Carbon nanotube films conduct electricity and transmit visible light, which makes them a popular material for the production of transparent electrodes – key elements of solar panels and touch screens that used to be made from indium tin oxide, a brittle and unsustainable material. At the same time, such properties of carbon nanotubes as conductivity, transparency and flexibility can be further improved by doping with a small amount of other substances.
“Doping is crucial. The technologies available today do not allow us to produce carbon nanotubes of such quality that they would have the requisite characteristics as is. But we can adjust the properties by adding impurities. The choice of the dopant is dictated by a compromise between conductivity increase, film transparency and stability of the effect. Previously, thin films of nanotubes could only satisfy two of the three criteria. But now, we have managed to achieve all three with one material,” Albert Nasibulin, professor at the Center for Photonic Science and Engineering, is quoted as saying by Skoltech.
Among the most common impurities are hydrogen tetrachloroaurate, which improves the electrical conductivity and transparency of carbon nanotube films, and copper bromide, which provides a good combination of stability and conductivity. However, hydrogen tetrachloroaurate provides only a short-term effect, while copper bromide reduces the transparency of the films. A similar dilemma is typical of most dopants known today.
The Skoltech scientists managed to solve this dilemma by using as a dopant gaseous nitrogen dioxide, which is also called “foxtail” due to its bright orange colour. “We were actually studying another, unstable and undesirable modification of carbon nanotubes, which is caused by this gas at much lower temperatures. But we ended up finding another temperature range, in which very stable modifications occur. Since our dopant is a gas, the doping process is fast, scalable and waste-free. Nitrogen dioxide is easy to integrate into existing synthesis process chains, and excess of this reagent is removed from the mixture because it liquefies when cooled to 20 °C,” Dmitry Krasnikov, assistant professor at the Center for Photonic Science and Engineering, is quoted as saying by Skoltech.
While the effect of hydrogen tetrachloroaurate decreases by three times in due course, the effect of nitrogen dioxide subsides by no more than one and a half times. Since nitrogen dioxide is a gas, it does not settle on the film in several layers, but stops at the molecular thickness of the coating. This will make it easier to use in the production of electrodes for photovoltaics, touch screens and cars.
