Defect Healing of Carbon Nanotubes by Rapid Vacuum Arc Annealing
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Defect Healing of Carbon Nanotubes by Rapid Vacuum Arc Annealing Jeff Tsung-Hui Tsai, Jason Li, and Andy Tseng Graduate Institute of Electro-Optical Engineering, Tatung University, No.40, Sec. 3, Jhongshan N. Rd., Taipei, 10452, Taiwan ABSTRACT A rapid thermal annealing process is demonstrated for healing the defects in carbon nanotubes using a DC vacuum arc discharge system. Multi-walled carbon nanotubes (MWCNTs) grown by chemical vapor deposition at a relatively low temperature (~650 °C) showed structural imperfections inside the tubes which are known as "bamboo-like" defects. These defects can be thermally annealed to reconstruct the graphitic structure. A vacuum arc discharge system was used to generate high temperatures (~1800 °C) followed by rapid cooling. The MWCNTs can be rapidly annealed in such a system by several heating and cooling cycles. The annealed samples were characterized by Raman spectroscopy and transmission electron microscopy. The defects were found to be healed when the environment contained water vapor, indicating that oxygen may play an important role in breaking the imperfect graphitic structure and removing the weakly bonded defects during the rapid heating cycles. After breaking the "bamboo" segment, the graphene shell was then reconstructed during the cooling process to produce multi-shell perfection. This method produces effective defect healing and bamboo structure removal from MWCNTs. INTRODUCTION Thermal growth of different carbon nanostructures such as nanocones [1], bundles of double-walled (DWCNT) [2], and single-walled carbon nanotubes (SWCNTs) [3] in high temperatures have been demonstrated by pulsed laser vaporization and also by the electric arc discharge process. Development of novel fabrication methods to produce high quality products or to control the growth of specific type of nanotubes has attracted intense research efforts recently. By high temperature (1600~2000 °C) treatment, the morphological transformation in bundled SWCNTs into MWCNTs was also demonstrated [4]. In this temperature regime, original tubes undergo massive bond rearrangement, transforming into different carbon nanostructures such as graphitic nanoribbons [5]. The purification of MWCNT can be achieved by thermal annealing the defect-contained nanotubes at graphitization temperature [6]. In general, the CNT fabricated by very high temperature (~2800 °C ) through the vaporsolid (VS) growth mechanism such as laser vaporization or electric arc discharge technique, the nanotubes presenting highly structure order and less formation of amorphous carbon on the surface of the outer shell. This can be reveled from its Raman spectra which the intensity of G band (IG, ~1600 cm-1) is much larger than the intensity of D band (ID ~1350 cm-1). Although these fabrication methods provide good quality of CNTs, however the product usually combines
with large amount of graphite clusters or graphite powders. It requires additional purification process to remove these external impurities. On the contrary, when
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