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Colloidal Chemistry

Colloidal synthesised QDs are flexible in comparison to their counter parts of epitaxially growth ones as they are "substrate free" dots. Their economic production cost, quality, the ability to assemble to form a superlattice and the possibility of embedding them in a conductive pours polymer matrix make them a promising candidate for many applications

Organometalic route of synthesis is one of the most successful methods of nanocrystal preparation in terms of quality and monodispersity. The process is carried out at temperature in the range (120oC -360oC). In a three neck flask.  Precursor reagents are injected rapidly into a hot  solvent with centrifugation. A nucleation starts immediately and the growth become faster as the time progressing. However, Ostwald ripening (the process of smaller particles are being consumed by larger particles)  and the long chain ligands tend to slow the aggregation process and eventually  mono-disperse dots are formed. 

In this kind of chemical reactions temperature plays an important role in dot formation. It is responsible on decomposing the reagents and creating the supersaturation of these species that is ease through nucleation of nanocrystals. 

On the other hand, the same result can be achieved by mixing the reagents at low temperature then increase it in a controllable way in order to accomplish the supersaturation case. 

Choosing the right precursor reagents, capping ligands and controlling other parameters: time, temperature and concentration are the key of controlling the size and shape of the nanocrystals.    

colloidal


A schematic of a three neck flask used in the preparation of colloidal QDs,
and a representation of a QD and its capping ligands

Following is a video shows the preparation of colloidal CdSe quantum dots in the laboratory



References:

  1. Klimov, V. (2004). Semiconductor and Metal Nanocrystals: Synthesis and Electronic and Optical properties . New York: Marchel Dekker, Inc.. 1-64.

  2. Murray C. B., Sun S., Gaschler W., Doyle H., Berley T. A. and Kagan C. R.. (2001). Colloidal Synthesis of nanocrystals and nanocrystal superlattice. IBM Journal of Research and Development. 45 (1), 47-55.