A. Schilder

Boron-doping effects in carbon nanotubes

When nanotubes form in a carbon arc, the presence of boron results in long boron-doped carbon nanotubes which are generated as dominant zigzags. Metallic behaviour is observed, in contrast to carbon nanotubes, which are semi-conducting.

Metallic behaviour of boron-containing carbon nanotubes

Abstract Paramagnetism in boron-containing carbon nanotubes was detected by SQUID measurements, and a g -value of 2.0020, corresponding to the free carrier spin, was observed over a wide temperature range (108–455 K). This value implies intrinsic metallic behaviour and contrasts with thermally activated semiconduction which multi-walled carbon nanotubes usually exhibit.

Ferromagnetic versus molecular ordering in C60 charge transfer complexes

Abstract [TDAE]C60 (TDAE, tetrakis(dimethylamino)ethylene) was the first and is so far the only C60 charge transfer complex, which exhibits molecular ferromagnetism with a Curie temperature of about 16 K. Though extensively studied a convincing access to the origin of the low-temperature phase is still missing. Three mechanisms for the ferromagnetism will be compared: the classical Heisenberg ferromagnet, configuration interaction and spin polarization. The role of molecular ordering and of the Jahn-Teller distortion of C60 will be discussed.

The role of TDAE for the magnetism in [TDAE]C60

The mechanism leading to the outstanding magnetic property of the organic charge-transfer salt is still a matter of discussion. The scope of this paper is to explore the role of for the magnetic ordering below TC = 16 K and to find a model which is in accordance with the experimental findings. Therefore besides we synthesized slightly altered compounds by either perdeuterating TDAE or replacing C60 by C70 in various concentrations. The magnetic ordering and the electric transport properties in these systems are investigated between 4 and 300 K by ac-susceptibility, ESR, NMR and microwave conductivity. As a result, itinerant magnetic moments can be excluded in the whole temperature range and the radical ion is coupled with by strong spin-exchange and has to be taken into account for a description of the magnetic ordering. Microscopic models assuming a second charge transfer seem to be an appropriate way to describe the experimental results.

Preparation and characterisation of the C60 charge transfer complex C60−.[1,1′,3,3′-tetramethyl-Δ2,2′-bi(imidazolidine)]+.

Abstract Ferromagnetism in the C 60 charge transfer salt C 60 −. [1,1′,3,3′-tetramethyl-Δ 2,2′ -bi(imidazolidine)] +. (C 60 [TMBI]) with Curie temperatures T c above 140 K and coercive forces H c of about 1000 Oe were reported by Wang and Zhu (J. Phys. Chem. Solids 55 (1994) 437). This dramatic increase of T c and H c by a small variation of the counterion compared to the first C 60 ferromagnet C 60 −. [tetrakis(dimethylamino)ethylene] +. (Allemand et al., Science 253 (1991) 301) triggered us to elucidate the charge transfer from TMBI to C 60 by absorption measurements in the VIS/NIR. We found that the reaction kinetics is much more complex than in C 60 [TDAE]. Solid state investigations of C 60 [TMBI] with EPR and microwave conductivity are in clear disagreement with the results published by Wang and Zhu. We found instead strong evidence that C 60 [TMBI] is rather a diamagnetic C 60 CT complex with a lot of structural defects than a high temperature molecular ferromagnet.

Far-infrared study of C60-tetraphenylphosphoniumiodide

We report a far-infrared vibrational study of (Ph 4 P) 2 IC 60 which reveals a room temperature Jahn-Teller distortion of the C 60 monoanion to either a D 5d or D 3d reduced-symmetry point group. Temperature dependences of the intramolecular C 60 and counterion vibrations indicate a weak phase transition of the bulk material occurring between 125 and 150 K.

Perdeuteration of TDAE in [TDAE]C60: A study by ESR and NMR

The mechanism responsible for the feromagnetic-like ordering at 16 K of the organic compound [TDAE]C60 is still a matter of discussion. Especially, the consequence of the donor TDAE for the magnetic ordering is not yet understood. All attempts to replace TDAE by similar organic donors failed in giving a system with such an outstanding magnetic property. Perdeuteration of TDAE is the smallest possible change of the system. AC-susceptibility measurements show that deuteration does not suppress the magnetism. ESR and NMR measurements on [TDAE-d24]C60 give a further evidence that TDAE+ is involved in electron spin exchange processes.