Diamond and Related Materials 11(2002) 1081–1085

0925-9635/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0925-9635Ž01.00643-4

From straight carbon nanotubes to Y-branched and coiled carbonnanotubes

L.P. Biro *, R. Ehlich , Z. Osvath , A. Koos , Z.E. Horvath , J. Gyulai , J.B. Nagya, b a a a a c´ ´ ´ ´

Research Institute for Technical Physics and Materials Science, H-1525 Budapest, P.O. Box 49, Hungarya

Max Born Institut fur Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Str. 2A, 12489 Berlin, Germanyb ¨Facultes Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61 Namur, B-5000, Belgiumc

Abstract

Straight carbon nanotubes, carbon nanotube ‘knees’, Y-branches of carbon nanotubes, and coiled carbon nanotubes were grownon a graphite substrate by the decomposition of fullerene under moderate heating(450 8C) in the presence of transition metalparticles. The grown structures were investigated without any further manipulation by STM. The observed coiled carbon nanotubesare tentatively identified with the theoretically predicted haeckelite.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Carbon nanotube growth; Y-branched nanotubes; Coiled nanotubes; Defects

1. Introduction

Soon after the discovery of straight carbon nanotubesw1x — constituted of perfectly rolled, defect free gra-phitic layers — the idea of branchedw2–4x and coiledw5x carbon nanostructures emerged. The branched nan-otubes can constitute valuable building blocks for nan-oelectronicsw6,7x — for three-terminal devices and 2-Dnetworks — while the nano-coils may be promisingfrom the point of view of nano-electromechanical sys-tems and nano-actuatorsw8x.There are two distinct ways to produce branched

carbon nanostructures: by(i) ‘forced’ branching intemplates with branched nano-channelsw9x; and by(ii)spontaneous branching during growthw10–12x. The firstprocedure yields structures with non-equal anglesbetween the branches, while the procedures from thesecond group usually yield structures with branchesoriented at 1208, like the one shown in Fig. 1. However,the graphene layers in the Y-junctions grown using themethod reported by Satishkumarw12x, are oriented in afishbone-like way, i.e. these layers are not parallel withthe axes of the structure.

*Corresponding author. Tel.:q36-1-2754-993; fax:q36-1-3922-226.

E-mail address: biro@mfa.kfki.hu(L.P. Biro).´

Multiwall coiled carbon nanotubes were first observedby transmission electron microscopy(TEM) w13x. Coilswith different pitch values were observed, in some casesthe neighboring coil elements are touching with an inter-spire distance close to the van der Waals distancebetween the layers of graphitew14x.Although sustained efforts were invested to find single

wall carbon nanotube coils by high resolution transmis-sion electron microscopy(HRTEM), up to now noconclusive evidence was found during TEM investiga-tion of samples in which multiwall coils were found.The first experimental observation of tightly wound,single wall coiled carbon nanotubes was achieved byscanning tunneling microscopy(STM) w15x. In a samplein which multiwall coils were found — prepared usingthe procedure given by Amelinckx et al.w13x — severalsingle wall coils of different pitches were found.In the present work we report the observation of

nanotube knees, Y-branched and coiled single wallcarbon nanotubes grown on a graphite substrate, usinga similar growth procedure as the one reported by Biro´

et al. w16x for straight carbon nanotubes. The relation ofthe observed carbon nanostructures with the theoreticallypredicted haeckelite-type structuresw17x proposedrecently is discussed.

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Fig. 1. Ball and stick model of the Y-branching of carbon nanotubesproposed by Scuseriaw3x. Heavy line marks the six heptagons.(Usedwith permission from Scuseria.)

Fig. 3. Constant current topographic STM image in top view pres-entation with artificial illumination from top, showing a Y-branchedcarbon nanotube image(I s200 pA,U s1 V, PtIr tip). The undulatedt t

line from A to B is step edge, while the structure labeled C is anextended defect. The white lines make an angle of 1218.

Fig. 2. Top view, constant current, topographic STM image(I s1 nA,t

U s1 V, PtIr tip) of carbon nanotubes grown by fullerene decom-t

position. The gray scale included in the image shows the extensionin a direction perpendicular to the plane of the image: object C is atwo atomic layer thick graphite stripe with a measured height of 0.7nm.

2. Experimental results and discussion

The room temperature growth of nanotubes was car-ried out under similar conditions as reported earlierw16x.Nickel particles of 200 nm in size, or Ni and Co

particles (Co particles of 60 nm) were mechanicallymixed with C , or with C powder; a 1:2 metal to60 70

fullerene mass ratio was used. The mixture was loadedin a quartz ampoule placed in an electric furnace. Thevacuum chamber was pumped down to a pressure of2=10 mbar. During deposition the ampoule wasy6

heated at 4508C. The flux of material emerging fromthe ampoule was directed onto a highly oriented pyroliticgraphite (HOPG) substrate at a distance of 30 cm,situated near a thickness monitor. During heating up todeposition temperature and cooling down, the substratewas protected by a shutter. In room temperature exper-iments — as measured by a thermocouple — thetemperature of the substrate was not modified signifi-cantly during deposition. In other experiments the sub-strate was resistively heated during deposition, or afterdeposition, to perform an in situ annealing for removingthe fullerenes that were not reacted. The thickness ofthe deposited layers was in the range of 3–10 nm ofequivalent fullerene thickness, the deposition timeranged from 3 to 10 min.After deposition the HOPG substrates were examined

by STM under ambient conditions. Commercially avail-able, mechanically cut PtIr tips were used. Constantcurrent topographical images were acquired using typicaltunneling currentsI s500 pA, and tunneling biasU st t

1 V. Usually low scan frequencies in the range of 1 Hzwere preferred.

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Fig. 4. Constant current topographic images mounted together showing a carbon nanotube making knees and random curvatures produced bydefects, branching into a straight and a coiled tube. The detail shown on the left-hand side is a higher magnification image taken of the boxindicated in the composed image. In each image, the gray scales indicate the corresponding extension in a direction perpendicular to the planeof the image.

Fig. 2 shows several nano-objects grown from thedecomposition products of C . After deposition, without70

breaking vacuum, the sample was annealed at 5158Cfor 25 min. The two carbon nanotubes labeled B1 andB2 grew side by side, except the region marked by awhite arrow. There, they cross a cleavage defect seen asa dark feature(labeled D). A nanotube ‘knee’, labeledA, is formed when two tubes of different diametersgrow together. Structures of this kind were found byTEM in samples of multiwall carbon nanotubesw18x,their possible electronic structure was investigated the-oretically w19,20x. The two tubes composing object Ahave different diameters, as one can deduce from theapparent height values measured by STM, hs0.27A1

nm and h s0.70 nm, respectively. These values cannotA2

be directly converted to geometric diameters because ofdifferences in electronic structure and due to the exis-tence of two tunneling gaps: STM tipynanotube; andnanotubeysubstrate w21,22x. The diameter ratio ofapproximately 3 may originate from one tube beingmetallic while the other tube is semiconducting. It isworth pointing out that the tube A1 seems to havestarted growth in the region where B2 crosses thecleavage defect. It is frequently observed that surface

defects influence or initiate the growth of carbon nano-tubes under the conditions used in the present work.As proposed earlier in theoretical model structures

w18–20,23x, a knee, like object A, can be built byincorporating one or more pentagon–heptagon pairs inthe graphitic network. More complex structures, like T-or Y-junctions can be constructed too, using a largernumber of heptagons, with or without pentagonsw2–4,6,7x. The spontaneous and free branching of singlewall carbon nanotubes grown by fullerene decomposi-tion on graphite, yields Y-like single wall structures,like the ones already reported by Nagy et al.w11x andBiro et al. w24x. This branching may be related to the´three axes, oriented at 1208, for which the interactionenergy of a carbon nanotube and of the graphite supportis minimal w25,26x. In Fig. 3, a branching is shown intop view presentation. This Y-junction was grown bythe decomposition of C . The growth by fullerrene70

decomposition usually yields Y-branches with angles of1208. A possible structure for such a branching is shownin Fig. 1. The apparent heights of all three branches arein the range of 0.2 nm.When a larger number of defects is inserted in the

hexagonal network building up the carbon nanotubes,

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Fig. 5. Coiled carbon nanotube crossing several cleavage steps on graphite.(a) Top view, constant current, topographic STM images of carbonnanotube coil(I s300 pA,U s400 mV, PtIr tip); (b) STS spectrum showing the different electronic structures of the coil and of the HOPGt t

substrate.

randomly curved tubes are obtained, like in the upperhalf of Fig. 4. This figure shows a carbon nanotubegrown by C decomposition, which originates from a60

region of extended defects. After a number of knees —without diameter change — the tube starts meandering,and finally it branches into a straight tube(left) andinto a coiled tube(right). The measured height valuesare as follows: 0.83 nm for the tube before branching;0.33 nm for the straight tube after the branching; and0.3 nm for the coil. The spires of the coiled tube in Fig.4 do not ‘touch’ at the van der Waals distance corre-sponding for graphene layers. A coil with touchingspires is shown in Fig. 5a. This coil traverses severalcleavage steps, it was measured over a length of 1.8mm. The scanning tunneling spectroscopy(STS) per-formed on the coil and on the HOPG far from the coil,Fig. 5b, indicates a much larger gap value — of theorder of 1.5 eV — for the coil than for HOPG.Earlier experimentalw27x and theoreticalw27,28x work

has showed periodic ‘stripe-like’ features in atomicresolution STM images of single wall carbon nanotubes.These features have a much shorter horizontal periodic-ity — comparable to the distance of neighboring atomicpositions — and much smaller corrugation amplitude,typically on the 0.1-nm scale as compared with the coilsreported in the present work.The structural models for regularly coiled carbon

nanotubes are based on the regular incorporation ofpentagon–heptagon pairs in the hexagonal networkw5,15,18x. According to these models, the coil may beregarded as the succession of numerous short identical

knees with the same tube diameter on both sides of theknee. However, the mechanism by which these defectpairs are inserted regularly, has not been clarified yet.On the other hand, if one does not regard the pentagonaland hexagonal rings as defects, but as regular buildingblocks, then their very precise positioning is an inherentproperty of the structure. Such structures, named hae-ckelites, constituted from pentagons, hexagons and hep-tagons, or only from pentagons and heptagons havebeen proposed recentlyw17x. According to theoreticalcalculations, the stability of tubular structures built fromhaeckelite is situated between that of regular carbonnanotubes and that of Cw17x. In particular, a layered60

structure built only from pentagons and heptagons hasa fairly good stability(0.307 eVyatom above graphene)as compared to fullerene(0.419 eVyatom above graphe-ne) w17x. The coiling of tubular structures resulted fromsuch haeckelite layers may be attributed to the relaxationof bond strain, as shown by recent calculationsw29x. Inthis way the formation of carbon nanotube coils has astructural origin and the coils may be regarded as thefirst experimental evidence for the existence of haecke-lite structures.The production of numerous defect-based structures

during the growth of carbon nanostructures on HOPGfrom the decomposition products of fullerenes is partlyattributed to the low growth temperature, ranging downto room temperature. Under these conditions, the Catoms and clusters have low mobility, which mayenhance the quenching in of the pentagonal and heptag-onal rings in the growing structure. When a stable

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haeckelite nucleus is formed, this will continue to growas a coiled structure.

3. Conclusions

Novel carbon nanostructures: knees; Y-branched car-bon nanotubes; and coiled carbon nanotubes were pro-duced directly on the HOPG substrate and examined bySTM without any further manipulation. The formationof the carbon nanostructures containing non-hexagonalrings is attributed partly to the templating effect of theHOPG and partly to the growth at room temperature,which enhances the probability of quenching-in for non-hexagonal rings.It is proposed that coiled carbon nanotubes may be

regarded as being built from a theoretically predictedmaterial, the haeckelite, in which pentagons, hexagonsand heptagons are equally considered as regular buildingelements.

Acknowledgments

This work was supported by the EU5, contractsNANOCOMP, HPRN-CT-2000-00037 and EU5 Centreof Excellence ICAI-CT-2000-70029, and by OTKAgrants T 30435 and T 25928 in Hungary. Financialsupport in Germany by BMBF Internationales Buro¨(HUN 98y034) and in Hungary by TeT Grant D-44y98is gratefully acknowledged.

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