Direct growth of graphene on rigid and flexible substrates: progress, applications, and challenges

Viet Phuong Pham *abc, Hyeon-Sik Jang ab, Dongmok Whang *abd and Jae-Young Choi *a
aSchool of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon, Gyeonggi-do 440-746, Republic of Korea. E-mail: dwhang@skku.edu; jy.choi@skku.edu
bSKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University (SKKU), Suwon, Gyeonggi-do 440-746, Republic of Korea
cCenter for Multidimensional Carbon Materials, Institute for Basic Science, Ulsan 44919, Republic of Korea. E-mail: pvphuong85@ibs.re.kr
dResearch Center for Time-Domain Nano-Functional Devices, Samsung Advanced Institute of Technology (SAIT), Yongin 449-712, Republic of Korea

Received 28th March 2017

First published on 31st August 2017


Graphene has recently been attracting considerable interest because of its exceptional conductivity, mechanical strength, thermal stability, etc. Graphene-based devices exhibit high potential for applications in electronics, optoelectronics, and energy harvesting. In this paper, we review various growth strategies including metal-catalyzed transfer-free growth and direct-growth of graphene on flexible and rigid insulating substrates which are “major issues” for avoiding the complicated transfer processes that cause graphene defects, residues, tears and performance degradation in graphene-based functional devices. Recent advances in practical applications based on “direct-grown graphene” are discussed. Finally, several important directions, challenges and perspectives in the commercialization of ‘direct growth of graphene’ are also discussed and addressed.


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Viet Phuong Pham

Dr Viet Phuong Pham has been a Postdoctoral Researcher at the School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon (2016–2017) and is a Research Fellow at Center of Multifunctional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan (2017–now), Republic of Korea. He received his PhD degree at SKKU Advanced Institute of Nanotechnology (SAINT), SKKU, Republic of Korea (2016). His research interest focuses on nanomaterials (CNTs, graphene, nanowires, MoS2, h-BN, and other TMDs), dispersion of CNTs–graphene flakes–AgNWs, 2D material synthesis (CNTs, graphene), graphene transfer techniques (PMMA, gold film, mechanical, electrochemical, etc.), doping strategies (wet and dry) with dopants (TFSA, AuCl3, HNO3, FeCl3, benzyl viologen (BV), F4-TCNQ, Cl2, BCl3, N2, CF4, O2, etc.) on 2D materials, plasma (doping, etching, cleaning, pulse, low-energy, free-damage) on nanomaterials, self-assembly of block copolymers (BCPs), laser-nanomaterial interaction, and applications (composites, solar cell, catalysts, HER, water splitting, wearable electronics, nanogenerator, energy storage, OLED, transistor, sensor, flexible display).

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Hyeon-Sik Jang

Hyeon-Sik Jang is a PhD student at the School of Advanced Materials Science & Engineering and SKKU Advanced Institute of Nanotechnology (SAINT), SKKU, Republic of Korea. His current research interest is the synthesis of 2D materials and their related applications.

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Dongmok Whang

Dr Dongmok Whang is a Full Professor at the School of Advanced Materials Science & Engineering and SKKU Advanced Institute of Nanotechnology (SAINT), SKKU, Republic of Korea. He received a PhD degree from the Department of Chemistry, Pohang University of Science and Technology (POSTECH), Republic of Korea (1997). Prior to joining SKKU, he was a Senior Research Fellow at Harvard University (2001–2004) and a Postdoctoral Research Associate, POSTECH (1997–2001). He has co-authored over 100 peer-reviewed publications and his current number of citations is about 10[thin space (1/6-em)]000. His research interest is on graphene, semiconductor nanowires, porous nanostructures, hybrid nanostructures, and energy harvesting for industrial applications.

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Jae-Young Choi

Dr Jae-Young Choi is an Associate Professor, School of Advanced Materials Science and Engineering, SKKU located at Suwon in Republic of Korea. Prior to joining SKKU, he was a Vice President of Samsung Advanced Institute of Technology (SAIT), Samsung Electronics, and a Director of Graphene Center of SAIT. He earned his PhD in Materials Science and Engineering from Korea Advanced Institute of Science and Technology (KAIST) in 1998, and he was a Research Fellow in 1998–1999 at Ames Lab, Iowa, US. He has co-authored over 100 peer-reviewed publications and over 400 patents related to nanomaterials and their device applications, and his current number of citations is over 15[thin space (1/6-em)]000. Particularly for a number of patents related to graphene, he was ranked as the 1st in the world as reported in Nature Materials 2012. His current research interests are innovative synthesis and device applications of 2D materials and new nanostructured materials.


1. Introduction

Single-layer graphene (SLG) and few-layer graphene (FLG) films have been regarded as ideal materials for electronics and optoelectronics due to their excellent electrical properties and their ability to integrate with current top-down device fabrication technology.1–30 Since the beginning of the 21st century, the interest in graphene materials has drastically increased, which is apparent in the number of annual publications on graphene (Fig. 1). To date, various strategies, including chemical vapour deposition (CVD),31 liquid and mechanical exfoliation from graphite,26,32,33 epitaxial growth on a crystal substrate,34–37 or solution-based processes on graphene oxides38–44 have been investigated for obtaining graphene layers. In particular, recent advances in CVD growth have successfully led to large-scale graphene production on metal substrates,1,31,45–51 driven by the high demand for utilizing graphene in possible applications of current complementary metal-oxide-semiconductor (CMOS) technology such as radio-frequency transistors, optical devices, and deposition processes.2
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Fig. 1 Publications on Graphene from 2000 to 2016. Source: ISI Web of Science (search: Topic = Graphene).

Nowadays, high-quality large-area graphene can be well synthesized on conducting metal substrates by using the catalytic CVD growth approach, which has promoted a wide range of graphene-based device applications.1,31,45–51 However, graphene grown on a metal substrate needs to be transferred onto dielectric substrates for electronic applications. Although various approaches, such as wet etching/transfer,48 mechanical exfoliation/transfer,26,32,33 bubbling transfer,53 and electrochemical delamination,54–56 for transferring graphene from the catalytic metal growth substrates to dielectric device substrates have been developed, none of these approaches are free from degradation of the transferred graphene. For example, ‘wet etching and transfer’, the most widely used tranfer approach, is a serial process, which includes encapsulation of the graphene surface with a polymer supporting layer, subsequent chemical etching of the underlying metal substrate, transfer of the polymer/graphene film onto a dielectric substrate and removal of the polymer supporting layer.52 Unfortunately, the transfer process is not just inconvenient but also causes various chemical and mechanical defects in the transferred graphene layer. CVD graphene grown on arbitrary substrates contains various defects, such as point defects, dislocation-like defects, cracks, wrinkles, and grain boundaries.57 Because carbon atoms at such defect sites are chemically less stable than the carbon in defect-free graphene,57 the defect sites exposed to unavoidable surface/interface contaminants during the graphene transfer process are chemically damaged by bonding to oxygen, hydrogen, etc. Similarly, CVD graphene can be contaminated by metallic impurities from the growth substrate, which influences the electrochemical and electronic properties of graphene.58,59 CVD graphene never has 100% coverage and there are defects and holes which can be determined electrochemically.60 In addition, transfer of ultrathin graphene layers onto target substrates leaves unavoidable mechanical defects in the transferred graphene, such as cracks, tears and wrinkles. Thus, it leads to high deterioration in the performance of the resulting graphene-based devices, such as inducing a gradual reduction in the electrical conductivity of the devices or reducing the stability or increasing the leakage current of the devices.

Recently, significant efforts have been made to obtain graphene on semiconductor and dielectric substrates to avoid the problematic wet-transfer process. For example, graphene was directly grown on a quartz substrate using a thin layer of Cu (100–450 nm thick) on the substrate as a catalytic layer.61 After growth on the Cu layer, the graphene layer could be transferred directly onto the underlying dielectric surface through de-wetting and evaporation of the Cu layer. Furthermore, the graphene on Cu was patterned for facile transfer to the underlying dielectric substrate after etching of the metal layer underneath the graphene.62 However, the above transfer methods may only be suitable for small-size lateral graphene. Recently, Byun et al. have obtained FLG directly on SiO2 using organic-polymer-coated insulators and thermal encapsulation of the Ni layer.63 Lee et al. have observed the formation of FLG at the interfaces of Ni and SiO2 using plasma-enhanced CVD (PECVD); however, the interface FLG was defective and thick.64 The graphene formation on Ni is due to the dissociation and precipitation processes of the carbon species in Ni.64 Consequently, the carbon precipitation is a non-equilibrium process and might be a major challenge for obtaining homogeneous graphene based on an Ni catalyst.46,65 The drawback of the metal catalyzed-CVD process is that defects and tears are unavoidable during the transfer process.66 To avoid these drawbacks, two growth approaches have been suggested for the direct formation of graphene on flexible and rigid insulating substrates: (1) metal-catalyzed direct growth without transfer to external substrates,61 and (2) direct growth of graphene on a dielectric substrate without a metal catalyst.67–69 A recent report demonstrated graphene synthesized by a metal-free CVD process on sapphire for forming large-scale highly crystalline SLG; however, it still has high wrinkles or ripples,70 similar to the situation in the case of graphene pads/exfoliated h-BN.71 Consequently, metal-free CVD growth is not yet applicable to amorphous flexible and rigid substrates. A promising approach for increasing the graphene quality and minimizing the amount of metal catalyst is by evaporation and reaction with carbon-based gas precursors on the substrate surface. The formation of SLG and FLG on amorphous oxide substrates has been described in a previous report.72 However, there are still many drawbacks that need to be addressed, particularly the amount of defects and non-uniform graphene due to imperfect nucleation and catalytic reactions. To the best of our understanding, there is no uniform high-quality SLG directly grown on dielectric substrates.

Considering that the mechanical transfer of graphene to the device substrates inevitably causes serious degradation in the performance of the resulting graphene devices, direct growth of graphene in a simple way on flexible organic (e.g. PI, PDMS, and Willow glass) or rigid inorganic (e.g. glass, AlN, GaN, sapphire, quartz, mica, Si, textured Si, SiO2, SiC, fused silica, MgO, h-BN, MnO2, TiO2, and HfO2) insulating substrates is highly desirable. The direct growth approach for device applications enables the avoidance of complex transfer processes and transfer-induced defects. Moreover, it enables conformal growth of graphene on three-dimensional (3D) surfaces, which is necessary for various functional devices, such as sensors,73–75 black silicon solar cells,76 cambered micro-optics,77 3D microelectromechanical system (MEMS),78 or CMOS technology-based applications.2 Here, we present an overview of various recently reported strategies for direct graphene growth on rigid and flexible substrates. In addition, a wide-range of applications as well as the perspectives and challenges are also addressed.

2. The general growth mechanism of CVD-based graphene

CVD growth of graphene is a chemical process for the formation of SLG or FLG on an arbitrary substrate by exposing the substrate to the gas-phase precursors under controlled reaction conditions.79 Owing to the versatile nature of CVD, intricately mixed homogeneous gas-phase and heterogeneous surface reactions are involved.80 In general, as the partial pressure and/or temperature in the reaction substances are increased, homogeneous gas-phase reactions and the resulting homogeneous nucleation become significant.80 To grow a high-quality graphene layer, this homogeneous nucleation needs be minimized.80 A general mechanism for CVD-based graphene growth on catalytic metal substrates, for the growth of a uniform and highly crystalline graphene layer on the surface, includes eight steps as follows: (1) mass transport of the reactant, (2) reaction of the film precursor, (3) diffusion of gas molecules, (4) adsorption of the precursor, (5) diffusion of the precursor into the substrate, (6) surface reaction, (7) desorption of the product, and (8) removal of the by-product (Fig. 2).81
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Fig. 2 Diagram of the general growth mechanism of CVD-based graphene: transport and reaction processes. Reproduced with permission.81 Copyright 2011, Freund Publishing.

Typically, CVD growth of 2D materials (e.g. graphene) involves catalytic activation of chemical reactions of precursors at the growth substrate surface/interface in a suitably designed environment. Generally speaking, the roles of precursors, conditions (e.g. fast growth rates, large domain size, or very high crystalline quality), atmosphere, substrates and catalysts are the key factors affecting the final quality of the grown 2D materials. So far, significant efforts have been made to prepare highly crystalline 2D materials (e.g. graphene), but many challenges are still ahead. For example, due to the rough feature of the catalytic metal surface, growth of uniform and high quality graphene is considerably difficult. The 2D material research community is also interested in new precursors (e.g. solid precursor only, gas precursor or solid precursor mixed with certain solvents) that could induce the formation of high-quality uniform graphene with minimal defect density. Another question is the effect of growth rate on the catalytic metal surface on the quality of graphene. Currently, it is difficult to give an exact answer, as investigations are progressing at an exponential rate.

However, non-catalytic direct-growth of graphene on semiconducting and dielectric substrates follows different mechanisms according to our best insights. To date, the understanding of the concept of the general mechanism of the direct growth of graphene is still not yet adequate, either experimentally or theoretically, with many proposed possible growth mechanisms, e.g. vapour–solid–solid,82 or vapour–solid83 or solid–liquid–solid.84 There have been arguments on the direct-growth mechanism of graphene domains on dielectric substrates or non-catalytic substrates in previous reports,69,70,85 but the mechanism for the entire process of the carbon precursor transformation to the crystalline graphene structures has not yet been fully understood. Thus, understanding the graphene growth mechanism and the effect of various growth conditions will be of significant interest to the 2D material research community to obtain large-scale, high-quality graphene.

3. Transfer-free direct growth of graphene layers on dielectric substrates

To avoid the problems arising in the graphene transferring process, two growth approaches have been suggested for the direct formation of graphene on flexible and rigid insulating substrates without additional transfer processes: (1) catalytic growth with the help of an external metal catalyst,61 and (2) non-catalytic direct growth of graphene on a dielectric substrate without a metal catalyst.67–69

The direct growth of graphene is a process on flexible substrates (PI, PDMS and Willow glass) and rigid substrates (glass, AlN, GaN, sapphire, quartz, mica, Si, textured Si, SiO2, SiC, fused silica, MgO, h-BN, MnO2, TiO2, and HfO2) without transfer processes7,62–73,84,86–110 compared with conventional indirect growth processes on metal substrates (Cu, Ni, Ge, etc.) which need additional transfer processes onto arbitrary substrates.1–6,8–61 Using this method, we can avoid the complicated transfer process, which induces the defects, residues, and tears that degrade the performance of graphene-based devices. Various approaches for direct growth of graphene were classified into three major types: (i) catalyst-free and polymer-free, (ii) based on both catalyst and polymer, and (iii) based on a metal catalyst (Fig. 3).


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Fig. 3 General methods for direct-growth of graphene onto a target insulating substrate.

3.1. Catalytic direct growth of graphene

3.1.1. Metal-catalyzed graphene growth without additional mechanical transfer. In this part, direct growth of graphene based on the catalyst is considered to be a direct growth process in a CVD chamber with the support of metal catalysts, apart from the common carbon-containing gas sources (CH4, C2H2, C8H8, C3H3N, C4H6 and C6H14) and additional gases (O2, H2, GeH4 and Ar) as a simplification in the classification with respect to other cases. Direct growth of graphene assisted by metal catalysts (Cu, Ni, W and SiH4) on quartz, sapphire, Si, SiO2, fused silica and h-BN/SiO2 has been thoroughly investigated both experimentally and using simulations.1,61,62,64,72,84,89–91,93,96,98–100,102,108

In 2009, the first pioneering study by Levendorf et al. introduced metal-catalyzed direct growth on a SiO2 substrate. As the initial step, graphene was grown on a Cu/Ni layer deposited on the SiO2 substrate using a CVD process, followed by patterning of the grown graphene and subsequent chemical etching of the Cu/Ni layer to obtain a patterned graphene layer on the SiO2 substrate (Fig. 4).62 However, the carrier mobility of the FET device obtained by Levendorf et al. was not high enough (700 cm2 V−1 s−1).62 In another report, the direct growth of graphene on quartz, sapphire, SiO2 and fused silica substrates using a catalytic Cu layer deposited on the substrates was carried out by Ismach et al. (Fig. 5a–d).61 The further controlling of evaporation and the de-wetting process may lead to directly patterned graphene and other 2D materials on dielectric substrates for large-scale electronics. In a recent report by Marchena et al. in 2016, using a low growth temperature (670 °C), graphene was directly grown on a flexible glass substrate (Corning Willow glass) and on SiO2, using a CVD approach assisted by a catalytic Ni layer on the substrates (Fig. 6).108 The Ni layer between the graphene and the underlying substrate was removed by chemical etching similar to the previous report. The low temperature and short growth process used in this work are essential for achieving the direct growth of graphene on flexible substrates, for potential applications in transparent flexible electronics without the expensive transfer process.


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Fig. 4 Cu/Ni-catalyzed direct-growth of graphene on SiO2 for an FET device. Reproduced with permission from ref. 62, copyright 2009, American Chemical Society.

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Fig. 5 (a–d) Diagram of Cu-catalyzed direct-growth of graphene on a quartz substrate. (a–d) Reproduced with permission from ref. 61, copyright 2010, American Chemical Society.

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Fig. 6 Diagram of Ni-catalyzed CVD direct-grown graphene on (a) SiO2 and (b) flexible Willow glass. (a and b) Reproduced with permission from ref. 108, copyright 2016, OSA Publishing.

The advantage of this metal-catalyzed direct graphene growth on dielectric substrates is that the catalytic metal layer enables direct growth of a high-quality graphene layer on a dielectric substrate without transfer to external substrates and metal catalysts. However, this approach cannot completely eliminate the chemical residues and mechanical defects induced during the chemical etching of the catalytic metal layer.

3.1.2. Catalytic growth of graphene at the interface between a catalytic metal layer and an underlying growth substrate. As an efficient approach for direct graphene growth on dielectric substrates, catalytic conversion of organic layers (self-assembly layers or carbon-contained organic thin layers) to graphene layers at the interface between the metal catalyst film and dielectric inorganic substrates has been studied.7,63,101,109 This method not only allows direct growth of a graphene layer with good crystallinity on the dielectric substrate due to effective metal-to-catalytic conversion, but is also free from transfer-related defects, unlike the metal-catalyzed direct graphene growth described in the previous section.

Shin et al. presented a simple synthesis approach for transfer-free FLG on an SiO2 substrate, through pyrolysis of an aliphatic-self-assembled monolayer (aliphatic-SAM) polymer of carbon materials and Ni metal as a catalyst, as shown in Fig. 7.7 Among the various carbon sources, SAM is very effective and promising as the uniform carbon layer can be introduced with a controlled thickness. The structure of the Ni/SAM/substrate provided advantages for preventing the evaporation of the SAM during high-temperature heating and for removing the top-most catalyst layer after synthesis, to form high-quality graphene without transfer-related defects. In 2011, Byun et al. used Ni and Cu metals for catalytic conversion of polymer films (polystyrene, polyacrylonitrile (PAN) and polymethylmethacrylate (PMMA)) for investigating the direct growth of graphene on an SiO2 substrate, as shown in Fig. 8a.63 These polymers are generally inexpensive, safe to handle and suitable for a scalable growth process. Byun et al. obtained FLG via catalytic pyrolysis (1000 °C in 1 min) using a capping Ni film (50 nm thick) as a catalyst.63 More recently, using Cu-assisted catalytic conversion of a phenyl-SAM polymer (PhSi(OMe)3), Yang et al. successfully synthesized FLG on the desired substrates (SiO2, quartz, GaN and textured Si) (Fig. 7b–g), which showed an 84.3% optical transmittance and a sheet resistance of 3.5 kΩ sq−1 on a quartz substrate. In addition, the directly grown graphene showed good chemical sensitivity with NO2 and NH3 gases.101


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Fig. 7 (a) Diagram of Ni and SAM-catalyzed transfer-free growth of graphene on SiO2. (b) AFM image of graphene referred from octyl-SAM on SiO2. (c) TEM image of BLG after octyl-SAM pyrolysis. (d) AFM image of graphene/SiO2 after etching of Ni. (a–d) Reproduced with permission from ref. 7, copyright 2011, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Fig. 8 (a) Transfer-free graphene growth process based on SAM polymers (PS, PAN, PMMA) onto a SiO2 substrate. (a) Reproduced with permission from ref. 63, copyright 2011, American Chemical Society. (b–g) Diagram of the transfer-free graphene growth: (b) target substrate, (c) phenyl-SAM coating on an arbitrary substrate, (d) evaporation of a Cu thin film on the SAM-coated substrate, (e) graphene growth between the Cu layer and the target substrate by thermal annealing, (f) selective Cu etching, and (g) the graphene-based device after contact metal deposition. (b–g) Reproduced with permission from ref. 101, copyright 2016, American Chemical Society.

Direct growth of FLG using catalytic conversion of an amorphous carbon layer on a sapphire substrate with Ni and tungsten (W) as catalysts was carried out by Yamada et al. (Fig. 9a).89 They investigated the graphene layer precipitated between upper W or Ni layers and an underlying sapphire substrate by Raman and differential interference micrographs.89 In another report, in 2011, Su et al. used CVD for directly forming a wafer-scale graphene layer on quartz and SiO2 based on a Cu catalyst, as the first large-scale synthesis report (Fig. 9b).90 Large-scale graphene on dielectric substrates was appropriately formed by the optimization of CVD growth parameters with a moderate mobility (672 cm2 V−1 s−1) for FET performance. The %T and Rs obtained in this study were 94% and 2 kΩ sq−1, respectively.


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Fig. 9 (a) Diagram of W and Ni-catalyzed direct-growth of graphene on a sapphire substrate. (a) Reproduced with permission from ref. 73, copyright 2016, IOP Publishing. (b) Raman spectra of the as-grown top layer graphene on Cu and bottom layer graphene at the other side of Cu. (b) Reproduced with permission from ref. 90, copyright 2011, American Chemical Society.

A pioneering study for high-quality large-scale direct-growth of bilayer graphene (BLG) on SiO2 using a Ni catalyst and various carbon sources (PMMA, C8H8, C3H3N, C4H6) has been demonstrated by Peng et al. (Fig. 10).91 At 1000 °C, the carbon sources on the top of Ni were decomposed and diffused into the Ni layer. When cooled to room temperature, BLG was suitably formed between the Ni layer and SiO2. The Ni films were etched, and then BLG was obtained directly on SiO2, without a transfer process. The evidence for the formation of BLG on SiO2 substrates was observed by Raman and TEM images.91


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Fig. 10 (a) Diagram of the Ni-catalyzed direct-growth of CVD BLG on a SiO2 substrate. (a) Reproduced with permission from ref. 91, copyright 2011, American Chemical Society.

In 2011, Liu et al. chose h-BN as a dielectric substrate for the direct synthesis of highly oriented pyrolytic graphite, and mechanically exfoliated graphene using a two-step CVD approach (Fig. 11a–c).93 h-BN seems to be an ideal dielectric substrate because of its ultra-smooth and ultra-flat surface compared to the SiO2 wafer surface; according to AFM images of graphene/h-BN and graphene/SiO2, the values of roughness were 0.21 and 0.94 nm, respectively.112 Liu et al. used Cu foil, hexane solution (C6H14), and ammonia borane (NH3-BH3) as precursor catalysts for a controllable large-scale synthesis.93 This approach can set the course in exploration of nanoelectronic applications based on heterostructural graphene/h-BN.


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Fig. 11 (a–c) Diagram of Cu, hexane, and ammonia borane-catalyzed direct-growth of CVD graphene on an h-BN substrate located on SiO2. (a–c) Reproduced with permission from ref. 93, copyright 2011, American Chemical Society. Diagram of Ni-catalyzed direct-growth of CVD graphene on a SiO2 substrate at 460 °C < T < 600 °C (d) and T < 260 °C (e). (d and e) Reproduced with permission from ref. 96, copyright 2012, Nature Publishing Group.

Due to the disadvantage of high cost and limited substrates in CVD graphene growth at the very high temperature zone, graphene synthesis was attempted at the low temperature zone both by direct growth and indirect growth. However, this attempt faces many significant challenges due to the imperfect growth nature of graphene formation at very low temperature. Recently, Ni-catalyzed direct growth on SiO2 at near room-temperature (25–160 °C) has been carried out by Kwak et al., as shown in Fig. 11d.96 However, the quality of the resulting graphene was inferior to that of the metal-catalyzed graphene grown at high temperature. More recently, in 2017, Vishwakarma et al. reported transfer-free graphene grown directly on SiO2 at very low temperature (250 °C) by a CVD approach using tin (Sn) as a metal catalyst and amorphous carbon (a-C) as a carbon source in SLS reactions (Fig. 12).84 However, low quality multi-layer graphene with a high number of defects and contaminants was obtained due to the low growth temperature (250 °C).


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Fig. 12 Schematic of multilayer graphene direct-growth on SiO2 based on an Sn catalyst by the CVD approach at 250 °C via annealing and chemical etching processes. Reproduced with permission from ref. 84, copyright 2017, Nature Publishing Group.

In 2014, based on a Ni-catalyzed solid phase reaction of a polyvinyl alcohol (PVA) polymer layer, graphene was grown directly at an interface between a catalytic Ni layer and an underlying growth substrate (SiO2, sapphire), as shown in Fig. 13.109 As a result, a high surface coverage of graphene layer was achieved at an optimized thickness of the Ni (>200 nm) and PVA polymer layer (9.2 nm). This report also presented additional monolayer graphene growth obtained in a H2 atmosphere only, at a lower temperature (850 °C) than that of an Ar atmosphere, using H2 diffusion in the catalyst/substrate interface to activate carbon atoms for graphene growth.109


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Fig. 13 (a) Graphene directly grown on SiO2 using a Ni catalyst and spin coated PVA layer via solid phase reaction. (b) Growth and cooling temperature sequences for graphene synthesis. (a and b) Reproduced with permission from ref. 109, copyright 2014, The Royal Society of Chemistry.
3.1.3. Vapour-catalytic growth of graphene. In principle, direct growth of graphene on dielectric surfaces by using a gas-phase catalyst is a promising approach because of the possibility of low-temperature catalytic growth of low-defect graphene without additional etching of the catalyst. Unlike metal-catalyzed CVD growth and SiC epitaxial growth, this strategy is more compatible with Si-based techniques in electronic device fabrication, such as TCFs or FET.

In addition, in 2011, Chen et al. reported an interesting strategy using O2 gas as an additional catalyst for CVD direct growth of polycrystalline graphene on SiO2 and quartz substrates (Fig. 14).69 However, Chen et al. obtained low carrier mobility values for FET devices (531 cm2 V−1 s−1 in air and 472 cm2 V−1 s−1 in N2).69 Teng et al. described the direct growth of graphene layers on SiO2 using thermally evaporated Cu atoms as catalysts (Fig. 15).72 The crystallinity of the obtained graphene was comparable to that of graphene grown on a Cu catalyst substrate, though the carrier mobility of the obtained graphene (100–600 cm2 V−1 s−1) was low.72 In a more recent report, Lee et al. used germane gas (GeH4) as a gas phase catalyst in direct graphene growth on SiO2 (Fig. 16).107 Although the achieved electron density in this study was high (3 × 1012 cm−2), the carrier mobility of the fabricated-FET device was still very low (160 cm2 V−1 s−1) compared with the performance of the conventional indirect growth method on metal foils. Other exciting reports by Kim et al.,98 Li et al.,99 and Tang et al.100 describe the assembly of Cu foil and SiH4 catalysts inside the CVD chamber as assistive materials for the direct growth of graphene on h-BN and h-BN/SiO2 insulating surfaces (Fig. 17). The best results were obtained by Tang et al. with a mobility of ∼20[thin space (1/6-em)]000 cm2 V−1 s−1, and a graphene domain size of ∼20 μm at a growth rate of ∼1 μm min−1.100


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Fig. 14 Diagram of oxygen-catalyzed CVD direct-growth of graphene on SiO2 and quartz substrates. Reproduced with permission from ref. 69, copyright 2011, American Chemical Society.

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Fig. 15 Diagram of Cu-catalyzed direct-growth of graphene films on SiO2 and quartz and their Raman data at various locations in a CVD chamber. Reproduced with permission from ref. 72, copyright 2012, American Chemical Society.

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Fig. 16 Diagram of GeH4-catalyzed CVD direct-growth of graphene on SiO2. Reproduced with permission from ref. 107, copyright 2016, AIP Publishing.

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Fig. 17 (a) Photograph and diagram of Cu-catalyzed direct-growth of CVD graphene on a SiO2 substrate. (a) Reproduced with permission from ref. 98, copyright 2013, American Chemical Society. (b) Schematic of SiH4 and Cu-catalyzed direct-growth of graphene on SiO2. (b) Reproduced with permission from ref. 99, copyright 2016, AIP Publishing. (c) Diagram of the growth process of SiH4-catalyzed growth of SLG onto an h-BN substrate located above an SiO2 wafer. (c) Reproduced with permission from ref. 100, copyright 2015, Nature Publishing Group.

3.2. Non-catalytic direct growth of graphene

3.2.1. Non-catalytic direct growth on rigid inorganic substrates. Compared with the conventional catalytic growth approach, the direct growth of graphene on a dielectric substrate without any external catalysts has various advantages, such as low process cost, and shorter experimental processes. However, the drawback of this method is that without using catalysts, the chemical reactions for the excitation of the kinetic energy of the graphene growth process is not sufficient to obtain high quality direct-grown graphene for commercialization, compared with the catalytic direct growth described in previous sections.

In 2011, Zhang et al. studied the direct growth of graphene without an external catalyst at low-temperature (500 °C) by using a remote-PECVD system on various inorganic dielectric substrates (sapphire, quartz, mica, Si, SiC, glass) (Fig. 18a).68 This method is simple, cost-effective, scalable (4-inch wafer) and compatible with various devices such as gas sensors, nanoresistors and TCFs. In another report, Song et al. demonstrated catalyst-free direct CVD growth of large-scale single-layer graphene (SLG) on sapphire at a higher temperature (950 °C) without a transfer process (Fig. 16b).70 Son et al. reported catalyst-free direct growth of graphene as a horse-pad shape on a mechanically exfoliated h-BN surface located on a SiO2 substrate in an ambient pressure CVD (APCVD) process (Fig. 19a–h).71 As a result, a direct-grown graphene pad with a diameter of up to ∼110 nm was achieved.


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Fig. 18 (a) Diagram of the PECVD system for nanographene film growth. (a) Reproduced with permission from ref. 68, copyright 2011, Springer and Tsinghua University Press. (b) Diagram of the catalyst-free direct-growth of graphene on sapphire via a CVD process. (b) Reproduced with permission from ref. 70, copyright 2012, The Royal Society of Chemistry.

image file: c7cs00224f-f19.tif
Fig. 19 (a–h) Catalyst-free direct-growth of a graphene pad on an h-BN/SiO2 substrate. (a–h) Reproduced with permission from ref. 71, copyright 2011, The Royal Society of Chemistry. (i) Diagram of catalyst-free bottom-up graphene growth on an SiO2 substrate. (i) Reproduced with permission from ref. 92, copyright 2016, The Royal Society of Chemistry.

Most recently, Min et al. reported a new technique for catalyst-free bottom-up growth of graphene nanoribbons (GNRs) and nanoporous sheets, which is based on the pyrolysis of a self-assembled monolayer of ferric stearate molecules on an SiO2 substrate (Fig. 17i).92 In this method, the GNRs containing pyrrolic N-enriched edges exhibited p-type semiconductor characteristics, whereas the nanoporous graphene sheets containing inhomogeneous pores and graphitic N-enriched basal planes exhibited n-type semiconductor characteristics. It is expected that this direct graphene growth technique on an SiO2 substrate can be applied in various graphene-based electronic applications, without the graphene transfer process. However, the drawback of this method is the high-density defects of the GNRs, which were indicated by a high D peak in their Raman spectra.

In 2016, Song et al. reported the direct growth of conformal graphene films on a 3D structured quartz surface (Fig. 20), which can be efficiently applied in pressure sensor applications.73 Here, the graphene 3D layer conformally grown on the quartz surface reveals reasonably good electrical and optical properties with Rs ∼ 2000 Ω sq−1 and %T ∼ 80% at 550 nm, which can be attached to a flat graphene film on a PDMS substrate, and then can work as a pressure-sensitive sensor.


image file: c7cs00224f-f20.tif
Fig. 20 (a) Schematic of direct-growth of 3D conformal graphene films on a quartz substrate. (b) Cross-section view of graphene/quartz. (c) Photograph of graphene/quartz with Ag electrodes. (a–c) Reproduced with permission from ref. 73, copyright 2016, American Chemical Society.

Furthermore, without catalyst support, Trung et al.,94 Xu et al.,95 and Bi et al.97 directly grew graphene on Si, SiO2, quartz, AlN and h-BN substrates. Xu et al. used a CVD approach with two-temperature zones for direct growth of uniform and continuous graphene layers, as shown in Fig. 21a.97 As a result, the directly grown graphene on a transparent quartz substrate shows a reasonably good transmittance (%T) and sheet resistance (Rs) of 97% and 650 Ω sq−1, respectively. These values, reported by Xu et al. are almost similar to the values in Bi et al., which were 94% and 425 Ω sq−1 obtained using the PECVD method (Fig. 19b–e). Certainly, CVD and PECVD will provide a highly transfer-free technique for future high-performance electronic devices. In another study, Muñoz et al. successfully reported direct-grown graphene on insulating substrates (quartz, SiO2), using remote electron cyclotron resonance plasma assisted chemical vapor deposition r-(ECR-CVD) at a low temperature (650 °C).104 Through the sophisticated control of the growth parameters (e.g. nucleation density, edge growth, thickness), Muñoz et al. obtained highly crystalline SLG, exhibiting a %T of ∼92% and an Rs of ∼900 Ω sq−1.104


image file: c7cs00224f-f21.tif
Fig. 21 (a) Diagram of the direct-growth of graphene on SiO2 by two temperature zone-based CVD. (a) Reproduced with permission from ref. 95, copyright 2013, The Royal Society of Chemistry. (b–e) Direct-growth of few-layer graphene (FLG) on SiO2 as follows: (b) Raman spectra of direct-grown graphene/SiO2 at 1100 °C, 1150 °C and 1200 °C, (c) Raman 2D peaks, and (d and e) SEM images. (b–e) Reproduced with permission from ref. 97, copyright 2012, The Royal Society of Chemistry.

In early 2017, Pang et al. reported a new method, based on self-terminating confinement for the growth of large-scale uniform SLG directly onto two sandwiched Si/SiOx substrates (Fig. 22).105 This result is a significant improvement for transfer-free graphene growth, considering that the transfer-free synthesis of “extremely uniform” graphene as a monolayer or bilayer has had significant challenges.


image file: c7cs00224f-f22.tif
Fig. 22 Diagram of the direct-growth of CVD-based large area uniform SLG on SiO2. Reproduced with permission from ref. 105, copyright 2017, American Chemical Society.

In 2015, Sun et al. reported catalyst-free direct growth of uniform graphene on various solid glasses (quartz, borosilicate, sapphire) by using a cost-effective APCVD process, and its potential for everyday applications such as heat resistant devices, photocatalytic plates, and energy-saving smart windows, as shown in Fig. 23.106 Sun et al. have also shown that the layer thickness of graphene is easily tunable. This is one of the pioneering research studies on the direct well-controlled large-scale growth of high-quality graphene on insulating solid glasses.


image file: c7cs00224f-f23.tif
Fig. 23 Catalyst-free APCVD growth of uniform graphene on various solid glasses (quartz, borosilicate, sapphire). Reproduced with permission from ref. 106, copyright 2015, American Chemical Society.

In 2010, Rummeli et al. investigated a novel facile strategy for CVD direct growth using hexane (C6H14) vapour as the carbon source for obtaining nanocrystalline graphene and FLG on an MgO dielectric substrate at a very low temperature (325 °C) (Fig. 24a and b).67


image file: c7cs00224f-f24.tif
Fig. 24 SEM images of direct-grown graphene by CVD on an MgO substrate. Reproduced with permission from ref. 67, copyright 2010, American Chemical Society.

One of the advantages of the direct growth using a low temperature PECVD process is that graphene films can be formed on arbitrary glass substrates. To verify this, Sun et al. carried out the direct graphene growth using RF-PECVD at 600 °C (Fig. 25a–d).110 The Raman spectra showed good characteristics of graphene at 1349 cm−1 (D band), 1591 cm−1 (G band) and 2690 cm−1 (2D band) (Fig. 25d).110 A strong defect-related D band was observed at high-power treatment, but the intensity of the D band was reduced at lower power and by controlling the growth parameters. These results indicate that low-temperature catalyst-free PECVD growth can lead to a higher-quality direct-grown graphene of low fabrication cost with multi-functional electrodes for versatile applications, such as solar cells, transparent electronics and smart windows.


image file: c7cs00224f-f25.tif
Fig. 25 (a) Direct growth of graphene by a remote RF-PECVD system on various glasses (FTO glass, borosilicate glass, mica, quartz glass, sapphire, normal green glass, normal white glass). Photograph of the PECVD system (b) and of various used glasses (c). (d) Raman spectra of direct-grown graphene on various glasses. (a–d) Reproduced with permission from ref. 110, copyright 2015, Springer and Tsinghua University Press. (e) Direct-growth of graphene by microwave-PECVD assisted by NH3 gas precursor. (f) Raman spectra of graphene grown on various insulating substrates with 30% NH3 at 700 °C for 3 h. (e and f) Reproduced with permission from ref. 111, copyright 2017, The Royal Society of Chemistry.

In the latest report, Zheng et al. revealed an innovative technique for synthesizing graphene directly based on NH3-assisted microwave-PECVD by inserting a protective shield in order to protect the samples from the ion-induced damage of the plasma ball, while the energetic and reactive species could pass through the protective shield and reach the substrates to synthesize graphene (Fig. 25e and f).111 Unfortunately, this protective shield was not good enough, and still the significant damage-inducing ions passed through the protective shield to reach the graphene. Consequently, the D bands of the Raman spectra were still high.111 To completely remove this ion-induced damage, the protective shield needs to be re-designed as in recent reports.20–25

3.2.2. Direct growth of graphene on flexible organic polymer substrates at low temperature. Direct growth of graphene on flexible organic substrates has huge potential in applications related to flexible and stretchable electronics, such as e-skin and health monitoring on the human body.113–115 However, the limited thermal stability of organic substrates, which can easily be melted, deformed or damaged at high temperatures (>300 °C), leads to a serious limitation in direct growth of graphene on the flexible substrates, because the quality of graphene grown at low temperatures (<400 °C) is much lower than that of the one at high temperatures (∼1000 °C). Owing to these constraints, graphene growth on flexible substrates is only recently being studied.87,88 To reduce the process temperature, most of the studied growth methods involve the catalytic conversion of organic precursors to graphitic layers on the flexible organic substrates with the help of catalytic metal layers.

In 2012, Kim et al. reported a low-temperature (300 °C) growth of graphene-graphitic carbon (G-GC) films on a Cu layer deposited on a polyimide (PI) substrate using inductively coupled plasma-enhanced CVD (ICP-CVD), and direct transfer of the G-GC films onto a underlying flexible PI substrate using wet etching of the Cu layer (Fig. 26a–f).87 The optical and electrical characteristics of G-GC are affected by varying the growth temperature, plasma power and growth time. More recently, in 2016, Seo et al. revealed a simple, inexpensive, scalable and patternable process to synthesize graphene-dielectric bi-layer (GDB) films on solution-processed polydimethylsiloxane (PDMS) under a Ni capping layer (Fig. 26g).88 Seo et al. deposited a Ni film as a catalyst and encapsulation layer on a PDMS layer that was a few micrometer thick; this layer enabled direct growth of GDB between the substrate and Ni layer. PDMS (4 μm)/Ni (400 nm) films on the substrate were thermally annealed under vacuum, forming a PDMS/MLG/Ni/MLG structure. At the interface of the PDMS layer and the Ni film, the carbon atoms in the PDMS surface diffused into the Ni layer under high temperature, and carbon atoms were released to form MLG on both sides of the Ni layer during cooling. With this method the GDB structure was fabricated simultaneously and directly on the substrate, by thermal conversion of the PDMS without using additional graphene transfer and patterning processes or formation of an expensive dielectric layer, which makes the device fabrication process much easier.


image file: c7cs00224f-f26.tif
Fig. 26 Diagram of the direct-growth of graphene onto flexible substrates: (a–f) PI, and (g) PDMS. (a–f) Reproduced with permission from ref. 87, copyright 2012, IOP Publishing. (g) Diagram of the direct-growth of bilayer graphene (BLG) on a PDMS substrate based on a Ni catalyst. (g) Reproduced with permission from ref. 88, copyright 2017, IOP Publishing.

4. Applications of direct-grown graphene

A wide range of functional devices (transistors, solar cells, sensors, resistors, diffusion barriers, heat-resistant devices, photocatalytic plates and energy-saving smart windows) of graphene directly grown on various dielectric substrates using different growth methods, catalysts and device performances to date have been introduced, as briefly classified in Table 1.
Table 1 A brief classification of direct-grown graphene on various substrates and their applications to date. Note that “NA” means “not applicable”
Substrate Property Method Catalyst Applications of direct-grown graphene Results Ref.
PI Flexible ICP-CVD Cu Strain sensor Transmittance (%T) (77% at 550 nm)

Sheet resistance (Rs) (80 kΩ sq−1)

87
PDMS Flexible Spin coat and thermal annealing, CVD Ni FET Electron mobility μe = 0.01 cm2 V−1 s−1

On/off ratio (1.1 × 104)

88
MgO Rigid CVD Hexane (C6H14) NA NA 7
Rigid glass Rigid PECVD NA Two-terminal resistor %T = 85%

Rs = 7 kΩ sq−1

8
Flexible willow glass Flexible CVD Ni Graphene pattern High flexibility pattern 108
FTO glass Rigid PECVD NA DSSC Efficiency η = 3.86% 110
Green glass
White glass
Blue glass
Brown glass
Cobalt glass
Tinted glass
AlN Rigid APCVD NA NA NA 97
GaN Rigid Thermal evaporation and annealing, CVD Cu/Ni and polymer (SAM) Chemical sensor Sensing of NO2, NH3 gas 101
Sapphire Rigid Thermal evaporation and annealing, CVD Cu NA NA 61
PECVD NA NA NA 68
CVD NA Top-gated FET Hole mobility μh = 277 cm2 V−1 s−1, μe = 227 cm2 V−1 s−1 70
Thermal evaporation and annealing W and Ni NA NA 89
APCVD NA Heat-resistant Excellent heat resistant 106
Thermal evaporation and annealing Ni and PVA NA NA 109
PECVD NA DSSC Efficiency η = 3.86% 110
Microwave

PECVD

NH3 NA NA 111
Sapphire (randomly stepped (0001)) Rigid CVD NA NA NA 85
Sapphire (phase separated (0001)) Rigid CVD NA NA NA 85
Sapphire (randomly stepped (1120)) Rigid CVD NA NA NA 85
Al2O3 Rigid PECVD NA NA NA 8
Microwave

PECVD

NH3 Back-gated FET Rs = 3.8–6.6 kΩ sq−1

μe = 16 cm2 V−1 s−1

111
Quartz Rigid Thermal evaporation and annealing, LPCVD Cu NA NA 61
Thermal evaporation and annealing, CVD Cu NA NA 66
PECVD NA Two-terminal resistor %T = 92%

Rs = 40 kΩ sq−1

68
CVD O2 NA NA 69
CVD Cu and H2 FET μe = 100–600 cm2 V−1 s−1 78
CVD NA Pressure sensor Pressure sensitivity (−6.524 kPa−1) at 0–200 Pa 73
CVD Cu Bottom-gated FET %T = 94%

Rs = 2 kΩ sq−1

90
CVD NA NA %T = 97%

Rs = 650 Ω sq−1

95
Thermal evaporation and annealing, CVD Cu/Ni and polymer (SAM) Chemical sensor (NO2, NH3 gas) %T = 84.3%

Rs = 3.5 kΩ sq−1

101
ECR-CVD NA NA %T = 92%

Rs = 900 Ω sq−1

104
APCVD NA Heat-resistant Excellent heat resistant 106
PECVD NA DSSC Efficiency η = 3.86% 110
Microwave

PECVD

NH3 NA NA 111
Mica Rigid PECVD NA NA NA 68
PECVD NA DSSC Efficiency η = 3.86% 110
NA Transparent circuit for LED High transparent and flexibility 110
Si Rigid Thermal evaporation and annealing, LPCVD Cu NA NA 62
PECVD NA NA NA 68
APCVD NA NA NA 97
Si(111) Rigid Thermal evaporation and annealing NA NA NA 94
Textured Si Rigid Thermal evaporation and annealing, CVD Cu/Ni and polymer (SAM) Chemical sensor Sensing of NO2, NH3 gas 101
SiC Rigid PECVD NA NA NA 68
Fused silica Rigid Thermal evaporation and annealing, CVD Cu NA NA 61
Borosilicate Rigid APCVD NA Heat-resistant Excellent heat resistant 106
PECVD NA DSSC Efficiency η = 3.86% 110
Pyrolysis, thermal evaporation, annealing, and APCVD Ni and polymer (SAM) FET μe = 4400 cm2 V−1 s−1

n = 2 × 1012 cm−2

7
Thermal evaporation and annealing, LPCVD Cu NA NA 62
Pyrolysis and thermal annealing, CVD Ni/Cu and polymers (PS, PAN, PMMA) NA NA 63
PECVD Ni NA NA 64
Thermal evaporation and annealing, CVD Cu NA NA 61
PECVD NA Two-terminal resistor Resistance reduced 20% 68
CVD O2 FET μe = (531 cm2 V−1 s−1 in air, 472 cm2 V−1 s−1 in N2) 69
CVD Cu and H2 FET μe = 100–600 cm2 V−1 s−1 72
CVD Sn and a-C NA NA 94
CVD Cu Bottom-gated FET μe = 670 cm2 V−1 s−1 90
Pyrolysis, thermal evaporation and annealing, LPCVD Ni, C8H8, C3H3N, C4H6 NA NA 91
Langmuir-Blodgett

Pyrolysis

NA FET μe = 78.7 cm2 V−1 s−1

On/off ratio (200–220)

93
CVD NA NA NA 95
Diffusion-assisted synthesis Ni and graphite powder Back-gated FET μh = 667 cm2 V−1 s−1 96
APCVD NA Photovoltaic (DSSC) Rs = 63 Ω sq−1

μ = 201.4 cm2 V−1 s−1

Photovoltaic efficiency (4.25%)

97
CVD Cu Bottom-gated FET μh = 800 cm2 V−1 s−1

μe = 700 cm2 V−1 s−1

98
CVD Cu and SiH4 NA NA 99
Thermal evaporation and annealing, CVD Cu/Ni and Polymer (SAM) Chemical sensor Sensing of NO2, NH3 gas 101
Thermal evaporation and annealing, CVD Ni Back-gated FET Rs = 700–2100 Ω sq−1

On/off ratio ∼3

μh = 655 cm2 V−1 s−1

102
PECVD NA Diffusion barrier An excellent Cu/FLG barrier/SiO2 structure 103
ECR-CVD NA NA NA 104
CVD NA Back-gated FET μh = 410–760 cm2 V−1 s−1 105
CVD GeH4 FET μe = 160 cm2 V−1 s−1

n = 3 × 1012 cm−2

107
CVD Ni Graphene pattern High flexibility pattern 108
Thermal evaporation and annealing Ni and PVA NA NA 109
Al2O3/SiO2 Rigid Microwave

PECVD

NH3 Back-gated FET Rs = 3.8–6.6 kΩ sq−1

μe = 16 cm2 V−1 s−1

111
h-BN/SiO2 Rigid APCVD NA NA NA 71
CVD Cu, hexane, ammonia borane NA NA 93
APCVD NA NA NA 97
CVD SiH4 FET μe = 20[thin space (1/6-em)]000 cm2 V−1 s−1

Gr domain ∼20 μm

Growth rate

∼1 μm min−1

100
MnO2 Rigid Microwave

PECVD

NH3 NA NA 111
TiO2 Rigid Microwave

PECVD

NH3 NA NA 111
HfO2 Rigid Microwave

PECVD

NH3 NA NA 111


4.1. Transistors (FETs)

The transfer-free growth of graphene will provide a new way to fabricate FET-based electronic devices from graphene simply and inexpensively, while avoiding the transfer process. In general, FETs based on transfer-free direct graphene growth on various substrates (PDMS, sapphire, quartz, SiO2 and h-BN), have been investigated thoroughly in previous studies.1,7,62,69,70,72,88,92,96,98,100,102,105,107 In particular, direct-grown graphene on an h-BN substrate using an SiH4 catalyst has emerged as the most promising research direction, with the best mobility of an FET device of up to 20[thin space (1/6-em)]000 cm2 V−1 s−1 for a graphene domain size of ∼20 μm obtained using the CVD method (Fig. 27a and b).100 Utilizing another synthesis method, Zheng et al. revealed an FET device fabrication technique based on microwave-PECVD that involves inserting a protective shield which can be used for graphene direct-growth on insulating substrates (Fig. 27c and d).111 However, the obtained results from the FET devices are still of poor quality such as contact resistance (3–8–6.6 kΩ sq−1) or field-effect mobility (16 cm2 V−1 s−1). Therefore, CVD is still the best method for direct-growth of graphene currently.
image file: c7cs00224f-f27.tif
Fig. 27 (a) OM image of an FET device based on SiH4-catalyzed direct-growth of SLG on an h-BN substrate located above an SiO2 wafer. The inset is the AFM image of the FET device. (b) OM image of a h-BN flake and single-crystal graphene/h-BN. (a and b) Reproduced with permission from ref. 100, copyright 2015, Nature Publishing Group. Electrical properties of direct-grown graphene-based FET devices by an NH3-assisted microwave-PECVD method with output characteristics at various voltages (c), and transfer characteristics of five NH3-doped FETs and an FET device using only H2, and C2H2. (c and d) Reproduced with permission from ref. 111, copyright 2017, The Royal Society of Chemistry.

4.2. Dye-sensitized solar cells (DSSCs)

Highly conductive graphene films are particularly suitable for photovoltaic (PV) applications. Usually, graphene layers are grown on a catalytic metal substrate and transferred to the device substrate to be used as electrodes for organic photovoltaic cells (OPV),116 Schottky solar cells,117 or CdTe solar cells,118 to replace the conventional transparent conducting films (TCFs), such as In2O3:Sn (ITO) and SnO2:F (FTO). However, their highest efficiency is only 4.17%,118 which could not compete with conventional solar cells because a complex wet transfer process of the graphene films results in the degradation of the transferred graphene electrode and a high Rs of the PV device. Therefore, it is highly desirable to develop a direct growth approach for highly conductive graphene films on device substrates, to replace conventional TCFs for PV applications. Bi et al. studied the fabrication of a DSSC device that includes a graphene/Pt electrode, an electrolyte between the two electrodes and a dye-sensitized porous TiO2 photoanode, as shown in Fig. 28a and b and Table 1, based on direct-grown graphene on SiO2 using the APCVD approach at 1100–1200 °C.97 In this way, the DSSC showed a new JV efficiency of ∼4.25% as the counter electrode, similar to the performance (4.32%) of the SnO2:F (FTO) electrode device, which shows the potential of the direct-grown graphene as a novel electrode material in highly conducting PV applications. To pave the way for graphene application in cost-effective solar cells, in 2015, DSSC prototype devices were fabricated using directly grown graphene/glass samples (Fig. 28c–e).110 These DSSCs included a dye-sensitized nanocrystalline TiO2 working electrode, an electrolyte solution, containing a dissolved I/I3 redox couple and a counter electrode prepared using graphene on white float glass. The results of this work suggested that graphene directly grown on a glass substrate displays higher catalytic activities with regard to the reduction of the I3 ion than that of the transferred Cu-grown graphene (Fig. 28e). The overall power conversion efficiency (η) of the direct-grown graphene/glass-based DSSC devices reached 3.86%, which is comparable to those based on graphene nanosheet films on FTO (3.29–6.81%),119 and APCVD-grown graphene/SiO2 (2.95–4.25%),97 but superior to that assembled by reduced graphite oxide on FTO (0.2–3.7%).120 Such results offer evidence that the directly grown graphene can serve as a low-cost electrode material to substitute for cost-ineffective FTO in future photovoltaic applications.
image file: c7cs00224f-f28.tif
Fig. 28 (a) Diagram and (b) JV curves of DSSC devices originated by FTO/Pt and graphene/Pt electrodes. (a and b) Reproduced with permission from ref. 95, copyright 2013, The Royal Society of Chemistry. (c) Diagram of a DSSC device based on a PECVD direct-grown graphene/glass counter electrode. (d) JV curves of DSSC devices based on FTO/Pt and graphene/Pt counter electrodes. (e) CV curves of the directly grown graphene-coated glass electrode (red) and the transferred graphene (grown on Cu)-coated electrode (green). (c–e) Reproduced with permission from ref. 110, copyright 2015, Springer and Tsinghua University Press.

4.3. Sensors (pressure, strain, chemical)

Graphene can be used as a 3D structured electrode in multifunctional devices, such as pressure sensors,69 black silicon solar cells,76 cambered micro-optics,77 and MEMS sensors.78 In general, it is almost impossible to conformally transfer graphene grown on a catalytic metal substrate onto a 3D structural surface without mechanical damage.121 Therefore, the direct growth of graphene on the 3D structured device surface can be a potential way to solve the limitations and problems above.

In 2016, Song et al. demonstrated a simple and effective method to directly produce conformal graphene films on the surface of 3D micro-structured quartz substrates using the CVD system, which could be efficiently applied to a pressure sensor (Fig. 29).73 As a result, this device showed a high-pressure sensitivity of −6.524 kPa−1 in the range of 0–200 Pa with high-reliability (∼5000 cycles), and an ultrafast response (∼4 ms). Therefore, this pressure sensor might have high potential in practical applications, e.g. a wind pressure sensor. In 2012, by using CVD growth at low-temperature (300 °C) for G-GC films on a dielectric PI substrate, Kim et al. successfully fabricated a G-GC-based strain sensor on a PI substrate, and demonstrated the resistance modulation at different strains.87 The resistance of the G-GC films showed a gradually increasing tensile strain of ∼0.8% for 340 s (Fig. 30a). Particularly, it linearly increased in the range of 31.64–31.69 MΩ with the applied strains of 0.1–0.8% (Fig. 30b). In 2016, Yang et al. reported a chemical sensor based on transfer-free graphene growth, for sensing NO2 and NH3 gas molecules with good sensitivity.101 The sensitivity of the transfer-free graphene was 11.5%/200 ppm (ΔR/R0), which is lower than those of the previous reports such as CVD SLG (32%/200 ppm and 40%/200 ppm),122,123 CVD graphene (51%/40 ppm),124 and ozone-treated CVD graphene (19.7%/200 ppm).125 Nevertheless the sensitivity results by Yang et al. are higher than those of other reports, such as CVD graphene (4%/100 ppm),126 epitaxially grown graphene on 6H-SiC (2.25%/500 ppm)127 and 3D-rGO hydrogel (0.19%/2 ppm).128


image file: c7cs00224f-f29.tif
Fig. 29 (a) Schematic of the working mechanism of a pressure sensor. (b) Equivalent circuit diagram of the conformal graphene pressure sensor. (c) Typical IV curves of the PDMS/Gr/Gr/GQ sensor with different applied pressures. (d) Pressure response curves for the flat Gr/Q and conformal Gr/GQ films, respectively. The PDMS/Gr/Gr/GQ sensor with grating structures exhibits much higher pressure sensitivity than the PDMS/Gr/Gr/Q sensor. (e) Magnified pressure–response curves for the PDMS/Gr/Gr/GQ sensor at 0–900 Pa. Reproduced with permission from ref. 73, copyright 2016, American Chemical Society.

image file: c7cs00224f-f30.tif
Fig. 30 (a) Resistance changes in G-GC films at various treatment times. The inset is a strain-applied sensor. (b) Resistance changes in G-GC films under applied strains. (a and b) Reproduced with permission from ref. 87, copyright 2012, IOP Publishing. (c) Schematic of the chemical sensor. (d) Sensitivity with NH3 and NO2 gas. Sensitivity with increasing NO2 concentration: (e) time-resolved change of sensitivity and (f) maximum sensitivity after 120 s NO2 gas exposure and 120 s UV irradiation. (c–f) Reproduced with permission from ref. 101, copyright 2016, American Chemical Society.

4.4. Two-terminal resistors

Owing to the simple, cost-effective and scalable nature of catalyst-free direct growth of graphene on various substrates, the direct-grown graphene could have high potential in industrial applications, such as gas sensors, electrode materials and thin film resistors. In 2011, Zhang et al. used a PECVD system for the direct growth of a nanocrystalline graphene film on various substrates (sapphire, quartz, mica, Si, SiC, glass) without using any catalyst.68 They also fabricated a two-terminal resistor from direct-grown graphene on a SiO2 substrate (Fig. 31 and Table 1). Despite the poor crystallinity of the grown graphene, the resistance of the device was fairly low (2.5 kΩ).68 This facile, inexpensive, and scalable growth method is very compatible with semiconductor technologies; therefore, it could be applied for practical applications in gas sensors, nanoresistors, TCFs, and electrode-based materials.
image file: c7cs00224f-f31.tif
Fig. 31 (a) IV curves of two-terminal resistor devices as the inset. (b) Resistance values at various temperatures of resistors. The inset depicts the IV curve. (c) Optical image of direct-grown nanographene on 4-inch wafer glass. (d) Transmittance and sheet resistances of resistors. (a–d) Reproduced with permission from ref. 68, copyright 2011, Springer and Tsinghua University Press.

4.5. Diffusion barrier

Graphene is an excellent ultra-thin barrier against undesired mass transport. However, transfer-related chemical and mechanical defects of graphene have limited the practical application of conventional transferred graphene as a diffusion barrier.129 Recently, Mehta et al. reported the investigation of direct-grown multi-layer graphene (MLG) that acted as a diffusion barrier (Cu/MLG barrier/SiO2) as shown in Fig. 32.103 The results of this study have shown that direct-grown graphene membranes can efficiently block Cu diffusion in integrated circuit chips, photovoltaic cells and flexible electronic devices. In general, a defect-free graphene membrane is also an ideal ultra-thin barrier blocking ion and molecular transport.
image file: c7cs00224f-f32.tif
Fig. 32 (a) Fabrication process of a diffusion barrier based on direct-grown MLG on SiO2. (b) Bias-temperature stress (BTS) setup for characterizing Cu cation transport. (c) Raman spectra of direct-grown MLG on SiO2 with/without BTS. (d) AFM image and the height of etched MLG/SiO2. (a–d) Reproduced with permission from ref. 103, copyright 2017, The Royal Society of Chemistry.

4.6. Electrical heating devices

Electrically and thermally conductive graphene is an ideal heating element in heat modulation devices. In 2015, Sun et al. reported catalyst-free direct growth of uniform graphene on various solid glasses (quartz, borosilicate, sapphire), for constructing transparent heating electrodes used in thermal-induced, light modulating windows (Fig. 33).106 These windows made of electrochromic or thermochromic materials are located between two TCFs based on directly grown graphene glass plates. As a result, these are excellent transparency heat modulating devices.
image file: c7cs00224f-f33.tif
Fig. 33 Applications of catalyst-free APCVD direct-grown graphene glasses as heating devices on various solid glasses (quartz, borosilicate, sapphire). (a) Structural diagram of a direct-grown graphene glass heater. (b) Cycling performance of the thermochromic display made with graphene/quartz glass. (c) Fog removal time and defogger resistance as a function of input voltage; the inset shows the different defogging behaviors of the DG-based and TG-based defoggers before and after experiencing 2 min ultra-sonication treatment. (d) Optical transmittance of initial, bleached, interval and colored final states of the light modulating window; the inset shows the photographs of the window in initial, interval and colored final states. (a–d) Reproduced with permission from ref. 106, copyright 2015, American Chemical Society.

4.7. Photocatalytic plates and energy-saving smart windows

The direct-grown graphene on glasses can be highly promising in constructing photocatalytic plates and energy-saving smart windows, due to its transfer-free process. Sun et al. also revealed other possible applications of the catalyst-free direct-grown graphene solid glasses for photocatalytic plates and energy-saving smart windows (Fig. 34a–c).106 By integrating a photocatalyst BiOF with graphene glasses, photocatalytic plates useful for dye waste water treatment can be constructed (Fig. 34a). The performance of these devices was evaluated by the photocatalytic degradation of Rhodamine B (RhB) under irradiation by natural sunlight. In addition, these graphene glasses also show excellent performance in the construction of energy-saving smart windows, when combined with thermochromic VO2 coatings (Fig. 34d–f).
image file: c7cs00224f-f34.tif
Fig. 34 Applications of catalyst-free APCVD direct-grown graphene glasses in photocatalytic plates and energy-saving smart windows on various solid glasses (quartz, borosilicate, sapphire). (a) Schematic of graphene glass based photocatalytic plates for degradation of dye wastewater in a recyclable manner. (b) Comparison of the degradation efficiencies of RhB-containing wastewater by using chemically coated BiOF/graphene glass (red), physically coated BiOF/graphene glass (blue), physically coated BiOF/pure glass (pink), pure graphene glass (green), and pure glass (black) as photocatalysts under natural sunlight irradiation; the inset displays a photograph of prototype reactors after photo-degradation by employing the marked catalysts. (c) Cycling photo-degradation of a RhB-containing solution using chemically coated BiOF/graphene glass plates. (d) Schematic of an energy-saving window based on VO2/graphene glass. (e) Transmission spectrum of VO2/graphene glass samples taken at 25 and 90 °C, respectively, displaying a prominent thermochromic behavior; the inset shows the typical Raman spectrum of VO2 film on graphene glasses. (f) Transmittance at λ = 2000 nm of VO2/graphene glasses and VO2/bare glasses. (a–f) Reproduced with permission from ref. 106, copyright 2015, American Chemical Society.

4.8. Graphene patterns

To demonstrate the potential for application of Ni-catalyzed direct-grown graphene on Willow flexible glass and SiO2, Marchena et al. successfully produced ribbons and a square pattern of graphene (Fig. 35).108 In particular, growing graphene directly on an ultrathin flexible Willow glass substrate has potential in future flexible electronic and optoelectronic devices.
image file: c7cs00224f-f35.tif
Fig. 35 Sequences of direct-grown graphene pattern fabrication on Willow flexible glass and on SiO2 using a Ni catalyst by UV lithography. Reproduced with permission from ref. 108, copyright 2016, OSA Publishing.

4.9. Transparent circuit for a green LED device

A directly grown graphene/glass sample, for use in a range of transparent conductive applications (like transparent circuit) in industry, requires uniformity, low-cost, flexibility and good quality graphene on transparent substrates (Corning Willow glass). It is currently in high demand to explore the novel functions of circuits on flexible glass, which may have application potential in optoelectronics, gas/moisture/bio sensors etc. For instance, the resistances of patterned graphene within circuit devices would show a noticeable change without the harmful wet transfer graphene process applied directly on devices. In addition, regarding the potential for integration into flexible electronic devices, mechanical durability of the directly grown graphene is an important factor. To the best of our knowledge, such properties have not been studied so far.

Recently, Sun et al. fabricated and designed a transparent circuit based on PECVD direct-grown graphene/ultrathin Corning Willow flexible glass by utilizing photolithography, as shown in Fig. 36a.110 The results showed that the patterned graphene electrode on glass can light up a green light-emitting diode (LED) indicator (Fig. 36b). They also synthesized graphene directly on flexible mica glass and measured the change in the resistance using bending tests, with a bending variation of ∼45%, and the full recovery after bending indicates good mechanical stability and flexibility of the graphene electrode (Fig. 36c).


image file: c7cs00224f-f36.tif
Fig. 36 (a) Photograph of transparent circuit based on PECVD direct-grown graphene/glass. The inset shows the OM image of this device. (b) Photograph of patterned PECVD graphene on white glass showing the transparent conductivity to light up a green LED device. (c) Resistance with various bending values of a direct-grown graphene film on mica glass. The inset shows the bending test. (a–c) Reproduced with permission from ref. 110, copyright 2015, Springer and Tsinghua University Press.

5. Conclusions, perspectives, and challenges

Strategies for direct graphene growth on arbitrary dielectric flexible and rigid substrates using the CVD method with/without a metal catalyst have been briefly reviewed. In addition, a wide range of device applications of the direct-grown graphene have also been discussed. The prospects of direct-grown transfer-free graphene are bright and currently receiving considerable attention from the 2D material research community. By discovering new methods for obtaining transfer-free graphene, the direct fabrication of a wide-range of various hetero-structured devices can be achieved. However, understanding of the growth process and conditions that affect the quality of graphene is still very poor. So far, graphene grown directly on an insulating substrate is generally of low quality (Table 1). Because the direct growth relies on the thermal decomposition of carbon resources, the growth rate is usually low and the size of the graphene domain is small, resulting in growth of a defective graphene layer. Currently, many challenges remain in this direction and large-area high-quality graphene production is still very difficult. In order to obtain more advanced results, an in-depth understanding of the mechanism of graphene growth on insulating substrates is essential.

Given the practical applications of graphene, several directions to pursue in direct graphene growth include low-temperature growth, high-speed growth of highly crystalline graphene, and direct growth on other two-dimensional materials such as h-BN. Direct graphene growth at low temperatures is an important research issue, because high growth temperatures are not allowed on many device substrates, such as flexible polymer and Si substrates. Direct growth of graphene at low- or near-room temperature,67,87,96 has been carried out. However, the results to date have not yet met the expectations. The direct growth of large-scale graphene on h-BN is also an attractive topic.71,93,98–100,130–132 Graphene on h-BN can have excellent electrical properties, because h-BN is an ideal dielectric substrate for graphene devices, owing to its ultra-smooth, ultra-flat surface, insulation properties, chemical inertness and small lattice misfit compared to SiO2.112 In theory, graphene synthesized on as-grown h-BN exhibits better properties than that grown on transferred h-BN because of the transfer-induced contamination and defects on h-BN substrates. The transfer-free grown graphene on ultra-flat h-BN, which could preserve the pristine properties of graphene, enables further promising device applications based on vertically stacked 2D materials.

Ultrafast direct growth of single-crystal graphene on insulating substrates is another fascinating and challenging topic, as ultrafast conventional indirect growth on copper has been investigated thoroughly and has progressed in recent years.133 Further studies are required for obtaining faster direct growth and larger graphene single crystals on insulating substrates. Several possible strategies are proposed: (i) to explore a more efficient way to reduce the reaction barrier for graphene direct growth; (ii) to obtain epitaxial, direct-grown well-aligned graphene domains and seamlessly stitch them together into a complete single-crystal film. One potential way to realize strategy (i) is probably to introduce a gas phase catalyst enhancing the catalytic conversion of carbon precursors to a graphene layer. In fact, strategy (ii) has been implemented for conventional indirect-grown graphene on catalytic single-crystal substrates such as Ge(110),6 or Cu(111).134,135 However, the single-crystal graphene layer has a limited size due to the limited single-crystal substrate size, and often the quality is less satisfactory owing to the imperfect alignment of individual graphene islands. Thus, to achieve a large-area, high-quality direct-grown graphene film, preparation of a large-area substrate for releasing larger graphene nucleation seeds and an improvement in graphene alignment are critical issues in future studies.

In addition, direct-growth of graphene assisted by metal powder precursors (solid, diluted solution) contained inside a sub-chamber for a direct evaporation process into an innovative and re-designed-CVD main-chamber in order to allow graphene formation on dielectric substrates is an attractive topic and currently under investigation.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1402-12.

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