Nanotechnology is designing, developing or manipulating at nanometer (a billionth of a meter) scale. The dealing object size should be less than hundred nanometers at least in one dimension to call something to be nanotechnology. There are two design approaches in nanotechnology known as top-down and bottom-up. Both approaches are useful in different types of applications.
Top-down Approach
In top-down approach, nano-scale objects are made by processing larger objects in size. Integrated circuit fabrication is an example for top down nanotechnology. Now it has been grown to the level of fabricating nano electromechanical systems (NEMS) where tiny mechanical components such as levers, springs and fluid channels along with electronic circuits are embedded to a tiny chip. The starting materials in these fabrications are relatively large structures such as silicon crystals. Lithography is the technology which has enabled making such tiny chips and there are many types of them such as photo, electron beam and ion beam lithography.
In some applications larger scale materials are grinded to the nanometer scale to increase the surface area to volume aspect ratio for more reactivity. Nano gold, nano silver and nano titanium dioxide are such nano materials used in different applications. Carbon nanotube manufacturing process using graphite in an arc oven is another example for top-down approach nanotechnology.
Bottom –up Approach
Bottom-up approach in nanotechnology is making larger nanostructures from smaller building blocks such as atoms and molecules. Self assembly in which desired nano structures are self assembled without any external manipulation. When the object size is getting smaller in nanofabrication, bottom-up approach is an increasingly important complement to top-down techniques.
Bottom-up approach nanotechnology can be found from nature, where biological systems have exploited chemical forces to create structures for cells needed for life. Scientists and engineers perform research to imitate this quality of nature to produce small clusters of specific atoms, which can then self assemble into more complex structures. Manufacturing of carbon nanotubes using metal catalyzed polymerization method is a good example for bottom-up approach nanotechnology.
Molecular machines and manufacturing is a concept of bottom-up nanotechnology introduced by Eric Drexler in his book Engines of Creation in 1987. It has given early views of how can nano-scale mechanical systems be used to build complex molecular structures.
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Difference between Top-down and Bottom-up approach in nanotechnology 1. Manufacturing process starts from larger structures in top-down approach where starting building blocks are smaller than the final design in bottom-up approach 2. Bottom-up manufacturing can produce structures with perfect surfaces and edges (not wrinkly and does not contain cavities etc.) though surfaces and edges resulted by top-down manufacturing are not perfect as they are wrinkly or containing cavities. 3. Bottom-up approach manufacturing technologies are newer than top-down manufacturing and expected to be an alternative for it in some applications (example: transistors). 4. Bottom-up approach products have a higher precision accuracy (more control over the material dimensions) and therefore can manufacture smaller structures compared to top-down approach. 5. In top-down approach there is a certain amount of wasted material as some parts are removed from the original structure contrast to bottom-up approach where no material part is removed.
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