Question

Explain with examples the bottom-up and top-down techniques. What are their merits and demerits?

Answer 1

Bottom-up and top-down methods of synthesis 

• There are two approaches to the synthesis of nanomaterials: bottom-up and top-down. In the bottom-up approach, molecular components arrange themselves into more complex assemblies atom-by-atom, molecule-by-molecule, cluster-bycluster from the bottom (e.g., growth of a crystal). 
• In the top-down approach, nanoscale devices are created by using larger, externally-controlled devices to direct their assembly. The top-down approach often uses the traditional workshop or microfabrication methods in which externally-controlled tools are used to cut, mill and shape materials into the desired shape and order. Attrition and milling for making nanoparticles are typical top-down processes. 
• Bottom-up approaches, in contrast, arrange molecular components themselves into some useful conformation using the concept of molecular self-assembly. Synthesis of nanoparticles by colloid dispersions is an example of the bottom-up approach. 
• The bottom-up approach generally produces nanostructures with fewer defects as compared to the nanostructures produced by the top-down approach. The main driving force behind the bottom-up approach is the reduction in Gibbs free energy. Therefore, the materials produced are close to their equilibrium state. 
• In top-down techniques such as lithography, significant crystallographic defects can be introduced to the processed patterns. For example, nanowires made by lithography are not smooth and can contain a lot of impurities and structural defects on its surface. Since the surface area per unit volume is very large for the nanomaterials, these defects can affect the surface properties, e.g., surface imperfections may cause reduced conductivity and excessive generation of heat would result. 
• In spite of the defects, the top-down approach plays an important role in the synthesis and fabrication of nanomaterials. The present state of nanoscience can be viewed as an amalgamation of bottom-up chemistry and top-down engineering techniques.

Comments

Will u please give some short answer or edit important point from above answer which u have mentioned?

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I have taken steps to make the answer short, the answer is now  clear and follows your question