Wolfgang Bacsa

High specific surface area carbon nanotubes from catalytic chemical vapor deposition process

A carbon nanotube specimen with a carbon content of 83 wt.% (95 vol.%) and a specific surface area equal to 790 m2/g (corresponding to 948 m2/g of carbon) is prepared by a catalytic chemical vapor deposition method. The nanotubes, 90% of which are single- and double-walled, are individual rather than in bundles. High-resolution electron microscopy shows a diameter distribution in the range 0.8–5 nm and Raman spectroscopy shows a high proportion of tubular carbon. Both techniques reveal a maximum in the inner wall diameter distribution close to 1.2 nm.

Introduction to Carbon Nanotubes

Carbon nanotubes are remarkable objects that look set to revolutionize the technological landscape in the near future. Tomorrowʼs society will be shaped by nanotube applications, just as silicon-based technologies dominate society today. Space elevators tethered by the strongest of cables; hydrogen-powered vehicles; artificial muscles: these are just a few of the technological marvels that may be made possible by the emerging science of carbon nanotubes.

CCVD synthesis of carbon nanotubes from (Mg, Co, Mo)O catalysts: influence of the proportions of cobalt and molybdenum

Carbon nanotubes have been synthesised by catalytic chemical vapour deposition of a H2–CH4 mixture (18 mol% CH4) over (Mg,Co,Mo)O catalysts. The total amount of cobalt and molybdenum has been kept constant at 1 cat% and the proportion of molybdenum with respect to cobalt has been varied from x(Mo) = 0.25–1.0. This variation has important effects on both the yield and the nature (number of walls, straight walls or bamboo-like structures) of the carbon nanotubes. It also has an influence on the purity of the samples (amount of encapsulated metal particles, presence or not of amorphous carbon deposits). For x = 0.25, the nanotubes were mainly double- and triple-walled (inner diameter less than 3 nm); samples prepared from catalysts with higher molybdenum ratios contained larger multi-walled carbon nanotubes (inner diameter up to 9 nm), having up to 13 concentric walls. It is proposed that different growth mechanisms may occur depending on the initial composition of the catalyst.

Controlled laser heating of carbon nanotubes

We investigate laser heating of double wall carbon nanotubes deposited on surfaces and immerged in liquids as a function of laser wavelength. Observing the Raman spectrum we find that laser heating of agglomerated double wall carbon nanotubes is six times larger at 488nm than at 647nm. The wavelength dependence of the Raman G band is linear in the visible spectral range. The frequency shift of the Raman G band obtained in methanol as a function of temperature is close to what is observed for graphite.

Chirality of internal metallic and semiconducting carbon nanotubes

We have assigned the chirality of the internal tubes of double walled carbon nanotubes grown by catalytic chemical vapor deposition using the high sensitivity of the radial breathing ~RB! mode in inelastic lightscattering experiments. The deduced chirality corresponds to several semiconducting and only two metallic internal tubes. The RB modes are systematically shifted to higher energies when compared to theoretical values. The difference between experimental and theoretical energies of the RB modes of metallic tubes and semiconducting tubes are discussed in terms of the reduced interlayer distance between the internal and the external tube and electronic resonance effects. We find several pairs of RB modes corresponding to different diameters of internal and external tubes.

Spectroscopic detection of carbon nanotube interaction with amphiphilic molecules in epoxy resin composites

Incorporation of carbon nanotubes into epoxy resin composites has the effect of increasing electrical conductivity at low percolation levels. An amphiphilic molecule such as palmitic acid has been used to increase the surface contact area and to improve the dispersion of the carbon nanotube bundles in the prepolymer. The chemical environment of the dispersed nanotubes has been probed using vibrational Raman spectroscopy. Spectroscopic Raman maps on sample surfaces (60×60μm2) with ratios of nanotubes to palmitic acid varying from 1:2 to 2:1 by weight, have been recorded to test the uniformity of the dispersion. Substantial spatial inhomogeneities have been observed in the G-band shift and an additional spectral band at 1450cm−1. The 1450cm−1 band has been attributed to the CH3 group of the amphiphilic molecules adsorbed onto the nanotube surface. The maps are correlated with the measured electrical conductivity values. The highest conductivity has been observed for the best dispersed nanotubes and nanotube...

Properties of Carbon Nanotubes

After a brief reminder of the basics of carbon nanotubes regarding their morphology, structure, texture, and nanotexture, this chapter attempts to summarize the knowledge gathered on every aspect of their properties as the result of the extensive investigation carried out in this field since the 1990s. The properties covered include electrical, thermal, optical, electronic, and adsorptive (chemical reactivity) completed by a summary of the behavior of carbon nanotubes in biological environment, both from the point of view of eco- and cytotoxicity and that of positive (e.g., therapeutic) interactions. An abundant literature is cited for enabling the reader to deepen any selected topic among those addressed here.

Relating elasticity and graphene folding conformation

Variational calculus is employed to determine the folding behaviour of a single graphene sheet. Both the elastic and van der Waals energies are taken into account, and from these considerations the shape of the curve is determined. By prescribing that the separation distance between the folded graphene in the parallel region is 3.32 A, an arbitrary constant arising by integrating the Euler–Lagrange equation is determined, and the full parametric representations for the folding conformation are derived. Using typical values of the bending rigidity in the range of 0.800–1.60 eV, the shortest stable folded graphene sheets are required to be at least 6.5–10 nm in length.

Sodide and Organic Halides Effect Covalent Functionalization of Single-Layer and Bilayer Graphene

The covalent functionalization of single and bilayer graphene on SiO2 (300 nm)/Si was effected through sequential treatment with the alkalide reductant [K(15-crown-5)2]Na and electrophilic aryl or alkyl halides, of which the iodides proved to be the most reactive. The condensation reactions proceeded at room temperature and afforded the corresponding aryl- or alkyl-appended graphenes. For each sample, Raman and X-ray photoelectron spectroscopies were used to evaluate the degrees and uniformities of functionalization. Statistical analyses of the Raman data revealed that the introduction of the organic moieties was accompanied by sp3-rehybridization of the basal plane atoms. When bilayers consisting of 13C and 12C layers were treated, both the top and bottom sheets were decorated with organic groups. The reaction was followed using Raman spectroscopy, and the mechanism was studied by theoretical calculations. Indicative of its structure and reactivity, 4-pyridyl-decorated single-layer graphene was readily benzylated and appears to be an ideal platform to develop functional materials.

Optical Interference Substrates for Nanoparticles and Two-Dimensional Materials

Interference substrates are useful in enhancing Raman and luminescence signals and in increasing the optical contrast of nanoparticles and atomically thin layers. Interference substrates played a crucial role in the discovery of the electric field effect on electronic conduction in graphene a few years ago. They are now used for a wide range of two-dimensional materials and recently for the heterostructures of atomically thin films. The same or similar substrates can be employed for the exploration of a large variety of nanoparticles. Although optical interference has been known to occur within the proximity of surfaces for more than one century, optical interference has been only gradually used over the last two decades to enhance the optical response of nanoparticles and layered materials. We review the different forms of interference substrates used over time. While multiple interference effects are frequently put forward to explain the enhancement in interference substrates, we show here that the formation of optical surface standing waves near strongly reflecting surfaces is the main cause of field enhancement. In addition, we demonstrate how a metal layer improves optical field enhancement.