ELECTRONIC-APPLICATIONS-OF-CARBON-NANOTUBES

8/9/2019 ELECTRONIC-APPLICATIONS-OF-CARBON-NANOTUBES

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Introduction

The remarkable properties of carbon nanotubes may allow them to play a crucial role in therelentless drive towards miniaturization at the nanometre scale.

Nanotechnology is predicted to spark a series of industrial revolutions in the next two decadesthat will transform our lives to a far greater extent than silicon microelectronics did in the 20thcentury. Carbon nanotubes could play a pivotal role in this upcoming revolution if their remarkable electrical and mechanical properties can be exploited.

Since the first measurements were made in 1997, these rolled up sheets of graphite have capturedthe imagination of researchers around the world. Progress in understanding the basic physics andchemistry of nanotubes has advanced at a phenomenal rate - and shows no signs of slowing.

Carbon nanotubes can be considered as a single sheet of graphite rolled in the form of a tube,though it is not actually made by rolling one. They were first observed by a Japanese scientistSumio Iijima in the early part of the 1990s. The tubes that consist of a single layer of graphite istermed as Single walled nanotubes and the Multi-walled tubes are those consisting of morethan a single layer in the form of concentric cylinders.

Both these types have their respective fields of applications in the industry and the scientific

scenario. Nanotubes have an impressive list of attributes. They can behave like metals or semiconductors, can conduct electricity better than copper, can transmit heat better than diamond,and they rank among the strongest materials known - not bad for structures that are just a fewnanometres across. Several decades from now we may see integrated circuits with componentsand wires made from nanotubes, and maybe even buildings that can snap back into shape after anearthquake.


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Electronic Structure of nanotubes .

The remarkable electrical properties of single wall carbon nanotubes stem from the unusualelectronic structure of graphene- the 2-D material from which they are made. (Graphene issimply a single atomic layer of graphite.). The band structure of graphene is not same as that of ametal or a semiconductor. Instead it is in between these two extremes. In most directions,electrons moving at the Fermi energy are backscattered by atoms in the lattice whereas in someothers they dont. Graphene therefore can be considered as a semi-metal, since it is metallic inthese special directions and semiconducting in the others. Thus a nanotube can be either a metalor a semiconductor, depending on how the tube is rolled up.

Whereas the multiwall nanotubes were tens of nanometres across, the typical diameter of asingle-wall nanotube was just one or two nanometres. The past decade has seen an explosion of research into both types of nanotube. The multi walled carbon nanotubes should behave slightlydifferent to their single walled relatives due to the interaction of the adjacent layers. Thoughvarious theories can be incorporated, many of them may not hold true for such microscopicmaterials.

Nanotubes as one dimensional metals

Solid state devices in which electrons are confined to two-dimensional planes have providedsome of the exciting scientific and technological breakthroughs of the past many decades.However, 1-D systems are also proving to be very exciting. Studies of quasi 1-D systems, such as

conducting polymers , study of ballistic systems, electron waveguides and many other fields thatmay transform the face of electronics fall into this category. The 1-d systems on which these

phenomena can be studied have been limited by the fact that they are inherently complex tomake. What has been lacking is the perfect model system for exploring one dimensional transport

a 1-d conductor that is cheap and easy to make. , can be individually manipulated andmeasured, and has little structural disorder. Single walled carbon nanotubes fit this billremarkably well.

Nanotubes are ideal systems for studying the transport of electrons in one dimension, and havecommercial potential as nanoscale wires, transistors and sensors. For many years, studies of quasi-one-dimensional systems, such as conducting polymers, have provided a fascinating insightinto the nature of electronic instabilities in one dimension. In addition, 1-D devices such as

"electron waveguides" - in which electrons propagate through a narrow channel of material - have been created. Experiments on these devices have shown, for example, that the conductance of "ballistic" 1-D systems - in which electrons travel the length of the channel without beingscattered - is quantized in units of the charge on the electron squared divided by the Planck constant.


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One-dimensional conductors are also predicted to have unusual electronic properties that cannot be explained by Fermi-liquid theory - the theory that can predict the properties of most materials.Here, we will concentrate on two recent experiments that address the question of whether multiwall nanotubes are ballistic or diffusive conductors.

An ingenious way to measure the electrical conductance of multiwall nanotubes is as follows. Amacroscopic fibre of multiwall nanotubes was gently lowered into a drop of liquid metal. Becauseindividual nanotubes stick out from the fibre, it is possible, by dipping the nanotubes to differentdepths, to determine the resistance of individual nanotubes. This technique also allows anyvariation of resistance with length to be detected.

In various experiments multiwall nanotubes appeared to be ballistic conductors, despite theinteractions expected between the different layers. Moreover, the electrical current that could be

passed through a multiwall nanotube corresponded to a current density in excess of 10 7 amps per square centimetre. If nanotubes were classical resistors, the power dissipated by such a currentwould heat the nanotube so much that it would vaporize. The fact that this does not happensuggests that the electrons in nanotubes are strongly decoupled from the lattice. "Hot" electronsare efficiently removed by the liquid-metal contact rather than being converted into latticevibrations.

Since multiwall nanotubes consist of several concentrically arranged single-wall nanotubes, onewould not expect them to behave as one-dimensional conductors. If adjacent carbon layersinteract as in graphite, electrons would not be confined to one layer. The results from de Heer andco-workers suggest, however, that the current mainly flows through the outermost layer. Itappears therefore that the inner layers only provide mechanical support, although this mightchange if we were able to make electrical contact with all the layers. However, the question of whether the electron transport is ballistic or diffusive remains unresolved

The unique mechanical and electronic properties of multiwall nanotubes are proving to be arich source of new physics and could also lead to new applications in materials and devices .

More on Electronic properties

Carbon nanotubes are giant molecular wires in which electrons can propagate freely, just as theydo in an ordinary metal. This contrasts strongly with conventional "conducting" polymers inwhich the electrons are localized. These molecules are actually insulators and only becomeconductors if they are heavily doped. Graphite, on the other hand, can conduct electricity becauseone of the four valence electrons associated with each carbon atom is delocalized and cantherefore be shared by all the carbon atoms.

However, it turns out that a single sheet of graphite (also known as graphene) is an electronic

hybrid: although not an insulator, it is not a semiconductor or a metal either. Graphene is a"semimetal" or a "zero-gap" semiconductor.

This peculiarity means that the electronic states of graphene are very sensitive to additional boundary conditions, such as those imposed by rolling the graphene into a tube. It can be shownthat a stationary electron wave can only develop if the circumference of the nanotube is a multipleof the electron wavelength. This boundary condition means that a nanotube is either a true metalor a semiconductor - a fact that has been confirmed in experiments with single-wall nanotubes.


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One would expect to find more complex behaviour for multiwall nanotubes because of interactions between adjacent layers, and this is the subject of ongoing research. Moreover, bycombining different nanotubes, and supplementing them with gate electrodes, there is the

potential to make a wide variety of electronic devices, ranging from quantum wires to field effecttransistors.

On the fundamental side, a perfect metallic nanotube should be a ballistic conductor: in other words, every electron injected into the nanotube at one end should come out the other end.Although a ballistic conductor does have some resistance, this resistance is independent of itslength, which means that Ohm's law does not apply. Indeed, only a superconductor (which has noelectrical resistance whatsoever) is a better conductor.

A defect-free carbon nanotube is like an optical fibre. Fibres with large cores are called multi-mode fibres because several wavelengths (or eigenmodes) are allowed to propagate, usually atdifferent speeds, along the fibre. For data transmission, so-called single-mode fibres are preferred

because they allow for higher data rates. A single-wall nanotube is almost a single-mode fibre for electrons. Theory predicts the existence of two propagating eigenmodes for a single-wallnanotube, independent of its diameter. The electric conductance (the inverse of the resistance) isthen expected to be twice the fundamental quantum of conductance, G 0 = 2 e 2/h , where e is thecharge on the electron and h is the Planck constant. This means that nanotubes are predicted tohave a minimum resistance of about 6500 Ohms, independent of their length.

Field emission

The small diameter of carbon nanotubes is very favourable for field emission - the process bywhich a device emits electrons when an electric field or voltage is applied to it. Field emission isimportant in several areas of industry, including lighting and displays, and the relatively low

voltages needed for field emission in nanotubes could be an advantage in many applications.However, as with all new technologies, there are formidable obstacles to be overcome.

To make a field-emission source with just one nanotube, individual multiwall nanotubes weremounted onto a gold tip. The nanotubes were kept in place by van der Waals forces alone (i.e.adhesive was not used). The field emissions from multiwall nanotubes with open and closed endswere compared. Nanotubes grown in arc discharges are normally closed, but they can be opened

by applying a very large electric field, or by treating them with oxygen at high temperature. Fieldemission occurred when a potential of a few hundred volts was applied to the gold tip. Both openand closed nanotubes were capable of emitting currents as high as 0.1 mA, which represents atremendous current density for such a small object.

Surprisingly, closed nanotubes were much more efficient than open ones. This was surprising because the smaller effective curvature of the open nanotubes was expected to lead to a larger field amplification. It is now thought that other species (such as oxygen atoms) attach themselvesto the free dangling bonds at the end of the nanotube, resulting in localized electron states. Sincethese states lie well below the Fermi energy in the nanotube, they cannot emit electrons.Localized states are also thought to form at the tips of closed nanotubes. However, these statescouple to so-called -orbitals in the nanotube and this effectively enhances the emission of electrons. This also has the advantage of narrowing the energy distribution of the emittedelectrons. Electron microscopy is one application in which this effect would be very useful.


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Measuring Conductance of nanotubes

Before we can measure the conducting properties of a nanotube, we have to wire up the tube by

attaching metallic electrodes to it. The electrodes, which can be connected to either a single tubeor a bundle of tubes, are usually made using electron-beam lithography. This can be done in manyways and many others are on the way to becoming feasible in the lab. These include the

possibility of growing the tubes between electrodes, or by attaching the tubes to the surface in acontrollable fashion using either electrostatic or chemical forces. Other relatively conventionalmethods include the making of the electrodes and dropping the nanotubes onto them. Another isto deposit the tubes on the substrate, locate them with a scanning probe microscope, and thenattach leads to the tubes using lithography.

The source and the drain electrodes allow the conducting properties to be measured. Inaddition a third terminal gate is often used. The gate and the tube act like the two plates of acapacitor, which means that the gate can be used to electrostatically induce carriers on the tube.

When the conductance of the tubes are measured as a function of the gate voltage, two types of behaviour are observed, corresponding to metal and semiconductor tubes.

Applying the conductance properties.

1.Nanotube Transistors

Semiconducting nanotubes can work as transistors. The tube can be made to conduct by applyinga negative bias to the gate and turned off by a positive bias. A negative bias induces holes on the

tube and makes it conduct whereas a positive bias depletes holes and decreases conductance. This behaviour is analogous to p-type metal oxide silicon field effect transistor (MOSFET).Thechemical species adsorbed on the tube dope the tube to be p-type. By changing the tubeschemical environment we can change the level of doping.


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We see that the conduction of the semiconductor initially rises linearly as the gate voltage isreduced, conducting better as more holes are added from electrode to nanotube. The conductanceis limited only by barriers as holes traverse the tube. These barriers may be caused by structuraldefects in the tube, by atoms adsorbed on the tubes.

2.Nanotube rectifier

It is created by the intersection of two nanotubes such as a metallic tube crossing over asemiconducting tube. The metallic tube locally depletes the holes in the underlying p-typesemiconductor tube. That is an electron traversing the semiconducting tube must overcome the

barrier created by this metal tube. Biasing one end of the semiconducting tube relative to themetal tube leads to rectifying behaviour.

Nanotubes as model 1-Dimensional systems

The conductance of some nanotubes are near room temperature are not noticeably affected by theaddition of a few carriers. This behaviour is typical of metals which have a large number of

carriers and have conducting properties that are not significantly affected by the addition of a fewmore carriers. The conductances of these metallic nanotubes are much larger than thesemiconducting nanotubes. It implies that electrons can travel for distances of several micronsdown a tube before they are scattered. The experiments also showed that electrons can travel for long distances in nanotubes without being backscattered. Whereas in striking contrast is the

behaviour of metals in which scattering length from lattice vibrations are typically only severalnanometers at room temperature.The main reason for this difference is that an electron in a 1-Dsystem can only scatter by completely reversing its direction whereas electrons in a 2-D or 3-Dmaterial can scatter by simply changing changing direction through a tiny angle.


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Nano-electromechanical devices

By combining nanotube growth on surfaces with microfabrication methods, we will soon be ableto create novel nanotube devices for a variety of new studies and applications. For instance, thevariation of electrical properties of the tubes while undergoing mechanical deformation is

intriguing, and is important for potential applications in which nanotubes form the building blocks of nanoscale electromechanical devices. The most important observation in this regard isthe result that the conductance of the tube drops sharply as the tube is deformed and is restoredinto its original value when the deformation is retracted. This property and several other exclusivefeatures are applied in many theoretical development of highly sophisticated devices and manyalready realized applications such as in MEMS(Micro Electro Mechanical Systems).

Nanotubes for chemical sensors

Nanotubes could be used to detect small concentrations of gas molecules with ultra highsensitivity at room temperature. Gas sensing is important in environmental monitoring, thecontrol of chemical processes, and in agriculture and medical applications. Carbon nanotubes are

essentially large conjugated systems where electrons are delocalized and hence can conduct.Chemical sensors made of nanotubes can detect chemicals such as nitrous oxide and ammonia.Sensors made from single-walled nanotubes have high sensitivity and a fast response at roomtemperature (important advantage for gas detection).

Other related applications.

Another breakthrough in the electronic properties of nanotubes was the recent demonstration of "spin transport" by Kazuhito Tsukagoshi of the RIKEN laboratory in Japan, Bruce Alphenaar of Hitachi in Cambridge and Hiroki Ago of Cambridge University. Spin transport will be a keyfeature in "spintronic" devices that exploit the spin rather than the charge of electrons.Tsukagoshi and colleagues attached layers of cobalt, a magnetic metal, to opposite ends of amultiwall nanotube, and showed that the resistance of the nanotube depended on the relativeorientation of the magnetization in the two cobalt layers. For this to happen, the direction of theelectron spins must be maintained as they move along the nanotube, a property that could proveto be very useful in spintronics.

An Insight into the Exclusive Applications

..Into the future

Although there is no signs to believe that the nanotube based solid state devices would ever compete or pose threat to the much developed silicon based industry inspite of having an edge inthe drive towards miniaturization, these cons will almost surely die with the present.

In the predicted futuristic technological revolution involving the nanomachines and theassemblers-disassemblers brotherhood, silicon based electronics will surely be a misfit and


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nanotubes would fit the bill as easily than any other technology developed so far envisaged or atthe far sight.

The following is a birds view into the role of these tubes in the nanomachine controlledtechnology:-

As the counterpart of the silicon-based electronic devices.As building blocks of these nanoscale devices.As the fundamental part of the manipulating mechanism in an assembler or a disassembler As a part of observing medium such as in electron microscope(The field emission characteristics

of nano tubes are ideal for it)As part of manipulating and observing mechanisms like an Atomic force Microscope(Na

notubes are the ideal material for the manufacture of the tips of these microscopes.)

Applications and challenges

Industry has begun to notice the unique properties of carbon nanotubes. The first commercialdevice that uses multiwall nanotubes may be a lamp that operates on the field-emission principle.Moreover, the field-emitting characteristics of carbon-nanotube films have attracted seriousinterest from the giants of the display industry. Samsung, for example, plans to market a flat-

panel colour display made from multiwall nanotubes within two years. Meanwhile, research atIBM indicates that nanotubes transistors should be competitive with state-of-the-art silicondevices. Nanotubes could also be used to store hydrogen to power electric vehicles.

However, many technological hurdles need to be overcome before large-scale applications reachthe marketplace. For example, the techniques that are used to build electronic components fromnanotubes are painstaking and utterly inappropriate for mass production. But perhaps the mostsevere limitation is that high-quality nanotubes can only be produced in very limited quantities -

commercial nanotube soot costs 10 times as much as gold!

Although there are many challenges ahead, nanotubes appear destined to open up a host of new practical applications and improve our understanding of basic physics at the nanometre scale.

Industry and nanotubes

THE numerous extraordinary properties of carbon nanotubes are now well known and it is clear that nanotubes differ from ordinary molecules and solids in many respects. In fact, nanotubeshave an ambiguous identity: they have reasonably well defined structures - albeit a large varietyof them - like molecules, but their relatively large length and width means that they also resemblesolids. Indeed, nanotubes can be extended to macroscopic lengths and widths to ultimately mergewith bulk graphite, which has a familiar layered structure.


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The bond that connects every atom to its three neighbours in the graphite plane is one of thestrongest found in nature. This strength is reflected in the hardness of diamond, although thegraphitic bond is even stronger. This property gives carbon nanotubes exceptional strength. Instark contrast, the bonding between the graphite planes is very weak, which means that the layerscan easily slide over each other -- a property that makes graphite a good lubricator.

We are only beginning to understand the uniqueness of the electrical properties of carbonnanotubes, which can be semiconductors or metals depending on the way the graphene sheets arerolled up into a cylinder. In their metallic form, nanotubes have exceptional current-carryingcapabilities, which may be related to "ballistic transport". This form of electrical transport had

previously only been observed at very low temperatures, but may occur at room temperature incarbon nanotubes, albeit in a modified form. Meanwhile, the hollow interiors of carbon nanotubesand the high chemical inertness of the graphite suggest that nanotubes could be used as containersfor gases and chemicals.

Outlook

Carbon nanotubes exhibit a wealth of properties and phenomena. While many of these areunderstood, others remain controversial, and nanotubes are sure to remain an exciting area of condensed-matter physics for years to come. The amazing structural and electronic properties of nanotubes are not in doubt. Like any new technology, however, nanotubes will have tooutperform current technology to gain a foothold in commercial markets. All these challengeswill keep nanotube researchers busy for a long time to come.

IN THE past ten years or so, the remarkable electrical and mechanical properties of carbonnanotubes have captured the attention of researchers worldwide. This is largely because thesenovel structures could lead to a huge range of potential applications worth billions of dollars.These range from nanoscale electronics and tools to manipulate individual atoms, toexceptionally strong materials, flat-panel displays and hydrogen fuel cells.

However, to turn nanoscience into a technology, we need to be able to grow carbon nanotubesand fabricate nanometre-sized devices on a large scale. We also need a thorough understanding of the properties of nanotubes. Early efforts to characterize carbon nanotubes were hindered by theinability to make sufficiently pure samples, and the difficulty in assembling "addressable"structures from individual nanotubes.

In the future, integrated circuits that have components or wires made from nanotubes willunavoidably rely on some sort of chemical "self-assembly" in which the chemical properties of the constituent molecules cause them to form regular structures, or on methods to control thegrowth of nanotubes on surfaces. Developing these chemical approaches will undoubtedly benefitfundamental studies of quasi-one-dimensional systems and their practical applications.

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ELECTRONIC-APPLICATIONS-OF-CARBON-NANOTUBES