Origins, Properties, and Applications of Carbon Nanotubes and Fullerenes

Transcription

1 Origins, Properties, and Applications of Carbon Nanotubes and Fullerenes by Mario Anthony Rosso Submitted in partial fulfillment of the requirements for the graduate course IT 283 Advance Materials and Processes California State University Fresno Spring 2001

2 TABLE OF CONTENTS List of Tables...iii List of Figures...iv INTRODUCTION... 1 CARBON... 2 GRAPHITE... 2 GRAPHITE RELATED MATERIALS... 3 SINGLE CRYSTAL GRAPHITE FLAKES AND KISH... 3 GRAPHITE FIBERS AND WHISKERS... 4 GRANULAR CARBON MATERIALS... 4 CARBYNES AND CARBOLITES... 4 MISCELLANEOUS FORMS OF CARBON... 5 DIAMOND... 5 DIAMOND RELATED MATERIALS... 6 ORIGINS OF FULLERENES... 7 ORIGINS OF NANOTUBES... 9 FULLERENE STRUCTURES NANOTUBE STRUCTURE MULTI-WALLED NANOTUBES (MWNT) SINGLE-WALLED NANOTUBES (SWNT) PROPERTIES OF CARBON-BASED NANOTUBES MECHANICAL PROPERTIES Tensile Strength...19 Elasticity...19 PROPERTIES OF THERMAL TRANSFER PROPERTIES OF CONDUCTANCE AND RESISTANCE PRODUCTION OF MULTI-WALLED AND SINGLE-WALLED NANOTUBES GRAPHITE ARC-EVAPORATION CHEMICAL VAPOR DEPOSITION (CVD) LASER VAPORIZATION APPLICATIONS MEDICINE APPLICATIONS IN THE CONSTRUCTION OF MOLECULAR-SCALE STRUCTURES AND NANOMACHINES SPRINGS AND BEARINGS GEARS VELCRO i

3 COMPOSITES ENERGY STORAGE CATALYTIC AGENTS FILMS AND COATINGS MICROELECTRONICS AND COMPUTING Switching Devices...32 Memory...33 Field-Emission Devices...34 High-Temperature Superconductors...34 OTHER APPLICATIONS CONCLUSION REFERENCES CITED ii

4 Tables TABLE 1 PROPERTIES OF SINGLE WALLED NANOTUBES...18 Figures FIGURE 1 CARBON PHASE DIAGRAM (DRESSELHAUS, 1996)... 3 FIGURE 3 IDEAL CRYSTALLINE LATTICE STRUTURE FOR DIAMOND (DRESSELHAUS, 1996)... 6 FIGURE 4 3D VISUALIZATION OF THE BUCKMINSTER FULLERENE MOLECULE (DRESSELHAUS, 1996)... 7 FIGURE 5 MASS SPECTROMETRY DAT A ILLUSTRATING THE P ROPONDERANCE OF C60. (DRESSELHAUS, 1996)... 8 FIGURE 6 MICROGTRAPHIC IMAGES OF MULTI-WALLED NANOTUBES (HARRIS, 1999) FIGURE 7 VARIOUS CLOSED-CAGED FULLERENE MOLECULES (DRESSELHAUS, 1996) FIGURE 8 R. BUCKMINSTER FULLER (BUCKMINSTER FULLER INSTITUTE, 2001) FIGURE 9 BACTERIOPHAGE DEMONST RATING NATURAL ICOSAHEDRAL SYMMETRY (DRESSELHAUS, 1996) FIGURE 10 ILLUSTRATION OF A TRUNCATED ICOSAHEDRON CIRCA 1500 (DRESSELHAUS, 1996) FIGURE 11 KAGOME PATTERN IN JAPANESE BASKET ART (HARRIS, 1999) FIGURE 12 MICROGRAPH ILLUSTRATING MWNT STRUCTURE (HARRIS, 1999) FIGURE 13 SWNT LATTICE STRUCTURE (HARIS, 1999) FIGURE 14 ILLUSTRATION OF CHIRAL CLASIFICATION OF SWNT (NANOTUBE IMAGE GALLERY, 2001) FIGURE 15 ELASTIC BUCKLING MECHANISMS OF SWNT (HARRIS, 1999) FIGURE 16 THERMAL CONDUCTIVITY (TOMÁNEK, 2001) FIGURE 17 ARC-EVAPORATION NANOTUBE PRODUCTION APPARTATUS (HARRIS, 1999) FIGURE 18 LASER APPARATUS USED IN THE PRODUCTION OF NANOTUBES (NANOTUBE GALLERY, 2001) FIGURE 19 CONCENTRICALLY ALIGNED NANOTUBE SPRING AND BEARING ASSEMBLY (HARRIS, 1999) FIGURE 20 FULLERENE GEARS (HARRIS, 1999) iii

5 INTRODUCTION Developments in manufacturing periodically call for the discovery or invention of new and exotic materials. Such has been the case with structural composites used in the aerospace industry, ceramics, and high-temperature superconductors. As the millennium dawns, mankind is once again faced with the necessity to develop unique and novel materials that move beyond the barriers of current technology and take advantage of the opportunities not yet imagined. Soon, engineers will be faced with physical limits to the amount of micro-miniaturization possible in electronic components using traditional silicon and metallic materials. Conversely, the advent of nanotechnology and its promise of self-replicating, nanometer-sized machines will require construction materials that cannot bee viewed with the human eye. In the future, it will be the chore of remarkably tiny machines, perhaps having some stiff and strong nanotube components to carry out the pick and place of material to build other little machines, devices, cars, houses, roads and so on, as pre-programmed. In the future, we will be giving up control, and instruction sets embedded in the nanotechnological devices will be taking over on the local level. (AScribe Newswire, January 2000) This report seeks to introduce the reader to a particular type of material that will surely serve as the picayune building blocks of these future machines. Based on recently discovered, stable form of crystalline carbon, these building materials of the future exhibit properties, both familiar and exotic, at a scale thousands of times smaller than that of a cross-section of human hair. This breakthrough - the discovery of the carbon-based family of fullerenes - happened quite by accident at Rice University in Houston, Texas. While some discussion will be made of the buckminsterfullerene molecule, the bulk of this report is focused on the nanotube and its miraculous properties and potential applications. 1

6 The research contained within this document seeks to describe carbon, the elementary atom on which all of the structures discussed here are based. Attention has been paid to the electrical, mechanical, and thermal properties of fullerenes and carbon nanotubes. Methods of production, as well as applications in areas of computing technology, field emission, medicine, and nanotechnology are expressed in detail within. CARBON Carbon, element six on the periodic table, is one of the most abundant substances on earth. Existing alone or in combination with other elements, compounds of carbon are prevalent in both solid and gaseous phases at room temperature and ambient pressures. It is the central element in essential organic compounds such as proteins, lipids, hydrocarbons, and polymers. Throughout history, carbon has been one of the most thoroughly studied elements; and it is this study that forms the basis of our understanding of organic chemistry. Despite mankind s age old familiarity with the element, only two forms of pure crystalline carbon allotropes were known until These, of course are diamond and graphite. The equilibrium phase diagram for Carbon is presented in Figure 1. This diagram illustrates the temperatures and pressures at which pure carbon exists in graphitic and diamond forms. GRAPHITE Graphite is the most common, natural equilibrium form of pure carbon in solid structure. It is stable at both ambient pressures and temperatures and is universally found in pencil lead and lubricants. Thought to be the stiffest material in existence, graphite is anisotropic and possesses characteristics of high thermal conductivity. Graphite s crystal structure is illustrated in Figure 2. Solid graphite is formed by stacking sheets of carbon atoms, called graphenes, in a honeycomb lattice. Each carbon atom is covalently bonded to three neighboring carbon atoms. 2

7 In the horizontal plane, within a single graphene layer, the structure is extremely strong and is consider a semimetal that easily conducts electrical current. In the perpendicular plane, between lattice layers, graphite is a poor conductor and is very weak. It is those frail Van der Waals bonds that form the weak interplanar adhesion between graphite sheets that make graphite well suited for use as pencil lead and lubricant. These weak bonding structures also make the use of epoxy necessary in order to bond graphite fibers together for use in high-strength composite materials. (Crespi, 2001) Figure 2 Honeycomb Cryslatlline Lattice for Graphite (Dresselhaus, 1996) Figure 1 Carbon Phase Diagram (Dresselhaus, 1996) GRAPHITE RELATED MATERIALS SINGLE CRYSTAL GRAPHITE FLAKES AND KISH Graphite Flakes are naturally occurring single crystals of graphite. Kish is a synthetic, single crystal form of graphite formed by melting metals with high carbon content. Highly Oriented Pyrolytic Graphite (HPOG) is formed through the pyrolysis of hydrocarbons at 3

8 temperatures above 2000 C and stress annealed near 3300 C. Using this method, flakes as thick as 5mm are possible. HPOG possesses properties near that of single crystal graphite and is used primarily for X-Ray and neutron spectrometry. (Dresselhaus, 1996) GRAPHITE FIBERS AND WHISKERS Carbon fibers have long been used as structural materials in industrial composites. Known for their extreme strength, carbon fibers are generally only about 10µm in width but have very long lengths. Graphite fibers are grown in configurations that approximate those of tree rings or onionskin. Graphite whiskers are graphene sheets rolled up into a scroll-like cylinder. Similar to the nanotubes discussed throughout this document, graphite whiskers have a high modulus of elasticity and are highly conductive. (Dresselhaus, 1996) GRANULAR CARBON MATERIALS Glassy carbon is a unique material that is produced by heating certain polymers slowly to close to 1000 C. A granular substance that is stable at high temperatures, glassy carbon can be polished to a shiny black finish. Compounds of glassy carbon are good conductors of heat, as well as being impermeable, biocompatible, and possessing moderate hardness. Carbon black material is produced through the dehydrogenation of hydrocarbons. It finds use as a filler material and is added to other materials to modify their properties. Carbon black exists in a granular state as fine carbon particles. Carbon-Coated Carbide Particles are approximately 100Å in diameter and non-reactive. These particles are typically formed near grains of cementite like those found in carbon steels. (Crespi, 2001) CARBYNES AND CARBOLITES Carbynes are all-carbon polymers that have a molecular structure of repeating carbons [ C = C ] n. Stable at room temperatures and pressures, carbynes have a silver-white 4

9 appearance and demonstrate hardness values between those of diamond and graphite. Carbynes form during very rapid transformations of liquid carbon into solid. Carbolites are transparentorange crystals of carbon with a hexagonal lattice structure. (Dresselhaus, 1996) MISCELLANEOUS FORMS OF CARBON Amorphous carbon is formed by irradiation of carbon materials and possesses various degrees of disorder. This unique form of carbon is used to add semiconducting properties to graphite sheets. Porous carbon contains nanometer sized pores with high surface area. The most common forms of porous carbon are activated carbon, ex-foliated graphite, and carbon aerogels. Liquid carbon is a produced by melting diamond or graphite solids. Since carbon has the highest melting point of any element, the crucible used to liquefy the diamond or graphite must also be made of carbon. Lasers are used to obtain the high temperatures necessary to transform solid carbon into liquid. (Dresselhaus, 1996) DIAMOND Graphite, when exposed to extreme heat and pressures, forms diamond. This process can be aided significantly by the introduction of a seed particle as catalyst. Graphite s isotropic sibling, diamond is well known as the hardest material on earth. Commonly found in industrial abrasives and gemstones, diamond will decompose to its equilibrium graphite state over time. This transformation can be greatly accelerated by the introduction of high temperatures. Along with graphite, diamond possesses the highest thermal conductivity and the highest melting point of any known substance. Diamond also boasts the highest atomic density of any solid, with a cubic lattice of pure carbon atoms. Figure 3 depicts the highly symmetrical cubic crystal structure found in diamond. Each carbon atom is covalently bonded to four neighboring carbons in a tetrahedral configuration. Unlike graphite, which possesses great strength within its two- 5

10 dimensional bonding structure, diamond forms strong bonds in three-dimensions. (Dresselhaus, 1996) Possessing a 56% higher atomic density than graphite, diamond is composed of two interpenetrating face centered cubic (FCC) lattice structures. While rare in its natural form, diamond can be grown synthetically, especially when assisted by the presence of a seed particle. It should be noted, however, that the thermal and electrical properties of diamond can be adversely influenced by the introduction of impurities. The purest forms of diamonds, however, have shown to have semiconducting properties. (Dresselhaus, 1996) Figure 3 Ideal Crystalline Lattice Struture for Diamond (Dresselhaus, 1996) DIAMOND RELATED MATERIALS The two most common forms of diamond-related materials are chemical vapor deposited diamond films and cage hydrocarbon molecules. Vapor deposited films are formed at high temperatures and pressures in the presence of an atomic hydrogen catalyst. These films are useful in coating Si, quartz, Ni, and W substrates. Among the most common diamond-like cage hydrocarbons are Adamantane, Cubane, and Diamantane. (Dresselhaus, 1996) 6

11 ORIGINS OF FULLERENES Carbon has been thoroughly studied and its properties known and documented for many years. Readily combining with many different elements, carbon is found in innumerable simple and complex molecular compounds. Previous to 1985, however, pure carbon was thought to exist in only two naturally occurring, solid forms: diamond and graphite. This all changed in 1985 and scientists were forced to alter their perception of the natural world with the discovery of a third allotropic form of solid carbon, the buckminsterfullerene. Buckminsterfullerenes, or the more affectionately named, buckyball, are symmetrical, ball-shaped molecules consisting of sixty carbon atoms arranged in an icosahedral geometric structure resembling a soccer ball. Figure 4 illustrates the icosahedral symmetry of the buckminster fullerene molecule. Buckminsterfullerenes are so named because of their resemblance to the geodesic dome structures created by famed architect R. Buckminster Fuller. (Harris, 1999) Figure 4 3D Visualization of the Buckminster Fullerene Molecule (Dresselhaus, 1996) Buckyballs were discovered by accident in August of 1985 at Rice University in Houston Texas by Richard Smalley and Harry Kroto. Smalley, a professor and researcher at Rice, was 7

12 working on the study of laser vaporization and its application in synthesizing clusters of semiconducting materials such as Silicon and Gallium Arsenide. Kroto, a University of Sussex professor was visiting Rice in hopes of recreating, in the laboratory, processes that take place on the surface of Red Giant stars. Together, with the assistance of a team of graduate students, Kroto and Smalley vaporized clusters of carbon atoms from a graphite substrate using a highpowered laser. Employing a mass spectrometer to measure carbon clusters in the condensing plasma, the team discovered, quite unexpectedly, that clusters of C 60 occurred repeatedly in large quantities. A graph of the mass spectrometer reading is included in figure 5. Figure 5 Mass spectrometry data illustrating the preponderance of C60. (Dresselhaus, 1996) After developing a number of hypotheses in regards to the geometry of the newly discovered molecule, Kroto and Smalley agreed on a conceptual structure resembling a 8

13 symmetrical, closed cluster where the carbon atoms are bonded to each other in a spheroidal molecule of great stability. The two researchers published their findings in November of 1985 in the journal Nature. (Harris, 1999) Kroto and Smalley later shared the 1996 Nobel Prize for Chemistry with Robert Curt of Rice University for their discovery of the buckminsterfullerene. (Browne, 1998) After the momentous discovery, the next step in the ultimate quest to produce commercial applications for this new form of carbon was to develop a method to produce large quantities of fullerenes. Originally, only minute amounts of buckyballs could be produced using the laser vaporization methods established by Smalley and Kroto. The breakthrough came from yet another duo of scientists, Wolfgang Krätschmer of the Max Planck Institute at Heidelburg and Donald Huffman of the University of Arizona. The two researchers and their associates used a carbon arc to vaporize graphite in a helium atmosphere, thus producing fullerene rich soot particles. Dissolving the soot in benzene produced a red solution containing fullerene crystals (90% C 60 and 10% C 70 ). Using this improved method, large quantities of pure fullerene crystals could be precipitated out of the benzene solution after drying. (Harris, 1999) ORIGINS OF NANOTUBES While the discovery of fullerenes and their remarkable properties excited researchers, practical applications for buckyballs seemed decades off. In again, quite by accident - a new class of fullerenes were discovered which possess revolutionary properties that should prove useful for both short and long term applications. Discovered in 1991 at NEC laboratories in Japan, nanotubes are elongated fullerenes that resemble nanometer-sized ropes and test tubes. The discovery of nanotubes was made by electron microscopist, Sumio Iijima. Iijima was inspired by an article in Nature written by Krätschmer and Hoffman. The accidental discovery 9

14 was made as Iijima was studying soot deposits from arc-evaporation experiments with a TEM microscope. Results of Iijima s initial studies focused on soot deposited on the walls of the experimental apparatus and were disappointing. Soot from the vessel walls was amorphous and uninteresting. Success came only after Iijima began to look at soot collected on the cathode of the arc apparatus. Here interesting structures including long, hollow fibers that had never been seen before were discovered. The first structures observed were multi-walled tubes: essentially coaxial collections of successive tubes enclosing other tubes. Single-walled nanotubes were observed two years later. A TEM micrograph of Iijima s nanotubes is included in Figure 6. Iijima published his findings in the October, 1991 Nature, and thus began a huge movement in the scientific community to find out more about these miniscule tubes and their properties, structures, and applications. (Harris, 1999) FULLERENE STRUCTURES Buckminster fullerenes are closed-caged, highly symmetrical molecules. Buckyballs exhibiting 5-fold symmetry much like that found in numerous biological organisms. All fullerenes have an even number of carbon atoms arranged in a convex-shaped cage. The C 60 molecule is a geodesic spheroid consisting of 20 hexagons and 12 pentagons. C 60 has 60 vertices topped by a single carbon atom each in a truncated icosahedral shape. (Dresselhaus,1997) The buckyball molecule follows Euler s Theorem, which specifies that any convex closed-caged structure can be made up of any number of hexagons but must include exactly 12 pentagons in order to provide the appropriate curvature necessary to close the cage. Stated mathematically, Euler s Theorem is: f + v = e + 2 (where f is he number of faces, v is the number of vertices, and e is the number of edges in the icosahedron). According to Euler s theorem, the smallest fullerene that can exist is C 20. (Dresselhaus, 1996) 10

15 Figure 6 Microgtraphic images of multi-walled nanotubes (Harris, 1999) Although C 20 is theoretically possible, it is a highly unlikely structure due to the fact that two pentagons do not go together well structurally. This is due to added strain on the geometry. This phenomenon of pentagons not existing well in pairs is known as the Isolated Pentagon Rule. The smallest fullerene to fit the Isolated Pentagon Rule is C 60. Fullerenes with less than 60 carbon atoms are very rare. The next smallest fullerene is C 70, a rugby-ball shaped molecule that is commonly found in soot. Figure 7 presents a number of fullerene molecules including C 60 and C 70. (Dresselhaus, 1996) 11

16 Figure 7 Various closed-caged fullerene molecules (Dresselhaus, 1996) Similar to structures found in metallurgy, buckyballs can be assembled into ordered FCC crystal structures, where C 60 molecules take the place of traditional atoms in the lattice. The FCC lattice allows enough room for other atomic elements to interstitially lodge themselves in with C60 molecules. The resulting crystalline compounds have been proven to have interesting characteristics such as high temperature superconductivity. (Mihaly) SIMILARITY TO NATURALLY OCCURRING STRUCTURES AND ARCHITECTURE Buckminsterfullerenes have many natural and architectural parallels. The most notable are the geodesic domes created by architect, R. Buckminster Fuller after which buckyballs are named (see Figure 8). The truncated icosahedral shape, facilitated by the intermingling of pentagons and hexagons, occurs repeatedly in biology as well. This unique structure affords the 12

17 largest amount of volume in the smallest surface area. Its five-fold symmetry is found in organisms ranging from the F?174 Bacteriophage as shown in Figure 9, to the curved shell of the tortoise. Leonardo DaVinci marveled at this unique shape at the beginning of the 16 th. century (see Figure 10). The use of pentagonal shapes to curve an otherwise flat surface of polygons goes back to early Japanese basket making. The Kagome pattern weaved with bamboo reeds allows concave and convex geometries like those found in Figure 11. (Dresselhaus, 1996) Figure 8 R. Buckminster Fuller (Buckminster Fuller Institute, 2001) Figure 9 Bacteriophage demonstrating natural icosahedral symmetry (Dresselhaus, 1996) 13

18 Figure 10 Illustration of a truncated icosahedron circa 1500 (Dresselhaus, 1996) Figure 11 Kagome pattern in Japanese basket art (Harris, 1999) NANOTUBE STRUCTURE In regards to the potential applications in industry, medicine, and research; nanotubes are by far the most important fullerene discovered to date. Nanotubes are all carbon, crystalline macromolecules that resemble strands of rope or string. They are built around one of the strongest bonding forces in nature: the covalent bond between carbon atoms. (Physics World, June 2000) Rolled into a seamless cylinder of carbon atoms and bonded in a repeating hexagonal lattice, nanotubes can grow up to microns or more in length but are only about one nanometer across. Nanotubes can best be described as rolled up sheets of graphite terminated at each end by one-half of a buckminster-fullerene molecule. They exhibit remarkable symmetry with slight variations in regards to the angle upon which the graphene sheets line up along the tube s axis. (Dresselhaus, 1998). Nanotubes, while single molecules, take on the characteristics of monoelemental polymers and solids. Buckyballs consist of only sixty atoms, hence C 60. Nanotubes, on the other 14

19 hand, theoretically, may possess millions of carbon atoms, making rope like structures of C 1,000,000. Richard Smalley, the co-discoverer of the buckminsterfullerene and colleague Boris Yakobson probably put it best in their published description of the scale of nanotubes: One could stare at a chip of graphite as Richard Feynman once stared at a drop of water. After magnifying our chip a billion times, creating a metal-gray rocky landscape about the size of Texas, we might spot a three-foot-diameter pipeline stretching from horizon to horizon. This is the nanotube. Actually, a one nanometer-wide pipeline occupies almost no space even over a substantial length. In fact, nanotubes sufficient to span the 250,000 miles between the earth to the moon at perigee could be loosely rolled into a ball the size of a poppy seed. (Smalley, 1997) MULTI-WALLED NANOTUBES (MWNT) The first nanotube structures discovered where multi-walled collections of concentric cylinders resembling Russian Matryoshka dolls. These nested, all-carbon structures where created by the arc-evaporation experiments discussed above. Instead of perfectly formed buckyball caps at the end of each tube, a variety of fullerene shapes terminated the honeycombed cylinders. Figure 12 depicts the concentric nature of MWNT s. The space between each carbon cylinder is approximately 0.352nm. Other nanotube-like structures of rolled graphene where present in early experiments as well. Most of these structures resembled Swiss rolls, or more appropriately, a graphite sheet rolled up into a scroll. (Harris, 1999) SINGLE-WALLED NANOTUBES (SWNT) Much more uniform in structure, single-walled nanotubes hold far more promise in terms of industrial applications than their multi-walled cousins. SWNT s are narrower, approximately 0.7nm in diameter, and nave fewer defects than MWNT s. Like MWNT s, however, they are almost always closed at each end by a fullerene cage. The caps closing a SWNT, overwhelmingly, are one half of a buckyball molecule at each end. They are far more rare and 15

20 more desirable than MWNT s; possessing ideal structure for future applications. At present, most SWNT s are found in curled and curved strands rather than straight lines. A perfect rendering of SWNT can be seen in Figure 13. (Harris, 1999) Figure 12 Micrograph illustrating MWNT structure (Harris, 1999) Figure 13 SWNT lattice structure (Haris, 1999) Variations in the angle of the graphite planes that make up the bulk of a single-walled nanotube form the basis of how they are classified. This characteristic twist is referred to as 16

21 the tube s chiral angle. There are three distinct classifications based on the chirality of a carbon nanotube. Zigzag nanotubes are so named because the angle at which the garphene sheet is rolled up makes it parallel to the row of zigzag bonds in the hexagonal structure. Armchair nanotubes are so named because the graphene sheet rolls up at an angle that is perpendicular to the bonds in the hexagonal lattice. The last classification, chiral nanotubes, have sheets aligned along the cylinder at some chiral angle other than armchair or zigzag, that is to say somewhere between 0 and 30. Figure 14 depicts the system used in identifying the chiral angle of singlewall nanotubes. (Smalley, 1997) Figure 14 Illustration of chiral clasification of SWNT (Nanotube Image Gallery, 2001) PROPERTIES OF CARBON-BASED NANOTUBES Because nanotubes are the most likely fullerene candidates for use in industry, this section will focus primarily on their unique properties. Table 1 below summarizes the major properties of single-walled nanotubes. 17

22 Table 1: The Physical Properties of Carbon Nanotubes (Adams, 2000) Equilibrium Structure Average Diameter of SWNT's nm Distance from opposite Carbon Atoms (Line 1) Analogous Carbon Atom Separation (Line 2) Parallel Carbon Bond Separation (Line 3) Carbon Bond Length (Line 4) C - C Tight Bonding Overlap Energy 2.83 Å Å 2.45 Å 1.42 Å ~ 2.5 ev Group Symmetry (10, 10) C 5V Lattice: Bundles of Ropes of Nanotubes Triangular Lattice (2D) Lattice Constant 17 Å Lattice Parameter: (10, 10) Armchair Å (17, 0) Zigzag Å (12, 6) Chiral Å Density: (10, 10) Armchair 1.33 g/cm 3 Interlayer Spacing:. Optical Properties (17, 0) Zigzag 1.34 g/cm 3 (12, 6) Chiral 1.40 g/cm 3 (n, n) Armchair (n, 0) Zigzag (2n, n) Chiral Fundamental Gap: For (n, m); n-m is divisible by 3 [Metallic] For (n, m); n-m is not divisible by 3 [Semi- Conducting] 3.38 Å 3.41 Å 3.39 Å 0 ev ~ 0.5 ev Electrical Transport Conductance Quantization (12.9 k O )-1 Resistivity 10-4 O -cm Maximum Current Density A/m 2. Thermal Transport Thermal Conductivity Phonon Mean Free Path Relaxation Time. Elastic Behavior Young's Modulus (SWNT) Young's Modulus (MWNT) Maximum Tensile Strength ~ 2000 W/m/K ~ 100 nm ~ s ~ 1 TPa 1.28 TPa ~ 100 GPa 18

23 MECHANICAL PROPERTIES Far lighter than steel, nanotubes are also between 10 and 100 times stronger. They have been described as the strongest fibers known to man. (Crespi, 2001) Tensile Strength Micro Newton s of force are required to break a single nanotube. No other known material posses a higher tensile strength. Because of their picayune size, scientists have only been able to test the tensile strength of MWNT s. The experiments proved that it would take 63Gpa of pulling strength to break apart MWNT s. In theory, single wall tubes may actually have a tensile strength hundreds of times stronger than steel. (AScribe Newswire, January 2000) Nanotubes get their strength from the strong carbon-to-carbon bonds that hold together its fullerene lattice. Each carbon is covalently bonded to three other carbons. This strong bonding mechanism, coupled with the stability of the geometric structure of nanotubes, makes them remarkably strong. (Physics World, June 2000) Elasticity Elasticity in both single and multi-walled nanotubes is calculated according to Young s Modulus. The elasticity modulus of MWNT s is measured with Transmission Electron Microscopes (TEM). Scientists using the TEM observe and measure the thermal vibrations at each end of the tubes and record their findings. MWNT s have been found to possess N/m 2 or approximately 1Tpa. This measurement is five times that of steel. As expected, the stiffness of each tube is related to that found in individual graphite sheets. It should be noted here that nanotubes grown with catalyst particles have lower elastic modulus due to defects introduced by the catalytic agents. 19

24 When exposed to great axial compressive forces, nanotubes have been shown to bend, twist, kink, and finally buckle. The tubes, however, do not break under the compressive loads. This distinctive behavior is illustrated in Figure 15. When tested under great axial compression, it has been found that nanotubes appear to behave consistent with the Euler Limit. The Euler Limit specifies the point at which a straight tube will buckle. Since the deformation in a nanotube is elastic, the tube returns to its original shape when the load is removed. (Physics World, June 2000) Figure 15 Elastic buckling mechanisms of SWNT (Harris, 1999) While all experimental evidence has been overwhelmingly positive in regards to the elastic strength of nanotubes, there does exists a degree of variability between different experiments and different types of nanotubes. In 1997 a team of scientists including W. Goddard III, G. Gao, and T. Cagin performed experiments to measure Young s Modulus for a series of nanotubes. For each iteration of the experiment, the researchers varied the chiral angle of the tubes being tested. It has been found that the (10,10) armchair variety of nanotubes possess a 20

25 modulus of Gpa. (17,0) zigzag tubes have been measured at Gpa. Finally, (12,6) chiral nanotubes top the list at Gpa. Previous tests at NEC s Princeton lab and the University of Illinois put the average modulus at 18Tpa. Most recently, in 1999, E. Hernandez and Angel Rubio tested SWNT s and found elastic modulus values between 1.22Tpa for (10,10) and (6,6) tubes; and 1.26Tpa for (20,0) chiral nanotubes. Rubio s group believes that the variations in modulus are directly related to the disorder present in the tube (Adams, 2000). Others, however, believe that stiffness increases with tube diameter. (Harris, 1999) Although there has not been a high degree of agreement in regards to the actual elastic modulus values, it is widely accepted that nanotubes behave similar to rubber tubing when buckling under heavy axial compressive loads. (Adams, 2000) PROPERTIES OF THERMAL TRANSFER Nanotubes are extremely stable at high temperatures. They can withstand 2800 C in a vacuum and up to 750 C at normal atmospheric pressures. It is these thermal characteristics as well as other factors that make nanotubes so well suited to serve as electrical conductors. Typical metallic conductors melt at temperatures between 600 C and 1000 C. In addition, nanotubes can transmit as much as 6000 W/mK at room temperature. Diamond can only transmit 3320 W/mK. (Avouris, 2000) Thermal transport properties are dependant upon temperature. Nanotubes conduct heat in an essentially linear fashion at temperatures ranging from 7K to 25K. The slope of the conduction curve rises at temperatures between 25K and 40K, and then levels off near room temperature. Figure 16 illustrates this relationship of thermal conductivity to temperature. (U) 21

26 PROPERTIES OF CONDUCTANCE AND RESISTANCE Nanotubes demonstrate a variety of properties and characteristics in regards to their ability to conduct electricity. In some instances, nanotubes behave like low resistance conductors while in other instances they behave like silicon-based semiconductors. While most polymers are considered to be insulators, nanotubes, like graphite sheets, can be characterized as semi-metals or zero-gap conductors. The factor determining which state a nanotube will operate in is related to its chirality. Chirality, again, describes the angle at which the graphene sheets roll up to form the cylindrical section of the nanotube. (Crespi, 2001) If a nanotube is rolled up evenly, like a sheet of paper with the top and bottom edges lined up, then it acts like a metallic conductor, efficiently carrying electricity. If a nanotube is rolled up askew, like a misbuttoned shirt, then its electrical properties change to those of a silicon-like semiconductor where current can be switched on and off. (Change, 2001) Figure 16 Thermal conductivity (Tománek, 2001) 22

27 A flat sheet of graphite is considered to be a semi-metal. It possesses properties of both metals and semiconductors like copper wire and silicon chips, respectively. In its metallic form, electrons freely move in the sheet, while in its silicon state, electron movement is retarded. Since a nanotube is in essence a rolled up sheet of graphite, it possesses many of these same characteristics. When the seams of the sheet are mated together at an angle that allows each side to match evenly, the quantum mechanical wave functions will similarly match. This evenly matched configuration limits the types of wave functions the electrons are allowed to exhibit and thus affects the motion of the electrons. In certain cases, if a particular defect is present in the honeycomb structure of the tube lattice, it will cause a part of the tube to conduct electricity while a different section of the tube will act as a semiconductor. This phenomenon is referred to in electronics as a Shottky Barrier, and is a common, necessary component in many electronic devices. (Crespi, 2001) When the structure of an SWNT is perfectly aligned, in other words the ends of the graphite sheet match up in a straight line, the tube takes on the characteristics of a ballistic conductor. A ballistic conductor ensures that each electron that enters one end exits the conductor at the opposite end. This highly efficient system possesses very little resistance. Only superconducting materials have resistance values lower than ballistic conductors. Additionally, it has been demonstrated that resistance in nanotubes, contrary to their copper wire counterparts, is independent of length. (Physics World, June 2000) While the length of nanotubes does not appear to be a factor in their resistance to current flow, the diameter of the tube, in addition to the tube s chirality, does indeed affect its conductive properties. 23

28 The geometry of nanotubes limits the electrons to a elect few slices of graphite s energy states. Depending on the diameter of the tube, one of these slices can include the narrow path that joins electrons with conduction states. This special point, called the Fermi point, makes two thirds of the nanotubes metallic. Otherwise, if the slices miss the Fermi point, the nanotubes semiconduct. (Avouris, 2000) Electrical conductance of a nanotube is equal to the inverse of its resistance; and is twice the fundamental quantum of conductance. Therefore, conductance can be expressed as G 0 = 2 e 2 /h where h = Plank s constant and e is the electron charge value. It can be concluded then that resistance should be approximately 6500 Ohms in SWNT regardless of their lengths. MWNT also have been shown to have a conductance of G 0 = 2 e 2 /h. making them ballistic conductors as well. While confirmation of these properties is difficult due to the miniscule size of nanotubes and the relatively gargantuan size of modern laboratory equipment, tests have demonstrated that MWNT s can conduct currents as high as 10 2 A/m 2 without signs of damage. Other resistive conductors such as copper wire would vaporize under similar conditions. (Physics World, June 2000) Some research scientists theorize that nanotubes may be able to carry currents as high as 1013A/cm 2 without risking destruction. (Adams, 2000) PRODUCTION OF MULTI-WALLED AND SINGLE-WALLED NANOTUBES Producing nanotubes for research purposes, and in the future for industrial applications, is performed in numerous ways; most of which rely on the heating of graphite to a sufficient temperature that a plasma state is reached. The vaporized carbon atoms produced in the plasma condense in the form of soot, which is then collected and processed to extract the desired fullerene products. The three methods most commonly used are arc-plasma evaporation, chemical vapor deposition (CVD), and laser vaporization. The single exception to these heat- 24

29 intensive methods involves a ball milling process that produces a large number of nanotubes without vaporization. (Chen) GRAPHITE ARC-EVAPORATION The earliest method employed in the production of nanotubes, arc evaporation involves the use of two opposing graphite rods separated in an apparatus about 2mm apart. One hundred Amps of electricity are then arced across the thin gap in a helium atmosphere thus vaporizing the graphitic ends in a hot plasma. Figure 17 depicts the most common laboratory apparatus necessary for plasma arc evaporation. Approximately 30% of the carbon soot produced contain nanotubes. Tubes produced using this procedure are usually no longer than 50 microns in length and are random in shape. Use of catalytic agents aid in the production and increases the volume of nanotubes produced significantly. (Avouris, 2000) Figure 17 Arc-evaporation nanotube production appartatus (Harris, 1999) 25

30 CHEMICAL VAPOR DEPOSITION (CVD) In CVD, a carbon substrate is heated slowly to approximately 600 C. When the temperature is at the optimum level, methane or other carbon rich gases are circulated inside the chamber. This method, when used with a catalyst, can convert most of the gaseous carbon to nanotubes. CVD is the easiest of all nanotube-producing methods and holds the most promise for making long-length nanotubes. The primary disadvantage of CVD is that it produces primarily multi-walled nanotubes with many defects. These poor quality MWNT s have lower tensile strength than those produced by arc plasma or laser vaporization. (Smalley, 1997) LASER VAPORIZATION Currently the most promising, albeit the most expensive, method for producing single walled nanotubes is laser vaporization. SWNT s are prized by researchers and industry alike. This method of producing nanotubes was originally developed by Richard Smalley and his tem at Rice University. It focuses a laser onto a graphite specimen to produce a plasma. The apparatus used in this process is depicted in Figure 18 below. Using a metallic catalyst such as cobalt, nickel, or iron, nanotubes are condensed from the plasma at about 1200 C. The resultant products are extremely uniform, long (~0.1mm) ropes of SWNT s in yields of approximately 70%. (Smalley, 1997) The success of Smalley s laser evaporation process has led to the creation of the first commercial organization to produce and distribute large quantities of nanotubes. Carbon Nanotechnologies, Inc., founded by Smalley and former Lyondel Chemical CEO, Bob Gower, is currently producing gram quantities of nanotubes daily, with production expected to increase to 20lbs. Per day by Currently, the price for a gram of nanotubes is $2,500. However, as 26

31 process improvements are made and demand grows, prices should drop to approximately $500 within a few years. (The Houston Chronicle, 2000) Figure 18 Laser apparatus used in the production of nanotubes (Nanotube Gallery, 2001) APPLICATIONS The properties of nanotubes are certainly amazing; but even more remarkable are the countless applications that may be possible when these newly discovered materials are integrated into mainstream products. Applications for nanotubes span the breadth of numerous disciplines including construction, manufacturing, medicine, electronics, nanotechnology, and so on. The obvious benefits of size are only the tip of the iceberg in terms of the miraculous properties and qualities that fullerenes and nanotubes will impart to future products and technology. 27

32 MEDICINE Nanotubes and other fullerenes may one day serve as transportation molecules for a variety of drugs and therapeutic compounds. Certain molecules can either be attached to the exterior of fullerenes or carried within the closed-cage structure. In the future, buckyballs might be able to attack diseases such as HIV by carrying drugs directly to the virus. (Hogan, 1998) The open cage of a carbon nanotube might carry biological agents to individual cells in the body, like a targeted, nanometer-sized test-tube. Potentially, nanotubes could be used as probes for atomic force microscopes in he task of identifying chemical characteristics of individual genes. (Avouris, 2000) Since, C 60 molecules are not digestible by the body, they could be a potentially safe alternative to other methods for the introduction of drugs into the body. Buckyballs might also be used to safely introduce heavy metals into the body for medical treatments that involve radiation. By enclosing a radioactive heavy metal particle inside of a C 60 cage, Rice University professor, Lon J. Wilson states that, you would have to break carbon carbon bonds in order to free the metal into the body. (Hogan, 1998) Buckballs are similar in size and shape to many viruses. It has been theorized that C 60 or a related fullerene molecule might be useful as a protease inhibitor for the treatment of the HIV immunodeficiency virus. (Bleeke, 1997) APPLICATIONS IN THE CONSTRUCTION OF MOLECULAR-SCALE STRUCTURES AND NANOMACHINES Nanotechnology is a new and exciting field, just beginning to unfold. Accepted in theory, nanotechnology promises to revolutionize the daily lives of every man woman and child on earth. One of the major obstacles yet to be hurdled in the quest for the inevitable 28

33 development of atomic-sized machines and structures is the availability and suitability of construction materials. Nanotubes appear to have the most potential in overcoming this obstacle. SPRINGS AND BEARINGS Experiments conducted at the University of California, Berkeley in July of 2000, revealed that concentrically configured nanotubes demonstrate mechanical properties akin to both springs and bearings. When two tubes are arranged coaxially, one inside of the other, they can be made to rotate independently with no signs of friction or wear. The concentrically configured tubes also can move axially, up and down, like a collapsible telescope. Again, this linear movement shows no signs of wear of fatigue. Thus these coaxially arranged nanotubes operate as linear and rotational bearings. The Van der Wals forces that lubricate the surfaces of the tubes also exert small forces that serve to retract the inner tube back into the outer tube when it is pulled in a linear direction. This allows the telescoping nanotubes to behave like tiny springs. A nanotube configured as both a bearing and spring is pictured below in Figure 19. These are two essential components necessary to build the nanometer scale machines envisioned for the future. (AScribe Newswire, 2000) Figure 19 Concentrically aligned nanotube spring and bearing assembly (Harris, 1999) 29

34 GEARS The symmetry and shape of fullerene molecules and nanotubes make them perfect candidates for the construction of nano-scale gears. Michael R. Falvo of the Universtiy of North Carolina at Chapel Hill envisions nanotube gears with atom-sized teeth. A conceptual diagram of nanometer gears is included below in Figure 20. (P.W., 2000) Figure 20 Fullerene gears (Harris, 1999) VELCRO The common loop-and-hook configuration of Velcro could be mimicked at the atomic level by using nanotubes. The ultra-high temperature resistance and super strength of nanotube Velcro would make them extremely powerful fasteners. (AScribe, February 2000) Nano Velcro could be used to manufacture anything from shuttles to micro-robots and would require the same force necessary to form diamond to pull the two sides apart. (AScribe, February 2000) 30

35 COMPOSITES While carbon fibers are already in widespread use in composite materials, nanotube fibers have distinct advantages over their longer cousins. The 1000-fold difference in size of traditional carbon fibers makes them more susceptible to breakage. (Bleeke, 1997) The reason for this is that the breaking strength of brittle materials decreases as the size of the longest internal flaw increases. A 30nm diameter fiber cannot have a transverse flaw bigger than 30nm. (Bleeke, 1997) ENERGY STORAGE Fullerenes, including nanotubes, may have potential as energy storage devices similar to batteries and fuel cells. Like graphite, nanotubes can store ions of lithium a commonly used material in certain batteries. Unlike graphite, however, nanotubes may be able to hold the lithium ions both inside and outside its tubular structure. His would prove a marked improvement in weight to potential energy ratio. It may also be possible to store hydrogen for use in fuel cells. For fullerenes to become useful energy carriers in application such as electric cars, the efficiency ratio of hydrogen-to-carbon by weight must exceed 6.5% hydrogen. Currently, fullerenes are only expected to be able to hold 4% hydrogen. (Mirsky, 2000) CATALYTIC AGENTS The stable structure of fullerenes, combined with their highly reactive nature, makes the closed-cage molecules well suited for use as catalysts. A C 60 molecule impregnated with an alkali metal demonstrates catalytic properties similar to those of platinum. (Dunford) FILMS AND COATINGS Buckyballs can be combined with various elements such as silicon and thermally treated to create coatings and layers for surfaces. These films and coatings impart a variety of electrical, 31

36 mechanical, and optical properties to the host materials they are used to bind and cover. For example, nanotubes can be added to plastic parts allowing them to be electrified, thus facilitating better adhesion of paint. Such a process has already been successfully implemented by General Motors. (Avouris, 2000) MICROELECTRONICS AND COMPUTING By far the most intriguing and abundant applications for nanotubes and fullerenes lie in the field of microelectronics and computing. Numerous applications ranging from molecular wire conductors to flat panel displays, to nanotransistors are currently being envisioned by scientists on every continent. Today, although we are at the dawn of this new technology, scientists have proven that it is possible to build entire nanometer-scaled circuits that consist of switches, memory, and conducting paths entirely out of nanotubes. Switching Devices Moore s Law states that microelectronic devices reduce in size and increase in capacity by a factor of two every 18 to 24 months. Theoretically, there is a point in the future at which Moore s Law breaks down and can no longer be extrapolated using the conventional silicon and metallic-based materials currently employed in the manufacture of microelectronics. The smallest devices currently being produced using such materials are over 200nm wide. Engineers are already beginning to worry about the possibility of further miniaturizing silicon based semiconductors. Microchips of the future will require smaller wires and transistors than photolithography can produce today. Electrically conductive macromolecules of carbon that self-assemble into tubes are being tested as ultra fine wires and as channels in experimental fieldeffect transistors. (Avouris, 2000) 32

37 Nanotubes possess electrical properties that allow them to perform as molecule-sized diodes, wires, and transistors. A team of research scientists led by Cees Dekker has discovered that nanotubes with kinks in their cylindrical bodies allow current flow in single directions only. This characteristic opens the door for future nano diodes and can ultimately lead to the production of transistors and integrated circuits with areas the size of a few atoms. (Hellemans, 1999) Correspondingly, the ballistic conductive properties of armchair nanotubes make plausible the idea of carbon-based nanowires that could conduct electricity between molecular computing devices that are thousands of times smaller than the tiniest circuits available today. (Bleeke, 1997) More exotic technologies are being developed at RIKEN laboratory in Japan, where switching devices based on electron spin rather than electron charge are being explored. These so called spintronic devices may one day replace traditional silicon based semiconductors. (Crespi, 2001) Memory Just as switching components such as diodes and transistors can be fabricated from nanotubes, so too can memory and storage devices. Nanotubes have been shown to have application in both electronic and electromechanical random access memories. Memory devices orders of magnitude smaller than anything in existence today could be assembled by placing C 60 atoms inside of nanotubes and assigning binary 1s and 0s to each end of the tube. As the buckyball moves to one side, it takes on the 0 state, as the tube moves to the opposite end of the tube, it takes on the 1 state. (AScribe, February 2000) 33

38 Field-Emission Devices One of the most immediate and promising applications for nanotubes concerns their use in Field Emission Displays (FED). Engineers at Ise Electronics Corporation in Japan are currently testing nanotube FED devices that function similar to modern light bulbs. Color prototypes of these light sources have proved to be ten times more efficient than conventional bulbs while providing twice the amount of light and with a greatly extended life. Preliminary work is currently being done at Stanford, Georgia Institute of Technology, and other universities on nanotube based FED s that may one day replace the common CRT and LCD screens we use today. (Avouris, 2000) Field-emission displays promise to be brighter and more colorful than the standard desktop monitor, with a viewing angle that s almost 180 degrees and the capability to support full-motion video. (Hogan, 1998) The idea behind using nanotubes in FED devices stems from the fact that they can be made to eject electrons from their ends at relatively low voltages with minimum power requirements. Nanotubes numbering in the millions can be spread evenly on a flat surface and electrical fields can control their emission of electrons. In addition to the aforementioned flat panel displays, FED technology may find applications in communications. Researchers at the University of North Carolina at Chapel Hill and at Lucent Technologies are working on nanotube based field emissions technologies that emit microwaves for use in cellular and wireless telecommunications. (Mirsky, 2000) High-Temperature Superconductors C 60 molecules can also be made to function as high temperature superconductors. Buckyballs can form FCC and octahedral crystalline solid structures. Within these lattices, there is abundant room for constituent elements such as Cesium (Cs), Potassium (K) and Rubidium 34