Artur Böttcher

C58 on HOPG: Soft-landing adsorption and thermal desorption

Electron-impact induced dissociation/ionization of C60 molecules was used to produce an intense beam of C58+ ions. This was directed towards a HOPG (highly oriented pyrolytic graphite) substrate under nominally perpendicular impact conditions in order to generate deposits by soft-landing (kinetic energy < 0.1 eV atom−1). Deposited C58 molecules could subsequently be thermally desorbed intactly. Thermal desorption mass spectra of the deposits exhibit only C58. Surface deposited C58 can react with background gases to generate hydride derivatives C58Hn which are also desorbable. The apparent desorption energy of C58 and C58Hn molecules from the HOPG surface varies with increasing adsorbate coverage from 2 ± 0.1 to 2.2 ± 0.1 eV as determined by a Redhead analysis. These values are 0.7 ± 0.1 eV larger than found for C60 desorbed from the same substrate.

Failure mechanism of free standing single-walled carbon nanotube thin films under tensile load

The mechanical properties of free standing films of as prepared and acid purified single-walled carbon nanotubes were studied by means of tensile stress measurements as well as electron microscopy. Films appear to fail by crack propagation involving rupture or matrix pull-out of bridging nanotube bundles.

IR Absorptions of C60+ and C60– in Neon Matrixes

C60(+) ions were produced by electron-impact ionization of sublimed C60, collimated into an ion beam, turned 90° by an electrostatic deflector to separate them from neutrals, mass filtered by a radio frequency quadrupole, and co-deposited with Ne on a cold 5 K gold-coated sapphire substrate. Infrared absorption spectroscopy revealed the additional presence of C60 and C60(-) in the as-prepared cryogenic matrixes. To change the C60(+)/C60(-) ratio, CCl4 or CO2 electron scavengers were added to the matrix gas. Also taking into account DFT calculations, we have identified nine new previously unpublished IR absorptions of C60(+) and seven of C60(-) in Ne matrixes. Our measurements are in very good agreement with DFT calculations, predicting D5d C60(+) and D3d C60(-) ground states. The new results may be of interest regarding the presence of C60 and C70 (as well as ions thereof) in Space.

Increase of the mean inner Coulomb potential in Au clusters induced by surface tension and its implication for electron scattering

Electron holography in a transmission electron microscope was applied to measure the phase shift {delta}{phi} induced by Au clusters as a function of the cluster size. Large {delta}{phi} observed for small Au clusters cannot be described by the well-known equation {delta}{phi}=C{sub E}V{sub 0}t (C{sub E}, interaction constant; V{sub 0}, mean inner Coulomb potential (MIP) of bulk gold; and t, cluster thickness). The rapid increase of the Au MIP with decreasing cluster size derived from {delta}{phi} can be explained by the compressive strain of surface atoms in the cluster.

Properties of non-IPR fullerene films versus size of the building blocks

This perspective focuses on the cage size dependent properties of novel solid fullerene nanofilms grown by soft-landing of mass-selected C(n)(+) (48, 50, 52, 54, 56, 58, 62, 64, 66 and 68) onto room temperature graphite surfaces under ultra-high vacuum conditions. Such non-isolated-pentagon-ring (non-IPR) fullerene materials are not accessible to standard fullerene preparation methods. The component molecular building blocks of non-IPR films were generated by electron impact induced ionization/fragmentation of sublimed IPR-C(70)(D(5h)) (-->C(n) (n = 68, 66, 64, 62)) or IPR-C(60)(I(h)) (-->C(n) (n = 58, 56, 54, 52, 50)). Non-IPR fullerene films on graphite grow via formation of dendritic C(n) aggregates, whereas deposition of IPR fullerenes under analogous conditions (via deposition of unfragmented C(60)(+) and C(70)(+)) leads to compact islands. The latter are governed by weak van der Waals cage-cage interactions. In contrast, the former are stabilized by covalent intercage bonds as mediated by the non-IPR sites (primarily adjacent pentagon pairs, AP). A significant fraction of the deposited non-IPR C(n) cages can be intactly (re)sublimed by heating. The corresponding mean desorption activation energies, E(des), increase from 2.1 eV for C(68) up to 2.6 eV for C(50). The densities of states in the valence band regions (DOS), surface ionization potentials (sIP) and HOMO-LUMO gaps (Delta) of semiconducting non-IPR films were measured and found to vary strongly with cage size. Overall, the n-dependencies of these properties can be interpreted in terms of covalently interconnected oligomeric structures comprising the most stable (neutral) C(n) isomers-as determined from density functional theory (DFT) calculations. Non-IPR fullerene films are the first known examples of elemental cluster materials in which the cluster building blocks are covalently but reversibly interconnected.

Nanostructuring the graphite basal plane by focused ion beam patterning and oxygen etching

Ga+ focused ion beam (FIB) patterning was used to structure highly oriented pyrolytic graphite surfaces with square, periodic arrays of amorphous carbon defects (mesh sizes: 300 nm–2 µm). Controlled oxygen etching of these arrays leads to matrices of uniform, orientationally aligned, nm-sized, hexagonal holes. The properties of the resulting hole assembly (hole depths and lateral hole dimensions) have been investigated by means of atomic force microscopy, scanning electron microscopy and FIB sectioning. The hole dimensions and uniformity both depend on the FIB parameters and etching conditions. Etching temperatures from 500 to 700 °C were applied. Initial etch rates of up to 106 C s−1 per individual hole were observed when using oxygen pressures of 200 mbar. For an etch temperature of 590 °C the rate of etching of individual holes was found to depend measurably on the inter-hole separation. This confirms that the associated reaction kinetics is mediated by the finite diffusion length of reactive oxygen species along the graphite basal plane. Prolonged etching results in hole–hole contact and generation of mesa arrays of controllable size and shape.

Solid C58 films

A new solid material has been created in ultra high vacuum by utilizing the aggregation process of C58 molecules deposited onto highly oriented pyrolytic graphite from a mass selected low-energy ion beam comprising C58+. Cluster fluxes of up to 3x10(11) ions s-1 cm-2 with impinging kinetic energies of 6+/-0.5 eV were typically applied. Growth of the solid C58 phase proceeds according to the cluster-aggregation-based Volmer-Weber scenario where initially ramified 2D islands transform into 3D pyramid-like structures at higher coverages. The C58 films created exhibit much higher thermal stability than the C60 solid phase. Sublimation of C58 sets in at a temperature of 700 K. Ultraviolet photoionization spectra (He I, 21.2 eV) yield a molecular ionization potential in the range between 6.6 and 7 eV. Density functional and Hartree-Fock theories suggest that the formation of C58 dimers and higher multimers upon deposition/aggregation gives rise to the high thermal stability and unique electronic properties of this material.

Non-IPR C60 solids.

Films comprising predominantly novel isomers of C(60) [=C(60)(nIPR)] have been generated by low energy ion beam deposition of vibronically excited C(60)(+) onto graphite followed by selective sublimation of C(60)(I(h)) from the deposited isomer mixture. The incident ions were generated by electron impact ionization/fragmentation of sublimed C(70). Images of the C(60)(nIPR) films obtained by applying atomic force microscopy show aggregates, which we attribute to covalently interlinked C(60)(nIPR) units. The covalent bonds are inferred from the significantly higher thermal stability of the C(60)(nIPR) films compared to the C(60)(I(h)) van der Waals solid-as measured by thermal desorption with mass spectrometric detection of the C(60) mass channel (the only desorbable species). In contrast to the characteristic doublet structure of the occupied valence band in the ultraviolet photoelectron spectrum of pure C(60)(I(h)), the valence band of C(60)(nIPR) films exhibits a triplet feature with the additional peak occurring at a binding energy of approximately 2.6 eV. This is an indicator of the electronic modifications induced by intermolecular bonding. C(60)(nIPR) films exhibit a narrower band gap than found for C(60)(I(h)). They also have significantly different chemical reactivity toward incorporation of thermal energy deuterium atoms. In order to model the experimental photoelectron spectra, various covalently linked oligomers of (#1809)C(60)(C(2v)), the second most stable conventional 60-atom fullerene cage, were calculated by means of the density functional theory. These spectral predictions together with analogous previous observations on related fullerene solids such as C(58) lead us to infer that C(60)(nIPR) films consist of fullerene cage isomers containing one or more adjacent pentagon pairs, which mediate covalent cage-cage interconnection.

C58 on Au(111): A scanning tunneling microscopy study

C58 fullerenes were adsorbed onto room temperature Au(111) surface by low-energy (~6 eV) cluster ion beam deposition under ultrahigh vacuum conditions. The topographic and electronic properties of the deposits were monitored by means of scanning tunnelling microscopy (STM at 4.2 K). Topographic images reveal that at low coverages fullerene cages are pinned by point dislocation defects on the herringbone reconstructed gold terraces (as well as by step edges). At intermediate coverages, pinned monomers act as nucleation centres for the formation of oligomeric C58 chains and 2D islands. At the largest coverages studied, the surface becomes covered by 3D interlinked C58 cages. STM topographic images of pinned single adsorbates are essentially featureless. The corresponding local densities of states are consistent with strong cage-substrate interactions. Topographic images of [C58]n oligomers show a stripe-like intensity pattern oriented perpendicular to the axis connecting the cage centers. This striped pattern becomes even more pronounced in maps of the local density of states. As supported by density functional theory, DFT calculations, and also by analogous STM images previously obtained for C60 polymers [M. Nakaya, Y. Kuwahara, M. Aono, and T. Nakayama, J. Nanosci. Nanotechnol. 11, 2829 (2011)], we conclude that these striped orbital patterns are a fingerprint of covalent intercage bonds. For thick C58 films we have derived a bandgap of 1.2 eV from scanning tunnelling spectroscopy data confirming that the outermost C58 layer behaves as a wide band semiconductor.

IR, NIR, and UV Absorption Spectroscopy of C60(2+) and C60(3+) in Neon Matrixes.

C60(2+) and C60(3+) were produced by electron-impact ionization of sublimed C60 and charge-state-selectively codeposited onto a gold mirror substrate held at 5 K together with neon matrix gas containing a few percent of the electron scavengers CO2 or CCl4. This procedure limits charge-changing of the incident fullerene projectiles during matrix isolation. IR, NIR, and UV-vis spectra were then measured. Ten IR absorptions of C60(2+) were identified. C60(3+) was observed to absorb in the NIR region close to the known vibronic bands of C60(+). UV spectra of C60, C60(+), and C60(2+) were almost indistinguishable, consistent with a plasmon-like nature of their UV absorptions. The measurements were supported by DFT and TDDFT calculations, revealing that C60(2+) has a singlet D5d ground state whereas C60(3+) forms a doublet of Ci symmetry. The new results may be of interest regarding the presence of C60(2+) and C60(3+) in space.