Physics and applications of aligned carbon nanotubes

127 Pages·2011·4.88 MB·English

This article was downloaded by: [Boston College]
On: 04 August 2011, At: 05:44
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Advances in Physics
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/tadp20
Physics and applications of aligned
carbon nanotubes
Yucheng Lan a , Yang Wang b & Z. F. Ren a
a Department of Physics, Boston College, Chestnut Hill, MA,
02467, USA
b School of Physics and Telecommunication Engineering, South
China Normal University, Higher Education Mega Center,
Guangzhou, 510006, China
Available online: 04 Aug 2011
To cite this article: Yucheng Lan, Yang Wang & Z. F. Ren (2011): Physics and applications of aligned
carbon nanotubes, Advances in Physics, 60:4, 553-678
To link to this article: http://dx.doi.org/10.1080/00018732.2011.599963
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-
conditions
This article may be used for research, teaching and private study purposes. Any
substantial or systematic reproduction, re-distribution, re-selling, loan, sub-licensing,
systematic supply or distribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation
that the contents will be complete or accurate or up to date. The accuracy of any
instructions, formulae and drug doses should be independently verified with primary
sources. The publisher shall not be liable for any loss, actions, claims, proceedings,
demand or costs or damages whatsoever or howsoever caused arising directly or
indirectly in connection with or arising out of the use of this material.
AdvancesinPhysics
Vol.60,No.4,July–August2011,553–678
REVIEWARTICLE
Physicsandapplicationsofalignedcarbonnanotubes
YuchengLana,YangWangbandZ.F.Rena*
aDepartmentofPhysics,BostonCollege,ChestnutHill,MA02467,USA;bSchoolofPhysicsand
TelecommunicationEngineering,SouthChinaNormalUniversity,HigherEducationMegaCenter
Guangzhou510006,China
(Received7January2011;finalversionreceived21June2011)
Ever since the discovery of carbon nanotubes (CNTs) by Iijima in 1991, there have been
extensiveresearcheffortsontheirsynthesis,physics,electronics,chemistry,andapplications
duetothefactthatCNTswerepredictedtohaveextraordinaryphysical,mechanical,chemi-
1 cal,optical,andelectronicproperties.AmongthevariousformsofCNTs,single-walledand
1 multi-walled,randomandaligned,semiconductingandmetallic,alignedCNTsareespecially
0
2 importantsincefundamentalphysicsstudiesandmanyimportantapplicationswillnotbepos-
ust siblewithoutalignment.EventhoughtherehavebeensignificantendeavorsongrowingCNTs
ug inanalignedconfigurationsincetheirdiscovery,littlesuccesshadbeenrealizedbeforeourfirst
A reportongrowingindividuallyalignedCNTsonvarioussubstratesbyplasma-enhancedchemi-
04 calvapordeposition(PECVD)[Science282(1998)1105–1108].Ourreportspearheadedanew
4 fieldongrowth,characterization,physics,andapplicationsofalignedCNTs.Uptonow,there
4
5: havebeenthousandsofscientificpublicationsonsynthesizing,studying,andutilizingaligned
] at 0 CthNeTphsyinsivcsarfioorutshaesipreaclitgs.nImnetnhti,stchoemirmapupnliiccaattiioonn,swinefireevldieewmtihsesicounr,reonpttisctaaltuasnotefnanlaigsn,esudbCwNavTes-,
ge lengthlighttransmissioninCNT-basednanocoaxstructures,nanocoaxarraysfornovelsolar
Colle cellTsthreucfotucruess,oefttch.isreviewistoexaminevariousalignedCNTsystems,eitherasanindividual
n or as an array, either the orientation is vertical, parallel, or at other angles to the substrate
o
st horizon,eithertheCNTcorestructuresaremostlyhollowchannelsorarecomposedofcomplex
o compartments.Majorfabricationmethodsareillustratedindetail,particularlythemostwidely
B
[ usedPECVDgrowthtechniqueonwhichvariousdeviceintegrationschemesarebased,followed
y
b byapplicationswhereascurrentlimitationsandchallengeswillalsobediscussedtolaydown
d thefoundationforfuturedevelopments.
e
d
a
o PACS: 61.48.De Structure of carbon nanotubes, boron nanotubes, and other related sys-
wnl tems,85.35.KtNanotubedevices,81.07.-bNanoscalematerialsandstructures:fabricationand
o characterization,01.30.RrSurveysandtutorialpapers;resourceletters
D
Keywords: carbonnanotubes;alignedarrays
Contents PAGE
1.Introduction 556
1.1.DiscoveryofCNTs 556
1.2.StructuresofCNTs 561
1.2.1.Graphite 561
1.2.2.Single-walledCNTs 561
1.2.3.Double-walledCNTs 562
1.2.4.Multi-walledCNTs 563
1.2.5.Bamboo-likeCNTs 563
1.2.6.Othercarbonnanomaterials 564
*Correspondingauthor.Email:[email protected]
ISSN0001-8732print/ISSN1460-6976online
©2011Taylor&Francis
DOI:10.1080/00018732.2011.599963
http://www.informaworld.com
554 Y.Lanetal.
1.3.HighanisotropicpropertiesofCNTs 564
1.3.1.Anisotropicmechanicalproperties 565
1.3.2.Anisotropicelectricalproperties 565
1.3.3.Anisotropicthermalproperties 565
1.3.4.Otheranisotropicphysicalproperties 566
1.4.GrowthtechniquesofCNTs 567
1.4.1.Arcdischarge 567
1.4.2.Laserablation 568
1.4.3.Chemicalvapordeposition 569
1.4.4.Othermethods 570
2.TechnologiestoachieveCNTalignment 571
2.1.InsitutechniquesforCNTalignment 572
2.1.1.ThermalCVDwithcrowdingeffect 572
2.1.2.ThermalCVDwithimposedelectricfields 573
1 2.1.3.VerticallyalignedCNTarraysbyPECVD 577
01 2.1.3.1 One-dimensionallyorderedCNTarrays 578
2
st 2.1.3.2 Three-dimensionallyorderedCNTarrays 581
u 2.1.4.ThermalCVDgrowthundergasflowfields 584
g
u
A 2.1.4.1 Fastheatingunderhighgasflow 584
4 2.1.4.2 Buoyanteffectatlowgasflow 584
0
4 2.1.4.3 Trenchstructure 585
4
5: 2.1.4.4 Two-dimensionalCNTnetworks 586
0
at 2.1.5.ThermalCVDgrowthwithepitaxy 587
e] 2.1.5.1 Lattice-directedgrowth 587
g
e 2.1.5.2 Ledge-directedgrowth 587
Coll 2.1.5.3 Graphoepitaxy 588
n 2.1.5.4 Two-dimensionalCNTnetworks 590
o
st 2.1.6.ThermalCVDundermagneticfields 591
o
B 2.2.ExsitutechniquesforCNTalignment 591
[
y 2.2.1.Electricfields 591
b
d 2.2.2.Magneticfields 592
e
d
a 2.2.3.Mechanicalmethods 594
o
nl 2.2.3.1 Stretchingmethod 594
w
o 2.2.3.2 Spinningmethods 595
D
2.2.4.Otherexsitumethods 596
3.Direct-currentPECVD 596
3.1.Equipmentsetupandgrowthprocedure 596
3.2.Substrateandunderlayer 597
3.3.Growthtemperature 598
3.4.Plasmaheatingandetchingeffects 599
3.5.Plasmastates 600
3.6.Catalystcrystalorientation 601
3.7.Electricfieldmanipulation 602
4.PropertiesandapplicationsofalignedCNTarrays 603
4.1.Field-emissiondevices 603
4.1.1.FieldemissionofalignedCNTarrays 604
4.1.2.CNTarrayemitters 607
4.1.3.High-intensityelectronsources 607
4.1.4.Lighting 607
AdvancesinPhysics 555
4.1.5.Field-emissionflat-paneldisplays 610
4.1.6.Incandescentdisplays 611
4.1.7.X-raygenerators 612
4.1.8.Microwavedevices 613
4.2.Opticaldevices 613
4.2.1.Photoniccrystals 613
4.2.2.Opticalantennae 615
4.2.3.Opticalwaveguides 616
4.2.4.Solarcellsbasedonnanocoaxes 619
4.3.Nanoelectrode-basedsensors 620
4.3.1.Nanoelectrodearrays 620
4.3.2.Ionsensors 623
4.3.3.Gassensors 625
4.3.3.1 Hydrogengassensors 627
1 4.3.3.2 Nitrogengassensors 628
01 4.3.3.3 Nitrousoxidegassensors 629
2
st 4.3.3.4 Ammoniagassensors 629
u 4.3.3.5 Othergassensors 631
g
u
A 4.3.4.Biosensors 631
4 4.3.4.1 Glucosesensors 631
0
4 4.3.4.2 DNAsensors 633
4
5: 4.3.4.3 Proteinsensors 635
0
at 4.3.5.Catalyst 636
e] 4.4.Thermaldevices:thermalinterfacematerials 637
g
e 4.5.Electricalinterconnectsandvias 639
Coll 5.PotentialapplicationsofCNTarrays 641
n 5.1.Mechanicaldevices 641
o
st 5.1.1.Carbonnanotuberopes 642
o
B 5.1.2.TEMgrids 644
[
y 5.1.3.Mechanicaltapes 645
b
d 5.2.Electricaldevices 645
e
d
a 5.2.1.Lowκ dielectrics 646
o
nl 5.2.2.Randomaccessmemory 646
w
o 5.2.3.Transistors 646
D
5.3.Thermoacousticloudspeakers 647
5.4.Electrochemical/chemicalstoragedevices 649
5.4.1.Fuelcells 650
5.4.2.Supercapacitors 653
5.4.3.Lithiumionbatteries 657
5.4.4.Hydrogenstorage 658
5.5.Electromechanicaldevices 658
5.5.1.Actuators 658
5.5.2.Artificialmuscles 658
5.6.Terahertzsources 659
5.7.Otherapplications 659
6.Conclusions 659
Note 660
Acknowledgements 660
References 660
556 Y.Lanetal.
1. Introduction
Carbon nanotubes (CNTs) and related nanostructures have been one of the most scientifically
studiedmaterialsystemsintherecentyearseversincetheirwell-knownexperimentaldiscovery
[1].Alargevarietyofpotentialapplicationshavebeenenvisioned,someofwhichhavealready
beenreducedtopractice,whileothersarestillunderstudy.ThemostintriguingpropertiesofCNTs
lie in their unique quasi-one-dimensional nanoscale structures that are intrinsically anisotropic:
propertiesinthelongitudinaldirectionaredrasticallydifferentfromthoseintheazimuthaldirec-
tions. It is critical in most applications to know and control how a CNT is oriented either as a
stand-alone individual or in a group of many.And it is obviously more challenging and more
desirable to obtain a CNT ensemble with all its members having a common orientation, some-
thing usually referred to as an aligned CNT array. Such an array, in many ways, preserves the
anisotropicpropertiesofindividualCNTs,andinthemeantimecanbemorerobustandlargerin
its physical size as a whole, which greatly facilitates their integration into practical devices.As
a necessary introduction, we will first review the fundamental structures of CNTs, their unique
1 anisotropicproperties,andthegeneralgrowthmechanismsbychemicalvapordeposition(CVD),
1
0 withoutspecifyingtheirorientationoralignment.Thiswillcarryoutthenecessarypreparationfor
2
st thefollowingin-depthreviewofthestate-of-the-artdiscoveriesandprogressesinthefabrication
u
g andapplicationsofalignedCNTarrays.
u
A
4
0
4 1.1. DiscoveryofCNTs
4
5:
0 The well-known allotropes of carbon are diamond, graphite, amorphous carbon, and fullerenes
at discoveredin1985[2].Fullereneisentirelycomposedofcarbonintheformofahollowsphere
]
ge (buckyball)orellipsoid.Figure1showsthecrystallographicstructuresofthefourallotropesof
e
oll carbon. CNTs (also called buckytubes in earlier days) are elongated cylindrical fullerenes with
C diameters of subnanometer to tens of nanometers depending on the number of graphitic layers
n
o andlengthsofsubmicrontohundredsoforeventhousandsofmicrons.
st
o ThelengthoftheCNTrangesfromlessthanamicrontoseveralmillimeters.Thecarbonatoms
B
[ inaCNTarebondedtrigonallyinacurvedsheet(graphiticlayer)thatformsahollowcylinder.
y
b Suchuniquenanostructuresresultinmanyextraordinarypropertiessuchashightensilestrength,
d
e
d
a
o
nl
w
o
D
Figure 1.Allotropesofcarbon.(a)Diamondwherethecarbonatomsarebondedtogetherinatetrahedral
latticearrangement;(b)graphitewherethecarbonatomsarebondedtogetherinsheetsofahexagonallattice
withvanderWaalsforcesbondingthesheetstogether;(c)amorphouscarbonwiththerandomcarbonatoms;
(d–f)fullereneswherethecarbonatomsarebondedtogetherinsphericalformation(C60)(d),inellipsoidal
formations(C70)(e),andintubularformations(CNTs)(f).
AdvancesinPhysics 557
high electrical and thermal conductivities, high ductility, high thermal and chemical stability,
whichmakesthemsuitableforvariousapplicationsasdiscussedinSections4and5.
CNTsaretypicallycategorizedassingle-walled(SWCNTs),double-walled(DWCNTs),and
multi-walled(MWCNTs)withrespecttothenumberofgraphiticlayers.Thenatureoftheatomic
bondinginaCNTisdescribedbyappliedquantumchemistryor,specifically,orbitalhybridization.
ThechemicalbondsinCNTsareallsp2bonds,similartothoseofgraphite.Thedetailsofatomic
structureofindividualCNTsarewelldescribedinpreviousreviews.
TheCNTshaveanarguablylonghistoryofdiscovery.Itwasspeculated[3]thatthefirstcarbon
filamentwaspossiblysynthesizedasearlybackas1889[4]atThomasAlvaEdison’serawhena
lightbulbfilamentwassearchedforincandescentlamps.Carbonfilamentswerethenproducedby
athermaldecompositionofgaseoushydrocarbon(methane)tomakelightbulbfilaments.Inthat
era,carbonfilamentssmallerthanafewmicronsindiametercouldnotbeobservedbecauseofthe
lowresolutionoftheopticalmicroscopesusedatthetime.Basedontheexperimentalmethodand
conditionsdescribedinthecorrespondingpatent,itisconceivablethathollowcarbonfilaments
1 werepossiblyproducedthen[3],althoughnoimageswererecordedasdirectevidences.
01 Thefirsttransmissionelectronmicroscopy(TEM)evidenceforthetubularnatureofnanoscale
2
st carbonfilamentswaspublishedin1952[5].ClearTEMimagesofhollowcarbonfilamentswere
u published,asshowninFigure2(a),whichclearlyillustratesthatthecarbonfilamentsarehollow
g
u
A tubes with diameters of about 50nm. The graphitic walls are clearly observed from the TEM
4 image contrast.The structures seem to be multi-walled with 15–20 layers [3]. In 1973, Boehm
0
4 reportedhollowcarbonfibersbycatalyticdisproportionationofcarbonmonoxideat480–700◦C
4
5: [7].In1976,hollowcarbonfiberswithnanometer-scalediametersweresynthesizedusingavapor-
0
at growthtechnique[6].ThereportedTEMimage(Figure2(b))clearlyshowsthatthehollowfiber
e] consistsofasinglegraphiticlayer.In1979,hollowcarbonfiberswereproducedoncarbonanodes
g
e duringarcdischargeandpresentedinaconference[8].Thesehollowtubularnanostructureswere
Coll growninanitrogenatmosphereatlowpressures.In1987,apatenttoproduce“cylindricaldiscrete
n carbonfibrils”witha“constantdiameterbetweenabout3.5andabout70nanometers...,length
o
st 102timesthediameter,andanouterregionofmultipleessentiallycontinuouslayersofordered
o
B carbon atoms and a distinct inner core ...” was issued [9]. Some scientists even believed that
[
y hollowcarbonnanostructureswerelikelyproducedinancientforging(ad900–1800)although,of
b
d course,nobodynoticedthematthattime[10].Mostoftheworkbefore1991wereunfortunately
e
d
a notwellknownbythebroaderscientificcommunity,norwerethehollowcarbonstructurescalled
o
nl “carbonnanotubes”,andthereforenotcreatingasignificantimpact.
w
o
D
Figure 2.Typicalhollowcarbonfiberimagesinthehistory.(a)FirstTEMimagesofpossibleMWCNTs
publishedin1952;(b)firstTEMimageofpossibleSWCNTspublishedin1976.(a)ReprintedfromCarbon,
44,M.MonthiouxandV.L.Kuznetsov,pp.1621–1623[3].Copyright(2006),withpermissionfromElsevier.
(b)ReprintedfromJournalofCrystalGrowth,32,M.Oberlinetal.,pp.335–349[6].Copyright(1976),with
permissionfromElsevier.
558 Y.Lanetal.
Sincebuckminsterfullerene(showninFigure1(d))wasdiscoveredbyarcdischargein1985[2],
moreandmorescientistsexpressedinterestsinnanomaterials.Shortlyafter,Iijima,usingthesame
method(arcdischarge)toproduceC ,foundthatthecentralcoreofthecathodicdepositcontained
60
avarietyofclosedgraphiticstructuresincludingnanoparticlesandnanotubes[1].Theobtained
CNTswereMWCNTsasshowninFigure3(a).TheworkwaspublishedinNaturein1991and
hasbeennoticedworldwideinthescientificcommunity.ItisfairtosaythatthereportofCNTs
by Iijima strengthened the scientific community’s pursuit to nanoscience and nanotechnology.
Readers interested in the CNT discovery stories are referred to the related literatures [3,13,14],
especiallytheeditorialpaperwrittenbyMonthiouxandKuznetsovinCarbon[3].
FollowingtheresearchofMWCNTsbyIijimain1991,SWCNTswereindependentlysynthe-
sizedusingarc-dischargetechniques(seeFigure3(b)and(c))byaddingtransition-metalcatalysts
(Fe[11]orCo[12]).ThesynthesisofSWCNTsisanimportantmilestoneinthedevelopmentof
CNTresearchbecausemanytheoreticalpredicationsofCNTpropertiescanbemoreconveniently
testedonthesimplestSWCNTstructures.
1 Thearc-discharge-producedmaterialsaremixturesofamorphouscarbon,graphite,andCNTs
01 withalowCNTyield.PureCNTsareusuallyobtainedbyvariouspurificationmethods[15–20].
2
st Althoughtheyieldislowandthepurificationprocedureiscomplex,theproducedCNTshavegood
u crystallinityandveryfewdefects.Sothemethodisstillwidelyusedtoday.In1995,analternative
g
u
A methodofpreparingSWCNTs,laservaporizationofgraphite,wasdiscoveredbySmalley’sgroup
4 [21].ThismethodresultedinahighyieldofSWCNTswithunusuallyuniformdiameters[22].
0
4 Besides the above classic CNT structures, hybrid CNTs, such as CNTY-junctions [23–26]
4
5: (Figure4(a)and(b))andCNTnanobuds[27,29](Figure4(c)),arealsodiscovered.CNTnanotorus
0
at (Figure4(d))isalsopredicted.Theyare,however,beyondthecontentofthisreview.
e] AlongwiththesynthesisofindividualCNTs,CNTarraysconsistingofanumberofindividual
g
e CNTswerealsofabricatedatthesametime.CNTropesconsistingof100–500alignedSWCNTs
Coll in a closely stacked two-dimensional triangular lattice arrangement were reported as early as
n in 1996 using a laser ablation method (Figure 5) [22]. SWCNT ropes were synthesized from a
o
st carbon–nickel–cobalt mixture at 1200◦C. These SWCNTs are nearly uniform in diameter and
o
B self-organizeinto“ropes”consistingof100to500SWCNTswithaSWCNTspacingof17Å.
[
y Later,large-scaleMWCNTswerefabricatedfromironnanoparticlesembeddedinmesoporous
b
d silicabypyrolysisofacetyleneusingaCVDmethod(Figure6)[30].TheMWCNTsgrownfrom
e
d
a
o
nl
w
o
D
Figure 3.High-resolutionTEM(HRTEM)imagesofthemostwell-known(a)MWCNTspublishedin1991
[1],(b)SWCNTspublishedin1993grownfromironcatalyst[11],and(c)SWCNTspublishedsimultaneously
with(b)usinganothercatalyst,cobalt[12].AlltheseCNTsweresynthesizedbyDCarc-dischargeevaporation
ofcarbon.(a)ReprintedbypermissionfromMacmillanPublishersLtd:Nature[1],copyright(1991).(b)
ReprintedbypermissionfromMacmillanPublishersLtd:Nature[11],copyright(1993).(c)Reprintedby
permissionfromMacmillanPublishersLtd:Nature[12],copyright(1993).
AdvancesinPhysics 559
1
1
0
2
st
u
g
u
A
4 Figure 4.(a)TEMimageofCNTY-junction;(b)SEMimageofCNTY-junctionwithasmoothsurface;
0 (c)TEMimageofCNTnanobudsinwhichafullereneiscombinedwithanSWCNTandattachedtothe
4
4 surface of the SWCNT. Inset is the structure of the nanobud on an SWCNT. (d)A predicted CNT nan-
05: otorus. (a) Reprinted by permission from Macmillan Publishers Ltd: Nature [23], copyright (1999). (b)
at Reprinted with permission from W.Z. Li et al.,Applied Physics Letters 79, pp. 1879–1881, 2001 [24].
e] Copyright (2001), American Institute of Physics. (c) Reprinted by permission from Macmillan Pub-
g
e lishers Ltd: Nature Nanotechnology [25], copyright (2007). (d) Reprinted figure with permission from
oll L.Liuetal.,PhysicalReviewLetters88,p.217206,2002[26].Copyright(2002)bytheAmericanPhysical
C
n Society.
o
st
o
B
[
y
b
d
e
d
a
o
nl
w
o
D
Figure 5.TEMimagesofanSWCNTropepreparedbylaservaporizationofgraphitein1996.(a)Asingle
SWCNTropemadeupof∼100SWCNTsasitbendsthroughtheimageplaneofthemicroscope,showing
uniformdiameterandtriangularpackingofthetubeswithintherope.(b)Sideviewofaropesegment.From
A.Thessetal.,Science,273,pp.483–487,1996[22].ReprintedwithpermissionfromAAAS.
the iron nanoparticles embedded in mesoporous silica are approximately perpendicular to the
surface of the silica and form an aligned array of isolated tubes with a tube spacing of about
100nm[30].Itisbelievedthatsomecatalyticironnanoparticleswereembeddedinthevertical
cylindrical pores. When CNTs grew in these vertical pores, they became perpendicular to the
surfaceofthesilicasubstrate.Thoseformedonironnanoparticlesembeddedininclinedcylindrical
pores were tilted along the axes of the pores. The growth direction of the nanotubes can be
controlledbytheporesfromwhichthenanotubesgrow.TheseMWCNTswererandomlygrown
560 Y.Lanetal.
Figure 6.(a)SEMimageofafilmcomposedofalignedCNTspreparedin1996.Thisfilmwithathickness
of50μmwasobtainedbygrowingfor2h.(b)High-resolutionTEMimageofaCNTcomposingtheCNT
film in (a), consisting of about 40 concentric shells of graphitic sheets with a sheet spacing of 0.34nm.
Theinnerandouterdiametersofthetubeare4and34nm,respectively.FromW.Z.Lietal.,Science,274,
pp.1701–1703,1996[30].ReprintedwithpermissionfromAAAS.
1
1
0
2
st
u
g
u
A
4
0
4
4
5:
0
at
]
e
g
e Figure 7. (a) Low-magnification SEM image of an MWCNT array grown by PECVD in 1998. (b)
oll High-magnificationSEMimageofMWCNTsshownin(a).TheCNTsareverticallygrownonthesubstrate
C
n andtheCNTsitesarerandom.FromZ.F.Renetal.,Science,282,pp.1105–1107,1998[32].Reprintedwith
o
st permissionfromAAAS.
o
B
[
y onthesubstrates,notverystraight,andveryoftenentangledtogether.Large-scaleMWCNTswere
b
d alsofabricatedfromthinfilmsofcobaltcatalystspatternedonasilicasubstratebypyrolysisof
e
d
a 2-amino-4,6-dichloro-s-triazine[31].
o
nl Straight,well-aligned,andseparatedCNTarrayswerenotsuccessfullyfabricateduntil1998
ow usingaplasma-enhancedchemicalvapordeposition(PECVD)methodbelow666◦C(Figure7)
D
[32].CNTswerealignedoverareasuptoseveralsquarecentimetersonnickel-coatedglass.The
diameterandlengthofthealignedCNTsarecontrollablefrom20to400nmandfrom0.1to50μm,
respectively.TheCNTswereverystraight(Figure7(b)).Becausethecatalyticnickelnanoparticles
werefabricatedbyradiofrequencymagnetronsputtering,thenanoparticlesdistributedrandomly
ontheglasssubstrate.TheverticalCNTswerethenrandomlygrownontheglasssurface.
Later,largeperiodicarraysofCNTsweregrownbyplasma-enhancedhotfilamentCVDon
periodic arrays of nickel dots prepared by e-beam lithography [33] (Figure 8). The sites of the
CNTsdependonthesitesofcatalyticnickelnanodots.Thenanotubegrowthprocessiscompatible
withsiliconintegratedcircuitprocessing,andCNTdevicesrequiringfreestandingverticalCNTs
canbereadilyfabricatedsincethen.
Inordertoobtainperiodiccatalyticnanoparticlescheaply,polystyrenemicrospherelithography
wasdeveloped[34,35].Figure9(a)showsalignedCNTsinahoneycomblatticepattern.TheCNTs
growfromtheperiodicallypatternedcatalystspreparedbymicrospherelithography(Figure9(b)).
Fromthenon,theverticallygrownCNTsaresite-controlled.These3D-alignedCNTshavewide
applications,suchasfield-emissiondisplays,physicalandbiologicalsensors,etc.
AdvancesinPhysics 561
Figure 8.SEMimagesofhighlyorderedarrayswiththeassistanceofe-beamlithographyformakingthe
catalyticNidots.(a)ArepeatedCNTarraypatternand(b)aCNTarraypattern.Reprintedwithpermission
fromZ.F.Renetal.,AppliedPhysicsLetters75,pp.1086–1088,1999[33].Copyright(1999),American
InstituteofPhysics.
1
1
0
2
st
u
g
u
A
4
0
4
4
5:
0
at
]
e
g
e Figure 9.(a)SEMimagesofahoneycombarrayofalignedCNTsgrownbyPECVDin2003[34].(b)AFM
oll imageofNicatalyticdotsusedtogrowCNTsin(a).TheNicatalyticdotswerepreparedbypolystyrene
C
n microspheremasks.Inset:highermagnificationimageofnickeldots.ReprintedwithpermissionfromZ.P.
sto Huangetal.,AppliedPhysicsLetters82,pp.460–462,2003[34].Copyright(2003),AmericanInstituteof
Bo Physics.
[
y
b
d At the same time, superlong CNT arrays were also grown vertically to the substrate surface
e
d
a usingathermalCVDmethod[36],especiallythewater-assistedCVDmethod.
o
nl AmongthevariousCNT-relatedstructuresdiscovered,wemainlydiscussherearraysofaligned
w
o CNTswhichmaybeSWCNTs,DWCNTs,MWCNTs,andbambooorfiber-like.Meanwhile,we
D
recommendahandfulofavailablebookstointerestedreadersforfurtherreadingandunderstanding
onindividualCNTsofmorevarieties[37–40].
1.2. StructuresofCNTs
1.2.1. Graphite
Agraphiticlayerisaone-atom-thickplanarsheetofsp2-bondedcarbonatomswithahoneycomb
crystallatticestructure(Figure10(a)).Thecarbon–carbonbondlengthis0.142nm.Graphitelayer
isthebasicstructuralelementofCNTs.Itsuniquephysicalpropertiesarereviewedinacollection
ofbooksandinavastamountofliteratures.
1.2.2. Single-walledCNTs
SWCNTs can be synthesized by CVD [6], arc discharge [11,12], and thermocatalytical dispro-
portionationofcarbonmonoxide[42,43].NowmassiveamountofSWCNTsisproducedmainly