Halogenated Graphenes Rapidly Growing Family of Graphene Derivatives

8/9/2019 Halogenated Graphenes Rapidly Growing Family of Graphene Derivatives

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KARLICKÝ ET AL . VOL. 7 ’ NO. 8 ’ 6434–6464 ’ 2013www.acsnano.org

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June 28, 2013

C 2013 American Chemical Society

Halogenated Graphenes: Rapidly Growing Family of GrapheneDerivativesFranti sek Karlick y, Kasibhatta Kumara Ramanatha Datta, Michal Otyepka, * and Radek Zbo ril*

Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palack y University, 17. listopadu 1192/12,771 46 Olomouc, Czech Republic

Graphene, comprising a single layerof sp2-bonded carbon atoms ar-ranged in a honeycomb lattice,

was the rst two-dimensional atomic crys-tal to be identi ed. Since its initial isola-tion in 2004, many studies have shownthatthis one-atom-thick materialof carbonuniquely combines superior mechanicalstrength, remarkably high electronic andthermal conductivities, highsurface area, andimpermeability to gases, in addition to manyother desirableproperties,all ofwhich makeithighly attractive for numerous applications.1

Many of the unique physical properties of graphene stem from its unusual electronicstructure near the Fermi level.2 The abilityto tailor its properties, especially the openingofa bandgap, is critical for its potential use inseveral (opto)electronic applications.3

In the past few years, covalently modi-ed graphene derivatives prepared by at-

tachment of hydrogen, halogens, or otheratoms have attracted considerable interestfor their potential applications (e.g., in elec-tronic devices).4 6 The relative simplicity of

atomic adsorbates allows them to be alsowell described by theoretical calculations.7Theidea to fully hydrogenategraphene wasput forward in 2003 ahead of graphene'sisolation, and several possible geometricalstructures were considered,8 even thoughat that stage it was clearly a hypotheticmaterial. In 2007, fully hydrogenated gra-phene was named graphane by Sofo et al .,9

who also discussed its electronic propertiesand its uorinated counterpart. Two yearslater, graphane was synthesized as a stablematerial under ambient conditions by expo-

sureofgraphenetocoldhydrogenplasma.10,11

Thefully uorinatedgraphenecounterpart,uorographene (graphene uoride, C1F1),

was prepared experimentally in 2010(refs 12 14) by chemical and mechanicalexfoliation of graphite uoride, that is, byapproaches that had been successfully ap-plied to prepare graphene from graphite.Graphite uoride is a well-known graphitederivative(uorine-intercalatedcompound)15

with covalent C F bonds. Although bulk graphite uoridehasbeenused asa lubricant

* Adress correspondence [email protected],[email protected].

Received for review May 13, 2013and accepted June 28, 2013.

Published online10.1021/nn4024027

ABSTRACT Graphene derivatives containing covalently bound halogens

(graphene halides) represent promising two-dimensional systems having inter-

esting physical and chemical properties. The attachment of halogen atoms to sp2

carbons changes the hybridization state to sp3 , which has a principal impact on

electronic properties and local structure of the material. The fully uorinated

graphene derivative,

uorographene (graphene

uoride, C1 F1), is the thinnestinsulator and the only stable stoichiometric graphene halide (C1X1 ). In this

review, we discuss structural properties, syntheses, chemistry, stabilities, and

electronic properties of uorographene and other partially uorinated, chlorinated, and brominated graphenes. Remarkable optical, mechanical,

vibrational, thermodynamic, and conductivity properties of graphene halides are also explored as well as the properties of rare structures including

multilayered uorinated graphenes, iodine-doped graphene, and mixed graphene halides. Finally, patterned halogenation is presented as an interesting

approach for generating materials with applications in the eld of graphene-based electronic devices.

KEYWORDS: graphane .graphene semiconductor. uorination.doping .magneticgraphene .band gapopening .chlorographene.chlorination . graphene oxide . graphene dot

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for nearly 100 years and has also been exploited as anexcellent electrode material in primary lithium batteries,graphene uoride was not experimentally realizeduntil 2010. Alternative ways of preparing uorinatedgraphene12,16 or graphite17,18 by cold plasma weredeveloped simultaneously. However, this approachoften generatesnonstoichiometricuorinatedgraphenes(C1Fa , a < 1). The attachment of uorine atoms to sp2carbons changes the hybridization state to sp3 (Figure 1aand Figure 1b), which signicantly aff ects the electronicpropertiesandlocalstructureof thematerialbutpreservesthe 2D hexagonal symmetry. Such structural changesinduce opening of the zero band gap of graphene atthe K point and lead to loss of the π -conjugated electroncloud above and below the graphene plane. The chargecarrier mobility has been shown to be 3 orders of magni-tude smaller for uorographene in comparison to gra-phene,and uorographenebehaves like an insulator witha minimal direct band gap at theΓ point (Figure 1c,d).

To date, dozens of experimental and theoreticalpapers on halogenated graphenes have been pub-lished. Besides the stoichiometric uorographene, nu-merous partially uorinated grapheneshave also beensynthesized,19 24 where the degree of uorinationenables control over the electronic properties, whichmight be utilized in band gap engineering of 2Dcarbon-based materials. Contrary to uorographene,the fully chlorinated counterpart has not been yet pre-pared,whilepartiallychlorinated25 28 or brominated27,28

graphene derivatives have been reported very recently.Halogenatedgraphenes exhibit a plethoraof remarkableand interesting electronic, optical, thermal, electrocataly-

tic, magnetic, mechanical, biological, and chemical prop-erties in comparison with their graphene counterparts.Here, we classify the graphene halides and review

preparation approaches, properties, and applications of halogenated graphenes. The review provides a complexoverview as it considers both experimental and theore-tical aspects of graphene halogenation. First, the synthe-sis of graphene halides is discussed. Next, properties of uorographene and partially uorinated graphenes are

summarized including insightful theoretical studies.Thesesectionsfocus onstructuralandvibrationalproper-ties, which can be used as ngerprints of the consideredmaterials, as well as electronic, optical, and mechanicalpropertiesofgraphenehalides, whichare importantfroman application perspective. Next, we summarize recentprogresson chlorinatedandbrominatedgraphenes fromboth theoretical and experimental standpoints. Finally,we discuss patterned structures on graphene.

Synthesis of Graphene Halides. Synthesis of Fluorogra- phene and Fluorinated Graphenes. There are twomain methods to prepare fluorographenes.12,13,16

One approach involves transformation of graphenesto fluorographenes by fluorination using an appro-priate fluorinating agent (see below).12,16 The otherprocedure utilizes chemical or mechanical exfoliation

of pristine graphite fluoride, i.e., the same techniquesthat have been successfully applied for the preparationof graphene from graphite.29,30

Fluorographene can be prepared by uorinatinggraphene using XeF2 at various temperatures under aninert atmosphere (Figure 2a)12,31 orat room temperature(30 C).16 The room temperature preparation of uoro-graphene involves uorinating graphene supported ona silicon-on-insulator substrate using XeF2 gas, whichselectively etches the Si underlayer and uorinates bothsides of thegraphene to form fullyuorinatedgraphenewith a dominant stoichiometry of C1.0F1.0 .16 In addition,uorination of highly oriented pyrolitic graphite (HOPG)

by uorine gas under high temperature (600 C) andsubsequentchemicalexfoliationhasbeen showntoyielda nonstoichiometric uorographene (C1F0.7) of low qual-ity owing to numerous structural defects caused bythe harsh preparation conditions.17 It should be noted,that uorination of graphene grown by chemical vapordeposition (CVD) on copperhasbeen reported to yield asingle side partiallyuorinatedgraphene with dominantstoichiometry of CF0.25 .16

VOCABULARY:: graphene derivatives a class of materials having two-dimensional scaff old, which are de-rived from graphene by covalent attachment of atoms,functional groups or molecular moieties. Graphane, gra-phene oxide, and uorographene represent the typicalexamples of graphene derivatives; uorographene stoichiometric C1F1 derivative of graphene with the uor-ine atom attached to each carbon atom thus having sp3

hybridization. Fluorographene can be prepared by gra-phene uorination or exfoliation of graphite uoride.Fluorographene represents still the only stoichiometricgraphene halide stable at ambient conditions;haloge-nated graphenes graphene derivatives, in which somecarbon atoms are covalently linked with halogen atoms.The carbon atoms linked with halogens have sp3 hybridi-zation and others have sp2 hybridization. The physico-chemical properties of graphene halides are stronglydependent on a degree of halogenation;patterned halo-genation the outcome of placing masks or metal grids

over graphene during halogenation process is known aspatterned halogenation. Uncovered regions of the gra-phene become halogenated and the masked regionsremain intact and can be used as conductive pathwaysfor device fabrication or construction of graphene super-structures;bandgap engineering graphenelacksabandgap because its valence and conduction bands touch eachother,andit islabeledas a semimetal.Thelacking bandgaplimits usage of graphene in contemporary electronic de-vices. The band structure of graphene can be modied toopen the band gap by many strategies, e.g. , halogenation,oxidation, hydrogenation or noncovalent attachment of various moleculesandspecies.A complex wayhowto open

and tune the band gap in graphene and its derivatives istermed as band gap engineering;

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Stoichiometric uorographene canalsobe preparedby a top-down approach employing mechanical12,14 orchemical13,23,32 exfoliation of graphite uoride. Gra-phene uoride in the form of colloidal suspensionscan be obtained by chemically etching bulk graphiteuoride via sonication in the presence of sulfolane,13

dimethylformamide(DMF),32 orN -methyl-2-pyrrolidone(NMP)23 (Figure 2b). In this process, the solvent mol-

ecules intercalate within the interlayers, weakening thevan der Waal's interactions between neighboring layersand facilitating the exfoliation of graphite uoride intocolloidal uorographenes. In the case of mechanicalexfoliation, the as prepared uorographene mono-layers are of high quality and are suitable for physicalexperiments. However, such an approach is difficultto scale-up for potential applications. On the otherhand, chemical exfoliation enables preparation of largeamountsof graphene uoride.However, polydispersivesystems are formed containing one- and few-layeruorographenes. Nonetheless, the solutionprocessabil-

ityof uorographene colloids could beadvantageous inthe eld of coatings, polymer nanocomposites, etc.

Fluorinated graphene with diff erent uorineloadings can be achieved by uorinating grapheneor chemically etching nonstoichiometric graphene uo-ride. Briey, uorination of graphene or reduced gra-phene oxide sheets (mono- and multilayered) is usuallycarried out in plasmas containing CF4 (refs 33, 34) andSF6,35 XeF2,36,37 uoropolymers20 or Ar/F2 (ref 38) asuorinating agents. The uorine content of the result-

ing uorinated graphenes can be varied by changingthe plasma treatment time as well as the uorinatingagent.33,36,39

Recently, Ruoff and co-workers20 have developeda versatile and environmentally friendly approach forselective or patterned uorination of graphene (on aSiO2/Si substrate) using the uoropolymer CYTOPcombined with laser irradiation. In this method, directcontact between CYTOP and the graphene surface isachieved by transferring graphene lms on Cu foilonto a SiO2/Si substrate coated with CYTOP. After laser

irradiation, photon-induced decomposition of CYTOPgeneratesmany activeintermediates, suchasCF x andFradicals, which react with sp2 hybridized grapheneforming C F sp3 bonds (Figure 2c). This process yieldssingle side uorination (25% coverage) as the activeuorine species cannot penetrate through the SiO2/Si

substrate.Similar to the isolation of uorographenes from

bulkgraphite uoridevia chemical etching, uorinatedgraphenes canbeexfoliatedfrom graphiteuoridewithdiff erent uorine contents in various solvents with/ without surfactants via sonication19,24,40 or mechanicalexfoliation.41 Fluorinated graphenes with compositionsCF0.25 and CF0.5 have been obtained by exfoliatinggraphite uoride with uorinated ionic liquids, asdescribed by Zheng et al .19 Very recently, multilayersemi-ionicallyuorinated graphenewaspreparedusinga one-pot synthesis employing liquid ClF3 and graphite(5 h). The obtained sample was subsequently mechani-cally exfoliated to single and bilayer lms.42,43

Fluorine-doped multilayered graphene (10 wt %,i.e., 6.6 atom %) has been synthesized using an arcdischarge process in which a hollow graphite rod islled with powdery graphite uoride.22 The uorine

content of the graphene matrix can also be tuned

Figure 1. (a) Hexagonal structure of sp2 carbons in graphene. (b) Transformation to sp3 carbons by uorination in the chairconformation of uorographene. Electronic band structures of (c) graphene and (d) uorographene. The original graphenezero band gap at the K point is opened to a band gap with minimal value of several eV at theΓ point (red circles).

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by the chemical reaction of graphene oxide withhydro uoric acid.21 Fluorination of graphene oxidecanbe done byexposinggrapheneoxidetoanhydrousHF vapors at various temperatures44 or photochemi-cally at room temperature using HF solution.45

Synthesis of Other Halogenated Graphenes. Re-cently, a nondestructive and patternable photochemi-cal chlorination treatment of single- and few-layergraphene has been used to produce a material withstoichiometry CCl0.08 .25 The chemical patterning wasachieved using a chlorine-resistant Al/Ti mask to pro-tect selected regions of graphene from photochlorina-tion. In this approach, chlorine radicals covalentlyattach to the basal carbon atoms of the graphene(C Cl), transforming from a sp2 to sp3 configurationand creating high structural disorder. The chlorineplasma technique allows controlled p-type doping

of graphene sheets and graphene nanoribbons.26

The chlorination of graphene occurs in two stages: inthe first stage, chlorination occurs rather nondestruc-tively and reversibly, whereas in the second stageat longer exposure times (>2 min), larger-area defectsbegin to form irreversibly. Graphene nanoribbons(GNR) supported on SiO2/Si and exposed to Cl plasma(1 min) showed a 1.32.2 fold increase in conductance(in ambient air). Similarly, after Cl plasma treatment for10 s, graphene sheets showed a slight increase (withrespect toGNR) inconductance,whereas longerexposureresulted in reduced conductivity. Compared to fluorina-tionand hydrogenation, thechlorineplasmareactionwithgraphene exhibits the slowest kinetics, showing only aslow increase in disorder with reaction time.

Recently, Rao and co-workers28 prepared few-layerchlorinated andbrominatedgraphenesup to30atom %

Figure 2. (a) Various steps involved in the uorination of graphene (PMMA poly(methyl methacrylate)). Reprinted withpermission fromref12.Copyright 2010 Wiley. (b) Schematic of the NMPintercalationandexfoliation fabrication processes usedtoprepareCFdispersions.Reprintedwith permission fromref23.Copyright 2012 Royal SocietyofChemistry. (c) Scheme showing themechanism of uorination using CYTOP and laser irradiation. Reprinted from ref 20. Copyright 2012 American Chemical Society.

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(CCl0.43 ) and 5 atom % (CBr0.05 ), respectively, usingultraviolet (UV) irradiation in liquid chlorine or liquidbromine media. A microwave-spark (MiW-S) assistedreaction has also been developed that allows thedirectproduction of Cl andBr-functionalized monolayergraphene sheets from graphite.27 During microwaveirradiation, graphite was shown to expand to 200 timesits original volume accompanied by luminous sparksand moderate ionization of the halogens (Xþ ). Theionized halogen atoms readily attacked the π -ring of graphene, resulting in halogenation, probably via anelectrophilic substitution mechanism. The amount of Clatoms on the graphene sheets was up to 21 atom %(CCl0.27 ),whereasBrwas4atom%(CBr0.04 ),asmeasuredby XPS analysis.27 Chlorination was shown to be moreeff ective than bromination as liquidCl2 is more reactivethan Br2.27,28 Thermal annealing or laser irradiation of thepreparedchlorinatedgraphene samples completelyremoved the chlorine to obtain pristine graphene.27,28

Interestingly, the halogen atoms in the resulting CXcould be substituted/modied by other organic func-tional groups under conventional organic reaction con-ditions, which opens up other possibilities forpreparinggraphene derivatives.27

Iodine-doped graphenes with an iodine loadingof 3 atom % have been prepared by direct heating of camphor and I2.46 In addition, exfoliation of grapheneoxide and I2 under ultrasonic treatment followedby thermal annealing at various temperatures (5001100 C) has been shown to generate iodine-dopedgraphenes (1.2 0.8 wt %, i.e., ∼0.1 atom %).

47 Veryrecently, halogenated graphenes have been prepared

by the thermal exfoliation of graphite oxide underdiff erent gaseous halogen atmospheres (chlorine, bro-mine, or iodine),48 generating halogenated grapheneswith doping levels of 5.9, 9.93, and 2.3 wt % (2.1, 1.6,and 0.2 atom %) for Cl, Br, and I, respectively.

Fluorographene (Graphene Fluoride, C1.0 F1.0 ). Structural Properties of CF. The structure of graphene can bederived from the structure of the 3D pristine materialbulk graphite, which comprises stacks of graphenelayersthat areweaklycoupled by vanderWaals forces.Similarly, the geometrical structure of fully fluorinatedgraphene (fluorographene, graphene fluoride, CF) canbe deduced from the structure of bulk graphite mono-fluoride (CF)n . Graphite fluoride has been shown toconsist of weakly bound stacked fluorographene layers,and its most stable conformation (predicted for themonocrystal) contains an infinite array of trans-linkedcyclohexane chairs with covalent CF bonds in an ABstacking sequence49,50 (Figure 3a), in agreement withthe model proposed from X-ray powder diffraction ex-periments by Touhara et al . (Figure 3b, AA0 stacking).51

However, for many years, it was also believed to containstacked structures with cistrans-linked cyclohexaneboats, as predicted from NMR second momentmeasure-ments(Figure3c,d).52 Variousstackingsequencesof(CF)n

arevery close on the energy scale, suggesting that in thestructure of (CF)n prepared, for example, by fluorination,a statistical distribution of various sequences can occur.

To gain a better understanding of the struc-tural properties of uorographene, prototype stoichio-metric con gurations for uorographene, for example,chair, boat, zigzag (stirrup), and armchair (Figure 4),have been studied theoretically. On the basis of rst-principles density functional theory (DFT) calculations,the chair conformation was predicted to be moststable,53,54 analogous to fully hydrogenated graphene(graphane). The zigzag (stirrup) conguration wasfound to be more stable than the boat and armchaircon gurations,and its formationenergy waspredictedto be only slightly higher than that of the chair cong-uration. The energy diff erences between the variouscon gurations were more pronounced for uoro-graphene than for fully hydrogenated graphene, but

they were of the same order of magnitude. The resultssuggestedthat theexperimentallyobserved uorogra-phene is unlikely to be in a single crystal form of chair,boat, or stirrup because each of those structures hasonly one or two distinctive in-plane lattice spacings,in contrast to the wide range of distributions observedexperimentally. Wide distributions of lattice parametersfor both uorographene54 and graphane55 have beenpredicted by simulations with large supercells contain-ing randomly distributed adsorbates, suggesting otherlocally stable congurations (e.g., twist-boat-chair orepoxy-pair conformations) are possible.

Nonetheless, in calculations of electronic, optical,and other properties of uorographene (see below),the chair conformation is usually considered themost stable (Figure 4), taking advantage of the highsymmetry. It corresponds to a unit cell of two uorineandtwocarbonatoms(P-3M1 (164) space group orD3dpoint group) with translation vectorsa1 =d (√ 3/2,1/2,0),and a2 = d (√ 3/2, 1/2, 0), and lattice parameter pre-dicted from standard generalized gradient approxi-mation (GGA) DFT calculations of d = 2.61 Å; thecorresponding C C and C F bond lengths are 1.58 Åand 1.38 Å, respectively.53,56 59 The uorographene lat-tice structure and the real and reciprocal space elements

Figure 3. Structural models of graphite uoride optimizedby GGA DFT calculations (gray and cyan colors indicatecarbon and uorine atoms, respectively). The numbersunderneath each structure show the calculated heats of formation (in kcal/mol). Reprinted from ref 49. Copyright2010 American Chemical Society.

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are shown Figure 5. The calculated structural parametersare not very sensitive to density functional choice (e.g.,the C C and C F bond lengths are predicted to be1.57 Å and 1.36 Å, respectively, using the hybridHSE06 functional).58 However, DFT calculations aregenerally sensitive to the choice of exchange-correla-tion functional, and therefore it is highly desirable to

cross-check the calculated data.Electron diff raction analysis of uorographene hascon rmed the existence of a hexagonal crystallinestructure12,13,17 and stoichiometry13 equivalent tothat of graphite uoride (Figure 6). It has been shownthat the retention of hexagonal crystalline order foruorographene is similar to that of graphene with 1%

expansion of the unit cell (Figure 7):12,17 compare theC1F1 experimental lattice constant of 2.48 Å versus 2.46Å for graphene. The increased unit cell and in-planelattice constant of CF is expected as the carbon atomsmaking C C bonds are converted from a sp2 to sp3

con guration during the uorination process accom-panied by an increase in the CC bond length.The bonding and composition of uorographenes hasalso been characterized by X-ray photoelectron spec-troscopy (XPS) and Raman spectroscopy. XPS analysisshowed the majority of bonding in this material is CF(86%), with smaller fractions of CF2 andC F3 species(owing to defects at the free edges).16 These defectsare believed to originate during the graphene transferprocess and vary from one support to another; forexample, larger quantities of CFn (n > 1) specieswere observed for uorinated graphene samples ona silicon-on-insulator substrate compared to those on

copper.16 Notably, the support also seems to aff ect theuorine content. Early experiments suggested that the

lateral dimensions of uorographene sheets typicallyrange from 200 nm to 2 μm (see Figure 6).12,13

De nitive proof of the presence of uorographenemonolayers has been obtained by atomic force micro-scopy (AFM) experiment (Figure 8), which revealed thatthe monolayer is 0.670.87 nm thick, although multi-layered sheets ca. 2 4 nm thick were also detected.13,17

Theexperimentalvalues(allowingfortheeff ectsof signalnoise and the presence of solvent impurities on thesurface or support), were in agreement with theoreticalestimates of the thickness of a single CF layer of 0.62 nmand were certainly less than the predicted thickness of a two-layer graphene uoride system of 1.24 nm.13

Electronic Properties of CF. The I V characteristicsof fully fluorinated graphene are strongly nonlinear

Figure 5. (a) Honeycomb lattice structure of uorogra-phene,made out of two interpenetrating triangular lattices(a 1 and a 2 arethelatticeunitvectors,andδ i , i =1,2,3arethenearest-neighborvectors) (b)corresponding to theBrillouinzone, the reciprocal basis vectors and high-symmetrypoints Γ , M, and K. Reprinted with permission from ref 2.Copyright 2009 American Physical Society.

Figure 4. Four diff erent con gurations of uorographene: (a) chair, (b) boat, (c) zigzag (stirrup), and (d) armchaircon gurations. The diff erent colors (shades) represent uorine atoms above and below the graphene plane. The supercellused to calculatethe elastic constantsis indicatedby thedashedbox (see sectionsbelow). Reprintedwith permission from ref 53. Copyright 2010 American Physical Society.

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with a nearly gate-independent resistance greater than 1GΩ , suggesting the presenceofa bandgap (whichisalsoexpected because of the sp3 carbon net, Figure 1).12,14,17

Devices fabricated from fully fluorinated grapheneshowed no leakage current at biases up to 10 V.The possibility of fabricating a transistor structureusing fluorinated monolayer graphene has also beendemonstrated.14 Fluorination has been shown to causea considerable increase in the resistance in the electro-neutralityregionowingto the creation ofa mobility gap

in the electronic spectrum where electron transportoccurs through localized states.

The density of states of uorographene has beeninvestigated by near edge X-ray absorption spectros-copy (NEXAFS).31 Pure graphene showed peaks at285.5 and 291.5 eV, corresponding to transitions tothe π * and σ * conduction states, respectively. The π *feature (characteristic of sp2 bonding) gradually de-creased with increasing uorination, providing directevidence of the formation of sp3 bonds in uorogra-phene (Figure 9). A broader hump at 288.4 eV wasinterpreted as the uorographene conduction bandedge and a much sharper peak at 287.4 eV wasattributed to an exciton absorption line. The changein energy diff erence between the leading edges of the NEXAFS spectra (dashed lines in Figure 9) andthe corresponding C 1s core level binding energiesof pure graphene and uorographene, a lower limitof 3.8 eV was estimated for the uorographene bandgap.31 We note that for partially uorinated graphene(CF0.25 ) a band gap of 2.9 eV was obtained from dI /dV measurement.60

Theoretical predictions of the electronic propertiesof uorographene have mainly focused on the bandstructure, namely band gap. Despite extensive re-search eff orts, questions remain regarding the bandstructure of uorographene.59 The band gap is usually

Figure 6. (a,c) Transmission electron microscopy (TEM) images of CF sheets obtained after graphite uoride exfoliation insulfolane. (b) Arrows in the HRTEM image indicate highly transparent graphene uoride monolayers. (d) The selected areaelectron diff raction (SAED) pattern conrms the stoichiometry and structure of the layers corresponding to the originalgraphite uoride. Reprinted with permission from ref 13. Copyright 2010 Wiley.

Figure 7. Transmission electron microscopy of CF sheetsobtained from grapheneexposed to atomicF. (a)Diff ractionpattern from a CF membrane. (b) Lattice constantd measuredusing microscopy images, such as that shown in panel a. Forcomparison,similarmeasurementsweretaken formembranesbefore uorination(lefthistogram). Thedotted lineindicatesd forgraphite.Reprinted withpermission fromref12.Copyright2010 Wiley.

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considered for the chair conformation of uorogra-phene with a unit cell containing four atoms (seeFigure 5 showing the unit cell and Brillouin zone).Traditional local density approximation (LDA) or gen-eralized gradient approximation (GGA) to DFT predictsthat uorographene is a direct band gap material: thebottom of theconduction band and topof thevalenceband arelocatedat theΓ pointinthe rstBrillouinzone

(Figure 10). The top of the valence band is doublydegenerate and the maximum band gap is located atthe K points. The minimal direct band gap of uoro-graphene of about 3.1 eV 53,56 59,61 63 predicted fromGGA DFT indicates that uorographene possessesinsulating properties, in agreement with early cal-

culations on graphite uoride.50,64

Band gap valuesof other uorographene conformations (Figure 4) arerather similar or slightly higher up to 4.2 eV for thearmchair con guration.53,54 However, one main limita-tion of the DFT approach is it is an inherently ground-state theory. LDA and GGA functionals systematicallyunderestimate Kohn Sham band gaps (comparedto experimentally determined values), whereas theHartree Fock method systematically overestimatesthem. Hybrid functionalscontain a fraction of HartreeFock exchange, and their computationally accessibleshort-rangevariants (asbyHeyd,Scuseria,and Ernzerhof (HSE)65 ) are often eff ective for predicting band gapsof solids and low-dimensional carbon materials:66the HSE06 functional67 predicts a band gap of about5 eV for uorographene.58 The high-level many-bodyGW approximation (GWA), which includes electronelectron interactions beyond DFT, predicts a band gapof 7.0 8.3 eV 53,54,56,59,68 70 (depending on GW leveland orbitals used),59 that is, around two times largerthan the value obtained with GGA DFT (Figure 10).Any agreement between GGA DFT and experimental“ optical band gap” values (>3.0 or >3.8 eV, see below)is coincidental. On the other hand, accurate GWelectronic band gaps and the energies of electron

Figure 8. (a,b) Two independent AFM images of graphene uoride monolayers and (c,d) their height proles, providingevidence that the layers are


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transitions derived from optical spectra do not exactlymatch each other, because the electron transitionsobserved in optical spectra involve formation of anexciton. Therefore DFT and GW band gaps shouldnot be directly compared with absorption peaks inoptical spectra and reasonable agreement with opticalmeasurement can be achieved only after inclusionof electron hole correlations into calculations (seebelow for BSEGWA spectra).59,70 Eff ect of variousdefects on discussed electronic properties is probablyrather small.59 GWApredicts that thebandgapof uoro-grapheneis greater thanthatofgraphane, whereasGGAfunctionals suggest the opposite trend. Recent calcula-tions have shown that it is possible to obtain the same

order of bandgaps with DFT as from GWif the screenedhybrid functional HSE06 is used.58,59

Recently, spin orbit couplings (SOCs) of uorogra-phene have been calculated by DFT.61 The resultssuggested that SOC-induced band splittings near theirFermi energies in uorographene are signicantlyhigher(of the order of102 eV)than forpure graphene(of the order of 106 eV) and are comparable to valuesfor diamond and archetypal semiconductors.61

Optical Properties of CF. Studies of the absorptionspectra of pristine, partially fluorinated or fully fluori-nated graphene have revealed that they exhibitdramatically different optical properties (Figure 11).Graphene shows an absorption spectrum that is rela-tively flat for light energies


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uorination(1dayof uorination), anadditionalemissionpeak at 2.88 eV was observed, which was also accom-panied by a second peak located 157 meV (Figure 12).The optical band gap of uorographene is nearly 3.8 eV,wide enough foroptoelectronic applicationsin theblue/ UV region.31

The rst exciton peaks in the uorographene ab-sorption spectrum have been predicted theoreticallyusing Bethe-Salpeter equation (BSE) on top of GWA,

whichaccountsforelectronelectron(e e)andelectronhole (e h) correlations, and compared with spectrafrom therandom-phase approximation (RPA)on topof GWA (without eh).54,59,68,70 For the single-particleresult (Figure 13a, the blue curve, without eh) theonset of the spectrum was located at the GWA bandgap and the absorption pro le increased very slowlyuntil near 10 eV. This was interpreted as small spatialoverlapping of the wave functions of the valence bandmaximum (VBM) and the conduction band minimum(CBM), implying that the lowest optical transition fromVBM to the CBM (Figure 13b, transition v1c1) is weak.Thenotable optical absorption starting from 10 eV wasmainly attributed to transitions from high energyvalence bands to the second conduction bands withenergies varying from 8 to 11 eV (Figure 13b, transi-tion v1 c2). When e h interactions were taken intoaccount, a prominent peak was observed, which origi-nated from a few strong resonant excitonic states thatappeared at 9 10 eV 54,59,68,70 (Figure 13a). The majorcontribution to this feature was related to transitionsfrom the top two valence bands and the lowest fourconduction bands (Figure 13b, two dominant transi-tions v1 c2 and v2 c1). In the low-frequency opticalabsorption spectrum, a bound exciton at 3.8 eV

(or alternatively 5.4 eV 54 and 5.1 eV 59,70 ) was identied

with a huge binding energy of 3.5 eV (or alternatively2 eV) and comparatively weak optical activity (inset of Figure13a). Thelarge occupationprobability associatedwith the low energy of this excitonic state suggests itmight play a vital role in the photoluminescence of thismaterial and explains the prominent ultraviolet lumi-nescence peak reported experimentally by Jeon et al.31

Vibrational Properties of CF. Theoretical analysisof vibrational modes can provide valuable insightsinto the experimental Raman and infrared spectra of fluorographene CF. In particular, since Raman spectraconvey information on a particular structure, andhence can be viewed as its signature, observed peaksare compared to the calculated Raman-active modes.Theoretical analysis has revealed that the phononspectra and density of states (DOS) of the most stablechair conformation do not show clearly separatedgroups of phonons (Figure 14). The dominant contri-bution to the acoustic modes has been attributed tothe fluorineatoms, whereas thehigh-frequencymodesaround 1200 cm 1 have a clear carbon character.71

Twelve phonon modes of the chair conformation of fluorographene, which has P 3m1 symmetry (D3d pointgroup), have been shown to belong to four irreduciblerepresentations: Eg and A1g are Raman active modes,

Figure 12. Photoluminescence emission spectra recordedat room temperature (excitation at 290 nm, 4.275 eV) of graphene (blue) and uorographene (green,1 day uorina-tion; red, 5 days). Reprinted from ref 31. Copyright 2011American Chemical Society.

Figure13. (a)Optical absorption spectra of uorographenewith (red line, BSE-GWA) or without eh interactions (blueline, RPA-GWA). (b) GW band structure of uorographene.Reprinted with permission from ref 68. Copyright 2011 TheMaterials Research Society.

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while Eu and A2u are infrared active modes (Table 1).Some of the phonon branches of the CF boat con-formation have imaginary frequencies, and hence it ispredicted to be unstable in spite of the fact that such astructure can be optimized.69 However, the possibilitythat this unstable structurecan occurat finiteand smallsizes cannot be excluded.

Because Raman spectroscopy provides a wealth of useful information, it hasbeenactively used duringtheuorination process of graphene (Figure 15).12,14,16,17

However, because the energies of the lasers usedwere lower than the band gap (uorographene is

wide band gap material), no signature Raman activityfrom fully uorinated regions was observed in thoseexperiments12,14,16 and the only Raman peaks wereassociated with graphene. The main features in theRaman spectra of pristine graphene are the G- and 2D-bands, which occur around 1580 and 2680 cm1,respectively (Figure 15). The G-band is associated withthedoubly degenerateE2g phononmode of graphene,whereas the 2D mode (also called G0) originates froma second-order process, involving two phonons nearthe K point without the presence of any kind of disorders or defects. Conversely, the presence of de-fects in the sample activates additional peaks in theRaman spectra of graphene.TheD, D0,andDþ D0peaksinvolve phonon modes from graphene, and henceoccur at the same frequencies in partially uorinatedsamples (at 1350, 1620, and 2950 cm1, respectively;Figure 15a).12,14,16 These Raman peaks originate fromdouble resonance processes at the K point in thepresence of defects. The D and G peaks of grapheneprovide valuable information on thedensityof defects.These peaks disappear for almost fully covered sam-ples, which has been found for uorographene12,14,16

but not in previous experiments on graphane orgraphite uorides.

The rst report on Raman signatures of CF in thelow-frequency region by using a UV laser (with energyof 5.08 eV) was by Wang et al.18 Two Raman activemodes were detected at 1270 and 1345 cm1, whichare absent under lower laser energies. Even thoughthe CF samples also contained multilayer regions, theexperimental values correlated with the aforemen-tioned DFTphonon frequencies (Figure14): thehigher-frequency mode as A1g mode with out-of-planemotions of F against C (at 1312 or 1305 cm1)69,71

and the lower-frequency mode as a 2-fold degeneratein-plane E g vibration (at 1244 or 1264 cm1).69,71

FTIR spectra obtained in transmission mode(Figure 16a) have shown that uorographene exhibitsmuch stronger IR bands than graphene due to thegreater transparency of the former.31 Furthermore, aprominent feature at1260 cm1 characteristic of cova-lent C F bond stretching72 has been observed. Foruorographene exfoliated by N -methyl-2-pyrrolidone

(NMP), peaks at 1212 and 1084 cm1 were observed

Figure 14. (a) Phonon dispersion of CF in the chair conformation and the corresponding phonon DOS. Reprinted withpermission from ref 71. Copyright 2011 American Institute of Physics. (b) Symmetries, frequencies, and descriptions of Raman-active modes of CF. Reprinted with permission from ref 69. Copyright 2011 American Physical Society.

TABLE 1. List of Symmetries and Phonon Frequencies atDifferent k -points for Fluorographene. R and I IndicateRaman and Infrared Active Modes, Respectively 71

symmetry (activity) Γ M K

A2u (R) 0 211 260Eu (I) 0 260 260Eu (I) 0 291 298Eg (R) 250 318 356Eg (R) 250 459 510Eu (I) 294 580 510Eu (I) 294 633 961A1g (R) 686 1039 962

Eg (R) 1244 1106 962Eg (R) 1244 1116 1114A2u (I) 1219 1117 1114A1g (R) 1312 1188 1150

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corresponding to the stretching vibration of the CFcovalent bonds and the semi-ionic stretching vibrationofC F bonds, respectively(Figure 16b).23 Studies ofFTIRspectra with increasing uorination time (Figure 16c)have revealed an absorption band at 1112 cm1 relatedto the 'semi-ionic' CF, which gradually changes intoa strong band at 1211 cm1 attributable to covalentC F bonding.73 An experimental infrared active modeat 1204 cm 1 has also been reported.18 The aforemen-tioned frequencies of the experimental infrared activemode corresponding to C F stretching agree with the

calculated infrared active mode A2u at the Γ point(at 1219 cm 1 , Table 1).71

The experimental determination of the phonondispersion relations of uorographene can be veryuseful in thecharacterization of this material.Asuoro-graphene is a wide band gap material, this allowsdiscrimination of the Raman activity originating fromthefully uorinatedregionsduring synthesisby usingalaser with appropriate energies.71

Mechanical, Thermodynamical Properties and Stabi-lity of CF. Graphene and its derivatives graphane and

Figure15. Raman signatures of CF.(a) Evolution of thespectra fora graphene membrane exposed to atomicF andmeasuredasa function of time under thesame Raman conditions.Thecurvesare shifted forclarity.(b) Intensities of theD and2D peaks(normalized with respect to the G peak intensity) as a function of uorination time. The solid curves are for guidance only.(c)Comparison of CF membraneswith graphiteuorideand itsmonolayer. Reprinted with permission from ref12. Copyright2010 Wiley.

Figure16. (a) FTIR spectra of CFprepared byuorinatinggraphene,pristinegraphene, andgraphite uoride.Reprintedfromref 31. Copyright 2011 American Chemical Society. (b) FTIR spectra of CF obtained at diff erent sonication times for graphiteuorideexfoliated byNMP.Reprintedwithpermission from ref23.Copyright2012RoyalSocietyof Chemistry. (c)Evolution of

CFphaseson graphene with increasing exposure time to XeF2 characterized by FTIRspectra. Reprinted withpermissionfromref 31. Copyright 2012 Wiley.

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fluorographene can be isolated and made into free-hanging membranes. This makes it possible to mea-sure the elastic constants of these materials fromnanoindentation experiments by an atomic force mi-croscope. The experimental elastic constants can becompared to first-principles calculations, giving infor-mation about the purity and structural crystallinityof the experimental samples. Nair et al.12 have con-ducted a nanoindentation experiment on fluorogra-phene by recording the bending of an AFM cantileveras a function of its displacement, and hence calculatedthe force acting on the membrane (typical loadingcurves and breaking forces are shown in Figure 17),giving a value of 100 ( 30 N m1 or 0.3 TPa for the 2D

Young's modulus, (three times less stiff than graphenedue to the longer sp3 type bonds in CF). However, theexperimentalYoung'smodulus wasapproximatelyhalf thetheoreticalvalue (226 N m1).53 Thiswasattributedto an appreciable number of defects in the experi-mental samples. Large scale atomistic simulationsusing the reactive force field approach have shown74

that fluorographene remains a flat sheet (unrippled)similar to graphane even at high temperature, that is,upto900K,incontrasttothethermalripplingbehaviorof graphene.

The variation of the strain energy, E s, and itsderivative with respect to the applied uniform strain,dE s/dε, have been predicted theoretically (Figure 18a;comparison with CH and CCl).75 The results showedthat the derivative was linear for small ε in the harmo-nic range. Elastic deformation continued until themaximum ofdE s/dε, whereuponthestructurerevertedto its initial state when the applied strain was lifted.Beyond the maximum the structural instability sets inwith irreversible deformations. The region beyondthe maximum is called the plastic region. The eff ectof elastic strain on the band gap of CCl, CF, and CHwas also calculated, and the results are presented inFigure 18b. CH and CCl were predicted to have similar

response to elastic strain: their band gaps increasedfor small strain followed by a rapid decrease for largestrain. In contrast, the band gap of CF did not initiallyshow anysignicant increasewith increasingstrain: forsmall strain it was almost unaltered, but decreasedrapidly for large strain.

Superiornanoscalefrictiononuorinatedgraphenehas been reported by Park et al .60,76 using ultrahighvacuum friction force microscopy. The measured fric-tion on uorinated graphene with C4F compositionwas∼

6 times largerthan on pure graphene forappliednormal forces up to 150 nN, whereas uorinationslightly reduced (by about 25%) the adhesion forcebetween theAFM tipandgraphene.60 DFT calculations

con rmed reduction of the adhesive properties andshowed friction force on graphene mainly governedby out-of-plane bending.76 In contrast, low interlayerfriction was reported for multilayer CF from dispersioncorrected DFT calculations.77

Electrical measurements (Figure 19) have beenutilized to study the thermal stability of CF in moredetail than possible with Raman spectroscopy.12 Thethermal stability of CF was observed to be higher thanthat of graphene, graphene oxide, and even graphiteuoride. Under similar conditions, graphite uoride

began decomposing at 300 C. The higher stabilityof CF can be due to the absence of structural defectsandstrain. The thermal stabilityandchemical inertnessofCFwas comparable tothatof the fullyuorinated1Dcarbonchainof Teon.12,23 CF underwentslow decom-position at T > 260 C and rapidly decomposed onlyabove 400 C. These properties in principle makecolloidal CF derived from graphite uoride a favorablecandidate to replace Teon in various protective coat-ings. CF has also been shown to be resistant to varioussolvents under ambient conditions.

Chemistry of CF and Applications. Hydrophobic gra-phene fluoride (CF0.5) nanosheets can be made waterdispersible by stabilizing them with fluoro-surfactants.

Figure 17. Mechanical properties of CF. (a) Examples of the loading curves for graphene (blue) and CF (red) membranes.Fracture loads are marked by the circled crosses. Up until these breaking points, the curves were nonhysteretic. Top andbottom insets: AFM images of a CF membrane before and after its fracture, respectively. (b) Histogram of the breaking forcefor graphene (hashed) and CF (solid color). All the membranes (15 of each type) were mounted on identical Quantifoils andcontacted with the same AFM tip. Reprinted with permission from ref 12. Copyright 2010 Wiley.

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Recently, our group reported nonlinear optical responseof graphene fluoride via aqueous phase exfoliation of

graphite fluoride with the aid of a fluorosurfactant.78

The uorination process is reversible as uorinecan be easily removed from CF by thermally annealingthe sample (de uorination) in the presence of hydro-gen gas or anhydrous hydrazine vapor. Robinsonand co-workers have shown that chemical reductionof uorographenes by means of hydrazine vapor ismore eff ective for de uorination than thermal anneal-ing via Raman spectroscopy.16 Thermal de uorination(400 600 C) of graphene uoride has been shown toresult in the removal of carbons and evolution of CFproducts (e.g., CF4, C2F4, C2F6).16,79

The addition of KI to a colloidal CF dispersionforming graphene is also an interesting approach forde uorination.13 In this reaction CF transforms intounstable graphene iodine (iodographene), which ra-pidly disproportionates into graphene and iodine:

CFþ KIf KFþ CIf Cþ KFþ 12 I2 (1)

The structure and electronic properties of uorogra-phene doped with metals such as K, Li, Au atoms andC12 N4F4 (F4-TCNQ) have been investigated by densityfunctional theory. It was shown that adsorption of K orLi atoms results in the electron doping of uorogra-phene, whereas Au atoms and F4-TCNQ introducedeep levels inside the band gap.80

Furthermore, the strong polarity of the CF bondsstimulates interesting biological responses. Wangand co-workers73 observed thatbone marrow derivedmesenchymal stem cells (MSCs) cultured on fullyuorinated graphene proliferated faster and were

more con uent after a week than cells culturedon partially uorinated graphene and graphene(Figure 20a c and Figure 20d). The fully uorinatedgraphene was associated with a nearly 3-fold increase incell density, showing that the introductionof CF bondson the surface of graphene facilitates cell adhesion and

proliferation. To understand the eff ect of texture andwettability on the controlled growth of MSCs on thesurface uorinatedgraphenes, water contact anglemea-surementswereperformed.The surfaceroughnessof thegraphene increased with increasing uorine content,thereby reducing the water contact angle from 83 to

∼1 (grapheneto fully uorinatedgraphene; theinsets inFigure 20e g). Inaddition,selective attachmentofMSCsover microchannels of CF was achieved via patterningCF with PDMS. This work opens up interesting avenuesfor partially and fully uorinated graphenes in tissueengineering related applications.

Quantum Monte Carlo simulations predicted quali-tative diff erent behavior of helium lms on uoro-graphene and graphane with respect to behavior ongraphite because of the diff erent surface composition,symmetry, and spacing of the adsorption sites.81 Thecommensurate state analogous to the√ 3 √ 3 R30

Figure18. (a)Variation ofthe strainenergyE s (curves on therighthandside)and itsderivativewith respect to applied uniformstrain ε, i.e., dE s/dε (curves on the left-hand side) calculated for CCl, CH, and CF. After the maxima, these structures becomeunstableandundergoplastic deformation.(b) Variationof thebandgapwithuniform strain. Reprintedfromref 75.Copyright2012 American Chemical Society.

Figure 19. Characteristics of highly stable 2D insulator. (a)Changes in uorographene's resistivity F induced by an-nealing. No changes were detected at T A below 200 C. Athigher T A, F decreased below 1 TΩ and was experimentallymeasurable. Owing to nonlinear I V characteristics, theplotted F values were recorded for a xed bias V SD of 1 V (circles).For anygivenT A,approximately1hwasrequiredtoreacha saturatedstate. Thesolidlineshows theexponentialdependence yielding E des ≈ 0.65 eV. (b) I V characteristicsfor partially uorinated graphene obtained by reduction at350 C. The curves from attest to steepest were measuredat T = 100, 150, 200, 250, and300K, respectively. Thescalingfactor Γ is plotted in the inset. The solid line shows the best

ttothefunctionexp(E h /T ). Reprinted withpermissionfromref 12. Copyright 2010 Wiley.

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on graphite was unstable, while a superuid groundstate for 4He and a uid ground state for 3He werefound on uorographene and graphane.

Fluorinated Graphenes. Basic Properties of Partially

Fluorinated Graphenes ( C 1 F n , n < 1). The introductionof fluorine modifies the electronic properties of gra-phene by reducing the charge in the conducting π orbitals.16,17 The resistance of the fluorinated gra-phenescanbe tuned by theextentof fluorine content.For example, fluorinated graphene with a compositionof CF0.25 has been shown to have a 6-fold higherresistance than pristine graphene; in addition, CF0.25has been shown to be a wide band gap material thatexhibits optical transparency.16 Furthermore, defluor-ination of these samples either by heating or exposingto hydrazine vapors restored the conductivity andthe ambipolar nature similar to that of graphenes.For various applications related to the fabrication of devices, single-side fluorination of graphene would besufficient as it opens a significant band gap.20 Single-side fluorination can easily lead to a random surfacecoverage of up to CF0.5 .82 Band gaps of fluorinatedgraphenes (obtained by exfoliating graphite fluoridevia fluorinated ionic liquids) with compositions CF0.25and CF0.5 have been measured by diffuse reflectancespectroscopy as nearly 1.8 and 2.2 eV, respectively;19

however,a bandgapof2.9 eVwasobtainedfromdI /dV measurement on CF0.25 (XeF2 fluorination of CVDgraphene as grown on a copper foil).60

From thetheory, themost stablesingle-sideconfor-mation of C1Fa , where a < 1, is predicted to be CF0.25 .16

Density of states calculations for graphene with in-creasing F coverage showedwideningof theband gap

and lowering of the Fermi level in the valence band.16

Tuning the band gap (ca . 0 3 eV at DFT level) canbe achieved by precise adsorption of uorine as wellas a transformation from nonmagnetic semimetal(graphene) to either a nonmagnetic/magnetic metalor a magnetic/nonmagnetic semiconductor/insulatorwith a direct band gap.83 Band gaps of 2.93 eV (LDA) or 5.99 eV (GW0) for CF0.25 and 1.57 eV (LDA) or5.68 eV (GW0) for boat CF0.5 have been predicted(Figure 21a c).69 In theoretical studies, the total en-ergies and/or binding energies have often been usedas criteria forwhethera givenCFn structureexists.Evenif a CFn structure seems to be in a minimum on thepotential energy surface, its stability should be meti-culously examined by calculating frequencies of allphonon modes in the Brillouin zone (BZ). Sahin et al.69

found that the CF0.25 ,theCF0.5 boat, and the CF0.5 chairstructures have positive frequencies throughout theBZ, indicating theirstability. Optical absorption spectraofCF0.25 calculatedfromBethe-Salpeterequation (BSE)have shown strong excitonic eff ects (analogically withCF, see subsection “ Optical properties of CF” above).70

The rst exciton peak locates at 4.21 eV (Figure 21d,black line); that is, the onset of absorption spectra issigni cantly shifted to lower energies with respect to

Figure20. (a c) Fluorescent imagesof the actin cytoskeletonof MSCs culturedon graphene,partiallyuorinated grapheneoruorographene stained with rhodamine phalloidin on day 7 (scale bar = 100 μm). (d) Proliferation of MSCs cultured on the

graphene lms,showing thecontrolled growth ofMSCs onuorinated graphenewithdiff erent coverage of uorine. (e g)AFMimages of graphene, partially uorinated graphene, and uorographene, respectively, showing the surface of uorinatedgraphene (scale bar = 5 μm).Theinsets showdata for thewater contactangle.Reprinted withpermissionfrom ref73.Copyright2012 Wiley.

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electronic GW band gap (6 eV 69 from GW0, 5.5 eV 70

from G0W0). It is worthnoting that large scalemoleculardynamics simulationshaveshown a rollingof grapheneas a consequence of strain when graphene is uori-nated/hydrogenated from a single side.84 Therefore, inthe absence of a substrate, free-standing single-sidedhalogenated graphene will not be a at atomic layer.

Recently, uorinated graphene obtained by coop-erative exfoliation of graphite uoride using cetyl-trimethyl-ammonium bromide (CTAB) and dopaminehave been shown to exhibit a full-color emission fromviolet to red light (3.101.65 eV) when excited at365 nm.24 Band gap energies of uorinated graphene(however, containing also oxygen) for several uorinecoverageshavebeenmeasured (Figure 22).21 Thereisablue shiftobservedforthe uorinatedgraphenesheetsas compared to that of graphene sheets (i.e., from269 nm of graphene to 251 and 247 nm in Figure 22a)indicating the opening of the band gap. Moreover, thehigher coverage of uorine in graphene sheets de-creases the electrical conductivity (Figure 22b) andband gap widening (from 1.8 to 2.9 eV for CF0.10 andCF0.48 , respectively) was observed (Figure 22c,d).

Fluorine-doped reduced graphene oxide (RGO)is reportedly a better substrate for surface enhancedRaman spectroscopy of molecules than unmodiedRGO.33 In addition,it hasbeenshownthat thechemicalenhancement factor can be tuned by changing theuorine/carbon ratio (17 27 atom %) due to the pre-

sence of a strong local electric eld induced by the localdipoles of F-containing groups on the RGO surface.33

Thomas et al .85 have reported that uorinated graphene

oxide has high nonlinear absorption and nonlinear scat-tering, and its optical limiting threshold is about an orderof magnitude better than that of graphene oxide (GO).

Raman spectroscopy of uorinated graphenes hasbeen mainly discussed in the uorographene sec-tion. Single side uorination leads to the appearanceof a D peak at 1350 cm1 and broadening of theG (1580 cm1) and D0 peaks (1620 cm1), as well as adecrease in the 2D Raman peaks which is similarto that of graphene oxide.16 Sun and co-workers35

have studied layer-dependent uorination of n-layergraphenes by SF6 plasma treatment. They observedthat uorination of monolayer graphenes is easier andfaster than multilayer graphenes because of the form-er's high surface reactivity due to corrugations whichbecome notably smaller for bilayers and disappearfor thicker graphenes. During uorination of mono-layer graphene, three new peaks at 1350, 1620, and2920cm 1 were observedin theRamanspectra,whichwere ascribed to D, D0, and Dþ G bands, respectively.The presence of D, D0, and Dþ G bands indicates thatdefects were introduced into the graphene latticeduringtheplasma treatment, whichoccursmore easilyfor single-layer graphene than thicker graphenes.The same group also observed that uorination by

CF4 plasma treatment inducesa lower number of latticedefects andhighermagnitudeof p-dopingto graphenethan CHF3 plasma treatment.34

Other Unusual Properties of Fluorinated Graphenesand Applications. Dilute fluorinated graphene sheets(F/Cratioof1to2000or0.05%)producedbyCF4 plasmain a reactive ion etching system have been shown toexhibit anisotropic, colossal negative magnetoresis-tance and unusual “ staircase” behavior at low tempera-tures (


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spectroscopy and scanning tunneling microscopy(STM).

Theresistanceat thechargeneutrality point increasedby3 orders of magnitude from 25 k Ω at 200 K to 2.5 MΩat 5 K, displaying a strong insulating behavior at dilute Fconcentration.86 Moreover, the presence of adatom-induced local magnetic moments has been reported indilute fluorinated graphene due to observed spin-flipscattering.87 The spin-flip rate was tunable via fluorinecoverage and carrier density. Modification of graphenewith fluorine may offer a platform for studying magnet-ism in this unusual two-dimensional electron gas anda gate-controllable, lithography-compatible, approachto control spins in graphene spintronics devices.

Diluted F atoms chemisorbed on graphene havealso been investigated theoretically.88 It was shownthat the nature of the chemical bonding of an F atomadsorbed on top of a C atom in graphene stronglydepends on carrier doping. In neutral samples, the Fimpurities make a sp3-like bonding of the C atombelow, generating a local deformation of the hexago-nal lattice. As the graphene is electron-doped, the Catom withdraws back to the graphene plane and forhigh doping its electronic structure corresponds to anearly pure sp2 con guration. This sp3 sp2 doping-induced crossover provides a new facility for control-ling graphene's electronic properties.

Fluorination of graphene has been shown to result

in the development of strong paramagnetism withincreasing uorine coverages, that is, in CF x sampleswith x increasingfrom0.1to1(uorographene), aswellas more than an order of magnitude increase in low-T saturation magnetization (Figure 23a).89 For the stoi-chiometric uorographene C1F1, a signi cant decreasein themagnetization wasobservedcompared to CF0.75or CF0.9 , even though the material showed strongparamagnetism. The number of spins N increasedmonotonically with x up to x ≈ 0.9, then decreasedslightly for the fully uorinated samples (Figure 23b).A plot of the number of Bohr magnetons, μB, perattached F atom (inset in Figure 23b) clearly showedthat the initial increase (up to x ≈ 0.5) in the numberof paramagnetic centers was proportional to x , buta more complicated relation between the number of atoms and N applied at higher x .

Recently, a method to obtain ultrathin, uniform,high-κ , and top-gate dielectrics required to realizethe full potential of graphene-based device technolo-gies was developed.37 A 15 nm atomic layer deposi-tion (ALS) Al2O3 was uniformly deposited on epitaxialgraphene functionalized by uorine atoms. The under-lying graphene propertieswere notdegraded,and6.7%of the surface of epitaxial graphene was converted to

Figure 22. (a) UV Visible absorption spectra of graphene oxide (GO), hydrothermally reduced graphene sheets (HGS), andtwo typical uorinated graphene sheets (FGS) samples (FGS-150 and FGS-180). (b) Electrical conductivities of FGS samplesaccompanying thevariation in C/Fratio.The inset shows a schematicmodel of the four-probe instrumentused. (c)Arrheniusplot for the logarithm of conductivities of uorinated graphenes series versus the inverse of temperature. (d) Experimentalband gap energies of uorinated graphene (obtained from slopes of lines in panel c subplot) for several F coverages.Reprinted with permission from ref 21. Copyright 2012 Elsevier.

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C F bonds which provided enough ALD nucleationsites to obtain conformal lms without pinholes.

NEXAFS spectroscopy has been demonstrated tobe a powerful technique for evaluating the anisotropyin the chemical bonding of semiuorinated graphite(using a gaseous mixture of BrF3 and Br2).90 The CK edge NEXAFS spectra of uorinated HOPG measuredat incidence angles of 20 and 90 are compared inFigure 24. This gure shows that the π -system of C2Fis retained after uorination, as seen from the intensepeak near 285 eV. The peak B at 288.5 eV with ashoulder C around 290 eV is related to 1s f σ * transi-tions within carbon atoms bonded with uorine.All three peaks are signicantly reduced in intensitywhen the spectra are recorded at normal incidence(Figure 24d), implying that thesepeaks (A, B, andC) are

largely due to carbon electrons from orbitals perpen-dicular to the basal plane. The intensity of D0 and Dare independent of the beam direction and are asso-ciated withtransitions involvingstatesof carbon atoms(σ * states) that arenonbonded or bondedwith uorineatoms. The strong dependence of the intensity of peaks F and G (measured by FK edge spectra) on theincidence of X-ray radiation (Figure 24e and Figure 24f)shows that the C F bonds have a predominantly per-pendicular orientation to the basal plane. This spectro-scopic technique helps to shed light on the anisotropicstructure of semiuorinated graphite, in which half thecarbonatoms are covalently bonded withuorine, whilethe rest of the carbon atoms preserve π electrons.

The thermal conductivity of uorinated graphenehas also been studied theoretically by classical none-quilibrium molecular dynamics.91 The thermal conduc-tivity was found to depend strongly on the coverageand distribution of uorine atoms. For random uorina-tion, the thermal conductivity decreased rapidly withincreasing uorine coverage from 0 to 20%, stabilizedbetween 20and 70%, and then increased rapidlyas thecoverage approached 100%. The thermal conductivityof graphene uoride was much less sensitive to tensilestrain than that of pristine graphene.

MonteCarlo calculationshavesuggestedthatuoro-graphenes reduce the binding energies of adsorbates(such as CH4,CO2, N2, O2, H2S, SO2) with respect to puregraphene.92 In most cases, the adsorption selectivitywasgreatest forunmodiedgraphenesurfacesandwasreduced by uorination.92

The surface chemistry associated with CF4- and

Cl2-based inductively coupled plasmareactive ionetching (ICP RIE) of the6H-SiC (0001)surface followedby thermal annealing at 970 C has the potential toyield large area graphene-on-insulator lms.93

On the basis of the partially uorinated graphenewith composition CF0.25 , a novel class of “ acceptortype” uoride intercalated graphite compounds hasbeen proposed.94 According to theoretical predictionsandexperimental evidence, these types of compoundsexhibit signicantly higher isosteric heats of adsorptionfor H2 than previously demonstrated for commonlyavailable, porouscarbon-basedmaterials. Theunusuallystrong interaction with H2 arises from the semi-ionicnature of the C F bonds. Although high H2 storagecapacity (>4 wt %) at near-ambient temperatures maynotbe feasible due todiminishedheats ofadsorptionatveryhigh H2 densities, enhanced storage propertiescanbe envisaged by doping the graphitic host with appro-priatespecies(e.g., nitrogen) topromotemoreextensivecharge transfer from graphene to Fanions.

Multilayered Fluorinated Graphenes( C a F b , a < b). Yanet al .22 have shown that fluorine-doped multilayeredgraphene (10 wt %, ∼6.6 atom %, synthesized by arcdischarge process) sheets exhibit superhydrophobicproperties (CAg 150 ) comparable to that of graphene

Figure 23. (a) Magnetic moment Δ M (after subtractinglineardiamagneticbackground)as a function of the paralleleld strength H for diff erent F/C ratios. (b) Main panel:number of spins N extracted from the ts in panel a as a

functionoftheF/Cratio.Thesolidcurveisforguidanceonly.Inset: the same N normalized to the concentration of adatoms in each sample ( μB/F atom is obtained by dividingthe number of momentsN , assuming that each carries 1 μB,by the number of F atoms per g of uorinated graphene).Error bars indicate the accuracy of determination of theuorineconcentrations. Reprinted with permissionfromref

89. Copyright 2012 Nature Publishing Group.

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sheets. The sliding angle of an F-doped graphene sheetfilmwasabout 13comparedtoabout38 for a pureGSfilm, indicatingthat rollingof thedropletontheF-dopedGS film was easier than on pure graphene. In addition,the fluorine content in the graphene matrix can betuned by chemical reaction of graphene oxide withhydrofluoric acid.21

Lee et al.42 have prepared multilayer semi-ionically

uorinated graphene with high insulating prop-erties, which upon reduction with acetone showeda 109 times increase in current from 1013 to 10 4 A,indicating a transition from insulator- to graphene-likebehavior (Figure 25a). The current increase of thereduced uorinated graphene was much higher withrespect to reduced graphene oxide (GO). The reducedmultilayered C2F lm showed p-type doped electricalcharacteristics (Figure 25b). The reduction in acetoneproceeds as 2C2F(semi ‑ionic) þ CH3C(O)CH3(l) f HF þ2C(s) þ C2F(covalent) þ CH3C(O)CH2(l) at low tempera-tures. The reduction of multilayered semi-ionicallyuorinatedgraphene lm using acetone is well proven

by Raman, XPS, and transport measurement analysis(Figure 25c,d).42 It should be noted that the XeF2treatment is an eff ective way for high quality uorina-tion of suspended graphene. However, it is insufficientfor multilayered and even bilayered uorographene.12

Thesynthesis proposedbyLeeetal .42 providedseveralgrams of multilayer semi-ionically uorinated gra-phene via a single step uorination process for 5 h,which not only generates highly insulating uorinatedgraphene (Figure 25a) but could also be used to massproduce uorinated graphenes for industrial applica-tions. The wetting characteristics of GO changes upon

uorination due to the low surface free energy of the C F bonds.95 The extent of oxygen coverage ongraphene uoride can be controlled by diff erent reac-tion conditions to oxidize uorinated graphite. Thematerial with high uorine content (23 atom %) inGOwasusedtocreateamphiphobic inks forcoatingonsteel, porous substrates.

Bilayer uorographene C2F (Figure 26) has been

found to be a much more stable material than bilayergraphane.96 This is evident by comparison of the for-mation energies of the nal structures and is accentu-ated by the fact that the formation energy of partiallyuorinated bilayer graphane is always negative, in

contrast to partially hydrogenated bilayer graphene.The creation of interlayer chemical bonds occursat higher amounts of uorination compared withhydrogenation. The electronic band structure of C2Fhas been shown to be similar to that of monolayeruorographene, but its band gap is signicantly

larger (about 1 eV).96 Furthermore, it has been foundthat bilayer uorographene is almost as strong asgraphene, as its 2D Young's modulus is approximately300 N m 1.96 Structural stability and electronic andmagnetic properties of uorinated bilayer graphenewere studied also for various uorine coverages.96,97

Infrared spectral signatures of bilayered surface-uorinatedgraphene havebeen studiedbymolecular

dynamics.98,99

Density functional theory (DFT) computations haverevealed the existence of considerable CH3 3 3F Cbonding between graphane and a uorinated gra-phene bilayer with a small energy gap (of 0.5 eV atPBE GGA level), much lower than those of individual

Figure24. NEXAFSspectrameasurednear theCK edge of HOPG at an incidenceangleθ of radiationof 15 (a)and90 (b). Theinset shows theorientation of theelectric eldvectorof synchrotron radiationrelative to thebasalplaneof graphite.NEXAFSspectra of the uorinated HOPG measured near the CK edge at grazing (c) and normal(d) incidence, and near the FK edge atgrazing (e) and normal (f) incidence. Reprinted from ref 90. Copyright 2012 American Chemical Society.

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graphane and uorographene. The binding strengthof this bilayer can be signicantly enhanced by anappropriate external electrical eld. Changing the di-rection and strength of the electric eld can eff ectivelymodulate the energygap of the G/CF bilayer(03.0eV)and correspondingly causes a semiconductormetaltransition.100 In sharp contrast, theelectronicpropertiesof separated graphane or uorographene monolayerare rather robust in response to an electric eld withcorresponding negligible modulation of the bandstructures.

Halogenated Graphenes (Ca X b , X = Cl , Br, I ; a e b).Chlorographene. The fully chlorinated graphene(chlorographene, graphene chloride, or CCl) has beenbroadly theoretically discussed,13,56 59,75 but not yet

prepared.Thepristineparent material, graphitechloride,is unstable at temperatures >0 C, but stable at lowertemperatures;15 thereforeit is reasonable toassumethatCCl is unstable under ambient conditions. On the otherhand, its instability has not yet been proven experimen-tally. Moreover, partially chlorinated graphene deriva-tives have been reported recently.25 28 However, thecoverageofchlorineatomswasmaximallyupto30atom% (CCl0.43 ). The stability of such compounds has beenconfirmed from room temperatures to 500 C.28 There-fore, the existence of chlorographene is still uncertainand further experiments are needed to verify whetherCCl is stable at room temperature.

The lattice constant and CC distance in the chairconformation of chlorographene (2.91 Å and 1.76 Å)

Figure 25. (a) I V characteristics of multilayered semi-ionically uorinated graphene lms before and after reduction.(b)I V g characteristicsof a corresponding FETdevice before(inset) andafterthe reductionprocess. (c)Plotshowing variationof current on the reduction time. (d) Raman spectroscopyresults accordingto the reduction time. Reprinted with permissionfrom ref 42. Copyright 2013 Wiley.

Figure 26. (a) Top and (b) side views of bilayer uorographene. The carbon atoms in the two layers are shown in diff erentcolorsfor clarity, andthe uorine atomsareindicated by light green (open circles with thesmallestdiameter).Reprinted fromref 96. Copyright 2012 American Chemical Society.

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have been predicted at the GGA DFT level to benotably larger than the equivalent parameters foruorographene (2.61 Å and 1.58 Å) and graphane

(2.54 Å and 1.54 Å),57 59 and the C C distance issigni cantly deviated from typical values for a singleC(sp3) C(sp3) bond (

∼1.54 Å). This is because chlorine

atoms attached to the perturbed graphene sp3 latticepartially overlap with each other and the whole latticemust balance C C bonding with ClCl repulsion.58

Consequently, CCl is less stable then CH and CF.Surprisingly, initial theoretical calculations of the

electronic structure of CCl (ref 56) suggested metallic

behavior. This was probably due to “

nonbonded”

struc-tures (composed of carbon sp2 net and adsorbedchlorineatoms) reported later57 andcon rmedrecentlyby consideration of the reaction mechanisms of Clatoms and graphene101 or Cl atom adsorption.26,102

The reported chlorographene GGA DFT band gap of 1.4 eV 57 59 corresponding to an sp3 “ bonded” chairstructure indicates semiconducting or insulating prop-erties. However, the GGA band gap values may beseriously underestimated. More realistic is the value of 2.8eV 58,59 obtained using thehybrid(HSE06)functional.The bottom of the conduction-band and top of thevalence-band are located at the Γ point in the rstBrillouin zone, the top of the valence band is doublydegenerate, and the maximum band gap is locatedat theK point (Figure 27). Similar touorographene, thehigh-level many-body GW approximation which in-cludeselectron electron interactionsbeyondDFT,pre-dicted a dramatically increased band gap of 4.35.2 eV (depending on the GW level and orbitals used).59,75

It hasbeen reported75 that all the phononmodes of chairlike CCl have positive frequencies (Figure 28a),and hence the predicted structure of chlorographeneis stable at T = 0 K. Although chlorographene belongsto the same space group, D3d , as CH and CF, the

phonon frequencies are lowered (softened) due tosaturation of the C atoms with heavy Cl atoms(cf. Figure 28a versus Figure 14a for CF). Group theoryanalysis has shown that the decomposition of thevibration representation at the Γ -point is Γ = 2A1g þ2A2u þ 4Eg þ 4Eu. Among these,the modes at105,398,751, and 1042 cm1 (Figure 28a) are bond stretchingmodes and are Raman-active. The Raman mode A1g at1042 (398) is entirely due to the out-of-plane vibrationof C and Cl atoms moving in the same (opposite)direction with respect to each other (cf . Figure 14b).Further observation of these Raman active modes is

expected to shed light on the Cl coverage and thestructure of chlorinated graphene.Comparison of the optical absorption spectrum of

CCl obtained at the DFT þ RPA and G0W0þ RPA levelsshows (Figure 28b) that inclusion of electronelectroninteraction leads to blue shift of the absorption dueto quasi-particle corrections; however, spectra shapeis preserved.59 On the other hand, the inclusion of the electron hole correlations yields a signicant redshift of the absorption spectrum (insets in Figure 28b)similar to uorographene (Figure 13). A prominentphysical eff ect of the electron hole interactions isapparent for some bound excitons below the G0W0gap (see insets in Figure 28b), which are completelymissing in the G0W0þ RPA. Therst exciton peak of theCCl spectra is located at 2.82 eV with a correspondingbinding energy of 1.25 eV.59

Properties of Chlorinated Graphenes. Adsorption of the Cl atom on graphene has been mainly studiedtheoretically.26,75,101,102 The adsorption of a single Clatom was shown to be significantly different from thatof H andF. A charge-transfercomplex (whereC orbitalsmaintain a planar sp2 net) was predicted. The CCldistance was notably larger than that of CF and CH(covalent bond), suggesting that the carbon net is not

Figure 27. (a) Electronic band structure of chlorographene CCl. The LDA band gap is shaded yellow. The GW0 correctedvalence and conduction bands are shown by a dashed line and red balls. The zero of energy is set to the Fermi level, E F.(b) Density of states projected to variousorbitals (PDOS).Reprinted from ref 75. Copyright 2012 American Chemical Society.(c) Comparison of CCl band structures calculated using the PBE functional (black line), HSE06 functional (blue line), GW overPBE (red circles), and GW over HSE06 (green dots). The Fermi level was set at zero energy. Reprinted from ref 59. Copyright2013 American Chemical Society.

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as buckled (Figure 29).101

Various adsorption configura-tions were possible during the adsorption of otherchlorinemolecules(Figure30), However,notallpossibleconfigurations or patterns obtained by the chlorina-tion of one surface of graphene were stable.75 Whenboth sides of the graphene sheet were exposed, the Clatoms preferred to form homogeneous patterns, andthe optimal coverage was 25%, in contrast to the valueof 100% in hydrogenation and fluorination.101 Interest-ingly, DFT calculations have shown that chlorine bindsto the graphene edges more easily than adsorbingon the graphene plane and graphene could be brokeninto edges by chlorine.102 Those results could serve asan alternative explanation of the photochemical chlor-ination experiment.25

The presence of the characteristic D-peak at1330 cm 1, the G-peak at 1587 cm1, and the 2D-peak at 2654 cm 1 (Figure 31a; cf. 1580 and 2680 cm1 forgraphene) from chlorinated graphene indicates alow coverage of Cl (expected

∼8 atom%).25 For higher

coverage (∼

21 atom %), the obvious D peak locatedat 1359 cm 1 in the Raman spectrum of CCl0.21(Figure 31b) indicates that during the chlorinationprocess, largerquantities of covalent CCl bondswereformed in graphene sheets, and thus a high degree of

crystalline disorder was generated by the transformation

from sp2

to sp3

con guration.27

The covalent attach-ment of Cl atoms on the graphene sheets (CClbonding) has been con rmed by XPS and IR spectro-scopic analysis. Since the Raman measurements wereperformed in the range from 1000 or 1250 cm1

to higher frequencies, possible low-frequency Raman-active peaks originating from chlorine atoms couldnot be observed (vibrational modes up to 1050 cm1

are theoreticallypredictedforCCl (Figure28)).However,lower frequencies were recorded in the infrared spectraof chlorinated samples (Figure 31c). A band at 790 cm1

(930 cm 1) was reportedfor∼

30atom%28 (∼

5.9wt%48)coverage of chlorine atoms, which was assigned tothe C Cl stretching vibration.28 Only one vibrationalmode was suggested by theoretical calculations of CCl whichmay correspond to thisband:the A2u modeat∼

850 cm 1, which is infrared active (Figure 28).75

The chlorinated graphene samples were shownto be stable under ambient conditions, and thereforecan be stored for long periods. The thermal stabilityof CCl0.43 was examined in detail by IR and energydispersive X-ray analysis (EDAX) spectra.28 A decreaseintheC Cl band intensitywasobservedupon heatingprogressively, and it completely disappeared above500 C (Figure 31c).

Figure 28. (a) Phonon bands of chlorographene calculatedusing the SDM (small displacement method) and DFPT(density functionalperturbation theory) methods. Inset:struc-ture of chair CCl. Reprinted from ref 75. Copyright 2012American Chemical Society. (b) Absorption spectra of chloro-graphene forlightpolarizationparallel to thesurface plane.Theinsetscontainampli edabsorptionspectra in thevicinityof therst exciton peak and G0W0(PBE) gap (dashed line). Reprinted

from ref 59. Copyright 2013 American Chemical Society.

Figure 29. Adsorption of a single (a) Cl, (b) F, or (c) H atomon graphene from LDA. Structures with partial parametersare shown in the upper panels, and electron localizationfunction (ELF) plots with an isosurface value of 0.70 aredepicted in the bottom panels. Reprinted from ref 101.Copyright 2012 American Chemical Society.

Figure 30. Schematic representation of how various adsorp-tion con gurations evolve during chlorination of graphene.Reprinted from ref 101. Copyright 2012 American ChemicalSociety.

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The Cl coverage can be used to tune the grapheneband gap. Typically, the partially chlorinated grapheneexhibited a nonzero band gapand displayeda 4 ordersof magnitude higher sheet resistance than graphene(8% coverage of chlorine atoms).25 The higher resis-tance can be explained by perturbation of the gra-phene π -conjugated systems upon the chlorinationprocess. Furthermore, the dielectric constant (ε) of thechlorinated reduced graphene oxide composite lm(ε = 169) was found to be much higher than untreatedreduced graphene oxide composite lm (ε = 24).103

The large enrichment in the dielectric constant isattributed to the interfacial polarization between thechlorinated reduced graphene oxide platelets and thepolymer, along with the polar and polarizable CCl

bonds. These chlorinated reduced graphene oxideplatelets also displayed a93% increase in conductivity,due to p-type doping eff ect created by Cl atoms.

Properties of Brominated Graphenes. Fully bromi-nated graphene (graphene bromide) is unstable, andmetallic behavior is predicted from calculations(Figure 32).56 58 The predicted lattice constant, CC,and C Br distances are 3.11, 1.86, and 1.92 Å, respec-tively, at the GGA DFT level.57 However, preparationsof a few-layer graphene brominated up to 4.8 atom %by UV irradiation in liquid-bromine medium28 andgraphenebrominated up to 4 atom % usingmicrowave-spark assisted reaction27 have been reported. TheD peak (∼1350 cm

1) of brominated graphene(Figure 33a) has been shown to be weaker than thatof chlorinated graphene, possibly due to the lowerdegree of modification. Compared with the bandfor pristine graphene, the strong and symmetric 2Dpeak of CBr0.04 was blue-shifted (8 cm1) and becamebroader, consistent with the small extent of modi-fication by Br27 The infrared spectrum of the bromi-nated graphene (9.9 wt %, 1.6 atom %) obtained fromthermally reduced graphene oxide (TRGO) showed avibration at 600 cm1 (Figure 33b), which is indicativeof a C Br bond in TRGO-Br.48

Changes in the electronic properties of few-layer

graphenes dopedby adsorption andintercalationof Br2vapors have been studied by Raman spectroscopy.104

The π conjugation network of graphene remainedunaff ected after halogen doping. Adsorption of bro-mine on single-layer graphene created a high dopedhole density, well beyond that achieved by electricalgating with an ionic polymerelectrolyte. In addition, the2D Raman band was completely quenched. The bilayerspectra indicated that doping by adsorbed Br2 wassymmetrical on the top and bottom layers. Br2 inter-calated into 3-layer and 4-layer graphenes. The combi-nation of both surface and interior doping with Br2 inthree and four layers created a relatively constantdoping level per layer. Forgraphenewith three or morelayers, a resonant Raman signal was observed for Br2 ataround 238 cm 1; for monolayer and bilayer graphene,no bromine signal was observed (Figure 33c).104 Thiscould be an orientation eff ect, since, if the Br2 sitsperpendicular to the surface as LDA DFT calcula-tions suggest10

Halogenated Graphenes Rapidly Growing Family of Graphene Derivatives

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