H. Kuzmany

Resonance Raman and infrared spectroscopy of carbon nanotubes

We present a comparative analysis of the vibrational and structural properties of carbon nanotubes. The first-order Raman spectrum exhibits two lines at 1582 cm−1 and at 1350 cm−1. The observed ratio of the integrated intensity of these lines was found to be different as compared to polycrystalline graphite. The position and intensity of the line around 1350 cm−1 strongly depend on the energy of the exciting laser line. This dispersion effect is again different from the dispersion in nanocrystalline graphite. It is discussed in terms of a photoselective resonance process. Transmission infrared spectra of the nanotubes show one broad and asymmetric line at 1575 cm−1 and a weaker line at 868 cm−1.

Raman spectroscopy of small-diameter nanotubes

Results based on Raman measurements of small-diameter nanotubes (NTs) are presented and discussed in this paper. The NTs with diameters from 1 nm down to 0.4 nm were produced either as the inner tubes in the double-wall carbon NTs (DWCNTs) or as tubes embedded in the channels of the zeolite crystals. While analysing the Raman spectra attention was paid to the radial breathing mode (RBM), the D line and the G band. For both NT systems the RBM frequency was found to follow the same functional diameter dependence as the tubes with larger diameters. However, in contrast to the latter, the diameters of the thin tubes obtained from density functional theory calculations must be taken into account to explain satisfactorily the observed line positions. The resonance behaviour of the RBM intensities was recorded for the tubes in zeolites. It allows us to ascribe a position of the RBM to a particular NT. This result also demonstrates the breakdown of a simple tight-binding approach to the electronic structure but agrees with predictions from ab initio calculations. The D line of the outer tubes in DWCNTs is dispersive, similar to the single-wall carbon NTs. However, the rate of dispersion is reduced for the inner tubes in DWCNTs. This is attributed to the fact that the inner and outer tubes are probed with the same laser excitation. The linear shift due to the increasing laser energy is compensated by the negative shift due to the NT diameter. The latter is smaller for the inner NTs which leads to a stronger compensation of their dispersive behaviour. This effect is even stronger for the NTs in zeolites. In the extreme case, the strong Raman lines are not dispersive at all. This unexpected behaviour was explained by the detailed ab initio calculation of the phonon structure. The G bands of the inner semiconducting tubes were observed as new features in the Raman spectra of DWCNTs. On the other hand, no lines of metallic inner tubes were found. G bands of semiconducting as well as metallic NTs were detected for the zeolite samples. In either case, Raman lines due to the recently proposed Peierls-like mechanism for the thin metallic tubes were not indentified. This mechanism must therefore cause a significant reduction in Raman intensity.

Evidence for trans-polyacetylene in nano-crystalline diamond films

Abstract In CVD deposited diamond films a Raman mode at approximately 1150 cm−1 is often used as a simple criterion for a nano-crystalline diamond phase in the sample. Since this line shows a dispersion of approximately 25 cm−1/eV it was suggested to originate from trans-polyacetylene segments in the grain boundaries. We calculated the position of the ν 1 mode of trans-polyacetylene using a vibronic model for resonance Raman scattering and compared the results with the measured positions of the 1150 cm−1 mode for diamond samples on different substrates and found a very good agreement. Within the calculations we were able to estimate the mean number of uninterrupted conjugated bonds in the chains. Additionally, the response for the 1150 and 1480 cm−1 lines turned out to be unstable vs. exposure to elevated temperatures which is further evidence for its origin from conjugated segments.

Structural and electronic transitions in polyaniline: A Fourier transform infrared spectroscopic study

Fourier transform infrared (FTIR) spectroscopic studies are carried out on different, potentiometric well defined oxidation states of polyaniline in aqueous acidic and organic electrolytes. During the oxidation process ring structures transform from benzenoid into quinoid states. The fully reduced state of polyaniline shows differences in the anion contents in acidic and organic electrolytes. The 400 mV vs saturated calomel electrode (SCE) oxidized state has the maximum number of the intercalated anions in aqueous acidic media in accordance with supporting potentiometric titration experiments. This conducting form of polyaniline shows similar FTIR spectra in organic as well as in acidic media. For the oxidized state at 800 mV vs SCE, a deintercalation of anions in aqueous acidic, or further intercalation in organic electrolyte is observed. Beyond 800 mV vs SCE, polyaniline shows degradation processes in aqueous acidic media which are found to proceed via formation of benzoquinone‐like structures and final...

Electronic Properties of Polymers and Related Compounds

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Studies of chain conformational kinetics in poly(di‐n‐alkylsilanes) by spectroscopic methods 2. Conformation and packing of poly(di‐n‐hexylsilane)

Unoriented and highly oriented films of poly(di‐n‐hexylsilane) have been studied by Raman scattering, wide angle x‐ray diffraction and optical absorption measurements. From a comparison of group theoretical predictions with those bands observed in the Raman experiments and from the x‐ray layer line spacing from an oriented sample, a planar zig–zag conformation for the silicon backbone was deduced. X‐ray reflections were suitably indexed by a monoclinic unit cell containing two molecules. Polarized Raman studies on uniaxially oriented samples also revealed that the hexyl side chains are not orthogonal to the silicon backbone but may be slightly tilted in order to minimize intramolecular steric interactions.

Raman spectroscopy of fullerenes and fullerene-nanotube composites

The discovery of fullerenes in 1985 opened a completely new field of materials research. Together with the single–wall carbon nanotubes (SWCNTs) discovered later, these curved carbon networks are a playground for pure as well as applied science. We present a review of Raman spectroscopy of fullerenes, SWCNTs and composite materials. Beginning with pristine C60, we discuss intercalated C60 compounds and polymerized C60, as well as higher and endohedral fullerenes. Concerning SWCNTs, we show how the diameter distribution can be obtained from the Raman spectra and how doping modifies the spectra. Finally, the Raman response of C60 encapsulated into SWCNTs (C60 peapods) is discussed.

A double-temperature-gradient technique for the growth of single-crystal fullerites from the vapor phase

Fullerite single crystals were prepared by a sublimation-condensation method in a closed evacuated glass tube situated in a double-temperature-gradient furnace. Crystals of various size and up to 9 mg weight with well expressed smooth and shiny faces were obtained. X-ray analysis, interfacial angle measurements and observed morphological habits of selected crystals of C60 confirm the theoretically predicted and experimentally well established fcc structure at room temperature with two types of morphological faces, namely {111} and {100}. A strong tendency to twinning was observed. In the case of C70 crystals, a pure fcc structure was observed. Information on growth kinetics and on instability versus exposure to air and light were obtained from surface studies. Characteristic changes in a thin surface layer were observed when crystals were exposed to air and light. A new phase of C60 stabilized by oxygen was characterized.

Structure and stability of endohedral fullerene Sc3N@C80: A Raman, infrared, and theoretical analysis

Structure and stability of endohedral fullerene Sc3N@C80 were studied by temperature-dependent Raman and infrared spectroscopy as well as by quantum-chemical [density-functional-based tight-binding] calculations. The material showed a remarkable thermal stability up to 650 K. By both theory and experiment, translational and rotational Sc3N modes were found. These modes give a direct evidence for the formation of a Sc3N–C80 bond which induces a significant reduction of the ideal Ih–C80 symmetry. From their splitting pattern a crystal structure with more than one molecule in the unit cell is proposed. According to our results: (i) a significant charge transfer from the Sc3N cluster to the C80 cage; (ii) the strength of three Sc–N bonds; (iii) the chemical bond between triscandium nitride cluster and C80 cage; and (iv) a large HOMO–LUMO gap are responsible for the high stability and abundance of Sc3N@C80.

Infrared spectroscopy of fullerenes

This review summarizes the present status of the application of IR spectroscopy to the field of fullerene science. In accord with the amount of published work the report concentrates to a large extent on the C60 molecule in its neutral and charged form in a crystalline environment. After a short introduction to the basic terminologies and properties of fullerenic materials a detailed description of the response of pristine C60 the IR radiation is given. It includes a review on the vibrational fine structure, the isotope effects, and the influence of the structural phase transitions. For the case of the doped material the response of the free carriers and their interaction with the vibrational modes as observed in the IR spectra is reviewed, including the case of high-temperature superconductivity. Special attention is paid to the colourful behaviour of the phases AC60 and to the air stability of this compounds. In the final part of the review selected examples are given for the application of IR spectroscopy to higher fullerenes and to fullerene-derived materials.