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theory was proven by DiVincenzo and Mele in 1984.84,91 They reported that Wallace’s electrons hopping conduction could be recasted as massless spin −1/2 particles in a DiracWeyl formalism, an equation used to model neutrinos. The theory explains that electrons behave as if they have an extra degree of freedom, known as pseudospin in the unit cell of the graphene. Furthermore, Semenoff was able to predict anomalous behaviors of graphene with respect to the quantum Hall Top-Down Method The principle of the top-down method is to exfoliate graphite that is used as starting material.121,123 Mechanical exfoliation, chemical/ electrochemical exfoliation and chemical/electrochemical fabrications are classified in the top-down methods. The top-down technique is very easy to apply for large-scale graphene production. However, conventional top-down methods, such as Hummer graphite oxidation, requires a controlled reaction and always provide abundant structural defects that cause low electrical conductivity.89,125,126 Below are some of the top-down techniques that are usually used by researchers to synthesize graphene: Exfoliation method.—Exfoliation is a simple and common technique that can be used for graphene synthesis from graphite or other carbon sources. There are several types of exfoliation such as mechanical exfoliation, chemical exfoliation and/or electrochemical exfoliation. Mechanical exfoliation or more commonly known as the effect.91–93 Further observations have been reported by Konstantin 94 Novoselov and Andre Geim in 2004, where their experiment included innovative methods in providing new important information in relation to graphene. This useful information caused K. Novoselov and A. Geim to receive the Nobel Prize in Physics in 2010. Since then, graphene has been known as an impressive material with exceptional physical properties (Table I) in terms of its mechanical, optical, electronic strength and electrochemistry properties.9,11,12,95,96 The timeline of events in the history of graphene is summarized in Fig. 2. In the future the research will emphasize to find easier route of synthesis with better characteristic. Furthermore, in the industrial perspective, one will consider cost of production and reproducible product to have a standard quality. Graphene can be produced in various forms.85 In 2013, Bianco et al. proposed the first nomenclature for naming graphene and its derivatives with the aim of making it easier for other researchers to Scotch tape method, is a traditional method that has been applied for 1,21,89 85,94,97 study graphene (Fig. 3). decades. This method is made famous by K. Novoselov and Andre Geim since they both won the Nobel Prize in Physics due to the results obtained by implementing this method. Mechanical exfoliation (Fig. 5a) is the first method to obtain one layer of graphene.21,89,127 Examples of mechanical peels are micromecha- nical peels as shown in Table II. Dasari et al. in 2017 showed a representation of micromechanical stripping of graphene sheets using Scotch tape method.41 Graphite was placed on the substrate and repeatedly peeled using adhesive tape until a monolayer sheet is obtained. Although this process is simple but the main challenges with this method is that the product that obtained is small and contains some structural defects.41 Figure 3 is various type of graphene The synthesis method can hugely affect the form of the graphene 113–116 Liu et al., (2015) have studied the detail of defects in the graphene 116 Structural defects greatly affect the chemical and electronic properties of the graphene produced. For the chemical properties of graphene, defects associated with dangling bonds can increase the graphene reactivity. Simulation studies show that every functional group that is perfectly bound to the small binding energy in graphene causes an increase in its reactivity, thus being the reason why its formation must be controlled.119,120 General Synthesis Methods to Produce Graphene Several methods for producing graphene have been reported. In general, graphene production can be done by two types of methods, which are top-down and bottom-up84,121,122 (Fig. 4). More informa- tion about all methods are discussed as follow: Table I. The extraordinary properties of graphene. equipped with SEM images. as the synthesis process can frequently cause defects in the graphene products. However, not all defects will deteriorate the properties of the graphene as some defects proved to be beneficial. Defects that can be controlled could be useful for some application. Defects that are found in the graphene structures have been studied by several researcher. Defects can be divided into two types, which are point defects and one-dimensional line defects. product structure. The examples of point defects are Stoke-Wales dis- abilities, single vacancies, and pooled vacancies. Meanwhile, the one-dimensional defect is a line defect as slope boundary that separates two domains in different lattice orientations from the normal tilt axis in the plane. These defects can be considered as point defect lines that are reconstructed with or without dangling ties.113,116,117 Defects can also occur at the edges of graphene. These damaged edges can arise due to local changes in the type of reconstruction or due to the continuous removal of carbon atoms from the backbone.116,118 Chemical peel is an effective way to produce large amounts of graphene. However, this method has several disadvantages, such as it involves complex chemical processes and it also generally produces sheet-shaped graphene that has low conductivity.147–149 Liquid phase exfoliation (LPE) is a new top-down method that only involves flaking natural graphite through high-shear mixing or sonication.21,128,150 Illustration of liquid phase exfoliation is shown in Fig. 5b. Until now, there are two different graphite peeling techniques using LPE, and these are cavitation in sonication and high-shear mixing. In practice, the LPE operating conditions are very mild and do not need a vacuum or high temperature system. For large scale applications, the high-shear mixing method or micro- fluidizer is more recommended than the sonification assisted LPE method.100,151 This is due to the low graphene product and high energy consumption for process with LPE sonication method. Meanwhile, high-shear mixing or microfluidizer method can ex- foliate graphite better than the LPE sonication method.100,151,152 Electrochemical method.—The electrochemical method is car- ried out using a minimum of four main components, and these are anode, cathode, electrolyte, and power supply (Fig. 6a). Anode, being a source of carbon, will be oxidized and exfoliated to produce graphene. The cathode can be varied by using either a graphite (Figs. 6b and 6c) or a platinum (Fig. 6d). The experiment is usually arranged as shown in Fig. 6. The anode and cathode are immersed into the electrolyte at a certain distance. Positive or negative voltage is applied to the anode depending on the desired peeling mechanism.129,153,154 The choice of anode, cathode and electrolyte solution used is a crucial factor for the electrochemical process as it can affect the graphene product obtained. Liu et al. used a pencil graphite as a source of graphite, which subsequently used it as anode and cathode (Fig. 6b).155 The anode and cathode are immersed in 1 M H3PO4 with an applied potential between +7 V and −7 V. The product obtained is not homogeneous, and the thickness and size distribution were quite wide. Parvez et al. in 2013 used graphite as anode and platinum (Pt) as cathode.156 The electrodes were immersed in sulfuric acid (electrolyte solution) with ECS Journal of Solid State Science and Technology, 2020 9 093013 Properties Optical transparency Electron mobility Thermal conductivity Specific surface area Breaking Strength Young’s modulus Details References 97.7% 11 200,000 cm2 V−1 s−1 5000 W m−1 K−1 2630 m2 g−1 42 N m−1 0.25–1.0 TPa 8, 17 68, 95, 98 99 100, 101, 102 9PDF Image | Progress in Graphene Synthesis
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