Carbon Nanotubes Under Pressure

Abstract

Carbon nanotubes (CNTs) have extraordinary mechanical properties due to the stiff sp ² bond resulting in the exceptionally high Young’s moduli in the tera-pascal range, together with their tube structure (Treacy et al., 1996). They have unique electronic properties; they can be either metallic or semiconducting depending on the chirality — the direction along which a tube is rolled up (Odom et al., 1998). Pressure modifies these properties. The sp ² bond stiffens further, and the band gap in semiconducting CNTs changes with pressure (Yang and Han, 2000). To characterize and understand the behaviour of CNTs under pressure, the shift rates of the phonon frequencies with pressure are very interesting, as they directly reflect the mechanics and are closely related to the electronic properties. They can also be used as strain sensors.

In this chapter, we will focus on the shift with pressure of the graphite mode (GM) and the radial breathing mode (RBM). The GM is an in-plane vibrational mode, coming from graphite and characteristic of sp ²-hybridized carbon (Tuinstra and Koenig, 1970). The study of the GM pressure coefficients of CNTs thus provides a direct approach to understand the sp ² bond. It links closely to the high pressure study of other sp ²-bonded materials such as graphene and fullerene. The RBM, though related to the GM, is a unique signature of CNTs (Rao et al., 1997). Its vibrational frequency is diameter-dependent and therefore of critical importance to the study of features, which are related to the tube structure, including the GM pressure coefficients. We will briefly mention other modes, such as the 2D-mode, the second order D-mode from defects, which reflects the change in the electron bands, essential to characterizing graphene (Ferrari et al., 2006).