Philipp Gruene

Structures of neutral Au7, Au19, and Au20 clusters in the gas phase.

The catalytic properties of gold nanoparticles are determined by their electronic and geometric structures. We revealed the geometries of several small neutral gold clusters in the gas phase by using vibrational spectroscopy between 47 and 220 wavenumbers. A two-dimensional structure for neutral Au7 and a pyramidal structure for neutral Au20 can be unambiguously assigned. The reduction of the symmetry when a corner atom is cut from the tetrahedral Au20 cluster is directly reflected in the vibrational spectrum of Au19.

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.

The adsorption of CO on group 10 (Ni, Pd, Pt) transition-metal clusters

The adsorption of a single CO molecule on clusters of the Group 10 transition metals is characterized by infrared multiple photon dissociation (IR-MPD) spectroscopy. The cationic, neutral, and anionic carbonyl complexes contain between 3 and up to 25 metal atoms. The C-O stretching frequency nu(CO) shows that while both nickel and platinum clusters adsorb CO only in atop positions, palladium clusters exhibit a variety of binding sites. These findings can be rationalized by considering the increasing role relativistic effects play in the electronic structure of the cluster complexes going down the group. Conclusions for the cluster-support interactions for size-selected supported particles are drawn from the charge dependence of nu(CO) for the gas-phase species.

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.

Probing the structures of gas-phase rhodium cluster cations by far-infrared spectroscopy

The geometric structures of small cationic rhodium clusters Rh(n)(+) (n = 6-12) are investigated by comparison of experimental far-infrared multiple photon dissociation spectra with spectra calculated using density functional theory. The clusters are found to favor structures based on octahedral and tetrahedral motifs for most of the sizes considered, in contrast to previous theoretical predictions that rhodium clusters should favor cubic motifs. Our findings highlight the need for further development of theoretical and computational methods to treat these high-spin transition metal clusters.

H2 adsorption on 3d transition metal clusters: a combined infrared spectroscopy and density functional study.

The adsorption of H2 on a series of gas-phase transition metal (scandium, vanadium, iron, cobalt, and nickel) clusters containing up to 20 metal atoms is studied using IR-multiple photon dissociation spectroscopy complemented with density functional theory based calculations. Comparison of the experimental and calculated spectra gives information on hydrogen-bonding geometries. The adsorption of H2 is found to be exclusively dissociative on Sc(n)O+, V(n)+, Fe(n)+, and Co(n)+, and both atomic and molecularly chemisorbed hydrogen is present in Ni(n)H(m)+ complexes. It is shown that hydrogen adsorption geometries depend on the elemental composition as well as on the cluster size and that the adsorption sites are different for clusters and extended surfaces. In contrast to what is observed for extended metal surfaces, where hydrogen has a preference for high coordination sites, hydrogen can be both 2- or 3-fold coordinated to cationic metal clusters.

Infrared induced reactivity on the surface of isolated size-selected clusters: dissociation of N2O on rhodium clusters.

Multiple photon infrared excitation of size-selected Rh(6)N(2)O(+) clusters drives surface chemistry resulting in partially oxidized clusters.

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-

Nature of Ar bonding to small Co(n+) clusters and its effect on the structure determination by far-infrared absorption spectroscopy.

Far-infrared vibrational spectroscopy by multiple photon dissociation has proven to be a very useful technique for the structural fingerprinting of small metal clusters. Contrary to previous studies on cationic V, Nb, and Ta clusters, measured vibrational spectra of small cationic cobalt clusters show a strong dependence on the number of adsorbed Ar probe atoms, which increases with decreasing cluster size. Focusing on the series Co(4) (+) to Co(8) (+) we therefore use density-functional theory to analyze the nature of the Ar-Co(n) (+) bond and its role for the vibrational spectra. In a first step, energetically low-lying isomer structures are identified through first-principles basin-hopping sampling runs and their vibrational spectra are computed for a varying number of adsorbed Ar atoms. A comparison of these fingerprints with the experimental data enables in some cases a unique assignment of the cluster structure. Independent of the specific low-lying isomer, we obtain a pronounced increase in the Ar binding energy for the smallest cluster sizes, which correlates nicely with the observed increased influence of the Ar probe atoms on the IR spectra. Further analysis of the electronic structure motivates a simple electrostatic picture that not only explains this binding energy trend but also rationalizes the stronger influence of the rare-gas atom compared to the preceding studies by the small atomic radius of Co.

Infrared-Induced Reactivity of N2O on Small Gas-Phase Rhodium Clusters

Far- and mid-infrared multiple photon dissociation spectroscopy has been employed to study both the structure and surface reactivity of isolated cationic rhodium clusters with surface-adsorbed nitrous oxide, Rh(n)N(2)O(+) (n = 4-8). Comparison of experimental spectra recorded using the argon atom tagging method with those calculated using density functional theory (DFT) reveals that the nitrous oxide is molecularly bound on the rhodium cluster via the terminal N-atom. Binding is thought to occur exclusively on atop sites with the rhodium clusters adopting close-packed structures. In related, but conceptually different experiments, infrared pumping of the vibrational modes corresponding with the normal modes of the adsorbed N(2)O has been observed to result in the decomposition of the N(2)O moiety and the production of oxide clusters. This cluster surface chemistry is observed for all cluster sizes studied except for n = 5. Plausible N(2)O decomposition mechanisms are given based on DFT calculations using exchange-correlation functionals. Similar experiments pumping the Rh-O stretch in Rh(n)ON(2)O(+) complexes, on which the same chemistry is observed, confirm the thermal nature of this reaction.