Carbon nanotube

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Carbon nanotubes are cylindrical carbon molecules with novel properties that make them potentially useful in a wide variety of applications (e.g., nano-electronics, optics, materials applications, etc.). They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Inorganic nanotubes have also been synthesized.

A nanotube (also known as a buckytube) is a member of the fullerene structural family, which also includes buckyballs. Whereas buckyballs are spherical in shape, a nanotube is cylindrical, with at least one end typically capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is on the order of a few nanometers (approximately 50,000 times smaller than the width of a human hair), while they can be up to several centimeters in length. There are two main types of nanotubes: single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs).

Nanotubes are composed entirely of sp² bonds, similar to graphite. Stronger than the sp³ bonds found in diamond, this bonding structure provides them with their unique strength. Nanotubes naturally align themselves into "ropes" held together by Van der Waals forces. Under high pressure, nanotubes can merge together, trading some sp² bonds for sp³ bonds, giving great possibility for producing strong, unlimited-length wires through high-pressure nanotube linking. [1]

While it has long been known that carbon fibers can be produced with a carbon arc, and patents were issued for the process, it was not until 1991 that Sumio Iijima, a researcher with the NEC Laboratory in Tsukuba, Japan, observed that these fibers were hollow. This feature of nanotubes is of great interest to physicists because it permits experiments in one-dimensional quantum physics.

An electronic device known as a diode can be formed by joining two nanoscale carbon tubes with different electronic properties.
An electronic device known as a diode can be formed by joining two nanoscale carbon tubes with different electronic properties.

Contents

Single-walled nanotubes

The (n,m) nanotube naming scheme can be thought of as a vector (Ch) in an infinite graphene sheet that describes how to 'roll up' to graphene sheet to make the nanotube. T denotes the tube axis, and a1 and a2 are the unit vectors of graphene in real space.
The (n,m) nanotube naming scheme can be thought of as a vector (Ch) in an infinite graphene sheet that describes how to 'roll up' to graphene sheet to make the nanotube. T denotes the tube axis, and a1 and a2 are the unit vectors of graphene in real space.

Most SWNTs have a diameter of close to 1nm, with a tube length that can be many thousands of times larger. SWNTs with length up to orders of centimeters have been produced (Zhu, et al., 2002). The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite (called graphene) into a seamless cylinder. The way the graphene sheet is wrapped is represented by a pair of indices (n,m) called the chiral vector. The integers n and m denote the number of unit vectors along two directions in the honeycomb crystal lattice of graphene. If m=0, the nanotubes are called "zigzag". If n=m, the nanotubes are called "armchair". Otherwise, they are called "chiral".


SWNTs are a very important variety of carbon nanotube because they exhibit important electric properties that are not shared by the multi-walled carbon nanotube (MWNT) variants. SWNTs are the most likely candidate for miniaturizing electronics past the microelectromechanical scale that is currently the basis of modern electronics. The most basic building block of these systems is the electric wire, and SWNTs can be excellent conductors. (Dekker, et al., 1999) One useful application of SWNTs is in the development of the first intramolecular field effect transistors (FETs). The production of the first intramolecular logic gate using SWNT FETs has recently become possible as well (Derycke, et al., 2001). To create a logic gate you must have both a p-FET and an n-FET. Because SWNTs are p-FETs when exposed to air and n-FETs when unexposed to oxygen, they were able to protect half of a SWNT from oxygen exposure, while exposing the other half to oxygen. The result was a single SWNT that acted as a NOT logic gate with both p and n-type FETs within the same molecule.

SWNTs are still very expensive to produce, and the development of more affordable synthesis techniques is vital to the future of carbon nanotechnology. If cheaper means of synthesis cannot be discovered, it would make it financially impossible to apply this technology to commercial-scale applications.

Multiwalled carbon nanotubes

Properties

Mechanical

Strength

Carbon nanotubes are one of the strongest materials known to man, both in terms of tensile strength and elastic modulus. This strength results from the the covalent sp^2 bonds formed between the individual carbon atoms. In 2000, a SWNT was tested to have a tensile strength of 63 GPa. In comparison, high-carbon steel has a tensile strength of approximately 1.2 GPa. CNTs also have very high elastic modulus, in the order of 1 TPa [2]. Since carbon nanotubes have relatively low density, the strength to weight ratio is therefore truly exceptional.

Under excessive tensile strain, the tubes will undergo plastic deformation, which means the deformation is permanent. This deformation begins at strains of approximately 5% [Qian et al, 2002] and can increase the maximum strain the tube undergoes before fracture by releasing strain energy.

CNTs are not nearly as strong under compression. Due to their hollow structure, they tend to undergo buckling, when placed under compressive, torsional or bending stress.

Dynamic properties

Multiwalled carbon nanotubes, multiple concentric nanotubes precisely nested within one another, exhibit a striking telescoping property whereby an inner nanotube core may slide, almost without friction, within its outer nanotube shell thus creating an atomically perfect linear or rotational bearing. This is one of the first true examples of molecular nanotechnology, the precise positioning of atoms to create useful machines. Already this property has been utilized to create the world's smallest rotational motor and a nanorheostat. Future applications such as a gigahertz mechanical oscillator are envisioned.


Electrical

Due to the symmetry and unique electronic structure of graphene, the structure of a nanotube strongly affects its electrical properties. For a given (n,m) nanotube, if 2n + m=3q (where q is an integer), then the nanotube is metallic, otherwise the nanotube is a semiconductor. Thus all armchair (n=m) nanotubes are metallic, and nanotubes (5,0), (6,4), (9,1), etc. are semiconducting. An alternative (equivalent) representation of this condition is if (n - m) /3=integer, then the SWNT is metallic. In theory, metallic nanotubes can have an electrical current density more than 1,000 times stronger than metals such as silver and copper.

Thermal

All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a property known as "ballistic conduction," but good insulators laterally to the tube axis.

Defects

As with any material, the existence of defects affect the material properties. Defects can occur in the form of atomic vacancies. High levels of such defects can lower the tensile strength by up to 85% [3]. Due to the almost one-dimensional structure of CNTs, the tensile strength of the tube is dependent on the weakest segment of it in a similar manner to a chain, where a defect in a single link will greatly diminish the strength of the entire chain.

In terms of the tube's electrical properties, they too are affected by the presence of defects. A common result is the lowered conductivity through the defected region of the tube. Some defect formation in armchair type tubes (which are metallic) can cause the region surrounding that defect to become semiconducting. Futhermore single monoatomic vacancies induce magnetic properties.

Synthesis

Techniques have been developed to produce nanotubes in sizeable quantities, including arc discharge, laser ablation, high pressure carbon monoxide (HiPco), and chemical vapor deposition (CVD). Of these, the CVD method has shown the most promise in terms of its price/unit ratio. It generally involves reacting a carbon-containing gas (such as acetylene, ethylene, ethanol, etc.) with a metal catalyst particle (usually cobalt, nickel, iron or a combination of these such as cobalt/iron or cobalt/molybdenium) at temperatures above 600°C. Unfortunately, although these methods can produce large quantities of nanotubes, their cost still precludes any large-scale applications.

Fullerenes and carbon nanotubes are not necessarily products of high-tech laboratories; they are commonly formed in such mundane places as candle flames. However, these naturally occurring varieties, due to the highly uncontrolled environment in which they're produced, are highly irregular in size and quality, lacking the high degree of uniformity necessary to meet the needs of both research and industry.

Applications

The strength and flexibility of carbon nanotubes makes them of potential use in controlling other nanoscale structures, which suggests they will have an important role in nanotechnology engineering. The highest tensile strength an individual SWNT has been tested to be is 63 GPa [4]. In Earth's upper atmosphere, atomic oxygen erodes the carbon nanotubes, but other applications of carbon nanotubes rarely need the surface to be protected. Though it is debatable if nanotube materials can ever be made with a tensile strength approaching that of individual tubes, composites may still yield incredible strengths potentially sufficient to allow the building of such things as space elevators, artificial muscles, ultrahigh-speed flywheels, and more. MIT is working on combat jackets utilizing carbon nanotubes for ultrastrong fibers and for monitoring its wearer's condition.

Carbon nanotubes have already been used as composite fibers in polymers and concrete to improve the mechanical, thermal and electrical properties of the bulk product. Researchers have also found that adding them to polyethylene increases the polymer's elastic modulus by 30%. In concrete, they increase the tensile strength, and halt crack propagation.

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.

A recent 2005 Science paper notes that drawing transparent high strength swathes of SWNT is a functional production technique. These conductive elastic materials are among the many applications listed here of photovoltaic active structures as well as load structures.

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.

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). 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. Nanotubes have been shown to be superconducting at low temperatures.

Nanotubes can be opened and filled with materials such as biological molecules, raising the possibility of applications in biotechnology. They can be used to dissipate heat from tiny computer chips.

Carbon nanotube fiber & 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. 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 burned out of them to make them purely 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 in 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. In comparison, the bullet-resistant fiber Kevlar is 27-33J/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 [5]. 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. [6] [7]

Current progress

  • In April of 2001, IBM announced it had developed a technique for automatically developing pure semiconductor surfaces from nanotubes.
  • On September 19, 2003, NEC Corporation, Japan, announced stable fabrication technology of carbon nanotube transistors.
  • High purity (80%) nanotubes were reported in June 2003 with metallic properties can be extracted with electrophoretic techniques. [8]
  • As of 2003, nanotubes cost from 20 euro per gram to 1000 euro per gram, depending on purity, composition (single-wall, double-wall, multi-wall) and other characteristics.
  • In June 2004 scientists from China's Tsinghua University and Louisiana State University demonstrated the use of nanotubes in incandescent lamps, replacing a tungsten filament in a lightbulb with a carbon nanotube one.
  • In 2004, Nature published a photo of an individual 4 cm long single-wall nanotube (SWNT).
  • In August 2005, GE announced the development of an ideal carbon nanotube diode that operates at the "theoretical limit," or best possible performance. The company also observed a photovoltaic effect in the nanotube diode device that could lead to breakthroughs in solar cells that make them more efficient and a more viable alternative in the mainstream energy market.[9]
  • In September of 2005 Texas-based Applied Nanotech, in conjunction with six Japanese electronics firms, have created a prototype of a 25-inch TV using carbon nanotubes. The prototype TV does not suffer from "ghosting," as some types of digital TVs do.
  • In September 2005 researchers at Lawrence Livermore National Laboratory demonstrated that ignition by a conventional flashbulb takes place when a layer of 29% iron enriched SWNT is placed on top of a layer of explosive material such as PETN. With ordinary explosives optical ignition is only possible with high powered lasers [10].
  • In September 2005 researchers demonstrated a new way to coat MWNT's with magnetite which after orientation in a magnetic field were able to attract each other over a distance of at least 10 micrometres. [11]. The nanotubes were functionalized with negatively charged carboxylic acid groups in an AIBN type free radical addition. Magnetite nanoparticles prepared by the Massart method were given a positive charge by washing with nitric acid which made them stick to the nanotubes by electrostatic forces.
  • In September 2005, Korean scientists lead by Pohang University of Science and Technology Professor Kim Kwang-Soo succeeded in pulling out a nested tube from a multiwalled nanotube (MWNT), extracted 1 millimeter.

Carbon nanotubes in electrical circuits

Carbon nanotubes have many properties from their unique dimensions to an unusual current conduction mechanisms 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 engineers solution is needed if nanotubes are to become feasible for commercial circuits.

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References

Deepak Srivastava and Chenyu Wei (2003). Nanomechanics of Carbon Nanotubes and Composite. Applied Mechanics Review 56 (2): 215–230.

Dong Qian et al. (2002). Mechanics of Carbon Nanotubes. Applied Mechanical Review 55 (6): 495–533.

Dekker, C., Carbon Nanotubes as Molecular Quantum Wires, Phys. Today, 1999, May, 22-28.

Derycke, V., Mertel, R., Appenzeller, J., Avouris, Ph., Carbon Nanotube Inter- and Intramolecular Logic Gates, Nano Lett., 2001, 1, 453-456.

Zhu, H. W., Xu, C. L., Wu, D. H., Wei, B. Q., Vajtai, R., Ajayan, P. M., Direct Synthesis of Long Single-Walled Carbon Nanotubes Strands, Science, 2002, 296, 884.