1. Science and Technology of Nanomaterials

2. Nanotechnology – not new to India Nanotechnology might be of raging interest to scientists world over now. But Indians had used nanomaterials in the 16 th century and enabled Arab blacksmith’s in making “ Damascus Steel Sword” which was stronger and sharper. For the Damascus Sword, Indians produced the raw material – mined iron ore and exported it.. ---- Courtesy Science and Technology column of “ The Hindu” ---- Courtesy Science and Technology column of “ The Hindu”

3. Nanoscience : It is defined as the study phenomena and manipulation of materials at atomic, molecular and macromolecular states, where properties differ significantly from those at a larger scale. Nanotechnology : It is defined as the design, characterization, production and application of structures, devices and systems by controlling shape and size at the nanometer scale.

4. Why the properties of Nanoparticles are different? The properties of nanoscale materials are very much different from those at a larger scale. Two principal factors cause the properties of nanomaterials to differ significantly from other materials : The properties of nanoscale materials are very much different from those at a larger scale. Two principal factors cause the properties of nanomaterials to differ significantly from other materials : - increased surface area - quantum effects 1. Increase in surface area to volume ratio: Nanomaterials have a relatively larger surface area when compared to the same volume (or mass) of the material produced in a larger form. Nanomaterials have a relatively larger surface area when compared to the same volume (or mass) of the material produced in a larger form. Let us consider a sphere of radius ‘r’, Its surface area is 4  r 2 Volume is 4  r 3 /3 Surface area to its volume ratio= (3/r) Thus when the radius of the sphere decreases, its surface area to its volume ratio increases. Area= 6 x 1m 2 =6 m 2 Area = 6 x(1/2) 2 x 8 =12 m 2

5. 2. Quantum Confinement Effect When atoms are isolated the energy levels are discrete. When very large number of atoms are closely packed to form a solid, the energy levels split and form bands. Nanomaterials represent intermediate stage. When atoms are isolated the energy levels are discrete. When very large number of atoms are closely packed to form a solid, the energy levels split and form bands. Nanomaterials represent intermediate stage. From the study of particles in a potential box or well, when the dimensions of such wells or boxes are of the of de- Broglie wavelength of electrons or mean free path of electrons (i.e., within few tens of nanometer), energy levels of electrons change. This effect is called Quantum confinement. From the study of particles in a potential box or well, when the dimensions of such wells or boxes are of the of de- Broglie wavelength of electrons or mean free path of electrons (i.e., within few tens of nanometer), energy levels of electrons change. This effect is called Quantum confinement. Specifically, the phenomenon results from electrons and holes being squeezed into a dimension that approaches a critical quantum confinement, called the “exciton Bohr radius”. Specifically, the phenomenon results from electrons and holes being squeezed into a dimension that approaches a critical quantum confinement, called the “exciton Bohr radius”. These can affect the optical, electrical and magnetic behaviour of materials. These can affect the optical, electrical and magnetic behaviour of materials.

6. Variation of Properties of Nanomaterials 1. Physical Properties : The change in interparticle spacing and the large surface- to -volume ratio in particles have a combined effect on material properties. The melting point decreases with size and at very small sizes the decrease is faster. The interatomic spacing decreases with size. 2. Chemical Properties: The large surface- to -volume ratio, the variation in geometry and the electronic structure have a strong effect on catalytic properties. Another important possible application is hydrogen in metals. It is well known that most metals do not absorb hydrogen, and even among those that do, hydrogen is typically absorbed dissociatively on the surfaces with a hydrogen- to- metal atom ratio of 1. For a small positively charged clusters of Ni, Pd, and Pt containing between 2 and 60 atoms can absorb up to 8 H- atoms per metal atom. The number of absorbed atoms decreases with increasing cluster size.

7. 3. Electrical Properties : Ionization potential at small sizes are higher than that for the bulk and show marked fluctuations as a function of size. Due to quantum confinement the electronic bands in metals become narrower. In nanoceramics and magnetic nanocomposites, the electrical conductivity increases with reduction in particle size whereas in metals electrical conductivity decreases. 4. Optical Properties : Suppose we have a suspension of nanoparticles in a host. Depending on the particle’s size, different colours are seen. Gold nanospheres of 100 nm appears orange in colour, while 50 nm nanospheres appear green in colour. The particles can be made to emit or absorb specific wavelengths (colours) of light, merely by controlling their size.

8. 5. Magnetic Properties : The strength of a magnet is measured in terms of coercivity and saturation magnetization values. These values increase with a decrease in grain size and an increase in the specific surface area (surface area per unit volume) of the grains. Small particles are more magnetic than the bulk material. Nanoparticles of even non-magnetic solids are found to be magnetic. In addition to free clusters, clusters of nonmagnetic elements supported on metal substrates have also been proposed to be magnetic. Ferromagnetic and antiferromagnetic multilayer have been found to exhibit giant-magneto resistance (GMR). 6. Mechanical Properties : Most metals are made up of small crystalline grains, if these grains are nanoscale in size, the interface area (grain boundary) within the material greatly increases, which enhances its strength. For e.g. : Nanocrystalline Nickel is as strong as hardened steel. Because of the nanosize, many of their mechanical properties such as hardness and elastic modulus, fracture toughness, scratch resistance, fatigue strength and hardness are modified.

9. Production Techniques of Nanomaterials : TOP DOWN APPROACH   BALL MILLING METHOD BOTTOM UP APPROACH  ARC DISCHARGE  LASER VAPOURIZATION  CHEMICAL VAPOUR DEPOSITION (CVD)  FLAME SYNTHESIS  SOL-GEL METHOD

10. BALL MILLING METHOD (MECHANICAL CRUSHING METHOD) Process: In this method the material is taken in a container which contain small balls of either silicon carbide or Tungsten carbide balls. The container is rotated at high speed. The material gets crushed due to impact of the blls and its size reduces to nano size. Materials prepared: Elemental and oxide powders like Fe (10-30nm), Fe nitriles, metal oxides, CeO2 and ZnO etc,.

11. Methods of Production 1. wo carbon rods are placed end to end, separated by approximately 1mm, in an enclosure filled with inert gas (helium, argon) at low pressure (between 50 and 700 mbar). A direct current of 50 to 100 A at 20 V creates a high temperature discharge between the two electrodes. The discharge vaporizes one of the carbon rods and forms a small rod shaped deposit on the other rod. Production of nanotubes in high yield depends on the uniformity of the plasma arc and the temperature of the deposit form on the carbon electrode. 1. ARC DISCHARGE : T wo carbon rods are placed end to end, separated by approximately 1mm, in an enclosure filled with inert gas (helium, argon) at low pressure (between 50 and 700 mbar). A direct current of 50 to 100 A at 20 V creates a high temperature discharge between the two electrodes. The discharge vaporizes one of the carbon rods and forms a small rod shaped deposit on the other rod. Production of nanotubes in high yield depends on the uniformity of the plasma arc and the temperature of the deposit form on the carbon electrode.

12. 2. LASER VAPOURIZATION In 1995, Smalley's group at Rice University reported the synthesis of carbon nanotube by laser vaporisation. A pulsed, or continuous laser is used to vaporise a graphite target in an oven at 1200 °C. Samples were prepared by laser vaporization of graphite rods with a 50:50 catalyst mixture of Cobalt and Nickel at 1200 o C in flowing argon, followed by heat treatment in a vacuum at 1000 o C to remove the C60 and other fullerenes. The initial laser vaporization pulse was followed by a second pulse, to vaporize the target more uniformly. The use of two successive laser pulses minimizes the amount of carbon deposited as soot. The second laser pulse breaks up the larger particles ablated by the first one, and feeds them into the growing nanotube structure. The material produced by this method appears as a mat of “ropes”, 10-20nm in diameter and up to 100um or more in length. In 1995, Smalley's group at Rice University reported the synthesis of carbon nanotube by laser vaporisation. A pulsed, or continuous laser is used to vaporise a graphite target in an oven at 1200 °C. Samples were prepared by laser vaporization of graphite rods with a 50:50 catalyst mixture of Cobalt and Nickel at 1200 o C in flowing argon, followed by heat treatment in a vacuum at 1000 o C to remove the C60 and other fullerenes. The initial laser vaporization pulse was followed by a second pulse, to vaporize the target more uniformly. The use of two successive laser pulses minimizes the amount of carbon deposited as soot. The second laser pulse breaks up the larger particles ablated by the first one, and feeds them into the growing nanotube structure. The material produced by this method appears as a mat of “ropes”, 10-20nm in diameter and up to 100um or more in length.

13. 3. CHEMICAL VAPOUR DEPOSITION (CVD) Chemical vapour deposition (CVD) synthesis is achieved by putting a carbon source in the gas phase and using an energy source, such as a plasma or a resistively heated coil, to transfer energy to a gaseous carbon molecule. Commonly used gaseous carbon sources include methane, carbon monoxide and acetylene. The energy source is used to "crack" the molecule into reactive atomic carbon. Then, the carbon diffuses towards the substrate, which is heated and coated with a catalyst (usually a first row transition metal such as Ni, Fe or Co) where it will bind. Carbon nanotubes will be formed if the proper parameters are maintained.

14. 4. FLAME SYNTHESIS This method is based on the synthesis of SWNTs in a controlled flame environment, that produces the temperature, forms the carbon atoms from the inexpensive hydrocarbon fuels and forms small aerosol metal catalyst islands. On these metal islands the SWNTs are grown in the same manner as in laser ablation and arc discharge. These metal catalyst islands can be made in three ways. The metal catalyst (cobalt) can either be coated on a mesh 50, on which metal islands resembling droplets were formed by physical vapour deposition. These small islands become aerosol after being exposed to a flame. The second way 52, is to create aerosol small metal particles by burning a filter paper that is rinsed with a metal-ion (e.g. iron nitrate) solution. The third way, is the thermal evaporating technique in which metal powder (e.g. Fe or Ni) is inserted in a trough and heated.

15. 4. SOL-GEL METHOD

16. 6. ELECTRODEPOSITION METHOD Process: This phenomenon is similar to electroplating where, by controlling the current and other parameters, single layer atoms can be made to deposit on cathode. This phenomenon is similar to electroplating where, by controlling the current and other parameters, single layer atoms can be made to deposit on cathode. Advantages: Materials prepared by this method are strong, highly flat and uniform, large surface area (favourable for electrical conduction). Applications: Batteries, Fuel Cells, Solar cells, magnetic read heads, etc.,

17. CARBON NANOTUBE (CNT’s)

18. Allotropy The property of existence of elements in different physical forms with similar chemical composition

19. ALLOTROPES OF CARBON

20. These are Tiny tubes about 10,000 times thinner than a human hair, consist of rolled up sheets of graphite with a hexagonal honeycomb structure. Their diameters are normally in the range of 1/1,000,000,000 meter. They are known to have excellent mechanical, electrically selective, highly efficient hydrogen storage properties and almost defect-free. Carbon Nanotubes are an allotrope of carbon, which exists abundantly on earth. What are Carbon Nanotubes ?

22. Properties of CNTs - High Electrical Conductivity - Very High Tensile Strength - Highly Flexible- can be bent considerably without damage -Very Elastic ~18% elongation to failure -High Thermal Conductivity -Low Thermal Expansion Coefficient -Good Field Emission of Electrons -Highly Absorbent -High Aspect Ratio (length = ~1000 x diameter)

23. METHODS OF PREPARATION OF CARBON NANOTUBES  ARC DISCHARGE METHOD  LASER VAPORISATION METHOD METHOD  CHEMICAL VAPOUR DEPOSITION METHOD (CVD) DEPOSITION METHOD (CVD)

24. Method Arc discharge method Chemical vapour deposition Laser ablation (vaporization) Who Ebbesen and Ajayan, NEC, Japan 1992 Endo, Shinshu University, Nagano, Japan Smalley, Rice, 1995 How Connect two graphite rods to a power supply, place them a few millimetres apart, and throw the switch. At 100 amps, carbon vaporises and forms a hot plasma. Place substrate in oven, heat to 600 o C, and slowly add a carbon-bearing gas such as methane. As gas decomposes it frees up carbon atoms, which recombine in the form of NTs Blast graphite with intense laser pulses; use the laser pulses rather than electricity to generate carbon gas from which the NTs form; try various conditions until hit on one that produces prodigious amounts of SWNTs Typical yield 30 to 90%20 to 100 % Up to 70% SWNT Short tubes with diameters of 0.6 - 1.4 nm Long tubes with diameters ranging from 0.6-4 nm Long bundles of tubes (5-20 microns), with individual diameter from 1-2 nm. M-WNT Short tubes with inner diameter of 1- 3 nm and outer diameter of approximately 10 nm Long tubes with diameter ranging from 10-240 nm Not very much interest in this technique, as it is too expensive, but MWNT synthesis is possible. Pro Can easily produce SWNT, MWNTs. SWNTs have few structural defects; MWNTs without catalyst, not too expensive, open air synthesis possible Easiest to scale up to industrial production; long length, simple process, SWNT diameter controllable, quite pure Primarily SWNTs, with good diameter control and few defects. The reaction product is quite pure. Con Tubes tend to be short with random sizes and directions; often needs a lot of purification NTs are usually MWNTs and often riddled with defects Costly technique, because it requires expensive lasers and high power requirement, but is improving

25. Applications of Carbon Nanotubes Carbon nanotubes have various physical properties, they show their unlimited applicabilities, such as  Electron emitter  VFD (Vacuum Fluorescent Display)  FED (Field Emission Display)  Hydrogen storage fuel cell  Nano-wires, nano tweezers

26.  Flat panel displays, such as LCD (Liquid Crystal Display)  LED (Light Emitting Diode)  PDP (Plasma Display Panel)  FED (Field Emission Display)