Nanoscale technologies: carbon nanotubes

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INTRODUCTION

The history of mankind has been at times interrupted by major scientific and technological revolutions that changed the course of history, and sometimes the behavior and habits of a man. In a medium-term future we will live next electronics revolution , where we spend from microelectronics to nanoelectronics, where the processing speed and size of the devices will decrease significantly. In May 2002 IBM announced that it has improved its carbon nanotube transistor (CNT) in such so that their performance is greater than the most advanced silicon transistors. One of the researchers , said, "It's as if we had previously developed a new type of light bulb that worked, but needed high voltages, it was not very bright and one had to turn all the lights in the house at once. Now, we can make the lamps brighter, use less power, and turned on and off individually. "

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In short we are a gateway to a new opportunity, a BOOM on miniaturization, and therefore all systems high performance and fidelity, when the chip development reaches physical barrier that silicon can no longer be smaller. That is, within approximately 10 to 15 years estimated by Moore's law.

Around 20 years ago, Sumio Iijima, sitting at his electron microscope laboratory in the NEC Fundamental Research in Tsukuba, watching strange nanoscale fibers deposited on a speck of soot. Consisting of carbon, and so regular and symmetric as crystals, these macromolecules exquisite refinement and impressive length soon called nanotubes. They have since been the subject of intense basic research. He has led a step further. Now also interested in engineering. Many of the extraordinary properties of nanotubes, superlative resilience, tensile strength and thermal stability, have triggered the imagination, who dreams of microscopic robots, auto body dent-resistant and earthquake-proof buildings. However, the first products that incorporate nanotubes do not because of these attributes, but by virtue of their electrical properties. Some cars from General Motors include plastic parts to which nanotubes are added, the plastic material is electrically charged during the painting so that it adheres well. Soon will come to market two lighting products and nanotube-based display.

In the long term, the most valuable applications will draw most from the unique electronic properties of nanotubes. In principle, carbon nanotubes can play the same role of the silicon in electronic circuits, but on a molecular scale where silicon and other semiconductors stop working. Although the electronics industry is leading to the critical dimensions of transistors in commercial chips below 200 nanometers, about 400 atoms wide, engineers are faced with major obstacles to further miniaturization. Between now and 10 years, the materials and processes that have guided the computer revolution will begin to reach its physical limit impassable. There are still huge economic incentives to further reduce the devices, because the velocity, density and performance of micro-refineries will increase with the reduction of the minimum size of components. The experiments of recent years have given scientists hope that nanotubes could be fabricated with connections and the active devices as small as ten nanometers or less. Nanotubes embedded in electronic circuits, they operate more quickly and without consuming much energy as today's.

The first carbon nanotubes that Iijima observed in 1991 were termed multi-wall Tube, each containing a number of hollow cylinders of carbon atoms nested in the manner of a sponge. Two years later, Iijima and Donald Bethune, this one from IBM, created by hand each single-walled nanotubes, composed exclusively of a layer of carbon atoms. Both types of tubes, made similarly, have many similar properties, the obvious, the length and narrowness, enormous. The single-pair model, approximately one nanometer in diameter, can span thousands of nanometers long.

The tube that gives these remarkable stability is the intensity with which the carbon atoms bind to each other, a property that explains the hardness of diamond. In this mineral, carbon atoms are joined in a four-sided tetrahedron. However, in nanotubes the atoms are arranged in hexagonal rings, the same structure that characterizes the figure in fact a nanotube looks like a plate (or several plates stacked) chart rolled into a cylinder without seams. No one knows for sure why atoms condense into tubes, but it seems that can grow by adding atoms to their ends, like a weaver will add points to the sleeve of a sweater.

CARBON NANOTUBES

DEFINITION.

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Carbon nanotubes are considered a great promise due to their exceptionally strong mechanical properties, its ability to efficiently carry high densities of electric current, and other electrical and chemical properties. Nanotubes, which are approximately 10,000 times thinner than a human hair, can be made almost perfectly straight in special chambers of gas plasma. They are the strongest fibers known. A single perfect nanotube is 10 to 100 times stronger than steel per unit weight and have very interesting electrical properties, conducting electric current hundreds of times more efficiently than traditional copper wires.

Graphite is composed of carbon atoms in the form of structured panel. These panel-type layers are placed one above the other. A single layer of graphite is very stable, strong and flexible. Since a graphite layer is so stable single, weakly adheres to the layers at hand, For this is used in pencils - because as you type, fall small graphite flakes. In carbon fibers, the individual layers of graphite is much larger than in pencils, and form a structure long, wavy, thin-type spiral. You can paste these fibers one to another and form a substance so strong, lightweight (and expensive) used in airplanes, tennis rackets, racing bikes and so on. But there is another way of structuring the layers that produces a stronger material still wrapping the panel-type structure to form a graphite tube. This tube is a carbon nanotube.

Carbon nanotubes, besides being extremely hardy, have interesting electrical properties. A layer of graphite is a semi-metal. This means it has properties intermediate between semiconductors (such as silicon in computer chips, when electrons move with restrictions) and metals (like copper used in cables when electrons move without restriction). When a layer of graphite rolled into a nanotube, besides having to align the carbon atoms around the circumference of the tube, also the wave functions of quantum mechanical style of the electrons must also comply. This setting restricts the types of wave function which can be electrons, which in turn affects their motion. Depending on the exact form in which rolls along the nanotube can be a semiconductor or a metal.

COMPOSITION AND GEOMETRY.

The development achieved by scientific areas known as nanoscience and nanotechnology is due in part to the discovery and subsequent developments force microscope (AFM) and scanning tunneling (STM). Both microscopes are indispensable tools have been configured to interrogate the properties of nano-sized systems. The locality and the precise control of electromagnetic interactions of these techniques allows the investigation of the chemical, mechanical or electrical nanometric structures, irrespective of the nature of the nanostructures. These can be either semiconducting organic molecules or biological molecules. The properties mentioned force microscope and scanning tunneling can be harnessed to develop new techniques for surface modification and manipulation at the nanoscale. These methods can form the basis for the development of new techniques of lithography below 10nm.

The composition and geometry of carbon nanotubes generate, regardless of their training, a unique electronic complexity. This is due in part to size, then do not forget to send quantum physics on the nanoscale. But the graphite itself constitutes, of itself, a very special material. If most of the electrical conductors are either metals or semiconductors, graphite belongs to the restricted group of semimetals, installed in a delicate balance in the transition zone between them. Combining the semimetallic properties of graphite with the rules of quantum energy levels and electron waves, carbon nanotubes are emerging as exotic drivers. Imposes certain rule of the quantum world that electrons behave as particles, the electron waves amplify or cancel each other. Therefore, an electron is distributed around the circumference of a nanotube can autocancelarse wholly, and will only electrons with equal right wavelength. Of all the possible wavelengths electronic or quantum states that are in a flat sheet of graphite, only a small set will be permitted when it rolls into a nanotube.

The set will depend on the circumference of the nanotube, as well as whether the nanotube is twisted in the manner of the neon signs of a barbershop. When slicing a few electronic states of a single metal or a semiconductor there are no surprises. The semimetals, however, are much more sensitive. And of those, more interesting carbon nanotubes. On a sheet of graph, the Fermi point, some specific electronic state, gives the conductivity graph almost all wielding, in any other state shall enjoy freedom of movement of electrons. Only a third of all carbon nanotubes combines the correct diameter and the corresponding degree of twist to include this special Fermi point in his set of allowed states. These are authentic nanofilaments metallic nanotubes.

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Carbon nanotubes are not all the same band interval, because for each circle there is a unique set of allowed states of valence and conduction. Carbon nanotubes are not all the same band interval, because for every circle there is a unique set of allowed states of valence and conduction. Smaller diameter nanotubes have very few states widely separated in energy. By increasing the diameter of the nanotubes, increasingly allowed states and shortens the distance between them. It happens then that nanotubes of different sizes can have zero band intervals (zero, like a metal), the size range of silicon band or almost any value between. No other known material can be sharpened with such ease. But the growth of nanotubes occurs still a lot of different geometries. so researchers are busy in finding ways that we ensure specific types of nanotubes.

Nanotubes multiple wall thicknesses can develop even more complex behavior. Each layer of the tube has a slightly different geometry. If we can custom design the composition of each, would have fulfilled the dream of making multi-walled tubes that are autoaislantes or carrying multiple signals at once, as nanoscale coaxial cables. Our knowledge and control of growth of nanotubes are far from such goals. However, to incorporate nanotubes into circuits have begun operating, at least, to unravel their basic properties.

PROPERTIES.

  • Size. 0.6 to 1.8 nanometers in diameter. Electron beam lithography can create lines of 50 nm wide.
  • Density. 1.33 to 1.40 g/cm3. Aluminum has a density of 2.7 g/cm3
  • Tensile. Strength of 45 billion pascals. Alloys of high-strength steel to break around 2 billion of pascals.
  • Elasticity. They can be bent to large angles and return to their original state without damage. Metals and carbon fibers fracture at similar efforts.
  • Current carrying capacity. Estimated at billion amps per square centimeter of copper wires to melt a million amperes per square centimeter or so.
  • Field emission. They can control phosphors with 1 to 3 volts if electrodes are spaced one micron. Using regular methods with molybdenum tips of fields require 50 to 100 V / m and have very limited lifetimes.
  • Heat transfer. It is predicted as high as 6.000 watts per meter per kelvin at room temperature. The nearly pure diamond transmits 3.320 W / mK
  • Thermal Stability. Stable to 2,800 degrees Celsius even in vacuum, and 750 ° C in air. The metal wires in microchips melt between 600 and 1000 ° C.

NANOCIRCUITS: How to make "transistors and wire interconnects".

Several research groups have successfully constructed operating electronic devices from carbon nanotubes. In some field-effect transistors (FET) using simple semiconductor nanotube between two metal electrodes to create a channel through which the electrons circulate. The current flowing through it may be activated by applying voltage to a third electrode immediately. Nanotube-based devices operate at room temperature with electrical characteristics strikingly similar to commercial silicon devices. Other research groups have found that the gate electrode can change the conductivity of the nanotube channel in an FET by a factor of a million or more, comparable to silicon FET. Because of its small size, however, commuted the nanotube FET without mistake and consume much less power than a silicon device. In theory, a switch made at the nanoscale could work at speeds of a chronological or more terahertz, a thousand times faster than the available processors.

Given the wide range of band intervals and behaviors of nanotubes, there are multiple possibilities open to additional nanodevices. In the laboratory, measuring junctions of metallic and semiconductor nanotubes has been observed that these behave as diodes, allowing electricity to flow in one direction. In line of principle, combinations of nanotubes with different band ranges may become light-emitting diode lasers and perhaps the nanoscale. Nothing now seems to hamper the development of a gifted nanochip connections, switches and memory elements made with nanotubes and other molecules. This molecular engineering could be obtained, at last, not just tiny versions of devices to use but also others that explore the quantum effects.

Until now, the nanotube circuits are manufactured one by one and with great effort. Although not yet determined a single protocol for the construction of nanotubes and each research group follows its own protocol to establish a nanotube to traditional metal electrodes in all resource is the combination of traditional lithography tools for the electrodes with high resolution as atomic force microscopes, to place the nanotubes. Needless to say that is a long way to industrial production, complex, automated parallel silicon microchips as it sits on the computer industry.

MANUFATURING PROCESSES:

We presume that we can think about making a circuit structure, based on nanotubes, but we must find ways to develop nanotubes positions, orientations, shapes and specific sizes. At Stanford University and other institutions has shown that placing particles of nickel, iron or other catalyst on a substrate, nanotubes are obtained that grow where they want. At Harvard he has found a way to unite nanofilamentos nanotubes with silicon, weaving connections to the circuits produced by the usual means.

  • Volatilize, Bake or Bombing. While Sumio Iijima was the first to see a nanotube, if others are ahead in manufacturing. Unknowingly, Neanderthals manufacture tiny amounts of nanotubes in the fires that warmed their caves. Separated by heat, carbon atoms recombine in soot, some cells generate amorphous other spheres called "buckyballs" and other long cylindrical capsules, the "buckytubes" or nanotubes. Science has discovered three ways to produce soot that contains a significant proportion of nanotubes. So far, however, all three methods suffer from some important limitations: they all produce mixtures of nanotubes with a wide range of lengths, many defects and a variety of twists.
  • Big spark. In 1992 a Pullickel Thomas Ebbesen and M. Ajayan, Fundamental Research Laboratory of NEC, published in the first method of making macroscopic quantities of nanotubes. It consists of connecting two graphite rods to a power source, separate a few millimeters and a switch. A spark to 100 amps of current between the bars, the carbon is evaporated in a hot plasma. Part of it is re-condense in the form of nanotubes. Normal Yield: Up to 30 percent by weight. Advantage: higher temperatures and metallic catalysts added to the bars can produce single-walled nanotubes and multi with few structural defects, if one. Limitations: The tubes tend to be short (50 microns or less) and placed in random shapes and sizes.
  • A hot gas. Morinubo Endo of Shinshu University in Nagano, introduced in the manufacture of nanotubes, the method of chemical vapor deposition (CVD). Substrate is placed in an oven heated to 600 grams C and slowly added methane gas releases carbon atoms, that can be recombined in the form of nanotubes. Normal Yield: 20 to almost 100 percent. Advantages: CVD technique is the simplest of the three methods for application to industrial scale. It could be used for long nanotubes needed in the fibers used in composite materials. Limitations: The nanotubes thus produced are usually multiple and sometimes walls are riddled with defects. Hence, the tubes have only one tenth of the tensile strength over those produced by arc discharge.
  • A laser bombardment. The bombing of a metal with intense laser pulses to produce more extravagant metallic molecules when they received the news of the discovery of nanotubes. On your device replaced the metal graphite rods. Soon they produce carbon nanotubes using laser pulses instead of electricity to generate hot carbon gas from which are formed naotubos. Tested with several catalysts and succeeded finally, conditions that occur in prodigious amounts of single-wall nanotubes. Normal Yield: Up to 70 percent. Benefits: They produce single-wall nanotubes with a diameter range that can be controlled by varying the reaction temperature. Limitations: This method requires expensive lasers.
    • We talk about timid steps. Nevertheless, enough to glimpse the application of carbon nanotubes as transistors and wires for interconnects in microchip circuits. These filaments, of about 250 nanometers wide, are metallic. The engineers would love to achieve much lower, so as to integrate more devices in the same section. But the further miniaturization of the wires must overcome two difficulties imposing. First, we have not yet developed an effective method to dissipate heat generated by the devices, if you squeeze more rapidly would cause overheating. Second, to sharpen the wires and, in short, the strands will be degraded, as blown fuses. Nanotubes could theoretically solve both problems. Carbon nanotubes conduct heat almost to the diamond or sapphire, a notion that seems to be confirmed in experiments provisional. The nanotubes could then efficiently cool very dense series of devices. Additionally, since the bonds between carbon atoms are much stronger than any other metal nanotubes can carry large amounts of electric current. Recent measures show that a bundle of nanotubes of a square centimeter of cross section might lead one billion amperes. These currents as high vaporized copper or gold.

FULLERENES: Carbon cages.

Known carbon allotropic forms are: graphite, diamond and fullerenes. In 1985 he discovered the third allotropic form of carbon, it was a substance where each molecule had sixty atoms of carbon, this discovery was a prelude and an incentive to search for new materials coming to the coincidental discovery of nanotubes. Fullerenes are much more abundant than we think, may even be more abundant than graphite and the diamond can be found in the smoke and soot from combustion, we find by studying the stars and interstellar space or in the earth layers that show us the planet's geological, fullerenes have also been found in meteorites that fall to earth. Recent studies also indicate that every living organism has a certain amount of fullerenes in composition, all these facts, give us an idea of the vast field of study and the numerous lines of investigation that may arise around the study of fullerenes.

Fullerenes are large molecules as spheres. The most common is the C60, but as there are more carbon, C70, C84, C240, C540 ... and there are also less, which usually have a quasi-geometric arrangement of spherical or ellipsoidal shape. In 1991 he found another form of carbon, "nanotube". A nanotube is a very large fullerene in a linear fashion.

APPLICATIONS.

NANOTECH TOOLS.

Nanoclips. Two nanotubes attached to electrodes on a glass rod, open and close through a voltage change. These tweezers are used to imprison and move objects 500 nanometers in size.

Scanning Microscope higher resolution. Attached to the tip of a scanning probe microscope, nanotubes can amplify the lateral resolution of the instrument a factor of ten or more, allowing clear representations of proteins and other molecules.

MEDICAL PURPOUSES.

Chemical and genetic probes. "DNA strand" A microscope with a nanotube tip can locate a strand of DNA and identify the chemical markers that reveal which of the possible variables of a gene provides the thread. This process is not easy because, as DNA can not penetrate cell membranes, requires the help of a protractor. A team of European researchers has developed a new method of introducing DNA into mammalian cells by modified carbon nanotubes. The carbon nanotubes are tiny structures shaped needle, made of carbon atoms.

To use nanotubes as gene carrier was necessary to modify them. The research team was able to bind to the outside of carbon nanotubes made of multiple chains of carbon and oxygen atoms whose side is a positively charged amino group (- NH3 +). This small change makes the nanotubes are soluble. Furthermore, positively charged groups attract the negatively charged phosphate groups on the backbone of DNA. By using these electrostatic attractive forces, the researchers succeeded in setting a solid plasmid outside of the nanotubes. He contacted the nanotube-DNA hybrids with cell culture of mammalian cells. The result was that carbon nanotubes, along with DNA cargo, came within the cell. Electron microscope images showed the way in which the nanotubes penetrated the cell membrane. Nanotubes not damage cells because, unlike the previous transport systems genetics, do not destabilize the membrane to penetrate. Once inside the cell, genes found to be functional. Using carbon nanotubes as a carrier is not limited to transplant genes. Using carbon nanotubes as a carrier is not limited to transplant genes. New scientific achieve possible the transport of medicines and development of new medical techniques.

ELECTRONICS.

Memory mechanical "non-volatile RAM" It has screen tested nanotubes deposited on a support block as a function of binary memory device, with voltages forcing contact between tubes ( "on" state) or separation ( "off" state).

Superconductivity at room temperature with nanotubes. According to some experiments recently carried, carbon nanotubes can conduct electricity without resistance at temperatures above the boiling point of water. If confirmed this would be the first superconductors that would operate at a "normal" temperature, without machinery specially heat.

Researchers at the University of Houston found tracks of superconductivity in these nanotubes. There is zero resistance, but the closest anyone has ever approached. Currently there is no evidence of any superconductor that operates at a temperature less than 143 degrees, but did find a material capable of conducting electricity without resistance at room environment and not lose heat energy, which would mean much faster electronic circuits. Technically they are studying the effects of magnetic fields in hollow fibers of carbon called carbon nanotubes multipair. It is believed that resistance to the conductivity does not become zero because the connections between tubes are not superconducting.

Ultrathing speakers. Shoushan Fan and his team at Tsinghua University in Beijing, have managed to create a surface that can attach to a gadget and is composed of coiled nanotubes with a diameter of 10 nanometers (we would notice that are not tubes but actually a completely flat surface) than when subjected to an audible frequency, they act as speakers. Almost magic to our eyes. The basis of operation of these carbon nanotubes as speakers have to look at the temperature difference that material on the environment can achieve when we apply a sound signal. This difference in temperature causes changes in air pressure that surrounds the coated surface of nanotubes and the sound is produced. The sound generated is very low in power, but the particular structure of the nanotubes allows the sound heard is powerful enough.

Nanotube bulbs. Two companies are engaged in the manufacture of goods that use carbon nanotubes as field emitters. The Japanese Ise Electronics has tested compounds for manufacturing nanotube bulbs prototype vacuum tubes in six colors, which doubles the brightness of bulbs, has a tenfold greater durability and energy saving. The first prototype has worked well for more than 10,000 hours and has not failed. The engineers at Samsung in Seoul nanotubes spread in a thin film on the control electronics and then placed on top phosphor-coated glass for display a prototype plant. When the panel held a demonstration in 1999, was optimistic that the company may have the device that shine like a cathode ray tube and consume a tenth of power ready for production in the current year. The third area which carbon nanotubes are special electronic properties is very small scale, where the effects are of interest depending on the size.

INDUSTRIAL MATERIALS.

Materials for maximum strength. Embedded in a composite material, the nanotubes enjoy enormous elasticity and tensile strength. They may be used in cars that bounce in an accident or buildings shaking in an earthquake instead of cracking.

CHEMISTRY.

The nanotubes secure method of storing and transporting hydrogen.The future development of systems with hydrogen as fuel may depend on whether or not to develop a secure method of transporting and storing hydrogen. A car that runs through the combustion of hydrogen with oxygen to produce only water as waste. On paper, it is the perfect environmentally friendly car. But its implementation is facing many challenges, including the availability of a safe way to transport and store hydrogen. Today, hydrogen is stored and transported at low temperatures and air bottles to be treated carefully, since this gas is very unstable and any stroke can be dangerous. Carbon nanotubes have been proposed as candidates to store large quantities of hydrogen safely. At the Institute of Material Science of Barcelona (ICMAB), Laboratory equipment Electronic Structure of Materials, is working on a project with the U.S. company Air Products to find out how to store hydrogen in carbon nanotubes.

They store hydrogen like sponges, although it is unclear how "has been shown that carbon nanotubes store hydrogen, though not really know how," In several experiments, the researchers explain, it was found that when deposited carbon nanotubes inside a pressure chamber and hydrogen is admitted into the chamber, later, again leaving out the hydrogen in the chamber, the amount of gas is less outgoing than incoming. The difference is that corresponding to hydrogen has been incorporated into the nanotube, so as comparable to a liquid would be trapped in a sponge. The fullerene, a C60 molecule has the shape of a truncated icosahedron, equal to that of a ball football. In whom it is believed that hydrogen could be stored, with a stability greater than that provided by the nanotubes.

Chemical Sensors with nanotubes. Two research groups, one from the University of California at Berkeley and a Stanford University have shown that carbon nanotubes could be optimal chemical sensors capable of detecting very small concentrations of toxic gases. The detection of gas molecules is critical in environmental monitoring, control of chemical processes in space missions and in agriculture and medicine. Without going any further, the detection of nitrogen dioxide (NO2) plays an important role in the analysis of pollution resulting from combustion or automotive emissions, detection of ammonia (NH3) receives a special interest in industrial environments, biological and doctors. nanotubes synthesized by different techniques, creating single-walled tubes, tubes layers walls, plus "strings" attached braided tube laterally to each other by weak van of Waals forces. The electric character, metallic or semiconducting of the nanotubes depends on structural details like the precise form of cut and roll it up graphitics, and the presence of defects and impurities. Each different configuration is sensitive to a specific gas molecule.The extreme sensitivity of electrical properties to the presence of gas molecules absorbed on the tube are the reason that has led to propose the use of carbon nanotubes as novel chemical sensors. The substantial change in resistance and drastic change thermoelectric power show that the nanotubes act as highly sensitive oxygen sensors.