It’s time for an update on graphene, that super material of the future! Scientists have come up with some new ways of making it that are easier and cheaper than ever before.
“Fascination with this material stems from its remarkable physical properties and the potential applications these properties offer for the future. Although scientists knew one atom thick, two-dimensional crystal graphene existed, no-one had worked out how to extract it from graphite.”
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Scientists cook up material 200 times stronger than steel out of soybean oil
“Many production techniques involve the use of intense heat in a vacuum, and expensive ingredients like high-purity metals and explosive compressed gases. Now a team of Australian scientists has detailed how they turned cheap everyday ingredients into graphene under normal air conditions. They said the research, published today in the journal Nature Communications, may open up a new avenue for the low-cost synthesis of the highly sought-after material.” Click on the Link below to read more:
In 2016, annual global semiconductor sales reached their highest-ever point, at $339 billion worldwide. In that same year, the semiconductor industry spent about $7.2 billion worldwide on wafers that serve as the substrates for microelectronics components, which can be turned into transistors, light-emitting diodes, and other electronic and photonic devices.
A new techniquedeveloped by MIT engineers may vastly reduce the overall cost of wafer technology and enable devices made from more exotic, higher-performingsemiconductormaterials than conventional silicon.
The new method, reported today in Nature, uses graphene—single-atom-thin sheets of graphite—as a sort of “copy machine” to transfer intricate crystalline patterns from an underlying semiconductor wafer to a top layer of identical material.
The engineers worked out carefully controlled procedures to place single sheets of graphene onto an expensive wafer. They then grew semiconducting material over the graphene layer. They found that graphene is thin enough to appear electrically invisible, allowing the top layer to see through the graphene to the underlying crystalline wafer, imprinting its patterns without being influenced by the graphene.
Graphene is also rather “slippery” and does not tend to stick to other materials easily, enabling the engineers to simply peel the top semiconducting layer from the wafer after its structures have been imprinted.
Jeehwan Kim, the Class of 1947 Career Development Assistant Professor in the departments of Mechanical Engineering and Materials Science and Engineering, says that in conventional semiconductor manufacturing, the wafer, once its crystalline pattern is transferred, is so strongly bonded to the semiconductor that it is almost impossible to separate without damaging both layers.
“You end up having to sacrifice the wafer—it becomes part of the device,” Kim says.
With the group’s new technique, Kim says manufacturers can now use graphene as an intermediate layer, allowing them to copy and paste the wafer, separate a copied film from the wafer, and reuse the wafer many times over. In addition to saving on the cost of wafers, Kim says this opens opportunities for exploring more exotic semiconductor materials.
“The industry has been stuck on silicon, and even though we’ve known about better performing semiconductors, we haven’t been able to use them, because of their cost,” Kim says. “This gives the industry freedom in choosing semiconductor materials by performance and not cost.”
Kim’s research team discovered this new technique at MIT’s Research Laboratory of Electronics. Kim’s MIT co-authors are first author and graduate student Yunjo Kim; graduate students Samuel Cruz, Babatunde Alawonde, Chris Heidelberger, Yi Song, and Kuan Qiao; postdocs Kyusang Lee, Shinhyun Choi, and Wei Kong; visiting research scholar Chanyeol Choi; Merton C. Flemings-SMA Professor of Materials Science and Engineering Eugene Fitzgerald; professor of electrical engineering and computer science Jing Kong; and assistant professor of mechanical engineering Alexie Kolpak; along with Jared Johnson and Jinwoo Hwang from Ohio State University, and Ibraheem Almansouri of Masdar Institute of Science and Technology.
Since graphene’s discovery in 2004, researchers have been investigating its exceptional electrical properties in hopes of improving the performance and cost of electronic devices. Graphene is an extremely good conductor of electricity, as electrons flow through graphene with virtually no friction. Researchers, therefore, have been intent on finding ways to adapt graphene as a cheap, high-performance semiconducting material.
“People were so hopeful that we might make really fast electronic devices from graphene,” Kim says. “But it turns out it’s really hard to make a good graphene transistor.”
In order for a transistor to work, it must be able to turn a flow of electrons on and off, to generate a pattern of ones and zeros, instructing a device on how to carry out a set of computations. As it happens, it is very hard to stop the flow of electrons through graphene, making it an excellent conductor but a poor semiconductor.
Kim’s group took an entirely new approach to using graphene in semiconductors. Instead of focusing on graphene’s electrical properties, the researchers looked at the material’s mechanical features.
“We’ve had a strong belief in graphene, because it is a very robust, ultrathin, material and forms very strong covalent bonding between its atoms in the horizontal direction,” Kim says. “Interestingly, it has very weak Van der Waals forces, meaning it doesn’t react with anything vertically, which makes graphene’s surface very slippery.”
Copy and peel
The team now reports that graphene, with its ultrathin, Teflon-like properties, can be sandwiched between a wafer and its semiconducting layer, providing a barely perceptible, nonstick surface through which the semiconducting material’s atoms can still rearrange in the pattern of the wafer’s crystals. The material, once imprinted, can simply be peeled off from the graphene surface, allowing manufacturers to reuse the original wafer.
The team found that its technique, which they term “remote epitaxy,” was successful in copying and peeling off layers of semiconductors from the same semiconductor wafers. The researchers had success in applying their technique to exotic wafer and semiconducting materials, including indium phosphide, gallium arsenenide, and gallium phosphide—materials that are 50 to 100 times more expensive than silicon.
Kim says that this new technique makes it possible for manufacturers to reuse wafers—of silicon and higher-performing materials—”conceptually, ad infinitum.”
An exotic future
The group’s graphene-based peel-off technique may also advance the field of flexible electronics. In general, wafers are very rigid, making the devices they are fused to similarly inflexible. Kim says now, semiconductor devices such as LEDs and solar cells can be made to bend and twist. In fact, the group demonstrated this possibility by fabricating a flexible LED display, patterned in the MIT logo, using their technique.
“Let’s say you want to install solar cells on your car, which is not completely flat—the body has curves,” Kim says. “Can you coat your semiconductor on top of it? It’s impossible now, because it sticks to the thick wafer. Now, we can peel off, bend, and you can do conformal coating on cars, and even clothing.”
Going forward, the researchers plan to design a reusable “mother wafer” with regions made from different exotic materials. Using graphene as an intermediary, they hope to create multifunctional, high-performance devices. They are also investigating mixing and matching various semiconductors and stacking them up as a multimaterial structure.
“Now, exotic materials can be popular to use,” Kim says. “You don’t have to worry about the cost of the wafer. Let us give you the copy machine. You can grow your semiconductor device, peel it off, and reuse the wafer.”
” … this new discovery makes it possible to mass-produce graphene and graphene membranes to improve a host of products, from fuel cells to solar cells to supercapacitors and sensors.”
University of Arkansas researchers have discovered a simple and scalable method for turning graphene oxide into a non-flammable and paper-like graphene membrane that can be used in large-scale production.
“Due to their mechanical strength and excellent charge and heat conductivities, graphene-based materials have generated enormous excitement,” said Ryan Tian, associate professor of inorganic chemistry in the J. William Fulbright College of Artsand Sciences. “But high flammability jeopardizes the material’s promise for large-scale manufacturing and wide applications.”
Graphene’s extremely high flammability has been an obstacle to further development and commercialization. However, this new discovery makes it possible to mass-produce graphene and graphene membranes to improve a host of products, from fuel cells to solar cells to supercapacitors and sensors. Tian has a provisional patent for this new discovery.
Using metal ions with three or more positive charges, researchers in Tian’s laboratory bonded graphene-oxide flakes into a transparent membrane. This new form of carbon-polymer sheet is flexible, nontoxic and mechanically strong, in addition to being non-flammable.
Further testing of the material suggested that crosslinking, or bonding, using transition metals and rare-earth metals, caused the graphene oxide to possess new semiconducting, magnetic and optical properties.
For the past decade, scientists have focused on graphene, a two-dimensional material that is a single atom in thickness, because it is one of the strongest, lightest and most conductive materials known. For these reasons, graphene and similar two-dimensional materials hold great potential to substitute for traditional semiconductors. Graphene oxide is a common intermediate for graphene and graphene-derived materials made from graphite, which is a crystalline form of carbon.
The researchers’ findings were published in The Journal of Physical Chemistry C.
More information: Hulusi Turgut et al. Multivalent Cation Cross-Linking Suppresses Highly Energetic Graphene Oxide’s Flammability, The Journal of Physical Chemistry C (2017). DOI: 10.1021/acs.jpcc.6b13043
In October 2004, University of Manchester’s Andre Geim, along with his colleague Kostya Novoselov, discovered published their discovery that when a block of graphite is broken down to just 10 or 100 layers thick, a material known as graphene emerges1.
With substantial material properties involving its superior strength as well as both heat and electricity conductibility, while remaining such a thin material, graphene has become one of the most studied materials to date.
While graphene is most often employed in disciplines such as bioengineering, composite materials, energy technology and nanotechnology, its ability to be interjected with other elements allows for its applications to be limitless.
One of the most pressing challenges that the graphene industry faces is a lack of pure production of the material. A recent research report conducted by the Centre for Advanced 2 D Materials (CA2DM) at the National University of Singapore has found that most graphene production companies generate a material that is comprised of a graphene content of only 2-10%2.
Canadian based company Elcora Advanced Materials Corporation has become one of the leading graphene producers in the world, while also maintaining products comprised of 55% graphene content. As its unique designed processing technology not only works towards achieving the purest form of graphene possible, Elcora ensures the cost effective production of graphene from natural graphite in a green and efficient manner.
By minimizing the need for using harsh chemicals, while also eliminating the environmentally damaging byproducts or waste that often follow graphene production, Elcora is one of the most environmentally safe graphene plants available today3.
After acquiring control of the Ragedera graphite mine located in Colombo, Sri Lanka, Elcora Advanced Material has been able to successfully produce an estimated 18,000 tonnes of high quality graphite per year.
With production bases located in both South Wales and Seoul, Korea, Haydale GrapheneIndustries is one of the numerous companies working towards enhancing the carbon fiber composites for specific aerospace and automotive needs. In doing so, research conducted by both Haydale and scientists from the School of Engineering at Cardiff University have investigated how the addition of graphene nanoplaatelets (GP) and carbon nanotubes (CNT) into the composites can allow for reinforcing benefits of the technology.
These benefits include an increased resistance and tolerance to damage of the vehicle, while also showing a 13% increase in the compression strength following impact performance studies. By positively influencing composites such as aircraft wings and automobile parts, Haydale has improved these important properties that are required for maintaining such high performance structures4.
In addition to achieving such impressive material improvements, the techniques employed during this process were performed in a much more cost effective, green and efficient manner. The process employed in the development of this composite involved treating the surface of the nanomaterials with Haydale’s low temperature and low energy HDPlas ® plasma process4.
This plasma functionalization process not only produces high integrity materials, but also avoids the typical waste production associated with functionalization processes while simultaneously promoting homogenous dispersion and chemical bonding. Haydale researchers are hopeful that this newly developed material can allow for lighter and stronger wings to be implemented into aircraft deisgns that can simultaneously reduce the amount of carbon dioxide emissions released by these aircrafts.
Rahul Nair from the University of Manchester in the United Kingdom has recently developed a method involving the use of graphene oxide in order to effectively desalinate water. Considered to be the oxidized form of graphene, graphene oxide membranes have recently emerged as an excellent membrane material that is capable of separating multiple different types of molecules and ions present in an aqueous solution5.
The sieving potential of graphene oxide membranes has been successful in removing small nanoparticles, organic molecules and large salts from solution; however, their ability to filter out common salts has not been documented until now. Previous attempts at employing graphene oxide membranes in the filtration of smaller salts in water have caused the membranes to expand and prevent the flow of water from entering the pores of the membrane.
By placing walls composed of a substance known as epoxy resin that is typically used in glues and coatings on either side of the graphene oxide membrane, the team of researchers led by Dr. Nair was able to successfully prevent the swelling of the membranes upon its immersion in water6.
With a uniform pore size within the membrane of only 0.9 nm in width, this highly selective graphene oxide membrane has several advantages as compared to its bulk counterpart, graphene7. As a much more inexpensive option coupled with a long operational lifetime, graphene oxide membranes have a spectacular separation potential that could have a significant impact in a wide variety of energy reduction and environmental conservation industries around the world.
Outside of its potential for water purification purposes, researchers believe that this technology could also provide a useful addition in the dehydration and purification of biofuels. In most biofuel processes, water is formed as a byproduct, and its presence in the biofuel can affect the final product in a detrimental way. Therefore, the hope is that the application of graphene oxide membranes in this industrial process could have an advantageous use.
Similarly, graphene oxide membranes have a well-documented gas separation ability that prevents any vapor molecules from passing through the membrane. In one of the first studies illustrating this property, researchers measured the loss of weight within a containing initially filled with alcohol before and after it was sealed with a graphene oxide membrane8.
Following the membrane sealing, researchers found that no noticeable variation in the weight of or the pressure within the container was detected. As a result of this remarkable gas separation property, researchers are hopeful the use of graphene oxide membranes can be applied to the controlling of greenhouse gas emissions, as well as the purification of hydrogen-related clean energy gases, in future real world applications.
The University of Cambridge has recently developed a highly conductive ink known as ‘Graphene – IPA Ink.” Composed of powdered graphite dissolved in alcohol, this ink has the potential to be used in inkjet printers that print electrical circuits onto paper.
By forcing the ink through a micrometer-scale capillary at an extremely high pressure, the resulting product is a smooth and conductive material9. Researchers are hopeful that devices such as Radio Frequency Identification (RFID) antennas, passports, electronic tags, and similar everyday items can be printed at a much cheaper rate with the application of electronic circuits printed using this graphene ink.
While graphene may appear to be a single product, it has developed into several different types of applications in its short 13-year live span since its first entrance into the scientific world. Its wide range of uses allow for this material to have a promising future, in which its varying and impressive properties of transparency, strength and conductivity can improve almost every industry of the world.
The world of two-dimensional materials, like graphene, have allowed for researchers to manipulate different geometries and combinations of these compounds to create wonderful new products of the future. As research and development projects continue to work on graphene and its numerous applied products, new two-dimensional materials continue to be discovered each day in continuance of this revolutionary pathway that has set by graphene.
Researchers at MIT have found a way to make graphene with fewer wrinkles, and to iron out the wrinkles that do appear. They found each wafer exhibited uniform performance, meaning that electrons flowed freely across each wafer, at similar speeds, even across previously wrinkled regions.
New technique produces highly conductive graphene wafers.
” … electrons can blitz through graphene at velocities approaching the speed of light, far faster than they can travel through silicon and other semiconducting materials.
” … Graphene, therefore, has been touted as a promising successor to silicon, with the potential to enable faster, more efficient electronic and photonic devices.”
” … pristine graphene — a single, perfectly flat, ultrathin sheet of carbon atoms, precisely aligned and linked together like chickenwire — is extremely difficult.”
” … which can derail an electron’s bullet-train journey, significantly limiting graphene’s electrical performance.”
From an electron’s point of view, graphene must be a hair-raising thrill ride. For years, scientists have observed that electrons can blitz through graphene at velocities approaching the speed of light, far faster than they can travel through silicon and other semiconducting materials.
Graphene, therefore, has been touted as a promising successor to silicon, with the potential to enable faster, more efficient electronic and photonic devices.
But manufacturing pristine graphene — a single, perfectly flat, ultrathin sheet of carbon atoms, precisely aligned and linked together like chickenwire — is extremely difficult. Conventional fabrication processes often generate wrinkles, which can derail an electron’s bullet-train journey, significantly limiting graphene’s electrical performance.
Now engineers at MIT have found a way to make graphene with fewer wrinkles, and to iron out the wrinkles that do appear. After fabricating and then flattening out the graphene, the researchers tested its electrical conductivity. They found each wafer exhibited uniform performance, meaning that electrons flowed freely across each wafer, at similar speeds, even across previously wrinkled regions.
In a paper published today in the Proceedings of the National Academy of Sciences, the researchers report that their techniques successfully produce wafer-scale, “single-domain” graphene — single layers of graphene that are uniform in both atomic arrangement and electronic performance.
“For graphene to play as a main semiconductor material for industry, it has to be single-domain, so that if you make millions of devices on it, the performance of the devices is the same in any location,” says Jeehwan Kim, the Class of 1947 Career Development Assistant Professor in the departments of Mechanical Engineering and Materials Science and Engineering at MIT. “Now we can really produce single-domain graphene at wafer scale.”
Kim’s co-authors include Sanghoon Bae, Samuel Cruz, and Yunjo Kim from MIT, along with researchers from IBM, the University of California at Los Angeles, and Kyungpook National University in South Korea.
A patchwork of wrinkles
The most common way to make graphene involves chemical vapor deposition, or CVD, a process in which carbon atoms are deposited onto a crystalline substrate such as copper foil. Once the copper foil is evenly coated with a single layer of carbon atoms, scientists submerge the entire thing in acid to etch away the copper. What remains is a single sheet of graphene, which researchers then pull out from the acid.
The CVD process can produce relatively large, macroscropic wrinkles in graphene, due to the roughness of the underlying copper itself and the process of pulling the graphene out from the acid. The alignment of carbon atoms is not uniform across the graphene, creating a “polycrystalline” state in which graphene resembles an uneven, patchwork terrain, preventing electrons from flowing at uniform rates.
In 2013, while working at IBM, Kim and his colleagues developed a method to fabricate wafers of single-crystalline graphene, in which the orientation of carbon atoms is exactly the same throughout a wafer.
Rather than using CVD, his team produced single-crystalline graphene from a silicon carbide wafer with an atomically smooth surface, albeit with tiny, step-like wrinkles on the order of several nanometers. They then used a thin sheet of nickel to peel off the topmost graphene from the silicon carbide wafer, in a process called layer-resolved graphene transfer.
In their new paper, Kim and his colleagues discovered that the layer-resolved graphene transfer irons out the steps and tiny wrinkles in silicon carbide-fabricated graphene. Before transferring the layer of graphene onto a silicon wafer, the team oxidized the silicon, creating a layer of silicon dioxide that naturally exhibits electrostatic charges. When the researchers then deposited the graphene, the silicon dioxide effectively pulled graphene’s carbon atoms down onto the wafer, flattening out its steps and wrinkles.
Kim says this ironing method would not work on CVD-fabricated graphene, as the wrinkles generated through CVD are much larger, on the order of several microns.
“The CVD process creates wrinkles that are too high to be ironed out,” Kim notes. “For silicon carbide graphene, the wrinkles are just a few nanometers high, short enough to be flattened out.”
To test whether the flattened, single-crystalline graphene wafers were single-domain, the researchers fabricated tiny transistors on multiple sites on each wafer, including across previously wrinkled regions.
“We measured electron mobility throughout the wafers, and their performance was comparable,” Kim says. “What’s more, this mobility in ironed graphene is two times faster. So now we really have single-domain graphene, and its electrical quality is much higher [than graphene-attached silicon carbide].”
Kim says that while there are still challenges to adapting graphene for use in electronics, the group’s results give researchers a blueprint for how to reliably manufacture pristine, single-domain, wrinkle-free graphene at wafer scale.
“If you want to make any electronic device using graphene, you need to work with single-domain graphene,” Kim says. “There’s still a long way to go to make an operational transistor out of graphene. But we can now show the community guidelines for how you can make single-crystalline, single-domain graphene.”
“Using electrons like photons could provide the basis for a new electronic device that could carry current with virtually no resistance, even at room temperature.”
A new way to grow narrow ribbons of graphene, a lightweight and strong structure of single-atom-thick carbon atoms linked into hexagons, may address a shortcoming that has prevented the material from achieving its full potential in electronic applications.
Graphene nanoribbons, mere billionths of a meter wide, exhibit different electronic properties than two-dimensional sheets of the material. “Confinement changes graphene’s behavior,” said An-Ping Li, a physicist at the Department of Energy’s Oak Ridge National Laboratory. Graphene in sheets is an excellent electrical conductor, but narrowing graphene can turn the material into a semiconductor if the ribbons are made with a specific edge shape.
Previous efforts to make graphene nanoribbons employed a metal substrate that hindered the ribbons’ useful electronic properties. Now, scientists at ORNL and North Carolina State University report in the journal Nature Communications that they are the first to grow graphene nanoribbons without a metal substrate. Instead, they injected charge carriers that promote a chemical reaction that converts a polymer precursor into a graphene nanoribbon.
At selected sites, this new technique can create interfaces between materials with different electronic properties. Such interfaces are the basis of semiconductor electronic devices from integrated circuits and transistors to light-emitting diodes and solar cells. “Graphene is wonderful, but it has limits,” said Li.
“In wide sheets, it doesn’t have an energy gap–an energy range in a solid where no electronic states can exist. That means you cannot turn it on or off.” When a voltage is applied to a sheet of graphene in a device, electrons flow freely as they do in metals, severely limiting graphene’s application in digital electronics.
“When graphene becomes very narrow, it creates an energy gap,” Li said. “The narrower the ribbon is, the wider is the energy gap.” In very narrow graphene nanoribbons, with a width of a nanometer or even less, how structures terminate at the edge of the ribbon is important too. For example, cutting graphene along the side of a hexagon creates an edge that resembles an armchair; this material can act like a semiconductor. Excising triangles from graphene creates a zigzag edge–and a material with metallic behavior.
To grow graphene nanoribbons with controlled width and edge structure from polymer precursors, previous researchers had used a metal substrate to catalyze a chemical reaction. However, the metal substrate suppresses useful edge states and shrinks the desired band gap. Li and colleagues set out to get rid of this troublesome metal substrate. At the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL, they used the tip of a scanning tunneling microscope to inject either negative charge carriers (electrons) or positive charge carriers (“holes”) to try to trigger the key chemical reaction. They discovered that only holes triggered it. They were subsequently able to make a ribbon that was only seven carbon atoms wide–less than one nanometer wide–with edges in the armchair conformation.
“We figured out the fundamental mechanism, that is, how charge injection can lower the reaction barrier to promote this chemical reaction,” Li said. Moving the tip along the polymer chain, the researchers could select where they triggered this reaction and convert one hexagon of the graphene lattice at a time. Next, the researchers will make heterojunctions with different precursor molecules and explore functionalities. They are also eager to see how long electrons can travel in these ribbons before scattering, and will compare it with a graphene nanoribbon made another way and known to conduct electrons extremely well.
Using electrons like photons could provide the basis for a new electronic device that could carry current with virtually no resistance, even at room temperature.
“It’s a way to tailor physical properties for energy applications,” Li said. “This is an excellent example of direct writing. You can direct the transformation process at the molecular or atomic level.” Plus, the process could be scaled up and automated.
Source and top image: Oak Ridge National Laboratory
“By 2025 the UN expects that 14% of the world’s population will encounter water scarcity.”
Graphene-oxide membranes have attracted considerable attention as promising candidates for new filtration technologies. Now the much sought-after development of making membranes capable of sieving common salts has been achieved.
New research demonstrates the real-world potential of providing clean drinking water for millions of people who struggle to access adequate clean water sources.
The new findings from a group of scientists at The University of Manchester were published today in the journal Nature Nanotechnology. Previously graphene-oxide membranes have shown exciting potential for gas separation and water filtration.
Graphene-oxide membranes developed at the National Graphene Institute have already demonstrated the potential of filtering out small nanoparticles, organic molecules, and even large salts. Until now, however, they couldn’t be used for sieving common salts used in desalination technologies, which require even smaller sieves.
Previous research at The University of Manchester found that if immersed in water, graphene-oxide membranes become slightly swollen and smaller salts flow through the membrane along with water, but larger ions or molecules are blocked.
The Manchester-based group have now further developed these graphene membranes and found a strategy to avoid the swelling of the membrane when exposed to water. The pore size in the membrane can be precisely controlled which can sieve common salts out of salty water and make it safe to drink.
As the effects of climate change continue to reduce modern city’s water supplies, wealthy modern countries are also investing in desalination technologies. Following the severe floods in California major wealthy cities are also looking increasingly to alternative water solutions.
When the common salts are dissolved in water, they always form a ‘shell’ of water molecules around the salts molecules. This allows the tiny capillaries of the graphene-oxide membranes to block the salt from flowing along with the water. Water molecules are able to pass through the membrane barrier and flow anomalously fast which is ideal for application of these membranes for desalination.
Professor Rahul Nair, at The University of Manchester said: “Realisation of scalable membranes with uniform pore size down to atomic scale is a significant step forward and will open new possibilities for improving the efficiency of desalination technology.
“This is the first clear-cut experiment in this regime. We also demonstrate that there are realistic possibilities to scale up the described approach and mass produce graphene-based membranes with required sieve sizes.”
Mr. Jijo Abraham and Dr. Vasu Siddeswara Kalangi were the joint-lead authors on the research paper: “The developed membranes are not only useful for desalination, but the atomic scale tunability of the pore size also opens new opportunity to fabricate membranes with on-demand filtration capable of filtering out ions according to their sizes.” said Mr. Abraham.
By 2025 the UN expects that 14% of the world’s population will encounter water scarcity. This technology has the potential to revolutionize water filtration across the world, in particular in countries which cannot afford large scale desalination plants.
It is hoped that graphene-oxide membrane systems can be built on smaller scales making this technology accessible to countries which do not have the financial infrastructure to fund large plants without compromising the yield of fresh water produced.
Quantum dots are very small particles that exhibit luminescence and electronic properties different from those of their bulk materials. As a result, they are attractive for use in solar cells, optoelectronics, and quantum computing. Quantum computing involves applying a small voltage to quantum dots to regulate their electron spin state, thus encoding information.
While traditional computing is based on a binary information system, electron spin states in quantum dots can display further degrees of freedom because of the possibility of superposition of both states at the same time. This feature could increase the density of encoded information.
A scanning electron microscope image of a quantum dot experimental setup
A scanning electron microscope image of the quantum dot used in this research. We formed the quantum dot by applying voltage to surface gate electrodes. Electron spin states can be read out by measuring the electric current flowing nearby the dot (white arrow). (Image: Osaka University)
Readout of the electron spin of quantum dots is necessary to realize quantum computing. Single-shot spin readout has been used to detect spin-dependent single-electron tunneling events in real time. The performance of quantum computing could be improved considerably by single-shot readout of multiple spin states.
A Japanese research collaboration based at Osaka University has now achieved the first successful detection of multiple spin states through single-shot readout of three two-electron spin states of a single quantum dot. They reported their findings in Physical Review Letters (“Single-Shot Ternary Readout of Two-Electron Spin States in a Quantum Dot Using Spin Filtering by Quantum Hall Edge States”).
Comparison Between Binary Spin Readout and Ternary Spin Readout
This is a comparison between binary spin readout and ternary spin readout. (Image: Osaka University)
To read out multiple spin states simultaneously, the researchers used a quantum point contact charge sensor positioned near a gallium arsenide quantum dot. The change in current of the charge sensor depended on the spin state of the quantum dot and was used to distinguish between singlet and two types of triplet spin states.
“We obtained single-shot ternary readout of two-electron spin states using edge-state spin filtering and the orbital effect,” study first author Haruki Kiyama says.
That is, the rate of tunneling between the quantum dot and electron reservoir depended on both the spin state of the electrons and the interaction between electron spin and the orbitals of the quantum dot. The team identified one ground state and two excited states in the quantum dot using their setup.
The researchers then used their ternary readout setup to investigate the spin relaxation behavior of the three detected spin states.
“To confirm the validity of our readout system, we measured the spin relaxation of two of the states,” Kiyama explains.
“Measurement of the dynamics between the spin states in a quantum dot is an important application of the ternary spin readout setup.”
Binary Spin Readout
This is the results of binary spin readout using previous and new schemes, and that of ternary spin readout by combining these two binary-readout schemes. (Image: Osaka University)
The spin relaxation times for the quantum dot measured using the ternary readout system agreed with those reported, providing evidence that the system yielded reliable measurements. This ternary readout system can be extended to quantum dots composed of other materials, revealing a new approach to examine the spin dynamics of quantum dots and representing an advance in quantum information processing.
A single cell can contain a wealth of information about the health of an individual. Now, a new method developed at MIT and National Chiao Tung University could make it possible to capture and analyze individual cells from a small sample of blood, potentially leading to very low-cost diagnostic systems that could be used almost anywhere.
The new system, based on specially treated sheets of graphene oxide, could ultimately lead to a variety of simple devices that could be produced for as little as $5 apiece and perform a variety of sensitive diagnostic tests even in places far from typical medical facilities.
The material used in this research is an oxidized version of the two-dimensional form of pure carbon known as graphene, which has been the subject of widespread research for over a decade because of its unique mechanical and electrical characteristics. The key to the new process is heating the graphene oxide at relatively mild temperatures. This low-temperature annealing, as it is known, makes it possible to bond particular compounds to the material’s surface.
These compounds in turn select and bond with specific molecules of interest, including DNA and proteins, or even whole cells. Once captured, those molecules or cells can then be subjected to a variety of tests.
Mild heating of graphene oxide sheets makes it possible to bond particular compounds to the sheets’ surface
Mild heating of graphene oxide sheets makes it possible to bond particular compounds to the sheets’ surface, a new study shows. These compounds in turn select and bond with specific molecules of interest, including DNA and proteins, or even whole cells. In this image the treatedgraphene oxide on the right is nearly twice as efficient at capturing cells as the untreated material on the left. (Image courtesy of the researchers)
The findings are reported in the journal ACS Nano (“Enhanced Cell Capture on Functionalized Graphene Oxide Nanosheets through Oxygen Clustering”).
Other researchers have been trying to develop diagnostic systems using a graphene oxide substrate to capture specific cells or molecules, but these approaches used just the raw, untreated material. Despite a decade of research, other attempts to improve such devices’ efficiency have relied on external modifications, such as surface patterning through lithographic fabrication techniques, or adding microfluidic channels, which add to the cost and complexity. The new finding offers a mass-producible, low-cost approach to achieving such improvements in efficiency.
The heating process changes the material’s surface properties, causing oxygen atoms to cluster together, leaving spaces of bare graphene between them. This makes it relatively easy to attach other chemicals to the surface, which can interact with specific molecules of interest. The new research demonstrates how that basic process could potentially enable a suite of low-cost diagnostic systems, for example for cancer screening or treatment follow-up.
For this proof-of-concept test, the team used molecules that can quickly and efficiently capture specific immune cells that are markers for certain cancers. They were able to demonstrate that their treated graphene oxide surfaces were almost twice as effective at capturing such cells from whole blood, compared to devices fabricated using ordinary, untreated graphene oxide, says Bardhan, the paper’s lead author.
The system has other advantages as well, Bardhan says. It allows for rapid capture and assessment of cells or biomolecules under ambient conditions within about 10 minutes and without the need for refrigeration of samples or incubators for precise temperature control. And the whole system is compatible with existing large-scale manufacturing methods, making it possible to produce diagnostic devices for less than $5 apiece, the team estimates. Such devices could be used in point-of-care testing or resource-constrained settings.
Existing methods for treating graphene oxide to allow functionalization of the surface require high temperature treatments or the use of harsh chemicals, but the new system, which the group has patented, requires no chemical pretreatment and an annealing temperature of just 50 to 80 degrees Celsius (122 to 176 F).
While the team’s basic processing method could make possible a wide variety of applications, including solar cells and light-emitting devices, for this work the researchers focused on improving the efficiency of capturing cells and biomolecules that can then be subjected to a suite of tests. They did this by enzymatically coating the treated graphene oxide surface with peptides called nanobodies — subunits of antibodies, which can be cheaply and easily produced in large quantities in bioreactors and are highly selective for particular biomolecules.
The researchers found that increasing the annealing time steadily increased the efficiency of cell capture: After nine days of annealing, the efficiency of capturing cells from whole blood went from 54 percent, for untreated graphene oxide, to 92 percent for the treated material.
The team then performed molecular dynamics simulations to understand the fundamental changes in the reactivity of the graphene oxide base material. The simulation results, which the team also verified experimentally, suggested that upon annealing, the relative fraction of one type of oxygen (carbonyl) increases at the expense of the other types of oxygen functional groups (epoxy and hydroxyl) as a result of the oxygen clustering. This change makes the material more reactive, which explains the higher density of cell capture agents and increased efficiency of cell capture.
“Efficiency is especially important if you’re trying to detect a rare event,” Belcher says. “The goal of this was to show a high efficiency of capture.” The next step after this basic proof of concept, she says, is to try to make a working detector for a specific disease model.
In principle, Bardhan says, many different tests could be incorporated on a single device, all of which could be placed on a small glass slide like those used for microscopy.
“I think the most interesting aspect of this work is the claimed clustering of oxygen species on graphene sheets and its enhanced performance in surface functionalization and cell capture,” says Younan Xia, a professor of chemistry and biochemistry at Georgia Institute of Technology who was not involved in this work. “It is an interesting idea.”
Nanoclusters of magnesium oxide sandwiched between layers of graphene make a compound with unique electronic and optical properties, according to researchers at Rice University who made computer simulations of the material. Credit: Lei Tao/Rice University
Rice University researchers have modeled a nanoscale sandwich, the first in what they hope will become a molecular deli for materials scientists.
Their recipe puts two slices of atom-thick graphene around nanoclusters of magnesium oxide that give the super-strong, conductive material expanded optoelectronic properties.
Rice materials scientist Rouzbeh Shahsavari and his colleagues built computer simulations of the compound and found it would offer features suitable for sensitive molecular sensing, catalysis and bio-imaging. Their work could help researchers design a range of customizable hybrids of two- and three-dimensional structures with encapsulated molecules, Shahsavari said.
The research appears this month in the Royal Society of Chemistry journal Nanoscale.
The scientists were inspired by experiments elsewhere in which various molecules were encapsulated using van der Waals forces to draw components together. The Rice-led study was the first to take a theoretical approach to defining the electronic and optical properties of one of those “made” samples, two-dimensional magnesium oxide in bilayer graphene, Shahsavari said.
“We knew if there was an experiment already performed, we would have a great reference point that would make it easier to verify our computations, thus allowing more reliable expansion of our computational results to identify performance trends beyond the reach of experiments,” Shahsavari said.
Graphene on its own has no band gap – the characteristic that makes a material a semiconductor. But the hybrid does, and this band gap could be tunable, depending on the components, Shahsavari said. The enhanced optical properties are also tunable and useful, he said.
“We saw that while this single flake of magnesium oxide absorbed one kind of light emission, when it was trapped between two layers of graphene, it absorbed a wide spectrum. That could be an important mechanism for sensors,” he said.
Shahsavari said his group’s theory should be applicable to other two-dimensional materials, like hexagonal boron-nitride, and molecular fillings. “There is no single material that can solve all the technical problems of the world,” he said. “It always comes down to making hybrid materials to synergize the best features of multiple components to do a specific job. My group is working on these hybrid materials by tweaking their components and structures to meet new challenges.”