Graphene is a hexagonal lattice of carbon-containing single layers. It is an excellent form of carbon. There are several forms of graphene and graphene sheet films are one of those excellent forms. 

They have excellent chemical and physical properties which are essential in carrying out all their uses. All the applications of graphene sheet films are highly authentic in their nature and are considered as an excellent source of building up the industries and the economy of the states. Nonetheless, graphene is an excellent product and all the forms of graphene are excellent in their nature too due to the commendable properties that graphene as an individual holds.

Introduction

Carbon atoms single layer that are bonded in a hexagonal honeycomb lattice leads to a structure possessing numerous required characteristics that seek the attention of various applications is known as graphene. High thermal conductivity and high charge carrier mobility are among the unique characteristics of graphene, and its large surface area makes graphene suitable for catalysis or sensing applications.

Although, effective usage of graphene in various technological applications is dependent on the production of the suitable type of graphene-based material varying from few-layer graphene sheets and large area single sheets to laminate films sintered from various micro- or nano-meter-sized graphene platelets.

Quality

Vacuum-based deposition technologies like CVD (chemical vapor deposition) can be used to successfully produce high-quality few- and single-layer graphene of -10 layers thickness. A carrier gas made up of CH4 that under high temperatures, forms a reaction with the surface for promoting graphene growth are capable of being used to grow CVD graphene on the metal substrates that are lattice-matched to the graphene lattice-like Cu and Ni.

CVD is used by both substrate types for fabricating few- and single-layer graphene of high quality, however, the substrate material determines the growth method as it varies. Remarkable electrical characteristics are seen by the graphene that's grown by the CVD methods and they can nearly almost the graphene sheet's estimated theoretical performance.

Outcomes

Although, graphene of high quality that's produced by CVD is expensive to produce, and it shows sensitivity towards defects and contamination, resulting in it being difficult for being utilized efficiently in various device applications. Graphene should be transferred to other substrates like Si/SiO2 as the substrates that are utilized in the growth of graphene are not suitable for the architectures of all devices. A solution-based method using a sacrificial polymer like a cross-linked polymer or poly-methyl-methacrylate (PMMA) is commonly used to accomplish it but it can introduce contamination and defects.

GO/rGO’s Preparation/Synthesis

Graphite oxide is different as graphite is a 3-D carbon-based material that is made up of millions of layers of graphene. The oxygenated functionalities are introduced into the structure of graphite when strong oxidizing agents oxidize graphite, which didn't only expand the separation of the layers but made the material hydrophilic too which means that they are dispersible in water. This characteristic allows graphite oxide's exfoliation into the water by utilizing sonication, and finally forming few-layers graphene or single-layer graphene.

Hummers first reported a method on which many of the modern procedures for GO’s synthesis are based. In that method, potassium permanganate’s solution oxidizes graphite in sulfuric acid. General usage of hydrazine is for GO’s reduction.

Graphene oxide’s Functionalization

Although it's due to these problems that hydrazine is extremely toxic and is capable of potentially functionalizing graphite oxide with the nitrogen heteroatoms, alternatives to hydrazine like HI, ascorbic acid, and NaBH4, among others, have been utilized for GO's reduction. Reduction of GO is possible in an aqueous solution or a thin film. This oxidation has GO as its effective by-product as when graphite forms a reaction with the oxidizing agents, the interplanar spacing between graphite’s layers is increased. Then, the totally oxidized compound can be dispersed in water and other base solutions, thus producing GO. Chemically, GO and graphite oxide are the same but they are very different when it comes to the structure.

The interplanar spacing between the compound's individual atomic layers is the major difference between GO and graphite oxide, and its reason is water intercalation. The sp2 bonding network also disrupts because of this increased spacing that's caused by the process of oxidation, meaning that both the GO and graphite oxide are usually known as electrical insulators. Despite being a poor conductor, GO’s treatment with chemical reduction, heat, or light can restore most of the characteristics of the famed pristine Graphene. Few methods are possible for turning graphite oxide into GO.

Most common approaches

Some of the most usual approaches are utilizing stirring, sonication, or both in combination. Sonication forms a broad range of sizes of graphene platelets. It can damage the Graphene flakes thus lessening them in the size of the surface from microns to nanometers. Sonication has been very successful in the exfoliation of graphene and it can be an extremely time-efficient method to exfoliate graphite oxide.

The number of layers is the major difference between GO and graphite oxide. However in GO dispersion, graphite oxide is a multilayer system, one can find a monolayer of flakes and some layers of flakes. Producing rGO by reducing GO is a very vital process as it makes a large influence on the produced-rGO’s quality. Thus, determining how close rGO will be the same in structure to the pristine graphene.

Large Scale Operations

It is due to comparative ease in the production of enough qualities of graphene with the required quality levels that rGO is the most obvious solution in large-scale solutions where scientific engineers want to use graphene in large quantities for industrial applications like energy storage. The reduction can be attained in various ways, however, all of those ways are based on electrochemical, thermal, or chemical means. Some methods can form extremely high-quality rGO, same as pristine graphene, but their performance can be time-consuming or complex.

Thermally Reducing GO

At 1000 or more temperatures, thermally reducing GO forms rGO that possesses an extremely high surface area, similar to the surface area of pristine graphene according to findings. The graphene platelet's structure gets damaged by the heating process as the carbon dioxide is released and the pressure builds.

Characteristics of rGO and GO

GO has many benefits and one of them is its easy dispersion in the organic solvents and water, along with various matrixes due to the oxygen functionalities' presence. When trying to enhance the mechanical and electrical characteristics of theirs, or when mixing the material with polymer matrixes or ceramic, this stays an extremely major characteristic. Although, when it comes to electrical conductivity, due to disruption of its sp2 bonding networks, GO Is usually known as an electrical insulator.

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Electrical Conductivity

GO’s reduction should be achieved for recovering the electrical conductivity and the honeycomb hexagonal lattice. It should be known that the attained rGO would be more difficult to disperse once most oxygen groups are removed, and that's due to its tendency of creating aggregates. GO's characteristics can be fundamentally changed by GO's functionalization. The chemically modified graphene that is produced then has the potential to be adaptable for various applications. GO can be functionalized in various ways, it is the desired application that determines its way of functionalization.

Optoelectronics

It is possible for biodevices, optoelectronics, or as a drug-delivery material to substitute amines for graphene’s organic covalent functionalization for increasing chemically modified graphene’s dispersibility in the organic solvents. According to findings, fullerene-functionalized secondary amines and porphyrin-functionalized primary amines can attach themselves to GO platelets, thus increasing nonlinear optical performance.

Significantly, we develop a process of oxidation and reduction that can separate the individual layers of carbon and isolate them then while causing no modification in their structure to make GO usable as an intermediary in the production of a few layers or monolayer graphene sheets. Completing the production of graphene sheets of similar quality like mechanical exfoliation but on a bigger scale has been very difficult for scientists. Until now, GO's chemical reduction is the most appropriate way to mass-produce graphene but still, its production is a difficult task. Graphene is expected to become more broadly utilized in industrial and commercial applications once they overcome this issue.

Graphene thin film fabrication

Device applications have a great interest in studying Graphene's fundamental physics and the usage of vacuum techniques for the fabrication of high-quality single-layer graphene's large area sheets. Cu and Ni are the most usually utilized metal substrates as they have a crystal structure with similarly matching lattice spacing to graphene. Ni has a lattice structure reminiscent of graphene's hexagonal lattice with similar lattice constants, making Ni extremely ideal for graphene growth. First, graphene is fabricated on a polycrystalline Ni substrate that is annealed at 800-1000 C of high temperature in an Ar/H2 atmosphere for increasing the size of the grains.

Polycrystalline Substrate

As compared to a single crystal, the cost of a polycrystalline substrate is lower, but it possesses grain boundaries and they restrict graphene grain’s maximum size. Then, the heated Ni substrate is made to contact with an H2/CH4 gas mixture and on contact, carbon atoms dissolve into the Ni film as the hydrocarbons decompose, thus producing a solid solution. Using argon gas to cool the sample causes the atoms of carbon to diffuse out from the Ni-C solid solution and precipitate on the surface of Ni in graphene films' form.

Same carrier gases are involved in the process in which Cu is used as a substrate. Although, at increased temperature, the solubility of carbon is much lower in Cu whereas the solubility of Ni is more. Instead of dissolving, hydrocarbons decompose on Cu's surface into a layer of Graphene as Cu is well lattice-matched to graphene too. Multilayer graphene can be easily produced by this technique by simply enabling the proceeding of the reaction for a longer time so that a Graphene multilayer can be built. CVD Graphene on Ni or Cu can be transferred to other substrates for becoming a device architecture’s part or being used to process further with chemical functionalization or metal nanoparticles.

Advantageous for Applications

Applications needing a few- or single-layer Graphene can get various advantages from this procedure. CVD graphene is ideally suitable for being utilized as a potential alternative to more expensive transparent conducting applications like ITO (indium tin oxide) in solar cell devices as they are extremely conductive (100–1000 Ω/square) and transparent (98% transparency to visible light) which makes them a perfect material. Although, the requirement of a single-crystal, large substrate restricts the single-grain graphene sheet’s ultimate size.

CVD Graphene is integrated into devices by using the solution transfer process and it limits CVD graphene’s usage by introducing a PMMA residue and defects that are difficult enough to remove. Such factors restrict the defect-free graphene grain’s ultimate size that is attained by utilizing a process of fabrication of CVD graphene.

Graphene Sheet Films

Graphene and graphene sheet films are the 2-D exfoliated graphite with remarkable and excellent chemical and physical characteristics. Graphene sheets are technically and specifically made up of carbon atoms single layer with carbon rings arranged into a hexagonal configuration that's the same as the honeycomb lattice. Graphite and graphene sheet films possess a similar atomic structure as graphite's thousand of these single layers stacked and bound to one another by utilizing Van Der Waals forces. Graphene possesses 2 or more layers with incommensurate adjacent Graphene sheets.

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Monolayer graphene films

Free monolayer graphene films retain Graphene’s ultra-high thermal conductivity characteristic with the thermal conductivity value as much as 3000 W/m.K. Lower density and remarkable flexibility are maintained by them whereas the most usually utilized metals for the dissipation of heat can't maintain as such. Different macro-scale morphology yet same micro-scale morphology were seen to be possessed by the graphene sheet films that were made by various advised methods because of the annealing at higher temperatures. Shiny metallic color with 7.5 micrometers of thickness is possessed by graphene sheet films. Their color is shiny metallic.

Graphene Sheet Films Characteristics

Graphene is the center of attention of many scientists because of its excellent structure and remarkable characteristics, especially keeping in mind the one-atom-thick planar sheet with carbon-carbon binding's sp2 hybridization. Single-layer graphene sheets with a single-layer nature have remarkable electronic characteristics because of the 2-D structure of the graphene sheets. Graphene sheet films' electronic characteristics are because of their features like specific and ballistic electron transport, the half-integer quantum Hall effect, and bandgap width.

Factors like the number of carbon rings along graphene sheets length or its minimum length, and the size of the graphene sheets determine the characteristics of graphene for instance its electrical conductivity and Young's Modulus. In order to fit into criteria 1 of nanoscale homogenization, the length of the graphene sheets should be very long so it could be favorable enough. The major electric and electronic characteristics among fullerenes and carbon nanotubes are originated from graphene sheets.

Preparation and Synthesis of Graphene Sheet Films

Various kinds of methods are being utilized right now for graphene sheet film's preparation and synthesis namely micromechanical cleavage, graphene oxide reduction, chemical vapor deposition (CVD), direct ultrasound sonication, scotch tape, vacuum thermal annealing, chemical, and thermal exfoliation, with each of the method possessing its benefits and losses along with their applicability-based compatibilities. Although, applying these methods to mass-produce industrial scales and purposes is kind of not so common. Graphite is oxidized to attain graphene oxide and it is utilized for achieving exfoliated 2-dimensional graphene sheet films.

Expandable Graphite’s Exfoliation

In another method, expandable graphene is exfoliated at more than 1000 °C for 60 seconds to prepare single-layer graphene sheet films. NaCl is applied in the next step for grinding graphite for three minutes or some for exfoliating it and it leads to a homogenous grayish mixture. Then, NaCl’s aqueous dissolution collects the exfoliated graphite and treats it for 24 hours at room temperature with oleum. The end sample is ultra-sonicated for five minutes in N, N'-dimethylformamide. Graphene sheet films are Generally made through carbon evaporation on an organic precursor for instance the controlled deposition and Polymethyl methacrylate (PMMA) on a substrate. Direct evaporation method, vacuum filtration method, and other assembling methods are the other methods that should be mentioned too.

Graphene Sheet Films application

There has been a massive increase in the interest in graphene sheet films for their application in a broad range and practically because of graphene sheet films’ huge contribution to numerous studies and thermal management areas, namely thermal interface materials and heat dissipation materials with really promising results. For example, there has been a demonstration of the graphene sheet film's thermal management capabilities by using them in 7w high-bright LEDs with the recorded lower temperatures as hot spots, whereas in the graphene sheet's absence, an increase in the temperature was seen.

Due to the lightweight, stretchable, and ultra-flexible graphene sheet films application cloth, there is a fast increase in the temperature and then a consequently fast temperature decrease in 5 minutes to the room temperature. Various applications like antibody-functionalized graphene sheets for diagnostic devices and mammalian and microbial detection as potential agents have been found by the graphene sheet films because of graphene sheet films’ remarkable structural, mechanical, and electrical features, large surface area per unit volume, and thermal characteristics.

Unique Transparency

In the development of biosensors, graphene sheet films' excellent transparency makes graphene sheet films the perfect candidates. In doing so, sensor-coated graphene can function as an uncoated sensor while they are mechanically and chemically protected and they can make as much contact in aqueous solutions.

We can solve the bottleneck's problem for nanopore-based DNA sequences with a single molecule by integrating 0.4 nm or less of graphene sheets into a nanopore. Their application is in the fabrication of condoms and elastic composite materials too. Brain tissue is viewed in a recent application by using implantable sensors based on transparent and flexible graphene with 90% of optical transparency.

Transformation into a Liquid Crystal

One can transform graphene sheet films into liquid crystal droplets by the placement of droplets in a changing pH solution. In this way, the structure of the graphene droplets changes while the application of a magnetic field, making it a drug carrier. Detection of disease is graphene sheet films' another application as there are demonstrations of it that when graphene is exposed to some of the specific disease markers, graphene changes shape.

Graphene sheets are also known as flying carpets in medical applications for functioning as a drug delivery platform for delivering two anticancer drugs to the lung tumor cells. Here, doxorubicin drug is encapsulated in the graphene sheets and when it is reached to tumor cells, it is released

Waterproof Transistor Designing

Graphene sheets are used in different applications in designing and fabrication of the waterproof transistor with graphene sheets being grown on metal. A digital switch can be made with a 105 ratio at 0.5 V of turn-on voltage by using boron nitride nanotubes for the perforation of a graphene sheet. In short, one can use graphene sheet films as conductive ink, spintronics, organic electronics, graphene quantum dots, Hall Effect sensors, optoelectronics, frequency multiplier, and transparent conducting electrodes.

Inheritances

Due to mother graphite, graphene sheet films possess remarkable electronic and thermal characteristics, remarkable conductivity, and they have found applications because of their such intensified characteristics as biosensing platforms, semiconductors, and transistors, and in cancer therapy and drug delivery. Graphene is at the center of attention of numerous applications and research due to various preparation and synthesis method along with graphite's huge source as the raw material.

Conclusion

Graphene sheet films are being used at a great scale nowadays as they possess properties that are authentic in their nature and promote the credibility of the product. Their production as well has increased massively over the past years which has eventually increased their applications as well. However, graphene sheet films are excellent products that are proving their originality by contributing a lot in the form of various applications and uses.

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References

https://www.intechopen.com/chapters/50810

https://www.sciencedirect.com/science/article/pii/S2590049820300072

https://www.graphene-info.com/graphene-applications

https://cutt.ly/BHBy4dW

https://www.sciencedirect.com/topics/engineering/graphene-sheet