Fabrication, post magnetic annealing treatment and property analyses of the Fe-encapsulated carbon nanostructures

• Main Authors: Yi-Fen Lin, 林怡芬
• Other Authors: Cheng-Tzu Kuo
• Format: Others
• Language: zh-TW
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/97377712620216155872

碩士 === 國立交通大學 === 材料科學與工程系所 === 93 === To develop the potential applications as magnetic media with nano-resolution, the well-aligned Fe-encapsulated carbon nanostructures were synthesized on Si wafer by ECR-CVD with H2 and CH4 as gas sources and Fe as the catalyst and under 875 Gauss magnetic field to maintain the ECR condition. The as-grown nanostructures were further post-annealed in vacuum (10-3 Torr) under a magnetic field of 875 Gauss at 640℃ for 4 hr. The main process parameters include the source gas ratio of H2/CH4 and the substrate bias. The morphologies, structures, bonding structures and magnetic properties of the nanostructures at each processing step were characterized by SEM, HRTEM, XRD, Raman spectroscopy, EDS, AFM, MFM, SQUID and field emission J-E measurements. From the experimental results, the following conclusions can be drawn. Regarding effects of the source gas ratio of H2/CH4, the results show that an increase in H2 flow rate at constant CH4 flow rate or a decrease in CH4 flow rate at constant H2 flow rate are more favor to produce CNTs with shorter length and smaller tube diameter. The effect of H-plasma is essentially to etch preferentially the amorphous carbon than graphite layers of CNTs. Therefore, at higher H-concentration, it implies a lower carbon concentration, lower growth rate and stronger etching effect; thus it results in formation of a shorter and smaller CNTs. In terms of memory density and rigidity of CNTs in perpendicular direction, the smaller and shorter CNTs or particle-like nanostructures are better to be applied in applications for magnetic storage media. Effect of the substrate negative bias is basically to accelerate the positive species, such as H-plasma, to bombard the substrate and also to align the carbon species in an ordered fashion vertically. Therefore, a higher negative bias (> -150 V) will result in a greater etching effect and forming a shorter and smaller well-aligned CNTs or particle-like nanostructures. On the contrary, under the conditions of no or smaller negative bias, the results indicate that carbon films or carbon nanostructures in non-uniform granularity may be formed. Under the present process conditions, the maximum tube number density of CNTs and the maximum number density of nano-particles can go up to 20.6 Gtubes/inch2 with 550 nm in length and 23.2 G/inch2 with 178 nm in height, respectively. Instead of CNTs, it is interesting to note that the particle-like Fe-encapsulated nanostructures could be melted together to become a big particle under focusing electron beam during TEM examination. This may be due to magnetic attraction among Fe catalysts at the tips and melting point decrease of nano-sized particles. Regard to effect of post magnetic annealing treatment at 640oC, the SEM examination shows that the CNTs become cleaner after treatment, which can be manifested from a decrease in Raman ID/IG ratio of CNTs. Furthermore, the XRD patterns indicate that the as-grown CNTs with diffraction signals of simple orthorhombic Fe3C, bcc-Fe and fcc-diamond will become CNTs without Fe signals after annealing treatment, so it gives rise to a decline in coercive force. The results also depict that the coercive force and the shift in hyseresis loop are a decline function of the measuring temperature and are linearly related to each other for both as-grown CNTs and CNTs after annealing treatment. In contrast, the magnetization intensity is also a decline function of the measuring temperature but is not linearly related to each other. The shift of hyseresis loop due to temperature is related to the exchange anisotropy. The average size of the encapsulated Fe catalysts in carbon nanostructures is 66 nm, which is much larger than the critical size (~ 14 nm) for the maximum coercive force of Fe particles. In other words, there are many spaces to improve the process to enhance the coercive force further by decreasing the sizes of CNTs or nano-particles. Although the post annealing treatment is not good for magnetic property improvement, however, it could benefit the field emission properties of CNTs with an increase in current density of three-order in magnitude. This may relate to a decrease in amorphous carbon after magnetic annealing in vacuum. From the AFM and MFM micrographs, it indicates that the Fe-encapsulated nanostructures can be imaged by MFM, indicating the higher possibility to be used for magnetic storage media applications with nano-resolution.