Darwin on a chip

Researchers of the MESA+ Institute for Nanotechnology and the CTIT Institute for ICT Research at the University of Twente in The Netherlands have demonstrated working electronic circuits that have been produced in a radically new way, using methods that resemble Darwinian evolution. The size of these circuits is comparable to the size of their conventional counterparts, but they are much closer to natural networks like the human brain. The findings promise a new generation of powerful, energy-efficient electronics, and have been published in the leading British journal ("Evolution of a Designless Nanoparticle Network into Reconfigurable Boolean Logic"). Learning from Nature One of the greatest successes of the 20th century has been the development of digital computers. During the last decades these computers have become more and more powerful by integrating ever smaller components on silicon chips. However, it is becoming increasingly hard and extremely expensive to continue this miniaturisation. Current transistors consist of only a handful of atoms. It is a major challenge to produce chips in which the millions of transistors have the same characteristics, and thus to make the chips operate properly. Another drawback is that their energy consumption is reaching unacceptable levels. It is obvious that one has to look for alternative directions, and it is interesting to see what we can learn from nature. Natural evolution has led to powerful ‘computers’ like the human brain, which can solve complex problems in an energy-efficient way. Nature exploits complex networks that can execute many tasks in parallel. Moving away from designed circuits The approach of the researchers at the University of Twente is based on methods that resemble those found in Nature. They have used networks of gold nanoparticles for the execution of essential computational tasks. Contrary to conventional electronics, they have moved away from designed circuits. By using 'designless' systems, costly design mistakes are avoided. The computational power of their networks is enabled by applying artificial evolution. This evolution takes less than an hour, rather than millions of years. By applying electrical signals, one and the same network can be configured into 16 different logical gates. The evolutionary approach works around - or can even take advantage of - possible material defects that can be fatal in conventional electronics. Powerful and energy-efficient It is the first time that scientists have succeeded in this way in realizing robust electronics with dimensions that can compete with commercial technology. According to prof. Wilfred van der Wiel, the realized circuits currently still have limited computing power. “But with this research we have delivered proof of principle: demonstrated that our approach works in practice. By scaling up the system, real added value will be produced in the future. Take for example the efforts to recognize patterns, such as with face recognition. This is very difficult for a regular computer, while humans and possibly also our circuits can do this much better." Another important advantage may be that this type of circuitry uses much less energy, both in the production, and during use. The researchers anticipate a wide range of applications, for example in portable electronics and in the medical world.

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Exploring catalytic reactions at the nanoscale

The National Physical Laboratory (NPL) has used a novel imaging capability - tip-enhanced Raman spectroscopy - to map catalytic reactions at the nanoscale for the first time. Catalysts are substances that facilitate chemical reactions without being consumed, enabling industry to produce chemicals which would otherwise be uneconomic or even impossible. Catalysts are used in over 90% of industrial chemical processes, from the production of pharmaceuticals to energy generation, and are thought to contribute to over 35% of global GDP. Pressure for greener, cheaper and more sustainable chemistry in industry is driving the search for new catalysts with improved efficiency and selectivity. Rational design of catalyst materials with tailored properties relies on our ability to identify active sites at reacting surfaces in order to understand structure-performance relationships. However, conventional analytical techniques often lack the required sensitivity at the necessary length-scales for this to be achieved. Tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful and reliable technique for characterising surfaces at the nanoscale, combining the high chemical sensitivity of surface-enhanced Raman spectroscopy and nanoscale spatial resolution of scanning probe microscopy Together, these properties make TERS ideally-suited to the characterisation of catalytic reactions at the nanometre length-scale. Schematic of the TERS apparatus and the catalytic reaction studied. A team from NPL has taken the lead in using TERS to identify catalytic nanoparticles on a surface and has achieved nanoscale mapping of catalytic activity for the first time. The nanometre resolution of this reactive spectroscopic imaging, published in the Royal Society of Chemistry journal ("Nanoscale mapping of catalytic activity using tip-enhanced Raman spectroscopy"), has yet to be matched by any other analytical technique. The team's work is hoped to pave the way for the routine use of TERS to study catalytic reactions with nanoscale resolution. In the future, the spatial variations identified using this technique could provide powerful new insights into molecular adsorption and reaction dynamics at surfaces, ultimately enabling improved control and efficiency of chemical processes through informed catalyst optimisation.

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How to make large 2-D sheets

Sheets of graphene and other materials that are virtually two-dimensional hold great promise for electronic, optical, and other high-tech applications. But the biggest limitation in unleashing this potential has been figuring out how to make these materials in the form of anything larger than tiny flakes. Now researchers at MIT and elsewhere may have found a way to do so. The group has determined a way to make large sheets of one such material, called molybdenum telluride, or MoTe2. The team says their method is also likely to work for many similar 2-D materials, and could make widespread applications feasible. The findings have been published in the ("Large-Area Synthesis of High-Quality Uniform Few-Layer MoTe2") by a team including MIT postdoc Lin Zhou; professors Mildred Dresselhaus, Jing Kong, and Tomás Palacios; and eight others at MIT, the China University of Petroleum, Central South University in China, the National Tsing-hua University in Taiwan, and Saitama University and Tohoku University in Japan. Millie Dresselhaus and Lin Zhou. “This material has a similar bandgap to silicon” — a characteristic needed in order to make transistors and solar cells — “and in single-layer form it has a direct bandgap,” Zhou says, which allows better light emission. “It also has strong absorption for solar radiation,” which is key to making practical solar cells, she says. Molybdenum telluride can exist in two different forms; one is metallic, meaning it conducts electricity well, and the other is a natural semiconductor, lending itself to applications in electronics. Controlling how the material is made allows the researchers to create whichever form is needed for a particular use. The new method is based on chemical vapor deposition (CVD), and makes it possible to create sheets of any thickness, and of a size limited only by the dimensions of the CVD chamber used for deposition. One challenge the team had to overcome was that the atoms of molybdenum telluride are very weakly bound to each other, so the tendency of the two precursor materials to form molybdenum telluride is low. “This makes it more challenging to make, compared to other similar materials,” Zhou says. The researchers were able to overcome this by using several stages of deposition, beginning with a layer of pure molybdenum. “This method makes it easy, because you only need to control one material,” Zhou says. This step is followed by oxidation of that layer; this material is then removed and powdered tellurium is added, vaporized in a carrier gas of hydrogen and argon, at a temperature of 700 degrees Celsius. The use of hydrogen in the process, the team found, is crucial to producing a uniform MoTe2 film. The material should be immediately usable to create electronic devices including field-effect transistors, which the team has already demonstrated in the lab. “Our process can grow sheets that have a very large area, are very homogeneous, and have high quality,” Zhou says. The team now aims to explore adapting this process to create large sheets of other promising thin materials: “2-D materials are a big family with different properties,” Zhou says. She and her colleagues will examine whether versions of the process can work with other compounds. Molybdenum telluride also lends itself to applications in spintronics, Zhou says, an emerging technology based on the spins of electrons rather than their charge, as in conventional electronics. Physicist Ado Jorio of the Federal University of Minas Gerais, Brazil, who was not involved in this work, says, “What is most impressive is that this group has been able to consecutively develop new formulae to produce almost any low-dimensional material they want, always scalable with the highest quality worldwide.” And Vincent Meunier, a physicist at Rensselaer Polytechnic Institute who was also not associated in this research, adds, “One of the many advantages of the proposed approach stems from its simplicity. The consequences of this development are likely to be numerous, as it provides a versatile and scalable technique to develop macroscopic amounts of atomically thin films, thereby surmounting major roadblocks faced by layered-materials based research so far.”

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