ABSTRACT
Single-wall carbon nanotube (SWCNT) networks have been potentially envisioned as one of promising semiconducting materials for transistors in logic circuit applications. The advantage of using SWCNT random networks as field-effect transistors (FET) is the ease of fabrication, which practically makes cost-effective solution processable and printable electronics feasible. It is challenging to fabricate SWCNT FET on a large scale with reasonably high mobility and full yield of semiconductor devices owing to the co-existence of metallic (M) and semiconducting (S) tubes in its networks or films. This chapter will introduce several major approaches, including specific synthesis of S-SWCNTs, selective elimination or destruction of M-SWCNTs, and photolithography-assisted stripping to intentionally improve the semiconducting characteristics from its networks. Meanwhile, tremendous efforts have been devoted
to sort out M-and S-SWCNTs for preparing S-SWCNT rich ink for solution-based fabrication, including surfactant wrapping, aromatic extraction, amine reaction, selective oxidation, dielectrophoresis, bundled tubes removal, DNA or polymer wrapping, density gradient ultracentrifugation (DGU) and gel-based separation techniques. In addition, methods such as selective modification of SWCNTs with diazonium reagents or organic free radicals are adopted to successfully demonstrate full semiconductor device yield and reasonably high carrier mobility. Fundamental issues which limit the performance of SWCNT networks are also discussed. 8.1 IntroductionSWCNT, which is considered a rolled-up hollow cylinder of graphite monolayer with ~1 nm diameter, are widely regarded to be an outstanding candidate for field-effect transistors (FETs) owing to its unique properties. Based on their roll-up chirality, the electronic structure of SWCNTs is either metallic (M) or semiconducting (S). Due to the extremely high aspect ratio and outstanding nearly defect-free electronic structure, electronic transporting property of SWCNTs is ballistic, resulting in superior field-effect behavior on semi-conducting SWCNTs and its intrinsic carrier mobility can achieve about 100,000 cm2/V s. Owing to its outstanding electrical properties, SWCNTs have been widely anticipated as one of excellent potential candidates for electronic applications. However, the variations of chirality and diameter in SWCNT assembly lead to inconsistence of electrical properties from single tube devices. In addition, the integration of single-tube devices into a circuit is another techno-logical hurdle for realizing its electronic applications. Therefore the development of FETs based on SWCNT networks is intensified with tremendous efforts from researchers. There are several remarkable advantages from SWCNTs devices based on random networks such as ease of production, high reproducibility with consistent electrical properties and high scalability as well as high compatibility with conventional Si technology. Hence, Random SWCNT networks or thin films are considered as ideal semiconducting materials for low-cost carbon based electronics. Despite the great promise, realization of low-cost carbon-based electronics still remains difficult owing to inevitably technological challenges on device fabrication. There are
several major challenges on using SWCNT random networks or thin films for low-cost high-performance electronic applications, such as the following: (1) Coexistence of M-and S-SWCNTs leads to high off-current,
resulting in low on-off current ratio in FET configuration. (2) Relatively low effective field-effect mobility from SWCNT-
FETs is widely attributed to the contact resistance of internanotube junctions existing within SWCNT networks or thin films.
