connecting the dots

PerplexedjenlucCelebrity tabloids and similar gossip rags are filled with unnamed sources — you know, "Sources close to [Insert favorite Celebrity (TM) here] report that…." I've always wondered who these loose-lipped people are: a source standing close to the Celebrity (TM) on line at Starbucks, perhaps? A casual diner at the next table in a stylin' Hollywood eatery? Or maybe a disgruntled former employee, or pathetic former hanger-on who's bitter because s/he didn't become the Celebrity's (TM) new BFF? Because anyone who was truly close to a Celebrity (TM), wouldn't maintain that position for very long by talking to unscrupulous tabloid reporters, now, would they?

Tabloids aside, we generally expect the average news story to cite its sources by name, with rare exceptions, like when Woodward and Bernstein broke the Watergate scandal. So imagine my surprise when I noticed this little news item on CNET a couple of days ago about a start-up company called Stion with the stated mission of developing thin-film solar cells, mentioning unnamed "sources" in the third graph down. They've received a whopping $15 million in venture capital, which means their technological approach must be pretty promising, but Stion's manager of business development was rather coy about what, exactly, that material might be. He had plenty to say about what it was not — not silicon based, not cadmium telluride, and not a copper-indium-gallium-selenide compound, either — but otherwise kept mum, promising to reveal all "in due time," i.e., around 2010, when the first products are likely to be announced.

Sheesh, what a media tease! Clearly, the frustrated CNET reporter, Michael Kanelios, had no choice save to turn to his own Deep Throat, or two. Which, BTW, is not a criticism: he did a commendable job (far better than the celebrity tabloids) of assembling various pieces of circumstantial evidence in favor of the new material being… (drum roll, please)… quantum dots! I'm not sure his evidence is solid enough to warrant announcing this in the headline ("Harnessing quantum dots for solar panels") without a question mark, unless his unnamed sources are very reliable indeed. But it is a matter of public record that Stion's Chief Technology Officer, Howard Lee, worked as a solar researcher (specifically using quantum dots) at Lawrence Livermore National Lab for several years, and accumulated numerous patents relating to quantum dots during his stint at another start-up called Ultradot. And Stion's CEO, Chet Farris, is a former president of Shell Solar. Hmmm. The plot thickens. Kanelios, for one, can connect the dots.

No doubt some of you are wondering what the heck these quantum dot thingies are, and why they're such a big fat deal. It just so happens that I wrote about quantum dots way back in 2003 for The Industrial Physicist magazine — the closing of which I still mourn, because I got to cover so many cool cutting-edge topics as a contributing editor there. (I wasn't always about the pop culture physics; I used to work for the Dark Side, i.e., Very Serious Science Journalism, or VSSJ.) Quantum dots are essentially tiny bits of semiconductors — sometimes called nanocrystals, which just doesn't carry the same panache — just a few nanometers in diameter. It's like taking a wafer of silicon and cutting it in half over and over again (a semiconducting Zeno's Paradox?) until you have just one tiny piece with about a hundred to a thousand atoms. That's a quantum dot. (I think. It's not like you can actually see one with the naked eye. Billions of them could fit on the head of a pin.)

Size matters when it comes to semiconductors: smaller is usually better. Because they're so tiny, quantum dots have some unusual materials properties — specifically, the all-important electrical and optical ones — thanks to the quantum effects that kick in at smaller size scales, so they are of enormous interest to researchers. It's interesting physics fundamentally, and it offers an impressive sampling of potentially lucrative practical applications. Trust me, quantum dots are hot, even if they're currently simmering on the back burner in the news-hook-oriented media.

I must confess to finding it easier to write about applications of physics rather than the basic science. But when I started covering the quantum dots area, I learned some useful things about the "electrons and holes" effect that is critical not just to quantum dots, but also lasers and other semiconductor physics. This is not an easy thing for a lay person to visualize, although physicists toss those terms around like high school slang. So here's my attempt at the 411 on electrons and holes (scientific commenters, feel free to add your own take on the subject):

It helps to place semiconductors in general in the appropriate context, i.e., right smack between insulators and conductors. Insulator atoms hoard their electrons greedily, like misers or overprotective parents, and rarely part with them, while conductor atoms are like spendthrifts or exceedingly permissive parents, letting their electrons run amok all over the place (and a good thing, too, otherwise we'd never enjoy the benefits of electrical current). Semiconductor atoms are juuuust riiiight. They don't fling their electrons around all willy-nilly, but neither do they hang onto to them too tightly. It takes a bit of an energy boost to knock an electron loose in a semiconductor, and when the electron breaks free, it leaves behind a "hole" in the atom's electronic structure — a vacancy, if you will, that another electron, sooner or later, will come along to fill. So a photon strikes a semiconductor atom and creates an electron-hole pair, which physicists call an exciton — because we need more confusing technical jargon in physics, don't we? Anyway, this enables the electrons to flow as a current. And current = power.

Much of the excitement over quantum dots stems from a decades-long quest to make silicon emit light efficiently in the visible spectrum. Back in 1990, European researchers managed to get porous silicon to emit red light, and figured it came about because of "quantum confinement" relating to the dot's small size. Basically, at 10 nanometers or less, the electrons and holes are being squeezed into such small dimensions that this alters the electronic and optical properties; it's the critical feature of most nanoscale materials, frankly. (Special bonus for Physics-Philes: a 2003 paper in Nature reported that shape might matter as much as size when it comes to quantum confinement.) Things snowballed from there, with scientists making more silicon dots (and, later, germanium dots) that emitted light in lots of bright, pretty colors, especially the highly desirable green and blue ranges. Basically, the bigger the dot, the redder the light, and the emitted light becomes shorter and shorter in wavelength — and higher in energy — as the dots shrink in size. This is called "tunability" because you can pretty much tailor the dots to emit whatever frequency of visible light you happen to need for a given application, simply by altering the size of the dots. Believe me, high-tech industries go nuts for anything with tunability. Plus, colors = pretty! Check out the pic! Doesn't it make you want to buy some quantum dots?

The most obvious application is using quantum dots as an alternative to the organic dyes used to tag reactive agents in fluorescence-based biosensors. You know, the dyes start to glow when, say, a harmful toxin is present. But the number of colors available using organic dyes is limited, and they tend to degrade rapidly. Quantum dots offer a broader spectrum of colors and show very little degradation over time. Having all those colors also means you can make light-emitting diodes (LEDs) from quantum dots, precisely tuned in the blue or green range. You can also build quantum dot LEDs that emit white light for laptop computers or interior lighting in cars. As for electronics, the possibilities are endless: all-optical switches and logic gates, for instance, with a millionfold increase in speed and lower power requirements, or, further in the future, quantum dots could be used to make teensy transistors for nanoelectronics.

But the current news is about quantum dots and their potential application to solar cells. As I mentioned earlier, Stion's CTO, Lee, did a lot of work in this area during his stint at Livermore, as have many other researchers. (A more technical overview of this research area by Science News' Peter Weiss can be found here, for those who are interested.)  As the CNET story reports, "Most solar cells on the market today extract electricity from sunlight with silicon and are integrated into glass substrates, which is relatively heavy." A company called First Solar uses a similar structure, but replaces the silicon with cadmium telluride, which is cheaper. As for the CIGS (copper, indium, gallium and selenide) version of the technology, there's several companies working on that, although products have yet to hit the market. It's expected that CIGS solar cells will be cheaper than silicon ones, but not quite as efficient: we're talking mid- to low teens, percentage wise, compared to 22% efficiency (and sometimes as much as 29%) for silicon solar cells. (A fossil fuel like gasoline can show 30-40% efficiency, so even silicon solar cells need to show some improvement in this area for broad practical application.)

So what can quantum dots bring to the solar cell table? The CNET article doesn't go into much detail:

"Partly because of their small size, quantum dots can be highly sensitive to physical phenomena and can be used to trap electrons. Since solar panels work by wiggling electrons out of sunlight and transferring them to a wire, quantum dots in theory could work well in solar panels."

This doesn't really say anything meaningful, to a non-scientist, about the actual process that's taking place — because the process is very complicated and hard to explain in a short news article. Those electron-hole pairs known as excitons I mentioned earlier? Usually, photons from sunlight that strike the semiconductor material used in solar cells unleash only one electron. In theory, they should be able to loosen more than one, thereby giving rise to several excitons, but for reasons relating to heat loss in atomic collisions, or some such thing (paging Chad Orzel for a better explanation!), there's usually only a 1-to1 ratio. That's why solar cells are limited in their energy efficiency. But last year, researchers working with quantum dots made of lead selenide found they could produce as many as seven excitons (electron-hole pairs) from a single high-energy photon of sunlight. This could boost solar cell efficiencies to as much as 42% — enough to be competitive with the more common fossil fuel energy sources.

So Stion is doing good work, and with any luck, they'll be rewarded with some healthy profit margins in due time. But all this hard-core quantum physics talk has made me long for the comparable simplicity of the celebrity tabloids. Intellectual stimulation is all very well and good, but sometimes my brain just needs a break. And the latest unscrupulous unnamed sources are telling me there's trouble in some Celebrity's (TM) fairytale paradise…

5 thoughts on “connecting the dots”

  1. “It’s like taking a wafer of silicon and cutting it in half over and over again (a semiconducting Zeno’s Paradox?) until you have just one tiny piece with about a hundred to a thousand atoms. That’s a quantum dot. (I think. It’s not like you can actually see one with the naked eye. Billions of them could fit on the head of a pin.)”

    lol Jennifer, so how many angels can you get dancing on a pinhead. Maybe they are as small as quantum dots – or smaller still and in another dimension too.

    But manipulating quantum dots, that is hot!

  2. Thank you for this very interesting post!

    Quantum dots are cool stuff indeed! I learned about them essentially from the first volume I worked on as an editor in my current job, and that was fascinating – it’s a little bit of “quantum engineering”, since you can try to make a band structure that fits your needs.

    Moreover, these dots and their one-dimensional cousins, the quantum wires, are beautiful real-world implementations of the quantum mechanics textbook situations of “particles in a box”. They definitly deserve to be wider known.

    Best, stefan

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