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The first fully functional microprocessor logic devices based on few-atom-thick layered materials have been demonstrated by researchers from the Graphene Flagship, working at TU Vienna in Austria. The processor chip consists of 115 integrated transistors and is a first step toward ultra-thin, flexible logic devices. Using transistors made from layers of molybdenum disulphide (MoS2), the microprocessors are capable of 1-bit logic operations and the design is scalable to multi-bit operations.

With the drive towards smart objects and the Internet of Things, the microprocessors hold promise for integrating computational power into everyday objects and surfaces. The research is published this week in Nature Communications.

The Graphene Flagship is developing novel technologies based on graphene and related materials (GRMs) such as transition metal dichalcogenides (TMDs) like MoS2, semiconductor materials that can be separated into ultra-thin sheets just a few atoms thick. GRMs are promising for compact and flexible electronic devices due to their thinness and excellent electrical properties.

The ultra-thin MoS2 transistors are inherently flexible and compact, so this result could be directly translated into microprocessors for fully flexible electronic devices, for example, wearable phones or computers, or for wider use in the Internet of Things. The MoS2 transistors are highly responsive, and could enable low-powered computers to be integrated into everyday objects without adding bulk. “In principle, it’s an advantage to have a thin material for a transistor. The thinner the material, the better the electrostatic control of the transistor channel, and the smaller the power consumption,” said Thomas Mueller (TU Vienna), who led the work.

Mueller added “In general, being a flexible material there are new opportunities for novel applications. One could combine these processor circuits with light emitters that could also be made with MoS2 to make flexible displays and e-paper, or integrate them for logic circuits in smart sensors. Our goal is to realise significantly larger circuits that can do much more in terms of useful operations. We want to make a full 8-bit design – or even more bits – on a single chip with smaller feature sizes.”

Talking about increasing the computing power, Stefan Wachter (TU Vienna), first author of the work, said “Adding additional bits of course makes everything much more complicated. For example, adding just one bit will roughly double the complexity of the circuit.”

Compared to modern processors, which can have billions of transistors in a single chip, the 115-transistor devices are very simple. However, it is a very early stage for a new technology, and the team have concrete plans for the next steps: “Our approach is to improve the processing to a point where we can reliably make chips with a few tens of thousands of transistors. For example, growing directly onto the chip would avoid the transfer process, which would give higher yield so that we can go to more complex circuits,” said Dmitry Polyushkin (TU Vienna), an author of the work.

Semiconductors are used for myriad optoelectronic devices. However, as devices get smaller and smaller and more demanding, new materials are needed to ensure that devices work with greater efficiency. Now, researchers at the USC Viterbi School of Engineering have pioneered a new class of semiconductor materials that might enhance the functionality of optoelectronic devices and solar panels–perhaps even using one hundred times less material than the commonly used silicon.

Researchers at USC Viterbi, led by Jayakanth Ravichandran, an assistant professor in the Mork Family Department of Chemical Engineering and Material Sciences and including Shanyuan Niu, Huaixun Huyan, Yang Liu, Matthew Yeung, Kevin Ye, Louis Blankemeier, Thomas Orvis, Debarghya Sarkar, Assistant Professor of Electrical Engineering Rehan Kapadia, and David J. Singh, a professor of physics from University of Missouri, have developed a new class of materials that are superior in performance and have reduced toxicity. Their process, documented in “Bandgap Control via Structural and Chemical Tuning of Transition Metal Perovskite Chalcogenide,” is published in Advanced Materials.

Ravichandran, the lead on this research, is a materials scientist, who has always been interested in understanding the flow of electrons and heat through materials, as well as the how electrons interact within materials. This deep knowledge of how material composition affects electron movement was critical to Ravichandran’s and his colleagues’ most recent innovation.

Computers and electronics have been getting better, but according to Jayakanth Ravichandran, the principal investigator of this study, “the performance of the most basic device–the transistors –are not getting better.” There is a plateau in terms of performance, as noted by what is considered the “end of Moore’s law.” Similar to electronics, there is a lot of interest to develop high performance semiconductors for opto-electronics. The collaborative team of material scientists and electrical engineers wanted to develop new materials which could showcase the ideal optical and electrical properties for a variety of applications such as displays, light detectors and emitters, as well as solar cells.

The researchers developed a class of semiconductors called “transition metal perovskite chalcogenides.” Currently, the most useful semiconductors don’t hold enough carriers for a given volume of material (a property which is referred to as “density of states”) but they transport electrons fast and thus are known to have high mobility. The real challenge for scientists has been to increase this density of states in materials, while maintaining high mobility. The proposed material is predicted to possess these conflicting properties.

As a first step to show its potential applications, the researchers studied its ability absorb and emit light. “There is a saying,” says Ravichandran of the dialogue among those in the optics and photonics fields, “that a very good LED is also a very good solar cell.” Since the materials Ravichandran and his colleagues developed absorb and emit light effectively, solar cells are a possible application.

Solar cells absorb light and convert it into electricity. However, solar panels are made of silicon, which comes from sand via a highly energy intensive extraction process. If solar cells could be made of a new, alternative semiconductor material such as the one created by the USC Viterbi researchers– a material that could fit more electrons for a given volume (and reducing the thickness of the panels), solar cells could be more efficient–perhaps using one hundred times less material to generate the same amount of energy. This new material, if applied in the solar energy industry, could make solar energy less expensive.

While it is a long road to bring such a class of materials to market, the next step is to recreate this material in an ultra-thin film form to make solar cells and test their performance. “The key contribution of this work,” says Ravichandran, “is our new synthesis method, which is a drastic improvement from earlier studies. Also, our demonstration of wide tunability in optical properties (especially band gap) is promising for developing new optoelectronic devices with tunable optical properties.”

Graphene Flagship researchers from AMBER at Trinity College Dublin have fabricated printed transistors consisting entirely of layered materials. Published today in the leading journal Science, the team’s findings have the potential to cheaply print a range of electronic devices from solar cells to LEDs with applications from interactive smart food and drug labels to next-generation banknote security and e-passports.

Led by Professor Jonathan Coleman from AMBER (the Science Foundation Ireland-funded materials science research centre hosted in Trinity College Dublin), in collaboration with the groups of Professor Georg Duesberg (AMBER) and Professor Laurens Siebbeles (TU Delft, Netherlands), the team used standard printing techniques to combine graphene flakes as the electrodes with other layered materials, tungsten diselenide and boron nitride as the channel and separator (two important parts of a transistor) to form an all-printed, all-layered materials, working transistor.

All of these are flakes are a few nanometres thick but hundreds of nanometres wide. Critically, it is the ability of flakes made from different layered materials to have electronic properties that can be conducting (in the case of graphene), insulating (boron nitride) or semiconducting (tungsten diselenide) that enable them to create the building blocks of electronics. While the performance of these printed layered devices cannot yet compare with advanced transistors, the team believe there is a wide scope to improve the performance of their printed TFTs beyond the current state-of-the-art.

Professor Coleman, who is an investigator in AMBER and Trinity’s School of Physics, said, “In the future, printed devices will be incorporated into even the most mundane objects such as labels, posters and packaging. Printed electronic circuitry will allow consumer products to gather, process, display and transmit information: for example, milk cartons will send messages to your phone warning that the milk is about to go out-of-date. We believe that layered materials can compete with the materials currently used for printed electronics.”

All of the layered materials were printed from inks created using the liquid exfoliation method previously developed by Professor Coleman and already licensed. Using liquid processing techniques to create the layered materials inks is especially advantageous in that it yields large quantities of high quality layered materials which helps to enable the potential to print circuitry at low cost.

Carbon nanotubes can be used to make very small electronic devices, but they are difficult to handle. University of Groningen scientists, together with colleagues from the University of Wuppertal and IBM Zurich, have developed a method to select semiconducting nanotubes from a solution and make them self-assemble on a circuit of gold electrodes. The results were published in the journal Advanced Materials on 5 April.

The results look deceptively simple: a self-assembled transistor with nearly 100 percent purity and very high electron mobility. But it took ten years to get there. University of Groningen Professor of Photophysics and Optoelectronics Maria Antonietta Loi designed polymers which wrap themselves around specific carbon nanotubes in a solution of mixed tubes. Thiol side chains on the polymer bind the tubes to the gold electrodes, creating the resultant transistor.

This is an artist's impression of carbon nanotubes wrapped in polymers with thiol side chains (yellow spheres) and assembled on gold electrodes. Credit: Arjen Kamp

This is an artist’s impression of carbon nanotubes wrapped in polymers with thiol side chains (yellow spheres) and assembled on gold electrodes. Credit: Arjen Kamp

Patent

‘In our previous work, we learned a lot about how polymers attach to specific carbon nanotubes’, Loi explains. These nanotubes can be depicted as a rolled sheet of graphene, the two-dimensional form of carbon. ‘Depending on the way the sheets are rolled up, they have properties ranging from semiconductor to semi-metallic to metallic.’ Only the semiconductor tubes can be used to fabricate transistors, but the production process always results in a mixture.

‘We had the idea of using polymers with thiol side chains some time ago’, says Loi. The idea was that as sulphur binds to metals, it will direct polymer-wrapped nanotubes towards gold electrodes. While Loi was working on the problem, IBM even patented the concept. ‘But there was a big problem in the IBM work: the polymers with thiols also attached to metallic nanotubes and included them in the transistors, which ruined them.’

Solution

Loi’s solution was to reduce the thiol content of the polymers, with the assistance of polymer chemists from the University of Wuppertal. ‘What we have now shown is that this concept of bottom-up assembly works: by using polymers with a low concentration of thiols, we can selectively bring semiconducting nanotubes from a solution onto a circuit.’ The sulphur-gold bond is strong, so the nanotubes are firmly fixed: enough even to stay there after sonication of the transistor in organic solvents.

The production process is simple: metallic patterns are deposited on a carrier , which is then dipped into a solution of carbon nanotubes. The electrodes are spaced to achieve proper alignment: ‘The tubes are some 500 nanometres long, and we placed the electrodes for the transistors at intervals of 300 nanometres. The next transistor is over 500 nanometres away.’ The spacing limits the density of the transistors, but Loi is confident that this could be increased with clever engineering.

‘Over the last years, we have created a library of polymers that select semiconducting nanotubes and developed a better understanding of how the structure and composition of the polymers influences which carbon nanotubes they select’, says Loi. The result is a cheap and scalable production method for nanotube electronics. So what is the future for this technology? Loi: ‘It is difficult to predict whether the industry will develop this idea, but we are working on improvements, and this will eventually bring the idea closer to the market.’

It would be difficult to overestimate the importance of silicon when it comes to computing, solar energy, and other technological applications. (Not to mention the fact that it makes up an awful lot of the Earth’s crust.) Yet there is still so much to learn about how to harness the capabilities of element number fourteen.

The most-common form of silicon crystallizes in the same structure as diamond. But other forms can be created using different processing techniques. New work led by Carnegie’s Tim Strobel and published in Physical Review Letters shows that one form of silicon, called Si-III (or sometimes BC8), which is synthesized using a high-pressure process, is what’s called a narrow band gap semiconductor.

What does this mean and why does it matter?

Metals are compounds that are capable of conducting the flow of electrons that makes up an electric current, and insulators are compounds that conduct no current at all. Semiconductors, which are used extensively in electronic circuitry, can have their electrical conductivity turned on and off–an obviously useful capability. This ability to switch conductivity is possible because some of their electrons can move from lower-energy insulating states to higher-energy conducting states when subjected to an input of energy. The energy required to initiate this leap is called a band gap.

The diamond-like form of silicon is a semiconductor and other known forms are metals, but the true properties of Si-III remained unknown until now. Previous experimental and theoretical research suggested that Si-III was a poorly conducting metal without a band gap, but no research team had been able to produce a pure and large enough sample to be sure.

By synthesizing pure, bulk samples of Si-III, Strobel and his team were able to determine that Si-III is actually a semiconductor with an extremely narrow band gap, narrower than the band gap of diamond-like silicon crystals, which is the most-commonly utilized kind. This means that Si-III could have uses beyond the already full slate of applications for which silicon is currently used. With the availability of pure samples, the team was able to fully characterize the electronic, optical, and thermal transport properties of Si-III for the first time.

“Historically, the correct recognition of germanium as a semiconductor instead of the metal it was once widely believed to be truly helped to start the modern semiconductor era; similarly, the discovery of semiconducting properties of Si-III might lead to unpredictable technological advancement,” remarked lead author, Carnegie’s Haidong Zhang. “For example, the optical properties of Si-III in the infrared region are particularly interesting for future plasmonic applications.”

Hydrogen is both the simplest and the most-abundant element in the universe, so studying it can teach scientists about the essence of matter. And yet there are still many hydrogen secrets to unlock, including how best to force it into a superconductive, metallic state with no electrical resistance.

“Although theoretically ideal for energy transfer or storage, metallic hydrogen is extremely challenging to produce experimentally,” said Ho-kwang “Dave” Mao, who led a team of physicists in researching the effect of the noble gas argon on pressurized hydrogen.

It has long been proposed that introducing impurities into a sample of molecular hydrogen, H2, could help ease the transition to a metallic state. So Mao and his team set out to study the intermolecular interactions of hydrogen that’s weakly-bound, or “doped,” with argon, Ar(H2)2, under extreme pressures. The idea is that the impurity could change the nature of the bonds between the hydrogen molecules, reducing the pressure necessary to induce the nonmetal-to-metal transition. Previous research had indicated that Ar(H2)2 might be a good candidate.

Surprisingly, they discovered that the addition of argon did not facilitate the molecular changes needed to initiate a metallic state in hydrogen. Their findings are published by the Proceedings of the National Academy of Sciences.

The team brought the argon-doped hydrogen up to 3.5 million times normal atmospheric pressure–or 358 gigapascals–inside a diamond anvil cell and observed its structural changes using advanced spectroscopic tools.

What they found was that hydrogen stayed in its molecular form even up to the highest pressures, indicating that argon is not the facilitator many had hoped it would be.

“Counter to predictions, the addition of argon did not create a kind of ‘chemical pressure’ on the hydrogen, pushing its molecules closer together. Rather, it had the opposite effect,” said lead author Cheng Ji.

Transition metal oxides (TMO) are extensively studied, technologically important materials, due to their complex electronic interactions, resulting in a large variety of collective phenomena. Memory effects in TMO’s have garnered a huge amount of interest, being both of fundamental scientific interest and technological significance.

Dr. Amos Sharoni of Bar-Ilan University’s Department of Physics, and Institute of Nanotechnology and Advanced Materials (BINA), has now uncovered a new kind of memory effect, unrelated to memory effects previously reported.

Dr. Sharoni, together with his student Naor Vardi, and supported by theoretical modelling by Yonatan Dubi of Ben-Gurion University in the Negev, utilized a simple experimental design to study changes in the properties of two TMOs, VO2 and NdNiO3, which undergo a metal-insulator phase-transition. Their results, just published in the journal Advanced Materials, not only demonstrate a new phenomenon but, importantly, also provide an explanation of its origin.

Ramp reversal memory

Metal-insulator transitions are transitions from a metal (material with good electrical conductivity of electric charges) to an insulator (material where conductivity of charges is quickly suppressed). These transitions can be achieved by a small variation of external parameters such as pressure or temperature.

In Sharoni’s experiment, when heated the studied TMOs transit from one state to another, and their properties undergo a change, beginning in a small area where “islands” develop and then grow, and vice-versa during cooling, similar to the coexistence of ice and water during melting. Sharoni cooled his samples while transition was in process, and then examined what happened when they were reheated. He found that when the reheated metal-oxide reached the temperature point at which re-cooling had occurred, that is, in the phase coexistence state – an increase in resistance was measured. And this increase in resistance was observed at each different point at which cooling was initiated. This previously unknown and surprising phenomenon demonstrates the creation of a “memory”.

Sharoni explains: “When the temperature ramp is reversed, and the sample is cooled rather than heated, the direction change creates a “scar” wherever there is a phase-boundary between the conducting and insulating islands. The ramp reversal sequence “encrypts” in the TMO a “memory” of the reversal temperature, which is manifested as increased resistance”. Moreover, it is possible to create and store more than one “memory” in the same physical space.

Sharoni likens the creation of a “scar” to the motion of waves on the seashore. A wave rushes up the beach and as it recedes it leaves a small sandy mound at the furthest point that it reached. When the wave returns it slows and brakes as it reaches the mound obstacle in its path. However, if a strong wave follows, it rushes over the mound and destroys it. Similarly, Sharoni found that further heating the TMO enables it to complete transition and to cross the scarred boundaries, “healing” the scars and immediately erasing the memory. In contrast cooling does not erase them.

Technology and security

The results of Sharoni’s work will have important impact on additional research, both experimental and theoretical, and the simplicity of the experimental design will enable other groups studying relevant systems to perform similar measurements with ease.

The multi-state nature of the memory effect, whereby more than one piece of information can coexist in the same space, could be harnessed for memory technology. And while deleted computer data is not secure and can be recovered, at least partially, by talented hackers, the “erase-upon-reading” property of this system could make an invaluable contribution to security technologies.

 Imec, the research and innovation hub in nano-electronics and digital technology, announces that Jan Genoe, one of its distinguished scientists, has been awarded an ERC Advanced Grant. With the grant of 2.5 million euros for a five-year period, Genoe’s team will develop and integrate the breakthrough technology needed to prove the possibility of high-quality video-rate holographic projection. ERC Advanced Grants are awarded by the European Research Council to allow outstanding scientists to pursue ground-breaking, high-risk projects.

Today, despite many efforts by researchers worldwide, there are no holographic projectors that allow video-rate electronically controlled projection of complex holograms. Optically rewriteable holograms exist, but they are too slow; acoustically-formed holograms can be switched fast but the image complexity is very limited. With a breakthrough combination of smart electronics, optics and materials, imec’s Jan Genoe aims to clear the roadblocks and enable next-generation video holography.

Jan Genoe: “At imec, we have most of the underlying technologies and expertise that are needed to advance holography. Advanced CMOS technologies enable to write huge hologram patterns at data rates beyond 10 Gbit/s, we can design a front end that can control charges and voltage patterns at sub-wavelength resolution. Moreover,  we can grow the necessary waveguides, couple laser light into them, and integrate transparent semiconducting oxides to bring charges close to a waveguide. This grant offers us the opportunity to merge all the necessary technology to make this giant leap in holography.”

The ERC Advanced Grants are earmarked for scientists who are leaders in their field of research with at least a decade of significant achievements. Imec’s CTO Jo De Boeck comments “Adding to the other ERC grants that our researchers already received, this one again proves that we are investing in long-term, high-quality research needed to solve this generation’s R&D challenges. This radical combination of innovation in architecture, materials and driving schemes will be the driver for many future innovations and applications in domains such as augmented reality, automotive, optical metrology, mobile communication, education, or safety, innovations with a high economic and social impact.”

Jan Genoe is a Distinguished Member of Technical Staff of imec’s Large Area Electronics (LAE) department and part-time professor at KU Leuven (ESAT, Technology Campus Diepenbeek). He received an M.S. degree in Electrical Engineering and a Ph.D. from KU Leuven in 1988 and 1994 respectively. Before joining imec, Jan Genoe worked at the High Magnetic Field Laboratory in Grenoble (France) as a Human Capital and Mobility Fellow of the European Community. His current research interests are with designing circuits with organic and oxide transistors, but also with organic photovoltaics and piezo-electric devices. Jan Genoe is the author and co-author of more than 150 papers in refereed journals. He is reviewer for a broad range of journals and is member of the Technology Directions international program committee of the ISSCC.

For several years, a team of researchers at The University of Texas at Dallas has investigated various materials in search of those whose electrical properties might make them suitable for small, energy-efficient transistors to power next-generation electronic devices.

They recently found one such material, but it was nothing anyone expected.

In an article published online March 10 in the journal Advanced Materials, Dr. Moon Kim and his colleagues describe a material that, when heated to about 450 degrees Celsius, transforms from an atomically thin, two-dimensional sheet into an array of one-dimensional nanowires, each just a few atoms wide.

An image caught in mid-transformation looks like a tiny United States flag, and with false colors added, is arguably the world’s smallest image of Old Glory, Kim said.

This tiny US flag -- just a few nanometers wide and invisible to the naked eye -- is arguably the world's smallest image of Old Glory, according to its creators at the University of Texas at Dallas. In an experiment, the nanoflag pattern emerged unexpectedly as sheets of the "stripe" material -- molybdenum ditelluride -- were heated to about 450 degrees Celsius, at which point its atoms began to rearrange and form new structures -- the 'stars' in this false-color image. Each star consists of six central atoms of molybdenum surrounded by six atoms of tellurium. Stacked on top of one another, the stars form nanowires that might power advanced electronics. The transformation from stripes to stars is reported in the journal Advanced Materials. Credit: University of Texas at Dallas

This tiny US flag — just a few nanometers wide and invisible to the naked eye — is arguably the world’s smallest image of Old Glory, according to its creators at the University of Texas at Dallas. In an experiment, the nanoflag pattern emerged unexpectedly as sheets of the “stripe” material — molybdenum ditelluride — were heated to about 450 degrees Celsius, at which point its atoms began to rearrange and form new structures — the ‘stars’ in this false-color image. Each star consists of six central atoms of molybdenum surrounded by six atoms of tellurium. Stacked on top of one another, the stars form nanowires that might power advanced electronics. The transformation from stripes to stars is reported in the journal Advanced Materials. Credit: University of Texas at Dallas

“The phase transition we observed, this new structure, was not predicted by theory,” said Kim, the Louis Beecherl Jr. Distinguished Professor of materials science and engineering at UT Dallas.

Because the nanowires are semiconductors, they might be used as switching devices, just as silicon is used in today’s transistors to turn electric current on and off in electronic devices.

“These nanowires are about 10 times smaller than the smallest silicon wires, and, if used in future technology, would result in powerful energy-efficient devices,” Kim said. The lead authors of the study are Hui Zhu and Qingxiao Wang, graduate students in materials science and engineering in the Erik Jonsson School of Engineering and Computer Science.

Just a Phase?

When certain materials are subjected to changes in external conditions, such as temperature or pressure, they can undergo a phase transition. A familiar example is when liquid water is cooled to form a solid (ice), or heated to form a gas (steam).

For many materials, however, a phase transition means something a little different. As external temperature and pressure change, these materials’ atoms rearrange and redistribute to make a material with a different structure and composition. These changes can affect the new material’s properties, such as how electrons move through it. For scientists interested in new applications for materials, understanding such transitions is paramount.

In most cases, a type of graphic called a phase diagram helps researchers predict structural and property changes in a material when it undergoes a phase transition.

But nothing predicted what Kim’s team observed as it conducted experiments on a material called molybdenum ditelluride.

Nanoflags and Nanoflowers

Using a transmission electron microscope, the researchers started with atomically thin, two-dimensional sheets of molybdenum ditelluride, a material made up of one layer of molybdenum atoms and two layers of tellurium atoms. The material belongs to a class called transition metal dichalcogenides (TMDs), which show promise in replacing silicon in transistors.

“We wanted to understand the thermal stability of this particular material,” Kim said. “We thought it was a good candidate for next-generation nanoelectronics. Out of curiosity, we set out to see whether it would be stable above room temperature.”

When they increased the temperature to above 450 degrees Celsius, two things happened.

“First, we saw a new pattern begin to emerge that was aesthetically pleasing to the eye,” Kim said. Across the surface of the sample, the repeating rows, or stripes, of molybdenum ditelluride layers began to transform into shapes that looked like tiny six-pointed stars, or flowers with six petals.

The material was transitioning into hexa-molybdenum hexa-telluride, a one-dimensional wire-like structure. The cross section of the new material is a structure consisting of six central atoms of molybdenum surrounded by six atoms of tellurium.

As the phase transition progressed, part of the sample was still “stripes” and part had become “stars.” The team thought the pattern looked like a United States flag. They made a false-color version with a blue field behind the stars and half of the stripes colored red, to make a “nanoflag.”

Not in the textbooks

“Then, when we examined the material more closely, we found that the transition we were seeing from ‘stripes’ to ‘stars’ was not in any of the phase diagrams,” Kim said. “Normally, when you heat up particular materials, you expect to see a different kind of material emerge as predicted by a phase diagram. But in this case, something unusual happened — it formed a whole new phase.”

Each individual nanowire is a semiconductor, which means that electric current moving through the wire can be switched on and off, Kim said. When many of the individual nanowires are grouped together in bulk they behave more like a metal, which easily conducts current.

“We would want to use the nanowires one at a time because we are pushing the size of a transistor as small as possible,” Kim said. “Currently, the smallest transistor size is about 10 times larger than our nanowire. Each of ours is smaller than 1 nanometer in diameter, which is essentially an atomic-scale wire.

“Before we can put this discovery to use and make an actual device, we have many more studies to do, including determining how to separate out the individual nanowires, and overcoming technical challenges to manufacturing and mass production,” Kim said. “But this is a start.”

In electronics, the race for smaller is huge.

Physicists at the University of Cincinnati are working to harness the power of nanowires, microscopic wires that have the potential to improve solar cells or revolutionize fiber optics.

University of Cincinnati physicist Hans-Peter Wagner is exploring nanowire semiconductors to harness the power of light at the nano level. Credit: Andrew Higley/UC Creative Services

University of Cincinnati physicist Hans-Peter Wagner is exploring nanowire semiconductors to harness the power of light at the nano level. Credit: Andrew Higley/UC Creative Services

Nanotechnology has the potential to solve the bottleneck that occurs in storing or retrieving digital data – or could store data in a completely new way. UC professors and their graduate students presented their research at the March 13 conference of the American Physical Society in New Orleans, Louisiana.

Hans-Peter Wagner, associate professor of physics, and doctoral student Fatemesadat Mohammadi are looking at ways to transmit data with the speed of fiber optics but at a significantly smaller scale.

Wagner and lead author Mohammadi are studying this field, called plasmonics, with researchers from three other universities. For the novel experiment, they built nanowire semiconductors with organic material, fired laser pulses at the sample and measured the way light traveled across the metal; technically, the excitations of plasmon waves.

“So, if we succeed in getting a better understanding about the coupling between the excitations in semiconductor nanowires and metal films, it could open up a lot of new perspectives,” Wagner said.

The successful harnessing of this phenomenon — called plasmon waveguiding — could allow researchers to transmit data with light at the nano level.

Universities around the world are studying nanowires, which have ubiquitous applications from biomedical sensors to light-emitting diodes or LEDs. Four UC papers on the topic are among more than 150 others by nanowire researchers around the world to be presented at the March conference.

“You’re trying to optimize the physical structure on something approaching the atomic scale. You can make very high efficiency devices like lasers,” said Leigh Smith, head of UC’s Department of Physics. Smith and UC Physics Professor Howard Jackson also presented papers on nanowires at the conference. Virtually everyone benefits from this line of research, even if the quantum mechanics underlying the latest biosensors exceed a casual understanding. For example, home pregnancy tests use gold nanoparticles – the indicator that turns color. People use technologies all the time that they don’t understand,” Smith said.

Gordon Moore, co-founder of Intel Corp., observed that the number of transistors used in a microchip has roughly doubled every two years since the 1970s. This phenomenon, now called Moore’s Law suggests that computer processing power improves at a predictable rate.

Some computer scientists predicted the demise of Moore’s Law was inevitable with the advent of microprocessors. But nanotechnology is extending that concept’s lifespan, said Brian Markwalter, senior vice president of research and technology for the Consumer Technology Association. His trade group represents 2,200 members in the $287 billion U.S. tech industry.

“It’s not a race to be small just to be the smallest. There’s a progression of being able to do more on smaller chips. The effect for consumers is that every year they get better and better products for the same price or less,” he said.

Nanotechnology is opening a universe of new possibilities, Markwalter said.

“It’s almost magical. They get better, faster, cheaper and use less power,” he said.

Markwalter said UC professor Wagner’s research is exciting because it shows promise in using optical switches to address a bottleneck in data transmission that occurs whenever you try to store or remove data.

“It’s really a breakthrough area to merge the semiconductor world and the optical world,” Markwalter said. “[Wagner’s] working at the intersection of fiber optics and photonics.”

But even nanotechnology has its limits, Smith said.

“We’re running toward the limits of what’s physically possible with present technologies,” Smith said. “The challenges are pretty immense. In 10 or 20 years there has to be a fundamental paradigm shift in how we make structures. If we don’t we’ll be caught at the same place we are now.”

How one UC experiment works:

UC graduate student Fatemesadat Mohammadi and Associate Physics Professor Hans-Peter Wagner fire laser pulses at semiconductor nanowires to excite electrons (called excitons) that potentially serve as an energy pump to guide plasmon waves over a coated metal film just a few nanometers thick without losing power, a nettlesome physical property called resistivity

They measure the resulting luminescence of the nanowire to observe how light couples to the metal film. By sending light over a metal film, a process called plasmonic waveguiding, researchers one day could transmit data with light at the nano level.

“The luminescence is our interest. So we coat them and see: How does the photoluminescence characteristic change?” Mohammadi said.

To make the semiconductor, they use a technique called high-vacuum organic molecular beam deposition (pictured above) to spread organic and metal layers on gallium-nitride nanorods.

The use of organic film is unique to the UC experiment, Wagner said. The film works as a spacer to control the energy flow between excitons in the nanowire and the oscillation of metal electrons called plasmons.

The organic material has the added benefit of also containing excitons that, arranged properly, could support the energy flow in a semiconductor, he said.

Coating the nanorods with gold significantly shortens the lifetime of the exciton emission resulting in what’s called a quenched photoluminescence. But by using organic spacers between the nanorod and the gold film, the researchers are able to extend the emission lifetime to nearly the equivalent of nanorods without a coating.

Once the gold-coated sample is prepared, they take it to an adjacent lab room and subject it to pulses of laser light.

Mohammadi said it took days of painstaking work to arrange the small city of mirrors and beam splitters bolted at precise angles to a workbench for the experiment (pictured above left).

The reactions in the nanowire take just 10 picoseconds (which is a trillionth of a second.) And the laser pulses are faster still — 20 femtoseconds (a figure that has 15 zeros following it or a quadrillionth of a second.)

The UC project used a gold coating so that experiments could be replicated at a later date without risk of oxidation. But traditional coatings such as silver, Mohammadi said, hold even more promise.