Diameter-control of single-walled carbon nanotubes produced by magnetic field-assisted arc discharge Yanjie Su, Yaozhong Zhang, Hao Wei, Zhi Yang, Eric Siu-Wai Kong, Yafei Zhang * Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, Research Institute of Micro/Nanometer Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, China ARTICLE INFO Article history: Received 2 December 2011 Accepted 7 February 2012 Available online 14 February 2012 ABSTRACT We have demonstrated a scalable approach to synthesize single-walled carbon nanotubes (SWCNTs) with selected diameter distributions by applying a magnetic field perpendicular to the electric field in the arc plasma. It is found that the purity and orientation of SWCNTs can be controlled by the magnetic field. SWCNTs with different diameter distributions can be separated into two different regions by the applied magnetic field, and the diameter- selection efficiency is improved by modifying the direction of the magnetic field. Our find- ings suggest that the motion of the catalyst particles with different sizes, positive carbon ions and electrons are significantly influenced by Lorentz forces, resulting in the difference in the growth processes of the SWCNTs due to the collective interactions between the arc plasma and the magnetic field. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Single-walled carbon nanotubes (SWCNTs) can be semicon- ducting or metallic depending on their diameter and chiral angle [1–2], which are identified as the chiral indices (n,m). Moreover, the bandgap energies of semiconducting SWCNTs are inversely proportional to their diameters. However, the as-grown SWCNTs always come as a mixture of nanotubes with different chiralities, resulting in the heterogeneous per- formance of SWCNT-based devices. This presents a major obstacle to many advanced applications of SWCNTs (such as field-effect transistors [3], solar cells [4,5], and optical sen- sors [6]). Although specific SWCNTs with controlled chiralities can be successfully achieved through selective oxidation [7], density gradient ultracentrifugation [8,9], and gel chromatog- raphy [10], these methods still suffer from setbacks such as low yield and high cost. In order to obtain SWCNT devices with homogeneous properties, one must be able to prepare SWCNTs with tailor-made chiral angle or diameter. Within the last decade, controllable synthesis of SWCNTs with nar- row-chirality distribution has been widely investigated by chemical vapor deposition (CVD) with flexible control of reac- tion parameters (such as catalyst type and particle size, growth conditions, and carbon source feedstock/concentra- tion) [11–14]. The as-synthesized SWCNTs, however, are still known to contain some structural defects. Direct current (DC) arc discharge method, generally used to synthesize defect-free SWCNTs, is having difficulty to be utilized for controllable synthesis of SWCNTs with a narrow-chirality distribution, due to the complicated nucleation and growth issues [15,16]. Our earlier investigation suggested that low-pressure CO can influence the catalyst formation and carbon precipita- tion, resulting in controllable synthesis of SWCNTs with nar- row diameter distribution [17]. Recently, we have demonstrated that the arc plasma morphology responds strongly to transverse magnetic field due to electromagnetic interactions, i.e., arc plasma parameters can be easily con- trolled by the transverse magnetic field. Volotskova et al. [18] reported that the chirality distribution of SWCNTs 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2012.02.013 * Corresponding author: Fax: +86 21 3420 5665. E-mail address: yfzhang@sjtu.edu.cn (Y. Zhang). CARBON 50 (2012) 2556 – 2562 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon