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Tuesday, June 5, 2012

Material science nanomaterials

Introduction to nanoscience and nanotechnology
Nanoscience is the study of atoms and molecular structures whose size, at least in one dimension is between 1-100 nm. Nanotechnology is the design and fabrication of devices using such nanostructures.
Size a few nanometers to a few hundred nanometers. Both surface effects and size effects plays crucial role in the properties of nanomaterials. Exhibit properties strikingly different from those of bulk materials. The state of matter around nanosize is referred to as mesoscopic state. The physics of nanotechnology is also called as mesoscopic physics
Shapes of nanomaterials
Nanomaterials are classified as quantum wells, quantum wires and quantum dots. In a three dimensional structure, if one dimension, say thickness, is of nanosize, then the structure is called quantum well. If two dimensions are of nanosize, then it is called a quantum wire. If all the three dimensions are of nanosize, then it is called quantum dot.     The word quantum is associated with the structures because the properties exhibited by them are described by quantum mechanics.
Density of states
The density of states is defined as the number of available electron states per unit volume per unit energy range centered at a certain energy level E.
Density of states for nanostructures
            For a quantum well, density of states varies as a step function.
            In a quantum wire, density of states has peaks.
            For quantum dots, permitted energy values are almost discrete.
Nanomaterials- examples
    Fullerenes, carbon nanotubes and nanowires are examples of inorganic nanomaterials.
    Artificially synthesized DNA strands are examples of organic nanomaterials.
Methods of preparation of nanomaterials
There exist many methods for the preparation of nanomaterials. These methods are broadly classified into two: top-down approach and bottom-up approach.
Top-down methods
Milling (mechanical grinding) and lithography are examples of top-down methods. During milling, balls in the vial impart energy to powder material splitting them further to nanoscale. Lithography deals with the nanoelectromechanical systems which can be developed by using light, electron beam or ion beam.
Bottom-up methods
Wet chemical methods, plasma chemical methods, sol gel methods, physical vapour deposition, chemical vapour deposition, sputtering, Laser ablation, Molecular beam epitaxy, thermolysis, electrodeposition, Self assembly method etc are examples of bottom-up methods.
Wonders of nano technology
Fullerene and carbon nanotubes
Carbon molecule having spherical, ellipsoidal or tubular form is known as fullerene. Spherical fullerenes are called buckyballs and cylindrical fullerenes are called buckytubes. In buckyball clusters, 20 to 60 carbon atoms are accommodated. Nanotubes with small dimensions and megatubes with larger dimensions are extensively used in electronics industry. Spherical particles based on multiple carbon layers surrounding a buckyball core are known as nano-onions. If  two bucky balls are linked by a carbon chain, it is called ‘linked ball and chain dimers’.
BuckyballIt contains sixty carbon atoms with 20 hexagonal and 12 pentagonal faces symmetrically arranged to form a molecular ball of carbon atoms.

Properties of fullerene
Showing aliphatic behaviour in chemical reactions.     Sparingly soluble in many solvents. Stable. If we heat fullerenes without air at very high temperature, it turns into graphite. Thin layers of fullerenes are coloured from yellow to yellow-green. Different species of fullerenes are fullerites, fullerides, endohedral fullerenes, exohedral fullerenes, heterofullerenes and metcars.
Carbon nanotubes-Synthesis methods
  Chemical vapour deposition, Carbon arc methods and laser evaporation
Applications of carbon nanotubes
1.In the construction of electronic devices like transistors, logic gates etc.
2. In the construction of nanowires.
3. In flat panel displays.
4. In scanning probe microscopes.      
5. In fuel cells/batteries.
6. As catalysts in chemical reactions.
7. As chemical sensors.
8. In air pollution filters and water filters.

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