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US 8,696,938B2 56 (S)Jang,etal.3.4Succeededinisolatingsingle-layerand multi-layergraphenestructuresfrompartialycarbon ized or graphitized polymeric carbons, which were obtainedfromapolymerorpitchprecursor.Carboniza tion involves linking aromatic molecules or planar cyclicchainstoformgraphenedomainsorislandsinan esentialy amorphous carbon matrix. For instance, polymericcarbonfiberswereobtainedbycarbonizing polyacrylonitrile(PAN)fiberstoadesiredextentthatthe fiberwascomposedofindividualgrapheneshetsiso10 latedorseparatedfromeachotherbyanamorphous (X)X.Yang,etal.44 synthesizednanographeneshets withlengthsofupto12nmusingamethodthatbegan withSuzuki-Miyauracouplingof1,4-diodo-2,3,5,6- tetraphenyl-benzenewith4-bromophenylboronicacid. Theresultinghexaphenylbenzenederivativewasfurther derivatizedandring-fusedintoSmallgrapheneshets. Thisisaslowprocessthatthusfarhasproducedvery Smallgrapheneshets. Thereareseveralmajorisuesassociatedwiththeafore mentionedproceses.Theseinclude,forinstance: (1)Inmostofthesemethodsofgraphiteintercalationand carbonmatrix.Theresultingfiberswerethensubjected exfoliation,undesirablechemicalsareused.Consequently, toasolventextraction,orintercalation/exfoliationtreat atediouswashingstepisrequired,whichproducescon ment.Grapheneplateletswerethenextractedfromthese fibersusingabalmilingprocedure. (t)Mack,Viculis,andco-workers38.39developedalow 15 taminatedwastewaterthatrequirescostlydisposalsteps. (2)TheGOnanoplateletspreparedbyApproach1exhibitan electricalconductivitytypicalyseveralordersofmagni tudelowerthantheconductivityofpristineNGPs.Even temperatureprocessthatinvolvedintercalatinggraphite withpotassiummeltandcontactingtheresultingK-in tercalatedgraphitewithalcohol,producingviolently exfoliatedgraphitecontainingmanyultra-thinNGPs. Theprocessmustbecarefulyconductedinavacuumor anextremelydrygloveboxenvironmentsincepure alkali metals. Such as potassium and Sodium, are extremelysensitivetomoistureandposeanexplosion danger.Itisquestionableifthisprocessiseasilyame 25 nabletothemassproductionofnano-scaledplatelets. Onemajoradvantageofthisprocessisthenotionthatit producesnon-oxidizedgrapheneshetssincenoacid/ normallypristinegrapheneandhighlyconducting.How oxidizerintercalationorahightemperatureisinvolved. ever,otherthanthedirectultrasonicationmethoddevel (u)In2004,Novoselov,Geim,andco-workers 1.2 pre 30 opedbytheaplicantsearlier54,theseprocessesarenot paredsingle-shetgraphenebyremovinggraphenefrom amenabletolarge-scaleproductionofNGPswithareason agraphitesampleoneshetatatimeusinga"Scotch ablecost. tape'method.Althoughthismethodisnotamenableto Inanattempttoaddresstheseisues,theaplicantshave large-scaleproductionofNGPs,theirworkdidspur globalyincreasinginterestinnanographenematerials,35 mostlymotivatedbythethoughtsthatgraphenecouldbe usefulfordevelopingnovelelectronicdevices. (v)ZhamuandJang54developedaveryefectivewayof exfoliating/separatingNGPsfromnaturalgraphiteand otherlaminargraphiticmaterialsbyexposingthemate40 rial(withoutanyintercalationoroxidation)toanultra Sonicationtreatment.Thisprocessmaybeconsideredas peelingoffgraphenelayersatarateof20,000layersper second(iftheultrasonicfrequencyis20kHz)orhigher (ifhigherfrequency).TheresultingNGPsarepristine45 graphenewithoutanyintentionalyaddedorbonded OXygen. Approach3:EpitaxialGrowthandChemicalVaporDeposi tionofNanoGrapheneSheetsonInorganicCrystalSurfaces previouslydisclosedaprocessforexfoliatingalayeredmate rialtoproducenano-scaledplateletshavingathickness smallerthan100nmRef.57.Theprocesscomprises:(a) chargingalayeredmaterialtoanintercalationchambercom prisingagaseousenvironmentatafirsttemperatureandafirst pressureSuficienttocausegasspeciestopenetrateintoan interstitialspacebetweenlayersofthelayeredmaterial,form ingagas-intercalatedlayeredmaterial;and(b)operatinga dischargemeanstorapidlyejectthegas-intercalatedlayered materialthroughanozzleintoanexfoliationZoneatasecond pressureandasecondtemperature,allowinggasspecies residingintheinterstitialspacetoexfoliatethelayeredmate rialforproducingtheplatelets.Oneadvantageofthisprocess is the notion that itdoes not involve the utilization of un desirableacids(suchasSulfuricacidornitricacid)oroxidiz ers(suchasSodiumchlorateorpotassiumpermanganate). ThegasenvironmentusedintheprocesscanincludeaSuper criticalfluid,buttheintercalatedlayeredmaterialmustbe rapidlydischargedoutoftheintercalationchamber. (w)Small-scaleproductionofultra-thingrapheneshets50 onasubstratecanbeobtainedbythermaldecomposi tion-basedepitaxialgrowth40andalaserdesorption ionizationtechnique41.Ascanningprobemicroscope Inarelatedtopic,Kaschak,etal.55proposedamethodof afterchemicalreduction,theGO stilexhibitsamuch lowerconductivitythanpristineNGPs.Itappearsthatthe preparationofintercalatedgraphite,whichinvolvestheuse ofanoxidizingagentsuchasnitricacidorpotassium permanganate,typicalyandnecesarilyrequiresgraphite tobeheavilyoxidized.Completereductionofthesehighly oxidizedgraphiteplateletstorecovertheperfectgraphene structurehithertohasnotbeensucesfulyatained. (3)TheNGPsproducedbyApproach2andApproach3are modifyinggraphitebyintroducingaSupercriticalfluidinto intersticesofchemicallyintercalatedorintercalated/oxi dizedgraphite(ratherthantheoriginalnaturalgraphite).The intersticesofintercalatedand/oroxidizedgraphitehadbeen expandedandchemicallymodifiedduetothepresenceof intercalantspecies(suchasSulfuricacid)oroxidation-in ducedfunctionalgroups(suchascarboxyl).Kaschak,etal. wasusedbyRoyetal.42andbyLuetal.43to manipulategraphenelayersatthestepedgesofgraphite55 andetchedHOPG,respectively,withthegoaloffabri catingultra-thinnano-structures.Itwasnotclearif singlegrapheneshetswereobtainedusingthistech niquebyeithergroup.Epitaxialfilmsofgraphitewith onlyoneorafewatomiclayersareoftechnologicaland60 Scientificsignificanceduetotheirpeculiarcharacteris 55didnotteachabouttheapproachofdirectlyintercalating ticsandgreatpotentialasadeviceSubstrate.The grapheneshetsproducedaremeanttobeusedforfuture nano-electronicaplications,ratherthancomposite reinforcements. Approach4:TheBottom-UpApproach(Synthesisof GraphenefromSmallMolecules) 65 theun-treatednaturalflakegraphitewithasupercriticalfluid; nordidtheyteachabouttheapproachofintercalatingand exfoliatinggraphiteusingthesameSupercriticalfluid.The modifiedgraphiteasproposedbyKaschak,etal.55stil requiredahightemperatureexposurestep,typicalyat700 1200°C.,toexfoliatetheintercalatedandmodifiedgraphite.PDF Image | SCO2 Producing Nanographine Platelets
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