Gas Aggregation Technique Research Articles

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.

Magnetic properties of L1 FePt and FePt:Ag nanocluster films

A sputtering gas-aggregation technique has been used to prepare FePt and FePt:Ag nanocluster films. The cluster size was controlled in a range from 3 to 6 nm. FePt cluster films were directly deposited onto Si substrate; FePt:Ag cluster films were fabricated by depositing a FePt cluster layer between a Ag underlayer and overlayer. Nanostructure and magnetic properties of the samples were characterized by x-ray diffraction, transmission electron microscopy, and magnetometry. The high magnetic anisotropy L10 fct phase was realized in the films annealed at a temperature of 550 °C and above. The orientation of clusters is random. The coercivity increases with an increase of annealing temperature; high in-plane and out-of-plane coercivities, exceeding 10 kOe, were achieved in both FePt and FePt:Ag cluster films after annealing. For FePt:Ag films, the coercivity increases with Ag underlayer thickness, tAg, and reaches about 17 kOe at room temperature for tAg=5 nm after annealing at 650 °C for 10 min. The high coercivity is closely correlated with the degree of L10 ordering and nanostructure of the films.

Comparison of oxidized polycrystalline copper foil with small deposited copper clusters on their behavior in the ammonia oxidation: an investigation by means of in situ NEXAFS spectroscopy in the soft X-ray range

The oxidation of ammonia to nitrogen or nitric oxide was investigated using on the one hand a polycrystalline copper foil and on the other hand deposited copper clusters prepared with the inert gas aggregation technique. The behavior in the oxidation of ammonia of both model catalysts was studied using in situ near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in the soft X-ray range and mass spectrometry. It is shown that the copper foil reacts in a similar way to the copper clusters. Differences appear only with respect to the reaction temperature required, which is lower for the cluster sample. It can be concluded that the results obtained in experiments with polycrystalline copper foil are exemplary for and can be transferred to a supported copper catalyst consisting of small copper particles.

Preparation of vanadium and vanadium oxide clusters by means of inert gas aggregation

Vanadium clusters were prepared by the inert gas aggregation technique by evaporating V of high purity. Structural characterization was performed by high-resolution transmission electron microscopy. Apart from vanadium clusters of bcc structures in different orientations, crystalline VO clusters of NaCl structures were observed, which was attributed to the reaction with free oxygen present on the amorphous carbon substrates. The latter could be detected by electron energy-loss spectroscopy. The exposure of the samples to air caused a change to amorphous structures, which re-crystallized under the electron beam. This effect was interpreted as a reaction with water present in the air.

Growth of Rodlike Silver Nanoparticles by Vapor Deposition of Small Clusters

A beam of silver clusters of 3 nm average diameter is produced by the gas-aggregation technique and deposited onto the NaCl (100) surface. Subsequent agglomeration leads in part to the growth of rodlike particles with the structure of truncated decahedra of typical dimensions 70 nm long and 10 nm diameter and a fivefold symmetry, as shown in the picture. A growth mechanism is inferred from a detailed analysis of electron microscopy data.

Growth of Rodlike Silver Nanoparticles by Vapor Deposition of Small Clusters

A beam of silver clusters of 3 nm average diameter is produced by the gas-aggregation technique and deposited onto the NaCl (100) surface. Subsequent agglomeration leads in part to the growth of rodlike particles with the structure of truncated decahedra of typical dimensions 70 nm long and 10 nm diameter and a fivefold symmetry, as shown in the picture. A growth mechanism is inferred from a detailed analysis of electron microscopy data.

Physical and chemical interface effects on Mie plasmon absorption of sodium nanoclusters passivated with CH 4− nCl n ( n=1–4) molecules

We have examined the metal/medium interface effects on the optical absorption due to Mie plasmon resonance of sodium nanoclusters passivated with CH 4− n Cl n ( n=1–4) molecules. The redshifted bands are observed around 600 nm in each Na/CH 4− n Cl n aerosol generated by the gas-aggregation technique with a reactant nozzle (GATRN). The peak energies are also redshifted with increasing the reactant-gas flow rates. The dielectric coated metal-particle model predicts that the sodium nanoclusters passivated with CH 4− n Cl n form in the aerosols and that the peak redshifts are due to the dielectric-shell growth and static charge transfer at metal/medium interface.

Preparation and characterization of nanometre-size InSb particles

Abstract The alloying behaviour of antimony molecules Sb4 into nanometre size indium clusters deposited on amorphous carbon has been studied by transmission electron microscopy and selected area electron diffraction for morphological and structural characterizations, and by energy dispersive X-ray spectroscopy for analysis of composition. First, large indium clusters containing about 1700 atoms (mean diameter = 4.4 nm) formed by the gas aggregation technique in a thermal source are deposited on amorphous carbon. Nucleation and growth mechanisms lead to the formation of indium particles randomly dispersed on the substrate. Then antimony molecules evaporated at various thicknesses are deposited on to the indium aggregates. These two successive depositions have been performed at room temperature. A dissolution of antimony atoms into the indium particles takes place and InSb compound is successfully formed. InSb particles as produced have the zinc-blende structure (sphalerite type) which is the structure of t...

AuSb2 alloy formation on HOPG by successive deposition of Sb clusters and Au atoms

The behaviour of gold atoms that alloy into amorphous antimony (aSb) islands is studied by transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS). In a first step, antimony thin films are produced by low-energy cluster beam deposition (LECBD). The antimony clusters (of diameter ≈5 nm, with 2300 atoms) are generated in a thermal source by the gas aggregation technique and deposited onto highly-oriented pyrolitic graphite (HOPG). For cluster submonolayer coverage (<0.2 M.L.), the diffusion and interaction of clusters on substrate lead to the formation of amorphous ramified Sb islands, formed by the aggregation of spherical particles about 10 nm in diameter. In a second step, gold is vapour-deposited onto the aSb–HOPG substrate. The influence of gold atom deposition on the morphology of the islands is studied. Moreover, varying the gold thickness, we characterize the AuSb2 alloy formation.

Optical absorptions of the X-center-type sodium nanoclusters in the gas-aggregation technique with a reactant nozzle

The effects that the interaction of Na nanoclusters with an electron-affinity reactant Cl 2 influences on their optical absorptions have been investigated using the gas-aggregation technique with a reactant nozzle (GATRN). In addition to the surface plasmon band of the Na nanoclusters, extra bands with large red-shift were observed at the growth/interaction region of GATRN. Spectral analogy with X-center bands in NaCl crystals suggests the formation of NaCl-coated Na nanoclusters. The red-shift peak of the extra bands has been explained by the dielectric coated metal-particle model and charge transfer from the metal-cluster core to the dielectric shell.

Spontaneous alloying on HOPG by successive deposition of Sb clusters and Au atoms

Alloying of gold atoms into nanometer-sized antimony clusters deposited on highly oriented pyrolitic graphite (HOPG) has been studied by transmission electron microscopy (TEM) for morphological and structural characterisations and by energy dispersive X-ray spectroscopy (EDX) for analysis of composition. First, large antimony clusters (≈5.4 nm) generated by the gas aggregation technique in a thermal source were deposited on HOPG. These antimony clusters diffused on graphite and formed, by collision and irreversible sticking, ramified islands the density of which can be predicted, prior to deposition, with suitable cluster flux and substrate temperature. Then, the gold atoms were evaporated at various mean thicknesses on the same Sb N –HOPG substrate. For this post-deposition of gold, the morphology of the deposit was quite different, depending on the evaporation method (Knudsen cell heated by electron bombardment or spiral tungsten filament heated by the Joule effect): ramified islands of Sb–Au alloys plus gold supported particles when using the first method and only ramified Sb–Au islands for the second one. In this paper, we give an explanation of these two different morphologies and we study the alloy formation versus the gold content.

Reactive nucleation of silver clusters with oxygen and water

The nucleation of silver atoms by the gas aggregation technique when adding O 2 and H 2O molecules in the carrier gas was investigated by photoionization mass spectrometry. No change appeared in the bare silver cluster mass spectra when only water was added to the nucleation carrier gas. In the presence of oxygen and water, reactive nucleation occurred simultaneously. At low reactive gas concentration (unsaturated regime), Ag n O x H y compounds with x=2, 3 and 4 are produced for n⩾6. For a larger reactive gas concentration (saturated regime) two series of cluster bunches having Ag 3O 3H y as the building block were observed. The precursors of these series are (Ag 4O 3H y ) + and (Ag 5O 5H y ) +, respectively. This behavior differs totally from the results obtained for reactive nucleation of an alkaline vapor but can be viewed as a precursor of the surface silver hydroxidation mechanism.

In situ surface electron energy loss fine structure investigations of copper clusters deposited on pyrolytic boron nitride

Abstract Copper clusters were prepared by using the inert gas aggregation technique. In situ surface electron energy loss fine structure investigations above the M 2,3 edge show a sizeable dilatation of the first nearest-neighbour distance provided the mean diameter of the clusters is decreased to 40 A (corresponding to a number N 0 in the cluster of approximately 2800 atoms). Below this value our results are in good agreement with the predicted contraction according to the liquid drop model. The latter, however, strongly contradicts the results recently found using high resolution transmission electron microscopy and electron diffraction. It is believed that this contraction can be explained by inharmonic surface crystal potentials for small particles.

Structural properties of granular Bi-SiO x films prepared by low-energy cluster beam deposition

In this paper, we propose a new technique for the elaboration of size controlled nanoparticles embedded in a matrix: the low-energy cluster beam deposition (LECBD). Bi-SiO x nanocermets have been prepared by codeposition of Bi neutral clusters and SiO x molecules. We present a study by transmission electron microscopy (TEM) of the structural properties of these new cermets. The clusters are generated by the gas-aggregation technique in a thermal source which produces clusters with a low energy. Then, the incident clusters do not break at the impact on the surface and conserve their size and structure. Consequently, the cermets are composed of Bi nanograins with a size distribution comparable to that of the free clusters, at low metallic concentrations. This result has been obtained for three different size distributions of the free clusters showing that the LECBD technique allows the control of the size of the embedded particles in the nanometric range. Moreover, the size and the concentration of grains are independent at metallic concentrations below 10%. This result is very important for studying the influence of these two parameters on the physical properties of granular materials.

Formation of antimony oxide clusters by gas aggregation

SbxOy clusters are produced by using a gas aggregation technique. Antimony vapor is mixed with He/O2 or He/N2O and cooled in a reaction channel. After photoionisation with a KrF (248 nm) or ArF (193 nm) excimer laser the products are mass analyzed in a time of flight mass spectrometer. In the presence of N2O no oxide clusters besides SbO+ can be detected, while with oxygen under similar experimental conditions dramatic changes can be observed. At low oxygen partial pressure the obtained spectra are dominated by the pure Sb x + clusters with low intensity of SbxO y + , whereas at high oxygen partial pressure antimony oxides following the general sequence SbO+(Sb2O3)n are most abundant. The same stable species can furthermore be produced via aggregation of vaporised solid antimony oxide (Sb2O3). Within these experiments another new Series of antimony oxides tentatively assigned to (Sb2O3) n + appeared in the mass spectra.

The CO stretch frequency of fully CO covered Ag-clusters in dependence on cluster size

Silver clusters Agn prepared either by the gas aggregation technique (GAT) or by matrix aggregation of Ag were embedded in CO and CO/Ar-matrices. The vibrational CO stretch frequency at the monolayer coverage, which is found on bulk silver at 2110 cm-1, preserves its value with decreasing particle size, Agn n≥300. For smaller clusters a decrease of this frequency with decreasing size was observed. For the smallest investigated clusters Agn, n∼30, a value of 2080 cm-1 was obtained. The weakening of the CO bond is an indication of a charge transfer from the cluster to the CO monolayer. This result is in accordance with the surface plasmon absorption measurements of Ag-clusters isolated in CO matrices.

Reactions of sodium/antimony‐clusters with molecular chlorine in a flow‐tube‐reactor coupled to a time‐of‐flight mass spectrometer

AbstractA gas aggregation technique is used to produce NaxSby, clusters. After passing a flow‐tube reactor, the clusters are ionized with unfocused high fluence KrF‐excimer laser radiation and then mass analyzed in a time‐of‐flight mass spectrometer. Dramatic changes are observed in the cluster ion distribution upon adding high purity chlorine gas into the flow tube. As a result of reactions with Cl2, all NaxSby, clusters disappear while pure antimony cluster ions arise. The distribution patterns of the produced Sbx+ clusters are the same as those observed in experiments with pure antimony.

Sputtering rates and nanoscale cluster production in a hollow-cathode apparatus

Abstract By combining a hollow-cathode plasma sputter device with the gas aggregation technique, an intense source of nanoscale metal clusters has been developed. Net sputtering rates have been measured as a function of pressure, flow velocity, electric current and cathode geometry, and in most instances they were found to behave differently than their planar-electrode sputtering counterparts. Deposition rates on substrates located away from the hot plasma region have been also measured. Clusters of Cu, Ag, Au and Ta have been produced and their size has been determined via XRD and TEM. The size of Cu n clusters is found to increase with the sputter source discharge current.

HREM studies of the structure and the oxidation process of copper clusters created by inert gas aggregation

Structure and reactivity with oxigen of Cu clusters in the size range of 4.5±2.5 nm created by the inert gas aggregation technique were studied by HREM. The pure Cu clusters investigated under clean conditions show the structures of MTP's with a small lattice dilatation of the (111) plane of 1.25%. For icosahedral and decahedral particles this dilatation corresponds to a splitting of the nearest neighbour distance showing two different values, i.e. dilatation of 2.2% and contraction of 2.8% for the two edges of the deformed tetrahedral subunits, respectively. Oxidation at room temperature and air pressure of 1 bar only begins after a few minutes of exposure to air, after having undergone creation of probably non-stoichiometric intermediate states in the cuprite Cu2O structure with the bulk values of the bond lengths.

Shell effects in fission of small doubly charged lithium clusters.

The Coulombic fission of small doubly charged lithium and potassium clusters has been investigated around their critical size of stability ${\mathit{n}}_{\mathit{c}}$=25 and ${\mathit{n}}_{\mathit{c}}$=21, respectively. Neutral clusters are produced by the gas aggregation technique, photoionized by a pulsed laser source and mass analyzed in a tandem time-of-flight mass spectrometer. Individual fragmentation channels due to the unimolecular dissociation of size-selected parents are analyzed by using the retarding-field method. In this size range it is found that the fission has an asymmetric character. The shell effects govern the preferred output channels which include in any case the singly charged trimer. The absolute values of the fission energy barriers are estimated.