Fig. 1: Dielectric response of a metallic carbon nanotube before and after cutting. White arrows indicate the cut tube and its fragments. The dielectric response is almost linearly dependent on length up to 2 μm.

Increasing demand for smaller and smaller electronic devices has necessitated research into materials that can transport information efficiently at these ever-decreasing scales. Much effort has been focused on carbon nanotubes for their novel electronic properties. It is known that these properties are dependent on the chirality and diameter of the tubes — some nanotubes show metallic behavior, while others are semiconducting. The nanotubes therefore need to be separated and sorted for specific device applications. These two types of nanotubes are usually separated based on differences in dielectric properties — the capacity to repel electric fields — because metallic nanotubes show a stronger response than semiconducting carbon nanotubes of comparable diameter. Now, Liwei Chen and colleagues1 in China and the US have found that the dielectric response of metallic nanotubes decreases with decreasing length, and becomes indistinguishable from that of semiconducting nanotubes below 200 nm. The results demonstrate that length also needs to be taken into consideration when separating carbon nanotubes.

It is challenging to measure the properties of very short nanotubes and obtain statistically meaningful data. However, using a contactless approach — electrostatic force microscopy (EFM) — Chen and colleagues managed for the first time to measure the length effect of dielectric properties for individual nanotubes. They found that whereas the dielectric response of metallic nanotubes shows a strong length dependence, the length response of the semiconducting tubes was less evident.

Chen and colleagues investigated the source of this dependence of metallic nanotube length. They concluded that defects at the ends of the nanotubes reduce the electrical conductivity and dielectric response, and because the ends play a proportionally greater role at shorter tube lengths, the dielectric effect is reduced as the nanotube become shorter.

“The implications of these results go beyond separation,” says Chen. “It’s highly relevant in various problems such as the downscaling of channel lengths in carbon nanotube transistors, or the thickness in carbon-nanotube-based nanofiltration membranes and so on.”