Dreiecksstruktur: Die ungewöhnlichen magnetischen Gesamtmomente der endohedralen Clusterfullerene Ho3N@C80 und Tb3N@C80 werden durch starke Ligandenfelder innerhalb der (M3N)-Cluster erklärt. Als Ergebnis dieser Wechselwirkungen sind die einzelnen magnetischen Momente m der Metallionen (M=Ho oder Tb) nicht parallel oder antiparallel zueinander, sondern parallel zu den M-N-Bindungen ausgerichtet (siehe Schema).

Magnetization M(H, T) measurements on R3N@C-80 (R = Ho and Tb) for magnetic fields mu(0)H up to 5 T and temperatures T down to 1.8 K are presented. For both fullerenes, the M(H,T) curves depend on the ratio HIT only and are well described by Brillouin functions with J = 16 and g = 1.25 for Ho3N@C-80 and J = 12, g = 1.5 for Tb3N@C-80. The corresponding moments per cage of 20 and 18 mu(B), respectively, cannot be explained by a collinear ferromagnetic or antiferromagnetic alignment of the three R magnetic moments in the encaged cluster (R3N). The observed moments are explained by strong ligand fields acting on the individual, ferromagnetically coupled R moments within the (R3N) cluster. (c) 2004 Published by Elsevier B.V.

New clusters in endohedral fullerenes can stabilize fullerene cages which are not to be isolated in the empty form like C-72 or C-74 and even the non-IPR structures C-66 and C-68. On the other hand fullerenes can provide an ideal isolated environment with its internal space, which enables the stabilization of clusters such as Sc2C2 (Sc2C2@C-84) or Sc3N (Sc3N@C-80). To open the route for new endohedral fullerene structures we report on the influence of the type of the atmosphere in the arc burning process as well as on new nitride cluster fullerenes like Ho3N@C-80. In this way a synthesis of fullerenes is described for the first time where the endohedral structures are the main product (yield in the range of 70 to 90 %) of all fullerenes. The isolated endohedral structures were studied spectroscopically as well as by cyclic voltammetry. The results of spectroscopic data were compared with the data for other endohedral fullerenes and the interactions of the clusters with cage are analysed. As a conclusion of the synthesis of new endohedral fullerenes by variation of the atmosphere in the burning chamber a general outlook of the potentialities of this new route in fullerene production is given.

The adsorption of the endohedral fullerene, $\mathrm{La}@{\mathrm{C}}_{82},$ and the higher fullerene, ${\mathrm{C}}_{84},$ on $\mathrm{Si}(100)\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}1$ is investigated using a scanning tunneling microscope (STM) operating in ultrahigh vacuum. Both molecules are found to adsorb directly above the dimer rows that are formed on the $\mathrm{Si}(100)\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}1$ surface, as well as in trough sites midway between dimer rows. Adsorption above dimer rows, not observed for ${\mathrm{C}}_{60},$ is attributed to the larger radius of curvature of these fullerene cages. The response of $\mathrm{La}@{\mathrm{C}}_{82}$ to manipulation by the tip of the STM is also investigated. Molecules in either adsorption site may be manipulated with a threshold gap impedance \ensuremath{\sim}1.0 G\ensuremath{\Omega}. Owing to a near-commensurability between the molecular diameter of $\mathrm{La}@{\mathrm{C}}_{82}$ and the lattice constant of the Si(100) surface, close-packed arrangements of molecules may be formed.

focus of fundamental and applied materials research since the first preparation of macroscopic C60 quantities by Krätschmer et al. in 1990 [1]. The ability to encage atoms and small molecules opened the door to new structures which are highly reactive under standard conditions. Up to now low production yields and a time consuming multi stage separation hampered the search for applications of endohedral fullerenes. Fullerenes with a triscandium nitride cluster inside as for instance Sc3N@C80 (Fig. 1) have been discovered in 1999, when molecular nitrogen was entering a fullerene reactor accidentally [2]. In our present work the influence of the reactor atmosphere on the fullerene formation and distribution has been studied systematically. Various nitrogen sources in combination with different group 3 and rare earth metals have been investigated. A significant enhancement of the Sc3N@C80 yield in the fullerene extract was first obtained replacing N2 by calcium carbamide. Even a selective Sc3N@C80 formation with relative yields up to 90 % was achieved by the use of ammonia gas. Our „reactive atmosphere fullerene burning method“ has been successfully applied to produce other trimetal nitride fullerenes, e.g. Ho3N@C80, Er3N@C80 or Er2Sc@C80 as the main fullerene structure in the soot extract [3]. This is illustrated by the high-performance liquid chromatogram in Fig. 2, which is strongly dominated by the Ho3N@C80 peak. By a single extraction step to remove hydrocarbon byproducts these endohedrals are available in reasonable purities. To provide analytical grade fullerenes only one chromatographic separation step is required, compared to the multistage standard technique for the isolation of endohedral fullerenes. Our approach represents an important progress towards a low cost synthesis of these fascinating structures.

We report a study of the electronic structure and charge transfer in the metallofullerene Sc3N@C-80 using photoemission and x-ray absorption spectroscopy. Through a comparison of the x-ray absorption spectrum of Sc3N@C-80 at the Sc L-2,L-3 edge with atomic multiplet calculations, the Sc 3d electron count is determined to be 0.6, thus giving an effective Sc valency of 2.4. With the N atom gaining a full electronic shell by means of covalent bonding with the Sc (also involving the Sc 3d electron density observed in the x-ray absorption experiments), the remaining six valence electrons of the Sc3N cluster are then transferred to the carbon cage which stabilizes the C-80 cage structure with I-h symmetry, a structure which is not energetically favored in neutral C-80. The presence of the highly symmetric I-h cage structure is further supported by the observation of distinct fine structure in the valence band photoemission spectra of the endohedral, which results from the high degree of effective degeneracy of the electronic states in the molecule. Finally, the results of investigations of K-doped Sc3N@C-80 using photoemission give insight into the KxSc3N@C-80 phases that are formed upon intercalation.

Four rate-limiting processes for the formation of single-wall carbon nanotubes (SWCNT) could be identified by varying furnace temperature, gas type, and pressure in a pulsed-laser evaporation setup. One rate-limiting process accounts essentially for all relevant gas-pressure dependencies and can be quantitatively described using a single gas-specific constant. One thermally activated process is related to fullerene formation, whereas another process, following a T-2-law, is discussed in terms of the diffusion of carbon through molten catalyst nanoparticles. The data provide strong support for an "undercooled melt" mechanism of nanotube formation.

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Structure and stability of endohedral fullerene Sc3N@C80 were studied by temperature-dependent Raman and infrared spectroscopy as well as by quantum-chemical [density-functional-based tight-binding] calculations. The material showed a remarkable thermal stability up to 650 K. By both theory and experiment, translational and rotational Sc3N modes were found. These modes give a direct evidence for the formation of a Sc3N–C80 bond which induces a significant reduction of the ideal Ih–C80 symmetry. From their splitting pattern a crystal structure with more than one molecule in the unit cell is proposed. According to our results: (i) a significant charge transfer from the Sc3N cluster to the C80 cage; (ii) the strength of three Sc–N bonds; (iii) the chemical bond between triscandium nitride cluster and C80 cage; and (iv) a large HOMO–LUMO gap are responsible for the high stability and abundance of Sc3N@C80.

We have studied the electronic structure and charge transfer properties of the monometallofullerene Sc3N@C-80 using photoemission spectroscopy. In this metallofullerene the C-80 cage with I-h symmetry is stabilised by the inclusion of the Sc3N unit. Investigation of the pristine material using core level photoemission shows a strong, symmetric broadening of the C1s line compared to C-60 and a chemical shift of the Sc 2p doublet equivalent to that in Sc-2@C-84. This is a first hint for a similar charge transfer from the Sc ions in Se3N@C-80 and Sc-2@C-84. The valence band photoemission shows a distinct fine structure and an onset of the HOMO at about 1 eV binding energy. This clearly indicates a closed shell structure. Finally, we also present first photoemission studies of the combinational doping of the Sc3N@C-80 system, here realized via potassium intercalation.

Among the rare earth metals samarium is expected to form an ion of the two-valent redox state in endohedral fullerene structures. As compared to thulium and europium metallofullerenes a similar distribution of carbon cage structures is expected for samarium.The samarium fullerene structures produced by the Kratschmer-Huffman method were studied with respect to the influence of the Sm-carbon ratio on the type and yield of the metallofullerenes. The chromatographic separation carried out by a two step HPLC resulted in a larger family of Sm-fullerenes (C-2n, 2n = 74, 78, 82, 84, 86, 88, 90, 92) encapsulating one ion as detected by mass spectrometry. Only the Sm-C-76 structure was missing. No dimetallofullerenes of Sm were found.The samarium structures of C-74 and C-82 were characterised by UV-Vis and IR spectroscopy. The results were compared with those of Eu@C-74 and Tm@C-82. The redox state of the metal ion in the metallofullerene was shown to be Sm2+.

With respect to its redox state cerium is of high interest in metallofullerene research as its preferable redox states are 3+ and 4+. As representative structures of the cerium fullerene family both Ce-2@C-72 and Ce@C-82 were prepared by the Kratschmer are burning method. The metallofullerene Ce-2@C-72 was isolated for the first time using a two stage HPLC separation technique. The UV-Vis-NIR, IR and ESR spectra were compared with those of other C-72 cage metallofullerenes. The existence and stability of the Ce-2@C-72 structure supports the assumption that the C-72 carbon cage can be stabilised by metal ions.The endohedral fullerene Ce@C-82 was also isolated by two stage HPLC and characterised by W-Vis-NIR, IR and ESR spectroscopy for comparison with other endohedral C-82 fullerenes. The redox properties of this metallofullerene structure were studied by cyclic voltammetry.

While endohedral fullerenes of the type Me@C-82 were extensively investigated the structures with lower carbon cage sizes are of high interest to study the role of the carbon cage on their structure and physical properties. The existence of C-72 and C-74 metallofullerene structures differs from the availability of empty fullerenes which were not isolated up to now.As model structures La-2@C-72, TmC76 and TmC78 were chosen to study the correlation between the type of the metal incorporated and the metallofullerene cage size. The structures were prepared by the Kratschmer are burning method and isolated by a multistep HPLC separation technique. W-Vis-NIR and infrared spectroscopy were used at room temperature. It is shown that some metals which are present in the 2+ redox state in fullerenes (Tm) form stable exohedral structures of C-78.