Apparao M. Rao

Evidence for charge transfer in doped carbon nanotube bundles from Raman scattering

Single-walled carbon nanotubes (SWNTs) are predicted to be metallic for certain diameters and pitches of the twisted graphene ribbons that make up their walls. Chemical doping is expected to substantially increase the density of free charge carriers and thereby enhance the electrical (and thermal) conductivity. Here we use Raman spectroscopy to study the effects of exposing SWNT bundles to typical electron-donor (potassium, rubidium) and electron-acceptor (iodine, bromine) dopants. We find that the high-frequency tangential vibrational modes of the carbon atoms in the SWNTs shift substantially to lower (for K, Rb) or higher (for Br2) frequencies. Little change is seen for I2 doping. These shifts provide evidence for charge transfer between the dopants and the nanotubes, indicating an ionic character of the doped samples. This, together with conductivity measurements, suggests that doping does increase the carrier concentration of the SWNT bundles.

Continuous production of aligned carbon nanotubes: a step closer to commercial realization

High-purity aligned multi-walled carbon nanotubes MWNTs were synthesized through the catalytic decomposition of a ferrocene-xylene mixture at ; 6758C in a quartz tube reactor and over quartz substrates, with a conversion of ; 25% of the total hydrocarbon feedstock. Under the experimental conditions used, scanning electron microscope images reveal that the MWNT array grows perpendicular to the quartz substrates at an average growth rate of ; 25 mmrh. A process of this nature which does not require preformed substrates, and which operates at atmospheric pressure and moderate temperatures, could be scaled up for continuous or semi-continuous production of MWNTs. q 1999 Elsevier Science B.V. All rights reserved.

Molecular functionalization of carbon nanotubes and use as substrates for neuronal growth

Carbon nanotubes are strong, flexible, conduct electrical current, and can be functionalized with different molecules, properties that may be useful in basic and applied neuroscience research. We report the first application of carbon nanotube technology to neuroscience research. Methods were developed for growing embryonic rat-brain neurons on multiwalled carbon nanotubes. On unmodified nanotubes, neurons extend only one or two neurites, which exhibit very few branches. In contrast, neurons grown on nanotubes coated with the bioactive molecule 4-hydroxynonenal elaborate multiple neurites, which exhibit extensive branching. These findings establish the feasability of using nanotubes as substrates for nerve cell growth and as probes of neuronal function at the nanometer scale.

Model of carbon nanotube growth through chemical vapor deposition

Abstract This Letter outlines a model to account for the catalyzed growth of nanotubes by chemical vapor deposition. It proposes that their formation and growth is an extension of other known processes in which graphitic structures form over metal surfaces at moderate temperatures through the decomposition of organic precursors. Importantly, the model also states that the form of carbon produced depends on the physical dimensions of the catalyzed reactions. Experimental data are presented that correlate nanotube diameters to the size of the catalyst particles. Nanotube stability as a function of nanotube type, length and diameter are also investigated through theoretical calculations.

DISSOLUTION OF SINGLE-WALLED CARBON NANOTUBES

834 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 0935-9648/99/1007-0834

NANOTUBE COMPOSITE CARBON FIBERS

17.50+.50/0 Adv. Mater. 1999, 11, No. 10 be detected. Furthermore, the mechanical properties of the complex are solid-like at temperatures below the observable glass transition, which would not be the case for a phase separated material containing rubbery microdomains. Instead, we advance an explanation for the extraordinary properties of this liquid crystal complex by invoking a high degree of coupling between mesogenic side groups and the ethylene oxide backbone which thereby inhibits the formation of helical conformations favored by afreeo poly(ethylene oxide) chains. Within such helical arrangements the lithium ions would be tightly coordinated below Tg and effectively trapped. By suppressing the formation of such helical structures, an open ethylene oxide structure is obtained within which lithium ions are free to move and where empty sites exist for the ions to occupy. It is remarkable that a similar (but weaker) effect is observed in the amorphous material. It seems that insertion of rigid isophthalate units also inhibits helical formation, sufficient to provide a measure of ionic decoupling, but for the liquid crystal complex the open structure is further stabilized via the interactions between the liquid crystal side groups. It is worth noting that this view is supported by the continuity of behavior from the melt into the glassy state in both the heat capacity and electrical conductivity, indicating that the structure of the melt is not strongly influenced by temperature. In addition, the dissolution of the ions in the backbone does not swell the smectic layer and thus, the open network must be relatively unchanged at least in the direction normal to the smectic layers. This suggests that by careful engineering of the types of liquid crystal phase present, it should be possible to tailor the conductivity mechanism to particular applications. It is also remarkable that the conductivity in the MeOC6G6 complex increases strongly (by several orders of magnitude) as the AO:Li ratio is decreased from 10:1 to 3:1. We should now be able to employ more concentrated polymer electrolytes than is presently possible with conventional materials. Transport number data for these electrolytes are not yet available, but it is tempting to speculate that the elimination of acation trappingo within the ethylene oxide helix will lead to substantial increases in cation mobility. For many years this has been one of the principal goals of polymer electrolyte research.

Uptake, translocation, and transmission of carbon nanomaterials in rice plants.

Single walled carbon nanotubes (SWNTs) were dispersed in isotropic petroleum pitch matrices to form nanotube composite carbon fibers with enhanced mechanical and electrical properties. We find that the tensile strength, modulus, and electrical conductivity of a pitch composite fiber with 5 wt % loading of purified SWNTs are enhanced by ∼90%, ∼150%, and 340% respectively, as compared to the corresponding values in unmodified isotropic pitch fibers. These results serve to highlight the potential that exits for developing a spectrum of material properties through the selection of the matrix, nanotube dispersion, alignment, and interfacial bonding.

Selective gas detection using a carbon nanotube sensor

Recent development of nanotechnology has reshaped the landscape of modern science and technology, while in the meantime raised concerns about the adverse effects of nanomaterials on biological systems and the environment. Owing to their mutual interaction, carbon-based nanomaterials readily aggregate and are not considered potential contaminants in the liquid phase. However, when discharged into the environment, the hydrophobicity of nanomaterials can be averted through their interaction with natural organic matter (NOM), a heterogeneous mixture of decomposed animals and plants and a major pollutant carrier in nature. Consequently, mobile NOM-modified nanomaterials may pose a threat to ecological terrestrial species through further physical, chemical, and biological processes. The impact of nanomaterials on high plants has scantly been examined in the current literature. Among the studies available, none have used major food crops or carbon nanoparticles (a major class of nanomaterials) for their evaluations. Although both enhanced and inhibited growth have been reported for vegetations exposed to nanomaterials at various developmental stages, including seed germina-

Chemical Attachment of Organic Functional Groups to Single-walled Carbon Nanotube Material

A circular disk resonator is used to study the gas sensing properties of carbon nanotubes. It detects the presence of gases based on the change in the dielectric constant rather than electrical conductivity of single walled carbon nanotubes (SWNTs) upon gas exposure. A conducting circular disk is coated with electric arc prepared SWNTs and degassed by heating under a high vacuum. It exhibits noticeable shifts in resonant frequency to both polar (NH3 and CO) and nonpolar gases (He, Ar, N2, and O2). Gas concentrations as low as 100 ppm can be detected using this sensor configuration.

Carbon-nanotube-based resonant-circuit sensor for ammonia

We have subjected single-walled carbon nanotube materials (SWNTMs) to a variety of organic functionalization reactions. These reactions include radioactive photolabeling studies using diradical and nitrene sources, and treatment with dichlorocarbene and Birch reduction conditions. All of the reactions provide evidence for chemical attachment to the SWNTMs, but because of the impure nature of the staring materials, we are unable to ascertain the site of reaction. In the case of dichlorocarbene we are able to show the presence of chlorine in the SWNT bundles, but as a result of the large amount of amorphous carbon that is attached to the tube walls, we cannot distinguish between attachment of dichlorocarbene to the walls of the SWNTs and reaction with the amorphous carbon.