31-01-2010, 11:35 PM
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Abstract
Carbon nanotubes (CNTs) are a recently discovered allotrope of carbon. They take the form of cylindrical 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 materials science. 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
Introduction
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.
History
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 Iijimaâ„¢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.
Properties
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 we would 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 more applications.
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. Strength 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.
Kinetic
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
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.
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 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.
The mobility is higher than 100,000 cm2/Vs at room temperature, higher than any other known semiconductor. (The previous record, for InSb, was 77,000 cm2/Vs, set in 1955.) The mobility is a function of the gate voltage, and is higher when the gate voltage is low, i.e. when there are fewer charges in the devices. We don't know why this is yet, but we are studying this. The mobility is also rather independent of temperature, suggesting that the thermal vibrations of the lattice, called phonons, don't play much of a role in scattering the electrons.
Why is the mobility so high? Part of the reason is that graphite itself is a good conductor of electricity. The mobility of charges in graphite is around 20,000 cm2/Vs at room temperature. Graphite also has other excellent properties - it's strong, lightweight, and an excellent conductor of heat. But graphite isn't a semiconductor
- it doesn't have a bandgap - so it can't be used to make semiconductor devices like transistors. The nanotube can be thought of as a way to engineer a bandgap in graphite so we can use it for semiconductor devices (see Introduction to Carbon Nanotubes above). The mobility in nanotubes turns out to be even higher than in graphite. Part of the reason for this may lie in the one- dimensional nature of the nanotube - it's harder to scatter electrons in one-dimension, because they can only go forward or backward, not to the sides.
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). 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.
Arc discharge
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.
Laser ablation
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.). Nanotubes grow at the sites of the metal catalyst; the carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle, where it forms the nanotubes. The catalyst particles generally stay at the tips of the growing nanotube during the growth process, although in some cases they remain at the nanotube base, depending on the adhesion between the catalyst particle and the substrate.
If plasma is generated by the application of a strong electric field during the growth process (plasma enhanced chemical vapor deposition), then the nanotube growth will follow the direction of the electric field. By properly adjusting the geometry of the reactor it is possible to synthesize vertically aligned carbon nanotubes (i.e., perpendicular to the substrate), a morphology that has been of interest to researchers interested in the electron emission from nanotubes. Without the plasma, the resulting nanotubes are often randomly oriented, resembling a bowl of spaghetti. Under certain reaction conditions, even in the absence of a plasma, closely spaced nanotubes will maintain a vertical growth direction resulting in a dense array of tubes resembling a carpet or forest.
Of the various means for nanotube synthesis, CVD shows the most promise for industrial scale deposition in terms of its price/unit ratio. There are additional advantages to the CVD synthesis of nanotubes. Unlike the above methods, CVD is capable of growing nanotubes directly on a desired substrate, whereas the nanotubes must be collected in the other growth techniques. The growth sites are controllable by careful deposition of the catalyst. Additionally, no other growth methods have been developed to produce vertically aligned nanotubes. Natural, incidental, and controlled flame environments Fullerenes and carbon nanotubes are not necessarily products of high-tech laboratories; they are commonly formed in such mundane places as ordinary flames, produced by burning methane, ethylene, and benzene, and they have been found in soot from both indoor and outdoor air. However, these naturally occurring varieties can be highly irregular in size and quality because the environment in which they are produced is often highly uncontrolled. Thus, although they can be used in some applications, they can lack in the high degree of uniformity necessary to meet many needs of both research and industry. Recent efforts have focused on producing more uniform carbon nanotubes in controlled flame environments.
Device Fabrication
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 below. Atomic force microscope (AFM) image of crossed nanotubes (green) contacted by Au electrodes (yellow) using the "find 'em and wire 'em" technique. In this work, performed at UC Berkeley, the nanotubes were deposited onto the chip from solution, and located using the AFM. Scanning electron microscope (SEM) image of a long nanotube transistor fabricated at Maryland using the "find 'em and wire 'em" technique. The nanotube is the thin horizontal white line connecting the two gold leads (thicker vertical lines). Here the nanotubes were grown by chemical vapor deposition directly on the substrate, and located using the SEM.
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. Typically silicon chips (pieces of flat silicon wafer from the semiconductor industry) are used as the substrate material, with a layer of silicon dioxide (glass) grown on top of the silicon as an insulator. The catalyst can be obtained in several ways; the easiest is to dip the silicon chip into a solution of ferric nitrate in isopropanol, and then dip the chip into hexane to cause the ferric nitrate to come out of solution. This deposits nanocrystals of ferric nitrate on the chip, which can be reduced to iron with hydrogen in the growth furnace.
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 collected and amplified to produce the image signal. When the SEM beam hits an insulator, some electrons stick in the insulator and it becomes negatively charged. When the beam scans over the nanotube, the electrons are free to spread out along the nanotube, and thus the area around the nanotube is less negatively charged. The less negatively charged area allows more electrons from the substrate to escape and be detected, producing a signal when the beam scans across the nanotube. Examples of SEM and AFM images of nanotubes are seen below.
(FESEM) and atomic force microscope (AFM) images of nanotubes (two narrow lines) and Cr/Au alignment markers (squares and geometric shapes). The FESEM (a) images the conducting alignment marks and nanotubes, but is insensitive to the surface contamination visible in the AFM image (b). The FESEM image was acquired approximately 100 times faster than the AFM scan.
Once the nanotubes are located, they may be contacted electrically using electron-beam lithography (EBL). A thin layer of resist (a polymer) is spun onto the chip, and the SEM is used again, but this time the energetic electron beam is used to write a pattern in the resist where we want the electrodes to be. The resist which has been exposed to the beam is then washed away in a solvent, and metal (such as gold) is evaporated into the holes in the resist, forming wires which contact the nanotubes. The excess metal which is on top of the resist is lifted off of the chip using a second solvent which dissolves the remaining resist. The electrodes for both the crossed nanotube device and the long nanotube device shown above were fabricated using EBL.
Electrical measurements
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.
Once electrical contacts are made to the nanotubes, we can test their electrical properties. The simplest nanotube device has just two electrode, one at each end of the nanotube. There is actually a third electrode, called the gate, which is the silicon substrate underneath the nanotube. This electrode is not in electrical contact with the nanotube, since it is separated from the nanotube by an insulator (typically silicon dioxide). However, the capacitor formed by the nanotube and the gate can be charged by applying a voltage between nanotube and gate. This way we can change the amount of charge on the nanotube.
When we change the gate voltage (changing the amount of charge on the nanotube) and measure the conductance between the two contacts on the nanotube (conductance is the inverse of resistance) we see one of two types of behavior. Either the conductance stays constant as we change the gate voltage, or it drops dramatically as we make the gate voltage more positive (see below). We identify the first type of behavior with the metallic nanotubes - changing the charge on a metal does not change its conductance. The second type of behavior we associate with the semiconducting nanotubes - unless they are "doped", semiconductors don't have any charges which can carry current. The gate voltage allows us to add charge to the nanotube and make it conduct. Negative gate voltage adds "holes" (positive charges corresponding to the absence of an electron) to the nanotube, and it conducts better. Around zero gate voltage there are no holes, and the nanotube stops conducting. (The nanotube should conduct again at a positive enough voltage which would add negatively charged electrons to the nanotube, but it doesn't for reasons related to a barrier at the metal-nanotube interface.)
Conductance as a function of gate voltage for (a) a metallic nanotube, and (b) a semiconducting nanotube.
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 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. Structural
- clothes: waterproof tear-resistant cloth fibers
- combat jackets: MIT is working on combat jackets that use carbon nanotubes as ultrastrong fibers and to monitor the condition of the wearer.
- concrete: In concrete, they increase the tensile strength, and halt crack propagation.
- polyethylene: Researchers have found that adding them to polyethylene increases the polymer's elastic modulus by 30%.
- sports equipment: Stronger and lighter tennis rackets, bike parts, golf balls, golf clubs, golf shaft and baseball bats.
- space elevator: This will be possible only if tensile strengths of more than about 70 GPa can be achieved. Monoatomic oxygen in the Earth's upper atmosphere would erode carbon nanotubes at some altitudes, so a space elevator constructed of nanotubes would need to be protected (by some kind of coating). Carbon nanotubes in other applications would generally not need such surface protection.
- ultrahigh-speed flywheels: The high strength/weight ratio enables very high speeds to be achieved.
Electromagnetic
- artificial muscles
- buckypaper - a thin sheet made from nanotubes that are 250 times stronger than steel and 10 times lighter that could be used as a heat sink for chipboards, a backlight for LCD screens or as a faraday cage to protect electrical devices/aeroplanes.
- chemical nanowires: Carbon nanotubes additionally can also be used to produce nanowires of other chemicals, such as gold or zinc oxide. These nanowires in turn can be used to cast nanotubes of other chemicals, such as gallium nitride. These can have very different properties from CNTs - for example, gallium nitride nanotubes are hydrophilic, while CNTs are hydrophobic, giving them possible uses in organic chemistry that CNTs could not be used for.
- computer circuits: A nanotube formed by joining nanotubes of two different diameters end to end can act as a diode, suggesting the possibility of constructing electronic computer circuits entirely out of nanotubes. Because of their good thermal properties, CNTs can also be used to dissipate heat from tiny computer chips. The longest electricity conducting circuit is a fraction of an inch long.(Source: June 2006 National Geographic).
- conductive films: A 2005 paper in Science notes that drawing transparent high strength swathes of SWNT is a functional production technique (Zhang et. al., vol. 309, p. 1215). Additionally, Eikos Inc. of Franklin, Massachusetts is developing transparent, electrically conductive films of carbon nanotubes to replace indium tin oxide (ITO) in LCDs, touch screens, and photovoltaic devices. Carbon nanotube films are substantially more mechanically robust than ITO films, making them ideal for high reliability touch screens and flexible displays. Nanotube films show promise for use in displays for computers, cell phones, PDAs, and ATMs.
- electric motor brushes: Conductive carbon nanotubes have been used for several years in brushes for commercial electric motors. They replace traditional carbon black, which is mostly impure spherical carbon fullerenes. The nanotubes improve electrical and thermal conductivity because they stretch through the plastic matrix of the brush. This permits the carbon filler to be reduced from 30% down to 3.6%, so that more matrix is present in the brush. Nanotube composite motor brushes are better-lubricated (from the matrix), cooler-running (both from better lubrication and superior thermal conductivity), less brittle (more matrix, and fiber reinforcement), stronger and more accurately moldable (more matrix). Since brushes are a critical failure point in electric motors, and also don't need much material, they became economical before almost any other application.
- light bulb filament: alternative to tungsten filaments in incandescent lamps.
- magnets: MWNTs coated with magnetite
- optical ignition: A layer of 29% iron enriched SWNT is placed on top of a layer of explosive material such as PETN, and can be ignited with a regular camera flash.
- solar cells: GE's carbon nanotube diode has a photovoltaic effect. Nanotubes can replace ITO in some solar cells to act as a transparent conductive film in solar cells to allow light to pass to the active layers and generate photocurrent.
- superconductor: Nanotubes have been shown to be superconducting at low temperatures.
- bound to the charge plates of capacitors in order to dramatically increase the surface area and therefore energy storage ability.
- displays: One use for nanotubes that has already been developed is as extremely fine electron guns, which could be used as miniature cathode ray tubes in thin high-brightness low-energy low-weight displays. This type of display would consist of a group of many tiny CRTs, each providing the electrons to hit the phosphor of one pixel, instead of having one giant CRT whose electrons are aimed using electric and magnetic fields. These displays are known as field emission displays (FEDs).
- transistor: developed at Delft, IBM, and NEC.Chemical
- air pollution filter: Future applications of nanotube membranes include filtering carbon dioxide from power plant emissions.
- biotech container: Nanotubes can be opened and filled with materials such as biological molecules, raising the possibility of applications in biotechnology.
- water filter: Recently nanotube membranes have been developed for use in filtration. This technique can purportedly reduce desalination costs by 75%. The tubes are so thin that small particles (like water molecules) can pass through them, while larger particles (such as the chloride ions in salt) are blocked. Mechanical
- oscillator: fastest known oscillators (> 50 GHz).
- liquid flow array: Liquid flows up to five orders of magnitude faster than predicted through array.
- slick surface: slicker than Teflon and waterproof.In electrical circuits Carbon nanotubes have many propertiesâ€from their unique dimensions to an unusual current conduction mechanismâ€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 nanotubesâ€metallic, semiconducting, single-walled, multi-walledâ€produced. A chemical engineering solution is needed if nanotubes are to become feasible for commercial circuits. As fiber and film One application for nanotubes that is currently being researched is high tensile strength fibers. Two methods are currently being tested for the manufacture of such fibers. A French team has developed a liquid spun system that involves pulling a fiber of nanotubes from a bath which yields a product that is approximately 60% nanotubes. The other method, which is simpler but produces weaker fibers uses traditional melt-drawn polymer fiber techniques with nanotubes mixed in the polymer. After drawing, the fibers can have the polymer component burned out of them leaving only the nanotube or they can be left as they are.
Ray Baughman's group from the NanoTech Institute at University of Texas at Dallas produced the current toughest material known as of mid-2003 by spinning fibers of single wall carbon nanotubes with polyvinyl alcohol. Beating the previous contender, spider silk, by a factor of four, the fibers require 600 J/g to break In comparison, the bullet-resistant fiber Kevlar is 27“33 J/g. In mid -2005, Baughman and co-workers from Australia's Commonwealth Scientific and Industrial Research Organization developed a method for producing transparent carbon nanotube sheets 1/1000th the thickness of a human hair capable of supporting 50,000 times their own mass. In August 2005, Ray Baughman's team managed to develop a fast method to manufacture up to seven meters per minute of nanotube tape. 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.
Conclusion
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.
Reference
- http://www.pa.msu.edu/cmp/csc/ntproperties
- http://en.wikipediawiki/Nanotubes
- http://www.rdg.ac.uk/%7Escsharip/tubes.htm
Contents
1. Abstract
2. Introduction
3. History
4. Properties
Structure
Strength
Kinetic
Thermal
Electrical
Defects
Conductance & Mobility
Arc Discharge
Laser Ablation
Device Fabrication
Application
Structural
Electromagnetic
Chemical
Mechanical
In electrical circuits
Carbon Nanotubes
Abstract
Carbon nanotubes (CNTs) are a recently discovered allotrope of carbon. They take the form of cylindrical 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 materials science. 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
Introduction
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.
History
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 Iijimaâ„¢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.
Properties
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 we would 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 more applications.
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. Strength 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.
Kinetic
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
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.
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 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.
The mobility is higher than 100,000 cm2/Vs at room temperature, higher than any other known semiconductor. (The previous record, for InSb, was 77,000 cm2/Vs, set in 1955.) The mobility is a function of the gate voltage, and is higher when the gate voltage is low, i.e. when there are fewer charges in the devices. We don't know why this is yet, but we are studying this. The mobility is also rather independent of temperature, suggesting that the thermal vibrations of the lattice, called phonons, don't play much of a role in scattering the electrons.
Why is the mobility so high? Part of the reason is that graphite itself is a good conductor of electricity. The mobility of charges in graphite is around 20,000 cm2/Vs at room temperature. Graphite also has other excellent properties - it's strong, lightweight, and an excellent conductor of heat. But graphite isn't a semiconductor
- it doesn't have a bandgap - so it can't be used to make semiconductor devices like transistors. The nanotube can be thought of as a way to engineer a bandgap in graphite so we can use it for semiconductor devices (see Introduction to Carbon Nanotubes above). The mobility in nanotubes turns out to be even higher than in graphite. Part of the reason for this may lie in the one- dimensional nature of the nanotube - it's harder to scatter electrons in one-dimension, because they can only go forward or backward, not to the sides.
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). 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.
Arc discharge
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.
Laser ablation
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.). Nanotubes grow at the sites of the metal catalyst; the carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle, where it forms the nanotubes. The catalyst particles generally stay at the tips of the growing nanotube during the growth process, although in some cases they remain at the nanotube base, depending on the adhesion between the catalyst particle and the substrate.
If plasma is generated by the application of a strong electric field during the growth process (plasma enhanced chemical vapor deposition), then the nanotube growth will follow the direction of the electric field. By properly adjusting the geometry of the reactor it is possible to synthesize vertically aligned carbon nanotubes (i.e., perpendicular to the substrate), a morphology that has been of interest to researchers interested in the electron emission from nanotubes. Without the plasma, the resulting nanotubes are often randomly oriented, resembling a bowl of spaghetti. Under certain reaction conditions, even in the absence of a plasma, closely spaced nanotubes will maintain a vertical growth direction resulting in a dense array of tubes resembling a carpet or forest.
Of the various means for nanotube synthesis, CVD shows the most promise for industrial scale deposition in terms of its price/unit ratio. There are additional advantages to the CVD synthesis of nanotubes. Unlike the above methods, CVD is capable of growing nanotubes directly on a desired substrate, whereas the nanotubes must be collected in the other growth techniques. The growth sites are controllable by careful deposition of the catalyst. Additionally, no other growth methods have been developed to produce vertically aligned nanotubes. Natural, incidental, and controlled flame environments Fullerenes and carbon nanotubes are not necessarily products of high-tech laboratories; they are commonly formed in such mundane places as ordinary flames, produced by burning methane, ethylene, and benzene, and they have been found in soot from both indoor and outdoor air. However, these naturally occurring varieties can be highly irregular in size and quality because the environment in which they are produced is often highly uncontrolled. Thus, although they can be used in some applications, they can lack in the high degree of uniformity necessary to meet many needs of both research and industry. Recent efforts have focused on producing more uniform carbon nanotubes in controlled flame environments.
Device Fabrication
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 below. Atomic force microscope (AFM) image of crossed nanotubes (green) contacted by Au electrodes (yellow) using the "find 'em and wire 'em" technique. In this work, performed at UC Berkeley, the nanotubes were deposited onto the chip from solution, and located using the AFM. Scanning electron microscope (SEM) image of a long nanotube transistor fabricated at Maryland using the "find 'em and wire 'em" technique. The nanotube is the thin horizontal white line connecting the two gold leads (thicker vertical lines). Here the nanotubes were grown by chemical vapor deposition directly on the substrate, and located using the SEM.
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. Typically silicon chips (pieces of flat silicon wafer from the semiconductor industry) are used as the substrate material, with a layer of silicon dioxide (glass) grown on top of the silicon as an insulator. The catalyst can be obtained in several ways; the easiest is to dip the silicon chip into a solution of ferric nitrate in isopropanol, and then dip the chip into hexane to cause the ferric nitrate to come out of solution. This deposits nanocrystals of ferric nitrate on the chip, which can be reduced to iron with hydrogen in the growth furnace.
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 collected and amplified to produce the image signal. When the SEM beam hits an insulator, some electrons stick in the insulator and it becomes negatively charged. When the beam scans over the nanotube, the electrons are free to spread out along the nanotube, and thus the area around the nanotube is less negatively charged. The less negatively charged area allows more electrons from the substrate to escape and be detected, producing a signal when the beam scans across the nanotube. Examples of SEM and AFM images of nanotubes are seen below.
(FESEM) and atomic force microscope (AFM) images of nanotubes (two narrow lines) and Cr/Au alignment markers (squares and geometric shapes). The FESEM (a) images the conducting alignment marks and nanotubes, but is insensitive to the surface contamination visible in the AFM image (b). The FESEM image was acquired approximately 100 times faster than the AFM scan.
Once the nanotubes are located, they may be contacted electrically using electron-beam lithography (EBL). A thin layer of resist (a polymer) is spun onto the chip, and the SEM is used again, but this time the energetic electron beam is used to write a pattern in the resist where we want the electrodes to be. The resist which has been exposed to the beam is then washed away in a solvent, and metal (such as gold) is evaporated into the holes in the resist, forming wires which contact the nanotubes. The excess metal which is on top of the resist is lifted off of the chip using a second solvent which dissolves the remaining resist. The electrodes for both the crossed nanotube device and the long nanotube device shown above were fabricated using EBL.
Electrical measurements
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.
Once electrical contacts are made to the nanotubes, we can test their electrical properties. The simplest nanotube device has just two electrode, one at each end of the nanotube. There is actually a third electrode, called the gate, which is the silicon substrate underneath the nanotube. This electrode is not in electrical contact with the nanotube, since it is separated from the nanotube by an insulator (typically silicon dioxide). However, the capacitor formed by the nanotube and the gate can be charged by applying a voltage between nanotube and gate. This way we can change the amount of charge on the nanotube.
When we change the gate voltage (changing the amount of charge on the nanotube) and measure the conductance between the two contacts on the nanotube (conductance is the inverse of resistance) we see one of two types of behavior. Either the conductance stays constant as we change the gate voltage, or it drops dramatically as we make the gate voltage more positive (see below). We identify the first type of behavior with the metallic nanotubes - changing the charge on a metal does not change its conductance. The second type of behavior we associate with the semiconducting nanotubes - unless they are "doped", semiconductors don't have any charges which can carry current. The gate voltage allows us to add charge to the nanotube and make it conduct. Negative gate voltage adds "holes" (positive charges corresponding to the absence of an electron) to the nanotube, and it conducts better. Around zero gate voltage there are no holes, and the nanotube stops conducting. (The nanotube should conduct again at a positive enough voltage which would add negatively charged electrons to the nanotube, but it doesn't for reasons related to a barrier at the metal-nanotube interface.)
Conductance as a function of gate voltage for (a) a metallic nanotube, and (b) a semiconducting nanotube.
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 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. Structural
- clothes: waterproof tear-resistant cloth fibers
- combat jackets: MIT is working on combat jackets that use carbon nanotubes as ultrastrong fibers and to monitor the condition of the wearer.
- concrete: In concrete, they increase the tensile strength, and halt crack propagation.
- polyethylene: Researchers have found that adding them to polyethylene increases the polymer's elastic modulus by 30%.
- sports equipment: Stronger and lighter tennis rackets, bike parts, golf balls, golf clubs, golf shaft and baseball bats.
- space elevator: This will be possible only if tensile strengths of more than about 70 GPa can be achieved. Monoatomic oxygen in the Earth's upper atmosphere would erode carbon nanotubes at some altitudes, so a space elevator constructed of nanotubes would need to be protected (by some kind of coating). Carbon nanotubes in other applications would generally not need such surface protection.
- ultrahigh-speed flywheels: The high strength/weight ratio enables very high speeds to be achieved.
Electromagnetic
- artificial muscles
- buckypaper - a thin sheet made from nanotubes that are 250 times stronger than steel and 10 times lighter that could be used as a heat sink for chipboards, a backlight for LCD screens or as a faraday cage to protect electrical devices/aeroplanes.
- chemical nanowires: Carbon nanotubes additionally can also be used to produce nanowires of other chemicals, such as gold or zinc oxide. These nanowires in turn can be used to cast nanotubes of other chemicals, such as gallium nitride. These can have very different properties from CNTs - for example, gallium nitride nanotubes are hydrophilic, while CNTs are hydrophobic, giving them possible uses in organic chemistry that CNTs could not be used for.
- computer circuits: A nanotube formed by joining nanotubes of two different diameters end to end can act as a diode, suggesting the possibility of constructing electronic computer circuits entirely out of nanotubes. Because of their good thermal properties, CNTs can also be used to dissipate heat from tiny computer chips. The longest electricity conducting circuit is a fraction of an inch long.(Source: June 2006 National Geographic).
- conductive films: A 2005 paper in Science notes that drawing transparent high strength swathes of SWNT is a functional production technique (Zhang et. al., vol. 309, p. 1215). Additionally, Eikos Inc. of Franklin, Massachusetts is developing transparent, electrically conductive films of carbon nanotubes to replace indium tin oxide (ITO) in LCDs, touch screens, and photovoltaic devices. Carbon nanotube films are substantially more mechanically robust than ITO films, making them ideal for high reliability touch screens and flexible displays. Nanotube films show promise for use in displays for computers, cell phones, PDAs, and ATMs.
- electric motor brushes: Conductive carbon nanotubes have been used for several years in brushes for commercial electric motors. They replace traditional carbon black, which is mostly impure spherical carbon fullerenes. The nanotubes improve electrical and thermal conductivity because they stretch through the plastic matrix of the brush. This permits the carbon filler to be reduced from 30% down to 3.6%, so that more matrix is present in the brush. Nanotube composite motor brushes are better-lubricated (from the matrix), cooler-running (both from better lubrication and superior thermal conductivity), less brittle (more matrix, and fiber reinforcement), stronger and more accurately moldable (more matrix). Since brushes are a critical failure point in electric motors, and also don't need much material, they became economical before almost any other application.
- light bulb filament: alternative to tungsten filaments in incandescent lamps.
- magnets: MWNTs coated with magnetite
- optical ignition: A layer of 29% iron enriched SWNT is placed on top of a layer of explosive material such as PETN, and can be ignited with a regular camera flash.
- solar cells: GE's carbon nanotube diode has a photovoltaic effect. Nanotubes can replace ITO in some solar cells to act as a transparent conductive film in solar cells to allow light to pass to the active layers and generate photocurrent.
- superconductor: Nanotubes have been shown to be superconducting at low temperatures.
- bound to the charge plates of capacitors in order to dramatically increase the surface area and therefore energy storage ability.
- displays: One use for nanotubes that has already been developed is as extremely fine electron guns, which could be used as miniature cathode ray tubes in thin high-brightness low-energy low-weight displays. This type of display would consist of a group of many tiny CRTs, each providing the electrons to hit the phosphor of one pixel, instead of having one giant CRT whose electrons are aimed using electric and magnetic fields. These displays are known as field emission displays (FEDs).
- transistor: developed at Delft, IBM, and NEC.Chemical
- air pollution filter: Future applications of nanotube membranes include filtering carbon dioxide from power plant emissions.
- biotech container: Nanotubes can be opened and filled with materials such as biological molecules, raising the possibility of applications in biotechnology.
- water filter: Recently nanotube membranes have been developed for use in filtration. This technique can purportedly reduce desalination costs by 75%. The tubes are so thin that small particles (like water molecules) can pass through them, while larger particles (such as the chloride ions in salt) are blocked. Mechanical
- oscillator: fastest known oscillators (> 50 GHz).
- liquid flow array: Liquid flows up to five orders of magnitude faster than predicted through array.
- slick surface: slicker than Teflon and waterproof.In electrical circuits Carbon nanotubes have many propertiesâ€from their unique dimensions to an unusual current conduction mechanismâ€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 nanotubesâ€metallic, semiconducting, single-walled, multi-walledâ€produced. A chemical engineering solution is needed if nanotubes are to become feasible for commercial circuits. As fiber and film One application for nanotubes that is currently being researched is high tensile strength fibers. Two methods are currently being tested for the manufacture of such fibers. A French team has developed a liquid spun system that involves pulling a fiber of nanotubes from a bath which yields a product that is approximately 60% nanotubes. The other method, which is simpler but produces weaker fibers uses traditional melt-drawn polymer fiber techniques with nanotubes mixed in the polymer. After drawing, the fibers can have the polymer component burned out of them leaving only the nanotube or they can be left as they are.
Ray Baughman's group from the NanoTech Institute at University of Texas at Dallas produced the current toughest material known as of mid-2003 by spinning fibers of single wall carbon nanotubes with polyvinyl alcohol. Beating the previous contender, spider silk, by a factor of four, the fibers require 600 J/g to break In comparison, the bullet-resistant fiber Kevlar is 27“33 J/g. In mid -2005, Baughman and co-workers from Australia's Commonwealth Scientific and Industrial Research Organization developed a method for producing transparent carbon nanotube sheets 1/1000th the thickness of a human hair capable of supporting 50,000 times their own mass. In August 2005, Ray Baughman's team managed to develop a fast method to manufacture up to seven meters per minute of nanotube tape. 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.
Conclusion
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.
Reference
- http://www.pa.msu.edu/cmp/csc/ntproperties
- http://en.wikipediawiki/Nanotubes
- http://www.rdg.ac.uk/%7Escsharip/tubes.htm
Contents
1. Abstract
2. Introduction
3. History
4. Properties
Structure
Strength
Kinetic
Thermal
Electrical
Defects
Conductance & Mobility
Arc Discharge
Laser Ablation
Device Fabrication
Application
Structural
Electromagnetic
Chemical
Mechanical
In electrical circuits
