Cadmium

Make Quantum Dots (Cadmium Selenide Type)

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• published: 07 May 2012
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In this video we make cadmium selenide quantum dots. WARNING: This experiment uses toxic and carcinogenic reagents and directly handles extremely hot liquids. Gloves, protective clothing and a fume hood should be used. This should be performed by, or under the direct supervision of, an experienced chemist. Make trioctylphosphine selenide solution: Combine 30mg of pure selenium powder, 5mL of 1-octadecene and 0.4mL of trioctylphosphine. Gently heat it until all the selenium dissolves into a clear liquid. Once it's ready, take off heating, seal it and let it cool. Make quantum dots: Combine 13mg of cadmium oxide, 0.6mL of oleic acid and 10mL of 1-octadecene. Heat the mixture until the cadmium oxide completely dissolves to form cadmium oleate. After the cadmium oxide dissolves keep heating until the mixture hits 225 degrees celsius. Inject 1mL of the trioctylphosphine selenide solution from before and shake. Quickly withdraw small ~0.5mL portions of liquid and quench by placing it into vials at room temperature. A more narrow particle size distribution can be obtained if the vials are cooled on dry ice. The first several portions should be removed as fast as possible. The remaining portions may be withdrawn when there is a visible color change. What's happening is the cadmium oleate is reacting with the trioctylphosphine selenide to form cadmium selenide. These particles start small but grow in size the longer the solution reacts. Now this growth only continues if the temperature is maintained so withdrawing it at regular intervals and placing it in a room temperature vial stops the reaction and locks the particles into their current size. The oleic acid surrounds, or "caps", the particles and keeps them from aggregating. The quantum dots will fluoresce under ultraviolet light. It is recommended to use a fluorescence spectrometer to observe the differences in wavelength emitted. But a good digital camera can also distinguish the differences and exaggerate them for human observation. References: Boatman, E. M.; Lisensky, G. C.; Nordell, K. J. J. Chem. Educ. 2005, 82, 1697--1699.