Porous, 3-D forms of graphene developed at MIT can be 10 times as strong as steel but much lighter.
A team of researchers at MIT has designed one of the strongest lightweight materials known, by compressing and fusing flakes of graphene, a two-dimensional form of carbon. The new material, a sponge-like configuration with a density of just 5 percent, can have a strength 10 times that of steel.
In its two-dimensional form, graphene is thought to be the strongest of all known materials. But researchers until now have had a hard time translating that two-dimensional strength into useful three-dimensional materials.
The new findings show that the crucial aspect of the new 3-D forms has more to do with their unusual geometrical configuration than with the material itself, which suggests that similar strong, lightweight materials could be made from a variety of materials by creating similar geometric features.
The findings are being reported today in the journal Science Advances, in a paper by Markus Buehler, the head of MIT’s Department of Civil and Environmental Engineering (CEE) and the McAfee Professor of Engineering; Zhao Qin, a CEE research scientist; Gang Seob Jung, a graduate student; and Min Jeong Kang MEng ’16, a recent graduate.
Other groups had suggested the possibility of such lightweight structures, but lab experiments so far had failed to match predictions, with some results exhibiting several orders of magnitude less strength than expected. The MIT team decided to solve the mystery by analyzing the material’s behavior down to the level of individual atoms within the structure. They were able to produce a mathematical framework that very closely matches experimental observations.
Two-dimensional materials — basically flat sheets that are just one atom in thickness but can be indefinitely large in the other dimensions — have exceptional strength as well as unique electrical properties. But because of their extraordinary thinness, “they are not very useful for making 3-D materials that could be used in vehicles, buildings, or devices,” Buehler says. “What we’ve done is to realize the wish of translating these 2-D materials into three-dimensional structures.”
The team was able to compress small flakes of graphene using a combination of heat and pressure. This process produced a strong, stable structure whose form resembles that of some corals and microscopic creatures called diatoms. These shapes, which have an enormous surface area in proportion to their volume, proved to be remarkably strong. “Once we created these 3-D structures, we wanted to see what’s the limit — what’s the strongest possible material we can produce,” says Qin. To do that, they created a variety of 3-D models and then subjected them to various tests. In computational simulations, which mimic the loading conditions in the tensile and compression tests performed in a tensile loading machine, “one of our samples has 5 percent the density of steel, but 10 times the strength,” Qin says.
Buehler says that what happens to their 3-D graphene material, which is composed of curved surfaces under deformation, resembles what would happen with sheets of paper. Paper has little strength along its length and width, and can be easily crumpled up. But when made into certain shapes, for example rolled into a tube, suddenly the strength along the length of the tube is much greater and can support substantial weight. Similarly, the geometric arrangement of the graphene flakes after treatment naturally forms a very strong configuration.
The new configurations have been made in the lab using a high-resolution, multimaterial 3-D printer. They were mechanically tested for their tensile and compressive properties, and their mechanical response under loading was simulated using the team’s theoretical models. The results from the experiments and simulations matched accurately.
The new, more accurate results, based on atomistic computational modeling by the MIT team, ruled out a possibility proposed previously by other teams: that it might be possible to make 3-D graphene structures so lightweight that they would actually be lighter than air, and could be used as a durable replacement for helium in balloons. The current work shows, however, that at such low densities, the material would not have sufficient strength and would collapse from the surrounding air pressure.
But many other possible applications of the material could eventually be feasible, the researchers say, for uses that require a combination of extreme strength and light weight. “You could either use the real graphene material or use the geometry we discovered with other materials, like polymers or metals,” Buehler says, to gain similar advantages of strength combined with advantages in cost, processing methods, or other material properties (such as transparency or electrical conductivity).
“You can replace the material itself with anything,” Buehler says. “The geometry is the dominant factor. It’s something that has the potential to transfer to many things.”
The unusual geometric shapes that graphene naturally forms under heat and pressure look something like a Nerf ball — round, but full of holes. These shapes, known as gyroids, are so complex that “actually making them using conventional manufacturing methods is probably impossible,” Buehler says. The team used 3-D-printed models of the structure, enlarged to thousands of times their natural size, for testing purposes.
For actual synthesis, the researchers say, one possibility is to use the polymer or metal particles as templates, coat them with graphene by chemical vapor deposit before heat and pressure treatments, and then chemically or physically remove the polymer or metal phases to leave 3-D graphene in the gyroid form. For this, the computational model given in the current study provides a guideline to evaluate the mechanical quality of the synthesis output.
The same geometry could even be applied to large-scale structural materials, they suggest. For example, concrete for a structure such a bridge might be made with this porous geometry, providing comparable strength with a fraction of the weight. This approach would have the additional benefit of providing good insulation because of the large amount of enclosed airspace within it.
Because the shape is riddled with very tiny pore spaces, the material might also find application in some filtration systems, for either water or chemical processing. The mathematical descriptions derived by this group could facilitate the development of a variety of applications, the researchers say.
“This is an inspiring study on the mechanics of 3-D graphene assembly,” says Huajian Gao, a professor of engineering at Brown University, who was not involved in this work. “The combination of computational modeling with 3-D-printing-based experiments used in this paper is a powerful new approach in engineering research. It is impressive to see the scaling laws initially derived from nanoscale simulations resurface in macroscale experiments under the help of 3-D printing,” he says.
This work, Gao says, “shows a promising direction of bringing the strength of 2-D materials and the power of material architecture design together.”
Graphene Flagship researchers from Trinity College Dublin in collaboration with the National Graphene Institute (NGI) at The University of Manchester, have used graphene to make a polysilicone polymer, known commonly as the novelty children’s material Silly Putty®, conduct electricity. Using this conductive polymer they found that they were about to create sensitive electromechanical sensors.
The team’s findings have been published in the journal Science*.
This research, led by Professor Jonathan Coleman, Trinity College Dublin, in collaboration with Professor Robert Young of The University of Manchester, potentially offers exciting possibilities for applications in new, inexpensive devices and diagnostics in healthcare and other sectors.
Professor Coleman, Investigator in AMBER and Trinity’s School of Physics along with postdoctoral researcher Conor Boland (both seen in the image below), discovered that the electrical resistance of putty infused with graphene (‘G-putty’) was extremely sensitive to the slightest deformation or impact, having a gauge factor >500. They mounted the G-putty onto the chest and neck of human subjects and used it to measure breathing, pulse and even blood pressure. It showed unprecedented sensitivity as a sensor for strain and pressure, hundreds of times more sensitive than current sensors. The G-putty also works as a very sensitive impact sensor, able to detect the footsteps of small spiders.
Professor Coleman said: “What we are excited about is the unexpected behaviour we found when we added graphene to the polymer, a cross-linked polysilicone. This material is well known as the children’s toy Silly Putty. It is different from familiar materials in that it flows like a viscous liquid when deformed slowly but bounces like an elastic solid when thrown against a surface. When we added the graphene to the silly putty, it caused it to conduct electricity, but in a very unusual way. The electrical resistance of the G-putty was very sensitive to deformation with the resistance increasing sharply on even the slightest strain or impact. Unusually, the resistance slowly returned close to its original value as the putty self-healed over time.”
He continued, “While a common application has been to add graphene to plastics in order to improve the electrical, mechanical, thermal or barrier properties, the resultant composites have generally performed as expected without any great surprises. The behaviour we found with G-putty has not been found in any other composite material. This unique discovery will open up major possibilities in sensor manufacturing worldwide.”
In their paper the team show that following the addition of graphene to the polymer it not only became conductive (reaching approximately 0.1 S/m at approximately 15 volume % of graphene) but importantly, it retained its viscoelasticity characteristics. The graphene sheets are able to respond to polymeric deformation in a time dependant manor, forming networks that break and reform during mechanical deformation. This changes the conductivity of the polymeric material, enabling it to sense by deformation.
Following the initial development work at Trinity College Dublin scientists at the NGI at The University of Manchester analysed the structure of the material and were able to develop a mathematical model of the deformation of the material which explains the effect of its structure upon its mechanical and electrical properties.
Robert Young, Professor of Polymer Science and Technology at the NGI said: “The endless list of potential applications of graphene, never ceases to amaze me. We have now developed a new high-performance sensing material, ‘G-putty’, that can monitor deformation, pressure and impact at a level of sensitivity that is so precise that it allows even the footsteps of small spiders to be monitored.
“It will have many future applications in sensors, particularly in the field of healthcare. The collaboration has been undertaken under the umbrella of the European Graphene Flagship, in which Trinity College Dublin and The University of Manchester both play a prominent role. It is an excellent example of what is being achieved in the Flagship programme.”
Learn more: G-Putty sensors – an unexpected breakthrough
Graphene-based device could accelerate the development of 2D photoelectronics
The Center for Integrated Nanostructure Physics, within the Institute for Basic Science (IBS) has developed the world’s thinnest photodetector, that is a device that converts light into an electric current. With a thickness of just 1.3 nanometers – 10 times smaller than the current standard silicon diodes – this device could be used in the Internet of Things, smart devices, wearable electronics and photoelectronics. This 2D technology, published on Nature Communications, uses molybdenum disulfide (MoS2) sandwiched in graphene.
Graphene is a fantastic material: It’s conductive, thin (just one-atom thick), transparent and flexible. However, since it does not behave as a semiconductor, its application in the electronics industry is limited. Therefore, in order to increase graphene’s usability, IBS scientists sandwiched a layer of the 2D semiconductor MoS2 between two graphene sheets and put it over a silicon base. They initially thought the resulting device was too thin to generate an electric current but, unexpectedly, it did. “A device with one-layer of MoS2 is too thin to generate a conventional p-n junction, where positive (p) charges and negative (n) charges are separated and can create an internal electric field. However, when we shine light on it, we observed high photocurrent. It was surprising! Since it cannot be a classical p-n junction, we thought to investigate it further,” explains YU Woo Jong, first author of this study.
To understand what they found, the researchers compared devices with one and seven layers of MoS2 and tested how well they behave as a photodetector, that is, how they are able to convert light into an electric current. They found that the device with one-layer MoS2 absorbs less light than the device with seven layers, but it has higher photoresponsitivity. “Usually the photocurrent is proportional to the photoabsorbance, that is, if the device absorbs more light, it should generate more electricity, but in this case, even if the one-layer MoS2 device has smaller absorbance than the seven-layer MoS2, it produces seven times more photocurrent,” describes Yu.
Why is the thinner device working better than the thicker one? The research team proposed a mechanism to explain why this is the case. They recognized that the photocurrent generation could not be explained with classical electromagnetism, but could be with quantum physics. When light hits the device, some electrons from the MoS2 layer jump into an excited state and their flow through the device produces an electric current. However in order to pass the boundary between MoS2 and graphene, the electrons need to overcome an energy barrier (via quantum tunnelling), and this is where the one-layer MoS2 device has an advantage over the thicker one.
The monolayer is thinner and therefore more sensitive to the surrounding environment: The bottom SiO2 layer increases the energy barrier, while the air on top reduces it, thus electrons in the monolayer device have a higher probability to tunnel from the MoS2 layer to the top graphene (GrT). The energy barrier at the GrT/MoS2 junction is lower than the one at the GrB/MoS2, so the excited electrons transfer preferentially to the GrT layer and create an electric current. Conversely, in the multi-layer MoS2 device, the energy barriers between GrT/MoS2 and GrB/MoS2 are symmetric, therefore the electrons have the same probability to go either side and thus reduce the generated current.
Imagine a group of people in a valley surrounded by two mountains. The group wants to get to the other side of the mountains, but without making too much effort. In one case ( the seven-layers MoS2 device), both mountains have the same height so whichever mountain is crossed, the effort will be the same. Therefore half the group crosses one mountain and the other half the second mountain.
In the second case (analogue to the one-layer MoS2 device), one mountain is taller than the other, so the majority of the group decide to cross the smaller mountain. However, because we are considering quantum physics instead of classical electromagnetism, they do not need to climb the mountain until they reach the top (as they would need to do with classical physics), but they can pass through a tunnel. Although electron tunneling and walking a tunnel in a mountain are very different of course, the idea is that electric current is generated by the flow of electrons, and the thinner device can generate more current because more electrons flow towards the same direction.
Actually, when light is absorbed by the device and MoS2 electrons jump into an excited state, they leave the so-called holes behind. Holes behave like positive mobile charges and are essentially positions left empty by electrons that absorbed enough energy to jump to a higher energy status. Another problem of the thicker device is that electrons and holes move too slowly through the junctions between graphene and MoS2, leading to their undesired recombination within the MoS2 layer.
For these reasons, up to 65% of photons absorbed by the thinner device are used to generate a current. Instead, the same measurement (quantum efficiency) is only 7% for the seven-layer MoS2 apparatus.
“This device is transparent, flexible and requires less power than the current 3D silicon semiconductors. If future research is successful, it will accelerate the development of 2D photoelectric devices,” explains the professor.
Learn more: The Thinnest Photodetector in the World
Researchers at the Center for Multidimensional Carbon Materials (CMCM), within the Institute for Basic Science (IBS) have demonstrated graphene coating protects glass from corrosion. Their research, published in ACS Nano, can contribute to solving problems related to glass corrosion in several industries.
Glass has a high degree of both corrosion and chemical resistance. For this reason it is the primary packaging material to preserve medicines and chemicals. However, over time at high humidity and pH, some glass types corrode. Corroded glass loses its transparency and its strength is reduced. As a result, the corrosion of silicate glass, the most common and oldest form of glass, by water is a serious problem especially for the pharmaceutical, environmental and optical industries, and in particular in hot and humid climates.
Although there are different types of glass, ordinary glazing and containers are made of silicon dioxide (SiO2), sodium oxide (Na2O) along with minor additives. Glass corrosion begins with the adsorption of water on the glass surface. Hydrogen ions from water then diffuse into the glass and exchange with the sodium ions present on the glass surface. The pH of the water near the glass surface increases, allowing the silicate structure to dissolve.
▲ Corrosion mechanism on the glass surface (via Bin Wang et al, ACS Nano 2016. American Chemical Society).
Scientists have been looking at how to coat glass to protect it from damage. An ideal protective coating should be thin, transparent, and provide a good diffusion barrier to chemical attack. Graphene with its chemical inertness, thinness, and high transparency makes it very promising as a coating material. Moreover, owing to its excellent chemical barrier properties it blocks helium atoms from penetrating through it. The use of graphene coating is being explored as a protective layer for other materials requiring resistance to corrosion, oxidation, friction, bacterial infection, electromagnetic radiation, etc.
IBS scientists grew graphene on copper using a technique previously invented by Prof. Rodney S. Ruoff and collaborators, and transferred either one or two atom-thick layers of graphene onto both sides of rectangular pieces of glass. The effectiveness of the graphene coating was evaluated by water immersion testing and observing the differences between uncoated and coated glass. After 120 days of immersion in water at 60°C, uncoated glass samples had significantly increased in surface roughness and defects, and reduced in fracture strength. In contrast, both the single and double layer graphene-coated glasses had essentially no change in both fracture strength and surface roughness.
“The purpose of the study was to determine whether graphene grown by chemical vapor deposition on copper foils, a now established method, could be transferred onto glass, and protect the glass from corrosion. Our study shows that even one atom-thick layer of graphene does the trick,” explains Prof. Ruoff, director of the CMCM and Professor at the Ulsan National Institute of Science and Technology (UNIST). “In the future, when it is possible to produce larger and yet higher-quality graphene sheets and to optimize the transfer on glass, it seems reasonably likely that graphene coating on glass will be used on an industrial scale.”
An environmentally friendly, efficient and low-cost method for hydrogenation of graphene with visible light has been developed by researchers at Uppsala University and AstraZeneca Gothenburg, Sweden. The research study is presented in an article in Nature Communications.
The study shows that the two-dimensional and atom-thin carbon material graphene reacts with formic acid in a water solution upon irradiation with visible light. In the reaction, formic acid acts as masked hydrogen and a material is produced where hydrogen extensively has been added to graphene. One says that graphene has been hydrogenated. The study was performed by Assoc. Prof. Henrik Ottosson’s research group at the Department of Chemistry – Ångström Laboratory, together with colleagues in Chemistry, Physics and Engineering at Uppsala University and at AstraZeneca Gothenburg.
“The reaction is convenient and cheap, and hydrogenated graphene may be applied within areas such as hydrogen storage. Additionally, upon functionalization of graphene one can open a band gap and this fact is of high relevance for electronics applications”, says Henrik Ottosson.
Yet, graphene research is a side-project in Henrik Ottosson’s group. The group normally studies the behaviours of various aromatic hydrocarbons upon irradiation, and they apply a rule, the so-called Baird’s rule, which can be derived through chemically applied quantum mechanics.
Chemical compounds that are aromatic have an inherently high stability and often they are not easy to degrade. Benzene is the most well known aromatic compound and more than half of all known chemical compounds contain aromatic groups.
The high stability of aromatic compounds is explained by Hückel’s ‘4n+2’ rule, but this rule is only valid for compounds in their electronic ground states. Upon exposure to light of a certain wavelength, the aromatic compounds reach electronically excited states. According to Baird, compounds that are aromatic in the ground state become antiaromatic and reactive in the excited state. The rule, neglected for decades, can now be used to describe various behaviours of aromatic compounds when irradiated.
Using Baird’s rule, Henrik Ottosson’s group develops new light-initiated reactions. First, they studied addition of hydrosilanes to benzenes, naphthalene and gradually larger polycyclic aromatic hydrocarbons (hydrosilanes are compounds that can be regarded as heavy analogues of hydrogen). Despite the fact that it is not possible to explain if, and how, Baird’s rule can be applied to graphene (an essentially infinitely large polycyclic aromatic hydrocarbon), the group explored graphene chemistry and found a very efficient addition reaction when using formic acid.
At AstraZeneca one sees interesting possibilities for the future:
“It has become more common to apply light-initiated reactions during the development of new molecules in our drug research programs. We challenge ourselves to continuously develop more efficient and environmentally friendly chemical methods. The recent progress we have seen in photochemistry, highlighted by the results herein, will increase our opportunities to access chemistry that no one thought possible a few years ago. In addition, graphene based materials have exceptional inherent properties. There is a wealth of possible applications that could result in the next biomedical revolution”, says Joakim Bergman, Innovative Medicines and Early Development Biotech Unit AstraZeneca Gothenburg.
Researchers have confirmed the existence of a naturally occurring exotic property in which a material becomes thicker when stretched – the opposite of most materials – a discovery that could lead to new studies into the fundamental science of nano-materials behavior.
The counterintuitive phenomenon, called auxetic behavior, has been extensively studied in engineered structures that have potential applications in medicine, tissue engineering, body armor and “fortified armor enhancement.”
However, until now the behavior has not been confirmed in natural materials, said Peide Ye, Purdue University’s Richard J. and Mary Jo Schwartz Professor of Electrical and Computer Engineering.
The auxetic behavior was discovered in a material called black phosphorous.
The phenomenon is governed by a fundamental mechanical property of materials called the Poisson’s ratio, which characterizes how a material behaves when stretched. Most materials when stretched become thinner and when compressed become thicker, and they are said to have a positive Poisson’s ratio.
“A negative Poisson’s ratio is theoretically possible but until now has not, with few exceptions of man-made structures, been experimentally observed in any natural materials,” Ye said. “Here, we show that the negative Poisson’s ratio exists in the natural material black phosphorus.”
Findings are detailed in a research paper that appeared on Sept. 23 in the journal Nano Letters.
“Until now, there has been a lack of experimental evidence since the measurement of internal deformation in auxetic materials, in particular at the atomic level, is extremely difficult,” Ye said.
Researchers used a technique called Raman spectroscopy to document the negative Poisson’s ratio in extremely thin, individual layers of black phosphorous called phosphorene. The research was based at the Birck Nanotechnology Center in Purdue’s Discovery Park.
The Nano Letters paper was authored by doctoral student Yuchen Du; former postdoctoral research associate Jesse Maassen; graduate students Wangran Wu and Zhe Luo; Xianfan Xu, the James J. and Carol L. Shuttleworth Professor of Mechanical Engineering and professor of electrical and computer engineering; and Ye. Du carried out most of the experiments. Maassen performed the theoretical work critical to the research. He is now an assistant professor of physics at Dalhousie University in Nova Scotia, Canada.
The researchers focused on the material’s uniquely puckered crystal structure in which atoms are arranged in a wavy pattern. Like silicon, the material possesses a bandgap, a trait essential for a semiconductor’s ability to switch on and off in electronic circuits. The material also has a relatively high “carrier mobility,” meaning it is very conductive and could be useful for technological applications.
Future research will include work to investigate whether the negative Poisson’s ratio exists in other so-called “two-dimensional” materials, including extremely thin layers of graphite called graphene.
Researchers discover that electrons mimic light in graphene, confirming a 2007 prediction – their finding may enable new low power electronics and lead to new experimental probes.
A team led by Cory Dean, assistant professor of physics at Columbia University, Avik Ghosh, professor of electrical and computer engineering at the University of Virginia, and James Hone, Wang Fong-Jen Professor of Mechanical Engineering at Columbia Engineering, has directly observed—for the first time—negative refraction for electrons passing across a boundary between two regions in a conducting material. First predicted in 2007, this effect has been difficult to confirm experimentally. The researchers were able to observe the effect in graphene, demonstrating that electrons in the atomically thin material behave like light rays, which can be manipulated by such optical devices as lenses and prisms. The findings, which are published in the September 30 edition of Science, could lead to the development of new types of electron switches, based on the principles of optics rather than electronics.
“The ability to manipulate electrons in a conducting material like light rays opens up entirely new ways of thinking about electronics,” says Dean. “For example, the switches that make up computer chips operate by turning the entire device on or off, and this consumes significant power. Using lensing to steer an electron ‘beam’ between electrodes could be dramatically more efficient, solving one of the critical bottlenecks to achieving faster and more energy efficient electronics.”
Dean adds, “These findings could also enable new experimental probes. For example, electron lensing could enable on-chip versions of an electron microscope, with the ability to perform atomic scale imaging and diagnostics. Other components inspired by optics, such as beam splitters and interferometers, could additionally enable new studies of the quantum nature of electrons in the solid state.”
While graphene has been widely explored for supporting high electron speed, it is notoriously hard to turn off the electrons without hurting their mobility. Ghosh says, “The natural follow-up is to see if we can achieve a strong current turn-off in graphene with multiple angled junctions. If that works to our satisfaction, we’ll have on our hands a low-power, ultra-high-speed switching device for both analog (RF) and digital (CMOS) electronics, potentially mitigating many of the challenges we face with the high energy cost and thermal budget of present day electronics.”
Light changes direction – or refracts – when passing from one material to another, a process that allows us to use lenses and prisms to focus and steer light. A quantity known as the index of refraction determines the degree of bending at the boundary, and is positive for conventional materials such as glass. However, through clever engineering, it is also possible to create optical “metamaterials” with a negative index, in which the angle of refraction is also negative. “This can have unusual and dramatic consequences,” Hone notes. “Optical metamaterials are enabling exotic and important new technologies such as super lenses, which can focus beyond the diffraction limit, and optical cloaks, which make objects invisible by bending light around them.”
Electrons travelling through very pure conductors can travel in straight lines like light rays, enabling optics-like phenomena to emerge. In materials, the electron density plays a similar role to the index of refraction, and electrons refract when they pass from a region of one density to another. Moreover, current carriers in materials can either behave like they are negatively charged (electrons) or positively charged (holes), depending on whether they inhabit the conduction or the valence band. In fact, boundaries between hole-type and electron-type conductors, known as p-n junctions (“p” positive, “n” negative), form the building blocks of electrical devices such as diodes and transistors.
“Unlike in optical materials”, says Hone, “where creating a negative index metamaterial is a significant engineering challenge, negative electron refraction occurs naturally in solid state materials at any p-n junction.”
The development of two-dimensional conducting layers in high-purity semiconductors such as GaAs (Gallium arsenide) in the 1980s and 1990s allowed researchers to first demonstrate electron optics including the effects of both refraction and lensing. However, in these materials, electrons travel without scattering only at very low temperatures, limiting technological applications. Furthermore, the presence of an energy gap between the conduction and valence band scatters electrons at interfaces and prevents observation of negative refraction in semiconductor p-n junctions. In this study, the researchers’ use of graphene, a 2D material with unsurpassed performance at room temperature and no energy gap, overcame both of these limitations.
The possibility of negative refraction at graphene p-n junctions was first proposed in 2007 by theorists working at both the University of Lancaster and Columbia University. However, observation of this effect requires extremely clean devices, such that the electrons can travel ballistically, without scattering, over long distances. Over the past decade, a multidisciplinary team at Columbia – including Hone and Dean, along with Kenneth Shepard, Lau Family Professor of Electrical Engineering and professor of biomedical engineering, Abhay Pasupathy, associate professor of physics, and Philip Kim, professor of physic at the time (now at Harvard) – has worked to develop new techniques to construct extremely clean graphene devices. This effort culminated in the 2013 demonstration of ballistic transport over a length scale in excess of 20 microns. Since then, they have been attempting to develop a Veselago lens, which focuses electrons to a single point using negative refraction. But they were unable to observe such an effect and found their results puzzling.
In 2015, a group at Pohang University of Science and Technology in South Korea reported the first evidence focusing in a Veselago-type device. However, the response was weak, appearing in the signal derivative. The Columbia team decided that to fully understand why the effect was so elusive, they needed to isolate and map the flow of electrons across the junction. They utilized a well-developed technique called “magnetic focusing” to inject electrons onto the p-n junction. By measuring transmission between electrodes on opposite sides of the junction as a function of carrier density they could map the trajectory of electrons on both sides of the p-n junction as the incident angle was changed by tuning the magnetic field.
Crucial to the Columbia effort was the theoretical support provided by Ghosh’s group at the University of Virginia, who developed detailed simulation techniques to model the Columbia team’s measured response. This involved calculating the flow of electrons in graphene under the various electric and magnetic fields, accounting for multiple bounces at edges, and quantum mechanical tunneling at the junction. The theoretical analysis also shed light on why it has been so difficult to measure the predicted Veselago lensing in a robust way, and the group is developing new multi-junction device architectures based on this study. Together the experimental data and theoretical simulation gave the researchers a visual map of the refraction, and enabled them to be the first to quantitatively confirm the relationship between the incident and refracted angles (known as Snell’s Law in optics), as well as confirmation of the magnitude of the transmitted intensity as a function of angle (known as the Fresnel coefficients in optics).
“In many ways, this intensity of transmission is a more crucial parameter,” says Ghosh, “since it determines the probability that electrons actually make it past the barrier, rather than just their refracted angles. The transmission ultimately determines many of the performance metrics for devices based on these effects, such as the on-off ratio in a switch, for example.”
The Science study was supported by Semiconductor Research Corporation’s NRI Center for Institute for Nanoelectronics Discovery and Exploration (INDEX). The collaboration was made possible through the support of the NRI program, which has brought together some of the country’s best groups in the study of 2D materials to focus on novel device architectures that may outperform conventional silicon-based technologies.
For the first time researchers succeeded to place a layer of graphene on top of a stable fatty lipid monolayer. Surrounded by a protective shell of lipids graphene could enter the body and function as a versatile sensor.
The results are the first step towards such a shell, and have been published in the journal Nanoscale on 28 September 2016.
In contrast to previous work, the researchers observed a stable structure when placing graphene on a single layer of lipids. A patent has been submitted for these findings. PhD candidate Lia Lima and co-workers made this discovery under the supervision of chemist Grégory Schneider.
Graphene is a surface material that consists of a single layer of carbon atoms. It is extremely thin, strong and flexible. In addition, graphene is wanted in the technological world for its effective conduction of electricity. The applications of graphene vary widely. ‘Graphene is particularly sensitive and can respond to its environment in the body’, says Schneider. Therefore, future applications for the body are for example biosensors and systems that allocate the right spot for performing diagnosis.
Bonding with graphene
To make graphene suitable for these applications, hard inorganic materials are often used as a support. However, these hard materials are not ideal for the use of graphene in the body. For this reason scientists are looking for soft, organic molecules to bind with graphene, in this case lipids.
Lipids on graphene
Lipids are fats that can be found in the protective layer of a cell – the cell membrane. This membrane consists of a double layer of lipids. When graphene could be placed between these two layers, it could travel through the body freely. ‘A method that is already used with cancer medicines,’ explains Schneider. ‘We made a single layer of lipids in the lab and transferred graphene on top: a first step towards mimicking the cell membrane.’
In their research the scientists discovered that a layer of lipids provides good support to graphene. The researchers used infrared measurements to prove the stability of the lipid layer. They also found that the lipids improve the electrical conduction of graphene. This effect of lipids is promising for future applications. Improvements of electrical conduction make it possible to measure the electrical signals of graphene in the body. These signals tell something about the environment of graphene, like the acidity or the presence of certain proteins.
Eventually graphene could travel through the body when it is stabilised by lipids. ‘However, we still have a long way to go’, says Schneider. ‘The next step is to place a lipid layer on both sides of graphene, like a sandwich.’
Learn more: Travelling through the body with graphene
New analysis finds way to safely conduct heat from graphene to biological tissues
In the future, our health may be monitored and maintained by tiny sensors and drug dispensers, deployed within the body and made from graphene — one of the strongest, lightest materials in the world. Graphene is composed of a single sheet of carbon atoms, linked together like razor-thin chicken wire, and its properties may be tuned in countless ways, making it a versatile material for tiny, next-generation implants.
But graphene is incredibly stiff, whereas biological tissue is soft. Because of this, any power applied to operate a graphene implant could precipitously heat up and fry surrounding cells.
Now, engineers from MIT and Tsinghua University in Beijing have precisely simulated how electrical power may generate heat between a single layer of graphene and a simple cell membrane. While direct contact between the two layers inevitably overheats and kills the cell, the researchers found they could prevent this effect with a very thin, in-between layer of water.
By tuning the thickness of this intermediate water layer, the researchers could carefully control the amount of heat transferred between graphene and biological tissue. They also identified the critical power to apply to the graphene layer, without frying the cell membrane. The results are published today in the journal Nature Communications.
Co-author Zhao Qin, a research scientist in MIT’s Department of Civil and Environmental Engineering (CEE), says the team’s simulations may help guide the development of graphene implants and their optimal power requirements.
“We’ve provided a lot of insight, like what’s the critical power we can accept that will not fry the cell,” Qin says. “But sometimes we might want to intentionally increase the temperature, because for some biomedical applications, we want to kill cells like cancer cells. This work can also be used as guidance [for those efforts.]”
Qin’s co-authors include Markus Buehler, head of CEE and the McAfee Professor of Engineering, along with Yanlei Wang and Zhiping Xu of Tsinghua University.
Typically, heat travels between two materials via vibrations in each material’s atoms. These atoms are always vibrating, at frequencies that depend on the properties of their materials. As a surface heats up, its atoms vibrate even more, causing collisions with other atoms and transferring heat in the process.
The researchers sought to accurately characterize the way heat travels, at the level of individual atoms, between graphene and biological tissue. To do this, they considered the simplest interface, comprising a small, 500-nanometer-square sheet of graphene and a simple cell membrane, separated by a thin layer of water.
“In the body, water is everywhere, and the outer surface of membranes will always like to interact with water, so you cannot totally remove it,” Qin says. “So we came up with a sandwich model for graphene, water, and membrane, that is a crystal clear system for seeing the thermal conductance between these two materials.”
Qin’s colleagues at Tsinghua University had previously developed a model to precisely simulate the interactions between atoms in graphene and water, using density functional theory — a computational modeling technique that considers the structure of an atom’s electrons in determining how that atom will interact with other atoms.
However, to apply this modeling technique to the group’s sandwich model, which comprised about half a million atoms, would have required an incredible amount of computational power. Instead, Qin and his colleagues used classical molecular dynamics — a mathematical technique based on a “force field” potential function, or a simplified version of the interactions between atoms — that enabled them to efficiently calculate interactions within larger atomic systems.
The researchers then built an atom-level sandwich model of graphene, water, and a cell membrane, based on the group’s simplified force field. They carried out molecular dynamics simulations in which they changed the amount of power applied to the graphene, as well as the thickness of the intermediate water layer, and observed the amount of heat that carried over from the graphene to the cell membrane.
Because the stiffness of graphene and biological tissue is so different, Qin and his colleagues expected that heat would conduct rather poorly between the two materials, building up steeply in the graphene before flooding and overheating the cell membrane. However, the intermediate water layer helped dissipate this heat, easing its conduction and preventing a temperature spike in the cell membrane.
Looking more closely at the interactions within this interface, the researchers made a surprising discovery: Within the sandwich model, the water, pressed against graphene’s chicken-wire pattern, morphed into a similar crystal-like structure.
“Graphene’s lattice acts like a template to guide the water to form network structures,” Qin explains. “The water acts more like a solid material and makes the stiffness transition from graphene and membrane less abrupt. We think this helps heat to conduct from graphene to the membrane side.”
The group varied the thickness of the intermediate water layer in simulations, and found that a 1-nanometer-wide layer of water helped to dissipate heat very effectively. In terms of the power applied to the system, they calculated that about a megawatt of power per meter squared, applied in tiny, microsecond bursts, was the most power that could be applied to the interface without overheating the cell membrane.
Qin says going forward, implant designers can use the group’s model and simulations to determine the critical power requirements for graphene devices of different dimensions. As for how they might practically control the thickness of the intermediate water layer, he says graphene’s surface may be modified to attract a particular number of water molecules.
“I think graphene provides a very promising candidate for implantable devices,” Qin says. “Our calculations can provide knowledge for designing these devices in the future, for specific applications, like sensors, monitors, and other biomedical applications.”
Materials researchers at North Carolina State University have developed a technique that allows them to integrate graphene, graphene oxide (GO) and reduced graphene oxide (rGO) onto silicon substrates at room temperature by using nanosecond pulsed laser annealing. The advance raises the possibility of creating new electronic devices, and the researchers are already planning to use the technique to create smart biomedical sensors.
In the new technique, researchers start with a silicon substrate. They top that with a layer of single-crystal titanium nitride, using domain matching epitaxy to ensure the crystalline structure of the titanium nitride is aligned with the structure of the silicon. Researchers then place a layer of copper-carbon (Cu-2.0atomic percent C) alloy on top of the titanium nitride, again using domain matching epitaxy. Finally, the researchers melt the surface of the alloy with nanosecond laser pulses, which pulls carbon to the surface.
If the process is done in a vacuum, the carbon forms on the surface as graphene; if it is done in oxygen, it forms GO; and if done in a humid atmosphere followed by a vacuum, it forms as rGO. In all three cases, the carbon’s crystalline structure is aligned with the underlying copper-carbon alloy.
“We can control whether the carbon forms one or two monolayers on the surface of the material by manipulating the intensity of the laser and the depth of the melting,” says Jay Narayan, the John C. Fan Distinguished Chair Professor of Materials Science and Engineering at NC State and senior author of a paper describing the work.
“The process can easily be scaled up,” Narayan says. “We’ve made wafers that are two inches square, and could easily make them much larger, using lasers with higher Hertz. And this is all done at room temperature, which drives down the cost.”
Graphene is an excellent conductor, but it cannot be used as a semiconductor. However, rGO is a semiconductor material, which can be used to make electronic devices such as integrated smart sensors and optic-electronic devices.
“We have already patented the technique and are planning to use it to develop smart biomedical sensors integrated with computer chips,” Narayan says.
A device made of bilayer graphene, an atomically thin hexagonal arrangement of carbon atoms, provides experimental proof of the ability to control the momentum of electrons and offers a path to electronics that could require less energy and give off less heat than standard silicon-based transistors. It is one step forward in a new field of physics called valleytronics.
“Current silicon-based transistor devices rely on the charge of electrons to turn the device on or off, but many labs are looking at new ways to manipulate electrons based on other variables, called degrees of freedom,” said Jun Zhu, associate professor of physics, Penn State, who directed the research. “Charge is one degree of freedom. Electron spin is another, and the ability to build transistors based on spin, called spintronics, is still in the development stage. A third electronic degree of freedom is the valley state of electrons, which is based on their energy in relation to their momentum.”
Think of electrons as cars and the valley states as blue and red colors, Zhu suggested, just as a way to differentiate them. Inside a sheet of bilayer graphene, electrons will normally occupy both red and blue valley states and travel in all directions. The device her Ph.D. student, Jing Li, has been working on can make the red cars go in one direction and the blue cars in the opposite direction.
“The system that Jing created puts a pair of gates above and below a bilayer graphene sheet. Then he adds an electric field perpendicular to the plane,” Zhu said.
“By applying a positive voltage on one side and a negative voltage on the other, a bandgap opens in bilayer graphene, which it doesn’t normally have,” Li explained. “In the middle, between the two sides, we leave a physical gap of about 70 nanometers.”
Inside this gap live one-dimensional metallic states, or wires, that are color-coded freeways for electrons. The red cars travel in one direction and the blue cars travel in the opposite direction. In theory, colored electrons could travel unhindered along the wires for a long distance with very little resistance. Smaller resistance means power consumption is lower in electronic devices and less heat is generated. Both power consumption and thermal management are challenges in current miniaturized devices.
“Our experiments show that the metallic wires can be created,” Li said. “Although we are still a long way from applications.”
Zhu added, “It’s quite remarkable that such states can be created in the interior of an insulating bilayer graphene sheet, using just a few gates. They are not yet resistance-free, and we are doing more experiments to understand where resistance might come from. We are also trying to build valves that control the electron flow based on the color of the electrons. That’s a new concept of electronics called valleytronics.”
Li worked closely with the technical staff of Penn State’s nanofabrication facility to turn the theoretical framework into a working device.
“The alignment of the top and bottom gates was crucial and not a trivial challenge,” said Chad Eichfeld, nanolithography engineer. “The state-of-the-art electron beam lithography capabilities at the Penn StateNanofabrication Laboratory allowed Jing to create this novel device with nanoscale features.”
FAU researchers make key break-through
Graphene is one of the most promising new materials. However, researchers across the globe are still looking for a way to produce defect-free graphene at low costs. Chemists at FAU have now succeeded in producing defect-free graphene directly from graphite for the first time. They recently published their findings in the journal Nature Communications (DOI: 10.1038/ncomms12411).
Graphene is two dimensional and consists of a single layer of carbon atoms. It is particularly good at conducting electricity and heat, transparent and flexible yet strong. Graphene’s unique properties make it suitable for use in a wide range of pioneering technologies, such as in transparent electrodes for flexible displays.
However, the semi-conductor industry will only be able to use graphene successfully once properties such as the size, area and number of defects – which influence its conductivity – can be improved during synthesis. A team of FAU researchers led by Dr. Andreas Hirsch from the Chair of Organic Chemistry II has recently made a crucial break-through in this area. With the help of the additive benzonitrile, they have found a way of producing defect-free graphene directly from a solution. Their method enables the graphene – which is of a higher quality than ever achieved before – to be cut without causing defects and also allows specific electronic properties to be set through the number of charge carriers. Furthermore, their technique is both low-cost and efficient.
A common way of synthesising graphene is through chemical exfoliation of graphite. In this process, metal ions are embedded in graphite, which is made of carbon, resulting in what is known as an intercalation compound. The individual layers of carbon – the graphene – are separated using solvents. The stabilised graphene then has to be separated from the solvent and reoxidised. However, defects in the individual layers of carbon, such as hydration and oxidation of carbon atoms in the lattice, can occur during this process. FAU researchers have now found a solution to this problem. By adding the solvent benzonitrile, the graphene can be removed without any additional functional groups forming – and it remains defect-free.
‘This discovery is a break-through for experts in the international field of reductive graphene synthesis,’ Professor Hirsch explains. ‘Based on this discovery we can expect to see major advancements in terms of the applications of this type of graphene which is produced using wet chemical exfoliation. An example could be cutting defect-free graphene for semi-conductor or sensor technology.’
The method devised by FAU researchers has another advantage: the reduced benzonitrile molecule formed during the reaction turns red as long as it does not come into contact with oxygen or water. This change in colour allows the number of charge carriers in the system to be determined easily through absorption measurements. This could previously only be done by measuring voltage and means that graphene and battery researchers now have a new way of measuring the charge state.
Learn more: Low-cost and defect-free graphene