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Converting graphene into diamond film without high pressure

Posted By Graphene Council, The Graphene Council, Wednesday, December 11, 2019
Can two layers of graphene be linked and converted to the thinnest diamond-like material? Researchers of the Center for Multidimensional Carbon Materials (CMCM) within the Institute for Basic Science (IBS, South Korea) have reported in Nature Nanotechnology ("Chemically Induced Transformation of CVD-Grown Bilayer Graphene into Fluorinated Single Layer Diamond") the first experimental observation of a chemically induced conversion of large-area bilayer graphene to the thinnest possible diamond-like material, under moderate pressure and temperature conditions.

This flexible, strong material is a wide-band gap semiconductor, and thus has potential for industrial applications in nano-optics, nanoelectronics, and can serve as a promising platform for micro- and nano-electromechanical systems.

Diamond, pencil lead, and graphene are made by the same building blocks: carbon atoms (C). Yet, it is the bonds’ configuration between these atoms that makes all the difference. In a diamond, the carbon atoms are strongly bonded in all directions and create an extremely hard material with extraordinary electrical, thermal, optical and chemical properties. In pencil lead, carbon atoms are arranged as a pile of sheets and each sheet is graphene. Strong carbon-carbon (C-C) bonds make up graphene, but weak bonds between the sheets are easily broken and in part explain why the pencil lead is soft. Creating interlayer bonding between graphene layers forms a 2D material, similar to thin diamond films, known as diamane, with many superior characteristics.

Previous attempts to transform bilayer or multilayer graphene into diamane relied on the addition of hydrogen atoms, or high pressure. In the former, the chemical structure and bonds’ configuration are difficult to control and characterize. In the latter, the release of the pressure makes the sample revert back to graphene. Natural diamonds are also forged at high temperature and pressure, deep inside the Earth. However, IBS-CMCM scientists tried a different winning approach.

The team devised a new strategy to promote the formation of diamane, by exposing bilayer graphene to fluorine (F), instead of hydrogen. They used vapors of xenon difluoride (XeF2) as the source of F, and no high pressure was needed. The result is an ultra-thin diamond-like material, namely fluorinated diamond monolayer: F-diamane, with interlayer bonds and F outside.

For a more detailed description; the F-diamane synthesis was achieved by fluorinating large area bilayer graphene on single crystal metal (CuNi(111) alloy) foil, on which the needed type of bilayer graphene was grown via chemical vapor deposition (CVD).

Conveniently, C-F bonds can be easily characterized and distinguished from C-C bonds. The team analyzed the sample after 12, 6, and 2-3 hours of fluorination. Based on the extensive spectroscopic studies and also transmission electron microscopy, the researchers were able to unequivocally show that the addition of fluorine on bilayer graphene under certain well-defined and reproducible conditions results in the formation of F-diamane. For example, the interlayer space between two graphene sheets is 3.34 angstroms, but is reduced to 1.93-2.18 angstroms when the interlayer bonds are formed, as also predicted by the theoretical studies.

“This simple fluorination method works at near-room temperature and under low pressure without the use of plasma or any gas activation mechanisms, hence reduces the possibility of creating defects,” points out Pavel V. Bakharev, the first author and co-corresponding author.

Moreover, the F-diamane film could be freely suspended. “We found that we could obtain a free-standing monolayer diamond by transferring F-diamane from the CuNi(111) substrate to a transmission electron microscope grid, followed by another round of mild fluorination,” says Ming Huang, one of the first authors.

Rodney S. Ruoff, CMCM director and professor at the Ulsan National Institute of Science and Technology (UNIST) notes that this work might spawn worldwide interest in diamanes, the thinnest diamond-like films, whose electronic and mechanical properties can be tuned by altering the surface termination using nanopatterning and/or substitution reaction techniques. He further notes that such diamane films might also eventually provide a route to very large area single crystal diamond films.

Tags:  2D material  bilayer graphene  Center for Multidimensional Carbon Materials  chemical vapor deposition  Graphene  Institute for Basic Science  Ming Huang  nanoelectronics  Nature Nanotechnology  Pavel V. Bakharev  Rodney S. Ruoff  semiconductor  Ulsan National Institute of Science and Technology 

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Properties of graphene change due to water and oxygen

Posted By Graphene Council, The Graphene Council, Friday, December 6, 2019
We often find that food becomes rotten when we leave it outside for long and fruits turn brown after they are peeled or cut. Such phenomena can be easily seen in our daily life and they illustrate the oxidation-reduction reaction. The fundamental principle controlling physical properties of two-dimensional materials noted as next generation materials like graphene is found to be redox reactions.

The research team consisted of Professor Sunmin Ryu, Kwanghee Park, and Haneul Kang, affiliated with Department of Chemistry, POSTECH, discovered that the doping of two-dimensional materials with influx of charges from outside in the air is by an electrochemical reaction driven by the redox couples of water and oxygen molecules. Using real-time photoluminescence imaging, they observed the electrochemical redox reaction between tungsten disulfide and oxygen/water in the air. According to their study¸ the redox reaction can control the physical properties of two-dimensional materials which can be applied to bendable imaging element, high-speed transistor, next generation battery, ultralight material and other two-dimensional semiconductor applications.

Two-dimensional materials like graphene and tungsten disulfide are in the form of a single or few layers of atoms in nanometer size. They are thin and easily bended but hard. Because of these properties, they are used in semiconductors, display, solar battery and more and, they are called as a dream material. However, since all atoms exist on the surface of a material, it is limited to the ambient environment such as temperature and humidity which often causes them to modify or transform. Before the research team announced on the result of their study, it has been unknown why such phenomenon happens and has been difficult to commercialize, being unable to control material properties.

The research team used real-time photoluminescence imaging of tungsten disulfide and Raman spectroscopy of graphene. They demonstrated molecular diffusion through the two-dimensional nanoscopic space between two-dimensional materials and hydrophilic substrates. They also discovered that there was enough amount of water to mediate the redox reactions in the space. Furthermore, they proved that charge doping in the acid such as hydrochloric acid is also dictated by dissolved oxygen and hydrogen-ion concentration (pH) in the same way.

What they have accomplished in this research is the fundamental principle needed to govern electrical, magnetic, and optical properties of two-dimensional or other low-dimensional materials. It is anticipated that this method can be applied to improve pretreatment which is needed to prevent two-dimensional materials from being modified by surroundings and aftertreatment technology such as encapsulation for flexible and stretchable displays.

Professor Sunmin Ryu said, "Using the real-time photoluminescence, we were able to demonstrate that the electrochemical reaction driven by the redox couples of oxygen and water molecules in the air is the key and proved the fundamental principle for governing properties of materials. This reaction is applied to not only two-dimensional materials but also other low-dimensional materials such as quantum dot and nanowires. So, our findings will be an important steppingstone to development of nano technology based on low-dimensional materials."

Tags:  2D materials  Battery  Graphene  Haneul Kang  Kwanghee Park  POSTECH  semiconductor  Sunmin Ryu  transistor 

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Using electronics to solve common biological problems

Posted By Graphene Council, The Graphene Council, Wednesday, December 4, 2019
Researchers from multiple disciplines are working together at KAUST to develop bioelectronics that can detect diseases, treat cancers and track marine animals; they may even discover the next generation of computing systems.

Cancer-killing magnets

Jurgen Kosel is an electrical engineer who loves to play with magnets. His research group has developed a technique to fabricate unique magnetic iron-oxide nanowires that can kill cancer cells1.

“Certain kinds of iron-based magnetic nanoparticles were approved many years ago by the U.S. Food and Drug Administration for use inside the human body. They are regularly used as contrast agents in magnetic resonance imaging and as nutritional supplements for people with iron deficiency,” says Kosel.

The magnetic nanoparticles currently in use are spherical in shape. Kosel and his team developed wire-shaped magnetic nanoparticles that can be rotated like a compass needle, creating a pore in cancer cell membranes that induces natural cell death. These cancer-killing nanowires can be made even more effective when coated with an anti-cancer drug or heated with a laser. They are "eaten" by cancer cells, and once released inside, they can wreak havoc.

Kosel has been working closely with cell biologist Jasmeen Merzaban, and more recently, with organic chemist Niveen Khashab to "functionalize" the surfaces of his magnetic nanowires to ensure the body’s immune system does not treat them as foreign. They are also working on preventing the wires from sticking together and on targeting cancer cells more specifically by coating them with antibodies that recognize specific antigens on their cell membranes.

Kosel has also worked with electrical engineer Muhammad Hussain to use magnets for improving the safety of cardiac catheters. They have developed a flexible magnetic sensor that is sensitive enough to detect the Earth’s magnetic field. When these sensors are placed on the tip of a cardiac catheter, for example, clinicians can detect its orientation inside blood vessels. This enables them to direct it where it is needed in order to insert a stent, for example, to relieve blockage in a heart artery. This reduces the need for prolonged doses of X-rays and contrast dyes during procedures like coronary angioplasty.

Disease detection

“Over the past 50 years, the 500-billion-dollar semiconductor industry has mainly focused on two applications: computing and communications,” says KAUST electrical engineer, Khaled Salama. “But this technology holds a lot of promise for other areas, including medical research, as people are living longer and needing more care. We need a paradigm shift to leverage some of the technologies we’ve developed for use in this area.”

Salama has developed a sensor that can detect "C-reactive proteins," a biomarker of cardiovascular disease2. He’s done this by functionalizing electrodes with nanomaterials and gold nanoparticles to improve their sensitivity. The electrodes give a signal that is proportional to the amount of C-reactive protein in a blood sample. His group developed a unique process that 3D prints the microfluidic channels that deliver samples to the sensor for biological detection.

Elsewhere at KAUST, Sahika Inal is developing a device that can make life easier for diabetics.

Inal comes from a textile manufacturing background, but her studies on the electrical properties of polymers, which are biocompatible, have led her down the route of bioelectronics. 

Her team has developed inkjet-printed, disposable, polymer-based sensors that can measure glucose levels in saliva3. “We inkjet-print conducting polymers. The biological ink contains the enzymes used for glucose sensing, an encapsulation layer that protects the enzymes, a layer that only allows glucose penetration and an insulating layer to protect the electronics,” she explains. “And then you have a paper-based sensor within a few minutes!”

Inal is also developing other biochemical sensors that can generate their own energy from compounds already present in the body to power implantable devices, such as cardiac pacemakers.

“To conduct impactful bioelectronics work, I need to be in an environment where there are biologists, the people who can give me feedback on what I develop,” says Inal.

Bio-inspired computers and animal tracking
Bioelectronics not only encompass electronic devices designed to solve biological problems, they are also electronic solutions inspired by biology.

Khaled Salama is interested in a relatively new type of bio-inspired device called a "memristor"4. These are electrical components inspired by the neural networks and synapses of the brain. Researchers hope they will lead to the next generation of computing systems and that they will be better equipped to very rapidly process huge amounts of data. Salama has developed an approach that improves their computational efficiency while reducing power consumption in these typically energy-intensive devices.

Sensing data in harsh marine environments can be particularly challenging, says Kosel. Researchers have often resorted to electronic tags placed on large marine animals to track their movements. They also use electronic sensors to conduct flow, salinity, pressure and temperature measurements in the sea. Smaller, lighter, less power-hungry tags are needed to resist corrosion, and withstand biofouling, a bacterial crust that forms on almost anything that stays in the sea for too long.

Kosel’s solution was to develop graphene sensors fabricated with a single-step laser-printing technique for marine applications. These laser-induced graphene sensors are resistant to corrosion and can survive high temperatures. They are very light and flexible, making them suitable for attaching to smaller marine animals. They also developed a technique5 that involves conducting high-frequency measurements that allow them to withstand the effects of an accumulating biofouling layer.

The group have started a conference, which will be held annually at KAUST. Last year, among the many esteemed attendees was George Malliaras, a Prince Philip Professor of Technology at the University of Cambridge. Malliaras praised the university for its world-class instrumentation, access to excellent collaborations within the campus and mechanisms to collaborate with people abroad. He says, "Taken together, these attributes have made KAUST very successful at addressing some of the most important problems that humanity faces today." 

Tags:  Bioelectronics  George Malliaras  Graphene  Healthcare  Jasmeen Merzaban  Jurgen Kosel  Khaled Salama  King Abdullah University of Science and Technology  Muhammad Hussain  nanoparticles  Niveen Khashab  Sahika Inal  semiconductor  University of Cambridge 

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Microcavities save organic semiconductors from going dark

Posted By Graphene Council, The Graphene Council, Monday, December 2, 2019
More and more electronics manufacturers are favoring organic LED displays for smartphones, TVs and computers because they are brighter and offer a greater color range.

The organic semiconductors that drive these devices are highly flexible and easily controlled. They also have the potential to be mass produced more readily than inorganic semiconductors such as silicon, which require higher temperatures for processing.

But there is a dark side to purely organic LEDs: They can be incredibly wasteful, losing up to 75% of their energy because organic semiconductors have a tendency to enter “dark states” in which they don’t emit light. These states sometimes even lead to the devices breaking down. Researchers have been looking for ways to either harness these dark states or jettison them altogether.

A collaboration led by Andrew Musser, assistant professor of chemistry and chemical biology in the College of Arts and Sciences, and Jenny Clark of the University of Sheffield, United Kingdom, has found a way to keep these organic semiconductors from going dark. Musserused tiny sandwich structures of mirrors, called microcavities, to trap light and force it to interact with a layer of molecules, forming a new hybrid state, known as a polariton, that mixes light and matter.This approach could lead to brighter, more efficient LEDs, sensors and solar cells.

The team’s paper, “Manipulating Molecules with Strong Coupling: Harvesting Triplet Excitons in Organic Exciton Microcavities,” published in Chemical Science.

“In the LED world, people are putting huge efforts into designing these vast libraries of molecules and testing them in different device configurations to see if, by tweaking the bonds or changing energy levels, they can harvest these dark states more efficiently,” Musser said. “It’s a cumbersome, difficult battle because it’s really hard to design molecules. And you don’t necessarily know how to make them do what you want.

“So what we’ve done here is address that problem with a standard molecule, purely by putting it between these mirrors and tuning the way it interacts with light,” he said. “This suggests that, for some phenomena, we can bypass a lot of this cumbersome synthetic exploration and tune the molecules at a distance.”

Musser’s interest in polaritons began while he was studying the ways organic semiconductors can improve light-harvesting efficiency in solar cells. In that case, molecules undergo a process called singlet fission, in which they absorb one photon and split that energy into two “packets” – essentially two excited electrons – thereby doubling the photon current efficiency in the solar cell.

Musser began investigating how the reverse process can also occur, with two packets of energy combining into a single, high-energy state that can emit a high-energy photon. That led him to microcavities and the ways these simple optical structures can have a profound effect on organic material through light.

In addition to manipulating a molecule’s electronic properties for enhanced brightness, recent research has demonstrated that these structures also can be used to target specific bonds and change their chemical reactivity.

Musser said different molecules interact with light in the microcavities in different ways, and further research is needed to explore the rules that underpin their behavior.

“Right now, it serves to show that when you have these complex materials and you do something even more complicated with them – putting them between these mirrors – weird and wonderful things can happen,” Musser said.

“This work literally sheds light on dark states,” said Clark. “We’ve shown that we can use polaritons to force dark states to emit light. Apart from immediate applications for LEDs, this offers a new method for studying organic semiconductors more broadly, using previously unavailable techniques.” 

Tags:  Andrew Musser  Chemical Science  energy  Graphene  Jenny Clark  LED  Semiconductor  silicon  smartphones  solar cell  U.S. Department of Energy  University of California  University of Cambridge  University of Kentucky  University of Sheffield 

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Global Graphene Group, Taiwan Company Sign JDA

Posted By Graphene Council, The Graphene Council, Wednesday, November 20, 2019

Global Graphene Group (G3) and a major Taiwan-based manufacturer have signed a joint development agreement (JDA) to incorporate graphene-enhanced materials into polyetheretherketone (PEEK)-based products for the semiconductor industry in portions of Asia.

PEEK is a high-performance engineering thermoplastic. The addition of G3’s graphene will improve the thermoplastic’s mechanical, electrical and thermal properties.

As an excellent self-lubrication material, graphene can help lower the friction ratio of PEEK/Graphene devices and reduce its wear rate significantly. It can also improve its anti-corrosion properties against harsh environments by creating a barrier to the polymer matrix.

“G3 is excited to partner with this major Taiwanese company to develop enhancements to their PEEK-based products in the Asian market,” said Dr. Bor Jang, G3 CEO. “Our graphene and graphene-enhanced solutions will be able to greatly improve the performance of semiconductors.”

“This agreement helps strengthen our relationship with this company and will allow G3 to grow our business in the Asia,” said John Davis, G3 COO. G3 expects to begin development work with the Taiwan-based company starting immediately.

Tags:  Bor Jang  Global Graphene Group  Graphene  John Davis  Semiconductor 

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Graphene and layered materials boost silicon technologies

Posted By Graphene Council, The Graphene Council, Saturday, November 16, 2019
Updated: Friday, November 8, 2019
Silicon semiconductor technology has done marvels for the advancement of our society, which has benefited tremendously from its versatile use and amazing capabilities. The development of electronics, automation, computers, digital cameras and smartphones based on this material and its underpinning technology has reached skyrocket limits, downscaling the physical size of devices and wires to the nanometre regime. 

Although this technology has been growing since the late 1960s, the miniaturization of circuits seems to have reached a possible halt, since transistors can only be shrunk down to a certain size and not further beyond. Thus, there is a pressing need to complement Si CMOS technology with new materials and fulfil the future computing requirements as well as the needs for diversification of applications.

Graphene and related materials offer prospects of advances in device performance at the atomic limit.  They provide a possible solution to overcome the limitations of silicon technology, where the combination of layered materials with silicon chips promises to surpass the current technological limitations.

A team of researchers including Stijn Goossens and Frank Koppens, based at Graphene Flagship partner ICFO, and industrial leaders from Graphene Flagship partner IMEC and TSMC provided an in-depth and thorough review of opportunities, progress and challenges of integrating atomically thin materials with Si-based technology. They give insights on how and why layered materials could overcome current challenges posed by the existing technology and how they can enhance both device component function and performance, to boost the features of future technologies, in the areas of computational and non-computational applications.

For non-computational applications, they review the possible integration of these materials for future cameras, low power optical data communications and gas and bio-sensors. In particular, in image sensors and photodetectors, graphene and related materials could enable new vision in the infrared and terahertz range in addition to the visible range of the spectrum. These can serve for example in autonomous vehicles, security at airports and augmented reality.

For computational systems, and in particular in the field of transistors, they show how challenges such as doping, contact resistance and dielectrics/encapsulation can be diminished when integrating layered materials with Si technology. Layered materials could also improve memory and data storage devices with novel switching mechanisms for meta-insulator-metal structures, avoid sneak currents in memory arrays, or even push the performance gains of copper wire-based circuitry by adhering graphene to the ultrathin copper barrier materials and thus reduce resistance, scattering and self-heating.

The review provides a roadmap of layered material integration and CMOS technology, pinpointing the stage at which all challenges regarding growth, transfer, interface, doping, contacting, and design are currently standing today and what possible processes are expected to be resolved to achieve such goals of moving from a research laboratory environment to a pilot line for production of the first devices that combine both technologies. The layered materials-CMOS roadmap, as presented in this review, gives an exciting glimpse into the future, with pilot production expected to be just a few years from now.

Frank Koppens, Graphene Flagship Work Package Leader for Photonics and Optoelectronics and lead author of the study, says: "Now we have a clear industry-driven roadmap on layered material-silicon technologies and manufacturing. Complementing the established silicon technology with layered materials is key to combine the best of both worlds and enable a plethora of large volume and low-cost applications."

Marco Romagnoli, Graphene Flagship Work Package Leader for Wafer-Scale System Integration, comments: "This is an interesting paper complementing a previous one focused on graphene photonics for telecommunications that completes the range of applications in which graphene can be exploited for large scale production in CMOS environments. Also interesting is the type of application, in which graphene can best exploit its characteristics, from IR/THz cameras to low-power electronic switching and memories.

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "The integration of graphene and related materials with silicon and CMOS technology is the next goal for the Flagship. For this reason, we will fund the first foundry focussed on the integration of layered materials. This work clearly spells out the vision for the transformative technology that integration will enable."

Tags:  Andrea C. Ferrari  Frank Koppens  Graphene  Graphene Flagship  ICFO  Marco Romagnoli  optoelectronics  photonics  Semiconductor  Stijn Goossens  transistor 

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Clarification of a new synthesis mechanism of semiconductor atomic sheet

Posted By Graphene Council, The Graphene Council, Tuesday, September 24, 2019
In Japan Science and Technology Agency's Strategic Basic Research Programs, Associate Professor Toshiaki Kato and Professor Toshiro Kaneko of the Department of Electronic Engineering, Graduate School of Engineering, Tohoku University succeeded in clarifying a new synthesis mechanism regarding transition metal dichalcogenides (TMD)1), which are semiconductor atomic sheets having thickness in atomic order.

Because it is difficult to directly observe the aspect of the growing process of TMD in a special environment, the initial growth process remained unclear, and it has been desirable to elucidate a detailed mechanism of synthesis to obtain high-quality TMD.

An in-situ observing synthesis method2) has been developed by our research group to examine the growth aspect of TMD as a real-time optical image in a special high temperature atmosphere of about 800°C in the presence of corrosive gases. In addition, a synthesis substrate, which is a mechanism to control diffusion during the crystal growth of a precursor3), has been developed in advance; further, it has been clarified that the growing precursor diffuses a distance about 100 times larger than in conventional semiconductor materials. 

It was also demonstrated that nucleation occurs due to the involvement of the precursor in a droplet state. Furthermore, by utilizing this method, a large-scale integration of more than 35,000 monolayer single crystal atomic sheets has been achieved on a substrate in a practical scale (Figure 1).

Utilizing the results of the present research, the large-scale integration of atomic-order thick4) semiconductor atomic sheets can be fabricated and is expected to be put into practical use in the field of next-generation flexible electronics.

Tags:  Electronics  Graphene  Semiconductor  Tohoku University  Toshiaki Kato  Toshiro Kaneko 

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Graphene in Electronic Circuits

Posted By Graphene Council, The Graphene Council, Wednesday, July 31, 2019
Updated: Tuesday, July 30, 2019
Ever since graphene was discovered in 2004, researchers around the world have been working to develop commercially scalable applications for this high-performance material.

Graphene is 100 to 300 times stronger than steel at the atomic level and has a maximum electrical current density orders of magnitude greater than that of copper, making it the strongest, thinnest and, by far, the most reliable electrically conductive material on the planet. It is, therefore, an extremely promising material for interconnects, the fundamental components that connect billions of transistors on microchips in computers and other electronic devices in the modern world.

For over two decades, interconnects have been made of copper, but that metal encounters fundamental physical limitations as electrical components that incorporate it shrink to the nanoscale. “As you reduce the dimensions of copper wires, their resistivity shoots up,” said Kaustav Banerjee, a professor in the Department of Electrical and Computer Engineering. “Resistivity is a material property that is not supposed to change, but at the nanoscale, all properties change.”

As the resistivity increases, copper wires generate more heat, reducing their current-carrying capacity. It’s a problem that poses a fundamental threat to the $500 billion semiconductor industry. Graphene has the potential to solve that and other issues. One major obstacle, though, is designing graphene micro-components that can be manufactured on-chip, on a large scale, in a commercial foundry.

“Whatever the component, be it inductors, interconnects, antennas or anything else you want to do with graphene, industry will move forward with it only if you find a way to synthesize graphene directly onto silicon wafers,” Banerjee said. He explained that all manufacturing processes related to the transistors, which are made first, are referred to as the ‘front end.’ To synthesize something at the back-end — that is, after the transistors are fabricated — you face a tight thermal budget that cannot exceed a temperature of about 500 degrees Celsius. If the silicon wafer gets too hot during the back-end processes employed to fabricate the interconnects, other elements that are already on the chip may get damaged, or some impurities may start diffusing, changing the characteristics of the transistors.

Now, after a decade-long quest to achieve graphene interconnects, Banerjee’s lab has developed a method to implement high-conductivity, nanometer-scale doped multilayer graphene (DMG) interconnects that are compatible with high-volume manufacturing of integrated circuits. A paper describing the novel process was named one of the top papers at the 2018 IEEE International Electron Devices Meeting (IEDM),  from more than 230 that were accepted for oral presentations. It also was one of only two papers included in the first annual “IEDM Highlights” section of an issue of the journal Nature Electronics.

Banerjee first proposed the idea of using doped multi-layer graphene at the 2008 IEDM conference and has been working on it ever since. In February 2017 he led the experimental realization of the idea by Chemical Vapor Deposition (CVD) of multilayer graphene at a high temperature, subsequently transferring it to a silicon chip, then patterning the multilayer graphene, followed by doping. Electrical characterization of the conductivity of DMG interconnects down to a width of 20 nanometers established the efficacy of the idea that was proposed in 2008. However, the process was not “CMOS-compatible” (the standard industrial-scale process for making integrated circuits), since the temperature of CVD processes far exceed the thermal budget of back-end processes.

To overcome this bottleneck, Banerjee’s team developed a unique pressure-assisted solid-phase diffusion method for directly synthesizing a large area of high-quality multilayer graphene on a typical dielectric substrate used in the back-end CMOS process. Solid-phase diffusion, well known in the field of metallurgy and often used to form alloys, involves applying pressure and temperature to two different materials that are in close contact so that they diffuse into each other.

Banerjee’s group employed the technique in a novel way. They began by depositing solid-phase carbon in the form of graphite powder onto a deposited layer of nickel metal of optimized thickness. Then they applied heat (300 degrees Celsius) and nominal pressure to the graphite powder to help break down the graphite. The high diffusivity of carbon in nickel allows it to pass rapidly through the metal film.

How much carbon flows through the nickel depends on its thickness and the number of grains it holds. “Grains” refer to the fact that deposited nickel is not a single-crystal metal, but rather a polycrystalline metal, meaning it has areas where two single-crystalline regions meet each other without being perfectly aligned. These areas are called grain boundaries, and external particles — in this case, the carbon atoms — easily diffuse through them. The carbon atoms then recombine on the other surface of the nickel closer to the dielectric substrate, forming multiple graphene layers.

Banerjee’s group is able to control the process conditions to produce graphene of optimal thickness. “For interconnect applications, we know how many layers of graphene are needed,” said Junkai Jiang, a Ph.D. candidate in Banerjee’s lab and lead author of the 2018 IEDM paper. “So we optimized the nickel thickness and other process parameters to obtain precisely the number of graphene layers we want at the dielectric surface. “Subsequently, we simply remove the nickel by etching so that what’s left is only very high-quality graphene — virtually the same quality as graphene grown by CVD at very high temperatures,” he continued. “Because our process involves relatively low temperatures that pose no threat to the other fabricated elements on the chip, including the transistors, we can make the interconnects right on top of them.”

UCSB has filed a provisional patent on the process, which overcomes the obstacles that, until now, have prevented graphene from replacing copper. Bottom line: graphene interconnects help to create faster, smaller, lighter, more flexible, more reliable and more cost-effective integrated circuits. Banerjee is currently in talks with industry partners interested in potentially licensing this CMOS-compatible graphene synthesis technology, which could pave the way for what would be the first 2D material to enter the mainstream semiconductor industry.

Tags:  2D materials  CVD  Graphene  Graphite  Junkai Jiang  Kaustav Banerjee  Semiconductor  UC Santa Barbara 

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New method of synthesising nanographene on metal oxide surfaces

Posted By Graphene Council, The Graphene Council, Tuesday, March 5, 2019
Updated: Tuesday, March 5, 2019

Nanostructures based on carbon are promising materials for nanoelectronics.

However, to be suitable, they would often need to be formed on non-metallic surfaces, which has been a challenge – up to now. Researchers at FAU have found a method of forming nanographenes on metal oxide surfaces. Their research, conducted within the framework of collaborative research centre 953 – Synthetic Carbon Allotropes funded by the German Research Foundation (DFG), has now been published in the journal Science.

Two-dimensional, flexible, tear-resistant, lightweight, and versatile are all properties that apply to graphene, which is often described as a miracle material. In addition, this carbon-based nanostructure has unique electrical properties that make it attractive for nanoelectronic applications. Depending on its size and shape, nanographene can be conductive or semi-conductive – properties that are essential for use in nanotransistors. Thanks to its good electrical and thermal conductivity, it could also replace copper (which is conductive) and silicon (which is semi-conductive) in future nanoprocessors.

Nanographene on metal oxides

The problem: In order to create an electronic circuit, the molecules of nanographene must be synthesised and assembled directly on an insulating or semi-conductive surface. Although metal oxides are the best materials for this purpose, in contrast to metal surfaces, direct synthesis of nanographenes on metal oxide surfaces is not possible as they are considerably less chemically reactive. The researchers would have to carry out the process at high temperatures, which would lead to several uncontrollable secondary reactions.

A team of scientists led by Dr. Konstantin Amsharov from the Chair of Organic Chemistry II have now developed a method of synthesising nanographenes on non-metallic surfaces, that is insulating surfaces or semi-conductors.

It’s all about the bond

The researchers’ method involves using a carbon fluorine bond, which is the strongest carbon bond. It is used to trigger a multilevel process. The desired nanographenes form like dominoes via cyclodehydrofluorination on the titanium oxide surface. All ‘missing’ carbon-carbon bonds are thus formed after each other in a formation that resembles a zip being closed.

This enables the researchers to create nanographenes on titanium oxide, a semi-conductor. This method also allows them to define the shape of the nanographene by modifying the arrangement of the preliminary molecules. New carbon-carbon bonds and, ultimately, nanographenes form where the researchers place the fluourine atoms.

For the first time, these research results demonstrate how carbon-based nanostructures can be manufactured by direct synthesis on the surfaces of technically-relevant semi-conducting or insulating surfaces. ‘This groundbreaking innovation offers effective and simple access to electronic nanocircuits that really work, which could scale down existing microelectronics to the nanometre scale,’ explains Dr. Amsharov.

Tags:  Friedrich-Alexander-Universität Erlangen-Nürnberg  Graphene  Konstantin Amsharov  nanoelectronics  nanographene  Semiconductor 

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Graphene Interlayer Fixes the Schottky Diode

Posted By Dexter Johnson, IEEE Spectrum, Friday, February 10, 2017

Schottky diodes are the grand daddy of semiconductor devices. They are formed when a semiconductor material is combined with a metal and the junction between the two materials creates the Schottky diode. Despite being around since forever, it’s never been quite possible to make them into an ideal diode in which when a voltage is applied it acts as conductor and when the voltage is reversed it serves as a insulator.

Now researchers at the Ulsan National Institute of Science and Technology (UNIST) in Korea have been able to produce the ideal version of the Schottky diode by inserting a graphene layer between the semiconductor and the metal, and in the process have eliminated 50 years of head scratching over this issue. 

In research described in the journal Nano Letters, the UNIST researchers discovered that graphene serves to prevent the intermixing of atoms that occurs when the semiconductor and metal are touching each other directly.

“The space between the carbon atoms that make up the graphene layer has a high quantum mechanical electron density and therefore no atoms can pass through it,” said Kibog Park, a professor at UNIST and co-author of the paper, in a press release. “Therefore, by inserting the graphene layer between metal and semiconductor, it is possible to overcome the inevitable atomic diffusion problem.”

While the research solved this problem, it also confirmed a prediction that it didn’t matter what kind of metal was used to form the Schottky junction; the performance does not change significantly.

The applications for Schottky diodes are pretty broad, but the main use is that of a rectifier, which converts alternating current (AC) to direct current (DC). But so many electronic devices use these diodes that this research is expected to resolve what has been a long-standing issue within the electronic industry.

Tags:  electronics  graphene interlayer  metal  rectifier  Schottky diodes  semiconductor 

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