Carbon nanotubes (CNTs), comprising multiple concentric shells and termed multi-wall carbon nanotubes (MWNTs), were discovered by Iijima in 1991 (Iijima, Nature 1991, 354, 56). Subsequent to this discovery, single-wall carbon nanotubes (SWNTs), comprising a single graphene rolled up on itself, were synthesized in an arc-discharge process using carbon electrodes doped with transition metals (Iijima et al., Nature 1993, 363, 603; and Bethune et al., Nature 1993, 363, 605). These carbon nanotubes (especially SWNTs) possess unique mechanical, electrical, thermal and optical properties, and such properties make them attractive for a wide variety of applications. See Baughman et al., Science, 2002, 297, 787-792.
The diameter and chirality of CNTs are described by integers “n” and “m,” where (n,m) is a vector along a graphene sheet which is conceptually rolled up to form a tube. When n−m=3q, where q is an integer, the CNT is a semi-metal. When n−m=0, the CNT is truly metallic in its behavior and referred to as an “armchair” nanotube. All other combinations of n−m are semiconducting CNTs with bandgaps in the range of 0.5 to 1.5 eV (0.8-1.4 eV for HiPco® SWNTS). See O'Connell et al., Science, 2002, 297, 593. CNT “type,” as used herein, refers to such electronic types described by the (n,m) vector (i.e., metallic, semi-metallic, and semiconducting).
The main hurdle to the widespread application of CNTs, and SWNTs in particular, is their manipulation according to electronic structure (Avouris, Acc. Chem. Res. 2002, 35, 1026-1034). All known preparative methods lead to polydisperse materials of semiconducting, semimetallic, and metallic electronic types. See M. S. Dresselhaus, G. Dresselhaus, P. C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, 1996; Bronikowski et al., Journal of Vacuum Science & Technology 2001, 19, 1800-1805; R. Saito, G. Dresselhaus, M. S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998. Recent advances in the solution phase dispersion (Strano et al., J. Nanosci. and Nanotech., 2003, 3, 81; O'Connell et al., Science, 2002, 297, 593-596) along with spectroscopic identification using bandgap fluorescence (Bachilo et al., Science, 2002, 298, 2361) and Raman spectroscopy (Strano, Nanoletters 2003, 3, 1091) have greatly improved the ability to monitor electrically distinct nanotubes as suspended mixtures and have led to definitive assignments of the optical features of semiconducting (Bachilo et al., Science, 2002), as well as metallic and semi-metallic species (Strano, Nanoletters, 2003).
Techniques of chemically functionalizing CNTs have greatly facilitated the ability to manipulate these materials, particularly for SWNTs which tend to assemble into rope-like aggregates (Thess et al., Science, 1996, 273, 483-487). Such chemical functionalization of CNTs is generally divided into two types: tube end functionalization (Chen et al., Science, 1998, 282, 95-98), and sidewall functionalization (PCT publication WO 02/060812 by Tour et al.).
While separation of CNTs by electronic type has been reported (Smalley et al., PCT Publication No. WO 03/084869 A2; Krupke et al., Science, 2003, 301, 344-347), such methods do not appear to be amenable to scale-up. It would be extremely useful to have a separation technique that affords such separation on a bulk scale, particularly wherein such a technique additionally possesses the operational flexibility afforded it by selective functionalization.