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Concept
Selective chemistry of single-walled nanotubes
Summary
Selective chemistry of single-walled nanotubes is a field in Carbon nanotube chemistry devoted specifically to the study of functionalization of single-walled carbon nanotubes.
Optical properties of carbon nanotubes
Reactivity of fullerene molecules with respect to addition chemistries is strongly dependent on the curvature of the carbon framework. Their outer surface (exohedral) reactivity increases with increase in curvature. In comparison with fullerene molecules single-walled nanotubes (SWNTs) are moderately curved. Consequently, nanotubes are expected to be less reactive than most fullerene molecules due to their smaller curvature, but more reactive than a graphene sheet due to pyramidalization and misalignment of pi-orbitals. The strain of a carbon framework is also reflected in the pyramidalization angle (Өp) of the carbon constituents. Trigonal carbon atoms (sp2 hybridized) prefer a planar orientation with Өp=0° (i.e. graphene) and fullerene molecules have Өp= 11.6°. The (5,5) SWNT has Өp~6° for the sidewall. Values for other (n,n) nanotubes show a trend of increasing Өp (sidewall) with decrease in n. Therefore, generally the chemical reactivity of SWNT increases with decrease in diameter (or n, diameter increases with n). Apart from the curvature SWNT reactivity is also highly sensitive to chiral wrapping (n,m) which determine its electronic structure. Nanotubes with n - m = 3i (i is an integer) are all metals and rest are all semiconducting (SC).
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.
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