Synthesis of Nanomaterials

There are two general approaches to the synthesis of nanomaterials, they are

(a) The top-down approach
(b) The bottom-up approach

(a) The top-down approach

It is a approach in which smaller objects are made by carving or fabricating larger or bulk materials.

It is like stone made into pieces from mountains

(b) The bottom-up approach

It is a approach in which larger objects are build from smaller building blocks like atoms and molecules.

It is like building a wall using smaller bricks

Top-down methods

By nature aren’t cheap and quick to manufacture.
Slow and not suitable for large scale production.

Bottom-up methods

Fabrication is much less expensive.

Pulsed Laser Deposition (PLD)

Principle: Lasers can be used to evaporate materials. The evaporated material can then be deposited on to a suitable substrate to form thin films. This method was first reported by Smalley in 1995.

Working: Pulse or continuous laser beam is made incident on a graphite target in a furnance at $1200°C$ . The laser beam vaporises the graphite target. Helium or argon gas is taken inside the furnance in order to keep the pressure maintained. The material is expelled from the target and forms a plume which is in the ionized state and hence known as plasma plume.

A very hot vapour is then expanded and rapidly cooled. As the vaporised species cool, small carbon molecules and atoms quickly condense to form large clusters. From these initial clusters, it grow into single wall carbon nanotubes.

Chemical Vapour Deposition (CVD)

principle: CVD is a process in which a solid is deposited on a heated surface via a chemical reaction from the vapour or gas phase to form nano powder.

Working: Chemical Vapour Deposition (CVD) involves the flow of a gas with diffused reactants over a hot substrate surface. The gas that carries the reactants is called the carrier gas. While the gas flow over the hot solid surface, the heat energy provokes chemical reactions of the reactants that form film during and after the reactions.

The byproduct of the chemical reactions are then vented. The thin film of desired composition can thus be created over the surface of the substrate. Inert gas is used to cool the reactor. The CVD method is used for the preparation of carbon nano tubes. The growth process takes about 30 minutes. The by products of the chemical reaction are removed through the outlet. The flow of $H 2$ gas is maintained at a rate of $200 mbar /$ minute: