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Nanoelectromechanical sensors based on suspended 2D materials

Posted By Graphene Council, Wednesday, August 26, 2020
An international team of researchers have recently published a review article on nanoelectromechanical (NEMS) sensors based on suspended two-dimensional (2D) materials in the journal Research ("Nanoelectromechanical Sensors Based on Suspended 2D Materials"), an open-access multidisciplinary journal launched in 2018 as the first journal in the Science Partner Journal (SPJ) program.

The paper is an invited contribution to a special issue on “Progress and challenges in emerging 2D nanomaterials – preparation, processing, and device integration”, and has the purpose of contributing to the development of the field of 2D materials for sensor applications and to their integration with conventional semiconductor technology.

“I believe NEMS sensors based on 2D materials will be essential for satisfying the demand for integrated, high-performance sensors set by applications such as the Internet of Things (IoT) and autonomous mobility”, says Lemme, first author of the paper.

The review summarizes the many studies that have successfully shown the feasibility of using membranes of 2D materials in pressure sensors, microphones, mass and gas sensors – explaining the different sensor concepts and giving an overview of the relevant material properties, fabrication routes, and operation principles.

“Two-dimensional materials are ideally suited for sensors”, says Lemme, “as they allow realizing free-standing structures that are just one of a few atoms thick. This ultimate thinness can be a decisive advantage when it comes to nanoelectromechanical sensors, since the performance often depends critically on the thickness of the suspended part. Furthermore, many 2D materials have unique electrical, mechanical and optical properties that can be exploited for completely new concepts of sensor devices.”

The review – which includes contributions from RWTH Aachen University, AMO GmbH, Universität der Bundeswehr Munich, KTH Royal Institute of Technology, TU Delft, Infineon and the Kavli Institute of Nanoscience – discusses the different readout and integration methods of different sensors based on 2D materials, and provides comparisons against the state of the art devices to show both the challenges and the promises of 2D-materials based nanoelectromechanical sensing.

“Proof-of-concept sensor devices based on suspended 2D materials are almost always smaller than their conventional counterparts, show improved performances, and sometimes even completely novel functionalities”, says Peter G. Steeneken, leader of work-package 6 (Sensors) in the Graphene Flagship and co-author of the paper. “However, there are still enormous challenges to demonstrate that 2D material-based NEMS sensors can outperform conventional devices on all important aspects – for example, the establishment of high-yield manufacturing capabilities. The Graphene Flagship represents the ideal platform to address these challenges, as it fosters collaborations between world-leading groups to achieve a set of well-defined goals. This paper is an example of how, by bringing together complementary expertise, we can achieve more.”

Tags:  2D materials  AMO GmbH  Graphene  Graphene Flagship  Infineon  Kavli Institute of Nanoscience  KTH Royal Institute of Technology  Max Lemme  RWTH Aachen University  Sensors  TU Delft  Universität der Bundeswehr Munich 

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A scalable manufacturing-technology for highly sensitive photodetectors on flexible substrates

Posted By Graphene Council, Thursday, June 4, 2020
Researchers from AMO GmbH and RWTH Aachen University have successfully demonstrated high-responsivity molybdenum disulfide (MoS2) photodetectors on flexible substrates, realized with a scalable manufacturing technology. The work has been recently published in the journal ACS Photonics, and it is the result of a cooperation with the University of Siegen, Raith B.V., AIXTRON SE, and the University of Wuppertal.

Molybdenum disulfide is a two-dimensional material that is ideally suited for realizing flexible high-sensitivity photodetectors. However, most of the devices demonstrated so far are based on MoS2 crystals of only a few micrometers in size, obtained in a complex process, poorly compatible with an industrial-scale implementation.

In their recent work, Schneider and co-workers have demonstrated an approach scalable to large-volume production of high-performance phototedectors, starting from MoS2 deposited on sapphire wafers using Metal Organic Vapor Phase Epitaxy (MOVPE). The excellent cooperation between AMO, RWTH, and AIXTRON has allowed optimizing the tools for material-growth (a commercial AIXTRON Planetary Reactor), as well as the transfer processes and the technology for realizing highly-sensitive photodetectors on flexible substrates.

This work is an important step towards real-life applications of 2D materials for flexible electronics in the areas of the Internet of Things and medical devices. In particular, “blue light hazard” – a possible risk related to certain modern light sources – can be efficiently detected by the present sensor concept. The research work was funded by the European Union (QUEFORMAL, 829035) and Graphene Flagship (785219, 881603), European regional funds (HEA2D, NW-1-1-036), the German Research Foundation (MOSTFLEX, 407080863) and the German Ministry of Education and Research (NeuroTec, 16ES1134).

Tags:  AIXTRON  AMO GmbH  Graphene  Photonics  RWTH Aachen University 

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High-quality boron nitride grown at atmospheric pressure

Posted By Graphene Council, Wednesday, April 22, 2020
Graphene Flagship researchers at RWTH Aachen University, Germany and ONERA-CNRS, France, in collaboration with researchers at the Peter Grunberg Institute, Germany, the University of Versailles, France, and Kansas State University, US, have reported a significant step forward in growing monoisotopic hexagonal boron nitride at atmospheric pressure for the production of large and very high-quality crystals.

Hexagonal boron nitride (hBN) is the unsung hero of graphene-based devices. Much progress over the last decade was enabled by the realisation that 'sandwiching' graphene between two hBN crystals can significantly improve the quality and performance of the resulting devices. This finding paved the way to a series of exciting developments, including the discoveries of exotic effects such as magic-angle superconductivity and proof-of-concept demonstrations of sensors with unrivalled sensitivity.

Until now, the most widely used hBN crystals came from the National Institute of Material Science in Tsukuba, Japan. These crystals are grown using a process at high temperatures (over 1500°C) and extremely high pressures (over 40,000 times atmospheric pressure). "The pioneering contribution by the Japanase researchers Taniguchi and Watanabe to graphene research is invaluable", begins Christoph Stampfer from Graphene Flagship Partner RWTH Aachen University, Germany. "They provide hundreds of labs around the world with ultra-pure hBN at no charge. Without their contribution, a lot of what we are doing today would not be possible."

However, this hBN growth method comes with some limitations. Among them is the small crystal size, which is limited to a few 100 µm, and the complexity of the growth process. This is suitable for fundamental research, but beyond this, a method with better scalability is needed. Now Graphene Flagship researchers tested hBN crystals grown with a new methodology that works at atmospheric pressure, developed by a team of researchers led by James Edgar at Kansas State University, US. This new approach shows great promise for more demanding research and production.

"I was very excited when Edgar proposed that we test the quality of his hBN", says Stampfer. "His growth method could be suitable for large-scale production". The method for growing hBN at atmospheric pressure is indeed much simpler and cheaper than previous alternatives and allows for the isotopic concentration to be controlled.

"The hBN crystals we received were the largest I have ever seen, and they were all based either on isotopically pure boron-10 or boron-11" says Jens Sonntag, a graduate student at Graphene Flagship Partner RWTH Aachen University. Sonntag tested the quality of the flakes first using confocal Raman spectroscopy. In addition, Graphene Flagship partners in ONERA-CNRS, France, led by Annick Loiseau, carried out advanced luminescence measurements. Both measurements indicated high isotope purity and high crystal quality.

However, the strongest evidence for the high hBN qualitycame from transport measurements performed on devices containing graphene sandwiched between monoisotopic hBN. They showed equivalent performance to a state-of-the-art device based on hBN from Japan, with better performance in some areas.

"This is a clear indication of the extremely high quality of these hBN crystals," says Stampfer. "This is great news for the whole graphene community, because it shows that it is, in principle, possible to produce high quality hBN on a large scale, bringing us one step closer to real applications based on high-performance graphene electronics and optoelectronics. Furthermore, the possibility of controlling the isotopic concentration of the crystals opens the door to experiments that were not possible before."

Mar García-Hernández, Work Package Leader for Enabling Materials, adds: "Free-standing graphene, being the thinnest material known, exhibits a large surface area and, therefore, is extremely sensitive to its surrounding environment, which, in turn, results in substantial degradation of its exceptional properties. However, there is a clear strategy to avoid these deleterious effects: encapsulating graphene between two protective layers."

García-Hernández continues: "When graphene is encapsulated by hBN, it reveals its intrinsic properties. This makes hBN an essential material to integrate graphene into current technologies and demonstrates the importance of devising new scalable synthetic routes for large-scale hBN production. This work not only provides a new and simpler path to produce high-quality hBN crystals on a large scale, but it also enables the production of monoisotopic material, which further reduces the degradation of graphene when encapsulated by two layers."

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "This is a nice example of collaboration between the EU and the US, which we fostered via numerous bilateral workshops. Devising alternative approaches to produce high-quality hBN crystals is crucial to enable us to exploit the ultimate properties of graphene in opto-electronics applications. Furthermore, this work will lead to significant progress in fundamental research."

Tags:  Andrea C. Ferrari  Christoph Stampfer  Graphene  Graphene Flagship  Hexagonal boron nitride  Mar García-Hernández  ONERA-CNRS  optoelectronics  RWTH Aachen University  Sensors 

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Two new FLAG-ERA projects in Aachen

Posted By Graphene Council, The 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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World’s smallest accelerometer points to new era in wearables, gaming

Posted By Graphene Council, The Graphene Council, Wednesday, September 11, 2019
Updated: Friday, September 6, 2019
In what could be a breakthrough for body sensor and navigation technologies, researchers at KTH have developed the smallest accelerometer yet reported, using the highly conductive nanomaterial, graphene.

Each passing day, nanotechnology and the potential for graphene material make new progress. The latest step forward is a tiny accelerometer made with graphene by an international research team involving KTH Royal Institute of Technology, RWTH Aachen University and Research Institute AMO GmbH, Aachen.

Among the conceivable applications are monitoring systems for cardiovascular diseases and ultra-sensitive wearable and portable motion-capture technologies.

For decades microelectromechanical systems (MEMS) have been the basis for new innovations in, for example, medical technology. Now these systems are starting to move to the next level – nano-electromechanical systems, or NEMS.

Xuge Fan, a researcher in the Department for Micro and Nanosystems at KTH, says that the unique material properties of graphene have enabled them to build these ultra-small accelerometers.

“Based on the surveys and comparisons we have made, we can say that this is the smallest reported electromechanical accelerometer in the world,” Fan says. The researchers reported their work in Nature Electronics.

The measure by which any conductor is judged is how easily, and speedily, electrons can move through it. On this point, together with its extraordinary mechanical strength, graphene is one of the most promising materials for a breathtaking array of applications in nano-electromechanical systems.

“We can scale down components because of the material’s atomic-scale thickness, and it has great electrical and mechanical properties,” Fan says. “We created a piezoresistive NEMS accelerometer that is dramatically smaller than any MEMS accelerometers available today, but retains the sensitivity these systems require.”

The future for such small accelerometers is promising, says Fan, who compares advances in nanotechnology to the evolution of smaller and smaller computers.

“This could eventually benefit mobile phones for navigation, mobile games and pedometers, as well as monitoring systems for heart disease and motion-capture wearables that can monitor even the slightest movements of the human body,” he says.

Other potential uses for these NEMS transducers include ultra-miniaturized NEMS sensors and actuators such as resonators, gyroscopes and microphones. In addition, these NEMS transducers can be used as a system to characterize the mechanical and electromechanical properties of graphene, Fan says.

Max Lemme, professor at RWTH, is excited about the results: "Our collaboration with KTH over the years has already shown the potential of graphene membranes for pressure and Hall sensors and microphones. Now we have added accelerometers to the mix. This makes me hopeful to see the material on the market in some years. For this, we are working on industry-compatible manufacturing and integration techniques."

Tags:  AMO GmbH  Electronics  Graphene  KTH Royal Institute of Technology  Max Lemme  RWTH Aachen University  Sensors  Xuge Fan 

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