Graphene is increasingly attractive to researchers. If in 2007 there were 797 articles devoted to graphene, then in the first 8 months of 2008 there were already 801 publications. What are the most significant recent studies and discoveries in the field of graphene structures and technologies?

To date, graphene (Fig. 1) is the thinnest material known to mankind, only one carbon atom thick. It entered physics textbooks and our reality in 2004, when researchers from the University of Manchester, Andre Game and Konstantin Novoselov, managed to obtain it using ordinary adhesive tape to sequentially separate layers from ordinary crystalline graphite, familiar to us in the form of a pencil rod (see . Application). Remarkably, a graphene sheet placed on an oxidized silicon substrate can be viewed with a good optical microscope. And this is despite its thickness of only a few angstroms (1Å = 10 -10 m)!

The popularity of graphene among researchers and engineers is growing day by day as it has unusual optical, electrical, mechanical and thermal properties. Many experts predict in the near future a possible replacement of silicon transistors with more economical and high-speed graphene ones (Fig. 2).

Despite the fact that mechanical peeling with adhesive tape makes it possible to obtain high-quality graphene layers for basic research, and the epitaxial method of growing graphene can provide the shortest path to electronic microcircuits, chemists are trying to get graphene from solution. In addition to low cost and high productivity, this method opens the door to many widely used chemical techniques that would allow graphene layers to be embedded in various nanostructures or integrated with various materials to create nanocomposites. However, when obtaining graphene by chemical methods, there are some difficulties that must be overcome: first, it is necessary to achieve complete separation of graphite placed in a solution; secondly, to make sure that the exfoliated graphene in the solution retains the shape of the sheet, and does not curl up and stick together.

The other day in a prestigious magazine Nature two papers by independently working scientific groups were published, in which the authors managed to overcome the above difficulties and obtain good quality graphene sheets suspended in solution.

The first group of scientists - from Stanford University (California, USA) and (China) - introduced sulfuric and nitric acids between layers of graphite (an intercalation process; see Graphite intercalation compound), and then quickly heated the sample to 1000°C (Fig. 3a) . Explosive evaporation of intercalant molecules produces thin (a few nanometers thick) graphite "flakes" that contain many graphene layers. After that, two substances, oleum and tetrabutylammonium hydroxide (HTBA), were chemically introduced into the space between the graphene layers (Fig. 3b). The sonicated solution contained both graphite and graphene sheets (Fig. 3c). After that, graphene was separated by centrifugation (Fig. 3d).

At the same time, the second group of scientists - from Dublin, Oxford and Cambridge - proposed a different method for obtaining graphene from multilayer graphite - without the use of intercalants. The main thing, according to the authors of the article, is to use the "correct" organic solvents, such as N-methyl-pyrrolidone. To obtain high-quality graphene, it is important to choose such solvents so that the energy of the surface interaction between the solvent and graphene is the same as for the graphene–graphene system. On fig. 4 shows the results of the stepwise production of graphene.

The success of both experiments is based on finding the right intercalants and/or solvents. Of course, there are other techniques for obtaining graphene, such as converting graphite to graphite oxide. They use an approach called "oxidation-delamination-reduction" in which graphite basal planes are coated with covalently bonded oxygen functional groups. This oxidized graphite becomes hydrophilic (or simply moisture-loving) and can easily delaminate into individual graphene sheets under the action of ultrasound while in an aqueous solution. The obtained graphene has excellent mechanical and optical characteristics, but its electrical conductivity is several orders of magnitude lower than the conductivity of graphene obtained using the "adhesive tape method" (see Appendix). Accordingly, such graphene is unlikely to find application in electronics.

As it turned out, graphene, which was obtained as a result of the above two methods, is of higher quality (contains fewer defects in the lattice) and, as a result, has higher conductivity.

Another achievement of researchers from California came in handy, who recently reported high-resolution (resolution up to 1Å) low-energy electron microscopy (80 kV) for direct observation of individual atoms and defects in the graphene crystal lattice. For the first time in the world, scientists have managed to obtain high-definition images of the atomic structure of graphene (Fig. 5), where you can see with your own eyes the grid structure of graphene.

Researchers at Cornell University have gone even further. From a sheet of graphene, they managed to create a membrane just one carbon atom thick, and inflate it like a balloon. Such a membrane turned out to be strong enough to withstand a gas pressure of several atmospheres. The experiment was as follows. Graphene sheets were placed on an oxidized silicon substrate with preliminarily etched cells, which were tightly attached to the silicon surface due to van der Waals forces (Fig. 6a). In this way, microchambers were formed in which the gas could be retained. After that, the scientists created a pressure difference inside and outside the chamber (Fig. 6b). Using an atomic force microscope, which measures the amount of deflection force that a cantilever with a needle feels when scanning the membrane at a height of only a few nanometers from its surface, the researchers were able to observe the degree of concavity-concavity of the membrane (Fig. 6c–e) when the pressure changed to several atmospheres.

After that, the membrane was used as a miniature drum to measure the frequency of its vibrations with a change in pressure. It was found that helium remains in the microchamber even at high pressure. However, since the graphene used in the experiment was not ideal (it had defects in the crystal structure), the gas gradually seeped through the membrane. Throughout the experiment, which lasted more than 70 hours, a steady decrease in membrane tension was observed (Fig. 6e).

The authors of the study point out that such membranes can have a wide variety of applications - for example, they can be used to study biological materials placed in a solution. To do this, it will be enough to cover such a material with graphene and study it through a transparent membrane with a microscope, without fear of leakage or evaporation of the solution that supports the vital activity of the organism. It is also possible to make atomic-sized holes in the membrane and then observe, by studying diffusion processes, how individual atoms or ions pass through the hole. But most importantly, the study by scientists from Cornell University has brought science one step closer to the creation of single-atom sensors.

The rapid growth in the number of studies on graphene shows that this is indeed a very promising material for a wide range of applications, but many theories and dozens of experiments still need to be built before they are put into practice.

Impermeable Atomic Membranes from Graphene Sheets (full text available) // NanoLetters. V. 8. No. 8. P. 2458–2462 (2008).

Alexander Samardak

Until last year, the only way known to science to produce graphene was to apply the thinnest layer of graphite on adhesive tape and then remove the base. This technique is called the "scotch tape technique". Recently, however, scientists have discovered that there is a more efficient way to obtain a new material: as a base, they began to use a layer of copper, nickel or silicon, which is then removed by etching (Fig. 2). In this way, rectangular sheets of graphene 76 centimeters wide were created by a team of scientists from Korea, Japan and Singapore. Not only did the researchers set a kind of record for the size of a piece of a single-layer structure of carbon atoms, they also created sensitive screens based on flexible sheets.

Figure 2: Obtaining graphene by etching

For the first time, graphene "flakes" were obtained by physicists only in 2004, when their size was only 10 micrometers. A year ago, the team of Rodney Ruoff at the University of Texas at Austin announced that they had managed to create centimeter-sized "scraps" of graphene.

Ruoff and colleagues deposited carbon atoms on copper foil using chemical vapor deposition (CVD). Researchers in the laboratory of Professor Byun Hee Hong from Sunkhyunkhwan University went further and enlarged the sheets to the size of a full-fledged screen. The new “roll” technology (roll-to-roll processing) makes it possible to obtain a long ribbon from graphene (Fig. 3).

Figure 3: High-resolution transmission electron microscopy image of stacked graphene layers.

A layer of an adhesive polymer was placed on top of the graphene sheets of physics, the copper substrates were dissolved, then the polymer film was separated - a single layer of graphene was obtained. To give the sheets greater strength, scientists in the same way "grew up" three more layers of graphene. At the end, the resulting “sandwich” was treated with nitric acid to improve conductivity. A brand new graphene sheet is placed on a polyester substrate and passed between heated rollers (Fig. 4).

Figure 4: Roll technology for obtaining graphene

The resulting structure transmitted 90% of the light and had an electrical resistance lower than that of the standard, but still very expensive, transparent conductor, indium tin oxide (ITO). By the way, using sheets of graphene as the basis of touch displays, the researchers found that their structure is also less fragile.

True, despite all the achievements, the commercialization of technology is still very far away. Transparent carbon nanotube films have been trying to supplant ITO for quite some time, but manufacturers can't seem to get around the problem of "dead pixels" that appear on film defects.

The use of graphenes in electrical engineering and electronics

The brightness of pixels in flat panel screens is determined by the voltage between two electrodes, one of which is facing the viewer (Fig. 5). These electrodes must be transparent. Currently, tin-doped indium oxide (ITO) is used to produce transparent electrodes, but ITO is expensive and not the most stable material. Besides, the world will soon exhaust its reserves of indium. Graphene is more transparent and more stable than ITO, and a graphene electrode LCD has already been demonstrated.

Figure 5: Brightness of graphene screens as a function of applied voltage

The material also has great potential in other areas of electronics. In April 2008, scientists from Manchester demonstrated the world's smallest graphene transistor. A perfectly correct layer of graphene controls the resistance of the material, turning it into a dielectric. It becomes possible to create a microscopic power switch for a high-speed nano-transistor to control the movement of individual electrons. The smaller transistors in microprocessors, the faster it is, and scientists hope that graphene transistors in computers of the future will be the size of a molecule, given that modern silicon microtransistor technology has almost reached its limit.

Graphene is not only an excellent conductor of electricity. It has the highest thermal conductivity: atomic vibrations easily propagate through the carbon mesh of a cellular structure. Heat dissipation in electronics is a serious problem because there are limits to the high temperatures that electronics can withstand. However, scientists at the University of Illinois have found that graphene-based transistors have an interesting property. They manifest a thermoelectric effect, leading to a decrease in the temperature of the device. This could mean that graphene-based electronics will make heatsinks and fans a thing of the past. Thus, the attractiveness of graphene as a promising material for microcircuits of the future further increases (Fig. 6).

Figure 6: An atomic force microscope probe scanning the surface of a graphene-metal contact to measure temperature.

It was not easy for scientists to measure the thermal conductivity of graphene. They invented an entirely new way to measure its temperature by placing a 3-micron-long graphene film over exactly the same tiny hole in a silicon dioxide crystal. The film was then heated with a laser beam, causing it to vibrate. These vibrations helped to calculate the temperature and thermal conductivity.

The ingenuity of scientists knows no bounds when it comes to using the phenomenal properties of a new substance. In August 2007, the most sensitive of all possible sensors based on it was created. It is able to respond to one gas molecule, which will help to detect the presence of toxins or explosives in a timely manner. Alien molecules peacefully descend into the graphene network, knocking out electrons from it or adding them. As a result, the electrical resistance of the graphene layer changes, which is measured by scientists. Even the smallest molecules are trapped by the strong graphene mesh. In September 2008, scientists from Cornell University in the United States demonstrated how a graphene membrane, like the thinnest balloon, inflates due to a pressure difference of several atmospheres on both sides of it. This feature of graphene can be useful in determining the course of various chemical reactions and in general in studying the behavior of atoms and molecules.

Getting large sheets of pure graphene is still very difficult, but the task can be simplified if the carbon layer is mixed with other elements. At Northwestern University in the United States, graphite was oxidized and dissolved in water. The result was a paper-like material - graphene oxide paper (Fig. 7). It is very tough and quite easy to make. Graphene oxide is suitable as a durable membrane in batteries and fuel cells.

Figure 7: Graphene oxide paper

The graphene membrane is an ideal substrate for objects of study under an electron microscope. Flawless cells merge in images into a uniform gray background, against which other atoms stand out clearly. Until now, it was almost impossible to distinguish the lightest atoms in an electron microscope, but with graphene as a substrate, even small hydrogen atoms can be seen.

The possibilities of using graphene are endless. Recently, physicists at Northwestern University in the US figured out that graphene can be mixed with plastic. The result is a thin, super-strong material that can withstand high temperatures and is impervious to gases and liquids.

The scope of its application is the production of light gas stations, spare parts for cars and aircraft, durable wind turbine blades. Plastic can be used to pack food products, keeping them fresh for a long time.

Graphene is not only the thinnest, but also the most durable material in the world. Scientists at Columbia University in New York have verified this by placing graphene over tiny holes in a silicon crystal. Then, by pressing the thinnest diamond needle, they tried to destroy the graphene layer and measured the pressure force (Fig. 8). It turned out that graphene is 200 times stronger than steel. If you imagine a graphene layer as thick as cling film, it would withstand the pressure of a pencil point, at the opposite end of which an elephant or a car would balance.

Figure 8: Pressure on graphene diamond needle

Graphene belongs to a class of unique carbon compounds that have remarkable chemical and physical properties, such as excellent electrical conductivity, combined with amazing lightness and strength.

It is assumed that over time it will be able to replace silicon, which is the basis of modern semiconductor production. At present, the status of the “material of the future” has been securely assigned to this compound.

Material Features

Graphene, most often found under the designation "G", is a two-dimensional form of carbon that has an unusual structure in the form of atoms connected in a hexagonal lattice. At the same time, its total thickness does not exceed the size of each of them.

For a clearer understanding of what graphene is, it is advisable to familiarize yourself with such unique characteristics as:

• Record high thermal conductivity;
• High mechanical strength and flexibility of the material, hundreds of times higher than the same indicator for steel products;
• Incomparable electrical conductivity;
• High melting point (more than 3 thousand degrees);
• Impermeability and transparency.

The unusual structure of graphene is evidenced by such a simple fact: when 3 million sheet blanks of graphene are combined, the total thickness of the finished product will be no more than 1 mm.

To understand the unique properties of this unusual material, it is enough to note that in its origin it is similar to the usual layered graphite used in pencil lead. However, due to the special arrangement of atoms in the hexagonal lattice, its structure acquires the characteristics inherent in such a hard material as diamond.

When graphene is isolated from graphite, in the atom-thick film formed in this process, its most “wonderful” properties are observed, which are characteristic of modern 2D materials. Today it is difficult to find such an area of ​​the national economy, wherever this unique compound is used, and where it is not considered as promising. This is especially evident in the field of scientific developments, which aim to master new technologies.

How to get

The discovery of this material can be dated back to 2004, after which scientists have mastered various methods for obtaining it, which are presented below:

• Chemical cooling, implemented by the method of phase transformations (it is called the CVD process);
• The so-called "epitaxial growth", carried out in a vacuum;
• Method of "mechanical exfoliation".

Let's consider each of them in more detail.

Mechanical

Let's start with the last of these methods, which is considered the most accessible for independent execution. In order to obtain graphene at home, it is necessary to sequentially perform the following series of operations:

• First you need to prepare a thin graphite plate, which is then attached to the adhesive side of a special tape;
• After that, it folds in half, and then returns to its original state again (its ends are divorced);
• As a result of such manipulations, it is possible to obtain a double layer of graphite on the adhesive side of the tape;
• If you perform this operation several times, it will be easy to achieve a small thickness of the applied layer of material;
• After that, adhesive tape with split and very thin films is applied to a silicon oxide substrate;
• As a result, the film partially remains on the substrate, forming a graphene layer.

The disadvantage of this method is the difficulty in obtaining a sufficiently thin film of a given size and shape, which would be securely fixed on the parts of the substrate reserved for this purpose.

Currently, most of the graphene used in everyday practice is produced in this way. Due to mechanical exfoliation, it is possible to obtain a compound of fairly high quality, but this method is completely unsuitable for mass production conditions.

Industrial Methods

One of the industrial ways to obtain graphene is to grow it in a vacuum, the features of which can be represented as follows:

• For its manufacture, a surface layer of silicon carbide is taken, which is always present on the surfaces of this material;
• Then the pre-prepared silicon wafer is heated to a relatively high temperature (of the order of 1000 K);
• Due to the chemical reactions occurring in this case, the separation of silicon and carbon atoms is observed, in which the first of them immediately evaporate;
• As a result of this reaction, pure graphene (G) remains on the plate.

The disadvantages of this method include the need for high-temperature heating, which often causes technical difficulties.

The most reliable industrial method to avoid the difficulties described above is the so-called "CVD process". When it is implemented, a chemical reaction occurs that occurs on the surface of the metal catalyst when it is combined with hydrocarbon gases.

As a result of all the approaches discussed above, it is possible to obtain pure allotropic compounds of two-dimensional carbon in the form of a layer only one atom thick. A feature of this formation is the connection of these atoms into a hexagonal lattice due to the formation of the so-called "σ" and "π" bonds.

Electric charge carriers in the graphene lattice are characterized by a high degree of mobility, which is much higher than that of other known semiconductor materials. It is for this reason that it is able to replace the classical silicon traditionally used in the production of integrated circuits.

The possibilities of practical application of materials based on graphene are directly related to the features of its production. Currently, there are many methods for obtaining its individual fragments, which differ in shape, quality and size.

Among all known methods, the following approaches stand out:

1. Production of a variety of graphene oxide in the form of flakes used in the production of electrically conductive paints, as well as various grades of composite materials;
2. Obtaining flat graphene G, from which components of electronic devices are made;
3. Growing material of the same type used as inactive components.

The main properties of this compound and its functionality are determined by the quality of the substrate, as well as the characteristics of the material with which it is grown. All this ultimately depends on the method of production used.

Depending on the method of obtaining this unique material, it can be used for a variety of purposes, namely:

1. Graphene obtained by mechanical exfoliation is mainly intended for research, which is explained by the low mobility of free charge carriers;
2. When graphene is obtained by a chemical (thermal) reaction, it is most often used to create composite materials, as well as protective coatings, inks, and dyes. The mobility of free carriers is somewhat higher, which allows it to be used for the manufacture of capacitors and film insulators;
3. If the CVD method is used to obtain this compound, it can be used in nanoelectronics, as well as for the manufacture of sensors and transparent flexible films;
4. Graphene obtained by the "silicon wafer" method is used to manufacture such elements of electronic devices as high-frequency transistors and similar components. The mobility of free charge carriers in such compounds is maximum.

The listed features of graphene open up broad horizons for manufacturers and allow them to concentrate their efforts on its implementation in the following promising areas:

• In alternative areas of modern electronics, associated with the replacement of silicon components;
• In the leading chemical industries of production;
• When designing unique products (such as, for example, composite materials and graphene membranes);
• In electrical engineering and electronics (as an "ideal" conductor).

In addition, cold cathodes, storage batteries, as well as special conductive electrodes and transparent film coatings can be made on the basis of this compound. The unique properties of this nanomaterial provide it with a wide range of possibilities for its use in advanced developments.

Advantages and disadvantages

Advantages of products based on graphene:

• High degree of electrical conductivity, comparable to the same indicator for ordinary copper;
• Almost perfect optical purity, due to which it absorbs no more than two percent of the visible light range. Therefore, from the outside, it seems almost colorless and invisible to the observer;
• Mechanical strength superior to diamond;
• Flexibility, in which single-layer graphene is superior to elastic rubber. This quality makes it easy to change the shape of the films and stretch them if necessary;
• Resistance to external mechanical influences;
• Incomparable thermal conductivity, in terms of which it is ten times superior to the same copper.

The disadvantages of this unique carbon compound include:

1. The impossibility of obtaining in volumes sufficient for industrial production, as well as achieving the physicochemical properties required to ensure high quality. In practice, it is possible to obtain only small sheet fragments of graphene;
2. Industrial products are most often inferior in their characteristics to samples obtained in research laboratories. It is not possible to achieve them with the help of ordinary industrial technologies;
3. High non-labor costs, which significantly limit the possibilities of its production and practical application.

Despite all these difficulties, researchers do not abandon attempts to develop new technologies for the production of graphene.

In conclusion, it should be stated that the prospects for this material are simply fantastic, since it can also be used in the production of modern ultra-thin and flexible gadgets. In addition, on its basis, it is possible to create modern medical equipment and drugs that can fight cancer and other common tumor diseases.

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Graphene is a revolutionary material of the 21st century. It is the strongest, lightest and most electrically conductive version of carbon bonding.

Graphene was found by Konstantin Novoselov and Andrey Geim, working at the University of Manchester, for which Russian scientists were awarded the Nobel Prize. To date, about ten billion dollars has been allocated to research the properties of graphene for ten years, and there are rumors that it can be an excellent replacement for silicon, especially in the semiconductor industry.

However, a two-dimensional structure similar to this carbonaceous material has also been predicted for other elements of the Periodic Table of Chemical Elements, and very unusual properties of one of these substances have recently been studied. And this substance is called "blue phosphorus".

Russian natives working in Britain, Konstantin Novoselov and Andrey Geim, created graphene - a translucent layer of carbon one atom thick - in 2004. From that moment, almost immediately and everywhere, we began to hear laudatory odes about a variety of amazing properties of a material that has the potential to change our world and find its application in various fields, from the production of quantum computers to the production of filters for obtaining clean drinking water. 15 years have passed, but the world under the influence of graphene has not changed. Why?

All modern electronic devices use electrons to transmit information. Now the development of quantum computers is in full swing, which many consider the future replacement of traditional devices. However, there is another, no less interesting way of development. Creation of so-called photonic computers. And recently, a group of researchers from the University of Exeter () discovered a particle property that could help design new computer circuits.

Graphene fibers under a scanning electron microscope. Pure graphene is recovered from graphene oxide (GO) in a microwave oven. Scale 40 µm (left) and 10 µm (right). Photo: Jieun Yang, Damien Voiry, Jacob Kupferberg / Rutgers University

Graphene is a 2D modification of carbon formed by a layer one carbon atom thick. The material has high strength, high thermal conductivity and unique physical and chemical properties. It exhibits the highest electron mobility of any known material on Earth. This makes graphene an almost ideal material for a wide variety of applications, including electronics, catalysts, batteries, composite materials, etc. The point is small - to learn how to obtain high-quality graphene layers on an industrial scale.

Chemists from Rutgers University (USA) have found a simple and fast method for producing high-quality graphene by processing graphene oxide in a conventional microwave oven. The method is surprisingly primitive and effective.

Graphite oxide is a compound of carbon, hydrogen and oxygen in various proportions, which is formed when graphite is treated with strong oxidizing agents. To get rid of the remaining oxygen in the graphite oxide, and then get pure graphene in two-dimensional sheets, requires considerable effort.

Graphite oxide is mixed with strong alkalis and the material is further reduced. As a result, monomolecular sheets with oxygen residues are obtained. These sheets are commonly referred to as graphene oxide (GO). Chemists have tried different ways to remove excess oxygen from GO ( , , , ), but GO (rGO) reduced by such methods remains a highly disordered material, which is far from real pure graphene obtained by chemical vapor deposition (CVD) .

Even in its disordered form, rGO has the potential to be useful for energy carriers ( , , , , ) and catalysts ( , , , ), but in order to take full advantage of the unique properties of graphene in electronics, one must learn how to obtain pure high-quality graphene from GO.

Chemists at Rutgers University offer a simple and fast way to reduce GO to pure graphene using 1-2 second microwave pulses. As can be seen from the graphs, graphene obtained by “microwave reduction” (MW-rGO) is much closer in its properties to the purest graphene obtained using CVD.

Physical characteristics of MW-rGO compared to pristine graphene oxide GO, reduced graphene oxide rGO, and chemical vapor deposition (CVD) graphene. Shown are typical GO flakes deposited on a silicon substrate (A); X-ray photoelectron spectroscopy (B); Raman spectroscopy and the ratio of crystal size (L a) to the peak ratio l 2D /l G in the Raman spectrum for MW-rGO, GO and CVD.

Electronic and electrocatalytic properties of MW-rGO compared to rGO. Illustrations: Rutgers University

The technical process for obtaining MW-rGO consists of several stages.

1. Oxidation of graphite by the modified Hummers method and its dissolution to single-layer flakes of graphene oxide in water.
2. GO annealing to make the material more susceptible to microwave irradiation.
3. Irradiation of GO flakes in a conventional 1000W microwave oven for 1-2 seconds. During this procedure, GO is rapidly heated to a high temperature, desorption of oxygen groups and excellent structuring of the carbon lattice occurs.

Shooting with a transmission electron microscope shows that after treatment with a microwave emitter, a highly ordered structure is formed in which oxygen functional groups are almost completely destroyed.

Transmission electron microscope images show the structure of graphene sheets with a scale of 1 nm. On the left is a single layer rGO with many defects, including oxygen functional groups (blue arrow) and holes in the carbon layer (red arrow). In the center and on the right is a perfectly structured two-layer and three-layer MW-rGO. Photo: Rutgers University

The excellent structural properties of MW-rGO when used in field effect transistors allow the maximum electron mobility to be increased to about 1500 cm 2 /V·s, which is comparable to the outstanding performance of modern high electron mobility transistors.

In addition to electronics, MW-rGO is useful in the production of catalysts: it showed an exceptionally low value of the Tafel coefficient when used as a catalyst in the oxygen evolution reaction: about 38 mV per decade. The MW-rGO catalyst also remained stable in the hydrogen evolution reaction, which lasted over 100 hours.

All this suggests an excellent potential for the use of microwave-reduced graphene in industry.

Research Article "High-quality graphene via microwave reduction of solution-exfoliated graphene oxide" published September 1, 2016 in the magazine Science(doi: 10.1126/science.aah3398).