Growth in Field Emission Properties of Graphene Nanocomposites

Made by: Prince Arora (2K12/EP/048) Amish Popli (2K12/EP/009) Ali Danish (2K12/EP/008)ACKNOWLEDGEMENT

I take this opportunity to express my profound gratitude and deep regards to my mentor Prof. S.C. Sharma for his exemplary guidance, monitoring and constant encouragement throughout the course of this project. The blessing, help and guidance given by him time to time shall carry me a long way in the journey of life on which I am about to embark.

ABSTRACT

The proposed project is to solve a theoretical model for the field emission properties of graphene composite thin films. In this Project Report we study the basic synthesis, properties and structure of graphene. We extensively study the field emission properties of graphene which will be extremely helpful in our future goals related to the project. We have referred many research papers relating to our subject and made a note of our findings in this report.

INDEXS.No.TopicPage No.

1Introduction about graphene

2Basic Properties of graphene

3Advantages and uses

4Synthesis Of Graphene

5Graphene Composites

6Field Emission Property

7Field emission property of graphene composites

8Conclusion

9Bibliography

Introduction to graphene Molecular structure of graphene High resolution transmission electron microscope images(TEM) of graphene Brief History of Graphite Carbon takes its name from the latin word carbo meaning charcoal. This element is unique in that its unique electronic structure allows for hybridization to build up sp3, sp2, and sp networks and, hence, to form more known stable allotropes than any other element. The most common allotropic form of carbon is graphite which is an abundant natural mineral and together with diamond has been known since antiquity. Graphite consists of sp2 hybridized carbon atomic layers which are stacked together by weak van der Waals forces. The single layers of carbon atoms tightly packed into a two-dimensional (2D) honeycomb crystal lattice is called graphene. This name was introduced by Boehm, Setton, and Stumpp in 1994 . Graphite exhibits a remarkable anisotropic behavior with respect to thermal and electrical conductivity. It is highly conductive in the direction parallel to the graphene layers because of the in-plane metallic character, whereas it exhibits poor conductivity in the direction perpendicular to the layers because of the weak van der Waals interactions between them. The carbon atoms in the graphene layer form three bonds with neighboring carbon atoms by overlapping of sp2 orbitals while the remaining pz orbitals overlap to form a band of filled orbitals the valence band and a band of empty * orbitals the conduction band which are responsible for the high in-plane conductivity. The interplanar spacing of graphite amounts to 0.34 nm and is not big enough to host organic molecules/ions or other inorganic species. However several intercalation strategies have been applied to enlarge the interlayer galleries of graphite from 0.34 nm to higher values, which can reach more than 1 nm in some cases, depending on the size of the guest species. Since the first intercalation of potassium in graphite, a plethora of chemical species have been tested to construct what are known as graphite intercalation compounds (GICs). The inserted species are stabilized between the graphene layers through ionic or polar interactions without influencing the graphene structure. Such compounds can be formed not only with lithium, potassium, sodium, and other alkali metals, but also with anions such as nitrate, bisulfate, or halogens. In other cases the insertion of guest molecules may occur through covalent bonding via chemical grafting reactions within the interlayer space of graphite; this results in structural modifications of the graphene planes because the hybridization of the reacting carbon atoms changes from sp2 to sp3. A characteristic example is the insertion of strong acids and oxidizing reagents that creates oxygen functional groups on the surfaces and at the edges of the graphene layers giving rise to graphite oxide. Schafheutl first (1840) and Brodie, 19 years later (1859) were the pioneers in the production of graphite oxide. The former prepared graphite oxide with a mixture of sulfuric and nitric acid, while the latter treated natural graphite with potassium chlorate and fuming nitric acid. Staudenmaier proposed a variation of the Brodie method where graphite is oxidized by addition of concentrated sulfuric and nitric acid with potassium chlorate. A century later (1958) Hummers and Offeman reported the oxidation of graphite and the production of graphite oxide on immersing natural graphite in a mixture of H2SO4, NaNO3, and KMnO4 as a result of the reaction of the anions intercalated between the graphitic layers with carbon atoms, which breaks the aromatic character. The strong oxidative action of these species leads to the formation of anionic groups on graphitic layers, mostly hydroxylates, carboxylates, and epoxy groups. The out of planar CO covalent bonds increase the distance between the graphene layers from 0.35 nm in graphite to about 0.68 nm in graphite oxide. This increased spacing and the anionic or polar character of the oxygen groups formed impart to graphene oxide (GO) a strongly hydrophilic behavior, which allows water molecules to penetrate between the graphene layers and thereby increase the interlayer distance even further. Thus graphite oxide becomes highly dispersible in water. The formation of sp3 carbon atoms during oxidation disrupts the delocalized system and consequently electrical conductivity in graphite oxide deteriorates reaching between 103 and 107 cm depending on the amount of oxygen.

Graphene is a two-dimensional single-atom thick membrane of carbon atoms arranged in a honeycomb crystal. It is a perfect example of a two-dimensional electron system for a physicist, an elegant form of a two-dimensional organic macromolecule consisting of benzene rings for a chemist and a material with immense possibilities for an engineer due to its excellent electrical, magnetic, thermal, optical and mechanical properties. Bilayer graphene is also an important material as shown and has very unique electronic structure and transport properties. Another direction is of nanopatterned graphene structures, most notably grapheme nanoribbons consisting of one-dimensional stripes of the honeycomb arrangement, which lead to bandgap opening, edge functionalization, etc. Depending on the edge shape, two important nanoribbons are armchair grapheme nanoribbons and zigzag graphene nanoribbons shown in Fig. 1.1. Finally, when multiple graphene layers are stacked, one obtains graphitic materials, and multiple nanoribbons stacking leads to multilayer graphene nanoribbons. Historically, the word graphene comes from the Greek word graphein, which means to write one of the earliest uses of this material. In the 1800s, the name graphite was given to the bulk material used in pencils by the German chemistWagner. For some time, graphite was mistakenly thought to be a form of lead. The confusion of lead pencils comes from that misunderstanding. Nonetheless, grapheme and graphite have been of immense use to mankind both in physical sciences and in technology as well as in the art form. The inspiring arrangement of carbon atomsleads to the artistic and architectural lattice shell structures most notable perhaps is Bucky ball by Buckminster Fuller.The most important historical application of graphite was in the molds to make cannon balls. It was truly a strategic material. In fact, the British crown imposed embargo on graphite during the Napoleonic wars. Other historical uses of graphite include crucibles due to its refractory nature, lubrication because graphene planes can slide against each other with ease, electrodes and motor/generator brushes due to high conductivity, and materials processing e.g. steel and alloy making. The intercalation compounds of graphite were first reported in the 1840s and have been extensively studied since the 1930s. In recent history, the use of graphite as a neutron moderator to thermalize high energy neutrons in nuclear reactors has been of great significance. The fundamental breakthroughs towards the physical understanding of graphene and graphite were routed in the 1940s and 1950s. Modern derivatives also include carbon nanofibres (with diameters less than 10 nm) prepared and studied extensively in the 1970s and 1980s. Graphene can also be conceptually thought of as a mother material for Bucky ball molecules and carbon nanotubes. Their discoverers in the 1980s (by R. F. Curl Jr, H.W. Kroto, R. E. Smalley, J. R. Heath and co-workers) and 1990s (by S. Iijima), respectively, formed the basis of not only new fundamental research areas, but also exciting new set of applications.Since graphene is just an atomic plane of graphite, it was known to humans in the form of graphite deposits around the globe at least for few centuries and was effectively discovered since the invention of X-ray crystallography. It was important to isolate this atomic plane and, much more important, to show that this is a unique material worth further studying. Initial theoretical effort to study its 2D electronic structure was made by P. R.Wallace in 1947 followed by its extension to the electronic structure of 3D graphite by D. F. Johnston, J. W. McClure andM. Yamazaki. J. W. McClure also emphasized that the quasiparticles were Dirac-like, which was re-iterated by G.Semenoff.

Graphene Bilayer Graphene

Fundamental CharacteristicsBefore monolayer graphene was isolated in 2004, it was theoretically believed that two dimensional compounds could not exist due to thermal instability when separated. However, once graphene was isolated, it was clear that it was actually possible, and it took scientists some time to find out exactly how. After suspended graphene sheets were studied by transmission electron microscopy, scientists believed that they found the reason to be due to slight rippling in the graphene, modifying the structure of the material. However, later research suggests that it is actually due to the fact that the carbon to carbon bonds in graphene are so small and strong that they prevent thermal fluctuations from destabilizing it.Electronic PropertiesOne of the most useful properties of graphene is that it is a zero-overlap semimetal (with both holes and electrons as charge carriers) with very high electrical conductivity. Carbon atoms have a total of 6 electrons; 2 in the inner shell and 4 in the outer shell. The 4 outer shell electrons in an individual carbon atom are available for chemical bonding, but in graphene, each atom is connected to 3 other carbon atoms on the two dimensional plane, leaving 1 electron freely available in the third dimension for electronic conduction. These highly-mobile electrons are called pi () electrons and are located above and below the graphene sheet. These pi orbitals overlap and help to enhance the carbon to carbon bonds in graphene. Fundamentally, the electronic properties of graphene are dictated by the bonding and anti-bonding (the valance and conduction bands) of these pi orbitals.Combined research over the last 50 years has proved that at the Dirac point in graphene, electrons and holes have zero effective mass. This occurs because the energy movement relation (the spectrum for excitations) is linear for low energies near the 6 individual corners of the Brillouin zone.These electrons and holes are known as Dirac fermions, or Graphinos, and the 6 corners of the Brillouin zone are known as the Dirac points.Due to the zero density of states at the Dirac points, electronic conductivity is actually quite low. However, the Fermi level can be changed by doping (with electrons or holes) to create a material that is potentially better at conducting electricity than, for example, copper at room temperature.Tests have shown that the electronic mobility of graphene is very high, with previously reported results above 15,000 cm2 and theoretically potential limits of 200,000 cm2 (limited by the scattering of graphenes acoustic photons). It is said that graphene electrons act very much like photons in their mobility due to their lack of mass. These charge carriers are able to travel sub-micrometer distances without scattering; a phenomenon known as ballistic transport.However, the quality of the graphene and the substrate that is used will be the limiting factors. With silicon dioxide as the substrate, for example, mobility is potentially limited to 40,000 cm2.Mechanical StrengthAnother of graphenes stand-out properties is its inherent strength. Due to the strength of its 0.142 Nm-long carbon bonds, graphene is the strongest material ever discovered, with an ultimate tensile strength of 130,000,000,000 Pascals (or 130 gigapascals), compared to 400,000,000 for A36 structural steel, or 375,700,000 for Aramid (Kevlar).Not only is graphene extraordinarily strong, it is also very light at 0.77milligrams per square metre (for comparison purposes, 1 square meter of paper is roughly 1000 times heavier). It is often said that a single sheet of graphene (being only 1 atom thick), sufficient in size enough to cover a whole football field, would weigh under 1 single gram.What makes this particularly special is that graphene also contains elastic properties, being able to retain its initial size after strain. In 2007, Atomic force microscopic (AFM) tests were carried out on graphene sheets that were suspended over silicone dioxide cavities. These tests showed that graphene sheets (with thicknesses of between 2 and 8 Nm) had spring constants in the region of 1-5 N/m and a Youngs modulus (different to that of three-dimensional graphite) of 0.5 TPa.Again, these superlative figures are based on theoretical prospects using graphene that is unflawed containing no imperfections whatsoever and currently very expensive and difficult to artificially reproduce, though production techniques are steadily improving, ultimately reducing costs and complexity.

Optical Properties

Graphenes ability to absorb a rather large 2.3% of white light is also a unique and interesting property, especially considering that it is only 1 atom thick. This is due to its aforementioned electronic properties; the electrons acting like massless charge carriers with very high mobility. A few years ago, it was proved that the amount of white light absorbed is based on the Fine Structure Constant, rather than being dictated by material specifics. Adding another layer of graphene increases the amount of white light absorbed by approximately the same value (2.3%). Graphenes opacity of 2.3% equates to a universal dynamic conductivity value of G=e2/4 (2-3%) over the visible frequency range.Due to these impressive characteristics, it has been observed that once optical intensity reaches a certain threshold (known as the saturation fluence) saturable absorption takes place (very high intensity light causes a reduction in absorption). This is an important characteristic with regards to the mode-locking of fibre lasers. Due to graphenes properties of wavelength-insensitive ultrafast saturable absorption, full-band mode locking has been achieved using an erbium-doped dissipative soliton fibre laser capable of obtaining wavelength tuning as large as 30 nm.In terms of how far along we are to understanding the true properties of graphene, this is just the tip of the iceberg. Before graphene is heavily integrated into the areas in which we believe it will excel at, we need to spend a lot more time understanding just what makes it such an amazing material. Unfortunately, while we have a lot of imagination in coming up with new ideas for potential applications and uses for graphene, it takes time to fully appreciate how and what graphene really is in order to develop these ideas into reality. This is not necessarily a bad thing, however, as it gives us opportunities to stumble over other previously under-researched or overlooked super-materials, such as the family of 2D crystalline structures that graphene has born.

Applications and uses of grapheneGraphene, the well-publicised and now famous two-dimensional carbon allotrope, is as versatile a material as any discovered on Earth. Its amazing propertiesas the lightest and strongest material, compared with its ability to conduct heat and electricity better than anything else, mean that it can be integrated into a huge number of applications. Initially this will mean that graphene is used to help improve the performance and efficiency of current materials and substances, but in the future it will also be developed in conjunction with other two-dimensional (2D) crystals to create some even more amazing compounds to suit an even wider range of applications.To understand the potential applications of graphene, you must first gain an understanding of the basic properties of the material.The first time graphene was artificially produced; scientists literallytook a piece of graphite and dissected it layer by layer until only 1 single layer remained. This process is known as mechanical exfoliation. This resulting monolayer of graphite (known as graphene) is only 1 atom thick and is therefore the thinnest material possible to be created without becoming unstable when being open to the elements (temperature, air, etc.).Because graphene is only 1 atom thick, it is possible to create other materials by interjecting the graphene layers with other compounds (for example, one layer of graphene, one layer of another compound, followed by another layer of graphene, and so on), effectively using graphene as atomic scaffolding from which other materials are engineered.These newly created compounds could also be superlative materials, just like graphene, but with potentially even more applications.After the development of graphene and the discovery of its exceptional properties, not surprisingly interest in other two-dimensional crystals increased substantially. These other 2D crystals (such as Boron Nitride, Niobium Diselenide and Tantalum (IV) sulphide), can be used in combination with other 2D crystals for an almost limitless number of applications.So, as an example, if you take the compound Magnesium Diboride (MgB2), which is known as being a relatively efficient superconductor, then intersperse its alternating boron and magnesium atomic layers with individual layers of graphene, it improves its efficiency as a superconductor. Or, another example would be in the case of combining the mineral Molybdenite (MoS2), which can be used as a semiconductor, with graphene layers (graphene being a fantastic conductor of electricity) when creating NAND flash memory, to develop flash memory to be much smaller and more flexible than current technology, (as has been proven by a team of researchers at the cole Polytechnique Fdrale de Lausanne (EPFL) in Switzerland).The only problem with graphene is that high-quality graphene is a great conductor that does not have a band gap (it cant be switched off). Therefore to use graphene in the creation of future nano-electronic devices, a band gap will need to be engineered into it, which will, in turn, reduce its electron mobility to that of levels currently seen in strained silicone films. This essentially means that future research and development needs to be carried out in order for graphene to replace silicone in electrical systems in the future. However, recently a few research teams have shown that not only is this possible, it is probable, and we are looking at months, rather than years, until this is achieved at least at a basic level. Some say that these kinds of studies should be avoided, though, as it is akin to changing graphene to be something it is not.In any case, these two examples are just the tip of the iceberg in only one field of research, whereas graphene is a material that can be utilized in numerous disciplines including, but not limited to: bioengineering, composite materials, energy technology and nanotechnology.Biological EngineeringBioengineering will certainly be a field in which graphene will become a vital part of in the future; though some obstacles need to be overcome before it can be used. Current estimations suggest that it will not be until 2030 when we will begin to see graphene widely used in biological applications as we still need to understand its biocompatibility (and it must undergo numerous safety, clinical and regulatory trials which, simply put, will take a very long time). However, the properties that it displays suggest that it could revolutionise this area in a number of ways.With graphene offering a large surface area, high electrical conductivity, thinness and strength, it would make a good candidate for the development of fast and efficient bioelectric sensory devices, with the ability to monitor such things as glucose levels, haemoglobin levels, cholesterol and even DNA sequencing. Eventually we may even see engineered toxic graphene that is able to be used as an antibiotic or even anticancer treatment. Also, due to its molecular make-up and potential biocompatibility, it could be utilised in the process of tissue regeneration.Optical ElectronicsOne particular area in which we will soon begin to see graphene used on a commercial scale is that in optoelectronics; specifically touchscreens, liquid crystal displays (LCD) and organic light emitting diodes (OLEDs). For a material to be able to be used in optoelectronic applications, it must be able to transmit more than 90% of light and also offer electrical conductive properties exceeding 1 x 106 1m1 and therefore low electrical resistance.Graphene is an almost completely transparent material and is able to optically transmit up to 97.7% of light. It is also highly conductive, as we have previously mentioned and so it would work very well in optoelectronic applications such as LCD touchscreens for smartphones, tablet and desktop computers and televisions.Currently the most widely used material is indium tin oxide (ITO), and the development of manufacture of ITO over the last few decades time has resulted in a material that is able to perform very well in this application. However, recent tests have shown that graphene is potentially able to match the properties of ITO, even in current (relatively under-developed) states. Also, it has recently been shown that the optical absorption of graphene can be changed by adjusting the Fermi level.While this does not sound like much of an improvement over ITO, graphene displays additional properties which can enable very clever technology to be developed in optoelectronics by replacing the ITO with graphene. The fact that high quality graphene has a very high tensile strength, and is flexible (with a bending radius of less than the required 5-10mm for rollable e-paper), makes it almost inevitable that it will soon become utilized in these aforementioned applications.In terms of potential real-world electronic applications we can eventually expect to see such devices as graphene based e-paper with the ability to display interactive and updatable information and flexible electronic devices including portable computers and televisions.

UltrafiltrationAnother standout property of graphene is that while it allows water to pass through it, it is almost completely impervious to liquids and gases (even relatively small helium molecules). This means that graphene could be used as an ultrafiltration medium to act as a barrier between two substances. The benefit of using graphene is that it is only 1 single atom thick and can also be developed as a barrier that electronically measures strain and pressures between the 2 substances (amongst many other variables).A team of researchers at Columbia University have managed to create monolayer graphene filters with pore sizes as small as 5nm (currently, advanced nanoporous membranes have pore sizes of 30-40nm). While these pore sizes are extremely small, as graphene is so thin, pressure during ultrafiltration is reduced. Co-currently, graphene is much stronger and less brittle than aluminium oxide (currently used in sub-100nm filtration applications).What does this mean? Well, it could mean that graphene is developed to be used in water filtration systems, desalination systems and efficient and economically more viable biofuel creation.Composite MaterialsGraphene is strong, stiff and very light. Currently, aerospace engineers are incorporating carbon fibre into the production of aircraft as it is also very strong and light. However, graphene is much stronger whilst being also much lighter. Ultimately it is expected that graphene is utilized (probably integrated into plastics such as epoxy) to create a material that can replace steel in the structure of aircraft, improving fuel efficiency, range and reducing weight.Due to its electrical conductivity, it could even be used to coat aircraft surface material to prevent electrical damage resulting from lightning strikes. In this example, the same graphene coating could also be used to measure strain rate, notifying the pilot of any changes in the stress levels that the aircraft wings are under.These characteristics can also help in the development of high strength requirement applications such as body armour for military personnel and vehicles.Photovoltaic CellsOffering very low levels of light absorption (at around 2.7% of white light) whilst also offering high electron mobility means that graphene can be used as an alternative to silicon or ITO in the manufacture of photovoltaic cells. Silicon is currently widely used in the production of photovoltaic cells, but while silicon cells are very expensive to produce, graphene based cells are potentially much less so.When materials such as silicon turn light into electricity it produces a photon for every electron produced, meaning that a lot of potential energy is lost as heat. Recently published research has proved that when graphene absorbs a photon, it actually generates multiple electrons. Also, while silicon is able to generate electricity from certain wavelength bands of light, graphene is able to work on all wavelengths, meaning that graphene has the potential to be as efficient as, if not more efficient than silicon, ITO or (also widely used) gallium arsenide.Being flexible and thin means that graphene based photovoltaic cells could be used in clothing; to help recharge your mobile phone, or even used as retro-fitted photovoltaic window screens or curtains to help power your home.Energy StorageOne area of research that is being very highly studied is energy storage. While all areas of electronics have been advancing over a very fast rate over the last few decades (in reference to Moores law which states that the number of transistors used in electronic circuitry will double every 2 years), the problem has always been storing the energy in batteries and capacitors when it is not being used. These energy storage solutions have been developing at a much slower rate.The problem is this: a battery can potentially hold a lot of energy, but it can take a long time to charge, a capacitor, on the other hand, can be charged very quickly, but cant hold that much energy (comparatively speaking).The solution is to develop energy storage components such as either a super capacitor or a battery that is able to provide both of these positive characteristics without compromise.Currently, scientists are working on enhancing the capabilities of lithium ion batteries (by incorporating graphene as an anode) to offer much higher storage capacities with much better longevity and charge rate. Also, graphene is being studied and developed to be used in the manufacture of super capacitors which are able to be charged very quickly, yet also be able to store a large amount of electricity.Graphene based micro-super capacitors will likely be developed for use in low energy applications such as smart phones and portable computing devices and could potentially be commercially available within the next 5-10 years. Graphene-enhanced lithium ion batteries could be used in much higher energy usage applications such as electrically powered vehicles, or they can be used as lithium ion batteries are now, in smartphones, laptops and tablet PCs but at significantly lower levels of size and weight.

Synthesis of graphemeSynthesis of Single Layer and Few Layered Graphenes(SLG and FG) have been synthesized by several methods. In Table 1.1, we have listed some of these methods. The synthesis procedure can be broadly classified into exfoliation, chemical vapor deposition (CVD), arc discharge, and reduction of graphene oxide.Mechanical ExfoliationStacking of sheets in graphite is the result of overlap of partially filled pz or orbital perpendicular to the plane of the sheet (involving van der Waals forces). Exfoliation is the reverse of stacking; owing to the weak bonding and large lattice spacing in the perpendicular direction compared to the small lattice spacing and stronger bonding in the hexagonal lattice plane, it has been tempting to generate graphene sheets through exfoliation of graphite (EG). Graphene sheets of different thickness can indeed be obtained through mechanical exfoliation or by peeling off layers from graphitic materials such as highly ordered pyrolytic graphite (HOPG), single-crystal graphite, or natural graphite. Peeling and manipulation of graphene sheets have been achieved through AFM and STM tips. Greater control over folding and unfolding could be achieved by modulating the distance or bias voltage between the tip and the sample. Zhang obtained 10100 nm thick graphene sheets using graphite island attached to tip of micro machined Si cantilever to scan over SiO2/Si surface. Folding and tearing of the sheets arise due to the formation of sp3-like line defects in the sp2 graphitic network, occurring preferentially along the symmetry axes of graphite. Novoselov et al pressed patterned HOPG square meshes on a photo resist spun over a glass substrate followed by repeated peeling using scotch tape and then released the flakes so obtained in acetone. Some flakes got deposited on the SiO2/Si wafer when dipped in the acetone dispersion. Using this method, atomically thin graphene sheets were obtained. This method was simplified to just peeling off of one or a few sheets of graphene using scotch tape and depositing them on SiO2 (300 nm)/Si substrates. Although mechanical exfoliation produces graphene of the highest quality (with least defects), the method is limited due to low productivity. Chemical exfoliation, on the other hand, possesses the advantages of bulk-scale production.

Chemical Exfoliation Chemical exfoliation is a two-step process. The first step is to increase the interlayer spacing, thereby reducing the interlayer van der Waals forces. This is achieved by intercalating graphene to prepare graphene-intercalated compounds (GICs). The GICs are then exfoliated into graphene with single to few layers by rapid heating or sonication. A classic example of chemical exfoliation is the generation of single-layer graphene oxide (SGO) prepared from graphite oxide by ultrasonication. Graphene oxide (GO) is readily prepared by the Hummers method involving the oxidation of graphite with strong oxidizing agents such as KMnO4 and NaNO3 in H2SO4/H3PO4. On oxidation, the interlayer spacing increases from 3.7 to 9.5 A, and exfoliation resulting in SLG is achieved by simple ultrasonication in a DMF/water (9:1) (dimethyl formamide) mixture. The SGO so prepared has a high density of functional groups, and reduction needs to be carried out to obtain graphene-like properties.

Chemical Vapor DepositionSimply put, CVD is a way of depositing gaseous reactants onto a substrate. The way CVD works is by combining gas molecules (often using carrier gases) in a reaction chamber which is typically set at ambient temperature. When the combined gases come into contact with the substrate within the reaction chamber (which is heated), a reaction occurs that create a material film on the substrate surface. The waste gases are then pumped from the reaction chamber. The temperature of the substrate is a primary condition that defines the type of reaction that will occur, so it is vital that the temperature is correct.During the CVD process, the substrate is usually coated a very small amount, at a very slow speed, often described in microns of thickness per hour. The process is similar to physical vapour deposition (PVD), the only difference being that the precursors are solid compounds, rather than gases, and therefore the process is slightly different. The solid compound or compounds is/are vaporized, and then deposited onto a substrate via condensation.The benefits of using CVD to deposit materials onto a substrate are that the quality of the resulting materials is usually very high. Other common characteristics of CVD coatings include imperviousness, high purity, fine grained and increased hardness over other coating methods. It is a common solution for the deposit of films in the semiconductor industry, as well as in optoelectronics, due to the low costs involved compared to the high purity of films created.Although there are a number of different formats of CVD, most modern processes come under two headings separated by the chemical vapour deposition operating pressure: LPCVD, and UHVCVD. LPCVD (low pressure CVD) is the CVD procedure carried out under sub-atmospheric pressures. This low pressure helps to prevent unwanted reactions and produce more uniform thickness of coating on the substrate. UHVCVD (ultra-high vacuum CVD) is a process is which CVD is carried out under extremely low atmospheric pressures; usually in the region of 10-6 Pascals.The disadvantages to using CVD to create material coatings are that the gaseous by-products of the process are usually very toxic. This is because the precursor gases used must be highly volatile in order to react with the substrate, but not so volatile that it is difficult to deliver them to the reaction chamber. During the CVD process, the toxic by-products are removed from the reaction chamber by gas flow to be disposed of properly.Fundamental Processes in the Creation of CVD GrapheneCVD graphene is created in two steps, the precursor pyrolysis of a material to form carbon, and the formation of the carbon structure of graphene using the disassociated carbon atoms. The first stage, the pyrolysis to disassociated carbon atoms, must be carried out on the surface of the substrate to prevent the precipitation of carbon clusters (soot) during the gas phase. The problem with this is that the pyrolytic decomposition of precursors requires extreme levels of heat, and therefore metal catalysts must be used to reduce the reaction temperature.The second phase of creating the carbon structure out of the disassociated carbon atoms, also requires a very high level of heat (over 2500 degrees Celsius without a catalyst), so a catalyst is imperative at this stage to reduce the temperature needed for a reaction to occur to around 1000 degrees Celsius. The problem with using catalysts is that you are effectively introducing more compounds into the reaction chamber, which will have an effect on the reactions inside the chamber. One example of these effects is the way the carbon atoms dissolve into certain substrates such as Nickel during the cooling phase.What all this means is that it is vitally important that the CVD process is very stringently co-ordinated, and that controls are put in place at every stage of the process to ensure that the reactions occur effectively, and that the quality of graphene produced is of the highest attainable.

Problems Associated with the Creation of CVD GrapheneIn order to create monolayer or few layer graphene on a substrate, scientists must first overcome the biggest issues with the methods that have been observed so far.The first major problem is that while it is possible to create high quality graphene on a substrate using CVD, the successful separation or exfoliation of graphene from the substrate has been a bit of a stumbling block. The reason for this is primarily because the relationship between graphene and the substrate it is grown on is not yet fully understood, so it is not easy to achieve separation without damaging the structure of the graphene or affecting the properties of the material. The techniques on how to achieve this separation differ depending on the type of substrate used. Often scientists can choose to dissolve the substrate in harmful acids, but this process commonly affects the quality of the graphene produced, so other methods are currently being researched.One alternative method that has been researched involves the creation of CVD graphene on a copper (Cu) substrate (in this example, Cu is used as a catalyst in the reaction). During CVD a reaction occurs between the copper substrate and the graphene that create a high level of hydrostatic compression, coupling the graphene to the substrate. It has been shown to be possible, however, to intercalate a layer of copper oxide (which is mechanically and chemically weak) between the graphene and the copper substrate to reduce this pressure and enable the graphene to be removed relatively easily (also, in this instance, the substrate can be reused).Scientists have also been looking into using Poly(methyl methacrylate) (PMMA) as a support polymer to facilitate the transfer of graphene onto an alternate substrate. With this method, graphene is coated with PMMA, and the previous substrate is etched. Then, the coated graphene is strong enough to be transferred to another substrate without damaging the material. Other support polymers that have been tested include thermal release tape and PDMS (Polydimethylsiloxane). However, PMMA has been shown to be the most effective at transferring the graphene without excessive damage.Another major hurdle is creating a completely uniform layer of graphene on a substrate. This is difficult to achieve as the kinetic transport dynamics of gas is affected by diffusion and convection, meaning that these values change within the space of a reaction chamber, in turn affecting the chemical reactions on the substrate. Also, due to fluid dynamics, there might be a depletion of reactants by the time gas reaches the further ends of the substrate, meaning that no reaction will occur. Some scientists have reported overcoming this issue by modifying the concentration of gases and also by incorporating spin coating methods.Current and Potential SolutionsIn terms of overcoming these issues, scientists have been developing more complex techniques and guidelines to follow in order to create the highest quality of graphene possible. One introductory technique to reducing the effects of these issues is by treating the substrate before the reaction takes place. A copper substrate can be chemically treated to enable reduced catalytic activity, increase the Cu grain size and rearrange the surface morphology in order to facilitate the growth of graphene flakes that contain fewer imperfections.This point of treating the substrate prior to deposition is something that will continue to be researched for a long time, as we slowly learn how to modify the structure of graphene to suit different applications. For example, in order to enable graphene to be effectively used in superconductors, doping must be carried out on the material in order to create a band-gap. This process could potentially be something that is carried out on a substrate before deposition occurs rather than treating the material after CVD.

Arc Discharge

Synthesis of graphene by the arc evaporation of graphite in the presence of hydrogen has been reported. This procedure yields graphene arc discharge graphene in H2 atmosphere (HG) sheets with two to three layers having flake size of 100200 nm. This makes use of the knowledge that the presence of H2 during arc discharge process terminates the dangling carbon bonds with hydrogen and prevents the formation of closed structures. The conditions that are favorable for obtaining graphene in the inner walls are high current (above 100 A), high voltage (>50 V), and high pressure of hydrogen (above 200 Torr). In figure.TEM and AFM images of HG sample are shown, respectively. This method has been conveniently used to dope graphene with boron and nitrogen. To prepare boron-doped graphene (B-HG) and nitrogen-doped graphene (N-HG), the discharge is carried out in the presence of H2 + diborane and H2 + pyridine or ammonia, respectively. Later, based on these observations, some modifications in the synthetic conditions also yielded FG in bulk scale. Cheng et al. used hydrogen arc discharge process as a rapid heating method to prepare graphene from GO. Arc discharge in an air atmosphere resulted in graphene nanosheets that are 100200 nm wide predominantly with two layers. The yield depends strongly on the initial air pressure. Li et al. have synthesized N-doped multilayered graphene in He and NH3 atmosphere using the arc discharge method. Arc discharge carried out in a helium atmosphere has been explored to obtain graphene sheets with different number of layers by regulating gas pressures and currents

Graphene Composites

Composites-Composite materials(also calledcomposition materialsor shortened tocomposites) are materials made from two or more constituent materials with significantly differentphysicalorchemical properties, that when combined, produce a material with characteristics different from the individual components. The individual components remain separate and distinct within the finished structure. The new material may be preferred for many reasons: common examples include materials which are stronger, lighter or less expensive when compared to traditional materials.Typicalengineeredcomposite materials include: Composite building materials such ascements,concrete Reinforced plasticssuch asfiber-reinforced polymer MetalComposites Ceramic Composites (composite ceramic and metal matrices)

How are composite materials made?The three main factors that help mold the end composite material are the matrix, reinforcement and manufacturing process. As matrix, many composites use resins, which are thermosetting or thermo softening plastics (hence the name reinforced plastics often given to them). These are polymers that hold the reinforcement together and help determine the physical properties of the end composite.Thermosetting plastics begin as liquid but then harden with heat. They do not return to liquid state and so they are durable, even in extreme exposure to chemicals and wear. Thermo softening plastics are hard at low temperatures and but soften with heat. They are less commonly used but possess interesting advantages like long shelf life of raw material and capacity for recycling. There are other matrix materials such as ceramics, carbon and metals that are used for specific purposes.Reinforcement materials grow more varied with time and technology, but the most commonly used ones are still glass fibers. Advanced composites tend to favor carbon fibers as reinforcement, which are much stronger than glass fibers, but are also more expensive. Carbon fiber composites are strong and light, and are used in aircraft structures and sports gear (golf clubs and various rackets). They are also increasingly used to replace metals that replace human bones. Some polymers make good reinforcement materials, and help make composites that are strong and light.The manufacturing process usually involves a mould, in which the reinforcement is first placed and then the semi-liquid matrix is sprayed or poured in to form the object. Moulding processes are traditionally done by hand, though machine processing is becoming more common. One of the new methods is called pultrusion and is ideal for making products that are straight and have a constant cross section, like different kinds of beams. Products that of thin or complex shape (like curved panels) are built up by applying sheets of woven fiber reinforcement, saturated with matrix material, over a mould. Advanced composites (like those which are used in aircraft) are usually made from a honeycomb of plastic held between two sheets of carbon-fiber reinforced composite material, which results in high strength, low weight and bending stiffness.

Graphene and composite materials

As was stated before, graphene has a myriad of unprecedented attributes, any number of which could potentially be used to make extraordinary composites. The presence of graphene can enhance the conductivity and strength of bulk materials and help create composites with superior qualities. Graphene can also be added to metals, polymers and ceramics to create composites that are conductive and resistant to heat and pressure.

Graphene composites have many potential applications, with much research going on to create unique and innovative materials. The applications seem endless, as one graphene-polymer proves to be light, flexible and an excellent electrical conductor, while another dioxide-graphene composite was found to be of interesting photocatalytic efficiencies, with many other possible coupling of materials to someday make all kinds of composites. The potential of graphene composites includes medical implants, engineering materials for aerospace and renewables and much more.

Applications of Graphene-Based Nanocomposites

Graphene has a great number of applications encompassing engineering, electronics, medicine, energy, industrial, household design, and many more . A previous review search yielded several review papers that examined field-oriented and specific applications of graphene. Majority of the papers dealt with electronic/sensor-oriented applications, to generalize the broad applications of graphene and graphene-based nanocomposite into their respective disciplines. Shen et al. (2012) extensively reviewed the biomedical applications of graphene including drug delivery, gene delivery, cancer therapy, biosensing and bioimaging, GO-based antibacterial materials, and scaffolds for tissue/cell culturing. Similarly, Huang et al. (2011) and Choi et al. (2010) explained various phenomena associated with graphene and graphene-based materials and their applications in the field of memory devices for electronics, ranging from electrochemical sensors to instrumentation.

1. Biological Applications of Graphene and Graphene-Based Nanocomposites

Graphene in various derivatives and in its precursor form has also shown potential applications in biological/medical fields, especially related to toxicity. Hu et al. (2010) demonstrated the antibacterial activity of two types of water dispersible graphene againstE. coliwith minimum cytotoxic effects on the human participants. The group concluded that GO paper can one day be effectively used in various environmental and biological applications . Liao et al. (2011) demonstrated the cytotoxicity effect of graphene and graphene oxide (GO) materials under controlled physicochemical parameters. The results showed that GO was more severely hemolytic than graphene and showed high activity under extremely small size. They observed that, when chitosan was coated on GO, the hemolytic activity disappeared completely, showing the biocompatibility of the composite for erythrocytes. They concluded that the biological or toxicological responses of the material were dependent on the particle size, quality, and state, the surface charge, and the oxygen threshold. Similarly, Liu et al. (2011) compared four different types of graphene materials (graphite (Gt), graphite oxide (GtO), GO, and reduced graphene oxide (rGO)) againstE. coli, to study the toxicity effects. The membrane and oxidative stress signals were used to measure the intensity of toxicity. Their results showed that GO was the most severely toxic, followed in descending order by rGO, Gt, and GtO. Santos et al. reported the design, fabrication, and antimicrobial application of a graphene-poly-N-vinyl carbazole (P1VK) nanocomposite, resulting in more than 80% microbial inhibition and toxicity toward a broad array of bacteria. Carpio et al. (2012) studied the toxicity effects of PVK-GO nanocomposite onplanktonic microbial cells,E. coli,C. metallidurans B. subtilisandR. opacus, biofilms, and mammalian fibroblast cells (NIH 3T3). Their results showed that PVK-GO presented a stronger antimicrobial effect than pristine GO. They also found that the PVK-GO was significantly neutral toward the fibroblast cells, indicating a huge potential of the composite material in biomedical and industrial applications . Peng et al. (2012) studied an Mn-ferrite (MnFe2O4)-decorated GO nanocomposite for biomedical applications. They observed that the magnetic property of the ferrites can be effectively used as an ideal hyperthermia andcontrast MRI agent. The nanocomposite when PEGylated showed excellent biocompatibility. Recently, Liu et al. (2013) synthesized a hydroxyapatite-GO nanocomposite as biocompatible prosthetic. They found that the (300) and (002) plane hydroxyapatite nanorods in the graphene matrix played a crucial role in maintaining the composites mechanical properties. Given its superior mechanical property, the authors suggested the nanocomposites potential in composite and biomedical industries . Many other applications of graphene nanocomposites in the field of electronics and other disciplines have been reported. Generalized applications of various kinds of graphene-based nanocomposites have been described in the literature. These include sensors, Li-Ion batteries, fuel cells solar, field emission , super capacitors, thermal transport and stability , packaging industry , corrosion , fire packaging and resistance, and many more. We expect that, very soon, all these applications will be available from manufacturers to end users at common commercial stores.

2. Ceramic Reinforced Graphene Nanocomposites and Their ApplicationThe recent use of ceramics in grapheme-based nanocomposite has sparked a global interest. The introduction of ceramic materials in few-layered graphene results in the formation of a composite yielding exceptional electrochemical performance with high charge carrier properties. The exploitation of such properties is a boon to the energy industry . Several ceramic-graphene composites like SiC-Graphene , Si3N4-graphene , Al2O3-graphene , ZrB2-graphene , ZrO-Al2O3-graphene, BN-Graphene, and many more are known to enhance not only electrical properties but also thermal conductivity, refractory, mechanical, antifriction, anticorrosive and biocompatibility properties for diverse applications. Use of ceramics within graphene matrix can help overcome the brittle nature, lower fracture toughness, and limited thermal shock resistance in the composite industry. The use of ZrB2-graphene is presently known to be used in aerospace industry as a high temperature barrier for space vehicle during the reentry event. These materials (ultrahigh temperature ceramic composites) are consistently used as the primal infrastructure for the nose caps in space shuttles and military ballistic equipment. Several other ultrahigh temperature ceramic composites have shown promising results. A few ultrahigh temperature ceramic composites are known to exist, for example, carbides of Ta, Zr, Hf, Nb, and borides of Hf, Zr, and Ti, respectively. Recently, Lahiri et al. (2013) have shown that with the introduction of short CNTs as reinforcement within the TaC ceramics, one can induce the formation of mulitlayers of graphene within the host matrix during the spark plasma sintering. This procedure helps in offering high resistance to pullout which results in higher strength material with delayed fracture . Similarly, Pejakovi et al. (2010) reported the synthesis of carbon rich-hafnia thin films using PLD technique. The NMR results showed that the sample contained graphene aromatically bonded carbon atoms presumably in graphene phase . TiN-graphene composites, on other hand, have shown promising results as a selective permeable membrane for hydrogen. The composite material according to Kim and Hong (2012) was prepared by hot press process. The disc obtained was used to study the hydrogen gas permeability between 0.1 and 0.3MPa and at 473, 573 and 673K, respectively, using a Knudsen diffusion model. The results obtained showed that the hydrogen permeability of TiN-graphene composites was better than the Pd-Ag amorphous membrane at 1.67, 2.09, and 2.83 107mol/msPa1/2at 673K under 0.3MPa, respectively . Almost similar results (2.62 107mol/msPa1/2at 673K under 0.3MPa) of hydrogen permeation were obtained recently by Lee et al. (2013) with the use of Al2O3/CeO2/graphene (ACG) composite membranes prepared by hot-press method. By exploiting the pore size distribution, surface area, and elasticity, one can use such kinds of membranes for high purity separation and filtration of chemicals, biomolecules, petroleum products, and many more .

Field emission properties

Field emission(FE) (also known asfield electron emissionandelectron field emission) is emission ofelectronsinduced by an electrostatic field. The most common context is field emission from a solid surface into vacuum. However, field emission can take place from solid or liquid surfaces, into vacuum, air, a fluid, or any non-conducting or weakly conducting dielectric. The field-induced promotion of electrons from the valence to conduction band of semiconductors (theZener effect) can also be regarded as a form of field emission. The terminology is historical because related phenomena of surface photoeffect,thermionic emission(orRichardsonDushman effect) and "cold electronic emission", i.e. the emission of electrons in strong static (or quasi-static) electric fields, were discovered and studied independently from the 1880s to 1930s. When field emission is used without qualifiers it typically means "cold emission".Field emission in pure metals occurs in high electric fields: the gradients are typically higher than 1 gigavolt per metre and strongly dependent upon thework function. Electron sources based on field emission have a number of applications, but it is most commonly an undesirable primary source ofvacuum breakdown and electrical dischargephenomena, which engineers work to prevent. Examples of applications for surface field emission include construction of bright electron sources for high-resolutionelectron microscopesor to dischargespacecraftfrom induced charges. Devices which eliminate induced charges are termedcharge-neutralizers.Field emission was explained byquantum tunnelingof electrons in the late 1920s. This was one of the triumphs of the nascentquantum mechanics. The theory of field emission from bulk metals was proposed byRalph H. FowlerandLothar Wolfgang Nordheim. A family of approximate equations, "FowlerNordheim equations", is named after them. Strictly, FowlerNordheim equations apply only to field emission from bulk metals and (with suitable modification) to other bulkcrystalline solids, but they are often used as a rough approximation to describe field emission from other materials.In some respects, field electron emission is a paradigm example of what physicists mean bytunneling. Unfortunately, it is also a paradigm example of the intense mathematical difficulties that can arise. Simple solvable models of the tunneling barrier lead to equations (including the original 1928 FowlerNordheim-type equation) that get predictions of emission current density too low by a factor of 100 or more. If one inserts a more realistic barrier model into the simplest form of theSchrdinger equation, then an awkward mathematical problem arises over the resulting differential equation: it is known to be mathematically impossible in principle to solve this equation exactly in terms of the usual functions of mathematical physics, or in any simple way. To get even an approximate solution, it is necessary to use special approximate methods known in physics as "semi-classical" or "quasi-classical" methods. Worse, a mathematical error was made in the original application of these methods to field emission, and even the corrected theory that was put in place in the 1950s has been formally incomplete until very recentlyA consequence of these (and other) difficulties has been a heritage of misunderstanding and disinformation that still persists in some current field emission research literature. This article tries to present a basic account of field emission "for the 21st century and beyond" that is free from these confusions.

Practical applications: past and present

Field electron microscopy and related basics

After Fowler-Nordheim theoretical work in 1928, a major advance came with the development in 1937 byErwin W. Muellerof the spherical-geometryfield electron microscope(FEM)(also called the "field emission microscope"). In this instrument, the electron emitter is a sharply pointed wire, of apex radiusr. This is placed, in a vacuum enclosure, opposite an image detector (originally a phosphor screen), at a distanceRfrom it. The microscope screen shows a projection image of the distribution of current-densityJacross the emitter apex, with magnification approximately (R/r), typically 105to 106. In FEM studies the apex radius is typically 100nm to 1 m. The tip of the pointed wire, when referred to as a physical object, has been called a "field emitter", a "tip", or (recently) a "Mueller emitter".When the emitter surface is clean, this FEM image is characteristic of: (a) the material from which the emitter is made: (b) the orientation of the material relative to the needle/wire axis; and (c) to some extent, the shape of the emitter endform. In the FEM image, dark areas correspond to regions where the local work functionis relatively high and/or the local barrier fieldFis relatively low, soJis relatively low; the light areas correspond to regions whereis relatively low and/orFis relatively high, soJis relatively high. This is as predicted by the exponent of Fowler-Nordheim-type equations.Theadsorptionof layers of gas atoms (such as oxygen) onto the emitter surface, or part of it, can create surfaceelectric dipolesthat change the local work function of this part of the surface. This affects the FEM image; also, the change of work-function can be measured using a Fowler-Nordheim plot (see below). Thus, the FEM became an early observational tool ofsurface science.For example, in the 1960s, FEM results contributed significantly to discussions onheterogeneous catalysis.FEM has also been used for studies ofsurface-atom diffusion. However, FEM has now been almost completely superseded by newer surface-science techniques.A consequence of FEM development, and subsequent experimentation, was that it became possible to identify (from FEM image inspection) when an emitter was "clean", and hence exhibiting its clean-surface work-function as established by other techniques. This was important in experiments designed to test the validity of the standard Fowler-Nordheim-type equation .These experiments deduced a value of voltage-to-barrier-field conversion factorfrom a Fowler- Nordheim plot , assuming the clean-surfacevalue for tungsten, and compared this with values derived fromelectron-microscopeobservations of emitter shape and electrostatic modeling. Agreement to within about 10% was achieved. Only very recently has it been possible to do the comparison the other way round, by bringing a well-prepared probe so close to a well-prepared surface that approximate parallel-plate geometry can be assumed and the conversion factor can be taken as 1/W, whereWis the measured probe-to emitter separation. Analysis of the resulting Fowler-Nordheim plot yields a work-function value close to the independently known work-function of the emitter.Field electron spectroscopy (electron energy analysis)Energy distribution measurements of field-emitted electrons were first reported in 1939.In 1959 it was realized theoretically by Young,and confirmed experimentally by Young and Muellerthat the quantity measured in spherical geometry was the distribution of the total energy of the emitted electron (its "total energy distribution"). This is because, in spherical geometry, the electrons move in such a fashion thatangular momentumabout a point in the emitter is very nearly conserved. Hence anykinetic energythat, at emission, is in a direction parallel to the emitter surface gets converted into energy associated with the radial direction of motion. So what gets measured in an energy analyzer is thetotal energyat emission.With the development of sensitive electron energy analyzers in the 1960s, it became possible to measure fine details of the total energy distribution. These reflect fine details of thesurface physics, and the technique of Field Electron Spectroscopy flourished for a while, before being superseded by newer surface-science techniques.Field electron emitters as electron-gun sourcesTo achieve high-resolution inelectron microscopesand other electron beam instruments (such as those used forelectron beam lithography), it is helpful to start with an electron source that is small, optically bright and stable. Sources based on the geometry of a Mueller emitter qualify well on the first two criteria. The firstelectron microscope(EM) observation of an individual atom was made by Crewe, Wall and Langmore in 1970,using ascanning electron microscopeequipped with an early field emission gun.From the 1950s onwards, extensive effort has been devoted to the development of field emission sources for use inelectron guns.[e.g., DD53] Methods have been developed for generating on-axis beams, either by field-induced emitter build-up, or by selective deposition of a low-work-functionadsorbate(usuallyZirconium oxide- ZrO) into the flat apex of a orientedTungstenemitter.Sources that operate at room temperature have the disadvantage that they rapidly become covered with adsorbatemoleculesthat arrive from thevacuumsystem walls, and the emitter has to be cleaned from time to time by "flashing" to high temperature. Nowadays, it is more common to use Mueller-emitter-based sources that are operated at elevated temperatures, either in theSchottky emissionregime or in the so-called temperature-field intermediate regime. Many modern high-resolution electron microscopes and electron beam instruments use some form of Mueller-emitter-based electron source. Currently, attempts are being made to developcarbon nanotubes(CNTs) as electron-gun field emission sources.The use of field emission sources in electron optical instruments has involved the development of appropriate theories of charged particle optics,and the development of related modeling. Various shape models have been tried for Mueller emitters; the best seems to be the "Sphere on Orthogonal Cone" (SOC) model introduced by Dyke, Trolan. Dolan and Barnes in 1953.[42]Important simulations, involving trajectory tracing using the SOC emitter model, were made by Wiesener and Everhart.Nowadays, the facility to simulate field emission from Mueller emitters is often incorporated into the commercial electron-optics programmes used to design electron beam instruments. The design of efficient modern field-emission electron guns requires highly specialized expertise.Atomically sharp emittersNowadays it is possible to prepare very sharp emitters, including emitters that end in a single atom. In this case, electron emission comes from an area about twice the crystallographic size of a single atom. This was demonstrated by comparing FEM andfield ion microscope(FIM) images of the emitter.Single-atom-apex Mueller emitters also have relevance to thescanning probe microscopyandhelium scanning ion microscopy(He SIM).Techniques for preparing them have been under investigation for many years.A related important recent advance has been the development (for use in the He SIM) of an automated technique for restoring a three-atom ("trimer") apex to its original state, if the trimer breaks up. ApplicationsThe development of large-area field emission sources was originally driven by the wish to create new, more efficient, forms ofelectronic information display. These are known as "field emission displays" or "nano-emissive displays". Although several prototypes have been demonstrated,the development of such displays into reliable commercial products has been hindered by a variety of industrial production problems not directly related to the source characteristics [En08].Other proposed applications of large-area field emission sources includemicrowavegeneration, space-vehicle neutralization,X-ray generation, and (for array sources) multiplee-beam lithography. There are also recent attempts to develop large-area emitters on flexible substrates, in line with wider trends towards "plastic electronics".The development of such applications is the mission of vacuum nanoelectronics. However, field emitters work best in conditions of good ultrahigh vacuum. Their most successful applications to date (FEM, FES and EM guns) have occurred in these conditions. The sad fact remains that field emitters and industrial vacuum conditions do not go well together, and the related problems of reliably ensuring good "vacuum robustness" of field emission sources used in such conditions still await better solutions (probably cleverer materials solutions) than we currently have.

Field emission properties of graphene

Graphene has grabbed appreciable attention due to its exceptional electronic and optoelectronic properties .One of the potential applications of graphene is in fieldemission (FE) displays. Malesevic et al. synthesized verticallyaligned few-layer graphenes (FLGSs) using plasma-enhancedchemical vapor deposition (PECVD) on titanium substrate,and turn-on field of the field emission from the graphene layer was as low as 1 V/m . Qi et al. prepared FLGSs byradio-frequency PECVD on Si(100) substrates without anycatalyst, and turn-on field of its emission was 3.91 V/m .

Field emission characteristics

Fowler and Nordheim (FN) first derived a semiclassical theory of field emission currents from cold metals in 1928 (Fowler & Nordheim, 1928). In this theory, the system is simplifiedas a one-dimensional structure along the direction of the external field. The emission tip is modeled as a semi-infinite quantum well. By employing the Wentzel-Kramers-Brillouin approximation, following FN plot is given. ln (I/V2) -1/V (1) where I is in amperes per square centimeter of emitting surface and V is the applied voltage.

Current-voltage characteristics for a mechanically sharpened carbon rod (opencircles) and a GNS cathode (solid circles). Inset shows the Fowler-Nordheim plots (ln(I/V2)vs 1/V) for the I-V characteristics of the GNS cathode. (Reprinted with permission,Matsumoto et al. 2007, American Institute of Physics.)The above FN theory should not be applied to nanometer sized emitters such as GNS andCNT because the geometrical size of the tip is comparable to the electron wavelength. Much sophisticated emission theory is developed by several authors (He et al., 1991; Liang &Chen, 2008; Forbes, 2001), and these theories will explain the difference between the straight line obtained by the simple FN plot and the experimentally obtained slightly curved feature(dotted line) in the inset of Fig. 3. For the precise fitting by using these sophisticated theories, it is necessary to determine physical values such as a shape and a size as well as the electronic properties; e.g. the defect density and work function of the tip. However, as shown, it is difficult to determine these values for our tip. Therefore, here we estimate the field emission characteristics by using the FN plots. Shows typical logarithmic current-voltage (I-V) characteristic of the GNS field emitter. The figure also shows I-V characteristic of a sharpened graphite rod without nanostructure. The currents were collected on a 3 mm diameter aluminum anode, which was located at 100 m in front of the cathode. The measurements were carried out in a vacuum chamber with a residual pressure of 1 x 10-6 Pa. The mechanically sharpened graphite rod without nanostructure showed little field emission current (open circles), whilethe GNS emitter starts to emit electrons at an average electric field of about 3 V/m and the emission current exceeds 2 mA at an applied electric field of 11 V/m (solid circles). The FNplot of the emission current from the GNS emitter is shown in the inset of Fig. Linear dependence of the Fowler-Nordheim plot suggests that the electron emission is dominatedby the Fowler-Nordheim tunneling process as described .