Structure of unsupported bismuth nanoparticles

We present new results of electron diffraction experiments on unsupported nanometer-sized bismuth clusters. The high intensity cluster beam, necessary for electron diffraction, is provided by an inert-gas aggregation source. The cluster beam contains particles with average cluster sizes between 4.5 and 10 nm. When using Helium as a carrier gas we are able to observe a transition from crystalline clusters to a new structure, which we identify with that of amorphous or liquid clusters. Received 28 November 2000     

Fragmentation and reliable size distributions of large ammonia and water clusters

The interaction of large ammonia and water clusters in the size range from <n> = 10 to 3 400 with electrons is investigated in a reflectron time-of-flight mass spectrometer. The clusters are generated in adiabatic expansions through conical nozzles and are nearly fragmentation free detected by single photon ionization after they have been doped by one sodium atom. For ammonia also the (1+1) resonance enhanced two photon ionization through the state with v = 6 operates similarly. In this way reliable size distributions of the neutral clusters are obtained which are analyzed in terms of a modified scaling law of the Hagena type [Surf. Sci. 106, 101 (1981)]. In contrast, using electron impact ionization, the clusters are strongly fragmented when varying the electron energy between 150 and 1 500 eV. The number of evaporated molecules depends on the cluster size and the energy dependence follows that of the stopping power of the solid material. Therefore we attribute the operating mechanism to that which is also responsible for the electronic sputtering of solid matter. The yields, however, are orders of magnitude larger for clusters than for the solid. This result is a consequence of the finite dimensions of the clusters which cannot accommodate the released energy. Received 21 November 2001     

High energy collisions of protonated water clusters

We have measured attenuation cross sections and fragmentation cross sections for protonated water clusters H(H₂O)_n ⁺ (n = 1 to 100) colliding with noble gas atoms (He and Xe) at a laboratory energy of 50 keV. In collisions with He, a transparency effect in the attenuation cross section was observed. For the case of fragmentation in collisions with Xe, a strong enhancement of small clusters was observed which we attribute to multifragmentation. Received 30 November 2000     

Magnetism in Rh clusters under hydrostatic deformations

The magnetic behavior of rhodium clusters Rh_N (N = 4-38) under hydrostatic deformations was investigated. The starting cluster structures were obtained from an evolutionary search algorithm applied to a Gupta potential. The spin-polarized electronic structure and related magnetic properties were calculated using a self-consistent spd tight-binding Hamiltonian within the unrestricted Hartree-Fock approximation. The magnetic behavior was analyzed in terms of the interdependence between the geometrical parameters and the electronic structure. Anomalous magnetic effects were found in some cases. Received 5 August 2002 / Received in final form 10 January 2003 Published online 4 March 2003 RID="a" ID="a"e-mail: berlanga@dec1.ifisica.uaslp.mx     

Stacking faults in close-packed clusters

Ground state geometries of small hard sphere clusters were studied using two different type of contact interaction, a pair-potential and a many-atom interaction. Monte Carlo method in an FCC lattice with all possible (111) stacking faults was used to obtain the minimum energy geometries for clusters up to 59 atoms. Due to the surface energy, FCC packing is generally favoured as opposite to the HCP structure. However, in most cluster sizes the ground state obtained with the many-atom interaction has one or more stacking faults. The most symmetric geometry is usually not the ground state. Clusters with 59 and 100 atoms were studied due the possibility of a high symmetry cluster with stacking faults in all four directions. The size dependence of the total energy has similarities with that of the average moment of inertia. Received 6 February 2002 / Received in final form 11 April 2002 Published online 19 July 2002     

Theoretical study of small silicon clusters on a graphite layer

In a series of recent experiments, the HOMO-LUMO energy gaps of small Si clusters deposited on a graphite substrate have been determined by Scanning Tunneling Microscopy (STM). The values obtained were found to be substantially smaller than the energy gaps of corresponding passivated clusters. This work considers dimensional reduction as a possible mechanism for a sizeable energy gap narrowing by the example of the system Si₅. The impact of the graphite substrate on the deposited species is investigated in the framework of a pseudocluster model. Received 30 November 2000     

Structures and energetics of carbon bridged C 60 clusters

The structures and energetics of carbon bridged C₆₀ clusters (C ₆₀ ) _n C_m have been studied by simulated annealing technique within the tight-binding molecular-dynamics. The “sp² addition” ball-and-chain dimers exhibit odd-even alternations over the number of chain atoms, with the dimers containing even chain atoms more stable against dissociation than their immediate neighbors containing odd chain atoms. In addition to the usual “sp² addition” dimers, a pentagon-linked C₁₂₁ isomer and a hexagon-linked C₁₂₂ isomer are also found to be stable. Based on our tight-binding calculations, trimers and larger clusters can be simply regarded as being made up of independent or weakly interacting dimers, if the C-C₆₀ joints on a single cage are not too close to each other. Large C₆₀ clusters connected by chains each containing only one or two carbon atoms have similar stability to that of constituent dimers, indicating the possibility to form stable C₆₀-carbon polymers. Received 17 January 2001 and Received in final form 26 February 2001     

Photoemission spectra of deuterated silicon clusters: experiment and theory

We compare experimentally measured and ab initio computed photoelectron spectra of negatively charged deuterated silicon clusters ( , 4m10, 0n2) produced in a plasma environment. Based on this comparison, we discuss the kinetics and thermodynamics of the cluster formation and the effect of deuterium on the geometrical and electronic structure of the clusters.     

Formation and stability of large B 6 O clusters with icosahedral structure

In this paper, the formation and the stability of large B₆O icosahedral particles was discussed on the basis of elastic deformation theory. Our calculation illustrate the stability of macroscopic Mackay packing B₆O icosahedral particles at high pressure. The transition pressure from rhombohedral structure of B₆O particles to macroscopic B₆O icosahedral ones was calculated to be 6 GPa, which is in good agreement with the experimental data (4.0-5.52 GPa). The maximum diameter of B₆O icosahedral particles at low pressure is estimated to be 200-300 nm. Received 30 November 2000     

Assembling of hydrogenated aluminum clusters

The electronic and atomic structure of Al₁₃H has been studied using Density Functional Theory. Al₁₃H has closed electronic shells. This makes the cluster very stable and suggests that it could be a candidate to form cluster assembled solids. The interaction between two Al₁₃H clusters was analyzed and we found that the two units preserve their identities in the dimer. A cubic-like solid phase assembled from Al₁₃H units was then modeled. In that solid the clusters retain much of their identity. Molecular dynamics runs show that the structure of the assembled solid is stable at least up to 150 K. A favorable relative orientation of the clusters with respect to their neighbors is critical for the stability of that solid. Received 21 November 2000     

Study of the symmetry of single-wall nanotubes by electron diffraction

Determining precisely the atomic structure of single-wall carbon nanotubes is essential since it tailors electronic properties of this new carbon material. Here, we present a quantitative electron diffraction study of electric-arc produced single-wall carbon nanotube bundles, combined with simulations based on the kinematic theory and with real-space images. We stress the importance of the twist of the bundle in the interpretation of our data and we analyze both packing lattice parameters and chirality distribution. We show that, within a given bundle, no chirality is favoured whereas SWNT diameters are almost uniform. Received 5 February 1999     

Molecular dynamics simulations of the freezing of gold nanoparticles

A set of molten gold clusters, each with 1157 gold atoms, was studied by molecular dynamics simulations as the clusters underwent freezing at three different temperatures. Most of the clusters attained an icosahedral structure upon freezing, a structure found to be stable to mild annealing. Other structures observed were imperfect truncated decahedral, truncated octahedral and hexagonal close packed structures. The role of kinetics in the process of cluster solidification is discussed. Received 6 November 2000     

Control of island morphology by dynamic coalescence of soft-landed clusters

The deposition of preformed clusters on surfaces has been established as a new way for growing nano-suctures on surfaces. It has been shown that supported island morphology relies on the dynamics of clusters, during the growth, giving rise to shapes from compact to ramified types. This paper identifies and discusses, in the case of antimony cluster deposits, several processes responsible for the non-equilibrium island shapes: limited kinetic cluster aggregation, size dependent coalescence, “wetting-like behavior” of antimony clusters on antimony islands. Using successive predetermined cluster sizes during the deposition process to synthesize polymorphic structure involves the interplay of those mechanisms. Received 1st December 2000     

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.     

Vibrational spectroscopy of large water clusters of known size

The structure of large water clusters of known size distributions 〈n〉 = 20-2000 is investigated by vibrational spectroscopy of the OH stretch mode. The water clusters are predissociated by a pulsed tunable infrared optical parametrical oscillator (OPO) in the frequency range 2800-3800 cm^-1. Their fragments are detected off-axis by electron impact ionization and mass analyzed by a quadrupole mass spectrometer. The largest ion signal stems from the neutral water hexamer fragment. The ion yield is investigated at certain wavelengths while the size of the clusters is varied, and for certain sizes complete absorption spectra of the OH stretch motion are measured. Fingerprints of the different coordination types of the water molecules in the clusters are found and it turns out that our method is especially sensitive to amorphous structures with frequencies shifting in the range of 3300-3400 cm^-1.     

A computer simulation study of the ground-state configurations of Fe and Fe-Al clusters

Using the noncentral embedded atom model potential recently proposed by Besson and Morillo for bulk alloys (), we performed computer simulations to predict the ground-state configurations of and clusters (). The computed structures of clusters are in general agreement with such theoretical results as have been obtained by density functional calculations (i.e. for ). The results for Fe-rich clusters show surface segregation of Al, which is in keeping with the findings of a previous study of clusters. Received 29 April 1999     

Study of (Al2O3)n(Ox) clusters with n = 16 and x = 0, 1, 2 from first principles calculations

The ionic and electronic structure of (Al₂O₃)_n(O_x) clusters with n 16 and x = 0, 1, 2 is studied by means of first principles density functional calculations, norm-conserving pseudopotentials and a numerical atomic basis set. The equilibrium geometries have been determined by total energy minimization, starting with several initial geometries for each cluster size. The trends obtained for the atomic arrangements (structural isomers, coordination numbers, disordered versus ordered structures, etc.) and the electronic properties (binding energies, Homo-Lumo gap and dipole moments) are discussed. For most of the oxidized clusters studied here we find that the Homo-Lumo gap and the magnitude of dipole moment of isomeric species can vary drastically.     

Modelling gold clusters with an empirical many-body potential

Molecular Dynamics Simulated Annealing has been used to probe the structure of small Au clusters consisting of between 2 and 40 atoms. The interatomic interactions within these clusters are described using an empirical Murrell-Mottram many-body potential energy function. Four distinct structural motifs are present in the structures of the predicted global minima, based on octahedra, decahedra, icosahedra and hexagonal prisms. Received 30 September 1999 and Received in final form 23 March 2000     

Novel synthesis, structure and catalysis of inverted core/shell structured Pd/Pt bimetallic nanoclusters

The catalytic properties of Pd-core/Pt-shell (inverted core/shell) structured bimetallic nanoclusters, synthesized by a successive addition method using sacrificial hydrogen, were investigated for hydrogenation of methyl acrylate. Partial deposition of Pt atoms on the surface of Pd nanoclusters can enhance the catalytic activity of the Pd atoms remaining in the surface of the inverted core/shell structured Pd/Pt bimetallic nanoclusters. Received 29 November 2000