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Laser used to transfer graphene for tech applications

Posted By Graphene Council, 21 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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Physicists 'trick' photons into behaving like electrons using a 'synthetic' magnetic field

Posted By Graphene Council, Thursday, September 17, 2020
Scientists have discovered an elegant way of manipulating light using a "synthetic" Lorentz force -- which in nature is responsible for many fascinating phenomena including the Aurora Borealis.

A team of theoretical physicists from the University of Exeter has pioneered a new technique to create tuneable artificial magnetic fields, which enable photons to mimic the dynamics of charged particles in real magnetic fields.

The team believe the new research, published in leading journal Nature Photonics, could have important implications for future photonic devices as it provides a novel way of manipulating light below the diffraction limit.

When charged particles, like electrons, pass through a magnetic field they feel a Lorentz force due to their electric charge, which curves their trajectory around the magnetic field lines.

This Lorentz force is responsible for many fascinating phenomena, ranging from the beautiful Northern Lights, to the famous quantum-Hall effect whose discovery was awarded the Nobel Prize.

However, because photons do not carry an electric charge, they cannot be straightforwardly controlled using real magnetic fields since they do not experience a Lorentz force; a severe limitation that is dictated by the fundamental laws of physics.

The research team have shown that it is possible to create artificial magnetic fields for light by distorting honeycomb metasurfaces -- ultra-thin 2D surfaces that are engineered to have structure on a scale much smaller than the wavelength of light.

The Exeter team were inspired by a remarkable discovery ten years ago, where it was shown that electrons propagating through a strained graphene membrane behave as if they were subjected to a large magnetic field.

The major drawback with this strain engineering approach is that to tune the artificial magnetic field one is required to modify the strain pattern with precision, which is extremely challenging, if not impossible, to do with photonic structures.

The Exeter physicists have proposed an elegant solution to overcome this fundamental lack of tunability.

Charlie-Ray Mann, the lead scientist and author of the study, explains: "These metasurfaces, support hybrid light-matter excitations, called polaritons, which are trapped on the metasurface.

"They are then deflected by the distortions in the metasurface in a similar way to how magnetic fields deflect charged particles.

"By exploiting the hybrid nature of the polaritons, we show that you can tune the artificial magnetic field by modifying the real electromagnetic environment surrounding the metasurface."

For the study, the researchers embedded the metasurface between two mirrors -- known as a photonic cavity -- and show that one can tune the artificial magnetic field by changing only the width of the photonic cavity, thereby removing the need to modify the distortion in the metasurface.

Charlie added: "We have even demonstrated that you can switch off the artificial magnetic field entirely at a critical cavity width, without having to remove the distortion in the metasurface, something that is impossible to do in graphene or any system that emulates graphene.

"Using this mechanism you can bend the trajectory of the polaritons using a tunable Lorentz-like force and also observe Landau quantization of the polariton cyclotron orbits, in direct analogy with what happens to charged particles in real magnetic fields.

"Moreover, we have shown that you can drastically reconfigure the polariton Landau level spectrum by simply changing the cavity width."

Dr Eros Mariani, the lead supervisor of the study, said: "Being able to emulate phenomena with photons that are usually thought to be exclusive to charged particles is fascinating from a fundamental point of view, but it could also have important implications for photonics applications.

"We're excited to see where this discovery leads, as it poses many intriguing questions which can be explored in many different experimental platforms across the electromagnetic spectrum."

Tags:  2D materials  Charlie-Ray Mann  Eros Mariani  Graphene  photonics  University of Exeter 

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E-Beam atomic-scale 3D 'sculpting' could enable new quantum nanodevices

Posted By Graphene Council, Thursday, September 17, 2020

By varying the energy and dose of tightly-focused electron beams, researchers have demonstrated the ability to both etch away and deposit high-resolution nanoscale patterns on two-dimensional layers of graphene oxide. The 3D additive/subtractive “sculpting” can be done without changing the chemistry of the electron beam deposition chamber, providing the foundation for building a new generation of nanoscale structures.

Based on focused electron beam-induced processing (FEBID) techniques, the work could allow production of 2D/3D complex nanostructures and functional nanodevices useful in quantum communications, sensing, and other applications. For oxygen-containing materials such as graphene oxide, etching can be done without introducing outside materials, using oxygen from the substrate.

“By timing and tuning the energy of the electron beam, we can activate interaction of the beam with oxygen in the graphene oxide to do etching, or interaction with hydrocarbons on the surface to create carbon deposition,” said Andrei Fedorov, professor and Rae S. and Frank H. Neely Chair in the George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology. “With atomic-scale control, we can produce complicated patterns using direct write-remove processes. Quantum systems require precise control on an atomic scale, and this could enable a host of potential applications.”

The technique was described in the journal ACS Applied Materials & Interfaces ("High-Resolution Three-Dimensional Sculpting of Two-Dimensional Graphene Oxide by E-Beam Direct Write"). The work was supported by the U.S. Department of Energy Office of Science, Basic Energy Sciences. Co-authors included researchers from Pusan National University in South Korea.

Creation of nanoscale structures is traditionally done using a multistep process of photoresist coating and patterning by photo- or electron beam lithography, followed by bulk dry/wet etching or deposition. Use of this process limits the range of functionalities and structural topologies that can be achieved, increases the complexity and cost, and risks contamination from the multiple chemical steps, creating barriers to fabrication of new types of devices from sensitive 2D materials.

FEBIP enables a material chemistry/site-specific, high-resolution multimode atomic scale processing and provides unprecedented opportunities for “direct-write,” single-step surface patterning of 2D nanomaterials with an in-situ imaging capability. It allows for realizing a rapid multiscale/multimode “top-down and bottom-up” approach, ranging from an atomic scale manipulation to a large-area surface modification on nano- and microscales.

“By tuning the time and the energy of the electrons, you can either remove material or add material,” Fedorov said. “We did not expect that upon electron exposure of graphene oxide that we would start etching patterns.”

With graphene oxide, the electron beam introduces atomic scale perturbations into the 2D-arranged carbon atoms and uses embedded oxygen as an etchant to remove carbon atoms in precise patterns without introduction of a material into the reaction chamber. Fedorov said any oxygen-containing material might produce the same effect. “It’s like the graphene oxide carries its own etchant,” he said. “All we need to activate it is to ‘seed’ the reaction with electrons of appropriate energy.”

For adding carbon, keeping the electron beam focused on the same spot for a longer time generates an excess of lower-energy electrons by interactions of the beam with the substrate to decompose the hydrocarbon molecules onto the surface of the graphene oxide. In that case, the electrons interact with the hydrocarbons rather than the graphene and oxygen atoms, leaving behind liberated carbon atoms as a 3D deposit.

“Depending on how many electrons you bring to it, you can grow structures of different heights away from the etched grooves or from the two-dimensional plane,” he said. “You can think of it almost like holographic writing with excited electrons, substrate and adsorbed molecules combined at the right time and the right place.”

The process should be suitable for depositing materials such as metals and semiconductors, though precursors would need to be added to the chamber for their creation. The 3D structures, just nanometers high, could serve as spacers between layers of graphene or as active sensing elements or other devices on the layers.

“If you want to use graphene or graphene oxide for quantum mechanical devices, you should be able to position layers of material with a separation on the scale of individual carbon atoms,” Fedorov said. “The process could also be used with other materials.”

Using the technique, high-energy electron beams can produce feature sizes just a few nanometers wide. Trenches etched in surfaces could be filled with metals by introducing metal atoms contained in precursors.

Beyond simple patterns, the process could also be used to grow complex structures. “In principle, you could grow a structure like a nanoscale Eiffel Tower with all the intricate details,” Fedorov said. “It would take a long time, but this is the level of control that is possible with electron beam writing.”

Though systems have been built to use multiple electron beams in parallel, Fedorov doesn’t see them being used in high-volume applications. More likely, he said, is laboratory use to fabricate unique structures useful for research purposes.

“We are demonstrating structures that would otherwise be impossible to produce,” he said. “We want to enable the exploitation of new capabilities in areas such as quantum devices. This technique could be an imagination enabler for interesting new physics coming our way with graphene and other interesting materials.”

Tags:  2D materials  Andrei Fedorov  Graphene  graphene oxide  nanomaterials  Pusan National University 

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A multivalued optical memory composed of 2D materials

Posted By Graphene Council, Friday, September 11, 2020
The National Institute for Materials Science (NIMS) has developed a memory device capable of storing multiple values using both optical and voltage input values. This device composed of layered two-dimensional materials is able to optically control the amount of charge stored in these layers. This technology may be used to significantly increase the capacity of memory devices and applied to the development of various optoelectronic devices. The research was published in Advanced Functional Materials ("Laser-assisted multilevel non-volatile memory device based on 2D van-der-Waals fewlayer-ReS2/h-BN/Graphene Heterostructures").

Memory devices used to store information (e.g., flash memory) play an indispensable role in today’s information society. The recording density of these devices has substantially increased in the past 20 years. In anticipation of widespread adoption of IoT technologies in the near future, it is desirable to accelerate the development of higher speed, larger capacity memory devices.

However, the current approach to increasing memory capacity and energy efficiency through silicon microfabrication is about to reach its limits. Development of memory devices with different working principles therefore has been awaited.

To meet expected technology needs, this research group has developed a transistor memory device composed of layered two-dimensional materials, including rhenium disulfide (ReS2) – a semiconductor – serving as a channel transistor, hexagonal boron nitride (h-BN) used as an insulating tunnel layer and graphene functioning as a floating gate.

This device records data by storing charge carriers in the floating gate in a manner similar to conventional flash memory. Hole-electron pairs in the ReS2 layer are prone to excitation when irradiated with light. The number of these pairs can be regulated by changing the intensity of the light.

The group succeeded in creating a mechanism that allows the amount of charge in the graphene layer to gradually decrease as the exited electrons once again couple with the holes in this layer. This success enabled the device to operate as a multivalued memory capable of efficiently controlling the amount of stored charge in stages through the combined use of light and voltage.

Moreover, this device can operate energy efficiently by minimizing electric current leakage—an achievement made possible by layering two-dimensional materials, thereby smoothening the interfaces between them at an atomic level.

This technology may be used to significantly increase the capacity and energy efficiency of memory devices. It also may be applied to the development of various optoelectronic devices, including optical logic circuits and highly sensitive photosensors capable of controlling the amount of charge stored in them through combined use of light and voltage.

This project was carried out by a research group consisting of Yutaka Wakayama (Leader of the Quantum Device Engineering Group (QDEG), International Center for Materials Nanoarchitectonics (MANA), NIMS), Bablu Mukherjee (Postdoctoral Researcher, QDEG, MANA, NIMS) and Shu Nakaharai (Principal Researcher, QDEG, MANA, NIMS).

This study was conducted in conjunction with another project entitled “Development of a ultra-sensitive photosensor using two-dimensional atomic film layers” funded by the Grant-in-Aid for JSPS Fellows.

Tags:  2D materials  Bablu Mukherjee  Graphene  International Center for Materials Nanoarchitecton  optoelectronics  Semiconductor  Sensors  Shu Nakaharai  The National Institute for Materials Science  Yutaka Wakayama 

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First 2D neural network

Posted By Graphene Council, Tuesday, September 8, 2020
Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) in collaboration with Samsung Advanced Institute of Technology have developed the first neural network for artificial intelligence made using two-dimensional materials. Two-dimensional materials are substances with a thickness of a few nanometers or less, often consisting of a single sheet of atoms. This machine vision processor made from these materials can capture, store and recognize more than 1,000 images. 

“This work highlights an unprecedented advance in the functional complexity of 2D electronics,” said Donhee Ham, the Gordon McKay Professor of Electrical Engineering and Applied Physics at SEAS and senior author of the paper. “We have performed both front-end optical image sensing and back-end image recognition in one, 2D, material platform.”

This work highlights an unprecedented advance in the functional complexity of 2D electronics. DONHEE HAM, GORDON MCKAY PROFESSOR OF ELECTRICAL ENGINEERING AND APPLIED PHYSICS.

Since the discovery of graphene in 2004, researchers have been working to harness the unique electronic and optoelectronic properties of atomically thin, two-dimensional semiconductors for the basic building blocks of a range of applications. 

Transistors made from two-dimensional materials have been used for simple digital logic circuits and photodetectors but large-scale integration for complex computing, such as artificial intelligence, has remained out of reach. So far, researchers have only been able to integrate about 100 transistors made from two-dimensional materials onto a single chip.  To put that in perspective, standard silicon integrated circuits, such as those in your smartphone, contain billions of transistors. 

Now, Ham and his team have developed an integrated circuit with more than 1,000 two-dimensional material-based transistors.  

“Two-dimensional material-based devices exhibit various exciting properties, but low integration level has restricted their functional complexity,” said Houk Jang, a research associate at SEAS and first author of the paper. “With 1,000 devices integrated on a single chip, our atomically thin network can perform vision recognition tasks, which is a remarkably advanced functionality of two-dimensional material-based electronics.”

The research team used a two-dimensional material called molybdenum disulfide (MoS2), the three-atom thick semiconductor, which interacts well with light. They arranged these photosensitive transistors into what’s known as a crossbar array, which is inspired by neuronal connections in the human brain. This relatively simple set-up allows the device to act as both an eye that can see an image and a brain that can store and recognize an image. 

On the front end, the crossbar array acts like an image sensor, capturing an image just like an eye. The photosensitivity of the materials means that the image can be stored and converted into electrical data. On the back end, the same crossbar array can perform networked computing on that electrical data to recognize and identify the image.

To demonstrate the process, the researchers showed the device 1,000 images of handwritten digits. The processor was able to identify the images with 94 percent accuracy.

“Through capturing of optical images into electrical data like the eye and optic nerve, and subsequent recognition of this data like the brain via in-memory computing, our optoelectronic processor emulates the two core functions of human vision,” said Henry Hinton, a graduate student at SEAS and coauthor of the paper. 

“This is the first demonstration of a neural network with two-dimensional materials that can interact with light,” said Jang. “Because it computes in memory, you don’t need separate memory and the calculation can be done with very low energy.” 

Next, the team aims to scale up the device even further for two-dimensional material-based, high resolution imaging system.

Tags:  2D materials  Donhee Ham  Graphene  Harvard John A. Paulson School of Engineering  Samsung Advanced Institute of Technology  Sensors  transistors 

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New technology extracts potential to identify quality graphene cheaper and faster

Posted By Graphene Council, Wednesday, August 26, 2020
Engineers at Australia’s Monash University have developed world-first technology that can help industry identify and export high quality graphene cheaper, faster and more accurately than current methods.

Published in international journal Advanced Science ("A High Throughput and Unbiased Machine Learning Approach for Classification of Graphene Dispersions"), researchers used the data set of an optical microscope to develop a machine-learning algorithm that can characterise graphene properties and quality, without bias, within 14 minutes.

This technology is a game changer for hundreds of graphene or graphene oxide manufacturers globally. It will help them boost the quality and reliability of their graphene supply in quick time.

Currently, manufacturers can only detect the quality and properties of graphene used in a product after it has been manufactured.

Through this algorithm, which has the potential to be rolled out globally with commercial support, graphene producers can be assured of quality product and remove the time-intensive and costly process of a series of characterisation techniques to identify graphene properties, such as the thickness and size of the atomic layers.

Professor Mainak Majumder from Monash University’s Department of Mechanical and Aerospace Engineering and the Australian Research Council’s Hub on Graphene Enabled Industry Transformation led this breakthrough study.

“Graphene possesses extraordinary capacity for electric and thermal conductivity. It is widely used in the production of membranes for water purification, energy storage and in smart technology, such as weight loading sensors on traffic bridges,” Professor Majumder said.

“At the same time, graphene is rather expensive when it comes to usage in bulk quantities. One gram of high quality graphene could cost as much as $1,000 AUD ($720 USD) a large percentage of it is due to the costly quality control process.

“Therefore, manufacturers need to be assured that they’re sourcing the highest quality graphene on the market. Our technology can detect the properties of graphene in under 14 minutes for a single dataset of 1936 x 1216 resolution. This will save manufacturers vital time and money, and establish a competitive advantage in a growing marketplace.”

Discovered in 2004, graphene is touted as a wonder material for its outstanding lightweight, thin and ultra-flexible properties. Graphene is produced through the exfoliation of graphite. Graphite, a crystalline form of carbon with atoms arranged hexagonally, comprises many layers of graphene.

However, the translation of this potential to real-life and usable products has been slow. One of the reasons is the lack of reliability and consistency of what is commercially often available as graphene.

The most widely used method of producing graphene and graphene oxide sheets is through liquid phase exfoliation (LPE). In this process, the single layer sheets are stripped from its 3D counterpart such as graphite, graphite oxide film or expanded graphite by shear-forces.

But, this can only be imaged using a dry sample (i.e. once the graphene has been coated on a glass slide). “Although there has been a strong emphasis on standardisation guidelines of graphene materials, there is virtually no way to monitor the fundamental unit process of exfoliation, product quality varies from laboratory to laboratory and from one manufacturer to other,” Dr Shaibani said.

“As a result, discrepancies are often observed in the reported property-performance characteristics, even though the material is claimed to be graphene.

“Our work could be of importance to industries that are interested in delivering high quality graphene to their customers with reliable functionality and properties. There are a number of ASX listed companies attempting to enter this billion-dollar market, and this technology could accelerate this interest.”

Researchers applied the algorithm to an assortment of 18 graphene samples – eight of which were acquired from commercial sources and the rest produced in a laboratory under controlled processing conditions.

Using a quantitative polarised optical microscope, researchers identified a technique for detecting, classifying and quantifying exfoliated graphene in its natural form of a dispersion.

To maximise the information generated from hundreds of images and large numbers of samples in a fast and efficient manner, researchers developed an unsupervised machine-learning algorithm to identify data clusters of similar nature, and then use image analysis to quantify the proportions of each cluster.

Mr Abedin said this method has the potential to be used for the classification and quantification of other two-dimensional materials.

“The capability of our approach to classify stacking at sub-nanometer to micrometer scale and measure the size, thickness, and concentration of exfoliation in generic dispersions of graphene/graphene oxide is exciting and holds exceptional promise for the development of energy and thermally advanced products,” Mr Abedin said.

Professor Dusan Losic, Director of Australian Research Council’s Hub on Graphene Enabled Industry Transformation, said: “These outstanding outcomes from our ARC Research Hub will make significant impact on the emerging multibillion dollar graphene industry giving graphene manufacturers and end-users new a simple quality control tool to define the quality of their produced graphene materials which is currently missing.”

Tags:  2D materials  Australian Research Council  Dusan Losic  energy storage  Graphene  Mainak Majumder  Monash University  water purification 

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Dr. Lutz Waldecker starts at RWTH with a fellowship of the Alexander von Humboldt Foundation

Posted By Graphene Council, Wednesday, August 26, 2020
This August, the Aachen Graphene & 2D Materials Center has gained new valuable expertise with the arrival of Dr. Lutz Waldecker as senior researcher in the Stampfer Group (2nd Institute of Physics A).

Waldecker is an expert on exciton physics and ultrafast dynamics in solids, in particular two-dimensional (2D) semiconductors. On his background, he has a Ph.D. at the Fritz Haber Institute of the Max Planck Society in Berlin, under the supervision of Ralph Ernstorfer, and a post-doc at Stanford University, in the group of Tony Heinz, where he worked on the optical and electronic properties of 2D semiconductors.

Among Waldecker’s recent results is the demonstration that dielectric screening causes a rigid shift of the single particle bands of 2D semiconductors, with little changes of the electronic dispersion [1]. This observation suggests a new, noninvasive way of inducing nanoscale functionality in 2D semiconductors by acting on the substrate dielectrics – an approach that Waldecker is planning to explore as part of his research in Aachen.

Waldecker’s position is funded by a Feodor-Lynen return fellowship of the Alexander von Humboldt Foundation. “It is great to see that the Aachen Graphene & 2D Materials Center is getting increasingly competitive in attracting top-class researchers”, says Prof. Christoph Stampfer, “I’m looking very much forward to working together with Lutz!”

Tags:  2D materials  Aachen Graphene & 2D Materials Center  Graphene  Lutz Waldecker  Semiconductors  Stampfer Group 

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WPI-MANA Demonstrates First Fabrication of fBBLG/hBN Superlattices

Posted By Graphene Council, Wednesday, August 26, 2020
A team at the International Center for Materials Nanoarchitectonics (WPI-MANA) has demonstrated for the first time the fabrication of folded bilayer-bilayer graphene (fBBLG)/hexagonal boron nitride (hBN) superlattices. This achievement could pave the way for expanded applications of superlattices, such as in a variety of quantum devices.

Graphene superlattices represent a novel class of quantum metamaterials that have promising prospects. They have been generating a lot of attention recently, ever since the discovery of superconductivity in twisted bilayer graphene (BLG). This was followed by studies related to twisted bilayer-bilayer graphene. Bernal-stacked BLG has a parabolic energy dispersion with a four-fold spin and valley degeneracy.

A superlattice is a periodic structure of layers of two or more materials. Typically, the width of layers is orders of magnitude larger than the lattice constant, and is limited by the growth of the structure. The WPI-MANA team's superlattices are made up of vertically stacked ultrathin/atomic-layer quasi 2D materials.

The WPI-MANA team's results point to the emergence of a unique electronic band structure in the fBBLG, which could provide a way for investigating correlated electron phenomena by performing energy-band engineering with superlattice structures.

The results of this study indicate the emergence of a unique electronic band structure in fBBLG, which could be modified by the moire superlattice potential. Although a systematic way to fold graphene is still lacking, it should be a fruitful topic of future research, leading to 2D paper-folding engineering like "origami." The team's results suggest a possible way to engineer 2D electronic systems by mechanical folding, similar to "tear and stack" for twisted heterostructures.

This work could lead the way to expanded applications of superlattices, including quantum devices such as Bloch oscillators, quantum cascade lasers and terahertz source generators.

Tags:  2D materials  Graphene  hexagonal boron nitride  International Center for Materials Nanoarchitecton 

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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 new two-dimensional carbon allotrope -- semiconducting diamane film synthesized

Posted By Graphene Council, Thursday, August 20, 2020
Atomically thin diamond, also called diamane, is a two-dimensional carbon allotrope and has attracted considerable scientific interest due to its potential physical properties. However, previous studies suggest that atomically thin diamond films are not achievable in a pristine state because diamonds possess a three-dimensional crystalline structure and would lack chemical stability when thinned down to the thickness of diamond's unit cell due to the dangling sp3 bonds. Chemical functionalization of the surface carbons with specific chemical groups was considered necessary to stabilize the two-dimensional structure, such as surface hydrogenation or fluorination, and various substrates have also been used in these synthesizing attempts. But all of these attempts change the composition of diamond films, that is to say, the successful synthesis of a pristine diamane has up until now not been achieved.

Regulating the phase transition process of carbon materials under high pressure and high temperature is always a straightforward method for achieving diamondization. Here, a team of scientists led by Drs. Feng Ke and Bin Chen from HPSTAR (the Center for High Pressure Science and Technology Advanced Research) used this direct approach, diamondization of mechanically exfoliated few-layer graphene via compression, to synthesize the long-sought-after diamane film. The study is published in Nano Letters.

The diamondization process is usually accompanied by an opening of an energy gap and a dramatic resistance increase due to the sp2-sp3 rehybridization between carbon atoms. "The in-situ electrical transport measurements of few-layer graphene are difficult to carry out under high pressure," said Feng Ke. "However, using our recently developed photolithography-based microwiring technique to prepare film electrodes on a diamond surface for resistance measurements, we are able to study the pressure-induced sp2-sp3 diamondization transition of mechanically exfoliated graphene with layer thickness ranging from 12- to bilayer at room temperature."

Their studies demonstrate that pristine h-diamane could be synthesized by compressing trilayer and thicker graphene to above 20 GPa at room temperature, which once synthesized could be preserved to about 1.0 GPa upon decompression. "The optical absorption reveals that h-diamane has an energy gap of 2.8 ± 0.3 eV, and further band structure calculations confirm an indirect band gap of 2.7-2.9 eV," explained the co-frist-author Lingkong Zhang, a PhD student at HPSTAR. "Compared to gapless graphene, semiconducting h-diamane offers exciting possibilities for carbon-based electronic devices."

The XRD measurements have shown that the few-layer graphene to h-diamane transition is a gradual structural transition, which helps to understand the continuous resistance increase and absorbance decrease in trilayer and thicker graphene with pressure above the transition pressure. Theoretical calculations indicate that a (−2110)-oriented h-diamane is energetically stable and has a lower enthalpy than its few-layer graphene precursor above the transition pressure.

"Like the discovery of graphene, carbon nanotubes, fullerenes, and other novel carbon allotropes, the realization of a pristine diamane represents another exciting achievement in materials science," added Dr. Bin Chen, "Thermal treatment at high pressure may be helpful to preserve a pristine h-diamane to ambient pressure, as suggested from the high-temperature and high-pressure method to synthesize a pressure quenchable h-diamond. The challenges still remain to achieve the preservation and industrial applications of diamane."

Tags:  2D Materials  Bin Chen  carbon nanotubes  Feng Ke  Graphene  HPSTAR 

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