Defected and Substitutionally Doped Nanotubes: Applications in Biosystems, Sensors, Nanoelectronics, and Catalysis

Abstract

Carbon nanotubes (CNTs) have been the subject of intensive research since their discovery by Iijima in the early 1990s (Iijima 1991). Single-walled carbon nanotubes (SWCNTs, Iijima & Ichihashi 1993; Bethune et al. 1993) are of particular interest because the electronic properties of these nanomaterials can vary from semiconducting to metallic depending on its molecular structure. This contrasts multi-walled carbon nanotubes (MWCNTs), which are metallic and exhibit a zero bandgap. Fundamental understanding of these supramolecular carbon allotropes (Tasis et al. 2006; Ajayan 1999) is essential to the development of new nanomaterials for applications in biosystems, sensors (Wang & Yeow 2009; Hu & Hu 2009; Li et al. 2008), optics (Avouris 2008), nanomechanics (Li et al. 2008; Park 2004), nanoelectronics (Park 2004; Fuhrer 2003; Tsukagoshi 2002), and catalysis (Serp 2003, Tian et al. 2006, Yeung et al. 2011). The molecular structure of SWCNTs can be described as a cylindrical roll of an infinite graphene sheet and is characterized by a chiral circumferential vector AB = ma + nb, a linear combination of two unit lattice vectors a and b where m and n are integers (Figure 1-1, Ajayan 1999; Moniruzzaman & Winey 2006). The pair of indices (m,n) for any given nanotube determines its diameter, chirality, and electronic character. For all n = m, the nanotube is termed armchair and is metallic, exhibiting a zero bandgap. For n ≠ m and neither n and m are zero, the nanotube exhibits chirality and supramolecular helicity, having important implications in optical properties. For n = 0 or m = 0, the nanotube is termed zigzag. For n − m = 3p, where p is a non-zero integer, the nanotube is semimetallic with a band gap on the order of meV. For n − m ≠ 3p, where p is a non-zero integer, the nanotube is semiconducting with a band gap on the order of 1 eV; as a general rule of thumb, the observed band gaps are roughly proportional to the reciprocal of the tube radius. Each individual C atom in the sidewall of a CNT exhibits pyramidalization and partial sp3 hybridization as a result of sidewall curvature. This phenomenon leads to a weakening of the overall π-conjugation of the SWCNT and slight misalignment of π-orbitals between adjacent atoms. Curved π-conjugation can be quantified using Haddon’s π-orbital axis vector (POAV) method (Figure 1-2, Haddon & Scott 1986). In this analysis, the pyramidalization angle θp = θσπ − 90o, where θσπ is the angle between the π-orbital and the σ-bond of the C atom of interest. In contrast to planar (i.e., θp = 0o) and pyramidal (i.e.,