Quantum Dot Production

Semiconductors can be used to confine electrons and produce quantum dots with several different methods. These will be outlines below in further detail.

Colloidal Synthesis

Precursors, organic surfactants and solvents are all essential components in the process to synthesize colloidal quantum dots. Initially, precursors need to be converted into monomers by heating the medium to a point that causes this to occur. The nanocrystal begins to grow when the monomers are supersaturated, by way of a nucleation process.

Keeping the temperature at the optimal temperature is essential for rearrangement of atoms in the synthesis process and the ideal growth of quantum dots. Additionally, the concentration of the monomer in the reactor should be closely monitored to ensure the focus of growth and the critical size of the nanocrystals is uniform. As the size is inversely proportional to the emission of the quantum dot, it is important that these factors are maintained.

Various alloys can be used to produce quantum dots, including:

• cadmium selenide
• cadmium sulfide
• indium arsenide
• indium phosphide

Quantum dots produced via colloidal synthesis typically range from 2 to 10 nm in size due to the great variance in the number of atoms that compose them, from 100 to 100,000.

Colloidal synthesis allows multiple quantum dots to be produced simultaneously in large batches. This is a convenient aspect for industrial or commercial purposes, as their production can be expanded to a large scale. Of all methods to produce quantum dots it is the most widely practiced and accepted to be the least toxic.

Fabrication Method

It is possible for a quantum dot to be constructed of two separate materials with a core and a shell to manipulate band gap energies. An example of this is a quantum dot with a cadmium selenide core and a shell make of zinc sulfide.

When subjected to particular conditions, quantum dots can nucleate spontaneously, which create a substrate that does not match the lattice of the crystal structure. This can lead to what is referred to as a wetting-layer, a two-dimensional layer of coherently strained islands.

The fabrication method may be an appropriate choice of production in some cases, although often the cost and lack of control of dot position limits the extent to which it can be used in practice.

Viral Assembly

Bacteriophage viruses that have been genetically engineered have been used in research in the production of quantum dots. Viruses have been shown an ability to detect types of semiconductor surfaces and viral crystalline structures can be adjusted by controlling the concentration and ionic strength of the solution, as well as the external magnetic field.

Electrochemical Assembly

Another technique to produce quantum dots involves electrochemical processes. This uses an ionic reaction at the interface between a metal and an electrolyte to form a template of spontaneous nanocrystal formation. The metal that carries the template is then used in a process of mesa etching in the formation of the quantum dots.

This technique is more delicate than some other techniques used to produce quantum dots, as it does not cause damage due to use of radiation. It also has potential to be used on a large scale on a commercial or industrial basis and also may often provide a more cost-effective solution.

References

• http://scholar.lib.vt.edu/theses/available/etd-04262005-181042/unrestricted/Ch3History.pdf
• http://education.mrsec.wisc.edu/300.htm
• http://www.sciencemag.org/content/296/5569/892
• http://link.springer.com/article/10.1007%2Fs11664-999-0104-0#page-1

Further Reading

• All Quantum Dots Content
• Quantum Dots – What are Quantum Dots?
• Quantum Dots in Biology and Medicine
• Quantum Dot Optical Properties
• Quantum Dot and Computing

Written by

Yolanda Smith

Yolanda graduated with a Bachelor of Pharmacy at the University of South Australia and has experience working in both Australia and Italy. She is passionate about how medicine, diet and lifestyle affect our health and enjoys helping people understand this. In her spare time she loves to explore the world and learn about new cultures and languages.