Synthesis of Graphene and Its Applications: A Review Wonbong Choi a ; Indranil Lahiri a ; Raghunandan Seelaboyina a ; Yong Soo Kang b a

This article was downloaded by: [Choi, Won Bong] On: 5 February 2010 Access details: Access Details: [subscription number 919112994] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Critical Reviews in Solid State and Materials Sciences Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t713610945 Synthesis of Graphene and Its Applications: A Review Wonbong Choi a ; Indranil Lahiri a ; Raghunandan Seelaboyina a ; Yong Soo Kang b a Nanomaterials and Devices Laboratory, Florida International University, Miami, Florida, USA b Department of Energy Engineering, Hanyang University, Seoul, Korea Online publication date: 05 February 2010 To cite this Article Choi, Wonbong, Lahiri, Indranil, Seelaboyina, Raghunandan and Kang, Yong Soo(2010) 'Synthesis of Graphene and Its Applications: A Review', Critical Reviews in Solid State and Materials Sciences, 35: 1, 52 71 To link to this Article: DOI: 10.1080/10408430903505036 URL: http://dx.doi.org/10.1080/10408430903505036 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

Critical Reviews in Solid State and Materials Sciences, 35:52 71, 2010 Copyright c Taylor and Francis Group, LLC ISSN: 1040-8436 print / 1547-6561 online DOI: 10.1080/10408430903505036 Synthesis of Graphene and Its Applications: A Review Wonbong Choi, 1, Indranil Lahiri, 1 Raghunandan Seelaboyina, 1 and Yong Soo Kang 2 1 Nanomaterials and Devices Laboratory, Florida International University, Miami, Florida, USA 2 Department of Energy Engineering, Hanyang University, Seoul, Korea Graphene, one-atom-thick planar sheet of carbon atoms densely packed in a honeycomb crystal lattice, has grabbed appreciable attention due to its exceptional electronic and optoelectronic properties. The reported properties and applications of this two-dimensional form of carbon structure have opened up new opportunities for the future devices and systems. Although graphene is known as one of the best electronic materials, synthesizing single sheet of graphene has been less explored. This review article aims to present an overview of the advancement of research in graphene, in the area of synthesis, properties and applications, such as field emission, sensors, electronics, and energy. Wherever applicable, the limitations of present knowledgebase and future research directions have also been highlighted. Keywords electronic properties, growth techniques, sensors, transistors, electrodes, rechargeable battery Table of Contents 1. INTRODUCTION...53 2. PROPERTIES OF GRAPHENE...53 2.1. Properties of Single- and Bi-layer Graphene...53 2.2. Properties of Few-Layer Graphene...55 3. GRAPHENE SYNTHESIS METHODS...56 3.1. Exfoliation and Cleavage...56 3.2. Thermal Chemical Vapor Deposition Techniques...57 3.3. Plasma Enhanced Chemical Vapor Deposition Techniques...58 3.4. Other Processing Routes...58 3.4.1. Chemical Methods...58 3.4.2. Thermal Decomposition of SiC...59 3.4.3. Thermal Decomposition on Other Substrates...60 3.4.4. Un-zipping CNTs and Other Methods...60 4. APPLICATIONS OF GRAPHENE...61 4.1. Graphene Field Emission (FE)...61 4.2. Graphene Based Gas and Bio Sensors...61 4.3. Field Effect Transistors (FET)...64 4.4. Transparent Electrodes...65 4.5. Battery...66 5. SUMMARY...67 REFERENCES...67 E-mail: choiw@fiu.edu 52

SYNTHESIS OF GRAPHENE AND ITS APPLICATIONS 53 1. INTRODUCTION In recent years graphene, a one-atom-thick planar sheet of sp 2 -bonded carbon atoms densely packed in a honeycomb crystal lattice, has grabbed appreciable attention to be used as a next generation electronic material, due to its exceptional properties including high current density, ballistic transport, chemical inertness, high thermal conductivity, optical transmittance and super hydrophobicity at nanometer scale. 1,2 The first graphene was extracted from graphite using a technique called micromechanical cleavage. 3 This approach allowed easy production of high-quality graphene crystallites and further led to enormous experimental activities. 4 Intrinsic graphene is characterized as a semi-metal or zerogap semiconductor and its unique electronic properties produce an unexpectedly high opacity for an atomic monolayer, with a startlingly low absorption ratio of 2.3% of white light. 5 Electrical characterization has shown a remarkably high electron mobility at room temperature, with experimentally reported values in excess of 15,000 cm 2 V 1 s 1. 3 The corresponding resistivity of the graphene sheet would be 10 6 ohm cm, less than the resistivity of silver, the lowest resistivity substance known at room temperature. 6 Graphene nano ribbons (GNRs), with zigzag or armchair configuration, show different electrical property; the zigzag GNRs are metallic, while armchairs can be either metallic or semiconductor. The energy band gap of armchair GNRs are inversely proportional to the width. 6 Exceptional electrical properties of graphene have attracted applications for future electronics such as ballistic transistors, field emitter, components of integrated circuits, transparent conducting electrodes and sensors. Graphene has a high electron (or hole) mobility as well as low Johnson noise (electronic noise generated by the thermal agitation of the charge carriers inside an electrical conductor at equilibrium, which happens regardless of any applied voltage), allowing it to be utilized as the channel in a field effect transistor (FET). Combination of excellent electrical property and low noise make graphene an excellent sensor. Its entire volume is exposed to the surrounding due to its 2D structure, making it very efficient to detect adsorbed molecules. The high electrical conductivity and high optical transparency promote graphene as a candidate for transparent conducting electrodes, required for applications in touch-screens, liquid crystal displays, organic photovoltaic cells and organic light-emitting diodes (OLEDs). 6 Most of these interesting applications require growth of single-layer graphene on a suitable substrate, which is very difficult to control and yet to be achieved. Many reports are available on graphene synthesis and most of those are based on mechanical exfoliation from graphite or thermal graphitization of a SiC surface 6,8 11 and recently, by chemical vapor deposition. 12 This article begins with a brief overview of properties of graphene and then reviews synthesis technologies, together with a brief discussion of their feasibility and potential applications including field emission, electronics, sensors and energy. 2. PROPERTIES OF GRAPHENE Keeping in view of the scientific interest generated by graphene and its possible future involvement in electronics and sensing applications, lot of research effort are devoted in understanding the structure and properties of graphene. Detailed discussion of properties of graphene is out of the scope of this article, which may be found in some recent review articles. 13,14 This section aims to introduce the basic properties of graphene, so that the applications are well understood. The following subsections will discuss, in short, attractive properties of single-, bi- and few-layer graphene. 2.1. Properties of Single- and Bi-layer Graphene Before introducing the properties of graphene, it is imperative to understand the structure of graphene. A single-layer graphene is defined as a single two-dimensional hexagonal sheet of carbon atoms (Figure 1a). 15 Bi-layer and few-layer graphene has 2 and 3 to 10 layers of such two-dimensional sheets, respectively. Graphene structures consisting more than 10 such layers are considered as thick graphene sheet and are of less scientific interest. In bi- and few-layer graphene, C atoms can be stacked in different ways, generating hexagonal or AA stacking, Bernal or AB stacking and rhombohedral or ABC stacking (Figure 1b). Graphene has a hybridized sp 2 bonding. It shows three in-plane σ bonds/atom and π orbitals perpendicular to the plane (Figure 1c). While the strong σ bonds work as the rigid backbone of the hexagonal structure, the out-ofplane π bonds control interaction between different graphene layers. The simplest way to differentiate between different thicknesses of graphene is through optical microscopy on Si substrate with a 285 nm SiO 2 capping layer, by using contrast spectra. 16 The best way to understand the structure of graphene is by TEM. However, TEM, being a destructive analytical tool, may not be suitable always. Careful investigation of local Raman spectra, obtained from different portions of graphene can give an idea about the thickness of the graphene (Figure 2). Number of layers in a graphene film can be estimated from the intensity, shape and position of the G and 2D bands. While 2D-band changes its shape, width and position with increasing number of layers, G-band peak position shows a down-shift with number of layers (Figure 2b). Single-layer graphene (SLG) is unique in electronic structure, as it shows band-overlap in two conical points (K and K ) in the Brillouin zone (Figure 3). The charge carriers in this structure, known as mass-less Dirac fermions, are electrons losing their rest mass, m 0 and can best be described by (2 + 1)-dimensional Dirac equations. 13,14,18 Thus, SLG is expected to show some unusual properties, as compared with metals and semiconductors and typical of a semi-metal. SLG shows room-temperature ambipolar characteristics, i.e., the charge carriers can be alternated between holes and electrons depending upon the nature of the gate voltage. 3

54 W. CHOI ET AL. FIG. 1. (a) Graphene structure of single two-dimensional hexagonal sheet of carbon atoms, (b) three most common structures and stacking sequences of graphene and (c) Schematic of the in-plane σ bonds and the π orbitals perpendicular to the plane of the graphene sheets (b and c reprinted with permission from Hass et al. 15 Copyright 2008: IOP). Anomalous (half-integer) quantum Hall effect (QHE), at low temperature and room temperature has also been reported for this structure. 19,20 These unusual properties of SLG has made it suitable for applications in electronics, as well as one of the most suitable materials for studying basic quantum physics phenomena. Among other properties that have received consider- able interest, the most important is the gas sensing ability. It was found that adsorbed gas molecules modify the local carrier concentration and a subsequent change in the resistance. Using this property of SLG, Schedin and collegoues 21 prepared micron-level gas sensors, which can detect adsorption and desorption of single molecules of gases like CO, H 2 O, NH 3 and NO 2. Molecular sensing capability could be achieved in this material, as graphene is electronically a very good low-noise material. Low-noise electronic structure also helped in preparation of truly two-dimensional nano electromechanical systems (NEMS) from SLG. 22 Single-layer graphene is also being credited as one of the strongest materials. Strength of a defect-free, mono-layer graphene was measured by using nano-indentation technique and also modeled using atomistic simulation method. 23,24 FIG. 2. (a) TEM images of graphene films with different thicknesses, (b) typical Raman spectra obtained from different thickness regions of graphene film (with increasing number of layers from bottom to top). (Reprinted with permission from the Macmillan Publisher Ltd: Nature, 17 Copyright 2009.) FIG. 3. Electronic band structure of single-layer graphene. (Reprinted with permission from Rao et al. 13 Copyright 2009: Royal Society of Chemistry.)

SYNTHESIS OF GRAPHENE AND ITS APPLICATIONS 55 Young s modulus of this structure was predicted to be 1.0 TPa, in both the methods. Using such mechanically strong graphene sheets, Chen et al. have prepared graphene papers, which were found to be bio-compatible also. 25 Such exciting new properties of graphene are expected to open up new frontiers in graphene application. Bi-layer graphene shows a gapless state with parabolic bands touching at K and K, in contrast to conical bands of single-layer graphene. Thus, bi-layer graphene is considered as a gapless semiconductor. In contrast to single-layer graphene, charge carriers in bi-layer graphene have finite mass and called massive Dirac fermions. The structure also shows an anomalous QHE, but different from that of single-layer graphene and as a result, it remains metallic at the neutrality points. 26 However, application of a gate voltage can change the carrier concentration and introduces asymmetry between the two layers. This results in formation of a semiconducting gap and restoration of normal QHE. 27 However, Zhou et al. has shown that an energy band gap of 0.26 ev is produced in graphene, when it is epitaxially grown on SiC substrate. 28 Such a structure, with a finite bandgap, makes graphene more suitable for application in electronics industries. It was found that the band gap decreases with increasing number of layers and approaches zero, as the structure has more than four layers. This substrate-induced band gap opening was proposed to be caused by graphene-substrate interaction and breaking of sublattice symmetry. In a related study, it was claimed that graphene, epitaxially grown on C-terminated surface of 4H-SiC, has a different stacking sequence. 29 As a result, the structure, irrespective of its number of layers (up to 10 layers thick), shows an electronic structure similar to that of single-layer graphene and behaves like SLG. Thus, synthesis methods play an important role in determining the structure and properties of graphene. In an interesting application of gaseous molecule adsorption on epitaxially grown graphene surface, it was shown that a controlled molecular treatment (by gases like H 2 and NO 2 ) can trigger a reversible metal-to-insulator transition in single- and bi-layer graphene. 30,31 Treatment of graphene sheets by atomic hydrogen is known to produce insulating or semiconducting graphane, a two-dimensional hydrocarbon structure. Moreover, single- and bi-layer graphene shows very high transparency for light waves in the range of ultra-violet to infra-red, making it suitable for applications as transparent electrode in solar cells. 32 2.2. Properties of Few-Layer Graphene Analysis of the band structure of few-layer graphene (FLG) shows no gap. The structure becomes increasingly metallic with more number of layers in it. 33 FLG shows very high surface area, almost comparable to that of single-layer graphene. As a result, it shows good gas adsorption property, which is verified for H 2 and CO 2. 34 At 300K and 100 bar, FLG samples were found to uptake up to 3 wt% H 2, which is very high. FLG has also shown good capability to be functionalized by different covalent and non-covalent modifications, in order to solubilize them in various solvents. Amide-functionalized FLG was found to be soluble in organic solvents like carbon tetrachloride (CCl 4 ), dichloromethane (DCM) etc. 35 FLG was found to become water-soluble, after reacting with concentrated H 2 SO 4 and HNO 3. 36 However, all such kind of functionalization through covalent modification was found to affect the electronic structure and thus, properties of FLG. Hence, functionalization through non-covalent modification was necessary, which can be performed effectively by wrapping with polyethylene glycol (PLG), to make it water-soluble, without modifying its electronic structure. 36 Apart from functionalization, decoration with metallic nano-particles also become necessary to make the structure more suitable for usage in electronics, optics and biotechnology related applications. FLG was found to be easily decorated with Pt, Ag, and Au nano-particles, in a single-step chemical process. 13,37 Such decoration enhances its application in opto-electronics. Chemical modification of graphene also leads to change in magnetic properties of graphene, by changing the edge characteristics. 38 Edge-state of graphene has attracted considerable attention in recent years, as it can lead to new magnetic properties of graphene, including ferromagnetism. It was found that the edge-state of FLG can be modified in a number of ways, by varying the type of molecules adsorbed on graphene. Such structural modifications are expected to generate new magnetism based applications of graphene; the most important field being memory devices. The ease with which the structure of graphene could be modified, by functionalization or other chemical treatments, has prompted application in biotechnology related fields. Recently, Varghese et al. have conducted a detailed study to understand the interaction of graphene with DNA nucleobases and nucleosides. 39 The interaction energies were found to be almost similar to that of single wall carbon nanotubes. However, more detailed research efforts need to be concentrated in this area, before commenting on suitability of graphene in bio applications. FLG has also been used effectively as part of composite electrodes in new generation Li-ion batteries due to its ability to take part in electrochemical reactions. 40,41 Graphene showed similar or better kinetics in many electrochemical systems than that of graphite or activated carbon (widely used in this type of applications). Such composite electrodes were found to enhance the performance of the batteries. Application of graphene in energy storage devices, such as batteries and supercapacitors, are very recent; but the success of the reported studies are expected to attract more research efforts in this field. All varieties of graphene, single-, bi-, and few-layer, have found potential applications in fields of electronics, memory, biotechnology, sensor, energy storage devices etc. As synthesis methods control the structure and properties of graphene, a variety of processing techniques are used by researchers, especially for large scale production. The subsequent sections will discuss different synthesis techniques of graphene and its main applications, reported so far.

56 W. CHOI ET AL. 3. GRAPHENE SYNTHESIS METHODS Synthesis of monolayer graphite was tried as early as in 1975, when B. Lang et al. 42 showed formation of mono- and multi-layered graphite by thermal decomposition of carbon on single crystal Pt substrates. However, due to lack of consistency between properties of such sheets, formed on different crystal planes of Pt and failure to identify the beneficial applications of the product, the process was not studied extensively, at that period of time. After a long gap, scattered attempts to produce graphene were reported again from 1999. 43,44 However, Novoselov et al. has been credited for the discovery of graphene in 2004. 3,45 They have first shown repeatable synthesis of graphene through exfoliation. The technique has been and is being followed since then, along with efforts to develop new processing routes for efficient synthesis of large-scale graphene. The synthesis routes of graphene can be broadly categorized into different sections, as delineated below. 3.1. Exfoliation and Cleavage Graphite is stacked layers of many graphene sheets, bonded together by week van der Waals force. Thus, in principle, it is possible to produce graphene from a high purity graphite sheet, if these bonds can be broken. Exfoliation and cleavage use mechanical or chemical energy to break these week bonds and separate out individual graphene sheets. The first attempt in this direction was by Viculis et al., who have used potassium metal to intercalate a pure graphite sheet and then exfoliate it with ethanol to form dispersion of C sheets. 8 During sonication, the exfoliated nano-carbon sheets formed nanoscrolls. TEM analysis showed presence of 40 ± 15 layers in each sheet. Though these carbon nanoscrolls were much thicker than fewlayer graphene, the process showed that, in practice too, it is possible to separate layers of graphene from graphite. The process needed modification to finally produce FLG. It took almost 20 more months to find a solution, when Novoselov et al. reported FLG and even SLG formation by exfoliation. 3 Exfoliation is basically a repeated peeling process. In the study of Novoselov et al., a commercially available highly oriented pyrolytic graphite (HOPG) sheet of 1 mm thickness was subjected to dry etching in oxygen plasma to make many 5 µm deep mesas (of area 0.4 to 4 mm 2 ). This was then put on a photoresist and baked, to stick the mesas to the photoresist. Then, a scotch tape was used to peel off layers from the graphite sheet. Thin flakes, attached to the photoresist, released in acetone and transferred to a Si substrate, were found to have single- to fewlayer graphene sheets. The technique was later used to produce two-dimensional atomic crystals of many other materials, including BN, MoS 2 also. 45 This process of producing graphene sheets was found to be very reliable and easy and thus, attracted the immediate attention of the scientific community. 46 48 In slight variations of the original process, it was shown that large ( 10 µm) and flat graphene flakes can be produced by manipulating the substrate bonding of HOPG on Si substrate and controlled exfoliation. 49 In another approach, mm-sized singleto few-layer graphene was produced by bonding bulk graphite to borosilicate glass followed by exfoliation, to leave single or few layer of graphene on the substrate. 50 Both of these advancements stressed on modifying the bonding with the substrate to generate large-area graphene sheets. These methods show good promise to be scaled up to industrial level production for largesize graphene based electronic devices. In a little different approach of exfoliation in liquid phase, Stankovich et al. proposed to use hydrophobicity of graphite oxide (GO) and exfoliated GO nanosheets by ultrasonication in aqueous suspension and attempted reduction of the films in hydrazine hydrate at 100 C for 24 h. 51,52 The product, however, was not fully reduced and found to have some amount of oxygen left in the structure. Hence, it can be called as partially reduced exfoliated nano graphite oxide sheets. Acting on the pros and cons of this liquid phase exfoliation process, Hernandez et al. came up with a new way of producing single- to few-layer graphene sheets, through dispersion and exfoliation of pure graphite in N-methyl-pyrrolidone. 53 The monolayer yield was 1 wt%, which could be improved up to 12 wt%, with further processing. The success of the process is dictated by the fact that the energy required to exfoliate graphite into singlelayer graphene was countered by the solvent-graphene interaction, with the solvent having similar surface energy as that of graphene. This process of liquid based exfoliation of graphite in organic solvent has the promise of large-scale production of graphene. A similar kind of process was used by Lotya et al. to produce single- to few-layer graphene, by making dispersion of graphite powder in sodium dodecylbenzene sulfonate (SDBS), followed by sonication, to exfoliate the graphite into graphene. 54 Figure 4 presents images of graphene, produced by these methods. There were many such attempts to produce graphene either from graphite or graphite oxide powder and using different solvents. 55 58 The process of exfoliation shows good promise to synthesize large scale graphene and some of the recent modifica- FIG. 4. High resolution transmission electron microscopy (HRTEM) images (bright field) of (A) single-layer 53 and (B) bilayer graphene. 54 (Reproduced with permission from Macmillan Publishers Ltd: Nature Nanotechnol, 53 Copyright 2008, and Lotya et al. 54 Copyright 2009: American Chemical Society.)

SYNTHESIS OF GRAPHENE AND ITS APPLICATIONS 57 tions, through liquid phase exfoliation, have made it possible to produce large-size also. However, the liquid phase exfoliated products suffer from the limitation that the structure has lot of defects, due to oxidation and reduction processes, leading to much poor electrical properties of graphene. The future improvisations need to concentrate on controlling the number of layers and minimizing impurity levels to lead this process into an industrial scale production level. 3.2. Thermal Chemical Vapor Deposition Techniques Synthesis of graphene through thermal chemical vapor deposition (CVD) has been quite new. The first report on planar few layer graphene (PFLG), synthesized by CVD, was found in 2006. 59 In this work, a natural, eco-friendly, low cost precursor, camphor, was used to synthesize graphene on Ni foils. Camphor was first evaporated at 180 C and then pyrolyzed, in another chamber of the CVD furnace, at 700 to 850 C, using argon as the carrier gas. Upon natural cooling to room temperature, few-layer graphene sheets were observed on the Ni foils. Graphene, thus produced, was found to have multiple folds (in HRTEM images) and estimated to have approximately 35 layers of graphene sheets. This study opened up a new processing route for graphene synthesis, though several issues like controlling the number of layers, minimizing the folds etc, were yet to be solved. In another approach, 1 to 2 nm thick graphene sheet was reported to be grown on Ni substrate by thermal CVD, while the same treatment failed to synthesize graphene on Si. 60 The process used a precursor gas mixture of H 2 and CH 4 (92:8 ratio), a total gas pressure of 80 Torr and was activated by DC discharge. Nanometer thick (confirmed by Auger Electron Spectroscopy) graphitic films were found to have atomically smooth micrometer size regions, separated by ridges. While the ridge formation was proposed to be due to difference in thermal expansion coefficients of Ni and graphite, the nucleation process was attributed to heteroepitaxial growth of graphene on Ni(111). Later, Yu et al. have reported three to four layer graphene formation on polycrystalline Ni foils (of 500 µm thickness), through thermal CVD process. 61 A precursor gas mixture of CH 4,H 2 and Ar (0.15:1:2 ratio), at a total flow rate of 315 sccm, was used for the synthesis process, allowing 20 min at 1000 Cforthe synthesis process. HRTEM and Raman spectroscopic analyses confirmed formation of graphene on Ni under moderate cooling rates only, while high and low cooling rates were found to be detrimental to graphene synthesis process. This difference in graphene formation was attributed to solubility of C in Ni and the kinetics of C segregation. Ni has a good solubility for carbon atoms. At slow cooling rate, C atoms get sufficient time to diffuse into bulk Ni and no segregation is found on the surface. At a moderate cooling rate, C atoms segregate and forms graphene, while at a higher rate also C atoms segregate out of Ni, but form a less crystalline, defective graphitic structure. This study gave the necessary input about the growth mechanism of graphene in CVD process. Having understood the mechanism of graphene growth, the next thrust was on growing graphene on a large scale and several research groups concentrated their efforts on this issue. 62 Wang et al. proposed a new method of growing substrate-free few-layered graphene. 63 They used MgO-supported Co catalysts to grow graphene in a ceramic boat, at 1000 C for 30 min, under a gas envelope of CH 4 and Ar (1:4 volume ratio, total 375 ml/min flow rate). Product of reaction was further washed by concentrated HCl to remove MgO and Co, followed by several distilled water wash and drying at 70 C. It was claimed that 500 mg of catalyst powder mixture can produce 50 mg of graphene. SEM, HRTEM and Raman spectroscopic analysis of the structure confirmed presence of rippled graphene sheets, having at least five layers. Though the mechanism of graphene formation by this route is still under investigation, the process showed a new window for large scale production of graphene. Another recent report has shown growth of single to few layer graphene on polycrystalline Ni film of 1-2 cm 2, by thermal CVD. 12 The Ni film (500 nm thick) was evaporated on asio 2 /Si substrate and was annealed in Ar+H 2 atmosphere, at 900 to 1000 C, for 10 to 20 minutes. This annealing step created Ni grains of 5 to 20 µm in size. After CVD at 900 to 1000 C for 5 to 10 minutes, using 5 to 25 sccm CH 4 and 1500 sccm H 2, graphene was found to form on the Ni the size of each graphene being restricted by the Ni grain size. The graphene was later transferred to any substrate, keeping its electrical properties unchanged, thus making them suitable for various electronic applications. Graphene, thus synthesized and transferred onto a glass substrate, has shown 90% optical transmittance. High transmittance and electrical properties, along with the capability to be synthesized on a large size and even on a pre-patterned substrate, may find interesting applications in next generation solar cells. An independent study by de Arco et al. has also reported similar properties from graphene, following identical synthesis approach. 64 In an almost similar approach, Kim et al. have shown growth of graphene on Ni film (300 nm thick), deposited on SiO 2 /Si substrate. 17 It was claimed that the thickness of the Ni film was optimized for best quality of graphene. Graphene synthesis was performed in a CVD chamber, using a gas mixture of CH 4,H 2 and Ar (50:65:200 sccm), at a temperature of 1000 C, followed by a rapid cooling (10 Cs 1 ) in Ar envelope. It was found that the high cooling rate was important to minimize the number of layers and for efficient transfer of these layers onto other substrates. This graphene was later successfully transferred onto flexible, transparent substrate, made up of polydimethylsiloxane (PDMS) without affecting its properties and showing 80% transparency in visible spectrum. It was observed by the authors that application of an aqueous FeCl 3 solution etches the nickel layers in a mild ph value, without causing formation of gaseous products or precipitates. Gaseous products, as is produced by HNO 3 etching, damage the graphene structure. 17 In a very recent development, highly crystalline few-layer graphene was grown directly on 1 cm 2 area of polycrystalline

58 W. CHOI ET AL. FIG. 5. (a) SEM images of as-grown graphene films on thin (300-nm) Ni layers and thick (1-mm) Ni foils (inset) 17 ;(b) HRTEM image of few layered graphene, showing individual graphene sheets with the edges of the graphitized layers. 63 (Reprinted with permission from Macmillan Publishers Ltd: Nature, 17 Copyright 2009, and Wang et al. 63 Copyright 2009: Wiley-VCH Verlag GmbH & Co.) Ni substrates, by carefully controlling the gas ratio, growth time and temperature. 65 Figure 5 shows some SEM images of the graphene synthesized through this route. In further development of the process, graphene was found to be synthesized on a 1 cm 2 area of Cu foil by thermal CVD process. 66 Graphene, thus produced, was of high quality and uniformity. Graphene was also transferred by two different and simple techniques to different substrates, enabling their application in various fields. However, it was found that graphene growth on Cu substrate was self-limiting, probably due to limited solubility of C in Cu. The process was claimed to be a surface-catalyzed process rather than a precipitation process, as has been reported for Ni. 61 The recent achievements in graphene growth by thermal CVD has confirmed reproducibility of good quality graphene on a centimeter scale substrate and successful transfer to many other substrates including Si, glass and PDMS. These developments create new pathways for application of graphene in photovoltaic and flexible electronics. However, in near future, issues like growth of graphene on wafer size substrates, controlling efficiently the number of layers should be solved, for creating more interest in actual applications. 3.3. Plasma Enhanced Chemical Vapor Deposition Techniques Interest in synthesizing graphene through plasma enhanced chemical vapor deposition (PECVD) is contemporary to that of exfoliation. The earliest report, by Obraztsov et al., has proposed a dc discharge PECVD method to produce so called nanostructured graphite-like carbon (NG). 67 The process used Si wafer and Ni, W, Mo and some other metal sheets as substrates and a gas mixture of CH 4 and H 2 (0% to 25% CH 4 ), with a total gas pressure of 10 to 150 Torr. The NG film produced by this process looks thicker at most of the places, except at some twisted portions. The first report on single-to few-layer graphene by PECVD was found in 2004. 68,69 A radio frequency PECVD system was used to synthesize graphene on a variety of substrates (Si, W, Mo, Zr, Ti, Hf, Nb, Ta, Cr, 304 stainless steel, SiO 2,Al 2 O 3 ), without any special surface preparation operation or catalyst deposition. The graphene sheets, produced in a gas mixture of 5 100% CH 4 in H 2 (total pressure 12 Pa), at 900W power and 680 C substrate temperature, was found to have subnanometer thickness and erected from the substrate surface. Simplicity of the process immediately attracted attention of the scientific community and the same kind of process was followed by many research groups, worldwide. 68 72 In one of these publications, Zhu et al. have proposed a growth mechanism for the graphene in PECVD chamber. 71 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. In a slightly modified process, Shang et al. have shown synthesis of multilayer graphene nanoflake films (MGNF) on Si substrates, through microwave PECVD (MW-PECVD). 73 Graphene produced in this method had highly graphitized knife-edge structure, with 2 to 3 nm thickness at the sharp edges. Graphene sheets were roughly vertical to the substrate (Si) and reported to show excellent bio-sensing capability (for dopamine). The report also claimed a very high growth rate of graphene, 1.6 µm min 1, which was 10 times faster than other processes. In a very similar way, Yuan et al. have synthesized high quality graphene sheets, 1 to 3 layers thick, on stainless steel substrate at 500 C, by microwave PECVD. 76 The process used a gas mixture of CH 4 and H 2 (1:9 ratio, at a total pressure of 30 Torr and 200 sccm flow rate) and microwave power of 1200W. Graphene, produced in this method, was found to show better crystallinity, than any other method. Figure 6 and 7 show SEM and HRTEM images of graphene sheets produced by PECVD technique. PECVD method has shown the versatility of synthesizing graphene on any substrate, thus expanding its field of applications. Future developments of this method should bring out better control over the thickness of the graphene layers and large scale production. 3.4. Other Processing Routes 3.4.1. Chemical Methods Apart from these main processing routes of exfoliation and different CVD processes, there have been attempts to produce graphene through many other techniques. One of them is chemical based techniques, a part of which is already covered in section 3.2, as liquid phase exfoliation. Chemical methods have also been used to chemically extract graphene films from graphite, without the exfoliation step. Horiuchi et al. first showed the possibility of this route to produce graphene, when they produced carbon nano films (CNF) from natural graphite. 77 Nat-

SYNTHESIS OF GRAPHENE AND ITS APPLICATIONS 59 FIG. 6. Graphene sheets synthesized through radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD) technique, (a) SEM image of graphene directly grown on the curved surface of a Ni wire of a TEM grid, (b) SEM of an enlarged nanosheets edge with a thickness less than 1 nm, (c) HRTEM image of a single nanosheet with two graphene layers, as indicated by the two parallel fringes. (Reprinted with permission from Zhu et al. 71 Copyright 2004: Elsevier.) ural graphite was subjected to a series of oxidation and purification processes, followed by dilution in methanol and several centrifugation steps to extract the thinnest sheets from the dispersion. The thickness of the CNF was found to be directly related with the dilution factor and the product had 1 to 6 layers of graphene. In a different chemical method, sulphuric and nitric acid were intercalated between the layers of graphite, followed by rapid heating to 1000 C, so that explosive evaporation of the acid molecules could produce thin graphitic sheets. 76 In a second step intercalation, oleum and tetrabutylammonium hydroxide (TBA) were used to produce single- and few-layer graphene, after sonication in a surfactant solution. The choice of intercalants, in this study, ensured minimum damage of sp 2 network in the graphene and thus, a higher quality graphene. In another attractive study, Choucair et al. have reacted sodium and ethanol (1:1 molar ratio) to form a graphene precursor, which was then rapidly pyrolyzed. 79 Graphene was found in the suspended solid, giving a high production yield of 0.1 g graphene per 1 mol of ethanol (Figure 8a). The process shows capability to synthesize graphene in large volume. In a totally different approach, chemical exfoliation was used to produce graphene. 78 Graphite oxide was thermally expanded by rapidly heating it at 1050 C, followed by a two-stage reduction, using hydrogen gas and N-methylpyrolidone. It was also claimed in the study that the number of layers in the graphene is dictated by the lateral size and crystallinity of the input graphite. The study, thus, shows a possible way of controlling the number of graphene layers, which may have high impact in the electrical applications. Detailed information about the chemical methods to produce graphene sheets could be found in a recent review by Park et al. 81 3.4.2. Thermal Decomposition of SiC One of the highly popular techniques of graphene growth is thermal decomposition of Si on the (0001) surface plane of single crystal of 6H-SiC. 80 Graphene sheets were found to be formed when H 2 -etched surface of 6H-SiC was heated to temperatures of 1250 to 1450 C, for a short time (1 to 20 minutes). Graphene, epitaxially grown on this surface, typically had 1 to 3 graphene layers; the number of layers being dependent on the decomposition temperature. In a similar process, Rollings et al. have produced graphene films, as low as one atom thickness. 82 Continued success of this process has attracted attention of the semiconductor industries, as this process may be a viable technique in post-cmos age 9,15,82 84 (Figure 8b shows SEM image of graphene grown epitaxially on SiC surface). J. Hass et al. have presented a comprehensive review on this topic, covering the issues of graphene growth on different faces of SiC and their electronic properties. 15 In a significant development to this technology, continuous films (mm scale) of graphene were synthesized on a Ni thin film coated SiC substrate, at a quite lower FIG. 7. Graphene sheets produced by microwave plasma enhanced chemical vapor deposition technique, (a) SEM and (b) TEM image of graphene on Cu grid, (c) SAD pattern of the sheet, showing sharp and clear diffraction, resembling graphene. (Reprinted with permission from Yuan et al. 76 Copyright 2009: Elsevier.) FIG. 8. (a) TEM image of agglomerated graphene sheets produced by chemical route, 79 (b), (c) SEM images of the graphene sheets epitaxially grown on SiC(0001) surface (annealed for 4 h in vacuum at 1900 C); fracture surface in (c) shows that bright lines in (b) are wrinkles in graphite. 84 (Reprinted with permission from Macmillan Publishers Ltd: Nature Nanotechnol, 79 Copyright 2009, and Cambaz et al. 84 Copyright 2008: Elsevier.)

60 W. CHOI ET AL. temperature (750 C). 85 This process has the added advantage of continuity of graphene film over the entire Ni-coated surface. Large area production of graphene makes this route favorable for industrial application. In a similar process, Emtsev et al. have shown synthesis of large-size, monolayer graphene films at atmospheric pressure. 86 The method was predicted to produce wafer-size graphene films. The process of growing graphene on SiC looks attractive, especially for the semiconductor industries. However, issues like controlling thickness of the graphene layers, repeated production of large-area graphene have to be solved, before the process can be adopted at an industrial scale. Analysis of the available research works on epitaxial growth of graphene film on SiC surface has pointed out several other important issues. Graphene, grown on SiC(0001) and SiC(000 1) was found to have different structures. Unusual rotational stacking observed in multilayer graphene (up to 60 layers thickness) grown on the SiC(0001) surface could probably explain their behavior like isolated graphene sheets. Such kind of unusual behavior could not be observed in graphene grown on SiC(0001). Future research works need to be directed to understand the mechanisms of both the growth processes and apply the knowledge in developing practical devices. Another important issue is structure and electronic properties of the interface layer between graphene and substrate, since it is known to affect the properties of graphene. Till now, the effect of interface is not well understood and future research should be concentrated in understanding this issue. With the knowledge about growth mechanism and interface effects and the ability to effectively control the number of layers, this method is set to be used industrially to produce wafer scale graphene. 3.4.3. Thermal Decomposition on Other Substrates In a similar kind of approach, graphene monolayers were grown on single crystal Ru(0001) surface at ultra-high vacuum condition (4 10 11 Torr). 87 Before the synthesis process, Ru crystal was cleaned by repeated cycles of Ar + sputtering/annealing and exposure to oxygen and heating to high temperature. Graphene was found to be formed on the crystal surface, either by thermal decomposition of ethylene (preadsorbed on crystal surface at room temperature) at 1000K or by controlled segregation of carbon from bulk of the substrate. The single-layered graphene was of high purity, covered large space (more than several microns) and periodically rippled. A similar effort by Sutter et al., by thermal decomposition of preabsorbed C atoms on Ru(0001) surface, lead to formation of macroscopic (more than 200 µm) single-crystalline domains of single- to few-layer graphene. 88 The method was proposed for graphene synthesis on other transition metals also, enabling them for wide usage in electronics, catalysts or sensing applications. Following the trend, graphene was synthesized on many other single crystal transition metal surfaces, including Ir, Ni, Co, Pt, etc. A comprehensive understanding of all such efforts can be obtained from a topical review by Wintterlin et al. 89 FIG. 9. (a) TEM image of partially unzipped MWCNT structure, opened by Li intercalation, 90 (b) high magnification TEM image of graphene sheet produced by multistep oxidationreduction treatment; the inset is the SAED pattern, which confirms the crystalline nature of the graphene sheet. 93 (Reprinted with permission from Cano-Márquez et al. 90 Copyright 2009: American Chemical Society, and Kim et al. 93 Copyright 2009: Elsevier.) 3.4.4. Un-zipping CNTs and Other Methods A very recent method of graphene synthesis has used multiwall carbon nanotubes (MWNT) as the starting material. The process is popularly known as un-zipping of CNTs. Three groups, working independently on this issue, reported their findings in almost same time period. In the earliest of them, it was claimed that MWNTs can be opened up longitudinally by using intercalation of Li and ammonia, followed by exfoliation in acid and abrupt heating 90 (Figure 9a). The product, among nanoribbons and partially opened MWNTs, also contained graphene flakes. In another study, graphene nanoribbons were produced by plasma etching of MWNTs, partially embedded in a polymer film. 91 The etching treatment basically opened up the MWNTs to form graphene. In a different approach, MWNTs were unzipped by a multi-step chemical treatment, including exfoliation by concentrated H 2 SO 4,KMnO 4 and H 2 O 2, stepwise oxidation using KMnO 4 and finally reduction in NH 4 OH and hydrazine monohydrate (N 2 H 4.H 2 O) solution. 92 This new process route of unzipping MWNTs to produce graphene creates possibilities of synthesizing graphene in a substrate-free manner. In another recent approach of producing multi-layered graphene sheet, aluminum sulfide (Al 2 S 3 ) was calcined under a(co+ar) gaseous environment 93 (Figure 9b). CO was found to be reduced by Al 2 S 3 to form gaseous carbon and α-alumina. Graphene sheets were later crystallized on the alumina particles. Though the mechanism of this transformation is not yet well understood, the process seems to be very interesting due to its simplicity. In future developments, if the process parameters can be controlled efficiently to minimize and effectively control the number of graphene layers, then bulk production of graphene can get a big jump. Highly oriented pyrolytic graphite (HOPG) has been cleaved by using a micro-cantilever (such as an AFM tip) to form very thin graphitic sheets. 94 The product was 10 to 100 nm thick

SYNTHESIS OF GRAPHENE AND ITS APPLICATIONS 61 and hence can not be treated as graphene. However, the effort has shown a new possibility of producing graphene sheets. In another recent approach, conducting nano carbon films (thickness 1 nm) and membranes were produced through a complex processing route based on molecular self-assembly, electron irradiation and pyrolysis. 95,96 Though these processes are not simple enough to be adopted industrially, future research efforts should be aimed to understand the mechanism of thin carbon film growth through these techniques and proposing possible process modifications to synthesize graphene. Most of the graphene synthesis methods, available to date, have been discussed in this section. The techniques vary in a wide range, from mechanical exfoliation and chemical exfoliation to fully chemical based oxidation/reduction methods, epitaxial growth on SiC and on many transition metal single crystals and CVD/PECVD. Since the discovery of graphene in 2004, the last few years have seen huge progress in terms of graphene growth, controlling number of layers in the product, synthesizing larger samples and producing in bulk volume. The future research efforts on graphene synthesis should be directed towards wafer-scale graphene growth, with pre-determined control over number of layers, to bring a new revolution in the electronics and semiconductor industries. 4. APPLICATIONS OF GRAPHENE 4.1. Graphene Field Emission (FE) One of the potential applications of graphene is in field emission (FE) displays. FE is an electron emission process in which electrons are emitted from a material under the application of high electric field. The simplest way to create such a field is by field enhancement at the tip of a sharp object. To take advantage of high field enhancement, graphene sheets, i.e., single or few layers, need to be erected on the substrates. Except for graphene synthesis by MW-PECVD method, 97 almost all other methods results in flat graphene layers on substrates. 2 Eda et al. have recently fabricated a graphene/polymer composite thin film for achieving a field enhancing structure, required for FE applications. 98 The FE cathodes by Eda et al. were prepared from graphene, synthesized from graphite oxide (GO) dissolved in polystyrene by spin coating it on to silicon substrates. 98 The detailed process of graphene/polymer composite thin film synthesis could be found in Reference 98. The orientation of the graphene sheets in the composite thin films was varied by controlling the spin coating speeds. Relatively better FE was observed from films prepared at spin coating speed of 600 rpm; the turn-on electric field (E to ) in such sample was 4 V/µm and the field enhancement factor (β) was 1200. In another work by Wu et al. 99 single layer graphene film was prepared by electrophoretic deposition (EPD) method. Graphene films, prepared by exfoliation of graphite, were dispersed in isoprophyl alcohol and the resulting solution was deposited onto indium tin oxide (ITO) coated glass substrates by EPD (Figure 10a). Graphene cathodes prepared by this method demonstrated an E to of 2.3 V/µm and a β of 3700. Although these research works 98,99 have demonstrated methods to prepare graphene films for FE applications on flexible and other substrates, these methods may not be suitable to achieve high FE currents, in the order of few ma-a, required for high current applications. This issue may probably be addressed by MW-PECVD method demonstrated by Malesevic et al., 97 to fabricate vertically aligned few layer graphene (FLG) FE cathodes on titanium and silicon (Figure 10b). FLG was synthesized by MW-PECVD with H 2 and CH 4 precursor gases, at 700 C. The quality of FLG films was observed to be dependent on the ratio of H 2 /CH 4 gases; best quality was achieved when the ratio was 8:1. Graphene cathodes prepared by this method demonstrated an E to of 1 V/µm, β of 7500 and a current density of 14 ma/cm 2 (Figure 10c). The advantage of this method is direct synthesis of graphene on metallic substrates creating ohmic contact, which is essential for FE applications. Moreover, no further processing is required. The drawback of this method is the limited scope to control the FLG density, which can cause field-screening effect. Works by Watcharotone et al., 100 Masaaki et al. 101 and Babenko et al. 102 have also theoretically addressed filed emission from graphene films. The theoretical works describe the importance of field enhancement factor 100,102 and role of defects 101 in graphene field emission. In summary, the enthusiasm for FE of graphene is justified by its unique properties. However, it might take a while to demonstrate a real FE device, which can reach the market. 4.2. Graphene Based Gas and Bio Sensors One of the most promising applications of graphene is in sensors, including gas and bio sensors. The operational principle of graphene based gas or bio electronic sensors is based on the change of graphene s electrical conductivity (σ ) due to adsorption of molecules on graphene surface. 23 The change in conductivity can be attributed to the change in carrier concentration of graphene due to the absorbed gas molecules acting as donors or acceptors. Furthermore, some interesting properties of graphene aid to increase its sensitivity up to single atom or molecular level detection. First, graphene is a two-dimensional (2D) material and its whole volume i.e., all carbon atoms are exposed to the analyte of interest. 21 Second, graphene is highly conductive with low Johnson noise (electronic noise generated by the thermal agitation of the charge carriers inside an electrical conductor at equilibrium, which happens regardless of any applied voltage), therefore, a little change in carrier concentration can cause a notable variation of electrical conductivity. 21 Third, graphene has very few crystal defects [6,19,26,45,103] ensuring a low level of noise caused by thermal switching. 20 Finally, four-probe measurements can be made on single crystal graphene device with ohmic electrical contacts having low resistance. 21,104 Since the first report on graphene sensing by Schedin et al. 21 in 2007, there have been several reports on graphene based sensors. In the work by Schedin et al., 21 graphene demonstrated

62 W. CHOI ET AL. FIG. 10. (a) High-magnification SEM image of the graphene film deposited on the ITO-coated glass for I min at an applied field of 160 V by EPD, using a 0.1 mg ml 1 graphene suspension as electrolyte, 99 (b) SEM image showing top view of FLG synthesized by MW PECVD, 97 (c) Current density as a function of applied electric field for FLG grown on silicon with gas ratio H 2 /CH 4 = 8/1. The results are shown for five voltage cycles without exposure to air in between each cycle and maintaining a constant vacuum; the inset shows the same data, plotted according to the Fowler-Nordheim relation. 97 (Reprinted with permission from Malesevic et al. 97 Copyright 2008: American Institute of Physics, and Wu et al. 99 Copyright 2009: Wiley-VCH Verlag GmbH & Co.) (d) Diode schematic circuit with polymer-graphene composite film as cathode and metal plate as anode, d is the inter-electrode distance. good sensing properties towards NO 2,NH 3,H 2 O and CO. 21 Graphene sensing properties were fully recoverable after exposure to the analyte of interest, by vacuum annealing at 150 C or by illumination to UV for short time. 21 Furthermore, it was also demonstrated that chemical doping of graphene by both holes and electrons, in high concentration, did not affect the mobility of graphene. 21 In another work by Fowler et al. 104 in addition to NO 2 and NH 3, dinitrotoulene (DNT), a volatile

SYNTHESIS OF GRAPHENE AND ITS APPLICATIONS 63 FIG. 11. (left) NO 2 and (right) NH 3 detection using a graphene film. Both the sensors have gold electrodes and measurement used a four wire method with 500 µa driving current. The NO 2 and NH 3 concentration is 5 ppm in dry nitrogen. (Reprinted with permission from Fowler et al. 104 Copyright 2009: American Chemical Society.) compound found in explosives, was also detected. Figure 11 demonstrates the graphene sensors response to NO 2 and NH 3. The sensing mechanism of NO 2 was attributed to hole induced conduction, as it withdraws an electron from graphene and in NH 3, to electron induced conduction, as it donates an electron to graphene. By utilizing the four-point electrode system, Fowler et al. also proposed that the role of electrode electrical contacts was minimum in the sensing mechanism of graphene. The sensing mechanism was primarily attributed to charge transfer at the graphene surface. 104 DNT sensing mechanism was similar to that of NO 2 i.e., electron-withdrawing and the limit of detection of DNT was reported to be 28 ppb, which is well below the room temperature vapor pressure of DNT, i.e., 173 ppb. 104 In another related study by Sundaram et al. 105 graphene surface was chemically modified by electrodeposition of Pd nanoparticles. This procedure may be advantageous, as the attached catalyst particles are expected to impart sensitivity toward certain analytes, which cannot be directly detected with unmodified material, due to insignificant response. 105 The electrodeposition of Pd on graphene was observed to improve the response of graphene sensors to H 2 detection, as Pd has good affinity towards H 2 detection. 105 In addition to gas sensing, recently Shan et al. 106 has demonstrated biosensing, i.e., glucose properties of graphene. With glucose oxidase (GOD) as an enzyme model, Shan et al. and their group constructed a novel polyvinylpyrrolidone protected graphene/polyethylenimine-functionalized ionic liquid/ GOD electrochemical biosensor. Through the sensor, they reported direct electron transfer of GOD, demonstrating graphene s potential application for fabrication of glucose sensors. A linear response up to 14 mm of glucose was observed in their work. 106 In addition to biosensing application for glucose detection, recently Alwarappan et al. 107 has demonstrated that graphene based biosensors are more effective than carbon nanotubes (CNT) to detect catecholamine neurotransmitters, such as dopamine and serotonin. They demonstrated that graphene electrodes exhibited a superior biosensing performance than CNTs toward dopamine detection in the presence of common interfering agents, such as ascorbic acid and serotonin. 107 In another work by Li et al., 103 a nano composite film sensing platform, based on the Nafion graphene, was used for determination of Cd 2+ by anodic stripping voltammetry (ASV). 104 The nano composite film has demonstrated advantages of graphene and the cationic exchange capacity of Nafion, which enhanced the sensitivity of Cd 2+ assay. Since exposure to cadmium, used in several industries, can cause renal dysfunction, bone degeneration, lung insufficiency, liver damage and hypertension in humans with both acute and chronic toxicity, the authors have explored the use of graphene for sensing cadmium. 108 Some researchers have recently observed that inclusion of lithographic (photo or e-beam) steps in preparation of graphene, can cause some negative effects on the sensing properties of graphene, due to presence of residual polymers on the graphene surface. In a work by Dan et al., 109 a cleaning process was demonstrated to remove the contamination on the sensor device structure, allowing intrinsic chemical response of graphene based sensors. The contamination layer was removed by a high temperature cleaning process in a reducing (H 2 /Ar) atmosphere. 109 For most of the gas and bio electronic sensor applications, graphene synthesized by various methods 21,104 106 was deposited on Si or Si/SiO 2 substrates and electrical contacts were prepared with Au/Ti or other metals, which provide good adhesion and ohmic contact with graphene. In addition to the experimental studies of graphene based sensors, there have been numerous theoretical reports 110 114 on the sensing properties of graphene. Most of the theoretical studies provide an understanding on the effect of absorption of the gas or biomolecules and their influence on the mobility of graphene and the charge transfer between the molecules and the graphene surface. Furthermore, the theoretical reports also analyze the effect of doping graphene for sensing applications. 109 114

64 W. CHOI ET AL. FIG. 12. (a) SEM picture of GNR devices fabricated on a 200 nm SiO 2 substrate. The widths of the GNRs from top to bottom are 20 nm, 30 nm, 40 nm, 50 nm, 100 nm and 200 nm. (b) AFM image of a single layer graphene before lithographic process, (c) Cross-section measurement of the AFM, which provides the thickness of the graphene. When accounting the background noise and interaction between the graphene and substrate, we consider sheets thinner than 0.5 nm to be single layer graphene. (Reprinted with permission from Chen et al. 121 Copyright 2007: Elsevier.) 4.3. Field Effect Transistors (FET) One of the potential applications of graphene is in FET. 3 However, being a zero-gap semiconductor, graphene cannot be directly utilized for FET applications. The observation of electric field effect in graphene was first reported by Novoselov et al. in 2004. In that report, the researchers observed that graphene based FETs demonstrated ambipolar characteristics with an electron and hole concentrations of 10 13 sq-cm, with mobilites up to 10000 sq-cm/v-s at room temperature and ballistic transport up to sub micrometer distances. 3 For the transistor applications, graphene should be in the form of quasi-one dimensional (1D) structure, with narrow widths and atomically smooth edges. 115 121 Such structures, termed as graphene nanoribbons (GNRs), are predicted to exhibit band gaps useful for room temperature FET applications, with excellent switching speed and high carrier mobility. 115 122 In addition to the 2D confinement due to its structure, electrons in graphene are further confined by the formation of nanoribbons (for example quantizing in k y direction). The width confinement is expected to result in the split of original 2D energy dispersion of graphene into a number of 1D modes. Due to this splitting, some 1D modes may not pass through the intersection point of the conduction and valence bonds, depending on the boundary conditions. Thus, the qausi-1d GNRs become semiconductors with finite energy band gap. 115 121 Band gaps up to 400 mev have been introduced by patterning graphene into GNRs. 120 122 Figure 12 demonstrates GNR FETs fabricated on SiO 2 /Si substrates. 121 Although band gap have been demonstrated in GNRs, these were observed to be quite different from those of graphene, in terms of carrier mobility and fabrication challenges. 123 Band gap has also been achieved through application of electric fields to bilayer graphene structures. 124 126 However, these gaps have been observed to be less than 400 mev and lead to significant tunneling between bands. 124 126 For the FET application, several researchers have demonstrated various methods to fabricate GNRs including chemical and lithographic methods. Lithographic patterning has led to fabrications of GNRs with widths of 20 to 30 nm. 120,127 While chemical routes have produced 10 to 15 nm width GNRs. 122 Figure 13 shows the fabrication process of GNRs by nanowire etch mask technique. By utilizing this process, GNRs with widths down to 6 nm were demonstrated. 122 Chen et al. 122 have investigated the FET properties of GNRs as a function of their widths. Their experiments have shown that resistivity of GNRs increased as their width decreased, which could be attributed to the edge states. Furthermore, the electrical current noise of GNR devices at low frequency was found to be dominated by 1/f noise. 121 Since the first experimental demonstration of graphene based FETs, several theoretical studies have been reported to predict the performance of GNR FETs as functions of theirs edge roughness, 128 chirality, 129 chemical doping, 130 carrier scattering, 131 and contacts. 132 In addition, various models have also been developed to predict the performance of GNR FETs. 133 138 The theoretical studies provide a better understanding about the performance of GNR based FETs characteristics; furthermore, they can be valuable tools for designing efficient FIG. 13. (a f) Schematic fabrication process to obtain GNRs by oxygen plasma etch with a nanowire etch mask. (Reprinted with permission from Bai et al. 127 Copyright 2009: American Chemical Society.)

SYNTHESIS OF GRAPHENE AND ITS APPLICATIONS 65 FIG. 14. Transmittance of the graphene films on a quartz plate (with increasing exposure time from bottom to top). The upper inset shows the ultraviolet (UV)-induced thinning and the consequent enhancement of transparency. The lower inset shows the changes in transmittance, Tr, and sheet resistance, R s, as functions of ultraviolet illumination time. (Reprinted with permission from Macmillan Publishers Ltd: Nature, 17 Copyright 2009.) FETs. Even though the potential of graphene FETs have been demonstrated, it might take several years to fabricate commercially viable logic devices from graphene, due to its various intrinsic difficulties. 4.4. Transparent Electrodes ITO is widely used to make transparent conductive coatings for liquid crystal displays (LCD), flat panel displays, touch panels, solar cells and EMI shielding. However, high cost, limited supply and brittle nature of indium restricts its application in flexible substrate, motivating the search for highly transparent, high conductivity thin-film alternatives. Graphene is expected to be one of the mostly sought materials for future optoelectronic devices, including transparent electrodes for solar cells and LCD displays. 139 141 The extraordinary thermal, chemical and mechanical stability of graphene, combined with its high transparency and atomic layer thickness, makes it an ideal candidate for transparent conducting electrode applications. Therefore, graphene is considered as a next generation transparent electrode material. The high hole transport mobility, large surface area and inertness against oxygen and water make graphene a promising candidate for photovoltaic applications. Monolayer graphene is highly conductive and highly transparent (absorb only 2.3% of white light); 142 Figures 14 and 15 demonstrate the transmittance behavior of graphene transparent electrodes. Recently K. Kim et al. reported 80% transmittance from the graphene grown on a 300 nm thick nickel layer, corresponding to 6 to 10 graphene layers. 17 The transmittance was increased up to 93% by further reducing the growth time and nickel thickness, resulting in formation of thinner graphene film. Ultraviolet/ozone etching was also suggested to reduce the thickness FIG. 15. (a) HRTEM image of graphene films with corresponding SAED pattern (inset), (b) Transmittance of a 10 nm thick graphene film (almost flat curve), in comparison with that of ITO (top curve) and FTO (bottom curve), (c) schematic of dyesensitized solar cell using graphene film as electrode, the four layers from bottom to top are Au, dye-sensitized heterojunction, compact TiO 2, and graphene film. (Reprinted with permission from Wang et al. 141 Copyright 2008: American Chemical Society.) of graphene. However, sheet resistance increases with the ultraviolet/ozone treatment time, in accordance with the decreasing number of graphene layers. 17 Wang et al. have reported application of graphene based transparent electrodes for dye-sensitized solar cell (DSSC). 141 They presented a simple approach for fabrication of graphene films from exfoliated graphite oxide, followed by thermal reduction. The transparent electrodes demonstrated a transparency of 70% over 1000 to 3000 nm, with good conductivity of 550 S/cm. The characteristic of the DSSC, prepared by using graphene electrode, showed a short-circuit photocurrent density of 1.01 ma/cm 2, open-circuit voltage of 0.7 V, calculated filling factor of 0.36 and overall power conversion efficiency of 0.26%. The low efficiency of DSSC was claimed to be due to low quality of graphene film. Figure 14 shows the structure of graphene films and their transmittance properties as a function of wavelength. Composites of graphene and poly(3,4-ethylenedioxythiophene) (PEDOT-PSS) were used as a counter electrode to show high transmittance (>80%) and high electrocatalytic activity, resulting in 4.5% overall energy conversion efficiency. 143 Transparent graphene film was also reported to be used for organic solar cell. 144 A graphene-based film was prepared by the thermal reaction of synthetic nano-graphene molecules of giant polycyclic aromatic hydrocarbons, which were cross-linked with each other and further fused into larger graphene sheets. The film showed an ultra smooth surface and outstanding thermal and chemical stability, but the power conversion efficiency was comparable with that of the ITO-based cell under low intensity

66 W. CHOI ET AL. monochromic illumination and relatively lower efficiency under simulated solar light. Presently, graphene sheet synthesized on Ni substrates and through other methods are being transferred on to conductive substrates for transparent electrode applications. 139 141 This method might be suitable for smaller electrodes and scientific exploration purposes; however, the challenge of large-area graphene synthesis directly on transparent substrates still exists. It is known that the energy band gap of armchair GNRs is inversely proportional to the width. The energy gap of a 15 nm wide GNR is around 0.2 ev, whereas an epitaxial graphene thin film on a SiC substrate has a band gap of 0.26 ev. 28 Therefore, graphene were also used as a novel acceptor for bulk heterojunction polymer photovoltaic cells, showing remarkably reduced photoluminescence and consequently efficient energy transfer in P3OT/graphene interface. 145 For microelectronic application, high mobility and excellent mechanical properties of transparent graphene film are useful to make flexible and stretchable electrodes. Kim et al. evaluated the foldability of the graphene films, transferred to a polyethylene terephthalate (PET) substrate (thickness, 100 µm) coated with a thin PDMS layer (thickness, 200 µm), by measuring resistance as a function of bending radii. 17 The resistances show little variation up to the bending radius of 2.3 mm (approximate tensile strain of 6.5%) and are perfectly recovered after unbending, (Figure 16). Notably, the original resistance can be restored even for the bending radius of 0.8 mm (approximate tensile strain of 18.7%), exhibiting extreme mechanical stability. FIG. 16. Variation in resistance of a graphene film transferred to a 0.3 mm-thick PDMS/PET substrate for different distances between holding stages (that is, for different bending radii). The left inset shows the anisotropy in four-probe resistance, measured as the ratio, R y /R x, of the resistances parallel and perpendicular to the bending direction, y. The right inset shows the bending process. 16 (Reprinted with permission from Macmillan Publishers Ltd: Nature, 17 Copyright 2009.) FIG. 17. Schematic illustration of synthesis and structure of SnO 2 /GNS for battery application. (Reprinted with permission from Pack et al. 40 Copyright 2009: American Chemical Society.) 4.5. Battery Li-ion battery has been a key component of hand-held devices, due to its renewable and clean nature. Graphite is usually employed as anode material in Li-ion battery, due to its reversibility and reasonable specific capacity. However, to meet the increasing demand for Li-ion batteries with higher energy density and durability, new electrode materials with higher capacity and stability need to be developed. Among the carbonaceous materials, graphene-based anode has been proposed as one of the promising alternatives in Li-ion batteries, as graphene has superior electrical conductivity than graphitic carbon, high surface area and chemical tolerance. 122,141,146 147 Paek et al. has prepared graphene nanosheets decorated with SnO 2 nanoparticles by dispersing reduced graphene nanosheets in the ethylene glycol and reassembling in presence of SnO 2 nanoparticles, as shown in Figure 17. 40 The SnO 2 /graphene exhibits a reversible capacity of 810 mah/g and its cycling performance is drastically enhanced in comparison with that of the bare SnO 2 nanoparticle. Wang et al. have shown self-assembled TiO 2 -graphene hybrid nanostructure to enhance high rate performance of electro-chemical active material. 41 They used anionic sulfate surfactants to assist the stabilization of graphene in aqueous solutions and facilitate self-assembly of in situ grown nanocrystalline TiO 2 with graphene. The specific capacity was 87 mah/g, which is more than double the high rate capacity (35 mah/g) of the control rutile TiO 2. The specific capacity of the anatase TiO 2 -functionalized graphene at the rate of 30C is as high as 96 mah/g compare with 25 mah/g of control anatase TiO 2. Though few reports on application of graphene as