Large-Scale Synthesis of Rings of Bundled Single-Walled Carbon Nanotubes by Floating Chemical Vapor Deposition

Since their discovery, single-walled carbon nanotubes (SWNTs) have attracted considerable attention due to their unique chemical and physical properties, as well as their promise in the area of materials chemistry. Using previously reported synthetic methods, carbon nanotubes (CNTs) are always grown as long strings, and this string shape largely determines their properties. CNTs with annular geometries make for rather unusual superstructures, and these closed-ring systems have been found to exhibit interesting transport properties. Recently, several post-treatment methods, including chemical modification and physical treatment, have been developed to fabricate nanotube rings. Using ultrasonic irradiation, Avouris and co-workers have found that linear CNTs can be folded into nanotube rings with various diameters. Sano et al. have used covalent ring-closure reactions to produce a sizeable number of nanotube rings from etched straight nanotubes. Meanwhile, SWNT rings have also been accidentally observed in directly synthesized materials. Liu et al. have detected trace quantities of rings (0.01–0.1 %) by scanning force and transmission electron microscopy (TEM) in raw SWNT samples made by laser ablation. There has also been a report of SWNT rings formed by solvent evaporation. Most recently, a small number of double-walled carbon nanotube rings have been identified in the products of a solidsolution reaction. However, if these nanotube rings are to be used for practical applications, a key issue that needs to be resolved is their large-scale synthesis with high purity. In this paper, bundles of SWNT rings have been synthesized in high yields by thermally decomposing acetylene at 1100 °C in a floating iron catalyst system. The rings typically have a small average diameter with a narrow size distribution and appear to mostly consist of SWNT toroids. The rings can be deposited on different substrates with varying densities at relatively low temperatures, which is a significant advantage for potential applications such as electronic devices. Raman scattering indicates that the Raman G-mode of the SWNT rings is split into several peaks over a broad frequency range, which can be ascribed to the bending strain of SWNT bundles induced by the curvature of the ring. The synthesis of SWNT rings reported here paves the way for investigating transport, electronic, and optical phenomena in these annular nanotube structures. The typical morphology of the nanotube rings is shown in Figure 1. Figure 1a clearly illustrates the high yield of nanotube rings synthesized by the optimized floating chemical vapor deposition (CVD) method. From many scanning electron microscopy (SEM) images of the products, we estimate that the yield of the grown rings is greater than 70 %, which greatly surpasses the yields previously achieved by other direct-growth techniques. Also, we observe that some of the rings fall on the substrate, while others adhere to the edge. The SEM image in Figure 1b shows that these structures have a well-defined ring configuration. The rings typically have an average diameter of 120 nm and a narrow thickness distribution ranging from 15 to 30 nm, as shown in Figure 1c and d, respectively. It is worth noting that the diameter of the SWNT rings here is much smaller than the previously reported diameters of 300–500 nm observed for rings synthesized by laser ablation and 500–600 nm seen for post-treated nanotube solutions. The smaller diameter of the rings may lead to more interesting quantum effects and electronic properties. C O M M U N IC A IO N S