Q & A
Have you ever wondered what graphene is? Is it different from graphite? How it’s made? We’re here to answer those questions!
1. What is carbon?
Carbon is a very common nonmetallic element, located in the second periodic IVA family of the periodic table, widely present in the atmosphere, crust and creatures in a variety of forms.
2. What is graphite?
Graphite is an elemental element of elemental carbon, and each carbon atom is connected to another three carbon atoms to covalently bond to form a covalent molecule. Graphite is a very soft mineral and is often used to make pencils and lubricants.
3. What is graphene?
The layered structure of the carbon atoms was infinitely peeled off to obtain a two-dimensional material of atomic thickness, which was named as graphene.
4. What are the types of graphene, and how many layers does each type have?
Single-layer graphene (1 layer), few-layer graphene (2~5 layers), multi-layer graphene (< 10 layers), graphene nanosheets (>10 layers)
5. What is the thickness of a single layer of graphene?
The thickness of a single layer of graphene is the thickness of a single layer of carbon atoms, which is about 0.334nm, 1/200000 of the hair.
6. What are the differences and relationship between graphene and other carbon elements?
Graphene (two-dimensional) is the basic unit that forms other carbon elements. It can be folded into fullerenes (zero-dimensional), curled into carbon nanotubes (one-dimensional) and stacked into graphite (three-dimensional.) Therefore, graphene can be regarded as the parent of the other three materials.
7. What is the difference in structure between graphite and graphene?
Graphite is composed of many layers of hexagonal carbon atoms that are arranged in honeycomb typed form. Graphene is a flat film composed of carbon atoms with sp2 hybridized orbitals and six angles honeycomb lattices.
7.1. What is sp2 hybridization?
sp2 hybridization is the process of hybridization between a s orbital and two p orbitals in the same electronic shell of an atom. When the atoms undergo sp2 hybridization, the s orbitals and the p orbitals are transformed into three equivalent atomic orbitals, called the “sp2 hybrid orbital.”
7.2 What are the allotropes of carbon that are formed because by sp2 hybridization?
Graphite, graphene, fullerene.
8. What is the fundamental difference between graphite and graphene?
Different structures lead to dramatic changes in the nature of electrons’ motion. The electrons in graphene have no mass and travel at a constant rate, which is very similar to the behavior of photons. Since the electronic properties have changed, many other properties are different.
9. When was graphene discovered?
In 2004, University of Manchester physicist Andre Geim and Konstantin Novoselov, successfully isolated graphene from graphite. This confirmed that graphene can exist alone, disproving the theory that a “single layer graphene would be unstable.”
Scientists thought that the force of stripping graphene from graphite is sufficient enough to destroy the graphene. Also, melting points of solids decrease with particle sizes decreasing. Solid melts when the particle size is reduced to a few atomic shells. In addition, the existence of internal energy in two-dimensional crystal amplifies the vibration of atoms. Scientists hypothesized that the vibrations would cause the distance between the atom and the bonds (formed and unformed) to be nearly identical.
10. How was graphene first prepared?
With the highly oriented pyrolytic graphite (HOPG) as raw materials, Andre Geim and Konstantin Novoselov used transparent tape to peel off a layer of graphite from the HOPG. They stuck tape together to make the graphite thinner and thinner until it was only one carbon atom thick. Graphene was thus discovered.
11.Can the marks left by pencils be considered single layer graphene?
Carbon layers are peeled off when the pencil marks the paper, and it might produce single layer graphene
12.Is a single-layered graphene sheet flat?
Graphene is a two-dimensional structure. It is not flat, but wavy and has ripples with amplitude of one nanometer. In a two tier system, these fluctuations are not as obvious, and when we approach multi-tiers, the fluctuations disappear completely.
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Have you ever wondered about the properties of graphene? What it looks like? How strong it is? We’re here to answer those questions!
1. What are the electrical characteristics of graphene?
Due to its stable lattice structure, graphene has excellent electrical conductivity. In perfect graphene, each carbon atom is sp2 hybridized and contributes the remaining one electron on a p orbital to form a large π-bond. The electrons orbits the nucleus, and scattering will not occur due to the lattice defects and the presence of a new atom. The interatomic force is incredibly strong. At room temperature, even if the surrounding carbon atoms bumped into one another, it would not affect the electrons in the graphene. Furthermore, the electrons in graphene have no mass, so no matter how much energy they carry, the velocity remains constant (1/300 of a photon’s velocity), and is far greater than the velocity of electrons in other conductors.
1.1 What is the conduct mechanism of metals?
Metals are metal cations that are immersed in a sea of electrons that are tightly packed, and conduct electricity via directional migration of free electrons. However, metallic bonds are not strong. For example, the ductility of metal is the result of the translation of atomic layers. Thus, there are often holes, hetero-atoms or other crystal defects, destroying the regular crystal structure of metal. when electrons pass through these electronic defects, a phenomenon known as scattering happens, reducing the speed and affecting the conductivity.
1.2 Why is the structure of graphene relatively stable?
In graphene, carbon atoms are linked by covalent bonds, which have relatively high bond energy. compared to the intermolecular forces (hydrogen and metal bonds), covalent bonds are harder to break. the energy from the c-c bond is higher than even that of a diamond’s single bond. Therefore we can say that the graphene structure has greater stability than a diamond
1.3 Why do electrons in graphene have no mass?
Graphene is a semimetal or gapless two-dimensional material. When electrons are moving in its two-dimensional structure, linear relationship is shown between their energy and momentum, thus the active mass of electrons or holes are zero. The electrons or holes here are relativistic particles, which can be described using the Dirac equation for particles with spin 1/2 (rather than Schrodinger equation for the non-relativistic).
1.4 How would we define the band structure of graphene?
Graphene structures are generally described as Dirac cones. Dirac cones are unique band structures with a top to bottom cone at the fermi energy level, between filled and unfilled electrons. This energy band structure satisfies the energy-momentum equation of relativistic particles, and is called the Dirac cone.
1.5 Electron mobility (Silicon vs. Graphene)
The electron mobility of silicon is 1400cm/ (V.s), and that of graphene is experimentally measured above 15000cm/ (V.s), 100 times that of silicon. Between 10~100K, the electron mobility in graphene is almost unrelated with the temperature, indicating that the primary scattering mechanism in graphene is defect scattering. If the integrity of graphene is improved, the mobility can be increased to 200000cm/ (V.s).
1.6 How strong is the interaction between electrons?
Using the ALS (advanced light source) booster synchrotron, scientists found that electrons in graphene interacts strongly both with the honeycomb lattice as well as each other.
- 2.What are the characteristics of graphene?
Graphite is one of the softest minerals, but when prepared into single-layered graphene, the properties change drastically. The tensile strength and elastic modulus of graphene are 125GPa and 1.1TPa respectively, which is harder than diamond. Its breaking strength is 200 times greater than high quality steel. According to experiments, the maximum pressure that graphene can bear per 100nm distance is roughly 2.9uN. This is equivalent to applying 55N of pressure to break 1 meter of graphene. If a packing bag is made completely of graphene, it should be able to withstand 2 tons of weight.
2.1 Why is graphene hard?
The electrons of carbon nuclei exist and move only in the radial direction of carbon nuclei. There are no electrons moving in the other direction. If any external materials comes into contact with it, it would contact only the nucleus, and not the electrons.
2.2 Is graphene the hardest substance?
As mentioned earlier, graphene is harder than even diamonds. In fact, up until 2015, graphene was the hardest two-dimensional material in the world. The title of hardest material in the world now belongs to borophene.
2.3 How hard is graphene? What about its ductility?
The breaking point of graphene is incredibly high, but it also retains its ductility. It can stretch an additional 20% of its original size. If one square meter of graphene is used to make a hammock, it would weigh less than 1 milligram, but could still withstand the weight of a cat.
3.What does graphene look like?
Graphene is a single sheet. Although photons cannot penetrate the nucleus of the carbon atoms, it can penetrate the vast space between the nuclei. When these graphene molecules are stacked in order, light can easily pass through the gaps in this structure. Therefore, graphene is ostensibly a transparent material.
3.1 What is the light transmittance level of graphene?
Theoretical and experiment results show that visible light transmittance of single layered graphene is 97.7%. In addition, the transmittance has a negative correlation with increasing number of layers.
3.2 What is the albedo of graphene? In the visible spectrum, the reflectivity of single-layered graphene is less than 0.1%, and the reflectivity of 10-layered graphene is a meager 2%.
3.3 What is the saturable absorption mechanism in graphene? When strong light meets graphene, the absorption in graphene is no longer linear, but is determined by the intensity of light. This effect is called the saturable absorption of light. For intense visible light or near infrared, graphene is easily saturated due to its overall optical absorption and zero band gap.
4. What are the thermal characteristics of graphene?
In theory, each gram of graphene has a surface area of up to 2640m^2. At room temperature, the thermal conductivity is about 5300 W/(m·K), higher than that of carbon nanotubes and diamonds, and is many times greater than those of metals such as copper and aluminum.
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1. How many graphene preparation methods are there?
Generally speaking, the preparation methods of graphene materials can be divided into three types: solid phase method, liquid phase method and gas phase method.
2. What is solid phase method?
Solid phase method is when carbon source is supplied in its solid state to produce graphene. This includes micro-mechanical exfoliation, epitaxial growth, and carbon nanotube exfoliation.
2.1 What is the micro-mechanical exfoliation and what the advantages and disadvantages?
Micro-mechanical exfoliation is the separation of graphene from highly oriented pyrolytic graphite by external force. Graphene was first produced through the micro-mechanical exfoliation method in 2004, and this method is still one of the most common methods for preparing graphene in laboratories.
First, etch out grooves on the surface of pyrolytic graphite. Then use scotch tape to peel off the surface repeatedly. Put the glass substrate with graphene nano-platelets into acetone solution to undergo ultrasonic oscillation. Afterwards transfer the graphene onto a single crystal silicon substrate.
2. The quality of samples are high
3. Fewer defects on products
4. Main method to prepare high quality single-layer graphene.
1. Varied Results
2. Products have small size and great uncertainty.
3. Inefficient, high cost and unsuitable for large-scale production.
2.2 What is epitaxial growth and what are its advantages and disadvantages?
The epitaxial growth includes both SiC and metal catalyzed epitaxial growths.
SiC epitaxial growth happens through heating SiC crystals at high temperatures, so that the Si atoms are evaporated and separated from the surface, and the remaining C atoms are reconstructed into graphene.
Metal catalyzed epitaxial growth occurs through the transfer of hydrocarbons to the transition metal substrate under ultra high vacuum settings, and catalyzing and dehydrogenating the adsorbed gas through heat.
1. Can be used to prepare single-layer or multi-layer graphene
2. Suitable for large-scale production
1. In the process of heating, the surface of SiC crystal can be easily reconstructed, which makes the surface structure of graphene appear more complex.
2. The morphology and properties of graphene prepared by metal catalyzed epitaxial growth are affected by the metal substrate.
3. The requirements of high heat and ultra high vacuum makes the equipment design and manufacturing process more difficult.
3. What is a liquid phase method?
Liquid phase method is applying the concept of solid phase exfoliation to liquids, and the method includes oxidation reduction, ultrasonic dispersion, and organic synthesis.
3.1 What is oxidation reduction and what are its advantages and disadvantages?
Oxidation reduction corrodes natural graphite with strong acids and oxidizing substances to generate graphite oxide (GO). Then it undergoes ultrasonic dispersion to turn into graphene oxide (mono-layer graphite oxide), and adds reduction agents such as carboxyl, epoxy and hydroxyl to remove oxygen-containing groups on the surface of graphite oxide.
This operation uses a strong oxidizing agent such as concentrated sulfuric acid, concentrated nitric acid, or potassium permanganate to oxidize graphite into GO.
In the oxidation process, some oxygen-rich functional groups are intercalated into graphite layers, increasing the distance between graphite layers. Then, after undergoing ultrasonic dispersion, single-layered or multi-layered graphene oxide are produced. With a strong reducing agent such as hydrazine hydrate, or sodium borohydride, graphene oxide is reduced to graphene.
1. Lower cost and relatively easy to accomplish
2. Effective way for preparing graphene on a large scale
3. Produces stable graphene suspension
4. Makes graphene dispersion easier
1. Can lead to liquid waste contamination
2. The outcome may have defects. i.e. five membered ring and seven membered ring’s topological defects, or the structural defect of -OH group, will limit graphene applications
3.2 What is ultrasonic dispersion and what are its advantages and disadvantages?
Ultrasonic dispersion occurs through the placing of small amounts of graphite into a solvent, forming a low concentration dispersion, and then use ultrasonic waves to destroy the Van der Waals’ force within the graphite layers. Afterwards, solvent is inserted into these layers, and the graphite is exfoliated into layers of graphene.
1. Chance to keep the integrity of the graphene structure and avoid the defects on the surface of graphene. In short, this preparation method can produce high quality graphene.
4. What is the gas phase method?
Gas phase method allows graphene to grow directly in gaseous or plasma states, including chemical vapor deposition (CVD) and arc discharge.
4.1 What is chemical vapor deposition and what are its advantages and disadvantages?
CVD uses gaseous carbon such as hydrocarbon to undergo pyrolysis at high temperatures. C atoms forms into metal, and the thickness of graphene can be altered by controlling the cooling rate. Graphene film is obtained by separating the film from the metal by slight chemical etching.
CVD can be used to control the size and number of graphene layers by adjusting the types of catalyst, the flow rates of gas mixtures, the reaction temperature and the holding time.
1. Can be carried out at lower temperatures
2. Graphene is easy to separate, which is beneficial to subsequent processing.
3. Can be used for mass production to produce high quality graphene.
1. Uses gaseous carbon, which is uncontrollable
2. Can not accurately control the number of layers of graphene during the preparation process
3. Metal substrates are expensive and complicated
Previously, we stated that single-layer graphene has a thickness of 0.335nm with a 1nm fluctuation in the vertical direction. We also described various methods in preparing graphene. The number of layers and the structure of graphene differs from various preparation methods. So effectively identifying the number of layers and structure of graphene is one of the key steps in obtaining high quality graphene.
The primary methods for characterizing graphene include; scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy (Raman) and Fu Liye transforming infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS).
1 Why use SEM to characterize graphene?
SEM identifies graphene morphology by reflecting the number of graphene layers through the color and surface folds of SEM images. Single-layer graphene under SEM shows an uneven surface with certain thickness. In order to reduce the surface energy, the morphology of single-layer graphene’s will change from 2D to 3D, so the amount of surface folds is significantly greater than multi-layer graphene. In addition, the amount of wrinkles decreases with the increase of graphene layers.
1.1 How does SEM work?
When a hot cathode electron gun or field emission cathode releases an electron beam, it projects towards the drawtube, and due to acceleration voltage(1-50kV) between anode and cathode voltage, it undergoes the convergence between the condenser and the objective lens, and a thin electron beam with a certain energy, intensity, and diameter is formed on the surface of the sample.
1.2 What is a two electron signal?
Two electron denotes the process of the outer electron of the sample atom bombarding and leaving the sample surface. This electron is free within the vacuum. Because the bond between the nucleus and the outer valence electrons is very small, electrons on the outer layer are easier to separate from atoms.
2 Why use TEM to characterize graphene?
TEM uses electron diffraction to estimate the number and size of graphene sheets. When changing the beam’s incident direction, the strength of diffraction spots in single-layer graphene remains unchanged. In double and multi-layer graphene, obvious changes in the beam incident angle brings a change on the strength of diffraction spots due to the inter layer interference effect.
2.1 What’s the principle behind TEM?
TEM usually uses a thermionic cathode gun to project an electron beam as the illumination source. Electrons emitted by thermionic cathode are driven through the anode hole at high velocities and are then concentrated into a beam towards the sample.
Electron beams with certain energies will react with the sample, and produce a variety of information; sample micro thickness, average atomic number, crystal structure and orientation difference.
3 Why use AFM to characterize graphene?
As the AFM probe slowly creeps towards the sample surface, the force between atoms will rise. The sample surface height can be directly converted through the amount of force of microprobe, and allows the AFM to gather information on the topography. Furthermore, the AFM also produces information on the transverse dimension, area and thickness of the sample.
3.1 How does AFM work?
Fix the side of a cantilever that is sensitive to force. The other side should contain small needlepoint. Gently push the needlepoint to the sample surface, and by controlling the variability of force when scanning, the cantilever should move in a perpendicular direction to the surface of the sample. This is due to the presence of a weak repulsive force between the needlepoint and the atoms on the sample surface. The surface topography of the sample can then be obtained through measuring the cantilever scanning points using optical detection or tunnel current detection.
4 Why use Raman to characterize graphene?
Raman spectroscopy is a fast and nondestructive method for characterizing crystal structure, electron band structure, phonon energy dispersion and electron phonon coupling. The structural defects of graphene, the in-plane vibration of sp2 carbon atoms and the stacking modes of carbon atoms can be obtained from the Raman spectrum’s shape, position and intensity.
4.1 How does Raman work?
The Raman spectrum is a result of the inelastic scattering of photons and molecules of the monochromatic incident light. When inelastic collision occurs, there is an energy exchange between the molecules and photons. Photons not only change the direction of their movement, but also transfer part of their energy to the molecules. Occasionally, the molecules will also transfer their vibration and rotation energy to photons, changing the frequency of the photon scattering. This process is called Raman scattering.
4.2 What is inelastic collision?
During the collision, objects tend to deform, emit heat, and produce sounds. As a result, kinetic energy will be lost in the process. Simply put, the loss of kinetic and mechanical energy due to internal frictions makes this an inelastic collision.
5 Why use FT-IR to characterize graphene?
FT-IR is a molecular absorption spectrum. When a molecule is energized by the infrared radiation it produces a vibrational energy level (and a rotational energy level). At this point, vibrational absorption occurs and the dipole moment changes, resulting in the infrared photons being absorbed to form an infrared absorption spectrum. FT-IR is mainly used to characterize the chemical structure of graphene along with its derivatives and composites.
5.1 How does FT-IR work?
First radiation from the infrared light goes through the concave mirror and forms parallel light. Then it enters the interferometer. The fluctuating light beam that leaves the interferometer then projects into a swinging mirror, letting the light travel through either the sample cell or alternate reference cell. Then, another oscillating mirror is used to focus the beam on the detector. In the process of moving, the moving mirror collects data in a predetermined range of length; of a finite size and equidistant position. The infrared absorption spectra is obtained through interferograms created through the data, and using the Fourier transform to process the signals. of transmittance or absorbance with wave number or wavelength are obtained.
6 Why use XPS to characterize graphene?
Using X – ray to radiate samples can stimulate the inner or valence electrons of atoms and molecules. Electrons energized by photons are called photoelectrons. The energy distribution of photoelectrons and information about the sample are obtained from the photoelectron spectrum.
With the development of graphene research, characterization methods of graphene are more and more abundant. However, graphene is generally only a few atomic layers thick, and the difference of crystal defects, surface adsorption materials and the preparation methods lead to different characterization results. Characterization of the mentioned methods above can characterize graphene in a certain extent, but there are some limitations. Practical research often requires a combination of characterization methods to obtain accurate information on graphene.
1 How can graphene be used with lithium batteries?
Graphene is an excellent electrical conductor. If used as a conduit for lithium batteries, it can improve the charging speed and overall performance of lithium batteries.
1.1 How is graphene used as the negative pole of lithium batteries?
Graphene can be directly used as the negative pole of lithium ion battery, or it can combine with SnO2, Si, Fe2O3, or TiO2 to serve as the negative pole.
1.2 How can graphene be used as the positive pole of lithium batteries?
Graphene can compounded with lithium iron phosphate or lithium vanadium phosphate to serve as the positive pole of the battery.
2. How is graphene used on lead-acid batteries?
Lead acid batteries can use graphene as electrode materials, instead of the original graphite material. This can extend the life span of batteries and improve charge capacity.
3. What are the main applications of graphene on cellphones?
Graphene films are very flexible, so they can be used to make cellphones with collapsible screens. Graphene can also be used within cellphone batteries and thermal conductive membranes.
4 What is the function of graphene on plastic modification?
Graphene can improve the strength, modulus, surface glossiness and surface hardness of plastics, as well as offer antistatic properties, thermal conductivity, and heat dissipation capability.
5 How does graphene be used with rubber?
Graphene can improve the wear and tear, heat dissipation capability and antistatic ability of rubber.
6 What are the applications of graphene in coating?
Graphene has enormous potential in applied coating. Graphene can be used in anti-corrosion, flame retardation, thermal conduction, and high strength coatings. In addition, electromagnetic shielding and anti-static coatings are also possible.
7 How can graphene be used with ink?
Graphene can be used to create electrically conductive ink.
8 What are some applications of graphene in electronics?
As mentioned above, graphene can be used to create conductive ink for printed circuits, so it can be used for bar codes, radio frequency identification etc.
9 What are the functions of graphene on optic cables?
Adding graphene powder into silver paste can help reduce resistance. Graphene coated cables reduces energy consumption and increases photoelectric conversion.
10 What are the applications of graphene in the electronics industry?
Graphene can be used in the preparation of ultra-thin conductive materials. It has an advantage in reducing cost, weight, and volume, while also improving performance.
11 How can graphene be applied to LED?
Graphene has high thermal conductivity and can be used to prepare heat radiation materials for lamps and lanterns. It can also be used to produce flexible light sources on a large scale.
12 What are the applications of graphene in environmental protection?
Graphene has a high SSA (Specific surface area) and can adsorb pollutants in water. Graphene can also be used as a filter in water treatment, and in sensors to detect trace substances in the environment.
13 What are the applications of graphene in textiles?
Graphene can be combined with a variety of cellulose to produce graphene fiber, graphene cashmere, and graphene pore material. Graphene can also be made into graphene textile sensors that can directly attach to clothing fabric. With the advent of smart-wear, it may be possible to collect data on user health.
14 What are graphene’s applications in the auto parts industry?
Graphene can be used to prepare high performance composite materials and anti-static tires.
15 What are graphene’s applications in lubricants?
Adding a minuscule of graphene to lubricating oil can greatly improve its performance.
16 What are the applications of graphene in the medical field?
Graphene can be used to prepare sensors for a variety of medical tests. If you want more information about this, please check:???
17 What are the applications of graphene in military?
Graphene can be used to create a unique high strength composite armor, functioning as a bullet proof, lightweight, camouflage-able, and conductive material ~ perfect for electromagnetic shielding or wave absorbing. In addition, graphene can also be used in the preparation of a high entropy alloy that meets a series of military needs.
1 what is the current market price of graphene?
In the beginning, graphene films, powders and dispersions were all in the limited stage of R & D and production, so the prices were incredibly high. Currently, many companies have established graphene production lines, suitable for large-scale production. Therefore, prices have dropped decreased. But the price of high-quality graphene powder still reaches thousands of yuan per gram. With expanding market applications and production scales, the price of graphene will be more competitive in the future.
2 what are the national graphene organizations?
China graphene industry technology innovation strategic alliance (CGIA2013), was founded in 2013. Within China, the current graphene companies are mostly distributed in the southeast coast, especially in the Yangtze River Delta region. Then followed by the Sichuan basin and Shandong region, while there are also a few companies in Tianjin, Shanxi and Inner Mongolia. In addition, Jiangsu, Inner Mongolia, Shandong and a few other places collaborated to set up the graphene alliance, which aims to promote government, enterprise, production and combined research to construct graphene raw materials for research and development, along with the industrial preparation and application of graphene.
4 opportunities for the graphene industry
In 2015, China Ministry of industry and information technology, the national development and Reform Commission, and the Ministry of science and Technology issued a statement, regarding the acceleration of the graphene industry and the development of innovation, to be accomplished by 2020, including a complete industrial grading system of graphene, standardizing the quality, along with low cost. In 2016, many policies were introduced, including; “nano materials”, “13th Five-Year special focus”, and “accelerating innovation and development of new materials industry”.
5 graphene industry concerns
Presently the concept of graphene is not widely known, and misinformation plagues the institutions and companies within the graphene industry. This is mostly due to the speculation of graphene; some individual enterprises refer to the graphite as graphene to accomplish sale goals, deceiving customers, and even advocating that graphene is a-jack-of-all-trades, with wonderful application in various fields. In addition, truthful information on graphene is not uniform, resulting in false information, and skepticism around graphene.
6, Published graphene related articles in China
Published articles related to graphene in China were steady from its discover until 2007, when it had exponential growth. This increase in publishings would continue until this year.
7, Graphene related patents in China
Graphene related patents in China were stable until 2009, after which the number of patents has increased exponentially.
8 graphene patent technology distribution
Graphene patented technology is concentrated in areas of energy storage in the form of lithium-ion batteries, super capacitors, solar cells, lead-acid batteries, fuel cells and the like. From 2013 to 2015, a large number of patents were filed for composite fibers, coatings, functional films, and water filters. Since then, the number of applications relating to graphene has decreased annually.
9 overall developments in the graphene industry
As of 2015, China accounted for 46% of the global share of graphene related patents. This incredible number is mostly attributed to the government support in the industry. There have been many special projects which has prompted China’s research and development to turn their attention to graphene. Besides China, South Korea, the United States, and Japan are the pioneering countries of graphene related research and technology.
10 current status of the graphene industry
The renowned academic Liu Zhongfan mentioned in his report that the graphene industry in China has reached the pinnacle of basic research. After which there will be breakthroughs in industrial usage and applications concerning graphene. New research will yield advances in the industry, leading to new technological cycles revolving around graphene.
1. Graphene defects?
The defects of graphene can be divided into two kinds: intrinsic and impurity.
1.1 What is intrinsic defect?
This defect is composed of carbon atoms uninvolved in graphene’s sp2 orbital hybridization. The changes in the hybridization orbitals are usually due to an imbalance of carbon atoms in carbon six-membered rings.
1.2 What is impurity defect?
Impurity defects are caused by carbon atoms that are covalently bound to graphene atoms. Due to the different kinds of atoms, an atomic defect in N or O changes the charge distribution and properties of graphene.
2. How many types of intrinsic defects are there?
There are five types: point defects, single hole defects, multiple hole defects, line defects, and defects introduced by off ground carbon atoms.
2.1 What is a point defect?
Point defects are formed due to C-C bond rotations, thus the defect neither introduces nor removes carbon in graphene molecules and doesn’t produce carbon atoms with dangling bonds. Point defects are produced by electron beam bombardment or rapid cooling in high temperature environments.
2.2 What is a single hole defect?
If a single carbon atom is lost in an arranged carbon six-membered ring, a single hole defect is formed on the graphene. It is obvious that the loss of a carbon atom results in the cleavage of the three covalent bonds that were originally associated with it, resulting in the formation of three dangling bonds. In order to reduce the overall energy loss of the molecule, the graphene rearranges and the final two dangling bonds are connected to each other, while one dangling bond remains.
2.3 What is a multiple hole defect?
On the basis of single hole defect, multiple hole defects are induced when carbon atoms with dangling bonds are lost.
2.4 What is a line defect?
Through the process of chemical vapor deposition, graphene will begin to grow in different positions on the metal surface, causing random growth. When the graphene grows to a certain size, cross fusion will occur, and the resulting defect is usually linear due to the defects of the initial crystal orientation.
3. How many types of impurity defects are there?
They can be divided into two kinds: one is the introduction of defects by out of plane hetero atoms, and the other is the substitution of intra – Surface heteroatom for defects.
3.1 What are out-of-plane heteroatom induced defects?
In the case of chemical vapor deposition, functional groups are introduced into the surface of graphene due to the usage of metal elements or oxygenic oxidants.
These hetero-atoms are bonded with carbon atoms in graphene by strong chemical bonds or weak Van der Waals’ force, which form out of plane heteroatom defects (OOPHD). In addition to metal atoms, (OOPHD) causes defects in the intrinsic properties of graphene (such as electrical properties, mechanical properties, and molecular assembly properties) due to oxidation. In general, such defects include hetero-atoms of oxygen atoms, hydroxyl groups and carboxyl groups.
3.2 What are In-plane heteroatom substitutional defects (IPHSD)?
Some atoms, such as nitrogen and boron, can form three chemical bonds, and thus can replace the positions of carbon atoms in graphene.
4. Why do graphene defects exist?
There are three cases: particle beam bombardment, chemical treatment initiation, and crystal growth defects.
4.1 What is particle beam bombardment?
When electron beams bombard the surface of graphene, carbon atoms leave the carbon six-membered ring. These carbon atoms either disappear from the surface, or transfer to the surface, forming new defects, and creating hetero atom defects. If the energy is suitable, ion beams, and gamma rays may also act on the graphene and produce the same effect.
4.2 What is initiation of chemical treatment?
Chemical agents containing elements such as oxygen, nitrogen, or boron are sometimes used to treat graphene. These treatments introduce atom impurity defects into graphene. Some of these defects are due to the process of producing graphene, while others result from the modification of graphene, which are intentional (such as the introduction of nitrogen atoms to graphene or graphene boron atoms to improve performance).
4.3 What is crystal growth defect?
The use of chemical vapor deposition in the preparation of graphene is actually through carbon atoms depositing and assembling on the surface of the metal. Due to the randomness of deposition, the growth of graphene in different regions can not form a uniform crystal extension orientation. The result is that when the regional graphene grows to a certain size and begins to intersect, different crystal orientation will lead to the formation of graphene line defects. This kind of defect is longer, and the graphene cannot become uniform on a large scale.
1 What is defect density?
Defect density refers to the distribution density of defects on graphene sheets. The defects involved include single vacancy defects, double vacancy defects, topological defects and various combinations of these defects. The defect density will lead to changes in band structure of graphene. In addition, it will also affect the vibration modes of graphene materials.
2 what are the methods for characterizing graphene defects?
Transmission electron microscopy (TEM) and Raman spectroscopy (SERS) are two main methods to characterize graphene defects. While TEM is suitable for academic research, Raman spectroscopy is better suited for industrial purposes.
3 How can we interpret graphene’s Raman spectra?
In Raman spectra in graphene there are different Raman peaks, such as D peaks, G peaks, D’ peaks, 2D (G’) peaks, D+D peaks (also denoted as D+G peaks or S3 peaks), 2D’ peaks, and D+D ‘peaks. Among them, D peaks, G peaks, 2D peaks and D+D peaks are used the most.
3.1 What is a D peak?
A D peak is considered a defect or boundary peak of graphene. It appears at 1270 ~1450 cm-1, revealing an inelastic inter valley scattering of iTO phonons near a K point and an inter valley scattering of a defect. This means that the defect will activate the respiratory vibration modes of the six- membered ring, and lead to the corresponding Raman active peaks. This defect is either a point defect on the slice layer or the edge of a lamellar grain boundary, so D peak is often used to characterize the defect or edge in graphene sample.
3.2 What is a G peak?
The G peak is usually considered the characteristic peak of sp2 hybridized carbon atoms. It is near 1580 cm-1, involving two scattering process at the gamma point near double degeneracy iTO and iLO phonon in the valley. The peak is generally generated by in-plane vibration of sp2 carbon atoms, and the corresponding vibrational modes have E2g symmetry. With the increase in graphene layers n, red shift of G peak positions also occur, and the displacement is related to 1/n. In addition, when the graphene sheet is stressed, G peak is also affected.
3.3 What is a D’ peak?
D’ peaks, similar to D peaks, are generally considered to be boundaries or defect peaks of graphene. The peak appears near 1620 cm-1, but the intensity is weak. It is often difficult to pick out a D’ peak when there’s a strong G peak. The formation of this peak can be both inter valley scattering and intra valley scattering. The main process is an intra valley scattering, involving an inelastic inter valley scattering of iLO phonons near a K point and an inter valley scattering of the defect. Although the intensity of D’ peaks is weak, there are strong correlations between the D’ peak and defects, boundaries, or hybrid forms of graphene. Therefore, D’ peaks are often one of the peaks that researchers pay close attention to.
3.4 What is 2D peak?
2D peaks, also labeled as G’ peaks, are the characteristic peaks that has the greatest difference between graphene and graphite. The peak has a certain wavelength dependence, and for a wavelength of 514 nm-1, the 2D peak appears near 2700 cm-1. The peak is produced by two inelastic scatterings from an iTO phonon near a K point. Two phonon resonance Raman peaks have nothing to do with defects, but is instead correlated with the layer structure and the mode of deposition. Therefore, it is used as a characteristics peak that identifies graphene and graphite.
3.5 What is a D+D peak?
D+D peaks are also often labeled as “D+G peaks” or “S3 peaks”, and are often considered as composite peaks of D peaks and D’ peaks. They are highly related to the graphene defect density. The peak generally appears near 2960 cm-1 and is a two phonon process peak. It is formed by a valley scattering between phonons near the K point, a valley scattering between phonons near K’ point, and an intra valley scattering of a defect. With the increase of defect density, the intensity of the peak rises significantly. Unlike weak D’ peaks, this peak is very easy to observe and is closely related to defect states. Therefore, it can be used to measure defect densities.
4 How do we characterize the defect density of graphene?
In the past, D peak intensity or D/G intensity ratio of Raman spectra were researched, and were used as the basis for evaluating defect densities of graphene materials. However, this method is suitable for graphene thin films and not applicable to graphene powders. With graphene films, the spot is more easily focused on a single graphene sheet during Raman testing; in contrast, graphene powders gathers large amounts of graphene sheets. As a result, Raman test spots are easier to focus on a large number of graphene sheets and boundaries. D peaks are increased due to an increase of boundary signals in the range of the facula.
5 How can we characterize the defect density of graphene powders?
As mentioned above, the peak intensity signal of the D peak is derived from the defect signal in the graphene powder. It can also be derived from a large number of boundary signals in graphene powders. This doesn’t necessarily indicate a defect correlation with the D peak, so it is difficult to judge the defect density on the graphene by the simple D peak intensity or D/G intensity ratios. D’ peaks have different peak intensities due to different defects, so part of the work is also compared with the intensity ratio of D/D’ as a way to screen the defects. However, the strength of D’ peaks is too weak to be detected. Therefore, for the characterization of the defect density of graphene powders, it’s reasonable to investigate the D+D’ peaks (S3 peaks), while reference peaks can consider 2D peaks unaffected by boundary states.
1 Graphene’s treatment of spinal cord injury
A research team at Rice University has focused on the study of graphene nano-ribbons for nearly 10 years. They discovered a process to “unwind” graphene nano-ribbons from multi-walled carbon nanotubes. Their discoveries were published in Science Magazine in 2009. Since then, researchers have used this discovery to improve materials; aircraft deicing, batteries, and low permeable gas storage containers. Researchers had also discovered that graphene could stimulate nerve growth, which led to the development of Texas-PEG, a nano composite based on nano-ribbon and polyethylene glycol. This material can be applied as a conductive frame, which can promote the truncation of spinal cord repair and reconnection.
1.1 How can Texas-PEG be used to treat the spinal cord?
PEG is a biocompatible polymer gel often used in surgery, medicine, or other biological applications. Graphene nano-ribbons developed in medical applications are highly soluble in PEG. Graphene nano-ribbons are first activated with PEG chains, and then mixed with PEG to form composites with good electrical conductivity, which helps in breaking connections at the ends of the spinal cord.
1.2 Is graphene treating spinal cord injury just a theory?
This is not theoretical.The results come from many experiments. Researchers performed experiments on rats with severed spinal cords and treated them with Texas-PEG material, resulting in the rats regaining partial function within a 24 hours. After two weeks, the rats recovered completely and showed near perfect motor control.
2 Graphene for in-vitro detection
The structure of a two-dimensional graphene plane with single atom thickness provides specific surface area, which can be used to load a large variety of metal, biological, and fluorescent molecules along with a variety of drugs. It also has many potential applications in the detection, isolation and purification of biomolecules, as well as targeted delivery of drugs. Because graphene has specific chemical properties which can immobilize a variety of biomolecules, its application is beyond ordinary carbon electrodes. For example, a glucose oxidase based sensor can be made by through graphene and glucose oxidase. Bio-sensing is a rapidly developing new technology. In medical field, using graphene as a sensor is an important research and application direction.
In addition, graphene can also detect the specific particles released before a heart attack which can be used to predict heart attacks. Currently, these graphene based sensors have been developed and used primarily to detect a variety of diseases, toxins and biomarkers. The properties of graphene, such as morphology, number of lamellae, degree of oxidation, and the interface of metal-graphene in composites, can directly affect the overall performance of the sensor. With further understanding of the intrinsic structure, physical and chemical properties of graphene, it will become widely used in bio-sensing systems.
2.1 What is bio-sensing?
Biosensors are pieces of analysis equipment combined biometric components, signal conversion devices, data processing systems and display systems. It is capable of sensing particular measured substances and converting them into an identifiable electrical signals. By processing the electrical signal, the sample and its data are detected.
Biosensors are part of a new wave of rapidly developing technology, with potential applications in many medical fields. Graphene has a unique advantage in the detection of food toxins, environmental pollution, specific pathogenic bacteria and bacteria. For example, when graphene oxide is attached to a specific toxin like protein structure, an enhanced signal can be generated to detect toxins with high sensitivity, and its detection capability is 10 times greater than an ordinary sensor.
2.2 What are the advantages of graphene-based biosensors?
- Small size
- Large specific surface area
- High sensitivity
- Fast response
- Fast electron transport
- Easy immobilization of proteins and maintenance of their activity
- Reduction of surface contamination
2.3 How do graphene gas sensors work?
Gas molecules adsorbed on the graphene layer act as receptors due to changes in electrical conductivity. These molecules reflect the concentration of the gas by changing the device conductance. Graphene has a large surface area which makes it sensitive to its surrounding environment, even if the gas molecule is adsorbed or released. Nano-medicine and the application of graphene in the medical field are still in early stages of development. However, it has the potential to bring revolutionary changes.
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