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Con olled syn hesis o single-chi ali y ca bon nano ubes
Juan Ramon Sanchez-Valencia1†, Thomas Dienel1, Oli e G öning1, I an Sho ubalko2, And eas Muelle 3†, Ma in
Jansen3, Kons an in Amsha o 3†, Pascal Ru ieux1 & Roman Fasel1,4
1nano ech@su aces Labo a o y, Empa, Swiss Fede al Labo a o ies o Ma e ials Science and Technology, 8600
Dübendo , Swi ze land.
2Labo a o y o eliabili y science and echnology, Empa, Swiss Fede al Labo a o ies o Ma e ials Science and
Technology, 8600 Dübendo , Swi ze land.
3Max Planck Ins i u e o Solid S a e Resea ch, Heisenbe gs asse 1, 70569 S u ga , Ge many.
4Depa men o Chemis y and Biochemis y, Uni e si y o Be n, F eies asse 3, 3012 Be n, Swi ze land
†P esen add esses: Nano echnology on Su aces Labo a o y, Ins i u o de Ciencia de Ma e iales de Se illa (CSIC-US),
A da. Amé ico Vespucio 49, E-41092 Se illa, Spain (J.R.S.-V.); BASF SE, GVM/I–L 544, 67056 Ludwigsha en,
Ge many (A.M.); Uni e si y E langen-Nu embe g, Ins i u ü O ganische Chemie II, Henkes asse 42, 91054 E langen,
Ge many (K.A.).
O e he las wo decades, single-walled ca bon nano ubes (SWCNTs) ha e ecei ed much
a en ion because hei ex ao dina y p ope ies a e p omising o nume ous applica ions1,2.
Many o hese p ope ies depend sensi i ely on SWCNT s uc u e, which is cha ac e ized by
he chi al index (n,m) ha deno es he leng h and o ien a ion o he ci cum e en ial ec o in
he hexagonal ca bon la ice. Elec onic p ope ies a e pa icula ly s ongly a ec ed, wi h
sub le s uc u al changes swi ching ubes om me allic o semiconduc ing wi h a ious
bandgaps. Monodispe se ‘single-chi ali y’ ( ha is, wi h a single (n,m) alue) SWCNTs a e hus
needed o ully exploi hei echnological po en ial1,2. Con olled syn hesis h ough ca alys
enginee ing3–6, end-cap enginee ing7 o cloning s a egies8,9 and also ube so ing based on
ch oma og aphy10,11, densi y-g adien cen i uga ion, elec opho esis and o he echniques12
ha e deli e ed SWCNT samples wi h na ow dis ibu ions o ube diame e and en iched in a
pa icula ube ype. Bu an e ec i e pa hway o uly monodispe se SWCNTs emains
elusi e. The use o empla e molecules o unambiguously dic a e he diame e and chi ali y o
he esul ing nano ube8,13–16 holds g ea p omise in his ega d, bu has hi he o had only
limi ed p ac ical success7,17,18. He e we show ha his bo om-up s a egy can p oduce a ge ed
nano ubes: we con e molecula p ecu so s in o ul asho singly capped (6,6) ‘a mchai ’
nano ube seeds using su ace-ca alysed cyclodehyd ogena ion on a P (111) su ace, and hen
elonga e hese du ing a subsequen g ow h phase o p oduce single-chi ali y and essen ially
de ec - ee SWCNTs wi h leng hs up o a ew hund ed nanome es. We expec ha ou on-
su ace syn hesis app oach will p o ide a ou e o nano ube-based ma e ials wi h highly
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op imized p ope ies o applica ions such as ligh de ec o s, pho o ol aics, ield-e ec
ansis o s, and senso s2.
Recen wo k has p oduced non-plana ca bon-based nanos uc u es such as ulle enes, ca bon
py amids, and buckybowls om hei co esponding quasi-plana polycyclic a oma ic hyd oca bon
p ecu so s h ough su ace-ca alysed cyclodehyd ogena ion (CDH)19–21. We ha e ex ended he
me hodology o he syn hesis o ul asho singly capped SWCNTs, ha is, a SWCNT end cap wi h a
sho ube segmen a ached. Such molecules ep esen ideal seeds o subsequen epi axial
elonga ion in o isome ically pu e SWCNTs. Fo mally, his app oach mimics he con en ional
syn hesis o SWCNTs by he oo -g ow h mechanism, in which nano ube g ow h s a s by nuclea ion
o an end-cap agmen on a me al nanopa icle22. The key poin is o a oid uncon olled,
spon aneous nuclea ion o end caps by p o iding a omically p ecise ul asho nano ube seeds which
unambiguously dic a e he chi al index o SWCNTs o ming on epi axial elonga ion.
P ecu so P1 (C96H54; Fig. 1) was designed and syn hesized by mul i-s ep o ganic syn hesis
o ackle his challenge ( o de ails, see Me hods). Upon in amolecula CDH i a o ds seed S1, an
ul a-sho singly capped (6,6) SWCNT bea ing a ca bon nano ube segmen . The selec i e g ow h o
(6,6) SWCNTs is illus a ed in Fig. 1 and combines wo s eps: (1) o ma ion o seed S1, and (2)
subsequen epi axial elonga ion. The i s s ep is ealized by deposi ing p ecu so P1 on a P (111)
su ace ollowed by annealing o 770 K unde ul ahigh acuum condi ions o induce he su ace-
ca alysed CDH eac ion (Fig. 2a, b). The second s ep, epi axial elonga ion, is achie ed by he
inco po a ion o ca bon a oms o igina ing om he su ace-ca alysed decomposi ion o a ca bon
eeds ock gas (Fig. 3a–c).
Figu e 2c shows a scanning unnelling mic oscopy (STM) image acqui ed a e deposi ing P1
on P (111). No s ep deco a ion o island o ma ion is obse ed. In e ac ions wi h su ounding
molecules and s ep edges a e hus la gely supp essed and ensu e subsequen unpe u bed CDH o he
p ecu so 19,20. Fo he majo i y o he as-deposi ed p ecu so s, STM e eals a h ee- old symme ic
con o ma ion. Howe e , he in insic axial chi ali y o he benzo[c]phenan h ene agmen s
([4]helicene) (see Ex ended Da a Fig. 1) and he con igu a ional lexibili y o he pe iphe al biphenyl
agmen s p oduce a la ge a ie y o possible geome ies (Fig. 2c, Ex ended Da a Fig. 1). The
p esence o axial chi ali y in he [4]helicene moie ies esul s in ou possible s e eoisome s and hus
eigh possible adso p ion geome ies (Ex ended Da a Fig. 1). An example o a molecule wi h i s h ee
ou e biphenyl g oups loca ed closes o he su ace is shown in Fig. 2e, oge he wi h he
co esponding STM simula ion based on he ex ended lowes unoccupied molecula o bi al (LUMO;
see Me hods). The excellen ag eemen be ween STM image and simula ion (Ex ended Da a Fig. 1)
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indica es ha he di e en opog aphic ea u es obse ed o he adso bed p ecu so s can be
a ibu ed o he di e en adso p ion geome ies. Impo an ly, he s e eoisome ism does no a ec he
CDH p ocess, since all chi al cen es will disappea du ing in amolecula cycliza ion.
Al hough P1 is designed o yield seed S1, he con o ma ional lexibili y o he pe iphe al
biphenyl g oups leads pa ially o undesi ed adso p ion geome ies. In con as o he s e eoisome s
discussed abo e, hese molecules will ollow a di e en CDH pa hway, ending in he o ma ion o
undesi ed buckybowls (Ex ended Da a Fig. 2). A s a is ical analysis o mo e han 100 p ecu so
monome s obse ed by STM e ealed ha mo e han 50% adop he desi ed con igu a ions
(Ex ended Da a Fig. 1). Mos impo an ly, he condensa ion p oduc s o p ecu so molecules
exhibi ing ‘w ong’ con o ma ions canno ac as seeds o he subsequen CNT g ow h p ocess ia
epi axial elonga ion, and hus will no a ec he selec i i y o SWCNT o ma ion.
Su ace-ca alysed CDH o p ecu so s (P1) in o seeds (S1) is induced by annealing a 770 K
o 10 min. STM images (Fig. 2d) show ha he o iginally quasi-plana h ee- old symme ic
molecules ans o m in o dome-shaped species wi h a p ominen inc ease in appa en heigh om 2
o 4.5 Å (Fig. 2 ). Addi ional p oo o success ul dehyd ogena ion o P1 in o S1 de i es om he
good ag eemen o high- esolu ion STM images and simula ions o he on ie molecula o bi als o
S1 (Fig. 2g). Bo h esul s demons a e he success ul o ma ion o he a ge ed singly capped
ul asho (6,6) SWCNT S1.
In he second s ep, he su ace-ancho ed seeds S1 a e g own in o (6,6) SWCNTs ia epi axial
elonga ion by exposing hem o a ca bon eeds ock gas such as e hylene o e hanol a empe a u es
be ween 670 and 770 K (see schema ic illus a ion in Fig. 3a–c). The p e-syn hesized seeds S1 a e
ex ended by ca aly ic epi axial elonga ion, which consis s o a consecu i e inco po a ion o ca bon
a oms (schema ically shown as C2) o igina ing om he decomposi ion o e hanol o e hylene. The
open pa o S1 is al eady in he equi ed con ac wi h he P (111) su ace, which ca alyses u he
epi axial elonga ion. In o de o unambiguously demons a e he ac i i y o S1 in an epi axial
elonga ion p ocess, he esul s o low exposu es o ca bon eeds ock gas we e ollowed in si u by
STM. Low doses o e hylene a 770 K p oduce a subs an ial inc ease in appa en heigh om 4 o
20 Å (Fig. 3d–g). A e exposu e o 1 Langmui (L) o e hylene, abou 18% o he ini ially deposi ed
p ecu so s P1 ha e g own in o SWCNTs, a densi y ha emains cons an o highe exposu es o
5 L. Exposu e o ye highe doses o e hylene o e hanol p oduces s ong changes in he su ace
opog aphy and makes STM imaging inc easingly di icul (Fig. 3h). The opog aphy becomes ough
and he P su ace is no longe disce nible. Ca e ul examina ion o he STM images e eals he
p esence o one-dimensional s uc u es lying ac oss he su ace (Fig. 3h), wi h an abundance o abou
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5 pe m2 and obse ed leng hs exceeding 200 nm. High esolu ion images o he elonga ed
s uc u es (Fig. 3i) e eal an in e nal s uc u e o highe con as lines along he di ec ion o he ube
axis. A supe imposed s uc u al model clea ly shows ha hese lines a e indeed he ca bon posi ions
o he g aphene s uc u e in a (6,6) SWCNT. In all cases whe e a omic esolu ion o he ubes could
be achie ed, he s uc u e p o ed o be consis en wi h a (6,6) SWCNT. The ou s anding ag eemen
o bo h he o ien a ion and he pe iodici y o he g aphene la ice wi h hose expec ed demons a e
ha he one-dimensional s uc u es a e he a ge ed (6,6) SWCNTs, ben ho izon ally ac oss he
sample su ace.
To u he co obo a e he densi y and leng h o he ho izon ally aligned SWCNTs, he
p e ious sample was imaged wi h a scanning helium ion mic oscope (SHIM). Tubes longe han
300 nm and wi h diame e s below 2 nm ( he highes esolu ion achie ed) could be obse ed
(Ex ended Da a Fig. 3), wi h a densi y o 3–4 ubes pe m2 ha is simila o he densi y es ima ed
om STM images (5 pe m2). We no e ha bo h STM and SHIM can only image ho izon ally
aligned SWCNTs, bu do no gi e access o e ically aligned SWCNTs.
To shed ligh on he o ien a ion o he SWCNTs and o cha ac e ize he selec i i y o he
g ow h p ocess, Raman cha ac e iza ion was pe o med. Measu emen s wi h he illumina ing lase
beam a no mal incidence (pe pendicula o he su ace plane) yielded ex emely low in ensi ies; he
G band, he mos in ense ea u e a a ound 1,590 cm−1, is e y weak (g ey cu e in Fig. 4a).
Howe e , when samples we e measu ed unde an illumina ion angle o 30°, all bands inc eased
signi ican ly in in ensi y (black cu e in Fig. 4a), as expec ed o a dominan ac ion o SWCNTs
o ien ed pe pendicula o he su ace. The pola izabili y o hese e ically o ien ed ubes wi h he
illumina ing lase beam a no mal incidence – and hus he elec omagne ic ield ec o
pe pendicula o he ube axis – is d as ically educed, esul ing in weak Raman in ensi ies23. A
p edominan ly e ical alignmen o he SWCNTs is consis en wi h he obse a ion ha in SHIM
images some CNTs appea o shake unde he ion beam (Ex ended Da a Fig. 3), and wi h he ough
su ace seen in STM. I also implies ha he o e all SWCNT densi y is expec ed o be signi ican ly
highe han ha es ima ed om he STM and SHIM da a.
Mo e impo an ly, he Raman spec a demons a e he high selec i i y o ou g ow h p ocess,
which p oduces (6,6) SWCNTs exclusi ely. The spec um shown in Fig. 4b p esen s clea ly de ined
bands a he posi ions expec ed o (6,6) SWCNTs. The band a 295 cm−1 is associa ed wi h he
adial b ea hing mode (RBM) whose equency depends s ongly on he nano ube diame e 23.
Empi ical equa ions p edic an RBM equency o 280–295 cm−1 o (6,6) SWCNTs23.
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Expe imen ally, he RBM o (6,6) SWCNTs deposi ed on SiO2 has been epo ed a a ound
289 cm−1 ( e . 24), bu i is well known ha he subs a e plays a c ucial ole in he RBM posi ion,
explaining he small de ia ion obse ed25. Mos impo an ly, ou Raman spec a do no show any
u he bands wi hin he RBM ange (200–400 cm−1), which unde lines he ex emely high selec i i y
o he p ocess. Ano he no able cha ac e is ic o he RBM is he excep ionally small wid h obse ed
( he ull-wid h a hal -maximum, FWHM, is 3.5 cm−1, limi ed by he esolu ion o he ins umen ),
which is as small as ha om isola ed indi idual small diame e SWCNTs (an FWHM o 3 cm−1)26.
The Raman spec a epo ed he e a e age o e a la ge a ea (beam diame e 1.5-10 µm (inse in Fig.
4b)), and hus e lec he p ope ies o a la ge numbe o SWCNTs. The na ow RBM peak he e o e
demons a es a e y high deg ee o monodispe si y.
The G band appea s as a double peak a 1,518 and 1,591 cm−1. The signi ican cu a u e in
small diame e SWCNTs causes a shi o lowe equencies o he op ical ib a ions associa ed wi h
ans e se (pe pendicula o he ube axis) a omic displacemen s ( he G− band)27. Al hough, o ou
knowledge, his spli ing has no been obse ed expe imen ally o a (6,6) SWCNT, he G+ and G−
band spli ing (

G) has been p edic ed o be 83 cm−1 ( e . 28), in good ag eemen wi h he spli ing
ha we obse e (

G = 73 cm−1). The addi ional peaks in he ange om 400 o 1,200 cm−1 ha e
p e iously been obse ed and used as e idence o he p esence o a mchai SWCNTs, since no peak
in his ange is p esen o semiconduc ing ubes29. Finally, he absence o any D band in he Raman
spec a (Fig. 4b) u he unde lines he ex eme cleanliness o ou p ocess ha yields no only
p ede ined single-chi ali y bu also essen ially de ec - ee SWCNTs.
These indings clea ly illus a e ha he use o a plana me al su ace ins ead o me al
nanopa icles e ec i ely sup esses spon aneous cap o ma ion and ha his, in combina ion wi h he
use o a sui able p ecu so species o p oduce a desi ed cap, enables highly selec i e SWCNT
ab ica ion. The en i e in si u p ocess and he low empe a u es in ol ed in bo h he CDH s ep
(770 K) and he subsequen epi axial elonga ion (670 K) a e ully compa ible wi h complemen a y
me al oxide semiconduc o (CMOS) echnology, and ou me hods migh he e o e sol e wo pi o al
challenges in he ealiza ion o CNT-based in eg a ed ci cui s o digi al elec onics: o p o ide
SWCNTs wi h iden ical elec onic p ope ies, and o in eg a e hese SWCNTs in de ice a chi ec u es
(which may be achie ed by si e-speci ic deposi ion o he molecula p ecu so and/o he ca alys
ilm). Howe e , u he p og ess ha aims o be echnologically ele an equi es a p ocess wi h
highe g ow h yields ha ideally app oach uni y. While we see conside able scope o op imiza ion
o he p esen p ocess (in e ms o ca alys , empe a u es, p essu es, and so on), an al e na i e is o
eplace he subs a e-ca alysed epi axial elonga ion s ep by a mo e e icien p ocess, such as ho

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ilamen chemical apou deposi ion30. This would also allow mo e lexibili y in independen ly
op imizing seed o ma ion and ube elonga ion while simul aneously supp essing spon aneous
SWCNT g ow h a bo h s ages. Ano he logical ex ension o he p ocess is o o he SWCNTs whe e
sub le ies ela ed o he chi al index o he seed may come in o play.
Recei ed 17 Ap il; accep ed 17 June 2014; doi:10.1038/na u e13607.
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Acknowledgemen s This esea ch was suppo ed in pa by he Swiss Na ional Science Founda ion and by he S a e
Sec e a ia o Educa ion, Resea ch and Inno a ion ia he COST Ac ion MP0901 ‘NanoTP’. K.A. acknowledges
inancial suppo om Deu sche Fo schungsgemeinscha .
Au ho Con ibu ions K.A., M.J. and R.F. ini ia ed and concei ed his wo k. K.A. designed he p ecu so molecules
and he co esponding syn he ic ou es, K.A. and A.M. syn hesised p ecu so molecules and pe o med HPLC, NMR and
MS analyses. J.R.S.-V. ca ied ou on-su ace syn hesis wo k. J.R.S.-V. and T.D. pe o med STM and Raman
measu emen s, I.S. did he He ion mic oscopy analysis. O.G. pe o med he calcula ions. All au ho s pa icipa ed in
analysis and in e p e a ion o he esul s. J.R.S.-V. d a ed he manusc ip , wi h con ibu ions om P.R. and O.G. R.F.
and K.A. edi ed he manusc ip and coo dina ed he e o s o he esea ch eams.
Au ho In o ma ion Rep in s and pe missions in o ma ion is a ailable a www.na u e.com/ ep in s. The au ho s decla e
no compe ing inancial in e es s. Reade s a e welcome o commen on he online e sion o he pape . Co espondence
and eques s o ma e ials should be add essed o R.F. ( oman. [email p o ec ed]) o K.A. (kons an in.amsha o @ au.de).
Figu e 1 | Two-s ep bo om-up syn hesis o SWCNTs. (1) Fo ma ion o singly capped ul asho
(6,6) SWCNT seed S1 ia cyclodehyd ogena ion (CDH) o he sui ably designed polya oma ic
hyd oca bon p ecu so C96H54 (P1). (2) Nano ube g ow h ia epi axial elonga ion (EE) . Pa s o he
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p ecu so P1 in ol ed in he o ma ion o he SWCNT end cap and he ul asho CNT segmen o
he seed S1 a e highligh ed in yellow and blue, espec i ely. Red dashed lines indica e he new C-C
bonds o med upon CDH. Epi axial elonga ion occu s ia he successi e addi ion o ca bon species,
as indica ed in g een.
Figu e 2 | Fo ma ion o (6,6) SWCNT seeds S1. a, b, Illus a ion o he he mally induced su ace
ca alysed CDH o o m he (6,6) SWCNT seed S1 om he adso bed p ecu so P1. c, d, STM images
o p ecu so molecules as deposi ed on P (111) (c) and a e annealing o 770 K (d). e, Close-up
STM image o a p ecu so ( op) and he co esponding simula ion based on he ex ended LUMO
(g eyscale) wi h a s uc u al model o he molecule supe imposed (bo om). , Line p o iles
(posi ions indica ed in c, d) o e an as-deposi ed p ecu so P1 (g ey line) and he seed species S1
ob ained a e annealing o 770 K (black line). g, Close-up STM images aken a −1 V and 0.1 V o
(6,6) SWCNT seed S1 (le ) and he co esponding simula ions o HOMO and LUMO, espec i ely
(g eyscale, igh ).
Figu e 3 | Epi axial elonga ion o singly capped SWCNT wi h (6,6) chi al index de ined by he
seed S1. a–c, Schema ic illus a ion o he epi axial elonga ion o (6,6) SWCNT seeds S1 ia
su ace-ca alysed C2 inco po a ion a he nano ube/me al in e ace. The use o e hanol as a ca bon
eeds ock gas is illus a ed, which decomposes on he ho , eac i e P su ace in o C2 species and
binds ‘epi axially’ o he bay egion o he p e iously o med singly capped SWCNT, esul ing in an
elonga ion o he ube along i s axis. d– , STM images o he as-p epa ed (6,6) SWCNT seeds (d),
and a e exposu e o low doses o e hylene o 1 L (e) and 5 L ( ), espec i ely, a a empe a u e o
770 K. Fo di ec compa ison, he same heigh colou scale is used o d– . g, Line p o iles aken
ac oss he ea u es indica ed by a ow pai s in he co esponding STM images d– . h, i, STM images
o a sample exposed o a p essu e o 1 × 107 mba o e hanol o 1 h (270 L) a a empe a u e o
770 K. A long SWCNT is obse ed o lie on he ough su ace (h). A close-up STM image (i)
iden i ies i as a (6,6) SWCNT.
Figu e 4 | SWCNT o ien a ion de e mina ion and single chi ali y assessmen by Raman
spec oscopy. a, Raman spec a o epi axially elonga ed (6,6) SWCNTs ob ained by exposing seeds
S1 o a p essu e o 1 × 107 mba o e hanol o 30 min (140 L) a a empe a u e o 670 K. The
spec a we e acqui ed wi h he sample su ace pe pendicula (g ey cu e) and a an angle o 30°
(black cu e) o he lase beam. b, Raman spec um o longe SWCNTs (1 h a 1 × 107 mba o
e hanol a 670 K; 270 L) o a sho lase illumina ion ime (30 s), e ealing de ec - ee SWCNTs as
judged by he absence o a D peak. The inse s show u he de ails on he e y na ow RBM a
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comple ed singly capped SWCNT and do no ac as seeds o nano ube g ow h on epi axial
elonga ion.
Ex ended Da a Figu e 3 | Scanning helium ion mic oscopy images. SHIM images o epi axially
elonga ed SWCNTs ob ained by exposing seeds S1 o 1 × 107 mba o e hanol o 1 h (270 L) a
770 K. Long ca bon nano ubes can be obse ed o lie on he su ace, and in some cases o shake
unde he ion beam (indica ed by ligh g een a ows, down). Top le , lowe -magni ica ion iew o
su ace: numbe ed colou ed boxes a e shown a highe magni ica ion o he igh . The unnumbe ed
g ey scale images show di e en su ace loca ions a highe magni ica ions. The igh mos panel
gi es a highe magni ica ion image o he long SWCNT seen in panel 1.
Ex ended Da a Figu e 4 | Syn hesis o SWCNT p ecu so P1. De ails a e gi en in Me hods: he e
we desc ibe eac ion s eps a o j. a, PPh3, oluene, e lux, 95%; b, B Ph3PCH2PhB , KO Bu, E OH,
e lux, 81%; c, I2, h , p opylene oxide, cyclohexane, 72%; d, Pd(PPh3)4, Cs2CO3, oluene/MeOH,
110°C, 79%; e, NBS, DBPO, CCl4, e lux, 70%; , NaCN, DMSO, RT, 40%; g, H2SO4, H2O, HOAc,
e lux, 98%; h, SOCl2, 65 °C; i, AlCl3, CH2Cl2, RT, 57%; j, p opanoic acid, TsOH, o-DCB, 180 °C,
65%.
Ex ended Da a Figu e 5 | LDI mass spec a o p ecu so P1 (C96H54). a, b, LDI mass spec a o
P1 be o e (a) and a e (b) sublima ion. Compu ed and expe imen ally obse ed iso ope dis ibu ion
pa e ns o C96H54 a e gi en in he inse o b.

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