection of both outgoing electrons and the residual ion allows the discrimination between single ionization of atoms, dimers and non-mass-selected small clusters as well as between ionization and excitation within the same cluster. Comparison of fully and partly differential ionization cross sections for clusters with those of atoms reveal clear signatures of multiple-scattering reactions. For ionization with excitation, an almost isotropic electron emission pattern is observed.     

XPS study of bonding in ligated Au clusters

Ligated metal cluster compounds containing a core of metal atoms with well defined structure surrounded by a variety of organic and inorganic ligands are closely related to the bare metal clusters that are only now becoming available in uniform cluster size. The evolution of band structure and the development of metallic properties as a function of cluster size are of considerable interest. We report here a comparison of these two types of systems based on a study by X-ray photoelectron spectroscopy. The valence band spectra of ligated Au clusters compounds are similar in many respects to those of bare clusters, indicating significant participation of the d electrons in bonding. The core electron binding energy shifts of the central Au atom in Au₁₁(PPh₃)₇Cl₃ corresponds to the loss of approximation one 6 s electron. The total charge transferred to the halogens is accounted for by the shifts of the central and three halogenbonded Au atoms. No indication of metallic behaviour is found in the core of the ligated clusters.     

GaAs Electron Spectrum with Antisite AsGa Defect Clusters in the Semiempirical Tight-Binding Method

A computational procedure for calculating the band structure of a crystal with antisite-defect clusters is developed using a large unit-cell method. GaAs electron spectra containing clusters of more than 100 As atoms characterized by crystal point-group symmetry are calculated. The spread of energy levels in the band gap is examined. The origin of these energy levels is interpreted as being the result of splitting of energy levels of single defects.     

Transmission electron microscopy of thiol-capped Au clusters on C: Structure and electron irradiation effects

High-resolution transmission electron microscopy is used to study interactions between thiol-capped Au clusters and amorphous C support films. The morphologies of the clusters are found to depend both on their size and on the local structure of the underlying C. When the C is amorphous, larger Au clusters are crystalline, while smaller clusters are typically disordered. When the C is graphitic, the Au particles adopt either elongated shapes that maximize their contact with the edge of the C film or planar arrays when they contain few Au atoms. We demonstrate the influence of electron beam irradiation on the structure, shape and stability of the Au clusters, as well as on the formation of holes bounded by terraces of graphitic lamellae in the underlying C.     

Anions of xenon clusters bound by long-range electron correlations

In contrast with the single atom, atomic van der Waals clusters can form stable anions where the excess electron is bound due to long-range correlations with the electrons of the cluster. We report on extensive all-electron many-body ab initio studies on Xe clusters. Three-dimensional, planar, and linear structures of the clusters are investigated and compared. In particular, we find that the minimal number of Xe atoms in the cluster required to form a stable anion is 5 independently of the dimensionality of the cluster. We provide electron affinities for clusters made of 5, 6, and 7 atoms in all dimensions and find that the planar clusters form the most stable anions. The Dyson orbitals of the excess electrons are computed and analyzed.     

PtIrn0,±（n=1～5）团簇结构与稳定性的理论研究

采用密度泛函理论(DFT)中的b3lyp方法,在lanl2dz基组水平上对PtIrn0,±(n=1～5)团簇的各种可能构型进行几何参数全优化,得到它们的基态构型,并对其稳定性进行计算研究.结果表明:PtIrn0,±(n=1～5)团簇存在多种异构体,在原子数较少时均为二维平面结构,随着原子数的增加其基态结构变为三维结构;团簇的热力学稳定性随着原子个数的增加越来越好,PtIr3+团簇稳定性最好,与纯Ir团簇相比,PtIrn(n=1～5)团簇更易得到电子,非金属性增强;Ir原子对团簇的稳定性起到了主导作用.     

Electron-forbidden energy gap of hydrogen in a wide pressure interval

A simple model is used for estimating the bottom energy of the electron conduction band and the electron-forbidden gap energy. It is shown that electrons in liquid hydrogen are localized not in electron bubbles, as was considered previously, but in molecular negative ions surrounded by voids about 0.5 nm in radius. The conductivity of fluid hydrogen at not very high pressures is connected to transfer of positively charged clusters and negatively charged bubbles. As the pressure and density increase, molecular dissociation occurs and electron localization on atoms becomes more favorable, also with the creation of a void around atomic negative ions. At a sufficiently high concentration of atoms, the probability of tunnel transition of an electron from one atom to another becomes close to unity, the energy level of the negative ion degenerates in the band, and the conductivity is caused by the transfer of these quasifree electrons. It is supposed that this charge transfer mechanism may play an important role in the region of fluid hydrogen metallization.     

Electronic properties of transition-metal-atom doped Si cage clusters

We present a density-functional study of electronic structures of convex-caged Si clusters doped with transition-metal (TM) atoms. First, we show the reason for their peculiar geometries in terms of interplay among the electron orbitals of Si and TM atoms. Then we describe the potential ability of the clusters to serve as charge sources to other objects such as Si crystal surfaces. Millennium Research for Advanced Information Technology (MIRAI) Project.     

Fluorinated ferromagnetic metal clusters and their superhalogen behaviour

Density functional theory calculations on the ground-state geometries and spin multiplicities of neutral and anionic ferromagnetic metal fluoride clusters, MF_n (M = Fe, Co and Ni; n = 1–7), have been performed. The results show that in the case of FeF_n and CoF_n clusters, a maximum of five F atoms can be bound atomically to metal atoms while four in the case of NiF_n. The remaining F atoms bind either very weakly or molecularly. The stabilities of all MF_n clusters are discussed by calculating dissociation energies to F atoms and F₂ molecules. We notice that the anionic species are relatively more stable than corresponding neutrals. The electron affinities of these clusters are very large, reaching values as high as 7.98 eV. Therefore, these clusters can be regarded as superhalogens.     

Investigation of the charge distribution in small cluster ions Ar<Stack><Subscript>13</Subscript><Superscript>+</Superscript></Stack> and Ar<Stack><Subscript>19</Subscript><Superscript>+</Superscript></Stack>

The results of ab initio studies of the atomic and charge structure of small clusters and cluster ions formed by 13 and 19 argon atoms are reported. It was found that the icosahedral atomic structure is energetically the most favorable for such clusters. The calculations demonstrate that when a single electron is removed from a cluster, the excess positive charge is distributed primarily over the surface of the formed cluster ion.     

Magic rule for Al(n)H(m) magic clusters

Using the electronic shell closure criteria, we propose a new electron counting rule that enables us to predict the size, composition, and structure of many hitherto unknown magic clusters consisting of hydrogen and aluminum atoms. This rule, whose validity is established through a synergy between first-principles calculations and anion-photoelectron spectroscopy experiments, provides a powerful basis for searching magic clusters consisting of hydrogen and simple metal atoms.