ABSTRACT
Amazingly, Hersam’s group has adopted a sorting technique known as density gradient ultracentrifugation (DGU), which basically exploits a gradient density medium to create density gradient within the centrifuge tube and SWCNTs are exclusively separated by their buoyant density after ultracentrifugation.62 Theoretically, SWCNTs diffuse to their respective isopynic points, in which SWCNT density matches the density of the gradient within the gradient via ultracentrifugation resulting in accomplishment of SWCNTs separation. According to their buoyant density, SWCNTs are differentiated in several layers in a centrifuge tube. A hydrodynamic model is adopted to describe the motion of SWCNTs and surfactant encapsulation surrounding SWCNTs during DGU.63 Sorting SWCNTs by diameter via DGU separation technique was primarily demonstrated in DNA-SWCNT hybrids system.62 Despite high selectivity on SWCNT diameter, this system fundamentally has several issues such as irreversible wrapping DNA surrounding SWCNTs obstructing recovery of its intrinsic properties, high cost of DNA limiting the potential to be a promising separation technique in future applications, and limitation on inability to disperse large-diameter (>1.2 nm) SWCNTs. Therefore, it is crucial to search a replacement for DNA in dispersing SWCNTs, and the most widely used candidate is a surfactant such as sodium dodecyl sulfate (SDS), sodium dodecylbenzenesulphonate (SDBS) and sodium cholate (SC). The pioneer research group on discovery of DGU separation technique has reported that the achievement of SWCNTs diameter and electronic sorting via DGU can be realized with the aid of SC surfactant encapsulation and co-surfactant system mixtures of SC and SDS (4:1 wt % ratio), respectively64 as shown in Figure 8.15. For the surfactant assisted DGU, it is speculated that the sorting of SWCNTs is likely due to the non-uniform binding of the two surfactants as a function of the SWCNT polarizability. The fabricated CNT network FET (CN-FET) devices with transfer-print technique possess impressive electrical characteristic with its mobility 20 cm2 V-1 s-1 and on-off ratio greater than 4 orders,64 respectively, as shown in Figure 8.15.
