When carbon nanotubes are synthesized, they are always monochrome. They come in various shades of grey, light grey, dark grey, and black … until now. This is because a team of researchers from Aalto University in Finland have found a way to produce coloured nanotubes.

While previously, coloured carbon nanotubes were possible, the colour was added to them in a secondary process. However, these purifying and colouring techniques costs both time and money, and risks damaging the tubes. By inducing colour directly in the fabrication process, coloured carbon nanotubes will be available for many more applications.

The team have now published their results in the Journal of the American Chemical Society, where they describe the discovery as follows; “We have succeeded in the direct synthesis of colorful SWCNT [single-walled carbon nanotube] thin films using a conventional FC-CVD [floating catalyst chemical vapor deposition] process by introducing a certain amount of CO2 [carbon dioxide]. By controlling the flow rate of CO2 and optimizing the reaction conditions, the color of the SWCNT thin films can be tuned from gray, green, brown to gray. The colors of the SWCNT thin films are attributed to nanotube diameter distributions in particular ranges that give rise to absorption peaks in the visible region.”

The right side shows the SWCNT thin films on filters obtained from different CO2 concentration at 850 °C next to the corresponding absorption spectra on the left

“Growing carbon nanotubes is, in a way, like planting trees.” Explains Dr Hua Jiang, a co-author of the study and senior scientist at Aalto University, “We need seeds, feeds, and solar heat. For us, aerosol nanoparticles of iron work as a catalyst or seed, carbon monoxide as the source for carbon, so feed, and a reactor gives heat at a temperature more than 850 degrees Celsius. Carbon dioxide acts as a kind of graft material that we can use to tune the growth of carbon nanotubes to various colors.”

The Aalto team have already published a number of ground-breaking pieces of research that have pushed our understanding of carbon nanotubes ever-further. But by finding a direct route to coloured carbon nanotubes they have really excelled themselves, not only in improving the range of possibilities and applications for carbon nanotubes, but also in increasing the amount of control that carbon nanotube producers have over their products.

As the university press release explains, “To get carbon structures to display colours is a feat in itself. The underlying techniques needed to enable the colouration also imply finely detailed control of the structure of the nanotube structures. The team’s unique method, which uses aerosols of metal and carbon, allows them to carefully manipulate and control the nanotube structure directly from the fabrication process.”

By using an advanced electron diffraction technique, the researchers were able to learn more about the exact atomic scale structure of their nanotube material. In particular they saw that the films have very narrow chirality distributions, meaning that the pattern formed by the honeycomb lattice on the tube walls were almost uniform. Adjusting the pattern so that the tubes were synthesized with more or less uniformity allowed the team to adjust the tubes colour, as well as their electrical properties.

(a) Chirality and (b) diameter distribution of SWCNTs with 0.25 vol % CO2at 880 °C

“We want to understand the science of how the addition of carbon dioxide tunes the structure of the nanotubes and creates colours. Our aim is to achieve full control of the growing process so that single-walled carbon nanotubes could be used as building blocks for the next generation of nanoelectronics devices,” says Esko Kauppinen, a professor at Aalto University and lead author of the study.

It is expected that the coloured tubes will be highly sought after, with a potential use in the making of touch sensors with many different colours even when switched off. The nanotube process could also be adapted for use in solar cells giving them completely new types of optical properties. Alternatively, the nanotubes could also be applied in the manufacture of transistors and countless other electrical components.

While further research is ongoing, the simplicity of the process means that production is highly scalable, allowing for the fabrication of carbon nanotube thin films at an industrial level.

“Usually you have to choose between mass production or having good control over the structure of carbon nanotubes.” Explains Dr Qiang Zhang, a postdoctoral researcher and co-author. “With our breakthrough, we can do both.”

Photo credit: Aaltouniversity, Globalsources, & Interestingengineering