Preparation of Cadmium Selenide Quantum Dot Nanoparticles
Procedure is based on E. M. Boatman, G. C. Lisensky, and K.
J. Nordell, "A Safer, Easier, Faster Synthesis for CdSe Quantum Dot Nanocrystals,"
J. Chem. Educ., 82,
1697-1699 (2005) and W. William Yu and
Xiaogang Peng, "Formation of High Quality Semiconductor Nanocrystals
in Non-Coordinating Solvents," Angew. Chem. Int. Ed. 41,
2368-2370 (2002).
The visible absorption and photoluminescence of CdSe nanoparticles
depend on the size of the particle. Immediately after the reactants in hot
solution are combined, samples are withdrawn and quenched at room temperature
to produce a series of increasing particle sizes. Octadecene is used as a non-coordinating, high-boiling solvent.
Procedure
Wear eye protection
Chemical gloves recommended
Fumehood recommended
Add 30 mg of Se and 5 mL octadecene to a 10 mL round bottom flask over
a stirrer hot plate.
Warning: Se is an inhalation hazard and this operation should be done
in a fume hood.
Measure by syringe 0.4 mL trioctylphosphine from its Sure-Seal bottle
and add to the flask.
Add a magnetic stir bar. Stir and warm the solution as necessary to completely
dissolve the selenium. Cool to room temperature. This stock solution may
be prepared ahead of time, has enough Se precursor for five preparations,
and can be stored in a sealed container for at least a week.
Add 13 mg of CdO to a 25 mL round bottom flask clamped in a heating
mantle.
Warning: CdO is an inhalation hazard and this operation should be done
in a fume hood.
To the same flask, add by pipet 0.6 mL oleic acid and 10 mL octadecene.
Insert a thermometer capable of measuring 225 degrees C.
Heat the cadmium solution. When the temperature reaches 225 degrees
C, use a clean and dry pipet to quickly transfer 1 mL of the room temperature
selenium solution to the 225 degree C cadmium solution and start timing.
Remove approximately 1 mL samples at frequent intervals using a 9
inch glass Pasteur pipet as the CdSe particles grow in size. In the video,
the first five samples were removed at 10 second intervals.
Continue removing samples at longer intervals so there is a noticeable
color change. Ten samples are removed within 3 minutes of the initial
injection.
Record the absorbance spectra of the solutions to find the maximum wavelength peak.
Record the emission spectra of the solutions to find the maximum wavelength
peak. In the movie, sequential samples are placed in a beam of 400 nm
wavelength light.
Option: Samples show narrower peaks in the absorption spectrum when they are quenched
more quickly. Instead of collecting in test tubes, they can be collected
in flasks containing liquid nitrogen.
Materials
CAUTION: Avoid physical contact with cadmium oxide and cadmium
selenide as both are carcinogens.
