**3. Synthesis of QDs: aqueous route and mechanism**

The effective synthesis of QDs is a challenging task. Research is warranted in the mechanism of synthesis, leading to the reproducibility of size and shape of QDs, so that the growth of QDs at the time of nucleation can be controlled effectively [22–26]. There are several methods of synthesis of QDs: molecular beam epitaxy (MBE), vapor phase epitaxy (VPE), metal organic chemical vapor deposition (MOCVD), radio frequency sputtering (RFS), electron beam lithography (EBL), optical ablation, Ball milling, quenching methods, reduction with microorganisms or plants [14] and chemical method [27–34]. Among all, chemical method is the most widely used method for the synthesis of QDs due to its better accuracy and large scale production [35–56]. Various reducing agents such as sodium borohydride, hydrazine hydrate, etc. are used during the chemical reduction synthesis. These methods are broadly divided into non-aqueous and aqueous-mediated reactions. Non-aqueous method have received first remarkable development from Bawendi, Alivisatos and Peng [57–62], popularly known as TOP-TOPO (trioctylphosphine for TOP and TOP oxide) or hot injection method in 1990s. Although the reaction developed by Murray et al. [57] for the synthesis of cadmium chalcogenides is probably the first ever controlled synthesis of semiconducting CdE (E = S, Se, Te) QDs. However, due to the highly toxic, pyrophoric and unstable nature of main precursor dimethyl cadmium [(CH3)2Cd] at room temperature, the method was not preferred [63–67]. Peng et al. [68, 69] contributed a comparatively greener method for the synthesis of CdE, as the first most efficient process of synthesizing highly fluorescent QDs with excellent morphological uniformity. The formed QDs in this high temperature method involved long chain organic solvents with high value of boiling point. Resultant QDs were found to be less defective due to effective surface passivation by TOP/TOPO. Both, the choice of ligand as well

#### *Aqueous-Mediated Synthesis of Group IIB-VIA Semiconductor Quantum Dots: Challenges… DOI: http://dx.doi.org/10.5772/intechopen.82891*

as the growth temperature, allowed the lattice morphology to be modified in a better way toward the synthesis of QDs. Hot injection method processes through two steps, i.e., rapid nucleation, while the reaction is heated up to the utmost temperature, followed by slow growth of colloidal nano crystals (CNCs) due to the addition of another precursor, with much lowered temperature. Though this method was observed to be suitable for the synthesis of the CNCs/QDs, there are some unexpected drawbacks, which lead the scientists to rethink about this particular process, before they could propose this method for the up gradation of the end products at the industrial level. The primary issues with this method are (a) use of non-ecofriendly organic solvents, (b) cannot be scaled up for mass production, which require several and repetitive reactions to achieve the desired amount of product, (c) non-biocompatibility, as water is not a suitable solvent for them to get solubilized, (d) difficult purification and extraction methods of the QDs from its mother liquor, (e) high cost of solvents as well as the process and (f) non-cooperation of green chemistry principles, which leads the whole process of synthesis into an environmentally challenging problem. Adoption of non-water soluble surface passivators (i.e., TOP/TOPO) confirms the nonbiocompatibility mode for these high temperature synthesis methods; therefore the concept of biocompatible aqueous phase QDs synthesis is warranted. Aqueous route of synthesis have all the positive aspects and most importantly, they are easy to carry out and are environmentally benign. Additionally, one can use high fluorescent nature of QDs in various bio-medications due to their water solubility. One can modify the surface of the QDs as required, with the help of water soluble chiral capping molecules to make the dots size dependent chiral optical property enhancer [70, 71]. Moreover, utilization of short water soluble ligands boost the optoelectronic applications (including photodetectors, solar cells and photocatalysis) too, by reducing dielectric barrier and inter dot separation distances in a solid QD film. Hence the objective of preparation of QDs reach to another domain of methodology, i.e., water-mediated or water soluble routes of synthesis. In early 1980s, Brus [72] and Henglein [3] and their co-workers first pioneered the aqueous synthesis route. They reported the change of band gap of QDs with their size. Vossmeyer et al. presented their work on CdS QDs. They reported the variation of UV-vis absorption spectrum with the change of size of CdS QDs. However, none of them discussed the photoluminescence property. First aqueous based synthesis with fluorescence properties of QDs was introduced by Weller et al. [73]. They have reported the water based synthesis of CdTe QDs under reflux condition at 96°C for prolonged hours, to obtain photoluminescence emission. After that, several approaches have been made toward the aqueous synthesis of CdSe and CdTe QDs. Although, they have achieved a mile stone, but as the research progressed, researchers tried to find the best route of synthesis in which they were able to synthesize the same or better quality CNCs. In the way of harvesting betterment and refinement of the existing methodologies, present day world scientists are trying to reach to a mile stone, in which the QDs are prepared as multiuse molecules. Hence, the aqueous synthesis method is still under investigation and modification. Therefore warrants systematic studies under variable conditions. Presently, global scenario speaks about the understanding of the reaction mechanisms and the suitable greener conditions involved during the synthesis of better quality QDs with easy, cost effective and most importantly greener technological way. However, achievement is still under the margin.
