Graphene is a representative two-dimensional material composed of sp² hybridized carbon atoms organized in hexagonal lattices [1]. Since graphene was first isolated in 2004, many remarkable properties, such as high tensile strength, outstanding thermal conductivity, high stretchability, and high transparency, have been reported [1–5]. In particular, the valence and conduction bands of graphene touch at the six Dirac points in the first Brillouin zone, enabling charge carriers to behave as massless Dirac fermions [2]. Accordingly, various graphenebased applications, including high-frequency transistors, energy storage devices, and optoelectronic applications, have been demonstrated [1–5]. To achieve the practical application of graphene in the abovementioned devices, developing growth protocols to engineer graphene with specific purity, layer number, functionalization, and lateral size is critical, and research on graphene synthesis is essential for this development.

Generally, graphene is synthesized using top-down or bottom-up growth approaches, as shown in Fig. 1. Top-down approaches involve the synthesis of graphene by exfoliating graphite, the bulk form of graphene, and this exfoliation can be conducted using techniques such as adhesive tape exfoliation, chemical exfoliation, electrochemical exfoliation, liquid exfoliation, laser ablation, and ball milling [6,7]. Mechanical exfoliation-derived graphene is high quality and advantageous for lab-scale fundamental research and has made significant contributions to understanding many critical features and applications of graphene; however, the small size and limited yield of graphene produced by this approach limits its large-scale usefulness [1]. Recently, Moon et al. [8] introduced a modified mechanical exfoliation method to obtain millimeter-sized graphene with the assistance of deposited thin metal films. Chemical exfoliation methods such as Hummer’s method introduce oxygen-containing functional moieties in graphene using oxidizing agents such as potassium permanganate and a mixture of nitric and sulfuric acid, and then reduce graphene oxide using conventional reducing agents such as NaBH₄ or metal salts such as Mn (II) [7,9]. Chemical methods result in the formation of graphene with high electrical conductivity, which can be used for applications This is in energy storage devices. However, the usage of expensive chemicals and poor quality of graphene, restrict its use in commercial applications [7,9].

Figure 1. Various protocols for graphene synthesis.

Even though top-down strategies such as mechanical exfoliation produce high-quality graphene, the process cannot be applied on an industrial scale, making it necessary to develop synthetic protocols to achieve large-scale growth of high-quality graphene [1]. In contrast, as its name suggests, the bottom-up strategy achieves the growth of graphene using simple carbon precursors and is advantageous for manufacturing high-quality graphene for electronic applications [10]. The most relevant and promising technique for graphene growth via the bottom-up approach is chemical vapor deposition (CVD). CVD can be used to manufacture high-quality graphene with a large area and hence can be scaled up for industrial applications [4]. Different types of CVD techniques are available, as shown in Table I. CVD graphene formation is typically accomplished by depositing a gaseous carbon precursor on the surface of a transition metal substrate at high temperatures (>1,000 °C) [4].

Table 1 . Classification of CVD methods based on operating conditions..

Parameter	CVD technique
Temperature	Thermal CVD
	Low-temperature CVD
Substrate heating	Hot wall CVD
	Cold wall CVD
Pressure	Low-pressure CVD
	Atmospheric pressure CVD
	Ultrahigh vacuum CVD
Additional energy sources	PECVD
	Microwave plasma-assisted CVD
	Radiofrequency CVD

However, high-temperature settings are incompatible with complementary metal-oxide-semiconductor (CMOS) manufacturing processes because they severely damage the substrates supporting graphene, consume a lot of energy, and require sophisticated equipment, preventing graphene from becoming commercially viable [11]. To overcome the limitations of high-temperature growth and improve the prospects of future electronic applications based on graphene, it is highly desirable to reduce the growth temperature of graphene. Low-temperature CVD (LT-CVD) techniques are sustainable and less costly [12]. This review discusses the recent advancements in lowtemperature graphene synthesis using CVD. Subsequently, we summarize the types of precursors involved and briefly describe the properties of these low-temperature grown graphene specimens. Finally, we explore the applications of these materials in electronics, optoelectronics, energy, etc.

2.1. Growth mechanism of graphene by CVD

The typical protocol for the CVD growth of graphene on a metal substrate commences with thermal annealing of the substrate, followed by the introduction of a hydrocarbon precursor, which may lead to the adsorption of the precursor species on the surface or scattering back into the vapor phase [shown in Fig. 2(a)]. The adsorbed precursor species are dehydrogenated to produce active carbon species, which lead to nucleation and growth, resulting in a continuous graphene layer [13]. The quality and number of layers of asgrown graphene can be changed according to several parameters [14]. For example, metallic substrates are extensively used as catalysts in the dehydrogenation of carbon feedstocks, and according to their distinct properties, such as carbon solubility, the number of layers of asgrown graphene can be determined. Several transition metals, primarily Cu and Ni, and other metals, such as Co, Pt, Pd, Ir, Au, Ru, and semi-metals, such as Ge, have been examined as suitable catalytic substrates [4,6,15–19]. Since the carbon solubility of Cu is poor, when precursor molecules adhere to the Cu surface during graphene growth, dehydrogenation occurs predominantly on the Cu surface, enabling the formation of monolayer graphene on the Cu (111) plane [20]. However, because Ni has a higher carbon solubility, graphene development on its surface usually results in multilayers because the precursor species dissolve in the Ni surface until it reaches saturation, at which time graphene nucleation begins [21]. In addition, semimetals such as Ge have been proven to realize wafer-scale growth of single-crystal monolayer graphene by utilizing the anisotropic orientation of the Ge (110) substrate [18]. Hydrogen gas serves as a co-catalyst to produce active carbon species and their attachment to the catalyst surface and etch out weaker C-C bonds [22]. Therefore, to ensure the synthesis of high-quality graphene, the partial pressure of gaseous carbon precursors should typically be less than that of hydrogen supplied [23,24]. Maintaining a low growth pressure of less than 10 Torr and employing a high cooling rate larger than 10 °C per min are other key parameters for improving graphene quality [4,25]. However, each of the abovementioned steps requires a high synthesis temperature (typically > 850 °C) to induce reactions [6].

Figure 2. (a) General mechanism for graphene growth via CVD. Reproduced with permission from [13], Copyright 2013, American Chemical Society. (b) Graphene growth energy profiles with methane and benzene as carbon sources. Reproduced with permission from [12], Copyright 2011, American Chemical Society.

2.2. Energy demands for low-temperature growth

The LT-CVD growth of graphene is considered highly desirable owing to its low energy requirement, convenience, affordability, and scalability as well as environmentally benign nature. The energy demands for each growth step can help determine the critical factors for realizing efficient LT-CVD graphene growth. The adsorption energy of the precursor on the substrate surface is a metric in the first stage [12]. The energy required for dehydrogenation may be estimated based on the number of hydrogen atoms present, and as a consequence, typical gaseous precursors (e.g., methane, acetylene, and ethylene) require more energy than aromatic compounds because of more hydrogen atoms, and thus more intermediate species [26].

In the last step of graphene growth, the active carbon species undergo coalescence and nucleation to produce graphene film [12]. The graphene structure comprises a hexagonal carbon skeleton, and because aromatic compounds already possess a hexagonal framework it is energetically favorable for them to form graphene. However, gaseous precursors such as methane require more energy to form graphene because the active species containing fewer carbon atoms have to combine and form a six-membered ring structure [shown in Fig. 2(b)] [12]. Therefore, carbon precursors with low energy demands are required to obtain high-quality graphene under low-temperature conditions. In addition, the use of plasma sources will help produce activated carbon species at lower temperatures.

2.3. Low-temperature graphene growth from various hydrocarbon precursors

The growth of graphene using liquid carbon sources, such as benzene, pyridine, etc. has been reported [27,28]. A schematic of the liquid precursor-based CVD is shown in Fig. 3(a). Benzene is a well-explored precursor for graphene growth, and the formation of graphene has been observed at a growth temperature as low as 300 °C; it is not observed below this temperature, probably due to the insufficient energy for dehydrogenation [29]. The various liquid precursors, substrates, growth temperature, etc. are summarized in Table II.

Table 2 . Summary of liquid precursors in low-temperature CVD graphene growth..

No.	Synthesis method	Growth temp (°C)	Carbon source	Catalyst/Substrate	Graphene type	Ref.
1	Oxygen-free APCVD	400	Benzene or pyridine	Cu interconnects	Multilayer	[35]
2	Direct CVD	350, 450	Methanol, propylene	TiO2	Monolayer or double layer	[32]
3	CVD	464	Ethanol	SiO2/Si, Ni catalyst	Multilayer	[30]
4	APCVD	300	Pyridine	Cu foil	N-doped graphene	[31]
5	PECVD	RT	1,2-dichlorobenzene (and other aromatic precursors)	Ni, Cu	Graphene nanostripes	[33]
6	Cold wall PECVD	100	Benzene	Ni catalyst on SiO2/Si	N-doped graphene	[36]
7	Cold wall CVD	400	Benzene	Ni catalyst on SiO2/Si	Large area bilayer graphene	[34]
8	CVD	500	Benzene	Cu foil	Monolayer	[29]

Figure 3. (a) Schematic for liquid precursor-based CVD, (b) Optical Micrograph, and (c) Raman map intensity of I_2D/I_G, and (d) I_D/I_G of bilayer graphene grown on Ni/SiO₂/Si. Reproduced with permission from [34], Copyright 2021, American Chemical Society.

Monolayer graphene was obtained from benzene derived from coal using CVD technology, where the process is possible because of dehydrogenation and bonding (T = 500 °C), not the complete decomposition of benzene, as the latter requires high-temperature conditions. This process yields high-quality graphene even at low absorption energies using polycrystalline Cu foil as the catalyst and substrate [29]. The growth of multilayered graphene at temperatures as low as 464 °C was achieved when a current enhanced CVD process was used. The deposition was performed on a Ni-coated SiO₂/Si surface, and the process resulted in graphene layers with good crystallinity and low defect concentration. Multilayer graphene grown in this manner can be used as an interconnect material in large-scale integration [30].

Son et al. [31] investigated the generation of nitrogen-doped graphene via a two-step atmospheric pressure-CVD (APCVD) wherein pyridine was the feedstock for both carbon and nitrogen, and the process was performed at 300 °C on a catalytic substrate with different partial pressures of the carbon precursor in each step. The resultant N-doped graphene films were used to fabricate field effect transistors (FET) and exhibited n-type transport characteristics with electron mobility of 1,400 cm²V⁻¹s⁻¹ [31]. The synthesis of graphene layer over TiO₂ using two different precursors, namely methanol and propylene, was explored by Fitri et al. [32], and it was observed that at low growth temperatures, TiO₂ catalyzes the growth of nanometerthick graphene in its vicinity. Graphene nanostripes were grown on Ni or Cu catalysts at room temperature conditions by using an aromatic carbon feedstock, such as 1,2-dichlorobenzene [33]. In 2021, Haniff et al. [34] prepared graphene via cold wall CVD, wherein they utilized benzene’s low energy requirement and nickel’s catalytic ability to produce bilayer graphene at a temperature of 400 °C, as shown in Figs. 3(b)-(d).

Solid precursors are comparatively easier to handle than gaseous precursors and can be explored for large-scale growth owing to their low energy requirements. In 2011, Li et al. [12] used polymethyl methacrylate (PMMA) and polystyrene as precursors for centimeter-scale graphene growth on Cu substrate. Subsequently, large aromatic compounds such as 4″,5‴-dibromo- 1,1′:4′,1″:2″,1″:2‴,1⁗:4⁗,1′′′′′-sexiphenyl, 3′,6′-dibromo-1,1′:2′, 1″-terphenyl (DBTP), perylene tetracarboxylic anhydride, etc. have recently been explored for graphene synthesis [37–39]. These solid aromatic molecules have a hexagonal ring structure, similar to benzene, and using them to create graphene at low temperatures is comparatively easier [29]. However, due to the complex structure and characteristics of these compounds, the reaction would proceed via complex pathways and mechanisms, and understanding the mechanism and energetics involved is critical to ensure the efficient creation of graphene sheets. Therefore, future studies on the best solid precursors for high-quality graphene growth at low temperatures are required.

Graphene nanoribbons (GNRs), strips of 2D graphene, are important in electronic, optoelectronic, and other applications due to their tunable properties [40]. Most GNR synthesis protocols are based on a modified Ullman coupling reaction, where the metal substrate (Au) acts as a catalyst for the free radical generation followed by the cyclodehydrogenation of the precursor monomer, which is usually a halogenated polyphenylene molecule [37]. Armchair GNRs (AGNRs) are classified into three families based on their atomic number across the width of GNR (N = 3n, 3n + 1, and 3n + 2). Figure 4(a) shows types of GNRs based on edge orientation. The N of a given AGNR can be estimated from its Raman spectra by locating the position of the radial breathing-like mode (RBLM) peak, as shown in Fig. 4(b) [38]. The various solid precursors, substrates, growth temperature, etc. are summarized in Table III.

Table 3 . Summary of solid precursors in low-temperature CVD graphene growth..

No.	Synthesis method	Growth temp (°C)	Carbon source	Substrate/Catalyst	Graphene type	Ref.
1	CVD	400	3’,6’-dibromo-1,1’:2’,1”-terphenyl	Au/mica	Armchair graphene	[38]
2	PECVD	500	1,2,3,4-tetraphenylnaphthalene	Al2O3/PI	Four layers	[47]
3	CVD	500	Perylenetetracarboxylic dianyhydride	SiO2/Si	GNRs	[39]
4	2Z-CVD	250-500	4″,5‴-dibromo-1,1′:4′,1″:2″,1‴:2‴,1⁗:4⁗,1⁗′- sexiphenyl	SiO2/Si	GNRs	[37]
5	CVD	350-400	3,9-dibromoperylene and 3,10-dibromoperylene	Fused silica	Armchair graphene nanoribbon	[41]
6	AP CVD	300	Chitosan	Cu foil	N-doped graphene QDs	[45]
7	CVD	400	Polystyrene spheres	Cu nanocap array	Cu/Graphene double	[43]

Figure 4. (a) Types of GNRs based on the edge configuration. (b) Raman spectra of 9-AGNR and in the inset on the right side, the RBLM wavenumber dependence on the width of the AGNR plot. Reproduced with permission from [38], Copyright 2017, American Chemical Society. (c) Formation of 5-AGNRs and 10-AGNRS by CVD. Reproduced with permission from [41], Copyright 2017, American Chemical Society. (d) and (e) CVD growth of GNRs via CVD through the thermal decomposition of the PTCDA precursor. Reproduced with permission from [39], Copyright 2020, American Chemical Society.

In 2017, Sakaguchi et al. [37] reported the CVD growth of GNRs in the temperature range of 250-500 °C using a two-zone CVD setup. The growth of GNRs occurs via the radical polymerization of the carbon precursor (4″,5‴-dibromo-1,1′:4′,1″:2″,1″:2‴,1⁗:4⁗,1′′′′′- sexiphenyl), which results in biradical formation, followed by a conformation-controlled surface-catalyzed cascade dehydrogenation reaction on the Au (111) substrate to form acene-type GNRs.

The growth of N = 9 armchair GNRs (9-AGNRs) was reported in 2017 by Chen et al. [38], wherein they adopted CVD synthesis at a temperature below 400 °C using DBTP as the carbon source. The deposition process, which uses Au-coated mica as substrate, involves biradical generation via dehalogenation followed by coupling, and the oligomer formed is further dehydrogenated, yielding nanoribbons. In the same year, Chen et al. [38] reported the CVD synthesis of 5-AGNRs and the lateral fusion of 5-AGNRs to form wider AGNRs, such as 10-AGNR and 15-AGNR, using an isomeric mixture of 3,9-dibromoperylene and 3,10-dibromoperylene as carbon precursors. The temperature condition used for the 5-AGNR synthesis is 350 °C on Au/mica substrate. The mechanism for 5-AGNR involves the dehalogenation of the precursor via homolysis and intramolecular cyclodehydrogenation catalyzed by Au substrates, as shown in Fig. 4(c). Subjecting the 5-AGNRs to thermal annealing at 500 °C delivered 10- AGNRs and elevating the temperature by another 100 °C results in the formation of 15-AGNRs. The width of 5-AGNR is ~0.9 nm, and those of 10-AGNR and 15-AGNR are ~1.3 and ~1.8 nm, respectively [41].

In 2020, Shekhirev et al. [39] reported the CVD growth of 5- AGNRs using the commercially available perylene tetracarboxylic anhydride as carbon precursor and a variety of substrates (Au, Cu, and SiO₂/Si) at a temperature of 500 °C. The reaction proceeds via the thermal decomposition of the precursor, resulting in the elimination of carbon oxides and the tetra radicals of perylene hence obtained polymerizes and forms the AGNRs which have a 5-atom width armchair structure, as shown in Figs. 4(d) and 4(e). At a higher growth temperature of 600 °C, the AGNRs undergo cross fusion, and their structural degradation occurs. The 5-AGNRs were used to fabricate FETs and gas sensors for volatile organic compounds, such as alcohols and amines [39].

Kojima et al. [42] investigated the synthesis of cove-type graphene nanoribbon networks by interlinking GNRs using two-zone CVD with Au (111) coated on mica or glass as the substrate for application in thermoelectrics owing to their porous structure. 4′,5″-dibromo- 1,1′:2′,1″:2″,1‴-quaterphenyl (DBQP) was used as the starting material for the growth of 5-CGNR-1-1 at 500 °C and further thermal annealing at 600 °C yields the 2D cove type graphene nanoribbon network. As mentioned earlier, the initial reaction proceeds via an Ullman coupling reaction, and the formed 1D structures interconnect to produce 2D networks. The 2D networks have a high electrical conductivity of 188 Sm⁻¹ and low thermal conductivity of 0.11 Wm⁻¹K⁻¹ [42].

Polystyrene (PS) spheres were used as a solid carbon source as well as a template to develop a Cu/graphene double nanocap array through a CVD process at 400 °C. The nanocap array was found to be a good surface-enhanced Raman scattering (SERS) substrate for detection of rhodamine 6 G. The nanocaps exhibited good SERS performance owing to the synergistic interactions between Cu and graphene [43].

The Fermi level of pristine graphene is located at the Dirac point and therefore it has zero bandgap. Introducing dopants in the graphene network helps to fine-tune the bandgap, work function, and thereby the conductive behavior of graphene, which could increase the efficiency of optoelectronic devices that use CVD graphene [44]. N-doped graphene quantum dots were synthesized from chitosan (carbon and nitrogen source) using an atmospheric CVD setup with growth carried out at 300 °C on Cu foil. This fast and eco-friendly thermal decomposition mechanism of chitosan produces N-graphene QDs with strong luminescence, indicating good optoelectronic properties [45].

The CVD growth of graphene is an efficient method for producing high-quality films; however, wet chemical transfer is generally used for catalyst etching (shown in Fig. 5) [46]. The incomplete removal of the polymer support layer used during etching might result in p-type doping of graphene. The etchant solution might introduce contaminants, or water-based transfer might trap water between graphene and the desired substrate [24,46]. Therefore, the direct growth of graphene on the desired substrate is beneficial. The growth of graphene adhered to a flexible Al₂O₃/polyimide substrate through interfacial adhesive bonding was achieved using a plasma-enhanced CVD setup with 1,2,3,4- tetraphenylnaphthalene as carbon feedstock with embedded Cu ions which catalyzes the graphene growth. The resulting graphene has good bending stability and could be used as a wearable temperature sensor. It can also be applied to FETs owing to its compatibility with organic semiconductors [47].

Figure 5. Schematic diagram of graphene growth on Cu foil and the subsequent PMMA assisted wet chemical transfer of graphene onto a SiO₂/Si substrate. Reproduced with permission from [46], Copyright 2019, American Chemical Society.

Graphene synthesis from gaseous carbon feedstock is generally achieved with high growth temperatures, typically 1,000 °C. The most often employed gaseous precursors are methane, acetylene, and ethylene [48]. A schematic representation of the CVD growth of graphene from gaseous precursors is shown in Fig. 6(a) [46]. When the LTCVD growth of graphene using these precursors is carried out, the high energy requirement for C-H bond cleavage must be supplied by other sources such as plasma. These energy sources can produce active carbon species without raising the growth temperature. Plasma enhanced CVD (PECVD) has been effectively utilized for the growth of graphene on various substrates, namely Glass and FTO, Ti coated polymers, VO₂, NiO, Ag, Cu, carbon fibers, and SiO₂, and has resulted in the monolayer to multilayer morphologies with a variety of orientations such as vertical nanosheets. The PECVD obtained graphene has been studied for applications such as flexible thin-film capacitors, Li-S batteries, flexible electronics, high-performance supercapacitors, etc. [49–73]. The various gaseous precursors, substrates, growth temperature, etc. are summarized in Table IV.

Table 4 . Summary of gaseous precursors in low-temperature CVD graphene growth..

No.	Synthesis method	Growth temp (°C)	Carbon source	Substrate/Catalyst	Graphene type	Ref.
1	PECVD	400-550	Methane	Glass and FTO	vertically oriented (3D) graphene nanostructures	[49]
2	PECVD	500	Acetylene	SiO2/Si wafers, Ni/Cu catalysts	Few to monolayer	[61]
3	ICP-CVD	450	Acetylene	Ni/SiO2	Multilayer	[62]
4	CVD	150, 400	Methane	Eagle glass, polyethylene terephthalate, and Ti catalyst	Micrometer scale monolayer	[54]
5	Plasma assisted thermal CVD	100	Methane	Ti buffered PET and polydimethylsiloxane (PDMS) substrate	Monolayer	[56]
6	Microwave plasma CVD	300	Methane and CO2	Quartz, glass	Large area, few-layer	[59]
7	PECVD	500	Methane	VO2	Vertically erected nanowalls	[57]
8	PECVD	350	Methane, acetylene	NiO nanosheets	Vertical graphene nanosheets	[55]
9	Microwave plasma CVD	125	Methane and CO2	N-type Si	Large area graphene	[70]
10	Forced convection CVD	<400	Methane	Cu foil	Monolayer	[58]
11	Very high frequency- Hot wire cell PECVD	275	Methane	Ag thin film on a corning glass substrate	Flakes	[71]
12	ICP-CVD	300	Methane	SiO2/Si Cu film catalyst	Bilayer	[72]
13	CVD	450	Acetylene	SiO2/Si Ni/Au catalyst	Few layer	[63]
14	CVD	350-500	Acetylene	Porous Si	Graphene carbon nanocomposites	[65]
15	Microwave assisted CVD	<300	Methane	Si wafer	Vertically aligned graphene	[50]
16	Cold wall CVD	450	Acetylene	Ni-Au catalysts on SiO2/Si wafers	Few layer curved graphene films	[64]
17	Hot wire CVD	450	Methane	W nanoparticles coated c-Si and quartz	Large area graphene nanoplatelets	[60]
18	PECVD	450	Ethylene	Ni catalyst on Al/Si	Vertical graphene nanosheets	[67]
19	PECVD	400	Methane	Carbon fibers	Vertical graphene	[51]
20	PECVD	50	Methane	SiO2	N-graphene nanowalls	[73]
21	cold-wall ICPCVD	450	Acetylene	Ni/Au catalyst on SiO2/Si	monolayer	[66]
22	PECVD	100	Methane	Electroplated Cu thin film on Cu/Ti/Silicon nitride/polyimide	Bilayer	[53]
23	PECVD	500	Acetylene	Ni alloy catalyst with 1wt% Au and 1wt% Cu on SiO2/Si wafers	Monolayer	[69]
24	PECVD	160	Methane	Cu ink deposited on PI	Monolayer, bilayer	[52]
25	CVD	400-550	Ethylene	Ni2C/Ni(100)	Monolayer	[68]

Figure 6. (a) Schematic of the CVD growth of monolayer graphene on Cu foil. Reproduced with permission from [46], Copyright 2019, American Chemical Society. (b) Schematic for Graphene-NiO nanosheet formation. Reproduced with permission from [55], Copyright 2018, Wiley-VCH. (c) Raman spectra and (d) TEM image (cross-sectional) of the graphene grown on Cu foil. Reproduced with permission from [58], Copyright 2019, American Chemical Society. (e) AFM image of Graphene on SiO₂. Reproduced with permission from [59], Copyright 2019, American Chemical Society.

In 2019, Bayram demonstrated the synthesis of 3D graphene on FTO using PECVD at 480 °C, and the vertical application of electricity resulted in an upward oriented graphene growth with an optical transmittance of 95 % [49]. The growth of vertically aligned and electrochemically active graphene at temperatures as low as 300 °C has been reported on silicon substrates using plasma-enhanced CVD [50], while another report demonstrated the growth of vertical graphene on carbon fibers to improve the mechanical interaction between the fibers and polymer by controlling the height of the vertical graphene [51].

In 2021, Lu et al. [52] reported the PECVD growth of graphene using Cu ink catalyst on flexible substrates, which resulted in graphene forming a protective layer over Cu to prevent oxidation; this finding could be applied in inkjet printing applications. In the same year, another article published by the same author reported the synthesis of graphene on electroplated Cu over polyimide substrates using PECVD without any active heating. This composite material can be employed in hybrid flexible electronic devices [53].

The use of Ti as a catalyst for graphene growth in flexible polymers, such as PET, was reported in 2018, and the process progressed successfully at a low temperature of around 150 °C and the growth on flexible substrates is beneficial as the transfer process is not required in such situations and the hence obtained monolayer graphene was found to defect-free [54]. The synthesis and supercapacitor application of NiO-contained graphene were explored by Lin and coworkers in 2018 [shown in Fig. 6(b)], and they were able to achieve a high gravimetric capacitance of 1073 C/g using the PECVD-synthesized composite [55].

Han et al. [56] synthesized large-area graphene with a high hole mobility of 21,000 cm²V⁻¹s⁻¹ via plasma-enhanced thermal CVD on a Ti-buffered substrate. Graphene growth on V₂O₃ for application as a host in Li-S battery cathode was achieved at a temperature of 500 °C by Song et al. [57]. In 2019, Kim et al. [58] reported the LTCVD growth of graphene using a blowing plasma source which efficiently directs the reactive radicals for graphene growth on Cu foil [as shown in Figs. 6(c) and 6(d)].

Vishwakarma et al. [59] reported a catalyst-free CVD growth of graphene on insulators such as glass with the aid of CO₂ as an additional precursor to produce high-quality graphene with 80 % optical transmittance [shown in Fig. 6(e)]. The growth of graphene nanoplatelets was demonstrated by Anuar et al. [60] using W coated Si as the substrate, yielding mono-to few-layered graphene. Various PECVD parameters, including the plasma density, the ratio of methane to H₂, growth time, and distance between the substrate and plasma source, etc. have been well explored to study their impact on the graphene quality [60].

In 2020, Zietz et al. [61] reported a novel strategy for graphene formation on Ni/Cu thin films by manipulating the motion of the active carbon species via the application of bias. Shrestha et al. [62] reported the growth of graphene using Ni catalysts via an inductively coupled CVD technique at a growth temperature of 450 °C. Low-temperature growth of graphene on catalysts such as Ni/Au films and alloys has been reported, and the latter results in curved graphene growth at 450 °C [63,64]. In 2019, Tynyshtykbayev et al. [65] established the synthesis of graphene on the surface of porous silicon to form nanocomposites. However, the application of higher temperatures results in the formation of carbon nanotubes with SiC inclusions [65]. The design of the Au-Ni catalyst by co-deposition was used to synthesize thin graphene films using inductively coupled plasma CVD at a growth temperature of 450 °C [66].

Hussain et al. [67] reported the use of capacitively coupled plasmadriven CVD for the deposition of graphene on Si/Al substrates with Ni catalysts. The relationship between the synthesis temperature and graphene quality was assessed, and an increase in the temperature decreased the defects [67]. In 2020, Zou et al. [68] explored the possibility of using strain to fine-tune the properties of graphene films formed on Ni (100) facets.

The LT-CVD process is advantageous for synthesizing graphene for various applications because low-temperature conditions prevent damage to the underlying substrate. In addition, elevated temperatures demand the use of expensive and sophisticated instruments which are not required for LT-CVD, thereby reducing the manufacturing costs for the fabrication of large-area graphene-based devices [11]. The direct growth of graphene on flexible substrates (such as PET) is a hot topic currently, as it simplifies the device fabrication protocol and makes it more viable for large-scale applications by eliminating the transfer process generally required when graphene is grown on a metal substrate [27].

3.1. Electronic and optoelectronic applications

The ultra-high charge carrier mobilities (approximately 200,000 cm²V⁻¹s⁻¹), excellent mechanical properties, and high optical transmittance (about 97.7 %) of graphene have inspired extensive studies for nanoelectronic applications, such as in FETs and optoelectronic devices [74–76].

Park et al. [54] employed monolayer graphene grown on Ti as the top electrode in flexible and transparent capacitor applications. They observed dielectric constants of ~47 at 100 kHz, with a low dissipation factor [54]. In 2021, Han et al. [56] reported the applicability of graphene developed on in situ grown Ti layer on a flexible substrate for direct integration in electronic devices. FETs fabricated from the grown graphene showed hole mobility of ~21,000 cm²V⁻¹s⁻¹ and low sheet resistance of 6 Ω.sq⁻¹ [56]. In 2021, Son et al. [35] demonstrated the application of CVD-synthesized Cu-graphene heterostructures as interconnects with low resistivity, high breakdown current density, and enhanced electrical properties. N-doped graphene grown using pyridine precursor exhibited electron mobility of 1,400 cm²V⁻¹s⁻¹ and sheet resistance of ~450 Ω/sq [31]. The direct growth of largearea graphene on insulating glass substrates at low temperatures with a sheet carrier concentration of 5.09 × 10¹³ cm⁻², resistance of 1,259 Ω/sq and carrier mobility of 97.5 cm²V⁻¹s⁻¹ was reported by Vishwakarma et al. [59] and such a strategy could be beneficial in the field of optoelectronics.

Sakaguchi et al. [37] used GNRs obtained by the LT-CVD of 4″,5‴-dibromo-1,1′:4′,1″:2″,1″:2‴,1⁗:4⁗,1′′′′′-sexiphenyl on Au(111) substrate to fabricate FETs and demonstrated hole mobility of 0.26 cm²V⁻¹s⁻¹ and an on/off ratio of around 88. Shekhirev et al. [39] fabricated devices using 5-AGNRs and exhibited hole mobility of 4.3 × 10⁻⁴ cm²V⁻¹s⁻¹ and electron mobility of 2.1 × 10⁻⁴ cm²V⁻¹s⁻¹ with ambipolar behavior. Nitrogen-doped graphene quantum dots synthesized via CVD by Kumar et al. [45] exhibited photoluminescence at 448 nm, making them suitable for optoelectronic applications. Chen et al. [38] reported the applicability of 9-AGNR in electronic and optoelectronic devices owing to an intrinsic charge carrier mobility of ~350 cm²V⁻¹s⁻¹ and an optical band gap of ~1.0 eV.

3.2. Other applications

In 2019, Bayram et al. [49] reported the synthesis of vertically grown graphene nanowalls with an optical transmittance of 95 % on glass and FTO substrates and showed that it has potential applications in energy devices, such as supercapacitors and solar cells. In 2018, Song et al. [57] demonstrated the growth of graphene on a V₂O₃ substrate for use as a hybrid host in Li-S batteries to aid in the trapping and release of lithium polysulfides. Using the derived cathode, they achieved high-rate performance and low-capacity decay [57].

Graphene-enhanced Raman scattering (GERS) refers to the SERS exhibited when graphene is used as the substrate for Raman spectroscopy [77]. Bilayer graphene synthesized by Pekdemir et al. [72] could exhibit the GERS effect when used as a substrate for molecules such as Rhodamine 6G as a result of the charge transfer induced chemical enhancement. In 2017, Zhu et al. [43] demonstrated the synthesis of a Cu/graphene double nanocap array via CVD, and this material was observed to showcase the GERS effect on representative molecules like Rhodamine 6G, with very low noise.

In 2017, Fitri et al. [32] used the CVD technique to develop a composite material of TiO₂ and graphene for application as a photocatalyst for the photodegradation of pollutants, such as methylene blue and estradiol. In addition, the composite had antifouling properties because graphene acted as a protective layer over the TiO₂ surface [32].

Sha et al. [51] grew graphene on carbon fiber to improve mechanical properties such as wettability, roughness, and the interfacial shear strength of carbon fiber reinforced composites. Graphene nanoribbon networks fabricated by Kojima et al. [42] exhibited very low crossplane thermal conductivity (0.11 Wm⁻¹ K⁻¹) while having a crossplane electrical conductivity of 188 S m⁻¹, making it a competent material for thermoelectric studies.

CVD graphene synthesis results in the large-area synthesis of highquality graphene. Having said that, generally, graphene synthesis requires high-temperature conditions, which leads to excessive cost and impedes the electronic applicability of the 2D material. LT-CVD growth of graphene is a green, safe, and cheap and scalable approach that can be used for various applications.

Various precursors can be used for the LT-CVD of graphene, solid precursors include bihalogenated polyphenylene monomers, aromatic polycarboxylic anhydrides, polymers, biopolymers, and other polyphenylene compounds. The list of liquid precursors contains aromatics such as benzene, pyridine, halogenated benzene, and lower aliphatic alcohols. CH4 is the most commonly used gaseous precursor for CVD graphene growth at low temperatures, others include acetylene and ethylene. The use of aromatic precursors lowers the growth temperature because they require less energy to overcome their binding energies due to the hexagonal framework in phenylene molecules which is similar to that of graphene. Gaseous precursors possess high binding energies and cross this energy barrier at low temperatures; energy must often be supplied in another form using plasma-enhanced or microwave-assisted, or other modified CVD strategies.

Research developments over the past five years in LT-CVD graphene growth show that there is still much to be explored in this field and that employing solid or liquid aromatic precursors has significant potential due to their lower binding energies. The development of improved precursors will revolutionize the commercialization of graphene in electronic applications such as CMOS.

This research was supported by the National Research Foundation (NRF) of Korea (NRF-2020R1A4A4079397 and NRF-2021R1A2C2012649). J. H. Lee acknowledges the Ajou University Research Fund.

The authors declare no conflicts of interest.