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
The researchers in Jonathan Claussen’s lab at Iowa State University (who like to call themselves nanoengineers) have been looking for ways to use graphene and its amazing properties in their sensors and other technologies.
Graphene is a wonder material: The carbon honeycomb is just an atom thick. It’s great at conducting electricity and heat; it’s strong and stable. But researchers have struggled to move beyond tiny lab samples for studying its material properties to larger pieces for real-world applications.
Recent projects that used inkjet printers to print multi-layer graphene circuits and electrodes had the engineers thinking about using it for flexible, wearable and low-cost electronics. For example, “Could we make graphene at scales large enough for glucose sensors?” asked Suprem Das, an Iowa State postdoctoral research associate in mechanical engineering and an associate of the U.S. Department of Energy’s Ames Laboratory.
But there were problems with the existing technology. Once printed, the graphene had to be treated to improve electrical conductivity and device performance. That usually meant high temperatures or chemicals – both could degrade flexible or disposable printing surfaces such as plastic films or even paper.
Das and Claussen came up with the idea of using lasers to treat the graphene. Claussen, an Iowa State assistant professor of mechanical engineering and an Ames Laboratory associate, worked with Gary Cheng, an associate professor at Purdue University’s School of Industrial Engineering, to develop and test the idea.
And it worked: They found treating inkjet-printed, multi-layer graphene electric circuits and electrodes with a pulsed-laser process improves electrical conductivity without damaging paper, polymers or other fragile printing surfaces.
“This creates a way to commercialize and scale-up the manufacturing of graphene,” Claussen said.
The findings are featured on the front cover of the journal Nanoscale’s issue 35. Claussen and Cheng are lead authors and Das is first author. Additional Iowa State co-authors are Allison Cargill, John Hondred and Shaowei Ding, graduate students in mechanical engineering. Additional Purdue co-authors are Qiong Nian and Mojib Saei, graduate students in industrial engineering.
Two major grants are supporting the project and related research: a three-year grant from the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 11901762 and a three-year grant from the Roy J. Carver Charitable Trust. Iowa State’s College of Engineering and department of mechanical engineering are also supporting the research.
The Iowa State Research Foundation Inc. has filed for a patent on the technology.
“The breakthrough of this project is transforming the inkjet-printed graphene into a conductive material capable of being used in new applications,” Claussen said.
Those applications could include sensors with biological applications, energy storage systems, electrical conducting components and even paper-based electronics.
To make all that possible, the engineers developed computer-controlled laser technology that selectively irradiates inkjet-printed graphene oxide. The treatment removes ink binders and reduces graphene oxide to graphene – physically stitching together millions of tiny graphene flakes. The process makes electrical conductivity more than a thousand times better.
“The laser works with a rapid pulse of high-energy photons that do not destroy the graphene or the substrate,” Das said. “They heat locally. They bombard locally. They process locally.”
That localized, laser processing also changes the shape and structure of the printed graphene from a flat surface to one with raised, 3-D nanostructures. The engineers say the 3-D structures are like tiny petals rising from the surface. The rough and ridged structure increases the electrochemical reactivity of the graphene, making it useful for chemical and biological sensors.
All of that, according to Claussen’s team of nanoengineers, could move graphene to commercial applications.
“This work paves the way for not only paper-based electronics with graphene circuits,” the researchers wrote in their paper, “it enables the creation of low-cost and disposable graphene-based electrochemical electrodes for myriad applications including sensors, biosensors, fuel cells and (medical) devices.”
Rutgers experts discover easy way to make graphene for flexible and printable electronics, energy storage, and catalysis
Rutgers University engineers have found a simple method for producing high-quality graphene that can be used in next-generation electronic and energy devices: bake the compound in a microwave oven.
The discovery is documented in a study published online today in the journal Science.
“This is a major advance in the graphene field,” said Manish Chhowalla, professor and associate chair in the Department of Materials Science and Engineering in Rutgers’ School of Engineering. “This simple microwave treatment leads to exceptionally high quality graphene with properties approaching those in pristine graphene.”
The discovery was made by post-doctoral associates and undergraduate students in the department, said Chhowalla, who is also the director of the Rutgers Institute for Advanced Materials, Devices and Nanotechnology. Having undergraduates as co-authors of a Science paper is rare but he said “the Rutgers Materials Science and Engineering Department and the School of Engineering at Rutgers cultivate a culture of curiosity driven research in students with fresh ideas who are not afraid to try something new.”
Graphene – 100 times tougher than steel – conducts electricity better than copper and rapidly dissipates heat, making it useful for many applications. Large-scale production of graphene is necessary for applications such as printable electronics, electrodes for batteries and catalysts for fuel cells.
Graphene comes from graphite, a carbon-based material used by generations of students and teachers in the form of pencils. Graphite consists of sheets or layers of graphene.
The easiest way to make large quantities of graphene is to exfoliate graphite into individual graphene sheets by using chemicals. The downside of this approach is that side reactions occur with oxygen – forming graphene oxide that is electrically non-conducting, which makes it less useful for products.
Removing oxygen from graphene oxide to obtain high-quality graphene has been a major challenge over the past two decades for the scientific community working on graphene. Oxygen distorts the pristine atomic structure of graphene and degrades its properties.
Chhowalla and his group members found that baking the exfoliated graphene oxide for just one second in a 1,000-watt microwave oven, like those used in households across America, can eliminate virtually all of the oxygen from graphene oxide.
Small balloons made from one-atom-thick material graphene can withstand enormous pressures, much higher than those at the bottom of the deepest ocean, scientists at The University of Manchester report.
This is due to graphene’s incredible strength – 200 times stronger than steel.
The graphene balloons routinely form when placing graphene on flat substrates and are usually considered a nuisance and therefore ignored. The Manchester researchers, led by Professor Irina Grigorieva, took a closer look at the nano-bubbles and revealed their fascinating properties.
These bubbles could be created intentionally to make tiny pressure machines capable of withstanding enormous pressures. This could be a significant step towards rapidly detecting how molecules react under extreme pressure.
Writing in Nature Communications, the scientists found that the shape and dimensions of the nano-bubbles provide straightforward information about both graphene’s elastic strength and its interaction with the underlying substrate.
The researchers found such balloons can also be created with other two-dimensional crystals such as single layers of molybdenum disulfide (MoS2) or boron nitride.
They were able to directly measure the pressure exerted by graphene on a material trapped inside the balloons, or vice versa.
To do this, the team indented bubbles made by graphene, monolayer MoS2 and monolayer boron nitride using a tip of an atomic force microscope and measured the force that was necessary to make a dent of a certain size.
“One can now start thinking about creating these baloons intentionally to change materials or study the properties of atomically thin membranes under high strain and pressure.”
Sir Andre Geim
These measurements revealed that graphene enclosing bubbles of a micron size creates pressures as high as 200 megapascals, or 2,000 atmospheres. Even higher pressures are expected for smaller bubbles.
Ekaterina Khestanova, a PhD student who carried out the experiments, said: “Such pressures are enough to modify the properties of a material trapped inside the bubbles and, for example, can force crystallization of a liquid well above its normal freezing temperature’.
Electrons reveal their quantum properties when they are confined to small spaces. Scientists from TU Wien (Vienna), Aachen and Manchester have created tiny quantum dots in Graphene
In a tiny quantum prison, electrons behave quite differently as compared to their counterparts in free space. They can only occupy discrete energy levels, much like the electrons in an atom – for this reason, such electron prisons are often called “artificial atoms”. Artificial atoms may also feature properties beyond those of conventional ones, with the potential for many applications for example in quantum computing. Such additional properties have now been shown for artificial atoms in the carbon material graphene. The results have been published in the journal “Nano Letters”, the project was a collaboration of scientists from TU Wien (Vienna, Austria), RWTH Aachen (Germany) and the University of Manchester (GB).
Building Artificial Atoms
“Artificial atoms open up new, exciting possibilities, because we can directly tune their properties”, says Professor Joachim Burgdörfer (TU Wien, Vienna). In semiconductor materials such as gallium arsenide, trapping electrons in tiny confinements has already been shown to be possible. These structures are often referred to as “quantum dots”. Just like in an atom, where the electrons can only circle the nucleus on certain orbits, electrons in these quantum dots are forced into discrete quantum states.
Even more interesting possibilities are opened up by using graphene, a material consisting of a single layer of carbon atoms, which has attracted a lot of attention in the last few years. “In most materials, electrons may occupy two different quantum states at a given energy. The high symmetry of the graphene lattice allows for four different quantum states. This opens up new pathways for quantum information processing and storage” explains Florian Libisch from TU Wien. However, creating well-controlled artificial atoms in graphene turned out to be extremely challenging.
Cutting edge is not enough
There are different ways of creating artificial atoms: The simplest one is putting electrons into tiny flakes, cut out of a thin layer of the material. While this works for graphene, the symmetry of the material is broken by the edges of the flake which can never be perfectly smooth. Consequently, the special four-fold multiplicity of states in graphene is reduced to the conventional two-fold one.
Therefore, different ways had to be found: It is not necessary to use small graphene flakes to capture electrons. Using clever combinations of electrical and magnetic fields is a much better option. With the tip of a scanning tunnelling microscope, an electric field can be applied locally. That way, a tiny region is created within the graphene surface, in which low energy electrons can be trapped. At the same time, the electrons are forced into tiny circular orbits by applying a magnetic field. “If we would only use an electric field, quantum effects allow the electrons to quickly leave the trap” explains Libisch.
The artificial atoms were measured at the RWTH Aachen by Nils Freitag and Peter Nemes-Incze in the group of Professor Markus Morgenstern. Simulations and theoretical models were developed at TU Wien (Vienna) by Larisa Chizhova, Florian Libisch and Joachim Burgdörfer. The exceptionally clean graphene sample came from the team around Andre Geim and Kostya Novoselov from Manchester (GB) – these two researchers were awarded the Nobel Prize in 2010 for creating graphene sheets for the first time.
The new artificial atoms now open up new possibilities for many quantum technological experiments: “Four localized electron states with the same energy allow for switching between different quantum states to store information”, says Joachim Burgdörfer. The electrons can preserve arbitrary superpositions for a long time, ideal properties for quantum computers. In addition, the new method has the big advantage of scalability: it should be possible to fit many such artificial atoms on a small chip in order to use them for quantum information applications.
Learn more: “Artificial Atom“ Created in Graphene