New experiments by researchers at The University of Manchester have placed the best limits yet on impermeability of graphene and other two-dimensional materials to gases and liquids. The work has also revealed that the carbon sheet can act as a powerful catalyst for hydrogen splitting, a finding that promises cheap and abundant catalysts in the future.
Graphene theoretically boasts a very high energy for the penetration of atoms and molecule, which prevents any gases and liquids from passing through it at room temperature. Indeed, it is estimated that it would take longer than the lifetime of the Universe to find an atom energetic enough to pierce a defect-free monolayer graphene of any realistic size under ambient conditions, say the researchers led by Professor Sir Andre Geim. This hypothesis is supported by real-world experiments performed over a decade ago which found that one-atom-thick graphene was less permeable to helium atoms than a quartz film of a few microns in thickness. Although the film is 100,000 thicker than graphene, this is still very far from the theoretical limit.
Perfectly sealed containers
The Manchester team developed a measurement technique that is many billion times more sensitive to permeating gas atoms than any of the known methods. In their study, reported in Nature, they began by drilling micron-sized wells in monocrystals of graphite or boron nitride, which they covered with a one-atom-thick graphene membrane. Since the top surface of these containers is atomically flat, the cover provides a perfect air-tight seal. The only way that atoms and molecules can enter a container is through the graphene membrane. The membrane itself is flexible and responds to minor changes in pressure inside the container.
The researchers then placed the containers in helium gas. If atoms enter or exit a container, the gas pressure inside increases or decreases, respectively, and makes the surface of the cover bulge over some small distances. The team monitored these movements with angstrom precision using an atomic force microscope.
The new result backs up (and provides an explanation for) some of the previous reports in the literature on graphene’s unexpectedly high catalytic activity, which was particularly counterintuitive because of the extreme inertness of its bulk parent, graphite, Professor Sir Andre Geim.
Like a “one-kilometre-thick wall of glass”
From changes in the membrane position, the number of atoms or molecules penetrating through graphene can be calculated precisely. The researchers found that no more than a few helium atoms - if any - entered or exited their container per hour. “This sensitivity is more than eight to nine orders of magnitude higher than achieved in previous experiments on graphene impermeability, which themselves were a few orders of magnitude more sensitive than the detection limit of modern helium leak detectors. To put this into perspective, one-atom-thick carbon is less permeable to gases than a one-kilometre-thick wall of glass”, explains Geim.
Helium is the most permeating of all gases, because of its small weakly interacting atoms. Nonetheless, the researchers decided to repeat their experiments with other gases such as neon, nitrogen, oxygen, argon, krypton, xenon and hydrogen. All of them showed no permeation with the same accuracy as achieved for helium, except for hydrogen. In contrast to all the others, it permeated relatively rapidly through defect-free graphene. Dr Pengzhan Sun, the first author of the Nature paper, commented “This is a shocking result: A hydrogen molecule is much larger than a helium atom. If the latter cannot pass through, how on earth larger molecules can”.
Curved graphene for hydrogen dissociation
The team attributes the unexpected hydrogen permeation to the fact that graphene membranes are not completely flat but have a lot of nanometre-sized ripples. Those act as catalytically active regions and dissociate absorbed molecular hydrogen into two hydrogen atoms, a reaction that is usually hugely unfavourable. Graphene ripples favour the hydrogen splitting, in agreement with theory. Then, the adsorbed hydrogen atoms can flip to the other side of the graphene membranes with a relative ease, similarly to permeation of protons through defect-free graphene. The latter process was known before and explained by the fact that protons are subatomic particles, small enough to squeeze through the dense crystal lattice of graphene.
“The new result backs up (and provides an explanation for) some of the previous reports in the literature on graphene’s unexpectedly high catalytic activity, which was particularly counterintuitive because of the extreme inertness of its bulk parent, graphite,” says Geim.
“Our work provides a basis for understanding why graphene can work as a catalyst -- something that should stimulate further research on using the material in such applications in the future,” Dr Sun adds. “In a sense, graphene nanoripples behave like platinum particles, which are also known to split molecular hydrogen. But no one expected this from seemingly inert graphene”.
For more than a decade, two-dimensional nanomaterials, such as graphene, have been touted as the key to making better microchips, batteries, antennas and many other devices. But a significant challenge of using these atom-thin building materials for the technology of the future is ensuring that they can be produced in bulk quantities without losing their quality. For one of the most promising new types of 2D nanomaterials, MXenes, that’s no longer a problem. Researchers at Drexel University and the Materials Research Center in Ukraine have designed a system that can be used to make large quantities of the material while preserving its unique properties.
The team recently reported in the journal Advanced Engineering Materials that a lab-scale reactor system developed at the Materials Research Center in Kyiv, can convert a ceramic precursor material into a pile of the powdery black MXene titanium carbide, in quantities as large as 50 grams per batch.
Proving that large batches of material can be refined and produced with consistency is a critical step toward achieving viability for manufacturing. For MXene materials, which have already proven their mettle in prototype devices for storing energy, computing, communication and health care, reaching manufacturing standards is the home stretch on the way to mainstream use.
“Proving a material has certain properties is one thing, but proving that it can overcome the practical challenges of manufacturing is an entirely different hurdle — this study reports on an important step in this direction,” said Yury Gogotsi, PhD, Distinguished University and Bach professor in Drexel’s College of Engineering, who has pioneered the research and development of MXene and is a lead author of the paper. “This means that MXene can be considered for widespread use in electronics and energy storage devices.”
Researchers at Drexel have been making MXene in small quantities — typically one gram or less — since they first synthesized the material in 2011. The layered nanomaterial, which looks like a powder in its dry form, starts as a piece of ceramic called a MAX phase. When a mixture of hydrofluoric and hydrochloric acid interacts with the MAX phase it etches away certain parts of the material, creating the nanometer-thin flakes characteristic of MXenes.
In the lab, this process would take place in a 60 ml container with the ingredients added and mixed by hand. To more carefully control the process at a larger scale, the group uses a one-liter reactor chamber and a screw feeder device to precisely add MAX phase. One inlet feeds the reactants uniformly into the reactor and another allows for gas pressure relief during the reaction. A specifically designed mixing blade ensures thorough and uniform mixing. And a cooling jacket around the reactor lets the team adjust the temperature of the reaction. The entire process is computerized and controlled by a software program created by the Materials Research Center team.
The group reported successfully using the reactor to make just under 50 grams of MXene powder from 50 grams of MAX phase precursor material in about two days (including time required for washing and drying the product). And a battery of tests conducted by students at Drexel’s Materials Science and Engineering Department showed that the reactor-produced MXene retains the morphology, electrochemical and physical properties of the original lab-made substance.
This development puts MXenes in a group with just a handful of 2D materials that have proven they can be produced in industrial-size quantities. But because MXene-making is a subtractive manufacturing process — etching away bits of a raw material, like planing down lumber —
it stands apart from the additive processes used to make many other 2D nanomaterials.
“Most 2D materials are made using a bottom-up approach,” said Christopher Shuck, PhD, a post-doctoral researcher in the A.J. Drexel Nanomaterials Institute. “This is where the atoms are added individually, one by one. These materials can be grown on specific surfaces or by depositing atoms using very expensive equipment. But even with these expensive machines and catalysts used, the production batches are time-consuming, small and still prohibitively expensive for widespread use beyond small electronic devices.”
MXenes also benefit from a set of physical properties that ease their path from processed material to final product — a hurdle that has tripped up even today’s widely used advanced materials.
“It typically takes quite a while to build out the technology and processing to get nanomaterials in an industrially usable form,” Gogotsi said. “It’s not just a matter of producing them in large quantities, it often requires inventing completely new machinery and processes to get them in a form that can be inserted into the manufacturing process — of a microchip or cell phone component, for example.”
But for MXenes, integrating into the manufacturing line is a fairly easy part, according to Gogotsi.
“One huge benefit to MXenes is that they be used as a powder right after synthesis or they can be dispersed in water forming stable colloidal solutions,” he said. “Water is the least expensive and the safest solvent. And with the process that we’ve developed, we can stamp or print tens of thousands of small and thin devices, such as supercapacitors or RFID tags, from material made in one batch.”
This means it can be applied in any of the standard variety of additive manufacturing systems — extrusion, printing, dip coating, spraying — after a single step of processing.
Several companies are looking developing the applications of MXene materials, including Murata Manufacturing Co, Ltd., an electronics component company based in Kyoto, Japan, which is developing MXene technology for use in several high-tech applications.
“The most exciting part about this process is that there is fundamentally no limiting factor to an industrial scale-up,” Gogotsi said. “There are more and more companies producing MAX phases in large batches, and a number of those are made using abundant precursor materials. And MXenes are among very few 2D materials that can be produced by wet chemical synthesis at large scale using conventional reaction engineering equipment and designs.”
"Rock-chair" Li-ion battery (LIB) was discovered in the late 1970s and commercialized in 1991 by Sony, which has become the priority way we store portable energy today. To honor the contribution for "creating a rechargeable world", the 2019 Nobel Prize in chemistry was awarded to three famous scientists (John B. Goodenough, M. Stanley Whittingham, Akira Yoshino) who made the most important contributions to the discovery of LIBs. However, this technology is nearing its practical performance limits and extensive efforts are underway to replace LIBs with new electrochemical storage solutions, which are safe, stable, low cost and with higher energy density to power long-range electric vehicles and long-lasting portable electronics.
Replacing the traditional graphite-based anodes with Li metal, a "holy" anode with a high theoretical capacity of 3860 mAh/g, shows it a promising approach. At present, Li metal anode suffers from poor cycling efficiency and infinite volume change, raising operational safety concerns. Effective efforts include functional electrolyte additive, artificial solid-electrolyte interface and using host scaffolds to buffer the volume expansion have been taken to tackle its disadvantages. Among these, the method of using scaffolds continues to see rapid development.
Graphite, a classic Li anode, shows a great promising as an effective host scaffold, which possesses a low density and high electron conductivity. However, it is generally accepted that Li metal wets graphite poorly, causing its spreading and infiltration difficult. Previous methods to transforming graphite from lithiophobicity to lithiophilicity include surface coating with Si, Ag or metal oxide (lithiophobic indicates a large contact angle, while lithiophilic indicates a low contact angle between molten lithium and solid surface). However, such a change in liquid spreading behavior is due to the replacement of graphite by reactive coating. Consequently, it might be asked whether graphite is intrinsically lithiophobic or lithiophilic.
Herein, the wetting behavior of molten Li on different kinds of graphite-based carbon materials were systematically studied. Firstly, the highly oriented pyrolytic graphite (HOPG) was used as the test sample (Figure a). It is observed that HOPG substrate immediately allows an contact angle (CA) of 73° with Li metal (Figure b, c). To check this experiment against theory, ab initio molecular dynamics simulation was performed with a molten Li droplet (54 Li atoms)/graphite (432 C atoms, two-layered graphene) setup to prove that a clean (002) surface of graphite is intrinsically lithiophilic at 500K and the results also confirmed that lithium and graphite have good affinity.
However, the CA of Li metal on porous carbon paper (PCP, Figure d) is as high as 142°, which indicates PCP is lithiophobic (Figure e and f). This result which contradicted with previous conclusion that graphite is intrinsically lithiophilic prompted researchers to gain further understanding of the effect of surface chemistry to the wetting performance of Li metal and graphite. Compared with HOPG, it is found that PCP surface has a large number of oxygen-containing functional groups. These surface impurities will play a key role in pinning the contact line between Li metal and PCP, resulting in a lager apparent contact angle.
In order to demonstrate this assumption, the PCP was first lithiated by decreasing its electrochemical potential with molten Li metal (Figure g). During this process, the surface impurities of PCP will be eliminated as well. The following experiment shows that lithiated PCP exhibited a small CA of ~52°, which indicated a successful transition from lithiophobicity to lithiophilicity. Due to its porous structure of lithiated PCP, the Li metal rapid diffused through (Figure h and i). The DFT simulation revealed that lithiated graphite and graphite possessed similar wetting performance, demonstrating the elimination of the surface impurities would be the key reason for this transition of wetting performance from PCP to lithiated PCP. The graphite powder is further used to test its wettability with Li metal. After continue mixing, the graphite powder could be uniformly dispersed in the Li metal matrix, further confirming a lithiophilic property of graphite. Taking advantage of this discovery, a novel Li metal-graphite compositing method was proposed and Li-graphite composite anode with large area can be produced in a large scale.
This work not only systematically studies the wettability of Li metal and graphite-based carbon materials, but also provides a novel idea for the construction of Li-carbon composite anode materials, which is helpful for the development of high-energy Li metal batteries.
Silicon is abundant in nature with high theoretical capacity (4200 mAh g-1), which is an ideal anode material to improve the energy density of dual-ion batteries (DIBs). However, its application in DIBs has been restricted by the large volume expansion problem (>300%).
Rigid contacts between silicon and current collectors, commonly made with metal foils, lead to significant interfacial stress. As a consequence, interface cracking and even exfoliation of active materials occur, resulting in unsatisfied cycling performance.
A research group led by Prof. TANG Yongbing and his team members (Dr. JIANG Chunlei, XIANG Lei, MIAO Shijie etc.) from the Shenzhen Institutes of Advanced Technology (SIAT) of the Chinese Academy of Sciences, along with Prof. ZHENG Zijian from the Hong Kong Polytechnic University, proposed a flexible interface design to regulate the alloying stress of silicon anode in silicon-graphite DIBs.
The flexible interface design could modulate the stress distribution via the construction of a silicon anode on a soft nylon fabric modified with a conductive Cu-Ni transition layer, which endowed the silicon electrode with remarkable fexibility and stability over 50,000 bends.
Assembly of the fexible silicon anode with an expanded graphite cathode yielded a silicon–graphite DIB (SGDIB), which possessed record-breaking rate performance (up to 150 C) and cycling stability over 2000 cycles at 10 C with a capacity retention of 97%.
Moreover, the SGDIB showed a high capacity retention of about 84% after 1500 bends and a low self-discharging voltage loss of 0.0015% per bend after 10 000 bends, indicating high potential for high-performance fexible energy-storage applications.
The study entitled "Flexible Interface Design for Stress Regulation of a Silicon Anode toward Highly Stable Dual-Ion Batteries" was published online in Advanced Materials.
Graphene is 200 times stronger than steel and can be as much as 6 times lighter. These characteristics alone make it a popular material in manufacturing. Researchers at the University of Illinois at Urbana-Champaign recently uncovered more properties of graphene sheets that can benefit industry.
Doctoral student Soumendu Bagchi, along with his adviser Huck Beng Chew in the Department of Aerospace Engineering in collaboration with Harley Johnson from Mechanical Sciences and Engineering identified how twisted graphene sheets behave and their stability at different sizes and temperatures.
“We concentrated on two graphene sheets stacked on top of each other but with a twist angle,” said Bagchi. “We did atomistic simulations at different temperatures for different sizes of graphene sheets. Using insights from these simulations, we developed an analytical model—you can plug in any sheet size, any twist angle, and the model will predict the number of local stable states it has as well as the critical temperature required to reach each of those states.”
Bagchi explained that bilayer graphene exists in an untwisted Bernal-stacked configuration, which is also the repeated stacking sequence of crystalline hexagonal graphite. When bilayer graphene is twisted, it wants to untwist back to its original state because that’s the most stable state and placement of the atoms.
“When the twisted atomic structure is heated, it tends to rotate back, but there are certain magic twist angles at which the structure remains stable below a specific temperature. And, there is a size dependency as well. What’s exciting about our work is, depending upon the size of the graphene sheet, we can predict how many stable states you will have, the magic twist angles at these stable states, as well as the range of temperatures required for twisted graphene to transition from one stable state to another,” Bagchi said.
According to Chew, manufacturers have been trying to make graphene transistors, and twisted graphene bilayers are known to exhibit exciting electronic properties. In manufacturing these graphene transistors, it’s important to know what temperature will excite the material to achieve a certain rotation or mechanical response.
“They’ve known that a graphene sheet has certain electronic properties, and adding a second sheet at an angle yields new unique properties. But a single atomic sheet is not easy to manipulate. Fundamentally, this study answers questions about how twisted graphene sheets behave under thermal loading, and provides insights into the self-alignment mechanisms and forces at the atomic level. This could potentially pave the way for manufacturers to achieve fine control over the twist angle of 2D material structures. They can directly plug in parameters into the model to understand the necessary conditions required to achieve a specific twisted state.”
Bagchi said no one has studied the 2D properties of materials like this. It is a very fundamental study, and one that began as a different project, when he bumped into something unusual.
“He noticed that the graphene sheets showed some temperature dependence,” Chew said. “We wondered why it behaved this way—not like a normal material.
“In normal materials, the interface is typically very strong. With graphene, the interface is very weak allowing the layers to slide and rotate. Observing this interesting temperature dependency wasn’t planned. This is the beauty of discovery in science.”
The study, “Rotational stability of twisted bilayer graphene,” by Soumendu Bagchi, Harley Johnson, and Huck Beng Chew is published in Physical Review B. DOI: 10.1103/PhysRevB.101.054109
This research is supported by the AFOSR Aerospace Materials for Extreme Environment Program and a grant from the National Science Foundation.
Alltimes Coatings have worked in partnership with Applied Graphene Materials and successfully used their recently launched Advantage Graphene anti-corrosion sprayable coating in what could be the world’s first roofing application using a graphene enhanced coating system.
Nigel Alltimes, Managing Director, Alltimes Coatings Limited, said:
We believe that with the launch of Advantage Graphene, we are bringing to market a unique and revolutionary liquid roofing system for our industrial and commercial customers. Without doubt, Applied Graphene Materials' deep understanding of coating technology and how best to effectively integrate graphene into novel chemistry, has played a major role in the successful launch of this product. Early feedback from our customers has been very positive and we anticipate strong uptake as we extend the performance of our product range with graphene technology.
A UCLA-led research team has produced in unprecedented detail experimental three-dimensional maps of the atoms in a so-called 2D material — matter that isn’t truly two-dimensional but is nearly flat because it’s arranged in extremely thin layers, no more than a few atoms thick.
Although 2D-materials–based technologies have not yet been widely used in commercial applications, the materials have been the subject of considerable research interest. In the future, they could be the basis for semiconductors in ever smaller electronics, quantum computer components, more-efficient batteries, or filters capable of extracting freshwater from saltwater.
The promise of 2D materials comes from certain properties that differ from how the same elements or compounds behave when they appear in greater quantities. Those unique characteristics are influenced by quantum effects — phenomena occurring at extremely small scales that are fundamentally different from the classical physics seen at larger scales. For instance, when carbon is arranged in an atomically thin layer to form 2D graphene, it is stronger than steel, conducts heat better than any other known material, and has almost zero electrical resistance.
But using 2D materials in real-world applications would require a greater understanding of their properties, and the ability to control those properties. The new study, which was published in Nature Materials, could be a step forward in that effort.
The researchers showed that their 3D maps of the material’s atomic structure are precise to the picometer scale — measured in one-trillionths of a meter. They used their measurements to quantify defects in the 2D material, which can affect their electronic properties, as well as to accurately assess those electronic properties.
“What’s unique about this research is that we determine the coordinates of individual atoms in three dimensions without using any pre-existing models,” said corresponding author Jianwei “John” Miao, a UCLA professor of physics and astronomy. “And our method can be used for all kinds of 2D materials.”
Miao is the deputy director of the STROBE National Science Foundation Science and Technology Center and a member of the California NanoSystems Institute at UCLA. His UCLA lab collaborated on the study with researchers from Harvard University, Oak Ridge National Laboratory and Rice University.
The researchers examined a single layer of molybdenum disulfide, a frequently studied 2D material. In bulk, this compound is used as a lubricant. As a 2D material, it has electronic properties that suggest it could be employed in next-generation semiconductor electronics. The samples being studied were “doped” with traces of rhenium, a metal that adds spare electrons when replacing molybdenum. That kind of doping is often used to produce components for computers and electronics because it helps facilitate the flow of electrons in semiconductor devices.
To analyze the 2D material, the researchers used a new technology they developed based on scanning transmission electron microscopy, which produces images by measuring scattered electrons beamed through thin samples. Miao’s team devised a technique called scanning atomic electron tomography, which produces 3D images by capturing a sample at multiple angles as it rotates.
The scientists had to avoid one major challenge to produce the images: 2D materials can be damaged by too much exposure to electrons. So for each sample, the researchers reconstructed images section by section and then stitched them together to form a single 3D image — allowing them to use fewer scans and thus a lower dose of electrons than if they had imaged the entire sample at once.
The two samples each measured 6 nanometers by 6 nanometers, and each of the smaller sections measured about 1 nanometer by 1 nanometer. (A nanometer is one-billionth of a meter.)
The resulting images enabled the researchers to inspect the samples’ 3D structure to a precision of 4 picometers in the case of molybdenum atoms — 26 times smaller than the diameter of a hydrogen atom. That level of precision enabled them to measure ripples, strain distorting the shape of the material, and variations in the size of chemical bonds, all changes caused by the added rhenium — marking the most accurate measurement ever of those characteristics in a 2D material.
“If we just assume that introducing the dopant is a simple substitution, we wouldn’t expect large strains,” said Xuezeng Tian, the paper’s co-first author and a UCLA postdoctoral scholar. “But what we have observed is more complicated than previous experiments have shown.”
The scientists found that the largest changes occurred in the smallest dimension of the 2D material, its three-atom-tall height. It took as little as a single rhenium atom to introduce such local distortion.
Armed with information about the material’s 3D coordinates, scientists at Harvard led by Professor Prineha Narang performed quantum mechanical calculations of the material’s electronic properties.
“These atomic-scale experiments have given us a new lens into how 2D materials behave and how they should be treated in calculations, and they could be a game changer for new quantum technologies,” Narang said.
Without access to the sort of measurements generated in the study, such quantum mechanical calculations conventionally have been based on a theoretical model system that is expected at a temperature of absolute zero.
The study indicated that the measured 3D coordinates led to more accurate calculations of the 2D material’s electronic properties.
“Our work could transform quantum mechanical calculations by using experimental 3D atomic coordinates as direct input,” said UCLA postdoctoral scholar Dennis Kim, a co-first author of the study. “This approach should enable material engineers to better predict and discover new physical, chemical and electronic properties of 2D materials at the single-atom level.”
Other authors were Yongsoo Yang, Yao Yang and Yakun Yuan of UCLA; Shize Yang and Juan-Carlos Idrobo of Oak Ridge National Laboratory; Christopher Ciccarino and Blake Duschatko of Harvard; and Yongji Gong and Pulickel Ajayan of Rice.
The research was supported by the U.S. Department of Energy, the U.S. Army Research Office, and STROBE National Science Foundation Science and Technology Center. The scanning transmission electron microscopy experiments were conducted at the Center for Nanophase Materials Sciences, a DOE user facility at Oak Ridge National Laboratory.
The realistic mechanical properties of monolayer graphene have been successfully studied by a new method developed by a research team led by Dr Lu Yang, Associate Professor of Department of Mechanical Engineering at City University of Hong Kong (CityU). The groundbreaking discovery will promote the application of graphene in different areas, such as the touch monitor on flexible mobile phones.
Dr Lu’s research achievement has been published in the prestigious international journal Nature Communications, titled “Elastic straining of free-standing monolayer graphene.” This paper was also highlighted in “Editors' Choice” in the 21 February 2020 issue of Science.
A two-dimensional carbon substance, graphene is the strongest material known with excellent electrical and thermal conductivity. Hence, it is deemed a "super material", ideal for many fields, for example, transistors, biosensors and batteries.
Graphene’s structure as a single layer of atoms has made it extremely difficult for scientists to test its actual mechanical properties such as elasticity and tensile strength. The studies in this area so far have covered only its ideal limits by local indentation experiments and theoretical calculations.
“No one has really stretched a large-area, free-standing monolayer graphene and tested its elastic tensile properties,” Dr Lu said.
Over the years, Dr Lu has researched the mechanical properties of various nanomaterials. His research team has successfully developed a new method for transferring large-area graphene onto his unique nanomechanical testing platform, performing in situ tensile tests in a scanning electron microscope to study changes in stretching and shaping.
“One major challenge in our study is how to transfer and lay an extremely light and thin monolayer graphene sample onto a testing platform without damage, and apply the strain evenly when stretching it,” Dr Lu said.
The experiment showed that the tensile strength of chemical vapour deposition (CVD)-grown monolayer graphene can reach 50 to 60 GPa (gigapascal), with elastic strain up to 6%, and the measured Young’s modulus (or the “elastic modulus”) is 920 GPa, which is very close to the theoretical value of ~1,000 GPa. Pascals are units of measurement for stress.
“It took us nearly four years to overcome a lot of difficulties for the experiment, but our work has revealed the realistic mechanical properties of graphene for engineering relevance,” Dr Lu said.
Its strength and stretchability make graphene a suitable material for manufacturing flexible electronic devices, such as transistors with better robustness, organic light-emitting diodes, and other mechanical components.
It can also be used for the production of composite materials and in the areas of biomedical research, aviation and national defence.
Dr Lu said he was grateful to CityU for providing top-notch facilities for his team to conduct their research, such as the Nano-Manufacturing Laboratory at the CityU Shenzhen Research Institute, the Centre for Super-Diamond and Advanced Films, and the Centre for Advanced Structural Materials.
In addition, CityU’s emphasis on interdisciplinary collaboration helped his research. “Our experiment required experts from the disciplines of mechanics, materials science, chemistry and physics to work together, and the outstanding talents in these fields can be readily found at CityU,” Dr Lu said.
Members of the research team include PhD students Cao Ke and Han Ying in the Department of Mechanical Engineering and Dr Ly Thuc-hue, Assistant Professor in the Department of Chemistry, at CityU, as well as experts from Tsinghua University and Xidian University.
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.
Graphene is well-known for its remarkable electronic, mechanical and thermal properties, but industrial production of high-quality graphene is very challenging. A research team at Delft University of Technology has now developed a mathematical model that can be used to guide the large-scale production of these ultrathin layers of carbon. The findings were published this week in The Journal of Chemical Physics.
“Our model is the first to give a detailed view of what happens at the micro and nanoscale when graphene is produced from plain graphite using energetic fluid mixing,” says Dr. Lorenzo Botto, researcher at the department of Process & Energy at TU Delft. “The model will help the design of large-scale production processes, paving the way for graphene to be incorporated in commercial applications from energy storage devices to biomedicine”.
Graphite and graphene
Graphene can be made from graphite, which is a crystalline form of pure carbon, widely used for example in pencils and lubricants. The layers that make up graphite are called graphene and consists of carbon atoms arranged in a hexagonal structure. These extremely thin carbon layers possess remarkable electrical, mechanical, optical and thermal properties.
For example, a single layer of graphene is about 100 times stronger than the strongest steel of the same thickness. It conducts heat and electricity extremely efficiently and is nearly transparent. Graphene is also intrinsically very cheap, if scalable methods to produce it in large quantities can be found. Graphene has attracted much attention of the past decade as a candidate material for applications in a variety fields such as electronics, energy generation and storage, and biomedicine. In the near future we may replace the copper wiring in our houses with graphene cables, and develop all-carbon batteries that use graphene as the main building block. However, the fabrication of high quality graphene at industrial scale and affordable price remains a challenge. A new theoretical and computational model developed at TU Delft addresses this challenge.
Production of graphene
One of the most promising techniques to produce graphene from graphite is so-called liquid-phase exfoliation. In this technique, graphite is sheared in a liquid environment until layers of graphene detach from the bulk material. The liquid causes the graphene layers to detach gently, which is important to obtain high-quality graphene.
The process has already been successful in the production of graphene on laboratory scale, and, on a trial-and-error basis, on larger scales. It has the potential to be used on industrial scales, to produce tons of material. However, in order to increase the scale of graphene production, we need to know the process parameters that make the exfoliation work efficiently without damaging the graphene sheets.
A research team at TU Delft led by Dr. Lorenzo Botto has now developed the first rigorously derived and validated mathematical model to determine those parameters. This model can be embedded in large-scale industrial process optimisation software or used by practitioners to choose processing parameters.
“The exfoliation process is difficult to model,” explains Botto. “The adhesion between graphene layers is not easy to quantify and the fluid dynamical forces exerted by the liquid on the graphite depend sensitively on surface properties and geometry.” Team members Catherine Kamal and Simon Gravelle developed and tested the model against molecular dynamics simulations, and proved that that the model can be very accurate. Key to the success of the model is the inclusion of hydronamic slip of the liquid pushing against the graphite surface, and of the fluid forces on the graphene edges.
Botto: “The model forms the basis for better control of the technique at any scale. We hope it will pave the way to the large-scale production of graphene for all kinds of useful applications.”
Dr. Botto obtained his Ph.D. in Fluid Mechanics at Johns Hopkins University (USA), and has worked in the USA, UK and Switzerland. He recently moved to TU Delft. The mission of his research is to use fluid mechanics knowledge to support the large-scale, sustainable production of materials that can solve important societal challenges, from sustainable energy production to environmental remediation. His work on graphene exfoliation is funded by a 1.5M€ Starting Grant (Grant agreement ID: 715475) from the European Research Council (ERC). Read more about the project.
Botto: “Fluid forces can be used to produce and process graphene at the scale required by market applications. However, to reach market readiness we need control over quality and processes. By uncovering underlying fluid mechanical principles, I aim for a profound impact on our ability to produce two-dimensional carbon nanomaterials on large scales.