Gas Aggregation Technique Research Articles

Valence change in rare earth cluster oxides

Rare earth clusters of europium, thullium and ytterbium were generated by gas aggregation technique and probed by photoionization mass spectrometry. Their relative intensities in mass spectra have shown that their stabilities are governed by compact geometrical structures. The addition of oxygen gas in the nucleation region was used to produce the reactive nucleation. Several stages of oxidation were observed as a function of oxygen pressure up to saturation. For the maximal degree of oxidation the observed oxide ion compositions enable one to follow the valence of metal atom in its oxide as cluster size increases. This exhibits a divalent to trivalent valence change with cluster size. Moreover it emerges from the data that the divalent to trivalent transition for Tm, Yb, and Eu occurs at different size values.

An optical method for temperature measurement of silver clusters

Generating silver clusters by means of the gas aggregation technique, we have observed that in addition to larger clusters, a substantial amount of silver dimers is always present. This allows us to determine the vibrational population in the electronic ground state and, hence, the vibrational temperature of Ag2 optically. Temperatures between 70–300 K were measured in this experiment, dependent on cell parameters, e.g. evaporation rate, argon flow rate, length of the drift tube, etc. The high volume collisional rate of 1021 cm−3s−1 at the given pressures allows us to assume that the cluster temperature does not differ considerably from that of dimer.

Observation of size-dependent coalescence of Ag clusters

Clusters generated by the gas aggregation technique with Diameters D ⩽ 50 A ̊ are condensed at room temperature on thin carbon films and investigated by means of electron microscopy. Spontaneous coalescence of clusters occurs during deposition. Larger clusters ( D ⩾ 300 A ̊ ), on the other hand, do not tend to coalesce and mainly preserve their shape, exhibiting only small alterations due to sintering in the boundary regions. Surface area determination by the BET method using N 2 as the adsorbate gives a specific surface area of 3.6 m 2/g in the case of powder resulting from small clusters ( D κ 20 A ̊ ).

Structures and defects of Au particles (1?3 nm) by HREM

Gold particles produced by the inert gas aggregation technique were observed in HREM. Among the smallest ones (1–3 nm), four structures were recognized: decahedra, icosahedra, cuboctahedra and body-centered tetragonal or orthorhombic structures. The observation of the latter structure for Au clusters is reported for the first time. By improved image processing, their morphologies and defects were precisely determined. Most of them contained small distortions compared to the ideal structures.

Mass-spectrometric investigation of the stabilities and structures of Mn-O and Mn-Mg-O clusters.

Time-of-flight mass spectrometry was used to investigate Mn-O and Mn-Mg-O clusters produced by a gas aggregation technique. The primary stoichiometries observed were (MnO${)}_{\mathit{x}}^{+}$ (x=1--13) and (MnO${)}_{\mathit{x}}$${\mathrm{O}}^{+}$ (x=4--22) clusters, and also these series with partial substitutions of Mg for Mn. The mass spectral abundance patterns suggest that the most stable structures for the stoichiometric clusters are stacks of rings composed of (MnO${)}_{3}$ units, and that the oxygen-rich clusters prefer these structures with a single Mn atom vacancy. The partial substitution of Mg for Mn in the clusters appears to have little effect on these structural preferences. Interesting similarities and differences are observed when the stoichiometric and structural properties of these clusters are compared with the properties of the bulk materials, and also alkaline-earth oxide clusters.

Electronic Structures and Related Properties. Electron Impact Ionization Potentials of Gold and Silver Clusters Men, n ≤ 22

AbstractSmall noble metal clusters were produced by a gas aggregation technique and were investigated mass spectrometrically using electron ionizing energies ranging from 100 eV down to the threshold region. For energies above approximately 20 eV electronic effects are displayed, that characterize a stable cationic distribution, i.e. enhanced stability is found for odd numbered clusters posessing an even number of electrons, furthermore, shell closing appears at n = 3, 9, 21… On the other hand, the electronic effects found while investigating the threshold energy region have to be attributed to neutral clusters, i.e. even numbered clusters are more stable.–The vertical ionization potentials in this region clearly demonstrate the manifestations of shell effects as well as distinct even‐odd oscillations. For both Ag and Au a dramatical drop in the ionization potential is observed from dimer to trimer. The overall size‐dependence of the ionization energies differs from that of alkali clusters due to the presence of d‐electrons.

Structures and bonding properties of calcium oxide clusters inferred from mass spectral abundance patterns

Calcium oxide clusters were produced by a gas aggregation technique and investigated using time-of-flight mass spectrometry. Depending on the source conditions, either singly or doubly charged clusters dominate the mass spectrum. Singly charged (CaO) n Ca m + (m=0-5) clusters are observed for higher flows of inert carrier gas, whereas primarily doubly charged (CaO) n Ca 2+ (n=3-100) clusters are observed at lower flows. The mass spectral abundance patterns indicate that the clusters have cubic structures resembling pieces of the fcc lattice of solid CaO

Generation of rare earth metal clusters by means of the gas-aggregation technique

Generation of rare-earth clusters is achieved using a gas aggregation technique. Varying the nucleation conditions induces a change in the relative cluster abundances. This can be understood in terms of competing cubic versus icosahedron structures.

Shell effects in singly and multiply charged silver and gold clusters

The mass spectra of silver- and gold-clusters, generated by a gas aggregation technique and ionized by electron impact, reveal anomalies in the relative abundance of both singly and multiply charged clusters. Concentration maxima for singly charged species Ag n + and Au n + (n=3, 9, 19, (21), 35) are in agreement with experimental data of Katakuse and the predictions from the electronic shell model. The observed anomalies in the abundance spectra of doubly charged silver and gold clusters as well as triply charged silver cluster ions are explained in terms of electronic shell closing.

Mass spectrometric study of MgO clusters produced by the gas aggregation technique

Gas phase (MgO)n+ and (MgO)nMg+ clusters were produced in a gas aggregation source and studied by using laser-ionization time-of-flight mass spectrometry. A MgO molecule apparently serves as the nucleus for cluster growth, to which Mg and O atoms add. The heat generated by the formation of metal-oxygen bonds, and that added to the cluster by ionization leads to the production of clusters with the stoichiometry of the stable high-temperature oxide. The abundance maxima observed in the mass spectra indicate that the clusters form compact cubic structures similar to pieces of the MgO crystal lattice. The primary fragmentation channel responsible for the observed patterns is probably the loss of MgO monomers.

Silver-halogen cluster compounds Ag n X m (n?2; 0?m?n;X=F, Br)

Nonstoichiometric silver-halogen cluster compounds Ag n X m (0≤m≤n;X=F, Br) are generated by cocondensation of Ag atoms and AgX species using a slightly modified gas aggregation technique. The AgX molecules are produced by partial decomposition of SF6 and Br2 respectively at the surface of the hot silver containing crucible, followed by the reaction of halogen atoms with silver, giving rise to the formation of AgX molecules. In a heterogeneous nucleation between these molecules and evaporated Ag atoms the afore mentioned cluster compounds are formed. The degree of halogenation can either be controlled by the adjustment of the silver evaporation rate, or even more easily by controlling the partial pressure of the halogenating agent. The mass spectra of singly charged halogenated clusters, which are generated by electron impact ionization, reflect the stability of ions. These mass spectra demonstrate that there is an alternation in the intensity pattern up to a relatively high degree of halogenation (m) for each of the investigated compound series Ag n X m ,n≤8. This behavior is similar to the well-known odd-even effect for pure metal clusters, allowing us to postulate the existence of a “metallic” core which governs the stability of the cluster ion (at least for not too high degree of halogenation).

A new type of cluster and cluster ion source

Combining a magnetron gas discharge with the gas aggregation technique an intense source of clusters has been developed. A large part (up to 80%) of the clusters can be generated as ions without using additional electron impact ionisation.

A New Type of Cluster-Ion Source for Thin Film Deposition

ABSTRACTAtoms are gas discharge sputtered from a solid target. They are condensed to form clusters using the gas aggregation technique. An intense beam of clusters of all solid materials can be obtained. Up to 80 % of the clusters can be ionised without using additional electron impact ionisation. Total deposition rates vary between 1 and 1000 Å per second depending on cluster diameter, which can be varied between 3 and 500 nm. Thin films of Al, Cu, and Mo have been produced so far. For non accelerated beams a weakly adhering mostly coulored deposit is obtained. Accelerating the cluster ions this changes to a strongly adhering film, having a shiny metallic appearance, and a very sharp and plane surface as seen in an electron microscope. The advantages compared to Kyoto ICB-method are: easy control of the cluster size, no electron impact ionisation, high degree of ionisation, and sputtering is used instead of thermal evaporation, which allows the use of high melting point materials.

Structure of silver microclusters

We have studied the structure of Ag microclusters using EXAFS. The measurements were performed over a wide range of cluster sizes at the National Synchrotron Light Source (NSLS), beam line X-18B. The clusters were prepared using the gas aggregation technique and isolated in solid argon at 4.2 K. A strong contraction of the interatomic distances was observed for Ag dimers and multimers. Clusters larger than a mean diameter of 20 Å show a small contraction of the nearest-neighbor distance and a structure consistent with an fcc lattice. By contrast, microclusters between 9 and 16 Å show a small nearest-neighbor expansion and a strong reduction or absence of higher-order shells in the EXAFS spectra. This suggests a different crystallographic structure for these Ag microclusters.

Electronic-shell effects in small doubly charged lead clusters.

Lead-cluster mass spectra, the lead clusters being generated by the gas aggregation technique and ionized by electron impact, reveal, besides the well-known relative abundance maxima for singly charged species, relative concentration maxima for the doubly charged ones. Among the most intense peaks the following ones for ${\mathrm{Pb}}_{\mathrm{n}}^{2+}$: n=9, 15, 23, 27, and 35 were found. Since lead is tetravalent, the total numbers of valence electrons for the above-mentioned ions are 34, 58, 90, 106, and 138 per cluster (i.e., 4n-2), respectively. These numbers are exactly those for which the shell closing of the valence electrons within a spherically symmetric potential is achieved. This result demonstrates the importance of electronic effects on the stability, not only for neutral and singly charged metal clusters, but also for doubly charged ones.

High-resolution electron microscopy of supported silver clusters

Abstract Small silver clusters produced by the gas aggregation technique and supported by thin carbon films or asbestos fibres have been investigated by high-resolution electron microscopy. Single crystals and twins have been found, and the proportion of the latter increased with increasing particle size. Both decahedra and cuboctahedra coexist for particles ranging in size between 2 and 10 nm. The cuboctahedra could also be characterized by weak-beam dark-field electron microscopy. Both symmetries were often found to have a few structural defects, the particles with threefold symmetries always containing fewer defects than those with the fivefold symmetries. The influence of contamination by air was also studied.

EXAFS study of Ag, Fe, and Ge microclusters

We have determined the structure of microclusters of Ag, Fe, and Ge using EXAFS. The measurements were performed over a wide range of cluster sizes. The clusters were prepared using the gas aggregation technique and isolated in solid argon. The measurements were performed at beam line X-18B at the National Synchrotron Light Source (NSLS).

Structure of Ag, Fe and Ge microclusters

The structures of Ag, Fe and Ge microclusters were determined using EXAFS. The measurements were performed over a wide range of clusters sizes. The clusters were prepared using the gas aggregation technique and isolated in solid argon at 4.2 K. The measurements were performed at the National Synchrotron Light Source (NSLS) at beam line X-18B. A strong contraction of the interatomic distances was observed for Ag dimers and multimers. Silver clusters larger than 12 A mean diameter show a small contraction of thenn distance and a structure consistent with an fcc lattice. By contrast clusters smaller than 12 A show the presence of a small expansion and a strong reduction or absence ofnnn in the EXAFS signal. This points towards a different crystallographic structure for Ag microclusters with diameter less than 12 A. In iron clusters we observe a gradual reduction of thenn distance as the cluster size decreases. The interatomic distance for iron dimers was determined to be 1.94 A, in good agreement with earlier measurements. The iron microclusters show a bcc structure down to a mean diameter of 9 A. Iron clusters with 9 A mean diameter show a structure inconsistent with a bcc lattice. The new structure is consistent with an fcc or hcp lattice. The measurements on Ge clusters show the presence of only nearest neighbors. There was clear evidence of temporal annealing as determined by variations in the near edge structure of the K-absorption edge. Absorption edge measurements were also performed for free Ge clusters travelling perpendicular to the direction of the synchrotron radiation beam. The measurements performed on the free clusters were consistent with those obtained for matrix isolated clusters.

Stability of multiply positively charged homonuclear clusters.

Singly and multiply charged clusters of the elements Ag, Au, Bi, and Pb---prepared by the gas-aggregation technique and ionized by electron impact---are detected by a single-focusing magnetic mass spectrometer. Stable doubly and triply charged clusters ${X}_{n}^{m+}$ (m=2,3) are observed for values of n which are well below both of the previously reported ones and the theoretical values predicted in the open literature. The various critical values we obtain are as follows: ${\mathrm{Pb}}_{\mathrm{n}}^{2+}$ (${n}_{c}$=3); ${\mathrm{Bi}}_{\mathrm{n}}^{2+}$ (${n}_{c}$=5); ${\mathrm{Ag}}_{\mathrm{n}}^{2+}$, ${\mathrm{Au}}_{\mathrm{n}}^{2+}$ (${n}_{c}$=9), and ${\mathrm{Au}}_{\mathrm{n}}^{3+}$ (${n}_{c}$=22); ${\mathrm{Ag}}_{\mathrm{n}}^{3+}$ (${n}_{c}$=22); ${\mathrm{Bi}}_{\mathrm{n}}^{3+}$ (${n}_{c}$=38); ${\mathrm{Pb}}_{\mathrm{n}}^{3+}$ (${n}_{c}$=43). A strong dependence of the obtained values on the experimental formation conditions of the neutral clusters is observed.

Structural Changes in Ag,Fe,Mn, and Ge Microclusters

AbstractThe structures of Ag,Fe,Mn, and Ge microclusters were determined using EXAFS. The measurements were performed over a wide range of clusters sizes.The clusters were prepared using the gas aggregation technique and isolated in solid argon at 4.2 K. A strong contraction of the interatomic distances was observed for Ag dimers and multimers. Silver clusters larger than 17 A mean diameter show a small contraction of the nn distance and a structure consistent with an fcc lattice. By contrast smaller clusters show the presence of a small expansion and a strong reduction or absence of nnn in the EXAFS signal. This points towards a different crystallographic structure for Ag microclusters. In iron clusters we observed a gradual reduction of the nn distance as the cluster size decreases. The interatomic distance for iron dimers was determined to be 1.94 A, in good agreement with earlier measurements. The iron microclusters show a bcc structure down to a mean diameter of 9 A. Iron clusters with 9 A mean diameter show a structure consistent with an fcc or hcp lattice.Mn microclusters show a simpler crystallographic structure than α-Mn. The measurements on Ge clusters show the presence of only nearest neighbors. There was clear evidence of temporal annealing as determined by variations in the near edge structure of the K-absorption edge. Absorption edge measurements were also performed on free Ge clusters traveling perpendicular to the direction of the synchrotron radiation beam.