Chemical functionalization of SWCNTs is essential to take full advantage of these properties and realize the many potential applications. Such modifications are required to improve solubility in aqueous and non-aqueous solvents and to facilitate processing. Typically, the chemical modification of carbon nanotubes can be via covalent chemistry with the chemical group bound directly to the nanotube surface or via non-covalent adsorption of functional molecules to the surface. Covalent attachment of functional groups is particularly promising as the bonding is very robust. However, extensive covalent functionalization can significantly alter the intrinsic electronic and mechanical properties of SWCNTs and hence the ability to control the level of chemical functionalization is often desired.
Reversible result
In a recent SWCNT functionalization study, researchers in the UK have probed the attachment of six-membered aromatic rings to the surface of SWCNTs using X-ray and ultraviolet photoelectron spectroscopy, and near-edge X-ray absorption fine structure (NEXAFS) studies. NEXAFS and valence-band photoemission data obtained from the chemically modified material show that the electronic structure of SWCNTs are altered upon covalent functionalization. It was observed that the π electronic states, responsible for electrical conductivity in nanotube material, were suppressed. In line with such a reduction, the electrical resistivity of the chemically modified SWCNT films increased. The change in electronic structure was found to be reversible by heating the material to 500 °C under ultrahigh vacuum.
The researchers are currently looking at other types of covalent and non-covalent modifications of carbon nanotubes and examining how the functional groups influence the electronic properties of the material and the overall conductivity of thin films. The observation that the covalent functionalization of carbon nanotubes and the effect on properties is reversible may offer new ways of processing and assembling nanotube structures.