Mechanical exfoliation is the first established method of extraction of graphene flakes on a substrate [4] . In nanotechnology, it is a top-down method of graphene synthesis, by which a stress (longitudinal or transverse) is applied to layered graphite surface using a simple adhesive tape (scotch tape) or AFM tip. Graphite structure is composed of several single-atomic layers of graphene stacked and held together by weak van der Waals forces. Between each layer is an interlayer distance of 3.3 Å (0.33 nm) and interbond energy of 2 eV/nm². Single layer graphene (SLG) to few layer graphene (FLG) can be produced through mechanical cleaving by slicing down the layers from graphitic materials such as HOPG, single-crystal graphite or natural graphite [40] [41] . This method requires an external force of about ~300 nNμm⁻² to separate a single graphene layer from graphite [42] . Mechanical exfoliation can be achieved using several agents including scotch tape [4] , electric field [43] , and ultrasonication [44] . Prior to the first production of a single layer graphene by Novoselov et al., Ruoff et al. [41] first produced thin multilayered graphite with about 200 nm thickness by mechanical exfoliation using AFM tip (Figure 4(a)). Figure 4(b) shows the micro-graph of fabricated thin multilayered graphite consisting of 500-600 layers of monolayer graphene.

Novoselov et al. [4] utilized adhesive tape to produce single graphene layer (thickness < 10 nm) by exfoliation from 1 mm thick HOPG mesa. This involved compressing dry etched graphite mesa (by oxygen plasma) against a 1 mm thick wet photoresist layer over a glass substrate. Baking was done to attach the HOPG mesa to the layer of photoresist and exfoliation of the graphite flakes was achieved by using the scotch tape. Dispersed graphite flakes in acetone was transferred onto a cleaned SiO₂/Si substrate. Figure 4(c) shows optical micro-graph of multilayer graphene flakes produced via mechanical exfoliation on top of SiO₂/Si substrate while Figure 4(d) shows the identification of single graphene flakes with different number of layers.

The problem with the exfoliation process is scalability and reproducibility. Though it produces high quality graphene layers, there is still a need for further improvement to produce large-scale, high quality, and defect-free graphene for use in nanotechnology.

Chemical exfoliation is another route to obtaining monolayer graphene. This method involves production of suspension which changesgraphite to form graphene through the formation of graphene-intercalated compounds (GICs). This involves the introduction of alkali metals into a graphite solution to increase the interlayer spacing, thereby reducing the van der Waals forces between the layers and forming GICs which are then dispersed in a liquid medium through sonication to form single-layer graphene [45] . Alkali metals are the only choice of materials as they easily form GICs on reaction with graphite due to the difference in ionization potential between graphite and alkali metals. Chemical exfoliation methodology was used by Viculis et al. to obtain graphene nanoplatelets (GNP) using potassium alkali metal [46] . Potassium graphite (KC₈) intercalated compound was formed when potassium reacted with graphite at 200˚C under inert atmosphere. Dispersion of the GIC in aqueous ethanol (CH₃CH₂OH) resulted in an exothermic reaction as shown in (1) and produced GNP.

KC 8 + CH 3 CH 2 OH → 8 C + KOCH 2 CH 3 + 1 2 H 2 (1)

Potassium ions dissolve into the ethanol solution forming potassium ethoxide (KOCH₂CH₃) and hydrogen gas is formed to aid the separation of the layers of graphene. Precaution must be taken during this reaction as alkali metals vigorously react with water and alcohol, as such there is a need for an ice bath to dissipate heat during reaction. The main advantage of alkali metals, such as potassium, is their relatively small atomic radius which is smaller than the interlayer spacing in graphite and hence fits the interlayer spacing easily as shown in Figure 5(a).

Figure 5(b) shows SEM micrographs of the chemically exfoliated GNP with ~10 nm thickness and ~30 layers. Viculis et al. also carried out exfoliation via GIC formation using an alkali metal such as Cesium (Cs) and NaK₂ (sodium-potassium) alloy [32] . Work by Vallés et al. [48] utilized ternary potassium salt K(THF)_xC₂₄ tetrahydrofuran (THF) as the alkali metal to prepare potassium-intercalated GICs which was dispersed in N-methyl pyrrolidone (NMP) to yield a stable dispersion of negatively charged SLG as shown in Figure 5(c).

One major challenge in this work was the reassembly of produced graphene back into graphite. Later studies have incorporated surfactants in the solvent used and utilized immiscible liquids such as heptane and water [48] [49] . Other studies have utilized ferric chloride, supercritical carbon dioxide, and nitromethane as the intercalation chemicals [50] [51] [52] .

Graphene synthesis through chemical exfoliation is important and unique as it can produce a large amount of graphene at a low temperature. It is also scalable and can be extended to a wide range of functionalized graphene production.

Chemical reduction of graphite oxide (GO) is one of the conventional ways of preparing graphene in large quantities. The three routes available for GO synthesis; the Brodie method [5] , Staudenmaier method [53] , and Hummers method [54] consist of oxidation of graphite through strong acids and oxidants such as concentrated sulfuric acid (H₂SO₄), nitric acid, and potassium permanganate. Oxidation degree can be varied by the stoichiometry, reaction conditions (pressure, temperature, etc.), and the graphite type (HOPG, pure graphite, etc.). The works of Brodie et al. [5] in 1859 first demonstrated the synthesis of GO by adding potassium chlorate (KClO₃) to a graphite slurry in the presence of fuming nitric acid (HNO₃). However, the process was time consuming, involved several steps, and hazardous. This brought about a modification to the process by Staudenmaier et al. in 1898 [53] with the inclusion of concentrated H₂SO₄ to the process and resulted in the production of highly oxidized GO in a single step process. A popular method used today was developed in 1958 by Hummers et al. [54] . This involves the oxidation of graphite by mixing the graphite slurry in sodium nitrite (NaNO₃), concentrated H₂SO₄, and potassium permanganate (KMnO₄) without the use of HNO₃. Figure 6 shows a schematic diagram of GO synthesis from graphite method using the three methods explained.

When graphite turns to GO, the interlayer spacing is increased to between two to three times depending on the oxidation time. In the case of pristine graphite, interlayer spacing is increased from about 3.34 Å to 5.62 Å after an hour of oxidation, and then further expanded to about 7.35 Å upon prolonged oxidation of 24 hours [5] . Upon ultrasonication in a DMF/water (9:1) (dimethyl formamide), GO interlayer distance is further expanded, forming individual layer suspensions. With hydrazine hydrate treatment, GO reduces back to graphene or reduced graphene oxide (RGO). The chemical reduction process is done using dimethyl hydrazine or hydrazine in the presence of a surfactant or polymer to produce homogenous colloidal graphene suspensions [55] . Figure 7 shows the process for chemical synthesis of graphene from graphite. Reduction of GO has also been performed using sodium borohydride (NaBH₄) [56] , hydroxylamine [57] , hydroquinone [58] , ascorbic acid [59] , etc. as reducing agents.

The use of toxic chemicals such as HNO₃ and KClO₃ has prompted researchers to develop new methods to graphene oxide synthesis [60] [61] [62] . However, the Hummer’s method has been found to be quicker, safer, and more efficient method for graphene oxide synthesis used widely today with some modifications. Hummer’s method was modified and further improved by Marcano et al. with the use of a larger amount of KMnO₄, exclusion of NaNO₃, and a 9:1 reaction mixture of H₂SO₄/H₃PO₄ [61] . Compared to the Hummer’s method, this improved method shows higher oxidation efficiency and production of better hydrophilic oxidized graphene product [61] .

Various reports on the production of graphene by graphene oxide reduction exists. Some of the methods involve thermal reduction [63] [64] [65] , electrochemical reduction [66] [67] , photochemical reduction [68] , hydrothermal reduction [69] , microwave assisted reduction [70] , and chemical reduction using reducing agents [71] .

Paredes et al. [29] produced graphene nanosheet derived from graphene oxide using the Hummer’s method with the reduced and unreduced aqueous dispersion drop casted onto freshly cleaved HOPG substrate. It was reported that the chemically reduced graphene oxide nanosheets were hardly distinguishable from their counterparts in the topographic AFM images. However, they could be distinguished by their phase contrast with the unreduced graphene oxide showing a phase difference on the HOPG substrate while the RGO showed little phase contrast with the HOPG substrate (due to little defects). The thickness of the chemically reduced graphene oxide nanosheet was reported as ~1 nm using tapping mode AFM as shown in Figure 8. It was concluded that the chemically reduced graphene oxide still contained some oxygen functional groups [29] .

The chemical reduction method is the most useful method as it removes most of the functional groups present in the intermediate graphene oxide with the structure of the graphene surface restored after reduction. However, the produced graphene contains some defects due to the presence of remaining oxygen functional groups (such as −OH and −COOH) that causes some disorder in the graphene electronic structure. This results in reduced electrical conductivity of produced graphene as compared to that of pristine graphene (defect-free). These remaining functional groups on graphene and RGO can be used to tune their electrical and chemical properties for different applications.

This involves the solvothermal synthesis of graphene with the use of a solvent interacting with precursor [47] . This reaction is performed in a closed vessel under high pressure with a 1:1 molar ratio of ethanol and sodium. Using sonication, graphene sheets can be smoothly detached by pyrolysis of sodium ethoxide. The produced graphene sheets were determined to have dimensions up to 10 μm. Using Raman spectroscopy and transmission electron microscopy (TEM), the graphene sheets were characterized to determine the crystalline structure, number of layers, band structure, and the graphitic nature [73] . Raman spectra showed a broad D-band (defect) and G-band with the intensity ratio of ~1.16 (I_G/I_D), representative of single layer defective graphene. Though this process was less expensive, easy, and required low temperature, the presence of many defects reduced the graphene quality. This method of graphene production has not been widely explored.

Epitaxial thermal synthesis of graphene on the surface of single-crystalline silicon carbide (SiC) is one of the well-known synthesis techniques. The term “epitaxy” is a method that allows the deposition of a single-crystalline film (epitaxial film) on a single-crystal substrate. When an epitaxial film is of the same material as the substrate, it is known as homoepitaxial layer, and if the epitaxial film and the substrate have different material, then an heteroepitaxial film is formed. Epitaxial growth on SiC forms an heteroepitaxial layer of single-layer graphite or graphene. In 1975, Bommel et al. [74] first reported heat treatment between 1000˚C and 1500˚C under ultrahigh pressure produced graphite on both polar planes of SiC. Synthesis by de Heer et al. in 2004 [75] produced ultrathin (few layers between 1 to 3 monoatomic graphene layers) graphitic layers on Si-terminated (0001) face of single-crystal 6H-SiC. Using the same thermal decomposition method, they fabricated 1 to 2 layers of epitaxial graphene on the (0001) face of 6H-SiC [76] . Synthesized epitaxial graphene on SiC although had high quality, they could not be transferred to other substrates.

In 2012, Juang et al. [77] reported a breakthrough with the growth of epitaxial graphene on SiC substrate coated with Ni catalyst thin film. This produced graphene which was easily transferable to other substrate for versatile applications [77] . Epitaxial growth of graphene on SiC is very promising due to its scalability, best electronic properties, and especially due to its high-quality graphene.

Thermal chemical vapor deposition process involves exposure of a substrate to thermally decomposed precursors in gaseous state resulting in the deposition of the product onto the surface of the substrate at a high temperature. In many cases, deposition at high temperature is not desired, plasma-assisted reaction and deposition have been utilized to reduce the temperature. This process will be explained later in this paper. Depending on the quality, type of precursor, desired thickness of product, and structure, several types of CVD processes can be utilized including thermal, plasma enhanced (PECVD), reactive, hot wall, cold wall, etc. In the hot wall process, the walls of the reactor are kept relatively constant all around the reactor while in the cold wall process, the walls never get heated. Thermal CVD and PECVD are the most commonly used types of chemical vapor deposition to produce graphene. Figure 9 demonstrates the schematic of a thermal and plasma-enhanced CVD process.

1) Thermal chemical vapor deposition of graphene

This process of graphene production involves flowing of precursor gases such as methane, hydrogen and argon in certain ratios into a quartz tube containing a substrate such as Cu at high temperatures in a furnace (Figure 9(a)). Over time, there is a deposition of graphene comprising of either single layer, bilayer or multilayer depending on preset conditions such as gas flow rate, reaction time, pressure, and temperature. Deposition of high-quality graphene using CVD process is usually done on various transition-metal substrate (their surface acts as catalyst) such as Ni [78] , Pd [73] , Ru [79] , Ir [80] , and Cu [34] with different hydrocarbons such as methane, benzene, acetylene, and ethylene [34] . On exposure of the transition-metal to hydrocarbon gas at high temperature, there is carbon saturation on the substrate and a thin film of carbon is formed on cooling.

In 1966 [81] , Ni was exposed to methane at a temperature of 900˚C to form thin graphite. Formation of FLG was observed on the evaporation of carbon from a graphite rod in 1971 [82] . In 1975, Lang et al. [42] demonstrated the deposition of monolayer graphitic structure on Pt using ethylene for the first time. Eizenberg et al. [83] later reported the formation of graphitic layer on Ni (111) over a long reaction time. There was over two decades interval in single graphitic layer deposition research due to inadequacy in finding applications of thin graphite films. However, the discovery of graphene in 2004 [4] with potential application in graphene-based electronics, there arose wide interest in this material.

In 2006, the first attempt at synthesis of graphene on Ni foil via CVD method was found using camphor (terpenoid, a white transparent solid of chemical formula C₁₀H₁₆O) as the precursor [84] . When viewed with the TEM, the product was found to have a hexagonal planar few-layer graphite-like structure consisting of almost ~35 layers of stacked SLG with interlayer spacing of 0.34 nm. However, the first evidence of large-scale monolayer graphene using CVD growth was discovered by Obraztsov et al. [85] in 2007. The growth was achieved using precursors of methane and hydrogen under direct current (DC) discharge of ~0.5 Acm⁻². The as-deposited graphene structure shown in Figure 10 shows the anomalous surface ridges which can be described as graphene folding lines [85] . The ridge formation in the graphene was due to the thermal expansion coefficient difference between graphene and Ni. Raman spectroscopy and scanning tunneling microscopy (STM) confirmed the presence of about ~1 - 2 nm thick FLG [85] .

Successively, other reports have demonstrated synthesis of single-to few-layer graphene on Ni substrate. In 2009, Reina et al. [34] utilized e-beam evaporated Ni on SiO₂/Si substrate to grow graphene using diluted hydrocarbon gas. About 1 - 10 layers of graphene were observed using high resolution transmission electron microscopy (HRTEM) as shown in Figures 11(a)-(c). Raman spectroscopy was used to confirm the number of layers by the intensity ratio of the G’(2D) to G bands. The Raman signature were found around ~1342 cm⁻¹, ~1581 cm⁻¹, and ~2677 cm⁻¹ corresponding to the D, G, and G’(2D) bands of produced graphene respectively as shown in Figure 11(b) [34] .

Graphene growth on Cu substrate displays another method towards scalability of graphene growth with increased single layer coverage [86] . Li et al. in 2009 demonstrated large-scale graphene synthesis on copper foils by thermal CVD of methane at 1000˚C [86] . The deposition of carbon on copper was described as a high-temperature surface catalyzed process due to the limited solubility of carbon in copper [86] . Additionally, graphene growth on Cu occurs by segregation of supersaturated carbon and adsorbed carbon on copper surface during the rapid cooling process in CVD copper [86] [87] . Following growth of graphene on Cu foil, grown graphene was transferred onto another surface through copper etching yielding several chemically unaffected layers of graphene confirmed by Raman spectroscopy and HRTEM [86] .

Similarly, Lee et al. [87] developed optimized condition for synthesis of graphene produced by CVD process on Cu foil. It was observed that high pressure, low hydrogen to methane ratio, with reduced growth time produced graphene layers with improved quality [87] . As shown in Figure 12, there was formation of large Cu grains due to Cu grain coalescence during pre-annealing step, formation of copper grain boundary (Cu GB) separating the regions of small and big grains, and the presence of graphene folding lines like those obtained by Obraztsov et al. [85] [87] .

Furthermore, grown graphene was transferred to a 290 nm-thick SiO₂/Si substrate. The produced graphene on Cu substrate was first coated with polymethyl methacrylate (PMMA) on the graphene side to provide strength and serve as protection to avoid graphene damage during the transfer process. In order to etch away the Cu foil, the PMMA/graphene/Cu structure was placed in Fe(NO₃)₃ solution (Figure 13(b)) with the PMMA side facing up. PMMA/graphene heterostructure was removed from the solution (Figure 13(c)) and rinsed severally in deionized (DI) water to remove residual Cu etchant solution (Figure 13(d)). The SiO₂/Si substrate was dipped in DI water (Figure 13(e)) and the PMMA/graphene structure. Through van der Waals forces, the graphene surface adhered to the SiO₂/Si substrate and the PMMA support was rinsed with solvents (acetone and isopropyl alcohol) (Figure 13(f)). The transferred graphene was distinguishable by the difference in color and contrast compared to the SiO₂/Si substrate as observed by the naked eye (Figure 13(g)) [87] . Transferred graphene displaced some wrinkles introduced during the transfer process as evidenced by scanning electron microscopy [87] .

Numerous reports have shown graphene synthesis on other metal thin films and metal foils [34] . However, graphene growth by CVD method has been mostly done on Cu [27] [34] and Ni substrates [34] . The problem of producing graphene on Ni substrate included the time consuming non-self-limiting growth, and the production of large number of wrinkles and folds. Copper substrates have demonstrated more favorable CVD growth of graphene [86] .

2) Plasma-Enhanced Chemical Vapor Deposition

This process of graphene production involves chemical reactions of reacting gases inside a vacuum chamber in the presence of plasma resulting in deposits of thin film on the surface of the substrate (Figure 9(b)), thus, the name, plasma-enhanced chemical vapor deposition (PECVD). Sources of plasma in PECVD can include radio frequency (RF), microwave, and inductive coupling (electrical currents produced by electromagnetic induction). PECVD process of graphene synthesis is performed at a relatively lower temperature and shorter deposition time than other CVD techniques and as such is more feasible for large scale industrial applications. By controlling the process parameters, catalyst-free growth can be achieved [88] . RF plasma-enhanced chemical vapor deposition has been achieved using substrates such as Ti, SiO₂, Si, Al₂O₃, Mo, Hf, Zr, Nb, Cr, W, Ta, and 304 stainless steel [5] . This method reduces energy consumption and prevents the formation of unwanted products or amorphous carbon [89] .

Thin graphitic layer was first synthesized using PECVD by Obraztsov et al. [90] through DC discharge CVD of a methane and hydrogen mixture. Nanocrystalline graphite growth was observed on silicon wafer, Ni, Mo, and Ni substrates. A thick layer of graphene was produced by this process [90] . Wang et al. [91] deposited graphene on different substrates using PECVD method with RF power of 900 W while varying methane concentration at different temperatures. Increase in graphene growth rate with increase in temperature and methane concentration was observed [91] . Other studies on graphene synthesis using PECVD were done, such as atmospheric pressure process [92] , microwave plasma [93] , nitrogen-doped graphene/CNT hybrid structure [94] , and many others. PECVD method has been confirmed to produce high-crystalline graphene with high-purity; however, extensive research is ongoing in the production of uniform large-scale and single layer graphene through this technique.

Several other ways to produce graphene exists including: arc discharge of graphite [23] , electron beam irradiation of PMMA nanofibers [95] , and conversion of nanodiamond [96] . Graphene can be synthesized by unzipping of multi-walled carbon nanotubes (MWCNTs) using highly oxidative KMnO₂ and concentrated H₂SO₄ [38] , laser irradiation [38] , Li in liquid ammonia [10] , and plasma etching [39] to open up the carbon nanotubes for graphene nanoribbons (GNR) production. Arc discharge methods are also commonly used to synthesis boron- and nitrogen-doped graphene [97] [98] . Arc discharge between carbon electrodes and pyridine or ammonia in the presence of nitrogen has yielded nitrogen-doped graphene sheets [97] . Boron-doped graphene sheet have been synthesized in the presence of hydrogen-diborane vapor mixture or boron-stuffed graphite electrodes [98] .