Table Of ContentMaterialsScienceandEngineeringR70(2010)63–91
ContentslistsavailableatScienceDirect
Materials Science and Engineering R
journal homepage: www.elsevier.com/locate/mser
Controlled growth and modification of vertically-aligned carbon nanotubes
for multifunctional applications
Hao Chena, Ajit Royb, Jong-Beom Baekc, Lin Zhud, Jia Qua,*, Liming Daid,*
aWenzhouMedicalCollege,270XueyuanRoad,Wenzhou,Zhejiang325027,China
bAirForceResearchLaboratory,AFRL/RX,Wright-PattersonAFB,OH45433,USA
cInterdisciplinarySchoolofGreenEnergyandInstituteofAdvancedMaterialsandChemicals,UlsanNationalInstituteofScienceandTechnology(UNIST),#100,Banyeon,
Ulsan689-798,RepublicofKorea
dDepartmentofChemicalEngineeringandDepartmentofMacromolecularScienceandEngineering,CaseSchoolofEngineering,CaseWesternReserveUniversity,
10900EuclidAvenue,Cleveland,OH44106,USA
AR TI CLE I NFO ABS TRA CT
Articlehistory: Vertically-aligned carbon nanotubes possess many advantages for a wide range of multifunctional
Availableonline1July2010 applications.Alongwiththecontrolledgrowthofaligned/micropatternedcarbonnanotubes,surface
modificationofvertically-alignedcarbonnanotubesareessentialinordertomeetspecificrequirements
Keywords: demandedforparticularapplications.Whilemanyinnovativesyntheticmethodshavebeendeveloped
Carbonnanotube forcontrolledgrowthofvertically-alignedmultiwalledandsingle-walledcarbonnanotubes,various
Alignment interestingphysicalandchemicalapproacheshaverecentlybeendevisedforfunctionalizationofthe
Patterning
constituentcarbonnanotubesinvertically-alignedcarbonnanotubearrayswiththeiralignmentbeing
Functionalization
largely retained. In this article, recent developments in the controlled growth and modification of
Application
vertically-alignedcarbonnanotubesformultifunctionalapplicationsarereviewed.
(cid:2)2010ElsevierB.V.Allrightsreserved.
Contents
1. Introduction...................................................................................................... 64
2. Controlledgrowth................................................................................................. 64
2.1. Vertically-alignedmultiwalledcarbonnanotubes(VA-MWNTs) ....................................................... 64
2.1.1. VA-MWNTsbytemplate-synthesis....................................................................... 64
2.1.2. VA-MWNTsbytemplate-freegrowth..................................................................... 65
2.1.3. VA-MWNTswithspecialarchitectures.................................................................... 65
2.1.4. Superlongvertically-alignedcarbonnanotubes(SLVA-CNTs).................................................. 70
2.2. Vertically-alignedsingle-walledcarbonnanotubes(VA-SWNTs)....................................................... 70
2.2.1. VA-SWNTsbytemplate-freegrowth ..................................................................... 70
2.2.2. PreferentialgrowthofsemiconductingVA-SWNTs.......................................................... 70
2.3. VA-MWNTmicropatterns...................................................................................... 71
2.3.1. VA-MWNTmicropatternsbyphotolithography............................................................. 71
2.3.2. VA-MWNTmicropatternsbysoft-lithography.............................................................. 72
2.3.3. VA-MWNTmicropatternsbyplasmapatterning............................................................ 73
2.3.4. VA-MWNTmicropatternsbye-beamlithography........................................................... 74
2.3.5. VA-MWNTmicropatternswith3Darchitectures............................................................ 74
2.4. MulticomponentVA-CNTmicropatterns.......................................................................... 75
2.4.1. MulticomponentVA-CNTmicropatternsbydirectgrowth.................................................... 76
2.4.2. MulticomponentVA-CNTmicropatternsbycontacttransfer .................................................. 77
3. Controlledmodification............................................................................................. 77
3.1. ModificationofVA-CNTsbyplasmaandphotochemicalactivation..................................................... 77
3.2. ModificationofVA-CNTsbyelectrochemicaldeposition ............................................................. 79
3.3. ModificationofVA-CNTsbychemicalfunctionalizationanddoping.................................................... 81
* Correspondingauthors.
E-mailaddresses:[email protected](J.Qu),[email protected](L.Dai).
0927-796X/$–seefrontmatter(cid:2)2010ElsevierB.V.Allrightsreserved.
doi:10.1016/j.mser.2010.06.003
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4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Controlled growth and modification of vertically-aligned carbon
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nanotubes for multifunctional applications
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13. SUPPLEMENTARY NOTES
14. ABSTRACT
Vertically-aligned carbon nanotubes possess many advantages for a wide range of multifunctional
applications. Along with the controlled growth of aligned/micropatterned carbon nanotubes, surface
modification of vertically-aligned carbon nanotubes are essential in order to meet specific requirements
demanded for particular applications. While many innovative synthetic methods have been developed for
controlled growth of vertically-aligned multiwalled and single-walled carbon nanotubes, various
interesting physical and chemical approaches have recently been devised for functionalization of the
constituent carbon nanotubes in vertically-aligned carbon nanotube arrays with their alignment being
largely retained. In this article, recent developments in the controlled growth and modification of
vertically-aligned carbon nanotubes for multifunctional applications are reviewed.
15. SUBJECT TERMS
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF
ABSTRACT OF PAGES RESPONSIBLE PERSON
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Prescribed by ANSI Std Z39-18
64 H.Chenetal./MaterialsScienceandEngineeringR70(2010)63–91
3.4. ModificationofVA-CNTsbypolymermasking..................................................................... 84
4. Concludingremarks................................................................................................ 89
Acknowledgements................................................................................................ 89
References ....................................................................................................... 89
1. Introduction applications involving CNTs. However, it is very rare that CNTs
withdesirablebulkpropertiesalsopossessthesurfacecharacter-
Carbon nanotubes (CNTs), including single- and multi-walled istics required for certain specific applications. Apart from the
structures,areanattractiveclassofnanomaterials.Whileasingle- controlled growth of aligned/micropatterned CNTs, therefore,
walledcarbonnanotube(SWNT)maybeconceptuallyviewedasa surfacemodificationandinterfacialengineeringarealsoessential
graphene sheet that is rolled into a nanoscale tube form, a in making functional CNT materials of good bulk and surface
multiwalled carbon nanotube (MWNT) consists of additional propertiesasdemandedforspecificapplications[36–42]. Inthis
graphenecoaxialtubesaroundtheSWNTcore[1,2].SWNTsshow article, recent developments in the controlled growth and
electric properties that are not shared by their multi-walled modification of both VA-MWNTs and VA-SWNTs are reviewed,
counterparts.Forinstance,thebandgapofSWNTscanvaryfrom alongwithdiscussionoftheirmultifunctionalapplications.
zerotoabout2eVandtheycanexhibitmetallicorsemiconducting
behavior, whereas MWNTs are zero-gap metals [1,2]. This is 2. Controlledgrowth
because the graphene sheet in SWNTs can be rolled up with
varying degrees of twist along its length and SWNTs can have a Carbon nanotubes synthesized by conventional techniques
varietyofchiralstructures[1–6].Dependingontheirdiameterand usually exist in a randomly entangled form [1–6]. In view of
the chirality of the orientation of graphene rings along the additionaladvantagesassociatedwithVA-CNTsformanyapplica-
nanotubelength,SWNTsmayexhibitsemiconductingormetallic tions,severalchemicalvapordeposition(CVD)methodshavebeen
behavior. SWNTs also are more homogenous than their multi- developedforlarge-scaleproductionofvertically-alignedsingle-
walledcounterparts,atleastintermsoftheirdiameters((cid:2)1nm). walled, multiwalled, and super-long carbon nanotube arrays
Therefore, SWNTs are the promising candidate for micro-/nano- [5,6,43–46].TheseVA-CNTarrayscanbetransferredontovarious
electronics.Apartfromthesemiconductingproperties,character- substrates of particular interest in either a patterned or non-
istic of certain SWNTs, both SWNTs and MWNTs possess a high patternedfashion.Thewell-alignedstructureoffersadvantagesfor
surface area per unit weight, good mechanical properties, high notonlyanefficientdeviceconstructionbutalsocontrolledsurface
electrical conductivity at the metallic state, and high thermal modification. The aligned/micropatterned growth of VA-CNTs is
conductivity/stability [1–6]. These interesting properties make reviewedinthissection,whilethemodificationofVA-CNTswillbe
CNTs very attractive for a variety of potential applications, discussedinSection3.
including as conductive materials [7–11], electromagnetic and
microwaveabsorbingcoatings,high-strengthcomposites[12–14] 2.1. Vertically-alignedmultiwalledcarbonnanotubes(VA-MWNTs)
and fibers [15–22], sensors [23,24], field emission displays [25],
energystorageandenergyconversiondevices[26–32],radiation 2.1.1. VA-MWNTsbytemplate-synthesis
sourcesandnanometer-sizedsemiconductordevices[33,34],and Inordertoconstructananotubefieldemitter,deHeeretal.[47]
interconnects[33,35].Formostoftheabove-mentioned,andmany first made an ethanol dispersion of arc-produced carbon nano-
other applications, it is highly desirable to prepare aligned/ tubes.Theseauthorsthenpassedthenanotubedispersionthrough
micropatterned CNTs. Particularly, vertically-aligned CNTs (VA- an aluminum oxide micropore filter, leading to nanotubes
CNTs)canprovideawell-definedlargesurfacearea,andtheycan perpendicularly aligned on the filter surface. The resultant
be readily incorporated into device configurations. Besides, the perpendicularly-aligned nanotubes can be transferred onto the
alignedgrowthcanproduceCNTmaterialsfree-fromamorphous cathode substrate in a field emitting device. Similar porous
carbonswithaverynarrowrangeoftubelengthsanddiameters, membranes(e.g.mesoporoussilica,aluminananoholes)havealso
whichisanadditionaladvantageforsomeoftheaforementioned, beenusedasatemplatefortheso-calledtemplate-synthesisofVA-
andmanyother,applications[1,2]. CNTs [48–52]. Inparticular,Liet al.[53] prepared thefirst large
Ontheotherhand,ithasalsobeenrecognizedthatsurfaceand/ scale VA-MWNT array by CVD deposition of acetylene on iron
[(Fig._1)TD$FIG]
or interfacial properties are of paramount importance for most nanoparticlesembeddedinmesoporpoussilicaat7008C(Fig.1).
Fig.1.(A)High-magnificationscanningelectronmicroscope(SEM)imageofcarbonnanotubesgrowingoutfromthemesoporousiron/silicasubstrateandforminganarray.
Thesecarbonnanotubeshavediametersof(cid:2)30nm.Spacingsbetweenthenanotubesare(cid:2)100nm.Mostofthecarbonnanotubesarealignedperpendicularlyonthesilica
surface.(B)SEMimageofthemesoporousiron/silicasubstratebeforecarbondeposition.Pores(cid:2)30nmindiameteraredistributedonthesubstrateandareseparatedby
(cid:2)100nm.Manyporeshaverelativelyregularcircularopenings(adaptedfromRef.[53]).
H.Chenetal./MaterialsScienceandEngineeringR70(2010)63–91 65
Thegrowthdirectionofthenanotubescouldbecontrolledbythe sourcerequiredforthenanotubegrowth,isofparticularinterest,
orientationoftheporesfromwhichnanotubesgrow. as it is a one-step process involving no pre-preparation of
Similarly, Li and Xu [48] have also reported the controlled catalyst nanoparticles on the substrate used for the nanotube
growthofVA-MWNTsbythepyrolysisofhydrocarbononanickel growth. In this context, Dai and co-workers [67–71] have
catalyst embedded in a porous silicon substrate. In these cases, prepared large-scale aligned MWNTs perpendicular to the
porous silicons containingmicro-,meso-, and macro-poreswere substrate surface by pyrolysis of iron (II) phthalocyanine,
producedbyelectrochemicallyetchingthecrystallinesiliconwafer FeC N H (designated as FePc hereafter), in an Ar/H atmo-
32 8 16 2
(astheanode)inanaqueoushydrofluoricacid(HF)solution(using spherewithadualfurnacefittedwithindependenttemperature
a Pt wire as the cathode). Generally speaking, anodic aluminum controllers.Theas-synthesizednanotubesalignalmostnormalto
oxide films can be used for producing VA-CNTs with uniform the substrate surface and the constituent CNTs have a fairly
diametersandlengthsbythepyrolysisoforganicmoleculesinto uniform tubular length and diameter with a well-graphitized
the well-defined nanopores either with or without a catalyst multiwall structure [69]. Depending on whether or not the
[54,55]. catalytic particle is lifted up with the growing nanotube, two
growth mechanisms, namely ‘‘tip-growth’’ and ‘‘base-growth’’,
2.1.2. VA-MWNTsbytemplate-freegrowth have been proposed [1,2,69]. With the FePc CVD system, we
Withoutusingthetemplatepores,manygroupshavereported found that the growth of carbon nanotubes start from the iron
the growth of VA-MWNTs [2]. For instance, Rao et al. [56] have catalystparticles,whichconsistsoflargerparticlesandsmaller
prepared VA-MWNT arrays by high-temperature (ca. 9008C) ones [69]. While the smaller iron particles remaining on the
pyrolysis of ferrocene, which contains both metal and carbon substratearecatalyticallyactivetosupportthenanotubegrowth,
source required for the nanotube growth. In a separate but thelargerironparticlesaremainlyresponsibleforproducingthe
somewhat related study, Kamalakaran et al. [57] reported the carbon atomistic species from FePc vapors required for the
formation of VA-MWNT arrays by pyrolysis of a jet solution of growth of the nanotubes. The head-on contact between two
ferrocene and benzene in an argon atmosphereat relativelylow adjacent large iron particles in the growth front facilitates the
temperatures (e.g. 8508C). As reviewed by one of the pioneers verticalalignment[69].
(Meyyappanetal.)[58],plasmaenhancedCVD(PECVD)hasbeen WiththeavailabilityofVA-MWNTsinrelativelylargequanti-
widely used to produce VA-CNTs. The PECVD even allowed the ties, Zhang et al. [16,17] have recently produced CNT yarns and
growth of individually aligned, free-standing, vertical CNTs [58]. sheetsfromVA-MWNTforestsinscalablequantitiesbycontinuous
Ren et al. [59] synthesized large arrays of VA-MWNTs by radio- high-ratespinning(Fig.3).Onemeterofyarncantypicallybemade
frequency sputter-coating a thin nickel layer onto display glass, inafewminutes.TheCNTyarnsandsheetsarepotentiallyuseful
followedbyplasma-enhancedhotfilamentCVDofacetyleneinthe formakingtransparentand highlyconductingelectrodes,planar
presenceofammoniagasbelow6668C.Fig.2showsSEMimagesof sources of polarized broadband radiation, flexible organic light-
theresultantVA-MWNTarrays.Theseauthorshavealsoprepared emitting diodes, advanced sensors and actuators, microwave
VA-MWNTs on polished polycrystalline and single crystal nickel bondingofplastics,andthinfilmloudspeakers,tomentionbuta
substrates by the plasma enhanced hot filament CVD at fewapplications[17,44,72].CNTfibershavealsobeenpreparedby
temperatures below 6668C [60]. The plasma density, acetylene othermethods[15,73–75].
to ammonia gas ratio, and gas flow rates were found to play
importantrolesinregulatingthediameteranduniformityofthe 2.1.3. VA-MWNTswithspecialarchitectures
resulting aligned CNTs. In addition to the aforementioned and As can be seen from the above discussion, a large variety of
someotherplasmaenhancedCVDtechniques[61,62],microwave single-component VA-CNT materials have been reported for
plasmaenhancedCVDmethodhasalsobeenreportedforsynthesis variousmultifunctionalapplications[2,5,6].Owingtotheirunique
ofVA-MWNTs[63–65].UnliketheplasmaenhancedgrowthofVA- one dimensional electronic structure, CNTs offer particular
MWNTs,AvigalandKalishetal.[66]reportedanewmethodforthe advantagesasmolecularwiresforthedevelopmentofnanotube
aligned growth by applying an electric field to a Co-covered nanodevices,includingnanotubesensorsandotheroptoelectronic
substrateinaregularcold-wallchemicalvapordepositionreactor systems.Thereisalsoapressingneedtointegratemulticomponent
(withnoplasmaapplied)containingflowingmixtureofmethane nanoscale entities into multifunctional systems and to connect
andargonat8008C.TheyfoundthatVA-MWNTsformedwhena these nanosystems to the micro/macro world. Although the
positivebiasisappliedtothesubstrate. connection from the nanoworld to the outside world has been
The formation of aligned carbon nanotubes from organic- one of the long-standing problems in nanotechnology and still
metalcomplexes,containingboththemetalcatalystandcarbon remainsabigchallenge,afewinnovativeroutestotheintegration
[(Fig._2)TD$FIG]
Fig.2.(A)SEMmicrographofcarbonnanotubesalignedperpendiculartothesubstrateoverlargeareas.(B)Enlargedviewof(A)alongthepeelededgeshowingdiameter,
length,straightness,anduniformityinheight,diameter,andsitedensity(adaptedfromRef.[59]).
[(Fig._3)TD$FIG]
66 H.Chenetal./MaterialsScienceandEngineeringR70(2010)63–91
Fig.3.SEMimagesshowing(a)smallCNTbundlessimultaneouslypulledandtwistedfromtheVA-CNTforesttoformCNTyarns(adaptedfromRef.[16]).(b)(left)Photograph
ofatransparentself-supportingMWNTsheet,and(right)SEMimagesshowingtheconversionofaVA-MWNTforestintoasingleMWNTsheetandatwo-dimensionally
reinforcedstructurefabricatedbyoverlayingfournanotubesheetswitha458shiftinorientationbetweensuccessivesheets(adaptedfromRef.[17]).
ofCNTsintomultidimensionalandmulticomponentsystemshave supported by a SiO /Si substrate used for the nanotube growth,
2
recentlybeendevised[2]. was finger pressed from the Si side onto a vertically positioned
glassslide.Thenanotubesinthisfilmhavediametersrangingfrom
2.1.3.1. VA-MWNTarrayswithcurly-entangledend-segmentsatthe 10to15nmwithatubelengthofabout150mmandatubedensity
top. Along with others, Qu et al. [76] and Zhao et al. [77] have of(cid:2)1010–1011cm(cid:4)2(Fig.4BandC).Abookof1480gwasheldonto
synthesized VA-CNT arrays with a straight aligning body and a a thin wire that was pre-glued on the back side of the SiO /Si
2
curly-entangled end-segment at the top by a CVD process on a substrate.Anoveralladhesionforceof90.7N/cm2wascalculated
SiO /Siwafer.DuringthepyrolyticgrowthoftheVA-MWNTs,the fortheVA-MWNTdryadhesivefilmshowninFig.4A–whichis
2
initially formed nanotube segments from the ‘‘base growth’’ almosttentimesthatofageckofoot.Similaradhesionbehaviors
processgrewinrandomdirectionsandforma‘‘coiled/entangled’’ were observed for the VA-MWNT dry adhesive against other
nanotube top layer to which the underlying straight nanotube substrates with different flexibilities and surface characteristics,
arrays then emerged. Followingearlierattemptsto mimicgecko includinggroundglassplates,polytetrafluoroethylene(PTFE)film,
feet using microfabricated arrays of VA-CNTs [77–80], the VA- rough sandpaper, and poly(ethylene terephthalate) (PET) sheet
MWNTarraysproducedbyalow-pressureCVDprocesswithcurly- [76].
entangledend-segmentsatthetopweredemonstratedtobeideal AsshowninFig.4D,thenormaladhesionforceforVA-MWNT
for mimicking gecko-feet hairs [76]. In particular, Qu et al. [76] filmswiththetubelengthrangingfromapproximate10–150mm
havefoundthatthe‘‘coiled/entangled’’nanotubetoplayercould increasedslightlyfrom10to20N/cm2.However,thecorrespond-
createananisotropicadhesionforcethroughthesidewallcontact ingshearadhesionforceincreasedfrom10to100N/cm2overthe
withvarioussubstrates.Itisbelievedthatthedifferencebetween same range of nanotube lengths. The shear adhesion force is
normaladhesionandshearadhesionfacilitatesthegeckotoswitch typically several times stronger than the corresponding normal
betweenattachmentanddetachmentasitmoves. adhesionforceataconstantnanotubelengthoverabout10mm.
TodemonstratetheadhesionperformanceoftheVA-MWNTs,a The high shear adhesion force of the VA-MWNT dry adhesive
[(Fig._4)TD$FIG]small piece of the VA-MWNT film (4mm(cid:3)4mm, Fig. 4A), ensuresastrongadhesiontothetargetsurfaceforhangingheavy
Fig.4.(A)Abookof1480ginweightsuspendedfromaglasssurfaceusingVA-MWNTssupportedonasiliconwafer.Thetop-rightsquaredareashowstheVA-MWNTarray
film,4mm(cid:3)4mm.(BandC)SEMimagesoftheVA-MWNTfilmunderdifferentmagnifications.(D)Nanotubelength-dependentadhesionforceofVA-MWNTfilmsattached
ontothesubstratewithapreloadingof2kg.Theverticalandhorizontalbarsrepresentthedeviationsoftheforceandnanotubelengthmeasuredformorethan20samplesof
thesameclass,respectively.(E)AdhesionstrengthofVA-MWNTswithlength100(cid:5)10mmatdifferentpull-awaydirections.Theredarrowsrepresentstheaverageforces
measuredformorethan20samples,whilethetwoperpendicularbluedotlinesdefinepossibledeviationsoftheforcemeasuredfordifferentsamplesofthesameclass.The
nanotubesandsubstratesshownin(E)arenottoscale(adaptedfromRef.[76]).
[(Fig._5)TD$FIG]
H.Chenetal./MaterialsScienceandEngineeringR70(2010)63–91 67
Fig.5.(A)SEMand(B)schematicdiagramforthemorphologicalchangeofVA-MWNTarraysduringadhesionmeasurements:(A)Top(a–f)andside(g–l)viewsofVA-MWNT
filmswithdifferentlengthbefore(a–c,g–i)andafter(d–f,i–l)adhesionmeasurements.(a,d,g,j:(cid:2)5mm;b,e,h,k:(cid:2)70mm;c,f,i,l:(cid:2)150mm).Thearrowsunderneaththe
wordsof‘‘After’’indicatethesheardirectionduringtheshearadhesionforcemeasurements.(B)Preloading(a);attachmentoftheVA-MWNTarrayontotheglasssubstrate
(b);shearadhesionforcestretchingthenonalignednanotubesonthesubstratetoformthe‘‘line’’contact(c);normaladhesionforceleadingtothenonalignednanotubes
‘‘point-by-point’’peel-offfromthesubstrate(d).Insetshowsthestructuresimilaritybetweenthecross-sectionviewsoftheVA-MWNTsandgecko’salignedelastichairs
(adaptedfromRef.[76]).
objects along the shear direction, whilst a much weaker normal ‘‘point-by-point’’detachingprocess(Fig.5B(d)),requiringamuch
adhesionforceallowsthenanotubefilmtobereadilydetachedin lower force than that for pulling off the entire nanotube array
the normal direction. The VA-MWNT arrays were repeatedly (Fig.4D).Thisanisotropicforcedistributionensuresstrongbinding
attachedanddetachedfromtheglasssurface,andthesupported alongthesheardirectionandeasyliftinginthenormaldirection.
weightdidnotdecrease[76].Toelucidatetheangular-dependence Thisfindingshouldopenmanytechnologicalapplications,ranging
oftheadhesionforces,Quetal.[76]measuredthepull-offforcein fromnewtypesofathleticshoesandcartiresthathaveanunusual
various pull-away directions. The decrease in the pull-off force grip, to creating window-clinging suits like Spider-Man’s, to
withincreasingpull-awayangleshowninFig.4Eindicatesthatthe sealingpackagestobondingelectronicandevenaerospacevehicle
shearadhesionforceismuchstrongerthanthenormaladhesion parts in outer space where traditional polymer-based adhesives
force. wouldbedriedupandfailedunderthevacuumenvironment.
Qu et al. [76] further examined the morphology of the top
surface and cross-sectional area of the VA-MWNT films with 2.1.3.2. VA-MWNTarrayswithY-shapedatthetop. Usingbranched
different tube lengths before and after the shear adhesion nanochannel alumina templates, Li et al. [81] have produced Y-
measurements. As expected, randomly entangled nanotube seg- shapedcarbonnanotubesbythepyrolysisofmethaneovercobalt-
mentsarisingfromtheinitialstageofthe‘‘base-growth’’process coveredmagnesiumoxide(Fig.6).Inthiscase,theseauthorsfirst
wereobservedonthetopsurfaceoftheas-synthesizedVA-MWNT preparedthetemplatewithY-branchednanochannelsbyanodiz-
arrays(Fig.5A(a–c)).Aftertheshearadhesionforcemeasurements, ing a highly pure aluminum sheet in 0.3M oxalic acid at 108C
however,itwasfoundthatthetoplayeroftherandomlyentangled under a constant voltage of 50V for 15h, which resulted in an
nanotubesegmentsbecamehorizontallyaligned(Fig.5A(d–f)).The hexagonal array of pores near the aluminum surface. After
degreefortheshear-inducedhorizontalalignmentincreasedwith chemicallyremovingtheoriginalfilm,asecondanodizationwas
increasingthealignednanotubelength(Fig.5A(d–f)).Beforethe performed under the same conditions, typically for 30min. The
testing,thenanotube‘‘trunks’’areuniformlyaligned(Fig.5A(g–i)). anodizationvoltagewasthenreducedtoabout35V.Becausethe
However, after binding on the wall, the vertically-aligned pore cell diameter is proportional to the anodization voltage,
nanotube ‘‘trunks’’ were tilted along the shear direction reducingthevoltagebyafactorof1/2resultedintwiceasmany
(Fig.5A(j–l)).Thesignificantincreaseintheshearadhesionforce poresappearinginordertomaintaintheoriginaltotalareaofthe
withincreasingalignednanotubelengthobservedinFig.4Dseems template, and nearly all the pores branched into two, smaller-
tobedirectlyrelatedtothepresenceofthehorizontally-aligned diameterpores.Asaconsequence,theresultingtemplateconsisted
nanotube segments on the top surface of the VA-MWNT dry ofparallelY-branchedporeswithstemsabout90nmindiameter
adhesive films, which formed the tube-length-dependent hori- and branches about 50nm in diameter. Y-shaped CNTs have
zontally-alignedstructureundershear. potential applications in electronic and micro-/nano-fluidic
The SEM observations are consistent with the following devices(e.g.electroninterferometry)[82,83].
process. During the initial contact, the top nonaligned nanotube
segments(Fig.5B(a))adoptedrandomly-distributed‘‘line’’contact 2.1.3.3. VA-MWNT arrays with ZnO nanoparticles at the top. 1D
with the glass substrate (Fig. 5B(b)). Upon shear adhesion force heterojunction structures based on nanomaterials are of signifi-
measurement (Fig. 5B(c)), the applied shear force caused the cance to both fundamental nanoscience [2,84] and potential
nonalignednanotubesegmentstoalignalongthesheardirection applications in nanoscale systems, including various new elec-
on the glass substrate (Fig. 5B(c)) and the vertically-aligned tronic and photonic nanodevices [85–88]. Consequently, consid-
nanotube ‘‘trunks’’ to tilt along the shear direction (Fig. 5A(j–l)), erable effort has been made in recent years to devise and
leading to a predominant aligned ‘‘line’’ contact with the glass characterize various heterojunctions between different low-
surface(Fig.5A(d–f)).Duringthenormaladhesionforcemeasure- dimensional nanomaterials [89–91]. Examples include the syn-
ments,however,thetopnonalignednanotubesegmentscontacted theses of multiwalled CNT–zinc sulfide heterojunctions by a
withtheglasssubstratewerepeeledfromthesubstratethrougha combination of ultrasonic and heat treatments [92], CNT–silicon
[(Fig._6)TD$FIG]
68 H.Chenetal./MaterialsScienceandEngineeringR70(2010)63–91
Fig.6.Left:(a)SEMimageofaY-branchednanochanneltemplate(scalebar:1mm).TheinsetshowswheretheYbranchesstarttogrow.(b)Top-viewSEMimageofcarbon
nanotubesalignedinthetemplateafterion-millingofamorphouscarbononthesurface(scalebar,100nm).Thenanotubediameterislargerthantheoriginalporeowingto
thermalexpansionofthetemplateduringgrowth.Topinset,stempartoftheY-junctiontubes.Bottomleftinset,close-upoftheregionbetweenstemandbranchportionsstill
embeddedinthetemplate.Bottomrightinset,close-upofthetopofthenanotubeinitshexagonalcell.Right:transmissionelectronmicroscope(TEM)imageof(a)theY-
junctiontube(scalebar,50nm)withastemofabout90nmandbranches50nmindiameter.(b)Y-junctionformedbyusinghigheranodizationvoltages,resultinginstemsof
about100nmandbranches60nmindiameter(scalebar:200nm).(c)High-resolutionTEMimageofatypicalY-junctionnanotubewallshowinggraphiticmultiwall
structure(scalebar,5nm).Insetshowsthepartofthetubethatwasimaged(adaptedfromRef.[81]).
nanowireheterojunctionsbylocalizingasuitablemetalcatalystat CNTs [2,4,94]. However, the growth of VA-CNT heterojunction
the end of a preformed nanotube or nanowire [85], and SWNT– arrays was a big challenge. Liu et al. [95] have reported the
gold nanorod heterojunctions through the selective solution synthesis of large-scale vertically aligned CNT–ZnO heterojunc-
growthofAunanorodsonanSWNTstructure[93].Amongthem, tions simply by water-assisted chemical vapor deposition of
CNT-based1Dheterojunctionsareofparticularinterestbecauseof carbononazincfoil,whichactsasboththesubstratefortheVA-
the unique molecular geometry as well as excellent electronic, CNT growth and zinc source for the formation of the ZnO
thermal, and mechanical properties intrinsically associated with nanostructures. Water, as a weak oxidizer, provides the oxygen
[(Fig._7)TD$FIG]
Fig.7.(a)Low-magnificationSEMimageofaverticallyalignedCNT–ZnOheterojunctionarray.(bandc)Asfor(a),underhighermagnification.(d)TEMimageofatypicalCNT–
ZnOheterojunctionand(e)aselected-areaelectrondiffraction(SAED)patternofaZnOtip(adaptedfromRef.[95]).
[(Fig._8)TD$FIG]
H.Chenetal./MaterialsScienceandEngineeringR70(2010)63–91 69
Fig.8.Cross-sectional(A)andside-view(B)SEMimagesoftheCNT-CFs.Scalebarsfor(AandB):5mm.(C)Amperometricresponsestosuccessiveadditionsof3mMglucose
for(a)theglucose-oxidase-attachedalignedCNT–CFelectrode,and(b)theCFelectrodeafterhavingbeensubjectedtothesameelectrochemicaloxidationandglucose
oxidaseimmobilizationprocesses.TheinsetshowsaplotforcurrentincrementagainstglucoseconcentrationfortheCNT–CFglucosesensor(adaptedfromRef.[98]).
sourcefortheformationofZnOnanostructuresandalsoenhances pyrolysis of FePc, and demonstrated their potential applications
CNTgrowth[43]. in various electrochemical systems. Fig. 8A and B shows SEM
Fig. 7a shows a low-magnification SEM image of the as- images for CFs with a diameter of 7mm surrounded by FePc-
synthesized CNT–ZnO sample, revealing a large-scale vertically- generated VA-MWNTs in either a patterned or nonpatterned
aligned array. Although the corresponding SEM images under fashion. For these hybrid CF-CNT coaxial structures, the carbon
highermagnificationinFig.7bandcshowsomemisalignmentat microfiber can be used as a microelectrode to connect the
thetopofthenanotubearray,Fig.7bclearlyshowstheCNT–ZnO surrounding VA-CNTs to the outside world. Consequently, the
heterojunctionwithaZnOcrystal(brightspot)attachedonthetip resultant microsized CFs sheathed with VA-CNTs could offer a
of each of the constituent VA-CNTs. Fig. 7c further reveals that useful platform for the development of multidimensional and
approximately 40% of the ZnO nanoparticles are of hexagonal multifunctional nanomaterials and devices of practical signifi-
morphology.ThecorrespondingTEMimageinFig.7dshowsthat cance.TodemonstratethepotentialoftheCNT-sheathedCFsfor
the CNT thus prepared is multiwalled, and that the ZnO biosensing,Quetal.[98]haveattachedglucoseoxidasemolecules
nanoparticleisintimatelyconnectedtothealignedCNTstructure. onto electrochemically-induced carboxylic groups on the CF-
Fig.7eshowstheselected-areaelectrondiffraction(SAED)pattern supportedCNTsviaclassicalcarbodiimidechemistry.Theglucose
oftheZnOnanoparticle,whichindicatesthattheas-preparedZnO oxidase-grafted VA-CNTs on the CF were then used to detect
nanoparticleissinglecrystallineandcanbeindexedashexagonal glucose in a 0.1M phosphate buffer solution (pH 7.4) at room
wurtzite ZnO [96]. The intimately connected heterojunctions temperature by electrochemically probing hydrogen peroxide
betweenthealignedCNTsandZnOnanostructuresthusprepared generatedfromglucoseoxidationwithglucoseoxidase.Ampero-
possessinterestingoptoelectronicpropertiesattractiveformany metricresponsesfortheglucoseoxidase-immobilizedVA-CNTson
potentialapplications[95],includingtheiruseasfieldemittersand theCFsubstrateandthepristineCFelectrodetoeachsuccessive
electro-opticsensorsanddetectors. additionof3mMglucoseareshowninFig.8C.Whilethereisno
obvious current change with the addition of glucose for the CF
2.1.3.4. CarbonfiberssheathedwithVA-MWNTarrays. Thegrowth electrode(curve(b)ofFig.8C),astepwiseincreaseinthecurrent
of VA-CNTs around carbon-fibers provides a new class of signaluponeachsuccessiveadditionofglucosewasobservedfor
multidimensional and multicomponent nanomaterials with itsVA-CNTcoatedcounterpart(curve(a)ofFig.8C).Theinsetin
well-defined surface and interfacial structures attractive for a Fig.8Cshowsthecurrentincrementwiththeglucoseconcentra-
widerangeofpotentialapplications.Thostensonetal.[97]were tion for the CNT–CF electrode, which shows a pseudo-linear
the first to modify the surface of pitch-based carbon fiber by relationship indicating a high sensitivity and reliability. The
growing CNTs directly on carbon fibers. More recently, Qu et al. electrochemical detection of glucose using the glucose oxidase-
[98]havedevelopedaneffectiveapproachforthepreparationof containingCNTelectrodeinvolvesoxidationofhydrogenperoxide
multidimensional hybrid structures with individual microsized fromglucoseoxidation[98].Giventhatmanyenzymaticreactions
carbon fibers (CFs) sheathed by VA-MWNTs generated via areassociatedwiththegenerationofhydrogenperoxide,theabove
[(Fig._9)TD$FIG]
Fig.9.(A)SEMandTEMimagesofSLVA-SWNTforestspreparedbythewater-assistgrowthmethod(adaptedfromRef.[43]).(B)(a)Adigitalphotograph,(b)SEMimage,and
(c)TEMimageofthesuper-longdouble-walledCNTs(adaptedfromRef.[46]).
70 H.Chenetal./MaterialsScienceandEngineeringR70(2010)63–91
methodology should be applicable to sense many other biologi- witha10-nm-thickAllayer.TheAlcoatingwasusedtoeffectively
callyimportantsubstances. prevent the Fe catalyst from aggregation for supporting the PE-
CVDgrowthofVA-SWNTsbyquicklymoving(<5s)thecatalyst-
2.1.4. Superlongvertically-alignedcarbonnanotubes(SLVA-CNTs) coated SiO /Si wafer into the center of a plasma-enhanced tube
2
RecentadvancesinCVDtechniqueshavefacilitatedthegrowth furnace under a mixture gas of H /CH . Fig. 10a shows a typical
2 4
of super-long vertically aligned carbon nanotubes (SLVA-CNTs) SEMimageofVA-SWNTarraysproducedbythecombinedPE-CVD
with millimiter-order lengths (up to 10mm). Despite some and fast heating method. As can be seen in Fig. 10a, the as-
controversies in the role of H O molecules [99,101] in the synthesized nanotubes aligned almost normal to the substrate
2
water-assisted CVD reported by Hata et al. [43], several groups surfaceandhaveafairlyuniformtubularlength.Thecorrespond-
havereproducedvertically-alignedsuper-longmultiwalled(dou- ingcross-sectionalviewunderahighermagnificationshowsthat
ble-walled)andevensingle-walledCNTs,asexemplifiedinFig.9 theas-grownnanotubeswereverycleanandformedintoloosely-
[43,46,101–103]. packed ‘‘bundles’’ (Fig. 10b). A Raman spectrum of the as-
SLVA-CNTs offer significant advantages over their shorter synthesizedsamplerecordedwitha785nmlaser(Fig.10c)clearly
counterparts for various potential applications, including multi- showsthestrongresonantradialbreathingmodes(RBM)ofSWNTs
functional composites, sensors, and nanoelectronics [46]. In intherangeof130–280cm(cid:4)1.TheclearseparationoftheGpeaks
addition, their millimeter-order lengths provide novel opportu- atca.1570and1600cm(cid:4)1seeninFig.10cisalsoacharacteristic
nities for translating electronic, thermal, and other physical forSWNTs[99].Thehighvalueof(cid:2)12fortheG-bandtoD-band
propertiesthroughindividualnanotubesoverlargelengthscales. intensityratiosuggestsahighdegreeofgraphitizationfortheas-
synthesizedVA-SWNTarrays.QuandDai[45]havealsoperformed
2.2. Vertically-alignedsingle-walledcarbonnanotubes(VA-SWNTs) Ramanmeasurementsontheas-synthesizedSWNTsampleusing
the514nmlaser(Fig.10c),whichcanprobebothsemiconducting
2.2.1. VA-SWNTsbytemplate-freegrowth and metallic SWNTs and is often used to estimate their relative
As can be seen from theabove discussion, thegrowth of VA- contents[111–115].ThemuchstrongerRBMpeaksintherangeof
MWNTshasbeenstudiedforsomeyears.However,itisonlythe ca.150–210cm(cid:4)1,attributabletosemiconductingnanotubes,than
recent effort to grow VA-SWNTs [43,45,104,105]. The slow thoseoverca.210–280cm(cid:4)1,characteristicofmetallicnanotubes,
progressintheVA-SWNTgrowthislargelycausedbythedifficulty indicate that the as-synthesized SWNT sample contains a high
in preparing densely packed small catalyst particles ((cid:2)1nm in percentageofthesemiconductingnanotubes.Inviewofthefact
diameter) that do not aggregate into bigger ones at the high that SWNTs (either semiconducting or metallic) of different
temperature required for the aligned growth of SWNTs diameters resonate with laser beams of different excitation
[43,45,78,99–110]. Amama et al. [100] demonstrated that the wavelengths [2], multiple excitation wavelengths, including the
additionofH Ointothenanotube-growthCVDsystemprevented 633nmlaser[116],havebeenusedfortheRamanmeasurements
2
Fe catalyst particles from aggregation through Ostwald ripening on the as-synthesized SWNT sample [117]. A TEM image of
duetoretardeddiffusionratesofthecatalystatomsbythewater- individual nanotubes dispersed from an ethanol solution also
induced surface oxygen and hydroxyl species. After the seminal revealsthatthenanotubesthuspreparedareSWNTsmostlyfree
workreportedbyHataetal.[43,104],severalgroupshaverecently fromamorphouscarbon(Fig.10d).
reportedthegrowthofVA-SWNTs[45,99–110].Inparticular,Qu
andDai[45]haveexploitedthegrowthofVA-SWNTsbycombining 2.2.2. PreferentialgrowthofsemiconductingVA-SWNTs
PE-CVD with fast heating. This method facilitated the growth of One of the major hurdles for the widespread application of
SWNTsintheformoflargescalealignedarrayswithouttheneed SWNTsinsemiconductorelectronicsisthecoexistenceofmetallic
foranyadditionalH OorO .Inatypicalexperiment,athinfilmof and semiconducting carbon nanotubes in the as-synthesized
2 2
F[(Fig._10)TD$FIG] e(1nm)wassputter-depositedontoaSiO2/Siwaferpre-coated samples. Due to the presence of metallic nanotubes, field effect
transistor(FET)characteristics(e.g.theon/offratioandintegration
uniformity)becomepooranduncontrollable.Therefore,itwillbea
significant advancement if semiconducting VA-SWNT arrays
suitable for electronic applications (e.g. FETs) can be directly
synthesized. Indeed, certain CVD methods with and without the
plasma enhancement have been used to preferentially produce
nonaligned SWNTs with a high percentage of semiconducting
nanotubes ((cid:2)90%) [118,119] or SWNTs with a specific chirality
distribution[120–125].Duetotherandomfeatureofnonaligned
SWNTs,however,itisdifficulttofurtherincreasethepercentageof
semiconducting nanotubes necessary for the high-throughput
construction of electronic devices. Although the mechanism for
preferentiallygrowingthesemiconductingSWNTsisstillnotwell
understood, these pioneering studies indicate the possibility for
selective syntheses of semiconducting SWNTs by carefully
controlling the growth parameters. In this context, Qu et al.
[117]havedevelopedamodifiedPE-CVDandfastheatingmethod
for the preferential growth of semiconducting VA-SWNTs using a
low pressure (30mTorr) C H flow as carbon source without
2 2
additional carry gas. From Raman measurements using laser
Fig.10.SEMimages(a,b),Ramanspectra(c),andTEMimage(d)ofVA-SWNTs beams of multi-wavelengths, about 96% of semiconducting
synthesizedonaSiO2/Siwaferpre-coatedwitha1-nmFe/10-nmAlundera80W/ nanotubes in the as-grown VA-SWNT array was estimated. The
13.56MHzplasmaand0.14H2/CH4partialpressureratioat7508Cfor20min.The as-synthesizedsemiconductingVA-SWNTscanbedirectlyusedas
dashed rectangles in (c) define the approximate regions for metallic (M) and
semiconducting (S) Raman features of SWNTs. Inset of (d) shows a higher theelectronicallyactivematerialinFETdevices,evenwithoutany
magnificationTEMimageofanindividualnanotube(adaptedfromRef.[45]). purificationorseparation.
[(Fig._11)TD$FIG]
H.Chenetal./MaterialsScienceandEngineeringR70(2010)63–91 71
Fig.11.(a)AtypicalSEMimageofthepreferentiallysynthesizedSWNTbundlenetworkontwoelectrodes;Draincurrentvsdrainvoltagemeasuredatgatevoltagesranging
from(cid:4)30to(cid:4)5Vin5Vstepsfor(b)theas-synthesizedSWNTFETand(c)HiPconetworkdevice;and(d)draincurrentvsgatevoltageforboththeas-synthesizedSWNT(&)
andHiPco(*)networkdevicesmeasuredatadrainvoltageof0.02V(adaptedfromRef.[117]).
ToconstructthenanotubeFETs,Quetal.[117]dispersed0.1mg arrays should be attractive for the development of various
oftheas-synthesizedVA-SWNTsin1mLDMFunderultrasonica- optoelectronicnanodeviceswithhighperformance.
tionfor10min,followedbysolution-castingtheslightlydispersed
nanotubebundlesbetweenthedrainandsourceAuelectrodespre- 2.3. VA-MWNTmicropatterns
fabricated on a SiO /Si wafer (400-nm-thick SiO ) to form a
2 2
nanotubebundlenetworkFETwiththeSisubstrateasa bottom 2.3.1. VA-MWNTmicropatternsbyphotolithography
gateelectrode(Fig.11a).Forcomparison,theseauthorshavealso The preparation of micropatterned VA-CNTs is of paramount
preparedanetworkFETbasedonHiPcoSWNTsaccordingtothe importance to many potential applications related to nanotube
same procedure. They deliberately chose bundled nanotubes devices,rangingfromnewelectronfieldemittersinpaneldisplays
insteadofindividualnanotubesfortheFETinvestigationinorder [47], to nanotube sensor chips [132], to molecular filtration
todemonstratethehighpercentageofsemiconductingnanotubes membranes [51]. Along with many reported approaches for
intheas-grownVA-SWNTsample. fabricatingmicropatternsofrandomly-orientedcarbonnanotubes
AscanbeseeninFig.11b,FETsbasedonSWNTsgrownbythe [133–136], the preparation of carbon nanotube patterns with
preferential synthesis showeda typical field effectcharacteristic constituentnanotubesalignednormaltothesubstratesurfacehas
with the drain current increasing with increasing negative gate also been discussed [5,6,25,67,68] - albeit to a less extent. For
voltage, indicating a p-type semiconductor for the SWNTs. In example, Fan et al. [25] reported the first synthesis of regular
contrast,Fig.11cshowsalmostnofieldeffectforthecorresponding arrays of oriented nanotubes on Fe-patterned porous silicon by
HiPcodevice.Thevariationsofdraincurrentwiththegatevoltage pyrolysisofethylene(Fig.12).Thesewell-orderednanotubeswere
foradrainvoltageof0.02VgiveninFig.11dshowanon/offratioof demonstratedtobeusefulaselectronfieldemitters.
morethan100fortheas-synthesizedSWNTFETinairandaquasi- Ontheotherhand,Yangetal.[68]developedamicropatterning
linearplotwitharelativelysmallslopefortheHiPcodevice.The methodforphotolithographicgenerationoftheVA-MWNTarrays
on/offratioofmorethan100fortheas-synthesizedSWNTnetwork withresolutionsdowntoamicrometerscale.Fig.13showsthesteps
FET is comparable to that of planar FETs consisting of parallel ofthephotolithographicprocess(Fig.13A),alongwithatypicalSEM
SWNTsafterelectricalbreakdownofmetallicnanotubes[126]. imageoftheresultantVA-MWNTmicropattern(Fig.13B).
Due also to the similar length-scale between the nanotube These authors first photolithographically patterned a positive
lengthandchannelwidth,thepresenceofanymetallicnanotubes photoresist film of diazonaphthoquinone (DNQ)-modified cresol
in the semiconducting SWNT matrix could cause a short-circuit novolak(Fig.13C(a))ontoaquartzsubstrate.UponUVirradiation
problem for the FET. However, it did not happen in this case. through a photomask, the DNQ-novolak photoresist film in the
Therefore, these results clearly indicate the high yield of exposed regions was rendered soluble in an aqueous solutionof
semiconducting nanotubes in the as-synthesized VA-SWNT sodium hydroxide due to photogeneration of the hydrophilic
sample. The mobility (m) of the as-synthesized SWNTs was indenecarboxylic acid groups from the hydrophobic DNQ via a
estimated,accordingtothestandardformula[127–130],tobeca. photochemicalWolffrearrangement[137](Fig.13C(b)).Theythen
11.4cm2V(cid:4)1s(cid:4)1 in air, which is slightly higher than that of a carriedoutthepyrolysisofFePc,leadingtoregion-specificgrowth
planar FET after electrical breakdown of metallic nanotube of the aligned carbon nanotubes in the UV exposed regions
[131,132]. These preferentially-grown semiconducting VA-SWNT (Fig. 13B). In this case, the photolithographically patterned
