Plasma desorption mass spectroscopy of thiol-passivated gold nanoparticles

Plasma desorption mass spectrometry has been applied to characterization of dodecanthiol-passivated gold nanoparticles. An overview of the experimental set-up and mass analyses for the nanoparticles prepared in different conditions are shown. Mass distributions were found to shift to higher mass regions with increasing reaction temperature and reaction period. The results are consistent with those of transmission electron microscopy observations, UV-visible absorption spectra and also with a reported laser desorption mass spectrometry.     

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.     

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.     

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.     

Hyperthermal surface-collisions of water cluster cations

Size-selected, protonated water cluster cations (H₂O)_nH⁺, 4 n 32, are scattered at normal incidence from the surface of a diamond-coated silicon wafer at collision energies 0 E _coll 500 eV. The size distribution of collision-induced fragment-ions and the ion yield of scattered particles are analyzed, using a secondary time-of-flight mass spectrometer, as a function of the cluster size, n, and the collision energy, E _coll. Even at low impact energies only very small fragment-ions can be detected, with a maximum fragment size of 35% of the colliding parent cluster ions. For clusters consisting of more than 10 molecules, the protonated water dimer (H₂O)₂H⁺ becomes the predominant fragment-ion. The total charge survival yield obeys a nonlinear increase with cluster size; for the largest clusters investigated, more than 35% of the impacting ions survive the surface collision in the cationic charge state.     

Size dependence of the plasmon peak position in electron stimulated photon emission spectra of Ag clusters supported on amorphous carbon film

The plasmon energy of Ag clusters produced on an amorphous carbon substrate by gas-aggregation technique has been measured. It has been determined from the plasmon peak position in the light emission spectrum obtained during bombardment of Ag clusters by low-energy electrons. For Ag cluster films with maximum of the cluster size distribution at 30, 8 and 2.5 nm, the plasmon energy comprised 3.76, 4.13 and 4.28 eV (the wavelength was 330, 300 and 290 nm), respectively. The blue shift of the plasmon energy is probably related to the effect of confounding of collective and single-particle excitations.     

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     

Silicon-riched-oxide cluster assembled nanostructures formed by low energy cluster beam deposition

Free beam of silicon oxide nanoclusters is produced by a gas aggregation source from SiO precursor. Due to the disproportionation reaction during the condensation of SiO vapor the generated clusters are Si-riched. The clusters are collimated to be a fine beam and deposited on the substrate at room temperature. The microstructures of the cluster-based nanofilm are characterized by TEM. It is shown that with appropriate impacting parameters, Si-riched oxide nanofilms assembled from uniformly distributed isolated clusters can be obtained. And the clusters can self-organize into partially densely ordered packing within local domains. XPS spectra are taken to analyze the chemical components of the nanofilms. Photoluminescence from the Si-riched oxide nanofilms has also been observed.     

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.     

Time evolution of Ag-Cu and Ag-Pd core-shell nanoclusters

The possibility of obtaining core-shell nanoparticles by depositing adatoms of a different element B above a cluster of an element A (system B/A) is studied by Molecular Dynamics simulations. We consider the four cases Ag/Cu, Cu/Ag, Ag/Pd and Pd/Ag, which present very different behaviours, and investigate whether it is possible to build up of well-defined core-shell structures.     

Controlled synthesis of CdS nanowires using diamines

We demonstrate a new, simple, inexpensive process and systemic control over CdS nanowires using sulfur powders and organic diamines without any catalysts, surfactants, and templates under atmospheric benchtop conditions. By changing the kinds of amines and reaction temperatures can result in control of the shape and size of the nanocrystals, which are moderately monodispered with unique forms.     

Non-crystalline structures in the growth of silver nanoclusters

The growth of nanometer-size free Ag clusters is studied by Molecular Dynamics simulations. The morphology transition from the icosahedron at the magic size of N = 55 atoms to the Marks truncated decahedron at N = 75 is analyzed in details, in order to single out kinetic trapping and entropic effects. At very low T, the cluster is kinetically trapped in an icosahedral structure. At intermediate T the transition takes place sharply at N≃ 65. At higher T, the transition is smeared out and finally, around 550 K no transition is found because the 75 decahedron is melted. Received 28 November 2000     

Surface tensions and stress tensors of liquid and solid clusters by molecular dynamics

Surface tension and pressure (stress) tensors of Lennard-Jones clusters, in the size range 200 ~ 2700 atoms/cluster, formed from evaporating liquid droplets were calculated in a Molecular Dynamics simulation. Icosahedral clusters have a much larger surface tension than decahedral, fcc, and hcp ones, meanwhile asymmetric icosahedral clusters have a lower surface tension. Fcc and hcp clusters have a very small surface tension. Decahedral clusters have a surface tension closer to that of fcc and hcp ones than to that of icosahedral ones, though both icosahedral and decahedral structures have five fold symmetry axis. Binary component clusters have a higher surface tension than single component ones.     

The structure of 55-atom Cu–Au bimetallic clusters: Monte Carlo study

We have investigated segregation phenomena in Cu–Au bimetallic clusters with decahedral structures at 100 K and 300 K, based on the second-moment approximation of the tight-binding (TB-SMA) potentials by using Monte Carlo method. The simulation results indicate that there are three regions (split, three-shell onion-like and core-shell region) at 100 K and two regions (split and core-shell) at 300 K with the structure of decahedral clusters, as the chemical potential difference Δμ changes. It is found that the structure of decahedral clusters undergoes a division into smaller clusters in the split region. In the core-shell structure, Au atoms are enriched in surface and Cu atoms occupy the core of the clusters because of the different surface energy of Cu and Au. The Au atoms are enriched in the surface shell, and the Cu atoms are in the middle shell, while a single Au atom is located in the center to form the three-shell onion-like structure. The structure and binding energy of smaller clusters after splitting are also discussed. The Au atoms generally lie on the surface of the smaller clusters after splitting.     

Unsupported lead clusters and electron diffraction

A beam of Pb clusters is produced with the inert gas aggregation method and probed by electron diffraction. Analysis of the diffraction patterns indicates that average cluster size can vary between 3 and 7 nm, according to nucleation conditions. The diffraction patterns from beams with larger average cluster size are very similar to patterns calculated from model decahedron clusters, while those for smaller cluster size do not appear to have simple geometrical face-centred cubic, decahedral, or icosahedral structure. Received 30 November 2000     

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     

Surface coverage studies of the icosahedron by Li using density based molecular dynamics

Density based molecular dynamics has been used to investigate the ground state structures of heterogeneous binary clusters Al₁₃Li_n, n = 1, 2, 3, 4, 10, 19, 20, 21. Some of these structures have also been investigated by full Kohn-Sham based calculations. Our earlier investigations have shown that in the Al-Li cluster, the ground state configurations are the ones where the Al atoms form a core around which the Li atoms form a “cage”. In the present work, we have chosen the well-known Al₁₃ icosahedron as the surface over which we study the evolution of the surface coverage as the number of Li atoms increases. On the basis of the earlier work, we expect that the Al₁₃Li₂₀ cluster would be the most stable and indeed our simulations do yield such a novel fivefold symmetric stable structure formed out of purely metal atoms. This icosahedral substrate is also used to study the coverage of the aluminum surface by lithium atoms. For a small number of Li atoms, these studies suggest that the Li-Li dimerisation is not particularly favored. Received: 24 October 1997 / Revised: 7 April 1998 / Accepted: 29 June 1998     

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     

Stability and fragmentation processes of highly charged sodium clusters

Highly charged sodium clusters produced in collisions between neutral clusters and multiply charged ions are formed within a large range of temperatures and fissilities, and identified by means of a high-resolution reflectron-type time-of-flight mass spectrometer ( m/m 14000). The limit of stability of these charged clusters is experimentally investigated, and the time-of-flight spectra are compared with theoretical spectra based on Monte-Carlo simulations. The results indicate that the maximum fissility (X) of stable clusters is approaching the Rayleigh limit (X = 1) for larger clusters sizes. It is mainly limited by the initial neutral cluster temperature ( T 100 K) and the energy transfer in the ionizing collision. In addition, the comparison between the measured and simulated spectra suggests for high cluster charges a multi-fragmentation process, in which most of charge is emitted, creating low charged residual cluster ions.