An Easy Recipe For Quantum Dots 35
An anonymous reader writes "Semiconductor nanocrystals, better known as quantum dots, might find their way into solar cells, cancer tests, and all sorts of other products. Making them is surprisingly easy, if you have the right equipment, but it's not cheap. A team of reporters from Chemical and Engineering News visited Johns Hopkins and learned how to make the pricey particles (YouTube video). They have produced a slick video that explains the whole process."
Why we care about quantum dots (Score:5, Informative)
Quantum dot are semiconductors where their electrons and holes for electrons (which can be for most purposes thought of as particles themselves) are bound in special tight pairs that are unable to move much. One really nice is that their electronic properties can vary with the size and shape of the crystal. In particular, the band gap :, which is the energy range where electrons cannot live, can vary and be carefully controlled in a quantum dot. Insulators have really big band gaps, conductors have none or close to none, and things with medium band gaps are generally semiconductors. So being able to control your bandgap size means you can make semiconductors with essentially any properties you want.
The reason that quantum dots are so exciting for solar cells is that the way they transfer light to electricity can be fundamentally different than the standard process. For normal solar cells there's a theoretical maximum efficiency before which some of the energy has to go to waste heat. There are clever ways you can take advantage of some of this otherwise wasted heat, but by and large this is true waste heat. However, there are suggestions that the theoretical limit for quantum dot enabled solar cells should be larger.
This is not the only nice set of properties that quantum dots have. There's been suggestion that properly designed quantum dots could be used to do solid state quantum computing. If this does occur it will potentially allow quantum computers to be much more scalable and fault tolerant which currently are the primary problems preventing quantum computers from being more than lab curiosities. (Disclaimer: I'm not a physicist or an electrical engineer. Details here might be wrong.)
Re:What's So Expensive? (Score:5, Informative)
It's true that it's all so-called "wet chemistry" which is fairly simple. However there are many things that make these kinds of syntheses more difficult and complicated (and thus expensive) than other kinds. First of all, you'll notice how careful she had to be about allowing the reagents to get into contact with air. This is because many chemicals that are in air (especially oxygen and water) will kill the reaction. So you have to prepare reagents in an argon-filled glovebox, transfer reagents carefully into an argon-filled reaction flask, etc. Also note that to get good size uniformity, you need rather pure reagents, and you need to mix the reagents as homogeneously as possible (this is why she injects using two small syringes rather than one large syringe: it makes the addition faster and thus all the nanoparticles nucleate and grow at the same time and rate).
Now think about scaling this up to an industrial process. Most chemical plants don't have to worry too much about oxygen or moisture contamination (some of them do, and, of course, they are more expensive to build, operate, and repair). Also the whole 'rapid addition and homogeneous mixing' aspect inherently limits the ability to scale-up, which makes it harder to achieve industrial economies. And of course the ultra-pure reagents are more expensive.
Having said all that, like anything else if there is a pressing need for the material, industrial engineers will find clever ways to produce the material more and more cheaply and efficiently. (Microchips are horrendously complex to manufacture and yet are now remarkably cheap.) So I don't think this is an insurmountable problem... but it is more complicated, and thus expensive, than traditional chemical syntheses. (Actually there are various companies right now that will sell quantum dots of various sizes and kinds. They are mostly intended for use in research, thus are still fairly expensive, but it shows that there is already an industry developing around these materials.)