Iron-doped carbon aerogels: novel porous substrates for direct growth of carbon nanotubes

We present the synthesis and characterization of Fe-doped carbon aerogels (CAs) and demonstrate the ability to grow carbon nanotubes directly on monoliths of these materials to afford novel carbon aerogel-carbon nanotube composites. Preparation of the Fe-doped CAs begins with the sol-gel polymerization of the potassium salt of 2,4-dihydroxybenzoic acid with formaldehyde, affording K+-doped gels that can then be converted to Fe2+- or Fe3+-doped gels through an ion exchange process, dried with supercritical CO2, and subsequently carbonized under an inert atmosphere. Analysis of the Fe-doped CAs by TEM, XRD, and XPS revealed that the doped iron species are reduced during carbonization to form metallic iron and iron carbide nanoparticles. The sizes and chemical composition of the reduced Fe species were related to pyrolysis temperature as well as the type of iron salt used in the ion exchange process. Raman spectroscopy and XRD analysis further reveal that, despite the presence of the Fe species, the CA framework is not significantly graphitized during pyrolysis. The Fe-doped CAs were subsequently placed in a thermal CVD reactor and exposed to a mixture of CH4 (1000 sccm), H2 (500 sccm), and C2H4 (20 sccm) at temperatures ranging from 600 to 800 degrees C for 10 min, resulting in direct growth of carbon nanotubes on the aerogel monoliths. Carbon nanotubes grown by this method appear to be multiwalled (approximately 25 nm in diameter and up to 4 microm long) and grow through a tip-growth mechanism that pushes catalytic iron particles out of the aerogel framework. The highest yield of CNTs was grown on Fe-doped CAs pyrolyzed at 800 degrees C treated at CVD temperatures of 700 degrees C.     

Enhancement of weak anti-localization signatures in the magneto-resistance of bismuth anti-dot thin films

Anti-dot array thin films of bismuth were prepared by the e-beam deposition of this semi-metal on nano-porous substrates. The magneto-resistance measurements of bismuth thin films deposited under identical conditions on various substrates displayed signatures from both classical magneto-resistance and weak anti-localization effects. The relative intensity of the two effects could be altered by the choice of the substrate, with the anti-dot array morphology suppressing the classical magneto-resistance, and enhancing the weak anti-localization contribution to the measured magneto-resistance. As a result, the weak anti-localization effect can be traced to higher magnetic field strengths and higher temperatures than is possible in non-patterned films, improving the accuracy of the parameters extracted from the data. PACS 73.20.Fz; 73.23-b; 73.61.At     

Magnetoreflection studies of ion implanted bismuth

The effect of the implantation of Sb ions on the electronic structure of the semimetal bismuth is studied by the magnetoreflection technique. The results show long electronic mean free paths and large implantation-induced increases in the band overlap and L-point band gap. These effects are opposite to those observed for Bi chemically doped with Sb.     

X-ray characterization of graphite intercalation compounds

The understanding of electronic and lattice properties of graphite intercalation compounds depends critically on the model describing the structural properties. We report here results showing that well-staged as-grown samples do not exhibit the expected in-plane intercalant density, and that careful analysis of the 00? x-ray diffractograms reveals important information on the in-plane occupation probability.     

The temperature variation of the thermal conductivity of benzene-derived carbon fibers

The temperature variation of the in-plane thermal conductivity of benzene-derived carbon fibers (BDF) measured from 5 to 300 K is reported and discussed. Very high thermal conductivity values — comparable to that of the best HOPG heat treated at the same temperature — are found. The data confirms the high structural order previously reported for BDF.     

Landau levels in graphite intercalation compounds

The first calculation of the magnetic energy level structure of a graphite intercalation compound is presented. The calculational technique exploits the staging symmetry through the k_z-axis zone folding of the magnetic energy levels of the graphite π-bands. The results are applicable to the interpretation of the magnetoreflection and de Haas-van Alphen type experiments in intercalated graphite.     

Formation of graphitic structures in cobalt- and nickel-doped carbon aerogels

We have prepared carbon aerogels (CAs) doped with cobalt or nickel through sol-gel polymerization of formaldehyde with the potassium salt of 2,4-dihydroxybenzoic acid, followed by ion exchange with M(NO3)2 (where M = Co2+ or Ni2+), supercritical drying with liquid CO2, and carbonization at temperatures between 400 and 1050 degrees C under a N2 atmosphere. The nanostructures of these metal-doped carbon aerogels were characterized by elemental analysis, nitrogen adsorption, high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Metallic nickel and cobalt nanoparticles are generated during the carbonization process at about 400 and 450 degrees C, respectively, forming nanoparticles that are approximately 4 nm in diameter. The sizes and size dispersion of the metal particles increase with increasing carbonization temperatures for both materials. The carbon frameworks of the Ni- and Co-doped aerogels carbonized below 600 degrees C mainly consist of interconnected carbon particles with a size of 15-30 nm. When the samples are pyrolyzed at 1050 degrees C, the growth of graphitic nanoribbons with different curvatures is observed in the Ni- and Co-doped carbon aerogel materials. The distance of graphite layers in the nanoribbons is approximately 0.38 nm. These metal-doped CAs retain the overall open cell structure of metal-free CAs, exhibiting high surface areas and pore diameters in the micro- and mesoporic region.