Carbon nanotubes are metallic or semiconducting, based upon delocalized electrons occupying a 1-D density of states. However, any covalent bond on SWNT sidewall causes localization of these electrons. In the vicinity of localized electrons, the SWNT can no longer be described using a band model that assumes delocalized electrons moving in a periodic potential.
Two important addition reactions of SWNT sidewall are: (1) Fluorination, and (2) Aryl diazonium salt addition. These functional groups on SWNT improve solubility and processibility. Moreover, these reactions allow for combining unique properties of SWNTs with those of other compounds. Above all, the selective diazonium chemistry can be used to separate the semiconducting and metallic nanotubes.
Read more about this topic: Selective Chemistry Of Single-walled Nanotubes