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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
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 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.
The strength of the spÃ‚Â² 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 spÃ‚Â², 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/cmÃ‚Â³, 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.
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.
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 700Ã‚Â°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.
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.
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.
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.
- 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 27â€œ33 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.
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.
Conductance & Mobility
In electrical circuits
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CARBON NANO TUBES2.ppt (Size: 2.06 MB / Downloads: 741)
These nanoscopic structures are made of carbon atoms.
The carbon nano-tubes can be thought of as graphene planes 'rolled up' in a cylinder which are typically closed at their ends by semi-fullerene-like structures.
Depending upon the manner the graphene planes are cut before rolling , different types of carbon nano-tubes are obtained.
Based on the geometrical structures , carbon nano-tubes are classified into the following:
Chiral (helical) nano-tubes.
Depending upon the size of graphene area folded onto a cylinder , nano-tubes of different radii are obtained.
These tubes are extremely long and possess different symmetry.
STRUCTURE OF CNT
Nanotubes are formed by rolling up a graphene sheet into a cylinder and
capping each end with half of a fullerene molecule
A CNT is characterized by its Chiral Vector: Ch = n ÃƒÂ¢1 + m ÃƒÂ¢2,
Chiral Angle with respect to the zigzag axis.
The diameter of the nano-tubes depends on the values of n and m.
This helps to identify whether the carbon atoms are arranged in zig-zag , armchair or helical shape.
PROPERTIES OF CNT
Excellent field emitter :- higher aspect ratio and a smaller tip radius of curvature wich are ideal for field emission.
Very high current carrying capacity , electrical conductivity six orders higher than copper.
Thermal conductivity ~3000 W/mK in axial direction with small values in the radial direction
High mechanical and thermal stability
Carbon nano-tubes are one of the strongest materials both in terms of tensile strength and elastic modulus. and most flexible molecular material because of C-C covalent bonding and seamless hexagonal network architecture
These tubes have very high tensile strength ,multi-walled tubes have a tensile strength as high as 63GPa.
Laser Ablation Method
A laser is used to vaporize a graphite target in an oven at 12000C.
Then helium or argon gas is filled to keep the pressure in the oven at 500 Torr.
The graphite target gets vaporized to form small carbon molecules and atoms which condense to form single walled nano-tubes held together by Van der Waals forces.
MWNT would be synthesized in the case of a pure graphite.
Uniform SWNT could be synthesized if a graphite of a mixture of Co, Ni, Fe, Y are used instead of a pure graphite.
Laser vaporization are higher in yield than arc-discharge and can synthesize large and aligned high quality SWNTs.
By far the most costly, because expensive lasers required.
Thermal Chemical Vapor Deposition
Fe, Ni, Co, or alloy of the tree catalytic metals is initially deposited on a substrate.
To initiate the growth of nano-tubes, 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.).
The substrate is heated to approximately 700Ã‚Â°C.
Nano-tubes 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 nano-tubes.
Nano-structures provide interesting perspectives of applications because of their unique properties.
Carbon Nano-tubes possess many unique and remarkable properties (chemical, physical, and mechanical), which make them desirable for many applications.
Enhancements in miniaturization, speed and power consumption, size reduction of information processing devices, memory storage devices and flat displays for visualization are currently being developed.
The most immediate application for nano-tubes is in making strong, lightweight materials.
Carbon nano-tubes can emit a high electron field emission current from their tip, when submitted to a bias voltage. The emission from the nano-tubes are intense and very coherent, so they can be used in electron microscopes where electron sources are essential.
Apart from the above mentioned perspectives the carbon nano-tubes will find heir use in the following areas in near future
The phenomenal mechanical properties and the unique electronic properties make the carbon nano-tubes ,both interesting as well as potentially useful in the present and future technologies.
The electronics and mechanical properties of cnt depend on the growth conditions and the type of synthesis.
By controllable growth ,a significant improvement over the current state electronics can be achieved.
The CNTS have paved the way for miniaturizing the present technological systems providing greater strength, flexibility, agility, conductivity [electric & thermal] in a wide range of applications in different fields.
Though in the primary stage of development , the CNTs have great perspective in a very wide range of applications , thus revolutionizing & enhancing the field of science and technology in every possible manner.
Dai H (2002), Surface Science, 500, 218-241.
Chen R.J, Zhang Y, Wang D, Dai H (2001), Journal of American Chemical Society, 123, 3838-3839.
Functionalization of Single-Walled Carbon Nanotubes, Andreas Hirsch.
Topics in Applied Physics Carbon Nanotubes: Synthesis, Structure, Properties and Applications, M.S. Dresselhaus, G. Dresselhaus, Ph. Avouris.
Carbon Nanotubes: Introduction to Nanotechnology 2003, Mads Brandbyge.
Carbon Nanotubes: Single molecule wires Sarah Burke, Sean Collins, David Montiel, Mikhail Sergeev
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Carbon Nanotubes -- tiny tubes about 10,000 times thinner than a human hair -- consist of rolled up sheets of carbon hexagons.
Discovered in 1991 by researchers at NEC, they have the potential for use as minuscule wires or in ultrasmall electronic devices.
To build those devices, scientists must be able to manipulate the Nanotubes in a controlled way.
IBM researchers using an atomic force microscope (AFM), an instrument whose tip can apply accurately measured forces to atoms and molecules, have recently devised a means of changing a nanotube's position, shape and orientation, as well as cutting it.
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http://topicideas.org/how-to-electronic-...ad-seminar and presentation-report
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carbon nano tubes.ppt (Size: 3.19 MB / Downloads: 183) Are you looking for new power point presentation of carbon nanotubes,then here it is.......hope you are satisfied with the article....
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Carbon nanotu.pptx (Size: 723.27 KB / Downloads: 114)
Introduction to Carbon Nanotubes
Carbon nanotubes (CNTs; also known as buckytubes) are allotropes of carbon with a cylindrical nanostructure.
Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, which is significantly larger than any other material. These cylindrical carbon molecules have novel properties that make them potentially useful in many applications in nanotechnology, electronics, optics and other fields of materials science, as well as potential uses in architectural fields. They exhibit extraordinary strength and unique electrical properties, and are efficient thermal Conductors. Nanotubes are members of the fullerene structural family, which
also includes the spherical buckyballs. The ends of a nanotube might be 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 1/50,000th of the width of a human hair), while they can be up to
18 centimeters in length (as of 2010). Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs)
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Researchers Assemble Building Blocks of Nanocomputers
In a flurry of new research, scientists have begun to assemble the tiniest electronic elements into simple logic circuits—the building blocks of the electronic mazes that power computers. Three independent groups of scientists have worked on this and come up with fascinating results.
Chemist Charles Lieber and co-workers at Harvard University created simple logic circuits incorporating up to six transistors by crisscrossing nanometer-wide wires of silicon and gallium-nitride, each junction of which forms a transistor. This technique works by catalyzing the growth of the crystal wires from solutions of each material with the assistance of a laser.
Physicist Adrian Bachtold and colleagues at Delft University of Technology in the Netherlands carved aluminum strips from a layer of the metal and deposited carbon nanotubes on top. They then attached strips of gold to both ends of each nanotube, creating a transistor, and linked up to three such devices in various ways to make circuits that would execute simple logical functions: flipping a signal from off to on or vice versa, turning two off signals into an on, storing a unit of information or creating an oscillating signal.
Physicist Jan Hendrik Schön, with help from other researchers at Bell Laboratories, has refined a technique he recently described for making transistors out of a layer of small carbon molecules. Diluting these transistor molecules with insulating carbon chains, Schön found that just one was enough to turn a signal on or off, making a rudimentary circuit element.
However scientists still have to reduce the complete circuit to molecular size. But the fact that several groups have assembled basic circuits from molecule-scale parts is an indicator of how far molecular electronics and nanotechnology have come and is very encouraging for the future.
A quantum computer - a new kind of computer far more powerful than any that currently exist - could be made today say Thaddeus Ladd of Stanford University, Kohei Itoh of Keio University in Japan, and their co-workers. They have sketched a blueprint for a silicon quantum computer that could be built using current fabrication and measurement techniques.
Quantum and conventional computers encode, store and manipulate information as sequences of binary digits, or bits, denoted as 1s and 0s. In a normal computer, each bit is a switch, which can be either 'on' or 'off'.
In a quantum computer, switches can be on, off or in a superposition of states - on and off at the same time. These extra configurations mean that quantum bits, or qubits, can encode more information than classical switches.
Single Electron Memory
The single-electron memory is the latest development in the field of microelectronics called ‘single electronics’, in which electrons are shunted around circuits one by one like strollers through a maze. The electrons pass through turnstiles, hop between resting places, and as they go on their way they flip switches and perform ‘logic operations’ just like the electrical currents in ordinary computers. The difference is that the currents are minute so very little power is consumed. And because the electrons pass through the system one at a time, the electrical current is ‘granular’ -- like a trickle of sand from an hourglass, rather than a stream of liquid gushing through a lock gate.
This low-power, low-heat ‘granular’ electronics would manipulate information, not in the form of electrical pulses representing the binary digits ‘1’ and ‘0’ that encode information in today’s computers, but instead by using single electrons to represent a ‘bit’ of information. That is to say, the presence of an electron in a channel would signify a ‘1’, the absence a ‘0’.
IBM’s Molecular Computer
IBM researchers have built and operated the world's smallest working computer circuits in which individual molecules move across an atomic surface like toppling dominoes.
The new "molecule cascade" technique makes the digital-logic elements some 260,000 times smaller than those used in today's most advanced semiconductor chips.
The circuits were made by creating a precise pattern of carbon monoxide molecules on a copper surface. Moving a single molecule initiates a cascade of molecule motions, just as toppling a single domino can cause a large pattern to fall in sequence. The scientists then designed and created tiny structures that demonstrated the fundamental digital-logic OR and AND functions, data storage and retrieval, and the "wiring" necessary to connect them into functioning computing circuitry.
The most complex circuit they built -- a 12 x 17-nanometer three-input sorter -- is so small that 190 billion could fit atop a standard pencil-top eraser.
IBM's molecule cascade works because carbon monoxide molecules can be arranged on a copper surface in an energetically meta-stable configuration that can be triggered to cascade into a lower energy configuration, just as with toppling dominoes. The meta-stability is due to the weak repulsion between carbon monoxide molecules placed only one lattice spacing apart. What enables computation is that each cascade carries a single bit of information. By analogy, a toppled domino can be thought of as a logical "1," and an untoppled domino can be thought of as a logical "0." Similarly, a cascaded or non-cascaded molecular array can represent a logical "1" or "0," respectively.
Since there is no reset mechanism, these molecule cascades can only perform a calculation once.
One of the biggest discoveries to have aided the progress of electronics at the nano scale is that of carbon nanotubes. These are tiny tubular structures composed of a single layer of carbon atoms. Discovered in 1991 by Sumio Iijima of NEC Corporation, carbon nanotubes are an exotic variation of common graphite. The tubular structure imparts a number of mechanical and electronic properties that include super strength, combined with low weight, stability, flexibility, good heat conductance, large surface area and a host of intriguing electronic properties.
Carbon nanotubes are descendants of buckminsterfullerene, or "buckyball," the soccer-ball-shape molecule of 60 carbon atoms. It has been discovered that if a row of hexagons going down the tube's long axis were straight, the tube would behave as a metal and conduct electricity. If a line of hexagons formed a helix, however, the tube would act as a semiconductor. This gives rise to a wide variety of electronic applications in which the tubes can be used.
(To understand this completely refer our previous article on nano's like Nanotechnology )
Carbon Nanotubes Could Lengthen Battery Life
Carbon nanotubes could lengthen the life of batteries, according to new research. Recent findings suggest that the diminutive tubes can hold twice as much energy as graphite, the form of carbon currently used as an electrode in many rechargeable lithium batteries.
Conventional graphite electrodes can reversibly store one lithium ion for every six carbon atoms. By experiment it has been found that the tiny straws of carbon nanotubes manage to reversibly store one charged ion for every three carbon atoms. In explanation, the scientists note that the tubes' open ends facilitate the diffusion of lithium atoms into their interiors.
Carbon Nanotubes Could Serve as Ultrafast Oscillators
Carbon nanotubes are extremely small, measuring a few billionths of a meter in width. According to scientists these straws nestled inside one another with the inner set of tubes sliding in and out a billion times a second could constitute a gigahertz oscillator.
Scientists from the University of California have come up with the conclusion that if the inner core were pulled out of such a tube, it would not only retract back into the center of the tube, but it would also continue right out the other end. Nearly negligible friction between the tubes would enable a breakneck gigahertz oscillation frequency. And shorter tubes could move at even greater speeds.
However researchers have not yet figured out how exactly to excite the oscillator and how to couple it with the rest of the nanoscopic device. The actual implementation of this coupling represents another challenge in development.
Researchers Fashion the First Single Molecule Circuit
In a remarkable feat of engineering, researchers at IBM have wired up a working computer circuit within a single carbon nanotube. Building on earlier work, Phaedon Avouris and colleagues turned the nanotube—essentially a sheet of carbon atoms rolled into a supertiny straw—into a voltage inverter, or NOT gate, one of the three fundamental types of logic gates on which all computers rely. FIG below shows a view of the circuit that was fabricated
Of importance, the current in the carbon nanotube NOT gate comes out stronger than it goes in—a necessary criterion for any circuit design. And because this gain is by as much as a factor of 1.6, researchers believe that more complex single-nanotube circuits will be possible.
Nanotube 'Peapods' Exhibit Surprising Electronic Properties
In yet another small step toward building nanoscale devices, scientists have determined that nanotube peapods—minute straws of carbon filled with spherical carbon molecules known as buckyballs—have tunable electronic properties. Recent findings
suggest that stuffing the straws provide greater control over the electronic states of single-walled carbon nanotubes (SWNT).
Using a low-temperature scanning tunneling microscope, Ali Yazdani of the University of Illinois at Urbana-Champaign and colleagues imaged the physical structure of individual peapods (FIG ). They mapped the motion of electrons within the pipes and, as Yazdani explains, showed "that an ordered array of encapsulated molecules can be used to engineer electron motion inside nanotubes in a predictable way." Though the harbored buckyballs modify the electronic properties of the nanotube, the atomic structure of the straw remains unchanged
The researchers also utilized the microscope to move the buckyballs, which allowed them to compare the same section of a SWNT when it was filled and unfilled. "The encapsulated balls have a much stronger effect on the electronic structure of the tube than we had expected," says study co-author Eugene Mele of the University of Pennsylvania. Indeed, the authors conclude that their calculation not only shows how a peapod's electronic properties differ from those of its constituent parts, "it also provides possible design rules for proposing hybrid structures having a specific type of electronic functionality."
In addition to those listed some possible uses of carbon nanotubes in the future are:
• Field emitter for flat panel displays.
• Cellular phone signal amplifier.
• Ion storage for batteries.
• Materials strengthener.
In the future, these tubes could well replace silicon. Thus carbon nanotubes are of great importance in the field of nanoelectronics.
One nano step toward efficient LED lighting
Engineers at Kopin Corp. (Taunton, MA) are using nanotechnology (patent pending as NanoPockets) to produce "CyberLites" — blue light-emitting diodes (LEDs) smaller than a grain of sand . The new LEDs are as bright as 3.3V commercially available devices, yet can be driven by <2.9V (using 20mA of current) and still have 100 mC brightness. In addition, CyberLites have achieved ESD resistance >4000V compared to ~2000V resistance with commercially available LEDs; high ESD resistance is critical for industrial applications.
This work was done in cooperation with Jagdish Narayan of North Carolina State University and director of the NSF Center of Advanced Materials and Smart Structures. FIG 5 shows a ‘CyberLite’ on a US dime.NanoPockets is based on Kopin's patented wafer engineering process, already being used by the company for displays and HBT transistors; this process significantly reduces the number of natural atomic level defects when different semiconductor materials are combined. CyberLites are fabricated on gallium nitride grown — via organometallic chemical vapor deposition — on low-cost aluminum oxide. The process provides confinements ("NanoPockets") for production of light away from defects. The nanostructures, which are naturally formed as a result of internal strains, are spaced less than the separation of material defects, such as dislocations.
A blue CyberLite can be combined with yellow phosphor to create a white LED. Blue and white CyberLites are ideal for compact battery-powered portable light-using devices, such as wireless phones, games, camcorders, cameras and laptops.
Nano Solar Cells
nano solar cell :fraunhofer.
Paul Alivisatos, a chemist at the University of California, Berkeley, has an idea in which he aims to use nanotechnology to produce a photovoltaic material that can be spread like plastic wrap or paint.
His approach begins with electrically conductive polymers. To improve the efficiency, Alivisatos is adding a new ingredient to the polymer: nanorods, bar-shaped semiconducting inorganic crystals measuring just seven nanometers by 60 nanometers. The result is a cheap and flexible material that could provide the same kind of efficiency achieved with silicon solar cells. The prototype solar cells he has made so far consist of sheets of a nanorod-polymer composite just 200 nanometers thick. Thin layers of an electrode sandwich the composite sheets. When sunlight hits the sheets, they absorb photons, exciting electrons in the polymer and the nanorods, which make up 90 percent of the composite. The result is a useful current that is carried away by the electrodes. FIG 6 shows the hybrid nanocrystal-polymer solar cell which is made by blending CdSe nanocrystals with P3HT, a conducting polymer, to form a 200nm thick film sandwiched between an aluminum top contact and a transparent bottom contact.
By adjusting the diameter of the nanorods, Alivisatos' lab has tuned their cells' absorption spectrum to have as large an overlap with the solar energy spectrum as possible, enabling them to collect more light than typical plastic solar cells. This tuning will also enable the fabrication of nanocrystal / polymer/ nanoparticle combinations that absorb different wavelengths of light more efficiently. Multiple layers of varied composition can then be stacked on top of one another to form a more efficient cell. To further boost performance Alivisatos and his collaborators have switched to a new nanorod material, cadmium telluride, which absorbs more sunlight than cadmium selenide, the material they used initially. The scientists are also aligning the nanorods in branching assemblages that conduct electrons more efficiently than do randomly mixed nanorods.The nanorod solar cells could be rolled out, ink-jet printed, or even painted onto surfaces, so a billboard on a bus could be a solar collector.
Solar cells based on inorganic nanorods combine the processing advantages of small molecules and organic polymers with the performance advantages of bulk inorganic materials. Because of their solubility in various common solvents, nanorods can be used to make semiconductors using low cost processing techniques such as spin coating, blade casting, and screen printing on substrates of various flexibility, including plastic. They do not require a clean room, a vacuum chamber, or high temperatures for fabrication and the electrode and nanorod/polymer layers of the solar cell can be applied in separate coats for ease of production.
Drawback of working at the nano scale
The main drawback of nanoelectronics is that it is prone to damage due to external electric discharge. On a dry winter day, walking on a new carpet can generate a whopping 35,000-volt discharge. This high voltage does not harm us because the amount of charge that flows is puny. Still, it is large enough to destroy sensitive micro-electronic components. Modern microelectronics is extremely sensitive and can be ruined by the pulse of electricity of an electrostatic discharge (ESD) that can occur from mere handling of a chip.
ESD is an issue not only for finished products but also during their manufacture, from wafer fabrication to packaging to the assembly of complete systems. Each step has its own electrostatic hazards. The main cause of failure of electronics when ESD occurs is the heat generated by the electric current of the discharge, which can be enough to melt the material. Damage occurs even without melting. The properties of diodes and transistors are determined by the doping of the semiconductor: carefully introduced impurity atoms, or dopants, produce regions having specific electronic properties. Excessive heating can allow dopants to migrate, ruining the precise pattern of regions that is essential for the device to function properly.
Processes known as electro-current constriction and thermal runaway make matters worse by concentrating the heating in a hot spot: when one location of a semiconductor heats up significantly, its resistance falls, so that more of the current flows through the hottest place, heating it even more.
The solution to protect these delicate transistors is to include ESD protection circuits on the chip, to divert currents from discharges away from the transistors toward the ground. Since 1995 "smart" circuits known as ESD power clamps have been used to discharge the ESD current through the final stage, from the power rail to the ground. For example, some power clamps use a simple frequency-dependent filter to discriminate an ESD pulse from normal signals. Others detect the excess voltage of the discharge. Once the device senses the pulse, a signal powered by the pulse turns on robust transistor circuits to discharge the current safely to the ground.
David V. Cronin of Polaroid invented a mechanical solution to protect individual diodes when they are being handled: When the diode is not in its socket, conductive metal springs short the electrodes to the diode's metal casing (). Any ESD on the electrodes will flow to the casing instead of to the diode's semiconductor. When the laser diode is inserted into its socket, the metal spring disengages.
In years to come, traditional methods of ESD protection for semiconductors may not be acceptable with smaller, faster devices. Alternatively, designers might use new materials to make intrinsically sturdier transistors and rely on off-chip devices to prevent ESD pulses from reaching the nanocircuitry.
It appears that the future of electronics is ‘nano’ i.e. electronics at the nano scale could revolutionise the way the world works. Microelectronics has been miniaturized to such an extent that it now works at the molecular level to make use of fundamental properties, phenomena, and processes. Nanoelectromechanical systems (NEMS)--the smaller cousins of microelectromechanical systems (MEMS)--are paving the way for a revolution in applications such as sensors, medical diagnostics, displays and data storage. Current integrated circuits have minimum dimensions of the order of 0.35 microns. Based on current rates of development, people have project and implimentationed that around the year 2005 companies will be manufacturing in high volume integrated circuits that have dimensions around 0.1 micron.
Nanoelectronics will be a strategic branch of science and engineering for the current century, one that will fundamentally restructure the technologies currently used in computation , medicine, energy production, communication, and education.
As the twenty-first century unfolds, the impact of nanoelectronics on our society is expected to be as significant as that of the silicon chip, antibiotics, man made polymers or integrated circuits on the twentieth century.
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nanotechnology.ppt (Size: 6.71 MB / Downloads: 200)
Carbon nano-tubes “Nanotechnology”
At the end of applications
the session you will be able to describe the processing methods for carbon nanotubes and their
Learn basic experimental methods / tools used in nanotechnology
Get some idea on future nanotech applications
Advanced nanotech devices
Microscopes for nanotechnology
A nanometre (nm) is one thousand millionth of a metre.
For comparison, a single human hair is about 80,000 nm wide
Definition for Nanoscience: The study of phenomena and manipulation of materials at atomic, molecular and macromolecular scales, where properties differ significantly from those at a larger scale; and nanotechnologies as the design, characterisation, production and application of structures, devices and systems by controlling shape and size at the nanometre scale.
In some sense, nanoscience and nanotechnologies are not new. Chemists have been making polymers, which are large molecules made up of nanoscale subunits, for many decades and nanotechnologies have been used to create the tiny features on computer chips for the past 20 years. However, advances in the tools that now allow atoms and molecules to be examined and probed with great precision have enabled the expansion and development of nanoscience and nanotechnologies.
People are interested in the nanoscale (which we define to be from 100nm down to the size of atoms (approximately 0.2nm)) because it is at this scale that the properties of materials can be very different from those at a larger scale.
What is nanotechnology?
Materials properties can be different at nanoscale
Nanomaterials have a relatively larger surface area when compared to the same mass of material produced in a larger form. This can make materials more chemically reactive (in some cases materials that are inert in their larger form are reactive when produced in their nanoscale form), and affect their strength or electrical properties.
Quantum effects can begin to dominate the behaviour of matter at the nanoscale - particularly at the lower end - affecting the optical, electrical and magnetic behaviour of materials.
Materials can be produced that are nanoscale in one dimension (very thin surface coatings), in two dimensions (nanowires and nanotubes) or in all three dimensions (for example, nanoparticles).
Resistivity 10-4 W-cm
Maximum Current Density 1013 A/m2
Thermal Conductivity (Room Temperature) ~ 2000 W/m•K
Phonon Mean Free Path ~ 100 nm
Relaxation Time ~ 10-11 s
Young's Modulus (SWNT) ~ 1 TPa
Young's Modulus (MWNT) 1.28 TPa
Maximum Tensile Strength ~30 GPa
Superior strength & lightweight: Ropeway to outer space?
Expensive to produce
$54,000/1kg (year 2007)
Single-walled nanotubes (SWNTs) Purity: > 90 vol% (carbon nanotubes) > 50 vol% (single-walled nanotubes)
Diameter: 1-2 nm (from HRTEM)Diameter: 0.8-1.6 nm (from Raman spectra)Average diameter: 1.1 nm (from Raman spectra)Length: 5-15 umSSA: > 400 m2/g
Potential applications for carbon nanotubes
Additives in ploymers
Electron field emitters for
cathode ray lighting elements
flat panel display
gas-discharge tubes in telecom networks
Electromagnetic-wave absorption and shielding
Nanotube composites (by filling or coating);
STM, AFM, and EFM tips
Reinforcements in composites
Very thin coatings for electronics and active surfaces (self-cleaning windows).
In most applications the nanoscale components will be fixed or embedded but in some, such as those used in cosmetics and in some pilot environmental remediation applications, free nanoparticles are used.
The ability to machine materials to very high precision and accuracy (better than 100nm) is leading to considerable benefits in a wide range of industrial sectors, for example in the production of components for the information and communication technology (ICT), automotive and aerospace industries.
Range of products: silicon based electronics, displays, paints, batteries, micromachined silicon sensors and catalysts.
Composites that exploit the properties of carbon nanotubes – rolls of carbon with one or more walls, measuring a few nanometres in diameter and up to a few centimetres in length – which are extremely strong and flexible and can conduct electricity.
At the moment the applications of these tubes are limited by the difficulty of producing them in a uniform manner and separating them into individual nanotubes.
Lubricants based on inorganic nanospheres.
Magnetic materials using nanocrystalline grains; nanoceramics used for more durable and better medical prosthetics; automotive components or high-temperature furnaces; and nano-engineered membranes for more energy efficient water purification.
Current manufacturing standard for silicon chips in terms of the length of a particular feature in a memory cell is 90nm, but it is predicted that by 2016 this will be just 22nm.
Much of the miniaturisation of computer chips to date has involved nanoscience and nanotechnologies, and this is expected to continue in the short and medium term.
Alternatives to silicon-based electronics are already being explored through nanoscience and nanotechnologies, for example plastic electronics for flexible display screens.
Other nanoscale electronic devices currently being developed are sensors to detect chemicals in the environment, to check the edibility of foodstuffs, or to monitor the state of mechanical stresses within buildings.
Much interest is also focused on quantum dots, semiconductor nanoparticles that can be ‘tuned’ to emit or absorb particular light colours for use in solar energy cells or fluorescent biological labels.
Disease diagnosis, drug delivery targeted at specific sites in the body and molecular imaging are being intensively investigated and some products are undergoing clinical trials.
Nanocrystalline silver, which is known to have antimicrobial properties, is being used in wound dressingsin the USA.
The production of materials and devices such as scaffolds for cell and tissue engineering, and sensors that can be used for monitoring aspects of human health.
In the longer term, the development of nanoelectronic systems that can detect and process information could lead to the development of an artificial retina or cochlea.
Progress in the area of bio-nanotechnology will build on our understanding of natural biological structures on the molecular scale, such as proteins.
So far (2006), the relatively small number of applications of nanotechnologies that have made it through to industrial application represent evolutionary rather than revolutionary advances.
Current applications are mainly in the areas of determining the properties of materials, the production of chemicals, precision manufacturing and computing.
In mobile phones for instance, materials involving nanotechnologies are being developed for use in advanced batteries, electronic packaging and in displays. The total weight of these materials will constitute a very small fraction of the whole product but be responsible for most of the functions that the devices offer.
There will be significant challenges in scaling up production from the research laboratory to mass manufacturing.
In the longer term it is hoped that nanotechnologies will enable more efficient approaches to manufacturing which will produce a host of multi-functional materials in a cost-effective manner, with reduced resource use and waste.
Maybe possible to develop mechanical nano-machines which would be capable of producing materials (and themselves) atom-by-atom. Alongside such hopes for self-replicating machines, fears have been raised about the potential for these (as yet unrealised) machines to go out of control, produce unlimited copies of themselves, and consume all available material on the planet in the process.
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22-02-2011, 12:40 PM
Shishir-Rai-CarbonNanotubes.ppt (Size: 4.38 MB / Downloads: 210)
What is a Carbon Nanotube?
CNT is a tubular form of carbon with diameter as small as 1nm.
Length: few nm to microns.
CNT is configurationally equivalent to a two dimensional graphene
sheet rolled into a tube.
A CNT is characterized by its Chiral Vector: Ch = n â1 + m â2,
q ® Chiral Angle with respect to the zigzag axis.
Why do Carbon Nanotubes form?
Carbon Graphite (Ambient conditions)
sp2 hybridization: planar
Diamond (High temperature and pressure)
sp3 hybridization: cubic
Nanotube/Fullerene (certain growth conditions)
sp2 + sp3 character: cylindrical
Finite size of graphene layer has dangling bonds. These dangling
bonds correspond to high energy states.
Eliminates dangling bonds
Nanotube formation + Total Energy
Increases Strain Energy decreases
Types of CNTs
Single Wall CNT (SWCNT)
Multiple Wall CNT (MWCNT)
Can be metallic or semiconducting depending on their geometry
CNT: Implications for electronics
Carrier transport is 1-D.
All chemical bonds are
satisfied Þ CNT Electronics not bound to use SiO2 as an insulator.
High mechanical and thermal stability and resistance to electromigration Þ Current densities upto 109 A/cm2 can be sustained.
Diameter controlled by chemistry, not fabrication.
Both active devices and interconnects can be made from semiconducting and metallic nanotubes.
Nanotube Growth Methods
) Arc Discharge
b) Laser Abalation
Involve condensation of C-atoms generated from evaporation of solid carbon sources. Temperature ~ 3000-4000K, close to melting point of graphite.
Both produce high-quality SWNTs and MWNTs.
MWNT: 10’s of mm long, very straight & have 5-30nm diameter.
SWNT: needs metal catalyst (Ni,Co etc.).
Produced in form of ropes consisting of 10’s of individual nanotubes close packed in hexagonal crystals.
c) Chemical Vapor Deposition:
Hydrocarbon + Fe/Co/Ni catalyst 550-750°C CNT
• Dissociation of hydrocarbon.
• Dissolution and saturation
of C atoms in metal nanoparticle.
• Precipitation of Carbon.
Choice of catalyst material?
Base Growth Mode or Tip Growth Mode?
• Metal support interactions
Controlled Growth by CVD
a) SEM image of aligned nanotubes.
B) SEM image of side view of towers. Self-alignment due to Van der Walls interaction.
c) High magnification SEM image showing aligned nanotubes.
d) Growth Process: Base growth mode.
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12-03-2011, 08:43 PM
It is very useful stuff for technical person.
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27-04-2011, 11:21 AM
Seminar on cnts ppt.pptx (Size: 2.89 MB / Downloads: 67)
We can define nanocarbon as materials built at nanometer scale from sp2 hybridized carbon atoms similar to that of graphite.
This bonding structure, which is stronger than the sp3 bonds found in diamonds, provides the molecules with their unique strength.
Carbon nanotubes are wires of pure carbon with nanometer diameters and lengths of many microns.
They are the allotrope of carbon.
They take the form of cylindrical carbon molecules and have novel properties .
Which 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.
They are hollow cylinders composed of one or more concentric layers of carbon atoms in a honeycomb lattice arrangement.
C-60 clusters were the 3rd allotrope of carbon.
Japanese scientist Sumio Iijima discovered fullerene-related carbon nanotubes in 1991.
In the form of thread lying in a smear of sooth.
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.
It is important to note, however, that nasnoscale 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.
Several types of nanotubes exist; but they can be divided in two main categories:
single-walled (SWNT) :individual cylinders of 1-2nm.
multi-walled (MWNT):are collections of several concentric graphene cylinders
Armchair( n, n)
High mechanical stability and chemical inertness.
Carrier transport is 1-D.
Electrostatic behavior is different.
All chemical bonds of C atom are satisfied.
Thermal stabilty:3000 W/mK
Maximum strain:10% higher than any other material.
Very high current carrying capacity.
Excellent field emitter.
Chemical Vapour Deposition:
CNT’s in Electronics
Several types of devices can be made using SWCNT’s instead of conventional semiconductor such as silicon..
Quantum mechanical tunneling becomes important as the length of the transistor channel and thickness of gate insulator decreases.
This results in high leakage current which damages the transistor as a switch.
While scaling the width of metallic wire should also be scaled which increases the resistance.
Why we choose nanotubes?
SWCNT’s are one dimensional systems , so they do not allow the small angle scattering of electrons or holes.
There is no electromigration and metallic nanotubes carry current densities 2-3 orders of magnitude higher than metals such as copper or aluminum.
Some of the applications:
Micro-electronics / semiconductors
Controlled Drug Delivery/release
Field emission flat panel displays
Field Effect transistors
Single electron transistors
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.
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30-04-2011, 02:37 PM
CARBON NANOTUBES final ppt manohara gn.pptx (Size: 1.59 MB / Downloads: 50)
Folded version of two dimensional graphite sheets
Depending on the dimension of the graphite sheet and type of folding, different types can be created. Eg :SWNTS, MWNT’S
Sidewalls consist of only hexagonal carbon rings
TYPES OF CNT’S
There are two types of CNT’S
1. Single walled Nanotubes(SWNT’S)
2.Multi walled Nanotubes (MWNT’S)
1.SINGLE WALLED NANOTUBES
Most single-walled nanotubes (SWNT) have a diameter of close to 1 nanometer, with a tube length that can be many millions of times longer.
Single-walled nanotubes are an important variety of carbon nanotube because they exhibit electric properties that are not shared by the multi-walled carbon nanotube (MWNT) variants.
Multi-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tubes) of graphene.
MWNTs are zero-gap metals.
SYNTHISIS of CNT’S
There are three techniques to produced nanotubes
Arc discharge method
Laser Ablation method
Chemical Vapor Deposition (CVD) Method
a)Arc discharge method
c)Chemical Vapor Deposition
a)Electronic and Electrical property
Fast electron conductivity
Very small with high frequency and low power consumption.
Velocity of electrons in carbon nanotubes is equal to velocity of light.
Mobility of electron in carbon nanotubes is 200 times that of silicon
A) Flat Panel Display
Low power consumptio, High brightness
Wide viewing angle
Fast response rate
B) As Conductors and Transistors
High surface area to mass ratio of graphene.
Greater energy storage density than currently available.
Manufacture of Artificial muscles
60 percent of cars produced today now have fuel lines made with carbon nanotubes.
The carbon nanotubes inside the fuel lines are intended to reduce the risk of explosions by inhibiting static electricity.
Carbon nanotubes have already been added to sports equipment such as bats, bicycles, golf clubs, and hockey sticks in order to impart more strength and absorb vibrations.
Limitations of CNT’S
High-Quality nanotubes can only be produced in very limited quantities -commercial nanotube soot costs 10 times as much as gold!
Since these particles are very small, problems can actually arise from the inhalation of these minute particles.
In future carbon nanotubes replaces carbon fiber.
It is also used in VLSI interconnections and in water purification.
It replaces silicon based diodes and transisters in future.
LIST OF CARBON NANOTUBE SUPPLIERS
NIKKISO CO. LTD. (JAPAN)
CARBON DESIGNS, INC. (USA)
BAYER MATERIAL SCIENCE AG (GERMANY)
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.