The inset shows a schematic illustration of a SWCNT growing from a W–Co alloy particle. (b) Relative abundances of various helicities of SWCNTs grown from W–Co alloy catalyst particles. The color scale indicates the maximum reported abundance for that particular SWCNT. (a) Chiral index map showing the ( n, m) indices of all the SWCNTs that have been grown with high purity. Recent advances toward helicity-controlled SWCNT growth. An emerging application in this space is conductive SWCNT-based inks and thin film transistors for flexible electronic applications. (5) As can be seen in Figure 1, most of the existing technologies such as reinforced composites, Li-ion batteries, and electromagnetic (EM) shielding use randomly aligned, mixed helicity/diameter SWCNTs. Over the past decade, global CNT production, primarily centered on low-cost multiwalled CNTs (MWCNTs), has increased 10-fold and is predicted to rise to over 15 kilotons per year by 2020. These SWCNTs are mostly processed in powdered form and available from a number of suppliers (c.f. At one end lie mixed diameters and helicities as well as randomly aligned nanotubes. Figure 1 shows the SWCNT application space (with existing and emerging applications in square and oval boxes, respectively) in relation to their physical properties (diameter and helicity) and organizational architecture. Owing to their high aspect ratios, SWCNTs can be assembled in a variety of morphologies ranging from individual tubes to macroscopic architectures such as vertically aligned mats and fibers, making them useful in a wide variety of applications. Because of their symmetry, the tubes whose indices are such that n-m is a multiple of 3 are metallic SWCNTs, while all others are semiconducting, with bandgaps inversely proportional to their diameters. (4) The helicity of the “rolled-up” nanotube is determined by χ SWCNTs with χ equal to 0° and 30° are called zigzag ( m = 0) and armchair ( n = m) nanotubes, respectively, and tubes with all other chiral angles are called chiral SWCNTs ( n ≠ m). The structure of a SWCNT can be represented by a set of indices, ( n, m), corresponding to multiples of the graphene unit cell vectors that make up the chiral vector or the circumference of the tube.
Importantly, they can be semiconducting or metallic depending on their helical angle (χ), that is, the angle between the tube axis and the edge of the graphene lattice. (1−3) They possess a range of exceptional properties including high strength (∼37 GPa), thermal conductivity (∼3500 W/m/K), and ballistic electronic transport. Single-walled carbon nanotubes (SWCNTs), which can be considered as seamless cylinders of graphene, have been at the forefront of nanotechnology research for the past two decades.
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