R. Kamalakaran

Identification of electron donor states in N-doped carbon nanotubes

Nitrogen-doped carbon nanotubes have been synthesized using pyrolysis and characterized by scanning tunneling spectroscopy and transmission electron microscopy. The doped nanotubes are all metallic and exhibit strong electron donor states near the Fermi level. Using tight-binding and ab initio calculations, we observe that pyridine-like N structures are responsible for the metallic behavior and the prominent features near the Fermi level. These electron rich structures are the first example of n-type nanotubes, which could pave the way to real molecular heterojunction devices.

Pyrolytic production of aligned carbon nanotubes from homogeneously dispersed benzene-based aerosols

Aligned multi-walled carbon nanotubes (30–130 μm long, 10–200 nm outer diameter) have been prepared in high yield by pyrolysing homogeneously dispersed aerosols generated from benzene/ferrocene solutions, at 800°C or 950°C, using a compressed gas (Ar) driven atomiser. Scanning electron microscopy (SEM) and TEM studies reveal the presence of carpet-like flakes containing high yields of partly filled carbon nanotubes. X-ray diffraction confirms the presence of graphite-like structures, α-Fe and Fe3C (cementite). Nanotube and filling yields were found to be temperature and ferrocene concentration dependent. The preparative method opens up new avenues for nanotube synthesis based upon hydrocarbon/catalyst solutions.

Graphitic cones in palladium catalysed carbon nanofibres

Abstract High yields of graphitic conical nanofibres (5–70 nm OD; 5 μm long) are produced by pyrolysing palladium precursors under Ar at 850–1000°C. The fibres exhibit diamond-shaped Pd particles at their tips, which are responsible for the formation of stacked graphene cones (open, lampshade-type, or closed). The cones observed with apex angles of ca. 30°, 50° and 70° can be explained by an open cone approach, which considers different chiralities. Due to the presence of open edges (dangling bonds), we envisage that these novel nanofibres may find important applications in the fabrication of field emitters, gas storage components and composites.

Novel nanoscale gas containers: encapsulation of N2 in CNx nanotubes

Gaseous N2 has been efficiently introduced in the hollow cavities of aligned CNx nanotubes (15–80 nm od) by pyrolysing a jet (spray) solution of Fe(C5H5)2 and PhCH2NH2 in an Ar atmosphere at 850 °C; the aligned material consist of large arrays (2.5 cm2) of CNx (x < 0.05) ‘bamboo-like’ nanotubes (<100 μm in length); high resolution electron energy loss spectroscopy (HREELS) line-scans and elemental mapping studies reveal that N gas was encapsulated within the hollow compartments of the CNx nanotubes; these results demonstrate for the first time, that CNx nanotubes can be used to fabricate gas storage components.

Alloy nanowires: Invar inside carbon nanotubes

Invar (Fe65Ni35), a ‘zero’ thermal expansion alloy consisting of Fe and Ni, has been successfully introduced into carbon nanotubes by pyrolysing, at 800 °C, aerosols of NiCp2/FeCp2 mixtures dissolved in C6H6; scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) studies reveal the presence of flake-like structures (ca. 1–2 mm2) consisting of filled/aligned carbon nanotubes (⩽200 μm in length and ⩽80 nm in diameter) in a carpet pile-like configuration; analysis of the filling material (⩽500 nm in length and ⩽40 nm in diameter) by X-ray powder diffraction and high-resolution electron energy loss spectroscopy (HREELS) line scans, confirmed that Invar was formed; this appears, to the best of our knowledge, to be the first report of mixed metal alloy nanowires forming inside carbon nanotubes.

C-MoS2 and C-WS2 nanocomnposites

Composite C-MoS2 and C-WS2 nanostructures were produced by pyrolysing H2S/N2 over a mixture of MoO3 (or WO3) and carbon particles/nanotubes. MoS2 particles embedded in the nanotubes were also observed occasionally.

Doping and connecting carbon nanotubes

Self-assembly pyrolytic routes to arrays of aligned CN x nanotubes are described. The electronic properties and the density of states (DOS) of these N doped tubes characterized by scanning tunneling spectroscopy (STS) are also presented. Using tight-binding calculations, we confirm that the presence of N is responsible for introducing donor states near the Fermi Level. Finally, it will be shown that high electron irradiation during annealing at 700-800°C, is capable of coalescing single-walled nanotubes (SWNTs). We investigate the merge at the atomic level using tight-binding molecular dynamics (TBMD). Vacancies induce the coalescence via a zipper-like mechanism, responsible of a continuous reorganization of atoms on individual tube lattices within the adjacent tubes. The latter results pave the way to the fabrication of nanotube contacts, nanocircuits and strong 3D composites using irradiation doses under annealing conditions.

Metal and alloy nanowires: Iron and invar inside carbon nanotubes

Pyrolysis of hydrocarbons over metal catalysts has proved to be a very efficient and versatile technique for generating metal-filled carbon nanotubes. At reduced pressures Fe-filled nanotubes are formed efficiently from ferrocene and C60. Recently, alloys such as invar (Fe65Ni35) were successfully introduced in carbon nanotubes by pyrolysing aerosols of nickelocene/ferrocene (Cp2Ni/Cp2Fe) mixtures dissolved in benzene. With the introduction of Invar in carbon nanotubes it is now possible to study the physico chemical properties of these wires. Furthermore, the alloy formation notably reduces the formation of unwanted by-products such as amorphous carbon, or metal encapsulated particles. The results also confirm that the use of C60 or Ni/Fe also enhances the growth and crystallinty of the products. The products have been characterized by SEM, HRTEM, EELS, XRD and SQUID. The magnetic properties of metal- and alloy-filled nanotubes may be applied to the fabrication of high density magnetic storage devices, m...

Graphitic cones in carbon nanofibres

High yields of graphitic conical nanofibres (5-70 nm OD; <5 µm in length) are produced by pyrolysing various palladium precursors under an Ar atmosphere at 850-1000°C. The fibres exhibit diamond-shaped Pd particles at their tips, which are responsible for the carbon aggregation and its subsequent diffusion. This carbon replication phenomenon on the Pd particles results in the formation of stacked graphene cones, which grow aligned along a common axis, thus creating graphitic nanofibres. The cones within the fibres can be either open (lamp-shade type) or closed. The material has been analysed using sophisticated electron microscopy (HRTEM, SEM, ED) and spectroscopic techniques (Raman, EELS, EDX). Due to the large number of open edges, we envisage that these novel nanofibres may find important applications in the fabrication of field emitters, gas storage devices and composites.

Pure and aligned carbon nanotubes produced by the pyrolysis of benzene-based aerosols

The present work reports a method for preparing nanotubes involving pyrolysis of aerosols generated from benzene solutions of metallocenes. Different concentrations of ferrocene or ferrocene/nickelocene mixtures (65:35 and 25:75 Fe:Ni atomic ratios) were atomised with a compressed gas (Ar) driven sprayer, and pyrolysed at 800 or 950 °C. The black products, analyzed by SEM, (HR)TEM, and XRD, consisted of carpet-like aligned multi-walled nanotubes which are partly filled. The purity of the samples is generally high, but is drastically improved by using a ferrocene/nickelocene mixture 65:35 Fe:Ni. The changes in the nanotube size, and in the degree/size of filling, are reported as a function of synthesis parameters such as the nature and content of the metallocene, and the pyrolysis temperature.