Quantum Dots of Many Colors

Earlier this month, Technology Review reported that scientists from the University of Rochester have figured out how to use nanoscale crystals called quantum dots to enhance the longevity of artificial photosynthesis. Artificial photosynthesis is exactly what it sounds like ? a means of using the energy in sunlight, combined with water and air, to produce fuel, by incorporating light-absorbing nanoparticles into the mix.

But those nanoparticles tend to deteriorate very fast when exposed to sunlight, which frankly rather defeats the purpose of artificial photosynthesis. That?s where the quantum dots come in: they can absorb the light without deteriorating, and might be cheaper as well, since it doesn?t require precious metals.

That doesn?t mean there aren?t challenges: as the TR article points out, this system produces hydrogen but not oxygen ? and you need both for artificial synthesis. And when they finally do manage to generate both, they?ll need to figure out a way to do so in separate containers, since hydrogen and oxygen tend to explode ? not quite the kind of energy they?re looking for with artificial synthesis.

Colloidal quantum dots irradiated with a UV light. Wikimedia Commons. User: walkman16.

But this is just one of countless practical applications for these plucky little nanocrystals. As I wrote in 2007 and 2011, quantum dots are tiny bits of semiconductors just a few nanometers in diameter. It?s like taking a wafer of silicon and cutting it in half over and over again until you have just one tiny piece with about a hundred to a thousand atoms. That?s a quantum dot. 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.

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. This enables the electrons to flow as a current. And current = power.

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. 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.

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. 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

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.

Last year, engineers at Ohio State University ?invented a new kind of nano-particle that shines in different colors to tag molecules in biomedical tests.? The secret ingredient? quantum dots! This breakthrough ? described in the online edition of Nano Letters, in a paper by OSU?s Jessica Winter and Gang Ruan ? involved stuffing tiny plastic nanoparticles with even tinier quantum dots for use in biomedical tagging applications. It?s easier to see biological molecules under a microscope if they fluoresce, and quantum dots glow more brightly than other fluorescent molecules used for this purpose.

They also ?twinkle?, i.e., blink on and off, an effect that is less noticeable if there are many quantum dots congregated together. There are pros and cons to this behavior. Con: it ?breaks up the trajectory of a moving particle or tagged molecule? that one is trying to track under the microscope. Pro: when the blinking stops, scientists know they?ve reached a critical threshold of aggregated quantum dots.

What Winter and Ruan did to address this is to turn that ?con? into another ?pro? by stuffing quantum dots of different colors into the same micelle (a polymer (plastic) based spherical container commonly used in lan experiments). Their tests showed that doing show caused the micells to glow steadily. To wit:

?Those stuffed with only red quantum dots glowed red, and those stuffed with green glowed green. But those he stuffed with red and green dots alternated from red to green to yellow. The color change happens when one or another dot blinks inside the micelle. When a red dot blinks off and the green blinks on, the micelle glows green. When the green blinks off and the red blinks on, the micelle glows red.

If both are lit up, the micelle glows yellow. The yellow color is due to our eyes? perception of light. The process is the same as when a red pixel and green pixel appear close together on a television or computer screen: our eyes see yellow.?

The continuous glowing makes it easier to track tagged molecules with no breaks, and they could also use the color changes to determine when said tagged molecules congregate. The nanopartices would be great for microfluidic devices, and could one day be combined with magnetic particles to enhance medical imaging for, say, cancer detection. In fact, earlier this year, Nature Nanotechnology published the results of a study showing no ill effects over a one-year period in four rhesus monkeys injected with such tiny luminescent crystals.

Spin-cast quantum dot solar cell built by the Sargent Group at the University of Toronto. Credit: Lukasz Brzozowski, University of Toronto.

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

Another potential application is solar cells. Back in 2007, a CNET article reported that quantum dots were the likely ?secret ingredient? in thin-film solar cells being developed by a start-up company called Stion: ?Most solar cells on the market today extract electricity from sunlight with silicon and are integrated into glass substrates, which is relatively heavy.?

What can quantum dots bring to the solar cell table? According to that 2007? CNET article: ?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.?

And that?s on top of their potential to further artificial photosynthesis. It?s nice to see quantum dots getting a little love in the public sphere again.

References:

Ekimov, A. I. & Onushchenko, A. A. (1981) ?Quantum size effect in three-dimensional microscopic semiconductor crystals,? JETP Letters 34: 345?349.

Hoshino, K. et al. (2012) ?Nanoscale fluorescence imaging with quantum dot near-field electroluminescence,? Applied Physics Letters 101 (4).

Reed M.A. et al. (1988) ?Observation of discrete electronic states in a zero-dimensional semiconductor nanostructure,? Physical Review Letters 60 (6): 535?537

Ruan, G. and Winter,? J.O. (2011) ?Alternating color-quantum dot nanocomposites for particle tracking,? Nano Letters 11(3): 941-945.

Walling, M. A.; Novak, Shepard (2009-02) ?Quantum Dots for Live Cell and In Vivo Imaging,? International Journal of Molecular Science 10 (2): 441?491.

Ye, Ling, et al. (2012) ?A pilot study in non-human primates shows no adverse response to intravenous injection of quantum dots,? Nature Nanotechnology 7, 453?458.

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