A single-walled carbon nanotube (SWCNT) is a sheet of carbon just one atom thick rolled up into a tube that has a diameter of about 1 nm. The atoms in the sheet are arranged in a hexagonal lattice. The relative orientation of the lattice to the axis of the tube, or its chirality, determines whether the tube is a metal or a semiconductor and so what type of electronic properties it has.

SWCNTs are ideal for use in a variety of applications, such as sensors and transistors, thanks to their extremely high surface area and excellent charge transport properties. However, to make such devices, the tubes need to be rich in one chirality. This is difficult to achieve.

Increasing nanotube repulsion
Now, Alberto Striolo and colleagues say that surfactants, such as sodium cholate and flavin mononucleotide (FMN) could efficiently separate out SWCNTs based on diameter and chirality. Using molecular dynamics simulations, the researchers related the structure of the surfactant adsorbed on aqueous nanotubes to the effective repulsion/attraction between the tubes. They suggest that increasing the repulsion between individual nanotubes yields more stable dispersions of single-chiral tubes. Based on these theoretical simulations, Striolo says he and his team can now start designing molecules to make dispersions containing only the nanotubes they are interested in.

“Although the technique is not yet ready, our strategy is clear – we will design small molecules of surfactants that form aggregates with well-defined morphologies on nanotubes as a function of tube diameter and chirality,” he explained.

The researchers say that the aqueous surfactant aggregates should be rigid and that the hydrophilic parts of the surfactants (the parts that like to remain in contact with water) should be as far away as possible from the nanotubes. Such a recipe should produce nanotubes of the desired diameter and chirality.

The team is now busy designing such optimal surfactants. “We are building a library to describe the behaviour of several known surfactants,” Striolo told nanotechweb.org. “Some of these surfactants are known to be effective in stabilising nanotube dispersions – for example, FMN, sodium dodecyl benzene sulphonate and bile salts – while others are not (sodium dodecyl sulphate).”

The work was published in ACS Nano.