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石墨烯发展过程及展望

Graphene based materials:Past,present and future

Virendra Singh a ,b ,Daeha Joung a ,c ,Lei Zhai a ,d ,?,Soumen Das a ,b ,

Saiful I.Khondaker a ,c ,e ,?,Sudipta Seal a ,b ,?

a Advanced Materials Processing Analysis Center and Nanoscience Technology Center,University of Central Florida,Orlando,FL,USA b

Department of Mechanical,Materials and Aerospace Engineering,University of Central Florida,Orlando FL,USA

c Department of Physics,University of Central Florida,Orlando,FL,USA

d Department of Chemistry,University of Central Florida,Orlando,FL,USA

e School o

f Electrical Engineerin

g and Computer Science,University of Central Florida,Orlando,FL,USA a r t i c l e i n f o Article history:Received 22February 2011Accepted 30Marc

h 2011Available online 3April 2011

a b s t r a c t

Graphene,a two dimensional monoatomic thick building block of a

carbon allotrope,has emerged as an exotic material of the 21st cen-

tury,and received world-wide attention due to its exceptional

charge transport,thermal,optical,and mechanical properties.

Graphene and its derivatives are being studied in nearly every ?eld

of science and engineering.Recent progress has shown that the

graphene-based materials can have a profound impact on

electronic and optoelectronic devices,chemical sensors,nanocom-

posites and energy storage.The aim of this review article is to

provide a comprehensive scienti?c progress of graphene to date

and evaluate its future perspective.Various synthesis processes of

single layer graphene,graphene nanoribbons,chemically derived

graphene,and graphene-based polymer and nano particle compos-

ites are reviewed.Their structural,thermal,optical,and electrical

properties were also discussed along with their potential applica-

tions.The article concludes with a brief discussion on the impact of

graphene and related materials on the environment,its toxicological

effects and its future prospects in this rapidly emerging ?eld.

Published by Elsevier Ltd.Contents

1.

Introduction .......................................................................11812.History of graphene .................................................................11810079-6425/$-see front matter Published by Elsevier Ltd.

doi:10.1016/j.pmatsci.2011.03.003

?Corresponding authors at:Advanced Materials Processing Analysis Center and Nanoscience Technology Center,University of Central Florida,Orlando,FL,USA.

E-mail addresses:Lei.Zhai@https://www.sodocs.net/doc/817238985.html, (L.Zhai),saiful@https://www.sodocs.net/doc/817238985.html, (S.I.Khondaker),Sudipta.Seal@https://www.sodocs.net/doc/817238985.html, (S.Seal).

V.Singh et al./Progress in Materials Science56(2011)1178–12711179 3.Synthesis of graphene (1182)

3.1.Exfoliation and cleavage (1182)

3.1.1.Mechanical exfoliation in solutions (1183)

3.1.2.Intercalation of small molecules by mechanical exfoliation (1184)

3.2.Chemical vapor deposition(CVD) (1186)

3.2.1.Thermal CVD (1186)

3.2.2.Plasma enhanced CVD (1188)

3.2.3.Thermal decomposition on SiC and other substrates (1188)

3.3.Chemically derived graphene (1188)

3.3.1.Synthesis of graphene oxide and the reduction (1188)

3.3.2.Surface functionalization of graphene oxide(GO) (1191)

3.3.3.Structural and physical properties of reduced graphene oxide(RGO) (1192)

3.4.Other synthesis approaches (1195)

3.4.1.Total organic synthesis (1195)

3.4.2.Un-zipping carbon nanotubes(CNTs) (1198)

4.Graphene:characterization and properties (1201)

4.1.Characterization (1201)

4.1.1.Optical imaging of graphene layers (1201)

4.1.2.Fluorescence quenching technique (1201)

4.1.3.Atomic force microscopy(AFM) (1203)

4.1.4.Transmission electron microscopy(TEM) (1204)

4.1.5.Raman spectroscopy (1206)

4.2.Properties (1207)

4.2.1.Electrical transport property (1207)

4.2.2.Quantum Hall effect (1211)

4.2.3.Optical properties (1212)

4.2.4.Mechanical properties (1214)

4.2.5.Thermal properties (1215)

5.Chemically derived graphene:properties and applications (1216)

5.1.Assembly of GO/RGO for device applications (1216)

5.2.Electrical characterization of GO/RGO sheet (1218)

5.3.Defect density in chemically derived graphene (1220)

5.4.RGO as graphene quantum dot array:Coulomb blockade effect (1221)

5.5.Hopping conduction in RGO (1222)

5.5.1.Mott variable range hopping (1222)

5.5.2.Efros–Shklovskii(ES)VRH (1224)

5.6.Application of RGO for photodetector,phototransistor and emitter (1224)

5.7.Graphene thin film as transparent electrodes (1226)

5.8.Solar cell using GO/RGO (1228)

5.9.Electrochemical sensors and biosensors (1230)

6.Graphene based composites (1232)

6.1.Graphene–polymer composites (1232)

6.1.1.Synthesis of graphene reinforced polymer composite (1234)

6.1.2.Mechanical properties (1236)

6.1.3.Electrical properties (1238)

6.1.4.Thermal conductivity (1239)

6.1.5.Other properties (1240)

6.1.6.Applications (1241)

6.2.Graphene–nanoparticles composites (1241)

6.2.1.Synthesis (1241)

6.2.2.Applications (1245)

7.Toxicity of graphene/graphene oxide/reduced graphene oxide (1254)

8.Future prospects (1258)

Acknowledgements (1259)

References (1259)

1180V.Singh et al./Progress in Materials Science56(2011)1178–1271 Nomenclature

Acronyms

AFM atomic force microscopy

APTS3-aminopropyltriethoxysilane

ATRP atom transfer radical polymerization

CB Coulomb blockade

CCG chemically converted graphene

CNT carbon nanotube

CRG chemically reduced graphene oxide

CVD chemical vapor deposition

DCC N,N-dicyclohexylcarbodiimide

DGU density gradient ultracentrifugation

DMF N,N-dimethylformamide

DMSO dimethyl sulfoxide

DP dirac point

EBL electron-beam lithography

ECL electrogenerated chemiluminescence

EDC1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide

FET?eld effect transistors

FQM?uorescence quenching microscopy

GNR graphene nanoribbon

GO graphene oxide

GQD graphene quantum dot

HATU2-(7-aza-1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexa?uorophosphate HRTEM high resolution transmission electron microscopy

ITO indium tin oxide

LB Langmir–Blodgett

LED light emitting diode

MOSFET metal oxide semiconductor FET

NMP N-methyl-2-pyrrolidone

NMR nuclear magnetic resonance

NPs nanoparticles

P3HT poly(3-hexylthiophene)

PAH polyacyclic hydrocarbons

PAN Polyacrylonitrile

PEN poly(ethylene-2,6-naphthalate)

PET polyethylene terephthalate

PECVD plasma enhanced chemical vapor deposition

PL photoluminescence

PMMA poly(methylmethacrylate)

PS polystyrene

PSS poly(sodium4-styrenesulfonate)

PU poly urethane

PVA poly(vinyl alcohol)

PVC poly(vinyl chloride)

QD quantum dot

QHE quantum Hall effect

RGO reduced graphene oxide

SC sodium cholate

SEM scanning electron microscope

SLG single layer graphene

STEM scanning transmission electron microscopy

STM scanning tunneling microscopy

TBA Tetrabutylamomium

V.Singh et al./Progress in Materials Science56(2011)1178–12711181 TEM transmission electron microscopy

THF tetrahydrofuran

TRG thermally reduced graphene oxide

TSCuPc tetrasulfonate salt of copper phthalocyanine

VRH variable range hopping

XPS X-ray photoelectron spectroscopy

1.Introduction

The5th of October,2010was another beautiful day at Partin Elementary School in Oviedo.When Kaleb,a6year old kindergartener,took out his pencil and started writing letters on a piece of paper, he did not realize that he was using a material that caught the attention of all scienti?c community that same day.The Nobel Prize in Physics2010was awarded to Andre Geim and Konstantin Novoselov ‘‘for ground breaking experiments regarding the two-dimensional material graphene’’,a layer of graphite in the pencil.Graphene,one of the allotropes(carbon nanotube,fullerene,diamond)of ele-mental carbon,is a planar monolayer of carbon atoms arranged into a two-dimensional(2D)honey-comb lattice with a carbon–carbon bond length of0.142nm[1].Electrons in graphene behave like massless relativistic particles,which contribute to very peculiar properties such as an anomalous quantum Hall effect and the absence of localization[2,3].Graphene[2]has demonstrated a variety of intriguing properties including high electron mobility at room temperature(250,000cm2/Vs) [4,5]exceptional thermal conductivity(5000W mà1Kà1)[6]and superior mechanical properties with Young’s modulus of1TPa[7].Its potential applications include single molecule gas detection,trans-parent conducting electrodes,composites and energy storage devices such as supercapacitors and lith-ium ion batteries[7–20].In addition,a distinct band gap can be generated as the dimension of graphene is reduced into narrow ribbons with a width of1–2nm,producing semiconductive graphene with potential applications in transistors[8–10].There is no doubt that graphene has risen as a shining star in the horizon on the path of the scientists’searching for new materials for future electronic and composite industry.This review article narrates the brief history of graphene related research,and presents the synthesis of graphene and its derivatives and various characterization techniques per-taining to2D structure.Many extraordinary properties of graphene such as electrical,mechanical, anomalous quantum Hall effect,thermal,and optical are discussed.These properties have generated tremendous interest among material researchers.The recent applications in various?elds such as in large scale assembly and?eld effect devices,sensors,transparent electrodes,photodetectors,solar cells,energy storage devices,polymer composites,nanocomposites will be reviewed with a brief up-date on toxicology.The conclusion and outlook summarizes the research activities and presents the possible future research directions.

2.History of graphene

Although the usage of graphite started6000years ago,when Marican in Europe used it to decorate pottery,the research about graphene,essentially an isolated single-atom plane of graphite,dates back to the1960s when surprisingly higher basal-plane conductivity of graphite intercalation compounds were discovered compared to that of the original graphite[11–13].While the scienti?c community was excited about the discovery that might lead to a lighter,cheaper substitute for existing metal con-ductors,they were puzzled by the cause of the high conductivity of graphite intercalation compounds and cautious about the future applications.The research of graphene has grown slowly in late20th century with the hope to observe superior electrical properties from thin graphite or graphene layers while obtaining graphene was considered to be a formidable task in both theoretical and experimental aspect.In the graphite intercalation systems,large molecules were inserted between atomic planes, generating isolated graphene layers in a three-dimensional matrix.The subsequent removal of the lar-ger molecules produced a mixture of stacked or scrolled graphene layers without the control of the

1182V.Singh et al./Progress in Materials Science56(2011)1178–1271

structure.It was generally believed that,based on both theoretical calculation and experimental observation,2D materials did not exist without a3D base.AB initio calculations showed that a graph-ene sheet was thermodynamically unstable with respect to other fullerene structures if its size was less than about20nm(‘‘graphene is the least stable structure until about6000atoms’’and becomes the most stable one(as within graphite)only for sizes larger than24,000carbon atoms)[14].Various attempts were made to synthesize graphene including using the same approach for the growth of car-bon nanotubes(producing graphite with100layers of graphene)[15],chemical vapor deposition on metal surfaces(a few layers of graphene)[16,17],or the thermal decomposition of SiC[18].Although these approaches did not produce perfect monolayer graphene,the studies showed high-charge mobility of a few layers of graphene and the CVD approach has been optimized and become a major technique to produce graphene nowadays[19–21].It was until2004that Andre Geim and Konstantin Novoselov used a method to isolate graphene,a method similar to what young Kaleb did,drawing with a piece of graphite or peeling graphite with adhesive tape till the graphene is found.Such a‘‘kin-dergartner’’approach can provide high quality graphene with size in hundreds of microns[22].These high quality graphene crystals realize the investigation of their amazing properties.Since then,the re-search of graphene including the control of the graphene layers on substrates,functionalizing graph-ene and exploring the applications of graphene has grown exponentially.As shown in Fig.2.1,the number of publications on graphene(according to ISI Web of Knowledge SM)increases dramatically after2004.

The term of‘‘graphene’’was recommended by the relevant IUPAC commission to replace the older term‘‘graphite layers’’that was unsuitable in the research of single carbon layer structure,because a three-dimensionally(3D)stacking structure is identi?ed as‘‘graphite’’.The recent de?nition of graph-ene can be given as a two-dimensional monolayer of carbon atoms,which is the basic building block of

V.Singh et al./Progress in Materials Science56(2011)1178–12711183 excellent in-plane mechanical,structural,thermal and electrical properties of graphite[24].It is obvious that these excellent properties are relevant at the nanoscale and the manufacture of the con-ducting nanocomposites is highly dependent on the exfoliation of the graphite down to single graph-ene sheet in the matrices.However,the challenge remained to achieve complete and homogeneous dispersion of individual graphene sheets in various solvents[25].Like CNT and other nanomaterials, the key challenge in synthesis and processing of bulk-quantity graphene sheets is aggregation. Graphene,a one-atom-thick planar sheet of sp2-bonded carbon atoms densely packed in a honeycomb crystal lattice has very large speci?c surface area.Unless well separated from each other,graphene tends to form irreversible agglomerates or even restack to form graphite through Van der Waals inter-actions.The prevention of aggregation is essential for graphene sheets because most of their unique properties are only associated with individual sheets.

3.1.1.Mechanical exfoliation in solutions

Mechanical exfoliation is a simple peeling process where a commercially available highly oriented pyrolytic graphite(HOPG)sheet was dry etched in oxygen plasma to many5l m deep mesa(Fig.3.1). The mesa was then stuck onto a photoresist and peeled off layers by a scotch tape.The thin?akes left on the photoresist were washed off in acetone and transferred to a silicon wafer.It was found that these thin?akes were composed of monolayer or a few layers of graphene.While the mechanical exfo-liation of graphene used by Geim and coworkers[22]led to numerous exciting discoveries of graphene electronic and mechanical properties,such approach is limited by its low production.

On the other hand,although chemical oxidation of graphite and the subsequent exfoliation provide large amount of graphite oxide monolayer,the invasive chemical treatment inevitably generates structural defects as indicated by Raman spectroscopic studies[26,27].These structural defects dis-rupted the electronic structure of graphene and change it to semiconductive.The subsequent chemical reduction or thermal annealing(up to1000°C)are virtually impossible to regenerate the graphene structures as indicated by XPS studies[28].Therefore,physical exfoliation approaches are desirable where it is required to maintain the graphene structure.Blake et al.and Hernandez et al.have dem-onstrated that graphite could be exfoliated in N-methyl-pyrrolidone to produce defect-free monolayer Fig.3.1.Mechanical exfoliation of graphene using scotch tape from HOPG.

over more than 6weeks [31].Similarly,Green and Hersam have used sodium cholate as a surfactant to exfoliate graphite and moved further to isolate the resultant graphene sheets with controlled thick-ness using density gradient ultracentrifugation (DGU)(Fig.3.2).Since the exfoliation of the graphite yielded a dispersion of monolayer graphene and graphite with a few layers of graphene which have different buoyant density.DGU separations of such mixture produce graphene sheets with mean thicknesses that increase as a function of their buoyant density (Fig.3.3)[32].

3.1.2.Intercalation of small molecules by mechanical exfoliation

Agglomeration in graphite can be reduced appreciably by incorporating small molecules between the layers of graphite or by non-covalently attaching molecules or polymers onto the sheets,generat-ing graphite intercalation compounds (GICs).In GICs,the graphite layers remains unaltered with guest molecules located in the interlayer galleries.When the layers of graphite interact with the guest mol-ecules by charge transfer,the in-plane electrical conductivity generally increases but when the mol-ecules form covalent bonds with the graphite layers as in ?uorides or oxides the conductivity decreases as the conjugated sp 2system is disrupted.The ?rst graphite intercalation compound,(GIC),or commonly known as expandable graphite was prepared by Schafhautl in 1841while analyz-ing crystal ?ake of graphite in sulfuric acid solution.In the laboratory,?ake graphite was subjected to shear intensive mechanical stirring with ultrasonic solvent in the ultrasonic cleaning bath at

room

temperature to prepare expandable graphite.Experimental conditions could be tuned by changing ultrasonic solvent,ultrasonic power (nominal power of 500W and 250W)and ultrasonic time.After the treatment,the mixture was washed thoroughly with water to neutrality and dried below 60°C for 60min.The expandable graphite was then expanded at 900°C to obtain the expanded graphite (EG).The choice of ultrasonic solvent depended on the oxidation ability and water content of the solvents,which affected the volume of expanded graphite.Acetic acid,acetic acid anhydride,concentrated sul-furic acid and hydrogen peroxide were the examples of few ultrasonic solvents.Among all those,con-centrated sulfuric acid had been proved to be the best ultrasonic solvent to provide optimum condition for preparing the expandable graphite with ultrasound irradiation.Such sulfuric acid inter-calated graphite compound consisted of layers of hexagonal carbon structure within which H 2SO 4was intercalated.EG could be prepared either by oxidation with a chemical reagent or electrochemically in the intercalating acid [33,34].Graphite could expand up to a hundred times in volume at high temper-ature [35]due to the thermal expansion of the evolved gases trapped between the graphene sheets.So it was reasonably assumed that oxidants and other molecules could enter in the interlayer space of EG more easily compared to natural graphite.The in?uence of ultrasonic solvent and ultrasonic power on the volume of expanded graphite could be analyzed from the data listed in Table 3.1.

Li et al.reported the exfoliation–reintercalation–expansion of graphite to produce high quality sin-gle layer graphene sheets stably suspended in organic solvents [36].Commercial expandable graphite was subjected to brief heating (60s)at 1000°C in forming gas.It was then grounded with NaCl crystals and reintercalated with oleum.The exfoliated graphite was then dispersed in N,N-dimethylformamide (DMF)and treated with tetrabutylamomium (TBA).TBA could insert into and increase the distance be-tween adjacent layers of graphite facilitating the separation of graphene sheets in surfactant solutions.The reintercalation and the rapid,brief heating of EG enabled the preparation of highly conducting graphene sheets without functionalizing the

graphite.

1186V.Singh et al./Progress in Materials Science56(2011)1178–1271

Table3.1

The in?uence of ultrasonic solvent and ultrasonic power on the volume of expanded graphite.

Ultrasonic power(W)Ultrasonic solvents Ultrasonic time(min)Expanded volume(ml/g)

500Acetic acid60150

500Conc.sulfuric acid60500

500Alcohol60120

500Hydrogen peroxide60140

250Acetic acid60140

250Conc.sulfuric acid60380

250Alcohol6080

250Hydrogen peroxide60100

3.2.Chemical vapor deposition(CVD)

3.2.1.Thermal CVD

Besides mechanical exfoliation and chemical reduction methods to produce graphene sheets,sev-eral promising approaches including epitaxial growth from SiC,and chemical vapor deposition(CVD) on metal surfaces have been reported.Among them,the CVD growth appears to be the most promising technique for large-scale production of mono-or few-layer graphene?lms.Although the formation of ‘‘monolayer graphite’’was mentioned in early CVD studies on metal single crystals[37–39]the?rst successful synthesis of few-layer graphene?lms using CVD was reported in2006by Somani and coworkers using camphor as the precursor on Ni foils[40].This study opened up a new graphene syn-thesis route with several unsolved issues like controlling the number of layers,and minimizing the folding of graphene.Since then,much progress has been made to obtain graphene layers on several types of metal substrates with controlled thickness[26,41–47].After a chemical etching of the metal substrate,the graphene layers detach and can be transferred to another substrate,providing high quality graphene layers without complicated mechanical or chemical treatments.

The growth mechanism of graphene on substrates with mediate-high carbon solubility(>0.1atom-ic%)such as Co and Ni is through the diffusion of the carbon into the metal thin?lm at the growth temperature and the subsequent precipitation of carbon out of the bulk metal to metal surface upon the cooling[43,48].A typical CVD process(https://www.sodocs.net/doc/817238985.html,ing Ni as a substrate)involves dissolving carbon into the nickel substrate followed by a precipitation of carbon on the substrate by cooling the nickel.The Ni substrate is placed in a CVD chamber at a vacuum of10à3Torr and temperature below1000°C with a diluted hydrocarbon gas.The deposition process starts with the incorporation of a limited quantity of carbon atoms into the Ni substrate at relatively low temperature,similar to the carburization process. The subsequent rapid quenching of the substrate caused the incorporated carbon atoms to out-diffuse onto the surface of the Ni substrate and form graphene layers.Therefore,the thickness and crystalline ordering of the precipitated carbon(graphene layers)is controlled by the cooling rate and the concen-tration of carbon dissolved in the nickel which is determined by the type and concentration of the car-bonaceous gas in the CVD,and the thickness of the nickel layer.

In contrast,the graphene growth on low carbon solubility(<0.001atomic%)substrate like Cu mainly happens on the surface through the four-step process described by Li and coworkers as follow-ing[49]:

1.Catalytic decomposition of methane on Cu to form C x H y upon the exposure of Cu to methane and

hydrogen.In this process,the Cu surface is either undersaturated,saturated,or supersaturated with

C x H y species,depending on the temperature,methane pressure,methane?ow,and hydrogen par-

tial pressure.

2.Formation of nuclei as a result of local supersaturation of C x H y where undersaturated Cu surface

does not form nuclear.

3.Nuclei grow to form graphene islands on Cu surface saturated,or supersaturated with C x H y species.

4.Full Cu surface coverage by graphene under certain temperature(T),methane?ow rate(J Me),and

methane partial pressure(P Me).

V.Singh et al./Progress in Materials Science56(2011)1178–12711187 If the amount of available C x H y on the exposed Cu surface is insuf?cient to expand the C to the is-land edges,the Cu surface is only partially covered with graphene islands.Otherwise,if there is always enough methane to form suf?cient C x H y to drive the reaction between the C x H y at the surface and the edges of graphene islands,graphene islands would grow until to connect neighboring islands and fully cover the Cu surface.With the understanding of the graphene growth mechanism,various approaches were applied to control the graphene growth rate to obtain monolayer graphene.Lee and coworkers have used SiO2/Si substrates,coated with300nm thick Ni or700nm thick Cu to produce graphene layers[47].The average number of graphene layers grown on a Ni catalyst ranged from three to eight, depending on the reaction time and cooling rates.On the other hand,the mono-and bilayer graphene grows predominantly on a Cu catalyst.The low solubility of carbon in Cu is believed to induce the self-limiting graphene growth process[44].To prevent the formation of multilayer graphene on mediate-high carbon solubility(>0.1atomic%)substrates such as Co and Ni,the thin layers of Ni of thickness less than300nm were deposited on SiO2/Si substrates to produce graphene monolayer[43].Recently, Bae and coworkers reported a roll-to-roll production of30-inch.graphene?lms using the CVD ap-proach[50].Their fabrication process including three steps after the synthesis of graphene on copper substrates:(i)adhesion of polymer supports to the graphene on the copper foil;(ii)etching of the cop-per layers;and(iii)release of the graphene layers and transfer onto a target substrate(Fig.3.4).The obtained graphene monolayer?lms have sheet resistances as low as$125X/square with97.4%opti-cal transmittance,and exhibit the half-integer quantum Hall effect,indicating their high quality.In addition,a layer-by-layer stacking of doped four-layer?lm has sheet resistance as low as$30X/ square with$90%transparency,demonstrating great potential to replace commercial transparent electrodes such as indium tin oxides.

An interesting feature of the CVD approach to synthesize graphene is the possibility for substitu-

1188V.Singh et al./Progress in Materials Science56(2011)1178–1271

in lithium ion batteries is almost double compared to pristine graphene because the surface defects induced by nitrogen doping[53].

3.2.2.Plasma enhanced CVD

Plasma enhanced chemical vapor deposition(PECVD)offers another route of graphene synthesis at a lower temperature compared to thermal CVD.Thick graphite structures were observed during the fabrication of‘‘nanostructured graphite-like carbon’’using a dc discharge PECVD.The?rst report of the production of mono-and few layer of graphene by PECVD involved a radio frequency PECVD system to synthesize graphene on a various substrates where graphene sheets were produced from a gas mixture of5–100%CH4in H2(total pressure12Pa),at900W power and680°C substrate tem-perature[54,55].Since then,much effort have been devoted to understand the graphene growth mechanism and to optimize experimental conditions to control the thickness of graphene?lms [56–58].The advantages of the plasma deposition include very short deposition time(<5min) and a lower growth temperature of650°C compared to the thermal CVD approach(1000°C).The growth mechanism involved a balance between the graphene deposition through the surface diffu-sion 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 ?eld direction[56,58].

3.2.3.Thermal decomposition on SiC and other substrates

Producing graphite through ultrahigh vacuum(UHV)annealing of SiC surface has been an attrac-tive approach especially for semiconductor industry because the products are obtained on SiC sub-strates and requires no transfer before processing devices[18,59–61].When SiC substrate is heated under UHV,silicon atoms sublimate from the substrate.The removal of Si leaves surface carbon atoms to rearrange into graphene layers.The thickness of graphene layers depends on the annealing time and temperature.The formation of‘‘few-layer graphene’’(FLG)typically requires few minutes annealing of the SiC surface at temperature around1200°C[62].More recently,vapor phase annealing has been used to produce FLG on SiC.At the expense of a higher temperature(typically400°C above UHV tem-perature),[63]this method leads to the formation of FLG on SiC with an improved thickness homoge-neity[64].Although producing graphene on SiC substrates is attractive,several hurdles prevent the real application.For example,control the thickness of graphene layers in the routine production of large area graphene is very challenging.Another uncertainty involves the different epitaxial growth patterns on different SiC polar face(i.e.Si-face or C-face).Unusual rotational graphene stacking were observed in multilayers graphene grown on the C-face surface but not on Si-face surface.Such mis-match of graphene growth process has profound effects on the physical and electronic properties of epitaxial graphene.On the C-face,the‘‘twisted’’interface leads to the decoupling between different layers of graphene,each of which behaves as a single layer[65].However,the electronic properties of graphene multilayers on Si-face remains controversial[59].Future investigation is required to understand the mechanisms of the growth processes.The third issue is to understand the relationship between the structure and electronic properties of the interface layer between graphene and substrate [61].

The similar approach was applied to other metallic substrates to grow graphene layers.The(0001) faces of ruthenium(Ru)crystals were used under UHV to produce epitaxial graphene layers where a very sparse graphene nucleation at high temperatures allowed a linear dimensions growth of macro-scopic single-crystalline domains[66,67].It was found that the?rst graphene layer coupled strongly to the Ru substrate,while the second layer was free of the substrate interaction and had the similar electronic structure to free-standing graphene.Other metal substrates including Ir,Ni,Co,and Pt have been employed to produce graphene layers and was nicely reviewed by Wintterlin and Bocquet[68].

3.3.Chemically derived graphene

3.3.1.Synthesis of graphene oxide and the reduction

At present,chemical conversion of graphite to graphene oxide has emerged to be a viable route to afford graphene-based single sheets in considerable quantities[69–73].Graphite oxide(GO)is usually

V.Singh et al./Progress in Materials Science56(2011)1178–12711189 synthesized through the oxidation of graphite using oxidants including concentrated sulfuric acid,ni-tric acid and potassium permanganate based on Hummers method[74].Compared to pristine graph-ite,GO is heavily oxygenated bearing hydroxyl and epoxy groups on sp3hybridized carbon on the basal plane,in addition to carbonyl and carboxyl groups located at the sheet edges on sp2hybridized carbon.Hence,GO is highly hydrophilic and readily exfoliated in water,yielding stable dispersion con-sisting mostly of single layered sheets(graphene oxide).It is important to note that although graphite oxide and graphene oxide share similar chemical properties(i.e.surface functional group),their struc-tures are different.Graphene oxide is a monolayer material produced by the exfoliation of GO.Suf?-ciently dilute colloidal suspension of graphene oxide prepared by sonication are clear,homogeneous and stable inde?nitely.AFM images of GO exfoliated by the ultrasonic treatment at concentrations of 1mg/ml in water always revealed the presence of sheets with uniform thickness($1nm).The pristine graphite sheet is atomically?at with the Van der Waals thickness of$0.34nm,graphene oxide sheets are thicker due to the displacement of sp3hybridized carbon atoms slightly above and below the ori-ginal graphene plane and presence of covalently bound oxygen atoms.A similar degree of exfoliation of GO was also attained for N,N-dimethylformamide(DMF),tetrahydrofuran(THF),N-methyl-2-pyrrolidone(NMP)and ethylene glycol[75].Li et al.showed that the surface charges on graphene oxide are highly negative when dispersed in water by measuring the zeta potential due to the ioniza-tion of the carboxylic acid and the phenolic hydroxyl groups[76].Therefore,the formation of stable graphene oxide colloids in water was attributed to not only its hydrophilicity but also the electrostatic repulsion.

The chemical structure of graphene oxide such as the type and distribution of oxygen-containing functional groups have been studied using NMR13C-labelled graphene oxide[77,78]suggesting that the basal plane of the sheet is decorated with hydroxyl and epoxy(1,2-ether)functional groups with small amount of lactol,ester,acid and ketone carbonyl groups at the edge.These results are in good agreement with the model proposed by Lerf–Klinowski[79,80]and Dékány Models with small mod-i?cation[81].These functional groups provide reactive sites for a variety of surface-modi?cation reac-tions to develop functionalized graphene oxide-and graphene-based materials.On the other hand, due to the disruption of the conjugated electronic structure by these functional groups,graphene oxide was electrically insulating and contained irreversible defects and disorders[26,82],but chemical reduction of graphene oxide could partially restore its conductivity[26,82,83]at values orders of mag-nitude below that of pristine graphene.

Chemical reduction of graphene oxide sheets has been performed with several reducing agents including hydrazine[27,28,73,84],and sodium borohydrate[83,85].Hydrazine hydrate,unlike other strong reductants,does not react with water and was found to be the best one in producing very thin and?ne graphite-like sheets.During the reduction process,the brown colored dispersion of graphene oxide in water turned black and the reduced sheets aggregated and precipitated[26,82].The reduced graphene oxide became less hydrophilic due to the removal of oxygen atoms and thus precipitated. The reason of re-establishment of the conjugated graphene network could be attributed to the reac-tion pathway proposed by Stankovich et al.(Fig.3.5)[26].Hydrazine takes part in ring-opening reac-tion with epoxides and forms hydrazino alcohols[86].This initial derivative reacts further via the formation of an aminoaziridine moiety which undergoes thermal elimination of diimide to form a double bond.Li and coworkers demonstrated the preparation of stable aqueous suspension of reduced graphene oxide nanosheets by adjusting the pH(with ammonia solution)of the aqueous solution dur-ing reduction with hydrazine[76].The carboxylic acid groups were unlikely to be reduced by hydra-zine and thus remained intact after hydroxyl reduction.The adjustment of pH with ammonia solution deprotonated the carboxylic acid groups and thus the electrostatic repulsion among the charged groups on reduced graphene oxide enabled the formation of well-dispersed graphene colloids in water without any stabilizers.But,unless stabilized by selected surfactants,reduced graphene in organic sol-vent tend to agglomerate due to their hydrophobic nature[26,82].Another possible route to reduce GO was using sodium borohydride(NaBH4)[83]in aqueous solution where sodium borohydride is more effective than hydrazine as a reductant of graphene oxide although it can be slowly hydrolyzed by water.Such reduction produced reduced graphene oxide with sheet resistances as low as59k X/ square(compared to780k X/square for a hydrazine reduced sample,measured in the same study), and C:O ratios were as high as13.4:1(compared to6.2:1for hydrazine).The NaBH4treatment

1190V.Singh et al./Progress in Materials Science56(2011)1178–1271

eliminated all the parent oxygen containing groups and the resultant solid became IR inactive like pure graphite.Carbon elemental analysis revealed the evidence for the complete reduction of graph-ene oxide in this process[83,85].Other chemical reduction routes including using hydroquinone[87], gaseous hydrogen(after thermal expansion)[88],and strongly alkaline solutions[89]have also been investigated.While the reduction by hydrogen proved to be effective(C:O ratio of10.8–14.9:1),hydro-quinone and alkaline solutions were not as effective as hydrazine and sodium borohydride based on semi-quantitative results.

Thermal reduction is another approach to reduce GO to reduced graphene oxide that utilizes the heat treatment to remove the oxide functional groups from graphene oxide surfaces.Aksay’s group have exfoliate and reduce stacked GO by heating GO to1050°C where oxide functional groups were extruded as carbon dioxide[71,90].The authors reported that the exfoliation took place when the decomposition rate of the epoxy and hydroxyl sites of graphite oxide exceeded the diffusion rate of the evolved gases,thus yielding pressures that exceeded the Van der Waals forces holding the graph-ene sheets together.It was calculated that the pressure of2.5MPa was required to separate GO sheets by numerically evaluating the Hamaker constant while the pressure generated during the exfoliation was1–2orders of magnitude higher.Although the thermal reduction/exfoliation can produce80%sin-gle layer reduced graphene oxide according to the AFM studies,the removal of the oxide groups caused about30%mass loss and left behind vacancies and structural defects which may affect the mechanical and electrical properties of reduced graphene oxide.Nevertheless,the bulk conductivities

V.Singh et al./Progress in Materials Science56(2011)1178–12711191 of the products was measured to be1000–2300S/m,suggesting the effective reduction and restora-tion of electronic structures from GO[71].Recently,Dubin et al.reported a simple one-step,solvother-mal reduction method to produce reduced graphene oxide dispersion in organic solvent[91].The deoxygenation of GO resulted from both thermal deoxygenation at200°C when re?uxing GO in N-methyl-2-pyrrolidinone(NMP)along with a concomitant reaction of GO with NMP molecules.The solvothermally reduced graphene oxide layers remained in a stable dispersion after the reaction.

This approach provides a simple,low-temperature method to produce reduced graphene oxide.

3.3.2.Surface functionalization of graphene oxide(GO)

The surface functionalization of graphene oxide not only plays an important role in controlling exfoliation behavior of graphene oxide and reduced graphene oxide but also holds the key to the gate leads to various applications.The surface functionalization has taken two approaches:covalent func-tionalization and non-covalent functionalization.In covalent functionalization,oxygen functional groups on graphene oxide surfaces,including carboxylic acid groups at the edge and epoxy/hydroxyl groups on the basal plane can be utilized to change the surface functionality of graphene oxide.Graph-ene oxide had been treated with organic isocyanates to give a number of chemically modi?ed GO. Treatment of isocyanates reduced the hydrophilicity of graphene oxide by forming amide and carba-mate esters from the carboxyl and hydroxyl groups of graphene oxide,respectively.Consequently,iso-cyanate modi?ed graphene oxide readily formed stable dispersion in polar aprotic solvents giving completely exfoliated single graphene sheets with thickness of$1nm(Fig.3.6).This dispersion also facilitated the intimate mixing of the graphene oxide sheets with matrix polymers,providing a novel synthesis route to make graphene–polymer nanocomposites.Moreover,modi?ed graphene oxide in the suspension could be chemically reduced in presence of the host polymer to render electrical con-ductivity in the nanocomposites[92].

In order to use carboxylic acid groups on graphene oxide to anchor other molecules,the carboxylic acid groups have been activated by thionyl chloride(SOCl2)[93–96],1-ethyl-3-(3-dimethylaminopro-pyl)-carbodiimide(EDC)[97],N,N-dicyclohexylcarbodiimide(DCC)[98],or2-(7-aza-1H-benzotria-zole-1-yl)-1,1,3,3-tetramethyluronium hexa?uorophosphate(HATU)[99].The subsequent addition of nucleophilic species,such as amines or alcohols,produced covalently attached functional groups

1192V.Singh et al./Progress in Materials Science56(2011)1178–1271

on graphene oxide via the formation of amides or esters.The resultant amine functionalized graphene oxide has demonstrated various applications in optoelectronics[93,95,96],drug-delivery materials [97],biodevices[99],and polymer composites[98,100].The attachment of hydrophobic long,aliphatic amine groups on hydrophilic graphene oxide improved the dispersability of modi?ed graphene oxide in organic solvents[94],while porphyrin-functionalized primary amines and fullerene-functionalized secondary amines introduced interesting nonlinear optical properties[95,96].The amine groups and hydroxyl groups on the basal plane of graphene oxide have also been used to attach polymers through either grafting-onto or grafting-from approaches.To grow a polymer from graphene oxide,an atom transfer radical polymerization(ATRP)initiator(i.e.a-bromoiobutyrylbromide)was attached to graphene surfaces[101,102].The following living polymerization produced graphene oxide with poly-mers that enhanced the compatibility of solvents and other polymer matrices.Besides the carboxylic acid groups,the epoxy groups on graphene oxide can be used to attach different functional groups through a ring-opening reaction.Various amine ending chemicals such as octadecylamine[87],an io-nic liquid1-(3-aminopropyl)-3-methylimidazolium bromide[96]with an amine end group and3-aminopropyltriethoxysilane(APTS)have reacted with epoxy groups.

The non-covalent functionalization of graphene oxide utilizes the weak interactions(i.e.p–p inter-action,Van der Waals interactions and electrostatic interaction)between the graphene oxide and target molecules.The sp2network on graphene oxide provides p–p interactions with conjugated polymers and aromatic compounds that can stabilize reduced graphene oxide resulted from chemical reduction and produce functional composite materials.The conjugated polymers and aromatic compounds include poly(sodium4-styrenesulfonate)(PSS)[82],sulfonated polyaniline[103],poly(3-hexylthiophene)(P3HT)[104],conjugated polyelectrolyte[105],7,7,8,8-tetracyanoquinodimethane anion[106],tetrasulfonate salt of copper phthalocyanine(TSCuPc)[107],porphyrin[108,109],pyrene and perylenediimide decorated with water-soluble moieties[110],and cellulose derivatives[111]. During the chemical reduction of graphene oxide,reduced graphene oxide nanosheets are stabilized via the p–p interaction between aromatic molecules and reduced graphene oxide nanosheets.Aro-matic molecules have large aromatic plane and can anchor onto the reduced graphene oxide surface without disturbing its electronic conjugation,providing stability for reduced graphene oxide.For example,the sulfonate groups on TSCuPc introduce negative charges on reduced graphene oxide sheets and stabilize the RGO dispersion,providing single sheets of TSCuPc functionalized RGO for device fab-rication.In contrast,irreversible aggregation and precipitation of graphitic sheets occurred upon the reduction of graphene oxide without TSCuPc(Fig.3.7B inset).Atomic force microscopy(AFM)study (Fig.3.7)of reduced graphene oxide/TSCuPc composites provides detailed information about the indi-vidual layer of the reduced graphene oxide/TSCuPc composite sheets.The cross section analysis in the AFM height image indicates the thickness of the TSCuPc attached RGO sheet to be$1.9nm whereas the thickness of a single layer RGO was found to be approximately1nm.Therefore,the AFM height image con?rmed the non-covalent attachment of the aromatic molecules on the RGO basal plane through p–p interaction.The small dots in the AFM images are aggregates of TSCuPc[107].Dye-labeled DNA have also been used to functionalize graphene oxide to detect proteins and DNA[112].The?uorescence of the dye on the reduced graphene oxide was quenched by the substrate.In the presence of a target, the binding between the dye-labeled DNA and target molecule will alter the conformation of dye-labeled DNA,and disturb the interaction between the dye-labeled DNA and graphene oxide.Such inter-actions will release the dye-labeled DNA from the GO,restoring of dye?uorescence.

3.3.3.Structural and physical properties of reduced graphene oxide(RGO)

The optical and electrical properties of reduced graphene oxide depend on the spatial distribution of the functional groups and structural defects.For example,the electron mean free path is limited by the distance between two defective sites represented either by C A O or a vacancy[113].A giant-infrared-absorption band was observed in reduced graphene oxide attributed to the coupling of elec-tronic states to the asymmetric stretch mode of a structure consisting of oxygen atoms aggregated at the edges of defects[114].Therefore,understanding the molecular structure evolution of the GO structure during reduction is the key to obtain reduced graphene oxide with desired optical and electrical properties.The structures of RGO have been studied both theoretically[115]and experimen-tally[78,114–118].

Bagri and coworkers used molecular dynamics (MD)simulation to study the atomic structure evo-lution from graphene oxide to RGO during the thermal annealing process [115].As the author summa-rized in the paper,the carbonyl and ether groups on RGO formed from the hydroxyl and epoxy groups on graphene oxide during thermal annealing.Hydroxyl groups desorb at low temperature without altering the graphene basal plane.In contrast,isolated epoxy groups are relatively more stable,and substantially distort the graphene lattice on desorption.The removal of carbon from the graphene plane (generating structural defects)is more likely to occur when the initial hydroxyl and epoxy groups are close to each other.The reaction pathway during thermal annealing between two nearby functional groups leads to the formation of carbonyl and ether groups,which are thermodynamically very stable.These theoretical results are corroborated by FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS)experiments.

A systematic investigation of the electrical and chemical structure evolution was performed on the reduction process via thermal treatment in UHV and in an Ar/H 2reducing atmosphere on pristine GO thin ?lms and those that were previously treated with hydrazine vapor [116].The progressive loss of oxygen functional groups after each step of the reduction process was investigated by in situ XPS to reveal the change of carbon and oxygen bonds.It was found that the amount of carbon sp 2bonding increased with the loss of oxygen during the annealing process,reaching a maximum value of $80%at an oxygen content of $8%(C:O ratio 12.5:1).This suggests that the remaining oxygen

is AFM image of the RGO/TSCuPc composite on mica.Inset shows (A)Reducing RGO/TSCuPc composite ink.(B)Precipitate formed when GO was reduced without

1194V.Singh et al./Progress in Materials Science56(2011)1178–1271

responsible for$20%sp3bonding and annealing up to1100°C is not suf?cient to completely remove the oxygen from GO.Raman spectroscopy was also employed to reveal the structural evolution where the area ratio of the D and G bands was used as measure of the size of sp2ring clusters in a network of sp3and sp2bonded https://www.sodocs.net/doc/817238985.html,ing the empirical Tuinstra–Koenig relation[119]to obtain the lateral dimension of sp2ring clusters,an average graphitic domain size of$2.5nm in pristine GO was calcu-lated.After chemical(with hydrazine vapor)reduction and thermal annealing up to500°C,the change in the D/G peak area ratio was found to be negligible.A slight decrease in the full width half maxima (FWHM)of the D peak was observed only after annealing at1100°C,resulting in an increase in the size of the sp2cluster to$2.8nm.This observation suggests that even when the sp2carbon–carbon bonds are restored by de-oxidation,their spatial distribution in the honeycomb graphene lattice does not generate an expansion of a continuous sp2phase.This may be due to the fact that the sp2sites are isolated by disordered domains,which is indicated by the TEM studies[117,118].The conductivity was also measured vs.the sp2fraction.The data indicated that the presence of residual oxygen ($8%)signi?cantly hampered the carrier transport among the graphitic domains,and transport at the initial stages of reduction was dominated by hopping or tunneling amongst the sp2clusters.At lat-ter stages of reduction,newly formed smaller sp2domains connected the original sp2clusters so that charge could transport through percolation network.However,carrier transport above the percolation threshold was limited by those clusters that are not perfect graphene crystals.

Erickson et al.have investigated the local chemical structures on graphene oxide and reduced graphene oxide using the TEAM0.5TEM(a monochromated aberration-corrected instrument oper-ated at80keV)[117].GO was produced via a modi?ed Hummers method and drop cast onto lacey carbon TEM grids.For RGO specimens,GO-containing grids were reduced in a hydrazine atmosphere and then slowly heated to550°C under?owing N2.The TEM image of graphene oxide(Fig.3.8)clearly shows the oxidized area(A and B)and unoxidized graphene crystal area(Fig.3.8c).As explained in the ?gure caption,hydroxyl groups and epoxy were presented on the graphene oxide basal plane,in good agreement with Lerf–Klinowski[79,80]and Dékány Models[81].

On the other hand,the TEM image of RGO shows the disordered regions(Fig.3.9A)that are believed to result from the oxidized area being reduced by hydrazine and thermal annealing,and the unoxi-dized graphene regions.

Similarly,Gomez-Navarro and coworkers have used high resolution transmission electron micros-copy(HRTEM)to investigate the structure of reduced graphene oxide monolayers produced from chemical oxidation/reduction of graphite in atomic scale.Defect free graphene domains with sizes of a few nanometers were mixed with defect areas dominated by clustered pentagons and heptagons [120].The atomic structure of the RGO layers obtained from HRTEM is shown Fig.3.10with different regions of the image marked by colors in Fig.3.10b.The largest portion of the RGO layer is comprised of clean well crystallized graphene areas where the hexagonal lattice is clearly observed(light gray color in Fig.3.10b).The average size of the visible well-crystallized areas is from3to6nm,covering $60%of the surface.The formation of larger holes is caused by electron irradiation,similar to the TEM images of mechanically exfoliated graphene.In contrast to mechanically exfoliated graphene,RGO has a large amount of topological defects within the clean areas.These defects was classi?ed into isolated topological defects(pentagon–heptagon pairs,green),and extended(clustered)topological defects that appear as quasi-amorphous single layer carbon structures(marked in blue in Fig.3.10b).The ex-tended topological defects cover ca.5%of the surface and exhibit typical sizes of1–2nm in diameter.

According to the TEM studies,it is believed that isolated highly oxidized areas(few nm in size)are formed upon oxidation,while major graphene surface remains undisturbed.Upon reduction,the oxi-dized areas are restored to sp2-bonded carbon networks,which however lack the perfect crystallinity of intact graphene.The reduced disordered areas,which are best described as clustered topological defects,induce strain as well as in-plane and out-of-plane deformations in the surrounding RGO.Iso-lated topological defects,mostly dislocations,are also present and may have formed as a result of strain.The effects of these defects will have to be taken into account for any comprehensive study of the properties of RGO[120].

Gao et al.have investigated the structural change from graphene oxide to RGO using13C NMR[78].In their studies,RGO was produced from graphene oxide through a two-step reduction process—deoxygen-ation with NaBH4(chemically converted graphene,CCG1),followed by dehydration with concentrated

Aberration-corrected TEM image of a single sheet of suspended GO.The scale bar is2nm.Expansion

enlarged oxidized region of the material,then a proposed possible atomic structure

oxygen atoms in red,and?nally the average of a simulated TEM image of the

another structure where the position of oxidative functionalities has been

spot on the graphitic region.This spot moved along the graphitic region,but stayed

hydroxyl position(left portion of expansion(B))and for seven frames(14s)at a(1,

The ball-and-stick?gures below the microscopy images represent the proposed

simulated TEM image for the suggested structure agrees well with the TEM data.

from the exit plane wave reconstruction of a focal series of GO and the atomic

permission from[117].)

sulfuric acid(CCG2)and annealing of CCG2in Ar/H2at1100°C for15min(CCG3).The authors suggested that this process produced graphene with a very low number of remaining functional groups,high con-ductivity,larger crystallite size and good solubility.The13C NMR studies of graphene oxide illustrated the functional groups,as discussed in previous section while the13C NMR of different reduced materials, indicated that NaBH4reduction removed the epoxy groups on the basal plane with the regeneration of C@C bonds,the concentrated sulfuric acid treatment dehydrate hydroxyl groups to C@C bonds and the thermal annealing extrudes the carboxylic acid groups at the edge with the regeneration of C@C.By mea-suring the electric conductivity of the materials,the authors demonstrated the increased electrical con-ductivity(GO:4.08?10à1S/m,CCG1:8.23?101S/m,CCG2:1.66?103S/m,CCG3:2.20?104S/m) attributed to the restoration of conjugated electronic structures on RGO.

3.4.Other synthesis approaches

3.4.1.Total organic synthesis

Total synthesis of graphene-like polyacyclic hydrocarbons(PAHs),explored decades ago,has caught much attention as a possible alternative route to synthesize graphene.Although PAHs have some advantages including synthesis versatility and the capability of grafting aliphatic chains at the edge to modify solubility,the major challenge lies in preserving dispersibility and a planar geometry for large PAHs.While different PAHs synthesis routes are nicely reviewed by Mullen et al.[121],it is

important to note that Mullen’s group has made a major break-through in synthesizing two-dimension graphene ribbons with the size of 12nm through the Suzuki–Miyaura coupling of 1,4-diiodo-2,3,5,6-tetraphenylbenzene with 4-bromophenylboronic acid [122].The same group recently reported a bottom-up method to fabricate graphene nanoribbons (GNR)on gold surfaces from 10,100-dibromo-9,90-bianthryl precursor monomers [123].In the fabrication process,thermal deposi-tion of the monomers onto a gold surface removes the halogen substituents from the precursors,and provides the molecular building blocks for the targeted graphene ribbons (with a width of seven ben-zene molecules)in the form of surface-stabilized biradical species.During a ?rst thermal activation step,the biradical species diffuse across the surface and undergo radical addition reactions to form lin-ear polymer chains as imprinted by the speci?c chemical functionality pattern of the monomers.In a second thermal activation step,a surface-assisted cyclodehydrogenation establishes an extended fully aromatic system (Fig.3.11

).

TEM image of a monolayer of RGO.The scale bar is 1nm.Expansion micrograph,then a proposed possible structure for the region where functionalities on the sheets,and ?nally a simulated TEM image structure of a graphitic region.(Reproduced with permission from [117]

3.10.Atomic resolution,aberration-corrected TEM image of a single layer reduced graphene oxide membrane.(a)Original

and(b)with color added to highlight the different features.The defect free crystalline graphene area is displayed original light gray color.Contaminated regions are shaded in dark gray.Blue regions are the disordered single-layer networks,or extended topological defects,identi?ed as remnants of the oxidation reduction process.Red areas highlight individual ad-atoms or substitutions.Green areas indicate isolated topological defects,that is,single bond rotations dislocation cores.Holes and their edge reconstructions are colored in yellow.Scale bar1nm.(Reproduced with permission .)

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