Search

Our activities were focused on the preparation of WS2 films on sapphire substrates by sulfurization of different thick sputtered W films. The influence of very thin W films in the range from 4 nm to 12 nm on the structural, morphological and optical properties of WS2 was investigated. XRD measurements revealed a polycrystalline nature with hexagonal symmetry and randomly connected nanocrystals with grain size about 6 nm for all WS2 films. Using Raman spectroscopy with a 532 nm laser excitation, the presence of characteristic E12g and A1g vibration modes was recorded and the multilayered nature of the prepared WS2 films was confirmed. FESEM observations revealed randomly oriented lamellar and flake-shaped microstructures with the basal plane of the WS2 crystallites. Thinner WS2 films (20 and 24 nm) showed highly dense horizontally aligned flakes. On the other hand, thicker WS2 films (33 and 42 nm) indicated a granular surface and the WS2 crystallites grew perpendicularly to the substrate surface. All examined WS2 films were transparent from 30 to 78% in the spectral range of 500 to 900 nm and showed a direct bandgap of 2.3 eV.

With a continuous improvement of the production techniques for carbon nanofibers and carbon nanotubes along with an improvement of the available qualities of the materials, these reinforcements have been introduced into polymers, ceramics and metals. While in the field of polymers first success stories have been published on carbon nanofiller reinforcements, up to now metals containing these types of nanofillers are still a topic of intensive research. Basically a similar situation were found in those days, when micron sized carbon fibers came on the market. Today many applications of carbon fiber reinforced composites are existing, while metals reinforced with conventional carbon fibers are still only found in niche applications. Several reasons can be identified, why the introduction of carbon based nanofillers (nanofibers/nanotubes) into metallic matrices is a difficult task. Nevertheless it is worthwhile to carry out systematic studies in this field due to the excellent and promising thermal, electrical, mechanical or tribological properties of the nanofillers. This paper gives an overview and summarises the activities related to carbon nanotubes and nanofibers used as a reinforcement in metallic matrix materials. The main challenges and the potential with respect to material properties will be discussed.

Low energy vibrational excitations in single bonded fullerene dimeres (C59N)2 and (C60-)2 and in single wall carbon nanotubes were investigated by Raman spectroscopy. The two isoelectronic and isostructural dimeres exhibited very similar spectra with strongly resonance enhanced lines for the radial modes if excitation was with a red laser. For the biazafullerene for the first time all three geometrically allowed intercage modes could be identified. For the single wall carbon nanotubes the response from the radial breathing mode was analyzed for excitation with 20 different laser lines. The results were found to be consistent with contributions from all geometrically allowed tubes and a strong contribution to the force constant from intercage interaction.

Thin bundles of single wall carbon nanotubes were suspended between metal blocks deposited on a silicon substrate. SEM images confirm that the parts of bundles between the blocks are free standing and those outside the contacts are lying on the silicon substrate. Since the tubes are usually a few micrometers long Raman spectra for nanotubes with and without the contact could be taken. Raman spectra show differences in the position and shape of the RBM and G lines depending on the site of the measurement. The RBM mode is usually very weak as compared to the G line. The results showed that the geometry of the experiment should be taken into account for interpretation of single tube or thin bundle Raman experiments.

The hydrogen storage capacity in mechanically treated single wall carbon nanotubes (SWNTs) is investigated in this paper. In order to open the nanotubes three mechanical methods were applied: ball milling, ultrasonication and rubbing. Changes induced by the treatment were analyzed by Raman spectroscopy. The amount of hydrogen stored by the SWNTs reaches fractions of weight percents only. In the case of sonicated specimens, the hydrogen storage can be ascribed to titanium particles incorporated during the treatment.

The room temperature Raman spectra of the C84 isomers 22: D2 and 23: D2d are shown to resemble a downshifted and split C60 spectrum in the range of the radial cage modes below 450 cm−1. Additional Raman lines with a basically similar structure were found for three isomers of Sc2@C84 (Cs, C2v and D2d) below 200 and at 260 cm−1 and attributed to (Sc−C84)-vibrations. These modes are further specified with respect to their normal coordinates. Factor group splitting in the solid state of the Sc2@C84 D2d isomer explains for the larger number of (Sc−C84)-vibrations as found for the other isomers.

We report the isolation of nine isomers of [84]fullerene (C84). Of these minor isomers, six were sufficiently abundant to allow characterization by 13C NMR spectroscopy. These have, in order of decreasing abundance, D2(IV), D2d(II), Cs(V), D2d(I), D2(II), and C2(IV) molecular point group symmetry.

We present IR and Raman measurements of endohedral compounds Sc2@C84 with D2d symmetry and its empty counterpart C84. Our main task was to determine mutual carbon cage—Sc vibrations. The attention was therefore concentrated in the region below 300 cm−1 in which one expects this type of motions. Comparing spectra for empty and filled isomers with D2d symmetry we identified several candidates for the cage—Sc vibrations. The unexpected high number of such modes can be explained by site and correlation field splittings. Other modes observed in the region we attributed either to fundamental or combination modes of the cage. The temperature dependence of the spectra gave a better insight into the dynamics of the metallofullerene compounds. We observed a broadening of some modes in the mid-IR region with increasing temperature. From a model describing the interaction between cage modes and reorientational motion we obtained an activation energy for the reorientations.

We present IR transmission measurements for two isomers of the endohedral compound Sc2@C84 with D2d (No. 23) and D2 (No. 10) symmetry. The measurements were performed in the far- and mid-infrared region between 80 cm−1 and 4000 cm−1 and at temperatures from 80 K to 300 K. We identified several modes and investigated their dependence on the symmetry of the cage and on the temperature. The latter turned out to be rather strong and can be used to distinguish between modes of the metals in the cage and cage modes.

The hydrogen storage in purified single-wall carbon nanotubes (SWNTs), graphite and diamond powder was investigated at room temperature and ambient pressure. The samples were sonicated in 5 M HNO3 for various periods of time using an ultrasonic probe of the alloy Ti-6Al-4V. The goal of this treatment was to open the carbon nanotubes. The maximum value of overall hydrogen storage was found to be 1.5 wt %, as determined by thermal desorption spectroscopy. The storage capacity increases with sonication time. The sonication treatment introduces particles of the Ti alloy into the samples, as shown by X-ray diffraction, transmission electron microscopy, and chemical analysis. All of the hydrogen uptake can be explained by the assumption that the hydrogen is only stored in the Ti-alloy particles. The presence of Ti-alloy particles does not allow the determination of whether a small amount of hydrogen possibly is stored in the SWNTs themselves, and the fraction of nanotubes opened by the sonication treatment is unknown.