|Carbon nanotubes are already known as being extremely versatile and tough. They can be used for a host of medical, physical and chemical applications, among others. Carbon nanotubes could make many electronic devices cheaper and more efficient. But when nanotubes are manufactured, tubes that work for solar cells are mixed with tubes that work for batteries. The final product is a nanotube powder that is not ideal for any single commercial application. CNT manufacturing process itself is not specific to any of the potential applications the finished products are intended for. What this means is that nanotubes which may work for solar cells will not necessary work for transistors, and vice-versa. Existing technologies produce carbon nanotube powders that contain all sorts of structures. Separating those that are useful for a given application from those that are not is nearly impossible.
Zhenan Bao, Stanford associate professor of chemical engineering, and her colleagues at University of California-Davis and the Samsung Advanced Institute of Technology have discovered a technique to selectively sort semi conducting single-walled carbon nanotubes from the mixture. The semi conducting nanotubes could be used in flexible transistors for other technologies that Bao's group develops, including circuits printed on plastic, bendable display screens and stretchable electronics. The nanotubes could also close gaps in solar cell technology.
Up till now, sorting has been a major bottleneck for carbon nanotubes to be viable for practical electronics applications. This work solves the problem of separating the conducting from the semi conducting nanotubes. Conducting tubes are used in wires and electrodes but semi conducting tubes are the active material for transistors or solar cells. Mixtures of conducting and semi conducting tubes do not carry enough current for wires or battery electrodes. And when the mixture is used for semi conducting, as in a transistor, the excess current from the conducting nanotubes will short the device. The group uses a polymer that selectively sorts the mixture by wrapping around semi conducting nanotubes, and not conducting nanotubes. Mixing the polymer with commercially available carbon nanotubes in a solvent separates semi conducting tubes from conducting tubes. This is not the first time a polymer has been used to sort conducting and semi conducting nanotubes. However, past polymers have insulated the nanotubes and required extensive removal treatments to restore the conductivity of the nanotubes. The polymer in Bao's process does not need to be removed. The final product is a semi conducting nanotube and polymer ink that can be used to make printable electronics. The simple process allows easy building of useful devices.
The group tested nearly 200 individual nanotubes to confirm that the polymer only wraps around semi conducting tubes and not conducting tubes. The polymer has a long, rigid backbone, with regular arm-like molecular chains along each side. The side chains fit together like fingers, making a ribbon of polymer that wraps around the semi conducting nanotubes. Conducting nanotube component of CNT powders are very useful for creating energy-efficient wires, cables and electrodes, but explains that one of the semi conducting tubes can be used to create stretchable transistors or solar cells. The secret to the method the team developed is the use of a special polymer, which has an affinity for semi conducting carbon nanotubes alone.