**1. Introduction**

Nanoscience and nanotechnology is one of the highest potential applicative research fields in today's world. The current scenario of world depicts the valuation of nano sized materials (1–100 nm) [1] quite high in various areas of scientific applications, such as, photovoltaic solar cells [2–5], optoelectronics [6–10], biomedical [11–13], food and agriculture, industrial, textiles, etc. [14]. It has been observed that larger ratio of surface to volume of a catalyst would facilitate a better catalytic activity; therefore the size controlled synthesis of QDs to produce a larger ratio of surface to volume is of great importance. The enhanced surface to volume ratio would cause increase of surface states, which will change the activity of electrons and holes, thereby, affecting the chemical reaction dynamics as a whole. Studies reveal that the size quantization increases the band gap of photo catalysts to enhance the redox potential of conduction band electrons and valence band holes [15–17]. The resulting dominance of these tiny sized artificial atoms/molecules/materials reveals a prediction of market value of \$16.8 billion in upcoming future [18].
