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The electronic and the vibrational structure of carbon nanotubes with ultrahigh curvature was systematically studied by resonance Raman scattering, high-resolution transmission electron microscopy (HRTEM), molecular dynamics, and ab initio DFT calculations. The ultrahigh curvature tubes were grown inside commercial HiPco tubes after filling the latter with the small but carbon-rich molecule ferrocene. TEM showed partial filling of the outer tubes with inner tubes and mobility of the latter in the electron beam. The smallest analyzed tube was of (5,0) chirality and had a DFT determined diameter of 0.406 nm and a radial breathing mode frequency of 570 cm(-1). For all inner tubes which had transitions in the visible spectral range, transition energies and RBM frequencies were determined with a resonance width of only 45 meV. Experimentally determined transition energies revealed dramatic deviations up to several electronvolts compared to tight-binding calculations and a significant family spread of more than 2 eV but were in agreement with many electron contribution corrected extended tight-binding results and with results from DFT calculations.

A detailed study on the photoselective Raman response of p-type intercalated single-wall carbon nanotube (SWCNT) is presented. Here, we focus on the high doping region. The shift of the G-line is used to monitor the doping level of the p-type intercalated SWCNT using irontrichloride. Special emphasis is given on the changes in the dispersion of the D- and G'-lines as a function of doping. For very high doping levels a decrease of the slope is observed.

We studied peapods prepared with ferrocene (dicyclopentadienyl iron, Fe(Cp) 2 ) as filling species, which makes CNTs with diameters down to 1.1 nm accessible for filling. Two different single-wall carbon nanotube (SWCNT) samples with mean diameters of 1.1 nm (HiPco) and 1.6 nm (DIPS) were used. Filling was done from the gas phase at elevated temperatures. Sample analysis was done by multifrequency Raman spectroscopy. The Raman response from the encapsulated molecule could be observed through the wall of the CNTs, confirming the encapsulation of the employed molecular species. We observed a sharp decrease of the radial breathing mode intensity for the peapod samples compared to the unfilled CNTs. Ferrocene peapods were transformed to double wall carbon nanotubes (DWCNTs) in a heat induced growth process.

Raman scattering from double-walled carbon nanotubes is reported with particular emphasis on the response from the radial breathing mode (RBM) of the inner-shell tubes. The unexpected large number of very narrow lines observed is explained by the growth of one and the same inner tube type in different outer tubes in a highly shielded environment. The response of the RBM and of the G-line is used to analyze the transition from peapods to double-walled carbon nanotubes. During the transformation process the Raman response disappears for a short time, indicating the existence of some Raman dark matter. By preparing the starting peapods from heterofullerenes such as (C59N)2 or 13C-substituted fullerenes, hetero-nanotubes can be grown where nitrogen or the 13C atoms are incorporated into the inner tube wall. Copyright © 2007 John Wiley & Sons, Ltd.

We report on the preparation and subsequent Raman analysis of carbon nanotube peapods, using the fullerene C 60 and its heterofullerene derivative (C 59 N) 2 as filling materials. The filling with (C 59 N) 2 was done from liquid solution at room temperature and from the gas phase at elevated temperatures. The success of the encapsulation procedure is confirmed through the identification of fingerprint Raman modes and the conversion to double wall nanotubes through heating to 1250 °C. The 2D mode of double wall nanotubes made from (C 59 N) 2 peapods is observed to be downshifted compared to the same mode in double wall nanotubes made from C 60 peapods. We interpret this downshift as an evidence for the integration of the nitrogen into the lattice of the inner tube.

Raman scattering is used to study the filling of single wall carbon nanotubes with C60, C59N and anthracene. It is demonstrated how reversible opening and closing of the tubes can be used to check on the success of the filling process. In the case of C59N, filling was performed from the gas phase from 1:1 mixture between C60 and C59N. Comparing the Raman spectra from the encapsulated mixture with the spectra from C60 inside the tubes reveals that most of the molecules have reacted to form either the dimer (C59N)2 or the hetero-dimer C60C59N. Anthracene is demonstrated to enter the tubes but the molecule is subjected to a considerable geometrical change including some loss of hydrogens. Copyright © 2007 John Wiley & Sons, Ltd.

Raman spectra have been demonstrated repeatedly to be a very valuable tool for the analysis of new carbon phases such as fullerenes and single wall carbon nanotubes (SWCNTs). Recently it was demonstrated from TEM analysis that C 60 can be encapsulated into SWCNTs. The structures have been given the name ‘peapods’. The concentration of the encapsulated ‘peas’ and the bonding structure in the tube are still unknown but under heavy discussion. From experience with C 60 and SWCNTs, Raman spectroscopy is expected to be a key technique for the analysis of such structures. In our experiments, we found two modes in the region of the pentagonal pinch mode of C 60 . The resonance behavior for these two modes and their temperature dependence is shown in this paper.

We report on the evaluation of the distribution of diameters for nanotube samples with a wide variation of mean diameters. Such results were obtained from a detailed analysis of the radial breathing mode Raman response and compared to results obtained from an evaluation of optical spectra and X-ray diffraction pattern. The evaluation of the Raman data needs a well refined analysis as the experimental analysis exhibits a rather complicated and oscillating relation between response and exciting laser. Both, an exact calculation where the density of states was considered explicitly and an approximate calculation were applied. Both models used for the analysis are able to explain several unexpected results from the experiment such as the oscillating behavior of the spectral moments, unusual discontinuities in the first moments of the Raman response for excitation in the IR, a fine structure for the response in optics and Raman, and an up shift of the RBM frequency as compared to qualified ab initio calculations. In detail the first moment and the variance of the spectra were used for the evaluation of the diameter distribution. To obtain good results between experimental and theoretical oscillation pattern the transition energy between the first two van Hove singularities had to be scaled up which is considered as a result from coulomb interaction of the electrons in the tubular material. On the other hand the analysis does not only allow to determine the mean value and the width of the diameter distribution but yields also a value for the average bundle diameters or, alternatively, the strength of the tube-tube interaction. The model used for the analysis of the Raman data is also appropriate to analyze the optical response, at least for the spectral range from 0.5 eV to 3.5 eV. The fine structure in the response for the transitions between the three lowest van Hove singularities is well reproduced and the mean tube diameters and their distribution is obtained in very good agreement with the results from the Raman analysis. From the X-ray analysis the same mean values and comparable distributions for the tube diameters were received whereas the bundle diameters could not be retained with high precision in this case.

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.

Raman spectra are presented for the single bonded dimeric fullerene (C 60 - ) 2 and compared to optical spectra and Raman spectra of the isostructural and isoelectronic heterofullerene (C59N)2. The spectra of both materials exhibit strong correlations with respect to splitting, line position, and line intensity. This holds for non resonant excitation with blue and green lasers as well as for the strong resonances observed with red lasers. The latter observation is consistent with a downshift for the electronic transition energies as compared to C60. The absorption edge of thin films of (C59N)2 was found at 1.4 eV. The three intercage modes were observed at 82, 103, and 111, and at 88, 98, and 106 cm-1 for (C59N)2 and (C 60 - ) 2 , respectively. A surprising difference was found for the position of the pentagonal pinch modes in the two materials as they were observed at 1461 and at 1451 cm-1, for (C59N)2 and (C 60 - ) 2 , respectively. This is interpreted as a consequence of some characteristic differences in the electronic structure of the two compounds.

Tailoring the electronic properties of single-walled carbon nanotubes by chemical modification through incorporation of heteroatoms within the tube walls represents a key method for the applicability of these structures in semiconductor technology. For p-type doping experimental studies on the synthesis of B doped single-walled carbon nanotubes have been reported using substitution reactions, arc discharge and laser ablation. However, synthesis using the up-scalability of the widely used chemical vapor deposition approach has not been successful so far. In this work, we present an enhanced CVD approach using high vacuum and promoting the use of solely one liquid precursor containing B and C to synthesize B-doped single walled nanotubes with unique characteristics on the subject of controlled “low doping”. In comparison to non-doped single-walled carbon nanotubes, these tubes have exceptional stability. From Raman and TEM we observe a very low defect concentration concomitant with small nanotube bundles containing 3 to 5 tubes with a narrow diameter distribution (0.9 to 1.5 nm). The defect concentration is significantly lower than in pure carbon nanotube reference samples. The overall B content and the bonding environment of the incorporated B were probed in detail for the first time by XPS. We identified different B bonding environments at less than 1 at%. This highlights the applicability of a rigid band model improving the accessibility of these tubes with controlled p-type doping in nano- and optoelectronic devices.

A review is presented on the application of Raman scattering for the analysis of single wall carbon nanotubes. The Raman response from the tubes is discussed for different tube diameters and for different energies of the exciting laser. It is demonstrated, that the radial breathing mode is particularly useful since it exhibits a very strong photo-selective resonance scattering. The well known fine structure of this mode is demonstrated to be due to a clustering of the frequencies for all geometrically allowed tubes. From a quantitative analysis of the peak positions and of the first moments of the line pattern obtained for excitation with 27 different laser lines the ππ-overlap integral and the lifetime of the excited electronic state were determined to be 3.025 eV and 0.2 ps, respectively. From this fit also the contribution of the tube-tube interaction within the bundles to the frequency of the radial breathing mode was determined. The result was 7% for the set of tubes with larger diameter and 6% for the set of tubes with smaller diameter.

Recently, it was possible to fill single wall carbon nanotubes with C60 fullerenes resulting in carbon structures with new and interesting properties (so‐called C60 peapods). All Raman allowed modes of the free C60 were identified for the encaged C60. In this contribution we demosntrate how the concentration of the peas in the SWCNT pods can be obtained from a detailed analysis of the Raman spectra of the peapods.

Raman scattering of the radial breathing mode of single wall carbon nanotubes was measured with a large number of different laser lines. The first and the second moments of the spectral distribution were evaluated for all recorded spectra and were found to exhibit damped oscillations with the excitation energy. The oscillations originate from the confinement of the electrons into states distributed within the van Hove singularities. An extended as well as an approximate calculation of the moments yields very good agreement with the experiments and allows to determine the diameter of the nanotube bundles. Using the approximate calculation the mean diameter and the width of the diameter distribution can be evaluated from a single Raman experiment.

Conditional on the fabrication process, SWCNTs appear mostly as bundles of varying diameters. In this paper, the resulting distributions of bundle diameters, determined by Raman spectroscopy and by AFM tapping mode are compared directly.

A review is presented on optical and Raman characterization of carbon nanostructures. Particular attention is paid to the new phases of carbon such as fullerenes, poly-fullerenes and carbon nanotubes. For the case of the polyfullerenes the two isostructural dimeres (C59N)2 and (C60)2 are discussed in detail. Both structures exhibit a very strong resonance for red laser excitation of the radial modes. The frequency of the radial breathing mode of single wall carbon nanotubes is shown to exhibit an oscillating behavior as a function of the excitation energy. A quantitative analysis performed for a quasi-continuous distribution of diameters revealed 2.9 eV as the best value for the π-overlap γ0 and an 8% up shift of the mode due to the tube-tube interaction in the bundle.

In this contribution we present resent results on the phases of in situ rubidium intercalated C59N. Using Raman spectroscopy six distinct phases were identified by using the charge versus frequency relation of the Ag pinch mode. From additional in situ resistance measurements we find that all phases are semiconducting. Similar to RbxC60 the phase Rb3C59N has the minimum resistance.

Photoselective resonance Raman scattering from laser ablation grown carbon nanotubes is presented. The resonance excitation was found to exhibit a quasi-oscillatory behavior. Assuming a Gaussian distribution of diameters we extracted values for a mean diameter and a width of the distribution within a simple model. In a more expensive calculation the contribution from all geometrically allowed tubes in a certain diameter range was considered. To match experiments and calculations, the frequencies obtained from the latter must be upshifted by ≈10 cm−1. This is ascribed to the tube-tube interaction in the carbon nanotube bundles.