Ewald Janssens

Structures of silicon cluster cations in the gas phase.

We present gas-phase infrared spectra for small silicon cluster cations possessing between 6 and 21 atoms. Infrared multiple photon dissociation (IR-MPD) of these clusters complexed with a xenon atom is employed to obtain their vibrational spectra. These vibrational spectra give for the first time experimental data capable of distinguishing the exact internal structures of the silicon cluster cations. By comparing the experimental spectra with theoretical predictions based on density functional theory (DFT), unambiguous structural assignments for most of the Si(n)(+) clusters in this size range have been made. In particular, for Si(8)(+) an edge-capped pentagonal bypriamid structure, hitherto not considered, was assigned. These structural assignments provide direct experimental evidence for a cluster growth motif starting with a pentagonal bipyramid building block and changing to a trigonal prism for larger clusters.

Density functional study on structure and stability of bimetallic AuNZn (N⩽6) clusters and their cations

A systematic study on the structure and stability of zinc doped gold clusters has been performed by density functional theory calculations. All the lowest-energy isomers found have a planar structure and resemble pure gold clusters in shape. Stable isomers tend to equally delocalize valence s electrons of the constituent atoms over the entire structure and maximize the number of Au–Zn bonds in the structure. This is because the Au–Zn bond is stronger than the Au–Au bond and gives an extra σ-bonding interaction by the overlap between vacant Zn 4p and valence Au 6s(5d) orbitals. No three-dimensional isomers were found for Au5Zn+ and Au4Zn clusters containing six delocalized valence electrons. This result reflects that these clusters have a magic number of delocalized electrons for two-dimensional systems. Calculated vertical ionization energies and dissociation energies as a function of the cluster size show odd–even behavior, in agreement with recent mass spectrometric observations [Tanaka et al., J. Am. C...

Disparate effects of Cu and V on structures of exohedral transition metal-doped silicon clusters: a combined far-infrared spectroscopic and computational study.

The growth mechanisms of small cationic silicon clusters containing up to 11 Si atoms, exohedrally doped by V and Cu atoms, are described. We find that as dopants, V and Cu follow two different paths: while V prefers substitution of a silicon atom in a highly coordinated position of the cationic bare silicon clusters, Cu favors adsorption to the neutral or cationic bare clusters in a lower coordination site. The different behavior of the two transition metals becomes evident in the structures of Si(n)M(+) (n = 4-11 for M = V, and n = 6-11 for M = Cu), which are investigated by density functional theory and, for several sizes, confirmed by comparison with their experimental vibrational spectra. The spectra are measured on the corresponding Si(n)M(+)·Ar complexes, which can be formed for the exohedrally doped silicon clusters. The comparison between experimental and calculated spectra indicates that the BP86 functional is suitable to predict far-infrared spectra of these clusters. In most cases, the calculated infrared spectrum of the lowest-lying isomer fits well with the experiment, even when various isomers and different electronic states are close in energy. However, in a few cases, namely Si(9)Cu(+), Si(11)Cu(+), and Si(10)V(+), the experimentally verified isomers are not the lowest in energy according to the density functional theory calculations, but their structures still follow the described growth mechanism. The different growth patterns of the two series of doped Si clusters reflect the role of the transition metals 3d orbitals in the binding of the dopant atoms.

High Magnetic Moments in Manganese-Doped Silicon Clusters

We report on the structural, electronic, and magnetic properties of manganese-doped silicon clusters cations, Si(n)Mn(+) with n=6-10, 12-14, and 16, using mass spectrometry and infrared spectroscopy in combination with density functional theory computations. This combined experimental and theoretical study allows several structures to be identified. All the exohedral Si(n)Mn(+) (n=6-10) clusters are found to be substitutive derivatives of the bare Si(n+1)(+) cations, while the endohedral Si(n)Mn(+) (n=12-14 and 16) clusters adopt fullerene-like structures. The hybrid B3P86 functional is shown to be appropriate in predicting the ground electronic states of the clusters and in reproducing their infrared spectra. The clusters turn out to have high magnetic moments localized on Mn. In particular the Mn atoms in the exohedral Si(n)Mn(+) (n=6-10) clusters have local magnetic moments of 4 μ(B) or 6 μ(B) and can be considered as magnetic copies of the silicon atoms. Opposed to other 3d transition-metal dopants, the local magnetic moment of the Mn atom is not completely quenched when encapsulated in a silicon cage.

Scanning probe microscopy investigation of gold clusters deposited on atomically flat substrates

We systematically studied the influence of the substrate on the shape, mobility, and stability of deposited gold clusters. The Aun clusters were produced in a laser vaporization source and deposited with low kinetic energy (~0.4 eV/atom) on atomically flat substrates (graphite, mica, and gold and silver films on mica) under UHV conditions. Their size distribution is probed with time-of-flight mass spectrometry and ranges from dimers to several hundreds of atoms. Scanning probe microscopy is used to characterize the deposited clusters and the formation of islands by cluster aggregation. On all substrates, Aun islands can be clearly distinguished and the islands are flattened despite the small impact energy. The shape and size of the island configurations are strongly system dependent. Gold clusters deposited on Au(111) and Ag(111) films grown on mica do not aggregate, but deform due to strong cluster–substrate interactions. The clusters tend to grow epitaxially on these surfaces. On graphite and on mica, deposited clusters do diffuse and aggregate. On the graphite surface, large ramified islands are formed by juxtaposition of small islands and trapping of the clusters at the step edges. On the other hand, the diffusion of the clusters on mica results in a total coalescence of the Aun clusters into compact islands.

Carbon Monoxide Adsorption on Silver Doped Gold Clusters

Well controlled gas phase experiments of the size and dopant dependent reactivity of gold clusters can shed light on the surprising discovery that nanometer sized gold particles are catalytically active. Most studies that investigate the reactivity of gold clusters in the gas phase focused on charged, small sized clusters. Here, reactivity measurements in a low-pressure reaction cell were performed to investigate carbon monoxide adsorption on neutral bare and silver doped gold clusters (Au(n)Ag(m); n = 10-45; m = 0, 1, 2) at 140 K. The size dependence of the reaction probabilities reflects the role of the electronic shells for the carbon monoxide adsorption, with closed electronic shell systems being the most reactive. In addition, the clusters reaction probability is reduced upon substitution of gold atoms for silver. Inclusion of a single silver atom causes significant changes in the reactivity only for a few cluster sizes, whereas there is a more general reduction in the reactivity with two silver atoms in the cluster. The experimental observations are qualitatively explained on the basis of a Blyholder model, which includes dopant induced features such as electron transfer from silver to gold, reduced s-d hybrization, and changes in the cluster geometry.

Tuning the Geometric Structure by Doping Silicon Clusters

Ever since the discovery of C60, much effort has been expended in search of similar, finite-size stable clusters as building blocks for nanostructures. Apart from carbon, silicon has attracted much attention due to its vicinity to carbon in the periodic table as well as its importance in the semiconductor industry. In contrast to carbon, however, silicon favours sp hybridization and thus tetrahedral coordination, which leads to rather asymmetric and reactive structures for small, bare silicon clusters. 3] It has been argued that this deficiency can be solved by suitable doping of silicon clusters with transition metal ions. Following up on this idea, many theoretical studies have investigated SinM structures for various dopants and cluster sizes. [5, 6] Experimental information on doped silicon clusters has been obtained from mass spectrometry, photoelectron spectroscopy (PES), chemical probe methods, and photodissociation studies at fixed wavelengths. While there is no doubt that the structure of silicon clusters can be changed upon appropriate doping, detailed experimental studies on the growth mechanisms of doped silicon clusters are rather scarce, as it is difficult to investigate the structure of gas phase clusters experimentally. A deep knowledge about the influence of the dopant on the clusters’ structure, however, is necessary for the design and production of tailor-made silicon materials. It has recently been shown that infrared multiple photon dissociation (IR–MPD) of complexes of metal clusters with raregas atoms is a suitable experimental technique to obtain vibrational spectra for clusters in the gas phase. Comparison of experimental IR–MPD spectra of clusters with those obtained in calculations for different geometries, for example by using density functional theory (DFT), allows for the deduction of the cluster-size-specific structures. Herein we present the vibrational spectra of the small cationic copperand vanadium-doped silicon clusters SinCu + and SinV + (n=6–8). Copperand vanadium-doped silicon clusters show the same critical size for the transition from endohedral to exohedral structures, which has been rationalized by the similar atomic radii of the dopants. It is thus interesting to investigate whether doping with these two atoms will generate clusters with the same geometric structure. Figure 1 shows the vibrational spectra of Si8V + . The experimental spectrum (bottom panel) is obtained upon IR–MPD of its complex with one argon atom. In the case of resonant ab-

Identification of Conical Structures in Small Aluminum Oxide Clusters: Infrared Spectroscopy of (Al2O3)1−4(AlO)+

The vibrational spectroscopy of the electronically closed-shell (Al 2O 3) n (AlO) (+) cations with n = 1-4 is studied in the 530-1200 cm (-1) range by infrared predissociation spectroscopy of the corresponding ion-He atom complexes in combination with quantum chemical calculations. In all cases we find, assisted by a genetic algorithm, global minimum structures that differ considerably from those derived from known modifications of bulk alumina. The n = 1 and n = 4 clusters exhibit an exceptionally stable conical structure of C 3 v symmetry, whereas for n = 2 and n = 3, multiple isomers of lower symmetry and similar energy may contribute to the recorded spectra. A blue shift of the highest energy absorption band is observed with increasing cluster size and attributed to a shortening of Al-O bonds in the larger clusters. This intense band is assigned to vibrational modes localized on the rim of the conical structures for n = 1 and n = 4 and may aid in identifying similar, highly symmetric structures in larger ions.

The Cu7Sc Cluster is a Stable σ‐Aromatic Seven‐Membered Ring

Density functional theory calculations demonstrate that the global minimum of the Cu(7)Sc potential energy surface is a seven-membered ring of copper atoms with scandium in its center, yielding a planar D(7) (h) structure. Nucleus-independent chemical shifts [NICS(1)(zz) and NICS(2)(zz)] show that this cluster has aromatic character, which is consistent with the number of 4s electrons of copper and scandium plus the 3d electrons of scandium satisfying Hückels rule. According to a canonical MO decomposition of NICS(1)(zz) and NICS(2)(zz), the MOs consisting of the 4s atomic orbitals are mainly responsible for the aromatic behavior of the cluster. The electron localizability indicator (ELI-D) and its canonical MO decomposition (partial ELI-D) suggest that a localized basin is formed in Cu(7)Sc by the copper atoms whereas the two circular localized domains are situated below and above the ring. The planar Cu(7)Sc cluster can thus be considered as a sigma-aromatic species. These findings agree with the phenomenological shell model.

The structures of neutral transition metal doped silicon clusters, SinX (n = 6−9; X = V, Mn)

We present a combined experimental and theoretical investigation of small neutral vanadium and manganese doped silicon clusters Si(n)X (n = 6-9, X = V, Mn). These species are studied by infrared multiple photon dissociation and mass spectrometry. Structural identification is achieved by comparison of the experimental data with computed infrared spectra of low-lying isomers using density functional theory at the B3P86∕6-311+G(d) level. The assigned structures of the neutral vanadium and manganese doped silicon clusters are compared with their cationic counterparts. In general, the neutral and cationic Si(n)V(0,+) and Si(n)Mn(0,+) clusters have similar structures, although the position of the capping atoms depends for certain sizes on the charge state. The influence of the charge state on the electronic properties of the clusters is also investigated by analysis of the density of states, the shapes of the molecular orbitals, and NBO charge analysis of the dopant atom.