Make Quantum Dots (Cadmium Selenide Type)

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 The remaining portions may be withdrawn when there is a visible color change.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:

"A Safer, Easier, Faster Synthesis for CdSe Quantum Dot Nanocrystals"
Boatman, E. M.; Lisensky, G. C.; Nordell, K. J.
J. Chem. Educ. 2005, 82, 1697--1699.

"Formation of High-Quality CdTe, CdSe, and CdS Nanocrystals Using CdO as Precursor"
Peng, Z. Adam and Peng, Xiaogang J. Amer. Chem. Soc. 2001, 123 (1), 183-184

"Formation of High-Quality CdS and Other II-VI Semiconductor Nanocrystals in Noncoordinating Solvents: Tunable Reactivity
of Monomers"
Yu, W. Willian and Peng, Xiaogang Angew. Chem. Int. Ed. 2002, 41 (13), 2368.

Comments

[Owen2408]

What are the concentration of 1-Octadecene, Trioctylphosphine, and Oleic Acid?

[35764]

Thanks

[SilaC1]

Really powerful

[gryphongirl]

Were do u get the tinny bottles?Also so cool!!!

[PedroDaGr8]

As a nanotech chemist, nice to see more science on here. This topic is VERY close to home because at work I make CdSe nanocrystals (aka quantum dots). This is a very simple method for making CdSe nanocrystals and it pretty much works EVERY time. There are a lot of variants you can do in order to get dots with specific emission profiles and what not. Most of which I can discuss due to NDAs and all that fun stuff.

A few quick comments and notes:

First off, this uses the method developed and patented by Xiaogang Peng from the University of Arkansas. See the references I included at the end. Very specifically it uses the method developed in the second reference.

I notice your material appears to start degrading at the end. It goes from deep orange-red back down to yellowish. Did you add somethign to it or was it strictly oxidizing?

The ODE is unecessary other than as a dilutent, you can use strictly Trioctyl Phosphine (TOP) to dissolve the selenium powder, don't bother heating the TOP:Se solution. The dissolution reaction is exothermic itself. It will produce a good amount of heat on its own, with concentrated solutions producing enough heat to be uncomfortable to hold. Instead, just use vigorous stirring. Finer meshes of Selenium powder dissolve faster.

You don't need dry ice to cool the vials down if you are using small vials. Just use ice water slurry. Once the reaction drops below 100oC it basically ceases. So these should quench pretty rapidly.

Also, don't be surprised if your first few nanocrystal samples have a weird orangish or whitish emission. This is due to deep trap emission, which is VERY broad and and in very very small NCs can dominate the emission. This will dissapear as they NC's grow.

Lastly, don't be surprised if the NC's degrade overtime. These are unprotected and will be sensitive to moisture and oxygen.

I think I have given just about all of the advice I can give synthetically without running afoul of any of the NDAs I have signed at work. Cheers!



Reference:
"Formation of High-Quality CdTe, CdSe, and CdS Nanocrystals Using CdO as Precursor"
Peng, Z. Adam and Peng, Xiaogang J. Amer. Chem. Soc.2001, 123 (1), 183-184
"Formation of High-Quality CdS and Other II-VI Semiconductor Nanocrystals in Noncoordinating Solvents: Tunable Reactivity
of Monomers"
Yu, W. Willian and Peng, Xiaogang Angew. Chem. Int. Ed.2002, 41 (13), 2368.

[rcarney86]

Haha, cool. I did my undergraduate research with Dr. Peng at Arkansas. Nice job here, I've always wanted to see more and more 'real' science here at instructables.

Now I'm still working on nanoparticles (gold, not QDs) with Francesco Stellacci. I love it!

[PedroDaGr8]

Hehe, that's awesome. I did my undergraduate research in two essentially completely unrelated fields. While my MS was in yet another unrelated field. The first lab that I worked in did research in OLED materials. The second lab was purely applied inorganic. I did research on 5-coordinate aluminum species for the degradation of nerve agents and pesticides. It was a combination of the two that got me this job, despite having never seen, let alone synthesized, quantum dots in my life. Speaking of which those vials in my avatar are of quantum dots.

Gold-nanoparticles are such a crazy hot field. Very intersting and promising research being done. Though from a business perspective its a patent minefield. Kinda like stem cells.

[PedroDaGr8]

PS: Read some of the comments on the youtube page

1) I did an instructable on making different colors of LED's starting with a UV LED. If the person is interested in making lights with it.

2) QDot TV's will likely use one of the more common III-V semiconductors with the III being (In, Ga, Al, etc.) and the V being (P or N). This will allow a tunability of emission while getting around the hazardous materials being used. III-V's are much harder to make with controlled emission profiles though.

[NurdRage]

1) i'll be sure to inform the next person that asks :)

2) Sounds like a topic for a new video! ;D

[NurdRage]

1. Thanks for the references.

2. actually i think the heating was too high. The thermometer temperature controls to 225 but as i withdrew the solution the thermometer was no longer fully immersed so the temperature controller thought it was cooling down and increased heating. By the end the hot plate must have been pushing 300 and degraded the dots. I tried the experiment with a better thermometer and it didn't degrade. nonetheless, enough of the dots survived to still fluoresce.

3. i figured as much, i was pretty much copying the method in the reference. I had read in other references to use pure TOP but i figured since i had loads of ODE available i might as well save the precious little TOP i had and follow the instructions. I pay for these vids myself (chemicals included) so i try to be frugal where i can :)

4. The vials i had were so small that they floated or fell over in ice water, dry ice was easier to work with. But yeah, if longer tubes or some other restraint is used then ice water all the way ;)

5. that would explain some of my earlier attempts!

6. Gotcha. But i'm not too worried though, i've had these dots for a year now and they're still as bright as the day i made them. (if you check my watch and calculate the date, friday january 14th corresponds to 2011 ;)

Thanks for your insight and experience.
Now I'm trying to figure out what to do with these dots :)

[expertmagician]

Very cool, too complex for me to do at home :-)

But, do you know where I can buy some cheaper quantum DOTs ?

I would like then to have a stokes shift in the red range or near-IR range.

The larger the stokes shift the better.

Any ideas ?

DOTS which get excited in the 425nm range would also be interesting.

[PedroDaGr8]

Umm, most quatum dots are quite heavily excited in the 425nm range. QDs do not have the discrete spectrum of organic dyes.

See this image for an example of absorption and emission spectra of a quantum dot:
http://upload.wikimedia.org/wikipedia/commons/thumb/6/63/EF_605_spectra.png/500px-EF_605_spectra.png

The red is the emission while the blue is the absorption spectrum.

Also, I think you mean peak emission when you are saying stokes shift. Essentially the stokes shift refers to the difference in energy between the absorption peak and the emission peak.

As for their emission, red range is easy to find. 625-650nm. Near-IR gets harder as you have to branch into less common (and often harder to make) materials.

[allenamistral]

These are awesome! I did these as a project for a lab a while back (I'm an inorganic chemistry student), but I was never able to make the purple ones (likely because I had to make some chemical substitutions after my chemicals did not arrive on time). I'm so impressed!

[PedroDaGr8]

Quantum dots are VERY sensitive to composition because their emission is partly dependent on the bandgap of the semiconductor used. CdS is much easier to make to get those colors. Other options to mess with are CdZnSe and CdZnS.

[Spokehedz]

Whooooo. I am follower 600!

Love your stuff, keep up the good work. Clear and to the point as always.

[Monsterguy]

I really like that you told us that these are actually useful for something, excellent :)

[The Green Gentleman]

Shazam!  Upping your game.  Not that your game needed much upping.