Study of morphology and control of electric structure for carbon based hetero-nanotubes synthesized under high magnetic field

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Carbon nanotubes were synthesized by thermal CVD under high magnetic fields of 0T,1T,5T and 10T on the SiO_2 and Si substrates using a magnetic-field-proof furnace. The C_2H_2 gas pressure and temperature were 800℃ and 30 Torr, respectively. Although subtle differences between 0T and the presence of magnetic field were discernible in the SEM images, it was difficult by SEM to precisely evaluate changes of tube length and production yield under magnetic field. The growth directions of the nanotubes and nanowires were randomly oriented in all the cases. Differences in the presence of magnetic field were scarcely discernible by SEM for the deposits under low gas pressure (10 Torr) and low temperature (10 Torr and 700℃) using Ni Although an increase of onion-like structures was discernible in TEM images for the deposits under the magnetic field, obvious magnetic effects were not observed. Accordingly, synthesis was carried out under 700℃ and 10 Torr using the Fe catalyst. The production yield of the deposits under the magnetic field decreased in the case of Fe as the catalyst, whereas the Ni catalyst did not cause such a magnetic effect. A difference between Ni and Fe is Curie temperature, which is 358℃ for Ni and 769℃ for Fe. Since the synthesis temperature was 700℃, ferromagnetism of the Fe catalyst should be maintained at the synthesis temperature. It was suggested that the magnetic field influences on formation processes of nanotubes via magnetic effects on metal catalysts. For instance, a radical pair mechanism, is possible : a singlet-triplet(S-T) spin transition for precursors generated by thermal decomposition may occur under high magnetic field. This mechanism results in the magnetic effects due to the anisotropies in g-values and hyperfine interaction constants of radicals.