Alexander Kaiser

Adsorption of hydrogen on neutral and charged fullerene: Experiment and theory

Helium droplets are doped with fullerenes (either C60 or C70) and hydrogen (H2 or D2) and investigated by high-resolution mass spectrometry. In addition to pure helium and hydrogen cluster ions, hydrogen-fullerene complexes are observed upon electron ionization. The composition of the main ion series is (H2)(n)HC(m)(+) where m = 60 or 70. Another series of even-numbered ions, (H2)(n)C(m)(+), is slightly weaker in stark contrast to pure hydrogen cluster ions for which the even-numbered series (H2)(n)(+) is barely detectable. The ion series (H2)(n)HC(m)(+) and (H2)(n)C(m)(+) exhibit abrupt drops in ion abundance at n = 32 for C60 and 37 for C70, indicating formation of an energetically favorable commensurate phase, with each face of the fullerene ion being covered by one adsorbate molecule. However, the first solvation layer is not complete until a total of 49 H2 are adsorbed on C60(+); the corresponding value for C70(+) is 51. Surprisingly, these values do not exhibit a hydrogen-deuterium isotope effect even though the isotope effect for H2/D2 adsorbates on graphite exceeds 6%. We also observe doubly charged fullerene-deuterium clusters; they, too, exhibit abrupt drops in ion abundance at n = 32 and 37 for C60 and C70, respectively. The findings imply that the charge is localized on the fullerene, stabilizing the system against charge separation. Density functional calculations for C60-hydrogen complexes with up to five hydrogen atoms provide insight into the experimental findings and the structure of the ions. The binding energy of physisorbed H2 is 57 meV for H2C60(+) and (H2)2C60(+), and slightly above 70 meV for H2HC60(+) and (H2)2HC60(+). The lone hydrogen in the odd-numbered complexes is covalently bound atop a carbon atom but a large barrier of 1.69 eV impedes chemisorption of the H2 molecules. Calculations for neutral and doubly charged complexes are presented as well.

Methane Adsorption on Graphitic Nanostructures: Every Molecule Counts.

Bundles of single-walled nanotubes are promising candidates for storage of hydrogen, methane, and other hydrogen-rich molecules, but experiments are hindered by nonuniformity of the tubes. We overcome the problem by investigating methane adsorption on aggregates of fullerenes containing up to six C60; the systems feature adsorption sites similar to those of nanotube bundles. Four different types of adsorption sites are distinguished, namely, registered sites above the carbon hexagons and pentagons, groove sites between adjacent fullerenes, dimple sites between three adjacent fullerenes, and exterior sites. The nature and adsorption energies of the sites in C60 aggregates are determined by density functional theory and molecular dynamics (MD) simulations. Excellent agreement between experiment and theory is obtained for the adsorption capacity in these sites.

Methane Adsorption on Aggregates of Fullerenes: Site- Selective Storage Capacities and Adsorption Energies

Methane adsorption on positively charged aggregates of C60 is investigated by both mass spectrometry and computer simulations. Calculated adsorption energies of 118–281 meV are in the optimal range for high-density storage of natural gas. Groove sites, dimple sites, and the first complete adsorption shells are identified experimentally and confirmed by molecular dynamics simulations, using a newly developed force field for methane–methane and fullerene–methane interaction. The effects of corrugation and curvature are discussed and compared with data for adsorption on graphite, graphene, and carbon nanotubes.

Communication: Dopant-induced solvation of alkalis in liquid helium nanodroplets

Alkali metal atoms and small alkali clusters are classic heliophobes and when in contact with liquid helium they reside in a dimple on the surface. Here we show that alkalis can be induced to submerge into liquid helium when a highly polarizable co-solute, C60, is added to a helium nanodroplet. Evidence is presented that shows that all sodium clusters, and probably single Na atoms, enter the helium droplet in the presence of C60. Even clusters of cesium, an extreme heliophobe, dissolve in liquid helium when C60 is added. The sole exception is atomic Cs, which remains at the surface.

Building Carbon Bridges on and between Fullerenes in Helium Nanodroplets

We report the observation of sequential encounters of fullerenes with C atoms in an extremely cold environment. Experiments were performed with helium droplets at 0.37 K doped with C60 molecules and C atoms derived from a novel, pure source of C atoms. Very high-resolution mass spectra revealed the formation of carbenes of the type C60(C:)n with n up to 6. Bridge-type bonding of the C adatoms to form the known dumbbell C60=C=C60 also was observed. Density functional theory calculations were performed that elucidated the carbene character of the C60(C:)n species and their structures. Mass spectra taken in the presence of water impurities and in separate experiments with added H2 also revealed the formation of the adducts C60Cn(H2O)n and C60Cn(H2)n probably by H–OH and H–H bond insertion, respectively, and nonreactivity for the dumbell. So C adatoms that form carbenes C60(C:)n can endow pristine C60 with a higher chemical reactivity.

Electron-Induced Chemistry of Cobalt Tricarbonyl Nitrosyl (Co(CO)3NO) in Liquid Helium Nanodroplets.

Electron addition to cobalt tricarbonyl nitrosyl (Co(CO3NO) and its clusters has been explored in helium nanodroplets. Anions were formed by adding electrons with controlled energies, and reaction products were identified by mass spectrometry. Dissociative electron attachment (DEA) to the Co(CO)3NO monomer gave reaction products similar to those reported in earlier gas phase experiments. However, loss of NO was more prevalent than loss of CO, in marked contrast to the gas phase. Since the Co–N bond is significantly stronger than the Co–C bond, this preference for NO loss must be driven by selective reaction dynamics at low temperature. For [Co(CO)3NO]N clusters, the DEA chemistry is similar to that of the monomer, but the anion yields as a function of electron energy show large differences, with the relatively sharp resonances of the monomer being replaced by broad profiles peaking at much higher electron energies. A third experiment involved DEA of Co(CO)3NO on a C60 molecule in an attempt to simulate the effect of a surface. Once again, broad ion yield curves are seen, but CO loss now becomes the most probable reaction channel. The implication of these findings for understanding focused electron beam induced deposition of cobalt is described.

Isomeric Broadening of C60+ Electronic Excitation in Helium Droplets: Experiments Meet Theory

Helium is considered an almost ideal tagging atom for cold messenger spectroscopy experiments. Although helium is bound very weakly to the ionic molecule of interest, helium tags can lead to shifts and broadenings that we recorded near 963.5 nm in the electronic excitation spectrum of C60+ solvated with up to 100 helium atoms. Dedicated quantum calculations indicate that the inhomogeneous broadening is due to different binding energies of helium to the pentagonal and hexagonal faces of C60+, their dependence on the electronic state, and the numerous isomeric structures that become available for intermediate coverage. Similar isomeric effects can be expected for optical spectra of most larger molecules surrounded by nonabsorbing weakly bound solvent molecules, a situation encountered in many messenger-tagging spectroscopy experiments.

Magic Numbers for Packing Adamantane in Helium Droplets: Cluster Cations, Dications, and Trications

We report the first observation of cations, dications, and trications of large clusters of adamantane. Cluster formation was initiated near 0 K in helium droplets and ionization was achieved with one or more collisions with energetic He species (He*, He+, or He*–). The occurrence of Coulomb explosion appeared to discriminate against the formation of small multiply-charged clusters. High-resolution mass spectrometry revealed the presence of “magic number” m/z peaks that can be attributed to the packing of adamantane molecules into cluster structures of special stability involving preferred arrangements of these molecules. These abundance anomalies were seen to be independent of charge state. While some dehydrogenation of adamantane and its clusters was seen as well, no major transformations into adamantoids or microdiamonds were observed.

Highly Stable [C60AuC60]+/- Dumbbells

Ionic complexes between gold and C60 have been observed for the first time. Cations and anions of the type [Au(C60)2]+/– are shown to have particular stability. Calculations suggest that these ions adopt a C60–Au–C60 sandwich-like (dumbbell) structure, which is reminiscent of [XAuX]+/– ions previously observed for much smaller ligands. The [Au(C60)2]+/– ions can be regarded as Au(I) complexes, regardless of whether the net charge is positive or negative, but in both cases, the charge transfer between the Au and C60 is incomplete, most likely because of a covalent contribution to the Au–C60 binding. The C60–Au–C60 dumbbell structure represents a new architecture in fullerene chemistry that might be replicable in synthetic nanostructures.

Spin filter properties of armchair graphene nanoribbons with substitutional Fe atoms

ABSTRACT The spin filter capability of a (0,8) armchair graphene nanoribbon with Fe atoms at substitutional sites is investigated by density functional theory in combination with the non-equilibrium Greens function technique. For specific arrangements, a high degree of spin polarisation is achieved. These include a single substitution at an edge position or double substitution in the central sector of the transmission element. The possibility of switching between majority and minority spin polarisation by changing the double substitution geometry is predicted. Including the bias dependence of the transmission function proves to be essential for correct representation of the spin-resolved current-voltage profiles.