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A team of researchers, led by the University of Minnesota, have discovered a new nano-scale thin film material with the highest-ever conductivity in its class. The new material could lead to smaller, faster, and more powerful electronics, as well as more efficient solar cells.

The discovery is being published today in Nature Communications, an open access journal that publishes high-quality research from all areas of the natural sciences.

Researchers say that what makes this new material so unique is that it has a high conductivity, which helps electronics conduct more electricity and become more powerful. But the material also has a wide bandgap, which means light can easily pass through the material making it optically transparent. In most cases, materials with wide bandgap, usually have either low conductivity or poor transparency.

“The high conductivity and wide bandgap make this an ideal material for making optically transparent conducting films which could be used in a wide variety of electronic devices, including high power electronics, electronic displays, touchscreens and even solar cells in which light needs to pass through the device,” said Bharat Jalan, a University of Minnesota chemical engineering and materials science professor and the lead researcher on the study.

Currently, most of the transparent conductors in our electronics use a chemical element called indium. The price of indium has generally gone up over the last two decades, which has added to the cost of current display technology. As a result, there has been tremendous effort to find alternative materials that work as well, or even better, than indium-based transparent conductors.

In this study, researchers found a solution. They developed a new transparent conducting thin film using a novel synthesis method, in which they grew a BaSnO3 thin film (a combination of barium, tin and oxygen, called barium stannate), but replaced elemental tin source with a chemical precursor of tin. The chemical precursor of tin has unique, radical properties that enhanced the chemical reactivity and greatly improved the metal oxide formation process. Both barium and tin are significantly cheaper than indium and are abundantly available.

“We were quite surprised at how well this unconventional approach worked the very first time we used the tin chemical precursor,” said University of Minnesota chemical engineering and materials science graduate student Abhinav Prakash, the first author of the paper. “It was a big risk, but it was quite a big breakthrough for us.”

Jalan and Prakash said this new process allowed them to create this material with unprecedented control over thickness, composition, and defect concentration and that this process should be highly suitable for a number of other material systems where the element is hard to oxidize. The new process is also reproducible and scalable.

They further added that it was the structurally superior quality with improved defect concentration that allowed them to discover high conductivity in the material. They said the next step is to continue to reduce the defects at the atomic scale.

“Even though this material has the highest conductivity within the same materials class, there is much room for improvement in addition, to the outstanding potential for discovering new physics if we decrease the defects. That’s our next goal,” Jalan said.

No more error-prone evaporation deposition, drop casting or printing: Scientists at Ludwig-Maximilians-Universitaet (LMU) in Munich and FSU Jena have developed organic semiconductor nanosheets, which can easily be removed from a growth substrate and placed on other substrates.

Today’s computer processors are composed of billions of transistors. These electronic components normally consist of semiconductor material, insulator, substrate, and electrode. A dream of many scientists is to have each of these elements available as transferable sheets, which would allow them to design new electronic devices simply by stacking.

This has now become a reality for the organic semiconductor material pentacene: Dr. Bert Nickel, a physicist at LMU Munich, and Professor Andrey Turchanin (Friedrich Schiller University Jena), together with their teams, have, for the first time, managed to create mechanically stable pentacene nanosheets.

The researchers describe their method in the journal Advanced Materials. They first cover a small silicon wafer with a thin layer of a water-soluble organic film and deposit pentacene molecules upon it until a layer roughly 50 nanometers thick has formed. The next step is crucial: by irradiation with low-energy electrons, the topmost three to four levels of pentacene molecular layers are crosslinked, forming a “skin” that is only about five nanometers thick. This crosslinked layer stabilizes the entire pentacene film so well that it can be removed as a sheet from a silicon wafer in water and transferred to another surface using ordinary tweezers.

Apart from the ability to transfer them, the new semiconductor nanosheets have other advantages. The new method does not require any potentially interfering solvents, for example. In addition, after deposition, the nanosheet sticks firmly to the electrical contacts by van der Waals forces, resulting in a low contact resistance of the final electronic devices. Last but not least, organic semiconductor nanosheets can now be deposited onto significantly more technologically relevant substrates than hitherto.

Of particular interest is the extremely high mechanical stability of the newly developed pentacene nanosheets, which enables them to be applied as free-standing nanomembranes to perforated substrates with dimensions of tens of micrometers. That is equivalent to spanning a 25-meter pool with plastic wrap. “These virtually freely suspended semiconductors have great potential,” explains Nickel. “They can be accessed from two sides and could be connected through an electrolyte, which would make them ideal as biosensors, for example”. “Another promising application is their implementation in flexible electronics for manufacturing of devices for vital data acquisition or production of displays and solar cells,” Turchanin says.

As electronics become increasingly pervasive in our lives – from smart phones to wearable sensors – so too does the ever rising amount of electronic waste they create. A United Nations Environment Program report found that almost 50 million tons of electronic waste were thrown out in 2017–more than 20 percent higher than waste in 2015.

Troubled by this mounting waste, Stanford engineer Zhenan Bao and her team are rethinking electronics. “In my group, we have been trying to mimic the function of human skin to think about how to develop future electronic devices,” Bao said. She described how skin is stretchable, self-healable and also biodegradable – an attractive list of characteristics for electronics. “We have achieved the first two [flexible and self-healing], so the biodegradability was something we wanted to tackle.”

The team created a flexible electronic device that can easily degrade just by adding a weak acid like vinegar. The results were published May 1 in the Proceedings of the National Academy of Sciences.

A newly developed flexible, biodegradable semiconductor developed by Stanford engineers shown on a human hair. Credit: Bao Lab

A newly developed flexible, biodegradable semiconductor developed by Stanford engineers shown on a human hair. Credit: Bao Lab

“This is the first example of a semiconductive polymer that can decompose,” said lead author Ting Lei, a postdoctoral fellow working with Bao.

In addition to the polymer – essentially a flexible, conductive plastic – the team developed a degradable electronic circuit and a new biodegradable substrate material for mounting the electrical components. This substrate supports the electrical components, flexing and molding to rough and smooth surfaces alike. When the electronic device is no longer needed, the whole thing can biodegrade into nontoxic components.

Biodegradable bits

Bao, a professor of chemical engineering and materials science and engineering, had previously created a stretchable electrode modeled on human skin. That material could bend and twist in a way that could allow it to interface with the skin or brain, but it couldn’t degrade. That limited its application for implantable devices and – important to Bao – contributed to waste.

Bao said that creating a robust material that is both a good electrical conductor and biodegradable was a challenge, considering traditional polymer chemistry. “We have been trying to think how we can achieve both great electronic property but also have the biodegradability,” Bao said.

Eventually, the team found that by tweaking the chemical structure of the flexible material it would break apart under mild stressors. “We came up with an idea of making these molecules using a special type of chemical linkage that can retain the ability for the electron to smoothly transport along the molecule,” Bao said. “But also this chemical bond is sensitive to weak acid – even weaker than pure vinegar.” The result was a material that could carry an electronic signal but break down without requiring extreme measures.

In addition to the biodegradable polymer, the team developed a new type of electrical component and a substrate material that attaches to the entire electronic component. Electronic components are usually made of gold. But for this device, the researchers crafted components from iron. Bao noted that iron is a very environmentally friendly product and is nontoxic to humans.

The researchers created the substrate, which carries the electronic circuit and the polymer, from cellulose. Cellulose is the same substance that makes up paper. But unlike paper, the team altered cellulose fibers so the “paper” is transparent and flexible, while still breaking down easily. The thin film substrate allows the electronics to be worn on the skin or even implanted inside the body.

From implants to plants

The combination of a biodegradable conductive polymer and substrate makes the electronic device useful in a plethora of settings – from wearable electronics to large-scale environmental surveys with sensor dusts.

“We envision these soft patches that are very thin and conformable to the skin that can measure blood pressure, glucose value, sweat content,” Bao said. A person could wear a specifically designed patch for a day or week, then download the data. According to Bao, this short-term use of disposable electronics seems a perfect fit for a degradable, flexible design.

And it’s not just for skin surveys: the biodegradable substrate, polymers and iron electrodes make the entire component compatible with insertion into the human body. The polymer breaks down to product concentrations much lower than the published acceptable levels found in drinking water. Although the polymer was found to be biocompatible, Bao said that more studies would need to be done before implants are a regular occurrence.

Biodegradable electronics have the potential to go far beyond collecting heart disease and glucose data. These components could be used in places where surveys cover large areas in remote locations. Lei described a research scenario where biodegradable electronics are dropped by airplane over a forest to survey the landscape. “It’s a very large area and very hard for people to spread the sensors,” he said. “Also, if you spread the sensors, it’s very hard to gather them back. You don’t want to contaminate the environment so we need something that can be decomposed.” Instead of plastic littering the forest floor, the sensors would biodegrade away.

As the number of electronics increase, biodegradability will become more important. Lei is excited by their advancements and wants to keep improving performance of biodegradable electronics. “We currently have computers and cell phones and we generate millions and billions of cell phones, and it’s hard to decompose,” he said. “We hope we can develop some materials that can be decomposed so there is less waste.”

In the world of semiconductor physics, the goal is to devise more efficient and microscopic ways to control and keep track of 0 and 1, the binary codes that all information storage and logic functions in computers are based on.

A new field of physics seeking such advancements is called valleytronics, which exploits the electron’s “valley degree of freedom” for data storage and logic applications. Simply put, valleys are maxima and minima of electron energies in a crystalline solid. A method to control electrons in different valleys could yield new, super-efficient computer chips.

A University at Buffalo team, led by Hao Zeng, PhD, professor in the Department of Physics, worked with scientists around the world to discover a new way to split the energy levels between the valleys in a two-dimensional semiconductor.

The work is described in a study published online today (May 1, 2017) in the journal Nature Nanotechnology.

The key to Zeng’s discovery is the use of a ferromagnetic compound to pull the valleys apart and keep them at different energy levels. This leads to an increase in the separation of valley energies by a factor of 10 more than the one obtained by applying an external magnetic field.

“Normally there are two valleys in these atomically thin semiconductors with exactly the same energy. These are called ‘degenerate energy levels’ in quantum mechanics terms. This limits our ability to control individual valleys. An external magnetic field can be used to break this degeneracy. However, the splitting is so small that you would have to go to the National High Magnetic Field Laboratories to measure a sizable energy difference. Our new approach makes the valleys more accessible and easier to control, and this could allow valleys to be useful for future information storage and processing,” Zeng said.

The simplest way to understand how valleys could be used in processing data may be to think of two valleys side by side. When one valley is occupied by electrons, the switch is “on.” When the other valley is occupied, the switch is “off.” Zeng’s work shows that the valleys can be positioned in such a way that a device can be turned “on” and “off,” with a tiny amount of electricity.

Microscopic ingredients

Zeng and his colleagues created a two-layered heterostructure, with a 10 nanometer thick film of magnetic EuS (europium sulfide) on the bottom and a single layer (less than 1 nanometer) of the transition metal dichalcogenide WSe2 (tungsten diselenide) on top. The magnetic field of the bottom layer forced the energy separation of the valleys in the WSe2.

Previous attempts to separate the valleys involved the application of very large magnetic fields from outside. Zeng’s experiment is believed to be the first time a ferromagnetic material has been used in conjunction with an atomically thin semiconductor material to split its valley energy levels.

“As long as we have the magnetic material there, the valleys will stay apart,” he said. “This makes it valuable for nonvolatile memory applications.”

Athos Petrou, a UB Distinguished Professor in the Department of Physics, measured the energy difference between the separated valleys by bouncing light off the material and measuring the energy of reflected light.

“We typically get this type of results only once every five or 10 years,” Petrou said.

Extending Moore’s law

The experiment was conducted at 7 degrees Kelvin (-447 Fahrenheit), so any everyday use of the process is far in the future. However, proving it possible is a first step.

“The reason people are really excited about this, is that Moore’s law [which says the number of transistors in an integrated circuit doubles every two years] is predicted to end soon. It no longer works because it has hit its fundamental limit,” Zeng said.

“Current computer chips rely on the movement of electrical charges, and that generates an enormous amount of heat as computers get more powerful. Our work has really pushed valleytronics a step closer in getting over that challenge.”

MRAM lowers system power


April 28, 2017

BY BARRY HOBERMAN, CEO, Spin Transfer Technologies

ST-MRAM (spin-transfer magnetic RAM) is an extremely promising new technology with the potential to replace major segments of the market for flash, SRAM, and DRAM semiconductors in applications such as mobile products, automotive, IoT, and data storage. With ST MRAM technology, data is stored in minute magnetic nodes—a physical mechanism different from traditional non-volatile memory (NVM). MRAM technology fundamentally requires less energy to use, and features like byte-addressability that further contributes to energy efficiency.

Embedded MRAM primarily fills the role that is currently handled by embedded NOR flash: storage of code and data that must survive when the power is removed. Indeed, MRAM is challenging NOR flash due to overall lower power and byte-addressability.

Energy consumption starts with voltages and currents: their product yields the power of the device – that is, the rate of energy consumption. Lower voltages and currents mean lower power. Energy consumed is determined by how long that rate is sustained – power multiplied by operating time. Therefore speed, the ability to finish a job sooner, also contributes to lower energy consumption especially when devices can enter sleep mode after tasks are complete.

To understand how NOR flash consumes energy, we need to look at how it operates. Let’s say we have a 32-bit word whose value we wish to update. With NOR flash, you can store data only in locations that have been freshly erased. This means you have to erase the old value before you can write the new value.

But there’s a more significant challenge; you can’t just erase those 32 bits. NOR flash can only be erased in sectors. So, in order to update those 32 bits, you have to find a new place to write them. This means creating and maintaining pointers to keep track of stored data since, with each update, the data location will move. Eventually, you run out of fresh space, and must perform garbage collection to free up the space used by all the out-of-date instances.

By contrast, MRAM has none of these requirements. Because it is byte-addressable, you can read and write just as you would with SRAM. Those 32 bits that needed updating? You simply write the new value over the old value. MRAM consumes less energy for a number of reasons:

No erase before writing: NOR flash erasure is very slow. With MRAM, there’s really no notion of erasing data; you’re either writing 1s or 0s, in any combination. The need to erase is a key contributor to the energy consumption of a NOR flash device.
Faster, lower-power writing: Not only can MRAM devices be written more quickly than NOR flash (even without considering erasure), the power while writing is also lower. The fact that you can complete the operation sooner means you can put the device to sleep sooner, yet another advantage to lowering energy.

No charge pumps: NOR flash, unlike MRAM, needs high voltages internally – much higher than the voltages at the external power pins. Those voltages are generated by internal charge pumps. Ideally, power would stay the same, but real charge pumps aren’t ideal; their inefficiency means lost energy.
Charge pumps also take longer to power up, and settle after a sleeping device awakens. This increases wake-up times dramatically. MRAM wakes up in nanoseconds to micro- seconds; NOR flash in milliseconds.

No complex storage management: The lack of byte-addressability in NOR flash creates complexity that increases the time to store data and code. Data tables must be maintained, along with the occasional garbage collection. The CPU, or some other circuit, must manage this data storage. These other devices consume energy, so the more time spent managing data, the more energy consumed. This energy consumption doesn’t apply to MRAM technology.

Mixed read/write stream: NOR flash storage operations, due to complexity, mean long lock-out times during writes. No data reading is permitted during these times. If certain pieces of data are quickly needed, then further management may be required to anticipate this ahead of a data write, so the data can be cached. By contrast, MRAM can handle a stream of operations – reads and writes – in any combination.

Staggered writing: Data can be stored 32 bits at a time. While overall energy consumption in doing this is lower for MRAM than for NOR flash, it still might challenge the peak current capabilities of a battery-powered device. The ability for MRAM to break the write into four successive single-byte writes, a feature known as “staggered write,” reduces current demands on the battery.

Everything we experience is made of light and matter. And the interaction between the two can bring about fascinating effects. For example, it can result in the formation of special quasiparticles, called polaritons, which are a combination of light and matter. A team at the Center for Theoretical Physics of Complex Systems, within the Institute for Basic Science (IBS), modeled the behavior of polaritons in microcavities, nanostructures made of a semiconductor material sandwiched between special mirrors (Bragg mirrors). Published in Scientific Reports, this research brings new ideas to the emerging valleytronics field.

Minimal energy locations, called valleys, are shown with white crosses. Credit: IBS

Minimal energy locations, called valleys, are shown with white crosses. Credit: IBS

Emerging from the coupling of light (photons) and matter (bound state of electrons and holes known as excitons), polaritons have characteristics of each. They are formed when a light beam of a certain frequency bounces back and forth inside microcavities, causing the rapid interconversion between light and matter and resulting in polaritons with a short lifetime. “You can imagine these quasiparticles as waves that you make in water, they move together harmoniously, but they do not last very long. The short lifetime of polaritons in this system is due to the properties of the photons,” explains Mr Meng Sun, first author of the study.

Researchers are studying polaritons in microcavities to understand how their characteristics could be exploited to outperform the present semiconductor technologies. Modern optoelectronics read, process, and store information by controlling the flow of particles, but looking for new more efficient alternatives, other parameters, like the so-called ‘valleys’ could be considered. Valleys can be visualized by plotting the energy of the polaritons to their momentum. Valleytronics aims to control the properties of the valleys in some materials, like transition metal dichalcogenides (TMDCs), indium gallium aluminum arsenide (InGaAlAs), and graphene.

Being able to manipulate their features would lead to tunable valleys with two clearly different states, corresponding for example to 1 bit and 0 bit, like on-off states in computing and digital communications. A way to distinguish valleys with the same energy level is to obtain valleys with different polarization, so that electrons (or polaritons) would preferentially occupy one valley over the others. IBS scientists have generated a theoretical model for valley polarization that could be useful for valleytronics.

Although polaritons are formed by the coupling of photons and excitons, the research team modeled the two components independently. “Modeling potential profiles of photons and excitons separately is the key to find where they overlap, and then determine the minimal energy positions where valleys occur,” points out Sun.

A crucial feature of this system is that polaritons can inherit some properties, like polarization. Valleys with different polarization form spontaneously when the splitting of the transverse (i.e. perpendicular) electronic and magnetic modes of the light beam is taken into consideration (TE-TM splitting).

Since this theoretical model predicts that valleys with opposite polarization can be distinguished and tuned, in principle, different valleys could be selectively excited by a polarized laser light, leading to a possible application in valleytronics.

Researcher team led by Professor Takayuki Ohba at Tokyo Institute of Technology, ICE Cube Center, in collaboration with the WOW (Wafer-on-Wafer) Alliance(term 2), an Industry-academic collaborative research organization consisting of multiple semiconductor related companies aiming for practical applications of 3D IC technology, demonstrated the thermal resistance of the 3D stacked device can be reduced down to less than 1/3 relative to the conventional one bonded by bump(term 3) 3D IC in Through-Silicon-Via (TSV) wiring(term 4). Since semiconductor circuits are highly heat-generating bodies during operation, when heat is hard to be released, the temperature of the semiconductor results in highly rise, which leads to be a malfunction. The development of heat dissipation technology has been a big challenge.

To address this challenge, Ohba and colleagues analyzed thermal properties in 3D IC using finite element method (FEM)(term 5) and thermal network calculation method. The study identified three main factors of thermal resistance; the interconnection layers, dielectric layers and organic layers in the conventional bump type device. Contrary to the bump type, the thermal performance of a bumpless 3D IC was almost 150 times better than that of a conventional IC at the same TSV density. The researchers demonstrated to reduce the total thermal resistance to 0.46 Kcm2/W, whereas the conventional method is 1.54 Kcm2/W. This suggests that the bumpless enables lower temperature rise and three to four times further DRAM stacking.

This is a cross-sectional structure of micro bump and bumpless. Credit: Tokyo Institute of Technology

This is a cross-sectional structure of micro bump and bumpless. Credit: Tokyo Institute of Technology

Based on their demonstration experiments, the scientists will work toward practical use of large-capacity memory technology for mobile terminals and servers.

ClassOne Technology (classone.com), manufacturer of budget-friendly Solstice plating systems, announced it’s new CopperMax chamber — a design that is demonstrating major copper plating cost reductions for users of ≤200mm wafers.

ClassOne cited actual performance data from a CopperMax pilot installation on a Solstice tool at a Fortune 100 customer. Over a six-month period the customer tracked their actual production operating costs while using the new chamber for copper TSV, Damascene and high-rate copper plating. For the three processes with CopperMax they reported that operating costs were reduced between 95.8% and 98.4% compared with previously used conventional plating chambers.

“Many of our emerging market customers are starting to do copper plating,” said Kevin Witt, President of ClassOne Technology. “So we’ve spent a lot of time on the process, working to reduce customer costs and also increase performance. And the new CopperMax chamber is proving to do both.”

ClassOne pointed out that consumables are the largest cost factor in copper plating. Optimizing copper plating generally requires the use of expensive organic additives — which are consumed very rapidly and need to be replenished frequently.

CopperMax chamber

“We learned, however, that over 97% of those expensive additives were not being consumed by the actual plating process,” said Witt. “Most were being used up simply by contact with the anode throughout the process! So, we designed our new copper chamber specifically to keep additives away from the anode — and the results are pretty dramatic. Significant savings can be realized by high- and medium-volume users with high throughputs as well as by lower-volume and R&D users that have long idle times.”

The company explained that the CopperMax chamber employs a cation-exchange semipermeable membrane to divide the copper bath into two sections. The upper section contains all of the additives, and it actively plates the wafer. The lower section of the bath contains the anode that supplies elemental copper — which is able to travel through the membrane and into the upper section to ultimately plate the wafer. However, the membrane prevents additives from traveling down to the anode, where they would break down and form process-damaging waste products.

As a result, the CopperMax bath remains much cleaner, and bath life is extended by over 20x. This increases uptime, enables higher-quality, higher-rate Cu plating, and it reduces cost of ownership very substantially.

For example, a customer using a Solstice system with six CopperMax chambers and running TSV and high-rate copper plating will save over $300,000 per year just from additive use reductions.

In addition, the CopperMax also reduces Cu anode expenses. The chamber is designed to use inexpensive bulk anode pellets instead of solid machined Cu material, which cuts anode costs by over 50%. And since the pellets have 10x greater surface area they also increase the allowable plating rates.

“Like the rest of our equipment, this new chamber aims to serve all those smaller wafer users who have limited budgets,” said Witt. “Simply stated, CopperMax is going to give them a lot more copper plating performance for a lot less.”

Solstice plating system

In 2016, annual global semiconductor sales reached their highest-ever point, at $339 billion worldwide. In that same year, the semiconductor industry spent about $7.2 billion worldwide on wafers that serve as the substrates for microelectronics components, which can be turned into transistors, light-emitting diodes, and other electronic and photonic devices.

A new technique developed by MIT engineers may vastly reduce the overall cost of wafer technology and enable devices made from more exotic, higher-performing semiconductor materials than conventional silicon.

The new method, reported today in Nature, uses graphene — single-atom-thin sheets of graphite — as a sort of “copy machine” to transfer intricate crystalline patterns from an underlying semiconductor wafer to a top layer of identical material.

The engineers worked out carefully controlled procedures to place single sheets of graphene onto an expensive wafer. They then grew semiconducting material over the graphene layer. They found that graphene is thin enough to appear electrically invisible, allowing the top layer to see through the graphene to the underlying crystalline wafer, imprinting its patterns without being influenced by the graphene.

Graphene is also rather “slippery” and does not tend to stick to other materials easily, enabling the engineers to simply peel the top semiconducting layer from the wafer after its structures have been imprinted.

Jeehwan Kim, the Class of 1947 Career Development Assistant Professor in the departments of Mechanical Engineering and Materials Science and Engineering, says that in conventional semiconductor manufacturing, the wafer, once its crystalline pattern is transferred, is so strongly bonded to the semiconductor that it is almost impossible to separate without damaging both layers.

“You end up having to sacrifice the wafer — it becomes part of the device,” Kim says.

With the group’s new technique, Kim says manufacturers can now use graphene as an intermediate layer, allowing them to copy and paste the wafer, separate a copied film from the wafer, and reuse the wafer many times over. In addition to saving on the cost of wafers, Kim says this opens opportunities for exploring more exotic semiconductor materials.

“The industry has been stuck on silicon, and even though we’ve known about better performing semiconductors, we haven’t been able to use them, because of their cost,” Kim says. “This gives the industry freedom in choosing semiconductor materials by performance and not cost.”

Kim’s research team discovered this new technique at MIT’s Research Laboratory of Electronics. Kim’s MIT co-authors are first author and graduate student Yunjo Kim; graduate students Samuel Cruz, Babatunde Alawonde, Chris Heidelberger, Yi Song, and Kuan Qiao; postdocs Kyusang Lee, Shinhyun Choi, and Wei Kong; visiting research scholar Chanyeol Choi; Merton C. Flemings-SMA Professor of Materials Science and Engineering Eugene Fitzgerald; professor of electrical engineering and computer science Jing Kong; and assistant professor of mechanical engineering Alexie Kolpak; along with Jared Johnson and Jinwoo Hwang from Ohio State University, and Ibraheem Almansouri of Masdar Institute of Science and Technology.

Graphene shift

Since graphene’s discovery in 2004, researchers have been investigating its exceptional electrical properties in hopes of improving the performance and cost of electronic devices. Graphene is an extremely good conductor of electricity, as electrons flow through graphene with virtually no friction. Researchers, therefore, have been intent on finding ways to adapt graphene as a cheap, high-performance semiconducting material.

“People were so hopeful that we might make really fast electronic devices from graphene,” Kim says. “But it turns out it’s really hard to make a good graphene transistor.”

In order for a transistor to work, it must be able to turn a flow of electrons on and off, to generate a pattern of ones and zeros, instructing a device on how to carry out a set of computations. As it happens, it is very hard to stop the flow of electrons through graphene, making it an excellent conductor but a poor semiconductor.

Kim’s group took an entirely new approach to using graphene in semiconductors. Instead of focusing on graphene’s electrical properties, the researchers looked at the material’s mechanical features.

“We’ve had a strong belief in graphene, because it is a very robust, ultrathin, material and forms very strong covalent bonding between its atoms in the horizontal direction,” Kim says. “Interestingly, it has very weak Van der Waals forces, meaning it doesn’t react with anything vertically, which makes graphene’s surface very slippery.”

Copy and peel

The team now reports that graphene, with its ultrathin, Teflon-like properties, can be sandwiched between a wafer and its semiconducting layer, providing a barely perceptible, nonstick surface through which the semiconducting material’s atoms can still rearrange in the pattern of the wafer’s crystals. The material, once imprinted, can simply be peeled off from the graphene surface, allowing manufacturers to reuse the original wafer.

The team found that its technique, which they term “remote epitaxy,” was successful in copying and peeling off layers of semiconductors from the same semiconductor wafers. The researchers had success in applying their technique to exotic wafer and semiconducting materials, including indium phosphide, gallium arsenenide, and gallium phosphide — materials that are 50 to 100 times more expensive than silicon.

Kim says that this new technique makes it possible for manufacturers to reuse wafers — of silicon and higher-performing materials — “conceptually, ad infinitum.”

An exotic future

The group’s graphene-based peel-off technique may also advance the field of flexible electronics. In general, wafers are very rigid, making the devices they are fused to similarly inflexible. Kim says now, semiconductor devices such as LEDs and solar cells can be made to bend and twist. In fact, the group demonstrated this possibility by fabricating a flexible LED display, patterned in the MIT logo, using their technique.

“Let’s say you want to install solar cells on your car, which is not completely flat — the body has curves,” Kim says. “Can you coat your semiconductor on top of it? It’s impossible now, because it sticks to the thick wafer. Now, we can peel off, bend, and you can do conformal coating on cars, and even clothing.”

Going forward, the researchers plan to design a reusable “mother wafer” with regions made from different exotic materials. Using graphene as an intermediary, they hope to create multifunctional, high-performance devices. They are also investigating mixing and matching various semiconductors and stacking them up as a multimaterial structure.

“Now, exotic materials can be popular to use,” Kim says. “You don’t have to worry about the cost of the wafer. Let us give you the copy machine. You can grow your semiconductor device, peel it off, and reuse the wafer.”

The Department of Mechanical Engineering of The Hong Kong Polytechnic University (PolyU) has developed a novel technology of embedding highly conductive nanostructure into semiconductor nanofiber. The novel composite so produced has superb charge conductivity, and can therefore be widely applied, especially in environmental arena.

The innovation was awarded the Gold Medal with Congratulations of the Jury at the 45th International Exhibition of Inventions of Geneva, held on 29 March to 2 April this year.

A research team led by Prof. Wallace Leung develops novel semiconductor nanotubes with superb charge conductivity which can be widely used in different applications, especially in environmental arena. (PRNewsfoto/The Hong Kong Polytechnic Univer)

A research team led by Prof. Wallace Leung develops novel semiconductor nanotubes with superb charge conductivity which can be widely used in different applications, especially in environmental arena. (PRNewsfoto/The Hong Kong Polytechnic Univer)

Issues to address

Semiconductor made into nanofiber of diameter as small as 60nm (less than 1/1,000 of a human hair) have been widely used in modern daily life photonic devices (such as solar cells, photocatalyst for cleaning the environment), and non-photonic devices (such as chemical-biological sensor, lithium battery). However, electrons and holes generated by light or energy in semiconductor would readily recombine, thus reduce the current or device effectiveness. Such nature has limited the further development and applications of semiconductor nanofibers.

The novel technology developed by the research team led by Ir. Professor Wallace Leung, Chair Professor of Innovative Products and Technologies of the Department, have overcome such limitation. Applying electrospinning, the team succeeds in inserting highly conductive nano-structure (such as carbon nanotubes, graphene) into semiconductor nanofiber (such as Titanium Dioxide (TiO2 ). The novel nano-composite so produced thus provides a dedicated super-highway for electron transport, eliminating the problem of electron-hole recombination.

Amidst the potentially wide applications of the innovation in many spectrum, Professor Leung’s team has initially embarked on research of applying the novel nano-composite in two environmental aspects: solar cells, and photocatalysts for cleaning air.

Enhanced solar cell efficiency

The latest generation of solar cells (e.g. dye sensitized solar cell (DSSC), perovskite solar cell) are promising clean and renewable energy sources. Yet, for more wide applications, there are still much room for further enhancing their power conversion efficiency and producing in more cost-efficient ways.

By applying PolyU’s novel technology, carbon nanotube/graphene is embedded into the TiO2 component of DSSC and perovskite solar cell, boosting an increase of energy conversion from 40-66%. Compared to commercially available multi-crystalline silicon solar cell common in the market, with current price at US$0.25 (HK$1.94)/kWh, the cost of DSSC with carbon nanotube embedded is 12-32% higher (HK$2.18 – 2.56); while perovskite solar cell embedded with graphene is 28-40% lower (HK$1.17 – 1.40).

Given the superb charge conductivity of the novel semiconductor nanofiber, there is great potential for prompt development of more efficient solar cells, and at lower cost, than the silicon cells.

Enhanced photocatalyst performance in cleaning the air

TiO2 is the most commonly used photocatalyst material in commercially available air-purifying or disinfection devices in the market. However, TiO2 can only be activated by ultraviolet light (i.e. about 6% of solar energy), thus limiting its wider application as it is less effective in indoor environment. It is also relatively ineffective in converting nitric oxide (NO) into nitrogen dioxide (NO2), at a rate of less than 5%.

By applying PolyU’s novel technology, graphene roll is embedded into TZB composite (which mainly compose of TiO2). The novel semiconductor nanofiber so produced has superb conductivity, which provides a graphene superhighway for electrons to transport more quickly to oxide the absorbed pollutants. The technology also significantly increases the novel nano-fiber’s surface exposed for light absorption and trapping harmful molecules.

Such novel semiconductor nanofiber can convert about 90% of NO to NO2, a 35% increase compared to composite without graphene. If compared to high-standard TiO2 nano-particles commonly available in the market, the conversion rate is even 10 times more, yet 10 times more cost-efficient.

Readily available for wide applications

Given the wide uses of semiconductor nanofiber now and in the future, the PolyU groundbreaking technology that develops semiconductor nanofiber with superb charge conductivity has great potential for further development for different applications.

Besides in solar cells and photocatalysts, other obvious examples of making use of such novel technology include the development of biological-chemical sensors with enhanced sensitivity and sensing speed, and lithium batteries with lower impedance and increased storage.