Imaginehowmuchcontroloverresultantpropertiesyouwouldhaveifyouwereabletodepositandmaneuverindividualatomsintopredefinedarrangements,enroutetowardanewmaterial.Thisisfastbecomingareality,andistherealizationoftheultimatein“bottom-up”materialsdesign.Thusfar,oneisabletoeasilyfabricatematerialscomprisedofasmallnumberofatoms,withfeaturesonthenanometerscale(10−9m)–one-billionthofameter.Toputthisintoperspective,thinkofamaterialwithdimensionsapproximately1,000timessmallerthanthediameterofahumanhairfollicle!Aswewillsee,itisnowevenpossibletopushindividualatomsaroundasurfaceusingspecializedtechniques.
Weareatthecrossroadsofunprecedentedapplicationsthatwillonlybepossi-bleusingnanoscalebuildingblocks.Moreeffectivedeviceswillbeconstructedtoremovepollutantsfromtheenvironmentanddetect/deactivatechemicalandbiolog-icalwarfareagents.Integratedcircuitrywiththecapabilitiesofcurrentworkstationswillbethesizeofagrainofsandandwillbeabletooperatefordecadeswiththeequivalentofasinglewristwatchbattery.Roboticspacecraftsthatweighonlyafewpoundswillbesentouttoexplorethesolarsystem,andwidespreadspacetravelwillbepossibleforthemasses.Oh,yes–onethatisneartousall–inexpensivealterna-tiveenergysourceswillpowerourvehicles,ratherthandependingondwindlingoilreservesandthedailyfluctuationsof(soaring)gasprices![1]
Inordertogainrapidprogresstowardtheseintriguinggoals,thelevelofgovern-mentandprivatefundinginthenanosciencescontinuestosoar.Thishasspawnedanumberofinstitutesinrecentyears,focusedonresearch,development,andcom-mercializationofnanoscalediscoveries,aswellaspubliceducation/outreach.Somerecentexamplesare[2]:
–NationalNanotechnologyInitiative(FederalR&DProgram,Washington,DC);http://www.nano.gov
–RichardE.SmalleyInstituteforNanoscaleScienceandTechnology(RiceUniver-sity,Houston,TX);http://cohesion.rice.edu/centersandinst/cnst/index.cfm
–InstituteforNanotechnology(NorthwesternUniversity,Evanston,IL);http://www.nanotechnology.northwestern.edu/index.html
–NanoScienceandTechnologyInstitute(Cambridge,MA);http://www.nsti.org/about/
275
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–NationalCancerInstituteAllianceforNanotechnologyinCancer(Bethesda,MD);http://nano.cancer.gov/
–ASMENanotechnologyInstitute(NewYork,NY);http://www.nanotechnology-institute.org/about.html
–NanotechnologyInstitute(Philadelphia,PA);http://www.nanotechinstitute.org/nti/index.jsp)
–CenterforNanoscaleChemical–Electrical–MechanicalManufacturingSystems(Urbana,IL);http://www.nano-cemms.uiuc.edu/
–Nano/BioInterfaceCenter(UniversityofPennsylvania);http://www.nanotech.upenn.edu/
–CenteronNanotechnologyandSociety(Chicago-KentCollegeofLaw,IllinoisInstituteofTechnology);http://www.nano-and-society.org/
–TheNanoTechnologyGroup,Inc.(Nanoscienceeducationaloutreach,Wells,TX);http://www.thenanotechnologygroup.org/
–TheForesightInstitute(PaloAlto,CA);http://www.foresight.org
–TheInstituteforSoldierNanotechnologies(MassachusettsInstituteofTechno-logy);http://web.mit.edu/isn/aboutisn/index.html
Thefirstnationalnetworkfocusedonthedesign/fabrication/testingofnanomate-rialswasinstitutedin2004throughfundingfromtheNationalScienceFoundation.TheNationalNanotechnologyInfrastructureNetwork(NNIN)consistsofacon-glomerateof13sitesacrossthecountry(Figure6.1)thatarefocusedonallaspectsofnanomaterials.Since“nanotechnology”issuchaninterdisciplinaryfield,manymorenanorelatedresearchcenterswilllikelybeinstitutedinthenearfuture.However,aswithallscientificdisciplines,amajorroadblocktowardresearchprogressintheUSisdomesticstudentrecruitment.Thereareadecliningnumberofdegreesawardedinthescienceswithinrecentyears(fromB.S.toPh.D.levels)intheUS,relativetootherforeigncountries(e.g.,China,India).Thisrepresentsanominousforecast
Figure6.1.The13sitesoftheNationalNanotechnologyInfrastructureNetwork(FY2004-FY2009).Reproducedfromhttp://www.nnin.org.
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thatouradvancedtechnologyandwarfarecapabilitieswillgreatlylagbehindothercountries,threateningoureverydaywayoflifeandthe“superpower”statuslongenjoyedbytheUS.
Asexcitingasthefuturisticapplicationsmaysound,isitpossiblethatsuchtechno-logicalgrowthmaybeassociatedwithdiresocietalconsequences?InEricDrexler’sbook“EnginesofCreation,”[3]itwasforecastedthatself-replicatingnanomachineswouldtakeoveralllifeonEarth!Althoughthatnotionisfarfromreality,theremaybemoreseriousissuesthatarisethroughintroductionofnanomaterialsintothebiosphere.Historyhasrevealedthatchemistryisatwo-edgedsword,withbenefitsthatgreatlyimproveourlivesbutalsonegativeconsequencesforhumanhealthandourenvironment.Forexample,thinkofthechlorofluorocarbons(CFCs)thatwereonceusedforrefrigerants.Theirdiscoverywasheraldedasoneofthegreatesttri-umphsofmodernscience.Alas,manyyearsaftertheirworldwideadoption,itwasrealizedthattheycontributedtotheozoneholeandlikelyincreaseintheincidencesofskincancer.Likewise,emissionsfromfactoriesandautomobileswerenotconsid-eredasproblematicmanyyearsago,butwearenowawareofthedestructiveconse-quencesofthesesources(e.g.,acidrainandglobalwarming–theleadingcauseofcatastrophicchangesinourclimate/weatherpatterns).
Whatwillarisefromthewidescaleintroductionofnanoscalematerialsintoourworld?Areweonthevergeofupsettingthenaturalbalanceinwaysthatcannotbeoverturned?Theseareseriousquestionsthatmayonlybeansweredthroughfar-reachingresearchendeavors,manyofwhicharecurrentlybeinginvestigated.Inparticular,wemustfirstdeterminethetoxicological/environmentalconsequencesofnanoscalematerialswithcomparisonstoknowncontaminantssuchascolloids,aerosols/smokeparticulates,andasbestos.Someimportantquestionsthatmustbeaddressed[4]:
(i)Donanomaterialsbioaccumulate?
(ii)Whatisthedangerofexposureviaskinabsorption,ingestion,orinhalation
routes?
(iii)Whatisthefate,transport,andtransformationofnanosizedmaterialsafterthey
entertheenvironment?
Inadditiontothesetoxicological-basedquestions,anumberofethicalconsiderationsmustalsobeaddressed[5]:
(i)Equityissues–willnanotechnologybeutilizedtosolvethird-worldproblems,orwillitprimarilybeusedtoincreasetheprowessofindustriallyadvancedcountries?
(ii)Privacyissues–imagineaworldwhereyouhaveinvisiblesensors/microphones
–needwesaymore...
(iii)Security–howwillourcountryandothersdefenditselfagainstinvisible
nanoweaponry?
(iv)Human–machineinteractions–therearemanyreligiousandphilosophical
issuesassociatedwithembeddednanodeviceswithinthehumanbody.
Thefirstnewsreportonthepotentialdamagingeffectsofnanoscalematerialssurfacedaboutadecadeago,whenTiO2/ZnOnanoparticlesfromsunscreenwerefoundtocausefreeradicalsinskincells,damagingDNA.Sincethen,therehave
2786.1.Whatis“Nanotechnology”?
beenanincreasingnumberofsuchreportssuggestingthatnanostructuresareabletotraverseacrossmembranesinthebody,withanincreasingtoxicitywithdecreas-ingnanoparticulatedimensions.Perhapsthemostwidelyreportedstudysurfacedinmid-2004,whereitwasshownthatfullerenes,ananoscaleallotropeofcarbon,causebraindamageinaquaticspecies.Manyadditionalstudiesareneededtodeterminethefullimpactofnanomaterialsbeforetheirfullworldwideadoption.
Theintroductionofanewarchitecturesuchasnanomaterialsnecessitatestheneedfornewterminologyandmethodsofclassificationandcharacterization.Wemustalsounderstandthemechanismsbywhichindividualnanostructuresmayassembleintolargermaterials,asthiswillgreatlyaffectthepropertiesofthebulkdeviceforaparticularapplication.Thischapterwillfocusonalloftheseimportantissues,withanintroductiontothevarioustypesofnanomaterials,laboratorytechniquesusedfortheirsynthesis,and(perhapsmostimportantly)theirroleincurrent/futureapplications.
6.1.WHATIS“NANOTECHNOLOGY”?
Althoughthereismuchcurrentexcitementaboutnanomaterials,thereisreallynoth-ingnewaboutnanoscience.Infact,theearliestcivilizationsusednanoscalematerialsforavarietyofapplications.Forexample,theMayansusedamagnesiumaluminumsilicateclaycalledpalygorskite,whichcontainednanosizedchannelsthatwerefilledwithwater.TheMesopotamiancivilizationsusedcoloredglassfordecorativeappli-cationsthatcontainedembeddedmetallicnanoparticles.
PhysicsNobelLaureateRichardFeynmangavethefirstlectureregardingtheapplicationsfornanoscalematerials.Histalk,entitled“There’sPlentyofRoomattheBottom,”wasdeliveredon29December1959attheannualAmericanPhysicalSocietymeetingonthecampusofCaltech.Appendix2containsatranscriptofhisentiretalk,whichcontainsreferencestoafutureworldthatwasneverbeforeimagi-ned.Feynmanpointedoutthatdesigningmaterialsatom-by-atomisarealpossibility,asitwouldnotviolateanyphysicallaws.Healsopredictedsuchsci-fiaccomplish-mentsaswriting24volumesoftheEncyclopediaBrittanicaontheheadofapin,andevenmoreamazingly,thecompletereproductionofeverybookeverproducedtofitwithinasmallhandheldpamphletoflessthan40pages!Toputthesepropheticstate-mentsintocontext,atthetimehedeliveredthisspeech,computerssuchasUNIVAC1filledanentireroom(Figure6.2)andcarriedapricetagofover$1million.
Thefirstuseoftheterm“nanotechnology”wasbyNorioTaniguchiin1974attheInternationalConferenceonPrecisionEngineering(ICPE).Hisdefinitionreferredto“productiontechnologytogetextrahighaccuracyandultrafinedimensions,i.e.,theprecisenessandfinenessontheorderof1nm(nanometer),10−9minlength.”[6]Althoughmanydefinitionsfornanotechnologyhavebeensuggested,NASArecentlysuggestedthemostthoroughdescription:
Thecreationoffunctionalmaterials,devicesandsystemsthroughcontrolofmatteronthenanometerlengthscale(1–100nm),andexploitationofnovelphenomenaandproperties(physical,chemical,biological)atthatlengthscale.[7]
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Figure6.2.Photooftheroom-sizedUNIVAC1computersystemthatwasintroducedinthelate1950s.[8]
Figure6.3.ScanningtunnelingmicroscopeimageoftheplacementofindividualXeatomsonaNi(110)surface–nosurprise,byresearchersatIBM.ReproducedwithpermissionfromEigler,D.M.;Schweizer,E.K.Nature1990,344,524.Copyright1990MacmillanPublishersLtd.
AlthoughFeynmanandDrexlercertainlypopularizednanotechnology,theirinflu-encedidnotdirectlyleadtothedesignofnanoscalematerials.Rapidprogressinnanotechnologycouldonlytakeplaceafterthearrivalofsophisticatedinstru-mentation,capableofviewingandmanipulatingmaterialsonthenanoscale.Inthe1980s,scanningprobemicroscopy(SPM)wasdevelopedwhichallowedscientiststofulfillFeynman’svisionofpushingindividualatomsaroundasurface(Figure6.3).
2806.2.NanoscaleBuildingBlocksandApplications
Thistechniquewasco-inventedbyCalvinQuateandHemanthaKumarWickramasinghe.Interestingly,whenQuateandBinnigfirstsubmittedtheirworktothepeer-reviewedjournalPhysicalReviewLetters,itwasrejectedduetosuchfar-fetchedclaimsasbeingabletomeasureforcesonindividualatoms.However,theseresultswereeventuallypublished,directlyinfluencingthefutureofmolecularnanotechnology.The1986NobelPrizeinPhysicswasawardedtoGerdBinnigandHeinrichRohrertohonortheirdesignofthescanningtunnelingmicroscope(STM).TheysharedthePrizewithErnstRuska,theinventorofthefirstelectronmicroscope,anotheressentialtoolforthemodernnanomaterialsscientist.Infact,theresolutionofmodernelectronmicroscopesarenowhighenoughtoprovideimagesofindi-vidualatoms,andareoftenfittedwithdetectorsthatarecapableofdeterminingthechemicalcompositionand/oroxidationstateofthesurfaceatoms.Chapter7willdescribetheseandotherinstrumentsthatarecommonlyusedformaterials-relatedresearchanddevelopment.
6.2.NANOSCALEBUILDINGBLOCKSANDAPPLICATIONS
Thefirstquestionalmosteveryonenewtothenanoregimeasksis“whyarenano-materialssospecial?”Theleadingadvantageofthissizeregimeisthelargesur-facearea/volumeratioexhibitedbynanomaterials(Figure6.4).Accordingly,thistranslatestoaveryhighsurfacereactivitywiththesurroundingsurface,idealforcatalysisorsensorapplications.Further,sincebiologicalsystemsfeaturethesys-tematicorganizationofnanoscalematerials(e.g.,proteinsare1–20nminsize,thediameterofDNAisca.2.5nm),beingabletofabricatematerialsinthissizeregimeholdspromiseforartificialcomponentswithincells(thathaveca.10,000–20,000nm
Figure6.4.Comparisonofthesurfacearea/volumeratioofmacroscopicparticles(marbles)andnanoscopicaluminumoxideparticles.Sincenanoparticulescontainaproportionatelylargenumberofsurfaceatoms,thereareasignificantlygreaternumberofadsorption/reactionsitesthatareavailabletointeractwiththesurroundingenvironment.Further,whereasbendingofabulkmetaloccursviamove-mentofgrainsinthe>100nmsizeregime,metallicnanostructureswillhaveextremehardness,withsignificantlydifferentmalleability/ductilityrelativetothebulkmaterial.
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Figure6.5.Decreaseinthemeltingpointofgoldnanoparticleswithdecreasingdiameter.Itshouldbenotedthatthemeltingpointofbulkgoldis1,0◦C!AdaptedwithpermissionfromUnruh,K.M.etal.“MeltingBehaviorinGranularMetalThinFilms,”MaterialsResearchSocietySymposiumProceedings,vol.195.MaterialsResearchSociety,Apr16–20,1990.Copyright1990MaterialsResearchSociety.
diameters)todiagnose/fightdiseases,ilnesses,viruses,andothersuperficialweak-nesses(e.g.,artificialmuscles).
Anotherkeybenefitfornanomaterialsistheabilityofvaryingtheirfunda-mentalproperties(e.g.,magnetization,[9]opticalproperties(color),meltingpoint(Figure6.5),hardness,etc.),relativetobulkmaterialswithoutachangeinchemicalcomposition.Althoughbulkpropertiessuchasmeltingpointandhardnessarere-latedtotheenhancedsurfaceinteractionsamongnanoparticulates,thesize-tunableelectronicpropertiesareduetoquantumconfinementeffects,asdiscussedinlaterinthischapter.
Sinceweliveinamacroscopicworld,thenextgenerationofmaterialswillbeofsimilarphysicaldimensionsastoday’sconsumerproducts.Thatis,wehaveshrunkdownthesizeofcellphonesandcomputerstoalmosttheirusefullimits–anyfurther,andonewouldinconvenientlyneedtouseasharpstylustodialaphonenumber!However,asarticulatedinChapter4,althoughthesizeofelectronicdeviceswillremainsomewhatconstant,thespeedandcomputationalabilityofthesedevicesmustcontinuetoincrease.Thistranslatestomaterialsthatarebuiltfromthegroundup,onenanoscalebuildingblockatatime.However,itissyntheticallytooexpen-sive(andnotindustriallyscaleable)toarrangesuchsmallunitsintotheirdesiredpositionsbyhand.Consequently,materialschemistsarelargelyfocusedon“bottom-up”techniquesthataffordtheself-assemblyofnanoscalespecies.Aswewillseelaterinthischapter,paralleleffortsin“top-down”processingarebeingdeveloped
2826.2.NanoscaleBuildingBlocksandApplications
Figure6.6.Comparisonofthe“top-down”and“bottom-up”approachtonanomaterialssynthesis.
bymaterialsengineerstoyieldnanoscalebuildingblocksanddevicesthroughad-vancedlithographic,ablation,andetchingtechniques(Figure6.6).Inthisrespect,onecanconsiderananoscaleobjectasbeing“mesomolecular”or“mesoatomic”–anaggregateofsmallermolecular/atomicsubunits.
Therearetwoprimarytypesofnanoscalebuildingblocksthatmaybeusedforfurtherdevicefabricationandapplications:
(i)0D(e.g.,nanoparticles,nanoclusters,nanocrystals)(ii)1D(e.g.,nanotubes,nanofibers,nanowires)
Thedirectincorporationofthesenanoarchitecturesinexistingmaterialstoimprovetheirpropertiesisoftenreferredtoasincrementalnanotechnology.However,aswewillseelaterinthischapter,theself-assemblyofthesenanosizedbuildingblocksinto2Dand3Darchitecturesmayyieldentirelynewdevicesandfunctionalities–referredtoasevolutionarynanotechnology.6.2.1.Zero-DimensionalNanomaterials
Analogoustotheperiodinthissentence,a“zero-dimensional”structureisthesim-plestbuildingblockthatmaybeusedfornanomaterialsdesign.Thesematerialshave
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diameters<100nm,andaredenotedbynanoparticles,nanoclusters,ornanocrystals,whichareusedsynonymouslyintheliterature.However,inordertocontinueourrapidnanosciencedevelopments,theactiveparticipantsmustshareacommonlan-guage.Sincetherehasbeennobroadadoptionofsuchterminology,agoalofthischapteristoprovideexplicitdefinitionsandexamples(Figure6.7),inordertoavoidthecurrentnomenclatureambiguities.
Thetermnanoparticleisgenerallyusedtoencompassall0Dnanosizedbuildingblocks(regardlessofsizeandmorphology),orthosethatareamorphousandpos-sessarelativelyirregularshape.Herein,wewilldefinenanoparticlesasamorphousorsemicrystalline0Dnanostructureswithdimensionslargerthan10nm,andarel-ativelylarge(≥15%)sizedispersion.Foramorphous/semicrystallinenanostructuressmallerinsize(i.e.,1–10nm),withanarrowsizedistribution,thetermnanoclus-terismoreappropriate.Thisdistinctionisasimpleextensionoftheterm“cluster,”whichistypicallyusedininorganic/organometallicchemistrytoindicatesmallmole-cularcagesoffixedsizes(Figure6.7).Analogoustobulkmaterials,theagglomera-tionofnoncrystallinenanostructuralsubunitsshouldbestbetermedananopowder(Figure6.7).
Itisalsoimportantheretonotethedifferencebetweennanoparticles/nanoclustersandtraditionalcolloids,whichdatebacktotheearly1860s(Table6.1).Weareallfamiliarwiththetermcolloid,whichisusedtodescribesolid/liquidandsolid/gassuspensionssuchasmilk,paints,butter,smoke,andsmog.Althoughbothtypesofmaterialshavesizeswithinthenanoregime,theleadingdifferenceisthecontrolonehasovercompositionandmorphology.Aswewillseeshortly,inordertostabilizemetalnanostructures,astabilizingagentmustbeusedtopreventagglomerationintoalargerpowder.Thisisalsothecaseforcolloids,whichgen-erallyemploypolydispersedorganicpolymersandotherionicspeciesthatmayadsorbtothecolloidsurface.Suchavariationinthenatureoftheencapsulat-ingenvironmentleadstoalargedispersityinoverallmorphologyandpropertiesofcolloids.Bycontrast,inorderfornanomaterialstobeusedfor“bottom-up”design,theirsynthesisandresultantpropertiesmustbereproducible.Thisiseasilyaccomplishedthroughtheuseofstabilizingagentswithwell-definedstructuresthatdonotreactwith/surfacedeactivatetheentrainednanostructures(e.g.,dendrimers,polyoxoanions,etc.).
Thusfar,wehavedefinednomenclatureforamorphous0Dnanostructures.Anal-ogoustobulkmaterials,anynanomaterialthatiscrystallineshouldbereferredtoasananocrystal.Thistermshouldbereservedforthosematerialsthataresingle-crystalline;ifaparticleexhibitsonlyregionsofcrystallinity,itisbettertermedananoparticleornanoclusterdependingonitsdimensions.Transmissionelectronmicroscopy,especiallyintandemwithelectrondiffractionismostusefulindeter-miningthecrystallinityofanynanostructure(Figure6.8).
Aspecialcaseofnanocrystalthatiscomprisedofasemiconductorisknownasaquantumdot.[12]Typically,thedimensionsofthesenanostructureslieintherange1–30nm,basedonitscomposition(seebelow).Quantumdotscurrentlyfindappli-cationsassensors,lasers,andLEDs.Infact,newhigh-densitydisks(e.g.,HD-DVDandBlu-rayhigh-definitionDVDformats)mayonlybereadviabluelasers,which
2846.2.NanoscaleBuildingBlocksandApplications
Clusters
<< 1 nma)b)c)
NanoclustersNanocrystals
1 - 100 nm5 nm500 nm
5 nm
Nanoparticles
100 nm
Nanopowder
50 nm
100 nm - 1000 nmSub-Micron Particles
100 nm
1 - 100 µm: “microstructures” > 100 µm: “particulates”
Bulk Powder
Figure6.7.0Dnanostructurenomenclature.Shownarethewell-definedcagesizesofmolecularclusters(Os5(CO)16),(Os6(CO)18),[Os8(CO)22]2−.[13]Comparatively,nanoclustersshouldbeusedtodescribe0Dnanostructuresofahomogeneoussizedistribution.[14]Bycontrast,nanoparticlesexhibitagreaterrangeofsizes/shapes.[15]Nanocrystalsarecharacterizedbythepresenceofanorderedlatticearrayoftheconstituentsubunits,asillustratedbyasinglenanocrystalofCdSe.[16]Instarkcontrasttoananocrystal,anexampleofananopowderisshownthatconsistsofmicroscopicgrains,eachcomprisedofnanoscaleamorphousunits.[17]Thesizeregimethatisintermediatebetweenthenano-andmicroregimesisbestreferredtoassubmicron.[18]Thebulkpowderscalebaris200µm.
6Nanomaterials
Table6.1.Comparisonof0DNanoarchitectureswithTraditionalColloids[10]
285
Colloids
Typically>10nm
Poorlydefinedcompositions>15%Sizedispersion
Nonreproducible,uncontrollablemorphology/composition
Nonreproducibleproperties(esp.irreproduciblecatalyticactivities[11])TypicallyonlysolubleinpolarsolventsContainsurface-adsorbedspeciessuchas–OH,–X,–OH2,etc.
Nanoparticles/nanoclusters
Size:1–100nm(nanoclusters:1–10nm)Homogeneousmolecularcomposition≤15%Sizedispersion(less
polydispersityfornanoclustersrelativetonanoparticles)
Reproduciblesynthesis(controloversize,shape,andcomposition)Reduciblephysicalpropertiesandcatalyticactivity
Solubleinpolar/nonpolarorganic
solvents(dependingonstabilizingagent)Containcleansurfaces
a)
b)
Figure6.8.TEMimagesofamorphousnanoclusters(a),andnanocrystals,(b).Theinsetof(a)showsselectedareaelectrondiffraction(SAED);theabsenceofapatternindicatesanamorphousstructure.Thescalebaris20nm.Thehigh-resolutionTEMimageinsetin(b)showslatticespacingsofanindividualnanocrystal.Reproducedwithpermissionfrom(a)Liu,S.;Fooken,U.;Burba,C.M.;Eastman,M.A.;Wehmschulte,R.J.Chem.Mater.2003,15,2803.(b)Tirosh,E.;Shemer,G.;Markovich,G.Chem.Mater.2006,18,465.Copyright2003&2005AmericanChemicalSociety.
2866.2.NanoscaleBuildingBlocksandApplications
Table6.2.CalculatedExcitonBohrRadiiforVariousSemiconductors
MaterialSiCdSCdSeCdTeZnOZnSPbSPbSeInAsInSb
˚rB(A)55315611001850204460340540
arefabricatedfromquantumdots.Long-termapplicationsforthesestructureswilllikelyincludeopticalcomputingandhigh-efficiencysolarcells.
AsyoumayrecallfromChapter4,whenanelectronispromotedfromthevalencetoconductionbands,anelectron–holepairknownasanexcitoniscreatedinthebulklattice.ThephysicalseparationbetweentheelectronandholeisreferredtoastheexcitonBohrradius(rB)thatvariesdependingonthesemiconductorcompo-sition.Inabulksemiconductorcrystal,rBissignificantlysmallerthantheoverallsizeofthecrystal;hence,theexcitonisfreetomigratethroughoutthelattice.How-ever,inaquantumdot,rBisofthesameorderofmagnitudeasthediameter(D)ofthenanocrystal,givingrisetoquantumconfinementoftheexciton.Empirically,thistranslatestothestrongestexcitonconfinementwhenD≤2rB.
Analogoustothe“particle-in-a-box”modelfromintroductoryphysicalchemistry,excitonquantumconfinementresultsindiscreteenergylevelsratherthanthecontin-uousbandsofabulksemiconductorcrystal.Sincethegapbetweenadjacentenergylevelsisinfinitesimallysmallforabulksemiconductor,thebandgapisconsideredasafixedvalue.Incontrast,sincethedimensionsofaquantumdotareextremelysmall,theaddition/subtractionofasingleatomwillsignificantlychangethenanocrystaldimensionsandbandgap.Table6.2liststherBvaluesforcommonsemiconductorcrystalswhosebandgapmaybeeasilyfine-tunedbysimplychangingthediameterofthequantumdot,aslongasthedimensionsaresmallerthanrB.
Intheearly1980s,Efrosdescribedthesize-dependentelectronicpropertiesofquantumdots,firstdelineatingthatthebandgap,En,willincreasefromthebulkvaluebasedona1/R2confinementenergyterm(Eq.1).Accordingtoquantumcon-finementtheory,electronsintheconductionbandandholesinthevalencebandarespatiallyconfinedbythepotentialbarrierofthesurface.Duetoconfinementofbothelectronsandholes,thelowestenergyopticaltransitionfromthevalencetoconduc-tionbandwillincreaseinenergy,effectivelyincreasingthebandgap.(1)
h2π2
,En=Eg+
2µR26Nanomaterials287
whereEgisthebandgapofthebulksemiconductor;h,Planck’sconstant;R,theradiusofthequantumdot;andµisthereducedmassoftheexcitongivenbymemh/(me+mh).Heremeandmharemassesoftheelectronandhole,respectively.ThismodelwasexpandedbyBrusandcoworkerstoincludeCoulombicinterac-tionofexcitonsandthecorrelationenergy(Eq.2).(2)
h2π21.786e2
−+0.284ER,En=Eg+
εR2µR2µe4µ
ER==13.56,2h2ε2me2ε2ε0
whereε0isthepermittivityoffreespace;ε,thedielectricconstantofthebulksemi-conductor;andmeisthemassoftheelectron.
Hence,theabsorptionenergyofquantumdotswillshifttohigherfrequencywithdecreasingdiameterofthedots,withadependenceof1/R2.Thisisreadilyobservedfromthereflectedcolorsofquantumdotswithvaryingdiameters,shiftingfrombluetoredwithincreasingsize(Figure6.9).
Foranyonewhohasadmiredthebrightredcolorsofstainedglasswindows,itmaybesurprisingtonotethatthistooisananoparticlephenomenon.Infact,theredsandyellowsobservedintheseantiquatedglassesaretypicallycausedbythepresenceof
whereERistheRydberg(spatialcorrelation)energyofbulksemiconductor:
Figure6.9.Thesize-dependencyontheopticalpropertiesofCdSenanocrystals.Withdecreasingsize,thefluorescencepeakisshiftedtoshorterwavelengths.AlsoshownisthereducedphotobleachingexhibitedbyCdSenanocrystals–(top-bottomphotos):suspensionofnanocrystalsinsolution,beforeandafterirradiationwithUVlight,respectively.ReproducedwithpermissionfromPellegrino,T.;Kudera,S.;Liedl,T.;Javier,A.M.;Manna,L.;Parak,W.J.Small,2005,1,48.Copyright2005Wiley-VCH.
2886.2.NanoscaleBuildingBlocksandApplications
goldandsilvernanoparticles,respectively.However,formetallicnanoclusters/nanoparticleswithdiametersca.>2nm,theoperatingprincipleisdifferentfromsemiconductorquantumdots,sincethereisnobandgapbetweenvalenceandcon-ductionbands,andtheenergystatesformacontinuumanalogoustobulkmetal.Forthesemetallicnanostructures,anotherphenomenonknownassurfaceplasmonresonance(oftendenotedaslocalizedsurfaceplasmonresonance,LSPR)isactiveforthesestructures,involvingspecificscatteringinteractionsbetweentheimping-inglightandthenanostructures.Inparticular,theoscillatingelectricfieldoftheincominglightcausesthecoherentoscillationoftheconductionelectrons,result-inginaconcomitantoscillationoftheelectroncloudsurroundingthemetalnuclei(Figure6.10).[19]
Theleadingtheorythatdescribesthescatteringbehaviorofsmallsphericalparti-cleswithlightdatesbacktotheworkofMieintheearly1900s(Eq.3).[20]
33/224πNArεmεi(λ)
E(λ)=(3),
λln(10)(εr(λ)+2εm)2+εi2(λ)whereE(λ)istheextinction(sumofabsorptionandscattering);NA,thedensityof
thenanostructures;r,theradiusofthenanostructure;εm,thedielectricconstantofthemetallicnanostructure;λ,thewavelengthofabsorbingradiation;andεi,εraretheimaginaryandreal(respectively)portionsoftheλ-dependentdielectricfunctionofthenanostructure.
Thistheoryisstillrelevanttoday,alongsideRaleighscatteringtheory,todescribethecolorsofoursky,aswellastheappearanceofsuspensionssuchasmilkandlatexpaints.Itshouldbenotedthatfornonsphericalnanostructures,thedenominatorofthebracketedtermaboveisreplacedwith:
(εr(λ)+χεm)2,
whereχmayrangefromavalueof2(perfectsphere)to17(e.g.,fora5:1aspectrationanostructure)andbeyond.[21]
BasedonEq.3,thefactorsthatgoverntheoscillationfrequency(andtheobser-vedcolor)are:electrondensity(size/shapeofthenanostructures,Figure6.11),theeffectivenuclearchargeofthenuclei,andthesize/shapeofthechargedistribu-tion(polarizationeffects,stronglyaffectedbythedielectricconstantofthemetal).Asyoumightexpect,furthereffectstowardtheresonancefrequency/intensityare
E-field
Metalsphere
e− cloud
Figure6.10.SchematicofLSPRforananosphere,showingtheinducedoscillationoftheelectroncloudrelativetothenuclei.ReproducedwithpermissionfromKelly,K.L.;Coronado,E.;Zhao,L.L.;Schatz,G.C.J.Phys.Chem.B2003,107,668.Copyright2003AmericanChemicalSociety.
6Nanomaterials2
Figure6.11.Theinfluenceofsizeandshapeonthelight-scattering,andresultantcolors,ofsilvernanopar-ticles.ReproducedwithpermissionfromMirkin,C.A.Small2005,1,14.Copyright2005Wiley-VCH.
AB1200
1
Intensity800
2
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500550Wavelength (nm)
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Figure6.12.BiologicalsensingusingAgnanoparticles.Shownis(a)adark-fieldopticalimageofsurface-functionalized(biotinylated)Agnanoparticles,and(b)theshiftinwavelengthbefore(1)andafter(2)exposureto10nMstreptavidin.ReproducedwithpermissionfromHaes,A.J.;Stuart,D.A.;Nie,S.;Duyne,R.P.V.J.Fluoresc.2004,14,355.Copyright2004SpringerScienceandBusinessMedia.
observedfromthesolventandsubstratethatsurroundsthenanostructures.Assuch,noblemetalnanoparticleshavebeenusedastunableplatformsforbiologicalsensing(Figure6.12).
Forthesmallestofmetallicnanoclusterswithdimensionsca.<2nm,thesurfaceplasmonabsorptiondisappears.Sincesofewatomscomprisediscretenanoclustersofthissize,thespacingsbetweenadjacentenergylevels(referredtoastheKubo
290
Metal
6.2.NanoscaleBuildingBlocksandApplications
Size- Induced Metal-Insulator Transition
Insulator
Diameter : D >>104ÅNuclearity : N >>1010Kubo Gap : d <<10−6K Bulk MetalD ~ 104−102ÅN ~ 1010−104d ~10−6−1K D ~ 102−10ÅN ~ 104−10d ~1−103K D < 10ÅN < 10d > 103K Atoms & MoleculesMetal cluster /nanoparticleEnergyEFDensity-of-States
Figure6.13.Schematicofthedensityofstatesexhibitedbybulkmetalrelativetoincreasinglysmallernanoclusters.Theapproximatediameter,nuclearity,andKubogapforeachsizeregimeareindicated.Asthenanoclustersizedecreases,theenergycontinuumofthebulkmetalistransformedintodiscreteenergylevels,especiallyatbandedges.ReproducedwithpermissionfromRao,C.N.R.;Kulkarni,G.U.;Thomas,P.J.;Edwards,P.P.Chem.Soc.Rev.2000,29,27.Copyright2000RoyalSocietyofChemistry.
gap,δ,Eq.4)becomecomparabletothethermalenergy,kT–especiallyatlowertemperaturesandsmallernanoclusterdiameters.Thisresultsinashiftinconductivepropertiesofthenanocluster,frommetallictosemiconductingandinsulating,withdecreasingsize(Figure6.13).Inbandtheory,thebreadthofabandisdirectlyre-latedtothestrengthofinteractionsamongnearestneighbors.Foratoms/molecules(Figure6.13,farright)thisinteractionisweak,whichcorrespondstoanarrowband.However,asadditionalatomsareaddedtothesolid,strongerinteractionswillensue,resultinginagreaterdensityofstatesneartheFermilevel–additionalstatesnearthebandedgesdeveloplast.Hence,theinsulatingpropertiesofverysmallnanoclustersareduetothesmallnumberofneighboringatoms,whichareheldtogetherprimarilybynonmetallicinteractions(e.g.,vanderWaalforces).[22a]Asnanoclustersizeinc-reases,theatomics/panddorbitalsfromalargernumberofconstituentatomswillbroadenintobands,forminganenergycontinuum.
Quantumconfinementeffectsalsocauseachangeintheopticalpropertiesofmetallicnanoclusters.Forinstance,sincethespacingbetweenintrabandenergylevelsincreaseswithdecreasingnanoclustersize,the4s/pto5dfluorescentemis-sionforgold(e.g.,Au40–ca.1.1nmdiameter,Au8,etc.)willbecomeincreasinglyblue-shiftedwithdecreasingdimensions.[22]
6Nanomaterials(4)
δ=
291
3Ef
,2N
whereδistheKubogap,energyspacingbetweenadjacentElevels;N,thenuclearity,thenumberofatomsinthenanocluster;andEfistheenergyoftheFermilevel.
Mechanismforthenucleation/growth/agglomerationofmetalnanoclustersInordertomaintaincontroloverthecompositionandmorphologyofa0Dnanostruc-ture,itisessentialthatweunderstandtheself-assemblymechanismforthesestruc-tures.Onlywithinthelastdecadehavewefiguredoutwaystorepeatablycontrolthemorphologyandcompositionofnanoparticles/nanoclusters.Thisexplainswhythemechanisticdetailsofnanoclustergrowthhavenotsurfaceduntilrecently.Thefirstsystemtobeinvestigatedweremetalliciridiumnanoclusters,formedthroughthehydrogenationofapolyoxoanion-supportedIrcomplex(Figure6.14).[23]SinceIrdoesnothaveanobservablesurfaceplasmonresonanceprofile,therateofnanoclus-tergrowthwasdeterminedbyfollowingthehydrogenationofalkenesovertime.Thisispossibleinthissystemsincetheinitialcomplexisnotanactivetowardalkenehydrogenation;catalyticactivityarisesonlyfromthereducedmetal(Ir0).
Theoverallfour-stepmechanismfornanoclustergrowth/agglomerationisshowninFigure6.15.[24]Althoughthispathwayisbasedonmetallicnanoclusterstudies,othertypesofnanoclusters/nanoparticleswouldlikelyfollowasimilarroute.[25]Thefirststepinvolvestheslow,continuousnucleationofclustersthataremuchsmallerthan1nm.Whenanenergeticallyfavoredcriticalnucleussizeisreached(ca.15atomsforIr0;variesdependingonthemetal),additionalatomsrapidlyaggregatetothesurface–aprocessreferredtoasautocatalyticsurfacegrowth.Theterm“au-tocatalytic”isusedsincethenanoclustersformedfromthenucleationsteparealsoreactantsforsubsequentsurfacegrowth.Accordingly,theautocatalyticstepwillpro-ceedfasterasthereactionprogresses,effectivelyshuttingdownthenucleationstep–essentialtoachievemonodispersenanoclustergrowth.Thisisacrucialfinding,sincethisexplainsthestrictsizecontrolthatispossibleformetalnanoclustergrowth.Thatis,thesizeofthegrowingnanoclustermaybecontrolledbyvaryingtherelativevaluesofk1andk2,aswellastheavailabilityofadditionalprecursormolecules.Inthisfashion,metalnanoclustershavebeendesignatedas“livingmetalpolymers,”analogoustoorganiclivingpolymersdiscussedinChapter5.
Interestingly,ithasbeenshownthatthegrowthofnanoclustersproceedsthroughtheformationof“magicnumber”(orclosedshell)clustersthatexhibitunusualelec-tronicstability.Forfccorhcptransitionmetals,stableclusterscontain13,55,147,309,561,923,1,415,...metalatoms,wherethenumberofsurfaceatomsinthenthshellisgivenby10n2+2(n=1,2,3,4,5,...).[26]Itshouldbenotedthatin-termediatemagic-numbernanoclustersrepresentonlylocalminimainthepotentialenergysurface,relativetotheglobalminimumofabulkmetalwiththelowestpos-siblesurfacearea(Figure6.16).[27]Thehighyieldofmagicnumbernanoclustersisaconsequenceofkineticallycontrolledsurfacegrowth.Thatis,oncethesefavoredintermediatestructuresareformed,theyarelessreactivetowardautocatalyticsurfacegrowthrelativetononmagicnumberclusters.
2926.2.NanoscaleBuildingBlocksandApplications
9−
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H2
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W=OW-O-WNb=ONb-O-NbIr(0)
P2W15Nb3O629− Stabilizing Matrix
Figure6.14.Polyoxoanion-stabilizedIr0nanoclusterformation.Shownis(a)Polyhedralrepresentation
−
oftheα−1,2,3−P2W15Nb3O962stabilizinganion,and(b)aspace-fillingrepresentationofthe[(1,5−COD)Ir(P2W15Nb3O62)]8−complex.In(a),thethreeNbatomsareindicatedbythestripedoctahedrainpositions1–3.TheWO6polyhedraoccupythe4–18positions,andtheinternalPO4tetrahedralunitsareillustratedinblack.In(b),theblackspheresrepresentM–Oterminalgroups,andthewhitespheresrepresentM–O–Mbridginggroups.TheBu4N+andNa+counterionsareomittedforclarity.Images(a)and(b)reproducedwithpermissionfromFinke,R.G.;Lyon,D.K.;Nomiya,K.;Sur,S.;Mizuno,N.Inorg.Chem.1990,29,1784.ThebottomimageofthestabilizednanoclusterisreproducedfromWatzky,M.A.;Finke,R.G.Chem.Mater.1997,9,3083.Copyright1997AmericanChemicalSociety.
6Nanomaterials
A
k1H2k2H2k3
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n Pt''
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+
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00bulk
k4
Pt
0bulk
+
Figure6.15.Schematicofthefour-stepmechanismfortransitionmetal(e.g.,Pt)nanoclusterforma-tion.Shownare(i)nucleationtoadesiredclustersize;(ii)autocatalyticgrowthontotheclustersurface;(iii)diffusiveagglomerativegrowthoftwonanoclusters;and(iv)autocatalyticagglomerationintobulkmetalparticulates.ReproducedwithpermissionfromBesson,C.;Finney,E.E.;Finke,R.G.J.Am.Chem.Soc.2005,127,8179.Copyright2005AmericanChemicalSociety.
Figure6.16.Comparativeenergylevelsfor“magicnumber”noblemetalnanoclustersrelativetobulkmetal.Alsoshownaretheperfectfccarraysofthenanoclusters,inwhichmorethan75%oftheatomsarelocatedonthesurface.AdaptedwithpermissionfromFinke,R.G.inMetalNanoparticles:Synthesis,Characterization,andApplicationsFeldheim,D.L.;Foss,C.A.eds.,Dekker:NewYork,2002.Copyright2002Taylor&Francis.
2946.2.NanoscaleBuildingBlocksandApplications
Thelasttwostepsofthenanoclustergrowthmechanisminvolveagglomeration.Thisphenomenonwherebylarger(nano+)particulatesgrowattheexpenseofsmallernanoclustersisoftenreferredtoascoarseningorOstwaldripening.Whereasstep3illustratesabimolecularaggregationoftwonanoclusters,thelaststepinvolvestheformationofbulkmetalparticlesthroughasecondautocatalyticsurface-growthprocess.Thesetwostepsareobviouslyundesiredpathwaysduringnanoclustergrowth.However,itisessentialtohavethesemechanisticsteps(finally)stated,sincethiswillleadtofuturequantitativestudiesthatwillidentifythebestcondi-tionstopreventagglomeration(varyingratiosofk1–k4,stabilizingagents,metals,etc.).Thisdataiscurrentlylacking,andonesimply“hopesforthebest”regard-ingthelong-termstabilityofthegrownnanoclustersbasedonachosenstabilizingagent.AgeneralpredictionoftheoverallmechanismshowninFigure6.15isthatlowerconcentrationsandhighertemperaturesaremostconducivetoyieldnanoclus-tersratherthanbulkmetal(i.e.,k1k3andk4).Indeed,thispredictionisbackedupbyexperimentaldata.
Thefirst0Dnanoarchitecture:thefullerenes
Withoutquestion,ourmodern“nanotechnologyrevolution”wascatalyzedbythemid-1980sdiscoveryofcarbonnanoclustersknownasfullerenes(e.g.,C60,Figure6.17).[28]The1996NobelPrizeinChemistrywasawardedtoRichardSmalley,RobertCurl,andSirHaroldKrotoforthisdiscovery,whichfocusedtheworldwidespotlightonuniquenanoscalematerialsandtheirpossibleapplications.Todate,themostcommonapplicationsforfullerenesincludeMRIcontrastagents(exploitingitscontainerproperties),drug-deliveryagents(throughsurfacefunc-tionalization),fulleride-basedsuperconductors,andlight-activatedantimicrobialagents[29](Figure6.18).Itshouldbenotedthatthetermfullerenedoesnotsimply
Figure6.17.MolecularstructureofBuckminsterfullerene,C60,containingalternatingsix-andfive-memberedringsofsp2hybridizedcarbonatoms.ThisisonlyoneisomerforC60,outofastaggeringtotalof1,812possiblestructures.[34]
6Nanomaterials295
x
a)
10
b)
AcOOOH
OOO
O
O
O
OH
Ph
NH
OCOPh
OAc
OO
NH
O
c)
Ph
O
Figure6.18.Examplesoffullereneapplications.Shownarea)Gd3+@C60–[OH]x(x≈27)andGd3+@C60–[CCOOH]10usedasMRIcontrastagents,[35](b)theionicunitcellforthesuperconduc-tivealkalimetalfullerideCsxRbyC60,[36]and(c)theconjugatestructureofC60covalentlyboundtothelungcancerdrugPaclitaxel.[37]
meantheC60carbonallotropetermedBuckminsterfullerene(or“Buckyballs”),[30]butrathertheentireclassofclosed-cagecarbonclustersthatarecomprisedofexactly12pentagons,andavaryingnumberofhexagons(e.g.,20hexagonsforC60).
AswithothermajordiscoveriessuchasTeflonandnylon,[31]thesynthesisofC60wasserendipitous.Infact,theexperimentsthatledtothediscoveryoffullereneswereaimedatsimulatingtheenvironmentofacarbon-richredgiantstar.Assuch,thediscoveryofC60hasbeendubbed“thecelestialspherethatfelltoearth.”[32]Thesystemfeaturedthelaservaporizationofagraphitetargetintoaheliumcarriergaswhereintheatomsnucleatedintoclusters.Thegaswasthencooledusingsup-ersonicexpansion,andinjectedintoatime-of-flightmassspectrometerforanalysis(Figure6.19a).Reactivegasessuchashydrogenornitrogencouldalsobeaddedtothecarriergas,forthesynthesisofotherreactionproducts.Itshouldbenotedthatthistechniqueisnowusedtogeneratenanoclustersofawidevarietyofmetals,semiconductors(e.g.,Si),andinsulators(e.g.,Al2O3)dependingonthenatureofthetargetandco-reactantgasesemployed.
Thefirstlarge-scalesynthesisoffullereneswasdiscoveredin19byHuffmanandKratschmer;thisprovidedamacroscopicquantityofC60inordertoconfirmtheproposedicosohedralstructure.Theirtechniqueconsistedofthearc-evaporationofgraphiteelectrodesviaresistiveheatingwithinanatmosphereofca.100atm.helium
2966.2.NanoscaleBuildingBlocksandApplications
Vaporization laser
Supersonic Nozzlea)
He Gas Pulse
\"IntegratingCup\"
MassSpectrometer
Rotating Graphite
Disk
b)
He inlet
Electric ArcTo Vacuum
GraphiteRodElectrodesCurrent Source
Figure6.19.Schematicsofapparatifirstusedtosynthesizefullerenes.Illustratedare(a)theSmalley/Curlsupersoniclaserevaporationsystem,and(b)theHuffman/Kratschmerelectricarcapparatus.
(Figure6.19b).[33]Othermorerecenttechniquessuchashigh-temperaturecombus-tionofbenzeneandabenchtopgraphitearcprocess(Figure6.20)weredevelopedinordertoreducethecostandcomplexityassociatedwithfullerenesynthesis.Notonlywillsuchimprovementsallowformorewidespreadfabricationoffullerenesforresearch/applications(i.e.,synthesisnolongerlimitedtogroupswithsupersoniclaserandarcplasmasystems),butmayalsoresultinloweringthecostduetoamorestraightforwardindustrialscale-up.
BuckyballsrepresentthesmallestfullerenethatobeystheIsolatedPentagonRule(IPR)–i.e.,anenergeticrequirementthatpentagonsbesurroundedbyhexagons,sothatadjacentpentagonsdonotshareanedge.Calculationsshowthatπbondssharedbetweensix-memberedringshavelargepositivebondresonanceenergies(BREs)andbondorders,indicatingahighdegreeofaromaticityandstability/unreactivity.However,πbondsbetweenadjacentfive-memberedringshavelargenegativeBREswithverysmallbondorders,indicatingamuchlowerthermodynamicstability.Mostlikely,thisdifferenceisduetotheincreasedringstrainthatwouldbeimposedinthefullerenestructureasaresultoftwosmallerringsdirectlyadjacenttooneanother.TheoreticalcalculationsindicatethatthestrainenergyoftheicosohedralBuckminsterfullerenestructureisatleast2eVlowerthananyothernon-IPRisomer,ofwhichthereareover1,800possibilities.
6Nanomaterials
to pump
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Vacuum Ports
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manometer
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graphiteelectrode
1 LPyrexreactorvessel
Pyrexguidearm
SUBMERSIBLE
PUMP
SCALE:5 cm
AC METER
DC ARC WELDER
220V/50 AAC OUTLET
Figure6.20.Cross-sectionschematicsofreactorsusedforfullerenesynthesis.Shownare(a)areduced-pressurefuel-richpyrolyticchamber,and(b)abenchtopmodifiedarcevaporationsystem.Reproducedwithpermissionfrom(a)Hebgen,P.;Goel,A.;Howard,J.B.;Rainey,L.C.;VanderSande,J.B.Proc.Combust.Inst.2000,28,1397,Copyright2000Elsevier,and(b)Scrivens,W.A.;Tour,J.A.J.Org.Chem.1992,57,6932.Copyright1992AmericanChemicalSociety.
2986.2.NanoscaleBuildingBlocksandApplications
Interestingly,ithasrecentlybeenreportedthatadjacentpentagonscontainingatleastoneNatominsteadofC(e.g.,C58N2ratherthanC60),mayactuallybemorestablethanC60(Figure6.21).[38]ThisapparentanomalyisadirectcontradictionoftheIPR.Themostplausibleexplanationisthereductionofringstrainduetosp3hybridizationofN,aswellastheadditionofπ-electrondensity(fromtheNlone-pairelectrons)tothepentagons,resultinginanenhancedaromaticity/stability.Todate,onlyshort-livedazafullerenesC59NandC58N−2havebeenidentifiedexperi-mentally;thesearchcontinuesforstablestructures,sincethesewilllikelyresultindramaticallydifferentpropertiesandassociatedapplicationsrelativetotheirC-onlyanalogues.
Althoughfullereneshavebeenactivelyinvestigatedformorethantwodecades,thereisanongoingdebateregardingthegrowthmechanismofthesenanoclusters.Sincetheformationoffullerenesvialaser/arc/combustiontechniquesoccurstoorapidlytoisolateintermediatespecies,mostofthemechanisticproposalsarebasedontheoreticaltechniques(quantummechanicalandmoleculardynamics).Itwasoncethoughtthatfullereneswereformedfromthefoldingofpreformedgraphiticsheetsthatemanatedfromthetargetfollowinglaserablation.However,avarietyofexperimentshaveshownthatthegrowthprocessinitiatesfromsmalllinearchainsofcarbonatoms.Asthenumberofcarbonatomsincreases,thechainspreferentiallyconnectintoringstructuresduetotheirgreaterstabilities.Inparticular,forCnwheren<10(withtheexceptionofC6asdiscussedbelow),linearspeciesarethepre-ferredmorphologyratherthanrings(Figure6.22).Thepreferenceforringformationforn=6,andn≥10(especiallyforC10,C14,C18,etc.)isduetotheenhan-cedaromaticity/stabilityofplanarringswhenthereare4n+2πelectrons(wheren=1,2,3,...–theH¨uckelrule).
Figure6.23illustratesaproposedmechanismforthesubsequentstepsoffullerenegrowth,involvingtheself-assemblyofcarbonrings.Whenn≥30orso,themono-cyclicringsareproposedtoformgraphiticsheets.The“pentagonroad”mechanismproposedbySmalley[39]assumesthatthegraphiticsheetscontainbothhexagonandpentagonunits.ClosureofthesheetstoformBuckyballseffectivelyresultsingrowth
Figure6.21.IllustrationofC58N2that(a)satisfiestheIPRand(b)violatestheIPRwithadjacentpen-tagons.Thestructurewithadjacentpentagons,(b),ismorestablethan(a)by12.5kcalmol−1.ReproducedwithpermissionfromEwels,C.P.NanoLett.2006,6,0.Copyright2006AmericanChemicalSociety.
6Nanomaterials
8060Relative Stability [kcal/mol]40200−20−40−60−80−100
2
4
6
8101214Number of Carbon Atoms
16
18
20
monocyclic rings
linear chains
299
Figure6.22.Therelativestabilitiesoflinear-chaincarbonclustersvs.monocyclicringswithchangingclustersize.ReproducedwithpermissionfromHutter,J.;Luthi,H.P.;Diederich,F.J.Am.Chem.Soc.1994,116,750.Copyright1994AmericanChemicalSociety.
Figure6.23.Proposedmechanismforfullerenegrowth.ReprintedfromYamaguchi,Y.;Maruyama,S.Chem.Phys.Lett.1998,286,343.Copyright1998,withpermissionfromElsevier.87985734.
3006.2.NanoscaleBuildingBlocksandApplications
termination.Incontrast,the“fullereneroad”modelassumestheinitialformationofsmallernon-IPRfullerenes,whichundergothermalrearrangementtoyieldC60andhigherfullerenes.[40]
Asdiscussedearlier,pentagonunitsareessentialtothefullerenestructure,sincetheyallowtheplanargraphiticsheettocurl.Thedrivingforceforthisrearrange-mentislikelytheC–Cbondingofedgecarbonsthatsatisfiestheirunfilledvalences.Astheprevailingmechanismpointsout,adequateannealingisrequiredinordertoincorporateasufficientnumberofpentagons.Forinstance,ifthecoolingrateistoohigh,amorphoussootparticleswillbepreferentiallyformedratherthanfullerenes.Inaddition,anoveralllowgrowthtemperaturewillnotbesufficienttocausecageformation,yieldingplanargraphiticfragmentsinsteadoffullerenes.
Regardlessoftheproposedmechanism,afinalthermalannealingstepislikelyrequiredtoorganizethehexagonandpentagonsubunitsintothelowest-energyIPRarrangement.ThisrearrangementstepisknownastheStone–Wales(SW)transformation,andinvolvesaconcertedreorganizationofthehexagon/pentagonunits.WealreadysawanexampleofarareSWtransformationwherethenon-IPRN-containingspecieswasactuallylowestinenergy(Figure6.21).However,mostoftenthisrearrangementoccursintheoppositedirection–transformingadjacentpentagonsintoahexagon-isolatedstructure.ItshouldbenotedthattheStone–WalestransformationisactuallythermallyforbiddenviatheWoodward–Hoffmanrules;calculationsshowanenergybarrierofatleast5eVforthispathway.However,ithasbeenshownthatthisrearrangementmaylikelybecatalyzedbyadditionalcar-bonand/orhydrogenatomsthatarepresentduringlaser/arcorthermalcombustionsyntheses(Figure6.24).
Interestingly,afullerenestructuremayserveasanucleationsiteforadditionallayersofgraphiteenroutetowardmultishellfullerenes(Figure6.25).Thesearedenotedas“C60@C240”wherethe@symbolrepresentstheencapsulatedspecies.Thereareeventriple-layeredstructuressuchas“C60@C240@C560.”[41]Thoughverysmallquantities(<0.01%)ofmultilayeredfullerenesarefoundinthesootresultingfromlaservaporization,theyieldmaybeimprovedbyinvacuosublimationofthevaporphaseatahightemperature(ca.1,200◦C).
Althoughthe“bruteforce”methodsoflaser/arcandhigh-temperaturepyrolysisrepresentthemostcommontechniquesforgeneratingfullerenes,agoalofthesyn-theticorganicchemisthaslongbeenthesolution-phase,stepwisesynthesisofC60.In1999,apromisingstepinthatdirectionwasaccomplishedwiththefirstnonpy-rolyticsynthesisof“buckybowls.”[42]Thesestructureshadabowl-shapedstructure,andconsistedofthehexagon-isolatedpentagonbackboneexhibitedbyfullerenes(Figure6.26).Inearly2002,achlorinatedC60precursorwasreportedusingatra-ditional12-steporganicsynthesis.ThiscompoundwassubsequentlyconvertedtoBuckyballsusinghigh-temperaturevacuumpyrolysis(Figure6.27).TheyieldofC60was<1%–certainlynotusefulforcommercialproductionofBuckyballs!However,thenoveltyofthisapproachwasthatpyrolysisdidnotdecomposetheprecursorintosmallerunits,butratherservedtostitchtogetheradjacentarmsofthemolecularprecursor.Hence,thismethodprovidesatargetedroutetowardindividualfullerenes
6Nanomaterials301
Figure6.24.ThepotentialenergysurfaceoftheStone–Walestransformationbefore/aftertheadditionofcatalyzingmoietiessuchas(a)carbon,and(b)hydrogenatoms.Reproducedwithpermissionfrom(a)Eggen,B.R.;Heggie,M.I.;Jungnickel,G.;Latham,C.D.;Jones,R.;Briddon,P.R.Science1996,272,87,Copyright1996AAAS;and(b)Nimlos,M.R.;Filley,J.;McKinnon,J.T.J.Phys.Chem.A2005,109,96.Copyright2005AmericanChemicalSociety.
basedonthestructureoftheprecursor,ratherthanhigh-energymethodsthatalwaysresultinamixtureoffullereneproducts.
Inadditiontopristinefullerenestructures,ithasbeendiscoveredthatvariousmetalionsmaybeencapsulatedinsidethecagedstructuretoyieldendohedralfullerenes.Thusfar,avarietyofalkaliandlanthanidemetals,GroupVatoms,noblegases,andneutralmoleculessuchasCOandH2OhavebeensequesteredinsidetheC60struc-ture.Calculationshaveshownthattheencapsulationofnoblegasatomsandsmallions(e.g.,Li+,F−,Cl−)actuallystabilizethefullerenecage,whereaslargerspecies(e.g.,Rb+,Br−,I−)destabilizethecage.[43]Metallofullerenes(M@Cx)aretypicallygrownbyeitherlaserablationofmetal-dopedgraphitedisksathightemperature(ca.1,200◦C),orcarbonarctechniqueswithmetal-dopedgraphiterods.Anexampleofa
3026.2.NanoscaleBuildingBlocksandApplications
abcd
Figure6.25.ProposedschemefortheformationofthemultishellfullereneC60@C240.ReproducedfromMordkovich,V.Z.;Shiratori,Y.;Hiraoka,H.;Takeuchi,Y.SynthesisofMultishellFullerenesbyLaserVaporizationofCompositeCarbonTargets,foundonlineathttp://www.ioffe.rssi.ru/journals/ftt/2002/04/p581–584.pdf.
Figure6.26.Backbonestructureofsemibuckminsterfullerene,C30H12.
metallofullerene(Gd3+@C60)wasshowninFigure6.18a;thesestructuresarecom-monlyemployedasMRIcontrastagents.
Youmaybewondering“howdoestheiongetinsidethecage?”Thatis,doesthisoccurduringthegrowthofthefullerenestructureitself,ordoesthemetalioninsertafterthecageisalreadyformed?Ithasbeenshownthatthelatteroccurs,withtheexactentrancepathwaydependentonthesizeofthedopantspecies.SmalldopantssuchasHeorH+maydirectlypassthrougheitherhexagonorpentagonunitsofthecagetowardthevacantcore.However,forlargerions/atoms,someframeworkC–Cbondsmustbereversiblybrokeninordertoaccommodatetheincomingspecies–aptlyreferredtoasawindowmechanism(Figure6.28).Sincenon-IPRfullereneshaverelativelylargestrainenergiesduetofusedpentagons,thisprocessshouldoccurreadilyforthesestructures.Indeed,therehasbeenmuchrecentinterestinsynthe-sizing“unconventional”metallofullerenessuchasSc2@C66.[44]UnliketheemptyC66counterpart,thesestructuresarestablesincetheincomingmetalatomdonates
6Nanomaterials303
Figure6.27.SynthesisofaC60precursor.ReproducedwithpermissionfromScott,L.T.;Boorum,M.M.;McMahon,B.J.;Hagen,S.;Mack,J.;Blank,J.;Wegner,H.;deMeijere,A.Science2002,295,1500.Copyright2002AAAS.
electrondensitytotheC–Cbondbetweenadjacentpentagons,causingadecreaseinthelocalbondstrain.
Asyoumightimagine,relativelylargeionssuchasCs+,Y3+,orSc3+arelikelynotencapsulatedthroughasimplereversiblewindowingmechanism.Inorderforthistooccur,morethanoneC–Cbondwouldneedtobebroken(Figure6.28b),whichincreasestheenergeticbarrierforthistooccur.Recently,a“hole-repairingmecha-nism”wasproposedforY@C82metallofullerenes,inwhichcalculationspredictthecombinationofalargeC76open-cagefullereneandasmallerC6Yfragmentthateffectivelyrepairstheframeworkhole(Figure6.29).[45]Thesolution-phasesynthesisof0Dnanostructures
Mostofourdiscussionthusfarhasinvolvedsomeratherextremesyntheticenvironmentsoflaser,arc,orpyrolysis.However,apreferredroutetowardnano-clusters/nanoparticlesofmetalsandtheircompoundsisthroughuseofrelativelymildconditions–oftentakingplaceatroomtemperatureonthebenchtop.Thisisnotpossibleforcarbonnanoallotropes,sincetheprecursor(e.g.,graphite)contains
3046.2.NanoscaleBuildingBlocksandApplications
2.48Å
a)
b)
Figure6.28.Theoreticalintermediatesduringendohedralfullereneformation.Image(a)showsthenine-memberedringformedbyasinglepentagon–hexagonbond.Bycomparison,image(b)illustratestheformationofalarger13-memberedringbybreakingtwoframeworkbonds.ReproducedwithpermissionfromMurry,R.L.;Scuseria,G.E.Science1994,263,791.Copyright1994AAAS.
Figure6.29.Imagesofthecalculatedstructuresinvolvedinthe“hole-repairing”mechanismforendohe-dralfullerenegrowth.Shownfromlefttorightare:theC76opencage,topandsideviewsoftheC6Yfragment,andthefinalY@C82metallofullerene.ReproducedwithpermissionfromGan,L.-H.;Wang,C.-R.J.Phys.Chem.A2005,109,3980.Copyright2005AmericanChemicalSociety.
strongcovalentbonding,whichrequiresasignificantamountofenergyfordissoci-ationpriortoself-assembly.However,formetalnanostructuralgrowth,thesimplereductionofmetalsalts(usuallyviaNaBH4,H2,orhydrazineasreducingagents)isamenableformild,solution-phasegrowth(forexample,Eq.5forCumetal).Intheory,anymetalwithalargerstandardreductionpotential(E0)thanthereducingagent(e.g.,−0.481Vforborohydrideion)isacandidateforreductiontoitsmetallicform.Thisincludesmostofthefirst-rowtransitionmetalions,andmanyothersfromthemaingroup/transitionmetalseries.However,itshouldbenotedthatsolutionpH
6Nanomaterials305
andside-reactions(e.g.,metalionsbeingconvertedtoboridesbyBH−4ratherthanreduction)oftenprovideabarriertowardsuccessfulmetalionreduction.(5)Cu2++2Cl−+6H2O+2NaBH4→Cu0+2NaCl+7H2+2B(OH)3Iftheabovereactionweretobecarriedoutas-is,ametallicfilmorbulkpowderwouldbeformedratherthannanostructures.Thatis,asthemetalionsarereduced,theywouldinstantlyagglomeratewithoneanothertoformlargerparticulates.Hence,themostcrucialcomponentofnanostructuresynthesisisthestabilizingagentthatisolatesthegrowingnucleifromoneanother.Wesawanexampleofthisearlier,withpolyoxoanionsbeingusedtostabilizeIrnanoclusters(Figure6.14).Somedesirabletraitsofastabilizing(orentraining)agentare:
(i)Chemicallyunreactivetowardthegrowingnanocluster,renderinganunpassi-vatednanoclustersurface
(ii)Structurallywell-defined(size/shape),whichallowsforthecontrolledgrowth
oftheencapsulatednanocluster
(iii)Comprisedoflightelements(organic-based),soitsstructuredoesnotinter-ferewiththecharacterizationoftheentrainednanocluster.Thiswillalsofacil-itateitssacrificialremovalfromthenanoclusterbypyrolysisatrelativelylowtemperatures,ifdesired
(iv)Surface-modifiable,toallowfortunablesolubilityandselectiveinteractions
withexternalstimuli.Inaddition,toaffordcontrollableself-assemblyofent-rainednanoclustersonavarietyofsurfacesthroughchemisorption,ifdesiredInaqueoussolutions,themostcommonmethodusedtostabilizenanostructuresistheuseoforganic“capping”ligands.Forinstance,theTurkevichprocess,whichdatesbacktoearlycolloidalgrowthofthe1950s,usessodiumcitrate(I)toentrainthereducedgoldnuclei.
Particlediametersontheorderof10–20nmmaybesynthesizedusingthismethod.Inthiscase,sincegoldiseasilyreduced(E◦=1.00VforAuCl−4),thecitratereagentactsasboththereducingandstabilizingagent.Morerecently,acationicsta-bilizingagentbis(11-trimethylammoniumdecanoylaminoethyl)disulfidedibromide(TADDD,II)hasbeenutilizedtogrownanoclusterswithdiameters<10nm.IfNaBH4isusedasthereducingagent,thesulfidebondiscleavedresultingina–SHcappinggroup.Thethiolischemisorbedtothesurfaceofthegrowingnanostructuresurfacetopreventagglomeration(esp.effectivefor“thiol-philic”noblemetalssuchasPt,Ag,andAu).[46]
3066.2.NanoscaleBuildingBlocksandApplications
Recently,therehasbeenmuchinterestintheuseofstructurallyperfectden-drimerssuchaspoly(amidoamine)(PAMAM,Figure6.30)asstabilizingtemplatesfornanoclustergrowth.Byvaryingtheperipheralgroupsandnumberofrepeatbranchingunits(knownas“Generations”),oneisabletofine-tunethesizeofthe
M
n+
n+
H2N
H2N
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NH
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NHNHO
O
NNH
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NH2
NH
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O
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NH
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OO
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HO
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OH
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NH2
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NH
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n+
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ONH
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NH
OONH
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H2N
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ONHNH
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NH
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NH
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NHNHOONH
O
NH
NH2
NH2
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n+
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NHO
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1Њ
O
NH
1ЊNH2
2-SBD4-SBD6-SBD
3Њ amines1462254
1Њ amines
16256
NH
6-SBD pH 6
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++
++
+
NH
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pH 6
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NHO
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++
+++
+++
+
+
++
+
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+
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NH
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+
+
++++++
+
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+
+
+++
+++++
+
NH2
NH21Њ
+
Repeating unit:-(CH2CH2(CO)NHCH2CH2N)-pKa(1Њ) ~ 7-9pKa(3Њ) ~ 3-6
Figure6.30.MolecularstructureofasecondGeneration(G2)amine-terminatedPAMAMdendrimer,illustratingthepositionsofthemetalionschelatedtotheprimaryaminegroups(prereduction).Incom-parison,aG2hydroxyl-terminatedPAMAMdendrimerisshown,withthemetalionsnowpreferringtochelatetotheinteriortertiaryaminegroups.ShownonthebottomistheeffectofprotonationonG2/G6amine-terminatedPAMAMdendrimers.Aschematiconthelowerrightillustratesthepositionsofthepro-tonatedaminesatvaryingpHvalues.Asthegenerationsizeincreases,thesurfacedensityalsoincreaseswhichlimitstheaccessofprotons(orchelatingmetalions)tointeractwiththeinteriortertiaryaminegroups.ReproducedwithpermissionfromKleinman,M.H.;Flory,J.H.;Tomalia,D.A.;Turro,N.J.J.Phys.Chem.B2000,104,11472.Copyright2000AmericanChemicalSociety.
6Nanomaterials307
entrainednanocluster.Thoughamine-terminateddendrimersandhyperbranchedpolymersmaybeusedasatemplateforMn+chelationandsubsequentchemicalreduction,thesizeoftheresultantnanoparticleisrelativelylarge,withagreaterde-greeofagglomerationpossible.Thisisespeciallythecaseforhyperbranchedpoly-mersthatexhibitarandomstructure,whichresultsinamuchgreaternanoparticlepolydispersity.Ontheotherhand,iftheprimarysurfaceamines(–NH2)areeitherprotonated(–NH+3),orreplacedwithhydroxylgroups(–OH),theprereducedmetalionsareforcedfurtherintotheinteriorofthedendriticstructure(Figure6.30).Thisresultsinmuchsmallerdiametersandnarrowpolydispersitiesforthereducedmetalnanoclusters.[47]
InadditiontocontrollingthesurfacemoietiesandsolutionpH,thegenerationsizeofthedendrimerisalsoparamountforsuccessfulnanoclustergrowth.Asthedeg-reeofbranchingincreases,sodoesthesurfacedensity,whichpreventstheincomingmetalion(orH+duringprotonation)fromenteringtheinteriorofthedendriticarchi-tecture.Alternatively,forsmallergenerations,theentrainedspeciesbecomeseasilydislodgedfromtheinteriorduetoitsopenstructure.Hence,themosteffectivePA-MAMsizerangefornanoclustergrowthisbetweenthefourthandsixthgenerations(G4–G6),whichexhibitstrongcontainerproperties(Figure6.31).Asanillustra-tionoftheextremeflexibilityofthedendriticarchitecture,thecoremayalsobealteredtochangeitssolubilitycharacteristics,orallowthepenetrationofspeciesthroughtheperipheryathighgenerations(Figure6.32).Sincedendrimerscontain-inganalmostunlimitedrangeofcoresandperipheralgroupshavebeensynthesized,itisnowpossibletoeasilycontrolnanoclusterpropertiessuchascomposition,size,morphology,solubility,andencapsulation(e.g.,controlthereleaserateofentrainedmedicinalagents/sensorsbasedonstructuralorenvironmentalchanges,fortargeteddrugdeliveryorinsitumonitoring).
Figure6.31.RelativesizesandsurfacedensitiesofPAMAMdendrimers,showingthemostsuitablerangefornanoclustergrowthasGeneration4toGeneration6.ReproducedwithpermissionfromDendrimersandotherDendriticPolymers,Frechet,J.M.J.;Tomalia,D.A.eds.,Wiley:NewYork,2001.
3086.2.NanoscaleBuildingBlocksandApplications
Figure6.32.Molecularstructuresofdendrimersmodifiedwithlong-chainaliphaticcores.Unliketradi-tionaldendriticstructureswithsmallercores,asthegenerationsizeincreases,thereisanavailablechannelforexternalspeciestoenterthedendrimerinterior.ReproducedwithpermissionfromWatkins,D.M.;Sayed-Sweet,Y.;Klimash,J.W.;Turro,N.J.;Tomalia,D.A.Langmuir1997,13,3136.Copyright1997AmericanChemicalSociety.
Itshouldbenotedthatmetalnanoclustergrowthusingdendritictemplatesisstronglygovernedbythedegreeofcomplexationoftheprecursormetalions.Forinstance,silvernanoclustersarenotpossibleusinghydroxyl-terminatedPAMAMdendrimerssinceAg+isnotstronglychelatedtotertiaryaminegroups.However,ifCu0nanoclustersarefirstgeneratedwithinthestructure,followedbyAg+,aredoxreactionwillfacilitatethedisplacementofCu0withAg0withinthedendrimerinte-rior(Eq.7).(7)
Cu+2Ag+→Cu2++2Ag
Inadditiontosimplemetallicnanostructures,morecomplexintermetallicspecieshavealsobeensynthesizedthroughtheintroductionofmorethanonemetal.Forinstance,bimetallicnanoclustersmaybegeneratedviathreerouteswithinadendritichost(Figure6.33).Inadditiontoalreadybeingprovenforcore–shellnanoclusters,thisrouteshouldalsobeamenableforthegrowthoftrimetallicnanostructuresforinterestingcatalyticapplications.[48]
Otherpolymersmayalsoserveaseffectivestabilizingagentsfornanostructuralgrowth.Forinstance,poly(vinylpyrrolidone)(PVP,III),poly(styrenesulfonicacid)sodiumsalt(PSS,IV),andpoly(2-ethyl-2-oxazoline)(PEO,V)wererecentlyusedtogenerateanumberofintermetallicnanoalloysviaamildmetallurgyinabeakerapproachdevelopedbySchaakandcoworkers.[49]Sinceindividualmetalnanoparti-clesareinintimatecontactwithhighsurfacereactivityandlowmeltingpoints,theuseofhigh-temperatureannealingisnotrequired,unlikebulk-scalealloysynthe-sis.ThePVParchitecturehasalsobeenshowntofacilitatethegrowthofAu@Agcore–shellnanostructures,aswellasAgnanowiresandnanocubes.
6Nanomaterials
1. Co-complexation Method
MAp+MBq+
Gn-OH
2. Sequential Method
MAp+NaBH4
Gn-OH
3. Partial Displacement Method
MBq+
MAp+NaBH4
Gn-OH
More NobleMetal Salt
Alloy
Weak Reducing Agent (i.e H2)MBq+
or
NaBH4
309
Alloy
Core/Shell
Figure6.33.Schematicofthethreemethodsusedtogeneratebimetallicnanoclusterswithinadendritichost.ReproducedwithpermissionfromScott,R.W.J.;Wilson,O.M.;Crooks,R.M.J.Phys.Chem.B2004,109,692.Copyright2004AmericanChemicalSociety.
Forthesynthesisofnanostructureswithinnonpolarsolvents,oneusesstabili-zingagentsthatcontainalkylchainsratherthan–OHendgroups.Oneofthefirstcappingagentstobeusedfornoblemetalcolloidalgrowthwasalkylthiols(CH3–[CH2]X–SH).Inthissystem,the–SHendisboundtothesurfaceofthenanostructure,andthelongorganictailisresponsiblefordispersionwithintheorganicsolvent.Thoughthesecappingstabilizersworkedwellforcolloidalgrowthtopreventagglomeration,evenallowingsolventremoval/redispersionintoorganicsolvents,itwasrelativelydifficulttocontrolthesizedispersityofthenanoparticles.
3106.2.NanoscaleBuildingBlocksandApplications
Accordingly,systemsthatcontainananoreactortemplatehavebeenusedmostrec-entlyforcontrollednanostructuralgrowth.Infact,everyonewhohaswasheddishesorlaundryalreadyhassomeexperiencewiththesetypesofstabilizingagents,knownasmicelles.Thesecompoundscontainbothpolar(–OH,cationic/anionic)andnon-polar(aliphatic)ends.Soapsandsurfactantsworkbysurroundingthedirtparticlewiththenonpolarends,leavingthehydrophilicpolargroupsexposedtothesur-roundingwatermolecules.Thisresultsinpullingthedirtparticlefromthesurfaceoftheclothingfiber,forminganaqueoussuspension.Inasimilarfashion,anoil-in-watermicroemulsionmaybesetupusingcommonsurfactantssuchassodiumbis(2-ethylhexyl)sulfosuccinate(alsoreferredtoasAerosolOTorAOT,VI),orthenonionicsurfactantTriton-X(VII)inthepresenceofthebiphasicoil/watermixture.
Sincemostprecursorsforsolution-phasenanostructuralgrowthareionicmetalsalts,atypicalmicellewouldnotbeeffectivesincetheprecursorwouldnotbeconfinedtotheinteriorofthemicroemulsion.Hence,reversemicelles(orinversemicelles,Figure6.34)areusedtoconfinetheprecursorionstotheaqueousinterior,whicheffectivelyservesasananoreactorforsubsequentreduction,oxidation,etc.enroutetothefinalnanostructure.Notsurprisingly,eitherPAMAMOSdendrimers(Chapter5)ordodecyl-terminated(hydrophobic)PAMAMdendrimers(Figure6.35)havebeenrecentlyemployedforthisapplication.
a)b)
Figure6.34.Comparisonofatraditionalmicelleusedtoentrainorganicoils/dirtusingananionicsurfac-tant,(a),andareversemicelleusedtostabilizeaqueousnanoreactorswithinanonpolarsolvent,(b).
6Nanomaterials311
NONHHNCH2HOCH(CH2)9CH3
G4-C12
HNNHCH2HCOH(CH2)9CH3OONHNH(CH2)11CH3NHNONH(CH2)11CH3
G5-PPI-C12
Figure6.35.Hydrophobic-functionalizedPAMAM(G4-C12)andpoly(propyleneimine)(PPI)dendri-mers,whichserveastemplatesforAunanoclustergrowth.ReproducedwithpermissionfromKnecht,M.R.;Garcia-Martinez,J.C.;Crooks,R.M.Langmuir2005,21,11981.Copyright2005AmericanChemicalSociety.
Itshouldbenotedthatdendrimer-entrainednanoclusterssynthesizedwithinaque-oussolutionsmayalsobephase-transferredintoanorganicsolventbymixingwithalkylthiolsdissolvedinanonpolarsolvent.[50]Thisalsoresultsinmonodispersenanoclusters,withmuchlesspolydispersitythanearlycolloidalsynthesesthatemployedthiol-basedentrainingagents.Thatis,thenanoclustersizehasalreadybeencontrolledviaintradendrimerstabilization.Incontrast,theuseofalkylthiolsfromtheinitialstagesofgrowthisnotaseffectivetowardpreventingagglomerationduringthenucleationstep.
Itisnotalwaysnecessaryforthemetalionstobeencapsulatedwithinastabi-lizingpolymerduringchemicalreduction.Forinstance,thereducedmetalmaybeentrainedbypolymerizationprecursors,suchaspostreductionlivingradicalpoly-merizationthattakesplaceonthesurfaceofgoldnanoparticles(Figure6.36).Thisresultsinadense“polymerbrush”thatencapsulatesthemetallicnanoparticle,effectivelystabilizingthestructureagainstagglomeration.Subsequentalignmentandsurfacereactivityoftheresultantnanostructuresmaybefine-tunedbyvaryingthenatureofthepolymercoating.
3126.2.NanoscaleBuildingBlocksandApplications
Figure6.36.Schematicoftheformationofgoldnanoparticlescoatedwithfree-radicalpolymerizationinitiatorsthatsubsequentlyyieldAu@polymernanostructuresthroughasurface-controlledlivingpoly-merizationprocess.ReproducedwithpermissionfromOhno,K.;Koh,K.-M.;Tsujii,Y.;Fukuda,T.Macromolecules2002,35,.Copyright2002AmericanChemicalSociety.
Figure6.37.Syntheticschemeforthree-layercross-linkedmicelles.Shownisthemicellationofpoly[(ethyleneoxide)-block-glycerolmonomethacrylate-block-2-(diethylamino)ethylmethacrylate](PEO-GMA-DEA)triblockcopolymerstothefinal“onion-like”layerednanostructure.ReproducedwithpermissionfromLiu,S.;Weaver,J.V.M.;Save,M.;Armes,S.P.Langmuir2002,18,8350.Copyright2002AmericanChemicalSociety.
Increasinglycomplexstabilizingagentssuchascross-linkedmicelleshavebeendevelopedinrecentyears.Thesenanostructuresconsistofapolymericcorethatiscross-linkedwithsurroundingpolymerlayer(s)(Figure6.37).Incontrasttosolidnanoparticles,ahollownanoshellmaybesynthesizedbysacrificiallyremovingthecorematerialbychemicalorthermaldecomposition.Suchintriguingnanobuild-ingblockswilllikelybeofextremeimportancefornext-generationmedicaltreat-ment/sensing,hydrogenstorage,ion-exchange,andmicroelectronicsapplications.
6Nanomaterials
Ag nanoparticles
+
Co2+Ag
313
Co2+
Ag+
Co2+Ag+
Ag+
+
Co2+Ag
Co2+
Ag nanoshell
Ag+Co2+Ag+Co2+
Co core
Figure6.38.SchematicofAgnanoshellformationfromananostructuralCocore.DuetofavorableredoxcouplesbetweenAgandCo,ananoshellofmetallicsilverformsattheexpenseoftheinnerCocore.ReproducedwithpermissionfromChen,M.;Gao,L.Inorg.Chem.2006,45,5145.Copyright2006AmericanChemicalSociety.
Anotherstrategyfornanoshellgrowthconsistsofapplyingathinmetalliccoatingontosilicaorpolystyrenetemplatingspheres,withsubsequentsacrificialremovalofthetemplatebyhydrofluoricacid(HF)ortoluene,respectively.Thereversecaseofapolymer/ceramiccoatingontoremovablemetallicnanoparticleshasalsobeenexploitedtoyieldnonmetallicnanoshells.[51]However,theremovalofarelativelylargecore(i.e.,typically>200nm)fromananoscalecoatinggenerallyresultsinsignificantdeformationoftheresultantnanoshell.
Thestructuralrobustnessofnanoshellshasbeenrecentlyimprovedthroughtheuseofsacrificialcoresofsmallerdiameters.AnexampleofthisstrategyisthegrowthofsilvernanoshellsfromnanostructuralCotemplates(Figure6.38).ItshouldbenotedthatNaBH4reductionofligand-cappedCo2+ionspreferentiallyyieldsCo2Bratherthanmetalliccobalt.However,apostexposureofoxygenconvertstheborideintometalliccobaltnanostructures(Eq.8).[52]OncetheCocorewasformed,silverionswereintroducedintothesystemthroughadditionofAgNO3.SinceAg+(Ered◦=0.799V)ispreferentiallyreducedrelativetoCo2+(Ered=−0.377V),theexchangeofAgforCooccursspontaneouslyasAg+/Co0boundariesareformed.ThisprocesscontinuesasAg+ionsdiffusethroughthegrowingAg0layer,untilallofthemetallicCoisconsumedfromthecore.(8)
4Co2B+3O2→8Co+2B2O3
Ratherthanredox-governednanoshellgrowth,diffusion-governedroutesarealsopossible.Forexample,theexposureofcobaltnanoparticlestooxygen,sulfur,orsele-niumdoesnotsimplyformaCo-chalcogenidecoating,butratherhollownanoshellsofthecobaltchalcogenide(Figure6.39).[53]Thisinterestingresultisamodernextensionofaneffectthathasbeenstudiedsincethelate1940s–theKirkendallEffect.Thisphenomenondescribesthedifferentialdiffusionratesoftwospeciesindirectcontactwithoneanotheratelevatedtemperatures(e.g.,CuandZninbrass).Duringthegrowthofoxideorsulfidefilms,poresareformedinthesolidduetoavacancy-exchangemechanism.Thatis,theoutwardmovementofmetalionsthroughtheoxidelayerisbalancedwithaninwardmigrationoflatticevacancies
3146.2.NanoscaleBuildingBlocksandApplications
Figure6.39.HollownanoshellformationviatheKirkendallEffect.Shownistheevolutionofthehollownanoshellafterreactiontimesof(left–right):0s,10s,20s,1min,2min,30min.Aninterestingfeatureistheformationof“bridges”thatconnectthecoretothesulfideshell,facilitatingthefastoutwardmigrationofCo.ReproducedwithpermissionfromYin,Y.;Rioux,R.M.;Erdonmez,C.K.;Hughes,S.;Somorjai,G.A.;Alivisatos,A.P.Science2004,304,711.Copyright2004AAAS.
thatsettlenearthemetal/oxideboundary.Duetothelargenumberofdefectsandvolumeofbulksolids,theporesatthemetal–oxideinterfacedonotcoalesceintoanorderedarray.However,forananostructuralsystemthatisrelativelyfreeofdefectsandexhibitsalargesurface/volumeratio,theporesreadilyaggregateintoasinglehollowcore.
Notsurprisingly,0Dnanostructuralgrowthneednotbelimitedtometallicstructures,butmayalsoincludeothercompoundssuchasmetaloxides,sul-fides,etc.Thereareacopiousnumberofapplicationsfornanostructuralox-idessuchashigh-densitymagneticstorage,heterogeneouscatalysis,gassensors,electrolytesforlithiumbatteries,andfuelcells.Sunandcoworkersrecentlydes-cribedanovelmethodforthesynthesisofhexane-suspendedFe3O4(magnetite)nanoclustersthroughthereactionbetweenFe(acac)3,oleylamine,oleicacid,and1,2-hexadecanediolatca.200◦Cwithinphenylether.[54]Thisone-potsynthesisresultsinthethermalreplacementoftheacetylacetonateligands(recallChapter4–CVDprecursors)fromtheironcenter,followedbyoxideformationfromreactionwiththealcohol.Theoleylamineandoleicacidcondenseinsitutoformalong-chaininversemicellethatiscapableofsuspendingthenanoclustersinnonpolarmedia.Forpurification,onesimplyaddsanexcessofpolarsolvent(methanolorethanol)toprecipitatethenanostructures.Followingcentrifugation,thenanoclustersareresus-pendedinhexane,withthisprecipitation/resuspensionprocedurerepeated3–4timestoaffordmonodisperse,chemicallypureFe3O4nanoclusters.
Anothermorerecentstrategythatwedevelopedisthesynthesisofmetaloxidenanoparticlesfromthesimplereactionofinterdendriticstabilizedbasicmetalatesalts
6Nanomaterials315
HNH2N2HNH2N2H2NNH2N
N
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NH2N
NH2NN
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NH2NH2NH2NH2NH2NH2NH2N
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NH2H2N
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H2NH2NNH2NH2NH2NH2
NH2NH2NH2NH2
Na2CO3
Figure6.40.Schematicofthegrowthoftinoxidenanoclustersatroomtemperature.TheTEMimagesontherightillustrateinterdendriticstabilizednanoclustersusing(a)PAMAMand(b)amine-terminatedpoly(ethyleneimine)hyperbranchedpolymerhosts.ReproducedwithpermissionfromJuttukonda,V.;Paddock,R.L.;Raymond,J.E.;Denomme,D.;Richardson,A.E.;Slusher,L.E.;Fahlman,B.D.J.Am.Chem.Soc.2006,128,420.Copyright2006AmericanChemicalSociety.
withCO2(Figure6.40).[55]Thisstrategyshouldworkforanymetalatesaltthatistypicallyformedfromthereactionofthemetalhydroxidewithastrongbase(Eq.9,forsodiumhafnate).(9)
2NaOH+Hf(OH)4→Na2HfO3+3H2O
Inadditiontooxides,anumberofothercompositionsmaybesynthesized.Ingeneral,anystoichiometryis“fairgame”throughthesamereactionsthatonedoesinbulkscale.Forexample,sulfidesthroughreactionofprecursorswithH2S,nitridesthroughNH3exposure,etc.[56]Aslongasasuitableentrainingagentisused,oneisabletocontroltheresultantsize/morphologyofthenanostructures.Withcurrentadvancesincompoundandmetallicnanoclusters,withthepropertiesofeachbeingfine-tunedviaquantumconfinementeffects,onecanbegintoimagineintriguingdesignsforfuturemicroelectronicdevices.
Self-assemblyof0Dnanostructuresintoarrays
Nowthatyouunderstandhow0Dnanostructuresaresynthesizedandstabilized,itisworthwhilenotinghowthesestructuresarealignedintomorecomplexarrays.
3166.2.NanoscaleBuildingBlocksandApplications
OHOHTi(On-Bu)4OHOHOHOHOTiOOTiOOTiOOOOOOn-Bun-Bun-Bun-BuHydrolysisOHOTiOOOTiOHOOTiOHOOHOHHOOHOOTiOHOOOOHHOOHTiOOOHHOOTiOHOOHHOTiO2
GoldNanoparticlesOHHOHOHOHOHOHOOHOHOHOHOHFigure6.41.IllustrationofthechemisorptionofananostructureontoahydroxylatedTiO2surface.Thehydroxylgroupsontheperipheryofthenanostructurestabilizingagentprovidethehandleforsurfaceadsorption.ReproducedwithpermissionfromLiu,S.;Zhu,T.;Hu,R.;Liu,Z.Phys.Chem.Chem.Phys.2002,4,6059.Copyright2002PCCPOwnersSocieties.
Asyourecall,the“bottom-up”approachtomaterialsdesignfeaturesthepurposefulplacementofindividualnanoarchitectures,inordertobuildspecificfunctionaldevicesoneunitatatime.Thoughonewouldhavethegreatestcontroloverthedevicepropertiesthroughthegrowth/arrangementofindividualatoms,currentsyn-theticmethodologymosteasilyyieldsnanoclusters/nanoparticlesthatconsistofsmallgroupsofatoms(Natoms∼50+).
Aswehaveseen,astabilizingagentisusedtopreventagglomerationofgrow-ing0Dnanostructures.Infact,thiscomponentalsoprovidesaneffective“handle”tobindthenanostructuretoaparticularsurface.Onceappropriatereactivegroups(e.g.,–OH,NH2)areplacedonasurfacethroughmonolayerformation,thestabi-lizinggroupsurroundingthenanostructurespontaneouslybecomeschemisorbed(Figure6.41).Ifthenanostructuresareencapsulatedwithstabilizingagentsofwell-definedsizes,thenthespacingbetweenadjacentnanostructureswillalsobehighlyorderedandpredictable(Figure6.42).Thoughthismostoftenresultsina2Dmatrixofnanostructures,itisalsopossibletocreatealignmentinto1Dchains.Forinstance,Aunanostructuresthatarestabilizedbyalong-chainthiolatedpoly(ethyleneoxide)polymerformalineararrayviainteractionswithsulfate/acidgroupsofapolysac-charidecompound(chondroitinsulfatec,VIII)(Figure6.43).
6Nanomaterials317
Figure6.42.Schematicofthecontrolledspacingbetweenindividuallystabilizedgoldnanoparticlesbytheformationoffullereneinclusioncomplexes.ReproducedwithpermissionfromYonezawa,T.;Matsune,H.;Kunitake,T.Chem.Mater.1999,11,33.Copyright1999AmericanChemicalSociety.
Figure6.43.TEMimagesofa1DarrayofAunanoparticlesformedthroughtheinteractionofPEOandpolysaccharidechains.ReproducedwithpermissionfromTakagi,K.;Ishiwatari,T.Chem.Lett.2002,990.Copyright2002ChemicalSocietyofJapan.
3186.2.NanoscaleBuildingBlocksandApplications
Figure6.44.IllustrationofLbLgrowthofcyclodextrin-stabilizedAunanoparticles(darkspheres)andadamantyl-terminatedPPIdendrimers.ReproducedwithpermissionfromCrespo-Biel,O.;Dordi,B.;Reinhoudt,D.N.;Huskens,J.J.Am.Chem.Soc.2005,127,7594.Copyright2005AmericanChemi-calSociety.
Themostpopularmethodusedtoachieve3Darrayssuchaslayerednanostructuralthinfilms,isthelayer-by-layer(LbL)self-assemblypioneeredbyDecherintheearly1990s.[57]Thistechniqueisbasedonthesequentialadsorptionofspecieswithcom-plimentaryfunctionalgroups(e.g.,ioniccharges)onavarietyofsubstrates.TherearemanydeviceapplicationsforLbLthinfilms,suchasself-cleaningsurfaces,sur-facedeactivationofwarfareagents,solarenergy,drugdelivery,andoptoelectronics.AninterestingrecentprecedentforLbLnanostructuralfilmgrowthusessequen-tiallayeringofcyclodextrin-stabilizedAunanoparticlesandadamantyl-terminateddendrimers(Figure6.44).[58]Thefilmthicknesswasreportedas2nmperbilayer,allowingforstrictcontrolovertheresultantfilmthickness.
Onthetopicofself-assembly,wewouldberemissifwedidnotmentionarecentprecedentthatscoreshighinthe“cool”category.Wearereferringtothedevelopmentofsurface-rollingmoleculesthatareaptlytermednanocars/nanotrucks.[59]Ratherthanstabilizednanoparticles,thesenanovehiclesareorganicmoleculesthatcontainfullerenesaswheels(Figure6.45).Theplacementofthenanocarsontoagoldsurface
6Nanomaterials319
ab3.11nm1.98nmc
3.3nm2.1nmFigure6.45.ImagesanddimensionsofananocaronaAu(111)surface.Thebrightfeaturesarethefullerenewheels.ReproducedwithpermissionfromNanoLett.2005,5,2330.Copyright2005Ameri-canChemicalSociety.
isaseasyasspin-castingfromatoluenesuspension.Duetostrongadhesionforcesbetweenfullerenesandmetalsurfaces,thenanocarsspontaneouslydepositwithallfour“nanowheels”onthesubstrate.Theaxlesconsistoftriple-bondedalkynegroupsthatallowrotationofthefullerenewheels,transportingthenanocaracrossagoldsurfaceinresponsetochangesintemperature.[60]Thoughonlypivotingandtrans-lationalmotionarecurrentlypossible,thisworksetsanimportantsteptowardtherealizationofnanomachinesforanendlessnumberofpossibleapplications.6.2.2.One-DimensionalNanostructures
Thesecondclassofnanoscalebuildingblocks,referredtoas1Dnanostructures,isreservedforthosematerialsthathavenanoscaledimensionsthatareequivalentinallbutonedirection.Forinstance,letusconsiderthe2Darchitectureofthispage.Recallthata0Dnanostructureisanalogoustotheperiodfollowingthissentence(length=width);a1Dnanostructureisanalogoustothenumber“1”(length>width).
Sincewebeganthediscussionof0Dnanostructureswithnomenclature,wewillfollowsuitinthissection.Onceagain,itiseasytobeconfusedbythecommonsynonymoususeofthetermsnanotube,nanofiber,nanowire,andnanorod.How-ever,ifyouthinkoftheanalogousbulkmaterialswithouttheprefix“nano,”thereshouldbenoambiguityregardingtheproperuseofthesedescriptors(Figure6.46).Thecommonthreadamongallofthesestructuresisthattheirdiametersmustbewithinthe1–100nmrange;typically,theirlengthsarewithinthemicron(orlarger)regime.Ananotubeisa1Dstructurethatcontainsahollowcore,whereastheotherthreenanoarchitecturesaresolidthroughout.Thetermnanofibershouldbe
3206.2.NanoscaleBuildingBlocksandApplications
Figure6.46.Classificationsof1Dnanostructures.ThetopportionshowsSEMimagesofvariousnanostructures.[63]Whereasthemorphologyofnanowires,nanotubes,andnanofiberslookidenticalbySEM,nanorodsarenotablydifferent,withmuchshorterlengthsandstraightsidewalls.Thebottompor-tionshowshigh-resolutionTEMimages,whichprovidemorphologicaldetailsofthenanostructures.[]ThetopTEMimageshowscrystallinenanorodsgrownonthesurfaceofamorphousnanofibers.Thebot-tomtwoTEMimagesillustratethedifferencebetweennanowires/nanotubes–thelattercontainsahollowcore.Itshouldbenotedthatcrystallinenanorodsmayalsobetermed“nanocrystals,”astheirmorphologyresemblesthatofneedle-likebulkcrystallites.
6Nanomaterials321
reservedfor1Dnanostructuresthatareamorphous(andusuallynonconductive)suchaspolymersandothernongraphitizedcarbonaceousstructures.Bycontrast,ananowiredesignatesastructurethatiscrystalline,witheithermetallicorsemicon-ductiveelectricalproperties.
Ananorodistypicallyacrystalline1Dnanostructure,withanoveralllengthcom-parabletoitswidth(i.e.,bothdimensionsare<100nm).Astheirnameimplies,anotherfeatureofnanorodsistheirrigidsidewallstructures.However,sincecrys-tallinenanorodsexhibitthesameoverallshapeasneedle-likebulkcrystals,theterm“nanocrystal”isprobablymoreappropriateforthesestructures(or,moreexplicitly:“rod-likenanocrystals”).Whereasnanowires,nanofibers,andnanotubesexhibitaninterwovenarray,nanorodsarecompletelylinearinmorphology.Assuch,nanorodsarecapableofstackingontoeachothertoyieldinteresting2Dand3Darrays–notusuallyaseasytoperformwiththe“spaghetti-like”morphologyoftheother1Dnanostructures.Carbonnanotubes
Withoutquestion,themostwidelystudied1Dnanomaterialisthecarbonnanotube(CNT).ThesestructureswerefirstdiscoveredbyIijimain1991,[61]andconsistofagraphiticsheet(s)ofsp2hybridizedcarbonatoms(i.e.,graphene[62])rolledintoatubulararray.BasedonthelayersofgraphenesheetsthatcomprisetheCNT,thestructuresaredesignatedassingle-walled,double-walled,ormultiwallednanotubes(SWNTs,DWNTs,orMWNTs,respectively–Figure6.47).ThediametersofCNTsrangefrom1nm(SWNTs)to>30nm(MWNTs),withaspectratios(length:width)rangingfrom100togreaterthan1×106.EventhoughthediametersofCNTsare
Figure6.47.TEMimagesofMWNTs,formedfromthefolding(a)5-,(b)2-(i.e.,DWNT),and(c)7-stackedgraphenesheets.Thespacingbetweenindividualconcentriccylindersis0.34nm–thedistancebetweenadjacentplanesingraphite.ReprintedfromDresselhaus,M.S.;Dresselhaus,G.;Eklund,P.C.ScienceofFullerenesandCarbonNanotubes.Copyright1996,withpermissionfromElsevier.
3226.2.NanoscaleBuildingBlocksandApplications
ordersofmagnitudesmallerthanahumanhair,theirtensilestrengthisca.20timesgreaterthansteel–apropertyattributedtoextremelystrongsp2bondingbetweenneighboringhexagonalunits.
TheelectricalconductivityofSWNTsmayvaryfrommetallictosemiconducting,dependingonthewayagraphenesheetisfolded(Figure6.48).Inparticular,thediameterandhelicityofaSWNTareuniquelycharacterizedbythechiralityvector
whichconnectscrystallographicallyequivalentgraphenelat-(orHamadavector),C,
ticesites.Vectorindicesdesignatedby(n,m)areusedtoindicatethedirectionandlengthofthechiralityvector(Eq.10).Whenn=0,theSWNTisdenotedasthezigzagconformation;whenn=m,theSWNTisinitsarmchairform.Forallvaluesinbetweentheseextremes,thenanotubesaredesignatedsimplyaschiral.(10)
=na2,Ca1+m
wheren,maretheintegersdenotingthenumberofunitcellvectorsalongtwodirec-a2arethegrapheneunitcellvectors.tionsinthecrystalstructureofgrapheneanda1,
FormetallicSWNTs,theelectricalconductancemayexceedsilverorcopperbythreeordersofmagnitude.Calculationshaveshownthat(n,0)orzigzagSWNTsexhibitmetallicconductivitywhenn/3isaninteger,andsemiconductingpropertiesforallothervaluesofn.SimilarlyforchiralSWNTs,when(2n+m)/3isaninte-ger,thetubesaremetallic(otherwisearesemiconducting).Finally,armchairSWNTs,withn=m,exhibitmetallicconductivity(Figure6.49).Mostimportantly,electronicbandstructurecalculationsshowthatmetallicandsemiconductingCNTsaredepen-dentonlyon(n,m)–thatis,slightstructuralvariationsresultindramaticchangesintheirelectronicproperties.Forexample,thebandgapofsemiconductingSWNTsmaybefine-tunedfromca.10meVto1eV–withnorequiredadditionofdopants,unlikebulkSicounterparts.Ifsemiconductingnanotubesexhibitthesamechirality,thebandgapisinverselyproportionaltothediameter(e.g.,Eg(7,0)>Eg(10,0)).ThetunableelectronicpropertiesofCNTsarebeingexploredfornext-generationICarchitectures.AsyoumayrecallfromChapter4,traditionalSi-basedmicroelec-tronicdeviceswilllikelyreachafundamentallimitwithinthenextdecadeorso,necessitatingtheactivesearchforreplacementmaterials.Accordingly,anareaofintenseinvestigationismolecularelectronics–inwhichtheelectronicdeviceisbuiltfromtheplacementofindividualmolecules.[65]Notsurprisingly,theinterconnectsofthesedeviceswilllikelybecomprisedofCNTsandother(semi)conductive1Dnanostructuressuchasnanowires.
SinceCNTshaveahighelectricalconductivityandcontainsharptips,thesenano-materialsarethebest-knownfieldemitter[66]ofanymaterialtodate.Ingeneral,thesmallertheradiusofcurvatureofthetip,themoreconcentratedtheelectricfieldwillbe,whichcorrespondstoincreasedfieldemissionatlowrequiredvoltages.Thispropertyiscurrentlybeingexploitedforthedesignofflat-panelfieldemissiondis-plays.ThoughplasmaandLCDdisplaysarehotticketitemsatelectronicsstores,theybothpossessinherentdisadvantages.Plasmadisplaysareextremelyheavy,con-sumeasignificantamountofenergy,andareproneto“burn-in,”whichpermanentlydegradesthescreen.Ontheotherhand,LCDscreensareexpensivetoproduceandoftenlacktheresponsetimerequiredtoviewfast-pacedsportingevents/movies
6Nanomaterials323
Ј
Figure6.48.Illustrationofthehoneycomb2Dgraphenenetwork,withpossibleunitcellvectorindices(n,m).Thedottedlinesindicatethechiralityrangeoftubules,fromθ=0◦(zigzag)toθ=30◦(arm-chair).Forθvaluesbetween0and30◦,theformedtubulesaredesignatedaschiralSWNTs.Theelectricalconductivities(metallicorsemiconducting)arealsoindicatedforeachchiralvector.Thenumberappear-ingbelowsomeofthevectorindicesarethenumberofdistinctcapsthatmaybejoinedtothe(n,m)SWNT.Alsoshownisanexampleofhowa(5,2)SWNTisformed.ThevectorsABandAB,whichareperpendiculartothechiralvector(AA)aresuperimposedbyfoldingthegraphenesheet.Hence,thediameteroftheSWNTbecomesthedistancebetweenABandABaxes.ReprintedfromDresselhaus,M.S.;Dresselhaus,G.;Eklund,P.C.ScienceofFullerenesandCarbonNanotubes.Copyright1996,withpermissionfromElsevier.
3246.2.NanoscaleBuildingBlocksandApplications
(a)
Density of StatesArmchair nanotube(5,5)
metal
−10.0−5.0
Energy (eV)
0.05.010.0
(b)
Density of StatesZigzag nanotube
Semi-conductor
(7,0)
−10.0−5.00.05.010.0
Energy (eV)
Figure6.49.Comparisonofthedensityofstates,andresultantelectronicpropertiesofarmchairandzigzagSWNTs.ReproducedwithpermissionfromCharlier,J.-C.Acc.Chem.Res.2002,35,1063.Copy-right2002AmericanChemicalSociety.
withoutblur.Hence,asfaraspicturequalityisconcerned,traditionalCRTdisplaysarestillamongthefinestquality.ThereplacementofthistechnologywithCNTsisalogicalstepintheevolutionofdisplaypanels.Ratherthanasingleelectrongun,CNT-basedscreenswillcontainaseparatenanotubeelectrongunforeachindivid-ualpixelinthedisplay–dramaticallyenhancingtheresolutionandclarityofthepicture.Further,incontrasttocurrentlargeflat-paneltelevisions,theoverallweightofCNT-basedanalogueswillbesignificantlylower,andtheywillconsumefarlesspower.Thisconcepthasalreadybeenproveninprototypesandisscheduledtoreachthecommercialmarketwithin1–2years.
AnotherelectronicapplicationforCNTsisfornext-generationfield-effecttransis-tor(FET)design.The“proof-of-concept”forCNTFETswasdemonstratedinthelate1990s,withasimplebridgingoftwonoblemetalelectrodeswithaSWNT(Figure6.50–top).However,theelectricalcharacteristicsofthisnewFETdesignwerelessthandesirable,withhighcontactresistance(>1M)andlowdrivecur-rents.ResearchersatIBMhavesincemodifiedtheoriginaldesignwhereinthe
6Nanomaterials325
−5.0
Vgs − Vt−1.1 V−0.9 V−0.7 V−0.5 V−0.3 V−0.1 V
Id [A]−4.0
Id [µA]−3.0−2.0−1.00.0−1.6
−10−10−7
−8−10−10−9−10−10−10−11
−6
Vgs = −0.6V
−0.8
0.0
0.8
−1.6
Vgs [V]
−1.2
Vds [V]
−0.8−0.40.0
Figure6.50.Atomicforcemicroscope(AFM)topographicalimageofanoriginalCNTFET(top).Thesource/drainelectrodeswerefabricatedonaSiO2thinfilmgrownonaSiwafer.Thewaferitselfservedasthegateelectrode.Alsoshown(bottom)isanillustrationofatop-gatedCNTFETdesign,withanoxidethicknessof15nmandCNTlengthof300nm.Thedeviceelectricalcharacteristicsatroomtemperatureisalsoprovided.TheinsetshowsthetransfercharacteristicoftheFET(outputvoltageofthedeviceasafunctionoftheinputvoltage).ReproducedwithpermissionfromAvouris,P.Acc.Chem.Res.2002,35,1026.Copyright2002AmericanChemicalSociety.
semiconductingSWNTsareplaceddirectlyontoanoxidizedSiwafer,followedbythedepositionofthesourceanddrain(CoorTi)electrodes(Figure6.50–bottom).Throughsubsequentannealing,astrongerinteractionisaffordedbetweentheelec-trodesandCNTchannel,whichreducesthecontactresistance.
ThoughCNTFETsareinarelativelyearlystageofdevelopment,AvourisatIBMprovidedarecentcomparisonoftheoutputfromatop-gateCNTFET(Figure6.50–bottom)andexistingSi-basedFETs.[67]ForCNTswithanaveragediameterof1.4nm,theONcurrentforatop-gatedCNTFETisontheorderof2,100µAµm−1atVDS(drainvoltage)=VGS(gatevoltage)–VT(thresholdvoltage)=1.3V.Incomparison,thehighestdrivecurrentinap-CMOSunderthesameconditionsis650µAµm−1foragatelengthof50nm.Thetransconductance(ratiooftheoutputcurrentvariationtotheinputvoltagevariation)oftheCNTFETis2,300µSµm−1;thevalueforananalogousSip-CMOSis650µSµm−1.Hence,thevaluesobtainedthusfarshowthatCNTFETsoutperformSi-basedFETs.Asimprovementscontinue
3266.2.NanoscaleBuildingBlocksandApplications
tobemadetothedesignofnanotube-basedtransistors,thistechnologyshouldbereadyforthemarketjustasthe“glassceiling”ofSiCMOSdevicesisreached.ItshouldbenotedthattheextremelyefficientthermalconductivityofCNTswillalsobeexploitedforcoolingapplicationsforfuturecomputers–ofincreasingconcernasthechipdensitycontinuestosoar.
Inadditiontotheabovetunableconductiveproperties,CNTsarethestrongestandstiffestmaterialsknowntodate(Table6.3).Thehollow,closedmorphol-ogyofnanotubesresultsinratherintriguingdeformationmodesinresponsetoamechanicalstress(Figure6.51).Inparticular,ithasbeensuggestedthatStone–Walesdefectsmaybecomemobileunderstress,whichresultsinachangeintubediameter/chirality.[68]ThisalsocausesachangeintheelectronicpropertiesoftheCNT,openinguppossibilitiesforsensorapplications.However,forMWNTs,stud-ieshaveshownthatonlytheoutergraphiticshellisabletosupportstress,[69]andSWNTbundles/ropes(Figure6.52)exhibitsmallerYoung’smoduli,relativetoiso-latedSWNTs,duetoweakintertubecohesion.[70]Hence,althoughanindividualSWNThasanelasticmodulusof1.2TPa,thevalueforbundlesisca.100GPafordiametersintherangeof15–20nm.ToimprovetheloaddistributionthroughouttheCNTandimproveitsmechanicalstrength,thereareeffortstocreatecrosslinksbetweenindividualshellsofMWNTs,andamongSWNTswithinropes.
ThedesirablemechanicalpropertiesofCNTshavebeenexploitedinrecentyearsforthestructuralreinforcementofpolymers.[71]Thisisanaturalextensionoftra-ditionalcompositematerialsusedforapplicationsthatrequirebothhigh-strength
Table6.3.SpecificTensileStrengthsofVariousMaterials[72]
Material
Tensilestrength(MPa)101558075806002,0001,3003,4403,4004,3003,6206,2000
Density(kgdm−3)2.300.928.551.150.902.707.8.513.162.601.751.441.34
ConcreteRubberBrassNylon
PolypropyleneAluminumSteelTitanium
SiliconcarbideGlassfiberGraphiteKevlarcCNTs
Specificstrengtha
(kNmkg−1)4.3416.367.7.388.92222542881,0881,3072,4575,246,268
Breakinglengthb(km)0.441.666.919.929.0622.725.929.41101332505344,716
aThestrengthofamaterialdividedbyitsdensity.
bThelengthbeyondwhichastripofthematerial,ofuniformwidth,wouldbreakunderits
ownweightifsuspendedfrombothends.
cAsyntheticfiberusedinbullet-proofvests,comprisedofpoly(paraphenyleneterephthalamide).
6Nanomaterials327
Figure6.51.IllustrationofthedeformationmodesofSWNTs,resultinginahighelasticity.Thisbehaviorislikelyanartifactofin-planeflexibilityofagraphenesheet,andfacilerehybridizationofcarbonatomsfromsp2tosp3geometries.ReproducedwithpermissionfromAjayan,P.M.Chem.Rev.1999,99,1787.Copyright1999AmericanChemicalSociety.
Figure6.52.High-resolutionTEMimageofabundle/ropeofsingle-wallcarbonnanotubes,formedspon-taneouslythroughtheself-assemblyofindividualSWNTs(scalebaris10nm).Reproducedwithpermis-sionfromThess,A.;Lee,R.;Nikolaev,P.;Dai,H.;Petit,P.;Robert,J.;Xu,C.;Lee,Y.H.;Kim,S.G.;Rinzler,A.G.;Colbert,D.T.;Scuseria,G.E.;Tomanek,D.;Fischer,J.E.;Smalley,R.E.Science1996,273,483.Copyright1996AAAS.
andlightweightmaterials.Therearecurrentlyplentyofexamplesofcarbonfibercomposites–aircraftandspacecraftparts,racingcarbodies,golfclubshafts,bicy-cleframes,fishingrods,automobilesprings,sailboatmasts,andmanyothers–allincorporatingbulkcarbonfibers,ca.5–10µmindiameter.Thoughthistechnology
3286.2.NanoscaleBuildingBlocksandApplications
hasbeenaroundsincethe1950s,themuchlargeraspectratiosofCNTstranslatestoanevengreaterimpactonthefutureofmaterialsreinforcement.
InordertobroadenthescopeofCNTsfortextileapplications,itismostdesir-abletoconverttheas-formedpowdersintousefulfibersandyarns.Anintriguingrecentprecedentconsistsofshrinkingtheyarn-spinningprocess,usedbytheearliestcivilizations,tothenanoregime.Inthisprocess,MWNTsfromabamboo-likefor-estarrayweredrawnintofibers,andweavedintoyarns,thatwerebothstrongandhighlyflexible(Figure6.53).Onepossibleapplicationfortheseadvancednanotex-tilesisthedesignofa“supersuit”forthenextgenerationofsoldier.Thisuniformwillfeatureanumberoffunctionalitiesthatwillreactappropriatelytoitssurroundings(e.g.,deactivationofgaseouswarfareagents,“kickingin”artificialmuscles,climatecontrol,etc.–Figure6.53,bottom).
Inadditiontoincreasingstrengthandstiffness,theincorporationofCNTsalsoimpartsconductivitytothepolymermatrix.TheadditionofCNTsalsoenhancesthethermalconductivity/stability,solventresistance,andglasstransitiontemperatureofthenativepolymer.Further,duetothehighaspectratioofCNTs,muchlowerdopantlevelsarerequiredtoyieldthedesiredproperties,relativetostandardadditivessuchascarbonblackandlargergraphiticfibers.Thereareanendlessnumberofpoten-tialapplicationsforCNTcomposites–imaginefutureflat-paneldisplaysthatareflexible,exhibitingtheconsistencyofcommonfabrics;or,shirtsthatarecapableofmonitoringtheexternaltemperatureandautomaticallyheating/cooling.Theautomo-tiveindustrycurrentlyusesCNTcompositesforanumberofapplications,suchasincreasingthestrengthofsidemirrors.Inaddition,CNT-dopednylonisbeingusedforfuellinestoreducethebuildupofstaticelectricalcharges,andpreventfuellinerupturingduringanaccident.
Itshouldbenotedthatreinforcementapplicationsarenotlimitedtoorganicpoly-mers.Thoughceramicsarealreadyhardandchemically/thermallyresistant,thesematerialsmayalsobedopedwithCNTstoimprovetheirinherentbrittleness.Thishasalreadybeenprovenfortheincorporationof5–10%ofCNTswithinanaluminamatrix.TheresultantmaterialexhibitsfivetimesthefracturetoughnessandseventimesgreaterelectricalconductivityrelativetoundopedAl2O3,withanaddedprop-ertyofunidirectionalheatconductivity.[73]
InordertocontinuethedevelopmentofintriguingCNT-basedcomposites,threemainchallengesarebeingaddressed:
(i)Ensuringahomogeneousdispersionofnanotubesthroughoutthepolymermatrix
(ii)Controllingthedirection/orientationoftheCNTsinthematrix(iii)SeparatingindividualSWNTsfrombundles/ropes(exfoliation)
Inorderforefficientloaddelocalizationandstrengthenhancementofacompos-ite,theremustbeastronginteractionbetweenthedopantfibersandpolymermatrix.However,sinceCNTsareallotropesofcarbon,theyareinherentlyinsolubleinanysolvent.Hence,thereareanumberofstrategiesthathavebeenusedtomodifythesur-faceofCNTstofacilitatetheirinteractionswiththesurroundingmatrix.Notonlyarethesemethodsessentialforthedispersionandself-alignmentofnanotubesthrough-outapolymerhost(oronasurfaceformoleculardevices),butmayalsoassistinthe
6Nanomaterials329
Figure6.53.SEMimagesofspinningMWNTarraysintofibers,andsubsequentimagesofMWNTyarns.Shownbelowisanimageofafuturisticuniformtoequipthenextgenerationofwarfighter.TheSEMimageswerereproducedwithpermissionfromBaughman,R.H.Science2004,306,1358,Copy-right2004AAAS.TheimageofthefuturisticsoldierwasfurnishedbytheArmyNatickSoldierCenter(http://nsc.natick.army.mil)–refertotheAug.11,2003issueofChemicalandEngineeringNewsformoredetails.
3306.2.NanoscaleBuildingBlocksandApplications
purificationofCNTs(e.g.,separationofmetallicandsemiconductingCNTsthroughselectivefunctionalization,followedbytraditionalHPLC).
TherearetwotypesofsurfacemodificationsforCNTs:noncovalentinteractions(Figure6.54)andcovalentsidewall(Figure6.55)ordefect-site[74](Figure6.56)functionalization.[75]Bothmethods,aswellasphysicaltechniquessuchassonica-tion,aresuccessfulinseparatingindividualSWNTsfrombundles–aprocessknownasexfoliation.Ingeneral,itismostdesirabletoincorporateisolatedSWNTsinacompositeratherthanbundles,sincethelatterfeaturespoorintertubeinteractionsresultinginaloweroverallstrength–especiallyatlowCNTconcentrations.
AsillustratedinFigure6.54,noncovalentmodificationsconsistofeitherwrappingtheCNTwithpolymersorbiologicalmacromolecules,[76]orplacementofconju-gatedmacromoleculesonthesurfacethroughπ-stackinginteractions.Assuch,thesetypesofinteractionshavetheadvantageofnotalteringtheelectronicpropertiesof
a)
PolyA30
b)
PolyT30
Figure6.54.ExamplesofnoncovalentinteractionswithamacromoleculesolubilizingagentandCNTs.Shownis(a)thesolubilizationofSWNTswithPolyT30single-strandDNA,followedbythedisplacementofthebiomoleculethroughpreferredπ–πstackingbetweentheconjugateddyemethyleneblueandtheSWNTsurface–causingthecontrolledprecipitationofSWNTsfromsolution.Alsoshownisanothermethodforsolubilization/precipitationofSWNTsthroughuseofcomplimentarysingle-strandDNA.Re-producedwithpermissionfromChen,R.J.;Zhang,Y.J.Phys.Chem.B2006,110,54.Copyright2006AmericanChemicalSociety.
6Nanomaterials331
Figure6.55.SchematicofthevariousmethodstoperformsidewallfunctionalizationofSWNTs.Re-producedwithpermissionfromBanerjee,S.;Hemraj-Benny,T.;Wong,S.S.Adv.Mater.2005,17,17.Copyright2005Wiley-VCH.
332
D
HOOC
R
HOOC
COOH
RCOOH
6.2.NanoscaleBuildingBlocksandApplications
O
O
CICIO
NH2-RROHpyridine
OOO
R
R
RB
R
RCOOHCOOH
C
1) HNO3/H2SO4 or2) H2O2/H2SO4 or3) KMnO4/H2SO4 or
SOCI2
O
OOO
H
H
EDC / DCCH2N-R
NRRNHO
R=(CH2)nSHMetalColloidMO
NRRMO
NH
H
4) K2Cr2O7/H2SO4 or5) O2/H2OOxidized
-RA
Pristine
SWNTs
SWNTsO
Mn+Ln
A
O
ONH3R
−+
NH2OO
Mn+Ln
H
O
ONH3R
−+
OO
Mn+Ln
Figure6.56.IllustrationofpossibledefectsitesonthesurfaceofaSWNT,alongwithmethodsusedforcovalentfunctionalizationofSWNTsatdefectsites.Thetypesofdefectsshownare(a)5-or7-memberedringsintheframework,whichresultsinbending,(b)sp3-hybridizeddefects(R=–OH,–H),(c)oxidativesurfacedegradation,withholescappedby–COOHgroups,and(d)openendoftheSWNTshownwith–COOHcappinggroups(mayalsobeterminatedwith–OH,=O,–NO2,and–Hgroups).Reproducedwithpermissionfrom(a)Hirsch,A.Angew.Chem.Int.Ed.2002,41,1853and(b)Banerjee,S.;Hemraj-Benny,T.;Wong,S.S.Adv.Mater.2005,17,17.Copyright2002&2005Wiley-VCH.
theCNT(unlikecovalentmodifications).However,sincecovalentfunctionalizationplacesspecificchemicalgroupsontotheCNTsurface,thisrouteoffersagreaterpo-tentialfortheselectivetunabilityofCNTpropertiesaswellasmolecularcontrolovertheorganizationofCNTsfordevicefabrication.
ThedrivingforcebehindtheendcapandsidewallfunctionalizationofSWNTsisthestrainbroughtaboutfromthehighdegreeofsurfacecurvature.Thedeviationofthecarbongeometryfromplanarityisreferredtoasthepyramidalizationangle,θp,whichisdefinedas0◦forthesp2carbonatomsinC2H4,and19.5◦(i.e.,109.5◦–90◦)forthesp3carboninCH4(Figure6.57).SincetheθpofthecarbonatomsinC60is11.6◦,thedesiredgeometryisclosertotetrahedralratherthantrigonalplanar.Asaresult,fullerenesreadilyundergoadditionreactionstorelievethishighdegreeofsurfacestrain.SincetheendcapofaSWNTmaybeconsideredasahalf-fullerenestructure,withaθp≥9.7◦regardlessofthenanotubediameter,[77]thesepointsarehighlyreactivetowardelectrophilicaddition.
Incontrast,thesidewallpyramidalizationangleismuchlessthantheendcap,ontheorderof3–6◦,dependingonthediameter/chiralityoftheSWNT.Foranarmchair(5,5)SWNT,twotypesofC–Cbondsarepresentalongthesidewalls–eitherpar-allelorperpendiculartothenanotubeaxis.Eachofthesebondsexhibitsadifferent
6Nanomaterials333
Figure6.57.Schematicof(a)ametallic(5,5)SWNT,(b)thepyramidalizationanglesoftypicalsp2andsp3carbonatoms,(c)theπorbitalmisalignmentanglesalongC1–C4inthe(5,5)SWNTanditscap-pingfullerene,C60.ReproducedwithpermissionfromNiyogi,S.;Hamon,M.A.;Hu,H.;Zhao,B.;Bhowmik,P.;Sen,R.;Itkis,M.E.;Haddon,R.C.Acc.Chem.Res.2002,35,1105.Copyright2002AmericanChemicalSociety.
degreeofπorbitaloverlapwithadjacentcarbonatoms,asquantifiedbytheπorbitalmisalignmentangle,φ(Figure6.57).Whereasφ=0◦forfullerenes,themisalign-mentofneighboringπorbitalsinSWNTsmaybesignificant–aslargeas21.4◦for(5,5)species.Ingeneral,aSWNTofsmallerdiameterhasagreatersidewallreactiv-itythanlargeranaloguesduetorelativelylargeπorbitalmisalignmentangles(muchmoreinfluentialthanθp).ItshouldalsobenotedthattheoverallcurvatureofCNTsislessthanfullerenes,whichmakeSWNTssignificantlylessreactivethanfullerenes.Growthof1Dnanostructures
NowthatyouarefamiliarwiththepropertiesandapplicationsofCNTs,wemustnowconsiderthetechniquesusedfortheirsynthesis.InadditiontotheexperimentaldetailsofCNTgrowth,thissectionwillalsoprovidemechanisticdetailsonhowtheseinterestingnanostructuresform.Fortunately,recentstudieshaveshownthatthegrowthmechanismofCNTsisthesameforother1Dnanostructuressuchasnanowires.
TherearethreeprimarymethodsthatareusedtogrowCNTs:laserevaporationofgraphitictargets,arcdischargemethods,andchemicalvapordeposition(CVD).Youmightrecognizethefirsttwomethodsasalsobeingusedfortogeneratefullerenes;notsurprising,sinceCNTsareessentially1Dextensionsoffullereneclusters.TosynthesizeSWNTs,acatalyticamountofametal(e.g.,Co,Ni,Fe,Y,Mo,alloys,
3346.2.NanoscaleBuildingBlocksandApplications
etc.)mustbepresenttopreventtheleadingedgefromclosing,whichwouldgen-eratespheroidalfullerenes.ForMWNTgrowth,acatalystisnotrequired;however,theproductswillcontainalargenumberofothercarbonaceousproductssuchasfullerenesandamorphouscarbon.[78]
Whereasarc-basedmethodsgenerallycontainamixtureofproducts,withSWNTs/MWNTsofvaryingdiameters/morphologies,amorphoussoot,etc.,adual-pulsedlaserapproach,firstusedinthemid-1990s,resultsinSWNTropeswith>70%purity.[79]ThoughbothofthesemethodsmaybeusedtogeneratesmallquantitiesofCNTs,theyarenoteasilyscaleabletogenerateindustrialquantities.Further,theCNTsarisingfromvaporizationmethodsaretypicallyinatangledarray,withotherformsofcarbonintermixedwithremainingcatalyticmetal(esp.affectingsubsequentelectronicapplications).
Consequently,CVDisnowthemethod-of-choiceforthesynthesisofCNTs.AsdiscussedinChapter4,thesemethodsconsistofthedecomposition(typicallyther-mal)ofahydrocarbonprecursoronthesurfaceofcatalyticmetalnanostructures.Methaneandacetylenehavebeenusedmostextensivelyasprecursors;otheralter-nativesnowincludeCO,C2H4,andmethanol/ethanol.AswithanyCVDapproach,thismethodiseasilyscaleable,andisusedtogeneratekilogramquantitiesofCNTsforanever-increasinglaundrylistofapplications.
Mostimportantly,CVDismostamenableforthefacilealignedgrowthofCNTsfromsurface-immobilizedcatalystnanoclusters.Thisstrategy,pioneeredbyDaiandcoworkersatStanford,hasbeenusedtogroworderedarraysofbothMWNTsandSWNTsusingavarietyofexperimentalmodifications(Figure6.58).[80]CNTgrowthemanatesfromthecarefullyplacedcatalystparticle,withresultanttubediametersrelatedtothesizeoftheseedcatalyst.Theself-assemblyoftheCNTstakesplacethroughstrongintratube/intertubevanderWaalsinteractions(e.g.,MWNTarrays,Figure6.58a),andtube-patternedsubstrateinteractions(e.g.,SWNTarrays,Figure6.58b),orinducedbyelectricfields(e.g.,SWNTarrays,Figure6.58c).Thesestrategiessetanimportantprecedentofgrowingnanostructuresalongspecificgrowthdirectionsfromspecificsites–essentialforthefabricationoffutureintegratedcir-cuitsandotheradvancedelectronicdevices.
Aspreviouslyseenfornanotube-basedyarns,thegrowthofvertically-alignedCNTarrays,alsoknownas“forests,”areanotherexampleofself-assembly–throughintertubevanderWaalinteractions.Nanotubeforestsaretypicallygeneratedbyfirstdepositingcatalystspeciesontoasurface,followedbyCVD(Figure6.59).Therearealsoreportsoftheone-stepsynthesisofalignedMWNTbundlesandY-junctionCNTsusingacontrolledmixtureofmetallocene-basedprecursors(M(C5H5)2,M=Fe,Co,Ni)–precludingtheuseofsurface-immobilizedcatalystnanoclusters.[81]Sincetheprecursoractsasboththecarbonsourceandcatalyst,carbongrowthini-tiatesatapproximatelysametimefrommultiplemetalsites.Subsequentgrowthisself-directedintoaparallelarraybyinteractionsamongstneighboringCNTs.
Forsomeapplications,amaindrawbackofCVDisthatMWNTsareoftengeneratedalongsideSWNTs.Sincethesizeofthecatalystgovernsthediameterofresultanttubes,surface-immobilizednanoclustersofironoxide@PAMAMdendrimer,withwell-definedsizes,hasbeenshowntoyieldonlySWNTswith
6Nanomaterials335
Figure6.58.Examplesoftheorderedgrowthofcarbonnanotubes.Shownare(a)MWNTarraysgrownfromsquaredregionsofironnanoclusters;(b)Side-viewSEMimageofa“SWNTpowerlineonSiposts”;(c)alignedSWNTgrowththroughelectric-fieldinduction.ReproducedwithpermissionfromDai,H.Acc.Chem.Res.2002,35,1035.Copyright2002AmericanChemicalSociety.
anextremelynarrowdiameterdistribution.[82]TwoadvancedCVDprocesses,
RR
(fluidized-bedCVD)andHiPCO(high-pressureCOCVD),haveCoMoCAT
recentlybeendevelopedforthecommercialproductionofSWNTs(Figure6.60).Thoughtheexperimentalsetupofthesemethodsaresignificantlymorecomplexthanstandardhot-walledCVD,thesetechniquesarestillconsideredanextensionofCVD,astheprecursorisdecomposedonthesurfaceofthecatalyst.MostnoteworthyabouttheseandotherrecentCVDmethodsisthattheyoffertheabilitytofine-tuneresultantlengths,diameters,andevenchiralities[83]byvaryingtheoperatingcondi-tions.Thinkofthepossibilitiesforapplications,asitwouldnotbelonguntilyoucanplaceanordersuchas:“10gofpure(5,5)SWNTswithanaveragediameterof
336
quartz−−−−−−−−−+++++−−−−−6.2.NanoscaleBuildingBlocksandApplications
plasmaCH3COOHPSS−−−−−−−−−−−iron collioddeposition++−++−−+−+−repeating the LBLprocess−+−+−+−+−+−+−−−−−−−−−+++++−−−−−Figure6.59.Schematicandcross-sectionSEMimageofalayer-by-layer(LbL)approachtodepositironnanoclustersontoasurfacetoyieldverticallyalignedSWNTs.ReproducedwithpermissionfromLiu,J.;Li,X.;Schrand,A.;Ohashi,T.;Dai,L.Chem.Mater.2005,17,6599.Copyright2005AmericanChemicalSociety.
7nmandlengthof500nm.”However,thisdegreeofcontrolwillonlybepossibleoncetheexactgrowthmechanismisknown,ratherthancurrenteffortsthatinvoke“blind”modificationsofexperimentalvariables.
ThoughCNTgrowthdatesbacktotheearly1990s,detailsofthegrowthmech-anismarestillunclear.Themostcommonlyaccepteddescriptionisbasedonthevapor–liquid–solid(VLS)model,whichwasfirstproposedforthegrowthofsemi-conductorwhiskers(Figure6.61).[86]Thismodelassumesthatthecatalystliquefies,whichthenactsasapreferentialadsorptionsiteforgaseousprecursors.Subsequentgrowthofthenanotubes/nanowiresoccursbysupersaturationofthecatalystdroplet,andprecipitationattheliquid–solidinterface.However,theamountofcarboninasupersaturatedcatalystnanoclusterwillneverbesufficienttoaccountforthelengthornumberofnanostructuresobservedinpractice.Asaresult,additionalprecursoratomsmustbesuppliedcontinuouslytothecatalystnanoclusterinorderforsustainedgrowthtooccur.
Byexaminingthephasediagramforabinarysystem(precursorandcatalystspecies),onecaneasilydeterminethebesttemperaturerangeforVLSgrowth–anyvalueabovetheeutectictemperature,wherethecatalystremainsaliquid(Figure6.62).Aswesawearlierforgoldnanoparticles,asthediameterdecreases,themeltingpointissignificantlydecreasedrelativetothebulksolids.Asaresult,“size-correctedeutectics”mustbedeterminedforthespecificnanoparticlediameterinordertodetermineexperimentalconditionsfornanowiregrowth.
ThoughtheVLSmechanismisgenerallysufficienttomodelthegrowthofnanowires,therearemanyrecentreportsofCNTsbeinggrownattemperaturesbelowthesize-correctedeutectic.[87]Thoughsmallnanoparticlesmayexhibit“fluid-like”behaviorbyalteringtheirsurfacegeometry,empiricaldatahasshownthatthecatalystparticlemayremaincrystalline,ratherthananamorphousliquidphase,duringgrowth.Thissuggeststhatnucleationmaybedominatedbythecatalystsurface–notunlikemostheterogeneouscatalysisprocesses(e.g.,Ziegler–Nattapolymerization).[88]
reducin−+−+−+−+−+−+gCNT growth6Nanomaterials
a)
337
b)
Figure6.60.IllustrationoftwomethodsusedforthecommercialproductionofSWNTs.Shownare(a)theCoMoCatfluidizedbedmethodusingCOastheprecursorandaCo/Mobimetalliccatalyst,[84]and(b)theHiPco“floatingcatalyst”processusingthethermaldecompositionofironpentacarbonylatpressuresof1–10atm.[85]
Sincethegrowthof1Dnanostructuresisdifficulttostudyinsitu,anumberofcomputationaltechniqueshavebeenusedinanattempttodecipherthegrowthmech-anism(s).ForCNTgrowth,ithasbeenshownthatsmallgraphiticislandsformonthesupersaturatedcatalystsurface.Whentheislandcovershalfofthecatalystparticle,itliftsoffandformstheSWNT–withthesamediameterasthecatalyst.Basedonempiricaldataandtheoreticalpredictions,[]thecatalyticabilityofthemetal/alloys
3386.2.NanoscaleBuildingBlocksandApplications
VAPOR
VAPOR
SILICONCRYSTAL
111
Au-Si LIQUIDALLOY
SILICON SUBSTRATE
ab
Figure6.61.Theoriginalschematicusedtodescribevapor–liquid–solid(VLS)growthofsemiconductornanowires.ReproducedwithpermissionfromWagner,R.S.;Ellis,W.C.Appl.Phys.Lett.19,4,.Copyright19AmericanInstituteofPhysics.
forSWNTgrowthfollowstheorder:Ni/Mo>Ni/Cr>Ni/Co>Ni/Pt>Ni/Rh>Ni/Fe>Ni>Fe/Mo>Fe/Cr>Fe/Co>Fe/Pt>Fe/Rh>Fe>Ni/Mo>Fe/Mo>Co/Mo>Co>Pt>Cu.DependingonwhetherthemechanismproceedsviaVLSorsurface-additionmechanism,theaboveordermayberelatedtotheeaseofcar-bideformationandcarbondiffusionthroughthenanoclusterinterior,orthemorphol-ogy/compositionofthecatalystsurfaceandrateofcarbondiffusiononthecatalystsurface,respectively.Figure6.63showssomerecentmechanisticproposals,basedonexperimentalandtheoreticaldata.TheVLS-basedproposals(Figure6.63,topandmiddle)illustratethefollowingbasicsteps:
(i)Atinitialgrowthstages,carbondissolvesinthemoltencatalyticnanocluster.Calculationsindicatethatthereisadynamicprocessofcarbonprecipitationontothecatalystsurfaceandredissolution,untilahighlysupersaturatedcatalystisobtained.
(ii)Carbonprecipitatesonthesurfaceofthehighlysupersaturatedcatalystnano-clusters,formingcarbonstrings/polygons.Thiscausesadecreaseinthedis-solvedcarbonconcentration.
(iii)Thecarbonnucleiformgraphiticislandsonthesurfaceofthecatalyst,which
aggregateintolargergraphiticclusters.
(iv)Atlowtemperatures,thegraphiticislandsarenotabletoliftoffthecatalyst
surface,resultingingraphite-encapsulatedmetalnanoclusters.[90]
6Nanomaterials339
Figure6.62.Siliconnanowiregrowthfromagoldnanoclustercatalyst.Shownis(a)thephasediagramfortheAu/Sisystem,showingtheeutectictemperature/composition;(b)SEMimage;and(c)high-resolutionTEMimageofthenanowiresgrownatatemperatureof450◦C.Thedarktipofthenanowireisfromthegoldnanocluster.ReproducedwithpermissionfromHu,J.;Odom,T.W.;Lieber,C.M.Acc.Chem.Res.1999,32,435.Copyright1999AmericanChemicalSociety.
(v)Atrelativelyhightemperatures(ca.500◦C–1,200◦C),whenthediameterof
theislandbecomesca.1/2thatofthecatalyst,thegraphiticnucleusliftsoffthecatalystsurfacetoformtheSWNTendcap.SubsequentgraphitizationandgrowthpropagationoftheSWNTmayoccurthroughtworoutes(Figure6.63(middle)):
(i)“RootGrowth”(c–d):carbonatomsprecipitatefromthemoltencatalystandjointotheopenendofthegrowingSWNT.
(ii)“FoldedGrowth”(e–g):carbonatomsprecipitatedirectlyontothecatalystsur-face,andareaddedtothegraphiticendcap.
Incontrast,asurface-governedrouteischaracterizedbynegligiblecarbondissolu-tioninthecatalystbulk.Sincethegrowthisnotproposedtooccurthroughsupersat-uration/precipitation,anyspeciesthatarechemisorbedontothecatalystsurfacewillhaveadramaticinfluenceonSWNTgrowth.Figure6.63(bottom)showsaproposedrouteforasurface-basedSWNTgrowthmechanism,whichprogressesthroughthecontinualadditionofC2unitsontotheleadingedgeofthegrowingnanotube.As
3406.2.NanoscaleBuildingBlocksandApplications
Low Temperature
Intermediate Temperature
(A)
Initial tube
(B)
Figure6.63.ComparisonofaVLS-basedmechanism(topandmiddle)andasurface-mediatedroute[95](seetextfordetails).
UnsaturatedHighly
Supersaturated
Supersaturated
High Temperature
(a)
Fe
(b)
(e)
(c)
(f)
(d)
(g)
C60
(C)
(D)
(E)
Nucleation stage
Growth stage
6Nanomaterials341
indicatedinthisscheme,theabsenceofanactivecatalystwillresultintheforma-tionoffullerenesratherthanSWNTs.Inthisroute,nanotubegrowthisproposedtoinitiatefromsurfacemetalatomsaddingtotheedgeofapolyynering,whichissubsequentlyabletoaddadditionalcarbonatoms.
WhereaspathCofFigure6.63(bottom)showsthesuccessfuladditionofC2unitsatthegrowthedge,pathDshowstheformationofadefect,whichleadstotubeclosure.Itshouldbenotedthatthisrouteis1.3eVmorefavorablethanrouteCwhenthereisnometalliccatalystpresent,thusexplainingthepreferenceforclosed-cagefullerenes.Mostlikely,thisisaconsequenceofpentagonaldefectsthatareformedwhencarbonatomsaddtotheopenendofthegrowingSWNT.Hence,calculationshaveshownthatcatalyst-assisteddefectrepairisacrucialstepintheoverallmech-anism(Figure6.63(bottom,routeE).Thisisproposedtooccurthrougha“scootermechanism,”[91]wherebythecatalystatomsdiffusealongtheopen-endsurfaceoftheSWNT,facilitatingtheformationofhexagonsthatpreventprematurenanotubeclosure.
ItshouldbenotedthatanadditionalvariablemayberequiredtodescribethefullmechanismforCNTgrowth.Recentstudiessuggestthatthereislikelyanimpor-tantroleformetaloxidecatalysts,oroxygenatedco-reactants,intheoverallgrowthmechanismofSWNTs.Forexample,theuseofwatervaporasaco-reactantforCVDresultsinthegrowthofadenseforestofverticallyalignedSWNTs,withlengthsupto2.5mmafteronly10minofreactiontime(Figure6.a).[92]ThecatalyticenhancementofwatervaporisevidencedbytheSWNT/catalystweightratioof50,000%–morethan100timesgreaterthantheHiPCOprocess.Theeffectofanoxidizingspeciessuchaswatervaporseemslogicalfromthestandpointofreduc-ingtheamountofamorphouscarbondeposits,andcontrollingthedegreeofcarbonprecipitation(thuspreventingMWNTgrowth).However,itsrolemaybeevenmoresignificant–perhapsevenprovidinganewmodeofnucleation.Inparticular,itisproposedthatSWNTgrowthmaybeinitiatedfromoxygen-etchedgraphiticshellsthatencapsulatethecatalyst–arouterecentlytermed“nucleationviaetchedcarbonshells”(NECS),Figure6.b.[93]
Theformationofnanotubesisnotlimitedtocarbonaceousmaterials.Infact,anumberofinorganic-basednanotubeshavebeensynthesizedinrecentyears–suchasmetals,oxides,carbides,borides,etc.[94]Therearemanyapplicationsforthesemate-rials,suchascatalysis,sensors,andadvancedceramicnanostructures.Duetothelackofsp2hybridizationandnogrowthmodeanalogoustotherollingof“graphene”sheets,these1Dnanostructuresdonotself-assemblespontaneouslyfromacatalyticseed.Instead,themostcommonmethodtosynthesizethesestructuresisbycoatingaCNTwithathinfilmofthedesiredmaterial,followedbythesacrificialremovaloftheCNTtemplate.Aswesawearlierfornanoclustergrowth,templatingroutesofferaconvenientmodeofnanostructuralsynthesis,aswellassurfacealignmentofthenanoarchitecturesonasubstratesurface.
3426.3.Top-DownNanotechnology:“SoftLithography”
A
B
Catalyst particle after recalescence withgraphitic coating
Embroyonic
cap
Stableoxidizedring
CO
Large moltencatalystparticle
SWCNTgrowth
Oxygen etching
Step 2:
The solidified catalyst particle collides with the molten catalyst particle.Oxygen etches the cap andprevents MWCNT growth byoxidising precipitating
carbon.
CapFormation
Step 3:
A stable oxidized ring forms(c.f. preferential burning ofSWCNT caps). This forms theembryonic cap for SWCNT growth. The solid catalystparticle enters the molten
particle.
Caddition
by
substitutionStep 4:
Carbon subtitutes O at theoxidized ring interface andSWCNT growth occurs.
Step 1:
A solidified catalyst particlewith a graphite shell headstoward a molten catalyst
particle.
Figure6..(a)Photoofacm-scaleSWNTforest,shownnexttoamatchstickforcomparativepurposes.ReproducedwithpermissionfromHata,K.;Futaba,D.N.;Mizuno,K.;Namai,T.;Yumura,M.;Iijima,S.Science2004,306,1362.Copyright2004AAAS.(b)AproposedschemeforthelaserablativegrowthofSWNTsviametaloxidenanoclustercatalysts.ReproducedwithpermissionfromRummeli,M.H.;Borowiak-Palen,E.;Gemming,T.;Pichler,T.;Knupfer,M.;Kalbac,M.;Dunsch,L.;Jost,O.;Silva,S.R.P.;Pompe,W.;Buchner,B.NanoLett.2005,5,1209.Copyright2005AmericanChemicalSociety.
6.3.TOP-DOWNNANOTECHNOLOGY:“SOFTLITHOGRAPHY”AsdiscussedinChapter4,thedesignoftransistors,withfeaturesizescurrentlymuchlessthan100nm,willneedtomovebeyondtraditionalphotolithography(Figure6.65a)inordertocontinueMoore’sLawandtheminiaturizationofelectronicdevices.Inadditiontotheexorbitantcostofphotolithography,thistechniqueisnotamenableforthepatterningoflargeandnonplanarsubstrates–ofimportanceforfuture3Ddevices.ThoughadvancedtechniquessuchasdeepUV(DUV),extremeUV(EUV),andfocused-ionbeamlithographiesareabletopushtheresolutionlimitsofpatterningtowellbelow100nm,theyaremuchtooexpensiveforthelow-cost,high-volumeprocessingthatisrequiredforcommercialapplications.[96]
Withinthelastdecade,comparativelyinexpensiveandscaleabletchniquesknownas“softlithography”havebeenthefocusofmuchdevelopment.Patterningofasub-strateisaffordedbyusingamasterelastomericstampthatcontainsananostructuredpattern,knownasarelief,onitssurface.Contrarytophotolithography,theresolu-tionofthefinalpatternisnotlimitedbylightdiffraction,butonlydependsonthe
6Nanomaterials
Photoresist343
a)Si / QuartzWrite withscanning beam(or)Mask and expose to lightDevelopphotoresistSi / QuartzDry etchDepositmetal filmSi / QuartzSi / QuartzElectroplate metalRemovephotoresistMetalSi / QuartzSeparateSi / QuartzMetal(Master)PhotoresistSiCoat with PDMS prepolymerPDMSSiCurePeel off PDMS PDMS(Mold / Stamp)PDMSb)
Mold prepolymerPDMS(Replica)
Cure
Remove mold
Figure6.65.Comparisonof(a)conventionalphotolithography/electroplatingwith(b)softlithography.Shownin(b)isreplicamoldingwhichconsistsoftheformationofaPDMSstamp,andsubsequentreplicationofamasterinaphoto-orthermallycurableprepolymer.ReproducedwithpermissionfromGates,B.D.;Xu,Q.;Stewart,M.;Ryan,D.;Willson,C.G.;Whitesides,G.M.Chem.Rev.2005,105,1171.Copyright2005AmericanChemicalSociety.
3446.3.Top-DownNanotechnology:“SoftLithography”
dimensionsofthereliefstructures–typicallyfabricatedinthemasterbyelectron-beamlithography.Typically,themold(orstamp)iscomprisedofpoly(dimethylsiloxane)(PDMS),whichallowsforintimatecontactbetweenthemold/substratesurfaces,evenifnonplanarsubstratesareused.Morerecently,otherpolymershavebeendevelopedforthisapplicationsuchaspolyimides,polyurethanes,andavarietyofsubstitutedsiloxanes–especiallyfluorinatedana-loguesduetoeasyreleaseaftermolding,andlackofswellingbyorganicsolvents.Thetechniqueofreplicatingamasterpatternisaptlytermedreplicamolding(Figure6.65b).Intheory,theresolutionofthereplicawillbeidenticaltothemaster.However,duetothe“soft”natureofthemold,thenanoscalefeaturesofthereliefmaybecomedistortedduetopolymershrinkage(e.g.,solventevaporation,insitucross-linking,mechanicaldeformation),orinterfacialphenomenabetweenthemoldandmastersurfaces(e.g.,differingthermalexpansions,adhesiveforces[97]).Incontrast,ahardmoldofSiorquartzexhibitssignificantlylessdistortionduetotheirsolvent/chemicalresistance,andthermalstabilitiesattemperaturessufficienttocausepolymercross-linking.Hardmolds,usedforstep-and-flashimprintlithography(SFIL)andnanoimprintlithography(NIL),arecommonlyusedtopatternmaterialssuchasCDs,DVDs,andholographicimagesonthefrontofmostcreditcards.[98]Acommonapplicationforelastomericmoldsisformicro-ornanocontactprint-ing,whereaself-assembledmonolayer(SAM)isplacedonbothplanar[99]andcurved[100]surfacesviacontactwiththereliefsonthemold(Figure6.66).SAMswillbeanimportantarchitectureforthenextgenerationofnanostructuredmaterials.
(A) Nanotransfer Printng
PDMSAu layer
Octanedithiol
GaAs
Place stamp on substrate
150 nm
PDMSB
500 nm
C
GaAs
Remove stamp
Au layer
GaAs
Repeat process formultilayered structures
perpendicular channel200 nm
parallel channelFigure6.66.Schematicofthegeneralprocessofnanotransferprinting.SEMimageBillustratesa20-nmgroovedgoldlayertransferredontoaGaAssurface.ImageCshowsamultilayeredstackof20-nmthicklayersofparallelgrooves;thechannelsinadjacentlayersarealignedperpendiculartooneanother.[102]ReproducedwithpermissionfromGates,B.D.;Xu,Q.;Stewart,M.;Ryan,D.;Willson,C.G.;Whitesides,G.M.Chem.Rev.2005,105,1171.Copyright2005AmericanChemicalSociety.
6Nanomaterials
X(CH2)nSH + Au0X(CH2)nS−AuI + 1 2 H2345
ca. 30Њ
2-3 nm
Au
X(CH2)n\"S\"
0.288 nm
01.50 nmFigure6.67.Schematicofaself-assembledmonolayer(SAM),illustratingtheorganizationofthealkylchainsviavanderWaalinteractions,andtheclose-packedarrayofsulfuratomsonthegoldsurface.Re-producedwithpermissionfromXia,Y.;Whitesides,G.M.Angew.Chem.Int.Ed.1996,37,550.Copyright1996Wiley-VCH.
ThearchetypicalexampleofaSAMisthechemisorptionofalkylthiolsonagoldorsilversurface,whichresultsinself-assembly/alignmentintoa3Dforestarray(Figure6.67).ApplicationsforSAMsspananumberoffieldsfromsensorstohigh-densitystorage;arecentprecedentillustratestheselectiveadsorptionandsponta-neousalignmentofCNTs[101]anddirectedgrowthofnanowires[102]fromSAMs.Inordertoimprovethestampingresolutionoftheelasomericstamp,therehavebeenrecentimprovementsinboththestampand“molecularinks”(e.g.,alkylthi-ols,silanes).Inparticular,traditionalPDMSexhibitsarelativelyhighelasticitythatlimitspossiblerelieflinewidths;ontheotherhand,smallmolecularweightinksexhibitdiffusionduringpatterning.Hence,thefollowingcomplementarystrategieshavebeenemployed:
(i)Usingacompositetwo-layerstampcomprisedofa30µmhardenedPDMScoatingona2–3mmthickPDMSsupport[103];especiallyintandemwithsharp,V-shapedgrooves[96]
(ii)Usinghighmolecularweightinkssuchasdendrimers[104]andbiologicalmole-cules(e.g.,proteins[97])
Usingacombinationoftheabovemodificationshasnowextendednanocontactprint-ingtowellbelowthe30nmregime–evenaslowas2nm![105]
Oneofthemorerecenttechniquesfornanocontactprintingconsistsofthedirectwritingofamolecularinkontoanappropriatesubstrateviatheultra-finetipofanatomicforcemicroscope[106](Figure6.68).Theuseofsucha“nanofountainpen”isknownasdip-pennanolithography(DPN),firstdemonstratedbyMirkinandcowork-ersinthelate1990s.[107]ThoughtheearliestexamplesofDPNfeaturedalkylth-iolsastheinkontoAusurfaces,therearenowanincreasinglylargenumberofotherink/substratecombinationsthathavebeenreportedforDPN(Table6.4).[108]
3466.3.Top-DownNanotechnology:“SoftLithography”
AFM tip
CH3CSNH2
Cd(Ac)2
Writing direction
CH3CSNH2 + 2H2O → CH3COONH4 + H2S↑
H2S + Cd(Ac)2 → CdS↓ +2HAc
Water meniscus
CdS
substrate
Figure6.68.Illustrationofdip-pennanolithography,usedtowritenanofeaturesofCdSonmicaandSiOxsubstrates.ReproducedwithpermissionfromDing,L.;Li,Y.;Chu,H.;Li,X.;Liu,J.J.Phys.Chem.B2005,109,22337.Copyright2005AmericanChemicalSociety.
Table6.4.SummaryoftheInk-SubstrateCombinationsUsedtoDateforDPN[109]
Molecularink
Alkylthiols(e.g.,ODTaandMHAb)FerrocenylthiolsSilazanesProteins
ConjugatedpolymersDNA
FluorescentdyesSols
Metalsalts
ColloidalparticlesAlkynes
AlkoxysilanesROMPmaterials
Thioacetamide/cadmiumacetatec
SubstrateAuAu
SiOx,GaAsAu,SiOxSiOx
Au,SiOxSiOxSiOxSi,GeSiOxSiSiOxSiOx
SiOx,mica
a1-Octadecanethiol.
b16-Mercaptohexadecanoicacid,orthiohexadecanoicacid.
cDing,L.;Li,Y.;Chu,H.;Li,X.;Liu,J.J.Phys.Chem.B2005,109,22337.
AgeneralbenefitofDPNoverothersoftlithographictechniquesistheabilitytopatternnanostructures(includingbiologicalmaterials)byasinglestepwithoutcross-contamination,sincethedesiredchemistryoccursonlyinaspecificallydefinedlocationofthesubstrate.
ThemechanismofinktransportfromtheAFMtiptosubstrateiscurrentlyanitemofcontroversy.Recentmodelssuggestthatameniscusformsbetweenthetip
6Nanomaterials347
andsubstrate,whichaidsininktransport.Asaresult,thetransportrateisfoundtoincreaseconcomitantlywiththeambienthumidity–butonlyforinksthataresolubleinwater.Ingeneral,therateofinktransportisfoundtodecreasesignificantlywithincreasingcontacttime,duetothechangingsurfaceenergyofthesubstrate.Asyoumightimagine,therearemanyfactorsthatgoverninktransportandthefinalresolutionoftheprintednanostructure–tipshape,inkcomposition/concentration,substratesurfaceproperties,andambientconditions.Itisclearthatthecurrent15-nmresolutionofDPNmayonlybeimprovedonceafullpictureofinktransportisfullyunderstood.
IMPORTANTMATERIALSAPPLICATIONSV:
NANOELECTROMECHANICALSYSTEMS(NEMS)
Thepastdecadeorsohasbroughtabouttremendousgrowthinthefieldofmicroelectromechanicalsystems(MEMS)–devicessuchassensors,computers,electronics,andmachinesatthemicroscale.Thedesignofthesedevicesutilizesacombinationoftraditionalsemiconductorprocessingandmechanicalengineer-ing.Typically,theoutputofanelectromechanicaldeviceisthemovementofthemechanicalcomponent;atransducerisusedtoconvertthemechanicalenergyintoelectrical/opticalsignals,orviceversa.AsamplingofsomecurrentMEMSapplica-tionsinclude:
(i)Automotive(e.g.,airflowandtirepressuresensors,“smart”suspension,head-lightleveling,navigation,vehiclesecurity,automaticseatbeltrestraint,etc.)(ii)Micronozzlesforinkjetprinters(iii)Microtweezers
(iv)Aerospacenavigationalgyroscopes(v)Disposablebloodpressuretransducers
(vi)Portableskinanalysissensorsforcosmeticsapplications
TheextensionofMEMStothenanoregimeisreferredtoasNEMS–representingtheultimateinfuturedevices–withbenefitssuchaslowerpowerdissipationandultra-sensitiveandlocalizedresponses.Further,duetothesizeofNEMS,itwillbepossibletodirectlyincorporateanumberofauxiliaryfunctionalitiesalongsidetransistorswithinasinglechip.Indeed,theapplicationsforNEMSwillspanthefieldsofsensors,electronics,biotechnology,affectingvirtuallyeveryaspectofourlives.ThoughthetechnologyisinplacetofabricateNEMS,therearethreeprimarychallengesthatmustbeovercomepriortotherealizationofwidespreadcommercialapplications:
(i)Howtocommunicatesignalsfromthenanoscaletothemacroscopicworld(ii)Understandingandcontrollingmechanicalresponsesatthenanoscale
(iii)Developingmethodsforlow-cost,high-volume,andreproduciblenanofabrica-tion
OnerecentNEMSdeviceisamasssensorwitharesolutionatthezeptogramlevel(1zg=10−21g).[110]Thusfar,thelowestdetectionlimitforthisdeviceis7zg,orca.30Xeatoms.Incredibly,itissuggestedthatthistechnologymaybecombinedwithnanofluidicsforthegeneticanalysisoftheDNApresentwithinasinglecell!In
3486.3.Top-DownNanotechnology:ImportantMaterialsApplicationsV
comparison,currentmethodsutilizePCRamplification,wherebysmallsamplesofDNAarerepeatedlyreplicatedinordertofacilitatedetection.SuchanalyseswillbeofuseforthedetectionofgeneticmarkersforcancerorotherdiseasessuchasHIV.Theabilitytoidentifyproteinswillbeessentialforfuturediagnosticandforensicsapplications.
AnotherareaofNEMSthatisreceivingtremendousattentionisthemimicryofbiologicalsystems,aptlyreferredtoasbiomimetics.Forinstance,inthedevelopmentoflinearmolecularmusclesthatundergocontractionandextensionmovements.Initialworkinthisfieldutilizedtransitionmetalcomplexescontainingrotaxanesandcatenanes,duetothenondestructiveredoxprocessesoccurringonthemetalcenters.[111]Thoughthesecomplexeswereactuatedbyachemicalreaction,themovementwasinanoncoherentmanner.Inordertobettermimicskeletalmusclemovement,onehastolookatthemodeofmotionwithinthemostefficientmolecu-larmachines–inourhumanbodies.
Thecellularunitthatisactivetowardthecontractionofskeletalmuscles,knownasthesarcomere,iscomprisedofalternativelystackedfilamentsoftheproteinsactinandmyosin.Duringmusclecontraction,theproteinfilamentsslidepasteachotherasaresultofarowingactionofthesurfacemyosinheads(Figure6.69a).[112]Hence,
a)
Actin FilamentMyosin FilamentZ Disc
ContractionExtension
b)
CBPQT4+
O
O
Naphalene
TTF
+N
SO+N
S
SS
ON+
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N+
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+N
SS
SS
ON+
O
O
N+
Extension
(TTF/CBPQT4+couple preferred)
−4e
+N
S
O
O
O
S
SS
O
O
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+4e
+N
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S
O
+N
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SS
O
PPR8+
OO
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(Naphalene/CBPQT4+couple preferred)
PPR12+
1.4 nm
1.4 nm
1.4 nm
Figure6.69.Abiomimeticapproachtowardskeletalmusclemovement.Shownis(a)thestackedproteinfilamentsofthesarcomereand(b)aredox-controlledmolecularanalogue.AdaptedwithpermissionfromLiu,Y.;Flood,A.H.;Bonvallet,P.A.;Vignon,S.A.;Northrop,B.H.;Tseng,H.-R.;Jeppesen,J.O.;Huang,T.J.;Brough,B.;Baller,M.;Magonov,S.;Solares,S.D.;Goddard,W.A.;Ho,C.-M.;Stoddart,J.F.J.Am.Chem.Soc.2005,127,9745.Copyright2005AmericanChemicalSociety.
6Nanomaterials349
aneffectivebiomimeticapproachwouldentailthedesignofalineararchitecturethatfeaturesslidingcomponentsthatwillrespondtoachemicalstimulus.Thisapproachhasrecentlybeendemonstratedwiththedesignofarotaxanemoleculethatexhibitsredox-controlledcontractionandextensionofthemoleculararchitecture,inresponsetoachemicalorelectrochemicalstimulus(Figure6.69b).[113]
Thecontrolledmovementinthissystemiscontrolledbytheinteractionamongredox-activeunitsattheredox-activetetrathiafulvalene(TTF)units.Intheneu-tral,unperturbedstate,thetetracationiccyclophaneunits(cyclobis(paraquat-para-phenylene),CBPQT4+)aremoststablecoordinatedtotheTTFmoieties,duetoelectrondonationandπ-stackinginteractions.However,uponoxidationoftheTTFunits,theCBPQT4+ringsbecomeelectrostaticallyrepelled,migratingtothenapha-lenecomponent(Figure6.69b).Hence,the“rowing”actionexhibitedbyskeletalmusclesisemulatedbytheCoulombicrepulsionandπ-donationofthenaphalenerings.[114]ByattachingdisulfidetetherstotheCBPQT4+rings,theattachmentofthelinearmusclemaybefashionedtoagoldsurfaceenroutetowardabiomimeticNEMSdevice.[115]
Inthenextfewyears,therewillbemanyexcitingdevelopmentsrelatedtoNEMSdevices.Inparticular,replacingthecurrentchemicallydrivenmolecularmachineswiththosestimulatedbyopticalorelectricalpulses–dramaticallyextendingtherangeofapplications.Withsomanypossiblenanobuildingblocksatourdisposal,thescopeofdevicesandresultantapplicationsislimitedonlybyourimaginations–anexcitingareaofdiscoveryawaits!ReferencesandNotes
1Asamplingofsomeintriguingapplicationsthatarealreadypossibleusingnanomaterialsinclude:
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7
self-cleaningfabrics(viaTiO2nanoparticles),automobileclearcoatsthatpreventscratches(PPGnanoparticle-basedcoatings),carwashsolutionsthatpreventdirtfromadheringtoapaintedsurface,bandagesthatkillbacteria,drug-releaseagentsandtime-releasebiocidalcoatings,andtennisballsthatbouncetwiceaslongasconventionalballs.
OnlyUS-basedinstitutes/centersarelistedhere;foramorecomprehensivelistofworldwidenanotechnologyefforts,seehttp://sunsite.nus.sg/MEMEX/nanolink.html,acomprehensivelistingofnanorelatedwebsiteshostedbytheUniversityofSingapore.
NowavailableonlineatEricDrexler’s“ForesightInstitute”website:http://www.foresight.org/EOCFordetailsonthebiologicaleffectsofCNTs,see:Liu,Z.;Cai,W.;He,L.;Nakayama,N.;Chen,K.;Sun,X.;Chen,X.;Dai,H.Nat.Nanotechnol.2007,2,47,andreferencestherein.Thebiologi-caleffectsofdendriticpolymersisdescribedinBoas,U.;Heegaard,P.M.H.Chem.Soc.Rev.2004,33,43,andreferencestherein.Somecomprehensivewebsitesonthetoxicologicaleffectsofnanostructuresinclude:(a)http://orise.orau.gov/ihos/Nanotechnology/nanotechOSHrisks.html;(b)http://www.cdc.gov/niosh/topics/nanotech;(c)http://www.bnl.gov/cfn;(d)http://www.nanotox.com/nanomaterials-testing.htm;(e)http://membership.acs.org/c/ccs/nano.htmhttp://www.ethicsweb.ca/nanotechnology
http://en.wikipedia.org/wiki/UNIVACI.ThisimageisaworkofaUnitedStatesCensusBureauemployee,takenormadeduringthecourseofanemployee’sofficialduties.AsaworkoftheUSFederalGovernment,theimageisinthepublicdomain.
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Forexample,seePishko,V.V.;Gnatchenko,S.L.;Tsapenko,V.V.;Kodama,R.H.;Makhlouf,S.A.J.Appl.Phys.2003,93,7382.
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ForanicereviewregardingdefectsitesinCNTs,seeCharlier,J.-C.Acc.Chem.Res.2002,35,1063.Forathoroughrecentreviewofthesurfacechemistry(noncovalentandcovalent)ofCNTs,seeTasis,D.;Tagmatarchis,N.;Bianco,A.;Prato,M.Chem.Rev.2006,106,1105.
AinterestingrecentprecedentrelatedtothereversiblytunableexfoliationofSWNTsusingpoly(acrylicacid)atvaryingpHlevelsisreportedbyGrunlan,J.C.;Liu,L.;Kim,Y.S.NanoLett.2006,6,911.
Knupfer,M.;Reibold,M.;Bauer,H.-D.;Dunsch,L.;Golden,M.S.;Haddon,R.C.;Scuseria,G.E.;Smalley,R.E.Chem.Phys.Lett.1997,272,38.
Smalley,R.E.DiscoveringtheFullerenes,NobelLecture,1996.Maybefoundonlineat:http://nobelprize.org/chemistry/laureates/1996/smalley-lecture.pdf(alongwiththeNobellecturesfromCurlandKroto).
ItshouldbenotedthattheNationalInstituteofStandardsandTechnology(NIST)hasbeenrecentlyfocusedonthedevelopmentofstandardsynthesis,purification,andcharacterizationtechniquesforCNTs.Todate,thereareanumberofcompetingmethodsforSWNTs/MWNTs–allcitingpercentpurityvaluesthatappearratherarbitrary.Indeed,purchasinga“90%pureSWNT”samplefrommul-tiplevendorswillresultinverydifferentproducts!InordertocontinuetherapidprogressinCNTsynthesis/applications,itisessentialthatwesetupa“goldstandard”forCNTsthatwillimmedi-atelytelluswhatacertainpuritylevelmeans.Thatis,ifa“60%purity”valueiscited,clarifyingwhattheremaining40%consistsof(amorphouscarbon,remainingcatalyticmetal,othernanotubediameters/morphologies,etc.)
Dai,H.Acc.Chem.Res.2002,35,1035.
Rao,C.N.R.;Govindaraj,A.Acc.Chem.Res.2002,35,998,andreferencestherein.Forrecentinformationregardingtheroleofaluminaontheyield/morphologyofsupportedCNTcatalysts,see:Jodin,L.;Dupuis,A.-C.;Rouviere,E.;Reiss,P.J.Phys.Chem.B2006,110,7328.Itshouldbenotedthatthesupportednanoclustersmayresidewithinnanochannelstofacilitate1Dgrowth,examplesofthesemethods,whichincludebothtemplateand“closedspacesublimation”(CSS)are(andreferencestherein):(a)Li,J.;Papadopoulos,C.;Xu,J.M.;Moskovits,M.Appl.Phys.Lett.1999,75,367.(b)Kyotani,T.;Tsai,L.F.;Tomita,A.Chem.Mater.1996,8,2109.(c)Hu,Z.D.;Hu,Y.F.;Chen,Q.;Duan,X.F.;Peng,L.-M.J.Phys.Chem.B2006,110,8263.
Choi,H.C.;Kim,W.;Wang,D.;Dai,H.J.Phys.Chem.B2002,106,12361.ThefirstprecedentforSWNTgrowthfromgoldnanoclustershasbeenrecentlyreported:Bhaviripudi,S.;Mile,E.;Steiner,S.A.;Zare,A.T.;Dresselhaus,M.S.;Belcher,A.M.;Kong,J.J.Am.Chem.Soc.2007,129,1516.
Thefirstprecedentfor(n,m)controlofSWNTgrowthis:Lolli,G.;Zhang,L.;Balzano,L.;Sakulchaicharoen,N.;Tan,Y.;Resasco,D.E.J.Phys.Chem.B2006,110,2108.
Resasco,D.E.;Alvarez,W.E.;Pompeo,F.;Balzano,L.;Herrera,J.E.;Kitiyanan,B.;Borgna,A.J.Nanopart.Res.2002,4,131.
Nikolaev,P.;Bronikowski,M.J.;Bradley,R.K.;Rohmund,F.;Colbert,D.T.;Smith,K.A.;Smalley,R.E.Chem.Phys.Lett.1999,313,91.ItshouldbenotedthatFe(CO)5isnottheonlysysteminwhichtheprecursoractsasthemetalcatalystandcarbonsource.Anumberofmetallocenes
354References
(e.g.,ferrocene,cobaltocene,andnickelocene)havealsobeenused;however,theytypicallyresultinMWNTgrowthratherthanSWNTs.Thisismostlikelyduetothelargernumberofcarbonatomsfromcyclopentadienylgroupsthatmustself-assemble,relativetosmallercarbonprecursors(e.g.,CH4,C2H2,etc.)usedforSWNTgrowth.
Wagner,R.S.;Ellis,W.C.Appl.Phys.Lett.19,4,.Forarecentreviewofthesolid–liquid–solid(SLS)andsupercriticalfluid–liquid–solid(SFLS)mechanismsforsemiconductornanowiregrowth,see:Wang,F.;Dong,A.;Sun,J.;Tang,R.;Yu,H.;Buhro,W.E.Inorg.Chem.2006,45,7511.ArecentprecedentfortheepitaxialgrowthofZnOnanowiresatthejunctionofnanowalls:Ng,H.T.;Li,J.;Smith,M.K.;Nguyen,P.;Cassell,A.;Han,J.;Meyyappan,M.Science2003,300,1249.Theword“generally”isused,sincetherearealsoreportsofnanowiregrowthattemperaturesbelowtheeutectic.Forexample,see:Adhikari,H.;Marshall,A.F.;Chidsey,E.D.;McIntyre,P.C.NanoLett.2006,6,318.
Cantoro,M.;Hofmann,S.;Pisana,S.;Scardaci,V.;Parvez,A.;Ducati,C.;Ferrari,A.C.;Blackburn,A.M.;Wang,K.-Y.;Robertson,J.NanoLett.2006,6,1107.Deng,W.-Q.;Xu,X.;Goddard,W.A.NanoLett.2004,4,2331.
Graphite-encapsulatedmetalnanostructuresareofincreasingimportanceformagneticapplicationssuchashigh-densitymagneticrecordingmedia;forexample,see:Flahaut,E.;Agnoli,F.;Sloan,J.;O’Connor,C.;Green,M.L.H.Chem.Mater.2002,14,2553,andreferencestherein.Encap-sulationdominatesoverCNTgrowthatlowtemperaturessincethekineticenergyisnotsufficientforgraphiticislandstoliftoffthecatalystsurface.Hence,encapsulationmayeasilybelimited,whichenhancesCNTgrowth,bymaintainingelevatedtemperatures.Experimentalresultsalsoshowthatsmallcatalystnanoclusters(diameters<2nm)arefreeofgraphiteencapsulationsincetheydonotcontainasufficientnumberofdissolvedCatoms.However,formetalnanostructures>3nmindiameter,calculationssuggestthatgraphiteencapsulationisthermodynamicallypreferredoverSWNTgrowth.ThisisconfirmedbytheempiricalobservationthatSWNTsformonlyoncatalystparticleswithdiameters<2nm.
Lee,Y.H.;Kim,S.G.;Jund,P.;Tomanek,D.Phys.Rev.Lett.1997,78,2393.
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Forarecentreviewofinorganic-basednanotubes,see:Goldberger,J.;Fan,R.;Yang,P.Acc.Chem.Res.2006,39,239,andreferencestherein.
ThetopVLSmechanismwaspredictedusingmoleculardynamicscalculations.TheimagewasreproducedwithpermissionfromDing,F.;Bolton,K.;Rosen,A.J.Phys.Chem.B2004,108,17369.ThemiddleVLSmechanismshowsboth“rootgrowth”(c–d)and“foldedgrowth”(e–g).TheimagewasreproducedwithpermissionfromLee,D.C.;Mikulec,F.V.;Korgel,B.A.J.Am.Chem.Soc.2004,126,4951.Thebottommechanism,predictedbyquantummechanics/molecularmechanics,isoneoftherareexamplesofanatomic-levelpictureofCNTgrowth.TheimagewasreproducedwithpermissionfromDeng,W.-Q.;Xu,X.;Goddard,W.A.NanoLett.2004,4,2331.
http://www.asml.com–arecentpressreleaseindicatesthatEUVlithographywilllikelybeimple-mentedforhighvolumeproductionby2009,withfeaturesizeswellbelow32nm.
Inordertoreducetheadhesionbetweenapolymericmoldandasilicon/quartzmaster,themastersurfaceistypicallymodifiedwithafluorosilane(e.g.,CF3(CF2)6(CH2)2SiCl3(g)).Inaddition,thefinalremovalofthemoldmayalsobecarriedoutinthepresenceofaliquidwithalowviscositysuchasmethanol(solvent-assistedmicromolding(SAMIM)).
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Wewilldiscusstheoperatingprincipleofatomicforcemicroscopy(AFM)andotherscanningforcemicroscopiesinmoredetailinChapter7.Atthispoint,simplythinkofthistechniqueasanalogoustoanantiquatedrecordplayer,inwhichtheneedlegentlytouchesthesurfaceoftherecordtoproducemusic.Similarly,theAFMtipeithergentlytaps,orhoversimmediatelyabove,thesurfaceofaplanarsubstrate.
(a)Piner,R.D.;Zhu,J.;Xu,F.;Hong,S.;Mirkin,C.A.Science1999,283,661.(b)Hong,S.;Zhu,J.;Mirkin,C.A.Science1999,286,523.(c)Hong,S.;Mirkin,C.A.Science2000,288,1808.AverynicereviewofthevariousDPNmethodologiesisprovidedby:Ozin,G.A.;Arsenault,A.C.Nanochemistry:AChemicalApproachtoNanomaterials,RSC:Cambridge,UK,2005,pp.137–157.Zaumseil,J.;Meitl,M.A.;Hsu,J.W.P.;Acharya,B.R.;Baldwin,K.W.;Loo,Y.-L.;Roger,J.A.NanoLett.2003,3,1223.
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TopicsforFurtherDiscussion
1.Fromyourknowledgeofsemiconductorandmetallic0Dnanostructures,thinkabouthowyouwoulddesignacoatingthatwouldsensethesurroundingwallcolorsofwallsandadaptitscolortomatch(i.e.,color-adaptingfurniture!).
2.Mercedes-Benzandothermanufacturersfeaturescratch-resistantclearcoatsasstandardonnewvehicles.Whatarethesecoatingscomprisedof,andhowdoesthispreventsurfacescratching?
3.Carbonnanotubeshavebeentoutedasbeingusefultostorelargeamountsofhydrogengasforfuelcellapplications.Fromtheliterature,howisthiscontained–withintheinteriororadsorbedalongthesidewall?
4.Whataresomeexamplesof“self-cleaning”coatings?Howdothesework?
5.Whatfactorsgovernthetiltangle(betweenthesubstrateandalkylchain)ofaSAMonagoldorsilversurface?
6.ThegrowthmechanismofcarbonaceousnanostructuralmaterialsisgenerallythoughttobeviaVLS.Citesomerecentexamplesofcarbonnanotube/nanofibergrowthattemperaturesfarbelowthemelt-ingpointofthenanoparticulatecatalystspecies.
7.IntheFinkefour-stepmechanismfornanoclustergrowth,explainwhyhighertemperaturesaremostconduciveforthegrowthofmonodispersenanoclusters.
8.ThinkofanewdevicethatyoucouldfabricatebyDPNthatwouldbecomprisedofbothnanoclustersandnanotubes/nanowires.Whataresomepotentialapplicationsforthisdevice?
9.Howwouldyousynthesizefree-standingnanoringsusingboththetop-downandbottom-upapproaches?
10.Howwouldyousynthesizemetaloxidenanotubes,usingasacrificialtemplate?Citeanyrelated
precedentsfromtheliterature.
11.WhataresomeexamplesofNEMSdevicescurrentlyunderdevelopment?12.Whatarethemajordevelopmentaleffortsunderwaytopowernanodevices?
13.HowcouldyoudepositasquaregridofTiO2nanowireswithacontrollablespacingbetweenadjacent
nanowires?
14.DescribehowLbLmaybeusedtodepositcoatingsontocomplex,nonplanarsubstrates.Citeexam-plesforthisstrategyfromtheliterature.
356References
15.Citeexamplesofmaterialsdesignswhereboth“top-down”and“bottom-up”approacheswereused.16.TherearerecentreportsthatN-dopedCNTsarelesstoxicthanSWNTsorMWNTs(e.g.,NanoLett.
2006,6,1609).Providesomelikelyrationalesforthevaryingtoxicologicaleffectsforthesedopednanostructures.
FurtherReading
1.http://www.nanohub.org(freeregistration)
2.Ozin,G.A.;Arsenault,A.C.Nanochemistry:AChemicalApproachtoNanomaterials,RoyalSocietyofChemistryPublishing:Cambridge,U.K.,2005.
3.Dresselhaus,M.S.;Dresselhaus,G.;Eklund,P.C.ScienceofFullerenesandCarbonNanotubes,Academic:NewYork,1996.
4.DendrimersandDendriticPolymers,Frechet,J.M.J.;Tomalia,D.A.eds.,Wiley:NewYork,2001.5.AdvancedMaterials2004,16,15.SpecialissuededicatedtoGeorgeWhitesides.
6.MRSBulletin2006,31,5–May2006.Specialissueonmaterialsformagneticdatastorage.
7.MRSBulletin2005,30,12–December,2005.Specialissueonfabricationofsub-45-nmstructuresforthenextgenerationofdevices.
8.Madou,M.J.FundamentalsofMicrofabrication,2nded.,CRC:BocaRaton,2002.
9.AlternativeLithography–UnleashingthePotentialsofNanotechnology,Sotomayor-Torres,C.M.ed.,Kluwer/Plenum:NewYork,2003.
10.Wilson,M.;Kannangara,K.;Smith,G.;Simmons,M.Nanotechnology:BasicScienceandEmerging
Technologies,CRC:BocaRaton,2002.
11.CarbonNanotubes,O’Connell,M.J.ed.,CRC:BocaRaton,2006.
12.CarbonNanotubes:Synthesis,Structure,PropertiesandApplications,Smalley,R.E.;Dresselhaus,
M.S.;Dresselhaus,G.;Avouris,P.eds.,Springer:NewYork,2001.
13.D.Tomanek.http://www.pa.msu.edu/cmp/csc/nanotube.html.TheNanotubeSite(Internetrefer-ence).
14.Lyshevski,S.E.;Lyshevski,S.E.MEMSandNEMS:Systems,Devices,andStructures,CRC:Boca
Raton,2002.
15.Nanoparticles:FromTheorytoApplication,Schmid,G.ed.,Wiley:NewYork,2004.
16.Rotello,V.Nanoparticles:BuildingBlocksforNanotechnology(NanostructureScienceandTech-nology),Springer:NewYork,2004.
17.TheChemistryofNanomaterials:Synthesis,Properties,andApplications,Rao,C.N.R.;Muller,A.;
Cheetham,A.K.eds.,Wiley-VCH:Berlin,2004.
18.Wolf,E.L.NanophysicsandNanotechnology,Wiley-VCH:Berlin,2004.
http://www.springer.com/978-1-4020-6119-6
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