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Laser used to transfer graphene for tech applications

Posted By Graphene Council, 22 hours ago
Laser has been used to cut to shape and deposit graphene on a target substrate in a single step process, potentially lowering device fabrication time and cost. Graphene patches with diameters as small as 30 micrometers were transferred onto technologically relevant substrates.

The preferred method for production of large-area graphene is chemical vapour deposition (CVD), which allows roll-to-roll scalable production of good quality material. CVD is widely used to create graphene films and devices for industrial and research applications. The CVD process is most commonly restricted to growth on catalytic substrates, such as thin copper films.

In order to produce finished devices, such as field effect transistors, graphene needs to be transferred onto a technologically usable substrate, most commonly a silicon or silica wafer. The common methods of transferring graphene involve polymer intermediary overlayers, application of lithographic masking layers and chemical etching, steps that increase process complexity and reduce the quality of the pristine graphene. Laser-induced localized transfer bypasses all these steps, simplifying device fabrication.

Laser-induced transfer utilizes high power femtosecond laser pulses to “peel” graphene off a substrate. A possible explanation for the underlying physical mechanism is thermal expansion of the substrate, in this case nickel metal, which leads to a rupture of the graphene sheet at the edges of the laser-illuminated area. The research team, joining forces from the UK, Greece, Spain and Israel, having published their results in the journal Applied Surface Science, believes that laser transfer has the potential to eliminate many time-consuming lithographic processing steps, allowing precise, direct application of 2D materials with complex shapes to specific locations on a device, although they acknowledge that the process should be further refined to improve on the quality of the transferred material.

Tags:  2D materials  chemical vapour deposition  CVD  Graphene  Graphenea 

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How to stack graphene up to four layers

Posted By Graphene Council, Wednesday, July 29, 2020
Graphene, an atomically thin hexagonal structure of carbon atoms is a potential candidate for electronic and optoelectrical applications such as transparent electrodes and interconnect for integrated circuits. Yet, it is one thing to possess such useful properties and to induce an intended characteristic from this "wonder material" is another. In the face of the end of the "Moore's Law", chip makers have set their sight on multi-layered graphene for its scaling ability of integrated circuits to smaller physical dimensions and the electric-field induced bandgap, which is not affordable in monolayer graphene. Furthermore, owning to exotic physical properties controlled by its stacking orders (the arrangement of graphene layer along vertical direction) such as superconductivity and quantum Hall effect to name a few, multi-layer graphene is an interesting material for condensed matter physicists. Still, the unknown growth method for uniform single-crystalline multilayer graphene growth in a wafer scale presents a challenge.

Led by professor LEE Young Hee at the Center for Integrated Nanostructure Physics, the Institute for Basic Science (IBS) in Sungkyunkwan University, South Korea, an IBS research team reports a novel method to grow multi-layered, single-crystalline graphene with a selected stacking order in a wafer scale. They obtained four-layered graphene using chemical vapor deposition (CVD) via Cu-Si alloy formation.

There have been several approaches to control the number of graphene layers. Conventionally, the monolayer graphene, which is easily grown on Cu-substrate, can be detached from the Cu-substrate and transferred onto insulator substrates such as SiO2/Si. Therefore, the simplest method to make multilayer graphene is to stack them layer-by-layer via the transfer process. However, this transfer process may cause tearing, wrinkles, and/or polymer residues. Though such issues can be avoided via a direct method, i.e. CVD on Cu substrate, the low solubility of carbon (C) in copper (Cu) hampers the controlling of the number of graphene layers with high uniformity in a large area. By depositing Ni or Co to form Cu-Ni/ Cu-Co alloys or employing oxygen-rich Cu substrate, C solubility in Cu is boosted and thus stacks more layers of graphene. Nevertheless, a small portion of inhomogeneous multilayers occurs. Controlling the crystallographic stacking sequence of graphene films thicker than two layers with high uniformity has not been demonstrated to date.

Dr. Van Luan Nguyen, the first author of the study (now at Samsung Advanced Institute Technology) proposed to use silicon carbide (SiC) on the surface of Cu substrate alloy, via the sublimation of Si atoms at a high temperature. They controlled the C solubility in the Cu film by introducing Si content on Cu surface by heat treatment of Cu substrate with a constant H2 gas flow inside the quartz tube of CVD chamber. "The formation of a homogeneous Cu-Si alloy, as a role of catalyst, was critical to control the number layers of graphene film in a wafer scale with methane gas. With the presence of Cu-Si alloy, SiC can be formed when methane gas is injected and the following sublimation process of Si atoms leaves C atoms behind to form multilayer graphene. Si amount is fixed at 28.7 % for uniform multilayer graphene film. Depending the concentrations of argon (Ar)-diluted methane gas, the number of graphene layers varies," says Dr. Van.

Growing in a large scale of this much-hyped graphene has seen much progress over a decade, but building multi-layered graphene is just in its early stages. Our study offers a novel approach to upgrade the conventional CVD method by introducing an intermediate process of in-situ formation of SiC film," notes Dr. LEE Sang Hyub, coauthor of the study. Importantly, this study provides a new platform to synthesize graphene multilayer towards the uniform large-area single-crystalline layer-tunable multilayer graphene as well as graphite thin film. This is an initial step to incorporate multilayer graphene to display panels and integrated circuits such as via-holes and replacement of Cu electrodes as well as photoelectronic and photovoltaic devices.

"Deposition of Si by conventional methods at low temperatures such as thermal evaporation or sputtering does not work for uniform multilayer graphene growth. The key in our new approach is to form uniform Cu-Si alloy on quartz tube chamber in which Si is sublimated at high temperature of 900 ? with H2 gas flow in a controllable manner," explains Director LEE Young Hee, the corresponding author of the study. Although the substantial achievement has been demonstrated in our current work, Director Lee cautions that the method to deposit Si at high temperature during the growth process is not practical and can be harmful for the quart tube for long-term use. They are searching for a solution to replace the current one for mass product.

Tags:  chemical vapor deposition  CVD  Graphene  LEE Young Hee  monolayer graphene  Samsung Advanced Institute Technology  Sungkyunkwan University  Van Luan Nguyen 

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Rice lab turns trash into valuable graphene in a flash

Posted By Graphene Council, Tuesday, January 28, 2020
A new process introduced by the Rice University lab of chemist James Tour can turn bulk quantities of just about any carbon source into valuable graphene flakes. The process is quick and cheap; Tour said the “flash graphene” technique can convert a ton of coal, waste food or plastic into graphene for a fraction of the cost used by other bulk graphene-producing methods.

“This is a big deal,” Tour said. “The world throws out 30% to 40% of all food, because it goes bad, and plastic waste is of worldwide concern. We’ve already proven that any solid carbon-based matter, including mixed plastic waste and rubber tires, can be turned into graphene.”

As reported in Nature, flash graphene is made in 10 milliseconds by heating carbon-containing materials to 3,000 Kelvin (about 5,000 degrees Fahrenheit). The source material can be nearly anything with carbon content. Waste food, plastic waste, petroleum coke, coal, wood clippings and biochar are prime candidates, Tour said. “With the present commercial price of graphene being $67,000 to $200,000 per ton, the prospects for this process look superb,” he said.

Tour said a concentration of as little as 0.1% of flash graphene in the cement used to bind concrete could lessen its massive environmental impact by a third. Production of cement reportedly emits as much as 8% of human-made carbon dioxide every year.

“By strengthening concrete with graphene, we could use less concrete for building, and it would cost less to manufacture and less to transport,” he said. “Essentially, we’re trapping greenhouse gases like carbon dioxide and methane that waste food would have emitted in landfills. We are converting those carbons into graphene and adding that graphene to concrete, thereby lowering the amount of carbon dioxide generated in concrete manufacture. It’s a win-win environmental scenario using graphene.”

“Turning trash to treasure is key to the circular economy,” said co-corresponding author Rouzbeh Shahsavari, an adjunct assistant professor of civil and environmental engineering and of materials science and nanoengineering at Rice and president of C-Crete Technologies. “Here, graphene acts both as a 2D template and a reinforcing agent that controls cement hydration and subsequent strength development.”

In the past, Tour said, “graphene has been too expensive to use in these applications. The flash process will greatly lessen the price while it helps us better manage waste.”

“With our method, that carbon becomes fixed,” he said. “It will not enter the air again.”

The process aligns nicely with Rice’s recently announced Carbon Hub initiative to create a zero-emissions future that repurposes hydrocarbons from oil and gas to generate hydrogen gas and solid carbon with zero emission of carbon dioxide. The flash graphene process can convert that solid carbon into graphene for concrete, asphalt, buildings, cars, clothing and more, Tour said.

Flash Joule heating for bulk graphene, developed in the Tour lab by Rice graduate student and lead author Duy Luong, improves upon techniques like exfoliation from graphite and chemical vapor deposition on a metal foil that require much more effort and cost to produce just a little graphene.

Even better, the process produces “turbostratic” graphene, with misaligned layers that are easy to separate. “A-B stacked graphene from other processes, like exfoliation of graphite, is very hard to pull apart,” Tour said. “The layers adhere strongly together. But turbostratic graphene is much easier to work with because the adhesion between layers is much lower. They just come apart in solution or upon blending in composites.

“That’s important, because now we can get each of these single-atomic layers to interact with a host composite,” he said.

The lab noted that used coffee grounds transformed into pristine single-layer sheets of graphene.

Bulk composites of graphene with plastic, metals, plywood, concrete and other building materials would be a major market for flash graphene, according to the researchers, who are already testing graphene-enhanced concrete and plastic.

The flash process happens in a custom-designed reactor that heats material quickly and emits all noncarbon elements as gas. “When this process is industrialized, elements like oxygen and nitrogen that exit the flash reactor can all be trapped as small molecules because they have value,” Tour said.

He said the flash process produces very little excess heat, channeling almost all of its energy into the target. “You can put your finger right on the container a few seconds afterwards,” Tour said. “And keep in mind this is almost three times hotter than the chemical vapor deposition furnaces we formerly used to make graphene, but in the flash process the heat is concentrated in the carbon material and none in a surrounding reactor.

“All the excess energy comes out as light, in a very bright flash, and because there aren’t any solvents, it’s a super clean process,” he said.

Luong did not expect to find graphene when he fired up the first small-scale device to find new phases of material, beginning with a sample of carbon black. “This started when I took a look at a Science paper talking about flash Joule heating to make phase-changing nanoparticles of metals,” he said. But Luong quickly realized the process produced nothing but high-quality graphene.

Atom-level simulations by Rice researcher and co-author Ksenia Bets confirmed that temperature is key to the material’s rapid formation. “We essentially speed up the slow geological process by which carbon evolves into its ground state, graphite,” she said. “Greatly accelerated by a heat spike, it is also stopped at the right instant, at the graphene stage.

“It is amazing how state-of-the-art computer simulations, notoriously slow for observing such kinetics, reveal the details of high temperature-modulated atomic movements and transformation,” Bets said.

Tour hopes to produce a kilogram (2.2 pounds) a day of flash graphene within two years, starting with a project recently funded by the Department of Energy to convert U.S.-sourced coal. “This could provide an outlet for coal in large scale by converting it inexpensively into a much-higher-value building material,” he said.

Tour has a grant from the Department of Energy to scale up the flash graphene process, which will be co-funded by the start-up company, Universal Matter Ltd.

Co-authors of the paper include Rice graduate students Wala Ali Algozeeb, Weiyin Chen, Paul Advincula, Emily McHugh, Muqing Ren and Zhe Wang; postdoctoral researcher Michael Stanford; academic visitors Rodrigo Salvatierra and Vladimir Mancevski; Mahesh Bhatt of C-Crete Technologies, Stafford, Texas; and Rice assistant research professor Hua Guo. Boris Yakobson, the Karl F. Hasselmann Chair of Engineering and a professor of materials science and nanoengineering and of chemistry, is co-corresponding author.

Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice.

Tags:  CVD  Graphene  James Tour  Rice University  Rouzbeh Shahsavari 

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Grain boundaries in graphene do not affect spin transport

Posted By Graphene Council, Wednesday, December 4, 2019
Graphene is a material that has been gaining fame in recent years due to its magnificent properties. In particular, for spintronics, graphene is a valuable material because the spins of the electrons used remain unaltered for a relatively long time. However, graphene needs to be produced on a large scale in order to be used in future devices. With that respect, chemical vapour deposition (CVD) is the most promising fabrication method.

CVD involves growing graphene on a metallic substrate at high temperatures. In this process, the generation of graphene starts at different points of the substrate simultaneously. This produces different single-crystal domains of graphene separated from one another through grain boundaries, consisting of arrays of five-, seven- or even eight-member carbon rings. The final product is, thus, polycrystalline graphene.

Is polycrystalline graphene as good as single-crystal graphene for spintronics? Grain boundaries are a significant source of charge scattering, increasing the electric resistance of the material. How do they affect spin transport?

Some experiments suggest that grain boundaries do not play a major role on spin transport. In this context, Dr Aron W. Cummings, from the ICN2 Theoretical and Computational Nanoscience Group, led by ICREA Prof. Stephan Roche, together with researchers from the Université catholique de Louvain (Belgium), have used first-principles simulations to study the impact of grain boundaries on spin transport in polycrystalline graphene. The study is published in Nano Letters.

The researchers have considered two different mechanisms by which spins could lose their original orientation (spin relaxation). One accounts for the randomisation of spins within the grains due to spin-orbit coupling, the other considers the possibility of the spins to flip due to scattering in a grain boundary. However, the researchers found that the latter case did not happen. Grain boundaries do not have any adverse effect on spin transport.

Therefore, spin diffusion length in polycrystalline graphene is independent of grain size and depends only on the strength of the substrate-induced spin-orbit coupling. Moreover, this is valid not only for the diffusive regime of transport, but also for the weakly localized one, in which quantum phenomena begin to prevail. This is the first quantum mechanical simulation confirming that the same expression for spin diffusion length holds in both regimes.

The research highlights the fact that single-domain graphene may not be a requirement for spintronics applications, and that polycrystalline CVD-grown graphene may work just as well. This puts the focus on other aspects to enhance in graphene production, such as the elimination of magnetic impurities.

Tags:  Aron W. Cummings  chemical vapour deposition  CVD  Graphene  Nano Letters  Nanoscience  polycrystalline  Stephan Roche  Universite catholique de Louvain 

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Industrial scale production of layer 2D materials via intermediate-assisted grinding

Posted By Graphene Council, Tuesday, November 26, 2019
The large number of 2D materials, including graphene, hexagonal boron nitride (h-BN), transition metal dichalcogenides (TMDCs) like MoS2 and WSe2, metal oxides (MxOy), black phosphorene (b-P), etc, provide a wide range of properties and numerous potential applications.

In order to realize their commercial use, the prerequisite is large-scale production. Bottom-up strategies like chemical vapor deposition (CVD) and chemical synthesis have been extensively explored but only small quantities of 2D materials have been produced so far.

Another important strategy to obtain 2D materials is from a top-down path by exfoliating bulk layer materials to monolayer or few layer 2D materials, such as ball milling, liquid phase exfoliation, etc. It seems that top-down strategies are most likely to be scaled-up, however, they are only suitable for specific materials.

So far, only graphene and graphene oxide can be prepared at the tons level, while for other 2D materials, they still remain in the laboratory state because of the low yield.

Therefore, it is necessary to develop a high-efficiency and low-cost preparation method of 2D materials to progress from the laboratory to our daily life.

The failure of solid lubricants is caused by the slip between layers of bulk materials, and the result of the slip is that the bulk materials will be peeled off into fewer layers. Based on this understanding, in a new research article published in the Beijing-based National Science Review ("Mass Production of Two-Dimensional Materials by Intermediate-Assisted Grinding Exfoliation"), the Low-Dimensional Materials and Devices lab led by Professor Hui-Ming Cheng and Professor Bilu Liu from Tsinghua University proposed an exfoliation technology which is named as interMediate-Assisted Grinding Exfoliation (iMAGE).

The key to this exfoliation technology is to use intermediate materials that increase the coefficient of friction of the mixture and effectively apply sliding frictional forces to the layer material, resulting in a dramatically increased exfoliation efficiency.

Considering the case of 2D h-BN, the production rate and energy consumption can reach 0.3 g h-1 and 3.01×106 J g-1, respectively, both of which are one to two orders of magnitude better than previous results.

The resulting exfoliated 2D h-BN flakes have an average thickness of 4 nm and an average lateral size of 1.2 µm. Besides, this iMAGE method has been extended to exfoliate a series of layer materials with different properties, including graphite, Bi2Te3, b-P, MoS2, TiOx, h-BN, and mica, covering 2D metals, semiconductors with different bandgaps, and insulators.

It is worth mentioning that, with the cooperation with the Luoyang Shenyu Molybdenum Co. Ltd., molybdenite concentrate, a naturally existing cheap and earth abundant mineral, was used as a demo for the industrial scale exfoliation production of 2D MoS2 flakes.

"This is the very first time that 2D materials other than graphene have been produced with a yield of more than 50% and a production rate of over 0.1g h-1. And an annual production capability of 2D h-BN is expected to be exceeding 10 tons by our iMAGE technology." Prof. Bilu Liu, one of the leading authors of this study, said, "Our iMAGE technology overcomes a main challenge in 2D materials, i.e., their mass production, and is expected to accelerate their commercialization in a wide range of applications in electronics, energy, and others."

Tags:  2D materials  Bilu Liu  CVD  Graphene  Hui-Ming Cheng  Tsinghua University 

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The Graphene Flagship Means Business

Posted By Graphene Council, Saturday, October 19, 2019

Leiden University spin-off, Crucell, is a world-famous biotechnology company. Johnson & Johnson acquired the business for close to $2.4 billion back in 2011. This mammoth sum of money highlights the potential of university or corporate spin-offs and their investment attraction.

There is a growing appetite for spin-offs and SMEs that have grown out of research. Here are four SMEs that are setting the pace in graphene and related materials (GRM) commercialisation in the Graphene Flagship.


Emberion develops and produces graphene photonics and electronics that revolutionise infrared photodetectors and thermal sensors. Applications include hyperspectral and thermal imaging, night vision and X-ray detection.

The business was a spin off company from Nokia. Following a long history of graphene research inside Nokia’s research organisation, the team joined the Graphene Flagship to take the work carried out on optoelectronics to the commercial market.

“Emberion was established in quite an early phase of product development,” explained Tapani Ryhänen, CEO of Emberion. “We had promising results and functional prototypes from our research and above all, we were able to get an agreement with venture capital investors.

"Emberion is focusing on various spectrometer and machine vision applications by producing novel image sensors. We provide products with broad wavelength range and low noise. Our image sensors can be used for example in agriculture, food processing and pharma industries."

During the next year, Emberion will start delivering its first imager products. The business will also commence with the Graphene Flagship GBIRCAM spearhead project, together with its partners, to bolster the readiness of graphene-enabled optoelectronics in industry.


World leading graphene producer Graphenea, founded in 2010, was one of the first industrial partners to join the Graphene Flagship program. Graphenea participated in the proposal stages of the Graphene Flagship, collaborating closely since the inception of the EU-funded program.Business is booming for Graphenea. In 2018, the business generated €1.6 million, and is on track to grow by 25% in 2019.

Graphenea’s facilities are located in San Sebastián, Spain and Boston, USA. The 25 employees at Graphenea contribute to the successful development of graphene applications, including supplying CVD Graphene films, Graphene Field-Effect-Transistors chips (GFETs), Graphene Foundry Services (GFAB) and Graphene Oxides. Graphenea’s operation spans across more than 60 countries and a wide range of sectors.

“The collaboration between Graphenea and the Graphene Flagship has evolved over the last six years,” explained Iñigo Charola, business development director at Graphenea. “Our work has become incredibly industry-orientated, with focused spearhead projects to bring applications to market quickly and effectively.”

“Today, we are focusing not only on the production of graphene, but also developing our processing capabilities of the material. Our partnership with the Graphene Flagship provides the support to help reach this goal.” 


BeDimensional produces and develops graphene and 2D crystals for the manufacturing and energy industries. Its main target applications relate to coatings and paints and material production for energy applications.

As a start-up, BeDimensional was created as a spin-off company of the Istituto Italiano di Tecnologica (IIT).

The research group started out their research in fundamental studies of electronic properties of two-dimensional semiconductor systems. They were also investigating some possibilities to tune the interaction of hydrogen and carbon by curving a graphene sheet. For this latter study, the team were approached in the initial stages of the Graphene Flagship project to set-up a work package on hydrogen storage.

After a two-year incubation period within the institute itself, BeDimensional moved onto the market after the acquisition of 51% of its shares by the Camponovo. The definitive push towards the path of industrialisation came at the end of 2018, after the €18 million investment from Pellan Group.

So, what’s next for BeDimensional?
After closing this round of investment, BeDimensional is now fully immersed in implementing its industrial and commercial strategies. The first production line of two-dimensional crystals is already operational and the business is in the process of setting up more laboratories for research and development.

“We need to strategically position ourselves at the right level of the industrial value chains linked to each specific application,” explained Vittorio Pellegrini, founder and scientific advisor for BeDimensional. “We believe it is crucial to establish partnerships and joint ventures with appropriate global players. At the same time, we will reinforce our investment in R&D by attracting the best people on board.”

BeDimensional has a clear mission and knows how it’s going to reach its business goals. Watch this space. 


Versarien, headquartered in Cheltenham, UK, is an engineering solutions company that delivers novel technologies for industrial applications. Through subsidiary companies, Versarien delivers targeted solutions as well as research and development into new, complementary technologies.

Versarien was founded in 2010 and has been an associate member of the Graphene Flagship since 2018, collaborating closely with researchers in the project.

The company has acquired multiple businesses under the Versarien group, including two spin-offs from high-profile universities carrying out graphene research.

In 2014, 2-DTech joined the group. 2-DTech was originally formed by the University of Manchester to manufacture and supply high quality graphene for research and development. It has now grown into a commercial operation not only supplying high grade graphene and other 2-D materials, but also working on the application of graphene in product development projects.

In 2017, Versarien acquired Cambridge Graphene Ltd. from the University of Cambridge. Cambridge Graphene develops inks, composites and supercapacitors based on graphene and related materials, using processes developed at the Cambridge Graphene Centre.

The Cambridge Graphene Centre’s mission is to investigate the science and technology of graphene and other carbon allotropes, layered crystals and hybrid nanomaterials. The spin-out company has commercialised graphene inks for novel technology applications.

These two Graphene Flagship spin-offs join the numerous other companies in the Versarien group, creating a real force to be reckoned with for graphene commercialisation and business growth.

Tags:  BeDimensional  Cambridge Graphene Centre  CVD  Emberion  Graphene  Graphene Flagship  graphene oxide  Graphenea  Iñigo Charola  Leiden University  Tapani Ryhänen  Versarien  Vittorio Pellegrini 

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Grolltex Graphene Closes Oversubscribed Private Placement Financing Round

Posted By Graphene Council, Wednesday, September 11, 2019
Updated: Tuesday, September 10, 2019

Grolltex (named for ‘graphene-rolling-technologies’) is the largest commercial producer of single layer, ‘electronics grade’ graphene and graphene sensing materials in the U.S. They have announced that it has closed a non-brokered, oversubscribed private placement financing, in the form of a convertible note, with local area private investors. 

The gross proceeds of the private placement will be used for general working capital purposes and for increasing the capacity and quality testing capabilities of the company’s production facility in San Diego, California.

The company is focused on delivering inexpensive and enabling solutions to advanced nano-device and graphene sensor makers by fabricating the highest quality single layer graphene attainable, via chemical vapor deposition (or ‘CVD’).

The company is now capable of producing monolayer graphene sensors on large area plastic sheets at a cost of pennies per unit, in a high throughput and sustainable way.  Further, Grolltex is helping customers that currently produce their graphene sensors on silicon wafers, to transition that production capacity to making their sensors on large area sheets of biodegradable plastic instead, at a >100X cost savings. 

Monolayer graphene films are today seen as the most promising futuristic sensing materials for their combination of surface to volume ratio (the film is only one atom thick) and conductivity (the most conductive substance known at room temperature). Markets that are commercializing advanced sensors made of graphene include DNA sensing and editing, new drug discovery and wearable bio-monitors for glucose sensing and autonomous blood pressure monitoring via patches or watch-like wearable bracelet devices.

No securities were issued and no cash was paid as bonuses, finders’ fees, compensation or commissions in connection with the private placement.

Tags:  Biosensor  CVD  Graphene  Groltex  Sensors 

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Synthesizing single-crystalline hexagonal graphene quantum dots

Posted By Graphene Council, Monday, August 5, 2019
A KAIST team has designed a novel strategy for synthesizing single-crystalline graphene quantum dots, which emit stable blue light. The research team confirmed that a display made of their synthesized graphene quantum dots successfully emitted blue light with stable electric pressure, reportedly resolving the long-standing challenges of blue light emission in manufactured displays. The study, led by Professor O Ok Park in the Department of Chemical and Biological Engineering.

Graphene has gained increased attention as a next-generation material for its heat and electrical conductivity as well as its transparency. However, single and multi-layered graphene have characteristics of a conductor so that it is difficult to apply into semiconductor. Only when downsized to the nanoscale, semiconductor's distinct feature of bandgap will be exhibited to emit the light in the graphene. This illuminating featuring of dot is referred to as a graphene quantum dot.

Conventionally, single-crystalline graphene has been fabricated by chemical vapor deposition (CVD) on copper or nickel thin films, or by peeling graphite physically and chemically. However, graphene made via chemical vapor deposition is mainly used for large-surface transparent electrodes. Meanwhile, graphene made by chemical and physical peeling carries uneven size defects.

The research team explained that their graphene quantum dots exhibited a very stable single-phase reaction when they mixed amine and acetic acid with an aqueous solution of glucose. Then, they synthesized single-crystalline graphene quantum dots from the self-assembly of the reaction intermediate. In the course of fabrication, the team developed a new separation method at a low-temperature precipitation, which led to successfully creating a homogeneous nucleation of graphene quantum dots via a single-phase reaction.

Professor Park and his colleagues have developed solution phase synthesis technology that allows for the creation of the desired crystal size for single nanocrystals down to 100 nano meters. It is reportedly the first synthesis of the homogeneous nucleation of graphene through a single-phase reaction.

Professor Park said, "This solution method will significantly contribute to the grafting of graphene in various fields. The application of this new graphene will expand the scope of its applications such as for flexible displays and varistors."

This research was a joint project with a team from Korea University under Professor Sang Hyuk Im from the Department of Chemical and Biological Engineering, and was supported by the National Research Foundation of Korea, the Nano-Material Technology Development Program from the Electronics and Telecommunications Research Institute (ETRI), KAIST EEWS, and the BK21+ project from the Korean government.

Tags:  CVD  Graphene  KAIST  O Ok Park  quantum dots  Sang Hyuk Im 

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Graphene in Electronic Circuits

Posted By 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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Step right up for bigger 2D sheets

Posted By Graphene Council, Thursday, March 7, 2019
Rice University researchers determined complementarity between growing hexagonal boron nitride crystals and a stepped substrate mimics the complementarity found in strands of DNA. The Rice theory supports experiments that have produced large, oriented wafers.

Very small steps make a big difference to researchers who want to create large wafers of two-dimensional material. Atom-sized steps in a substrate provide the means for 2D crystals growing in a chemical vapor furnace to come together in perfect rank. Scientists have recently observed this phenomenon, and now a Rice University group has an idea why it works.

Rice materials theorist Boris Yakobson and researcher Ksenia Bets led the construction of simulations that show atom-sized steps on a growth surface, or substrate, have the remarkable ability to keep monolayer crystal islands in alignment as they grow. If the conditions are right, the islands join into a larger crystal without the grain boundaries so characteristic of 2D materials like graphene grown via chemical vapor deposition (CVD). That preserves their electronic perfection and characteristics, which differ depending on the material.

Atom-sized steps in a substrate provide the means for 2D crystals growing in a chemical vapor furnace to come together in perfect rank. Scientists have recently observed this phenomenon, and now a Rice University group has an idea why it works.

Rice materials theorist Boris Yakobson and researcher Ksenia Bets led the construction of simulations that show atom-sized steps on a growth surface, or substrate, have the remarkable ability to keep monolayer crystal islands in alignment as they grow.

If the conditions are right, the islands join into a larger crystal without the grain boundaries so characteristic of 2D materials like graphene grown via chemical vapor deposition (CVD). That preserves their electronic perfection and characteristics, which differ depending on the material.

The Rice theory appears in the American Chemical Society journal Nano Letters.The investigation focused on hexagonal boron nitride (h-BN), aka white graphene, a crystal often grown via CVD. Crystals nucleate at various places on a perfectly flat substrate material and not necessarily in alignment with each other.

However, recent experiments have demonstrated that growth on vicinal substrates -- surfaces that appear flat but actually have sparse, atomically small steps -- can align the crystals and help them merge into a single, uniform structure, as reported on arXiv. A co-author of that report and leader of the Korean team, Feng Ding, is an alumnus of the Yakobson lab and a current adjunct professor at Rice.

But the experimentalists do not show how it works as, Yakobson said, the steps are known to meander and be somewhat misaligned.

"I like to compare the mechanism to a 'digital filter,' here offered by the discrete nature of atomic lattices," he said. "The analog curve that, with its slopes, describes a meandering step is 'sampled and digitized' by the very grid of constituent atomic rows, breaking the curve into straight 1D-terrace segments. The slope doesn't help, but it doesn't hurt. Surprisingly, the match can be good; like a well-designed house on a hill, it stands straight.

"The theory is simple, though it took a lot of hard work to calculate and confirm the complementarity matching between the metal template and the h-BN, almost like for A-G-T-C pairs in strands of DNA," Yakobson said.

It was unclear why the crystals merged into one so well until simulations by Bets, with the help of co-author and Rice graduate student Nitant Gupta, showed how h-BN "islands" remain aligned while nucleating along visibly curved steps.

"A vicinal surface has steps that are slightly misaligned within the flat area," Bets said. "It has large terraces, but on occasion there will be one-atom-high steps. The trick by the experimentalists was to align these vicinal steps in one direction."

In chemical vapor deposition, a hot gas of the atoms that will form the material are flowed into the chamber, where they settle on the substrate and nucleate crystals. h-BN atoms on a vicinal surface prefer to settle in the crook of the steps.

"They have this nice corner where the atoms will have more neighbors, which makes them happier," Bets said. "They try to align to the steps and grow from there.

"But from a physics point of view, it's impossible to have a perfect, atomically flat step," she said. "Sooner or later, there will be small indentations, or kinks. We found that at the atomic scale, these kinks in the steps don't prevent h-BN from aligning if their dimensions are complementary to the h-BN structure. In fact, they help to ensure co-orientation of the islands."

Because the steps the Rice lab modeled are 1.27 angstroms deep (an angstrom is one-billionth of a meter), the growing crystals have little trouble surmounting the boundary. "Those steps are smaller than the bond distance between the atoms," Bets said. "If they were larger, like two angstroms or higher, it would be more of a natural barrier, so the parameters have to be adjusted carefully."

Two growing islands that approach each other zip together seamlessly, according to the simulations. Similarly, cracks that appear along steps easily heal because the bonds between the atoms are strong enough to overcome the small distance.

Any path toward large-scale growth of 2D materials is worth pursuing for an army of applications, according to the researchers. 2D materials like conductive graphene, insulating h-BN and semiconducting transition metal dichalcogenides are all the focus of intense scrutiny by researchers around the world. The Rice researchers hope their theoretical models will point the way toward large crystals of many kinds.

The U.S. Department of Energy (DOE) supported the research. Computer resources were provided by the National Energy Research Scientific Computing Center, supported by the DOE Office of Science, and the National Science Foundation-supported DAVinCI cluster at Rice, administered by the Center for Research Computing and procured in partnership with Rice's Ken Kennedy Institute for Information Technology.

Tags:  Boris Yakobson  CVD  Graphene  Ksenia Bets  Rice University  U.S. Department of Energy (DOE) 

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