*5.2.1 Bottom-up method*

It is also known as a constructive method like building-up of material from an atom. Sol–gel, spinning, chemical vapor deposition (CVD), pyrolysis, and biosynthesis are the foremost methods in this technique. Nanoparticles, nanoshells, and nanotubes with narrow size distribution can be synthesized by this approach. Besides, in this method deposition parameters can be controlled. However large scale production is difficult and chemical purification is needed.



#### **Table 2.**

*Techniques in Top-Down and Bottom-Up approaches.*

