Many technologies that we take for granted today, such as smart devices, result from the giant steps obtained in materials science over the last decades. However, many interesting and potentially fruitful challenges still occupy the minds of pure and applied scientists of this field. The subfield of nanoscience results from the interdisciplinary contribution of quantum mechanics and chemistry and offers the opportunity for developing materials of interest. Here, Dr Benjamin Flavel is a strong contributor and leads an advanced lab at the Karlsruhe Institute of Technology. His work, and that of his collaborators, focuses on carbon nanotubes, a material that holds exciting potential in applications including solar cells, thermoelectrics, batteries, light sensitive elements, on-chip single photon light sources, transistors and more.
In simple words, carbon nanotubes are molecules formed by the arrangement of carbon atoms, prepared under special conditions, such that the atoms take a cylindrical structure. The particular geometry of their molecular structure plays a fundamental role in shaping the properties of the resulting material. Since their discovery, carbon nanotubes have generated great interest, due to their richly varying physical, electronic and optical properties. It is possible to have single, double and multiple carbon walls with each wall potentially being either semiconducting or metallic, and possessing unique optical transitions covering the ultraviolet to infrared spectral range.
Although the unique optical properties and versatility of carbon nanotubes offer countless potential applications, many of these are yet to come to fruition. This realisation is hindered by the characteristic that generated such interest in them in the first place: their inherent inhomogeneity and varying properties. At present, it is not possible to simply grow arbitrary types of carbon nanotubes with pre-determined properties (electronic/optical) on the atomic scale. Instead, a mixture of many different types of nanotubes is obtained. Ideally, a pure carbon nanotube must be grown. In order to become a true advanced material of the future, it is necessary to develop methods capable of preparing carbon nanotubes with defined length, wall number, diameter, electronic and optical properties. Additionally, such methods to sort carbon nanotubes must yield high purity levels, they must be produced on a large-scale and be compatible with subsequent integration into device architectures. This is where the research of Dr Flavel and his team comes in.
Carbon nanotubes in solar cells
Single-walled carbon nanotubes (SWCNTs) have interesting electrical properties. Depending on parameters like their diameter, or their chirality (the specific arrangement of carbon atoms), they make excellent conductors (like metals), or semiconductors (like silicon). Thus, SWCNTs have attracted the attention of the scientists working on organic solar cells. Notably, these nanotubes have high charge carrier mobility, and show a good stability towards degradation in ambient, humid, and hot conditions.
In relation to the utilisation of single-walled nanotubes in organic solar cells, Flavel’s team have proposed solutions for facing one of the biggest problems: the short exciton diffusion length of around 5–10 nm in organic solar cells, which leads to reductions in efficiency. They resolved this issue with the use of single-walled carbon nanotubes, which have been estimated to have an exciton diffusion length of 0.1–1 µm. Additionally, due to extraordinary optical and electronic properties the carbon nanotube can be used as the photoactive or light absorbing component. By tailoring the chirality and diameter of the SWCNT, the band gap and thereby wavelength of light required for exciton generation can also be fine-tuned. The outcome is carbon nanotubes that are ideal for use in light sensitive devices, where individual chiralities allow for wavelength specific light absorption, or broadband solar cell devices from controlled mixtures of chirality.
Double-walled carbon nanotubes
The team at the Karlsruhe Institute of Technology are also actively researching double-walled carbon nanotubes (DWCNTs) and are the only group in the world currently capable of preparing DWCNTs with defined inner and outer wall electronic type. A unique intermediate between SWCNTs and multi-walled carbon nanotubes (MWCNTs), DWCNTs have generated great interest, and have been predicated to be useful in developing advanced biosensors, where the outer wall is used for integration of a bio-sensitive element and the inner-wall for signal transduction. DWCNTs also offer additional advantages and theoretical calculations predict that given the correct combination of constituent walls an observation of superconductivity might even be possible.
Overcoming the barriers
Dr Flavel and his team have developed novel sorting techniques, to routinely yield milligram quantities of metallic and semiconducting carbon nanotubes, chirality pure SWCNTs and even DWCNTs sorted by their outer-wall and inner-wall electronic type. Using this technique to prepare large quantities, the team then explore ways to control the order and orientation of this one-dimensional nanomaterial on the macro scale. That is, how do you make a film from nanotubes and how can you ensure this film’s compatibility with surrounding layers in a device stack? To address this, the team prepare inks with liquid crystal concentrations of carbon nanotubes from which highly aligned nanotube thin films over large areas are possible. Controlling the material properties in this way opens up the potential of carbon nanotubes in the use of energy related applications, such as solar cells.
Dr Flavel’s research has made significant strides in the field of carbon nanotubes. The team’s pioneering techniques overcome some of the issues related to the synthesis, purification, processing and device integration. Driving forward their potential, carbon nanotubes are a truly exciting material for the future.
I am not sure that anything inspired me to do research on carbon nanotubes, it just occurred organically and I have been fortunate to work with many excellent researchers in this field. However, I continue to be interested in this field because carbon nanotubes are the perfect example of the strength of nanotechnology. Just by making miniscule changes to the arrangement of carbon atoms in a nanotube it is possible to completely changes its properties, despite being made out of the same material.
What advantages do carbon nanotubes have over silicon for solar applications?
With silicon solar cells becoming increasingly efficient and price effective it becomes increasingly difficult to justify why any other materials should be used or developed but there are some properties that silicon can never have and this is where many of the niche organic solar cells come in – carbon nanotubes included. For example, you are able to print organic materials, they are flexible and transparent, less energy intensive in their production and easy to dispose of at the end of the product life cycle. Additionally, specific to carbon nanotubes, they absorb in a spectral range which silicon cannot, namely the infrared, so you could think about efficiency improvements by collecting much of the infrared radiation from the sun that is otherwise lost in traditional silicon solar cells
What has been the biggest challenge with respect to your research thus far?
I think I speak for a lot of junior researchers here when I say that the biggest challenge in research is not actually the science itself – that is our passion and we enjoy the fact that it’s not always easy. Rather, the biggest challenge is when your position and those of your entire team are entirely dependent on third party funding and you have to ensure that you have enough new research grants to keep things going. That can sometimes be a real burden.
Single-walled and double-walled carbon nanotubes have a wide range of applications. In your opinion, which is more promising for future developments?
It is hard to say if single- or double-walled carbon nanotubes are more promising for future developments. Each have advantages and disadvantages and it will most likely be highly dependent upon the intended application. However, I can say from a research perspective we are currently very excited about double walled carbon nanotubes because the combination of two carbon walls and the interplay of their electronic structure can lead to properties not seen in single-walled carbon nanotubes. From this point of view DWCNTs are not as well understood or studied compared to their single-walled counterparts and we look forward to contributing to the field in this space in the future.
Can you envision further applications, beyond those already known in the area (e.g., solar cells), related to the materials resulting from your research work?
Good question! The sky is really the limit. Carbon nanotubes have so many fantastic properties and they are finding ever-increasing application in real world products. Sometimes it can be a little surprising where you can find carbon nanotubes these days: from cars, cell phones, sports equipment such as tennis racquets, bicycles and golf clubs all the way through to novel cancer treatments.
Dr Flavel’s translational research explores new forms of carbon nanotube purification, photovoltaics and electronics, as well as fundamental studies on the behaviour of carbon nanotubes in these and other systems.
- Emmy Noether-Program
- Deutsche Forschungsgemeinschaft (DFG)
- Dr Daniel Tune, (KIT)
- Dr Han Li, (KIT)
Dr Benjamin Flavel obtained both a BSc (Hons) and PhD in Nanotechnology at the Flinders University of South Australia. He received a research fellowship from the Australian Government’s Endeavour Program to work in New Zealand at the University of Canterbury. Dr Flavel took up a postdoctoral fellowship in 2011 from the Alexander von Humboldt Foundation to work at the Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT) in Germany, where he has led his own research group since 2013.
Dr Benjamin Flavel
Institute of Nanotechnology
Karlsruhe Institute of Technology (KIT)