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White graphene: high defect tolerance and elasticity unveiled by nanomechanics experts

Posted By Graphene Council, Friday, September 25, 2020
Because of their unique physical, chemical, electrical and optical properties, two-dimensional (2D) materials have attracted tremendous attention in the past decades.

After revealing the realistic strength and stretchability of graphene, researchers from City University of Hong Kong (CityU) have carried forward the success by unveiling the high defect tolerance and elasticity of hexagonal boron nitride (h-BN), another 2D material known as “white graphene”.

This follow-up study (Cell Reports Physical Science, "Large Elastic Deformation and Defect Tolerance of Hexagonal Boron Nitride Monolayers”") will promote future development and applications of strain engineering, piezoelectronics and flexible electronics.

Since British scientists exfoliated single-atom-thick crystallites from bulk graphite in 2004 for the first time, research on 2D materials has undergone rapid advances. Novel 2D materials have been discovered, including hexagonal boron nitride (h-BN), the focus of this article, transition metal dichalcogenides (TMDs) such as MoS2, and black phosphorus (BP).

Those successfully isolated 2D materials have different band gaps (from 0 to 6 eV), and range from conductors, semiconductors to insulators1, which illustrates their potential in electronic device applications.

Sometimes referred as "white graphene", h-BN shares a similar structure with graphene. The theoretical estimates of its mechanical properties and its thermal stability are also comparable to those of graphene. Due to its ultra-wide band gap of ∼6 eV, h-BN can serve in optoelectronics or as a dielectric substrate for graphene or other 2D materials-based electronics.

More importantly, its band gap could be modified via the elastic strain engineering (ESE) approach in which the material band structure can be significantly tuned by lattice straining or distortion.

It is worth mentioning that h-BN can improve the performance of graphene devices. Similar to graphene’s atomic structure, monolayer h-BN has a small lattice mismatch and ultra-flat surface, which can significantly enhance graphene's carrier density. Carrier density represents the number of carriers that participates in conduction, which is one of the key factors contributing to electrical conductivity.

In addition, the ultra-wide band gap makes h-BN an ideal dielectric substrate for graphene and other 2D material-based electronics. Having no centre of symmetry, monolayer h-BN is predicted to exhibit induced piezoelectric potential under mechanical strains.

However, these fascinating properties and applications always require relatively large and uniform deformations. In fact, all materials need to have reliable mechanical properties before they can be used in practical devices.

That is why researchers have tried different approaches to explore the mechanical responses of graphene and other 2D materials under various conditions. Yet, most of the tests use the nanoindentation technique based on atomic force microscopy (AFM), in which the size of the indenter tip limits the testing area of the sample, and the strain is highly non-uniform.

Moreover, research that involves transferring samples of 2D materials onto a flexible substrate to introduce stretching has faced certain limitations. Due to the weak adhesion between 2D materials and substrate interface, it is very challenging to apply large strain on the samples of 2D materials. Hence tensile stretching of large pieces of freestanding monolayer h-BN and the effects of naturally occurring defects on its mechanical robustness remain largely unexplored.

Over the past three years, the research team led by Dr Lu Yang, Associate Professor of the Department of Mechanical Engineering (MNE) at CityU worked tirelessly with another team from Tsinghua University to develop the world’s very first quantitative in-situ tensile testing technique for free-standing 2D materials. Recently, they have expanded their research efforts from monolayer graphene to h-BN.

Using the 2D nanomechanical platform previously developed by the team, the researchers successfully performed quantitative tensile straining on freestanding monolayer h-BN for the first time (see Figure 1). The experiment showed that its fully recoverable elasticity was up to 6.2% and the corresponding 2D Young's modulus was about 200 N/m.

Another focus of the research was to explore the effects of h-BN’s naturally occurring defects on structural integrity and mechanical robustness. The team discovered that, monolayer h-BN containing voids of ~100 nm can be even strained up to 5.8% (see Movie/GIF). The atomistic and continuum simulations showed that compared to the imperfections introduced during sample preparation, the elastic limit of h-BN is virtually immune to naturally occurring atomistic defects (such as grain boundaries and vacancies). Those sub-micrometre voids are not detrimental, only reducing the elastic limit of h-BN from ~6.2% to ~5.8%, which demonstrates its high defect tolerance.

"Based on our experimental platform, we managed to investigate the mechanical properties of another important 2D material. For the first time, we demonstrated the high stiffness and large uniform elastic deformation of monolayer h-BN. The encouraging results not only contribute to the development of h-BN applications in strain engineering, piezoelectronics and flexible electronics, but also propose a new way to improve the performance of 2D composites and devices. They also provide a powerful tool to explore the mechanical properties of other 2D materials," Dr Lu said.

Tags:  2D material  City University of Hong Kong  Electronics  Graphene  hexagonal boron nitride  Lu Yang  piezoelectronics 

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Towards chirality control of graphene nanoribbons embedded in hexagonal boron nitride

Posted By Graphene Council, Wednesday, September 23, 2020

Graphene nanoribbons (GNRs) are one-dimensional strips of graphene that can exhibit either quasi-metallic or semiconducting behavior, depending on its specific chirality, including width, lattice orientation, and edge structure. The unique properties of GNR make it a promising substitute to engineer prospective nano-electronics.

There are two groups of GNRs that differ by edge type: zigzag (ZZ) and armchair (AC), which have been extensively studied in terms of synthesis approach and properties. However, fabrication of edge-specific sub-5 nm GNRs on the insulating substrate still remains a significant challenge.

Recently, a team led by Prof. Haomin Wang in Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences (CAS), reported the successful control over the chirality of GNRs embedded in hexagonal boron nitride (h-BN) nano-trenches, whose direction can be modulated by different catalytic cutting particles.

Both ZGNRs and AGNRs narrower than 5 nm are successfully synthesized. STEM investigation shows that in-plane epitaxy was realized at the boundary of graphene and h-BN with controlled chirality at the edge along the GNR while developed laterally.

Further electrical investigation reveals that all narrow ZGNRs exhibit a bandgap larger than 0.4 eV while narrow AGNRs exhibit a relatively large variation in band-gap. Transistors made of GNRs with large bandgaps exhibit on-off ratios of more than 105 at room temperature with carrier mobilities higher than 1,500 cm2V-1s-1.

This integrated lateral growth of edge-specific GNRs in h-BN brings semiconducting building blocks to atomically thin layer, and will provide a promising route to achieve intricate nanoscale electrical circuits on high-quality insulating h-BN substrates.

This paper has been published online in Nature Materials ("Towards chirality control of graphene nanoribbons embedded in hexagonal boron nitride").

Tags:  Chinese Academy of Sciences  Electronics  Graphene  Graphene Nanoribbons  Haomin Wang  hexagonal boron nitride  Shanghai Institute of Microsystem and Information  

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Thresholdless Soliton Formation in Photonic Moiré Lattices

Posted By Graphene Council, Wednesday, September 2, 2020
Researchers at The Institute of Photonic Sciences (ICFO), a BIST centre, are part of a collaboration that has reported, in Nature Photonics, the observation of soliton formation with a power threshold dictated by geometry in photonic moiré lattices.

Take two identical layers of semi transparent material that have the same structure, put them one on top of the other, rotate them and look at them from above, and hexagonal patterns start to emerge. They are known as moiré patterns or moiré lattices.

Moiré lattices are used every day in applications such as art, textile industry, architecture, as well as image processing, metrology and interferometry. They have been a matter of major interest in science, since they are easily produced using coupled graphene–hexagonal boron nitride monolayers, graphene–graphene layers and graphene quasicrystals on a silicon carbide surface and have proven to generate different states of matter upon rotating or twisting the layers to a certain angle, opening to a new realm of richer physics to be investigated. A few years ago scientists at MIT let by Prof. Pablo Jarillo-Herrero found a new type of unconventional superconductivity in twisted bilayer graphene that forms a moiré lattice. Since then, an explosion of new physics has occurred, which includes several landmark contributions by the ICFO team led by Prof. Efetov that unveiled a new zoo of unobserved states in such structures.

In a different realm of Physics, a team of scientists in a long-standing collaboration between ICFO researchers Prof. Yaroslav Kartashov and Prof. Lluis Torner, the former having been post-doctoral researcher in the same group as Prof Fangwei Ye (currently full professor at the Shanghai Jiao Tong University, where the experiments were conducted), and Prof. Vladimir Konotop in Lisbon, reported early this year in Nature the observation of the transition from delocalisation to localisation in two-dimensional patterns, afforded by the properties of the moiré structures with fundamentally different geometries (periodic, general aperiodic, and quasicrystal).

Now moiré lattices optically-induced in a photorefractive nonlinear crystal have been employed to observe the formation of optical solitons under different geometrical conditions controlled by the twisting angle between the constitutive sublattices. The behavior of the soliton formation threshold was confirmed to be directly linked to the band structure of the moiré lattices resulting from the different twisting angles of the sublattices and, in particular, of the band-flattening associated to the geometry of the lattices. Similar phenomena are anticipated to occur in moiré patterns composed of sublattices of other crystallographic symmetries and in other physical systems where flat-bands induced by geometry arise. The results were published in Nature Photonics.

Tags:  Graphene  Hexagonal boron nitride  Institute of Photonic Sciences  Lluis Torner  Nature Photonics  Pablo Jarillo-Herrero  Shanghai Jiao Tong University  Vladimir Konotop  Yaroslav Kartashov 

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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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New study unveils ultrathin boron nitride films for next-generation electronics

Posted By Graphene Council, Friday, June 26, 2020
An international team of researchers, affiliated with UNIST has unveiled a novel material that could enable major leaps in the miniaturization of electronic devices. Published in the prestigious journal Nature, this study represent a significant achievement for future electronics.

This breakthrough comes from a research, conducted by Professor Hyeon Suk Shin (School of Natual Sciences, UNIST) and Principal Researcher Dr. Hyeon-Jin Shin from Samsung Advanced Institute of Technology (SAIT), in collaboration with Graphene Flagship researchers from University of Cambridge (UK) and Catalan Institute of Nanoscience and Nanotechnology (ICN2, Spain).

In this study, the team successfully demonstrated the synthesis of thin film of amorphous boron nitride (a-BN) with extremely low dielectric constant as well as high breakdown voltage and superior metal barrier properties. The research team noted that this newly fabricated material has great potential as interconnect insulators in the next-generation of electronic circuits.

In the ongoing process of miniaturization of logic and memory devices in electronic circuits, minimizing the dimensions of interconencts - metal wires that link the different device components on the chip - is crucial to guarantee improved performance and faster response of the device. Extensive research efforts have been devoted to decreasing the resistance of scaled interconnects because integration of dielectrics using complementary metal oxide semiconductor (CMOS) compatible processes has proven to be exceptionally challenging. According to the research team, the required interconnect isolation materials should not only possess low relative dielectric constants (referred to as k-values), but should also be thermally, chemically, and mechanically stable.

There has been an ongoing quest to obtain materials with ultra-low-k (relative permittivity around or below 2) avoiding the artificial addition of pores in the thin film in the semiconductor industry for at least the past 20 years. Several attempts had been made to develop materials with desired characteristics, yet those materials have failed to be successfully integrated in interconnects due to poor mechanical properties or poor chemical stability upon integration, causing reliability failures.

In this study, the joint research has succeeded in demonstrating a Back-End-ofthe-Line (BEOL) compatible approach to grow amorphous boron nitride (a-BN) with extremely low-k dielectrics. In particular, they synthesized approximately 3 nm thin a-BN on a Si substrate, using low temperature remote inductively coupled plasma-chemical vapour deposition (ICP-CVD). The resulting material showed an extremely low dielectric constant in the range of 1.78, which is 30% lower than the dielectric constant of currently available insulators.

In this study, the joint research has succeeded in demonstrating a Back-End-ofthe-Line (BEOL) compatible approach to grow amorphous boron nitride (a-BN) with extremely low-k dielectrics. In particular, they synthesized approximately 3 nm thin a-BN on a Si substrate, using low temperature remote inductively coupled plasma-chemical vapour deposition (ICP-CVD). The resulting material showed an extremely low dielectric constant in the range of 1.78, which is 30% lower than the dielectric constant of currently available insulators.

"We found that temperature was the most important parameter with ideal a-BN film deposition occurring at 400° C," says Seokmo Hong in the Doctoral program of Natural Sciences, the first author of the study. "This material with ultra-low-k also manifests a high breakdown voltage and likely superior metal barrier properties, making the film very attractive for practical electronic applications."

Angle-dependent near-edge X-ray absorption fine structure (NEXAFS) measured in partial electron-yield (PEY) mode at Pohang Light Source-II 4D beam line was also used to investigate the chemical and electronic structures of a-BN. Their findings indicated that the irregular, random atomic arrangement causes the dielectric constant value to drop.

The new material also manifests excellent mechanical properties of high strength. Moreover, when researchers tested the diffusion barrier properties of a-BN in very harsh conditions, they found it can prevent metal atom migration from the interconnects into the insulator. This result will help resolves a long-standing issue of interconnects in CMOS integrated circuit fabrication, enabling further miniaturaization of electronic devices.

"Development of electrically, mechanically and thermally robust low-k materials (k < 2) has long been technically challenging," says Dr. Hyeon-Jin Shin from Samsung Advanced Institute of Technology (SAIT). "Our research is also a great example that shows companies and academic institutions working together to create greater synergy."

"Our results demonstrate that the amorphous counterpart of two-dimensional hexagonal BN possesses the ideal low-k dielectric characteristics for high-performance electronics," says Professor Shin. "If they are commercialized, it will be a great help in overcoming the crisis looming over the semiconductor industry."

Tags:  boron nitride  Electronics  Graphene  hexagonal boron nitride  Hyeon Suk Shin  Hyeon-Jin Shin  Samsung Advanced Institute of Technology  UNIST 

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The Physics behind Twisted Bilayer Graphene

Posted By Graphene Council, Tuesday, May 26, 2020
An international team of researchers, including ICFO Prof. Dmitri Efetov, give a thorough review in Nature Physics on the status and prospects of the physics behind stacked monolayers of 2D materials.

Strongly correlated quantum materials are excellent testbeds for complex quantum phases of matter since they have shown to give rise to spectacular phenomenology, from high-temperature superconductivity to the emergence of states with long-range quantum entanglement. However, their complexity and the vast amount of system variables have so far hindered scientists to obtain a more complete, thorough understanding of its microscopic mechanisms.

Van der Waals heterostructures consist of individual layers of two-dimensional atomic materials such as graphene, hexagonal boron nitride, and transition metal chalcogenides, which are vertically stacked on top of each other. As the relative angle between the crystals can be freely chosen, this creates a new capability in physics – a concept which is called Twistronics. Scientists have recently found that by overlaying two individual layers of these 2D materials, complex heterostructures moiré structures can be created, which host an amazing realm of unexplored and undiscovered physical phenomena.

Recently and considered one of major scientific achievements in these last two years, researchers found that, when tuning one of these systems, specifically in a twisted bi-layer graphene system, it was possible to drive the system from exhibiting strongly correlated states to presenting clear superconductivity features. That is, by changing the electrical charge carrier density within the device with a nearby capacitor, the material could be tuned from behaving as an insulator, to behaving as a superconductor, or even an exotic orbital magnet with non-trivial topological texture – a phase never observed before. Sor far, there is still no theoretical approach that can precisely explain such complex and exceptionally rich physics, and in particular, how these all these states may be linked or connected with each other and why they occur in such order.

In a recent study published in Nature Physics, researchers Leon Balents, Cory R. Dean, ICFO Prof. Dmitri K. Efetov and Andrea F. Young give a thorough report on the status and the prospects of these systems and the physics that arises from them. They focus on understanding the patterns that are created when two individual monolayers of these materials, called moiré patterns, are stacked in a specific way, discuss the engineering in van der Waals heterostructures as well as explore how different phenomena emerge from the moiré flat bands that are formed.

Flat bands are advantageous because they guarantee a large density of states, which amplifies the effects of interactions. Bearing this in mind, the researchers have focused their study on moiré systems in Twisted Bi Layer Graphene (tBLG) in particular. They have studied various systems in which a small mismatch in periodicity of the pattern, introduced either by lattice mismatch or rotational misalignment, results in different physical scenarios, may it be correlating states, insulating states, superconductivity, etc. From these, they search, among other things, to understand and find answers to what may be the nature of the insulating states, what is the origin and nature of the observed superconductivity? How strong are analogies to other correlated electronic systems, such as high-temperature superconducting cuprates? This study is a step forward in understanding and setting the basis for the theory that may be capable of explaining in a future all the rich and very complex physics behind these novel materials and systems. Being able to control and manipulate such systems, and really understanding what is occurring inside them, will be a major advancement, if not a revolutionary shift, in the engineering of these materials and the development of applications for future innovative and disruptive technologies.

Tags:  2D Materials  Andrea F. Young  Cory R. Dean  Dmitri K. Efetov  Graphene  Hexagonal boron nitride  Leon Balents  quantum materials 

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Oriented hexagonal boron nitride foster new type of information carrier

Posted By Graphene Council, Tuesday, May 26, 2020
Valleytronics gives rise to valley current, a stable, dissipationless current which is driven by a pseudo-magnetic field, Berry curvature. This gives rise to valletronics based information processing and storage technology. A pre-requisite for the emergence of Berry curvature is either a broken inversion symmetry or a broken time-reversal symmetry. Thus two-dimensional materials such as transition metal dichalcogenides and gated bilayer graphene are widely studied for valleytronics as they exhibit broken inversion symmetry.

For most of the studies related to graphene and other two-dimensional materials, these materials are encapsulated with hexagonal boron nitride (hBN), a wide band gap material which has comparable lattice parameter to that of graphene. Encapsulation with hBN layer protects the graphene and other two-dimensional materials from unwanted adsorption of stray molecules while keeping their properties intact. hBN also acts as a smooth twodimensional substrate unlike SiO2 which is highly non-uniform, increasing the mobility of carriers in graphene. However, most of the valleytronics studies on bilayer graphene with hBN encapsulation has not taken into account the effect of hBN layer in breaking the layer symmetry of bilayer graphene and inducing Berry curvature.

This is why Japan Advanced Institute of Science and Technology (JAIST) postdoc Afsal Kareekunnan, senior lecturer Manoharan Muruganathan and Professor Hiroshi Mizuta decided it was vital to take into account the effect of hBN as a substrate and as an encapsulation layer on the valleytronics properties of bilayer graphene. By using first-principles calculations, they have found that for hBN/bilayer graphene commensurate heterostructures, the configuration, as well as the orientation of the hBN layer, has an immense effect on the polarity as well as the magnitude of the Berry curvature.

For non-encapsulated hBN/bilayer graphene heterostructure, where hBN is present only at the bottom, the layer symmetry is broken due to the difference in the potential experienced by the two layers of the bilayer graphene. This layer asymmetry induces a non-zero Berry curvature. However, encapsulation of the bilayer graphene with hBN (where the top and bottom hBN are out of phase with each other) nullifies the effect of hBN and drives the system towards symmetry, reducing the magnitude of the Berry curvature. A small Berry curvature which is still present is the feature of pristine bilayer graphene where the spontaneous charge transfer from the valleys to one of the layers results in a slight asymmetry between the layers as reported by the group earlier. Nonetheless, encapsulating bilayer graphene with the top and bottom hBN in phase with each other enhances the effect of hBN, leading to an increase in the asymmetry between the layers and a large Berry curvature. This is due to the asymmetric potential experienced by the two layers of bilayer graphene from the top and bottom hBN. The group has also found that the magnitude and the polarity of the Berry curvature can be tuned in all the above-mentioned cases with the application of an out-of-plane electric field.

"We believe that, from both theoretical and experimental perspective, such precise analysis of the effect of the use of hBN both as a substrate and as an encapsulation layer for graphene-based devices gives deep insight into the system which has great potential to be an ideal valleytronic material," Professor Mizuta said.

Tags:  Afsal Kareekunnan  Graphene  Hexagonal boron nitride  Hiroshi Mizuta  Japan Advanced Institute of Science and Technology  Manoharan Muruganathan 

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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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Substances trapped in graphene nanobubbles exhibit unusual properties

Posted By Graphene Council, Wednesday, April 15, 2020
Skolkovo Institute of Science and Technology (Skoltech) scientists modeled the behavior of nanobubbles appearing in van der Waals heterostructures and the behavior of substances trapped inside the bubbles. In the future, the new model will help obtain equations of state for substances in nano-volumes, opening up new opportunities for the extraction of hydrocarbons from rock with large amounts of micro- and nanopores.

The results of the study were published in the Journal of Chemical Physics ("Model of graphene nanobubble: Combining classical density functional and elasticity theories").

The van der Waals nanostructures hold much promise for the study of tiniest samples with volumes from 1 cubic micron down to several cubic nanometers. These atomically thin layers of two-dimensional materials, such as graphene, hexagonal boron nitride (hBN) and dichalcogenides of transition metals, are held together by weak van der Waals interaction only.

Inserting a sample between the layers separates the upper and bottom layers, making the upper layer lift to form a nanobubble. The resulting structure will then become available for transmission electron and atomic force microscopy, providing an insight into ? the structure of the substance inside the bubble.

The properties exhibited by substances inside the van der Waals nanobubbles are quite unusual. For example, water trapped inside a nanobubble displays a tenfold decrease in its dielectric constant and etches the diamond surface (Nature Communications, "A hydrothermal anvil made of graphene nanobubbles on diamond"), something it would never do under normal conditions. Argon which typically exists in liquid form when in large quantities can become solid at the same pressure if trapped inside very small nanobubbles with a radius of less than 50 nanometers.

Scientists led by professor Iskander Akhatov of the Skoltech Center for Design, Manufacturing and Materials (CDMM) built a universal numerical model of a nanobubble that helps predict the bubble’s shape under certain thermodynamic conditions and describe the molecular structure of the substance trapped inside.

“In a practical sense, the bubbles in the van der Waals structures are most often regarded as flaws that experimenters are eager to get rid of. However, from the standpoint of straintronics, the bubbles create strain, and its effect on the electronic structure can be used to create practical devices, such as transistors, logic elements and ROM,” Petr Zhilyaev, a senior research scientist at Skoltech, commented on the study.

“In our recent study, we created a model which describes a specific shape that flat nanobubbles assume in the subnanometer dimension range only. We discovered that the vertical size of these nanostructures can only take discrete values divisible by the size of the molecules trapped. In addition, the model enables changing the size of nanobubbles by controlling the temperature of the system and the physicochemical parameters of the materials,” explained a senior research scientist at Skoltech, Timur Aslyamov.

Tags:  Graphene  hexagonal boron nitride  Iskander Akhatov  Petr Zhilyaev  Skolkovo Institute of Science and Technology  Timur Aslyamov 

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An ultimate one-dimensional electronic channel in hexagonal boron nitride

Posted By Graphene Council, Wednesday, March 11, 2020
In the field of 2D electronics, the norm used to be that graphene is the main protagonist and hexagonal boron nitride (hBN) is its insulating passive support. Researchers of the Center for Multidimensional Carbon Materials (CMCM) within the Institute for Basic Science (IBS, South Korea) made a discovery that might change the role of hBN. They have reported that stacking of ultrathin sheets of hBN in a particular way creates a conducting boundary with zero bandgap. In other words, the same material could block the flow of electrons, as a good insulator, and also conduct electricity in a specific location. Published in the journal Science Advances, this result is expected to raise interest in hBN by giving it a more active part in 2D electronics.

Similarly to graphene, hBN is a 2D material with high chemical, mechanical and thermal stability. hBN sheets resemble a chicken wire, and are made of hexagonal rings of alternating boron and nitrogen atoms, strongly bound together. However, unlike graphene, hBN is an insulator with a large bandgap of more than five electronVolts, which limits its applications.

“In contrast to the wide spectrum of proposed applications for graphene, hexagonal boron nitride is often regarded as an inert material, largely confined as substrate or electron barrier for 2D material-based devices. When we began this research, we were convinced that reducing the bandgap of hBN could give to this material the versatility of graphene,” says the first author, PARK Hyo Ju.

Several attempts to lower the bandgap of hBN have been mostly ineffective because of its strong covalent boron-nitrogen bonds and chemical inertness. IBS researchers in collaboration with colleagues of Ulsan National Institute of Science and Technology (UNIST), Sejong University, Korea, and Nanyang Technological University, Singapore, managed to produce a particular stacking boundary of a few hBN layers having a bandgap of zero electronVolts.

Depending on how the hBN sheets are piled up, the material can assume different configurations. For example, in the so-called AA′ arrangement, the atoms in one layer are aligned directly on the top of atoms in another layer, but successive layers are rotated such that boron is located on nitrogen and nitrogen on boron atoms. In another type of layout, known as AB, half of the atoms of one layer lie directly over the center of the hexagonal rings of the lower sheet, and the other atoms overlap with the atoms underneath.

For the first time, the team has reported atomically sharp AA′/AB stacking boundaries formed in few-layer hBN grown by chemical vapor deposition. Characterized by a line of oblong hexagonal rings, this specific boundary has zero bandgap. To confirm this result, the research performed several simulations and tests via transmission electron microscopy, density functional theory calculations, and ab initio molecular dynamics simulations.

“An atomic conducting channel expands the application range of boron nitride infinitely, and opens new possibilities for all-hBN or all 2D nanoelectronic devices,” points out the corresponding author LEE Zonghoon.

Tags:  2D materials  Graphene  hexagonal boron nitride  Institute for Basic Science  PARK Hyo Ju 

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