Purification of semiconducting carbon nanotubes.

Since their discovery by Iijima in 1991, carbon nanotubes (CNTs) have attracted great interest in chemistry, physics, electronics, and materials science. As a result of their chemical and mechanical stability, high conductivity, high surfaceto-volume ratio, and other unique properties, CNTs have various potential applications in areas such as field-effect transistors (FETs), sensors, light emitters, and logic circuits, to name but a few. CNTs are cylindrical carbon-molecule structures and can be either semiconducting or metallic depending on their helicities. However, the technological hurdle of synthesizing pure semiconducting CNTs or separating metallic and semiconducting CNTs after growth has hampered the further development of CNT applications because all current synthesis methods for CNTs yield a mixture of semiconducting and metallic CNTs. Extensive studies have been carried out in order to selectively synthesize semiconducting CNTs or to utilize post-growth separation by using various approaches. Among all of them, the finely tuned methane plasma reaction developed very recently by Hongjie Dai and co-workers selectively etches and eliminates the metallic CNTs from a nanotube film without damaging semconducting CNTs with a diameter more than 1.4 nm. This process represents a great breakthrough in the potential scalable manufacture of high-performance nanotube-based electronic devices. Previously, a variety of strategies have been developed, with limited success, to attempt to obtain high-quality, purely semiconducting, and narrow-diameter-distribution (even a unique type) CNTs, including the selective synthesis of semiconducting CNTs and post-treatments after CNT growth. Solution-phase approaches have been developed but are only applicable to suspended nanotubes in solvents. In addition, electrical breakdown can selectively remove metallic nanotubes on the substrate after fabricating electrodes on CNTs. However, scaling up of the treatment is difficult due to the need to apply a gate potential on each device and the need of careful monitoring of the current for such processes. More recently, selective chemical modification has been shown to be effective in obtaining pure semiconducting nanotubes and can be applied to large-scale fabrication of semiconducting nanotube deACHTUNGTRENNUNGvices. Among them, diazonium salts in aqueous solution can selectively react with metallic nanotubes, leaving only semiconducting nanotubes as the conductive elements in thin-film devices. However, the method does not remove the metallic nanotubes physically and the reaction is reversible at higher temperatures. In their new study, Dai and co-workers have demonstrated that pure semiconducting CNTs could be obtained with electrical properties similar to pristine materials. The process involves a selective hydrocarbonation reaction between metallic carbon nanotubes and a methane plasma, followed by an annealing step to break the reacted metallic nanotubes. More importantly, the group discovered that the selectivity of the treatment is diameter dependent. After the CNTs, which had a diameter distribution of 1–2.8 nm, were treated first with methane plasma at 400 8C, they were then annealed under vacuum at 600 8C. The CNTs with diameters less than 1.4 nm were all removed (Figure 1A) because both metallic and semiconducting CNTs are particularly reactive due to their higher radius of curvature and the higher strain that exists in the C C bonding of such CNTs. In the intermediate diameter range (1.4 to 2 nm), only metallic CNTs were removed (Figure 1A), thus leaving only semiconducting CNTs without any damage incurred. Dai and co-workers explained that the semiconducting CNTs are less reactive because of lower formation energies due to the electronic energy gain in opening the bandgap, and due to the higher chemical reactivity of the metallic CNTs, which possess more abundant delocalized electronic states, a finding that was supported by a theoretical study. For large-diameter CNTs, both metallic and semiconducting CNTs survived after the treatment (Figure 1A). This nondiscrimination is due to the smaller difference in chemical reactivity that exists when the diameter of the semiconducting CNTs becomes significantly larger. [*] D. Yuan, Prof. J. Liu Department of Chemistry Duke University, Durham, NC 27708 (USA) Fax: (+1)919-660-1605 E-mail: [email protected]