CARBON NANOTUBES

Carbon nanotubes (CNTs; also known as buck tubes) are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1,^[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.

carbon nanotubes and related structures Types of:

Single-walled: 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. The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite called graphene into a seamless cylinder.

Armchair (n,n) Graphene nanoribbon

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. In particular, their band gap can vary from zero to about 2 eV and their electrical conductivity can show metallic or semiconducting behavior, whereas MWNTs are zero-gap metals.

Multi-walled: Multi-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tubes) of graphite. There are two models which can be used to describe the structures of multi-walled nanotubes. A single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled newspaper. The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in graphite, approximately 3.4 Å.

Properties:

Ø Strength: Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus respectively. This strength results from the covalent sp² bonds formed between the individual carbon atoms.

Ø Hardness: Diamond is considered to be the hardest material, and it is well known that graphite transforms into diamond under conditions of high temperature and high pressure.

Ø Electrical: Because of the symmetry and unique electronic structure of graphene, the structure of a nanotube strongly affects its electrical properties. For a given (n,m) nanotube, if n = m, the nanotube is metallic; if n − m is a multiple of 3, then the nanotube is semi conducting with a very small band gap, otherwise the nanotube is a moderate semiconductor.

Potential and current applications:

In electrical circuits nanotube based transistors have been made that operate at room temperature and that are capable of digital switching using a single electron. However, one major obstacle to realization of nanotubes has been the lack of technology for mass production.

In structural because of the carbon nanotube's superior mechanical properties, many structures have been proposed ranging from everyday items like clothes and sports gear to combat jackets and space elevators.

NANOTECHNOLOGY IN ELECTRONICS: NANOELECTRONICS

Nanotube transistors: To build the nano transistor, the researchers grew carbon nanotubes, each one measuring only 0.7 to 1.1 nm in diameter, in a controlled process. the world's smallest nanotube transistor, with a channel length of only 18 nm - the most advanced transistors currently in production are almost four times this size.

The nanotube transistor just unveiled can deliver currents in excess of 15 µA at a supply voltage of only 0.4 V (0.7 V is currently the norm). A current density some 10 times above that of silicon, today's standard material, has been observed.

Nanotubes, which resemble microscopic straws of rolled-up chicken wire, are widely viewed as the potential next generation of materials for enabling improved speed and energy efficiency of computer chips.

The transistors are grouped in the same "cascading" sequences needed to produce computational logic and memory, and the process used to make them is compatible with the industrial VLSI (very large scale integration) manufacturing standard.

Integrated circuits more practical:

The handful of nanotube transistors in the circuits the team fabricated can't compare to the hundreds of millions of transistors on a commercial microprocessor or memory chip, but their arrangement, the way they were made and their properties are much closer to commercial-grade than any nanotube devices made before. The transistors are grouped in the same "cascading" sequences needed to produce computational logic and memory, and the process used to make them is compatible with the industrial VLSI (very large scale integration) manufacturing standard.

OPTICAL LED’s: are now used in cell phones and MP3 displays and prototype television sets, but their production requires a complex process, and it is difficult to manufacture OLEDs that are small enough for high-resolution displays. Unlike conventional computer chips - called CMOS, for complementary metal oxide semiconductor chips - the nanowire thin-film transistors could be produced less expensively under low temperatures, making them ideal to incorporate into flexible plastics that would melt under high-temperature processing. Conventional liquid crystal displays in flat-panel televisions and monitors are backlit by a white light, and each pixel acts as a filter that turns on and off to create images. OLEDS, however, emit light directly, eliminating the need to backlight the screen and making it possible to create more vivid displays. The researchers used nanowires as small as 20 nanometers - a thousand times thinner than a human hair - to create a display containing organic light emitting diodes, or OLEDS. The OLEDS are devices that rival the brightness of conventional pixels in flat-panel television sets, computer monitors and displays in consumer electronics. at are thin and flexible.

NANO-TECHNOLOGYIN COMMUNICATION

Nowadays, many people are working on the move with the help of laptop, other electronic equipment or even mobile phones. These people are in the urge to combine all functions like talking, working, accessing files etc…in one device irrespective of their location. This can be made possible with the help of nanotechnology, which can improve versatility through larger data storage, more mobile processing and faster data transfer.

APPLICATIONS OF Nanotechnology

Medicine: This can be made possible with the help of nanotechnology, which can improve versatility through larger data storage, more mobile processing and faster data transfer. The integration of nanomaterials with biology has led to the development of diagnostic devices, contrast agents, analytical tools, physical therapy applications, and drug delivery vehicles. The biological and medical research communities have exploited the unique properties of nanomaterials for various applications (e.g., contrast agents for cell imaging and therapeutics for treating cancer).Magnetic nanoparticles, bound to a suitable antibody, are used to label specific molecules, structures or microorganisms. Gold nanoparticles tagged with short segments of DNA can be used for detection of genetic sequence in a sample.

ENERGY: The most advanced nanotechnology projects related to energy are: storage, conversion, manufacturing improvements by reducing materials and process rates, energy saving (by better thermal insulation for example), and enhanced renewable energy sources Currently used light bulbs only: convert approximately 5% of the electrical energy into light. Nanotechnological approaches like light-emitting diodes (LEDs) or quantum caged atoms (QCAs) could lead to a strong reduction of energy consumption for illumination. Today's best solar cells have layers of several different semiconductors stacked together to absorb light at different energies but they still only manage to use 40 percent of the Sun's energy. Commercially available solar cells have much lower efficiencies (15-20%). Nanotechnology could help increase the efficiency of light conversion by using nanostructures with a continuum of bandgaps.

Information and communication: Current high-technology production processes are based on traditional top down strategies, where nanotechnology has already been introduced silently. The critical length scale of integrated circuits is already at the nanoscale (50 nm and below) regarding the gate length of transistors in CPUs or DRAM devices. The production of displays with low energy consumption could be accomplished using carbon nanotubes (CNT). Carbon nanotubes are electrically conductive and due to their small diameter of several nanometers, they can be used as field emitters with extremely high efficiency for field emission displays (FED).

Nanotechnology Risks and References

Toxicologists have dealt with nanoparticles that are the result of modern human life such as carbon particles in combustion engine exhaust.

Environmental Issues :Two areas are relevant here: (1) In free form nanoparticles can be released in the air or water during production (or production accidents) or as waste byproduct of production, and ultimately accumulate in the soil, water or plant life. (2) In fixed form, where they are part of a manufactured substance or product, they will ultimately have to be recycled or disposed of as waste.

Nanotechnology and Regulators Studies of the health impact of airborne particles are the closest thing we have to a tool for assessing potential health risks from free nanoparticles. These studies have generally shown that the smaller the particles get, the more toxic they become.

References

1. ^ G. Binnig, H. Rohrer (1986). "Scanning tunneling microscopy". IBM Journal of Research and Development 30: 4.

2 Bai (2000). Scanning tunneling microscopy and its applications. New York: Springer Verlag.

3 wikipedia