**2.1 Defining QDs: physical and quantum mechanical concept**

QDs are having quite resemblance with the atomic confinements. Basically, they are the nanoparticles of bulk semiconductors and their average size range lies within 1–10 nm. The physics behind the size of a QD is the exciton Bohr radius. It states that if the size of a particle is less than twice the size of the exciton Bohr radius of a specific material, then, the particle behaves as a quantum confined matter. At this point, the particle acts as a particle within a box of quantum mechanics. Brus et al. were the first to use the term 'Quantum Dot' [20]. They defined these artificial atoms' behavior as molecule like. They realized that this particular class of particle has discrete and quantized energy states, due to which their electronic properties possess resemblance with atoms/molecules. Although, nanoparticles of the semiconductor materials are found to be within the quantum confined region, it is quite obvious to achieve bigger particles during the process of synthesis within the same solution of nanoparticles, than, the Bohr exciton radius. But, people are focusing on the production of smaller size, i.e., within quantum region. The idea behind targeting quantum confinement is their dramatic change in electronic properties within the quantum dimension, which cannot be achieved beyond that size. Hence, nanoparticles of semiconductors are synthesized within that confinement precisely. One of the primary reasons behind occasional achievement of bigger particles is the aggregation of individual particles. This can be overcome by effective encapsulation with suitable capping agents during the course of various physical and chemical synthesis processes.

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