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October 4, 2010

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Redoubled efforts in solar cells

Solar energy is obviously one of the key renewable energy resources available to us. At the same time researchers are hitting against a glass ceiling. A famous 1961 paper by William Shockley (who co-invented the transistor) and Hans Queisser comes to the conclusion that for a semiconductor such as silicon the maximum conversion efficiency of solar energy into electricity will never be more than about 30%.

Dye-sensitized solar cells. A design similar to these solar cells is now used to demonstrate the creation and extraction of multiple charge carriers per photons.

One reason for this limit is that each light particle only excites one electron. Even if the electron has enough energy to excite two electrons, all this energy is lost and only one electron is excited. And this is the case for pretty much all present commercial solar cell technology. Fortunately, however, there are possible exceptions. Bruce Parkinson and colleagues from the University of Wyoming in the USA have now built a photovoltaic cell that at certain wavelengths of light can generate more than one electron per photon of light. Their approach promises to beat the Shockley-Queisser limit and could lead to solar cells with considerably enhanced efficiency.

In silicon and other semiconductors, if a photon excites an electron all excess energy is predominantly lost as heat. Of course, there are attempts to harvest the heat generated in solar cells, and such approaches could beat the Shockley-Queisser limit. And so could nanostructured materials that use for example plasmonic effects. But a more direct solution would be if the excess energy could be used to excite more than one electron in the first place.

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September 27, 2010

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How to put the quantum into silicon computers

Quantum computers are so highly sought after because they can solve complex mathematical problems and parallel computer operations such as code breaking really fast. Attempts to build quantum computers come in many flavours and use different kinds of quantum states, ranging from trapped atoms, superconductors to semiconductors such as gallium arsenide or diamond. The approach with diamond is particularly promising as it uses the spin of an electron to perform quantum computing operations, which means that such devices could be nicely tied in with conventional electronics and its control of electron transport itself.

Reading single spins. The information of a single spin in a reservoir can be read by a silicon single electron transistor island. Reprinted by permission from Macmillan Publishers Ltd. Nature (2010). doi:10.1038/nature09392

Now,  Andrea Morello, Andrew Dzurak and colleagues from the University of New South Wales in Sydney in collaboration with researchers from Melbourne and Aalto University in Finland have achieved a major step towards such quantum computers using the most straightforward electronic material available: silicon. In a paper in Nature they demonstrate the reading of a single electron’s spin with a silicon electronic circuit. “Until this experiment, no-one had actually measured the spin of a single electron in silicon in a single-shot experiment,” says Morello. Ronald Hanson from the TU Delft, who works on the competing approach using impurities within diamond, agrees: “this is a result that the silicon quantum computing community has been waiting for for a long time.”

The principle of how the electron spin is read with a silicon circuit is relatively straightforward. It uses a charge reservoir, where the electrons are stored in phosphorus impurities that are implanted into the silicon. Next to the reservoir is a single electron transistor, which is a tiny silicon transistor that can detect the presence of a single electron.

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September 24, 2010

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Relaxing times for scanning tunnelling microscopes

Scanning tunnelling microscopes (STM) are a wonderful instruments that can not only image but also manipulate individual atoms on a surface. Developed by Gerd Binning and Heinrich Rohrer in 1981, STM and their derivatives revolutionized our understanding of what happens on the surface of materials.

The famous quantum corral, fabricated and imaged in 1993 by Don Eigler at IBM. Atoms (blue) are placed on a circle and consequently modify the electron density on the surface (red ripples). (c) IBM

Expanding on Binning and Rohrer’s work at IBM in Zurich, Don Eigler at IBM’s Almaden lab in particular pioneered the modification of individual atoms on a surface. In 1989 he famously accomplished moving atoms on a surface to form the letters IBM. For these accomplishments Eigler received this year’s Kavli Prize in Nanoscience.

IBM continues to pioneer STM research at Almaden. The group of Andreas Heinrich, for example, who started at IBM in 1998 as a postdoc with Don Eigler, is working on the study of magnetic atoms. He and Eigler have now accomplished another major advance in microscopy technology. So far, STM measurements were mainly static and little information of dynamic properties was available. In a study published in Science, they now have developed a method that is able to measure how long it takes for atoms to flip their electron spin. This technique opens the door to measurements of the dynamics of a number of properties, for example vibrational states.

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