Carbon nanotubes could provide the circuitry for the computers of the future. But they can be either semiconducting or metallic, and the difficulty of getting the right type of tube in the right place on a computer chip has so far prevented their commercial use.

Now Northwestern University researchers have developed a way to separate nanotubes by electrical conductivity and by diameter, another property that’s important for applications in electronics.

In the test tube at left, metallic nanotubes are shown in green and semiconducting tubes in orange. To separate them, the researchers add surfactants–chemicals common in detergents–to solutions of nanotubes. The surfactants cluster around the nanotubes in different concentrations and arrangements, depending on the nanotubes’ sizes and electronic properties. The clusters have different densities, so spinning the solution at ultrafast speeds produces layers containing specific kinds of nanotubes. While the researchers expected to be able to sort nanotubes by diameter, the sorting by electronic type came as a surprise, says Mark Hersam, a professor of materials science and engineering at Northwestern. Richard Martel, a chemistry professor at the University of Montreal, calls the Northwestern researchers’ new approach “a breakthrough in the field.”