Synthesis of Quantum Dots

Elements of quantum dot synthesis involves the combination of an appropriate metallic or organometallic precursors like zinc, cadmium or mercury species with corresponding chalcogen precursor for example sulfur, selenium or tellurium species in coordinating solvent at high temperature. Solvent used in production process must be stable at high temperatures in order to prevent aggregation of quantum dots by acting as surfactant molecule for the stabilization of quantum dot surfaces. The Tri-n-octylphosphine oxide or TOPO is most commonly used due to its high boiling point and its ability to coordinate both metal and chalcogen elements. The TOPO is frequently used in combination with other surfactans or co-solvents such as tri-noctylphosphine (TOP), hexadecylamine, or stearic acid. The TOPO molecule is shown in following figure. Under these conditions the particle nucleation takes place, followed by epitaxial growth and nanocrystal annealing at s low temperatures. During the growth period, the size of quantum dot can be monitored using a spectroscopic probe within the reaction flask or by examining fractions taken at various intervals.

Once the specific size of quantum dots is obtained, growth is quenched by lowering the temperature of the reaction mixture. Growth rate and maximum particle size values can be manipulated to a certain extent by controlling the following parameters:

• Initial precursor concentration,
• Growth temperature,
• Length of the growth period.

The most important fact in production of quantum dots is that production must be performed under an inert atmosphere due to the reactivity of the precursor species with the oxygen and water. The product or in this case quantum dots are stable in air. There is also an option to introduce additional precursor material into the reaction vessel during the growth period to obtain larger quantum dots and to improve the size distribution.