Sc In Nanotech

SEMI CONDUCTORS IN NANO TECHNOLOGY Introduction: Nanotechnology is a multitude of rapidly emerging technologies, based upon the scaling down of existing technologies to the next level of precision and miniaturization. The original vision for nano technology is sometimes termed “Molecular manufacturing” or “molecular manufacturing based technology”.

Nanotechnology is about building things atom by atom,

molecule by molecule. Nanotechnology is providing a nanometer scale at which hydrogen and carbon atoms are picked up and by assembling them a machine is built, called “nano machine”. A nanometer is a billionth of a meter, i.e. about 1/80,000th of the diameter of a human hair, or ten times the diameter of a hydrogen atom. If you measure the width of ten atoms placed side by side you would have a nanometer. By application of these machines bringing about a great improvement in all fields of human being. What is nano-technology? The goal of early nanotechnology was to produce the first nano-size robotic arm capable of manipulating atoms and molecules either into a useful product or copies of itself. This is because manufacturing is basically a method for arranging atoms. Most methods arrange atoms crudely – even the finest commercial microchips are grossly irregular at the atomic scale, and much of today’s nanotechnology faces the same challenge. The molecular assembler is the answer to this challenge.

Once

perfected, it will position the molecules, bringing them together to the specific location and at the desired time. By holding and positioning molecules in this way, the molecular assemblers will control with precision how the molecules react, building up complex structures that finally lead to the desired product. Atoms and molecules stick together because they have complementary shapes that lock together or charges that attract.

Just like magnets, a positively

charged atom will stick to a negatively charged atom. As millions of these atoms are pieced together by nano-machines, a specific product will begin to take shape. The goal of nanotechnology is to manipulate atoms individually and place them in a pattern to produce a desired structure. Supercomputing: Molecular technology has obvious application to the storage and processing of information. In the computer industry, the ability to shrink the size of transistors on silicon microprocessors is already reaching the limits. Nanotechnology will be needed to create a new generation of computer components. Molecular computers could contain storage devices capable of storing trillions of bytes of information in a structure the size of a sugar cube. Moore’s Law: holding ground. Gordon Moore made a prediction in 1965 that computer processing power, or the number of transistors on an integrated chip, would double every 18 months.

The visionary ‘Moore’s

Law’, as we call it, has managed to hold its ground to date. To sustain Moore’s Law, transistors must be scaled down to at least 9 nanometres by around 2016, according to the Consortium of International Semiconductor Companies. If this is achieved, future chips will have billions of transistors. One long-term approach to finding ways to make electronic components smaller is to make them from single molecules. Components made from molecules are likely to be smaller than those made using today’s integrated chip (IC) fabrication methods, and they can potentially selfassemble, which would allow for inexpensive manufacturing. A single module can be used as the semi-conductor channel in a fieldeffect transistor (FET) in three ways.

1. Using an electric field to change the molecule’s conductance, this is

how silicon transistors work. 2. Reversibly changing the molecule’s shape to break contact with electrodes. 3. Changing the molecule’s shape to alter its internal conductance. Functional nano-scale devices: Till now it was inaccessible to experimental researchers to develop devices having 0.1 – 50nm dia. But now the development of fabrication tools and techniques capable of constructing such diameter structures has opened up numerous possibilities for investigating new devices. Intensive study is being done to determine the exact point in dimensional scaling where it becomes either physically unfeasible or financially impractical to continue reducing the size while increasing the complexity of silicon chips. Many laboratories have shifted the focus of their research from Si to single-electron devices (SEDs).

Functional device scales While the current-voltage (I-V) characteristics provide a common basis for comparison of device performance, there are significant variations in the fabrication methods and device structures being considered by the different labs with significant SED activity. The research in the surveyed laboratories

spans electrical measurements from millikelvin to room temperature and from discrete electronic elements to integrated single-electron transistors (SETs). The materials used to form the active single-electron element range from charge clusters that are shaped by electric fields in a two-dimensional electron gas to metallic colloids to single oligomers. The field of magnetics has experienced increasing attention since giant magneto resistance (GMR) in multilayered structures was discovered in 1988. in these structures, ferromagnetic layers are quantum mechanically coupled across a 1-3 nm non-magnetic metallic layer. GMR structures are being studied for application in hard disk heads, random access memories (RAMs) and sensors. Fabrication of GMR : The approaches under consideration for fabricating the GMR structures are : 1. Magnetron or ionbeam sputter deposition. 2. Epitaxy for layered structures. 3. Rubber stamping of nano-scale wire-like patterns. 4. Electroplating into nano-scale pores in polymer membranes.

In RAM applications, a high magneto resistance combined with a small coercive switching field is key to density, speed and low power. Optical devices have already benefited from incorporation of nanostructured materials: commercially availably semiconductor lasers incorporate active regions comprising quantum wells, the presence of which modifies the electronic density of states and the localization of electrons and holes, resulting in more efficient laser operation. Extrapolating from these results, even greater improvements are predicted for lasers utilizing either quantumwire or quantum-dot active layers. Recent advances in ‘self-assembling’ of

quantum-dot structures have stimulated the fabrication and characterization of quantum-dot lasers in Japan, Europe and USA. Nanocomputer : A nanocomputer is a computer whose fundamental components measure only a few nanometers (less than 100 nm.) A nanometer is a billionth of a metre and spans approximately 10 atomic diameters. Today, over 10,000 nanocomputer components can fit in the area of a single modern microcomputer component, thereby offering tremendous speed and density. Atomic wires and molecular devices : The ultimate in miniatursiation of computer circuitry would be circuit elements made out of small assemblies of atoms or molecules (molecular electronics). The first step on this road is to study the properties of atomic wires, which are nothing but short chains of atoms that conduct electricity between two contacts.

Researchers have performed calculations on both

metallic and covalently bonded atomic wires connecting two metal electrodes.

Conductance vs number of C atoms

The conductance of a linear carbon-atom chain (a ‘cumulene’) versus the number of atoms in the chain. The conductance oscillates between one and two quantum units; one unit corresponds to a resistance of 12,900 ohms. The low-bias conductance can be related to the density of states at the Fermi level of the electrodes. At zero bias, there is a large transfer of electronic charge from the electrodes to the carbon wires, effectively providing doping without introducing scattering centres. The two barriers at the wire ends reflect that most of the charge transferred t the wire accumulates at the ends.

Electron density of carbon chain The difference between electron densities of a system consisting of two electrodes connected by a 7 – carbon-atom chain with an applied bias of 3 volts and with no applied bias. The primary involvement of the wire’s states is clear. There are complex changes in the metal-to-wire charge transfer, polarization by the electric field and screening by the electrodes.

The

electrostatic potential associated with this charge distribution shows the way in which the voltage drop between the electrodes is distributed spatially along the wire. In particular, in the vicinity of the wire, the potential variation extends

deep into the electrodes, and about half of the potential drop occurs over the wire itself. Some studies conclude that the drop occurs only at the wire ends. Carbon nanotubes : Carbon nanotubes were first synthesized and characterized in late 1991. the novel material contained a wide variety of multiwalled nanotubes (MWNT) containing 2 to 50 concentric cylindrical grapheme sheets with a diameter of a few nanometers and a length of up to 1 µ m. It was produced at the negative electrode of an arc discharge and appeared to be mixed with a large amount of other forms of carbon.

Carbon Tubes This encouraged many groups throughout the world to produce and purify nanotubes. The theoretical study of their electronic structure followed in the next year. Soon it became clear that nanotubes have unique electronic and mechanical properties that could lead to ground-breaking industrial applications. However, resistance is a serious problem when building electric circuits on a small scale. If you build a circuit on a small scale, its natural frequency goes up (since the wavelength goes down with the scale) but the skin depth only decreases with the square root of the scale ratio, and therefore

resistance is quite a big problem. Possibly, we can beat resistance through the use of superconductivity if the frequency is not too high or by some other tricks. Conclusion : Nanotechnology, with all its challenges and opportunities, is an unavoidable part of our future. The possibilities with nanotechnology are immense and numerous.

The researchers are filled with optimism, and

products based on this technology are beginning to make their mark. The extent to which nanotechnology will impact our lives only depends on the limits of human ingenuinity. It can rightly be said that nanotechnology is slowly but steadily ushering in the next industrial revolution.