Low-Energy Irradiation Damage in Single-Wall Carbon Nanotubes

Single-wall carbon nanotubes (SWCNTs) are one of the most promising materials for future nano-electronics, because of their unique quasi-one-dimensional structures and excellent electric and mechanical properties. They also have very high chemical stability, owing to their robust sp2-bonding carbon network (graphene) with no dangling bonds. Because of the structural robustness, low-energy (typically 10 eV-20 keV) electron and photon irradiation in a vacuum had been generally assumed not to cause damage to SWCNTs when the energy is smaller than the knock-on threshold. In fact, analytical tools that use low-energy electrons or photons, such as scanning electron microscopy (SEM), had been commonly used for characterization of SWCNTs without serious concerns. In 2004, however, we reported that electron irradiation in a SEM caused severe damage (low-energy irradiation damage) in SWCNTs produced by both thermal chemical vapor deposition and laser ablation methods (Suzuki et al., 2004b). Other techniques using lowenergy electrons and vacuum-ultraviolet (VUV) light or soft x-rays (especially highbrilliance synchrotron radiation light), such as low-energy electron microscopy (LEEM) and photoemission spectroscopy, also inevitably damage SWCNTs. Therefore, paying attention to the low-energy irradiation damage is practically important for those who study SWCNTs. For example, when we measure the Raman and photoluminescence (PL) spectra and electric properties and take SEM images of the same SWCNTs, the SEM observations should be done last. Doing the high-resolution SEM observation first would inevitably cause severe damage and tremendously affects the following measurements. The low-energy irradiation damage and its defect characteristics are also physically interesting. In this chapter, we will review the physical and chemical property changes induced by the damage, and the defect properties, which are significantly different from those of other types of damage. We will examine the defect-induced metal-semiconductor transition of the room-temperature electric properties and discuss its mechanism. We will also summarize other types of damage, which are often confused with the low-energy irradiation damage, focusing on the differences between them. Before continuing to the main text, I must briefly explain how I compare spectra obtained form the same SWCNT sample. In many studies of the physical or chemical treatment of SWCNTs and graphene, spectra are often normalized to the maximum peak height. In