Dielectrophoretic Deposition and Alignment of Carbon Nanotubes

The carbon nanotube (CNT) is a unique form of carbon material. Since its discovery, CNT has been intensively studied due to its remarkable electrical, mechanical, thermal, and chemical properties (Iijima, 1991; Katz & Willner, 2004). Based on the structures and dimensions, CNTs can be divided into two groups: single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). An SWNT is one tube of graphene capped at both ends and it consists of only surface carbon atoms; its diameter is in the range of 1-2 nm. An MWNT is composed of multiple coaxial tubes of SWNTs and its diameter is often in the range of 10-150 nm. CNTs have a high potential in a broad range of applications, especially in nanoelectronics and biomedical sensors. A wide variety of electronic devices based on individual CNTs or CNT thin films have been developed and used as sensors (Boul et al., 2009; Wang et al., 2009; Xue & Cui, 2008b), field-effect transistors (Xue & Cui, 2009; Xue et al., 2006), conductive interconnects (Robertson et al., 2008; Xue & Cui, 2008a), and energy storage systems (Hu et al., 2009; Kaempgen et al., 2009). A critical step to obtain these practical devices is to deposit well-organized and highly aligned CNTs in desired locations. Recently, researchers have developed a number of methods to align CNTs: using moving fluids to organize nanotubes (S. Li et al., 2007), introducing gas flows in reactors or channels (Liu et al., 2009), withdrawing microfluidic channels from solutions (Tsukruk et al., 2004), spin coating nanotube dispersions with controlled speeds (LeMieux et al., 2008; Roberts et al., 2009), and magnetic capturing of nanotubes (Shim et al., 2009). However, many of these techniques have limitations and restrictions because they require either intensive preparation processes or assisting materials with special properties. Therefore, their applications are relatively limited. By comparison, dielectrophoresis, a simple but versatile method, has proven to be effective in aligning CNTs in small and large scales (Gultepe et al., 2008; Mureau et al., 2006). This method can be conducted at room temperature with low voltages. In addition, a number of parameters such as solution concentration, deposition time, alternating current (AC) amplitude, and frequency can be adjusted to optimize the quality of the aligned CNTs. More importantly, dielectrophoresis can be easily incorporated into device fabrication and has the potential to be used in wafer-level deposition for the mass production of CNT-based devices (Monica et al., 2008; Stokes & Khondaker, 2008; Xiao & Camino, 2009).