Synthesis of Germanium/Multi-Walled Carbon Nanotube Core-Sheath Structures via Chemical Vapor Deposition

One-dimensional (1D) nanostructures such as nanotubes, nanowires, and nanobelts have been the focus of much recent attention, owing to the novel electronic and optical properties intrinsically associated with their low dimensionality and the quantum confinement effect. Such 1D nanostructures have potential applications in nanoelectronics, advanced composites, field emission devices, sensors, probes, optics and optoelectronics (Baughman et al., 2002; Agarwal & Lieber, 2006). Silicon nanowires have been preferentially studied since Si is of great technological importance in microelectronics (Morales & Lieber, 1998). Silicon nanowires exhibit significant differences in physical (Cui & Lieber 2001; Ma, et al., 2003; Sun et al., 2001) and chemical properties (Sun et al., 2003; Chen et al., 2005) from bulk Si, which have been exploited to fabricate nanoelectronic devices such as logic circuits (Huang et al., 2001), field effect transistors (Lieber, 2003), and sensors (Cui et al., 2001). Compared to Si, Ge nanostructures are of particular interest, since the exciton Bohr radius of bulk Ge (24.3 nm) (Maeda et al., 1991) is larger than that of Si (4.9 nm) (Cullis et al., 1997), resulting in more prominent quantum confinement effects. Ge also offers the advantage of lower processing temperatures with easier integration into conventional devices. Furthermore, Ge has much higher electron and hole mobility than Si (Sze, 1981), which is especially required when electronic devices are scaled down to the sub-100 nm regime. Several growth methods have been developed for the synthesis of Ge nanowires, including laser ablation (Morales & Lieber, 1998; Zhang et al., 2000), thermal evaporation (Gu et al., 2001; Nguyen et al., 2005; Sun et al., 2006; Das et al., 2007; Sutter et al., 2008), supercriticalfluid synthesis (Ryan et al., 2003; Polyakov et al., 2006; Ziegler et al., 2004; Erts et al., 2006), liquid-state synthesis (Heath & LeGoues, 1993; Song et al., 2009), molecular beam epitaxy (Omi & Ogino, 1997), and chemical vapor deposition (CVD) (Kodambaka et al., 2007; Ryan et al., 2003). CVD has been the most widely employed of these synthesis methods, with the aim of synthesizing Ge nanowires in a controllable way via the selection of suitable Ge