CARBON NANOTUBES IN MICROELECTRONIC APPLICATIONS
CNTs, also called buckytubes, are cylindrical carbon molecules with unique properties that make them potentially useful in a wide variety of applications. These include applications in nano-electronics, optics, and materials applications. CNTs exhibit extraordinary strength as well as unique electrical, mechanical and thermal properties. CNTs reportedly have extremely high surface areas, large aspect ratios, and remarkably high mechanical strength. The tensile strength of CNTs is 100 times greater than that of steel, and the electrical and thermal conductivities approach those of copper. CNTs are also good incorporating agents due to their unique electrical, mechanical and thermal properties.
Nanotubes in Interconnect Applications:
The transistors at the bottom make up only a fraction of the total chip, and already today, the speed and performance of such chips is mainly limited by the interconnects, i. e. the copper-based wiring of the transistors with different metal layers (wires) and the vertical connections between these layers, Predicts a current density of 3.3 106 A/cm2 in a via, a value which, to date, can only be supported by CNTs, where current densities exceeding 109 A/cm2 have been reported in nanotubes without heat sinks. At this ITRS technology node a MPU/ASIC half-pitch of 32 nm is predicted. On this scale, traditional interconnect schemes become problematic due to the increased wire resistances resulting from grain and surface scattering effects and the higher current densities which must be carried. Sufficient heat removal from the chip is already a problem in present day computers.
Carbon Nanotubes in Transistor Applications
If a semiconducting SWCNT of about 1 nm diameter is attached to two separated (metallic) contacts (source and drain), a near by third gate-electrode can modulate the conductivity of the tube by about 6 orders of magnitude at room temperature. This effect has been observed already in and has led to a kind of race in the scientific community to achieve the best performing CNT-device. Explained by the assumption of simple 1-dimensional electrostatics, which relates the charge in the tube by the capacitance of the tube- and gate structure and the applied gate-voltage. Based on this theory a best performance projection for CNT-transistors can be made and compared to the ITRS requirements. We propose a vertical, coaxially gated nanotube transistor, with a 1 nm diameter tube, 10 nm gate-length and a 1 nm thick silicondioxide as the effective gate-oxide. which are always scaled to device width, we make a parallel array of this device comprising 250 CNTs per micron. we can estimate the performance of the CNT-transistor and the results are tabulated and compared with the ITRS in.
In electronic devices as field-emission sources
CNTs can be used in electric devices as field-emission sources. This can be done when a potential is between the CNT surface and the anode. Electrons are easily emitted from their tips due to the curvature present in the CNTs in the form of pentagons or due to the presence of oxidized tips. CNTs can be used for the fabrication of multiple electronic devices, including flat-panel displays, intense light sources, bright lamps and X-ray sources. Although CNTs are good emitters, nanocomposites of CNTs are also excellent electron-emission surfaces which are vacuum stable. There are many advantages when using CNTs as an electron emitter. These include long lifetimes of the components, stable field-emission over prolonged time periods, low emission threshold potentials, the absence of the need for an ultra-high vacuum, and high current densities. It has been reported that large current densities as high as 4 A/cm2 can be achieved.
Batteries (Lithium ions batteries)
Lithium (Li) is a useful element, as it offers unique properties due to its lowest electronegativity and because electrons are easily donated from Li. Thus, it is the best candidate for the fabrication of lightweight and efficient batteries. However, despite the above advantages, the high reactivity of Li limits its applicability, as the metal loses its efficiency. This problem can be solved by the combining an application of CNTs and Li by intercalating Li ions within CNTs. This enables Li+ ions to migrate from a graphitic anode to the cathode. A separating medium is required between the anode and cathode, usually polyolefin. The theoretical Li storage capacity is expected to be 372 mAh/g when intercalated with CNTs. The charge and discharge phenomena in these batteries are controlled by the Li+ intercalation and de-intercalation rates. Recently, CNTs could be used in Li ion batteries. A high irreversible capacity is possible for CNTs. Determine the charging and discharging phenomenon in Li+ batteries. SWNT materials chemically or mechanically, one can increase the electrochemical storage of these batteries. On the basis of such possibilities, many electronics companies have begun to use carbon nanofibers and nanotubes as the electrodes in Li+ batteries to enhance their storage capacity and lifetimes.
CNTs in supercapacitors and actuators
Due to their large surface area as well as their high electrical conductivity, CNTs are excellent materials for use in electrochemical devices the first to demonstrate that with sheet electrodes of pyrolytically grown MWNTs, it is possible to achieve very high specific capacitances in individual cells in devices containing 38 wt% of H2SO4 as the electrolyte. The cells could reach power densities that exceeded 8,000 W/kg. Recently, demonstrated that MWNT-polypyrrole composites are able to reach specific capacitances of CNT supercapacitors are used in applications to devices that require high power capabilities and higher storage capacities. The power densities approach 20 kW/kg at energy densities of 7 Wh/kg. CNT-embedded supercapacitors can be used to provide fast acceleration and to store braking energy electrically for hybrid electric vehicles. Actuators are important devices, but the problem associated with them is that their efficiency decreases with an increase in the temperature. Therefore, CNT-modified actuators were prepared by several researchers. These work at relatively low voltages and at temperatures as high as 350oC. For example, the maximum stress observed in SWNT actuators was 26 MPa. This value is comparable to that of natural muscles, as it is 100 times larger than the value for natural muscle.
Sensors are important detecting devices that are now widely used in different fields. The efficiency of biosensors and molecular sensors can be enhanced by attaching CNTs onto them. With chemical force microscopy techniques, were the first to demonstrate that it is possible to sense functional chemical groups attached onto the ends of CNTs. Thus, it is possible to construct various types of sensors containing nanotube composite pellets, which are very sensitive to gases and which can be used to monitor leaks in chemical plants. working in a similar field that SWNTs are extremely sensitive to air and vacuum conditions by noting large variations in the electrical resistance levels of their SWNT samples. They also added that MWNTs can be used as efficient sensors for NH3, H2O, CO2 and CO. Detected changes in the resistance and capacitance levels of CNTs when the environment was slightly modified. Highly sensitive and fast-responsive microwave-resonant sensors were prepared for detecting NH3 using either SWNTs or MWNTs. In addition to gas sensing, CNTs and its composites can be used assensitive environmental pressure sensors. CNTs are very sensitive to liquid immersion or polymer-embedding processes, as the nanotubes slightly deform min the presence of different liquid media.
Gas and hydrogen storage
Due to their hollow cylindrical nature, CNTs can act as efficient gas and metal containers. It has been found that many important chemical species, such as metals, metal carbides, and oxides, can be introduced into CNT core by different methods, including chemical treatments, arc-discharge methods, solidstate reactions and electrochemical technique. However, the major issues pertaining to these techniques are related to the fact that CNTs are not able to encapsulate gaseous substances. Recent studies have revealed that H2 and Ar can be stored in SWNTs and MWNTs, respectively. Demonstrated experimentally that it is possible to introduce gaseous nitrogen inside MWNTs using a single-step process. They also reported that MWNTs can be loaded via the spray pyrolysis of ferrocene and benzyl amine solutions. Similar work was carried out, but they used a powder pyrolysis of ferrocene and camphor under an ammonia atmosphere. The storage of different chemical species in CNTs may be advantageous for the fabrication of fuel cells for use mainly to power electric vehicles. Unfortunately, there is some controversy regarding the high-pressure storage of hydrogen. It is important to emphasize that the hydrogen storage capacity of CNTs ranges from 0.1 to 66 wt % . The influence of impurities (coming from the CNT synthesis methods) may be responsible for reported previously results showing uptake levels as high as 7%. From a theoretical point of view, density functional theory has been used to calculate the H2 storage capacity in CNTs. Different mechanisms by which CNTs possibly adsorb hydrogen molecules were proposed, including chemisorption, adsorption at interstitial sites, and the swelling of the nanotube array .
Scanning probe tips
Scanning probe tips are of great importance as they can be used to obtain images with better resolutions. If a MWNT-bound scanning probe is used instead of a normal probe, it becomes possible to obtain a better image resolution as compared. Chemically modified nanotube tips can be used as sensors for detecting specific chemical and/or biological groups. These sensors are important to detect added or even illegal substances.determined experimentally that it is possible to nfabricate nanotube tweezers by attaching two nanotubes onto a probe tip. This nano-tool operates according to the electrostatic interactions between two carbon cylinders. These achievementsshow clear advance regarding the use of nanotubes in current technologies, and it is clear that in the near future, further advances will be achieved.
Electronic devices using CNTs
CNTs are important nanomaterials that can be used in electronic devices to improve the properties of these fabricated a three-terminal switch-able device based upon SWNTs. The SWNT molecule was semiconducting and connected to metal nanoelectrodes. The performance at a low capacitance level was excellent. The major problem associated with nanotubes is the difficulty in manipulating them; thus, another method was devised to control the length and the electronic properties of individual nanotubes by STM nanostructuring. Described that crossings and bends with SWNTs could be generated by means of AFM. This technique allows us to cut nanotubes into shorter sections using controlled voltages applied at the STM tips and to fabricate tiny nanotube devices that could be useful for the future construction of molecular machinery, nano-scale circuits and other nanoelectric materials. There are many examples of experimental evidence indicating that CNTs can carry current densities of 109 A/cm2; t ese values are much lower than those observed in metals (105 A/cm2). The very first time the fabrication of field-effect transistors that exhibit a high favorable gain, a large on-off ratio and room-temperature operation. The researchers showed that one-, two- and three-transistor circuits exhibit a range of digital logic operations. Examples include an inverter, a logic NOR, a static random-access memory cell, and an AC ring oscillator. A Harvard group in a similar way produced crossed nanowire p-n junctions and junction arrays that were configured as key OR and NOR logic-gate structures, showing substantial gain. They can be used for the development of novel computer technologies in the near future. The researchers also showed that pristine CNTs always behave as p-type transistors, whereas doped-nanotubes act as n-type devices. Both types could be integrated to fabricate voltage inverters. It is possible to generate logic rings as well as NOR and OR logic gates using arrays of p- and n-type nanotube FETs, as it is difficult to control the chirality of the tubes with the electronic properties. It is possible to peel the outer layers of arc-discharged-produced MWNTs until the desired electronic properties of the outer shell are obtained. However, controlled growth in order to achieve selective chiralities needs to be investigated and exploited formore electrical applications. similar hypothesis and demonstrated that the electronic properties of CNTs can be utilized within polymeric materials to realize desired electrical applications.
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