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Carbon nanotubes are a recently discovered allotrope of carbon. They take the formcylindrical carbon molecules and have novel properties that make them potentially useful in a wide variety of applications in nanotechnology, electronics, optics, and other fields of materialsscience. They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Inorganic nanotubes have also been synthesized.A nanotube 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
millimeters in length. There are two main types of nanotubes: single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs).
Manufacturing a nanotube is dependent on applied quantum chemistry, specifically, orbital hybridization. Nanotubes are composed entirely of sp2 bonds, similar to those of graphite. This bonding structure, stronger than the sp3 bonds found in diamond, provides the molecules 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
What is a CARBON NANOTUBE?
Carbon nanotubes are wires of pure carbon with nanometer diameters and lengths of many microns. A single-walled carbon nanotube (SWNT) may be thought of as a single atomic layer thick sheet of graphite (called graphene) rolled into a seamless cylinder. Multi -walled carbon nanotubes (MWNT) consist of several concentric nanotube shells.Understanding the electronic properties of the graphene sheet helps to understand the electronic properties of carbon nanotubes. Graphene is a zero-gap semiconductor; for most directions in the graphene sheet, there is a bandgap, and electrons are not free to flow along those directions unless they are given extra energy. However, in certain special directions graphene is metallic, and electrons flow easily along those directions. This property is not obvious in bulk graphite, since there is always a conducting metallic path which can connect any two points, and hence graphite
conducts electricity. However, when graphene is rolled up to make the nanotube, a special
direction is selected, the direction along the axis of the nanotube. Sometimes this is a metallic direction, and sometimes it is semiconducting, so some nanotubes are metals, and others are
semiconductors. Since both metals and semiconductors can be made from the same all-carbon system, nanotubes are ideal candidates for molecular electronics technologies. Three nanotubes of different chiralities.In addition to their interesting electronic structure, nanotubes
have a number of other useful properties. Nanotubes are incredibly stiff and tough mechanically - the world's strongest fibers. Nanotubes conduct heat as well as diamond at room temperature.
Nanotubes are very sharp, and thus can be used as probe tips for scanning-probe microscopes, and field-emission electron sources for lamps and displays.
The current huge interest in carbon nanotubes is a direct consequence of the synthesis of buckminsterfullerene, C60, and other fullerenes, in 1985. The discovery that carbon could form
stable, ordered structures other than graphite and diamond stimulated researchers worldwide to search for other new forms of carbon. The search was given new impetus when it was shown in 1990 that C60 could be produced in a simple arc-evaporation apparatus readily available in all laboratories. It was using such an evaporator that the Japanese scientist Sumio Iijima discovered fullerene-related carbon nanotubes in 1991. The tubes contained at least two layers, often many more, and ranged in outer diameter from about 3 nm to 30 nm. They were invariably closed at both ends. A transmission electron micrograph of some multiwalled nanotubes is
shown in the figure (left). In 1993, a new class of carbon nanotube was discovered, with just a single layer. These single-walled nanotubes are generally narrower than the multiwalled tubes, with diameters typically in the range 1-2 nm, and tend to be curved rather than straight. The image on the right shows some typical single-walled tubes It was soon established that these new fibres had a range of exceptional properties (see below), and this sparked off an explosion of research into carbon nanotubes. It is important to note, however, that nanoscale tubes of carbon, produced catalytically, had been known for many years before Iijima's discovery. The main reason why these early tubes did not excite wide interest is that they were structurally rather imperfect, so did not have particularly interesting properties. Recent research
has focused on improving the quality of catalytically-produced nanotubes.
The strength of the sp² carbon-carbon bonds gives carbon nanotubes amazing mechanical properties. The stiffness of a material is measured in terms of its Young's modulus, the rate of change of stress with applied strain. The Young's modulus of the best nanotubes can be as high as 1000 GPa which is approximately 5x higher than steel. The tensile strength, or breaking strain of nanotubes can be up to 63 GPa, around 50x higher than steel. These properties, coupled with the lightness of carbon nanotubes, gives them great potential in applications such as aerospace. It has even been suggested that nanotubes could be used in the "space
elevator", an Earth-to-space cable. The electronic properties of carbon nanotubes are also extraordinary. Especially notable is the fact that nanotubes can be metallic or semiconducting depending on their structure. Thus, some nanotubes have conductivities higher than that of copper, while others behave more like silicon. There is great interest in the possibility of constructing nanoscale electronic devices from nanotubes, and some progress is being made in this area. However, in order to construct a useful device wewould need to arrange many thousands of nanotubes in a defined pattern, and we do not yet have the degree of control necessary to achieve this. There are several areas of technology where carbon nanotubes are already being used. These include flat-panel displays, scanning probe microscopes and sensing devices. The unique properties of carbon nanotubes will undoubtedly lead to many
The bonding in carbon nanotubes is sp², with each atom joined to three neighbours, as in graphite. The tubes can therefore be considered as rolled-up graphene sheets (graphene is an individual graphite layer). There are three distinct ways in which a graphene sheet can be rolled into a tube, as shown in the diagram below. The first two of these, known as "armchair" (top left) and "zig- zag" (middle left) have a high degree of symmetry. The terms "armchair" and "zig-zag" refer to the arrangement of hexagons around the circumference. The third class of tube, which in practice is the most common, is known as chiral, meaning that it can exist in two mirror-related forms. An example of a chiral nanotube is shown at the bottom left. The structure of a nanotube can be specified by a vector, (n,m), which defines how the graphene sheet is rolled up. This can be understood with reference to figure on the right. To produce a nanotube with the indices (6,3), say, the sheet is rolled up so that the atom labelled (0,0) is superimposed on the one labelled (6,3). It can be seen from the figure that m = 0 for all zig-zag tubes, while n = m for all armchair tubes.
Carbon nanotubes are one of the strongest materials known to humans, both in terms of tensile strength and elastic modulus. This strength results from the covalent sp2 bonds formed between the individual carbon atoms. In 2000, an MWNT 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, on the order of 1 TPa. Since carbon nanotubes have
a low density for a solid of 1.3-1.4 g/cm³, its specific strength is the best of known materials.
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. Because of their hollow structure and high aspect ratio, they tend to undergo buckling when placed under compressive, torsional or bending stress.
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.
Because of 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. In theory, metallic nanotubes can have an electrical current density more than 1,000 times greater than metals such as silver and copper.
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.
As with any material, the existence of defects affects 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%. Another well-known form of defect that occurs in carbon nanotubes is known as the Stone Wales defect, which creates a pentagon and heptagon pair by rearrangement of the bonds. Because of 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 diminishes the strength of the entire chain.The tube's electrical properties are also affected by the presence of defects. A common result is the lowered conductivity through the
defective region of the tube. Some defect formation in armchair- type tubes (which are metallic) can cause the region surrounding that defect to become semiconducting. Furthermore single monoatomic vacancies induce magnetic properties.The tube's thermal properties are heavily affected by defects. Such defects lead to phonon scattering, which in turn increases the
relaxation rate of the phonons. This reduces the mean free path, and reduces the thermal conductivity of nanotube structures.
Conductance and Mobility:
Recently, much of our research has focused on semiconducting nanotubes, because of their utility for devices. Since the conductance of the semiconducting nanotube can be changed by the voltage on a third electrode (the gate), the nanotube acts like a switch. This type of switch is called a field-effect transistor (FET), and forms the basis of most computer chips used today. We are very interested in determining how well nanotubes perform as field-effect transistors, in order to gauge their prospects for future electronics applications. The first question one might ask is: How well do semiconducting nanotubes conduct? The figure below shows the conductance of a very long nanotube (about 1/3 of a millimeter long) as a function of gate voltage. The highest conductance observed is 1.6 micro- Siemens, which corresponds to a resistance of around 600 kilo-Ohms.
How does this compare to other materials? :
In order to compare, we need to consider the conductivity, conductance x length/area. This
takes into account the fact that we expect a long, thin wire to have lower conductance than a short, fat wire. The conductivity of the nanotube is around 2.6 micro-Ohm-centimeters. This is
comparable to good metals like copper (1.6 micro-Ohm-centimeters), which is very surprising. This means that this nanotube switch can be tuned from insulating, to conducting as well as copper, simply by changing the gate voltage! The top panel shows an SEM image of a long semiconducting carbon nanotube spanning between two gold electrodes (scale bar is 100
micrometers). The bottom graph shows the conductance of this nanotube as a function of the voltage applied to the back gate (silicon substrate) at temperatures of 300, 200, and 100 Kelvins.The above analysis also hints that conductivity isn't the best number to use when comparing one semiconductor to another, since the conductivity changes with charge density (in this case with gate voltage). It's fine for metals, like copper, where the charge density is very high and doesn't change much. The number that's used to indicate how well one semiconductor conducts compared to another is mobility. Mobility is the conductance divided by the density of charge carriers, so it can be used to compare the conductance of semiconductor samples with different amounts of charge to carry the current. We know the charge density in our nanotube devices, because we know the capacitance C between the nanotube and the gate electrode that
is producing the charge. The charge Q is proportional to the capacitance and to the amount of gate voltage V we have applied: Q = CV. So we know everything we need to find the mobility. The mobility of one of our long nanotube transistors is shown below. Mobility as a function of gate voltage for a semiconducting carbon nanotube. At low gate voltage (low charge carrier density) the mobility exceeds that of InSb (77,000 cm2/Vs), the previous
highest-known mobility at room temperature.
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). Most of these processes take place in vacuum or with process gases. CVD
growth of CNTs can take place in vacuum or at atmospheric pressure. Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth processes are making CNTs more commercially viable. The arc-evaporation method, which
produces the best quality nanotubes, involves passing a current of about 50 amps between two graphite electrodes in an atmosphere of helium. This causes the graphite to vaporise, some of it condensing on the walls of the reaction vessel and some of it on the cathode.
It is the deposit on the cathode which contains the carbon nanotubes. Single-walled nanotubes are produced when Co and Ni or some other metal is added to the anode. It has been known since the 1950s, if not earlier, that carbon nanotubes can also be made by passing a carbon-containing gas, such as a hydrocarbon, over a catalyst. The catalyst consists of nano-sized particles of metal, usually Fe, Co or Ni. These particles catalyse the breakdown of the
gaseous molecules into carbon, and a tube then begins to grow with a metal particle at the tip. It was shown in 1996 that single- walled nanotubes can also be produced catalytically. The perfection of carbon nanotubes produced in this way has generally been poorer than those made by arc-evaporation, but great improvements in the technique have been made in recent years. The big advantage of catalytic synthesis over arc-evaporation is that it can be scaled
up for volume production. The third important method for making carbon nanotubes involves using a powerful laser to vaporise a metal-graphite target. This can be used to produce single-walled tubes with high yield.
Nanotubes were observed in 1991 in the carbon soot of graphite electrodes during an arc discharge that was intended to produce fullerenes. During this process, the carbon contained in the negative electrode sublimates because of the high temperatures caused by the discharge. Because nanotubes were initially discovered using this technique, it has been perhaps the most
widely used method of nanotube synthesis.
In the laser ablation process, a pulsed laser vaporizes a graphite target in a high temperature reactor while an inert gas is bled into the chamber. The nanotubes develop on the cooler surfaces of the reactor, as the vaporized carbon condenses. A water-cooled surface may be included in the system to collect the nanotubes.
Chemical vapor deposition (CVD):
Nanotubes being grown by plasma enhanced chemical vapor deposition
The catalytic vapor phase deposition of carbon was first reported in 1959, but it was not until 1993 that carbon nanotubes could be formed by this process. During CVD, a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt, iron, or a combination. The diameters of the nanotubes that are to be grown are related to the size of the metal particles. This can be controlled by patterned (or masked) deposition of the metal, annealing, or by plasma etching of a metal layer. The substrate is heated to approximately 700°C. To initiate the growth of nanotubes, two gases are bled into the reactor: a process gas (such as ammonia, nitrogen, hydrogen, etc.) and a carbon-containing gas (such as acetylene, ethylene, ethanol, etc.).
Find 'em and wire 'em :
This is a technique for synthesizing carbon nanotubes directly on silicon substrates, locating individual nanotubes, and electrically contacting nanotubes with metallic electrodes. The general idea is to "find 'em and wire 'em", as opposed to attempting to self- assemble nanotubes in place, or deposit nanotubes or wires at random and hope to contact some nanotubes. The great advantage of the find 'em and wire 'em technique is that customized devices can be made. Some examples are Atomic force microscope (AFM) image of crossed nanotubes (green) contacted by Au electrodes (yellow) using the "find 'em and wire
The disadvantages of the find 'em and wire 'em scheme are that only a limited number of devices can be made, and the technique is not "scalable" - that is, making twice as many devices takes twice as much time. If nanotubes are to find electronic applications in industry, scalable fabrication techniques will be needed.
CVD growth of nanotubes:
Chemical Vapor Deposition (CVD) can be used to prepare carbon nanotubes. The basic ingredients needed for CVD growth of nanotubes are a small catalyst particle (typically iron or
iron/molybdenum) and a hot environment of carbon-containing gas (we use CH4 and C2H4). The metal particle catalyzes the decomposition of the carbon-containing gases, and the carbon dissolves in the catalyst particle. Once the catalyst particle is supersaturated with carbon, it extrudes out the excess carbon in the form of a tube. One catalyst particle of a few nanometers in diameter can produce a nanotube millimeters in length, about 1 million times the
size of the particle. Nanotubes grown by the CVD process on a silicon dioxide covered
silicon chip. The thin white lines are the nanotubes. The nanotubes here form a continuous conducting network, and thus are too dense to use for device fabrication..
Locating the nanotubes
Once the nanotubes are grown on the substrates, they need to be located. To do this, first a pattern of alignment marks on the substrate is deposited, using a conventional lithography technique. A method for locating nanotubes is to use an atomic force microscope (AFM). The AFM uses a tiny needle on the end of a diving-board-like cantilever to tap on a surface as it scans over that surface. It senses the amplitude of the tapping and uses that
to follow the height variations in the surface, making a topographical map of the area. The AFM is very sensitive, so it is able to image the nanometer-diameter nanotubes lying on the flat
substrate. However, AFM is very time consuming, taking 5 minutes or so to image a 10 x 10 micron square image.
Another technique is to image nanotubes using the scanning electron microscope (SEM). This imaging technique relies on the fact that the nanotubes are conducting, and the substrate on which they are lying is insulating. The SEM images by scanning a high-energy beam of electrons over the sample. Secondary electrons generated by the energetic beam are collectedand amplified to produce the image signal.
The wires on the chip are much bigger than the nanotube, but still fairly small - typically the largest parts of the wires on the chip are one or two tenths of a millimeter across. We make contact to the wires on the chip under a microscope, either by using a wire bonder which can attach larger wires to the chip to connect it to a rigid chip holder, or by using a probe station, which has sharp needles that can be used to temporarily make contact to the wires
on the chip.
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 MWNT has been tested to be is 63 GPa. Bulk nanotube materials may never achieve a tensile strength similar to that of individual tubes, but such composites may nevertheless yield strengths sufficient for many applications. 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.
â€¢ clothes: waterproof tear-resistant cloth fibers
â€¢ combat jackets: MIT is working on combat jackets that use carbon nanotubes as ultrastrong fibers and to monitor the condition of the wearer.
â€¢ concrete: In concrete, they increase the tensile strength, and halt crack propagation.
â€¢ polyethylene: Researchers have found that adding them to polyethylene increases the polymer's elastic modulus by 30%.
â€¢ sports equipment: Stronger and lighter tennis rackets, bike parts, golf balls, golf clubs, golf shaft and baseball bats.
â€¢ ultrahigh-speed flywheels: The high strength/weight ratio enables very high speeds to be achieved.
â€¢ 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.
â€¢ 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.
â€¢ 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. Nanotube films show promise for use in displays for computers, cell phones, PDAs, and ATMs.
â€¢ light bulb filament: alternative to tungsten filaments in incandescent lamps.
â€¢ magnets: MWNTs coated with magnetite
â€¢ 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.
â€¢ solar cells: GE's carbon nanotube diode has a photovoltaic effect. Nanotubes can replace ITO in some solar cells to act as a stransparent conductive film in solar cells to allow light to pass
to the active layers and generate photocurrent.
â€¢ superconductor: Nanotubes have been shown to be superconducting at low temperatures.
â€¢ 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.
â€¢ 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
â€¢ transistor: developed at Delft, IBM, and NEC.
â€¢ air pollution filter: Future applications of nanotube membranes include filtering carbon dioxide from power plant emissions.
â€¢ biotech container: Nanotubes can be opened and filled with materials such as biological molecules, raising the possibility of applications in biotechnology.
â€¢ 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.
â€¢ oscillator: fastest known oscillators (> 50 GHz).
â€¢ liquid flow array: Liquid flows up to five orders of magnitude faster than predicted through array.
â€¢ slick surface: slicker than Teflon and waterproof.
Carbon nanotubes are the next step in miniaturizing electronic circuits, replacing silicon transistors and diodes, which are fast reaching the theoretical limits of size and speed of operation. Using CNTs, nanochips can be made with entire circuits on it. Ideal diodes can be made from CNTs, resulting in highly efficient electronic circuits. Further, CNTs have a number of other uses
other than in the electronic industry, as seen here.