When single-walled carbon nanotubes are made, a mixture of both metallic and semiconducting nanotubes is produced. This is a problem for those trying to make electronic devices from nanotubes, who need pure samples of either semiconducting or metallic tubes (depending upon the application), not both.

Now, researchers in the US and South Korea have a developed a new and simple technique that not only efficiently separates the two types of nanotube but also allows them to be patterned onto a substrate as thin films. These films could be used to make electronic devices with desirable properties, and could even replace silicon as the material of choice for integrated circuits.

Single-walled nanotubes are essentially rolled up sheets of graphite just one atom thick and can be metallic or semiconducting depending on the direction in which the sheet has been rolled. They have enormous potential as the building blocks in nanoscale electronics, and are often touted as being the perfect alternatives to silicon thanks to their tiny size and their ability to carry large currents. Metallic tubes could function as transparent conducting leads, while semiconducting tubes could make good nanoscale transistors.

Selective molecules

Although researchers have already proposed several techniques to separate nanotubes, most of these have proved difficult to perform on an industrial scale. However, help may be at hand: recent experiments have shown that specific molecules tend to interact selectively with metallic or semiconducting tubes in solution. Now, new work, by Zhenan Bao of Stanford University and colleagues, builds on this work by using such molecules to create a special surface that interacts selectively with nanotubes (Science 10.1126/science.1156588 ).

Bao and co-workers obtained their results by treating silicon substrate surfaces with molecules containing amines that "grabbed" the semiconducting tubes and ignored the metallic ones. Once this surface modification step was complete, the researchers then created thin films of the nanotubes on the substrate using a method called spin coating, in which the nanotubes are placed on a rapidly spinning surface so that they spread out thanks to centrifugal forces.

The scientists found that the thin films behaved as excellent field-effect transistors, with on-off ratios as high as 900,000, which is very close to the value for transistors used in liquid crystal displays (LCDs), for example. "The sub-monolayer films can also be completely transferred to different substrates," team member Melburne LeMieux told physicsworld.com. "They could be used to better understand nanotube networks by electrical testing and in techniques such as scanning Kelvin probe microscopy in which every nanotube can be characterized (since none are 'buried')."

To selectively absorb metallic tubes from a mix, the researchers used phenyl-terminated silanes on the silicon substrates. This selectivity is possible because the nanotubes are extended pi-electron systems that interact with other such systems via a mechanism called pi-pi stacking. Metallic nanotube films are excellent transparent conducting materials and could find applications in solar cells and touch screens, said LeMieux.

The researchers now hope to be able to scale up their technique and increase the density of sorted nanotubes on a substrate. One way to do this is by multiple transfers onto a target substrate, added LeMieux.