Nicole Grobert

Controlled production of aligned-nanotube bundles

Carbon nanotubes might be usefully employed in nanometre-scale engineering and electronics. Electrical conductivity measurements on the bulk material, on individual multi-walled and single-walled nanotubes and on bundles of single-walled nanotubes have revealed that they may behave as metallic, insulating or semiconducting nanowires, depending on the method of production—which controls the degree of graphitization, the helicity and the diameter. Measurements of Youngs modulus show that single nanotubes are stiffer than commercial carbon fibres. Methods commonly used to generate nanotubes—carbon-arc discharge techniques, catalytic pyrolysis of hydrocarbons and condensed-phase electrolysis—generally suffer from the drawbacks that polyhedral particles are also formed and that the dimensions of the nanotubes are highly variable. Here we describe a method for generating aligned carbon nanotubes by pyrolysis of 2-amino-4,6-dichloro-s-triazine over thin films of a cobalt catalyst patterned on a silica substrate by laser etching. The use of a patterned catalyst apparently encourages the formation of aligned nanotubes. The method offers control over length (up to about 50 μm) and fairly uniform diameters (30–50 nm), as well as producing nanotubes in high yield, uncontaminated by polyhedral particles.

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.

Enhanced magnetic coercivities in Fe nanowires

We describe a way of generating films (<2 mm2; <40 μm thick) of aligned Fe-filled carbon nanotubes. These Fe nanowires are usually composed of single Fe crystals, and have dimensions from 5–40 nm outer diameter and <10 μm in length. The carbon tubes, which coat the wires, have external diameters of ∼20–70 nm and are <40 μm in length. High-resolution electron energy loss spectroscopy, x-ray powder diffraction, and elemental mapping of the tubular structures reveal only characteristic metallic signals and the effective absence of oxygen (or any other nonmetallic element) within the wires. The material exhibits coercivities in the 430–1070 Oe range, i.e., greater than those reported for Ni and Co nanowires.

Carbon nanotubes – becoming clean

Carbon nanotubes (CNTs) are now well into their teenage years. Early on, theoretical predictions and experimental data showed that CNTs possess chemical and mechanical properties that exceed those of many other materials. This has triggered intense research into CNTs. A variety of production methods for CNTs have been developed; chemical modification, functionalization, filling, and doping have been achieved; and manipulation, separation, and characterization of individual CNTs is now possible. Today, products containing CNTs range from tennis rackets and golf clubs to vehicle fenders, X-ray tubes, and Li ion batteries. Breakthroughs for CNT-based technologies are anticipated in the areas of nanoelectronics, biotechnology, and materials science. In this article, I review the current situation in CNT production and highlight the importance of clean CNT material for the success of future applications.

Efficient route to large arrays of CNx nanofibers by pyrolysis of ferrocene/melamine mixtures

We report a high-yield route to two-dimensional arrays (<400×400 μm2) of aligned C49Nx (x ⩽ 1) nanofibers (<100 nm o.d.; <60 μm length), by pyrolyzing mixtures of ferrocene and melamine at 950–1050 °C under an Ar flow. The fibers exhibit unusual interlinked stacked-cone morphologies, ascribed to the presence of nitrogen. High-resolution electron energy-loss spectroscopy of the individual fibers reveals a 2% nitrogen content with ionization energies mainly at ∼400.9 eV, corresponding to N bonded to three C atoms within a hexagonal framework. The nanofibers may be useful for the economic fabrication of field emission sources and robust composites.

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.

Tungsten oxide tree-like structures

Abstract An interesting micrometer scale tree-like structure has been generated by heating a W foil, partly covered by a SiO 2 plate, in an Ar atmosphere at ca. 1600°C. Upon sonication, the trees are broken into nanoneedles (ca. 5–50 nm wide and 20–200 nm long) and planar polyhedral nanoparticles (ca. 10–50 nm cross-section). Structural analysis, using ED, EDX, XRD, and HRTEM, showed that: (1) the trees consist of well-crystallised WO x phases ( x =0–3); (2) the nanoneedles are composed mainly of monoclinic W 18 O 49 phases; and (3) the nanoparticles consist primarily of WO 3 . The tree growth is thought to arise from the intrinsic crystalline feature of WO x , the planar defect or the shear structure of which is responsible for the breakdown of the trees.

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.

SiOx-coating of carbon nanotubes at room temperature

A room temperature colloidal method for coating carbon nanotubes with silicon oxide is described. For the first time, the morphology, chemical composition and SiOx/C interfaces of the coatings were investigated using state-of-the-art transmission electron microscopy and highly spatially resolved electron energy-loss spectroscopy. The amorphous SiOx coatings are <10 nm thick. Electron energy-loss fine structure analyses indicate that the SiOx is partly substoichiometric. In addition, coatings were also created by a high-temperature route. However, they tend to be more unstable and defoliate when compared to coatings deposited at room temperature.

Synthetic routes to nanoscale BxCyNz architectures

Abstract B x C y N z nanoscale materials, hybrids of h-BN and graphite, have been recently synthesised using various techniques. Here, we present the latest advances in the synthesis and characterisation of B–C–N nanotubes and nanofibres. In particular, we focus on layered BC 2 N, BN, BC and CN x systems, reviewing their production methods as well as their structural and electronic properties. These materials may find important applications in the fabrication of nanotransistors, robust nanocomposites, conducting polymers, storage components and field emission sources.