Top-down approach:
In Top-down approach a bulk solids are dis-assembled that is broken or dissociated into finer pieces until the particles are in the order of nanometre. The technique developed under this tille are modified or improved one to fabricate micro-processors, micro-electro-mechanical systems (MEMS). Top-down approach use larger macroscopic initial structures, which can be externally-controlled in the processing of nanostructures. Typical examples are Mechanical grinding, Erosion, Lithography, are some of the techniques used under the top-down approach for the production of nanomaterials. It begin with a pattern generated on a larger scale, then reduced to nanoscale. By nature, they are not cheap and quick to manufacture. It is slow and not suitable for large scale production. The most top-down approach is photolithography. It has been used for a while to manufacture computer chips and produce structures smaller than 100 nm.
Bottom-up approach:
In the bottom-up approach, the nanostructured materials are synthesised by assembling the atoms or molecules together to form the nano-materials. Many techniques have been developed for the synthesis and formation of nanostructured materials under the bottom-up process. The common process techniques used for the synthesis of nanostructured materials under bottom-up approach are Physical methods which include vapour phase deposition techniques and epitaxial techniques and Chemical methods which include Colloids and sol gel. Bottom-dowm approach include the miniaturization of materials components up to atomic level with further self-assembly process leading to the formation of nanostructures. During self-assembly the physical forces operating at nanoscale are used to combine basic units into larger stable structures.
Typical examples are Chemical precipitation, gas phase agglomeration and self assembly are used to obtain the nanomaterials under bottom-up approach. It starts with atoms or molecules and build up to nanostructures, fabrication is much less expensive.
