System Bits: Aug. 27

A ring of 18 carbon atoms
Scientists at IBM Research – Zurich and Oxford University write about allotropes of carbon – the many versions of atomic carbon formations, such as diamonds and graphite.

“Carbon, one of the most abundant elements in the universe, can exist in different forms – called allotropes – giving it completely different properties from color to shape to hardness. For example, in a diamond every carbon atom is bonded to four neighboring carbons, whereas in graphite, every carbon atom is bonded to three neighboring carbons.

“While these are well-studied forms of carbon, there are lesser-known forms and one in particular has been elusive – cyclocarbons, where the carbon atoms have only two neighbors, arranged in the shape of a ring. Discussed for many years, their structure was unknown, and two possibilities were debated, either with all the bonds in the ring of the same length or with alternating shorter and longer bonds. Adding to the drama, evidence for their existence was published in the gas phase, but due to their high reactivity, they could not be isolated and characterized – that is until now.

“Based on our previous successes in imaging molecules with atomic force microscopy (AFM) and creating molecules by atom manipulation, scientists from the University of Oxford’s Department of Chemistry and IBM Research attempted to find the answer to this debate. Our goal was to synthesize, stabilize and characterize cyclocarbon.

“And for the first time, we have succeeded in stabilizing and imaging a ring of 18 carbon atoms.

“Our approach was to generate cyclocarbon by atom manipulation on an inert surface at low temperatures (5 K) and to investigate it with high-resolution AFM. We started the collaboration between the groups of Oxford and IBM three years ago with this goal. Initially, we focused on linear segments of two-fold coordinated carbons, exploring possible routes for creating such carbon-rich materials by atom manipulation. We triggered chemical reactions by applying voltage pulses with the tip of the atomic force microscope. We found that such segments could be formed on a copper substrate covered by a very thin layer of table salt. Because the salt layer is chemically very inert, the reactive molecules did not form covalent bonds to it.

“After the successful creation of the linear carbon segments, we attempted to create cyclocarbon on the same surface. To this end, the Oxford group synthesized a precursor to cyclo[18]carbon that is a ring of 18 carbon atoms.

“Future applications are suggested by the fact that we could fuse cyclocarbons and/or cyclic carbonoxides by atom manipulation. This possibility of forming larger carbon rich structures by fusing molecules with atom manipulation opens the way to create more sophisticated carbon-rich molecules and new carbon allotropes. Eventually, custom-made molecular structures might be used as elements for molecular electronics, based on single electron transfer.”

From Damascene swords to carbon nanotubes
The legendary Damascene sabers forged in the Middle East may have benefited from impurities in the iron ore used to make the extremely sharp swords. There are theories that these weapons might owe to what we now call carbon nanotube technology.

These thin, hollow tubes are only a single carbon atom in thickness. Like their carbon cousin, graphene – in which the atoms lie flat, in a two-dimensional sheet – they are among the strongest, most lightweight and flexible materials known.
“Fast-forward centuries,” said Stephan Hofmann from the Department of Engineering, “and we now realize there is a whole family of these extraordinary origami forms of carbon… and how to make them.” In fact, the University of Cambridge has more than 25 years’ cutting-edge experience in carbon nanotechnology, from diamond to nanotubes, and from conducting polymers to diamond-like carbon and graphene.

What makes carbon nanoforms such as graphene and CNTs so exciting is their electrical and thermal properties. Their potential use in applications such as lighter electrical wiring, thinner batteries, stronger building materials, and flexible devices could have a transformational impact on the energy, transport, and health care industries. As a result, investment totaling millions of pounds is now underpinning research and development in carbon-based research across the university.

“But all of the superlatives attributed to the materials refer to an individual, atomically perfect, nanotube or graphene flake,” Hofmann added. “The frequently pictured elephant supported by a graphene sheet epitomizes the often non-realistic expectations. The challenge remains to achieve high quality on a large scale and at low cost, and to interface and integrate the materials in devices.”

These are the types of challenges that researchers in the Departments of Engineering, Materials Science and Metallurgy, Physics and Chemistry, and the Cambridge Graphene Centre have been working toward overcoming.

Professor Alan Windle from the Department of Materials Science and Metallurgy, for example, has been using a chemical vapor deposition process to ‘spin’ very strong and tough fibers made entirely of CNTs. The nanotubes form smoke in the reactor but, because they are entangled and elastic, fibers can be wound continuously out of the reactor like nano candy floss. The yarn-like texture of the fibers gives them extraordinary toughness and resistance to cutting, making them promising alternatives to carbon fibers or high-performance polymer fibers like Kevlar, as well as for building tailored fiber-reinforced polymers used in aerospace and sports applications.

It is on the electrical front that they meet their greatest challenge, as Windle explained: “The process of manufacture is being scaled up through a Cambridge spin-out, Q-Flo; however, electrical conductivity is the next grand challenge for CNT fibers in the laboratory. To understand and develop the fiber as a replacement for copper conductors will be world-changing, with huge benefits.”

In 2013, Windle’s colleague Dr Krzysztof Koziol succeeded in making electric wiring made entirely from CNT fibers and developing an alloy that can solder carbon wires to metal, making it possible to incorporate CNT wires into conventional circuits. The team now makes wires ranging from a few micrometers to a few millimeters in diameter at a rate of up to 20 meters per minute – no small feat when you consider each CNT is ten thousand times narrower than a human hair.

With funding from the Royal Society and the European Research Council (ERC), the research is aimed at using CNTs to replace copper and aluminum in domestic electrical wiring, overhead power transmission lines and aircraft. CNTs carry more current, lose less energy in heat and do not require mineral extraction from the earth.

Moreover, they can be made from greenhouse gases; Koziol’s team is working with spin-out company FGV Cambridge Nanosystems to become the world’s first company to produce high-grade CNTs and graphene directly from natural gas or contaminated biogas. The company is already operating at an industrial scale, with high-purity graphene being produced at 1 kg per hour. “The aim is to produce high-quality materials that can be directly implemented into new devices, or used to improve other materials, like glass, metal or polymers,” said Koziol.

Working directly with industry will be key to speeding up the transition from lab to factory for new materials. Hofmann is leading a large effort to develop the manufacturing and integrated processing technology for CNTs, graphene and related nanomaterials, with funding from the ERC and Engineering and Physical Sciences Research Council (EPSRC), and in collaboration with a network of industrial partners.

“The field is at a very exciting stage,” he said, “now, not only can we ‘see’ and resolve their intricate structures, but new characterization techniques allow us to take real-time videos of how they assemble, atom by atom. We are beginning to understand what governs their growth and how they behave in industrially relevant environments. This allows us to better control their properties, alignment, location and interfaces with other materials, which is key to unlocking their commercial potential.”

For high-end applications in the electronics and photonics industry, achieving this level of control is not just desirable but a necessity. The ability to produce carbon controllably in its many structural forms widens the ‘materials portfolio’ that a modern engineer has at their disposal. With carbon films or structures already found in products such as hard drives, razor blades and lithium ion batteries, the industrial use of CNTs is becoming increasingly widespread, driven, for instance, by the demand for new technologies such as flexible devices and our need to harvest, convert and store energy more efficiently.

Electronic devices are shielded with an atomic layer
Stanford University researchers came up with a heat shield that is 10 atoms thick, providing a new way to protect electronic devices from overheating.

Excess heat given off by smartphones, laptops and other electronic devices can be annoying, but beyond that it contributes to malfunctions and, in extreme cases, can even cause lithium batteries to explode.

To guard against such ills, engineers often insert glass, plastic or even layers of air as insulation to prevent heat-generating components like microprocessors from causing damage or discomforting users.

Now, Stanford researchers have shown that a few layers of atomically thin materials, stacked like sheets of paper atop hot spots, can provide the same insulation as a sheet of glass 100 times thicker. In the near term, thinner heat shields will enable engineers to make electronic devices even more compact than those we have today, said Eric Pop, professor of electrical engineering and senior author of a paper published Aug. 16 in Science Advances.

“We’re looking at the heat in electronic devices in an entirely new way,” Pop said.

This greatly magnified image shows four layers of atomically-thin materials that form a heat-shield just two to three nanometers thick. / Image courtesy of National Institute of Standards and Technology

The heat we feel from smartphones or laptops is actually an inaudible form of high-frequency sound. If that seems crazy, consider the underlying physics. Electricity flows through wires as a stream of electrons. As these electrons move, they collide with the atoms of the materials through which they pass. With each such collision an electron causes an atom to vibrate, and the more current flows, the more collisions occur, until electrons are beating on atoms like so many hammers on so many bells – except that this cacophony of vibrations moves through the solid material at frequencies far above the threshold of hearing, generating energy that we feel as heat.

Thinking about heat as a form of sound inspired the Stanford researchers to borrow some principles from the physical world. From his days as a radio DJ at Stanford’s KZSU 90.1 FM, Pop knew that music recording studios are quiet thanks to thick glass windows that block the exterior sound. A similar principle applies to the heat shields in today’s electronics. If better insulation were their only concern, the researchers could simply borrow the music studio principle and thicken their heat barriers. But that would frustrate efforts to make electronics thinner. Their solution was to borrow a trick from homeowners, who install multi-paned windows – usually, layers of air between sheets of glass with varying thickness – to make interiors warmer and quieter.

“We adapted that idea by creating an insulator that used several layers of atomically thin materials instead of a thick mass of glass,” said postdoctoral scholar Sam Vaziri, the lead author on the paper.

Atomically thin materials are a relatively recent discovery. It was only 15 years ago that scientists were able to isolate some materials into such thin layers. The first example discovered was graphene, which is a single layer of carbon atoms and, ever since it was found, scientists have been looking for, and experimenting with, other sheet-like materials. The Stanford team used a layer of graphene and three other sheet-like materials – each three atoms thick – to create a four-layered insulator just 10 atoms deep. Despite its thinness, the insulator is effective because the atomic heat vibrations are dampened and lose much of their energy as they pass through each layer.

To make nanoscale heat shields practical, the researchers will have to find some mass production technique to spray or otherwise deposit atom-thin layers of materials onto electronic components during manufacturing. But behind the immediate goal of developing thinner insulators looms a larger ambition: Scientists hope to one day control the vibrational energy inside materials the way they now control electricity and light. As they come to understand the heat in solid objects as a form of sound, a new field of phononics is emerging, a name taken from the Greek root word behind telephone, phonograph and phonetics.

“As engineers, we know quite a lot about how to control electricity, and we’re getting better with light, but we’re just starting to understand how to manipulate the high-frequency sound that manifests itself as heat at the atomic scale,” Pop said.