Potential applications of carbon nanotubes

Carbon nanotubes have many potential applications, here is a list of some of the most important:

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Structural

• clothes: waterproof tear-resistant cloth fibers
• combat jackets: MIT is working on combat jackets that use carbon nanotubes as ultrastrong fibers and to monitor the condition of the wearer. [1]
• concrete: In concrete, they increase the tensile strength, and halt crack propagation.
• polyethylene: Researchers have found that adding them to polyethylene increases the polymer's elastic modulus by 30%.
• sports equipment: Stronger and lighter tennis rackets, bike parts, golf balls, golf clubs, golf shaft and baseball bats.
• space elevator: This will be possible only if tensile strengths of more than about 70 GPa can be achieved. Monoatomic oxygen in the Earth's upper atmosphere would erode carbon nanotubes at some altitudes, so a space elevator constructed of nanotubes would need to be protected (by some kind of coating). Carbon nanotubes in other applications would generally not need such surface protection.
• ultrahigh-speed flywheels: The high strength/weight ratio enables very high speeds to be achieved.
• Bridges: For instance in suspension bridges (where they will be able to replace steel), or bridges built as a "horizontal space elevator".

Electromagnetic

• artificial muscles^[1]
• buckypaper - a thin sheet made from nanotubes that are 250 times stronger than steel and 10 times lighter that could be used as a heat sink for chipboards, a backlight for LCD screens or as a faraday cage to protect electrical devices/aeroplanes.
• chemical nanowires: Carbon nanotubes additionally can also be used to produce nanowires of other chemicals, such as gold or zinc oxide. These nanowires in turn can be used to cast nanotubes of other chemicals, such as gallium nitride. These can have very different properties from CNTs - for example, gallium nitride nanotubes are hydrophilic, while CNTs are hydrophobic, giving them possible uses in organic chemistry that CNTs could not be used for.
• computer circuits: A nanotube formed by joining nanotubes of two different diameters end to end can act as a diode, suggesting the possibility of constructing electronic computer circuits entirely out of nanotubes. Because of their good thermal properties, CNTs can also be used to dissipate heat from tiny computer chips. The longest electricity conducting circuit is a fraction of an inch long. (Source: June 2006 National Geographic).
• conductive films: A 2005 paper in Science notes that drawing transparent high strength swathes of SWNT is a functional production technique (Zhang et al., vol. 309, p. 1215). Additionally, Eikos Inc of Franklin, Massachusetts and Unidym Inc.^[2] of Silicon Valley, California are developing transparent, electrically conductive films of carbon nanotubes to replace indium tin oxide (ITO) in LCDs, touch screens, and photovoltaic devices. Carbon nanotube films are substantially more mechanically robust than ITO films, making them ideal for high reliability touch screens and flexible displays. Nanotube films show promise for use in displays for computers, cell phones, PDAs, and ATMs.
• electric motor brushes: Conductive carbon nanotubes have been used for several years in brushes for commercial electric motors. They replace traditional carbon black, which is mostly impure spherical carbon fullerenes. The nanotubes improve electrical and thermal conductivity because they stretch through the plastic matrix of the brush. This permits the carbon filler to be reduced from 30% down to 3.6%, so that more matrix is present in the brush. Nanotube composite motor brushes are better-lubricated (from the matrix), cooler-running (both from better lubrication and superior thermal conductivity), less brittle (more matrix, and fiber reinforcement), stronger and more accurately moldable (more matrix). Since brushes are a critical failure point in electric motors, and also don't need much material, they became economical before almost any other application.
• light bulb filament: alternative to tungsten filaments in incandescent lamps.
• magnets: MWNTs coated with magnetite
• optical ignition: A layer of 29% iron enriched SWNT is placed on top of a layer of explosive material such as PETN, and can be ignited with a regular camera flash.
• solar cells: GE's carbon nanotube diode has a photovoltaic effect. Nanotubes can replace ITO in some solar cells to act as a transparent conductive film in solar cells to allow light to pass to the active layers and generate photocurrent.
• superconductor: Nanotubes have been shown to be superconducting at low temperatures.
• ultracapacitors: MIT is researching the use of nanotubes bound to the charge plates of capacitors in order to dramatically increase the surface area and therefore energy storage ability.^[3]
• displays: One use for nanotubes that has already been developed is as extremely fine electron guns, which could be used as miniature cathode ray tubes in thin high-brightness low-energy low-weight displays. This type of display would consist of a group of many tiny CRTs, each providing the electrons to hit the phosphor of one pixel, instead of having one giant CRT whose electrons are aimed using electric and magnetic fields. These displays are known as field emission displays (FEDs).
• transistor: developed at Delft, IBM, and NEC.

Chemical

• air pollution filter: Future applications of nanotube membranes include filtering carbon dioxide from power plant emissions.^[4]
• biotech container: Nanotubes can be opened and filled with materials such as biological molecules, raising the possibility of applications in biotechnology.
• hydrogen storage: Research is currently being undertaken into the potential use of carbon nanotubes for hydrogen storage. They have the potential to store between 4.2 and 65% hydrogen by weight. This is an important area of research, since if they can be mass produced economically there is potential to contain the same quantity of energy as a 50l gasoline tank in 13.2l of nanotubes. See also, Hydrogen Economy.^[5]
• water filter: Recently nanotube membranes have been developed for use in filtration. This technique can purportedly reduce desalination costs by 75%. The tubes are so thin that small particles (like water molecules) can pass through them, while larger particles (such as the chloride ions in salt) are blocked.

Mechanical

• oscillator: fastest known oscillators (> 50 GHz).
• nanotube membrane: Liquid flows up to five orders of magnitude faster than predicted by classical fluid dynamics.
• slick surface: slicker than Teflon and waterproof.

In electrical circuits

Carbon nanotubes have many properties—from their unique dimensions to an unusual current conduction mechanism—that make them ideal components of electrical circuits. Currently, there is no reliable way to arrange carbon nanotubes into a circuit.

The major hurdles that must be jumped for carbon nanotubes to find prominent places in circuits relate to fabrication difficulties. The production of electrical circuits with carbon nanotubes are very different from the traditional IC fabrication process. The IC fabrication process is somewhat like sculpture - films are deposited onto a wafer and pattern-etched away. Because carbon nanotubes are fundamentally different from films, carbon nanotube circuits can so far not be mass produced.

Researchers sometimes resort to manipulating nanotubes one-by-one with the tip of an atomic force microscope in a painstaking, time-consuming process. Perhaps the best hope is that carbon nanotubes can be grown through a chemical vapor deposition process from patterned catalyst material on a wafer, which serve as growth sites and allow designers to position one end of the nanotube. During the deposition process, an electric field can be applied to direct the growth of the nanotubes, which tend to grow along the field lines from negative to positive polarity. Another way for the self assembly of the carbon nanotube transistors consist in using chemical or biological techniques to place the nanotubes from solution to determinate place on a substrate.

Even if nanotubes could be precisely positioned, there remains the problem that, to this date, engineers have been unable to control the types of nanotubes—metallic, semiconducting, single-walled, multi-walled—produced. A chemical engineering solution is needed if nanotubes are to become feasible for commercial circuits.

Metallic and semiconducting nanotubes

Most experimentally observed CNTs are multi-walled structures with outer most shell diameters exceeding 10 nm.^{[citation needed]} Since current conduction in a MWCNT is known to be mostly confined to the outermost single-walled nanotube and since band gap of a SWCNT varies inversely with its diameter, MWCNTs are metallic in nature. SWCNTs can be either metallic or semiconducting depending on the way the roll-up of the graphene sheet occurs - an aspect termed as Chirality, and if all the roll-up types are realized with equal probability, 1/3 of the SWCNTs end up being metallic and 2/3 semiconducting. Thus, when CNTs are fabricated either by arc growth, laser ablation or chemical vapor deposition (CVD), a mixture of metallic and semiconducting nanotubes is formed.

Carbon Nanotube Interconnects

Metallic CNTs have aroused a lot of research interest in their applicability as Very-large-scale integration (VLSI) interconnects of the future because of their desirable properties of high thermal stability, high thermal conductivity and large current carrying capacity. An isolated CNT can carry current densities in excess of 1000 MA/sq-cm without any signs of damage even at an elevated temperature of 250 degrees C, thereby eliminating electromigration reliability concerns that plague Cu interconnects. Recent modeling work comparing the performance, power dissipation and thermal/reliability aspects of CNT interconnect to scaled copper interconnects have shown that CNT bundle interconnects can potentially offer advantages over copper^[6] Additionally, the concept of hybrid CNT/Cu interconnects-employing CNT vias in tandem with copper interconnects has been shown to offer advantages from a reliability/thermal-management perspective. More information on state-of-the-art of CNT interconnects (including their fabrication) can be found in the literature.

Carbon Nanotube Transistors

Semiconducting CNTs have been used to fabricate field effect transistors (CNTFETs), which show promise due to their superior electrical characteristics over silicon based MOSFETs. Since the electron mean free path in SWCNTs can exceed 1 micrometer, long channel CNTFETs exhibit near-ballistic transport characteristics, resulting in high speed devices. In fact, CNT devices are projected to be operational in the frequency range of hundreds of GHz. Recent work detailing the advantages and disadvantages of various forms of CNTFETs have also shown that the tunneling based CNTFET offers better characteristics compared to other CNTFET structures. This device has been found to be superior in terms of subthreshold slope - a very important property for low power applications.

Challenges in Electronic Design and Design Automation

Although CNT devices and interconnects have been separately shown to be promising in their own respects, there have been few efforts to successfully combine them in a realistic circuit. Most CNTFET structures employ the silicon substrate as a back gate. Applying different back gate voltages might become a concern when designing large circuits out of these devices. Several top-gated structures have also been demonstrated, which can alleviate this concern. Recently, a fully integrated logic circuit built on a single nanotube has been reported. However, this circuit also employs a back-gate. Additionally, there are still several process related challenges that need to be addressed before CNT-based devices and interconnects can enter mainstream VLSI process. This makes it an exciting and open field for research. Problems like purification, separation of carbon nanotubes, control over nanotube length, chirality and desired alignment, low thermal budget as well as high contact resistance are yet to be fully resolved. Although these are serious technological challenges, innovative ideas have been proposed to build practical transistors out of nano-networks. Since lack of control on chirality produces a mix of metallic as well as semi-conducting CNTs from any fabrication process and it is difficult to control the growth direction of the CNTs, random arrays of SWCNTs (that are easily produced) have been proposed to build thin film transistors. This idea can be further exploited to build practical CNT based transistors and circuits without the need for precise growth and assembly.

As fiber and film

One application for nanotubes that is currently being researched is high tensile strength fibers. Two methods are currently being tested for the manufacture of such fibers. A French team has developed a liquid spun system that involves pulling a fiber of nanotubes from a bath which yields a product that is approximately 60% nanotubes^{[citation needed]}. The other method, which is simpler but produces weaker fibers uses traditional melt-drawn polymer fiber techniques with nanotubes mixed in the polymer. After drawing, the fibers can have the polymer component burned out of them leaving only the nanotube or they can be left as they are.

Ray Baughman's group from the NanoTech Institute at University of Texas at Dallas produced the current toughest material known as of mid-2003 by spinning fibers of single wall carbon nanotubes with polyvinyl alcohol. Beating the previous contender, spider silk, by a factor of four, the fibers require 600 J/g to break^[7] In comparison, the bullet-resistant fiber Kevlar is 27–33 J/g. In mid-2005, Baughman and co-workers from Australia's Commonwealth Scientific and Industrial Research Organization developed a method for producing transparent carbon nanotube sheets 1/1000th the thickness of a human hair capable of supporting 50,000 times their own mass. In August 2005, Ray Baughman's team managed to develop a fast method to manufacture up to seven meters per minute of nanotube tape.^[8] Once washed with ethanol, the ribbon is only 50 nanometers thick; a square kilometer of the material would only weigh 30 kilograms.

In 2004, Alan Windle's group of scientists at the Cambridge-MIT Institute developed a way to make carbon nanotube fiber continuously at the speed of several centimetres per second just as nanotubes are produced. One thread of carbon nanotubes was more than 100 metres long. The resulting fibers are electrically conductive and as strong as ordinary textile threads.^[9][10]

For some current applications of carbon nanotubes, see: Timeline of carbon nanotubes

References

See also

• List of nanotechnology applications

Literature on carbon nanotubes in VLSI

1. S. Iijima, "Helical Microtubules of Graphitic Carbon," Nature, Vol. 354, pp. 56-58, 1991.
2. ITRS, "International Technology Roadmap for Semiconductors-2005 edition," SIA, Available online: http://www.itrs.net 2005.
3. M.S. Dresselhaus, G. Dresselhaus and Ph. Avouris, Editors, Carbon Nanotubes: Synthesis, Structure, Properties and Applications, Springer-Verlag, 2000.
4. F. Kreupl, et al., "Carbon Nanotubes in Interconnect Applications," Microelectronic Engineering, 64, pp. 399-408, 2002.
5. J. Li, et al., "Bottom-up Approach for Carbon Nanotube Interconnects," Applied Physics Letters, Vol. 82, No. 15, pp. 2491-2493, April 2003.
6. N. Srivastava and K. Banerjee, "Performance Analysis of Carbon Nanotube Interconnects for VLSI Applications," ICCAD, 2005, pp. 383-390.
7. N. Srivastava, R.V. Joshi and K. Banerjee, "Carbon Nanotube Interconnects: Implications for Performance, Power Dissipation and Thermal Management," IEDM, 2005, pp. 257-260.
8. K. Banerjee and N. Srivastava, "Are Carbon Nanotubes the future of VLSI Interconnections?", ACM Design Automation Conference, 2006, pp. 809-814.
9. K. Banerjee, S. Im and N. Srivastava, "Can Carbon Nanotubes Extend the Lifetime of On-Chip Electrical Interconnections?" IEEE Nano Networks Conference, 2006.
10. P. Avouris, et al., "Carbon Nanotube Electronics," Proc. IEEE, Vol. 91, pp. 1772-1784, 2003.
11. S. Wind, J. Appenzeller, and P. Avouris, "Lateral scaling in CN fieldeffect transistors," Phys. Rev. Lett., Vol. 91, pp. 058 301-1-058 301-4, 2003.
12. S. Hasan, S. Salahuddin, M. Vaidyanathan and M. A. Alam, "High-Frequency Performance Projections for Ballistic Carbon-Nanotube Transistors," IEEE Transactions on Nanotechnology, Vol. 5, No. 1, pp. 14-22, 2006.
13. J. Appenzeller, et al., "Comparing Carbon Nanotube Transistors - The Ideal Choice: A Novel Tunneling Device Design," IEEE TED, Vol. 52, No. 12, pp. 2568-2576, 2005.
14. S. J. Wind, J. Appenzeller, R. Martel, V. Derycke and Ph. Avouris, "Vertical scaling of carbon nanotube field-effect transistors using top gate electrodes," Applied Physics Letters, Vol. 80, No. 20, 3817 - 3819, 2002.
15. D. V. Singh, K. A. Jenkins, J. Appenzeller, D. Neumayer, A. Grill and H.-S. P. Wong, "Frequency Response of Top-Gated Carbon Nanotube Field-Effect Transistors," IEEE Transactions on Nanotechnology, Vol. 3, No. 3, pp. 383-387, 2004.
16. Z. Chen, J. Appenzeller, Y.-M. Lin, J. Sippel-Oakley, A. G. Rinzler, J. Tang, S. J. Wind, P. M. Solomon and Ph. Avouris, "An Integrated Logic Circuit Assembled on a Single Carbon Nanotube," Science, Vol. 311, p. 1735, 2006.
17. E. S. Snow, J. P. Novak, P. M. Campbell, and D. Park "Random networks of carbon nanotubes as an electronic material," Applied Physics Letters, Vol. 82, No. 13, pp. 2145 - 2147, 2003.

This article includes material from a column in the ACM SIGDA e-newsletter by Prof. Kaustav Banerjee
Original text is available here.