Structural modifications of multiwalled carbon nanotubes and their effects on optical properties

Multiwalled carbon nanotubes (MWCNTs) with outermost diameters of ~5 to more than 100 nm are modified and investigated for structural and optical properties. MWCNTs are sonicated for a long time (100 min) to intentionally introduce defects and functional groups as well as annealed at a high temperature (2,000 °C) for further graphitization. They are examined for absorption and emission behavior and compared to the initial material. The MWCNTs are found to show plenty of highly-resolvable emission bands. Since they are observed from all the samples, the bands are considered to be intrinsic to MWCNTs. The mechanisms previously suggested for the emission of MWCNTs are compared using the structural features and the optical data. It was deduced that defects or functional groups are not involved in the luminescence. Rather, the emission bands imply interband transitions between van Hove singularities in the electronic band structure. These results not only help to understand inherent properties of MWCNTs but may also give insight into the luminescence phenomena of other carbon nanotube family such as double-walled carbon nanotubes and functionalized MWCNTs.