Print Page | Contact Us | Report Abuse | Sign In | Register
Graphene Updates
Blog Home All Blogs
The latest news and information on all aspects of graphene research, development, application and commercialization.

 

Search all posts for:   

 

Top tags: graphene  2D materials  Sensors  Nanomaterials  Electronics  University of Manchester  Batteries  Graphene Flagship  graphene oxide  Semiconductor  coatings  First Graphene  Graphite  Healthcare  CVD  Li-ion batteries  energy storage  carbon nanotubes  composites  optoelectronics  Versarien  Applied Graphene Materials  Battery  graphene production  nanoelectronics  photonics  The Graphene Flagship  Medical  polymers  Haydale 

'Atomic dance' reveals new insights into performance of 2D materials

Posted By Graphene Council, Monday, February 17, 2020
A team of Northwestern University materials science researchers have developed a new method to view the dynamic motion of atoms in atomically thin 2D materials. The imaging technique, which reveals the underlying cause behind the performance failure of a widely used 2D material, could help researchers develop more stable and reliable materials for future wearables and flexible electronic devices.

These 2D materials - such as graphene and borophene - are a class of single-layer, crystalline materials with widespread potential as semiconductors in advanced ultra-thin, flexible electronics. Yet due to their thin nature, the materials are highly sensitive to external environments, and have struggled to demonstrate long-term stability and reliability when utilized in electronic devices.

"Atomically thin 2D materials offer the potential to dramatically scale down electronic devices, making them an attractive option to power future wearable and flexible electronics," said Vinayak Dravid, Abraham Harris Professor of Materials Science and Engineering at the McCormick School of Engineering.

The study, titled "Direct Visualization of Electric Field induced Structural Dynamics in Monolayer Transition Metal Dichalcogenides," was published on February 11 in the journal ACS Nano. Dravid is the corresponding author on the paper. Chris Wolverton, the Jerome B. Cohen Professor of Materials Science and Engineering, also contributed to the research.

"Unfortunately, electronic devices now operate as a kind of 'black box.' Although device metrics can be measured, the motion of single atoms within the materials responsible for these properties is unknown, which greatly limits efforts to improve performance," added Dravid, who serves as director of the Northwestern University Atomic and Nanoscale Characterization (NUANCE) Center. The research allows a way to move past that limitation with a new understanding of the structural dynamics at play within 2D materials receiving electrical voltage.

Building upon a previous study in which the researchers used a nanoscale imaging technique to observe failure in 2D materials caused by heat, the team used a high-resolution, atomic-scale imaging method called electron microscopy to observe the movement of atoms in molybdenum disulfide (MoS2), a well-studied material originally used as a dry lubricant in greases and friction materials that has recently gained interest for its electronic and optical properties. When the researchers applied an electric current to the material, they observed its highly mobile sulfur atoms move continuously to vacant areas in the crystalline material, a phenomenon they dubbed, "atomic dance."

That movement, in turn, caused the MoS2's grain boundaries -- a natural defect created in the space where two crystallites within the material meet-- to separate, forming narrow channels for the current to travel through.

"As these grain boundaries separate, you are left with only a couple narrow channels, causing the density of the electrical current through these channels to increase," said Akshay Murthy, a PhD student in Dravid's group and the lead author on the study. "This leads to higher power densities and higher temperatures in those regions, which ultimately leads to failure in the material."

"It's powerful to be able to see exactly what's happening on this scale," Murthy continued. "Using traditional techniques, we could apply an electric field to a sample and see changes in the material, but we couldn't see what was causing those changes. If you don't know the cause, it's difficult to eliminate failure mechanisms or prevent the behavior going forward."

With this new way to study 2D materials at the atomic level, the team believes researchers could use this imaging approach to synthesize materials that are less susceptible to failure in electronic devices. In memory devices, for example, researchers could observe how regions where information is stored evolve as electric current is applied and adapt how those materials are designed for better performance.

The technique could also help improve a host of other technologies, from transistors in bioelectronics to light emitting diodes (LEDs) in consumer electronics to photovoltaic cells that comprise solar panels.

"We believe the methodology we have developed to monitor how 2D materials behave under these conditions will help researchers overcome ongoing challenges related to device stability," Murthy said. "This advance brings us one step closer to moving these technologies from the lab to the marketplace."

Tags:  2D materials  Akshay Murthy  Chris Wolverton  Electronics  Graphene  Northwestern University  Vinayak Dravid 

Share |
PermalinkComments (0)
 

A fast light detector made of two-dimensional materials

Posted By Graphene Council, Monday, February 17, 2020

Two research groups at ETH Zurich have joined forces to develop a novel light detector. It consists of two-dimensional layers of different materials that are coupled to a silicon optical waveguide. In the future, this approach can also be used to make LEDs and optical modulators.

Fast and highly efficient modulators as well as detectors for light are the core components of data transmission through fibre optic cables. In recent years, those building blocks for telecommunications based on existing optical materials have been constantly improved, but now it is getting increasingly difficult to achieve further improvements. That takes the combined forces of different specializations, as two research groups at ETH Zurich have now shown.

A group of scientists led by professors Jürg Leuthold of the Institute for Electromagnetic Fields and Lukas Novotny of the Institute for Photonics, together with colleagues at the National Institute for Material Science in Tsukuba (Japan), have developed an extremely fast and sensitive light detector based on the interplay between novel two-dimensional materials and nano-photonic optical waveguides. Their results were recently published in the scientific journal Nature Nanotechnology.

Two-dimensional materials

“In our detector we wanted to exploit the advantages of different materials whilst overcoming their individual constraints,” explains Nikolaus Flöry, a PhD student in Novotny’s group. “The best way of doing so is to fabricate a kind of artificial crystal – also known as heterostructure – from different layers that are each only a few atoms thick. Moreover, we were interested to know whether all the buzz about such two-dimensional materials for practical applications is actually justified.”

In two-dimensional materials, such as graphene, electrons only move in a plane rather than three spatial dimensions. This profoundly alters their transport properties, for instance when an electrical voltage is applied. While graphene is not the ideal choice for optics applications, compounds of transition metals such as molybdenum or tungsten and chalcogenes such as sulphur or tellurium (abbreviated as TMDC) are highly photosensitive and, on top of that, can be easily combined with silicon optical waveguides.

Interplay of different approaches

The expertise for the waveguides and high-speed optoelectronics came from the research group of Jürg Leuthold. Ping Ma, the group’s Senior Scientist, stresses that it was the interplay between the two approaches that made the new detector possible: “Understanding both the two-dimensional materials and the waveguides through which light is fed into the detector was of fundamental importance to our success. Together, we realized that two-dimensional materials are particularly suited to being combined with silicon waveguides. Our groups’ specializations complemented each other perfectly.”

The researchers had to find a way to make the ordinarily rather slow TMDC-based detectors faster. On the other hand, the detector had to be optimally coupled to the silicon structures used as an interface without sacrificing its high-speed performance.  

Speed through vertical structure
“We solved the speed problem by realizing a vertical heterostructure made of a TMDC – molybdenum ditelluride in our case – and graphene,” Flöry says. Differently from conventional detectors, in that way electrons excited by incoming light particles don’t need to first make their way through the bulk of the material before being measured. Instead, the two-dimensional layer of TMDC ensures that electrons can leave the material in a very short time either upwards or downwards.

The faster they leave, the larger is the bandwidth of the detector. The bandwidth indicates at what frequency data encoded in light pulses can be received. “We had hoped to get a few Gigahertz of bandwidth with our new technology – in the end, we actually reached 50 Gigahertz,” says Flöry. Up to now, bandwidths of less than a Gigahertz were possible with TMDC-based detectors.

Optimal light coupling, on the other hand, was achieved by integrating the detector into a nano-photonic optical waveguide. A so-called evanescent wave, which laterally protrudes from the waveguide, feeds the photons through a graphene layer (which has a low electrical resistance) into the molybdenum-ditelluride layer of the heterostructure. There, they excite electrons that are eventually detected as a current. The integrated waveguide design ensures that enough light is absorbed in that process.

Technology with multiple possibilities

The ETH researchers are convinced that with this combination of waveguides and heterostructures they can make not just light detectors, but also other optical elements such as light modulators, LEDs and lasers. “The possibilities are almost limitless,” Flöry and Ma enthuse about their discovery. “We just picked out the photodetector as an example of what can be done with this technology.”

In the near future, the scientists want to use their findings and investigate other two-dimensional materials. About a hundred of them are known to date, which gives countless possible combinations for novel heterostructures. Moreover, they want to exploit other physical effects, such as plasmons, in order to improve the performance of their device even further.

Tags:  2D materials  ETH Zurich  Graphene  Jürg Leuthold  LED  Lukas Novotny  Nikolaus Flöry  optoelectronics 

Share |
PermalinkComments (0)
 

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 

Share |
PermalinkComments (0)
 

The ACS Publishes a Chicken-Sh!T Article About Graphene

Posted By Graphene Council, Friday, February 14, 2020

A joke targeted at graphene research seems driven more by envy than providing a check on its excesses 

Last month, the venerable American Chemical Society (ACS) in its journal ACS Nano decided to take a step off the path of its mandate to promote scientific discovery and thought it might be fun to ridicule it. ("Will Any Crap We Put into Graphene Increase Its Electrocatalytic Effect?" by Lu Wang, Zdenek Sofer and  Martin Pumera*

Of course, the target of the ridicule was graphene, which over the last decade has been taking much of the research funding targeted for advanced materials. Let’s say it was an easy target to malign, especially for those who are not invested in graphene’s development.

The ridicule was formulated in this way: a lot of research papers are published on how to dope graphene (add impurities to it), so what if we published a paper on someone doping graphene with chicken guano. Hysterical, right?

There’s just one problem with this joke, one of the cornerstones of semiconducting engineering for at least the last half-century has been doping. Doping is used to enhance, or just tweak, the conductivity of semiconductors by intentionally introducing impurities into them. 

To forego a long explanation of solid-state physics and band gaps, suffice it to say that digital electronics (the kind of electronics that enables you to read this post) depends on doping of electronic materials to function.

In the decade-and-a-half since graphene was first isolated, researchers have been mesmerized by its extraordinary properties and by logical extension its enormous potential in electronics. However, graphene in its pure state is not a semiconductor, but rather a conductor. In order for it to be useful in electronic applications, especially digital electronics, it needs to behave as a semiconductor: possessing the capability of starting and stopping the flow of electrons through it thereby creating the on/off states for binary digital logic.

Of course, researchers have spent countless hours researching on how to best exploit graphene for electronics—attracted to its extraordinary electronic properties—and have often been funded handsomely to do so. This funding—which has come at the expense of other lines of research (making graphene research an easy target for envy)—has been so strong because of the hope that it would lead to some breakthrough that would stave off the end of Moore’s Law. 

Moore’s Law argued back in 1965 that the number of transistors placed in an integrated circuit (IC) or chip doubles approximately every two years.  It turns out graphene hasn’t saved Moore’s Law as of yet. And Moore’s Law may have seen its road come to a dead end two years ago when chip makers just threw up their hands at a 7-nm node and said, “No more.”

This means for the last decade there has been a pressing need and a fervent hope that graphene could come to the rescue of complimentary metal-oxide-semiconductor (CMOS) digital electronics. To meet this need and interest, graphene research had to devote much of its time to doping of the material.

It is a worthy argument to contest how effective this has all been in bringing graphene closer as a viable alternative in a post-CMOS world. However, it would be silly to argue that such research should never have been undertaken, or even taken on so aggressively and broadly. There was a need and the market pulled for such a use for the material. This was not an idle or wasted effort as the ACS Nano article insinuates.

The gist of the ACS Nano article in the form of a joke was to suggest that all doping of graphene research is nonsense because so much of it is performed and published. “Look how funny it would be to dope graphene with chicken droppings. That will really stick their noses in it.” While this may be funny to some, it’s highly detrimental to the spirit and inspiration for scientific inquiry.

And it certainly doesn’t do justice to the many innovative ways that graphene and a whole class of 2D materials are being applied to solve existing and new engineering challenges. 

Tags:  2D materials  American Chemical Society  Electronics  Graphene  Lu Wang  Martin Pumera  Semiconductors  Zdenek Sofer 

Share |
PermalinkComments (0)
 

Origami-inspired robots that could fit in a cell

Posted By Graphene Council, Thursday, February 6, 2020

Imagine robots that can move, sense and respond to stimuli, but that are smaller than a hair’s width. This is the project that Cornell professor and biophysicist Itai Cohen, who gave a talk on Wednesday, January 29 as a part of Duke’s Physics Colloquium, has been working on with and his team. His project is inspired by the microscopic robots in Paul McEuen’s book Spiral. Building robots at such a small scale involves a lot more innovation than simply shrinking all of the parts of a normal robot. At low Reynolds number, fluids are viscous instead of inertial, Van der Waals forces come into play, as well as other factors that affect how the robot can move and function. 

To resolve this issue, Cohen and his team decided to build and pattern their micro robots in 2D. Then, inspired by origami, a computer would print the 2D pattern of a robot that can fold itself into a 3D structure. Because paper origami is scale invariant, mechanisms built at one scale will work at another, so the idea is to build robot patterns than can be printed and then walk off of the page or out of a petri dish.

However, as Cohen said in his talk last Wednesday, “an origami artist is only as good as their origami paper.” And to build robots at a microscopic scale, one would need some pretty thin paper. Cohen’s team uses graphene, a single sheet of which is only one atom thick. Atomic layer deposition films also behave very similarly to paper, and can be cut up, stretch locally and adopt a 3D shape. Some key steps to making sure the robot self-folds include making elements that bend, and putting additional stiff pads that localize bends in the pattern of the robot. This is what allows them to produce what they call “graphene bimorphs.”

Cohen and his team are looking to use microscopic robots in making artificial cilia, which are small leg-like protrusions in cells. Cilia can be sensory or used for locomotion. In the brain, there are cavities where neurotransmitters are redirected based on cilial beatings, so if one can control the individual beating of cilia, they can control where neurotransmitters are directed. This could potentially have biomedical implications for detecting and resolving neurological disorders. 

Right now, Cohen and his lab have microscopic robots made of graphene, which have photovoltaics attached to their legs. When a light shines on the photovoltaic receptor, it activates the robot’s arm movement, and it can wave hello. The advantage of using photovoltaics is that to control the robot, scientists can shine light instead of supplying voltage through a probe—the robot doesn’t need any tethers. During his presentation, Cohen showed the audience a video of his “Brobot,” a robot that flexes its arms when a light shines on it. His team has also successfully made microscopic robots with front and back legs that can walk off a petri dish. Their dimensions are 70 microns long, 40 microns wide and two microns thick. 

Cohen wants to think critically about what problems are important to use technology to solve; he wants make projects that can predict the behavior of people in crowds, predict the direction people will go in response to political issues, and help resolve water crises. Cohen’s research has the potential to find solutions for a wide variety of current issues. Using science fiction and origami as the inspiration for his projects reminds us that the ideas we dream of can become tangible realities.

Tags:  2D materials  Cornell University  Graphene  graphene bimorphs  Itai Cohen 

Share |
PermalinkComments (0)
 

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 

Share |
PermalinkComments (0)
 

Method detects defects in 2D materials for future electronics, sensors

Posted By Graphene Council, Tuesday, January 28, 2020
Two-dimensional materials are atomically thin, the most well-known being graphene, a single-atom-thick layer of carbon atoms. "People have struggled to make these 2D materials without defects," said Mauricio Terrones, Verne M. Willaman Professor of Physics, Penn State. "That's the ultimate goal. We want to have a 2D material on a four-inch wafer with at least an acceptable number of defects, but you want to evaluate it in a quick way."

The researchers' -- who represent Penn State, Northeastern University, Rice University and Universidade Federal de Minas Gerais in Brazil -- solution is to use laser light combined with second harmonic generation, a phenomenon in which the frequency of the light shone on the material reflects at double the original frequency. They add dark field imaging, a technique in which extraneous light is filtered out so that defects shine through. According to the researchers, this is the first instance in which dark field imaging was used, and it provides three times the brightness of the standard bright field imaging method, making it possible to see types of defects previously invisible.

"The localization and identification of defects with the commonly used bright field second harmonic generation is limited because of interference effects between different grains of 2D materials," said Leandro Mallard, a senior author on a recent paper in Nano Letters and a professor at Universidade Federal de Minas Gerais. "In this work we have shown that by the use of dark field SHG we remove the interference effects and reveal the grain boundaries and edges of semiconducting 2D materials. Such a novel technique has good spatial resolution and can image large area samples that could be used to monitor the quality of the material produced in industrial scales."

Vincent H. Crespi, Distinguished Professor of Physics, Materials Science and Engineering, and Chemistry, Penn State, added, "Crystals are made of atoms, and so the defects within crystals -- where atoms are misplaced -- are also of atomic size.

"Usually, powerful, expensive and slow experimental probes that do microscopy using beams of electrons are needed to discern such fine details in a material," said Crespi. "Here, we use a fast and accessible optical method that pulls out just the signal that originates from the defect itself to rapidly and reliably find out how 2D materials are stitched together out of grains oriented in different ways."

Another coauthor compared the technique to finding a particular zero on a page full of zeroes. "In the dark field, all the zeroes are made invisible so that only the defective zero stands out," said Yuanxi Wang, assistant research professor at Penn State's Materials Research Institute.

The semiconductor industry wants to have the ability to check for defects on the production line, but 2D materials will likely be used in sensors before they are used in electronics, according to Terrones. Because 2D materials are flexible and can be incorporated into very small spaces, they are good candidates for multiple sensors in a smartwatch or smartphone and the myriad of other places where small, flexible sensors are required.

"The next step would be an improvement of the experimental setup to map zero dimension defects -- atomic vacancies for instance -- and also extend it to other 2D materials that host different electronic and structural properties," said lead author Bruno Carvalho, a former visiting scholar in Terrones' group.

Tags:  2D materials  Graphene  Leandro Mallard  Mauricio Terrones  Northeastern University  Penn State  Rice University  Universidade Federal de Minas Gerais in Brazil  Vincent H. Crespi  Yuanxi Wang 

Share |
PermalinkComments (0)
 

It’s closeness that counts: how proximity affects the resistance of graphene

Posted By Graphene Council, Tuesday, January 28, 2020
Graphene is often seen as the wonder material of the future. Scientists can now grow perfect graphene layers on square centimetre-sized crystals. A research team from the University of Göttingen, together with the Chemnitz University of Technology and the Physikalisch-Technische Bundesanstalt Braunschweig, has investigated the influence of the underlying crystal on the electrical resistance of graphene.

Contrary to previous assumptions, the new results show that the process known as the ‘proximity effect’ varies considerably at a nanometre scale. The results have been published in Nature Communications.

The composition of graphene is very simple. It is a single atomic layer of carbon atoms arranged in a honeycomb structure. The three-dimensional form is already an integral part of our everyday lives: we see it in the lead of an ordinary pencil for instance. However, the two-dimensional material graphene was not synthesized in the laboratory until 2004. To determine the electrical resistance of graphene at the smallest scale possible, the physicists used a “scanning tunnelling microscope”. This can make atomic structures visible by scanning the surface with a fine metal tip. The team also used the tip of the scanning tunnelling microscope to measure the voltage drop and thus the electrical resistance of the tiny graphene sample.

Depending on the distance that they measured, the researchers determined very different values for the electrical resistance. They cite the proximity effect as the reason for this. “The spatially varying interaction between graphene and the underlying crystal means that we measure different electrical resistances depending on the exact position,” explains Anna Sinterhauf, first author and doctoral student at the Faculty of Physics at the University of Göttingen.

At low temperatures of 8 Kelvin, which is around minus 265 degrees Centigrade, the team found variations in local resistance of up to 270 percent. “This result suggests that the electrical resistance of graphene layers epitaxially grown on a crystal surface cannot simply be worked out from an average taken from values measured at a larger scale,” explains Dr Martin Wenderoth, head of the working group. The team assumes that the proximity effect might also play an important role for other two-dimensional materials.

Tags:  2D materials  Anna Sinterhauf  Graphene  Martin Wenderoth  University of Göttingen 

Share |
PermalinkComments (0)
 

First Graphene's Strong Advances in VFD Development

Posted By Graphene Council, Tuesday, January 28, 2020

First Graphene Limited  is pleased to provide an update of the work conducted in conjunction with 2D Fluidics Pty Ltd on the Vortex Fluidic Device (VFD) at the Company’s facilities at the Graphene Engineering and Innovation Centre (GEIC) in Manchester, UK and Flinders University

Background Summary on Graphene Oxide
Graphene oxide (GO) is the chemically modified derivative of graphene, whereby the basal planes and edges have been functionalised with oxygen containing functional groups such as hydroxyl, epoxy and carboxyl groups. These oxygen functionalities make GO hydrophilic and therefore dispersible, forming homogenous colloidal suspensions in water and most organic solvents. This makes it ideal for use in a range of applications.

To date, the most widely used process for the synthesis of graphene oxide is Hummer’s method. This typically  requires strong acids and oxidants, such as potassium chlorate (KClO3), nitric acid (HNO3), concentrated sulfuric acid (H2SO4) and potassium permanganate (KMnO4). Much work has been done to improve the synthesis methods while maintaining high surface oxidation, however these all required strong acids and oxidants.

Through its subsidiary 2D Fluidics Pty Ltd, FGR is developing a more benign processing route for oxidised graphene. The objective is to provide controlled levels of surface oxygen functionality to give better easier compatibility in aqueous and organic systems. This will not incur the higher oxygen (and other defect) levels which result from Hummer’s method and its subsequent reduction steps. It will also provide the ability to “tune or optimise” the surface oxidation level to suit respective applications.

FGR’s method synthesises GO directly from bulk graphite using aqueous H2O2 as the green oxidant. Different energy sources have been used for the conversion of H2O2 molecules into more active peroxidic species, such as a combination of a pulsed Nd:YAG laser and/or other light sources. The irradiation promotes the dissociation of H2O2 into hydroxyl radicals which then leads to surface oxidation.

The technology has been successfully transferred to the FGR laboratories at the Graphene Engineering and Innovation Centre (GEIC) in Manchester where it has undergone further development and optimisation to identify, understand and resolve future upscaling issues.

XPS analysis showed that the use of pre-treatment step in combination with the near infrared laser gave oxidised graphene sheets with an average surface oxidation of ~30- 35%: this will enhance compatibility with aqueous systems.

Further trials have already demonstrated that the two-step process is reproducible and versatile, with the ability to process different starting materials of graphite. The multi- disciplinary team has identified that control of the feed rate and energy input will allow us to control the surface oxidation, providing a consistent material that can be tailored as required for a range of applications.

Figure 5 shows that increase in surface oxygen content for two starting materials: graphite ore (top) and PureGRAPH® graphene (bottom). As we go through the two-  stage process, in both cases the surface oxygen functionality increases. The end- product has a range of functional groups, including C-O, C=O and COOH.

Next Steps
Operating parameters will now be established to provide yield data for future use in scaling the system for commercial production. It will also commence examining the end applications including, but not limited to the use in electronic devices, testing levels of toxicity for biological applications, for water filtration membranes and incorporation in membranes for studying anti-fouling properties.

Craig McGuckin, Managing Director of FGR, said, “The complementary characterisation techniques used to confirm the synthesis of oxidised graphene gives us confidence we are on the right route towards fabricating a material which is comparable to  the historical GO fabricated using the conventional Hummers method. We are  now  reviewing end applications and thus exploring a number of avenues which include but are not limited to the use in devices, testing levels of toxicity for biological applications, for water filtration membranes and incorporation in membranes for studying anti-fouling properties.”

Tags:  2D Fluidics Pty Ltd  2D materials  Craig McGuckin  First Graphene  Flinders University  Graphene  Graphene Engineering and Innovation Centre  graphene oxide 

Share |
PermalinkComments (0)
 

Well-designed substrates make large single crystal bi-/tri-layer graphene possible

Posted By Graphene Council, Sunday, January 26, 2020
Researchers of the Center for Multidimensional Carbon Materials (CMCM) within the Institute for Basic Science (IBS, South Korea) have reported in Nature Nanotechnology the fabrication and use of single crystal copper-nickel alloy foil substrates for the growth of large-area, single crystal bilayer and trilayer graphene films.

The growth of large area graphene films with a precisely controlled numbers of layers and stacking orders can open new possibilities in electronics and photonics but remains a challenge. This study showed the first example of the synthesis of bi- and trilayer graphene sheets larger than a centimeter, with layers piled up in a specific manner, namely AB- and ABA-stacking.

“This work provides materials for the fabrication of graphene devices with novel functions that have not yet been realized and might afford new photonic and optoelectronic and other properties,” explains Rodney S. Ruoff, CMCM Director, Distinguished Professor at the Ulsan National Institute of Science and Technology (UNIST) and leading author of this study. Coauthor and Professor Won Jong Yoo of Sungkyunkwan University notes that “this paves the way for the study of novel electrical transport properties of bilayer and trilayer graphene.”

For example, the same IBS research group and collaborators recently published another paper in Nature Nanotechnology showing the conversion of AB-stacked bilayer graphene film, grown on copper/nickel (111) alloy foils (Cu/Ni(111) foils), to a diamond-like sheet, known as diamane. Coauthor Pavel V. Bakharev notes that: “Less than one year ago, we produced fluorinated diamond monolayer, F-diamane, by fluorination of exactly the AB-stacked bilayer graphene films described in this new paper. Now the possibility of producing bilayer graphene of a larger size brings renewed excitement and shows how fast this field is developing.”

The right choice of substrate is essential for the correct growth of graphene. Foils made only of copper limit the growth of bilayer graphene and favor uniform monolayer growth. It is possible to obtain multilayer graphene sheets on nickel film, but these are not uniform, and tend to have small “patches” with different thicknesses. Finally, the commercially available foils that contain both nickel and copper are not ideal. Therefore, IBS researchers prepared ‘home-made’ single crystal Cu/Ni(111) foils with desired features, building further on a technique reported by the group in Science in 2018. Nickel films are electroplated onto copper(111)-foils so that the nickel and copper interdiffuse when heated and yield a new single crystal foil that contains both elements at adjustable ratios. Ruoff suggested this method and supervised Ming Huang’s evaluations of the best concentrations of nickel to obtain uniform graphene sheets with the desired number of layers.

IBS researchers grew bi- and tri-layer graphene sheets on Cu/Ni(111) foils by chemical vapor deposition (CVD). Huang achieved AB-stacked bilayer graphene films of several square centimeters, covering 95% of the substrate area, and ABA-stacked trilayer graphene with more than 60% areal coverage. This represents the first growth of high coverage ABA-stacked trilayer graphene over a large area and the best quality obtained for AB-stacked bilayer graphene so far.

In addition to extensive spectroscopic and microscopic characterizations, the researchers also measured the electrical transport (carrier mobility and band gap tunability) and thermal conductivity of the newly synthesized graphene. The centimeter-scale bilayer graphene films showed a good thermal conductivity, as high as ~2300 W/mK (comparable with exfoliated bilayer graphene flakes), and mechanical performance (stiffness of 478 gigapascals for the Young’s modulus, and 3.31 gigapascals for the fracture strength).

The team then investigated the growth stacking mechanism and discovered it follows the so-called “inverted wedding cake” sequence as the bottom layers are positioned after the top one. “We showed with three independent methods that the 2nd layer for bilayer graphene, and the 2nd and 3rd layers of the trilayer sheet grow beneath a continuous top layer. These methods can be further used to study the structure and stacking sequence of other 2D thin film materials,” notes Huang.

Ruoff notes that these techniques for synthesizing and testing large-scale ultrathin films could stimulate worldwide interest in further experimenting with single crystal Cu/Ni alloy foils, and even in exploring fabrication and use of other single crystal alloy foils. This research was performed in collaboration with UNIST and Sungkyunkwan University.

Tags:  2D materials  Center for Multidimensional Carbon Materials  Graphene  Institute for Basic Science  nanomaterials  nanotechnology  Rodney S. Ruoff 

Share |
PermalinkComments (0)
 
Page 1 of 6
1  |  2  |  3  |  4  |  5  |  6