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Pixie Dust and Carbon Nanotubes
18 May, 2007 11:27 am
Predictions of the use of carbon nanotubes (CNT) are so overwhelming that some call it pixie dust. Bright prototype CNT based flat screens can be seen on tradeshows. How long will it take to see multiple large scale applications of Carbon Nanotubes?
Produced at large scale: CNTs can be fabricated when it is hot (>600C) and in the presence of gases catalyzing the formation and feed growth such as helium, hydrogen and methane, alcohol or carbon vapor. The growth of small diameter tubes is catalyzed by transition metal nano-particles which form stable carbides (Co, Ni, Fe). Much has been learned over the last ten years how to optimize their growth and to improve the yield. But much remains to be learned how to grow tubes of one particular diameter (helicity, chirality), a given length at a pre-defined position. It is finally not enough to have them in your hand; we need to place them or disperse them in a precise manner to make use of their exceptional properties.
Light and strong: The carbon bonds in a graphitic structure are very strong, stronger than in diamond. CNTs do not fracture; they are extremely resilient and stable. CNTs are ideal to mix into polymers to improve their mechanical property. However, this turns out to be challenging. The flat surface of CNTs does not interact strongly with the polymer matrix and much research will be needed to make CNT polymer composites strong.
Matallic and semiconducting: Depending on the orientation of the hexagon lattice with respect to the tube axis, CNTs can be semiconducting or metallic, a very unique property. Without doping, the system can be metallic or insulating by simply changing the tube structure (chiraity/helicity). The tube electronic properties are absolutely unique: the tube can serve as a wave guide for electrons and the quasi-one dimensional structure gives rise to a very special distribution of the electronic states as a function of energy, something in between what is found in molecules and solids. CNTs can be used to make transistors and it has been predicted that CNT based transistors will replace some of conventional silicon based devices in the future.
Electrodes: The electric field at the tube end is much enhanced due to the small tube diameter. This has the effect that the threshold voltage is much reduced for CNTs. Future electron microscopes might use CNT electrodes. Field emission flat screen displays have been demonstrated by several laboratories and electronics industries and will soon be in production (1). CNTs have in the addition the advantage that they are chemically stable in high electric fields and work at larger electrodes distances, a technological advantage for large screens and their life time. The metallic properties in CNTs can be used to make polymers conducting. This opens applications from reducing static charging of plastics to transparent electrodes for future LCD flat screens.
Light emission and detection: Excited electronic states have a very special dynamics in CNTs which can lead to light emission in the near infrared in semiconducting CNTs. Light emission can be used to determine the distribution of diameters in a given sample, important in the development of chirality/helicity dependent fabrication or processing techniques.
Heat conductors: CNTs are excellent heat conductors, nearly two times larger than in diamond. Their one dimensionality makes it possible to evacuate heat in a given direction. It is very well possible that CNTs will be used to cool integrated circuits in the future.
Single and many tube applications and demonstrations: We can distinguish between single and many tube applications. Flat screens and composites are many tube applications while in transistors single tubes are used. Many tube applications are far easier to realize than single tube applications. For single tube applications much remains to be learned how to fabricate the devices in parallel.
Challenges, processing: While we know now to produce CNTs in large quantities the challenges today are in the growth of CNTs with a given property such as diameter, length, metallic or semiconducting, position on substrate, their distribution in a matrix or their interaction with environment. Issues which need to be addressed further are standardization and biocompatibility.
What is limiting progress? We know that the time between discovery and large scale applications can take as long as 60 years. Is the innovation chain adapted to this reality? Much of fundamental research is carried out at universities by PhD students which take 3-8 years to work on their subject. Large turnover of students and little possibility for the PhD student to continue their research work or apply their knowledge are factors which clearly limit progress. In a world where educated and a trained workforce is getting more expensive compared with what computer supported machines produce, it is important to not neglect factors which slows the innovation process. Apart from this there is much to be improved how government funded laboratories interact with industry to promote new technology. The understanding of the way innovation finds its way into products and their promotion is crucial for any economy to prepare for the future.
1 K A Dean, Nature Photonics Technology Focus MAY 2007 p273