Chinese scientists have developed a method to separate mixtures of metallic and semiconducting single-walled carbon nanotubes (SWCNTs) by a two-step dispersion-centrifugation process.   

Hao-Li Zhang's team at Lanzhou University revealed that the key to their separation strategy is in the strong interactions between SWCNTs and aromatic molecules. The team found that by carefully choosing the aromatic molecule, this caused the two types of nanotube to disperse differently in an organic solvent. They then centrifuged the mixture, firstly extracting the metallic SWCNTs and then the semiconducting SWCNTs. They discovered that linear anthracene and pentacene derivatives were more efficient at extracting the metallic SWCNTs and the best results were obtained using solvents in which the nanotubes are moderately soluble, such as N-methylpyrrolidone or dimethylformamide.   

Current procedures for making carbon nanotubes result in the formation of both metallic and semiconducting types. These exhibit different properties: metallic nanotubes can be used to make electrical wires and semiconducting nanotubes have shown promise in nanoscale electronic devices. 'Because the metallic and semiconducting SWCNTs have different properties, the mixture cannot be used directly in many applications, such as electronic and photovoltaic devices. Therefore, it is very important to separate them,' explains Zhang. Previously reported methods for separating the nanotubes involved either the destruction of one type or complicated, low-yielding processes. 

Shanju Zhang, an expert in carbon nanotube-based nanocomposites at the Georgia Institute of Technology, Atlanta, US, comments that this is 'a simple method that provides a promising route to a way of separating SWCNTs. This method would be very useful for future nano-electronic applications.' 

Elizabeth Davies

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