Make Quantum Dots (Cadmium Selenide Type)


Uploaded by NurdRage on 07.05.2012

Transcript:
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.
Greetings fellow nerds.
In this video we’re going to make cadmium selenide quantum dots,
a type of nanoparticle with interesting properties.
First, we need to make our source of selenium.
Get 30mg of pure selenium powder and add to it a stir bar.
Then add in 5mL of 1-octadecene and turn on the stirring.
Now add 0.4mL of trioctylphosphine.
Keep stirring and gently heat it until all the selenium dissolves into a clear liquid.
What’s happening is the trioctylphosphine is reacting with the selenium
to form trioctylphosphine selenide.
Once it’s ready, seal it and let it cool.
This will provide us with about five experiments worth.
Now get 13mg of cadmium oxide in a flask and place in a glass thermometer.
Mine happens to be a glass encapsulated digital thermometer.
Now add to it 0.6mL of oleic acid and 10mL of 1-octadecene.
Now heat the mixture until the cadmium oxide completely dissolves.
What’s happening is the cadmium oxide is reacting with the oleic acid to form cadmium oleate.
After the cadmium oxide dissolves keep heating until the mixture hits 225 celsius.
While you wait, get everything set up.
Have a dozen or so small vials or test tubes
and prepare a syringe with 1mL of the trioctylphosphine selenide solution from before.
Be ready because you need to work quickly.
Once the cadmium solution reaches 225 celsius,
inject the selenium solution and give it a quick shake,
Now immediately start withdrawing small samples of solution,
about half a milliliter to a full milliliter, and put them into the vials.
For the first several samples try to do this one after another as fast as possible.
Then withdraw only when you see the color significantly 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.
So by controlling how long they stay in hot solution we can control their size.
The oleic acid surrounds or caps the particles and keeps them from aggregating.
Okay, now we’ve made quantum dots.
These particular quantum dots are fluorescent in the visible region
and if exposed to an ultraviolet light will fluoresce different colors.
Now scientifically we have to ask the question:
If all the samples have exactly the same chemical composition,
why do they have different colors?
The difference arises because the particles are so small
that the allowed quantum states of the electrons in them are partially determined by the particle’s
shape and size, not just on their composition like with bulk materials.
The best analogy I can think of actually is a stereo system.
Large speakers are really good at producing long wavelength low frequency base sounds.
While smaller speakers are really good at short wavelength higher frequency sounds.
This is due to differences in the size of the diaphragm and the frequencies it can support
Likewise for quantum dots the smaller the dot the smaller and bluer
the natural wavelength of light it will emit,
the bigger the dot, the longer and redder the light it will emit.
Quantum dots are being researched because there are many applications
such as making solar cells more efficient,
better display technologies
and even medical imaging applications.
So even if you can’t make them in the lab,
I’m very sure you’ll soon encounter them in your daily life.
Thanks for watching, please subscribe, like and comment.
The procedure in this video was based on this article published in the journal of chemical education.
Now an additional experimental note if you try this
is that you probably won’t actually see this broad range of color
with your eyes since the wavelength differences are actually very subtle.
You’ll probably only see yellow and green.
That’s all that I’m actually here seeing in real life.
But a good high definition camera will automatically color compensate
to give the appearance of this broad range you’re seeing in the video.
So to observe these differences if you don’t have fluorescence spectrometer
look at the samples through a good digital camera.