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Crystal with a Twist: Researchers Grow Spiraling New Material

Posted By Graphene Council, Monday, February 17, 2020

With a simple twist of the fingers, one can create a beautiful spiral from a deck of cards. In the same way, scientists have created new inorganic crystals made of stacks of atomically thin sheets. These stacks unexpectedly spiral like a nanoscale card deck. Their surprising structures may yield unique optical, electronic and thermal properties. These properties may even include superconductivity, the ability to conduct electricity without loss. These crystals in the shape of a helix are made of stacked layers of germanium sulfide. This is a semiconductor material that, like graphene, readily forms sheets that are only a few atoms thick. Such “nanosheets” are also called “2D materials.”

This is the first time that scientists have made 2D materials that form a continuously twisting shape in a structure that is thousands layers thick. The spiral structures could hold unique properties that aren’t observed in regularly stacked materials. Scientists could likely use this technique to grow layers of other materials that form atomically thin layers.

Summary

To create the twisted structures, the team took advantage of a crystal defect called a screw dislocation, a “mistake” in the orderly crystal structure that gives it a bit of a twisting force. This “Eshelby Twist”, named after scientist John D. Eshelby, has been used by others to create nanowires that spiral like pine trees. But this study is the first time the Eshelby Twist has been used to make crystals built of stacked 2D layers of an atomically thin semiconductor.

In a major discovery last year, scientists reported that graphene becomes superconductive when two atomically thin sheets of the material are stacked and twisted at what’s called a “magic angle.” While other researchers have since succeeded at stacking two layers at a time, this new work provides a recipe for synthesizing stacked structures that are hundreds of thousands or even millions of layers thick in a continuously twisting fashion.

By adjusting the material synthesis conditions and length, the researchers could change the angle between the layers, creating a twisted structure that is tight, like a spring, or loose, like an uncoiled Slinky.

Scientists performed X-ray analyses for the study at the Advanced Light Source and measured the crystal’s twist angles at the Molecular Foundry, both DOE Office of Science user facilities.

Funding
Y.L. and J.Y. are supported by the Samsung Advanced Institute of Technology. Work at the Molecular Foundry and the Advanced Light Source was supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy. H.S. and D.C.C. are supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering. within the Electronic Materials Program (KC1201). This work was performed, in part, at the Center for Nanoscale Materials, a US Department of Energy Office of Science User Facility. We thank C. So, C. Song, X. Wang, S. Yan, K. Bustillo and C. V. Stan for help with the experiments.

Tags:  2D materials  Electronics  Graphene  Samsung Advanced Institute of Technology  Semiconductor 

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Engineers mix and match materials to make new stretchy electronics

Posted By Graphene Council, Saturday, February 8, 2020
At the heart of any electronic device is a cold, hard computer chip, covered in a miniature city of transistors and other semiconducting elements. Because computer chips are rigid, the electronic devices that they power, such as our smartphones, laptops, watches, and televisions, are similarly inflexible.

Now a process developed by MIT engineers may be the key to manufacturing flexible electronics with multiple functionalities in a cost-effective way.

The process is called  “remote epitaxy” and involves growing thin films of semiconducting material on a large, thick wafer of the same material, which is covered in an intermediate layer of graphene. Once the researchers grow a semiconducting film, they can peel it away from the graphene-covered wafer and then reuse the wafer, which itself can be expensive depending on the type of material it’s made from. In this way, the team can copy and peel away any number of thin, flexible semiconducting films, using the same underlying wafer.

In a paper published today in the journal Nature, the researchers demonstrate that they can use remote epitaxy to produce freestanding films of any functional material. More importantly, they can stack films made from these different materials, to produce flexible, multifunctional electronic devices.

The researchers expect that the process could be used to produce stretchy electronic films for a wide variety of uses, including virtual reality-enabled contact lenses, solar-powered skins that mold to the contours of your car, electronic fabrics that respond to the weather, and other flexible electronics that seemed until now to be the stuff of Marvel movies.

“You can use this technique to mix and match any semiconducting material to have new device functionality, in one flexible chip,” says Jeehwan Kim, an associate professor of mechanical engineering at MIT. “You can make electronics in any shape.”

Buying time

Kim and his colleagues reported their first results using remote epitaxy in 2017. Then, they were able to produce thin, flexible films of semiconducting material by first placing a layer of graphene on a thick, expensive wafer made from a combination of exotic metals. They flowed atoms of each metal over the graphene-covered wafer and found the atoms formed a film on top of the graphene, in the same crystal pattern as the underlying wafer. The graphene provided a nonstick surface from which the researchers could peel away the new film, leaving the graphene-covered wafer, which they could reuse. 

In 2018, the team showed that they could use remote epitaxy to make semiconducting materials from metals in groups 3 and 5 of the periodic table, but not from group 4. The reason, they found, boiled down to polarity, or the respective charges between the atoms flowing over graphene and the atoms in the underlying wafer.

Since this realization, Kim and his colleagues have tried a number of increasingly exotic semiconducting combinations. As reported in this new paper, the team used remote epitaxy to make flexible semiconducting films from complex oxides — chemical compounds made from oxygen and at least two other elements. Complex oxides are known to have a wide range of electrical and magnetic properties, and some combinations can generate a current when physically stretched or exposed to a magnetic field.

Kim says the ability to manufacture flexible films of complex oxides could open the door to new energy-havesting devices, such as sheets or coverings that stretch in response to vibrations and produce electricity as a result. Until now, complex oxide materials have only been manufactured on rigid, millimeter-thick wafers, with limited flexibility and therefore limited energy-generating potential.

The researchers did have to tweak their process to make complex oxide films. They initially found that when they tried to make a complex oxide such as strontium titanate (a compound of strontium, titanium, and three oxygen atoms), the oxygen atoms that they flowed over the graphene tended to bind with the graphene’s carbon atoms, etching away bits of graphene instead of following the underlying wafer’s pattern and binding with strontium and titanium. As a surprisingly simple fix, the researchers added a second layer of graphene.

“We saw that by the time the first layer of graphene is etched off, oxide compounds have already formed, so elemental oxygen, once it forms these desired compounds, does not interact as heavily with graphene,” Kim explains. “So two layers of graphene buys some time for this compound to form.”

Peel and stack

The team used their newly tweaked process to make films from multiple complex oxide materials, peeling off each 100-nanometer-thin layer as it was made. They were also able to stack together layers of different complex oxide materials and effectively glue them together by heating them slightly, producing a flexible, multifunctional device.

“This is the first demonstration of stacking multiple nanometers-thin membranes like LEGO blocks, which has been impossible because all functional electronic materials exist in a thick wafer form,” Kim says.

In one experiment, the team stacked together films of two different complex oxides: cobalt ferrite, known to expand in the presence of a magnetic field, and PMN-PT, a material that generates voltage when stretched. When the researchers exposed the multilayer film to a magnetic field, the two layers worked together to both expand and produce a small electric current. 

The results demonstrate that remote epitaxy can be used to make flexible electronics from a combination of materials with different functionalities, which previously were difficult to combine into one device. In the case of cobalt ferrite and PMN-PT, each material has a different crystalline pattern. Kim says that traditional epitaxy techniques, which grow materials at high temperatures on one wafer, can only combine materials if their crystalline patterns match. He says that with remote epitaxy, researchers can make any number of different films, using different, reusable wafers, and then stack them together, regardless of their crystalline pattern.

“The big picture of this work is, you can combine totally different materials in one place together,” Kim says. “Now you can imagine a thin, flexible device made from layers that include a sensor, computing system, a battery, a solar cell, so you could have a flexible, self-powering, internet-of-things stacked chip.”

The team is exploring various combinations of semiconducting films and is working on developing prototype devices, such as something Kim is calling an “electronic tattoo” — a flexible, transparent chip that can attach and conform to a person’s body to sense and wirelessly relay vital signs such as temperature and pulse. “We can now make thin, flexible, wearable electronics with the highest functionality,” Kim says. “Just peel off and stack up.”

Tags:  Electronics  Graphene  Jeehwan Kim  MIT  Semiconductor 

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Crystal-stacking process can produce new materials for high-tech devices

Posted By Graphene Council, Saturday, February 8, 2020
The magnetic, conductive and optical properties of complex oxides make them key to components of next-generation electronics used for data storage, sensing, energy technologies, biomedical devices and many other applications.

Stacking ultrathin complex oxide single-crystal layers -- those composed of geometrically arranged atoms -- allows researchers to create new structures with hybrid properties and multiple functions. Now, using a new platform developed by engineers at the University of Wisconsin-Madison and the Massachusetts Institute of Technology, researchers will be able to make these stacked-crystal materials in virtually unlimited combinations.

Epitaxy is the process for depositing one material on top of another in an orderly way. The researchers' new layering method overcomes a major challenge in conventional epitaxy -- that each new complex oxide layer must be closely compatible with the atomic structure of the underlying layer. It's sort of like stacking Lego blocks: The holes on the bottom of one block must align with the raised dots atop the other. If there's a mismatch, the blocks won't fit together properly.

"The advantage of the conventional method is that you can grow a perfect single crystal on top of a substrate, but you have a limitation," says Chang-Beom Eom, a UW-Madison professor of materials science and engineering and physics. "When you grow the next material, your structure has to be the same and your atomic spacing must be similar. That's a constraint, and beyond that constraint, it doesn't grow well."

A couple of years ago, a team of MIT researchers developed an alternate approach. Led by Jeehwan Kim, an associate professor in mechanical engineering and materials science and engineering at MIT, the group added an ultrathin intermediate layer of a unique carbon material called graphene, then used epitaxy to grow a thin semiconducting material layer atop that. Just one molecule thick, the graphene acts like a peel-away backing due to its weak bonding. The researchers could remove the semiconductor layer from the graphene. What remained was a freestanding ultrathin sheet of semiconducting material.

Eom, an expert in complex oxide materials, says they are intriguing because they have a wide range of tunable properties -- including multiple properties in one material -- that many other materials do not. So, it made sense to apply the peel-away technique to complex oxides, which are much more challenging to grow and integrate.

"If you have this kind of cut-and-paste growth and removal, combined with the different functionality of putting single-crystal oxide materials together, you have a tremendous possibility for making devices and doing science," says Eom, who connected with mechanical engineers at MIT during a sabbatical there in 2014.

The Eom and Kim research groups combined their expertise to create ultrathin complex oxide single-crystal layers, again using graphene as the peel-away intermediate. More importantly, however, they conquered a previously insurmountable obstacle -- the difference in crystal structure -- in integrating different complex oxide materials.

"Magnetic materials have one crystal structure, while piezoelectric materials have another," says Eom. "So you cannot grow them on top of each other. When you try to grow them, it just becomes messy. Now we can grow the layers separately, peel them off, and integrate them."

In its research, the team demonstrated the efficacy of the technique using materials such as perovskite, spinel and garnet, among several others. They also can stack single complex oxide materials and semiconductors.

"This opens up the possibility for the study of new science, which has never been possible in the past because we could not grow it," says Eom. "Stacking these was impossible, but now it is possible to imagine infinite combinations of materials. Now we can put them together."

The advance also opens doors to new materials with functionalities that drive future technologies. "This advance, which would have been impossible using conventional thin film growth techniques, clears the way for nearly limitless possibilities in materials design," says Evan Runnerstrom, program manager in materials design in the Army Research Office, which funded part of the research. "The ability to create perfect interfaces while coupling disparate classes of complex materials may enable entirely new behaviors and tunable properties, which could potentially be leveraged for new Army capabilities in communications, reconfigurable sensors, low power electronics, and quantum information science."

Tags:  Chang-Beom Eom  Evan Runnerstrom  Graphene  Jeehwan Kim  Massachusetts Institute of Technology  Semiconductor  U.S. Army Research Office  University of Wisconsin-Madison 

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Two new FLAG-ERA projects in Aachen

Posted By Graphene Council, Friday, January 31, 2020

The Aachen Graphene & 2D Materials Center  has won two projects on basic research and innovation on graphene in the last FLAG-ERA Joint Transnational Call.

FLAG-ERA is a network of national and regional funding organizations in Europe that supports the two first FET Flagship projects of the European Commission: the Graphene Flagship and the Human Brain Project. On November 2018, FLAG-ERA announced its third Joint Transnational Call (FLAG-ERA JTC 2019), with an initial budget of 20 M€. This type of call presents a number of peculiarities. First, it funds only topics where synergies with the two Flagships are expected. Second, it funds only projects that involve partners form three or more different countries participating to the FLAG-ERA net. Third, while all projects are evaluated “centrally” by an independent evaluation panel, those recommended for funding are funded by the individual funding agencies − meaning that each partner of the project is funded by its national funding agency.

“It might seem a complicated way of financing research”, says Prof. Max Lemme from the chair of Electronic Devices at RWTH Aachen University, “but graphene is a topic that profits enormously from this kind of transnational collaborations.” Lemme is partner of the project 2D-NEMS, together with Prof. Christoph Stampfer − also at RWTH − and with colleagues from the Royal Institute of Technology in Sweden and from Graphenea Semiconductor in Spain.

The goal of the project is to explore the potential of heterostructures formed by graphene and other two-dimensional materials for realizing ultra small and ultra sensitive sensors, such as accelerometers. “We want to understand which combination of 2D-materials works better for a certain type of sensors and why”, says Lemme. “And, most importantly, we want to realize prototypes that are not only good for high-impact publications, but that can be of real interest for industry.”

Christoph Stampfer, head of II Institute of Physics A, is also involved in the FLAG-ERA project TATTOOS, together with colleagues from UC Louvain in Belgium and CNRS in Paris.  TATTOOS is a more exploratory project, dedicated to some of the most fascinating properties of bilayer graphene.

As the name says, bilayer graphene is a material formed by two layers of graphene. One of the big scientific surprises of 2018 was that for certain “magic angles” between the two layers  the system can exhibit superconductivity or other exotic properties. “In TATTOOS we’ll use a technique developed by our CNRS colleague, which should allow to rotate dynamically the angle between the layers with the tip of an atomic force microscope.”, explains Stampfer. “It’s a crazy idea! Typically, changing the angle requires making a new sample. If they hadn’t already demonstrated this approach on a similar system, I would not believe it can work. I’m really excited to see what new physics we can explore in this way.”

Lemme and Stampfer are both members of the Aachen Graphene and 2D Materials Center. “The fact that the Center is participating in two of the nine projects funded in the sub-call “Graphene – Basic Research and Innovation”, is a good example of the relevance of the research done here in Aachen”, says Stampfer, who is also the spokesperson of the Center.

Tags:  2D materials  Christoph Stampfer  Graphene  Graphene Flagship  Graphenea  Max Lemme  RWTH Aachen University  Semiconductor 

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Chemists have managed to stabilize the 'capricious' phosphorus

Posted By Graphene Council, Tuesday, January 21, 2020
An international team of Russian, Swedish and Ukrainian scientists has identified an effective strategy to improve the stability of two-dimensional black phosphorus, which is a promising material for use in optoelectronics.

The most effective mechanism of fluorination has been revealed. In addition to increased stability compared to previously proposed structures, the materials predicted by the researchers showed high antioxidative stability. The main results of the work have been presented in The Journal of Physical Chemistry Letters.

Black phosphorus is obtained from white phosphorus under conditions of high pressure and elevated temperature. The material has a layered structure and resembles graphite in appearance and properties. However, unlike graphite, it is a good semiconductor.

"Phosphorene is a monolayer of black phosphorus with interesting physical properties (high anisotropic electrical and thermal conductivity, flexible band gap variability depending on the number of layers), which makes it a promising material for use in various fields of optoelectronics (transistors, inverters, flexible electronics, solar panels). Unfortunately, one of its main problems is instability in the environment. Unlike its volumetric analogue, which is almost immune to external conditions, phosphorene quickly begins to attach oxygen from the air and degrades within a few hours. As one of the strategies for improving the stability of phosphorene, mechanism of fluorination was proposed. Over the past five years, scientists have proposed several theoretically possible options for such a "coupling". An experiment was conducted that showed a significant increase in the stability of phosphorus in ambient conditions after fluorination. However, the features of the obtained material structure remained unexplained.

Using various theoretical approaches, my colleagues and I showed that the previously proposed structures of "stabilized" phosphorus were actually unstable. It is known that phosphorus is able to form compounds with 3 or 5 fluorine atoms. Our calculations also confirmed that the characteristic coordination of the phosphorus atom in the PF system is 3 or 5. By sequential addition of atoms, it was possible to identify the most effective and really working mechanism by which fluorine atoms should attach to the surface of phosphorene. Thus, we have determined the type of structures that are likely to have been obtained by our predecessors in the above-mentioned experiment," -- said Artem Kuklin, a research fellow of SibFU.

Scientists note that the materials formed by the predicted mechanism are really stable and have increased antioxidant ability (that is, they are not quickly degradable) and their electronic properties, which do not differ much from the properties of pure phosphorus, provide the possibility of their practical application in optoelectronic devices, i.e. transistors, solar panels, flexible electronics, LEDs, photosensors, biomedical devices, optical devices for storing and transmitting information, etc.

Tags:  Artem Kuklin  Graphene  optoelectronics  photonics  Semiconductor  Siberian Federal University 

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Energy levels in electrons of 2D materials are mapped for the first time

Posted By Graphene Council, Thursday, January 9, 2020
Researchers based at the National Graphene Institute at The University of Manchester have developed an innovative measurement method that allows, for the first time, the mapping of the energy levels of electrons in the conduction band of semiconducting 2D materials.

Writing in Nature Communications, a team led by Dr Roman Gorbachev reports the first precise mapping of the conduction band of 2D indium selenide (InSe) using resonant tunnelling spectroscopy, to access the previously unexplored part of the electronic structure. They observed multiple subbands for both electrons and holes and tracked their evolution with the number of atomic layers in InSe.

Many emerging technologies rely on novel semiconductor structures, where the motion of electrons is restricted in one or more directions. Such confinement is in the nature of 2D materials and it is responsible for many of their new and exciting properties.

For instance, the colour of the emitted light shifts towards shorter wavelengths as they get thinner, analogous to quantum dots changing colour when their size is varied. As another consequence, the allowed energy available for the electrons in such materials, called conduction and valence bands, split into multiple subbands.

We hope this study will pave the way for exploration of intersubband transitions and lead to development of prototype optoelectronic devices with tuneable emission in the challenging terahertz range, Dr Roman Gorbachev.

Optical transitions between such subbands present a large potential for real-life applications as they provide optically active in terahertz and far-infrared ranges, which can be employed for security and communication technologies as light emitters or detectors.

Dr Roman Gorbachev said: “We hope this study will pave the way for exploration of intersubband transitions and lead to development of prototype optoelectronic devices with tuneable emission in the challenging terahertz range.”

Tags:  2D materials  Graphene  optoelectronics  Roman Gorbachev  Semiconductor  University of Manchester 

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Saving Moore’s Law

Posted By Graphene Council, Tuesday, December 31, 2019
It’s a well-known observation: The number of transistors on a microchip will double roughly every two years. And, thanks to advances in miniaturization and performance, this axiom, known as Moore’s Law, has held true since 1965, when Intel co-founder Gordon Moore first made that statement based on emerging trends in chip manufacturing at Intel. 

However, integrated circuits are hitting hard physical limits that are rendering Moore’s Law obsolete — elements on a dense integrated circuit (IC) can get only so small and so tightly packed together before they begin to interfere with each other and otherwise lose their functionality.

“Apart from fundamental physical limits to the scaling of transistor feature sizes below a few nanometers, there are significant challenges in terms of reducing power dissipation, as well as justifying the incurred cost of IC fabrication,” said Kaustav Banerjee, a professor of electrical and computer engineering at UC Santa Barbara. As a result, the very devices that we rely on for their steadily improving performance and versatility — computers, smartphones, internet-enabled gadgets — would also hit a limit, he said.

But according to Banerjee, one of world’s leading scientific minds in the field of nanoelectronics, there is a way to maintain Moore’s Law indefinitely, by taking advantage of relatively new and promising two-dimensional (2D) materials and combining them with monolithic 3D (M3D) integration practices to create ultra-compact, yet high-performing electronic chips that could overcome the challenges that face conventional integrated circuits. While Banerjee first disclosed this idea in a visionary article back in 2014, more detailed research evaluating this technology from his Nanoelectronics Research Lab was recently published in the IEEE Journal of the Electron Devices Society.

“Two-dimensional materials can be stable in their monolayer form with atomic scale thickness – 0.5 nanometer or 5 Angstroms for graphene (a conductor) and hexagonal-boron-nitride (an insulator), and ~6.5 Angstroms for 2D transition metal dichalcogenides (semiconductors) such as molybdenum-disulphide (MoS2) or tungsten-disulphide/diselenide (WS2/WSe2).” Banerjee said. “In addition, due to their layered nature, they offer pristine surfaces relatively free of defects and are excellent conductors of heat in the in-plane direction. All these properties, along with the possibility to directly synthesize these materials on top of prefabricated devices, offer unprecedented advantages over conventional 3D ICs that are already in the market or M3D integration with conventional electronic materials.”

The Benefits of Thinness 

According to the Banerjee Group’s study, there’s a limit to how thin conventional semiconductor materials can get before their desirable electronic properties begin to fade. 

“Thickness scaling of common semiconductor materials, such as Si, becomes challenging below a few nanometers due to rapid degradation of their mobility caused by the increase in electron scatterings from surface roughness,” Banerjee said. “In fact, below ~1 nm, conventional materials like Si or Ge may not be thermodynamically stable.”

On the other hand, atomically thin and stable 2D materials, such as graphene, hexagonal boron nitride (h-BN), and transitional metal dichalcogenides (MoS2, WS2, WSe2, etc) are highly space-efficient, thickness-wise. Moreover, due to their layered nature and pristine interfaces, the 2D semiconductors exhibit reasonably high mobilities and immunity against surface defects, according to the paper. In addition, 2D materials tend to be a lot more flexible than their conventional counterparts, which make them ideal for state-of-the-art electronics applications, such as flexible displays.  Stacked 2D materials, in contrast to their stacked 3D counterparts, meanwhile, can also minimize the inter-tier signal delays, thermal resistance, and reduce potential overheating.

By selecting certain 2D materials and stacking them, according to the researchers, not only does the monolithic 3D conserve precious space on the chip, but also allows for configuration based on the combined electronic properties of the materials.

For example, owing to the atomically-thin vertical dimensions of 2D materials, and carefully-designed inter-tier electrostatics with graphene shielding layer that also benefits from enhanced heat dissipation, aggressive scaling of tier thickness down to sub-μm can be achieved,” Banerjee said. “Such scaling allows over 10-folds higher integration density with respect to conventional 3D integration, and over 150% greater integration density with respect to conventional M3D integration, with plenty of room for further improvements.” 

“Thus, 2D materials can help realize the ultimate density scaling of integrated electronics — both laterally and vertically — which can usher an unprecedented era of innovation and economic growth for the worldwide semiconductor industry,” he added.

Manufacturing Outlook

As with many innovations with potential to become mainstream technologies, there are challenges to consider to pave the way toward their mass manufacturing. For monolithic 3D devices, the challenges are to be able to fabricate these components at relatively low temperatures (lower than 500 degrees Celsius) to avoid degradations and damages to prefabricated devices located in the lower tiers; electromagnetic interference; and heat dissipation.

Last year, Banerjee’s group demonstrated a CMOS compatible graphene synthesis method that essentially addressed the low-temperature and transfer-free synthesis challenge for graphene. Similar efforts are underway in his laboratory to synthesize other 2D materials directly on wafers at low temperatures.

“Additionally, careful design is needed to electrically shield the generated electromagnetic waves from affecting the operations of devices on adjacent or nearby tiers,” said Junkai Jiang, the lead author of the article and recent recipient of a doctoral degree in electrical and computer engineering from Banerjee’s laboratory. The researchers noted that by using a thin graphene shielding layer between tiers (preferably doped to enhance electromagnetic screening effect), interference can be prevented even as the vertical layers are scaled down. 

In terms of heat dissipation, the thinness of the material itself is conducive to allowing the heat from densely packed stacked components to dissipate efficiently. Kamyar Parto, a co-author of the study and a member of Banerjee’s lab, remarked that “the 2D materials have much higher in-plane thermal conductivity compared to thinned-down conventional materials like silicon, which helps fast lateral heat transport, thereby reducing the risks of any hot-spot formation.”  

“Ultimately, we envision heterogeneously integrated devices and technologies enabled by 2D materials to realize the world’s tallest and densest ‘chip-cities’ with unprecedented performance, storage capacity, and energy-efficiency,” he added.

Tags:  2D materials  Electronics  Graphene  Hexagonal boron nitride  Intel  Junkai Jiang  Kamyar Parto  Kaustav Banerjee  nanoelectronics  Semiconductor 

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Converting graphene into diamond film without high pressure

Posted By 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, 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, 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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