Graphene
Graphene is a two-dimensional material with a hexagonal honeycomb lattice composed of carbon atoms and sp2 hybrid orbitals. It is a two-dimensional material with the thickness of only one carbon atom. Graphene has always been considered a hypothetical structure and cannot exist independently. Until 2004, physicists Andre Heim and Konstantin Novoselov of the University of Manchester in the United Kingdom succeeded in experimenting with graphite. The graphene was separated from it, and it was confirmed that it can exist alone, and the two also won the 2010 Nobel Prize in Physics for "a pioneering experiment in two-dimensional graphene materials".
Graphene is currently the thinnest but hardest nanomaterial in the world. It is almost completely transparent and only absorbs 2.3% of light"; the thermal conductivity is as high as 5300 W/m·K, which is higher than carbon nanotubes and diamonds at room temperature. Its electron mobility exceeds 15000 cm2/V·s, which is higher than carbon nanotubes or silicon crystals, and its resistivity is only about 10-6 Ω·cm, which is lower than copper or silver. It is currently the world’s smallest resistivity material[ 5][1]. Because of its extremely low resistivity and extremely fast electrons, it is expected to be used to develop a new generation of thinner and faster conductive electronic components or transistors. Because graphene is essentially a This kind of transparent and good conductor is also suitable for making transparent touch screens, light panels, and even solar cells.
Another characteristic of graphene is the ability to observe the quantum Hall effect at room temperature.
Discover history
In essence, graphene is a single-atom-layer planar graphite separated. According to this statement, scientists have been exposed to graphene since the establishment of X-ray crystallography in the early 20th century. In 1918, V. Kohlschütter and P. Haenni described the properties of graphite oxide paper in detail. In 1948, G. Ruess and F. Vogt published the first few-layer graphene (graphene with 3 to 10 layers) taken with a transmission electron microscope.
Regarding the manufacture and discovery of graphene, initially, scientists tried to use chemical exfoliation to make graphene. They embedded large atoms or macromolecules into graphite to obtain graphite intercalation compounds. In its three-dimensional structure, each layer of graphite can be regarded as a single layer of graphene. After chemical reaction treatment, after removing the embedded large atoms or macromolecules, a pile of graphene sludge will be obtained. Because it is difficult to analyze and control the physical properties of this pile of mud, scientists did not continue research in this area. Some scientists use chemical vapor deposition to epitaxially grow graphene films on various substrates, but the initial quality is not good.
In 2004, two physics teams from the University of Manchester and the Chernogolovka Institute of Microelectronics Technology worked together to first isolate a single graphene plane. Heim and team members accidentally discovered a simple and easy way to prepare graphene. They placed the graphite sheet in the plastic tape, folded the tape to stick the two sides of the graphite sheet, and peeled off the tape. The sheet was divided into two. By repeating this process continuously, thinner and thinner graphite flakes can be obtained, and some of the samples are composed of only one layer of carbon atoms-they have made graphene. Of course, just preparation is not enough. Generally, graphene is hidden in a large pile of graphite residues, and it is very difficult to stick to the substrate as ideally; therefore, finding the experimental amount of graphene is like finding a needle in the East China Sea. Even in the area as small as 1 cm2, using the cutting-edge technology of that era, it can't be found. Heim’s secret is if the graphene is placed on a silicon wafer coated with silicon oxide at a certain thickness. Using the interference effect of light waves, you can effectively use optical microscopes to find these graphenes. This is a very accurate experiment; for example, if the thickness of silicon oxide differs by more than 5%, instead of the correct value of 300nm, but 315nm, single-layer graphene cannot be observed.
Recently, scholars have studied the visibility and contrast of graphene on a variety of different material substrates, and also provided a simple and easy visibility enhancement method [12]. In addition, the use of Raman microscopy techniques for preliminary identification can also increase the screening efficiency [13].
In 2005, the same Manchester University team and Columbia University researchers confirmed that the quasiparticles of graphene are massless Dirac fermions. Discoveries like this have caused an upsurge in the study of graphene. Since then, hundreds of talented and academic researchers have stepped into this new field.
Now, as everyone knows, whenever graphite is scraped, like when drawing a line with a pencil, tiny graphene fragments will be made, and a lot of residue will be produced at the same time. Before 2004 and 2005, no one noticed the usefulness of these debris fragments. Therefore, the discovery of graphene should be attributed to the Haim team, who discovered a shining new star for solid-state physics.
Preparation
Prepared by mechanical peeling
In 2008, graphene prepared by mechanical exfoliation was one of the most expensive materials in the world, and a tiny sample of the cross-sectional size of human hair cost $1,000. Gradually, with the scale of the preparation process, the cost is reduced a lot. Now, the company's line number can use metric tons as the unit of measurement to buy and sell graphene. On the other hand, the price of graphene crystal film grown on the surface of silicon carbide is mainly determined by the cost of the substrate, which was about $100/cm2 in 2009. Using chemical vapor deposition, carbon atoms are deposited on a nickel metal substrate to form graphene. After the nickel metal is etched away, the carbon atoms are deposited on other substrates. In this way, graphene films up to 30 inches wide can be prepared more cheaply.
Tear tape method/light rubbing method
The most common is the micromechanical separation method, which directly cuts graphene flakes from larger crystals. In 2004, Heim et al. used this method to prepare a single-layer graphene, which can exist stably in the external environment. The typical preparation method is to use another material to expand or introduce defects of pyrolytic graphite for friction. The surface of bulk graphite will produce flake-like crystals, and these flake-like crystals contain a single layer of graphene. However, the disadvantage is that this method uses the flakes obtained by rubbing the graphite surface to screen out single-layer graphene flakes, the size of which is not easy to control, and it is impossible to reliably manufacture graphite flake samples of sufficient length for supply.
Silicon carbide surface epitaxial growth
In this method, silicon is removed by heating single crystal silicon carbide, and graphene sheets are decomposed on the single crystal (0001) surface. The specific process is: heating the sample obtained by etching with oxygen or hydrogen under high vacuum by electron bombardment to remove oxides. After the Auger electron spectroscopy is used to determine that the oxides on the surface are completely removed, the sample is heated to raise the temperature to 1250~1450°C and kept at a constant temperature for 1min~20min to form a very thin graphite layer. After several years of exploration, Claire Berg and others have been able to controllably prepare single-layer or multilayer graphene. It is easier to get up to 100 layers of multilayer graphene on the C-terminated surface. The thickness is determined by the heating temperature, and it is difficult to prepare a large area of graphene with a single thickness.
Metal surface growth
The epitaxial method uses the atomic structure of the growth substrate to "seed" graphene. First, carbon atoms are infiltrated into ruthenium at 1150°C, and then cooled. After cooling to 850°C, a large number of previously absorbed carbon atoms will float on the surface of ruthenium. The single-layer "islands" of carbon atoms in the shape of the lens cover the entire surface of the substrate, and eventually they can grow into a complete layer of graphene. After the first layer covers 80%, the second layer begins to grow. The graphene at the bottom layer will interact strongly with ruthenium, and after the second layer, it is almost completely separated from ruthenium, leaving only weak electrical coupling. The performance of the obtained single-layer graphene sheet is satisfactory. However, the graphene sheets produced by this method are often uneven in thickness, and the adhesion between the graphene and the matrix will affect the characteristics of the carbon layer. In addition, the substrate used by Peter Sert et al. is the rare metal ruthenium.
Oxidation thinning of graphite flakes
Graphene can also be thinned layer by layer by heating and oxidizing to obtain single and double layers of graphene.
Hydrazine reduction method
Put the graphene oxide paper into a pure hydrazine solution (a compound of hydrogen and nitrogen atoms), which will reduce the graphene oxide paper to a single layer of graphene.
Sodium ethoxylate cracking
A paper published in 2008 described a procedure that can produce graphene in gram quantities. The ethanol is first reduced with sodium metal, and then the obtained ethoxide product is cracked, washed with water to remove the sodium salt to obtain the graphene that sticks together, and then vibrated with gentle sound waves to produce pure graphene in gram quantities.
Cutting carbon nanotube method
Cutting carbon nanotubes is also an experimental method for making graphene ribbons. One method uses potassium permanganate and sulfuric acid to cut multi-walled carbon nanotubes in solution. Another method uses plasma to etch a portion of nanotubes embedded in a polymer.
Sonication of graphite
This method involves dispersing graphite in a suitable liquid medium and then ultrasonically treating it. Through centrifugal separation, non-expanded graphite is finally separated from graphene. This method was first proposed by Hernandez et al. The graphene concentration he obtained reached 0.01 mg/ml in N-methylpyrrolidone. Then, the method was mainly improved by multiple research groups. In particular, it has been greatly improved by the Alberto Mariani team in Italy. Mariani et al. reached a concentration of 2.1 mg/ml in NMP (the highest in this solvent). The highest graphene concentration published by the same group has been reported so far in any liquid and obtained by any method. An example is the use of a suitable ionized liquid as a dispersion medium for graphite exfoliation; a very high concentration of 5.33 mg/ml was obtained in this medium.
Application field
Single molecule gas detection
Graphene's unique two-dimensional structure makes it a bright application prospect in the sensor field. The huge surface area makes it very sensitive to the surrounding environment. Even the adsorption or release of a gas molecule can be detected. This detection can be divided into direct detection and indirect detection at present. Through the transmission electron microscope, the adsorption and release process of single atoms can be directly observed. The adsorption and release process of single atoms can be detected indirectly by measuring the Hall effect method. When a gas molecule is adsorbed on the graphene surface, a local change in electrical resistance occurs at the adsorption position. Of course, this effect can also occur in other materials, but graphene has high conductivity and low noise, and can detect this tiny resistance change.
Graphene nanoribbons
In order to impart certain electrical properties to the single-layer graphene, the graphene will be cut according to a specific pattern to form graphene nanoribbons. The cut edge shape can be divided into zigzag shape and armchair shape. The calculations made by the tight-binding approximation model predict that the zigzag shape has metal bond properties, and the armchair shape has metal bond properties or semiconducting properties; which property depends on the width. However, recent calculations based on density functional theory show that the armchair shape has semiconductor properties, and its energy gap is inversely proportional to the bandwidth of the nanobelt. The experimental results indeed show that as the bandwidth of the nanoribbons decreases, the energy gap increases. However, as of February 2008, there has not been any experiment to measure the energy gap to try to identify the precise edge structure.
The structure of graphene nanoribbons has high electrical conductivity, high thermal conductivity, and low noise. These excellent qualities make graphene nanoribbons another choice for integrated circuit interconnection materials, which may replace copper metal. Some researchers try to use graphene nanobelts to make quantum dots. They change the width at certain locations of the nanobelts to form quantum confinement.
The low-dimensional structure of graphene nanoribbons has very important optoelectronic properties: population inversion and broadband optical gain. These excellent qualities encourage graphene nanoribbons to be placed in microcavities or nanocavities to form lasers and amplifiers. According to a study in October 2012, some researchers try to use graphene nanoribbons in optical communication systems to develop graphene nanoribbons lasers.
integrated circuit
Graphene has ideal properties as an excellent integrated circuit electronic device. Graphene has high carrier mobility and low noise, allowing it to be used as a channel in field effect transistors. The problem is that it is difficult to manufacture single-layer graphene, and it is even more difficult to make an appropriate substrate.
According to a report in January 2010, the quantity and quality of SiC epitaxially grown graphene are suitable for mass production of integrated circuits. At high temperatures, the quantum Hall effect in these samples can be measured. See also the section of IBM's Working Transistors in 2010. The fast transistor'processor' made 2 inches (51 mm) of graphene sheets.
In June 2011, IBM researchers announced that they had successfully created the first graphene-based integrated circuit-broadband wireless mixer. The circuit processing frequency is up to 10GHz, and its performance is not affected at temperatures up to 127°C.
Graphene transistor
In 2005, the Geim research group and Kim research group found that graphene has a high carrier mobility (about 10 am/V·s) that is 10 times that of commercial silicon wafers at room temperature, and is very affected by temperature and doping effects. It is small and exhibits room temperature submicron ballistic transmission characteristics (up to 0.3 m at 300 K). This is the most prominent advantage of graphene as a nanoelectronic device, making the room temperature ballistic field effect tube attractive in the field of electronic engineering possible. The larger Fermi speed and low contact resistance help to further reduce the switching time of the device. The operation response characteristics of ultra-high frequency are another significant advantage of graphene-based electronic devices. Under modern technology, graphene nanowires can prove to be generally able to replace silicon as a semiconductor.
Transparent conductive electrode
Graphene's good electrical conductivity and light transmission properties make it a very good application prospect in transparent conductivity electrodes. Touch screens, liquid crystal displays, organic photovoltaic cells, organic light-emitting diodes, etc. all require good transparent conductive electrode materials. In particular, the mechanical strength and flexibility of graphene are better than those of indium tin oxide, a commonly used material. Due to the high brittleness of indium tin oxide, it is easier to damage. The graphene film in the solution can be deposited on a large area.
By chemical vapor deposition, large-area, continuous, transparent, and high-conductivity graphene thin films with few layers can be made, which are mainly used for anodes of photovoltaic devices and achieve an energy conversion efficiency of up to 1.71%; and indium tin oxide materials Compared with the produced element, its energy conversion efficiency is about 55.2%.
Thermal Conductive Material/Thermal Interface Material
In 2011, scholars from Georgia Institute of Technology in the United States first reported the application of vertically arranged functionalized multilayer graphene three-dimensional structure in thermal interface materials and its ultra-high equivalent thermal conductivity and ultra-low interface thermal resistance.
Field emission source and its vacuum electronic device
As early as 2002, graphene nanowalls perpendicular to the surface of the substrate were successfully prepared. It is regarded as a very good field emission electron source material. Recently, the electric field-induced electron emission effect of a single sheet of graphene has also been reported.
Super capacitor
Since graphene has an extremely high surface area to mass ratio, graphene can be used for conductive electrodes in supercapacitors. Scientists believe that the stored energy density of this supercapacitor will be greater than that of existing capacitors.
Desalination
Studies have shown that graphene filters may significantly outperform other desalination technologies.
Solar battery
The laboratory of the Viterbi School of Engineering at the University of Southern California reported on the large-scale production of highly transparent graphene films by chemical vapor deposition in 2008. In this process, the researchers created ultra-thin graphene sheets by forming a graphene film from the first deposited carbon atoms on a nickel plate in methane gas. Then, they laid a protective layer of thermoplastic on top of the graphene layer and dissolved the nickel underneath in an acid bath. In the final step, they attached the plastic-protected graphene to a very flexible polymer sheet, which can be incorporated into an organic solar cell (OPV cell, graphene photovoltaic cell). Graphene/polymer sheets have been produced, with a size range of 150 square centimeters, and can be used to produce flexible organic solar cells (OPV cells). This may eventually make it possible to run cheap solar cells that can cover a wide area, just like the printing of newspapers on a newspaper printer.
In 2010, Xinming Li, Hongwei Zhu and others combined graphene and silicon to construct a new type of solar cell for the first time. In this simple graphene/silicon model, graphene can not only serve as a transparent conductive film, but also separate photo-generated carriers at the interface with silicon. This structure, which can be combined with traditional silicon materials, opens up new research directions for the promotion of graphene-based photovoltaic devices.
Graphene biodevices
Due to its modifiable chemical function, large contact area, atomic size thickness, molecular gate structure, etc., graphene is an excellent choice for use in bacterial detection and diagnostic devices.
Scientists hope to develop a fast and cheap electronic DNA sequencing technology. They believe that graphene is a material with this potential. Basically, they want to use graphene to make a nanohole about the width of DNA and allow DNA molecules to swim through the nanohole. Since the four bases of DNA (A, C, G, T) have different effects on the conductivity of graphene, as long as the small voltage difference generated when the DNA molecule passes through, you can know which base is currently Swim through the nano hole. In this way, the goal can be achieved.
Antibacterial substance
Scientists at the Shanghai Branch of the Chinese Academy of Sciences found that graphene oxide is super effective in inhibiting the growth of E. coli without harming human cells. If graphene oxide also has antibacterial properties against other bacteria, it may find a series of new applications, such as shoes that automatically remove odors, or packaging to keep food fresh.
Graphene photosensitive element
A group of scientists from Singapore who specialize in the research of graphene materials have now developed the latest technology of applying graphene to camera photosensitive elements, which is expected to completely subvert the future development of digital photosensitive element technology.
Scholars from Nanyang Technological University in Singapore have developed a new type of photosensitive element that uses graphene as the material of the photosensitive element. Through its special structure, it is expected to make the photosensitive element 1,000 times better than traditional CMOS or CCD, and consume energy. It only needs 1/10 of the original. This sensitivity is almost as high as the latest photosensitive element technology of the watch. According to the data, it is actually really powerful beyond the mid-infrared range visible to the human eye.
Like many new photosensitive element technologies, this technology will be the first to be applied in the field of monitors and satellite imaging. However, the study also pointed out that this technology will eventually be applied to general digital cameras/cameras, and also mentioned that if it does enter the consumer field, they promise that this latest photosensitive element made of graphene can also reduce the manufacturing cost. Today's 1/5 is low. In other words, after seeing the amazing technological progress of graphene applications in the battery field, it seems not difficult to imagine the technical impact it will bring to photography (higher sensitivity, more power saving, and cheaper).
Graphene is currently the thinnest but hardest nanomaterial in the world. It is almost completely transparent and only absorbs 2.3% of light"; the thermal conductivity is as high as 5300 W/m·K, which is higher than carbon nanotubes and diamonds at room temperature. Its electron mobility exceeds 15000 cm2/V·s, which is higher than carbon nanotubes or silicon crystals, and its resistivity is only about 10-6 Ω·cm, which is lower than copper or silver. It is currently the world’s smallest resistivity material[ 5][1]. Because of its extremely low resistivity and extremely fast electrons, it is expected to be used to develop a new generation of thinner and faster conductive electronic components or transistors. Because graphene is essentially a This kind of transparent and good conductor is also suitable for making transparent touch screens, light panels, and even solar cells.
Another characteristic of graphene is the ability to observe the quantum Hall effect at room temperature.
Discover history
In essence, graphene is a single-atom-layer planar graphite separated. According to this statement, scientists have been exposed to graphene since the establishment of X-ray crystallography in the early 20th century. In 1918, V. Kohlschütter and P. Haenni described the properties of graphite oxide paper in detail. In 1948, G. Ruess and F. Vogt published the first few-layer graphene (graphene with 3 to 10 layers) taken with a transmission electron microscope.
Regarding the manufacture and discovery of graphene, initially, scientists tried to use chemical exfoliation to make graphene. They embedded large atoms or macromolecules into graphite to obtain graphite intercalation compounds. In its three-dimensional structure, each layer of graphite can be regarded as a single layer of graphene. After chemical reaction treatment, after removing the embedded large atoms or macromolecules, a pile of graphene sludge will be obtained. Because it is difficult to analyze and control the physical properties of this pile of mud, scientists did not continue research in this area. Some scientists use chemical vapor deposition to epitaxially grow graphene films on various substrates, but the initial quality is not good.
In 2004, two physics teams from the University of Manchester and the Chernogolovka Institute of Microelectronics Technology worked together to first isolate a single graphene plane. Heim and team members accidentally discovered a simple and easy way to prepare graphene. They placed the graphite sheet in the plastic tape, folded the tape to stick the two sides of the graphite sheet, and peeled off the tape. The sheet was divided into two. By repeating this process continuously, thinner and thinner graphite flakes can be obtained, and some of the samples are composed of only one layer of carbon atoms-they have made graphene. Of course, just preparation is not enough. Generally, graphene is hidden in a large pile of graphite residues, and it is very difficult to stick to the substrate as ideally; therefore, finding the experimental amount of graphene is like finding a needle in the East China Sea. Even in the area as small as 1 cm2, using the cutting-edge technology of that era, it can't be found. Heim’s secret is if the graphene is placed on a silicon wafer coated with silicon oxide at a certain thickness. Using the interference effect of light waves, you can effectively use optical microscopes to find these graphenes. This is a very accurate experiment; for example, if the thickness of silicon oxide differs by more than 5%, instead of the correct value of 300nm, but 315nm, single-layer graphene cannot be observed.
Recently, scholars have studied the visibility and contrast of graphene on a variety of different material substrates, and also provided a simple and easy visibility enhancement method [12]. In addition, the use of Raman microscopy techniques for preliminary identification can also increase the screening efficiency [13].
In 2005, the same Manchester University team and Columbia University researchers confirmed that the quasiparticles of graphene are massless Dirac fermions. Discoveries like this have caused an upsurge in the study of graphene. Since then, hundreds of talented and academic researchers have stepped into this new field.
Now, as everyone knows, whenever graphite is scraped, like when drawing a line with a pencil, tiny graphene fragments will be made, and a lot of residue will be produced at the same time. Before 2004 and 2005, no one noticed the usefulness of these debris fragments. Therefore, the discovery of graphene should be attributed to the Haim team, who discovered a shining new star for solid-state physics.
Preparation
Prepared by mechanical peeling
In 2008, graphene prepared by mechanical exfoliation was one of the most expensive materials in the world, and a tiny sample of the cross-sectional size of human hair cost $1,000. Gradually, with the scale of the preparation process, the cost is reduced a lot. Now, the company's line number can use metric tons as the unit of measurement to buy and sell graphene. On the other hand, the price of graphene crystal film grown on the surface of silicon carbide is mainly determined by the cost of the substrate, which was about $100/cm2 in 2009. Using chemical vapor deposition, carbon atoms are deposited on a nickel metal substrate to form graphene. After the nickel metal is etched away, the carbon atoms are deposited on other substrates. In this way, graphene films up to 30 inches wide can be prepared more cheaply.
Tear tape method/light rubbing method
The most common is the micromechanical separation method, which directly cuts graphene flakes from larger crystals. In 2004, Heim et al. used this method to prepare a single-layer graphene, which can exist stably in the external environment. The typical preparation method is to use another material to expand or introduce defects of pyrolytic graphite for friction. The surface of bulk graphite will produce flake-like crystals, and these flake-like crystals contain a single layer of graphene. However, the disadvantage is that this method uses the flakes obtained by rubbing the graphite surface to screen out single-layer graphene flakes, the size of which is not easy to control, and it is impossible to reliably manufacture graphite flake samples of sufficient length for supply.
Silicon carbide surface epitaxial growth
In this method, silicon is removed by heating single crystal silicon carbide, and graphene sheets are decomposed on the single crystal (0001) surface. The specific process is: heating the sample obtained by etching with oxygen or hydrogen under high vacuum by electron bombardment to remove oxides. After the Auger electron spectroscopy is used to determine that the oxides on the surface are completely removed, the sample is heated to raise the temperature to 1250~1450°C and kept at a constant temperature for 1min~20min to form a very thin graphite layer. After several years of exploration, Claire Berg and others have been able to controllably prepare single-layer or multilayer graphene. It is easier to get up to 100 layers of multilayer graphene on the C-terminated surface. The thickness is determined by the heating temperature, and it is difficult to prepare a large area of graphene with a single thickness.
Metal surface growth
The epitaxial method uses the atomic structure of the growth substrate to "seed" graphene. First, carbon atoms are infiltrated into ruthenium at 1150°C, and then cooled. After cooling to 850°C, a large number of previously absorbed carbon atoms will float on the surface of ruthenium. The single-layer "islands" of carbon atoms in the shape of the lens cover the entire surface of the substrate, and eventually they can grow into a complete layer of graphene. After the first layer covers 80%, the second layer begins to grow. The graphene at the bottom layer will interact strongly with ruthenium, and after the second layer, it is almost completely separated from ruthenium, leaving only weak electrical coupling. The performance of the obtained single-layer graphene sheet is satisfactory. However, the graphene sheets produced by this method are often uneven in thickness, and the adhesion between the graphene and the matrix will affect the characteristics of the carbon layer. In addition, the substrate used by Peter Sert et al. is the rare metal ruthenium.
Oxidation thinning of graphite flakes
Graphene can also be thinned layer by layer by heating and oxidizing to obtain single and double layers of graphene.
Hydrazine reduction method
Put the graphene oxide paper into a pure hydrazine solution (a compound of hydrogen and nitrogen atoms), which will reduce the graphene oxide paper to a single layer of graphene.
Sodium ethoxylate cracking
A paper published in 2008 described a procedure that can produce graphene in gram quantities. The ethanol is first reduced with sodium metal, and then the obtained ethoxide product is cracked, washed with water to remove the sodium salt to obtain the graphene that sticks together, and then vibrated with gentle sound waves to produce pure graphene in gram quantities.
Cutting carbon nanotube method
Cutting carbon nanotubes is also an experimental method for making graphene ribbons. One method uses potassium permanganate and sulfuric acid to cut multi-walled carbon nanotubes in solution. Another method uses plasma to etch a portion of nanotubes embedded in a polymer.
Sonication of graphite
This method involves dispersing graphite in a suitable liquid medium and then ultrasonically treating it. Through centrifugal separation, non-expanded graphite is finally separated from graphene. This method was first proposed by Hernandez et al. The graphene concentration he obtained reached 0.01 mg/ml in N-methylpyrrolidone. Then, the method was mainly improved by multiple research groups. In particular, it has been greatly improved by the Alberto Mariani team in Italy. Mariani et al. reached a concentration of 2.1 mg/ml in NMP (the highest in this solvent). The highest graphene concentration published by the same group has been reported so far in any liquid and obtained by any method. An example is the use of a suitable ionized liquid as a dispersion medium for graphite exfoliation; a very high concentration of 5.33 mg/ml was obtained in this medium.
Application field
Single molecule gas detection
Graphene's unique two-dimensional structure makes it a bright application prospect in the sensor field. The huge surface area makes it very sensitive to the surrounding environment. Even the adsorption or release of a gas molecule can be detected. This detection can be divided into direct detection and indirect detection at present. Through the transmission electron microscope, the adsorption and release process of single atoms can be directly observed. The adsorption and release process of single atoms can be detected indirectly by measuring the Hall effect method. When a gas molecule is adsorbed on the graphene surface, a local change in electrical resistance occurs at the adsorption position. Of course, this effect can also occur in other materials, but graphene has high conductivity and low noise, and can detect this tiny resistance change.
Graphene nanoribbons
In order to impart certain electrical properties to the single-layer graphene, the graphene will be cut according to a specific pattern to form graphene nanoribbons. The cut edge shape can be divided into zigzag shape and armchair shape. The calculations made by the tight-binding approximation model predict that the zigzag shape has metal bond properties, and the armchair shape has metal bond properties or semiconducting properties; which property depends on the width. However, recent calculations based on density functional theory show that the armchair shape has semiconductor properties, and its energy gap is inversely proportional to the bandwidth of the nanobelt. The experimental results indeed show that as the bandwidth of the nanoribbons decreases, the energy gap increases. However, as of February 2008, there has not been any experiment to measure the energy gap to try to identify the precise edge structure.
The structure of graphene nanoribbons has high electrical conductivity, high thermal conductivity, and low noise. These excellent qualities make graphene nanoribbons another choice for integrated circuit interconnection materials, which may replace copper metal. Some researchers try to use graphene nanobelts to make quantum dots. They change the width at certain locations of the nanobelts to form quantum confinement.
The low-dimensional structure of graphene nanoribbons has very important optoelectronic properties: population inversion and broadband optical gain. These excellent qualities encourage graphene nanoribbons to be placed in microcavities or nanocavities to form lasers and amplifiers. According to a study in October 2012, some researchers try to use graphene nanoribbons in optical communication systems to develop graphene nanoribbons lasers.
integrated circuit
Graphene has ideal properties as an excellent integrated circuit electronic device. Graphene has high carrier mobility and low noise, allowing it to be used as a channel in field effect transistors. The problem is that it is difficult to manufacture single-layer graphene, and it is even more difficult to make an appropriate substrate.
According to a report in January 2010, the quantity and quality of SiC epitaxially grown graphene are suitable for mass production of integrated circuits. At high temperatures, the quantum Hall effect in these samples can be measured. See also the section of IBM's Working Transistors in 2010. The fast transistor'processor' made 2 inches (51 mm) of graphene sheets.
In June 2011, IBM researchers announced that they had successfully created the first graphene-based integrated circuit-broadband wireless mixer. The circuit processing frequency is up to 10GHz, and its performance is not affected at temperatures up to 127°C.
Graphene transistor
In 2005, the Geim research group and Kim research group found that graphene has a high carrier mobility (about 10 am/V·s) that is 10 times that of commercial silicon wafers at room temperature, and is very affected by temperature and doping effects. It is small and exhibits room temperature submicron ballistic transmission characteristics (up to 0.3 m at 300 K). This is the most prominent advantage of graphene as a nanoelectronic device, making the room temperature ballistic field effect tube attractive in the field of electronic engineering possible. The larger Fermi speed and low contact resistance help to further reduce the switching time of the device. The operation response characteristics of ultra-high frequency are another significant advantage of graphene-based electronic devices. Under modern technology, graphene nanowires can prove to be generally able to replace silicon as a semiconductor.
Transparent conductive electrode
Graphene's good electrical conductivity and light transmission properties make it a very good application prospect in transparent conductivity electrodes. Touch screens, liquid crystal displays, organic photovoltaic cells, organic light-emitting diodes, etc. all require good transparent conductive electrode materials. In particular, the mechanical strength and flexibility of graphene are better than those of indium tin oxide, a commonly used material. Due to the high brittleness of indium tin oxide, it is easier to damage. The graphene film in the solution can be deposited on a large area.
By chemical vapor deposition, large-area, continuous, transparent, and high-conductivity graphene thin films with few layers can be made, which are mainly used for anodes of photovoltaic devices and achieve an energy conversion efficiency of up to 1.71%; and indium tin oxide materials Compared with the produced element, its energy conversion efficiency is about 55.2%.
Thermal Conductive Material/Thermal Interface Material
In 2011, scholars from Georgia Institute of Technology in the United States first reported the application of vertically arranged functionalized multilayer graphene three-dimensional structure in thermal interface materials and its ultra-high equivalent thermal conductivity and ultra-low interface thermal resistance.
Field emission source and its vacuum electronic device
As early as 2002, graphene nanowalls perpendicular to the surface of the substrate were successfully prepared. It is regarded as a very good field emission electron source material. Recently, the electric field-induced electron emission effect of a single sheet of graphene has also been reported.
Super capacitor
Since graphene has an extremely high surface area to mass ratio, graphene can be used for conductive electrodes in supercapacitors. Scientists believe that the stored energy density of this supercapacitor will be greater than that of existing capacitors.
Desalination
Studies have shown that graphene filters may significantly outperform other desalination technologies.
Solar battery
The laboratory of the Viterbi School of Engineering at the University of Southern California reported on the large-scale production of highly transparent graphene films by chemical vapor deposition in 2008. In this process, the researchers created ultra-thin graphene sheets by forming a graphene film from the first deposited carbon atoms on a nickel plate in methane gas. Then, they laid a protective layer of thermoplastic on top of the graphene layer and dissolved the nickel underneath in an acid bath. In the final step, they attached the plastic-protected graphene to a very flexible polymer sheet, which can be incorporated into an organic solar cell (OPV cell, graphene photovoltaic cell). Graphene/polymer sheets have been produced, with a size range of 150 square centimeters, and can be used to produce flexible organic solar cells (OPV cells). This may eventually make it possible to run cheap solar cells that can cover a wide area, just like the printing of newspapers on a newspaper printer.
In 2010, Xinming Li, Hongwei Zhu and others combined graphene and silicon to construct a new type of solar cell for the first time. In this simple graphene/silicon model, graphene can not only serve as a transparent conductive film, but also separate photo-generated carriers at the interface with silicon. This structure, which can be combined with traditional silicon materials, opens up new research directions for the promotion of graphene-based photovoltaic devices.
Graphene biodevices
Due to its modifiable chemical function, large contact area, atomic size thickness, molecular gate structure, etc., graphene is an excellent choice for use in bacterial detection and diagnostic devices.
Scientists hope to develop a fast and cheap electronic DNA sequencing technology. They believe that graphene is a material with this potential. Basically, they want to use graphene to make a nanohole about the width of DNA and allow DNA molecules to swim through the nanohole. Since the four bases of DNA (A, C, G, T) have different effects on the conductivity of graphene, as long as the small voltage difference generated when the DNA molecule passes through, you can know which base is currently Swim through the nano hole. In this way, the goal can be achieved.
Antibacterial substance
Scientists at the Shanghai Branch of the Chinese Academy of Sciences found that graphene oxide is super effective in inhibiting the growth of E. coli without harming human cells. If graphene oxide also has antibacterial properties against other bacteria, it may find a series of new applications, such as shoes that automatically remove odors, or packaging to keep food fresh.
Graphene photosensitive element
A group of scientists from Singapore who specialize in the research of graphene materials have now developed the latest technology of applying graphene to camera photosensitive elements, which is expected to completely subvert the future development of digital photosensitive element technology.
Scholars from Nanyang Technological University in Singapore have developed a new type of photosensitive element that uses graphene as the material of the photosensitive element. Through its special structure, it is expected to make the photosensitive element 1,000 times better than traditional CMOS or CCD, and consume energy. It only needs 1/10 of the original. This sensitivity is almost as high as the latest photosensitive element technology of the watch. According to the data, it is actually really powerful beyond the mid-infrared range visible to the human eye.
Like many new photosensitive element technologies, this technology will be the first to be applied in the field of monitors and satellite imaging. However, the study also pointed out that this technology will eventually be applied to general digital cameras/cameras, and also mentioned that if it does enter the consumer field, they promise that this latest photosensitive element made of graphene can also reduce the manufacturing cost. Today's 1/5 is low. In other words, after seeing the amazing technological progress of graphene applications in the battery field, it seems not difficult to imagine the technical impact it will bring to photography (higher sensitivity, more power saving, and cheaper).
