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Reviews in Advanced Sciences and Engineering Vol. 2, pp. 1–21, 2013 (www.aspbs.com/rase)

Graphene: Synthesis, Properties and Application in Transparent Electronic Devices Pushpendra Kumar1 , Arun Kumar Singh2, 3 , Sajjad Hussain2, 4 , Kwun Nam Hui5 , Kwan San Hui6 , Jonghwa Eom2, 3 , Jongwan Jung2, 4, ∗ , and Jai Singh2, 4, ∗ 1

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 11529, Taiwan Graphene Research Institute, Sejong University, Seoul 143-747, Korea 3 Department of Physics, Sejong University, Seoul 143-747, Korea 4 Department of Nano Science and Technology, Institute of Nano and Advanced Materials Engineering, Sejong University, Seoul 143-747, Korea 5 Department of Materials Science and Engineering, Pusan National University, Geumjeong-gu, Busan 609-735, Korea 6 Department of Systems Engineering and Engineering Management, City University of Hong Kong, Kowloon Tong, Hong Kong 2

ABSTRACT Recently, two-dimensional (2D) nanomaterials have received huge attention because of their attractiveness for use in many electronic and optoelectronic devices. Graphene is the two-dimensional basic building block for carbon allotropes of any dimensionality, such as graphite, nanotubes and fullerenes. As we know, transparent electrodes are an important component in many modern electronic devices such as touch screen, liquid crystal display (LCD), light-emitting diode (LED) and solar cells. In addition, all of electronic appliances are growing in demand too much fast due to the rapid industrialization and growing human population. Right now, this role has been well used by doped metal oxide materials; most common are tin doped indium oxide (ITO) and fluorine doped tin oxide (FTO). In recent years many other transparent conducting materials (TCM) have also been developed such as carbon nanotubes (CNTs), graphene, metal nanowires and nanoparticles. Among the all these TCM, graphene has received greater attention due to advantages over other materials because of its very high electrical conductivity, optical transparency and flexibility. The flexibility of graphene-based devices goes beyond conventional transistor circuits and includes flexible and transparent electronics, optoelectronics, sensors, electromechanical systems, and energy technologies. This review article will explore the production of graphene by different methods, properties of graphene and also analyze the application in transparent conducting electronic devices. KEYWORDS: Graphene, CVD, Transparent Conducting Device, Optical Properties, Electrical Properties.

CONTENTS 1. Introduction . . . . . . . . . . . . . . . . . . 2. Properties of Graphene . . . . . . . . . . . 2.1. Electronic Properties . . . . . . . . . 2.2. Vibrational Properties . . . . . . . . . 2.3. Mechanical Properties . . . . . . . . 2.4. Optical and Electrical Properties of 3. Synthesis of Graphene . . . . . . . . . . . 3.1. Mechanical Exfoliation . . . . . . . . 3.2. Chemical Vapor Deposition . . . . . 3.3. Plasma Enhanced Chemical Vapor Deposition . . . . . . . . . . .

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3.4. Thermal Decomposition of SiC for Graphene Synthesis . . . . . . . . . . . . . . . . . 3.5. Chemical Methods . . . . . . . . . . . . . . . . . 3.6. Molecular Beam Epitaxial Growth . . . . . . . 3.7. Other Methods . . . . . . . . . . . . . . . . . . . . 4. Application of Graphene as Transparent Electrode 4.1. Light Emitting Diodes . . . . . . . . . . . . . . . 4.2. Touch Screen . . . . . . . . . . . . . . . . . . . . . 4.3. Flexible and Stretchable Electronic . . . . . . . 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . References and Notes . . . . . . . . . . . . . . . . . . . . .

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1. INTRODUCTION ∗

Authors to whom correspondence should be addressed. Emails: [email protected], [email protected] Received: 30 April 2013 Accepted: 24 June 2013

Rev. Adv. Sci. Eng. 2013, Vol. 2, No. 4

Carbon based nanomaterials are fascinating from the perspective of fundamental science and technology because they are non-toxic, chemically and thermally tolerant and

2157-9121/2013/2/001/021

doi:10.1166/rase.2013.1043

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Graphene: Synthesis, Properties and Application in Transparent Electronic Devices

Pushpendra Kumar et al.

Jonghwa Eom earned his B.S. and M.S. degree from Seoul National University, in Physics in 1989 and 1991, respectively. He got his Ph.D. degree in Physics from Northwestern University, USA, in 1998. He worked as Research Associate at University of Chicago (from 1998–1999) and Naval Research Lab (USA) (from 1999–2001), respectively. Currently, he is working as a Professor with the Department of Physics, Sejong University. He has published more than 50 papers. His research interests include Graphene devices, Magnetic devices and Nanostructures, Spintronics, Quantum Electronic Transport, Mesoscopic Physics and Nanoscale.

Jongwan Jung received the M.S. and Ph.D. degrees in electrical engineering from the Korea Advanced Institute of Science and Technology, Seoul, Korea, in 1991 and 1996, respectively. From 2001 to 2003, he worked as a Postdoctoral Researcher at the Massachusetts Institute of Technology, Cambridge, MA. He also worked for several years at Hynix and Samsung Electronics as a semiconductor device engineer. Since 2006, he has been working as an associate professor at Sejong University. His current research interests include the growth of 2D material such as graphene, MoS2 , WS2 , and its characterization, and device applications.

Arun Kumar Singh earned his M.Sc. Degree in Physics with specialization in Electronics from Banaras Hindu University, Varanasi, in 2004. He received his Ph.D. degree in Physics from School of Materials Science and Technology, Indian Institute of Technology, Banaras Hindu University, India. Currently, he is working as postdoctoral researcher at Graphene Research Institute, Sejong University, Seoul, South Korea. His current research interest includes the fabrication and characterizations of organic electronic devices, Graphene and MoS2 based electronic devices and its applications.

Sajjad Hussain is a Ph.D. candidate in graphene research institute Sejong University, Seoul Korea. He received his master degrees in Materials Physics and Nanotechnology, at Department of Physics, Linköping University, Sweden. He also have studied Master in Physics with specialization in electronic from the Punjab University, Lahore, Pakistan. His current research interest is in the spatially synthesis, characterization of two dimensional materials (Graphene, MoS2 and WS2 ) for Nanoelectronic devices.

Kwun Nam Hui is an assistant professor at Department of Materials Science and Engineering of Pusan National University, South Korea. He earned his B.Sc. degree in Physics from The Hong Kong University of Science and Technology in 2003, M.Phil. and Ph.D. degrees in Electrical and Electronic Engineering from The University of Hong Kong in 2007 and 2009, respectively. He then worked as postdoctoral associate at Institute of Advanced Materials, Devices and Nanotechnology, Rutgers, The State University of New Jersey. He has published 54 peer-reviewed journal papers, and hold 1 US and 5 Korea patents. His research interests focus on applications of nanomaterials for solar cells, solid-state lighting, Li-battery, supercapacitor, and fuel cells.

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Kwan San Hui is an Assistant Professor at Department of Mechanical Engineering of Hanyang University, South Korea. Prior, he was a Lecturer at Department of Systems Engineering and Engineering Management of the City University of Hong Kong, Hong Kong. He received his B.Eng. degree in 2002, M.Phil. degree in 2004 and Ph.D. degree in 2008 from the Department of Mechanical Engineering of Hong Kong University of Science and Technology. Over the past 10 years, he has conducted research in different areas related to energy and environment, including Li-battery, supercapacitor, and electrodes for direct methanol/direct hydrogen peroxide fuel cell, hydrogen production by methanol steam reforming, solid-state lighting, desiccant cooling, and air and water treatment. He has managed 20 research projects as Principal Investigator with a total research grant of USD 1.3 million. Currently, he has been collaborating with colleagues in USA, Australia, Singapore, Japan, Korea, China, and Hong Kong. Thus far, his research has led to 1 US patent application, 5 Korea patent applications, 58 journal publications, and 50 conference papers. He has been regularly invited as the reviewer for dozens of international journals, and to give lectures and seminars at universities in Hong Kong, China and South Korea. Jai Singh is presently working as a Research Professor in the Graphene Research Institute, Department of Nano Engineering, Sejong University, Seoul, South Korea. He did his Master’s in Physics from Gorakhpur University, Gorakhpur and Doctoral degree in the field of II-VI oxide nanomaterails from Department of Physics, Banaras Hindu University, Varanasi, India. Soon after completing his Ph.D., he joined Cologne University, Cologne, Germany to carry out post-doctoral research work in the area of Transparent Conducting Oxide (TCO) materials and then after Pusan National University, Busan, South Korea to work on the same field. Dr. Singh has published more than 24 research publications in refereed international journals of repute. He has been awarded number of prestigious National and International awards, like GATE Fellowship from Ministry of HRD, Government of India, CSIR-Junior Research Fellowship and Senior Research Fellowship (2004–2009) by the Council of Scientific and Industrial Research, Government of India, NSC Postdoctoral Fellowship Taiwan etc. Dr. Singh is also serving as a guest editor for few journals of international repute. Pushpendra Kumar is an Academia Sinica Postdoctoral Research Fellow at Institute of Atomic and Molecular Spectroscopy (IAMS), Academia Sinica, Taipei, Taiwan. He completed his Master’s and Ph.D. degrees in Physics from Department of Physics, Banaras Hindu University, Varanasi, India in the field of II–VI chalcogenide semiconductor nanostructures. Soon after completing his Ph.D. he received Dr. D. S. Kothari Postdoctoral Fellowship from U.G.C. India and joined Nanotechnology Application Centre, University of Allahabad, Allahabad as DSK Fellow to carry out post-doctoral research work in the area of II–VI chalcogenide based Diluted Magnetic Semiconductor. After that he was awarded an International project from the Government of Chile, to work on graphene and graphene nanoparticle composite materials to be used as supercapacitor materials at Department of Materials Engineering, University of Concepcion, Concepcion, Chile. His current research interest includes the fabrication and characterizations of transition metal di-chalcogenides (MoS2 , WSe2 etc.) for devices fabrication and its applications, and potential materials to be used as cathode materials in Li-ion battery. mechanically robust.1 In the last decades, the successive discoveries of carbon based materials (fullerenes, carbon nanotube) have opened a new era in materials science.2–4 In addition, the recently discovered new allotrope of carbon is graphene, which consists of a single layer of carbon atoms arranged in hexagons.5 Graphene presents very unusual and interesting electrical, optoelectrical and mechanical properties.5 6 Graphene is a made of carbon atoms, these atoms arranged in a regular hexagonal pattern similar to graphite, but in a 2D layer of sp2 hybridized carbon atoms bonded together.5 6 The quantum confinement of the electrons due to the lack of a third Rev. Adv. Sci. Eng., 2, 1–21, 2013

dimension in graphene gives various novel properties.7 8 It has been extensively studied in the last several years from discoveries to till date. Geim and Novoselov awarded the 2010 Nobel Prize in physics for their breakthrough work on graphene.9 They reported its synthesis via a scotch tape method in 2004.5 Fascinating properties of graphene, i.e., high surface area (2630 m2 /g), high thermal conductivity (∼ 5000 W/mK), very high charged carrier mobility (∼ 200,000 cm2 /Vs) and greater Young’s modulus (‘∼ 1 TPa) have been well documented.5 10 11 For the improvements in knowledge and understanding about graphene along with large scale production, research has 3


increased exponentially with their potential application in nano optoelectronic devices. Now days, transparent conducting materials are an essential part in the optoelectronic devices such as LCD, OLED, and photovoltaic.12 13 This material is transparent to visible light and is electrically conductive. Currently, tin doped indium oxide (ITO) and fluorine tin oxide (FTO) material are being used in optoelectronic devices because of their high conductivity and good transparency.12–15 ITO and FTO have also been widely used as window electrodes in optoelectronic devices. ITO alone is expected to grow to be a $3 billion dollar market in 2010, with a 20% growth rate through 2013.16 However, ITO has several problems relating both to various material properties, and cost. ITO can crack and fracture at very low strains because it is a ceramic material which is main limitation for flexible devices. In addition to that traditional ITO electrode material is limited due to the limited indium availability on the earth along with the instability in an acid environment with poor transparency in the near infrared region and their susceptibility to ion diffusion into polymer layers.17 ITO films is its relatively high index of refraction (n > 20) and very high cost.18 Many of the alternative transparent electrodes, therefore, have been developed in order to replace ITO. One of the first and most mature of these new materials falls under the general category of conducting polymers like poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and carbon nanotubes (CNTs).19–24 Among these materials, CNTs films exhibit significantly high transparency across the whole visible light spectrum. However, CNT films also have some limitations, the density of nanotubes in CNT films must be above the threshold for the formation of a percolation network and the high resistance at the nanotube–nanotube junctions limits the conductive pathway within the films.25 Thus, CNTs films are not suitable as transparent electrodes in electronic devices. Therefore, efforts are needed to identifying, new high quality transparent electrode material for good conductivity, better stability in acid medium, enhanced transparency with flexibility equaling or better than ITO and carbon nanotube. Amongst concentrated attempts, the graphene is claimed to be an ideal candidate based on the control to the optical and electrical properties.26 However the pristine graphene (un-doped) has high sheet resistance an order of few K ohms.27 28 The low sheet resistance and high optical transparency are essential for use of graphene as a transparent conducting electrode. Thus doping of graphene is necessary for reduction of sheet resistance. Recently, many groups have reported the doping of graphene while maintaining the transparency and electrical properties of graphene.29–31 Moreover, the combination of its high thermal and chemical stability, high flexibility and low contact resistance with organic semiconductors, offers remarkable advantages for using graphene as a promising transparent conducting electrode in organic electronic devices, 4


e.g., organic light emitting diodes (OLEDs), solar cells, touch screens, field effect transistors (FETs) and liquid crystal displays (LCDs).32–35 It is superb transparent conductor. There is growing competitive interest to produce graphene films for application in the areas of optoelectronic where transparent, flexible and conducting coating together with low temperature synthesis is required. There are only fewpapers/reviews on the topic of graphenesynthesishave been publishedin recent year.36–38 However, the reviews lacking a comprehensive overview of recent experimental results related to graphene synthesis and its application in transparent electronic devices.

2. PROPERTIES OF GRAPHENE 2.1. Electronic Properties Graphene is a 2D sheet of carbon atoms displayed in a hexagonal honeycomblattice. The honeycomb lattice is a bipartite lattice with 2 atoms per unit cells (see Fig. 1) and a lattice√constant of a = 2:46 Å. The C C bond distance is a/ 2 = 1:42 Å,39 making the sheet quite strong. Afew such layers stacked on top of each other are still considered graphene; it obtainsat least 10 layers before a sample becomes bulk graphite,there are about 3.35 Å spacing between stacked sheets.40 As an effect of the graphene structure, the first Brillouin zone has two conical points K and K where a band crossing occurs (Fig. 2).41 Near these crossing point, the electron energy, E, is linearly dependent on the wave vector. The tight-binding approach considering only the first nearest neighbor interaction provides the dispersion relation of the electrons near the K/K points.42 2.2. Vibrational Properties The vibrational properties are responsible for several interesting properties in graphene such as high thermal conductivities and optical properties. Since graphene is composed of a light carbon atom, where the in-plane bonding is very strong, graphene exhibits a very high

Fig. 1. Diagram showing the graphene lattice unit cell. Atoms from different sub lattices (A) and (B) are marked by different colors. Reprinted with permission from [41], M. I. Katsnelson, Materails Today 10, 20 (2007). © 2007, Elsevier. Rev. Adv. Sci. Eng., 2, 1–21, 2013



Fig. 2. Electronic band structure of single-layer graphene. Reprinted with permission from [41], M. I. Katsnelson, Materails Today 10, 20 (2007). © 2007, Elsevier.

sound velocity. This large sound velocity is responsible for the very high thermal conductivity of graphene that is useful for many applications. Moreover, vibrational properties are instrumental in understanding other graphene attributes, including optical properties via phonon–photon scattering (e.g., in Raman scattering) and electronic properties via electron-phonon scattering. The vibrational properties of graphene can be understood with the help of the phonon dispersion relation.43 Raman spectroscopy has been used to characterize graphene and several review papers have been published on discussing the optical phonon spectrum and Raman spectrum of graphene.44 Generally, Graphene produces two strong optical peaks in Raman spectra, the G and the 2D band. The G band is due to individual bonds stretching and compressing, while the 2D band is due to breathing modes of the hexagonal rings of carbon atoms. They occur at 1580 and 1360 cm−1 respectively.45 46 Peaks can also be observed at twice those values due to the next harmonic mode of the oscillation. The band position can also be effected even by small amounts of strain present on the sample, doping and temperature whereas the intensity is less susceptible to these factors. The Raman spectra of graphene includes the G peak located at ∼ 1580 cm−1 and 2D peak at ∼ 2700 cm−1 , caused by the in-plane optical vibration (degenerate zone center E2g mode) and secondorder zone boundary phonons, respectively. The D peak, located at ∼ 1350 cm−1 due to first-order zone boundary phonons, is absent from defect-free graphene, but exists in defected graphene. It was proposed that Raman could be used to distinguish the quality of graphene and to determine the number of layers for m-layer graphene (for m up to 5) by the shape, width, and position of the 2D peak.47 48 2.3. Mechanical Properties The mechanical properties of graphene with the Young’s modulus and fracture strength have been investigated by various groups.49 The Young’s modulus of few layer graphene was experimentally investigated with forcedisplacement measurements by atomic force microscopy (AFM).50 The mechanical behavior of graphene layers Rev. Adv. Sci. Eng., 2, 1–21, 2013

can be described macroscopically by continuum elasticity theory. In 2008, Lee et al. employed nano-indentation in an atomic force microscope to measure the mechanical properties of graphene.51 They found that the Young’s modulus of graphene to be 1.0 ± 0.1 TPa (by assuming the thickness as 0.335 nm) and its breaking strength to be approximately 40 N/m. The elastic modulus of monolayer graphene sheet performed via chemical reduction in graphene oxide has been determined to be about 0.25 TPa by indentation of an atomic force microscopy tip at the center of a suspended graphene sheet. The elastic modulus and the intrinsic strength of defect free monolayer graphene sheet were measured to be 1.0 TPa and 130 GPa, respectively.52 Macroscopic graphene membranes have demonstrated extraordinary stiffness. The superior mechanical and thermal properties of monolayer graphene sheets render them as promising candidates for electrochemical resonators, chemical sensorsand enhancing fillers for composite materials. Nanoelectrochemical resonators fabricated from graphene nanosheets exhibit vibrations with fundamental resonant frequencies in megahertz range.53 2.4. Optical and Electrical Properties of Graphene The transmittance (T ) of graphene can be derived by applying Fresnel equations, T = 1 + 05n−2 ≈ 1 − n ≈ 977%. Where is fine structure constant. The absorbance of single layer graphene can be calculated as A = 1 − T = = 23% and reflect 0.1% of the incident light in the visible region.54 The optical absorption of graphene layers varies linearly proportional to the number of layers and each layer absorb 2.3%54 55 in the visible region as shown in Figure 3.

Fig. 3. (a) Photograph of graphene in transmitted light. This one-atomthick crystal can be seen with the naked eye because it absorbs approximately 2.3% of white light. (b) Transmittance spectrum of single-layer graphene (open circles). Slightly lower transmittance for 1 < 500 nm is probably due to hydrocarbon contamination. The red line is the transmittance expected for two-dimensional Dirac fermions, whereas the green curve takes into account a non-linearity and triangular warping of graphene electronic spectrum. The gray area indicates the standard error for our measurements. (Inset) Transmittance of white light as a function of the number of graphene layers (squares). The dashed lines correspond to an intensity reduction by pa with each added layer. Reprinted with permission from [54], R. R. Nair, et al., Science 320, 1308 (2008). © 2008, American Association for the Advancement of Science.

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Theoretically, the using the value of fine structure constant, absorbance of graphene has been calculated and it is in excellent agreement with the experimentally value of mechanically exfoliated graphene, found to be 97.7%.56 The high quality graphene synthesized by CVD method also showed the almost same transparency.57 The transmittance of the graphene films greatly depend on their crystal quality. The transparency of thermally reduced graphene oxide is about < 99.0%57 and cross-linked graphene from aromatic molecules is about < 99.5%.58 The higher transparency of these graphene may be due to the presence of defects. As we discussed above, the sheet resistance (Rs is very important factor for electrode. The sheet resistance of graphene films strongly depends on their surface morphology and crystal quality, the graphene fabricated by different approaches have different sheet resistance. For example, graphene film made by liquid exfoliated graphene as the precursor showed the lower sheet resistance in compare to graphene film made by cross-linking of carbon-rich molecules.60 61 However these graphene still have very high sheet resistance in comparison to ITO. Nowadays, CVD method is the most promising way to obtain large area and highly conductive graphene films on metals substrate.54 62 63 This value is close to the ITO. The conductivity of graphene can be efficiently increase by doping and reduces the Rs of graphene films. The doping of organic molecules can tuned the density of electrons and holes in graphene film and it can be easily distinguished by transport and Raman measurements.64 However, these doping should be such that which does not affect transparency and maintain charge carrier mobility. The sheet resistance of graphene decreases with increasing with the number of layers. Film thickness dependence sheet resistance for different transparent conductors is shown in Figure 4.65–74


3. SYNTHESIS OF GRAPHENE Graphene was synthesized in 2004 by Geim and Novoselov using the Scotch tape method, there have been many methods developed to produce mono-, bi- and trilayergraphene. However, this method is not appropriate for the large-scale production of graphene necessary to fulfill the requirements in different applications. In order to overcome this shortcoming, several synthesis methods for graphene have been reported in the literature. Table I shows a summary of some of the most important synthesis methods. The typical number of graphene layers produced as well as currently achievable dimensions is given. 3.1. Mechanical Exfoliation Mechanical exfoliation is probably the most common and well-known technique to attain single and few layer graphene on desired substrates. This method produces graphene from natural graphite by repeated peeling/ exfoliation. It is the cheapest method to produce high quality graphene. Ruoff and co-workers in late 90’s tried for the isolating thin graphitic flakes on SiO2 substrates by mechanical rubbing of patterned islands on HOPG.80 Using a similar method this was later achieved in 2005 by Kim and co-workers and the electrical properties were reported.81 The first report of isolating graphene onto insulating SiO2 substrate by mechanical exfoliation was made by Geim and co-worker in 2004.5 In the process reported by Geimand Novoselovat ManchesterUniversity, natural graphite is placed on the sticky side of common adhesive tape, the tape is pressed on a chosen substrate and then striped away. Flakes of graphene with the literal dimension of about ∼ 10–50 m are left on the substrate. Figure 5 shows optical and atomic force miscopy images of the prepared graphene films, single-layer to multi-layer graphene by mechanical exfoliation method.5 The fragments of graphene can be recognized by an optical microscope due to thin film interference, appearing as a region of slight discoloration. The graphene produced by Table I. Comparison of graphene synthesis methods, transparency (Tr), sheet resistance (Rs ). Ref. [73] 74

75

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Fig. 4. Film thickness dependence of sheet resistance for different transparent conductors. Reprinted with permission from [72], C. Yang, et al., J. Mater. Chem. A 1, 770 (2013). © 2013, The Royal Society of Chemistry.

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Synthesis methods PECVD growth HOPG (monolayer) Reduced graphite-oxide Reduced graphite-oxide CVD graphene AAO membrane filter-assisted thermal reduction and transferring methods CVD graphene

Transparency (%)

Sheet resistance (/sq)

91.9 97.7 65 90 80 80

198 29.4 1800 1900 367 850

90

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Fig. 5. Optical image of a large multilayer graphene flake with thickness ∼ 3 nm on top of an oxidized SiO2 wafer (B) AFM image of 2 × 2 m area of this flake near its edge. Colors: dark brown, SiO2 surface (C) AFM image of single layer graphene. Reprinted with permission from [5], I. W. Frank, et al., J. Vac. Sci. Technol. B25, 2558 (2007). © 2007, American Vacuum Society.

the scotch tape method is pure and clean. The exfoliation can be carried out by using multiple methods i.e., ultra sonication,82 electric field83 and even by transfer printing technique84 85 etc. In some other ways natural graphite has also been glued to the substrate either by regular adhesives like epoxy resin86 87 or even by SAMs to get better yield of single and few layer graphene flakes. Since then numerous other techniques were also tried to establish to produce thin graphitic films and few layer graphene from natural graphite or HOPG. Despite the wide use of the micromechanical cleavage, the identification and counting of graphene layers is a major problem. Graphene mono-layers are a marginal surrounded by accompanying thicker flakes. They cannot be seen in an optical microscope on most substrates. Graphene nanosheets obtained by mechanical exfoliation methods are usually characterized by optical microscopy, Raman spectroscopy and AFM. AFM analysis is carried out to verify its thickness and number of graphene layers. However, finding a single layer graphene sheet is a matter of chance and the yield of single and few layer graphene obtained by this method is very poor and the graphene sheets are randomly dispersed on the substrate. The lateral dimension of thus obtained graphene sheets is very small and ranging from a few microns to a couple of millimetres in size. For graphene to be used as an electrode on a solar cell it must cover the entire surface area of the cell, which is much larger than the area of a single fake. Thus this method is practicable only on a laboratory scale and is not scalable to a commercial level. 3.2. Chemical Vapor Deposition Chemical Vapor Deposition (CVD) has become one of the most promising techniques for making thin and Rev. Adv. Sci. Eng., 2, 1–21, 2013

continuous films with precise thickness control in microelectronics. Even though mechanical exfoliation produces highly pure graphene with nearly ideal electrical and mechanical functionalities but still disadvantage. Exfoliated graphene flakes are small, only few microns in size and randomly scattered on a substrate, and most substrate remain uncovered.But for various applications e.g., transparent conducting electrodes for an organic solar cell, graphene film covering whole substrate is highly needed. To synthesize large area graphene films, chemical methods are highly needed. To fulfill these requirement chemical growth of graphene includes reduced graphene oxides, plasma-enhanced CVD, and chemical vapor deposition. The most promising, inexpensive approach for deposition of reasonably high quality graphene is chemical vapor deposition onto metal substrates such Ni, Pd, Ru, and Cu.88 In addition, graphene deposited on copper foil using chemical vapor deposition is a very popular method to prepare samples. Recently, Singh et al. have demonstrated the synthesis of graphene by hot filament thermal chemical vapor deposition on copper substrates.89 Kim et al. have prepared graphene films on a Ni substrate to use as transparent electrodes in flexible, stretchable, and foldable electronics.63 CVD growth of graphene has been mainly practiced on copper90 91 and nickel92–94 substrates. It is interesting to know that the growth mechanism on each of these metals is different. The first attempt to grow the large area graphene by CVD method was made by using Nickel substrate.56 92 93 The reason behind choosing Ni substrate for CVD process was the lattice mismatch of graphene is minimum along Ni(111) favouring the growth of graphene. These reports instantaneously stimulated amazing interests to produce graphene by CVD method. Afterward there were several reports on growth mechanisms and augmenting growth conditions and parameters. Studies on the effect of cooling rates on graphene thickness and quality have also been carried out and well documented.95 According to the reports, high cooling rates (100 C/s) vanishes the growth of graphene because all carbon gets trapped within the nickel lattice. On the other hand, less cooling rates tends to formation of thick graphitic films. It was suggested that cooling rate of ∼ 10–15 C/s is adequate for growth of single and double layer graphene films.92–96 Figure 6 represents scheme of preparation of graphene by chemical vapour deposition method and transfer via polymer support. The carbon solves into the Ni during the CVD and forms graphene on the surface after cooling. With a polymer support the graphene can be stamped onto another substrate, after etchingof the Ni layer. Patterning of the Ni layer allows a control of the shape of the graphene.93 Copper substrate have proven to be more promising, widely reported and well documented to catalyse the growth of carbon in almost all forms such as graphite,97 diamond,98 carbon nanotubes,99 100 and most recently 7


Fig. 6. Schematic presentation for the synthesis of graphene by CVD method on Ni substrate and transfer via polymer support. Reprinted with permission from [93], M. Losurdo, et al., Phys. Chem. Chem. Phys. 13, 20836 (2011). © 2011, The Royal Society of Chemistry.

graphene101 as shown in Figure 7(c). Initially single and multi-layered graphene were produced on different (100), (110), (111), and (210) copper surfaces via carbon grafting at pre-eminent temperatures and was out-diffusion via carbon dissolution precipitation mechanism.102–105 The low reactivity of copper with carbon can be accredited to the fact that copper has a filled 3d-electron shell and the electronic distribution is symmetrical which minimizes

Fig. 7. SEM images of (a) carbon nanotubes. Reprinted with permission from [99], L. Ding, et al., Nano Lett. 9, 800 (2009). © 2009, American Chemical Society; (b) Diamond on Cu (111). Reprinted with permission from [97], L. Constant, et al., Surf. Sci. 387, 28 (1997). © 1997, Elsevier; (c) Photographs of Cu foils covered with fully grown graphene (Cu/fGr), with partially grown graphene (Cu/pGr) and without graphene after annealing in air (160 C, 6 min). Reprinted with permission from [101], C. Jia, et al., Scientific Reports 2, 707 (2012). © 2012, Macmillan Publishers Ltd.

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reciprocal repulsions. As a result, Cu can practise only tolerant bonds with carbon through charge transference from the p electrons in the sp2 hybridized carbon to the empty 4s states of copper.106 Because of this unusual mismatch of very small similarity between carbon and copper accompanied by the capability to form intermediate bonds makes copper a promising choice as a catalyst for graphitic carbon growth. Growth of graphene on copper in principle is forthright, concerning the decay of carbon source over copper substrate at 1000 C.107 Growth of single layer graphene on Cu foil has recently been reported using hexane108 at 950 C to discover the prospect of using liquids precursors that could enable the doping of graphene during synthesis by using nitrogen and boron containing organic solvents.108 The mechanism for graphene growth on copper is different than for nickel. The growth of graphene on copper is mainly a surficial phenomenon basically depends on the concentration of carbon atoms and their vapour pressures. The quality of CVD grown graphene on copper foils by using methane as a precursor was found to be superior quality and helpful optimizing the growth parameters. It has been reported that the use of ambient pressures and low concentrations of methane empowered single layer graphene synthesis.109 Ruoff group brilliantly optimized the growth conditions using a two-step process to grow continuous, large domain size, single layer graphene films with very high motilities (12000–16000 cm2 /Vs).110 Modifying the approach at little higher temperature (1035 C) Ruoff and co-worker have demonstrated the growth of large area (up to 0.5 mm) graphene single crystals on copper foils.111 A Schematic representation of the three main stages of graphene growth on copper foil by chemical vapour deposition is given in Figures 8(A) and (B). Figure 8(B) shows SEM images of as grown graphene on

Fig. 8. (A) Schematic illustrating; Reprinted with permission from [112], A. W. Tsen, et al., Accounts of Chemical Research DOI: 10.1021/ar300190z; © 2012, American Chemical Society; and (B) SEM image the three main stages of graphene growth on copper by CVD; Reprinted with permission from [28], X. Dong, et al., Carbon 50, 1517 (2012). © 2012, Elsevier; (c) Optical image of transferred graphene on SiO2 of 300 nm thick. Reprinted with permission from [115], M. P. Levendorf, et al., Nano Lett. 9, 4479 (2009). © 2009, American Chemical Society. Rev. Adv. Sci. Eng., 2, 1–21, 2013



Cu foil.112 Figure 8(c) shows the image of the transferred graphene on SiO2 substrate. Figure 9, shows scanning electron microscopy (SEM) images of single layer graphene on copper at different times.113 Figure 9(a) shows graphene of finite size (one is indicated by the larger oval) in the form of dark irregularly shaped flakes. The nucleation site of one of the flakes is indicated by the smaller oval in Figure 9(a). As the growth time is increased, the graphene domains progressively increase in size until coalescing (Fig. 9(b)) into a continuous layer. Figure 9(b) is an image just prior to the formation of a completely homogeneous layer, as indicated by the presence of discontinuities 9(b). It is possible to control the nucleation thickness and the size of the initial graphene film by tuning the pre-treatment conditions, the partial pressure of CH4 and the total growth pressure as indicated by Figure 9(c).114 After the nucleation, growth, and formation of a continuous monolayer, further exposure to the carbon precursor for up to 60 min does not lead to deposition of multi-layered graphene as shown in Figure 9(d). Figure 10 shows Raman map of a single layer graphene grown on bulk Cu substrate transferred onto an insulating substrate. Figure 10(b) shows Raman spectra taken from a graphene film grown on a thin film of copper. Figure 10(c) shows Raman spectrum after subtraction of the background) and Figure 10(d) shows Raman spectra for 1-2-3-layers of graphene after transfer. An enhancement in the D-peak and a change in the G/2D peak ratio can be observed with transfer of additional layers.114 115 Recently, Gannet et al. has been synthesized graphene by chemical vapor deposited on boron nitride substrates. Using a boron nitride under layer, they achieve mobilities

Fig. 9. SEM images of graphene on Cu for different growth times: (a) 1 min; (b) 2.5 min, (c) 1 min; Reprinted with permission from [80], Y. Zhang, et al., Appl. Phys. Lett. 86, 073104 (2005). © 2005, American Institute of Physics; and (d) 10 min, Reprinted with permission from [113], X. Li, et al., Science 324, 1312 (2009). © 2009, American Association for the Advancement of Science; and Reprinted with permission from [114], H. Kim, et al., ACS Nano 6, 3614 (2012). © 2012, American Chemical Society. Rev. Adv. Sci. Eng., 2, 1–21, 2013

Fig. 10. (a) Raman map of a single layer graphene grown on bulk Cu substrate and transferred onto an insulating substrate, (b)–(d) Raman spectra for 1–2–3-layers of graphene; Reprinted with permission from [115], M. P. Levendorf, et al., Nano Lett. 10, 1542 (2010). © 2009 and 2010, American Chemical Society.

as high as 37000 cm2 /Vs, an order of magnitude higher than commonly reported for CVD graphene and better than most exfoliated graphene.117 Dean et al. also synthesized graphene on the h-BN substrate by the mechanical transfer process.118 In one another method reported recently, Tour and coworkers have efficiently synthesizing large area graphene on copper foils using spin coated PMMA films (100 nm) on copper foils by annealing these foils in Ar/H2 atmosphere in the range of 1073 K to 1273 K. They found that the layer thickness may be easily controlled by varying the flow rate of the Ar/H2 . The solid precursors also represent exceptional advantage of doping the graphene with premeditated atoms very much similar to liquid precursors, allowing tuning its electronic properties. Furthermore, modifying their method Tour and co-workers have also demonstrated successful synthesis of N -doped graphene that by spin coating melamine (C3 N6 H6 +PMMA mixture on the copper foils and annealing.119 Significant advancement has been taken place in CVD synthesis of graphene films on copper and nickel foils and has been scaled up to the synthesis up to wafer size production. Thus synthesized graphene have been successfully transferred to arbitrary substrates to be used as transparent conducting films and flexible electronics and strain sensors.120 Thus CVD synthesis mechanism of graphene on different metal surfaces have proven itself to be one of the most auspicious techniques for producing large scale graphene for electronics and other applications. 3.3. Plasma Enhanced Chemical Vapor Deposition The plasma improvement in CVD first emerged in microelectronics because certain methods cannot tolerate the high substrate temperatures of the thermal CVD. Most substrates cannot resist this high temperature process. Plasma CVD (PCVD) has advantages to achieve deposition at 9



Fig. 11. Graphene synthesized by radio frequency (RF) plasma enhanced chemical vapor deposition (PE-CVD) method, (a) SEM image of graphene directly grown on the curved surface of a Ni wire of a TEM grid, (b) SEM of an enlarged nano-sheets edge with a thickness less than 1 nm, (c) HRTEM image of a single nano-sheet with two graphene layers, Reprinted with permission from [126], M. Zhu, et al., Carbon 45, 2229 (2007). © 2007, Elsevier B.V.

low temperature by use of reactive species generated in the plasma, and has already been used for the lowtemperature growth of carbon nanotubes (CNTs) with aligned CNTs.121–125 The low temperature operation is possible because the precursor dissociation (necessary for the deposition of all common semiconductor, metal and insulator films) is enabled by the high energy electrons in otherwise cold plasma. PECVD as a simple, lowcost, and scalable approach to fabricate excellent multifunctional electrodes has very promising potential for future applications. Simplicity of the process immediately attracted attention of the scientific community and the same kind of process was followed by many research groups. In one of these publications, Zhu et al. have proposed a growth mechanism for the graphene in PECVD chamber.126 According to their scheme, atomically thin graphene sheets are synthesized by a balance between deposition through surface diffusion of C-bearing growth species from precursor gas and etching caused by atomic

hydrogen. The verticality of the graphene sheets, produced through this method, is caused by the plasma electric field direction. Yuan et al. have synthesized high quality graphene sheets, 1 to 3 layers thick, on stainless steel substrate at 500 C, by microwave PECVD.127 The process used a gas mixture of CH4 and H2 (1:9 ratio, at a total pressure of 30 Torr and 200 sccm flow rate) and microwave power of 1200 W. Graphene, produced in this method, was found to show better crystallinity, than any other method. Figures 11 and 12 show SEM and HRTEM images of graphene sheets produced by PECVD technique.126 127 Nandamuri et al. reported successful synthesis of singlelayer graphene by eliminating this electric field on the surface by remote-plasma enhanced CVD.128 Since then, various methods of plasma excitation, such as radiofrequency (RF) discharge ,and microwave (MW) plasma are employed in the PECVD process to achieve largearea graphene growth at reduced substrate temperatures (∼ 650 C).129–132 All of these processes include hydrogen

Fig. 12. Graphene films formed by microwave plasma enhanced chemical vapor deposition technique, (a) SEM and (b) TEM image of graphene on Cu grid, (c) SAD pattern of the graphene sheet, showing sharp and clear diffraction, Reprinted with permission from [127], G. D. Yuan, et al., Chem. Phys. Lett. 467, 361 (2009). © 2009, Elsevier B.V.

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gas in the mixture for the synthesis of graphene, but the roles of plasma and hydrogen in the synthesis of PECVD graphene have not been discussed in detail. Recently, Kim et al. report H2 -free synthesis of high-quality graphene on Cu foil by plasma enhanced chemical vapor deposition.133 They demonstrated that methane alone can provide sufficient amount of hydrogen species for single-layer graphene synthesis on Cu by PECVD. The amount of hydrogen species dissociated from methane can be controlled by the plasma power and affects the nucleation and the grain size of graphene. Plasma enhanced chemical vapor deposition method has shown the versatility of synthesizing graphene on metal and insulating substrates, thus expanding its field of applications. Future developments of this technique should bring out better control over the thickness of the graphene layers and large area production. 3.4. Thermal Decomposition of SiC for Graphene Synthesis Epitaxial graphene is a commonly used technique for creating high quality monolayer graphene. From electronic applications point of view, transfer of the as-grown graphene from metallic surfaces onto anticipated insulating substrates by numerous methods has been performed. The upfront technique for transporting graphene grown on metals is to chemically engrave the metal substrate to obtain free standing graphene membranes that can be lifted onto desired substrates. Chemical etching of substrates like Ni and Cu are viable but is puzzling for metals i.e., Ru, Ir, Pd, Pt. To overcome these difficulties growth of graphene on insulating substrates such as glass or SiO2 /Si wafers or on plastic foils prominently required. Graphene produced by epitaxial method on non-conducting SiC substrate is also viable by sublimation of silicon atoms and graphitization of residual C atoms through annealing at high temperature (1000–1600 C).134 135 Thus grown graphene on SiC (0001) has proven to unveil high mobility, particularly multilayered films. In recent times monolayer SiC transformed graphene over a large area has been reported and exposed to demonstrate exceptional electrical properties.136 Thermal decomposition of Si on the (0001) surface plane of single crystal of 6H-SiC is one of the most popular techniques for graphene growth. In this process controlled (one to many layers) graphene sheets were initiated to be formed when H2 -etched surface of 6H-SiC and was heated at the temperature range 1250–1450 C, for few minutes (1 to 20 min).137 The numbers of layers are dependent on the decomposition temperature.138 By using a similar mechanism, Rollings et al. have synthesized graphene films of the thickness of one atom.139 Presently this procedure has attracted considerable attention from the semiconductor industries, as this technique may be a viable method in post-CMOS age digital electronics, Rev. Adv. Sci. Eng., 2, 1–21, 2013

Fig. 13. SEM images of the graphene sheets epitaxially grown on SiC(0001) surface (a), (b) Images of mono-layer graphene, (c) early growth, (d) magnified image with wrinkles Reprinted with permission from [139], N. Camara, et al., Nanoscale Res. Lett. 6, 141 (2011). © 2011, Springer Science + Business Media.

(Figs. 13(a)–(d)) shows SEM image of graphene grown epitaxy on SiC surface).139 Recently, Hass et al. have summarised this issue in his review on graphene growth on different faces of SiC and their electronic properties.140 In addition to noteworthy expansion of this technology, mm scale graphene film was synthesized on a Ni coated SiC substrate, at lower temperature (750 C). It is realised that the thermal decomposition of SiC to produce few layer graphene has added improvement to grow graphene film over insulator substrate. Very recently employing a similar process, Emtsev et al. predicted to produce wafersize graphene films and get success to synthesize of largesize, monolayer graphene films at atmospheric pressure.135 In spite of some success, some issues like controlled thickness (number of graphene layers), repeated production of large-area graphene needed to be addressed broadly, well before the process can be implemented to industrial scale. Careful analysis of the available reports on epitaxial graphene on SiC surface has pointed out several other important issues like graphene, grown on SiC(0001) and ¯ were shown to have different structures and SiC(0001) unusual rotational stacking of graphene sheets.141 142 One more important issue to be taken care off is the structure and the electronic properties of the interface layer between graphene and substrate, since it affects the properties of graphene. Due to these undesirable constraints, 11


future research works prerequisite to be focussed to recognize the mechanisms of growth and apply the acquaintance in developing practical devices. 3.5. Chemical Methods During the last 2 decades bottom-up approach for the synthesis of different dimensional e.g., 0 D (quantum dots), 1 D (nanotubes and nanorods), 2 D (graphene sheets) and 3 D architectures of nanostructured materials with tailored high performance functionalities is the key method for realistic applications in optoelectronics, field emitters, sensors, biology and light emitting diodes.143 144 In the recent developments, solution or wet chemical methods for graphene synthesis open up new dimensions at elaborated growth condition.145–147 Up to now, among several solutions approach, the liquid-phase exfoliation method has been found to be the best and most economic for the mass production of graphene. Hernandez et al. have synthesized monolayer graphene by liquid-phase exfoliation of graphite in organic solvents such as N -methyl-pyrrolidone (NMP).148 Analyzing a large number of TEM images and paying handydevotion to the uniformity of the sample, Hernandez et al. reported that the appearance of stable and transparent graphene sheets in the TEM images indicates the presence of monolayer graphene as the final product. This is also observed that the edges of the suspended film always fold back, allowing for a cross-sectional view of the film (Fig. 14). Thus folded graphene sheet is locally parallel to the electron beam.


The carefully observation of thus folded edges by high resolution TEM be responsible for an accurate approach to quantify the exact number of layers at different locations on the film. In addition to this conclusive identification of graphene can be well predicted by the careful analysis of small area electron diffraction (SAED) patterns. Hernandez et al. successfully defined the SAED pattern of monolayer and bilayer graphene. Figure 14(e) shows the TEM images of monolayer and bilayer graphene and their corresponding SAED patterns. The SAED pattern of monolayer graphene (Fig. 14(e)) exhibits the typical sixfold graphene symmetry. There are few reports on chemical methods which have also been used to chemically extract graphene from graphite, without the exfoliating.149 150 First Horiuchi et al. presented the possibility of aforesaid route to produce graphene sheets, when they produced carbon nano films (CNF) from natural graphite.151 In their procedure, natural graphite was exposed to a series of oxidation and refinement processes, followed by thinning in methanol and several centrifugation steps to extract the thinnest sheets from the dispersion. The thickness of thus formed carbon nanosheets was found to be directly associated with the dilution factor and the product had 1 to 6 layers of graphene. In another chemical process, sulphuric and nitric acid were introduced between graphite layers, followed by speedy heating to 1000 C, so that explosive evaporation of the acid molecules could produce thin graphitic sheets.152 An entirely different technique of ‘chemical exfoliation’ was used to produce graphene. In this process graphite oxide was thermally expanded by rapid heating it at

Fig. 14. Electron microscopy of graphene (a)–(d) TEM, (e) SADP and (f) HR-TEM images of Graphene. Reprinted with permission from [148], Y. H. V. Nicolosi, et al., Nature Nanotechnology 3, 563 (2008). © 2008, Macmillan Publishers Ltd.

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(a)

(b)

Fig. 15. AFM images of 6H-SiC (0001) after initial surface preparation (a) MBE growth for 60 min at 1040 C (b) and after graphitization for 10 min at 1140 C. Reprinted with permission from [160], E. Moreau, et al., Appl. Phys. Lett. 97, 241907 (2010). © 2010, American Institute of Physics.

1050 C, followed by a two-stage reduction, using hydrogen gas and N -methylpyrolidone.153 In one another attractive studyto synthesize graphene in large volume, sodium and ethanol (1:1 molar ratio, as precursor) was rapidly pyrolyzedto form graphene.154 Graphene was found in the suspended solid, giving a high production yield of 0.1 g graphene per 1 mol of ethanol. It was also established from the study that the number of layers in thus synthesized graphene is dictated by the lateral size and crystallinity of the input graphite and thus, shows a possible way of controlling the number ofgraphene layers.155 3.6. Molecular Beam Epitaxial Growth Molecular beam epitaxial (MBE) is well known and widely used technique for the production of high quality and homogeneous wafer-scale epitaxial layers.156 Hackley et al. claimed graphitic (amorphous) carbon formation, which is not graphene,by e-beam evaporation of carbon rod onto a Si (111) surface.157 Further these experiments focused onthe evaporation of carbon from bulk graphite and C60 . Recently, Park et al. report the direct growth of epitaxial graphene on SiC substrates from C60 and bulk graphite filament carbon sources.158 A graphite lament is loaded into an ultra-high vacuum. As the Filament is heated, carbon atoms sublimate off of the graphite. These carbons form a molecular beam in the vacuum, traveling through free space without interacting until they land on a metallic substrate (such as iridium and rutheniu) and form a graphene layer.159 Moreau et al. have also grown graphene by molecular beam epitaxial on the carbon-face of SiC.160 The AFM images of a graphene sample obtained by 60 min and 10 min graphitization at 1040 and 1140 C, respectively [Fig. 15]. 3.7. Other Methods There are several other methods to produce graphene such as atomic layer deposition,161 electron beam irradiation of PMMA nanofibres,162 arc discharge of graphite,163 pulsed Rev. Adv. Sci. Eng., 2, 1–21, 2013

laser deposition,164 thermal fusion of PAHs,165 conversion of nano-diamond and many other.166–170

4. APPLICATION OF GRAPHENE AS TRANSPARENT ELECTRODE 4.1. Light Emitting Diodes The transparent conductive electrode is an important component of LEDs, through which light couples out of the devices. Recently, organic light-emitting diodes (OLEDs) are a promising electronic display because of their high luminous efficiency, flexibility, cheap and compatibility with a wide variety of substrates. After the introduction of the bilayer organic electroluminescent heterostructure diode by Tang,171 the extensive research efforts over the last two decades have resulted in improvements in the luminous efficiency, lifetime, and color gamut of both small molecular weight and polymer OLEDs.172–174 Most of these devices ITO were used as electrode. However, ITO has many disadvantages as we discussed earlier in this review. Due to the brittle in nature it is not suitable for flexible OLEDs. The high transparency, flexibility, large area and strong mechanical properties of graphene make it a suitable candidate for flexible and large area OLEDs electrode. However, practical application of single layer graphene as the anode of organic optoelectronic devices has been limited because of its high sheet resistance and low work function.175 Wu et al. has used solution processing of functionalized multi layer graphene thin film as transparent conductive anode in OLEDs72 Hwang et al. have also fabricated blue OLEDs with multi layer graphene anode showed outstanding external quantum efficiency of 15.6% and power efficiency of 24.1 lm/W at 1000 cd/m2 .176 More recently, Lee et al. made extremely efficient flexible organic light-emitting diodes using modified graphene anode.175 They have achieved extremely high luminous efficiencies (37.2 lm W−1 in fluorescent OLEDs, 102.7 lm W−1 in phosphorescent OLEDs), which are significantly higher than those of optimized devices 13



Fig. 16. Schematic illustrations graphene based flexible OLEDs. (a) A process of hole-injection from graphene anode to HTL (NPB), (b) process of hole-injection from graphene anode to HTL (NPB) (anode) via a self-organized HIL with work-function gradient (GraHIL) illustration of a holeinjection process from a graphene to the NPB layer, (c) schematic illustration of phosphorescent OLEDs (d) schematic illustration of phosphorescent OLEDs using modified graphene and (e) Schematic illustration of fabrication steps for graphene anode based flexible OLEDs, Reprinted with permission from [175], T.-H. Han, et al., Nature Photonic 6, 105 (2012). © 2012, Macmillan Publishers Ltd.

with an indium tin oxide anode (24.1 lm W−1 in fluorescent OLEDs, 85.6 lm W−1 in phosphorescent OLEDs)175 as shown in Figures 16 and 17. Thus graphene show the great potential of graphene anodes for use in OLEDS. 14

4.2. Touch Screen An electronic visual display that can detect the presence and location of a touch by a finger or other objects Rev. Adv. Sci. Eng., 2, 1–21, 2013



Fig. 17. (e) Current efficiencies as a function of current density of OLED devices made by using 4L-G-HNO3 and ITO anodes. Inset of the figure shows the luminance as a function of voltage in the OLEDs. (f) Luminous efficiencies of phosphorescent OLED devices made by using 4L-G-HNO3 and ITO anodes; Reprinted with permission from [175], T.-H. Han, et al., Nature Photonic 6, 105 (2012). © 2012, Macmillan Publishers Ltd.

within the display area is known as touch screen. There are a variety of touch screen technologies exist, many such as infrared, resistive, surface acoustic, capacitive, surface capacitance and projected capacitance.177 178 The most common touch screen device formats are analog resistive and capacitive, both of which require a transparent conductor. The sheet resistance is important in the overall circuit design and does influence performance aspects such as response time, touch representation accuracy, and activation force. The transparent conductors also need to have very good uniformity, which is strongly influenced and determined by the coated thickness uniformity. The mechanical durability of the transparent conductors is also very important factor for proper operation of resistive touch panels. These conditions are very well satisfied by graphene. The graphene based touch screen first proposed by Bae et al.28 A palm-sized touch screen was made with a CVD grown graphene sheet. The high transparency (∼ 97%), flexibility and no toxicity are very useful for touch screen. Recently, graphene produced by CVD, was reported to reach up to 30-inch in size and roll-toroll production possibility of graphene sheets on a large scale which satisfies the requirements for resistive touch screens.28

have been made. CVD-grown graphene based source–drain electrodes and a single-walled carbon nanotube (SWNT) as semiconducting channel was fabricated on a flexible PET substrate.182 This device showed the on/off ratio of 100 and mobility of 2 cm2 V−1 s−1 and very small variation in mobility after mechanical strains of up to 2.2%. Such devices present high mechanical strength and optical transmittance. One of the merits of graphene electrodes in organic electronics is the low contact resistance between graphene and organic materials. Recently, high-performance organic FETs were prepared with composed of a pentacene active channel, a cross-linked polyvinylphenol (PVP) dielectric layer, a PEDOT:PSS gate electrode and graphene source–drain electrodes.183 This device showed a carrier mobility of 054 ± 004 cm2 V−1 s−1 and an on/off ratio of 107 , which was much higher than the values for devices with Au electrodes (0.02 cm2 V−1 s−1 . The mobility difference between the two materials was explained in terms of the low contact resistance between the pentacene and graphene electrodes. Same device configuration was fabricated on a flexible PET substrate showed an average carrier mobility of 0.12 cm2 V−1 s−1 . The device performance are shown in

4.3. Flexible and Stretchable Electronic Recently, many research groups have reported the largearea high-quality graphene epitaxial growth or chemical vapor deposition (CVD) techniques.28 The large area graphene synthesized on metals substrate (Ni or Cu) can be easily transfer to any of the desire substrates. The CVD approach is attractive because it permits fabrication over large areas and expands the applicability of graphene to flexible or fully stretchable devices on thin plastic or elastomeric substrates61 179–181 as shown in figure. The electrical, optical and mechanical properties of graphene allow great applications in the next generation of optoelectronics. Graphene-based flexible and stretchable transistors Rev. Adv. Sci. Eng., 2, 1–21, 2013

Fig. 18. Graphene-based touch screen panel connected to a computer with controlling software; Reprinted with permission from [28], X. Dong, et al., Carbon 50, 1517 (2012). © 2012, Elsevier.

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Fig. 21. Schematic diagram showing the fabrication steps of ion gel gated graphene transistor array on a flexible plastic substrate. Reprinted with permission from [185], B. J. Kim, et al., Nano Lett. 10, 464 (2010). © 2010, American Chemical Society. Fig. 19. Single layer graphene synthesized by chemical vapor deposition technique transfer onto a variety of substrates, (a) transfer of an ultra-large-area graphene film on a 35 inch polyethylene terephthalate (PET) substrate. (b) Graphene film transferred on a rigid transparent substrate, (c) graphene film on stretchable PDMS substrate and (d) graphene film on a flexible PET substrate. Reprinted with permission from [28], X. Dong, et al., Carbon 50, 1517 (2012). © 2012, Elsevier; and Reprinted with permission from [61], Y. Lee, et al., Nano Lett. 10, 490 (2010). © 2010, American Chemical Society.

It can be prepared by the gelation of a block copolymer (poly(styrene-methyl methacrylate-styrene) (PS-PMMAPS)) in an ionic liquid and at room temperature. The capacitance of PS-PMMAPS film is 5.17 F/cm2 , which is much higher than that of the commonly used 300 nm

Figure 20. The stable operation of devices within the range of tested strain and repeated cycles indicated the graphene can be used as transparent electrodes in electronic devices without any structural deterioration. Graphene based flexible transistors have also been made and some time we need high-k gate dielectric materials like Al2 O3 and HfO2 . However, these dielectric materials are not suitable for use in GFETs fabricated on flexible plastic substrates because they require a high growth temperature.183 So processing at low temperature and good interface properties are required for high performance GFET. These requirements can be well fulfill by using solution-processable ion gel gate dielectric.

(a)

(b)

Fig. 20. (a) The Output characteristics of flexible organic FETs with graphene electrodes and the inset of figure show a schematic diagram of the cross-section of an organic FET. (b) The transfer characteristics of flexible organic FETs and the inset of figure showing the flexibility and transparency of pentacene FETs. Reprinted with permission from [184], W. H. Lee, et al., Adv. Mater. 23, 1752 (2011). © 2011, Wiley-VCH.

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Fig. 22. (a) Optical images of an array of devices on a PET substrate. (b) Transfer and output characteristics of graphene FETs on PET substrate. In output curve, the gate voltage was varied between +2 and −3 V in steps of −1 V and transfer characteristics at drain to source voltage 1 V. (c) Distribution of the hole and electron mobility of graphene FETs on PET substrate. (d) Normalized effective mobility (/o ) as a function of the bending radius. Reprinted with permission from [185], B. J. Kim, et al., Nano Lett. 10, 464 (2010). © 2010, American Chemical Society. Rev. Adv. Sci. Eng., 2, 1–21, 2013



thick SiO2 films at 10 Hz.184 Schematic diagram with fabrication steps used to the ion gel gated graphene transistor array on a plastic substrate is shown in Figure 21. The detail fabrication process is given in Ref. [184]. A photograph of flexible GFET arrays and an optical image of the single devices are shown in Figure 22(a). Figure 22(b) shows the transfer and output characteristics of flexible GFET. The GFETs showed excellent performances with hole and electron mobilities of 203 ± 57 and 91 ± 50 cm2 V−1 s−1 , respectively at a drain bias of −1 V.185 All measurements were done under ambient conditions. It is also noticeable that the low operating voltage is one of the great advantages of ion gel-gated GFETs. There were no significant differences in the mobility and on-current compared to that GFET fabricated on the Si wafer. Figure 22(c) shows the distribution (total ∼ 50 devices) of the electron and hole mobility of GFET arrays on PET. Mechanical flexibility of GFET was also checked. Symmetric bending test was performed to measure the mechanical flexibility of the ion gel-gated GFET arrays. The change in effective carrier mobility, normalized to the value of the graphene FETs under the unbent condition is shown in Figure 22(d). They observed only 20% changes in /o as the bending radius was changed from 6 to 0.6 cm.

5. CONCLUSIONS The remarkable properties of graphene have renewed interest in inorganic, two-dimensional materials with unique optical, mechanical, and electrical properties, making it an emerging material for novel optoelectronics, photonics, and flexible transparent electrode applications. Several other potential properties and applications for graphene are under development, and many more have been proposed. These include lightweight, transistors, memories, molecular junctions, touch screens, LCDs, LEDs, flexible electrode, durable display screens, and solar cells, as well as various medical, chemical, and industrial processes enhanced or enabled by the use of graphene materials. Continued research and development of advanced devices utilizing graphene is critical in furthering transparent nanoelectronics beyond existing capabilities. We expect that transparent graphene electrodes can be replaced indium tin oxide in the near future. Acknowledgments: This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012R1A1A2007211). This work was also supported by the Priority Research Centers Program (2012-0005859) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology and by the Technology Innovation Program (industrial strategic technology development program, 10035430, Rev. Adv. Sci. Eng., 2, 1–21, 2013

development of reliable fine-pitch metallization technologies) funded by the Ministry of Knowledge Economy (MKE, Korea).

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