A scanning tunnelling microscope image of a single-atom transistor during fabrication. The pink colours represent the areas where a single phosphorus atom (centre) as well as phosphorus source and drain contacts will be placed. The gate contacts that control the transistor action from the side are not visible here. Credit: Martin Fuechsle

When Gordon Moore made his observation in 1965 that the number of transistors integrated on a single silicon chip is doubling roughly every two years, the only logical end point for such a trend would be a transistor made from a single atom. This point has now been reached. Writing in Nature Nanotechnology, Michelle Simmons from the University of New South Wales in Sydney and colleagues report a single-atom transistor, the world’s smallest, on a silicon chip. The transistor is based on current flowing through a single atom of phosphorus embedded in a silicon wafer.

Phosphorus is a natural choice for such a transistor, as it is relatively easy to integrate into silicon. There it acts as an electron donor because it has an additional electron compared to silicon. This additional charge can be used for conventional electronic devices such transistor, but also more complex schemes are possible. For example, the magnetic property of this single excess electron, its spin, can be used for new types of quantum computing. Indeed, I have previously blogged about such efforts from another research group in Sydney using multiple phosphorus atoms for silicon-based quantum computing.

One of the key challenges in making a single-atom transistor is to place a single phosphorus atom into silicon in a controlled fashion. Here, this is achieved by the careful placement of three phosphine (PH₃) molecules on the surface of silicon using a scanning tunneling microscsope. In a number of reaction steps these molecules dissociate and cause the ejection of a silicon atom from the surface and the incorporation of a single phosphorus atom in its place.

The single-atom transistor structure. The electric current between source (S) and drain (D) contacts, through the phosphorus atom in the centre, is controlled by the two gate contacts (G1,G2) (c) 2012 Nature Nanotechnology

Once incorporated into silicon, the energy level of the phosphorus’ outer electron states lies below that of the surrounding silicon. The precise match between silicon and phosphorus energy levels can be controlled by the electrical potential applied between two gate electrodes on either side (see figure on the right). This voltage controls the electric current between source and drain, and through the atom. If the energy levels of silicon and the phosphorus atom do match up, it is possible for electrons to pass through the phosphorous atom one by one. If the voltage between the two gates is set so that the silicon and phosphorus energy levels do not match up, there is no electric current.

There are, however, a few drawbacks in the present approach. So far this transistor only works for really low temperatures, barely above absolute zero. And in the current implementation the contacting electrodes are still several tens of nanometres apart – hardly on the single atom scale. Also, the fabrication of the structure with a scanning tunnelling microscope doesn’t allow to make billions of them in the same efficient way as commercial computer chips are fabricated.

Still, such single atom transistors could already be of interest for the quantum computing schemes investigated for phosphorus atoms, where the low temperature is less of an issue. Either way, considering that according to Gordon Moore’s law commercial silicon transistors are only expected to reach single atom scales beyond 2020, this study certainly is ahead of the curve.

Reference:

Fuechsle, M., Miwa, J., Mahapatra, S., Ryu, H., Lee, S., Warschkow, O., Hollenberg, L., Klimeck, G., & Simmons, M. (2012). A single-atom transistor Nature Nanotechnology DOI: 10.1038/nnano.2012.21

This post was chosen as an Editor’s Selection for ResearchBlogging.org

Further information:

The University of New South Wales has produced a YouTube video explaining these findings.

A beautifully looking graphics, isn't it? But there is a major caveat. As its creators would agree, this image is only a very crude depiction of reality and shouldn't be used for any scientific purpose... (c) LANES, EPFL

Nanotechnology is a wonderful science that has pushed functional devices to sizes not far away from the size of atoms. So small that if you want to image such structures, even a conventional electron microscope wouldn’t get you far. There is no way to directly see what is going on. This is a common problem. Take condensed matter physics – it is impossible to directly visualize the various interactions and events taking place inside a crystal. Or photonics, where complex light fields interact with tiny nanostructures in ways that can be really difficult to visualize, especially in real-time.

So, no wonder that artificial graphics often serve to illustrate a scientific concept or a certain device. And with the prevalence of advanced computer graphics programs such illustrations are becoming more and more fancy. In my opinion, this is a dangerous trend, because such graphics can distort the underlying science they try to depict.

Atom balls emit beams of light. It might have been better to keep it simple and not go down to the atomic scale. (c) National Science Foundation

Before starting to outline just how wrong such fancy images can be, I like to emphasize that I don’t want to criticize individuals over this, this is just to highlight a general trend that most of us are guilty of following. I mean, I am guilty, too, and have published papers as an editor containing overly fancy graphics. And the image shown above, by the way, made it on the cover of one of the Nature family of journals, although of course other journals show plenty of such graphics, too.

But why are such nice graphics wrong? Here are some reasons:

• Like above, they often show a mix of atoms and dense material. The atoms of the gold contacts are not shown, yet they are of similar size as those depicted. Why is this wrong? Well, such an image suggests that the gold contacts are governed by bulk material properties, whereas atomic effects are only relevant for part of the structure. Yet, on such length scales all atoms of the device influence each other and can’t be isolated in this way. The same for the sticks that are supposed to be atomic bonds in such images. They stop at the interface, which isn’t scientific reality either – atoms across such interfaces influence each other.
• Both the atoms as well as the contacts reflect light. At these length scales there can’t be such reflections because these sizes are smaller than the length scale of light. Why is this wrong? Especially for illustrations of photonic devices this is a problem because it creates a wrong thinking about light on the nanoscale. Basically, you would have to consider local electromagnetic fields. On this scale the interaction of light fields with metals for example is much more sophisticated than simple reflection. And come on, such detailed, mirror-like reflections from atoms?
• On the nanoscale, laser beams are often shown as diffuse beams of light. I am always puzzled why devices that are on the scale of the wavelength of light are shown to ‘emit’ such beams in a way that a floodlights would do. Why is this wrong?As in the previous example, light is emitted as electromagnetic fields, and has a wave as well as particle character. Such images of laser beams on this scale give the illusion that geometrical optics/ray optics still prevails at these length scales. And this is simply not the case. It would be better to simply depict the modes of light, and to be careful about mixing local and global properties.
• Sharp interfaces. Notice how clean and well-defined the various parts of devices often are in such graphics? Well, guess what, on the nanoscale interfaces are never as sharp and clean, atoms never arranged perfectly. Why is this wrong? Rough interfaces influence considerably device performance and moreover, interactions of materials across interfaces are often key to their performance. Keeping everything clinically clean can suppress key device physics and divert from this crucial aspect.
• Wrong scales. In the illustration above, the wires that connect the device to the outside world are of the same width as the atoms. (note: the creators of this particular graphic above did make explicit note that the scales are off). Why is this wrong? Wrong scaling relations can also give the wrong impression of the physics of a device, and could distort the impression of device functionality.

Finally a better example: the figure shows the interface between two materials on the atomic scale. The "balls" depict atoms but are sufficiently abstract to illustrate nothing other than the point being made - the crystal structure as well as the arrangement of atoms at the interface. (c) J. Mannhart, A. Herrnberger

To scientists, these criticisms are probably nothing new. Often, such graphics are produced not to make into actual scientific publications, but as ‘sexy’ cover art or for news stories. However, I do notice a disturbing trend of wrong graphics making their way into a paper. Just look at the table of content illustrations that some journals have. I have seen examples where the majority of illustrations in a paper were based on such science fiction and not on hard, solid data. In some cases, such graphics were even used to underline a scientific argumentation in a paper, to make a scientific point. It can’t get much more wrong than that.

My plea is to scale back the arms war in scientific illustrations. It is of course great to be able to make a device look appealing. But do atoms have to reflect light mirror-like? Why these attempts to be superrealistic when in fact the outcome is the opposite? Illustrations can still be visually attractive even if they are scientifically more accurate, for example by being more abstract. Abstract graphics can look cool, too. Illustrators should keep it simple and stick to the main purpose of the illustration. That way science wins.

Update: In the initial version of the blog post as published today I used the words ‘wrong’ and ‘misleading’ at various instances of the text. To avoid misunderstandings I have now changed it all to ‘wrong’, because it was not my intention at all to create the impression that such graphics are created (or used) with bad intentions.

Well, as with so many other facts, Wikipedia would also give you the answer to these questions. But that’s not the point. What Petroski has now done is to collect and curate interesting facts related engineering, and published them in alphabetical order as “An Engineer’s Alphabet“.

There are plenty of gems to discover in the book. Many of them I would never have thought to even look up on the internet without being prompted, and in that respect the book is inspiring. I certainly enjoyed browsing through the text. Written by an American professor, it is more American Dream than Steampunk in character, although to be fair Isambard Brunel does appear in eight different entries. Herbert Hoover thirteen times. Robert Noyce only once, in passing.

The best way to go about reading this book is simply do flip through it and to read here and there. Or, to use the indispensable index at the end. Indeed, if £18.99 or $21.99 should be a bit too much of an expense it might make sense to consider the various ebook options, with the highly useful possibility of searching the book. On the Kindle or the Nook prices are about half of the hardcover ones. It doesn’t seem to be available on iBooks. Either way, if you love engineering and are interested in broadening in particular your historic knowledge of the profession, this book might be for you.

Reference:

Petroski, Henry. An Engineer’s Alphabet. Cambridge University Press, 2011. 268 pages. ISBN: 9781107015067. $21.99 / £18.99

A coaxial cable plug. The coaxial nanolaser is more than 15,000 times smaller. Photo by mikemol via flickr.

When Oliver Heaviside invented the coaxial cable in 1880 he could not have foreseen the implications of his idea on modern nanotechnology. His coaxial cables consist of three layers: an inner metallic core, surrounded by an insulator, surrounded by a metallic layer on the outside. The benefit of this design is that the outer metallic layer shields the electrical signal through the cable from outside interference. This makes coaxial cables very useful for information transfer, and coax cables are used for TV antenna cables or some computer network cables. Mercedeh Khajavikhan, Yeshaiahu Fainman and colleagues from the University of California, San Diego now present a completely new application: they have fabricated coaxial lasers on the nanoscale that turn on without the usual minimum threshold power of usual lasers. To do this they had to shrink the coaxial cables first. These lasers are more than 15,000 times smaller than typical coaxial cables.

The nanoscale coaxial laser. Similar to coaxial cables it consists of an inner metal pillar and an outer metal shield. The structure is also protected from interference from the top. Inside is a semiconductor light emitter (red; insulated from the top metal through a SiO2 plug). The laser light exits through the hole in the substrate. Figure by Mercedeh Khajavikhan and Aleksandar Simic.

The benefit of a coaxial cable is that between the core and the outer metal layer well-defined and controlled electromagnetic waves can propagate shielded from any outside influence. Furthermore, shrinking such a device to the nanoscale – to length scales comparable to the light used – means that only the smallest optical beam pattern for the wavelength of light, known as the fundamental mode, fits into the small space between the metal structures. The other modes would be too large.

If we now add a matching light-emitting semiconductor into the structure (see figure), light emission from this semiconductor is only possible if it emits light in the form of the fundamental mode. All other emission modes are forbidden because they are not sustained in this small space. Here, the coaxial structures have a diameter of about 300 nanometres, matched to the lasing wavelength of the InGaAsP semiconductor of about 1,400 nanometers.

The single mode has other implications, adds Khajavikhan, “by ensuring that cavity only supports one mode (the lasing mode) and all other channels for emission are absent, the emitters in the gain region only emit in the lasing mode thus resulting in thresholdless lasing.” Normally, when lasers are turned on they first need to overcome various internal losses before lasing can set it. But because there is only one emission channel that is furthermore shielded from outside losses, lasing sets in immediately.

As a proof of this principle, the authors fabricated two coaxial lasers. A large one in which several optical modes can exist, and a smaller one with only the fundamental optical mode. As expected, the larger structure only lases above a certain threshold whereas the smaller device starts lasing immediately. Xiang Zhang from the University of California, Berkeley, who works on nanoscale lasers, considers this a very interesting development, particularly for fundamental quantum physics research. “The demonstrated nanoscale coaxial laser could be an interesting platform for the investigation of plasmon-based cavity quantum electrodynamics and quantum metamaterials.”

At the same time, Zhang cautions that there are still obstacles to overcome before coaxial lasers could complement the wide-spread use of coaxial cables. Low operating temperatures as well as the fact that these lasers are optically excited and not electrically need to be addressed in future work.

However, the practical potential is certainly clear, says Khajavikhan. “In the fabrication of these lasers we have used standard nanofabrication processes. More importantly, the lasers are batch fabricable, that is many of them can be fabricated at simultaneously and in the parallel way. [...] No other nanolaser is so well adapted to the existing nano-fabrication tools.”

With their commercialization still a bit off, however, as Zhang suggests these lasers could be subject of plenty of fundamental research on quantum mechanical effects. And more possibilities will arise, adds Fainman. “We feel this is just a beginning of a new family of light emitters with superior characteristics and many advances in this new area is yet to come.”

Reference:

Khajavikhan, M., Simic, A., Katz, M., Lee, J., Slutsky, B., Mizrahi, A., Lomakin, V., & Fainman, Y. (2012). Thresholdless nanoscale coaxial lasers Nature, 482 (7384), 204-207 DOI: 10.1038/nature10840

This post was chosen as an Editor’s Selection for ResearchBlogging.org

But that’s the boring stuff. Far more interesting is that light can also strongly couple to matter, but without getting absorbed. The example I am discussing here is when the interaction between light and a molecule is so strong that it profoundly alters the molecule’s energy states themselves, and not merely lifts electrons from one state to another. In particular, what Thomas Ebbesen, Tal Schwartz, James Hutchison and colleagues at the University of Strasbourg have now shown is that such interactions could find exciting new applications: to control energy levels of molecules, and in this way to influence the kinetics of  chemical reactions in a new way that creates many new possibilities.

Strong coupling of light and matter. Light confined between two mirrors can strongly interact between matter that is also between the mirrors and has a matching energy level. The strong light-matter coupling then causes a splitting of the matching energy level into two separate states.

To see how this looks in practice it is necessary to understand what the strong coupling between light and molecules means. First of all, to achieve the necessary strong coupling, it is necessary to create a strong feedback mechanism between light and matter. This can be done by squeezing the light field between two closely spaced mirrors, with the desired molecules in-between. In addition, the energy levels of the light field between the mirrors and one of the energy levels of the molecule need to match up. If all these conditions are fulfilled, then the energy state in question is split into two separated states (see figure). This is called Rabi splitting. The stronger the coupling, the larger the energy separation between the two states. Because of the beauty of quantum mechanics this doesn’t even require light to be present, the mirrors are enough.

The impact of this effect on chemical reactions is now clear, because the change in energy states of a molecule also changes chemical reactions. Different energy levels enable new reaction pathways, or could accelerate or slow down chemical reactions. Of course, chemical reactions often can be influenced by light. But in many cases this is down to electrons being excited from one state into another. Here, to use the language of the authors, the entire energy landscape of the system is remodelled in a controlled fashion. In particular, both the energy position as well as the strength of the coupling can be tuned via the properties of the mirrors, so that specific energy states can be addressed.

For their experimental demonstration, Ebbesen and colleagues studied the well-known molecules spiropyran and merocyanine, which can be converted into each other by selective irradiation with light. A film containing spiropyran was placed between two silver mirrors. The energy levels of spiropyran are not matched to that of the light field between the mirrors, so this molecule doesn’t show any coupling effect. Then the sample was irradiated with UV light, which converts spiropyran into merocyanine. Because merocyanine’s excited energy state is aligned with that of the light states between the mirrors, a strong coupling is expected. And indeed, this is shown in the authors’ first paper (ref. 1), where clearly merocyanine shows a Rabi splitting, and spiropyran doesn’t. Having demonstrated the basic principle of the effect, Ebbesen and colleagues now show the implications of the effect on chemical reactions (ref. 2). by studying the reaction dynamics between spiropyran and merocyanine. They found that the conversion from spiropyran to merocyanine is slowed down in the presence of strong coupling.

Taken together, both papers provide a beautiful proof of principle how the energetics of molecular reactions can be influenced by the coupling to light fields. And given that there is complete control over selecting the right energy level and over the Rabi splitting itself,  this strong coupling approach could lead to a considerable freedom in modifying chemical reactions, possibly with profound implications for many areas of chemistry.

Of course, first there is still a lot to do. So far, a reaction involving only a single molecule has been studied, and in future it will be interesting to see how more complex reactions can be modified in this way. Also, the use of silver mirrors with only a very narrow space in-between could make more complex experiments difficult. But as the authors note, “strong coupling is not limited to the [...] configuration used here. Any photonic structure that provides a sufficiently sharp resonance can be used.” In other words, certain open systems, such as photonic crystals, could show the same effect. We’re certainly only at the beginning of this new research area.

References:

1. Schwartz, T., Hutchison, J., Genet, C., & Ebbesen, T. (2011). Reversible Switching of Ultrastrong Light-Molecule Coupling Physical Review Letters, 106 (19) DOI: 10.1103/PhysRevLett.106.196405

2. Hutchison, J., Schwartz, T., Genet, C., Devaux, E., & Ebbesen, T. (2012). Modifying Chemical Landscapes by Coupling to Vacuum Fields Angewandte Chemie International Edition DOI: 10.1002/anie.201107033

Information stored by a chain of magnetic atoms. Left: an STM tip measures the magnetic state of the iron atoms. Right: through increasing the current between tip and atoms the magnetic states can be switched. Peaks become valleys and vice versa. (c) Science Magazine

I now finally got the time to follow-up on last week’s paper in Science by Andreas Heinrich‘s group at IBM on magnetic storage elements that are only a few atoms in size. There have been a few misconceptions in some of the news reports with some being plainly wrong (‘smallest storage device ever made’), and many didn’t mention much about the scientific principles behind this study, although these are quite interesting. One of the better reports appeared in the New York Times, albeit again without going much into details. So I hope I can still add something useful with this blog post.

And actually, we’ve come across Andreas Heinrich’s previous research before, he does very innovative research with scanning tunneling microscopes (STM). In this latest Science paper he has now explored the limits of magnetic storage devices. Magnetism is of course the basis for storage such as magnetic hard drives. The problem in increasing the storage density in any magnetic storage device is that the magnetic regions begin to interfere with each other as they become smaller and are integrated closer together, because magnetic states on the order of just a couple of atoms are not very stable.

For this reason, Heinrich studies antiferromagnets. In antiferromagnets the magnetic fields of neighbouring atoms point in opposite directions (see figure above). The opposing fields of the atoms mean that on average an antiferromagnet has no overall magnetic field, which makes them far less susceptible to outside interference from magnetic fields. Unfortunately, this also makes measurement of the magnetic properties very difficult and is the reason antiferromagnets are not used in data storage at present.

The problem of measuring antiferromagnets, however, is solved when using a STM with its atomically sharp metal tip, because then antiferromagnet is not measured on a global scale, but atom by atom. This happens via a small electric current that flows between the tip and the iron atoms on the surface. The tip of the STM is magnetized, so that the current through the tip depends on the relative magnetic alignment between the tip and the atom. Low if tip and atom’s magnetism is opposite, and high if in the same direction. In the actual experiments, the current was actually held constant, but instead the height of the tip above the surface was varied, so topographic maps of magnetism are obtained, see the figure above. The alternating highs and lows of these images show that the Fe atoms indeed are antiferromagnetically ordered. Manganese atoms on the other hand do not show this kind of order – the energetics for them are unfavourable.

Anyway, the antiferromagnetism can also be controlled. To read out, the electrical voltage applied to the STM tip is less than 2 millivolts.  By increasing this to 7 millivolts the force on the magnetic orientation of the atom underneath the tip becomes too strong and it switches its orientation. The magnetic interaction of the switched atom with its neighbours means that they all swap around. This can be seen from the figure above, where the peaks and valleys are reversed. The mechanism works best when initiated at either end of the chain.

The alternative order of the peaks and valleys can be used to encode information as either 1 or 0. What is so appealing about this mechanism is that the switching as well as the reading of the information does not need a magnetic field applied to the sample (although the tip is magnetized in a magnetic field).  Switching times as short as 20 nanoseconds were reached, which would translate to a maximum of 50 million switches per second.

But the real appeal is the size of the device. Bits were encoded into chains of only 6 atoms long. Unfortunately, low operation temperatures are needed to avoid random switching of the atoms. The details depend on the size and shape of the atomic arrangement, but is typically below 5 Kelvin (degrees above absolute zero), which is -268 degrees Celsius. To keep an array stable for 17 hours even requires cooling down to 0.5 K. So clearly, such kind of memories aren’t going to take off in earnest any time soon. In comparison, Resistive Random Access Memory or Phase-change Memory both are much closer to the market and achieve impressive storage densities.

Still, scientifically this work will have a strong impact. Not only shows it a path towards a more systematic way of studying antiferromagnetic order of atoms on a surface, it also does re-emphasize new ways of utilizing magnetism on the atomic scale.

Reference:

Loth, S., Baumann, S., Lutz, C., Eigler, D., & Heinrich, A. (2012). Bistability in Atomic-Scale Antiferromagnets Science, 335 (6065), 196-199 DOI: 10.1126/science.1214131

This post was chosen as an Editor’s Selection for ResearchBlogging.org

The temporal cloak. A light beam of a single colour is directed at a split-time lens (STL) that converts it into different colours. As the beam then propagates through an optical fibre, the blue light components travel faster than the red ones (the vertical axis shows time), so that eventually a brief gap is formed in the travelling beam during which there is no light present. Therefore, any even taking place during that temporal gap will be concealed from the beam. Afterwards, a reverse process restores the original beam so that an observer does not notice the cloaking device. Reprinted by permission from Macmillan Publishers Ltd. Nature (2012). doi:10.1038/nature10695

Devices that conceal objects from an observer are called cloaks. Conceptually, the idea of cloaking devices has its roots in science fiction, but such devices have indeed been demonstrated in the past few years. These cloaks are based on tiny structures that are able to bend light on predetermined paths as it passes through the structure. This is like a lens, but consisting of manmade materials, and much more versatile and powerful.

A very different type of cloak has now been published in Nature by Alexander Gaeta and colleagues from Cornell University. Following earlier theoretical proposals, they have now demonstrated the first temporal cloak where events are hidden in time, not in space so that an event is concealed from a light beam travelling through the same space for a certain amount of time. To understand the difference of a temporal to a spatial cloak, Robert Boyd and Zhimin Shi from the University of Rochester make a very good comparison in their News and Views article on the paper:

“The distinction between temporal and spatial cloaking can be understood in terms of a metaphor involving automobile traffic. A spatial cloak acts like a junction in the form of a ‘cloverleaf’ interchange or flyover, in which the traffic is guided (by slip roads) to bend around a certain region of space. After passing through the junction, the traffic continues in the same direction as if the junction did not exist. By contrast, a temporal cloak behaves like a railway crossing. Traffic is stopped when a train passes, forming a gap in the traffic. After the train has passed the crossing, the stopped cars speed up until they catch up with the traffic in front of them, and the fact that a train has crossed the intersection cannot be deduced by observing the traffic flow.”

The realization of such a cloak requires a number of steps. First of all, it isn’t easy to completely halt the propagation of light and then carry on as before in a way similar to traffic at a crossing. But what is possible is to create a gap in the traffic whilst it is flowing by making some of the cars go slower and others go faster.

In the actual device, cloaking starts when a beam of a single colour is directed at a so-called split-time lens (STL), which basically broadens the colour range of the original beam. For example, a green light beam would be converted into blue and red colours. This is a key step for the device, because it is relatively easy to make the beam components with different colours go either faster or slower – this kind of manipulation is routinely done in optical fibres. Here, in the optical fibre blue rays are made to travel faster and red ones slower. Eventually, the blue ones race far enough ahead so that the red ones can’t keep up and a gap forms in the middle of the beam where there is no light. Any event happening within this gap is going to be unnoticed by the light beam. Afterwards, once the zone of interest it passed, an inverse process first brings the different colours back together and then reinstates the original, single-colour light beam. The temporal cloaking is complete.

However, before we get too excited here (and I imagine that these cloaks will get quite some attention in the media), here are a few things that the device as demonstrated here does not do: first of all, it only works for a single colour. Also, it only works for a short moment – about 110 picoseconds (around a ten billionth of a second), although temporal gaps ten times longer should be feasible. And finally, the cloak only works for a small volume in a complicated experimental setup. This is nothing to carry around, so it won’t make anyone disappear for certain amounts of time, and it certainly isn’t a time-machine either and never will be.

With these caveats out of the way,  scientifically this is certainly an intriguing demonstration because not only complement temporal cloaks nicely the spatial ones and realize a new concept of light manipulation, but they also could find some real-world applications – even if these probably will be less head-line grabbing. For example, the in initial operation of the STL needs to be triggered by a short optical pulse. This possibility to control the operation of the cloak can be used in optical communications and data processing to selectively turn the cloak on and off so that light pulses carrying data are either processed (cloak off) or left alone (cloak on). But this is certainly something left for future research. As with any new research field, we don’t really know where these cloaks will take us, and certainly this is the exciting part.

References:

1. Fridman, M., Farsi, A., Okawachi, Y., & Gaeta, A. (2012). Demonstration of temporal cloaking Nature, 481 (7379), 62-65 DOI: 10.1038/nature10695

2. Boyd, R., & Shi, Z. (2012). Optical physics: How to hide in time Nature, 481 (7379), 35-36 DOI: 10.1038/481035a

Still, there is a commonality underlying such complex systems, in many cases they are hierarchical, which means they’re made of different objects on different scales – instruments playing tunes, amino acids forming proteins and so on. As David Spivak, Markus Buehler and others from MIT have described in a recent paper, a mathematical approach, known as category theory, can be used as a versatile tool that is capable of modelling complex systems by using the underlying rules governing a structure’s components. This is a very powerful approach and there is a lot to be gained by using this mechanism in materials science, to describe biomolecules or other hierarchical materials. Moreover, their approach makes it easy to connect different complex system. To put it crudely, understanding a Beethoven symphony may also provide insights into the properties of a protein, because category theory helps us links various complex systems.

Photo by Wayne Dixon via flickr.

To understand how this works, let’s take a look at an example provided by Buehler and colleagues – spider webs. These are made of individual fibres, consisting of smaller fibrils. The fibrils are made of a nanocomposite of crystal-like structures connected by flexible links. These structures are in turn made of various amino acids.

The complex structural hierarchy of spider silk (and other systems) is of course well-known. The problem researchers face is, however, that knowing the individual components of a material doesn’t necessarily mean that the properties of  the full system are known. For example, even though the molecular composition of a protein may be known, predicting its three-dimensional shape is notoriously difficult. It is the behaviour of structural elements in the context of their use that can be so difficult to understand. And this is where category theory is useful.

Example of a structure used in category theory. It relates objects such as tennis balls to its properties and uses.

The aim is to reduce complex interactions between different objects to a network of interactions and relations based on certain rules. These are called ontology logs, or ologs. Each olog must be mathematically well-defined, expressing a clear relation between properties of a system. “Ologs offer means to reveal the origin of the described system property and connect them to previous results or other topics and fields,” write the authors (ref. 2).

So how does such an olog look in practice? Basically any structure needs to be broken down into various objects that make up a system on different levels, along with arrows describing their relation – as shown in the image here for one such relation concerning tennis balls. Ologs typically consist of many such objects linked together. An olog describing spider silk, for example would establish relations between all those objects I mentioned above, and also involve structural characteristics. It would also make quantitative comparisons, say to look at the amount of crystal-like structures in the silk protein and compare that to the number of flexible links embedded to decide whether the structure is flexible or not. In other words, a computer program would parse an olog in way that’s somewhat related to a complex flowchart, taking decisions at different nodes, which makes these really easy to implement with a computer – especially with object-oriented computer languages. The difficulty is of course to define an olog in the first place, which can quickly take quite complex forms.

Photo by Delexed via flickr.

The strength of ologs is that these could be applied to a different problem. In mathematics, the advantage of category theory is that findings in one area of mathematics can be directly applied to another if these areas are similarly structured. The same could turn out to be very important here as well. Buehler and colleagues show this by drawing an analogy between spider silk proteins and social networks. The amount of strong social connections (nanocrystals in spider silk) versus more loose ones (flexible links) provides direct clues on the strength of a social network. A computer can parse the different input parameters in the same way, so what is an olog for spider silk could also be seen as an olog for a social network.

While I don’t know about the immediate use of the conclusions that could be drawn by comparing materials with social networks or symphonies, the implications for more related scientific areas are clear – most hierarchically structured material follow similar design rules. By using ologs it might be possible to use structural concepts used in natural compounds and transfer these to artificial materials, to obtain stronger man-made fibres for example.

The problem is of course to come up with appropriate ologs, and this is the hard part. Still, we are only at the beginning to realize what could (or could not) be done with ologs, and it will be really interesting to see how this plays out.

References:

1. Spivak, D., Giesa, T., Wood, E., & Buehler, M. (2011). Category Theoretic Analysis of Hierarchical Protein Materials and Social Networks PLoS ONE, 6 (9) DOI: 10.1371/journal.pone.0023911

2. Giesa, T., Spivak, D., & Buehler, M. (2011). Reoccurring Patterns in Hierarchical Protein Materials and Music: The Power of Analogies BioNanoScience, 1 (4), 153-161 DOI: 10.1007/s12668-011-0022-5

And as we’ve discussed, the problem is basically efficiency. There have been a lot of advances in inorganics recently, with single films now easily reaching efficiencies above 20%. A thin film GaAs solar cell this year achieved a record efficiency of 28.2%! These highly efficient cells are only about 1 micrometre thick(!), which means they are also quite flexible and bendable. And what’s more, fabrication is also very cheap. To make a thin-film solar cell doesn’t even waste an expensive wafer any more, there are techniques to remove the devices from the substrate and to reuse the wafer for the fabrication of the next cell.

In contrast, organic solar cells are much less efficient, less than half what those record breakers achieve – whether it is dye-sensitized cells or polymer-based ones. In the official, verified solar cell efficiency tables (reference below), GaAs as said achieves 28.2%, silicon thin films 19.1%, silicon crystals 25%, CIGS (of Solyndra fame) 19.6%. On the other hand, dye-sensitized solar cells achieve 10.9% and organic polymers 8.3%. And if you’re wondering, the absolute record is held by the more expensive so-called inorganic multijunction cells at 43.5%, but for concentrated light, not normal light.

But such huge differences in efficiency are known. Typically, the argument made in favour of organic solar cells is cost. But is that so? As explained, the latest generation of inorganic thin-film cells are very cheap to make as well. Moreover, one of the most expensive parts of solar cells are the panels that hold the cells, as well as installation. Assuming that these costs are half of the costs of solar modules (a not unreasonable approximation), fabricating organic solar cells that even would be only 10% to 20% the cost of inorganic ones will cut the cost per panel by 40% to 45%. Yet, with efficiencies of less than half of the inorganic ones, you need twice the amount of panels, so it won’t come cheaper.

Another problem is lifetime. Solar installations on rooftops should last 25 years. That is the usual warranty solar cell producers are offering. I have doubts organic solar cells can deliver that reliably. But as a counterexample, the above-mentioned Eight19 apparently hope to produce off-grid, low-power solar installations for developing countries, which could be an interesting market as well. There, installation is cheaper and panels won’t have to last as long. But this really seems more for the lower end of the market, and not for the electrical grid.

In addition, there might also be a case for more short-lived applications. For example, the backside of a tablet computer could be powered by them. Or the roof of a car could be covered with solar cells. But as I said, those inorganic cells with record-breaking 28.2% efficiency are also bendable, and can be used for most curved surfaces as well. The only different is that organics can be stretched, which semiconductors can’t do. This means you could envision fibres and textiles that act like solar cells, although this looks like a niche application to me.

Of course, there are also positive news. Mitsubishi Chemicals has reportedly achieved 10% efficient organic solar cells already. And by using multilayers, they hope to get to 15%. But will this be enough? The inorganics, although a much more mature technology, have recently made considerable headway based on new design concepts, and will remain 2 to 3 times more efficient in the foreseeable future. Take the Solyndra example – they were caught wrong-footed when prices for silicon solar cells came down significantly. There certainly might be applications for organic solar cells such as those in developing countries. But the question is, can the organics really compete in the long-term? I’d be curious to hear your thoughts on this topic. Is it too early to jump to conclusions?

In the meantime, there is one consolation – organic light-emitting diodes are about to take the mobile computing market in a storm, those AMOLED displays in some mobile phones are simply stunning.

Reference:
Green, M., Emery, K., Hishikawa, Y., Warta, W., & Dunlop, E. (2011). Solar cell efficiency tables (Version 38) Progress in Photovoltaics: Research and Applications, 19 (5), 565-572 DOI: 10.1002/pip.1150

Permanent electric polarization. Left: the electron density of the rubidium Rydberg electron in one of the atoms of the molecule. The asymmetry is hardly visible. Right: The same electron density, but with the density of the Rydberg atom on its own subtracted. The difference clearly shows an asymmetric distribution of electrons roughly at the position of the second rubidium atom, causing an electric polarization. (c) 2011 Science Magazine

Apply an electric field to a material, and its positive and negative charges will separate, creating an electric polarization. This is the fundamental effect behind capacitors used in electronics as well as in ferroelectrics used in some computer memories. In the latter case, to achieve a permanent electric polarization, the positive and negative charges need to be shifted permanently. This is the case if in a crystal all positive and all negative ions in a crystal are shifted in the same way with respect to each other.

The separation of positive and negative ions in a crystal can lead to a permanent electric polarization. A similar effect has now been achieved with electrons.

In a paper in Science from last week, researchers from the group of Tilman Pfau at the University of Stuttgart in Germany with colleagues from a number of other institutions have now demonstrated an entirely new way of achieving permanent electric polarization – namely by using electrons and not ions. This effect is remarkable, because electrons usually are much more mobile than atoms. Normally, any charge imbalance in the electron distribution of a material or molecule is easily neutralized simply by shifting electrons in the molecule around. At the same time, looking far ahead, such electron-based effects could lead to applications where the electric polarization needs to switch ultrafast.

However, the molecule studied by the researchers is quite different to usual molecules. It is formed by two rubidium atoms, which means that normally it should not show any electric polarization, simply because both atoms in the molecule are identical and for symmetry reasons no positive or negative ions would form in the first place.

But although they are both rubidium atoms, here there is a crucial difference in the electronic states. One of the atoms is in its energetic ground state, while the other is a so-called Rydberg atom, which means that its outermost electron is excited into a very high energy state and circulates the atom’s nucleus at a large distance. Rydberg atoms are huge in comparison. Here, the rubidium atom is roughly about 50 nanometres in size, corresponding to about 1,000 times the size of a oxygen molecule - and is larger also than the transistors in modern computer chips.

Obviously, for molecules made of an atom in its ground state and one in one of these Rydberg states there is a large imbalance in the electronic states. But still, how is it possible that in such a molecule any electric polarization forms? With one electron being that far out, the interaction between the electrons of the two atoms is extremely small. The reason a polarization forms in the molecule is that the electronic state of the Rydberg atom extends so far out that it extends across the other rubidium atom. The presence of this atom alters the Rydberg atom’s electronic states at the location of the regular atom. That way, the symmetry of the electronic states of the molecule is broken,  and this imbalance is the origin of the permanent electric polarization. To detect such a small imbalance, the researchers measured the shift of the molecule’s optical resonances in an electric field, which clearly confirms the effect.

More generally, electron-based polarizations could be very interesting because the electrons of a molecule are much more mobile than atoms themselves, so that the switching of the polarization – for example turning it around by 180 degrees – should occur much faster than in conventional materials, where the polarization is based on ions. This could be interesting for applications such as ultrafast data storage, where up and down of the polarization would represent the 1 and 0 of a computer bit.

Of course, we are still far away from such dreams. The Rydberg atoms are stable only at ultralow temperatures a few millionth of a degree above absolute zero. And also, here this effect seems again down to atomic positions, even if somewhat indirectly; it is the position of the second atom that determines the electric polarization. On the other hand, there are theoretical predictions where some molecules could also show electron-based electric polarization effects at much higher temperatures. Therefore, it seems to me this study is only the beginning for electron-based polarization effects, and we should look forward to similar observations being made in other molecules, too.

Reference:
Li, W., Pohl, T., Rost, J., Rittenhouse, S., Sadeghpour, H., Nipper, J., Butscher, B., Balewski, J., Bendkowsky, V., Low, R., & Pfau, T. (2011). A Homonuclear Molecule with a Permanent Electric Dipole Moment Science, 334 (6059), 1110-1114 DOI: 10.1126/science.1211255