Applied Science and Convergence Technology 2022; 31(3): 63-70
Published online May 30, 2022
Copyright © The Korean Vacuum Society.
aDepartment of Energy Systems and Department of Materials Science and Engineering, Ajou University, Suwon 16499, Republic of Korea
bDepartment of Materials Science and Engineering, Chungnam National University, Daejeon 34134, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Since the realization of the prospect of synthesizing graphene via mechanical exfoliation, graphene has garnered considerable attention owing to its remarkable chemical, electrical, material, and various other properties, which makes it an ideal contender for electronic and optoelectronic applications. Many top-down and bottom-up strategies have been researched to fabricate graphene films, notably chemical vapor deposition (CVD), which has been proven to deliver large-area, high-quality graphene films with the additional advantage of being economical. However, graphene production utilizing gaseous predecessors, such as methane, typically requires high temperatures, which increases the production cost due to the high thermal budget. Recently, the prospect of employing low-temperature conditions for graphene growth has been explored, using various hydrocarbon precursors and alternative energy sources. This review discusses the low-temperature CVD growth of graphene and recent improvements in this area with a brief introduction to applications based on these graphene specimens.
Keywords: Graphene, Chemical vapor deposition, Low-temperature growth, Electronic, Optoelectronic applications
Graphene is a representative two-dimensional material composed of sp2 hybridized carbon atoms organized in hexagonal lattices . 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 . 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 . Recently, Moon
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 . 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 . 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 . 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) .
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 . 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 . 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.
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 . The quality and number of layers of asgrown graphene can be changed according to several parameters . 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 . 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 . 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 . 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 . 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 .
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 . 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 .
In the last step of graphene growth, the active carbon species undergo coalescence and nucleation to produce graphene film . 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)] . 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.
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 . The various liquid precursors, substrates, growth temperature, etc. are summarized in Table II.
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 . 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 .
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
Graphene nanoribbons (GNRs), strips of 2D graphene, are important in electronic, optoelectronic, and other applications due to their tunable properties . 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 . 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) . The various solid precursors, substrates, growth temperature, etc. are summarized in Table III.
In 2017, Sakaguchi
The growth of N = 9 armchair GNRs (9-AGNRs) was reported in 2017 by Chen
In 2020, Shekhirev
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 .
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 . 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 .
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) . 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 .
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 . A schematic representation of the CVD growth of graphene from gaseous precursors is shown in Fig. 6(a) . 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.
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 % . 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 , 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 .
In 2021, Lu
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 . 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 .
In 2020, Zietz
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 . 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 .
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].
In 2019, Bayram
Graphene-enhanced Raman scattering (GERS) refers to the SERS exhibited when graphene is used as the substrate for Raman spectroscopy . Bilayer graphene synthesized by Pekdemir
In 2017, Fitri
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.