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

在无磁场条件下，在SiO_2和Si衬底上分别施加0T、1T、5T和10T的强磁场，用热化学气相沉积法制备了碳纳米管。C_2H_2气体压力为800℃，温度为30Torr。虽然在扫描电子显微镜图像上可以看出磁场作用下管子长度和产量的细微差别，但很难准确地评价磁场作用下管子长度和产量的变化。在所有情况下，纳米管和纳米线的生长方向都是随机取向的。在低气压(10Torr)和低温(10Torr和700Torr)下使用镍的镀层在扫描电子显微镜下几乎看不到磁场存在的差异，虽然在磁场下的透射电子显微镜图像中可以看出洋葱状结构的增加，但没有观察到明显的磁效应。相应地，在700Torr和10Torr的条件下，使用Fe催化剂进行了合成。当Fe作为催化剂时，在磁场作用下沉积产物的产率降低，而Ni催化剂不会引起这种磁效应。Ni和Fe的差别在于居里温度，Ni为358℃，Fe为769℃。由于合成温度为700℃，Fe催化剂的铁磁性应保持在合成温度。结果表明，磁场通过对金属催化剂的磁效应影响纳米管的形成过程。例如，自由基对机制是可能的：在强磁场下，热分解产生的前体可能发生单重态-三重态(S-T)自旋转变。这一机制是由于自由基的g值各向异性和超精细相互作用常数引起的磁效应。

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.