S ince their discovery, single-walled carbon nanotubes (CNTs) have attracted major research interest due to their extraordinary mechanical, chemical, and electronic properties. 1 They are metallic or semiconducting depending on their chirality, and as-synthesized material is normally a mixture of both types. For many applications, however, purified samples of only a certain type are in high demand. Purified semicon-ducting tubes are required, for instance, to achieve a large on/off ratio and high carrier mobility in thin-film field-effect transistors 2À7 and high power conversion efficiency for photovoltaics. 8,9 Moreover, for optoelectro-nic applications working in a specific wavelength range, the sorting of semiconduct-ing CNTs according to diameter is of great importance. In view of such demands, methods for the selective synthesis of CNTs of a certain electronic type or chiralites have been developed. 10,11 A low-cost mass production of selected CNTs is yet to be achieved, however, and postsynthesis methods are often relied on. 12 A promising postsynthesis selection method discovered recently is based on the physisorption of polymers on the surface of CNTs, which has the advantage of leaving the electronic properties of the CNT nearly unperturbed. 12 There is a relatively long history of using polymers to disperse CNTs in aqueous or organic solutions. 13,14 A recent finding is that, by using suitable polymers, CNTs can be selectively dispersed either for a specific diameter range or for certain chiral angles. 12 Among those tested, the π-conjugated polymer group of polyfluorene derivatives shows the ability to selectively disperse semicon-ducting CNTs. 15À23 In particular, the dioctyl-substituted polyfluorene (PFO) used with toluene as solvent prefers to disperse small-diameter semiconducting nanotubes with chiral angles larger than about 20°. 15 With longer side chains, larger-diameter tubes can be dispersed but the chiral angle preference is gradually lost. 6,20 More recently, copolymers of polyfluorene with anthracene or pyridine groups were found ABSTRACT The mechanism of the selective dispersion of single-walled carbon nanotubes (CNTs) by polyfluorene polymers is studied in this paper. Using extensive