**2.7. Vapor deposition method**

As the name suggests, this method involves deposition of vapors of the required material. The material is vaporized from the source and condenses on the substrate. It may or may not involve chemical reactions. Vapors can be deposited on the substrates by two main processes:


Very little work has been done by these techniques to grow TiO<sup>2</sup> HNSs. By these techniques, we can grow hierarchical structures. The film cost, thickness, source material and compositions can be controlled by these processes.

In CVD process, precursors are introduced in the reaction chamber and flow of molecules is regulated by control values. The precursor molecules get deposited over the surface of substrates after chemical reactions take place. Heat energy is provided for chemical reactions to take place. While in PVD, deposition occurs by various routes like evaporation, sputtering and molecular beam epitaxy (MBE). **Table 8** shows different characteristics of HNSs prepared via these processes.
