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With properties that promise faster computers, better sensors and much more, graphene has been dubbed the ‘miracle material’. But progress in producing it on an industrial scale without compromising its properties has proved elusive. University of Groningen scientists may now have made a breakthrough. Their results will be published in the journal Nano Letters.

Graphene is a special material with crystals that are just one atom thick. Electrons pass through it with hardly any resistance at all, and despite being very flexible, it is stronger than any metal. The discoverers of graphene, Andre Geim and Konstantin Novoselov, famously made it by peeling graphite with Scotch tape until they managed to isolate a single atomic layer: graphene. It won them the 2010 Nobel Prize in Physics.

“The challenge is to find a substrate that not only preserves the properties of graphene, but also enables scalable production,” said Stefano Gottardi, PhD student at the University of Groningen Zernike Institute for Advanced Materials.

A good candidate is chemical vapor deposition. Here heat is used to vaporize a carbon precursor like methane, which then reacts with a catalytically active substrate to form graphene on its surface. A transition metal is normally used as the substrate. However, not only does the transition metal act as a support, but it also tends to interact with the graphene and modify – or even deteriorate – its outstanding properties.

Cumbersome

To restore these properties after growth on the metal, the graphene has to be transferred to a non-interacting substrate, but this transfer process is cumbersome and often introduces defects. Nevertheless, many scientists are trying to improve graphene growth on transition metals, mostly using copper foil as the substrate.

This is what the Surfaces and Thin Films group of Gottardi’s supervisors Meike Stöhr and Petra Rudolf did too. “When we analyzed a sample of graphene on copper, we made some strange observations,” Stöhr recalled. The observations suggested that alongside the copper some copper oxide was also present. Indeed, a nice graphene film appeared to have formed on the copper oxide, and as oxidized metals might leave the properties of graphene unaltered, this was a potentially important observation.

Achievement

The Groningen team began to study this possibility in more detail. That was three years ago. Since then, Gottardi and his colleagues have managed to successfully grow graphene on copper oxide. This achievement together with an in-depth characterization of graphene’s properties will be published in Nano Letters. The team also reports the remarkable finding that graphene on copper oxide is decoupled from the substrate, which means that it preserves its peculiar electronic properties.

The results could be far-reaching. Stöhr: “Other labs need to reproduce our findings, and quite a bit of work needs to be done to optimize growth conditions.”

The best case scenario would be that large single-domain crystals of graphene could be grown on copper oxide. If this proves to be the case, it should then be possible to use lithographic techniques to make all sorts of electronic devices from graphene in a commercially viable manner. An unexpected observation three years ago may thus prove to be the start of a new era of graphene electronics.

If you can’t find the ideal material, then design a new one.

Northwestern University’s James Rondinelli uses quantum mechanical calculations to predict and design the properties of new materials by working at the atom-level. His group’s latest achievement is the discovery of a novel way to control the electronic band gap in complex oxide materials without changing the material’s overall composition. The finding could potentially lead to better electro-optical devices, such as lasers, and new energy-generation and conversion materials, including more absorbent solar cells and the improved conversion of sunlight into chemical fuels through photoelectrocatalysis.

“There really aren’t any perfect materials to collect the sun’s light,” said Rondinelli, assistant professor of materials science and engineering in the McCormick School of Engineering. “So, as materials scientists, we’re trying to engineer one from the bottom up. We try to understand the structure of a material, the manner in which the atoms are arranged, and how that ‘genome’ supports a material’s properties and functionality.”

The electronic band gap is a fundamental material parameter required for controlling light harvesting, conversion, and transport technologies. Via band-gap engineering, scientists can change what portion of the solar spectrum can be absorbed by a solar cell, which requires changing the structure or chemistry of the material.

Current tuning methods in non-oxide semiconductors are only able to change the band gap by approximately one electronvolt, which still requires the material’s chemical composition to become altered. Rondinelli’s method can change the band gap by up to 200 percent without modifying the material’s chemistry. The naturally occurring layers contained in complex oxide materials inspired his team to investigate how to control the layers. They found that by controlling the interactions between neutral and electrically charged planes of atoms in the oxide, they could achieve much greater variation in electronic band gap tunability.

“You could actually cleave the crystal and, at the nanometer scale, see well-defined layers that comprise the structure,” he said. “The way in which you order the cations on these layers in the structure at the atomic level is what gives you a new control parameter that doesn’t exist normally in traditional semiconductor materials.”

By tuning the arrangement of the cations–ions having a net positive, neutral, or negative charge–on these planes in proximity to each other, Rondinelli’s team demonstrated a band gap variation of more than two electronvolts. “We changed the band gap by a large amount without changing the material’s chemical formula,” he said. “The only difference is the way we sequenced the ‘genes’ of the material.”

Supported by DARPA and the US Department of Energy, the research is described in the paper “Massive band gap variation in layered oxides through cation ordering,” published in the January 30 issue of Nature Communications. Prasanna Balachandran of Los Alamos National Laboratory in New Mexico is coauthor of the paper.

Arranging oxide layers differently gives rise to different properties. Rondinelli said that having the ability to experimentally control layer-by-layer ordering today could allow researchers to design new materials with specific properties and purposes. The next step is to test his computational findings experimentally.

Rondinelli’s research is aligned with President Barack Obama’s Materials Genome Initiative, which aims to accelerate the discovery of advanced materials to address challenges in energy, healthcare, and transportation.

“Today it’s possible to create digital materials with atomic level precision,” Rondinelli said. “The space for exploration, however, is enormous. If we understand how the material behavior emerges from building blocks, then we make that challenge surmountable and meet one of the greatest challenges today–functionality by design.”

New pathway to valleytronics


January 27, 2015

A potential avenue to quantum computing currently generating quite the buzz in the high-tech industry is “valleytronics,” in which information is coded based on the wavelike motion of electrons moving through certain two-dimensional (2D) semiconductors. Now, a promising new pathway to valleytronic technology has been uncovered by researchers with the Lawrence Berkeley National Laboratory (Berkeley Lab).

Feng Wang, a condensed matter physicist with Berkeley Lab’s Materials Sciences Division, led a study in which it was demonstrated that a well-established phenomenon known as the “optical Stark effect” can be used to selectively control photoexcited electrons/hole pairs – referred to as excitons -in different energy valleys. In valleytronics, electrons move through the lattice of a 2D semiconductor as a wave with two energy valleys, each valley being characterized by a distinct momentum and quantum valley number. This quantum valley number can be used to encode information when the electrons are in a minimum energy valley. The technique is analogous to spintronics, in which information is encoded in a quantum spin number.

“This is the first demonstration of the important role the optical Stark effect can play in valleytronics,” Feng says. “Our technique, which is based on the use of circularly polarized femtosecond light pulses to selectively control the valley degree of freedom, opens up the possibility of ultrafast manipulation of valley excitons for quantum information applications.”

Wang, who also holds an appointment with the University of California (UC) Berkeley Physics Department, has been working with the 2D semiconductors known as MX2 materials, monolayers consisting of a single layer of transition metal atoms, such as molybdenum (Mo) or tungsten (W), sandwiched between two layers of chalcogen atoms, such as sulfur (S). This family of atomically thin 2D semiconductors features the same hexagonal “honeycombed” lattice as graphene. Unlike graphene, however, MX2 materials have natural energy band-gaps that facilitate their use in transistors and other electronic devices.

This past year, Wang and his group reported the first experimental observation of ultrafast charge transfer in photo-excited MX2 materials. The recorded charge transfer time of less than 50 femtoseconds established MX2 materials as competitors with graphene for future electronic devices. In this new study, Wang and his group generated ultrafast and ultrahigh pseudo-magnetic fields for controlling valley excitons in triangular monolayers of WSe2 using the optical Stark effect.

“The optical Stark effect describes the energy shift in a two-level system induced by a non-resonant laser field,” Wang says.

“Using ultrafast pump-probe spectroscopy, we were able to observe a pure and valley-selective optical Stark effect in WSe2 monolayers from the non-resonant pump that resulted in an energy splitting of more than 10 milli-electron volts between the K and K? valley exciton transitions. As controlling valley excitons with a real magnetic field is difficult to achieve even with superconducting magnets, a light-induced pseudo-magnetic field is highly desirable.”

Like spintronics, valleytronics offer a tremendous advantage in data processing speeds over the electrical charge used in classical electronics. Quantum spin, however, is strongly linked to magnetic fields, which can introduce stability issues. This is not an issue for quantum waves.

“The valley-dependent optical Stark effect offers a convenient and ultrafast way of enabling the coherent rotation of resonantly excited valley polarizations with high fidelity,” Wang says. “Such coherent manipulation of valley polarization should open up fascinating opportunities for valleytronics.”

Graphene, a one-atom thick lattice of carbon atoms, is often touted as a revolutionary material that will take the place of silicon at the heart of electronics. The unmatched speed at which it can move electrons, plus its essentially two-dimensional form factor, make it an attractive alternative, but several hurdles to its adoption remain.

A team of researchers from the University of Pennsylvania; University of California, Berkeley; and University of Illinois at Urbana-Champaign has made inroads in solving one such hurdle. By demonstrating a new way to change the amount of electrons that reside in a given region within a piece of graphene, they have a proof-of-principle in making the fundamental building blocks of semiconductor devices using the 2-D material.

Moreover, their method enables this value to be tuned through the application of an electric field, meaning graphene circuit elements made in this way could one day be dynamically “rewired” without physically altering the device.

The study was a collaboration between the groups of Andrew Rappe at Penn, Lane Martin at UC Berkeley and Moonsub Shim at Illinois.

It was published in the journal Nature Communications.

Silicon is used for making circuit elements because its charge-carrier density, the number of free electrons it contains, can be easily increased or decreased by adding chemical impurities. This “doping” process results in “p-type” and “n-type” semiconductors, silicon that has either more positive or more negative charge carriers.

The junctions between p- and n-type semiconductors are the building blocks of electronic devices. Put together in sequence, these p-n junctions form transistors, which can in turn be combined into integrated circuits, microchips and processors.

Chemically doping graphene to achieve p- and n-type version of the material is possible, but it means sacrificing some of its unique electrical properties. A similar effect is possible by applying local voltage changes to the material, but manufacturing and placing the necessary electrodes negates the advantages graphene’s form factor provides.

“We’ve come up with a non-destructive, reversible way of doping,” Rappe said, “that doesn’t involve any physical changes to the graphene.”

The team’s technique involves depositing a layer of graphene so it rests on, but doesn’t bond to, a second material: lithium niobate. Lithium niobate is ferroelectric, meaning that it is polar, and its surfaces have either a positive or negative charge. Applying an electric field pulse can change the sign of the surface charges.

“That’s an unstable situation,” Rappe said, “in that the positively charged surface will want to accumulate negative charges and vice versa. To resolve that imbalance, you could have other ions come in and bond or have the oxide lose or gain electrons to cancel out those charges, but we’ve come up with a third way.

“Here we have graphene standing by, on the surface of the oxide but not binding to it. Now, if the oxide surface says, ‘I wish I had more negative charge,’ instead of the oxide gathering ions from the environment or gaining electrons, the graphene says ‘I can hold the electrons for you, and they’ll be right nearby.'”

Rappe suggested using lithium niobate, as it is already commonly used in optical engineering and has properties that would lend themselves to creating p-n junctions. The researchers took advantage of the fact that a certain type of the material, periodically poled lithium niobate, is manufactured so that it has “stripes” of polar regions that alternate between positive and negative.

“Because the lithium niobate domains can dictate the properties,” Shim said, “different regions of graphene can take on different character depending on the nature of the domain underneath. That allows, as we have demonstrated, a simple means of creating a p-n junction or even an array of p-n junctions on a single flake of graphene. Such an ability should facilitate advances in graphene that might be analogous to what p-n junctions and complementary circuitry has done for the current state-of-the-art semiconductor electronics.

“What’s even more exciting are the enabling of optoelectronics using graphene and the possibility of waveguiding, lensing and periodically manipulating electrons confined in an atomically thin material.”

Their experiments also involved adding a single gate to the device, which allowed for its overall carrier density to be further tuned by the application of different voltages.

By taking into account how the oxide balances out its surface charges on its own, or by binding ions from the aqueous solution, the researchers were able to show the relationship between the polarization of the oxide and the charge carrier density of the graphene suspended over it.

And because the oxide polarization can be easily altered, the type and extent of supported graphene doping can be altered along with it.

“You could come along with a tip that produces a certain electric field, and just by putting it near the oxide you could change its polarity,” Martin said. “You write an ‘up’ domain or a ‘down’ domain in the region you want it, and the graphene’s charge density would reflect that change. You could make the graphene over that region p-type or n-type, and, if you change your mind, you can erase it and start again.”

This ability would represent an advantage over chemically doped semiconductors. Once the atomic impurities are mixed into the material to change its carrier density, they can’t be removed. Future research will investigate the feasibility of designing dynamic semiconducting devices with this technique.

“We can’t currently do that, but that’s the direction we want to take it,” Rappe said, “There are some oxides that can be repolarized on the timescale of nanoseconds, so you could make some really dynamic changes if you needed to. This opens up a lot of possibilities.”

Soitec (Euronext) has introduced its eSI90 substrate, the newest high-end wafer in its radio-frequency silicon-on-insulator (RF-SOI) product family. The eSI90 is designed to improve the RF performance of mobile communication components such as high-linearity switches and antenna tuners that are integrated in high-end smart phones for LTE Advanced networks using carrier aggregation. This enables multiple LTE carriers to be used together, providing higher data rates to enhance user experience.

The new wafers are Soitec’s second generation of eSI substrates, based on engineered high-resistivity (HR) substrates. Today, eSI substrates have been widely adopted by leading RF semiconductor companies to address device cost and performance needs for the 3G and 4G/LTE mobile wireless markets. Soitec’s eSI90 product exhibits higher effective resistivity than first-generation eSI wafers, enabling a 10-decibel (dB) improvement in linearity performance in RF front-end modules to address the stringent new requirements of LTE Advanced smart phones.

“Soitec continues to be the innovation frontrunner in RF-SOI substrates for the mobile industry with the introduction of eSI90, enabling high-performance RF devices for LTE Advanced and next-generation smart phones,” said Dr. Bernard Aspar, senior vice president and general manager of Soitec’s Communication & Power Business Unit. “Today, we estimate that more than one billion RF devices are produced each quarter using our eSI wafers. We are pleased to help our customers, the leading RF IC companies, meet the booming demand from the LTE Advanced market.”

Soitec developed a new metrology standard, the Harmonic Quality Factor (HQF), to predict the expected RF linearity of finished ICs. HQF correlates with the second harmonic distortion value of a coplanar waveguide deposited on the substrate. The new eSI90 wafers’ HQF maximum value is set to -90 decibel-milliwatts (dBm) compared to -80 dBm for first-generation eSI substrates. The lower limit on eSI90 wafers enables chip makers to take advantage of design and process improvements to increase the RF performance of their semiconductor designs and to meet MIMO (Multi-Input Multi-Output) and Carrier Aggregation LTE Advanced requirements, providing faster data connections.

With its high electrical conductivity and optical transparency, indium tin oxide is one of the most widely used materials for touchscreens, plasma displays, and flexible electronics. But its rapidly escalating price has forced the electronics industry to search for other alternatives.

One potential and more cost-effective alternative is a film made with silver nanowires–wires so extremely thin that they are one-dimensional–embedded in flexible polymers. Like indium tin oxide, this material is transparent and conductive. But development has stalled because scientists lack a fundamental understanding of its mechanical properties.

Now Horacio Espinosa, the James N. and Nancy J. Farley Professor in Manufacturing and Entrepreneurship at Northwestern University’s McCormick School of Engineering, has led research that expands the understanding of silver nanowires’ behavior in electronics.

Espinosa and his team investigated the material’s cyclic loading, which is an important part of fatigue analysis because it shows how the material reacts to fluctuating loads of stress.

“Cyclic loading is an important material behavior that must be investigated for realizing the potential applications of using silver nanowires in electronics,” Espinosa said. “Knowledge of such behavior allows designers to understand how these conductive films fail and how to improve their durability.”

By varying the tension on silver nanowires thinner than 120 nanometers and monitoring their deformation with electron microscopy, the research team characterized the cyclic mechanical behavior. They found that permanent deformation was partially recoverable in the studied nanowires, meaning that some of the material’s defects actually self-healed and disappeared upon cyclic loading. These results indicate that silver nanowires could potentially withstand strong cyclic loads for long periods of time, which is a key attribute needed for flexible electronics.

“These silver nanowires show mechanical properties that are quite unexpected,” Espinosa said. “We had to develop new experimental techniques to be able to measure this novel material property.”

The findings were recently featured on the cover of the journal Nano Letters. Other Northwestern coauthors on the paper are Rodrigo Bernal, a recently graduated PhD student in Espinosa’s lab, and Jiaxing Huang, associate professor of materials science and engineering in McCormick.

“The next step is to understand how this recovery influences the behavior of these materials when they are flexed millions of times,” said Bernal, first author of the paper.

University of Wisconsin-Madison materials engineers have made a significant leap toward creating higher-performance electronics with improved battery life — and the ability to flex and stretch.

Led by materials science Associate Professor Michael Arnold and Professor Padma Gopalan, the team has reported the highest-performing carbon nanotube transistors ever demonstrated. In addition to paving the way for improved consumer electronics, this technology could also have specific uses in industrial and military applications.

In a paper published recently in the journal ACS Nano, Arnold, Gopalan and their students reported transistors with an on-off ratio that’s 1,000 times better and a conductance that’s 100 times better than previous state-of-the-art carbon nanotube transistors.

“Carbon nanotubes are very strong and very flexible, so they could also be used to make flexible displays and electronics that can stretch and bend, allowing you to integrate electronics into new places like clothing,” says Arnold. “The advance enables new types of electronics that aren’t possible with the more brittle materials manufacturers are currently using.”

Carbon nanotubes are single atomic sheets of carbon rolled up into a tube. As some of the best electrical conductors ever discovered, carbon nanotubes have long been recognized as a promising material for next-generation transistors, which are semiconductor devices that can act like an on-off switch for current or amplify current. This forms the foundation of an electronic device.

However, researchers have struggled to isolate purely semiconducting carbon nanotubes, which are crucial, because metallic nanotube impurities act like copper wires and “short” the device. Researchers have also struggled to control the placement and alignment of nanotubes. Until now, these two challenges have limited the development of high-performance carbon nanotube transistors.

Building on more than two decades of carbon nanotube research in the field, the UW-Madison team drew on cutting-edge technologies that use polymers to selectively sort out the semiconducting nanotubes, achieving a solution of ultra-high-purity semiconducting carbon nanotubes.

Previous techniques to align the nanotubes resulted in less-than-desirable packing density, or how close the nanotubes are to one another when they are assembled in a film. However, the UW-Madison researchers pioneered a new technique, called floating evaporative self-assembly, or FESA, which they described earlier in 2014 in the ACS journal Langmuir. In that technique, researchers exploited a self-assembly phenomenon triggered by rapidly evaporating a carbon nanotube solution.

The team’s most recent advance also brings the field closer to realizing carbon nanotube transistors as a feasible replacement for silicon transistors in computer chips and in high-frequency communication devices, which are rapidly approaching their physical scaling and performance limits.

“This is not an incremental improvement in performance,” Arnold says. “With these results, we’ve really made a leap in carbon nanotube transistors. Our carbon nanotube transistors are an order of magnitude better in conductance than the best thin film transistor technologies currently being used commercially while still switching on and off like a transistor is supposed to function.”

The researchers have patented their technology through the Wisconsin Alumni Research Foundation and have begun working with companies to accelerate the technology transfer to industry.

Princeton University researchers have built a rice grain-sized laser powered by single electrons tunneling through artificial atoms known as quantum dots. The tiny microwave laser, or “maser,” is a demonstration of the fundamental interactions between light and moving electrons.

The researchers built the device — which uses about one-billionth the electric current needed to power a hair dryer — while exploring how to use quantum dots, which are bits of semiconductor material that act like single atoms, as components for quantum computers.

“It is basically as small as you can go with these single-electron devices,” said Jason Petta, an associate professor of physics at Princeton who led the study, which was published in the journal Science.

The device demonstrates a major step forward for efforts to build quantum-computing systems out of semiconductor materials, according to co-author and collaborator Jacob Taylor, an adjunct assistant professor at the Joint Quantum Institute, University of Maryland-National Institute of Standards and Technology. “I consider this to be a really important result for our long-term goal, which is entanglement between quantum bits in semiconductor-based devices,” Taylor said.

The original aim of the project was not to build a maser, but to explore how to use double quantum dots — which are two quantum dots joined together — as quantum bits, or qubits, the basic units of information in quantum computers.

“The goal was to get the double quantum dots to communicate with each other,” said Yinyu Liu, a physics graduate student in Petta’s lab. The team also included graduate student Jiri Stehlik and associate research scholar Christopher Eichler in Princeton’s Department of Physics, as well as postdoctoral researcher Michael Gullans of the Joint Quantum Institute.

Because quantum dots can communicate through the entanglement of light particles, or photons, the researchers designed dots that emit photons when single electrons leap from a higher energy level to a lower energy level to cross the double dot.

Each double quantum dot can only transfer one electron at a time, Petta explained. “It is like a line of people crossing a wide stream by leaping onto a rock so small that it can only hold one person,” he said. “They are forced to cross the stream one at a time. These double quantum dots are zero-dimensional as far as the electrons are concerned — they are trapped in all three spatial dimensions.”

The researchers fabricated the double quantum dots from extremely thin nanowires (about 50 nanometers, or a billionth of a meter, in diameter) made of a semiconductor material called indium arsenide. They patterned the indium arsenide wires over other even smaller metal wires that act as gate electrodes, which control the energy levels in the dots.

To construct the maser, they placed the two double dots about 6 millimeters apart in a cavity made of a superconducting material, niobium, which requires a temperature near absolute zero, around minus 459 degrees Fahrenheit. “This is the first time that the team at Princeton has demonstrated that there is a connection between two double quantum dots separated by nearly a centimeter, a substantial distance,” Taylor said.

When the device was switched on, electrons flowed single-file through each double quantum dot, causing them to emit photons in the microwave region of the spectrum. These photons then bounced off mirrors at each end of the cavity to build into a coherent beam of microwave light.

One advantage of the new maser is that the energy levels inside the dots can be fine-tuned to produce light at other frequencies, which cannot be done with other semiconductor lasers in which the frequency is fixed during manufacturing, Petta said. The larger the energy difference between the two levels, the higher the frequency of light emitted.

Claire Gmachl, who was not involved in the research and is Princeton’s Eugene Higgins Professor of Electrical Engineering and a pioneer in the field of semiconductor lasers, said that because lasers, masers and other forms of coherent light sources are used in communications, sensing, medicine and many other aspects of modern life, the study is an important one.

“In this paper the researchers dig down deep into the fundamental interaction between light and the moving electron,” Gmachl said. “The double quantum dot allows them full control over the motion of even a single electron, and in return they show how the coherent microwave field is created and amplified. Learning to control these fundamental light-matter interaction processes will help in the future development of light sources.”

Organic semiconductors are prized for light emitting diodes (LEDs), field effect transistors (FETs) and photovoltaic cells. As they can be printed from solution, they provide a highly scalable, cost-effective alternative to silicon-based devices. Uneven performances, however, have been a persistent problem. Scientists have known that the performance issues originate in the domain interfaces within organic semiconductor thin films, but have not known the cause. This mystery now appears to have been solved.

Naomi Ginsberg, a faculty chemist with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory and the University of California (UC) Berkeley, led a team that used a unique form of microscopy to study the domain interfaces within an especially high-performing solution-processed organic semiconductor called TIPS-pentacene. She and her team discovered a cluttered jumble of randomly oriented nanocrystallites that become kinetically trapped in the interfaces during solution casting. Like debris on a highway, these nanocrystallites impede the flow of charge-carriers.

“If the interfaces were neat and clean, they wouldn’t have such a large impact on performance, but the presence of the nanocrystallites reduces charge-carrier mobility,” Ginsberg says. “Our nanocrystallite model for the interface, which is consistent with observations, provides critical information that can be used to correlate solution-processing methods to optimal device performances.”

Ginsberg, who holds appointments with Berkeley Lab’s Physical Biosciences Division and its Materials Sciences Division, as well as UC Berkeley’s departments of chemistry and physics, is the corresponding author of a paper describing this research in Nature Communications. The paper is titled “Exciton dynamics reveals aggregates with intermolecular order at hidden interfaces in solution-cast organic semiconducting films.” Co-authors are Cathy Wong, Benjamin Cotts and Hao Wu.

Organic semiconductors are based on the ability of carbon to form larger molecules, such as benzene and pentacene, featuring electrical conductivity that falls somewhere between insulators and metals. Through solution-processing, organic materials can usually be fashioned into crystalline films without the expensive high-temperature annealing process required for silicon and other inorganic semiconductors. However, even though it has long been clear that the crystalline domain interfaces within semiconductor organic thin films are critical to their performance in devices, detailed information on the morphology of these interfaces has been missing until now.

“Interface domains in organic semiconductor thin films are smaller than the diffraction limit, hidden from surface probe techniques such as atomic force microscopy, and their nanoscale heterogeneity is not typically resolved using X-ray methods,” Ginsberg says. “Furthermore, the crystalline TIPS-pentacene we studied has virtually zero emission, which means it can’t be studied with photoluminescence microscopy.”

Ginsberg and her group overcame the challenges by using transient absorption (TA) microscopy, a technique in which femtosecond laser pulses excite transient energy states and detectors measure the changes in the absorption spectra. The Berkeley researchers carried out TA microscopy on an optical microscope they constructed themselves that enabled them to generate focal volumes that are a thousand times smaller than is typical for conventional TA microscopes. They also deployed multiple different light polarizations that allowed them to isolate interface signals not seen in either of the adjacent domains.

“Instrumentation, including very good detectors, the painstaking collection of data to ensure good signal-to-noise ratios, and the way we crafted the experiment and analysis were all critical to our success,” Ginsberg says. “Our spatial resolution and light polarization sensitivity were also essential to be able to unequivocally see a signature of the interface that was not swamped by the bulk, which contributes much more to the raw signal by volume.”

The methology developed by Ginsberg and her team to uncover structural motifs at hidden interfaces in organic semiconductor thin films should add a predictive factor to scalable and affordable solution-processing of these materials. This predictive capability should help minimize discontinuities and maximize charge-carrier mobility. Currently, researchers use what is essentially a trial-and-error approach, in which different solution casting conditions are tested to see how well the resulting devices perform.

“Our methodology provides an important intermediary in the feedback loop of device optimization by characterizing the microscopic details of the films that go into the devices, and by inferring how the solution casting could have created the structures at the interfaces,” Ginsberg says. “As a result, we can suggest how to alter the delicate balance of solution casting parameters to make more functional films.”

Adam Khan, founder and CEO of AKHAN Semiconductor, Inc. was granted a US patent by the US Patent and Trademark Office today for a groundbreaking process that adheres diamond, the only truly transparent semiconductor, to metals and alloys (including transparent metals) in a way that allows for reliable wire bonding and high conductivity. The application improves the properties of everyday electronics in a multitude of ways, most significantly, enabling the creation of a fully transparent circuit.

“The patent enables designers and manufacturers to surpass the longstanding technological roadblock to create fully transparent electronics,” Khan said. “Diamond may now be integrated with fully transparent metals such as indium tin oxide (ITO) and fluorine doped tin oxide (FTO). This means transparent concepts for future consumer electronic designs, like the next generation of wearables you see today, will become a reality.”

The patent, first filed in 2012, qualified as “first to invent” as Adam Khan was the first to demonstrate and secure IP rights to reliable metallization of diamond surfaces at his company. Previous to this invention, there was no way to successfully bond necessary contact metals to semiconducting diamond so the application of the diamond material into next generation devices was virtually impossible.

From a technical perspective, the patent allows for the metallization of n-type diamond semiconductors, a process that gives a material the ability to conduct electricity. Previously, it was only possible to create n-type semiconductors with Silicon, Germanium, Silicon Carbide, and GaAs which are widely used today. Each of these materials are non-transparent, presenting a foundational design challenge for the next-generation of electronics. This new diamond technology surpasses this design challenge and will usher in the era of diamond electronics.

Possible consumer designs include transparent computers, mobile phones, wearable tech such as that seen in Google Glass and other heads up display applications seen in a variety of industries. More specifically, industrial applications include: commercial avionics, aerospace, military/defense, satellite/telecommunications power electronics where electronic components are most commonly construed on transparent and radiation-hard substrates such as Fused Silica (glass) and Sapphire.

Each of these applications consist of not just greater design capabilities, but are more cost effective, can operate at higher temperatures, allows for thinner devices and advanced design capabilities. The diamond material has widely been recognized as the holy grail of power and consumer electronics and Khan’s patent makes the application of diamond technology a reality.

“There is no transparent electronic system available today, so AKHAN Semiconductor will be able to clear a key technological barrier moving forward,” Khan said. “Additionally, the patent is a key strategic advantage for AKHAN Semiconductor, and will help provide years of growth as we lead diamond semiconductor technology into mainstream.”