Magnetic-field-induced diameter-selective synthesis of single-walled carbon nanotubes† Yanjie Su, Yaozhong Zhang, Hao Wei, Liling Zhang, Jiang Zhao, Zhi Yang and Yafei Zhang * Received 18th November 2011, Accepted 18th December 2011 DOI: 10.1039/c2nr11783e We report a facile and scalable approach to synthesize single-walled carbon nanotubes (SWNTs) with selected diameter distribution by applying a magnetic field perpendicular to the electric field in the arc plasma region. It is found that this magnetic field-induced diameter-selectivity strategy enables the control of the SWNTs with different diameter distributions in different regions, and the diameter- selective efficiency could be enhanced by modifying the direction of magnetic field. Our results indicate that the motions of the catalysts with different particle sizes, positive carbon ions and electrons are significantly influenced by the magnetic field and electromagnetic force, resulting in the different nucleation and growth processes of SWNTs due to the collective interactions between the magnetic field and arc plasma. This approach would enable a viable route towards the synthesis of SWNTs with desired diameter through the tuning of arc parameters in the arc discharge process. Introduction The superior electronic and optical properties of single-walled carbon nanotubes (SWNTs) strongly depend on their diameter and chiral angle, 1,2 which are identified as the chiral indices (n, m). All SWNTs are divided into semiconducting and metallic depending on all possible (n, m)s, and the bandgap of semi- conducting SWNTs is inversely proportional to the diameter. Therefore, synthesis of the SWNTs with desired chiral angle or diameter distribution is important for the advancement of SWNTs in nanoelectronic applications. 3–5 Based on chemical vapor deposition (CVD) techniques, extensive efforts towards the controllable synthesis of SWNTs with specific diameter distribution or chirality have been made by modifying the cata- lyst type, 6,7 particle size, 8 and growth conditions (such as the temperature, carbon source concentration, and gas flow rate). 6,9–11 However, it is still a great challenge to effectively control the diameter distribution during the growth process; and the as-synthesized SWNTs are known to contain some structural defects. The direct current (DC) arc discharge method is considered to be one of the most efficient techniques for large-scale synthesis of defect-free SWNTs. 12,13 Unfortunately, this method exhibits low tunability and controllability of the SWNT growth process as compared to other SWNT synthetic techniques. However, significant progress in the control of the SWNT growth process has been achieved through applying an axial external magnetic field to the arc plasma. 14–16 Recently, Volotskova et al. 17 demonstrated that the distribution of SWNT chiralities synthe- sized by DC arc discharge was affected by the application of an axial magnetic field (0.2–2 kG) to the region of the arc plasma. Although the plasma density and electron temperature have been changed after applying an axial magnetic field, the tunability of chirality-distribution of the SWNTs is quite limited due to the weak Lorentz force on the arc plasma. In this paper, we report a facile and efficient method for diameter-selective synthesis of SWNTs, in which a weak magnetic field (B) perpendicular to the electric field (E) was applied to the arc plasma in the inter-elec- trode gap. The plasma morphologies were controlled by modi- fying the direction and strength of the magnetic field, for the first time proving that the SWNTs with different diameter distribu- tions can be selectively obtained in different regions. Moreover, this diameter-selective efficiency can be controlled by changing the strength of the applied magnetic field. Experimental The synthesis of SWNTs was performed as described earlier. 12 In all experiments reported in this contribution, the SWNT samples were synthesized with a current of 90 A between the consumable graphite anode with Ni/Y catalyst in a Ni : Y ¼ 4.2 : 1 ratio and the pure graphite cathode under a He buffer gas at a pressure of 50 kPa. SWNT samples were synthesized with and without adding a transverse magnetic field. A permanent magnet with different strengths was used to create a transverse magnetic field perpendicular to the electric field, which was placed outside the arc chamber, the horizontal distance between the magnet and the Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai, 200240, PR China. E-mail: yfzhang@ sjtu.edu.cn; Fax: +86 21 3420 5665; Tel: +86 21 3420 5665 † Electronic supplementary information (ESI) available: Schematic diagram of SWNT sampling points, peak areas of S 22 and M 11 and relative ratios of S/M. See DOI: 10.1039/c2nr11783e This journal is ª The Royal Society of Chemistry 2012 Nanoscale, 2012, 4, 1717–1721 | 1717 Dynamic Article Links C < Nanoscale Cite this: Nanoscale, 2012, 4, 1717 www.rsc.org/nanoscale PAPER Downloaded by Shanghai Jiaotong University on 03 December 2012 Published on 23 December 2011 on http://pubs.rsc.org | doi:10.1039/C2NR11783E View Article Online / Journal Homepage / Table of Contents for this issue