Bottom-up and top-down methods of synthesis

 There are two approaches to the synthesis of nano materials: 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-by cluster from the bottom (e.g., growth of a crystal).
 In the top-down approach, nano scale devices are created by using larger, externally-controlled devices to direct their assembly. The top-down approach often uses the traditional workshop or micro fabrication 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 nano particles 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 nano particles by colloid dispersions is an example of the bottom-up approach.
 An approach where both these techniques are employed is known as a hybrid approach. Lithography is an example in which the growth of thin film is a bottom-up method whereas itching is a top-down method.
 The bottom-up approach has been well-known to the chemists for a long time. This approach plays a very important role in preparing nanomaterials having very small size where the top-down process cannot deal with the very tiny objects.
 The bottom-up approach generally produces nano structures 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, nano wires 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 nano materials, 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 nano materials. The present state of nano science can be viewed as an amalgamation of bottom-up chemistry and top-down engineering techniques.