An Easy Recipe For Quantum Dots

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."

Comments

Why we care about quantum dots

[JoshuaZ]

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.)

What's So Expensive?

[Doc Ruby]

That video showed a lot of mixing, boiling, separation. None of it looked very expensive. The presenter mentioned a second process that smooths the surface of the initial yield of dots which might be expensive. But that just makes the dots more efficient at processing light. If the initial process is really cheap, the lower quality might be a better value than continuing the second process for an improvement relatively small compared to its increased cost.

While we work in labs to cheapen the refinement process, we could get a lot of cheap dots to apply to other uses that will only improve when the refinement process becomes cheap enough. Getting the dots into industry will ramp up demand for the higher quality ones.

As a side effect, places like China that tolerate toxic products as we see being contained in that video could dominate the market for the initial products. But places like the US that are less tolerant of toxins in the workplace and in pollution, but are more geared towards specializing in higher quality products (refined in different ways for different properties), could refine the raw dots into more valuable and effective products. Leaving the US dependent on China for raw materials, but able to switch suppliers to some other place, like India, that is similarly tolerant - or make the raw dots ourselves if a crisis outweighed the protections from toxins we used in the course of normal business. All of which could make a proper market, with balanced protections, that gives the world lots of cheap, and sufficient amounts of quality, quantum dots. Whose efficiencies and effectiveness can eliminate toxins and pollutions these dots replace in the industries where they're adopted.

Re:What's So Expensive?

[JustinOpinion]

That video showed a lot of mixing, boiling, separation. None of it looked very expensive.

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.)

Re:What's So Expensive?

[Doc Ruby]

How much does a tight glove box cost? How much do the "characterizing instruments" cost?

It seems to me that characterizing the dots requires only a calibrated source of UV (cheap) and a calibrated spectrometer (pretty cheap). At least in characterizing their phosphorescence, which is the characteristic that seems of main importance to this cancer lab, and to most optical switching applications.

Maybe too expensive for most Slashdotters, but not too expensive for even a paint factory. Or an inkjet ink factory.

Re:What's So Expensive?

[JustinOpinion]

So how much more expensive is the second, "smoothing" phase than the original production phase?

It's another wet-chemistry phase. It's no more expensive than the first synthesis step. But each step of course adds to costs (in terms of manpower, chemicals needed, etc.).

Similarly, adding the lipid layer is just a a ligand exchange: you mix the quantum dots with ligand in the right solvent mixture and they become coated. Simple in principle, not too complicated in practice, but it adds another step to the process.

how much do the products of each of those phases currently cost

Quantum dots are fairly expensive, but they are similar in cost to speciality chemicals that don't have industrial uses and thus don't benefit from economies of scale. Some examples from companies that currently sell quantum dots:
Invitrogen 4 ml of 1 micro-molar QD solution (~15 mg of qdot solids) for $335 ~ $22 million/kg
Sigma-Aldrich CdSe QD, 5mg/mL, 10mL solution for $399 ~ $8 million / kg
SpectrEcology 50 mg CdSe/ZnS QDs for $449.00 ~ $9 million / kg

For comparison, ubiquitous chemicals like gasoline are ~$1/kg, common chemicals like acetone (reagent grade) are ~$30/kg, high-purity semi-rare materials (e.g. pure selenium) are ~$1,000/kg, and speciality chemicals (for which there is no industrial need) are typically $100-$1,000 for a 500 mg quantity, which means $1 million / kg. As you can see, it is much more expensive to synthesise a speciality chemical (basically requires a trained chemist to manually do a small-scale lab synthesis for each batch), as compared to industrial-scale manufacturing.

There's no doubt that quantum dots could be made more cheaply if there were a real need for them. There are huge challenges in terms of how to scale-up the synthesis, but nothing that couldn't be addressed with clever chemical engineering and automation.

Re:Named by the marketing dept?

[BoothbyTCD]

These structures are effectively 0-dimensional as far as electrons (and holes, since the places electrons should be are treated like particles) are concerned. There is no place for the electrons to go. This is as opposed to nanowires (1D) and thin films (2D)