Carbon nanotubes show a strong tendency to aggregate into parallel bundles or ropes. This bundling perturbs the electronic structure of the tubes and can affect their optical properties.
The researchers developed a technique that detaches the tubes from their bundles and applies a coating to stop them re-aggregating. They used ultrasound to agitate an aqueous dispersion of single-walled carbon nanotubes in sodium dodecyl sulphate surfactant. Then they centrifuged the liquid to remove any remaining tube bundles, ropes and residual particles of iron catalyst. The result was single nanotubes enclosed in a sodium dodecyl sulphate micelle.
The direct bandgap semiconducting tubes were found to fluoresce in the 800-1600 nm range, a region important for fibre-optic communications and bioimaging. Most of the nanotubes were 80-200 nm long: their average length was 130 nm. Tube diameters, meanwhile, were between 0.7 and 1.1 nm.
The nanotubes had a characteristic luminescence lifetime of less than 2 ns and the scientists estimated the quantum yield to be of the order of 10-3.
Aggregating the isolated nanotubes or acidifying the SDS micellar suspension significantly reduced the photoluminescence intensity. The scientists proposed that this was because the presence of a metallic nanotube within a bundle will quench electronic excitation on an adjacent semiconducting tube, thereby preventing its luminescence.