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Science and Technology of Nanotube-Based Materials

OTTO Z. ZHOU

Department of Physics and Astronomy and Curriculum in Applied and Materials Science University of North Carolina Chapel Hill, North Carolina

Carbon nanotubes (Iijima, 1991; Dresselhaus et al., 1996) are a specific type of one-dimensional nanomaterial that has sparked people's imaginations. A cover story in American Scientistmagazine a few years ago noted that carbon nanotubes could be used to build a space cable connecting the Earth and moon. News from NASA indicates that, in the near future, spacecraft may be based solely on carbon material—powered by either fuel cells based on carbon materials or lithium-iron batteries based on nanomaterials. Carbon materials also were featured very prominently in the recent national nanotechnology initiative and were mentioned in the President's State of the Union address, in which he referred to carbon nanotubes as a thousand times stronger than steel.

Are carbon nanotubes useful at all in the near term—within the next 5 years, rather than 20 or 30 years down the road? Let us go back to first-year chemistry to think about the materials that can be made from carbon. Carbon certainly is unique in the periodic table; it is truly multifunctional because of its ability to form either sp2or sp3bonds. It can form allotropes with very different structures. For example, diamond with sp3bonding has a closely-packed structure, a large elastic modulus, and is electrically insulating. Graphite with sp2bonding has a layer structure that is very strong within the layer due to the strong covalent bonds, but weak between the layers due to the weak van der Waalsbonds. As a result, graphite can be used as a lubricant and battery electrode because ions can be stored between the graphite layers.

Buckyballs can be thought of as zero-dimensional structures. They are a truly nanoscale material, because the diameter of the molecule is about 1 nm (10−9m). Assembled into a solid with weak intermolecular bonding, buckyballs have very interesting properties. For example, if C60 molecules are charged,



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Page 89 Science and Technology of Nanotube-Based Materials OTTO Z. ZHOU Department of Physics and Astronomy and Curriculum in Applied and Materials Science University of North Carolina Chapel Hill, North Carolina Carbon nanotubes (Iijima, 1991; Dresselhaus et al., 1996) are a specific type of one-dimensional nanomaterial that has sparked people's imaginations. A cover story in American Scientistmagazine a few years ago noted that carbon nanotubes could be used to build a space cable connecting the Earth and moon. News from NASA indicates that, in the near future, spacecraft may be based solely on carbon material—powered by either fuel cells based on carbon materials or lithium-iron batteries based on nanomaterials. Carbon materials also were featured very prominently in the recent national nanotechnology initiative and were mentioned in the President's State of the Union address, in which he referred to carbon nanotubes as a thousand times stronger than steel. Are carbon nanotubes useful at all in the near term—within the next 5 years, rather than 20 or 30 years down the road? Let us go back to first-year chemistry to think about the materials that can be made from carbon. Carbon certainly is unique in the periodic table; it is truly multifunctional because of its ability to form either sp2or sp3bonds. It can form allotropes with very different structures. For example, diamond with sp3bonding has a closely-packed structure, a large elastic modulus, and is electrically insulating. Graphite with sp2bonding has a layer structure that is very strong within the layer due to the strong covalent bonds, but weak between the layers due to the weak van der Waalsbonds. As a result, graphite can be used as a lubricant and battery electrode because ions can be stored between the graphite layers. Buckyballs can be thought of as zero-dimensional structures. They are a truly nanoscale material, because the diameter of the molecule is about 1 nm (10−9m). Assembled into a solid with weak intermolecular bonding, buckyballs have very interesting properties. For example, if C60 molecules are charged,

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Page 90 they become superconductors, with the highest superconducting transient temperature of any organic superconductor (Andreoni, 2000). Carbon nanotubes come with different chiral angles. Calculations and experimental results have shown that they can be either metallic or semiconducting depending on the chirality. This is very different from what we typically know of semiconductors, which have to be doped to make either a P-type or an N-type. Here, then, is a structure that intrinsically becomes either metallic or semiconducting. Why should we be interested in carbon nanotubes? For one thing, there are not very many materials that have structural perfection at a molecular level as ideal as a single carbon nanotube. One can think about using their aspect ratio and small diameter for imaging applications. Also, they have very good mechanical properties and thermal properties. Theoretical calculations and measurements performed on individual carbon nanotubes have shown that their elastic modulus is as high as that of diamond, on the order of one terapascal. Indeed, if we could make a defect-free cable—one as long as we wanted—then a cable to connect the Earth and the moon would be within the realm of possibility. With carbon nanotechnology and nanotubes, theory is ahead of experimental data. Because a carbon nanotube has a relatively simple structure, its properties can be readily calculated. For example, one can ask questions such as, “If I start with a perfect carbon nanotube and remove one carbon atom from the graphite lattice, what property would result?” Or, “How would twisting or bending change the electronic properties?” The results of these calculations are very interesting and suggest that the carbon nanotubes can function as the smallest sensors, transistors, and so on. However, the difficulty here is how to control materials structure and properties at the atomic and molecular level in actual experiments. Right now, there are three main techniques for making carbon nanotubes: laser ablation, arc-discharge, and chemical vapor deposition (Dresselhaus et al., in press). Each has its own advantages and disadvantages. Although rapid progress has been made in terms of synthesis of carbon nanotubes with selected diameters and orientation, there is still no effective method to control the nanotube chirality that is a prerequisite for utilizing carbon nanotubes in electronic devices. Nanotechnology is becoming a reality in no small part due to advances in imaging technology such as scanning probe microscopy and electron microscopy. Because of its sharp tip and large aspect ratio, a carbon nanotube attached to the tip of an atomic force microscope (AFM) can significantly enhance the microscope's resolution as demonstrated by researchers at Rice University (Dai et al., 1996). In addition, these functionalized AFM tips can be used to measure the binding energy of larger molecules—that is, the point at which they break or dissociate from the backbone structure (Wong et al., 1998). There are start-up companies now attempting to commercialize these kinds of AFM tips.

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Page 91 Carbon nanotubes are being investigated for chemical detection and storage applications. For example, automotive companies are looking at carbon nanotubes because of their reported (but controversial) high hydrogen storage capacities (Dillon et al., 1997). By replacing the carbon materials used as the anodes in current lithium-ion batteries with carbon nanotubes, it is possible to increase the battery lifetime. The advantage of carbon nanotubes is that, if processed such that their inner cores are accessible for diffusion, the storage capacity for both Li and H2can be significantly increased. Another application of carbon nanotubes is as an electron field emitter. There are two ways one can take electrons out of a metal: one is to apply heat and the second is to apply a bias voltage so that electrons can escape to the vacuum level, which is termed electron field emission. Because of its small diameter and large aspect ratio, a carbon nanotube is expected, and has been shown experimentally, to have a low threshold electric field for electron emission (Zhu et al., 1999). This means that devices such as field emission displays (FED) can operate under lower voltages if carbon nanotubes are used as the emitters. An additional advantage compared to conventional electron emissive materials is processibility and high current capacity. Recent research has demonstrated emission current density as high as 4A/cm2from a macroscopic nanotube film. This far exceeds the performance of other materials such as diamond and enables their application in devices with high current requirements such as microwave amplifiers. Because of the potential market size, the possibility of utilizing carbon nanotubes as cold cathode materials for field emission displays has attracted considerable commercial interest. Several major display companies have devoted substantial R&D effort in this area. A prototype 9-inch field emission display based on carbon nanotubes has recently been demonstrated. Compared to the lithography-based Spindt-type emitters, the fabrication process can be simplified significantly by using carbon nanotubes as the cold cathodes. However, engineering issues such as emission uniformity and stability have yet to be solved. Although the development of carbon nanotubes as practical engineering materials is still at an early stage, there are already indications of how they might be commercialized in the near future, particularly in devices that do not require a significant amount of material. For example, we have recently demonstrated applications of carbon nanotubes in gas discharge tubes used to protect houses and telephone lines from overvoltage (Rosen et al., 2000). This is a relatively simple device composed of two metal electrodes in an inert gas environment. In parallel with the electronic device to be protected, it acts as an insulator. The electronic device carries all the current, and it works fine. However, if there is a lightning strike or a voltage surge, electrons emitted from the metal electrode strike the inert gas and cause plasma breakdown, so a short is created that carries all the current density. The advantage of this device compared to a solid-state device is that it can carry very high current density, but it is not very reliable.

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Page 92 This is okay for telephone lines, but for high-speed internet data lines with more sensitive electronic components, this variation cannot be tolerated. We have shown that by coating the electrodes with carbon nanotubes, we can make gas discharge tubes that improve stability by a factor of 20. They are very stable to about a thousand surges, without much degradation. Carbon nanotubes are, indeed, very interesting in the sense that the material is not only an ideal system to study fundamental science in one dimension but also has promising properties that can lead to some very practical devices in the near future. REFERENCES Andreoni, W., ed. 2000. The Physics of Fullerene-Based and Fullerene-Related Materials. Boston: Kluwer Academic Publishers. Dai, H., J. H. Hafner, A. G. Rinzler, D. T. Colbert, and R. E. Smalley. 1996. Nanotubes as nanoprobes in scanning probe microscopy. Nature 384: 147–151. Dillon, A. C., K. M. Jones, T. A. Bekkedahl, C. H. Kiang, B. S. Bethune, and M. J. Heben. 1997. Storage of hydrogen in single-walled carbon nanotubes. Nature 386: 377–379. Dresselhaus, M. S., G. Dresselhaus, and P. C. Eklund. 1996. Science of Fullerenes and Carbon Nanotubes. San Diego: Academic Press. Dresselhaus, M. S., G. Dresselhaus, and P. Avouris, eds. In Press. Topics in Applied Physics, Vol.80. Heidelberg: Springer-Verlag. Iijima, S. 1991. Helical microtubules of graphite carbon. Nature 354(6348): 56–58. Rosen, R., W. Simendinger, C. Debbault, H. Shimoda, L. Fleming, B. Stoner, and O. Zhou. 2000. Application of carbon nanotubes as electrodes in gas discharge tubes. Applied Physics Letters 76(13): 1668–1670. Wong, S. S., E. Joselevich, A. T. Woolley, C. L. Cheung, and C. M. Lieber. 1998. Covalently functionalized nanotubes as nanometre-sized probes in chemistry and biology. Nature 394: 52–55. Zhu, W., C. Bower, O. Zhou, G. Kochanski, and S. Jin. 1999. Large current density from carbon nanotube field emitters. Applied Physics Letters 75(6): 873–875.