Electrons reveal their quantum properties when they are confined to small spaces. Scientists from TU Wien (Vienna), Aachen and Manchester have created tiny quantum dots in Graphene
In a tiny quantum prison, electrons behave quite differently as compared to their counterparts in free space. They can only occupy discrete energy levels, much like the electrons in an atom – for this reason, such electron prisons are often called “artificial atoms”. Artificial atoms may also feature properties beyond those of conventional ones, with the potential for many applications for example in quantum computing. Such additional properties have now been shown for artificial atoms in the carbon material graphene. The results have been published in the journal “Nano Letters”, the project was a collaboration of scientists from TU Wien (Vienna, Austria), RWTH Aachen (Germany) and the University of Manchester (GB).
Building Artificial Atoms
“Artificial atoms open up new, exciting possibilities, because we can directly tune their properties”, says Professor Joachim Burgdörfer (TU Wien, Vienna). In semiconductor materials such as gallium arsenide, trapping electrons in tiny confinements has already been shown to be possible. These structures are often referred to as “quantum dots”. Just like in an atom, where the electrons can only circle the nucleus on certain orbits, electrons in these quantum dots are forced into discrete quantum states.
Even more interesting possibilities are opened up by using graphene, a material consisting of a single layer of carbon atoms, which has attracted a lot of attention in the last few years. “In most materials, electrons may occupy two different quantum states at a given energy. The high symmetry of the graphene lattice allows for four different quantum states. This opens up new pathways for quantum information processing and storage” explains Florian Libisch from TU Wien. However, creating well-controlled artificial atoms in graphene turned out to be extremely challenging.
Cutting edge is not enough
There are different ways of creating artificial atoms: The simplest one is putting electrons into tiny flakes, cut out of a thin layer of the material. While this works for graphene, the symmetry of the material is broken by the edges of the flake which can never be perfectly smooth. Consequently, the special four-fold multiplicity of states in graphene is reduced to the conventional two-fold one.
Therefore, different ways had to be found: It is not necessary to use small graphene flakes to capture electrons. Using clever combinations of electrical and magnetic fields is a much better option. With the tip of a scanning tunnelling microscope, an electric field can be applied locally. That way, a tiny region is created within the graphene surface, in which low energy electrons can be trapped. At the same time, the electrons are forced into tiny circular orbits by applying a magnetic field. “If we would only use an electric field, quantum effects allow the electrons to quickly leave the trap” explains Libisch.
The artificial atoms were measured at the RWTH Aachen by Nils Freitag and Peter Nemes-Incze in the group of Professor Markus Morgenstern. Simulations and theoretical models were developed at TU Wien (Vienna) by Larisa Chizhova, Florian Libisch and Joachim Burgdörfer. The exceptionally clean graphene sample came from the team around Andre Geim and Kostya Novoselov from Manchester (GB) – these two researchers were awarded the Nobel Prize in 2010 for creating graphene sheets for the first time.
The new artificial atoms now open up new possibilities for many quantum technological experiments: “Four localized electron states with the same energy allow for switching between different quantum states to store information”, says Joachim Burgdörfer. The electrons can preserve arbitrary superpositions for a long time, ideal properties for quantum computers. In addition, the new method has the big advantage of scalability: it should be possible to fit many such artificial atoms on a small chip in order to use them for quantum information applications.
Learn more: “Artificial Atom“ Created in Graphene
Storing fluctuating and delivering stable electric power supply are central issues when using energy from solar plants or wind power stations. Here, efficient and flexible energy storage systems need to accommodate for fluctuations in energy gain. Scientists from the Leibniz Institute for Interactive Materials (DWI), RWTH Aachen University and Hanyang University in Seoul now significantly improved a key component for the development of new energy storage systems.
Redox flow batteries are considered a viable next generation technology for highly efficient energy storage. These batteries use electrolytes, chemical components in solution, to store energy. A vanadium redox flow battery, for example, uses vanadium ions dissolved in sulfuric acid. Being separated by a membrane, two energy-storing electrolytes circulate in the system. The storage capacity depends on the amount of electrolytes and can easily be increased or decreased depending on the application. To charge or discharge the battery, the vanadium ions are chemically oxidized or reduced while protons pass the separating membrane.
The membrane plays a central role in this system: On the one hand, it has to separate the electrolytes to prevent energy loss by short-circuiting. On the other hand, protons need to pass the membrane when the battery is charged or discharged. To allow efficient, commercial use of a redox flow batteries, the membrane needs to combine both these functions, which still remains a significant challenge for membrane developers so far.
The current benchmark is a Nafion membrane. This membrane is chemically stable and permeable for protons and is well known for H2 fuel cell applications. However, Nafion and similar polymers swell when exposed to water and loose their barrier function for vanadium ions. Polymer chemists try to prevent vanadium leakage by changing the molecular structure of such membranes.
The researchers from Aachen and Seoul came up with a completely different approach: “We use a hydrophobic membrane instead. This membrane keeps its barrier functions since it does not swell in water,” explains Prof. Dr.-Ing. Matthias Wessling. He is the vice scientific director at the Leibniz Institute for Interactive Materials and heads the chair of Chemical Process Engineering at RWTH Aachen University. “We were pleasantly surprised when we discovered tiny pores and channels in the hydrophobic material and they appear to be filled with water. These water channels allow protons to travel through the membrane with high speed. The vanadium ions, however, are too large to pass the membrane.” The diameter of the channels is less than two nanometers and the barrier function seems to be stable over time: Even after one week or 100 charging and discharging cycles vanadium ions could not pass the membrane. “We reached an energy efficiency of up to 99 percent, depending on the current. This shows that our membrane is a true barrier for the vanadium ions,” says Wessling. At all current densities tested, between 1 and 40 milliampere per square centimeter, the scientists reached 85 percent energy efficiency or more whereas conventional systems do not exceed 76 percent.
With over 40,000 students enrolled in 130 study programs, it is the largest technical university in Germany. The university maintains close links to industry and accounts for the highest amount of third-party funds of all German universities in both absolute and relative terms per faculty member.
National rankings regularly identify RWTH Aachen among the national top three in fields such as engineering (especially mechanical engineering and electrical engineering), computer science, physics, chemistry, and medicine. In the field of mechanical engineering, the university was ranked 17th in the world by QS World University Rankings in 2013.
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Removing tumors from within the inner ear is a very delicate matter that typically requires surgeons to remove the entire mastoid bone. However, in the future, all doctors will need to do is cut a tunnel of 5 mm in diameter through the bone using a miniature robot named NiLiBoRo.
The system is capable of adjusting its path while drilling through bone to steer around sensitive tissue such as blood vessels and nerves. Researchers will be displaying the new technology at the Compamed exhibition from November 16-19 in Düsseldorf (Hall 08a, Booth K38).
Surgery is unavoidable for treating inner ear tumors, but the inner ear is difficult to access. This is because it is covered by a cranial bone known as the mastoid, or petrosal bone. What’s more, the surrounding tissue contains lots of nerves and blood vessels. For this reason the surgeons will cut out as much of the mastoid bone as needed until they have located each one of these sensitive structures. Only then can they be sure not to damage them. What this entails most of the time is the removal of the entire bone. The hole thus created is filled in with fatty tissue taken from the abdomen after the completion of the procedure.
Such materials could seal blood vessels during surgery and re-open them subsequently.
Materials that self-assemble and self-destruct once their work is done are highly advantageous for a number of applications – as components in temporary data storage systems or for medical devices.
For example, such materials could seal blood vessels during surgery and re-open them subsequently. Dr. Andreas Walther, research group leader at DWI – Leibniz Institute for Interactive Materials in Aachen, developed an aqueous system that uses a single starting point to induce self-assembly formation, whose stability is pre-programmed with a lifetime before disassembly occurs without any additional external signal – hence presenting an artificial selfregulation mechanism in closed conditions. Their results are published as this week’s cover article in ‘Nano Letters’.
Biologically inspired principles for synthesis of complex materials are one of Andreas Walther’s key research interests. To allow the preparation of very small, elaborate objects, nanotechnology uses self-assembly. Usually, in man-made self-assemblies, molecular interactions guide tiny building blocks to aggregate into 3D architectures until equilibrium is reached. However, nature goes one step further and prevents certain processes from reaching equilibrium. Assembly competes with disassembly, and self-regulation occurs. For example, microtubules, components of the cytoskeleton, continuously grow, shrink and rearrange. Once they run out of their biological fuel, they will disassemble.
Future nanoelectronic information storage devices are also tiny batteries – astounding finding opens up new possibilities
Resistive memory cells (ReRAM) are regarded as a promising solution for future generations of computer memories. They will dramatically reduce the energy consumption of modern IT systems while significantly increasing their performance. Unlike the building blocks of conventional hard disk drives and memories, these novel memory cells are not purely passive components but must be regarded as tiny batteries. This has been demonstrated by researchers of Jülich Aachen Research Alliance (JARA), whose findings have now been published in the prestigious journal Nature Communications. The new finding radically revises the current theory and opens up possibilities for further applications. The research group has already filed a patent application for their first idea on how to improve data readout with the aid of battery voltage.
Conventional data memory works on the basis of electrons that are moved around and stored. However, even by atomic standards, electrons are extremely small. It is very difficult to control them, for example by means of relatively thick insulator walls, so that information will not be lost over time. This does not only limit storage density, it also costs a great deal of energy. For this reason, researchers are working feverishly all over the world on nanoelectronic components that make use of ions, i.e. charged atoms, for storing data. Ions are some thousands of times heavier that electrons and are therefore much easier to ‘hold down’. In this way, the individual storage elements can almost be reduced to atomic dimensions, which enormously improves the storage density.