Suspended Carbon Nanotubes: Applications in Physical Sensors and Actuators 375 Suspended Carbon Nanotubes: Applications in Physical Sensors and Actuators

Carbon is truly an extraordinary material with physical structures spanning threedimensional (3D) graphite, two-dimensional (2D) grapheme and zero-dimensional (0D) buckyballs or buckminster fullerine spheres. It is not surprising that the structural characteristics of carbon thus yield band diagrams displaying a diverse array of physical properties. When a 2D graphene sheet is rolled into a cylinder, a one-dimensional (1D) or quasi-1D form of carbon results, namely carbon nanotubes (CNTs), which have been one of the most extensively studied materials since their discovery (Ijima, 1991). A single rolled-up sheet of graphene results in a single-walled nanotube (SWNT) with a typical diameter of 1 – 2 nm. Multi-walled nanotubes (MWNTs) consist of concentric cylinders with an interlayer spacing of 0.3 – 0.4 nm, and diameters that are at least an order of magnitude larger than SWNTs, between 10 – 30 nm. The exceptional thermal, mechanical, electronic and optical properties of nanotubes (Dresselhaus, Dresselhaus, Avouris, 2001) has created a surge of applications, ranging from the use of CNTs as interconnects (Li et al., 2003), heat transport materials (Yu et al., 2006), novel transistors (Bachtold, et al. 2001), as well as optical materials (Homma et al., 2009). The focus of this chapter is on the nanoelectronic applications of suspended carbon nanotubes, in particular their use as physical sensors and actuators. Unlike 3D materials, when nanotubes are dispersed on a substrate, their properties are intimately influenced by the tube-to-substrate interactions, particularly those of SWNTs. For example, when diameter and helicity of SWNTs are controlled such that semiconducting tubes result (Odom et al., 1998), no luminescence is detected for SWNTs lying on a substrate eventhough semiconducting SWNTs have a direct band gap (Lefebvre et al., 2003). In addition, van der Waals interactions between CNTs and the substrate cause radial and axial deformations (Hertel et al., 1998) which affect the electron transport properties of the tubes. The presence of the substrate beneath the tube can also influence heat dissipation mechanisms, which is an underlying motivation for using suspended CNTs as thermal conductivity based pressure sensors, and will be described in Section 2.2. Such sensors are important for vacuum-based microcavity applications (vacuum microelectronics, microeletromechanical-systems (MEMS) such as gyroscopes and RF MEMS switches). The high