Review—Critical Considerations of High Quality Graphene Synthesized by Plasma-Enhanced Chemical Vapor Deposition for Electronic and Energy Storage Devices

There are six most common graphene production techniques that are crucial in determining the properties of the product; 1) bottom-up synthesis, 2) mechanical exfoliation, 3) synthesis on SiC, 4) reduction of graphene oxide, 5) liquid-phase exfoliation and 6) chemical vapor deposition (CVD).^11–13 The restricted utilize of graphene on basic research and ideal applications such as high-frequency transistors and touch screens is due to high production costs and finite scalability of methods such as mechanical exfoliation^14,15 synthesis on SiC^11,15 and bottom-up synthesis from organic precursors.^13,15

Similarly, CVD of hydrocarbons¹¹ for mass-production of graphene for energy storage seems generally unsuitable due to moderate product purity, high cost and rather low yield.¹⁵ Production of vertical graphene nanosheet electrodes¹⁷ is efficient by CVD technique even though it is known that the packing density of the as-obtained graphene is very low.¹⁶ Other than those techniques, two techniques are widely employed for the bulk production of graphene: 1) liquid-phase exfoliation, and 2) reduction of graphene oxide.

Thermal expansion of graphite intercalation compounds or also known as 'expandable graphite' to obtain pristine or expanded graphite particles is called liquid-phase exfoliation. In order to reduce the strength of the Van der Waals attraction between the graphene layers, it is first dispersed in a solvent. Then, the exfoliation of graphite is induced into high-quality graphene sheets^11,18 by an external driving force such as ultrasonication,¹⁸ electric field¹⁹ or shearing.²⁰ Unfortunately, the low yield of this process leaves a considerable amount of unexfoliated graphite, which must be removed.²⁰ Yet, the low cost and high scalability of this technique¹⁸ make it suitable for producing graphene in bulk quantities.²¹

Highly defective form of graphene with a disrupted sp2-bonding network which is called Graphene oxide (GO) is produced by strong oxidation of pristine graphite^22,23 followed by stirring or ultrasonication in liquid media.²⁴ In order to restore the π network, GO must be reduced.²⁵ The production of reduced graphene oxide (RGO) is commonly involved chemical, thermal and electrochemical processes.^15,25,26 The production of bulk quantities with high yield and contained costs is achievable despite the low-to-medium quality of the obtained material due to the presence of both extrinsic defects; O- and H-containing groups, and intrinsic defects; edges and deformations. Alternative techniques are available for producing commercially available graphene for EESDs such as carbon nanotube unzipping²⁷ or direct arc-discharge²⁸ rather than using liquid-phase exfoliation and reduction of GO.

Despite the unsuitability of CVD for mass-production, it is a rather easy approach to obtain graphene with the desired features as compared to other techniques. CVD is widely known to involve the decomposition of a carbon feedstock, with the aid of heat and metal catalysts. Various metals, such as Cu, Ni, Pt, Ru, and Ir, have been proven to catalyze the growth of graphene. The number of graphene layers can be controlled by the type, thickness and crystal orientation of the catalyst used, whereas the size area of the graphene corresponds to the size of the catalyst. According to Azam et al., 2013, there are five types of CVD; 1) water-assisted CVD(WACVD), 2) alcohol catalytic CVD (ACCVD), 3) thermal CVD(TCVD), 4) floating catalyst CVD (FCCVD) and 5) plasma-enhanced CVD (PECVD).²⁹ However, the most common CVD used for carbon nanotube (CNT) or graphene growth is ACCVD, TCVD and PECVD.

ACCVD is a technique that involves pyrolytic decomposition of hydrocarbon gases; commonly benzene, ethanol, acetylene, ethylene, etc., with the presence of metal catalysts at elevated temperatures in the range of 600–1,200°C.³⁰ It is a simple technique due to the need of simple equipment; an oven, a tubular reactor, and a set of mass flow controllers to feed the gas.³¹ The main parameters to be carefully handle that control the graphene structures are catalyst, carbon feedstock, flow rate, time, pressure, temperature, and gas flow rate.

Plasma-enhanced CVD (PECVD) is well-known as a main technique for graphene synthesis. Even though thermal CVD (T-CVD) is a good potential, PECVD is more popular because its merits in lower substrate temperature, higher growth selectivity, and better control in nanostructure ordering/patterning^32–36 due to the presence of energetic electrons, excited molecules and atoms, free radicals, photons, and other active species in the plasma region. Compared with T-CVD, PECVD growth is more complex.³⁷

Similar to other techniques, PECVD also uses gas sources. The main difference is that to activate the gas, thermal energy will be used, whereas the electron impact will activate molecules in the main chamber. The gas activation takes place in a non- equilibrium plasma, generally referred as a glow discharge.³⁸ To grow graphene, feedstock gas, catalyst nature, and substrate temperature are among parameters that require consideration.³⁹ The schematic of PECVD set-up is shown in Figure 1.

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Plasma sources

Various metal catalysts

Various metals such as ruthenium (Ru),⁴⁵ iridium (Ir),⁴⁶ nickel (Ni),^47–50 and copper (Cu)^47,51,52 have been used as substrate materials for graphene synthesis. Other than metals, semiconductors and insulators also frequently used for the synthesis such as Si^53–69 carbon particle,⁷⁰ crystal,⁷¹ GaAS,⁷² quartz,⁷³ SiO₂^53,59,74 and Al₂O₃ (sapphire).^53,59,72 Comparatively, CVD is a rather easy approach to obtain graphene with the desired features. CVD is widely known to involve the decomposition of a carbon feedstock, either hydrocarbons or polymers, with the aid of heat and metal catalysts. As mentioned earlier, the number of graphene layers can be controlled by the type and thickness of the catalyst used, whereas the size area of the graphene corresponds to the size of the catalyst.⁷⁵ Due to their non-filled d-shells, these transition metals are able to absorb and interact with the hydrocarbons that are used as the carbon sources.⁷⁵

Similarly, graphene growth requires the same transition metal group as CNT synthesis. It is means that any studies related on parameters for CNT growth could be useful for graphene growth. Fe, Co and Ni are the most common transition metals for the synthesis of CNTs. Different metals each have their own unique mechanism for growing graphene. The ability of the transition metal to facilitate hydrocarbon dehydrogenation and carbon segregation will affect the quality of the graphene growth. The as-grown graphene and different type of substrates would have different interaction strengths and growth mechanisms. There are 11 different transition metals that possess different interaction properties with graphene summarized by Batzill.⁷⁶

In order for researchers to predict the overall outcome of graphene growth, the study of the phase diagrams of the transition metals to carbon enables is very useful. These metals are commonly having relatively high carbon solubility. From the phase diagram in Figure 2, the solubility of carbon can reach a maximum of 4.1 at.% and 2.7 at.% for Co and Ni, respectively. Fe is estimated to have the carbon solubility of >25 at.%. Ni and Co would form metastable carbide at high temperature and the maximum is at ∼1600 K. For most of the CVD process, the temperature is operated at around 1000°C. According to the phase diagrams, the solubility of carbon is ∼0.6 at.% and ∼0.9 at.% for Ni and Co, respectively. During the cooling stage, the carbon solubility decreases and the carbon precipitation gives the formation of graphene.^77,78

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Large-area growth of graphene films using plasma-assisted CVD on Cu^79,80 and Ni^81–83 has been established recently. Generally, annealing temperature is ranging from 450 to 1000°C.^81–83 The former study most comparable to the current work is that of Kim et al.,⁷⁹ who employed a custom- designed, multi-slot microwave antenna system to prompt plasma over a large-area surface. In producing high quality graphene on a large scale, it was proved that CVD of hydrocarbons onto relatively inexpensive polycrystalline Ni and Cu substrates is a highly promising approach.^84–86 Since the wet etching of Ni and Cu is feasible and straightforward, the transfer of graphene grown on Ni and Cu onto arbitrary substrates can be readily achieved.^84–86

Commonly, in previous research, multilayered graphene is found to be grown on high solubility of carbon (>0.1 at.%) Ni substrate by CVD technique.^85,87–94 Even though Reina et al.⁸⁵ recently did show that their Raman spectrum was similar to that of ideal single layer graphene (SLG) and provided the fact that SLG could be synthesized on Ni substrates, the thickness control of large area uniform graphene growth still remains a challenge. The growth of multilayer graphene on Ni has been ascribed to the growth mechanism, which is suggested to proceed via a combination of CVD surface growth at the growth temperature, and precipitation of carbon from bulk to the surface of Ni upon cooling after CVD growth.^88,89,93

Cu is one of the most attractive substrates for graphene growth due to its inexpensive large-area synthesis. For better understanding in mechanism underlying thermal CVD of graphene on Cu, some studies reported that before introducing hydrocarbon gas mixtures, the substrates are typically preheated to approximately 900–1000°C.^95–97 As the demand for graphene is increased, CVD is a very helpful technique in rapid graphene growth at reduced substrate temperatures. As reported using various methods of plasma excitations, to activate graphene growth at reduced substrate temperatures, plasma energy is very helpful in the detachment process of hydrocarbons.^{79–82,98–100} These approaches resulted in producing graphene flakes and vertically standing graphene sheets.

Hydrocarbon gases can break down on transition metals that have low carbon solubility during this technique.¹⁰¹ A fraction of the carbon atoms in a metal at elevated temperatures (900–1100°C), are known to precipitate as a graphite film upon cooling. In producing high-quality graphene films with >95% as monolayer on Cu, this method has been practiced.^102,103 The growth of graphene is limited to the surface of Cu as the growth mechanism of SLG has been attributed primarily to the low solubility of C in Cu (<0.001 at.%). It is proved by Bae et al. that have been obtained SLG films up to 30 in grown on Cu by using low pressure CVD.¹⁰² The transferred films show a sheet resistance of ∼30 Ω/sq with ∼90% optical transmittance for a doped four-layer graphene film (layer-by-layer transfer) and mobilities up to 7350 cm² V⁻¹ s⁻¹ at low temperature. It is well known that CVD growth of graphene on Cu substrates is considered to be self-limiting and somewhat surface mediated, leading to the growth of predominantly single-layer films due to the extremely low carbon solubility in Cu.^84,97 Certain CVD conditions such as high growth pressure and large amount of carbon precursor has been reported for such self-limiting growth to be occured, where massive non-uniform multi-layer graphene films are grown.^104,105 For a better controlling of thickness and uniformity of CVD graphene films grown on Cu, a detailed understanding of the formation of the multi-layer feature is strongly required. Consisting of numerous grain boundaries, large scale CVD graphene films produced so far are found to be polycrystalline.^106,107

Effect of substrates on graphene growth

There are several important parameters involved in the graphene growth by PECVD such as type of precursors, growth pressure, substrate temperatures, flow rate and gas ratio. While the overall growth reaction is a relatively simple which breaking methane gas into solid carbon (C) and hydrogen gas (H₂), much is still left to learn about the exact mechanisms of graphene growth. Multiple growth mechanism has been suggested by various authors.

Different metal substrates

For differing metal substrates of varying carbon solubility, it is generally agreed there are 2 possible ways for graphene to grow. Through isotopic labelling of the methane,¹⁰⁸ switching from¹¹¹ CH₄ to¹¹⁰ CH₄ midway of a CVD reaction for a Cu substrate as well as a Ni substrate, different graphene growth mechanism was observed. One is the purely surface based reaction for low carbon solubility metals such as Cu. In such reaction, hydrocarbon is adsorbed onto the surface, decomposed by the Cu substrate to form the graphene. Alternatively, metals with high carbon solubility would have the carbon atoms diffuse into the substrate bulk till it reaches critical super-saturation from which it will then start to precipitate to the surface forming graphene thus creating a multi-layered graphene. These two mechanisms are illustrated in the following Figure 3. High carbon solubility metals such as Ni would diffuse both¹¹¹ Cand¹¹⁰ C into the metal bulk before forming graphene that is comprised of randomly mixed with¹¹¹ C and¹¹⁰ C. This is in contrast to the low carbon solubility Cu substrate where the resulting graphene has a center comprised of Ref. 111 C and outer ring of Ref. 110 C proving the growth is only surface mediated.

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Substrates morphology

Besides, substrates morphology also played an important role to grow high quality graphene. Han et al.¹⁰⁹ reported that the formation of nucleation sites and size of graphene domains was strongly influenced by substrates morphology. The comparison was done between polished and unpolished Cu and it was found that a lot of nucleation sites existed after 3 s on the unpolished Cu with average domain size of about 30 μm² compared to the polished Cu with average domain size of about 140 μm². These nucleation sites coalesced together to form a continuous monolayer graphene with increasing growth time. After 5 and 10 min growth, there was formation of stripe-like multilayer graphene islands on the unpolished sample and no transformation of the monolayer graphene was found on the polished sample. Figure 4 shows the optical micrographs of the graphene grown on the polished and unpolished Cu.

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Substrate surface orientation

Surface orientation also affects the quality of the graphene produced. Ishihara et al.¹¹⁰ had grown graphene on a polycrystalline Cu with several surface orientations of (111), (101) and (100) as verified by electron backscattered diffraction (EBSD) shown in Figure 5b. Analysis of I_2D/I_G ratio from Raman spectra as shown in Figure 5d reveals that single-layer graphene (SLG) or few-layer graphene (FLG) were preferentially formed on the Cu(111) surface than on Cu(100) surface. Higher I_2D/I_G ratio indicated smaller thickness of the graphene layer produced. There were no graphitic peaks recorded on the Cu(101) surface. Wood et al.¹¹¹ also reported that Cu(111) surface contributed to accelerated formation of high quality, monolayer graphene growth with higher coverage compared with other orientations due to higher carbon diffusion rate.

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It is commonly well known that in graphene, the carbon bonds with each other to form hexagons in a honeycomb lattice with a two-atom basis. Couple this with the fact that FCC-structured metals with (111) surface termination also have the hexagonal structure for its interstitial sites which matched with the structure of the graphene may explain the preferential growth of graphene on certain orientations. The other surface terminations like (100) and (110) tend to have square-shaped and rectangular-shaped of the interstitial sites.¹¹² This observation further strengthens the fact that the most suitable substrates to grow high quality graphene is FCC-structured metals with the (111) surface orientation.

Underlying mechanism of graphene growth

Current CVD mostly produces graphene with a polycrystalline structure. This type of graphene contains many defects causing a decrement of several of its superlative characteristics. In crystal materials, polycrystalline can be defined as the combination of single-crystalline domains that consist of different lattice orientations with grain boundaries separating each one of them.¹¹³ Mattevi, Kim and Chhowalla¹¹⁴ had illustrated the growth process of typical CVD done on copper substrate as in Figure 6. First, the oxide layer on the Cu substrate was reduced in hydrogen atmosphere which also leads to annealing process of Cu. At this stage, crystal grain growth and removal of crystal defects occurs. Then, there will be formation of many nucleation sites on the Cu surface and all of these sites will grow until all of them merge together and form a graphene film on the surface of the metals. Each nucleation site will form a domain with different lattice orientation and these differences will lead to the formation of grain boundaries on the graphene. Grain boundaries (GB) can be considered as surface defect as it will disrupt the characteristics of graphene especially the electrical and thermal conductivity. Due to that, recently much attention was given to grow graphene on single-crystal metal substrates in an attempt to reduce lattice orientation mismatch. Despite this, the formation of mismatched graphene domains still remains unresolved.

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Gao et al.¹¹⁵ suggested that any interaction between graphene and substrate can contribute to the formation of domains on the graphene. Graphene has a lattice parameter of 2.46 Å, while Cu(111) has 2.56 Å. Due to this small lattice different, there was formation of multiple Moiré patterns which indicate the weak interaction between graphene and Cu(111). Additionally, this also proves that grown graphene has preferred orientations which cause the formation of graphene domains. Contradictory to this, Ogawa et al.¹¹⁶ reported that Cu(111) surface had a single orientation of graphene while other Cu orientation such as (100) formed multi-domain structure with two preferential orientation. By this, the advantages of using single-crystal metal substrates still remain unclear.

Yet, a lot of researches need to be done to explore the mechanism of graphene growth on metal substrates. Clear understanding on the effect of metals surface orientation toward the formation of graphene might contribute to the formation of high quality graphene for commercial used. To produce a high quality graphene, the relationship between types of substrates, growth mechanism, surface morphology and orientation and lattice characteristic need to be fully understand.

Reaction temperature has been one of the most important factors to produce high quality graphene. In Ni substrate itself, varying temperatures would induce different graphene growth mechanisms.¹¹⁷ Ultrahigh vacuum graphene growth on single crystal nickel (111) from 10⁻⁵ Torr ethylene was observed by LEED and Auger electron spectroscopy for surface carbide and graphene. At temperatures from 460°C to 650°C, the mechanism of graphene growth is similar to that of other transition metal. However, at temperatures lower than 460°C, graphene formation is from the carbide surface that created upon exposure to hydrocarbons. Through DFT modelling, it was shown that graphene growth occurs by carbon segregation from bulk to the graphene-carbide interface.

Various observations on the effect of the methane partial pressure on the graphene growth was studied. In the case of Cu-Ni alloy,¹¹⁸ at lower methane partial pressure (>0.003 mbar) only the growth of monolayer graphene was possible. However, bilayer and tri-layer graphene forms when the partial pressure is increased to 0.5 mbar. Additionally, several groups have noted that the increase in partial pressure of methane also increases the nucleation density of graphene in the substrate. In order to achieve single crystal graphene, this presents a problem as an increased nucleation causes the formation of multiple graphene domains namely polycrystalline graphene.

Therefore, in one of the attempt to selectively grow single crystal graphene, a repeated etching and re-growing CVD technique was developed,¹¹⁹ resulting in single crystal graphene of up to ∼3 mm in size with electron mobility of 13000 cm² V⁻¹ s⁻¹ grown on Pt substrate. Through the application of hydrogen etching and regrowing cycle, unnecessary nucleation sites are removed thus reducing the nucleation sites as shown in Figure 7. The remaining nucleation site will then regrow forming large area single crystals as the cycle repeats. This was also reported to also have the additional benefit of healing any structural defect occurring during the growth.

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Other concerns are on the surface modification of the substrate in which graphene is grown by themselves. Mechanically exfoliated graphene grown on hydrogen terminated (110) germanium was shown to be a monolayer single crystal graphene that is wrinkle free.¹²⁰ This etch-free dry transfer was repeated for 5 times using the same substrate each time resulting in the same graphene quality showing the reusability of the H-terminated single crystal Ge/Si substrate.

The substrate single crystal (110) germanium was grown epitaxially on Si(110) wafer and then graphene was grown by low pressure CVD. H-terminated surface allows the formation of freestanding graphene islands meaning that when multiple graphene nucleation sites coalesce it happens without any grain boundary defects. Combining this with the minimal thermal expansion coefficient between the materials, wrinkle free growth is also made possible. Furthermore, the weak adhesion of graphene to the substrate allows next to no damage to the graphene and also the substrate without the need for hazardous and destructive etching process. Using the Drude model, the carrier mobility was determined to be 7250 ± 1390 cm² V⁻¹ s⁻¹(SD) with a maximum of 10620 cm² V⁻¹ s⁻¹.

The positive effect of hydrogen has been noted by various authors such as Memon et al.¹²¹ and Vlassiouk et al.¹²² For a copper substrate, the removal of the oxide surface is vital for graphene growth. The presence of hydrogen during the reaction itself is reported to assist in the formation of carbon active species, CH_x and also in removing unwanted C-C bonds through etching as was mentioned in the etching and regrowing method above. Vlassiouk et al. also notes that the resulting shapes of graphene differ greatly according to the hyrogen pressure. Higher pressure causes the preferential growth of hexagonal graphene instead of irregular shaped ones albeit at the reduction in the maximum domain sizes grown.

The parameters mentioned above, while not comprehensive, cover the main parameters that greatly affect graphene growth and its resulting quality. It has been shown that carbon solubility of the metallic substrate results thickness of the graphene grown such as the monolayer graphene on Cu and the multilayered graphene in Ni metal. For Cu substrate, a smoother surface allows the development of bigger domains which is a priority in producing high quality domain. Furthermore, certain crystal orientation of the surface was also shown to facilitate graphene growth better than others as seen by the preferential growth of graphene on Cu(111) rather than other surface orientation. Apart from affecting the growth mechanism and decomposition of hydrocarbons, temperature control is vital for the graphene domains itself by changing the carbon solubility of the metals. In the case of methane based CVD, its partial pressure also has the ability of causing the growth of multi layered graphene when it is too high as well as the formation of multiple nucleation sites which leads to undesirable multi-domain polycrystalline graphene. This was successfully overcome by developing a grow, etch and re-grow procedure where excess nucleation sites are removed allowing only select sites to develop into bigger domains.

Finally, hydrogen gas is found to be vital in many ways. Initially, it prevents the formation of surface oxides on the metallic substrate. During the reaction, hydrogen terminated substrate surface allows a wrinkle free graphene growth without any grain boundary. The hydrogen terminated surface was also shown to facilitate the facile recovery of graphene film even by mechanical exfoliation from the substrate. All in all, understanding the growth mechanism is the key in producing large area high quality graphene especially for a single crystal graphene. It could be clearly seen that changes in each of the reaction parameters brings a big change on how the reaction mechanism itself proceeds and much is yet still to be understood.

Due to unique band structures, the carriers in graphene are bipolar, with electrons and holes that can be continuously tuned by a gate electrical field.¹³⁰ Majority in the experimental values of the field effect mobility of graphene showed one order of magnitude higher than Si. Graphene grown on Cu films has been widely employed as a method to produce large scale transistor arrays with uniform electrical properties.¹³¹ One of the most potential applications of graphene is in field effect transistors (FETs). Basically, graphene should be in the form of quasi one dimensional structure as per requirement to be used in FET applications.¹³² Wei et al.¹³³ reported the routes to modulate the electronic characteristic of graphene by nitrogen doping using critical PECVD (Figure 8). This metal free growth of high quality hexagonal nitrogen doped graphene crystals or continuous nitrogen doped graphene films with atomically clean surfaces directly on SiO₂/Si, Al₂O₃, hexagonal boron nitride(h-BN), mica, highly oriented pyrolytic graphite substrates at 435°C. The FET mobility of nitrogen doped graphene resulted in the range of 100 to 400 cm² V⁻¹ s⁻¹. This study reveal that by doping the nitrogen with graphene can effectively produce high quality of material without metal catalyst. Also, it is atomically clean surface, as well as enhances the performance of electrical device and overcome the conventional post growth process. This value may be further improved by using boron nitride, other modified surfaces,¹³⁴ or metals¹³⁵ to improve the carrier injection from the electrodes and to reduce amount of charged impurities trapped in the SiO² surface.

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Herein, Wei et al.¹³⁶ described the production of graphene crystal growth by using critical PECVD. During the growth process, H₂ plasma was introduced to etch the graphene from the edges.¹³⁷ The FETs were fabricated by using hexagonal graphene crystals. The results showed a symmetrical linear output curves which display the good ohmic contacts between electrode and hexagonal graphene crystals. The value for FET mobility was in the range of 550–1600 cm² V⁻¹ s⁻¹ with the dirac point at a positive gate voltage around 10 to 30 V. Also, the comparable mobility of the hexagonal graphene crystals with graphene materials prepared by Cu-CVD and peel-off graphene materials indicates their high electrical quality for device applications with the FET mobilities were 600–1500 cm² V⁻¹ s⁻¹ and 850–2200 cm² V⁻¹ s⁻¹, respectively. The results are higher than those amorphous carbon nitrogen films (10 cm² V⁻¹ s⁻¹) produced by metal free PECVD and to those of nitrogen doped graphene (200 to 450 cm² V⁻¹ s⁻¹).

Moreover, microwave plasma CVD (MPCVD) was employed by Kim et al. to synthesize high quality monolayer graphene film.¹³⁸ Various synthesis temperatures in the range of 450 to 750°C were used to investigate the sheet resistance. From the findings, the high transmittance of graphene films was ∼89% at 450°C with good sheet resistances of 1855 Ω/sq. Meanwhile, the best sheet resistance of graphene film was at 750°C with sheet resistance of 590 Ω/sq. This high quality graphene production is potentially useful for manufacturing future large-area electronic devices using microwave plasma CVD at significantly lower temperatures than that in conventional thermal CVD processes.

In addition, Kim et al.¹³⁹ successfully demonstrated one-step approach to fabricate patterning graphene on SiO₂ substrate without using catalysts or lithography. A shadow mask composed of stainless steel is placed on the top of the SiO₂ (300 nm)/n-Si wafer substrate. The graphene growth was performed by using PECVD. O2 gas was purged into the chamber prior the graphene growth to remove any organic material on the top surface of substrate. The flow rate and RF power employed was 40 sccm and 50 W, respectively. The CVD and RF plasma parameter during graphene growth was 10 mTorr, 2 sccm, and 20 sccm for pressure, hydrogen and methane flow rates, respectively. After finishing the graphene growth, the sample was cooled immediately by turning off the heating power with the cooling rate of 3°C s⁻¹. This is a simple graphene method compared to other graphene growth processes such as graphene transfer, catalyst-etching, lithography, and metal catalysts which can be prepared in several hours by using different shapes of samples. From Hall bar measurements show lower mobility 105 cm² V⁻¹ s⁻¹ but higher compared with catalyst-free nanocrystalline graphene (1–40 cm² V⁻¹ s⁻¹) and chemically reduced graphene oxide films (∼1 cm² V⁻¹ s⁻¹). Further growth optimization is required to improve the high quality of graphene production for future graphene applications.

To achieve high field enhancement, graphene sheets either as single or few layers need to be erected on the substrates.¹³² Among the graphene growth techniques, PECVD showed the most promising process for field emission applications compared with other graphene growth techniques which resulted in flat graphene layers on substrates.¹⁴⁰ Jiang et al. have recently reported the preparation of vertically standing graphene (VSG) films on Cu foil by using a MPCVD.¹⁴¹ The result obtained show an excellent properties with atomically thin edges of graphene sheets and homogenous surface morphology¹⁴² may be attributed by many factors such as VSG film possess plenty of atomic-thick graphene edges, and the unique formation of VSG film growth with good interfacial contact¹⁴³ as well as low resistance between films and substrates. Moreover, the as-prepared VSG films demonstrated large field-enhancement factor, low threshold field and low turn-on electric field of 1.1 × 10⁴, 3.0 V μm⁻¹, and 1.3 V μm⁻¹, respectively.

Wu et al. successfully demonstrated a fabrication of sensitive and large scale manufacturing of selective FET biosensor by using PECVD technique to directly grown vertically-oriented graphene sheets on gold electrodes.¹⁴⁴ The direct growth of vertically-oriented graphene sheets on gold electrodes showed the turn-on electric field of 2.3 V mm⁻¹ and the threshold field of 5.2 V mm⁻¹ at 10 mA cm⁻² substantially lower than those of the graphene-powder coating with the turn-on electric field of 5.2 V mm⁻¹ and the threshold field of 9.6 V mm⁻¹.

Chockalingam et al.¹⁴⁵ demonstrated that at 1 mA cm⁻² current density the MWCNT-graphene-like nanocarbon hybrid film exhibit the minimum threshold field of 3.6 Vmm⁻¹ accompanied with field enhancement factor (β) is 3164 at 20 Torr. Meanwhile, the maximum current density and β at 5 Torr were found to be 0.12 mA cm⁻² with field enhancement factor of 3356. The good adhesion between MWCNT and Ni substrate show high density of material which produces good contact and lower the electrical resistance. Alternatively, multilayer graphene (MLG) on chemically prepared silicon nanowire (SiNW) were successfully fabricated by Deng et al.¹⁴⁶ using microwave PCVD. From the field emission study indicates that hybrid MLG–SiNW display remarkable and excellent properties of field emission performance compared with SiNWs. They have a low turn-on electric field of 2.42 V μm⁻¹, large current density of 3.49 mA cm⁻² and excellent field emission stability. Both materials possess remarkable properties which can contribute to an excellent field emission material in potential applications for vacuum electron sources.

To summarize this section, hexagonal graphene crystals with graphene materials prepared by Cu-CVD and peel-off graphene materials indicates their high electrical quality with the FET mobilities 600 to 1500 cm² V⁻¹ s⁻¹ and 850 to 2200 cm² V⁻¹ s⁻¹ compared with the amorphous carbon nitrogen films produced by metal free PECVD (10 cm² V⁻¹ s⁻¹) and nitrogen doped graphene (200 to 450 cm² V⁻¹ s⁻¹). Also, the graphene pattern on SiO₂ show lower mobility 105 (cm² V⁻¹ s⁻¹) but higher compared with catalyst-free nanocrystalline graphene (1–40 cm² V⁻¹ s⁻¹) and chemically reduced graphene oxide films (∼1 cm² V⁻¹ s⁻¹). The direct growth of vertically-oriented graphene sheets on gold electrodes showed the turn-on electric field of 2.3 V mm⁻¹ at 10 mA cm⁻² and the threshold field of 5.2 V mm⁻¹ at 10 mA cm⁻² substantially lower than those of the graphene-powder coating with the turn-on electric field of 5.2 V mm⁻¹ and the threshold field of 9.6 V mm⁻¹. These vertically-oriented graphene sheets showed higher threshold field compared with other material such as hybrid multilayer graphene–silicon nanowire (2.42 V μm⁻¹), MWCNT-graphene (3.6 V mm⁻¹), and vertically standing graphene (3.0 V μm⁻¹). The above results suggest that single-layer graphene have a great potential as high-performance field emitters.

Over the past decade, there is a high demand for advanced renewable electrodes for energy-storage applications.¹⁴⁷ Among various energy storage devices, supercapacitor has been extensively studied to be utilized as power sources because of high power density, long cycle life, high stability, and rapid charging/discharging rate.^148–151 The superb characteristic are widely used in energy management, memory back-up systems, consumer electronics, and industrial power.^149,152,153 The combination of high energy density fuel cells or batteries and high power density of supercapacitors to be used in various applications such as starters, generators, and electric braking assistance.¹⁵⁴ The 3D graphene and vertically oriented few-layer graphene (VFG) nanocup (3D VFG) was successfully demonstrated by Qi et al.¹⁵⁵ using PECVD. This as-fabricated 3D VFG was grown on alveolate Pt film display large specific area and good specific capacitance for high performance supercapacitor. From electrochemical measurements indicated as-fabricated 3D VFG display higher specific capacitance of 1052 mF cm⁻² compared to VFG-plane 337 mF cm⁻²) and excellent cycling stability of 93% capacitance retention after 3000 cycles which suggested that great opportunities in supercapacitor application (Figure 9). This excellent electrochemical performance of electrode material may be attributed from the 3D VFG-nanocup hybrid structured characteristic such as high specific surface area for ion transmission and storage.

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The critical issue of contact resistance in a supercapacitor has been solved by Bo et al.¹⁵⁶ by building bridges of vertical graphene (VG) nanosheets between the current collector and active materials. The "bridges" was constructed by directly grown VG nanosheets on the surface of nickel foam current collector by PECVD process. The constructed VG-bridged supercapacitor show superb rate performance proved from cyclic voltammetry measurement at scan rate from 20 to 1000 mV s⁻¹ and galvanostatic charge/discharge measurement at 1 to 100 A g⁻¹ with capacitance retention of ∼90%. In addition, this VG bridged graphene-film also display power capability of 112.6 kW kg⁻¹ at high current density of 600 A g⁻¹. By employing VG nanosheets will enhance supercapacitor performance due to their superior characteristic such as high chemical tolerance, excellent electrical conductivity, high chemical tolerance as well as unique growth orientation. Also, VG nanosheets can build up a short-cut and high speed bridge between the current collector and active materials to facilitate electron transport during the charge/discharge processes. They were expected that this work will open up new opportunities in advanced application.

Also, Bo et al. has reported on a one-step binder-free fabrication method for supercapacitor electrodes consisting of vertically-oriented graphene (VG) uniformly grown on a metallic current collector by PECVD.¹⁵⁷ The as-prepared vertically-oriented graphene exhibit a gravimetric specific capacitance of 129 and 112 F g⁻¹ in 6 M KOH and 1 M TEABF4/AN, respectively. The device work properly with the specific power was as 23.1 and 16.2 kW kg⁻¹ in in 6 M KOH and 1 M TEABF4/AN, respectively displaying higher energy density performance maybe due to higher potential window in 1 M TEABF4/AN (2.2 V) than that in 6 M KOH (0.9 V). Also, the high surface area and porous structure of VG-coated e is mainly from the VG interlayer and intersheet channels which give one order of magnitude higher than that of the bare stainless steel for charge storage performance. These result exhibit higher capacitance value compared those reported for horizontally oriented graphene and traditional activated carbon or carbon powders.

Vertical graphene nanosheets (VGN) with high surface area have been described by Ghosh et al.¹⁵⁸ using microwave PECVD process. From cyclic voltammetry analyses at various scan rates, the unaltered, mirror symmetric and quasi-rectangular shape for the VGN/Na₂SO₄, VGN/KOH and VGN/H₂SO₄ display the near ideal capacitance behavior with good electrochemical reversibility with areal specific capacitance of 44 μF cm⁻², 197 μF cm⁻², and 188 μF cm⁻², respectively. Among them, H₂SO₄ exhibited excellent specific areal capacitance (188 μF cm⁻²) and good capacitance retention up to 200 cycles with the capacitance retention of 96.8% followed by VGN/Na₂SO₄ (95.93%) and VGN/KOH (98.41%). This better conductivity and high surface area of the electrode material show excellent performance to be used in supercapacitor.

Besides, rechargeable lithium-ion batteries (LIBs) have been widely utilized in energy storage devices due to their excellent capacity, remarkable charging efficiency, and long cycling life. Instead of superior characteristic, synthesis and design of novel electrode materials must take a consideration for performance lithium storage behavior^159,160 to be used as main power sources in portable electronics, renewable energy systems, and hybrid electric vehicles.¹⁶¹

Cho et al. reported a simple fabrication process to develop a porous silicon nanofibers (Si NFs) using PECVD technique which involves three steps¹⁶² which are firstly, the fabrication of polymer/Si NFs by electrospinning, secondly, by reducing the as-spun fibers with Mg at 650°C and graphene coated at 400°C to get graphene-coated Si NFs to be used as anode material in LIBs. The as obtained anode show remarkable cycling performance with higher capacity retention compared with pure Si NFs. Also, the high cycling performance graphene-coated Si NFs may be attributed by the confinement of porous-thin Si NFs with graphene. The graphene would restrain the volume change of Si NFs and enhanced the electrode electrical conductivity. This graphene coated Si NFs exhibit a stable electrochemical performance up to 50 cycles (Figure 10) with specific capacity of 760 mAh g⁻¹ compared with Si NFs alone.

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In recent years, graphene has attracted tremendous attention which displays unique properties including thermal, electrical, and mechanical properties.¹⁶³ These features play an important role in various applications.¹⁶⁴ Realizing the shortage of graphene, 3D graphene has been explored due to easy preparation, economical devices, and high efficiency.¹⁶⁵ By controlling the structures of the carbon nanowires, Kim et al. have successfully synthesized scalable highly branched graphene nanosheets (HBGNs) electrodes using low cost DC PECVD under atmospheric pressure.¹⁶⁶ The as-prepared hybrid graphene–HBGN anode showed an excellent capacity of 500 mAh g⁻¹ with 10% irreversible capacity loss at a current density of C/5 even after 100 cycles. Furthermore, at high rate of 4 C, the BGN electrode retained a good specific capacity of 297 mAh g⁻¹ showing a promising electrochemical performance for a large scale as anode materials in LIBs. This electrode material exhibited promising electrochemical performance due to their large surface area, unique morphology, and excellent mechanical properties. Also, this unique morphology of 3D HBGNs provides efficient diffusion of Li-ions with large surface area and many voids leading to a large number of sites for Li-ion storage.

Furthermore, 3D Si thin film supported on a graphene scaffold (GSSSE) was successfully prepared by Wang et al.¹⁶⁷ to be used as an anode electrode for high energy LIBs by using MPCVD. At current density of 797 mA g⁻¹ the as-prepared Si anode exhibit a gravimetric capacity as high as 1560 mA h g⁻¹ with capacity retention of 84% after 500 cycles. Also, after 1200 cycles, the result show the specific capacities of 1083 and 803 mA h g⁻¹ at 2390 mA g⁻¹ and 7170 mA g⁻¹, respectively. These high specific capacities, excellent cyclability and rate performance could be due to the highly porous 3D architecture of the graphene scaffold, which possesses good electrical conductivity as well as mechanical flexibility. Also, the 3D architecture of the graphene scaffold 3D architecture of the graphene scaffold can essentially accommodate the volume changes of Si during lithium insertion and extraction processes. Meanwhile, for electrochemical performance, the excellent cyclic stability of the GSSSE may be attributed to the unique designed architecture structure: (1) large volume change of Si during charge–discharge processes may be attributed by the macro porosity of graphene scaffold. (2) large surface area of graphene scaffold provides more active sites for electrochemical reactions, which lead to faster kinetics and higher utilization of the active electrode material. (3) highly porous and excellent conductivity of graphene scaffold deliver a good pathway for lithium ions and electrons between the current collector and electrode.

For supercapacitor application, the cyclic voltammetry analyses of vertically-oriented graphene electrode in 6 M KOH and 1 M TEABF4/AN resulted in gravimetric specific capacitance of 129 F g⁻¹ and 112 F g⁻¹, respectively. These excellent specific capacitance value are higher compared with 3D VFG-nanocup (1052 mF cm⁻² which is three times than VFG-plane 337 mF cm⁻²) and vertical graphene nanosheets/H₂SO₄ (188 μF cm⁻² up to 200 cycles with the capacitance retention of 96.8%).

Meanwhile, vertical graphene-bridged supercapacitor displays power capability up to 112.6 kW kg⁻¹ at a high current density of 600 A g⁻¹ with capacitance retention of ∼90% (at 100 A g⁻¹) and the maximum specific power of vertically-oriented graphene electrode in 6 M KOH and 1 M TEABF4/AN were 23.1 and 16.2 kW kg⁻¹, respectively. The as-prepared 3D Si thin film supported on a graphene scaffold anode exhibit a gravimetric capacity as high as 1560 mA h g⁻¹ with capacity retention of 84% after 500 cycles at current density of 797 mA g⁻¹. Also, after 1200 cycles, the result shows the specific capacities of 1083 and 803 mA h g⁻¹ at 2390 mA g⁻¹ and 7170 mA g⁻¹, respectively. These result show high gravimetric capacity compared with graphene coated Si NFs (760 mAh g⁻¹, 50 cycles) and as-prepared hybrid graphene–HBGN anode with gravimetric capacity of 500 mAh g⁻¹ (100 cycles) and 297 mAh g⁻¹ at a current density of C/5 and 4 C, respectively. Table III in specific summarized the performance of electronic and energy storage devices utilizing graphene grown from PECVD technique.