DTIC ADA573890: Revealing the Impact of Catalyst Phase Transition on Carbon Nanotube Growth by in Situ Raman Spectroscopy

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Revealing the Impact of Catalyst Phase
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Transition on Carbon Nanotube
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in Situ
Growth by Raman Spectroscopy
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RahulRao,†,‡,r,*NealPierce,†,§DavidLiptak,†,^DaylondHooper,†,^GordonSargent,†,^S.LeeSemiatin,†
StefanoCurtarolo,zAvetikR.Harutyunyan,#andBenjiMaruyama†,*
†MaterialsandManufacturingDirectorate,AirForceResearchLaboratory,Wright-PattersonAirForceBase,Ohio45433,UnitedStates,‡NationalResearchCouncil,
Washington,D.C.20001,UnitedStates,§UniversityofDaytonResearchInstitute,Dayton,Ohio45469,UnitedStates,^UESInc.,Dayton,Ohio45433,UnitedStates,
zDukeUniversity,Durham,NorthCarolina27708,UnitedStates,and#HondaResearchInstitute,Columbus,Ohio43212,UnitedStates.rPresentaddress:Honda
ResearchInstitute,Columbus,Ohio43212,UnitedStates.
ABSTRACT Thephysicalstateofthecatalystanditsimpacton
thegrowthofsingle-walledcarbonnanotubes(SWNTs)isthesubject
ofalong-standingdebate.WeaddressedithereusinginsituRaman
spectroscopy to measure Fe and Ni catalyst lifetimes during the
growthofindividualSWNTsacrossawiderangeoftemperatures
(500(cid:2)1400 (cid:2)C). The temperature dependence of the Fe catalyst
lifetimesunderwentasharpincreasearound1100(cid:2)Cduetoasolid-
to-liquidphasetransition.Bycomparingexperimentalresultswith
themetal(cid:2)carbonphasediagrams,weprovethatSWNTscangrow
fromsolidandliquidphase-catalysts,dependingonthetemperature.
KEYWORDS: carbonnanotubes.CVDgrowth.phasetransition.Ramanspectroscopy.insitu
C
arbonnanotubesareimpressivebe- carbon nanotube to grow preferentially
causeoftheirwiderangeofpotential metallic10orsemiconductingnanotubes.11
applications,fromstructuralcompo- Diffusion mechanisms and solubility limits
sitestoenergystoragetosensorsandelec- inliquidversussolidstatesareverydiffer-
tronicdevices.1However,theirtransitionto ent, and are important considerations in
commercialuseisimpededbyourcontin- growth models; for example whether car-
uedinabilitytocontrolgrowth.Thelackof bon diffuses in the bulk of the catalyst or
control largely stems from holes in our along the surface.12 However, the most
fundamentalunderstandingofgrowthmech- interesting catalysts, that is, transition me-
anisms.Inthisregard,oneimportantissue talslikeFeorNihavesolid-to-liquidtransi-
thatremainslargelyunresolvedistherole tionswithinthetypicaltemperaturerange
of the physical state of the catalyst in of SWNT growth. Thus we wondered how
influencing the structure and yield of thestateofthecatalystimpactsthegrowth
SWNTs.Whilethestateofthecatalyst(i.e., ofcarbonnanotubesandwhattheimplica-
liquidorsolid)duringnanotubegrowthwas tionsforthisareongrowthmechanisms.
longdebatedintheliterature,growthfrom To directly address this issue, here we
oxides,2 carbides,3,4 and low-melting tem- used a uniquesystem, theAdaptive Rapid
*Addresscorrespondenceto
peraturemetals,5haveshownthatgrowthis Experimentation and Spectroscopy (ARES) [email protected],
possible from both liquids and solids. The system,whichincorporatedmicro-Raman [email protected].
stateofthecatalysthasimportantimplica- spectroscopywithcold-wallchemicalva-
ReceivedforreviewSeptember3,2012
tionsforboththeresultantgrowthinterms por deposition (CVD), to measure the andacceptedJanuary23,2013.
of yield or length,6,7 as well as our under- growth kinetics of individual SWNTs13
Publishedonline
standing of the growth mechanisms. For from two of the most popular transition
10.1021/nn304064u
example, in the solid state, the catalyst metal catalysts;Fe and Ni. In situ Raman
canformfacets,whichmaytemplate8,9the spectroscopy during SWNT growth has CXXXXAmericanChemicalSociety
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4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Revealing the Impact of Catalyst Phase Transition on Carbon Nanotube
5b. GRANT NUMBER
Growth by in Situ Raman Spectroscopy
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Directorate,Wright-Patterson AFB,OH,45433
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14. ABSTRACT
The physical state of the catalyst and its impact on the growth of single-walled carbon nanotubes (SWNTs)
is the subject of a long-standing debate. We addressed it here using in situ Raman spectroscopy to measure
Fe and Ni catalyst lifetimes during the growth of individual SWNTs across a wide range of temperatures
(5001400 C). The temperature dependence of the Fe catalyst lifetimes underwent a sharp increase around
1100 C due to a solidto- liquid phase transition. By comparing experimental results with the metalcarbon
phase diagrams, we prove that SWNTs can grow from solid and liquid phase-catalysts, depending on the
temperature.
15. SUBJECT TERMS
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF
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Figure1. (a)Schematicoftheadaptiverapidexperimentationandspectroscopy(ARES)systemusedforinsitustudiesof
SWNTcatalystlifetime.AmagnifiedviewofthepillarsisshownintheSEMimageonthebottomright.(b)Waterfallplot
showingtemporalevolutionofRamanspectrafromagrowingnanotubeintheDandGbandregion.Spectraaveragedevery
20sfromconsecutivetimewindowsareplottedfrombottomtotop.
previouslybeenusedsuccessfullytoevaluategrowth ofSWNTsfromwithinthelaserspot.Anexampleofa
kineticsofindividual13andensemblesofSWNTs.14(cid:2)17 time-seriesofRamanspectracollectedduringagrowth
Herewewereabletoperformmorethan100experi- experimentwithFecatalystisshownasawaterfallplot
ments over a wide range of growth temperatures in Figure 1b, where initiation of SWNT growth is
(500(cid:2)1400 (cid:2)C), enabling us to reveal striking differ- indicatedbytheappearanceandsubsequentincrease
encesbetweenFeandNiwithanunprecedentedlevel inintensityoftheGbandat∼1560cm(cid:2)1(downshifted
ofdetail.TheFecatalystlifetimeexhibitedadisconti- due to the elevated temperature). The G band areas
nuity with increasing growth temperatures and was fromeachspectrumwerecalculatedandplottedwith
accompaniedbyanincreaseinSWNTyield,indicating respecttotimeinordertoevaluatethegrowthkinetics
thefirstdirectobservationofgrowthacrossasolid(cid:2) oftheSWNTs.TheRamanspectrawerealsonormalized
liquid phase transformation. Ourresults suggest that with respect to the substrate (Si) peak intensities in
thelengthandnucleationefficiencyofSWNTscanbe ordertoenablefaircomparisonofthedataacrossall
controlled by engineering the phase states of the the growth experiments. The Raman spectrum col-
catalysts. lectedjustbeforetheappearanceoftheGbandwas
assignedtothetimecorrespondingtot=0.Wenote
RESULTSANDDISCUSSION
that a few previous in situ growth studies have re-
Figure 1a shows a schematic of the ARES system portedan“incubationtime”duringwhichtheGband
usedfortheSWNTgrowthstudies.Thesetupinvolved intensity increases slowly, followed by growth and
laser-induced heating (laser excitation = 2.33 eV) of terminationphases.15,18Suchanincubationtimewas
catalystfilms(FeandNi)onsiliconpillars,whichwere notobservedinthisstudy.
formedbyetchingSi-on-SiO wafers(seethemagni- The integrated G band areas (normalized to unity)
2
fiedscanningelectronmicroscopy(SEM)imageonthe from two different experiments (using Fe and Ni
bottom right in Figure 1a and the Methods section catalysts)areplottedwithrespecttotimeinFigure2.
below for more details).13 Thesmall thermal mass of ThegrowthcurvesinFigure2werefittedtoadecaying
thepillars(10μmdiameterandheight),combinedwith exponentialgrowthequationoftheform:G(t)=ντ[1(cid:2)
thermal isolation from the wafer due to the oxide exp((cid:2)t/τ)], whereG(t), ν,and τare the areaof the G
barrier, enabled rapid heating to nanotube growth band, initial growth rate, and time constant for the
temperatures ranging from 500 to 1400 (cid:2)C using reaction,respectively.13(cid:2)17Thegrowthcurvesshown
ethylene as the hydrocarbon source. The light scat- inFigure2panelsaandbcorrespondtoexperiments
teredfromthepillarswascollectedforRamanspectra, conducted at different growth temperatures. It is
whichwereacquiredcontinuouslyduringthegrowth clear from both Figure 2 that the growth curve
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1000 and 1100 (cid:2)C, thus implying a difference in the R
catalyticprocessatlowandhightemperatures.
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The shaded band between 1000 and 1100 (cid:2)C in
Figure 3a delineates the transition between low and I
C
hightemperaturegrowth,referredtoasregionsIandII,
respectively. The catalyst lifetimes decrease monoto-
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nicallywithincreasinggrowthtemperatureinregionI,
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followed by a transition between 1000 and 1080 (cid:2)C,
abovewhichthecatalystlifetimejumpsfrom∼100s
up to ∼600 s. Beyond 1100 (cid:2)C, the lifetime values
decreaseagainuptothemaximumgrowthtempera-
ture(∼1390(cid:2)C)inregionII.ThedecreaseinFecatalyst
lifetimes arealso indicatedby theexponential fitsto
the data in regions II and I in Figure 3a (also see
SupportingInformation,FigureS1andrelateddiscus-
sionregardingthedatafits).IncontrasttoFe,theNi
catalyst lifetimes decrease monotonically across the
fulltemperaturerange(Figure3c),anddonotexhibit
any discontinuity, suggesting the same phase of Ni
catalystatallgrowthtemperatures.Unlikethecatalyst
lifetimes, the initial growth rates did not display any
discontinuity for either Fe or Ni with increasing tem-
perature(SupportingInformation,FigureS2).
Tounderstandthenatureofthisdiscontinuityinthe
Figure 2. Typical growth curves at two different growth
Fe catalyst lifetime, we compare the Fe and Ni life-
temperaturesobtainedbyplottingthecalculatedGband
area(normalizedtounity)versustime,using(a)Fe,and(b) time(cid:2)temperatureplotsinFigure3panelsaandcto
Nicatalysts.Thegrowthcurvesarefittedtoanexponential their corresponding modified metal(cid:2)carbon binary
equationoftheformG(t)=ντ[1(cid:2)exp((cid:2)t/τ)]whereG(t),ν,
phase diagrams. It should be noted that our experi-
andτaretheareaoftheGband,initialgrowthrateandtime
constantofthereaction,respectively. mentalresultsarebasedonreactionkinetics,whilethe
phase diagrams correspond to thermodynamic equi-
correspondingtothehighertemperatures(990(cid:2)Cfor librium. Nevertheless,itisinstructive tocomparethe
Feand1200(cid:2)CforNi)hasasmallertimeconstant; evolution of the lifetimes with the thermodynamic
phase diagrams for a qualitative understanding of
thatis,growthsatthehighertemperaturesterminated
the catalyst behavior. In Figure 3 panels b and d we
faster than growth at lower temperatures, implying
showtheFe-andNi-richpartsofthemodifiedFe(cid:2)C
that the catalyst had a shorter lifetime at the higher
and Ni(cid:2)C phase diagrams, respectively. Both phase
temperature.Henceforth,thetimeconstantsobtained
diagramshavebeendownshiftedbyΔT≈125(cid:2)Cto
fromthefitstothegrowthcurveswillbereferredtoas
account for the Gibbs(cid:2)Thomson depression for a
thecatalystlifetimes.
supportedcatalystparticle.19,20Theinitialparticlesize
In general, the lifetime of a thermally activated
distributionsoftheas-depositedFeandNideposition
catalytic process is a function of the temperature.
on the pillars were quite similar (see Supporting In-
Lifetimevaluesobtainedfrommanydifferentexperi-
formationFigureS3),withameanparticlesizearound
ments using Fe and Ni catalysts are plotted against
2.5nm,hencetheshadedtransitionregionsinFigure3
growth temperatures in Figure 3 panels a and c, toaccountforsize-relatedeffects.
respectively. Every data point in Figure 3a,c corre- Whencomparingthelifetime(cid:2)temperatureplotin
sponds to a separate growth experiment; in all, over Figure 3a with the modified Fe(cid:2)C phase diagram
100 experiments were performed over a wide tem-
(Figure 3b), it becomes immediately apparent that
peraturerangefrom500to1400(cid:2)C.Notethesignifi- region I can be attributed to the phase fields below
cant difference between the behaviors of Fe and Ni: the eutectic temperature. This implies that the Fe-
WhiletheNicatalystlifetimesdecreasemonotonically catalystwasinthesolidstateforgrowthtemperatures
withtemperature(Figure3c),thereisadiscontinuityin upto1000(cid:2)C.Interestingly,thisrangeoftemperatures
the Fe case between 1000 and 1100 (cid:2)C (Figure 3a) is also the most common for SWNT growth. The Fe
where the catalyst lifetime increases from ∼100 s to catalyst lifetimes experience a sharp increase from
∼600 s. From a thermodynamic perspective, the dis- ∼100sto∼600sasthegrowthtemperaturegoesup
continuityinthelifetimeoftheFecatalystnear1100(cid:2)C to around 1100 (cid:2)C.As mentioned above,discontinu-
(Figure3a),thatis,anabruptincreasewithincreasing itiesinthermalprocessescanbeattributed tophase
temperature,issuggestiveofaphasetransitionbetween transitions.Indeedwhencomparedwiththemodified
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Figure 3. (a) Semilog plot of catalyst lifetimes vs growth temperatures for single SWNTs (circles) and multiple SWNTs
(triangles)grownfromFe,and(b)theFe-richpartofthemodifiedFe(cid:2)Cphasediagram.(c)Semilogplotofcatalystlifetimesvs
growthtemperaturesforNi,and(d)theNi-richpartofthemodifiedNi(cid:2)Cphasediagram.Thesolidlinesareexponentialfitsto
thedata.Theshadedhorizontalregionsinthefigurescorrespondtophasetransitionsinthemodifiedbinaryphasesystems.
Thehorizontalandverticalerrorbarsinpanelsaandcindicatea(5%variabilityincatalystlifetimevalues,anda(25(cid:2)C
uncertaintyintemperaturemeasurement,respectively.
phasediagram,onecanseethatregionIIisabovethe andcanbeexpectedtobeenhancedathighergrowth
eutectic,implying thattheFecatalystisintheliquid temperatures,causingincreasedmassloss(orgain)in
stateinthisregion.Wenotethatthereisalargedegree the active catalyst particle, and hence terminating
ofscatterintheFelifetimedata,whichoccursduetoa growthatanearliertimecomparedtogrowthatlower
distributionincatalystparticlesizes(SupportingInfor- temperatures.26Finally,wenotethatoursubstratewas
mation,FigureS3).Smallerdiametercatalystparticles siliconwithanativeoxidelayer.Featomsareknownto
arehighlyreactiveandcanbedeactivatedmoreeasily diffuse into the Si surface during SWNT growth.24
than larger particles,21 leading to shorter lifetimes. Additionally, Fe and Ni particles are known to form
Nonetheless,inspiteofthescatter,theincreaseinFe silicides at the elevated growth temperatures,27,28
catalystlifetimesbetweenregionsIandIIissignificant whichcouldcauseearlygrowthtermination.Whileit
andlikelyduetoaphasetransformation. is unclear which of these mechanisms was directly
WithinregionIorregionII,thedecreaseincatalyst responsible for growth termination, a detailed study
lifetimeswithtemperaturecouldoccurduetooneor wasbeyondthescopeofthispaper.
moreterminationmechanisms,namely,overcoatingof TheNi(cid:2)Cphasediagram(Figure3d)islesscompli-
thecatalystparticlebycarbon,22,23particlecoarsening,21 catedthantheFe(cid:2)Cphasediagram.Theeutecticpoint
orsubsurfacediffusion.24Themaximumgrowthtem- for bulk Ni(cid:2)C is ∼170 (cid:2)C higher than that of Fe(cid:2)C
perature in region I was around 1000 (cid:2)C, above the (itoccursat1326(cid:2)C,29andforournanoparticlesitis
thermal decomposition temperature of our hydro- expected to be ∼1180 (cid:2)C according to the G-T
carbon feedstock (ethylene).25 Thus the likelihood of depression). We conclude that the Ni catalyst used
excess carbon buildup at higher temperatures in- forSWNTgrowththereforeremainedinthesolidstate
creases and could cause early termination of growth over the entire growth temperature range, which
(short lifetime). Furthermore, particle coarsening via explainsthemonotonicdecreaseofcatalystlifetimes
Ostwaldripeningandsinteringisknowntoterminate withincreasingtemperatures(similartothedecrease
SWNTgrowth.21Coarseningisalsoathermalprocess inlifetimesinregionIofsolidFe,asseeninFigure3b)
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Figure4. (a)SEMimageand(b)postgrowthRamanspectrumofanindividualSWNTgrownunderthelaserspotfromaFe
catalystparticle.Asinglepeakinthelow-frequencyRBMregionandanarrowGbandidentifytheindividualSWNT.(c)SEM
imageand(d)postgrowthRamanspectrumfromapillarwheremultipleSWNTsgrewfromFecatalystparticleswithinthe
laserspot.ThepresenceofmultipleSWNTsisconfirmedbytheappearanceofseveralRBMs,aswellasabroaderGbandinthe
Ramanspectrum.
and the absence of any discontinuities (phase tran- radial breathing mode (RBM) region, a disorder in-
sitions)inNi.WhilewedidnotperformgrowthswithNi ducedpeakat∼1345cm(cid:2)1,andthegraphiticGband
attemperaturesbeyond1200(cid:2)C,itisworthnotingthat at∼1592cm(cid:2)1.30Theappearanceofasinglepeakat
thebehavioroftheNiservesasausefulreferencefor 143cm(cid:2)1intheRBMregioninFigure4bconfirmsthe
comparison with Fe and confirms the impact of the presenceofasingleSWNTonthepillarwithinthelaser-
peculiaritiesoftheirphasediagramsonSWNTgrowth. irradiatedzone.Additionalsupportisprovidedbythe
The decrease of lifetimes with temperature can be narrow linewidths of the RBM (∼4 cm(cid:2)1) and the G
attributed to reasons similar to the ones described band (∼8 cm(cid:2)1), which are characteristics of an in-
aboveforFe. dividual SWNT.31,32 In contrast, the Raman spectrum
WeperformedSEMandexsituRamanspectroscopy from the pillar that has multiple SWNTs (Figure 4c)
postgrowthtoinvestigatedifferencesbetweennano- exhibitsRBMsat165,177,and201cm(cid:2)1,aswellasa
tubes grown from solid and liquid Fe, and solid Ni. broaderGband(∼14cm(cid:2)1),confirmingthepresence
SEM images collected from the Fe catalyst pillars of multiple SWNTs. We note that growth of a single
with growth temperatures below 1000 (cid:2)C (region I, SWNT under our synthesis conditions implies low
Figure 3a) revealed that a majority contained only a nucleationefficiencies, thereasons forwhichareun-
single SWNT within the area heated by the laser,13 clear at present. One possible reason could be the
while SEM images from the pillars corresponding to largerinitialcatalystfilmthicknessesemployed,which
higher growth temperatures (region II) revealed the couldsignificantlyreducethenumberofcatalystparticles
presence of multiple SWNTs grown within the area. that are small enough to nucleate SWNTs. Neverthe-
Figure4panelsaandcshowexamplesofSEMimages less, it is important to stress that the low nucleation
of growth experiments from regions I and II. The efficienciesenabledustomeasurethegrowthkinetics
presenceofan individualSWNTand multipleSWNTs ofindividualSWNTs,therebygivingusuniqueinsights
can be clearly discerned at lower (Figure 4a) and comparedtothelargenumbersofnanotubesgrownin
higher(Figure4c)growthtemperatures,respectively. forests.
Further confirmation of this phenomenon was ob- We represent the occurrences of growth of single
tainedby postgrowth Ramanspectroscopy measure- SWNTs with circles and multiple growths of SWNTs
ments. Figure 4 panels b and d show postgrowth with triangles in Figure 3a. Interestingly, postgrowth
Ramanspectracollectedfromtheareascorresponding SEM and Raman spectroscopy analysis on the Ni
totheSEMimagesinFigure4panelsaandc,respectively. catalyst pillars showed only individual SWNTs on all
TheRamanspectra(laserexcitation2.33eV)exhibitpeaks the pillars (Supporting Information, Figure S4). The
typical of SWNTs, namely, peaks in the low frequency observationofsingleandmultipleSWNTsinregionsI
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and initial growth rates are from the ensemble (see R
SupportingInformation,FigureS5andrelateddiscussion).
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Finally, we point out another important difference
betweenFeandNi;SWNTgrowthfromFehaslonger I
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lifetimescomparedtothatofNi.Lifetimesupto2000s
wereobservedinSWNTsgrowingfromFe,whichisa
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factorof6higherthanthelongestlifetimeobservedforNi.
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Thediffusionofcarboninsolidγ-FeandsolidNiare
ofthesameorderofmagnitude;34,35however,thesolu-
bilitiesofcarboninFeandNiaredifferent.Althoughthe
carbonsolubilityhasbeenpredictedtoreduce(Fe(cid:2)C)36
orincrease(Ni(cid:2)C)37viadifferingmechanismsduetothe
size reduction of nanoparticles, the values for the bulk
systems can be used as a reasonable starting point.
AccordingtothebulkFe(cid:2)CandNi(cid:2)Cphasediagrams,29
the solvus of γ-Fe is ∼4 times higher than that of Ni,
makingFemoreefficientatgrowingSWNTs,andcould
Figure5. Histogramsoflengthscorrespondingtoindividual
SWNTs (bottom panel) grown at temperatures < 1100 (cid:2)C explainthelongerlifetimesobservedinFe.
(regionI,Figure3b)andmultipleSWNTs(toppanel)grown
attemperatures>1100(cid:2)C(regionII,Figure3b). CONCLUSIONS
andIIcorrelatesverywellwiththephasetransitionsof WeusedinsituRamanspectroscopytomeasurethe
thecatalystinregionsIandII,implyingthatthesolid lifetimes of Fe and Ni catalysts over a wide range of
catalyst produced single SWNTs for both Fe and Ni, growth temperatures. With our custom rapid experi-
whilethenucleationdensitywashigherwiththeliquid mentation (ARES) system, we carried out over 100
Fecatalyst,producingmultipleSWNTs.Indeed,liquid experiments,enablingustogleanimportantinsights
Fehasahighersolubilityforcarbon,andhenceprob- into the growth process that would have otherwise
ablyahigheractivityforSWNTgrowth.Furthermore, beenmissed.Ourresultsrevealedadiscontinuityinthe
thediffusionofcarbonthroughliquidFeisfasterthan Fe catalyst lifetimes around 1100 (cid:2)C due to a solid-
diffusionthroughsolidγ-Fe(by2ordersofmagnitude),33,34 to-liquidphasetransition. Incontrast, themonotonic
thustheSWNTsgrownfromliquidFeareexpectedto behavioroftheNilifetimesmeantthatNiwasinthe
be longer.6 SEM analysis revealed that the SWNTs in solid state through the whole range of growth tem-
regionIIweregenerallylongerthantheSWNTsfrom peratures. The behavior of the Fe catalyst was ex-
region I (see histograms in Figure 5). We note that a plained by comparison with the binary metal(cid:2)carbon
detailed analysis of diffusion and reduction rates in phasediagrams,aswellasexperimentalevidence,which
solidand liquid Fe catalyst particlesand their conse- showed dramatic increases in the nucleation densities
quencestowardnanotubegrowthwouldprovidevital andlongerlifetimeswhencrossingthephaseboundary
clues to prove the occurrence of a phase transition; fromsolidtotheliquidstate.OurresultsprovethatSWNT
such studies are currently in progress and will be growth is feasible from both solid and liquid catalyst
reported in the future. We also note that the in situ dependingonthegrowthtemperature,resolvingalong-
growth curves measured from experiments that pro- standingdebateinthefield.Asadirectconsequence,the
duced single and multiple SWNTs were fitted to a length and nucleation density of the SWNTs can be
singleexponentialcurve.Inotherwords,intheexperi- tailored for specific applications by engineering the
ments that produced multiple SWNTs, the lifetime correctcatalystandgrowthtemperature.
METHODS nanotubegrowth.Priortoeachgrowthexperiment,thecham-
GrowthoftheSWNTstookplacevialaser-inducedCVDinside berwaspumpeddowntoabasepressureof∼9(cid:3)10(cid:2)6Torr,
followedbybackfillingargontothegrowthpressureof25Torr.
ahighvacuumchambercoupledtoaRamanmicroscope,as
Atthistimeaconstantflowofargonandhydrogenwasinitiated
shown schematically in Figure 1. The samples consisted of
at25and10sccm,respectively,whilethechamberpressurewas
siliconpillars,10μmindiameter,spaced40μmapart,formed
maintainedat25Torr.TheRamanexcitationlaserwasfocused
bydeepreactiveionetchingofa10μmtoplayerofasilicon- oneachpillarthroughalongworkingdistance40(cid:3)objective
on-insulator substrate. The small thermal mass of the pillar
lensandheatingwasachievedbyincreasingthelaserpower.
allowedforefficientheatingtothegrowthtemperature(from Thetemperatureofthesiliconpillarswasestimatedusingthe
500to1400(cid:2)C)usingafewhundredmilliwattsoflaserpower. ratiooftheanti-StokestoStokespeaksintheRamanspectra
Theoxidebarrierlayerbelowthepillarsenhancedheatingby fromthepillars.38Oncethelaserwasfocusedonapillar,the
restrictingconductiveheattransferbetweenthepillarandthe powerwasincreasedtoheatituptothegrowthtemperature,
underlying bulk silicon substrate. Thin films (2(cid:2)3 nm) of Fe followedbytheintroductionof5sccmofethylenetoinitiate
andNi,sputteredonthesubstrateswereusedascatalystsfor nanotubegrowth.Ramanspectrabetween(cid:2)2000and3000cm(cid:2)1
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werecollectedandsavedatone-secondintervalsduringthe 15. Li-Pook-Than,A.;Lefebvre,J.;Finnie,P.PhasesofCarbon R
growth experiment. Nanotube nucleation and growth was NanotubeGrowthandPopulationEvolutionfrominSitu
detected by the appearance, and subsequent increase in RamanSpectroscopyDuringChemicalVaporDeposition. T
intensity,oftheGbandat1500(cid:2)1600cm(cid:2)1.Theethyleneflow J.Phys.Chem.C2010,114,11018–11025.
wasturnedoffaftertheGbandintensitystabilizedfollowingthe 16. Chiashi,S.;Murakami,Y.;Miyauchi,Y.;Maruyama,S.Cold I
initialincrease,thusindicatingterminationofgrowth. WallCVDGenerationofSingle-WalledCarbonNanotubes C
Conflict of Interest: The authors declare no competing andinSituRamanScatteringMeasurementsoftheGrowth
financialinterest. Stage.Chem.Phys.Lett.2004,386,89–94. L
17. Einarsson, E.; Murakami, Y.; Kadowaki, M.; Maruyama, S.
E
Acknowledgment. The authors gratefully acknowledge Growth Dynamics of Vertically Aligned Single-Walled
funding from the Air Force Office of Scientific Research Carbon Nanotubes from in Situ Measurements. Carbon
2008,46,923–930.
(AFOSR)and theNationalResearch Council. Wealsoexpress
18. Wako, I.; Chokan, T.; Takagi, D.; Chiashi, S.; Homma, Y.
ourgratitudetoJunLouforproducingthepillarsubstratesand
Direct Observation of Single-Walled Carbon Nanotube
thankOhadLevyforusefuldiscussions.
GrowthProcessesonSiO SubstratebyinSituScanning
SupportingInformationAvailable:Discussiononexponential ElectronMicroscopy.Chem2.Phys.Lett.2007,449,309–313.
fitstothedata,histograms,andSEMimagesofinitialandfinal 19. Jiang, A.; Awasthi, N.; Kolmogorov, A. N.; Setyawan, W.;
particlesizedistributions,SEMandRamanspectrumcollected Börjesson,A.;Bolton,K.;Harutyunyan,A.R.;Curtarolo,S.
from an individual SWNT grown from Ni. This material is Theoretical Study of the Thermal Behavior of Free and
availablefreeofchargeviatheInternetathttp://pubs.acs.org. Alumina-SupportedFe-CNanoparticles.Phys.Rev.B2007,
75,205426.
20. Harutyunyan,A.R.;Mora,E.;Tokune,T.;Bolton,K.;Rosén,
REFERENCESANDNOTES
A.;Jiang,A.;Awasthi,N.;Curtarolo,S.HiddenFeaturesof
1. Baughman,R.H.;Zakhidov,A.A.;deHeer,W.A.Carbon the Catalyst Nanoparticles Favorable for Single-Walled
Nanotubes;The Route toward Applications. Science CarbonNanotubeGrowth.Appl.Phys.Lett.2007,90,3120.
2002,297,787. 21. Amama,P.B.;Pint,C.L.;McJilton,L.;Kim,S.M.;Stach,E.A.;
2. Steiner, S. A.; Baumann, T. F.; Bayer, B. C.; Blume, R.; Murray,P.T.;Hauge,R.H.;Maruyama,B.RoleofWaterin
Worsley,M.A.; MoberlyChan, W.J.;Shaw,E.L.;Schlogl, SuperGrowthofSingle-WalledCarbonNanotubeCarpets.
R.;Hart,A.J.;Hofmann,S.;etal. NanoscaleZirconiaas NanoLett.2009,9,44–49.
a Nonmetallic Catalyst for Graphitization of Carbon 22. Puretzky,A.A.;Geohegan,D.B.;Jesse,S.;Ivanov,I.N.;Eres,
andGrowthofSingle-andMultiwallCarbonNanotubes. G.InSituMeasurementsandModelingofCarbonNano-
J.Am.Chem.Soc.2009,131,12144–12154. tubeArray GrowthKinetics During ChemicalVaporDe-
3. Yoshida,H.;Takeda,S.;Uchiyama,T.;Kohno,H.;Homma,Y. position.Appl.Phys.A:Mater.Sci.Process.2005,81,223–
Atomic-Scale in-Situ Observation of Carbon Nanotube 240.
GrowthfromSolidStateIronCarbideNanoparticles.Nano 23. Yamada,T.;Maigne,A.;Yudasaka,M.;Mizuno,K.;Futaba,
Lett.2008,8,2082–2086. D.N.;Yumura,M.;Iijima,S.;Hata,K.RevealingtheSecretof
4. Wirth,C.T.;Bayer,B.C.;Gamalski,A.D.;Esconjauregui,S.; Water-Assisted Carbon Nanotube Synthesis by Micro-
Weatherup, R. S.; Ducati, C.; Baehtz, C.; Robertson, J.; scopic Observation of the Interaction of Water on the
Hofmann, S. The Phase of Iron Catalyst Nanoparticles Catalysts.NanoLett.2008,8,4288–4292.
During Carbon Nanotube Growth. Chem. Mater. 2012, 24. Kim,S.M.;Pint,C.L.;Amama,P.B.;Zakharov,D.N.;Hauge,
4633–4640. R. H.; Maruyama, B.; Stach, E. A. Evolution in Catalyst
5. Rao,R.;Eyink,K.G.;Maruyama,B.Single-WalledCarbon MorphologyLeadstoCarbonNanotubeGrowthTermina-
NanotubeGrowthfromLiquidGalliumandIndium.Car- tion.J.Phys.Chem.Lett.2010,1,918–922.
bon2010,48,3971–3973. 25. Kamat,P.V.Graphene-BasedNanoarchitectures.Anchor-
6. Harutyunyan,A.;Tokune,T.;Mora,E.LiquidasaRequired ing Semiconductor and Metal Nanoparticles on a Two-
Catalyst Phase for Carbon Single-Walled Nanotube DimensionalCarbonSupport.J.Phys.Chem.Lett.2009,1,
Growth.Appl.Phys.Lett.2005,87,051919. 520–527.
7. Cervantes-Sodi,F.;McNicholas,T.P.;Simmons,J.G.,Jr.;Liu, 26. Picher,M.;Anglaret,E.;Arenal,R.;Jourdain,V.Processes
J.;Csányi,G.;Ferrari,A.C.;Curtarolo,S.ViscousStateEffect Controlling the Diameter Distribution of Single-Walled
on theActivity of FeNanocatalysts. ACS Nano 2010, 4, CarbonNanotubesDuringCatalyticChemicalVaporDe-
6950–6956. position.ACSNano2011,5,2118–2125.
8. Reich,S.;Li,L.;Robertson,J.ControltheChiralityofCarbon 27. Simmons, J. M.; Nichols, B. M.; Marcus, M. S.; Castellini,
NanotubesbyEpitaxialGrowth.Chem.Phys.Lett.2006, O.M.;Hamers,R.J.;Eriksson,M.A.CriticalOxideThickness
421,469–472. forEfficientSingle-WalledCarbonNanotubeGrowthon
9. Koziol,K.K.K.;Ducati,C.;Windle,A.H.CarbonNanotubes SiliconUsingThinSiO DiffusionBarriers.Small2006,2,
2
withCatalystControlledChiralAngle.Chem.Mater.2010, 902–909.
22,4904–4911. 28. Jung, Y. J.; Wei; Vajtai, R.; Ajayan, P. M.; Homma, Y.;
10. Harutyunyan,A.R.;Chen,G.;Paronyan,T.M.;Pigos,E.M.; Prabhakaran,K.;Ogino,T.MechanismofSelectiveGrowth
Kuznetsov,O.A.;Hewaparakrama,K.;Kim,S.M.;Zakharov, ofCarbonNanotubesonSiO/SiPatterns.NanoLett.2003,
2
D.;Stach,E.A.;Sumanasekera,G.U.PreferentialGrowthof 3,561–564.
Single-WalledCarbonNanotubeswithMetallicConduc- 29. Binary Alloy Phase Diagams, 2nd ed.; Massalski, T. B.,
tivity.Science2009,326,116–120. Okamoto,H.,Eds.;ASMInternationalMaterials:Park,OH,
11. Chiang,W.-H.;Sankaran,R.M.LinkingCatalystComposi- 1990.
tiontoChiralityDistributionsofas-GrownSingle-Walled 30. Jorio,A.;Dresselhaus,M.S.;Saito,R.RamanSpectroscopy
CarbonNanotubesbyTuningNixfe1-XNanoparticles.Nat. in Graphene Related Systems: Carbon Nanotubes, Nano-
Mater.2009,8,1–5. graphiteandGraphene;Wiley-VCH:Weinheim,Germany,
12. Hofmann,S.;Csányi,G.;Ferrari,A.C.;Payne,M.C.;Robertson, 2011.
J. Surface Diffusion: The Low Activation Energy Path for 31. Rao,R.;Menendez,J.;Poweleit,C.D.;Rao,A.M.Anharmo-
NanotubeGrowth.Phys.Rev.Lett.2005,95,036101. nicPhononLifetimesinCarbonNanotubes:Evidencefora
13. Rao,R.;Liptak,D.;Cherukuri,T.;Yakobson,B.I.;Maruyama, One-Dimensional Phonon Decay Bottleneck. Phys. Rev.
B.InSituEvidenceforChirality-DependentGrowthRates Lett.2007,99,047403–047404.
ofIndividualCarbonNanotubes.Nat.Mater.2012,11,1–4. 32. Jorio,A.;Fantini,C.;Dantas,M.;Pimenta,M.;SouzaFilho,A.;
14. Picher,M.;Anglaret,E.;Arenal,R.;Jourdain,V.Self-Deactivation Samsonidze,G.;Brar,V.;Dresselhaus,G.;Dresselhaus,M.;
ofSingle-WalledCarbonNanotubeGrowthStudiedbyinSitu Swan,A.;etal.LinewidthoftheRamanFeaturesofIndividual
RamanMeasurements.NanoLett.2009,9,542–547. Single-WallCarbonNanotubes.Phys.Rev.B2002,66,115411.
RAOETAL. VOL.XXX ’ NO.XX ’ 000–000 ’ XXXX G
www.acsnano.org
A
33. Goldberg,D.;Belton,G.TheDiffusionofCarboninIron- R
Carbon Alloys at 1560(cid:2)C. Met. Mater. Trans. B 1974, 5,
1643–1648. T
34. Tibbetts,G.G.DiffusivityofCarboninIronandSteelsat
HighTemperatures.J.Appl.Phys.1980,51,4813–4816. I
35. Xiang,Q.;Yu,J.;Jaroniec,M.SynergeticEffectofMoS and C
2
GrapheneasCocatalystsforEnhancedPhotocatalyticH
2
Production Activity of TiO Nanoparticles. J. Am. Chem. L
2
Soc.2012,134,6575–6578.
E
36. Harutyunyan, A.R.;Awasthi,N.;Jiang,A.; Setyawan, W.;
Mora, E.; Tokune, T.; Bolton, K.; Curtarolo, S. Reduced
CarbonSolubilityinFeNanoclustersandImplicationsfor
theGrowthofSingle-WalledCarbonNanotubes.Phys.Rev.
Lett.2008,100,195502.
37. Diarra,M.;Zappelli,A.;Amara,H.;Ducastelle,F.;Bichara,C.
ImportanceofCarbonSolubilityandWettingPropertiesof
Nickel Nanoparticles for Single Wall Nanotube Growth.
Phys.Rev.Lett.2012,109,185501.
38. Hart,T.;Aggarwal,R.;Lax,B.TemperatureDependenceof
RamanScatteringinSilicon.Phys.Rev.B1970,1,638–642.
RAOETAL. VOL.XXX ’ NO.XX ’ 000–000 ’ XXXX H
www.acsnano.org