Nanotechnology Ninad Mehendale

Synthesis of CNTs Arc Discharge Process: Simplest method of producing CNTs. However, crude product is obtained mixed with soot and catalytic metals.

Arc-vaporization of two carbon electrodes placed 1mm apart in an enclosure filled with inert gas at low pressure (300-760 torr). DC of 100A driven by approx. 20V creates high temperature discharge. The discharge vaporises one of the carbon electrodes and forms a small rod shaped deposit on the other .

For SWNT: anode doped with Fe, Co, Ni or Mo For MWNT: pure graphite electrodes The quantity and quality of the nanotubes obtained depend on various parameters such as the metal concentration, inert gas pressure, kind of gas, the current and system geometry. Diameter range for SWNT is 1.2-1.4 nm. MWNTs have outer dia. around 10nm.

Recent advances made in the arc discharge process: Synthesis in liquid nitrogen

Magnetic field synthesis

Rotating plasma-arc discharge These modifications have resulted in higher yields (>70%), higher purity (>90%) and lesser cost in some cases.

Laser Ablation Process: A continuous laser is used to vaporise a graphite target in an oven at 1200 °C. The oven is filled with helium or argon gas at 500 mmHg. The target is a pellet with graphite and Ni-Co catalyst in the ratio 1:1 and has length 10mm long and 3-5mm wide.

Laser synthesis

Laser ablation gives greater purity (>90%) with fewer defects compared to conventional arc discharge method. Very expensive method because of costly equipment and very high power requirement. TEM images of a bundle of SWNTs catalysed by Ni/Y (2:0.5 at. %) mixture, produced with a continuous laser.

Chemical Vapour Deposition (CVD): Achieved by using an energy source such as plasma or resistively- heated coil to transfer energy to (crack) a gaseous source of carbon such as methane, CO or acetylene. Excellent control over alignment, diameter and growth rate. Temperatures range from 650-900⁰C and yield is around 30%. Two-step process consisting of a catalyst preparation step followed by the actual synthesis of the nanotube. Catalyst is prepared by sputtering a transition metal onto a substrate and then using chemical etching to induce catalyst particle nucleation. Ammonia may be used as the etchant.

THERMAL CVD: In this method Fe, Ni, Co or an alloy of the three catalytic metals is initially deposited on a substrate. After the substrate is etched in a diluted HF solution , the specimen is placed in a quartz boat.

The boat is positioned in a CVD reaction furnace, and nanometre- sized catalytic metal particles are formed after an additional etching of the catalytic metal film using NH3 gas at a temperature of 750 to 1050⁰C. CNTs are grown on these fine catalytic metal particles in CVD. method.

Plasma-enhanced CVD: The plasma enhanced CVD method generates a glow discharge in a chamber or a reaction furnace by high frequency voltage applied to both electrodes. A substrate is placed on the grounded electrode. In order to form a uniform film, the reaction gas is supplied from the opposite plate.

Alcohol catalytic CVD: Possibility of large-scale production of high quality single wall nanotubes SWNTs at low cost. Evaporated alcohols like methanol and ethanol are utilised over iron and cobalt catalytic metal particles supported with zeolite. Generation is possible is possible at a relatively low temperature of about 550⁰C.