Quantum dots are semiconductors whose spatial dimensions in all three directions are very much reduced. The small size results in new quantum phenomena thereby changing the material’s properties, becoming very much different from its bulk form’s. One of these is the material’s optical properties which may give rise to a lot of applications.
Cadmium sulfide on the other hand, is a semiconductor whose main application is in light detection. It has gained a lot of attention as a quantum dot because of its diverse applications such as in lasers, light emitting diodes, and biological imaging. Cadmium Sulfide quantum dots show strong confinement effects near its Bohr radius (5.8nm) .
In an article by a team from the Institute of Chemistry in UP Diliman in Materials Letters , they have synthesized colloidal Cadmium Sulfide quantum dots using a three-component system. This enabled the team to control the size of the dots by varying the concentration of the precurosrs, the reaction time, and the temperature during the reaction process. The diameters of the quantum dots reported were from 3.368 nm to 8.411 nm.
The CdS were synthesized using a microemulsion process. Cetyltrimethylammonium bromide in hexane and n-pentanol, acts as micelles which encapsulate the CdS precursors. Upon mixing, the micelles collide and coalesce producing CdS quantum dots inside the micelles. Because the precursors’ release is controlled by the micelles, the CdS did not rapidly precipitate and coagulate. This make it easy for them to control the size of the quantum dots.
They observe the variation of the size of the quantum dots when they vary the concentrations of the precursors, the reaction time, and temperature during the process. They calculated the size of the particle using the Brus equation with the bulk semiconductors bandgap known and the quantum dots’ bandgap obtained from the absorption specstrocopy spectrum.
They noticed that there is an optimum concentration. They said, “at low concentration of the precursors the space inside the micelles are larger and it is more favorable to have several micelles to fuse resulting to an increase probability of contact between cadmium and sulfide ions thus increasing the particle size. On the other hand, at higher concentration of the precursors, the inner micellar space is saturated with the precursor ions and fusion readily leads to formation of larger particles.”
Moreover, particle size increases when the reaction time is increased as well as when the reaction temperature is increased. The increase of size however follows a linear trend only until some certain time or temperature. This made the authors hypothesize that when the quantum dots’ size is comparable to the Bohr radius, surface-controlled coagulation occurs to make larger sized quantum dots.
The authors conclude that the ease in the control of synthetic parameter which is important in fine tuning particle size and band gap energy of the material, is necessary for future application.
 I. H. J. Arellano, J. Mangadlao, I. Ramiro, K. Suazo, 3-component low temperature solvothermal synthesis of colloidal cadmium sulfide quantum dots, Materials Letters 64 (2010). doi: 10.1016/j.matlet.2010.01.021