All-carbon single-walled carbon nanotubes (SWCNTs) were successfully synthesized, nucleated using a fullerene derivative. A systematic investigation into the initial preparation of C60 fullerenes as growth nucleators for the SWCNTs was conducted. Enhancement in the yield of the produced SWCNT has been achieved with exploring different dispersing media for the fullerenes, the period, and environment of the initial thermal treatment of the fullerenes in addition to the use of different fullerene-based structures. The systematic studies significantly advance our understanding of the growth of the all-carbon catalyst-free single-walled carbon nanotubes. Field-effect transistors were fabricated using the catalyst-free SWCNT and then electrically characterized, showing current capacity as high as the well-studied catalyst-assisted nanotubes

Büchner, Bernd

Cuniberti, Gianaurelio

Dunsch, Lothar

Ibrahim, Imad

Popov, Alexey

Rümmeli, Mark H.

Zhang, Yang

Nanoscale Research Letters

English

PubMed

Imad Ibrahim

Yang Zhang

Alexey Popov

Lothar Dunsch

Bernd Büchner

Gianaurelio Cuniberti

Mark H Rümmeli

Springer - Publisher Connector

Growth of all-carbon horizontally aligned single-walled carbon nanotubes nucleated from fullerene-based structures

Repositorium für Naturwissenschaften und Technik  (TIB Hannover)

Ibrahim et al. Nanoscale Research Letters 2013, 8:265http://www.nanoscalereslett.com/content/8/1/265NANO EXPRESS Open AccessGrowth of all-carbon horizontally alignedsingle-walled carbon nanotubes nucleated fromfullerene-based structuresImad Ibrahim1,2*, Yang Zhang1, Alexey Popov1, Lothar Dunsch1, Bernd Büchner1,3, Gianaurelio Cuniberti2and Mark H Rümmeli1,3*AbstractAll-carbon single-walled carbon nanotubes (SWCNTs) were successfully synthesized, nucleated using a fullerenederivative. A systematic investigation into the initial preparation of C60 fullerenes as growth nucleators for theSWCNTs was conducted. Enhancement in the yield of the produced SWCNT has been achieved with exploringdifferent dispersing media for the fullerenes, the period, and environment of the initial thermal treatment of thefullerenes in addition to the use of different fullerene-based structures. The systematic studies significantly advanceour understanding of the growth of the all-carbon catalyst-free single-walled carbon nanotubes. Field-effecttransistors were fabricated using the catalyst-free SWCNT and then electrically characterized, showing currentcapacity as high as the well-studied catalyst-assisted nanotubes.Keywords: Horizontally aligned SWCNT, CVD, Catalyst-free SWCNT, Fullerene nucleate CNTBackgroundEnormous efforts have been invested towards the rea-lization of single-walled carbon nanotube (SWCNT)-based products due to their extraordinary properties[1,2]. One of the more attractive potential applicationsof these exciting nanostructures is as a building blockfor nanoelectronics. To this end, individual or parallel-aligned SWCNTs with tunable yield are important [3,4].For such applications, however, the reproducible controlof the nanotubes’ spatial orientation and chiral manage-ment still require further development [5]. Some successhas been achieved regarding the controlled fabrication ofwell-oriented nanotubes, especially when directly fa-bricating aligned tubes using chemical vapor deposition(CVD) [6,7]. Usually though, a catalyst particle (mostlymetal catalyst particles) are used to nucleate the growthof the nanotubes, and this has a drawback since thecatalyst particles may diffuse into the substrate or tubeand thus affect their intrinsic properties or that of a* Correspondence: i.ibrahim@ifw-dresden.de; m.ruemmeli@ifw-dresden.de1IFW-Dresden e.V., PF 270116, Dresden 01171, Germany3Department of Physics, Technische Universität Dresden, Dresden 01062,GermanyFull list of author information is available at the end of the article© 2013 Ibrahim et al.; licensee Springer. This isAttribution License (http://creativecommons.orin any medium, provided the original work is pdevice built around them [8,9]. Therefore, the synthesisof a catalyst-free-aligned SWCNT is very attractive. Dif-ferent all-carbon routes have been developed, for exam-ple, using diamonds as open-ended SWNT and fullerenesas SWCNT nucleators [10-12]. However, the yield of thegrown tubes is generally low. Moreover, this remains avery limited understanding of all-carbon SWCNT growth.In this study, we systematically investigate aspectsrelated to yield from metal-free horizontally orientedSWCNTs nucleated from pristine C60 fullerenes andexohedrally functionalized C60F18 fullerenes. Aside fromdirect comparisons between the two types of fullerenes,we also investigate the role of the dispersing solutionand pretreatment steps to functionalize and activatethem prior to CVD growth.MethodsNominal amounts of fullerene derivatives (C60 andC60F18), which will later serve as nanotube nucleators,were homogenously dispersed independently in toluene,acetone, and ethanol by overnight ultrasonication. Singlecrystal quartz substrates (10 × 10 × 0.5 mm, angle cut38° 00’, single side polished from Hoffman Materials, LLC,Carlisle PA, USA), were initially subjected to thermalan Open Access article distributed under the terms of the Creative Commonsg/licenses/by/2.0), which permits unrestricted use, distribution, and reproductionroperly cited.Figure 1 Characterization of as-produced carbon nanotubes.(a and b) Representative SEM images of CVD-grown horizontallyaligned CNT nucleated from pristine fullerenes (C60) and exohedrallyfunctionalized fluorofullerenes (C60F18), respectively. (c, d, and e)TEM micrograph, RBM of the Raman spectroscopy, and AFM imageof catalyst-free SWCNT, respectively. An inset is the height profile ofthe nanotube shown in panel e.Ibrahim et al. Nanoscale Research Letters 2013, 8:265 Page 2 of 6http://www.nanoscalereslett.com/content/8/1/265annealing in air at 750°C for 15 min prior to the chemicalvapor deposition (CVD) reaction for nanotube growth.This results in a smoother surface which helps providehigher yields [7]. The initial fullerenes were then placedon the quartz substrate prior to these treatments by dropcoating the dispersed fullerenes. The deposited fullerenesare opened (to form open caps that serve as nucleationcenters) and then activated by functionalization. Theseprocesses are accomplished by first heating the loadedsubstrates in various environments (air, synthetic air, Aror H2) for different periods (10 to 120 min) at tem-peratures between 400°C and 500°C in a 1-in purpose-built horizontal tube furnace. Thereafter, the activation isachieved by heating the samples at 900°C in water vapor(0.17 standard liter per minute (SLPM) Ar bubbledthrough water) for 2 min and then heating in hydrogen(0.75 SLPM) for the next 3 min. Later, the CVD reactionwas performed in a gaseous environment of hydrogen (4.5SLPM), Ar (0.2 SLPM), and Ar (0.32 SLPM) bubbledthrough ethanol, keeping the temperature stable at 900°Cfor 20 min.Atomic force microscopy (Digital InstrumentsNanoScope IIIa, Veeco, Plainview, NY, USA) operating inthe tapping mode was employed to characterize the fuller-enes after the different treatment steps and also assess theyield and diameter of the nanotubes after CVD growth.The length and alignment of the CNTs were determinedusing a scanning electron microscope (SEM; NOVA 200NanoSEM, FEI, Hillsboro, OR, USA; with typical accele-ration voltage of 2 to 3 kV), while the type and quality ofthe grown tubes were estimated by transmission electronmicroscopy (a double Cs-Corrected JEM-2010 F, JEOL,Akishima-shi, Japan; using an acceleration voltage of80 kV) and Raman spectroscopy (DXR SmartRamanThermo Scientific, Waltham, MA, USA; λ = 533 nm). Forthe electrical measurements, a set of source-drain elec-trode pairs (10 nm Cr, 40 nm Au) in addition to the gateelectrode (50 nm Al2O3, 10 nm Cr, 40 nm Au) were fabri-cated using standard e-beam lithography on the substrateswhere the nanotubes were as-grown.Results and discussionThe treated and activated fullerene derivatives were suc-cessfully used to nucleate the single-walled carbon nano-tubes grown by chemical vapor deposition. The CNTwere grown on very smooth single crystal quartz sub-strates, as this has been shown to aid high yields of hori-zontally aligned SWCNTs [7]. The fullerene derivativesused in this study were pure C60 and fluorofullerene(C60F18). These two were compared by dispersing themfirst in toluene. The fluorofullerene is a C60 surroundedby 18 fluorine atoms on the cage of the C60 and providesa useful way to investigate the role of surface-functiona-lized C60 against non-functionalized C60. Typical SEMmicrographs for the CNT nucleated from C60 andC60F18 are shown in Figure 1a,b, respectively. The grownCNTs are found to be single-walled, as shown in therepresentative transmission electron microscopy (TEM)micrograph (Figure 1c) and by the height profile ex-tracted from the atomic force microscopy (AFM) cha-racterization of the grown tubes (Figure 1e). Ramanspectroscopy studies confirm the presence of single-walled tubes by the existence of radial breathing modes(RBM) in the spectra (Figure 1d), which are a well-known signature for SWCNT and are frequently used toestimate the diameter of the investigated nanotubes [13].The grown SWCNT diameter distribution is in therange between 0.7 and 1.5 nm, as estimated from theRaman spectroscopy. A higher yield was achieved whenusing C60F18 as nucleators as compared to pristine C60,as shown in the representative SEM images provided inFigures 1a,b and 2a. We argue that this is due to thedramatic elongation of carbon atom bonds adjacent tothe fluorine atoms, which allows them to break moreeasily and hence make the formation of a spherical cap,which is appropriate for the tube nucleation and is moreefficient than the use of pristine C60 in the initial pre-synthesis step [14]. The higher yield (number of nano-tubes per unit area) of the grown tubes achieved with theC60F18 fullerenes is attractive on one side while otherwiseon the other because such exohedrally functionalizedFigure 2 Enhancement in the yield of the CVD-grown horizontally aligned SWCNT. (a) Variation in the yield of the nanotubes grownfrom C60 and C60F18. Yield of carbon nanotube dependency on (b) initial fullerene dispersing media, (c) the thermal oxidation environment, and(d) thermal oxidation period.Ibrahim et al. Nanoscale Research Letters 2013, 8:265 Page 3 of 6http://www.nanoscalereslett.com/content/8/1/265fullerenes are difficult to produce in large quantities,which make them economically unattractive in practicalterms. Hence, we now focus on efficient routes to growingCNT nucleated from pure C60 fullerenes. To do this, weexplore the role of the dispersing medium. We do this bycomparing the yield after dispersing the C60 in toluene,acetone, and ethanol. A dramatic increment in the tubeyield can be observed when using acetone as the disper-sing medium as seen in Figure 2b. The yield of the tubesFigure 3 Formation and size distribution of fullerene clusters formedas-deposited, after pretreatment for 75 min in (b) air, and (c) Ar. The upperrow shows those clusters originally dispersed in toluene. (d) Median and FWgrown from C60 dispersed in ethanol is less than found forthe dispersion in acetone but better than that for toluene.The reasons for this are discussed later. We now turn tothe influence of the pretreatment steps to open and acti-vate the fullerenes prior to exposing them to the CVDgrowth reaction. We first look at the opening of the fuller-enes. Different thermal pretreatment periods in air resultin different yields. The CNT yield increases with pretreat-ment time to a maximum at around 75 min, after whichon ST-cut quartz substrates. By visible light microscopy (a)row shows clusters originally dispersed in acetone while the lowerHM of the as-deposited and pretreated fullerene clusters.Figure 4 Characterization of clusters at the end of the grown SWCNT. Representative AFM images showing a globule at one end of theas-grown catalyst-free SWCNT along with the height profile of such globule feature to the right while the height profile of the grown CNT is tothe left of the AFM image.Ibrahim et al. Nanoscale Research Letters 2013, 8:265 Page 4 of 6http://www.nanoscalereslett.com/content/8/1/265the yield drops. This is because with excessive oxidation,most of the fullerene clusters are burnt away. Further en-hancement in the grown CNT yield was also achieved byoptimizing the oxygen environment. It was found that agas mixture of Ar or H2 with oxygen contents <0.1% wasbest. The variation in the CNT yield due to the change inthe thermal oxidation period is shown in Figure 2c whilethe effect of the thermal oxidation environment is pro-vided in panel d. The thermal oxidation step is required toFigure 5 Electrical characterization of the as-produced catalyst-free S(b) metallic SWCNTs. IV characteristic dependence on the gate voltage foropen up the fullerenes so as to provide hemispherical capswhich would later serve as the nucleation sites for contin-ued tube growth [12]. The oxidation process diminishesthe fullerene cluster size, as shown in Figure 3, in whichoptical micrographs for the as-deposited and thermallytreated fullerenes originally dispersed in acetone (upperrow) and in toluene (lower row) are provided. Panel b ofthe same figure presents the size distribution and fullwidth at half maximum of the formed fullerene clustersWCNTs. Two terminal IV characteristics of (a) semiconducting and(c) semiconducting and (d) metallic SWCNTs.Ibrahim et al. Nanoscale Research Letters 2013, 8:265 Page 5 of 6http://www.nanoscalereslett.com/content/8/1/265before and after treatment in different environments. Thecluster sizes increase markedly for ethanol and then acet-one. This trend is the same even for the thermally treatedclusters. A clear correlation between cluster size and yieldcan be observed (Figure 2b) larger cluster sizes lead to lar-ger SWCNT yields, and this explains the trend previouslyobserved for yield variation with dispersion medium. Theas-grown SWCNT on the host substrate were also in-vestigated by employing AFM, which reveals that thediameter distribution of the nanotubes is in the rangebetween 0.7 and 1.4 nm in good agreement with the TEMand Raman spectroscopy investigations. Often, we ob-served a globular-like feature at the end of a tube (seeFigure 4). We assume these are the clusters from whicha tube buds and grows from. The bulb heights are in therange between 2 and 10 nm and show no correlation tothe SWCNT diameters.We also electrically characterized the as-grown SWCNTroom temperature, firstly, by means of two terminal mea-surements and then they were gated and characterizedonce more. In the first step, source-drain electrode pairswere prepared by standard electron beam lithography. Tocharacterize the tubes, a potential VSD was applied acrossthe electrodes and the current, with the VSD measured.Typical two-terminal electrical characteristics from semi-conducting nanotubes are shown in panel a of Figure 4.The electrical characteristics of the SWCNTs vary as theyare dependent on the bandgap, which related to the nano-tube chirality (diameter). Figure 5b shows typical IV char-acteristics of metallic nanotubes. The devices exhibit aresistance less than 150 kΩ. This high resistance is attri-buted to backscattering and contact effects, which resultsin ISD saturation at high VSD [15]. Panels c and d of thesame figure show the IV characteristics of semiconductingand metallic SWCNTs with applied gate voltages, respec-tively. The metallic nanotubes show no dependence onthe gate voltage, as expected, the semiconducting nano-tube behavior depends strongly on the applied gate vol-tage. They are found to conduct well at negative gatevoltages while they are almost insulating at positive gatevoltages. This indicates they are p-type semiconductingtubes [16].ConclusionIn summary, we have systematically investigated thepretreatment steps and growth of catalyst-free growncarbon nanotubes using opened and functionalizedC60 and C60F18 as nucleation centers. The choice ofdispersion agent is also important such that largeclusters of initially deposited fullerenes lead to im-proved yields. Optimizing the thermal treatment stepsto open and functionalize the fullerene clusters arealso shown to improve the yield of the grown nanotubes.The as-synthesized tubes appear to be predominantlySWCNT. The high performance of the field-effect transis-tors fabricated using such catalyst-free SWCNTs makesuch tubes as promising candidates for future nanoelec-tronic applications.Competing interestsThe authors declare that they have no competing interests.Authors’ contributionsIIYZ, AP, LD, BB, GC, and MR researched data for the article, contributed tothe discussion of content, and reviewed and edited the manuscript beforesubmission. All authors read and approved the final manuscript.AcknowledgementsII thanks the DAAD; GC acknowledges support from the South KoreanMinistry of Education, Science, and Technology Program, Project WCU ITCEno. R31-2008-000-10100-0; and MHR thanks the EU (ECEMP) and the FreistaatSachsen.Author details1IFW-Dresden e.V., PF 270116, Dresden 01171, Germany. 2Institute forMaterials Science and Max Bergmann Center of Biomaterials, TechnischeUniversität Dresden, Dresden 01062, Germany. 3Department of Physics,Technische Universität Dresden, Dresden 01062, Germany.Received: 16 November 2012 Accepted: 5 April 2013Published: 6 June 2013References1. Tans SJ, Verschueren ARM, Dekker C: Room-temperature transistor basedon a single carbon nanotube. Nature 1998, 393:49–52.2. Kang SJ, Kocabas C, Ozel, Shim M, Pimparkar N, Alam MA, Rotkin SV, RogersJA: High-performance electronics using dense, perfectly aligned arrays ofsingle-walled carbon nanotubes. Nature Nanotech 2007, 2:230–236.3. Ibrahim I, Bachmatiuk A, Warner JH, Büchner B, Cuniberti G, Rümmeli MH:CVD grown horizontally aligned single wall carbon nanotubes: Synthesisroutes and growth mechanisms. Small 2012, 8:1973–1992.4. Kocabas C, Hur S-H, Gaur A, Meit MA, Shim M, Rogers JA: Guided growth oflarge-scale, horizontally aligned arrays of single-walled carbonnanotubes and their use in thin-film transistors. Small 2005, 1:1110–1116.5. Ishigami N, Ago H, Imamoto K, Tsuji M, Iakoubovskii K, Minami N: Crystalplane dependent growth of aligned single-walled carbon nanotubes onsapphire. J Am Chem Soc 2008, 130:9918–9924.6. Krupke R, Linden S, Rapp M, Hennrich F: Thin films of metallic carbonnanotubes prepared by dielectrophoresis. Adv Mater 2006, 18:1468–1470.7. Ibrahim I, Bachmatiuk A, Börrnert F, Blüher J, Zhang S, Wolff U, Büchner B,Cuniberti G, Rümmeli MH: Optimizing substrate surface and catalystconditions for high yield chemical vapor deposition grown epitaxiallyaligned single-walled carbon nanotubes. Carbon 2011, 49:5029–5037.8. Brukh R, Sae-Khow O, Mitra S: Stabilizing single-walled carbon nanotubesby removal of residual metal catalysts. Chem Phys Lett 2008, 459:149–152.9. Nel A, Xia T, Mädler L, Li N: Toxic potential of materials at the nanolevel.Science 2006, 311:622–627.10. Takagi D, Kobayashi Y, Homma Y: Carbon nanotube growth fromdiamond. J Am Chem Soc 2009, 131:6922–6923.11. Yao Y, Feng C, Zhang J, Liu Z: “Cloning” of single-walled carbonnanotubes via open-end growth mechanism. Nano Lett 2009,9:1673–1677.12. Yu X, Zhang J, Choi W, Choi J-Y, Kim JM, Gan L, Liu Z: Cap formationengineering: from opened C-60 to single-walled carbon nanotubes.Nano Lett 2010, 10:3343–3349.13. Jorio A, Saito R, Hafner JH, Lieber CM, Hunter M, McClure T, Dresselhaus G,Dresselhaus MS: Structural (n, m) determination of isolated Single-walledcarbon nanotubes by resonant raman scattering. Phys Rev Lett 2001,86:1118–1121.14. Ibrahim I, Bachmatiuk A, Rümmeli MH, Wolff U, Popov A, Boltalina O,Büchner B, Cuniberti G: Growth of catalyst-assisted and catalyst-freehorizontally aligned single wall carbon nanotubes. Status Solidi B 2011,248:2467–2470.Ibrahim et al. Nanoscale Research Letters 2013, 8:265 Page 6 of 6http://www.nanoscalereslett.com/content/8/1/26515. Lazzeri M, Mauri F: Coupled dynamics of electrons and phonons inmetallic nanotubes: current saturation from hot phonons generation.Phys Rev B 2006, 73(165419):1–6.16. Wang H, Luo J, Robertson A, Ito Y, Yan W, Lang V, Zaka M, Schäffel F,Rümmeli MH, Briggs GAD, Warner JH: High-performance field effecttransistors from solution processed carbon nanotubes. ACS Nano 2010,4:6659–6664.doi:10.1186/1556-276X-8-265Cite this article as: Ibrahim et al.: Growth of all-carbon horizontallyaligned single-walled carbon nanotubes nucleated from fullerene-basedstructures. 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Ibrahim et al. Nanoscale Research Letters 2013, 8:265http://www.nanoscalereslett.com/content/8/1/265NANO EXPRESS Open AccessGrowth of all-carbon horizontally alignedsingle-walled carbon nanotubes nucleated fromfullerene-based structuresImad Ibrahim1,2*, Yang Zhang1, Alexey Popov1, Lothar Dunsch1, Bernd Büchner1,3, Gianaurelio Cuniberti2and Mark H Rümmeli1,3*AbstractAll-carbon single-walled carbon nanotubes (SWCNTs) were successfully synthesized, nucleated using a fullerenederivative. A systematic investigation into the initial preparation of C60 fullerenes as growth nucleators for theSWCNTs was conducted. Enhancement in the yield of the produced SWCNT has been achieved with exploringdifferent dispersing media for the fullerenes, the period, and environment of the initial thermal treatment of thefullerenes in addition to the use of different fullerene-based structures. The systematic studies significantly advanceour understanding of the growth of the all-carbon catalyst-free single-walled carbon nanotubes. Field-effecttransistors were fabricated using the catalyst-free SWCNT and then electrically characterized, showing currentcapacity as high as the well-studied catalyst-assisted nanotubes.Keywords: Horizontally aligned SWCNT, CVD, Catalyst-free SWCNT, Fullerene nucleate CNTBackgroundEnormous efforts have been invested towards the rea-lization of single-walled carbon nanotube (SWCNT)-based products due to their extraordinary properties[1,2]. One of the more attractive potential applicationsof these exciting nanostructures is as a building blockfor nanoelectronics. To this end, individual or parallel-aligned SWCNTs with tunable yield are important [3,4].For such applications, however, the reproducible controlof the nanotubes’ spatial orientation and chiral manage-ment still require further development [5]. Some successhas been achieved regarding the controlled fabrication ofwell-oriented nanotubes, especially when directly fa-bricating aligned tubes using chemical vapor deposition(CVD) [6,7]. Usually though, a catalyst particle (mostlymetal catalyst particles) are used to nucleate the growthof the nanotubes, and this has a drawback since thecatalyst particles may diffuse into the substrate or tubeand thus affect their intrinsic properties or that of a* Correspondence: i.ibrahim@ifw-dresden.de; m.ruemmeli@ifw-dresden.de1IFW-Dresden e.V., PF 270116, Dresden 01171, Germany3Department of Physics, Technische Universität Dresden, Dresden 01062,GermanyFull list of author information is available at the end of the article© 2013 Ibrahim et al.; licensee Springer. This isAttribution License (http://creativecommons.orin any medium, provided the original work is pdevice built around them [8,9]. Therefore, the synthesisof a catalyst-free-aligned SWCNT is very attractive. Dif-ferent all-carbon routes have been developed, for exam-ple, using diamonds as open-ended SWNT and fullerenesas SWCNT nucleators [10-12]. However, the yield of thegrown tubes is generally low. Moreover, this remains avery limited understanding of all-carbon SWCNT growth.In this study, we systematically investigate aspectsrelated to yield from metal-free horizontally orientedSWCNTs nucleated from pristine C60 fullerenes andexohedrally functionalized C60F18 fullerenes. Aside fromdirect comparisons between the two types of fullerenes,we also investigate the role of the dispersing solutionand pretreatment steps to functionalize and activatethem prior to CVD growth.MethodsNominal amounts of fullerene derivatives (C60 andC60F18), which will later serve as nanotube nucleators,were homogenously dispersed independently in toluene,acetone, and ethanol by overnight ultrasonication. Singlecrystal quartz substrates (10 × 10 × 0.5 mm, angle cut38° 00’, single side polished from Hoffman Materials, LLC,Carlisle PA, USA), were initially subjected to thermalan Open Access article distributed under the terms of the Creative Commonsg/licenses/by/2.0), which permits unrestricted use, distribution, and reproductionroperly cited.Figure 1 Characterization of as-produced carbon nanotubes.(a and b) Representative SEM images of CVD-grown horizontallyaligned CNT nucleated from pristine fullerenes (C60) and exohedrallyfunctionalized fluorofullerenes (C60F18), respectively. (c, d, and e)TEM micrograph, RBM of the Raman spectroscopy, and AFM imageof catalyst-free SWCNT, respectively. An inset is the height profile ofthe nanotube shown in panel e.Ibrahim et al. Nanoscale Research Letters 2013, 8:265 Page 2 of 6http://www.nanoscalereslett.com/content/8/1/265annealing in air at 750°C for 15 min prior to the chemicalvapor deposition (CVD) reaction for nanotube growth.This results in a smoother surface which helps providehigher yields [7]. The initial fullerenes were then placedon the quartz substrate prior to these treatments by dropcoating the dispersed fullerenes. The deposited fullerenesare opened (to form open caps that serve as nucleationcenters) and then activated by functionalization. Theseprocesses are accomplished by first heating the loadedsubstrates in various environments (air, synthetic air, Aror H2) for different periods (10 to 120 min) at tem-peratures between 400°C and 500°C in a 1-in purpose-built horizontal tube furnace. Thereafter, the activation isachieved by heating the samples at 900°C in water vapor(0.17 standard liter per minute (SLPM) Ar bubbledthrough water) for 2 min and then heating in hydrogen(0.75 SLPM) for the next 3 min. Later, the CVD reactionwas performed in a gaseous environment of hydrogen (4.5SLPM), Ar (0.2 SLPM), and Ar (0.32 SLPM) bubbledthrough ethanol, keeping the temperature stable at 900°Cfor 20 min.Atomic force microscopy (Digital InstrumentsNanoScope IIIa, Veeco, Plainview, NY, USA) operating inthe tapping mode was employed to characterize the fuller-enes after the different treatment steps and also assess theyield and diameter of the nanotubes after CVD growth.The length and alignment of the CNTs were determinedusing a scanning electron microscope (SEM; NOVA 200NanoSEM, FEI, Hillsboro, OR, USA; with typical accele-ration voltage of 2 to 3 kV), while the type and quality ofthe grown tubes were estimated by transmission electronmicroscopy (a double Cs-Corrected JEM-2010 F, JEOL,Akishima-shi, Japan; using an acceleration voltage of80 kV) and Raman spectroscopy (DXR SmartRamanThermo Scientific, Waltham, MA, USA; λ = 533 nm). Forthe electrical measurements, a set of source-drain elec-trode pairs (10 nm Cr, 40 nm Au) in addition to the gateelectrode (50 nm Al2O3, 10 nm Cr, 40 nm Au) were fabri-cated using standard e-beam lithography on the substrateswhere the nanotubes were as-grown.Results and discussionThe treated and activated fullerene derivatives were suc-cessfully used to nucleate the single-walled carbon nano-tubes grown by chemical vapor deposition. The CNTwere grown on very smooth single crystal quartz sub-strates, as this has been shown to aid high yields of hori-zontally aligned SWCNTs [7]. The fullerene derivativesused in this study were pure C60 and fluorofullerene(C60F18). These two were compared by dispersing themfirst in toluene. The fluorofullerene is a C60 surroundedby 18 fluorine atoms on the cage of the C60 and providesa useful way to investigate the role of surface-functiona-lized C60 against non-functionalized C60. Typical SEMmicrographs for the CNT nucleated from C60 andC60F18 are shown in Figure 1a,b, respectively. The grownCNTs are found to be single-walled, as shown in therepresentative transmission electron microscopy (TEM)micrograph (Figure 1c) and by the height profile ex-tracted from the atomic force microscopy (AFM) cha-racterization of the grown tubes (Figure 1e). Ramanspectroscopy studies confirm the presence of single-walled tubes by the existence of radial breathing modes(RBM) in the spectra (Figure 1d), which are a well-known signature for SWCNT and are frequently used toestimate the diameter of the investigated nanotubes [13].The grown SWCNT diameter distribution is in therange between 0.7 and 1.5 nm, as estimated from theRaman spectroscopy. A higher yield was achieved whenusing C60F18 as nucleators as compared to pristine C60,as shown in the representative SEM images provided inFigures 1a,b and 2a. We argue that this is due to thedramatic elongation of carbon atom bonds adjacent tothe fluorine atoms, which allows them to break moreeasily and hence make the formation of a spherical cap,which is appropriate for the tube nucleation and is moreefficient than the use of pristine C60 in the initial pre-synthesis step [14]. The higher yield (number of nano-tubes per unit area) of the grown tubes achieved with theC60F18 fullerenes is attractive on one side while otherwiseon the other because such exohedrally functionalizedFigure 2 Enhancement in the yield of the CVD-grown horizontally aligned SWCNT. (a) Variation in the yield of the nanotubes grownfrom C60 and C60F18. Yield of carbon nanotube dependency on (b) initial fullerene dispersing media, (c) the thermal oxidation environment, and(d) thermal oxidation period.Ibrahim et al. Nanoscale Research Letters 2013, 8:265 Page 3 of 6http://www.nanoscalereslett.com/content/8/1/265fullerenes are difficult to produce in large quantities,which make them economically unattractive in practicalterms. Hence, we now focus on efficient routes to growingCNT nucleated from pure C60 fullerenes. To do this, weexplore the role of the dispersing medium. We do this bycomparing the yield after dispersing the C60 in toluene,acetone, and ethanol. A dramatic increment in the tubeyield can be observed when using acetone as the disper-sing medium as seen in Figure 2b. The yield of the tubesFigure 3 Formation and size distribution of fullerene clusters formedas-deposited, after pretreatment for 75 min in (b) air, and (c) Ar. The upperrow shows those clusters originally dispersed in toluene. (d) Median and FWgrown from C60 dispersed in ethanol is less than found forthe dispersion in acetone but better than that for toluene.The reasons for this are discussed later. We now turn tothe influence of the pretreatment steps to open and acti-vate the fullerenes prior to exposing them to the CVDgrowth reaction. We first look at the opening of the fuller-enes. Different thermal pretreatment periods in air resultin different yields. The CNT yield increases with pretreat-ment time to a maximum at around 75 min, after whichon ST-cut quartz substrates. By visible light microscopy (a)row shows clusters originally dispersed in acetone while the lowerHM of the as-deposited and pretreated fullerene clusters.Figure 4 Characterization of clusters at the end of the grown SWCNT. Representative AFM images showing a globule at one end of theas-grown catalyst-free SWCNT along with the height profile of such globule feature to the right while the height profile of the grown CNT is tothe left of the AFM image.Ibrahim et al. Nanoscale Research Letters 2013, 8:265 Page 4 of 6http://www.nanoscalereslett.com/content/8/1/265the yield drops. This is because with excessive oxidation,most of the fullerene clusters are burnt away. Further en-hancement in the grown CNT yield was also achieved byoptimizing the oxygen environment. It was found that agas mixture of Ar or H2 with oxygen contents <0.1% wasbest. The variation in the CNT yield due to the change inthe thermal oxidation period is shown in Figure 2c whilethe effect of the thermal oxidation environment is pro-vided in panel d. The thermal oxidation step is required toFigure 5 Electrical characterization of the as-produced catalyst-free S(b) metallic SWCNTs. IV characteristic dependence on the gate voltage foropen up the fullerenes so as to provide hemispherical capswhich would later serve as the nucleation sites for contin-ued tube growth [12]. The oxidation process diminishesthe fullerene cluster size, as shown in Figure 3, in whichoptical micrographs for the as-deposited and thermallytreated fullerenes originally dispersed in acetone (upperrow) and in toluene (lower row) are provided. Panel b ofthe same figure presents the size distribution and fullwidth at half maximum of the formed fullerene clustersWCNTs. Two terminal IV characteristics of (a) semiconducting and(c) semiconducting and (d) metallic SWCNTs.Ibrahim et al. Nanoscale Research Letters 2013, 8:265 Page 5 of 6http://www.nanoscalereslett.com/content/8/1/265before and after treatment in different environments. Thecluster sizes increase markedly for ethanol and then acet-one. This trend is the same even for the thermally treatedclusters. A clear correlation between cluster size and yieldcan be observed (Figure 2b) larger cluster sizes lead to lar-ger SWCNT yields, and this explains the trend previouslyobserved for yield variation with dispersion medium. Theas-grown SWCNT on the host substrate were also in-vestigated by employing AFM, which reveals that thediameter distribution of the nanotubes is in the rangebetween 0.7 and 1.4 nm in good agreement with the TEMand Raman spectroscopy investigations. Often, we ob-served a globular-like feature at the end of a tube (seeFigure 4). We assume these are the clusters from whicha tube buds and grows from. The bulb heights are in therange between 2 and 10 nm and show no correlation tothe SWCNT diameters.We also electrically characterized the as-grown SWCNTroom temperature, firstly, by means of two terminal mea-surements and then they were gated and characterizedonce more. In the first step, source-drain electrode pairswere prepared by standard electron beam lithography. Tocharacterize the tubes, a potential VSD was applied acrossthe electrodes and the current, with the VSD measured.Typical two-terminal electrical characteristics from semi-conducting nanotubes are shown in panel a of Figure 4.The electrical characteristics of the SWCNTs vary as theyare dependent on the bandgap, which related to the nano-tube chirality (diameter). Figure 5b shows typical IV char-acteristics of metallic nanotubes. The devices exhibit aresistance less than 150 kΩ. This high resistance is attri-buted to backscattering and contact effects, which resultsin ISD saturation at high VSD [15]. Panels c and d of thesame figure show the IV characteristics of semiconductingand metallic SWCNTs with applied gate voltages, respec-tively. The metallic nanotubes show no dependence onthe gate voltage, as expected, the semiconducting nano-tube behavior depends strongly on the applied gate vol-tage. They are found to conduct well at negative gatevoltages while they are almost insulating at positive gatevoltages. This indicates they are p-type semiconductingtubes [16].ConclusionIn summary, we have systematically investigated thepretreatment steps and growth of catalyst-free growncarbon nanotubes using opened and functionalizedC60 and C60F18 as nucleation centers. The choice ofdispersion agent is also important such that largeclusters of initially deposited fullerenes lead to im-proved yields. Optimizing the thermal treatment stepsto open and functionalize the fullerene clusters arealso shown to improve the yield of the grown nanotubes.The as-synthesized tubes appear to be predominantlySWCNT. The high performance of the field-effect transis-tors fabricated using such catalyst-free SWCNTs makesuch tubes as promising candidates for future nanoelec-tronic applications.Competing interestsThe authors declare that they have no competing interests.Authors’ contributionsIIYZ, AP, LD, BB, GC, and MR researched data for the article, contributed tothe discussion of content, and reviewed and edited the manuscript beforesubmission. All authors read and approved the final manuscript.AcknowledgementsII thanks the DAAD; GC acknowledges support from the South KoreanMinistry of Education, Science, and Technology Program, Project WCU ITCEno. R31-2008-000-10100-0; and MHR thanks the EU (ECEMP) and the FreistaatSachsen.Author details1IFW-Dresden e.V., PF 270116, Dresden 01171, Germany. 2Institute forMaterials Science and Max Bergmann Center of Biomaterials, TechnischeUniversität Dresden, Dresden 01062, Germany. 3Department of Physics,Technische Universität Dresden, Dresden 01062, Germany.Received: 16 November 2012 Accepted: 5 April 2013Published: 6 June 2013References1. Tans SJ, Verschueren ARM, Dekker C: Room-temperature transistor basedon a single carbon nanotube. Nature 1998, 393:49–52.2. Kang SJ, Kocabas C, Ozel, Shim M, Pimparkar N, Alam MA, Rotkin SV, RogersJA: High-performance electronics using dense, perfectly aligned arrays ofsingle-walled carbon nanotubes. Nature Nanotech 2007, 2:230–236.3. Ibrahim I, Bachmatiuk A, Warner JH, Büchner B, Cuniberti G, Rümmeli MH:CVD grown horizontally aligned single wall carbon nanotubes: Synthesisroutes and growth mechanisms. Small 2012, 8:1973–1992.4. 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Nanoscale Research Letters 2013, 8:265 Page 6 of 6http://www.nanoscalereslett.com/content/8/1/26515. Lazzeri M, Mauri F: Coupled dynamics of electrons and phonons inmetallic nanotubes: current saturation from hot phonons generation.Phys Rev B 2006, 73(165419):1–6.16. Wang H, Luo J, Robertson A, Ito Y, Yan W, Lang V, Zaka M, Schäffel F,Rümmeli MH, Briggs GAD, Warner JH: High-performance field effecttransistors from solution processed carbon nanotubes. ACS Nano 2010,4:6659–6664.doi:10.1186/1556-276X-8-265Cite this article as: Ibrahim et al.: Growth of all-carbon horizontallyaligned single-walled carbon nanotubes nucleated from fullerene-basedstructures. 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Growth of all-carbon horizontally aligned single-walled carbon nanotubes nucleated from fullerene-based structures

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