Controlled synthesis of single-chirality carbon nanotubes

Publisher: NPG; Journal: Nature: Nature; Article Type: Physics letter DOI: 10.1038/nature13607 Page 1 of 25 Controlled synthesis of single-chirality carbon nanotubes Juan Ramon Sanchez-Valencia1†, Thomas Dienel1, Oliver Gröning1, Ivan Shorubalko2, Andreas Mueller3†, Martin Jansen3, Konstantin Amsharov3†, Pascal Ruffieux1 & Roman Fasel1,4 1nanotech@surfaces Laboratory, Empa, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland. 2Laboratory for reliability science and technology, Empa, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland. 3Max Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany. 4Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland †Present addresses: Nanotechnology on Surfaces Laboratory, Instituto de Ciencia de Materiales de Sevilla (CSIC-US), Avda. Américo Vespucio 49, E-41092 Sevilla, Spain (J.R.S.-V.); BASF SE, GVM/I–L 544, 67056 Ludwigshafen, Germany (A.M.); University Erlangen-Nuremberg, Institut für Organische Chemie II, Henkestrasse 42, 91054 Erlangen, Germany (K.A.). Over the last two decades, single-walled carbon nanotubes (SWCNTs) have received much attention because their extraordinary properties are promising for numerous applications1,2. Many of these properties depend sensitively on SWCNT structure, which is characterized by the chiral index (n,m) that denotes the length and orientation of the circumferential vector in the hexagonal carbon lattice. Electronic properties are particularly strongly affected, with subtle structural changes switching tubes from metallic to semiconducting with various bandgaps. Monodisperse ‘single-chirality’ (that is, with a single (n,m) value) SWCNTs are thus needed to fully exploit their technological potential1,2. Controlled synthesis through catalyst engineering3–6, end-cap engineering7 or cloning strategies8,9 and also tube sorting based on chromatography10,11, density-gradient centrifugation, electrophoresis and other techniques12 have delivered SWCNT samples with narrow distributions of tube diameter and enriched in a particular tube type. But an effective pathway to truly monodisperse SWCNTs remains elusive. The use of template molecules to unambiguously dictate the diameter and chirality of the resulting nanotube8,13–16 holds great promise in this regard, but has hitherto had only limited practical success7,17,18. Here we show that this bottom-up strategy can produce targeted nanotubes: we convert molecular precursors into ultrashort singly capped (6,6) ‘armchair’ nanotube seeds using surface-catalysed cyclodehydrogenation on a Pt(111) surface, and then elongate these during a subsequent growth phase to produce single-chirality and essentially defect-free SWCNTs with lengths up to a few hundred nanometres. We expect that our onsurface synthesis approach will provide a route to nanotube-based materials with highly Publisher: NPG; Journal: Nature: Nature; Article Type: Physics letter DOI: 10.1038/nature13607 Page 2 of 25 optimized properties for applications such as light detectors, photovoltaics, field-effect transistors, and sensors2. Recent work has produced non-planar carbon-based nanostructures such as fullerenes, carbon pyramids, and buckybowls from their corresponding quasi-planar polycyclic aromatic hydrocarbon precursors through surface-catalysed cyclodehydrogenation (CDH)19–21. We have extended the methodology to the synthesis of ultrashort singly capped SWCNTs, that is, a SWCNT end cap with a short tube segment attached. Such molecules represent ideal seeds for subsequent epitaxial elongation into isomerically pure SWCNTs. Formally, this approach mimics the conventional synthesis of SWCNTs by the root-growth mechanism, in which nanotube growth starts by nucleation of an end-cap fragment on a metal nanoparticle22. The key point is to avoid uncontrolled, spontaneous nucleation of end caps by providing atomically precise ultrashort nanotube seeds which unambiguously dictate the chiral index of SWCNTs forming on epitaxial elongation. Precursor P1 (C96H54; Fig. 1) was designed and synthesized by multi-step organic synthesis to tackle this challenge (for details, see Methods). Upon intramolecular CDH it affords seed S1, an ultra-short singly capped (6,6) SWCNT bearing a carbon nanotube segment. The selective growth of (6,6) SWCNTs is illustrated in Fig. 1 and combines two steps: (1) formation of seed S1, and (2) subsequent epitaxial elongation. The first step is realized by depositing precursor P1 on a Pt(111) surface followed by annealing to 770 K under ultrahigh vacuum conditions to induce the surfacecatalysed CDH reaction (Fig. 2a, b). The second step, epitaxial elongation, is achieved by the incorporation of carbon atoms originating from the surface-catalysed decomposition of a carbon feedstock gas (Fig. 3a–c). Figure 2c shows a scanning tunnelling microscopy (STM) image acquired after depositing P1 on Pt(111). No step decoration or island formation is observed. Interactions with surrounding molecules and step edges are thus largely suppressed and ensure subsequent unperturbed CDH of the precursor19,20. For the majority of the as-deposited precursors, STM reveals a three-fold symmetric conformation. However, the intrinsic axial chirality of the benzo[c]phenanthrene fragments ([4]helicene) (see Extended Data Fig. 1) and the configurational flexibility of the peripheral biphenyl fragments produce a large variety of possible geometries (Fig. 2c, Extended Data Fig. 1). The presence of axial chirality in the [4]helicene moieties results in four possible stereoisomers and thus eight possible adsorption geometries (Extended Data Fig. 1). An example of a molecule with its three outer biphenyl groups located closest to the surface is shown in Fig. 2e, together with the corresponding STM simulation based on the extended lowest unoccupied molecular orbital (LUMO; see Methods). The excellent agreement between STM image and simulation (Extended Data Fig. 1) Publisher: NPG; Journal: Nature: Nature; Article Type: Physics letter DOI: 10.1038/nature13607 Page 3 of 25 indicates that the different topographic features observed for the adsorbed precursors can be attributed to the different adsorption geometries. Importantly, the stereoisomerism does not affect the CDH process, since all chiral centres will disappear during intramolecular cyclization. Although P1 is designed to yield seed S1, the conformational flexibility of the peripheral biphenyl groups leads partially to undesired adsorption geometries. In contrast to the stereoisomers discussed above, these molecules will follow a different CDH pathway, ending in the formation of undesired buckybowls (Extended Data Fig. 2). A statistical analysis of more than 100 precursor monomers observed by STM revealed that more than 50% adopt the desired configurations (Extended Data Fig. 1). Most importantly, the condensation products of precursor molecules exhibiting ‘wrong’ conformations cannot act as seeds for the subsequent CNT growth process via epitaxial elongation, and thus will not affect the selectivity of SWCNT formation. Surface-catalysed CDH of precursors (P1) into seeds (S1) is induced by annealing at 770 K for 10 min. STM images (Fig. 2d) show that the originally quasi-planar three-fold symmetric molecules transform into dome-shaped species with a prominent increase in apparent height from 2 to 4.5 Å (Fig. 2f). Additional proof of successful dehydrogenation of P1 into S1 derives from the good agreement of high-resolution STM images and simulations of the frontier molecular orbitals of S1 (Fig. 2g). Both results demonstrate the successful formation of the targeted singly capped ultrashort (6,6) SWCNT S1. In the second step, the surface-anchored seeds S1 are grown into (6,6) SWCNTs via epitaxial elongation by exposing them to a carbon feedstock gas such as ethylene or ethanol at temperatures between 670 and 770 K (see schematic illustration in Fig. 3a–c). The pre-synthesized seeds S1 are extended by catalytic epitaxial elongation, which consists of a consecutive incorporation of carbon atoms (schematically shown as C2) originating from the decomposition of ethanol or ethylene. The open part of S1 is already in the required contact with the Pt(111) surface, which catalyses further epitaxial elongation. In order to unambiguously demonstrate the activity of S1 in an epitaxial elongation process, the results of low exposures to carbon feedstock gas were followed in situ by STM. Low doses of ethylene at 770 K produce a substantial increase in apparent height from 4 to 20 Å (Fig. 3d–g). After exposure to 1 Langmuir (L) of ethylene, about 18% of the initially deposited precursors P1 have grown into SWCNTs, a density that remains constant for higher exposures of 5 L. Exposure to yet higher doses of ethylene or ethanol produces strong changes in the surface topography and makes STM imaging increasingly difficult (Fig. 3h). The topography becomes rough and the Pt surface is no longer discernible. Careful examination of the STM images reveals the presence of one-dimensional structures lying across the surface (Fig. 3h), with an abundance of about Publisher: NPG; Journal: Nature: Nature; Article Type: Physics letter DOI: 10.1038/nature13607 Page 4 of 25 5 per m2 and observed lengths exceeding 200 nm. High resolution images of the elongated structures (Fig. 3i) reveal an internal structure of higher contrast lines along the direction of the tube axis. A superimposed structural model clearly shows that these lines are indeed the carbon positions of the graphene structure in a (6,6) SWCNT. In all cases where atomic resolution of the tubes could be achieved, the structure proved to be consistent with a (6,6) SWCNT. The outstanding agreement of both the orientation and the periodicity of the graphene lattice with those expected demonstrate that the one-dimensional structures are the targeted (6,6) SWCNTs, bent horizontally across the sample surface. To further corroborate the density and length of the horizontally aligned SWCNTs, the previous sample was imaged with a scanning helium ion microscope (SHIM). Tubes longer than 300 nm and with diameters below 2 nm (the highest resolution achieved) could be observed (Extended Data Fig. 3), with a density of 3–4 tubes per m2 that is similar to the density estimated from STM images (5 per m2). We note that both STM and SHIM can only image horizontally aligned SWCNTs, but do not give access to vertically aligned SWCNTs. To shed light on the orientation of the SWCNTs and to characterize the selectivity of the growth process, Raman characterization was performed. Measurements with the illuminating laser beam at normal incidence (perpendicular to the surface plane) yielded extremely low intensities; the G band, the most intense feature at around 1,590 cm−1, is very weak (grey curve in Fig. 4a). However, when samples were measured under an illumination angle of 30°, all bands increased significantly in intensity (black curve in Fig. 4a), as expected for a dominant fraction of SWCNTs oriented perpendicular to the surface. The polarizability of these vertically oriented tubes with the illuminating laser beam at normal incidence – and thus the electromagnetic field vector perpendicular to the tube axis – is drastically reduced, resulting in weak Raman intensities23. A predominantly vertical alignment of the SWCNTs is consistent with the observation that in SHIM images some CNTs appear to shake under the ion beam (Extended Data Fig. 3), and with the rough surface seen in STM. It also implies that the overall SWCNT density is expected to be significantly higher than that estimated from the STM and SHIM data. More importantly, the Raman spectra demonstrate the high selectivity of our growth process, which produces (6,6) SWCNTs exclusively. The spectrum shown in Fig. 4b presents clearly defined bands at the positions expected for (6,6) SWCNTs. The band at 295 cm−1 is associated with the radial breathing mode (RBM) whose frequency depends strongly on the nanotube diameter23. Empirical equations predict an RBM frequency of 280–295 cm−1 for (6,6) SWCNTs23. Publisher: NPG; Journal: Nature: Nature; Article Type: Physics letter DOI: 10.1038/nature13607 Page 5 of 25 Experimentally, the RBM for (6,6) SWCNTs deposited on SiO2 has been reported at around 289 cm−1 (ref. 24), but it is well known that the substrate plays a crucial role in the RBM position, explaining the small deviation observed25. Most importantly, our Raman spectra do not show any further bands within the RBM range (200–400 cm−1), which underlines the extremely high selectivity of the process. Another notable characteristic of the RBM is the exceptionally small width observed (the full-width at half-maximum, FWHM, is 3.5 cm−1, limited by the resolution of the instrument), which is as small as that from isolated individual small diameter SWCNTs (an FWHM of 3 cm−1)26. The Raman spectra reported here average over a large area (beam diameter 1.5-10 µm (inset in Fig. 4b)), and thus reflect the properties of a large number of SWCNTs. The narrow RBM peak therefore demonstrates a very high degree of monodispersity. The G band appears as a double peak at 1,518 and 1,591 cm−1. The significant curvature in small diameter SWCNTs causes a shift to lower frequencies of the optical vibrations associated with transverse (perpendicular to the tube axis) atomic displacements (the G− band)27. Although, to our knowledge, this splitting has not been observed experimentally for a (6,6) SWCNT, the G+ and G− band splitting (  G) has been predicted to be 83 cm−1 (ref. 28), in good agreement with the splitting that we observe (  G = 73 cm−1). The additional peaks in the range from 400 to 1,200 cm−1 have previously been observed and used as evidence for the presence of armchair SWCNTs, since no peak in this range is present for semiconducting tubes29. Finally, the absence of any D band in the Raman spectra (Fig. 4b) further underlines the extreme cleanliness of our process that yields not only predefined single-chirality but also essentially defect-free SWCNTs. These findings clearly illustrate that the use of a planar metal surface instead of metal nanoparticles effectively supresses spontaneous cap formation and that this, in combination with the use of a suitable precursor species to produce a desired cap, enables highly selective SWCNT fabrication. The entire in situ process and the low temperatures involved in both the CDH step (770 K) and the subsequent epitaxial elongation (670 K) are fully compatible with complementary metal oxide semiconductor (CMOS) technology, and our methods might therefore solve two pivotal challenges in the realization of CNT-based integrated circuits for digital electronics: to provide SWCNTs with identical electronic properties, and to integrate these SWCNTs in device architectures (which may be achieved by site-specific deposition of the molecular precursor and/or the catalyst film). However, further progress that aims to be technologically relevant requires a process with higher growth yields that ideally approach unity. While we see considerable scope for optimization of the present process (in terms of catalyst, temperatures, pressures, and so on), an alternative is to replace the substrate-catalysed epitaxial elongation step by a more efficient process, such as hot Publisher: NPG; Journal: Nature: Nature; Article Type: Physics letter DOI: 10.1038/nature13607 Page 6 of 25 filament chemical vapour deposition30. This would also allow more flexibility in independently optimizing seed formation and tube elongation while simultaneously suppressing spontaneous SWCNT growth at both stages. Another logical extension of the process is to other SWCNTs where subtleties related to the chiral index of the seed may come into play. Received 17 April; accepted 17 June 2014; doi:10.1038/nature13607. 1. Jorio, A., Dresselhaus, G. & Dresselhaus, M. S. Carbon Nanotubes: Advanced Topics in the Synthesis, Structure, Properties, and Applications (Springer, 2008). 2. Jariwala, D., Sangwan, V. K., Lauhon, L. J., Marks, T. 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Vertical array growth of small diameter single-walled carbon nanotubes. J. Am. Chem. Soc. 128, 6560–6561 (2006). Acknowledgements This research was supported in part by the Swiss National Science Foundation and by the State Secretariat for Education, Research and Innovation via the COST Action MP0901 ‘NanoTP’. K.A. acknowledges financial support from Deutsche Forschungsgemeinschaft. Author Contributions K.A., M.J. and R.F. initiated and conceived this work. K.A. designed the precursor molecules and the corresponding synthetic routes, K.A. and A.M. synthesised precursor molecules and performed HPLC, NMR and MS analyses. J.R.S.-V. carried out on-surface synthesis work. J.R.S.-V. and T.D. performed STM and Raman measurements, I.S. did the He ion microscopy analysis. O.G. performed the calculations. All authors participated in analysis and interpretation of the results. J.R.S.-V. drafted the manuscript, with contributions from P.R. and O.G. R.F. and K.A. edited the manuscript and coordinated the efforts of the research teams. Author Information Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. Readers are welcome to comment on the online version of the paper. Correspondence and requests for materials should be addressed to R.F. (roman.f[email protected]) or K.A. ([email protected]). Figure 1 | Two-step bottom-up synthesis of SWCNTs. (1) Formation of singly capped ultrashort (6,6) SWCNT seed S1 via cyclodehydrogenation (CDH) of the suitably designed polyaromatic hydrocarbon precursor C96H54 (P1). (2) Nanotube growth via epitaxial elongation (EE) . Parts of the Publisher: NPG; Journal: Nature: Nature; Article Type: Physics letter DOI: 10.1038/nature13607 Page 9 of 25 precursor P1 involved in the formation of the SWCNT end cap and the ultrashort CNT segment of the seed S1 are highlighted in yellow and blue, respectively. Red dashed lines indicate the new C-C bonds formed upon CDH. Epitaxial elongation occurs via the successive addition of carbon species, as indicated in green. Figure 2 | Formation of (6,6) SWCNT seeds S1. a, b, Illustration of the thermally induced surface catalysed CDH to form the (6,6) SWCNT seed S1 from the adsorbed precursor P1. c, d, STM images of precursor molecules as deposited on Pt(111) (c) and after annealing to 770 K (d). e, Close-up STM image of a precursor (top) and the corresponding simulation based on the extended LUMO (greyscale) with a structural model of the molecule superimposed (bottom). f, Line profiles (positions indicated in c, d) over an as-deposited precursor P1 (grey line) and the seed species S1 obtained after annealing to 770 K (black line). g, Close-up STM images taken at −1 V and 0.1 V of (6,6) SWCNT seed S1 (left) and the corresponding simulations of HOMO and LUMO, respectively (greyscale, right). Figure 3 | Epitaxial elongation of singly capped SWCNT with (6,6) chiral index defined by the seed S1. a–c, Schematic illustration of the epitaxial elongation of (6,6) SWCNT seeds S1 via surface-catalysed C2 incorporation at the nanotube/metal interface. The use of ethanol as a carbon feedstock gas is illustrated, which decomposes on the hot, reactive Pt surface into C2 species and binds ‘epitaxially’ to the bay region of the previously formed singly capped SWCNT, resulting in an elongation of the tube along its axis. d–f, STM images of the as-prepared (6,6) SWCNT seeds (d), and after exposure to low doses of ethylene of 1 L (e) and 5 L (f), respectively, at a temperature of 770 K. For direct comparison, the same height colour scale is used for d–f. g, Line profiles taken across the features indicated by arrow pairs in the corresponding STM images d–f. h, i, STM images of a sample exposed to a pressure of 1 × 107 mbar of ethanol for 1 h (270 L) at a temperature of 770 K. A long SWCNT is observed to lie on the rough surface (h). A close-up STM image (i) identifies it as a (6,6) SWCNT. Figure 4 | SWCNT orientation determination and single chirality assessment by Raman spectroscopy. a, Raman spectra of epitaxially elongated (6,6) SWCNTs obtained by exposing seeds S1 to a pressure of 1 × 107 mbar of ethanol for 30 min (140 L) at a temperature of 670 K. The spectra were acquired with the sample surface perpendicular (grey curve) and at an angle of 30° (black curve) to the laser beam. b, Raman spectrum of longer SWCNTs (1 h at 1 × 107 mbar of ethanol at 670 K; 270 L) for a short laser illumination time (30 s), revealing defect-free SWCNTs as judged by the absence of a D peak. The insets show further details on the very narrow RBM at Publisher: NPG; Journal: Nature: Nature; Article Type: Physics letter DOI: 10.1038/nature13607 Page 16 of 25 completed singly capped SWCNT and do not act as seeds for nanotube growth on epitaxial elongation. Extended Data Figure 3 | Scanning helium ion microscopy images. SHIM images of epitaxially elongated SWCNTs obtained by exposing seeds S1 to 1 × 107 mbar of ethanol for 1 h (270 L) at 770 K. Long carbon nanotubes can be observed to lie on the surface, and in some cases to shake under the ion beam (indicated by light green arrows, down). Top left, lower-magnification view of surface: numbered coloured boxes are shown at higher magnification to the right. The unnumbered grey scale images show different surface locations at higher magnifications. The rightmost panel gives a higher magnification image of the long SWCNT seen in panel 1. Extended Data Figure 4 | Synthesis of SWCNT precursor P1. Details are given in Methods: here we describe reaction steps a to j. a, PPh3, toluene, reflux, 95%; b, BrPh3PCH2PhBr, KOtBu, EtOH, reflux, 81%; c, I2, hv, propylene oxide, cyclohexane, 72%; d, Pd(PPh3)4, Cs2CO3, toluene/MeOH, 110°C, 79%; e, NBS, DBPO, CCl4, reflux, 70%; f, NaCN, DMSO, RT, 40%; g, H2SO4, H2O, HOAc, reflux, 98%; h, SOCl2, 65 °C; i, AlCl3, CH2Cl2, RT, 57%; j, propanoic acid, TsOH, o-DCB, 180 °C, 65%. Extended Data Figure 5 | LDI mass spectra of precursor P1 (C96H54). a, b, LDI mass spectra of P1 before (a) and after (b) sublimation. Computed and experimentally observed isotope distribution patterns for C96H54 are given in the inset of b. P P ublisher: NP G ; Journal: N DOI: 1 N ature: Nature 10.1038/natu r Page 17 of 2 5 Figure 1 ; Article Typ e r e13607 5 e : Physics lett e er P P ublisher: NP G ; Journal: N DOI: 1 N ature: Nature 10.1038/natu r Page 18 of 2 5 Figure 2 ; Article Typ e r e13607 5 e : Physics lett e er P P ublisher: NP G ; Journal: N DOI: 1 N ature: Nature 10.1038/natu r Page 19 of 2 5 Figure 3 ; Article Typ e r e13607 5 e : Physics lett e er P P ublisher: NP G ; Journal: N DOI: 1 N ature: Nature 10.1038/natu r Page 20 of 2 5 Figure 4 ; Article Typ e r e13607 5 e : Physics lett e er Publisher: NPG; Journal: Nature: Nature; Article Type: Physics letter DOI: 10.1038/nature13607 Page 21 of 25 Extended Data Figure 1 Publisher: NPG; Journal: Nature: Nature; Article Type: Physics letter DOI: 10.1038/nature13607 Page 22 of 25 Extended Data Figure 2 Publisher: NPG; Journal: Nature: Nature; Article Type: Physics letter DOI: 10.1038/nature13607 Page 23 of 25 Extended Data Figure 3 Publisher: NPG; Journal: Nature: Nature; Article Type: Physics letter DOI: 10.1038/nature13607 Page 24 of 25 Extended Data Figure 4 Publisher: NPG; Journal: Nature: Nature; Article Type: Physics letter DOI: 10.1038/nature13607 Page 25 of 25 Extended Data Figure 5