**4. Factors responsible in aqueous synthesis of QDs**

There are four major parameters which are primarily responsible for precision of the end product during aqueous synthesis route. They are (a) the solubility product

of the semiconductor compounds in water, (b) binding affinity of the capping agent molecules toward the metal atom, (c) binding affinity of water and hydroxyl atom for the metal atoms and (d) pH of the reaction medium [74–81]. These enlisted thermodynamic parameters can be understood with the help of HSAB (hard and soft acids and bases) theory. HSAB states that hard-hard or soft-soft interactions are expected to have lower water solubility [82, 83]. Since chalcogenides ions (S<sup>−</sup><sup>2</sup> , Se<sup>−</sup><sup>2</sup> and Te<sup>−</sup><sup>2</sup> ) are soft bases and (Cd+2 and Zn+2) are borderline soft acids, it is expected that they readily form insoluble compounds. Soft-soft interaction leading to the low solubility of these compounds is the basis of their semiconducting nature. Their covalent nature increases in comparison to the hard-hard interactions. The pH of the medium also effects the formation of the desired compounds in a dramatic way. Since we know that ─OH (hydroxyl) ion is a hard base, therefore its binding affinity toward hard metal cations is more and hence ─OH can easily form bonds with Zn+2 than Cd+2 in basic conditions, thus effecting their solubility product constant (K*sp*). Therefore, careful pH management is very necessary for Zn chalcogenide than Cd chalcogenide compounds to prevent formation of metal hydroxide compounds. Because Cd can tolerate more basic condition than Zn due to its comparative more softness. Thus, one can understand that acidic condition may be more suitable for Zn chalcogenide compounds than Cd compounds, which can accommodate basicity up to pH = 12.5. Beyond this, Cd metal atoms will form Cd(OH)2 compounds. Hence, the conclusion can be drawn that, acidic condition is more suitable for the synthesis of these compounds. Another very important positive concept of utilization of acidic condition is that, it enhances the formation of smaller QDs than basic condition. The presence of ─OH radical in the medium is found to be responsible for this, due to the bigger size of the hydroxyl group than H<sup>+</sup> in acidic medium. The chemistry of the group IIB-VIA compounds overall reveals that aqueous medium is very easy and effective way of production of the QDs of those bulk compounds due to their solubility factor in water. We can easily extract them from the medium as after formation they show poor solubility in water. The band gap energy of these compounds can also be understood from the covalent nature of the bonding between the metal atom and the chalcogenide. It is observed that the pattern of band gap energy of metal (M) chalcogenides (S, Se and Te) is MS > MSe > MTe. The fact behind this order is the electronegativity of the anions. Since, the bigger sized anion (lower electronegative chalcogenides) does not hold their outermost electrons as strongly as the anions of their group possessing higher electronegativity, therefore covalent character is enhanced in these compounds. As a result, their bulk band gap (BG) gets decreased in a group with the same metal atom (CdS*BG* > CdSe*BG* > CdTe*BG*).
