W. Plank

Electronic structure of carbon nanotubes with ultrahigh curvature.

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.

Spectroscopic analysis of single-wall carbon nanotubes and carbon nanotube peapods

Abstract 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.

Raman spectrum and stability of (C59N)2

Raman spectra of (C59N)2 are presented for various laser lines and for temperatures between 300 K and 620 K. The material exhibits a reliable stability versus laser illumination for the full temperature range and a remarkable resonance enhancement of the cross section for red light excitation. The Raman intensity versus the used laserpower shows a linear dependence in the entire region observed. The spectra show a strong relation to those of C60, particularly in the spectral range of the tangential modes. Calculations of the mode frequencies with the semiempirical AM1 technique reveal good agreement with the observed Raman lines, in particular for the intermolecular modes observed at 82, 103 and 111 cm−1.

Oscillatory behaviour of the photoselective resonance scattering of single wall carbon nanotubes

The radial breathing mode around 190 cm -1 was investigated for excitation with 30 different laser lines. It exhibits an oscillatory behaviour with respect to excitation energy. This oscillations could be traced back to the periodicity of the van Hove singularities. Using the first and the second spectral moments parameters characterizing a diameter distribution can be evaluated.

Concentration of C60 Molecules in SWCNT

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.

Temperature Effects on the Raman Scattering of Single‐Wall Carbon Nanotubes

Measurements on the temperature dependency were carried out for Raman spectra of the radial breathing mode (RBM) of single‐walled carbon nanotubes (SWCNT) in the range between 200 K and 700 K. The observed shifts in lines could be compensated by varying the energy of the exciting laser. The changes are assumed to originate from the temperature dependence of the carbon π‐overlap integral γ0 and the factor of proportionality between RBM frequency and inverse tube diameter C1. The results can be reproduced within a tight‐binding calculation and thus allow to determine the relevant temperature coefficients

Transition of the heterofullerene (C59N)X to the monomeric phase of C59N

The stability of the azafullerenes of the type (C59N)X was studied by using infrared, optical and Raman experiments. X represents either hydrogen or an other cage of C59N. The solid phase of the dimer was shown to be stable up to 650 K followed by very slow degradation extending beyond 700 K. This is compared to our very latest results on the temperature stability of C59HN which was found to be stable only up to 540 K. A sudden change in the spectra at this temperature gives evidence for a transition to a new air stable phase which is claimed to be monomeric C59N. When heated in vacuum to about 700 K this phase dimerizes into (C59N)2.

Quantum oscillations for the spectral moments of Raman spectra from SWCNT

Photoselective resonance Raman scattering is demonstrated to exhibit quantum oscillations for the spectral moments if the spectra are excited with a large number of different laser energies. The oscillations originate from the small extension of the tubes in transversal direction. An appropriate model was constructed which allows to provide a quantitative analysis of the oscillations. Assuming a Gaussian distribution of diameters values for a mean diameter and the width of the distribution were obtained. The fine structure in the Raman response is demonstrated to be due to a clustering of nanotube diameters.

Thermal stability of the heterofullerene (C59N)X for X=C59N,H.

The stability of the azafullerenes of the type (C59N)X was studied by using IR, mass spectroscopy, optical and Raman experiments. X represents either hydrogen or an other cage of C59N. Experimental data indicate thermal stability of the solid phase of the dimer up to 600 K followed by very slow degradation extending beyond 700 K. This is compared to our latest results on the temperature stability of C59HN which was found to be stable only up to 540 K. A sudden change in the spectra at this temperature gives evidence for a transition to a new air stable phase. Since the mass spectra show no loss of hydrogen the new phase is claimed to be polymerized C59HN. When heated in vacuum to about 700 K this phase dimerizes into (C59N)2.

The phases of quenched fullerenes RbC60

RbC 60 is known to exist in several stable phases as a function of temperature. Particularly interesting systems are obtained if the material is warmed up from a quenched low temperature cubic phase. Subsequently, a dimeric, a monomeric and a polymeric phase are obtained and were analysed by Raman spectroscopy. The dimeric phase is of particular interest, since it exhibits a textbook-like splitting of the degenerated H g modes and an unusual but very well expressed lack of any splitting for the G g modes. This allows a very straight correlation of the spectra to those of C 60 .