Ch. Kramberger

Interaction between concentric tubes in DWCNTs

Abstract.A detailed investigation of the Raman response of the inner tube radial breathing modes (RBMs) in double-wall carbon nanotubes is reported. It revealed that the number of observed RBMs is two to three times larger than the number of possible tubes in the studied frequency range. This unexpected increase in Raman lines is attributed to a splitting of the inner tube response. It originates from the possibility that one type of inner tubes may form in different types of outer tubes. In this case, a splitting of lines results since the inner tube RBM frequency depends on the diameter of the outer tube. Finally, a comparison of the inner tube RBMs and the RBMs of tubes in bundles gave clear evidence for a stronger interaction between tubes in a bundle as compared to the interaction between inner and outer tubes.

Defect Free Inner Tubes in DWCNTs

By annealing fullerene peapods at high temperatures in a dynamic vacuum it is possible to produce double wall carbon nanotubes (DWCNTs). A Raman investigation revealed that the inner SWCNTs are remarkably defect free showing very strong and very narrow radial breathing modes (RBMs). Lorentzian line widths scale down to about 0.35cm−1 which is almost 10 times smaller than reported for single tubes so far. Also, the scattering intensities for the inner RBMs can be more than 10 times higher than those for the outer RBMs. Additionally, all radial modes of the inner tubes are split into at least two components.

Oscillatory behavior of Raman modes in SWCNT

Raman experiments are reported for seven samples of SWCNT with different diameters. The characteristic oscillations in the first and second moment of the radial breathing mode are used to determine the mean diameter of the tubes. These results are compared to results obtained by optical absorption and x-ray experiments. Oscillating behavior was also observed for frequencies of other modes such as the D and D* mode and for the intensities of the RBM, the D band, the G band and the D* band. Oscillations could be ascribed to the resonance of tubes with different diameters which are present in one sample.

Physics and Chemistry Inside Nanotubes

Abstract The interior of single‐wall carbon nanotubes (SWCNTs) is investigated by filling it with fullerenes and PbO. In the former case, maximum filling concentrations are evaluated for various fullerenes as a function of tube diameter. For filling with C60 a calibration table is provided that allows the determination of the filling concentration from Raman experiments. Smart filling from solution, from isotope‐labeled fullerenes, and filling from nonfullerenic carbon sources is discussed. In the case of filling with PbO, an influence of lateral confinement on the phonon wave functions, an instability of the tubes versus laser irradiation, and the persistent formation of PbO nano‐wires are observed.

Assignment of Chiral Vectors in Carbon Nanotubes

Double wall carbon nanotubes (DWNTs) were derived from peapods by annealing at high temperature. We report on the Raman response of the RBM of the inner tubes. As a consequence of their small diameter the Raman spectra of the inner components of the DWNTs show well distinguishable RBM lines of the different inner tubes, besides the usual response from the outer tubes. The evaluation of these spectra has led to a full assignment of the chiral vectors to the spectroscopic lines. We found vRBM = 235.5/d + 13.8 ⋅ d ,v in cm−1, d in nm.

Interaction between Inner and Outer Tubes in DWCNTs

By annealing fullerene peapods at high temperatures in a dynamic vacuum it is possible to produce double‐wall carbon nanotubes. A Raman investigation revealed that the inner single‐wall carbon nanotubes are remarkably defect free showing very strong and very narrow radial breathing modes (RBMs). The number of observed RBMs is larger than the number of geometrically allowed inner tubes. This splitting is caused by the interaction of one type of inner tube with several types of outer tubes the inner tube may grow in.

Catalyst Free Growth of Single‐Wall Carbon Nanotubes (SWCNTs)

We report on the growth process of single wall carbon nanotubes from a catalyst free carbon source. The carbon for the growth process is provided from C60 fullerenes encapsulated into the cage of the tubes (peapods). The growth process is studied by Raman scattering at various stages for the transformation of the C60 peas to a new SWCNT inside the primary tube. The growth process was found to start for tubes with the smallest geometrically allowed diameters. At the same time the response from the C60 molecules started to disappear. Eventually the signal from the latter was gone while only the thin inner tubes have reached their final concentration.

Listing of Raman Lines from Double‐Walled Carbon Nanotubes

Double walled carbon nanotubes were derived from C60 peapods by annealing at 1280 °C for 2 hours in a dynamic vacuum. Resonance Raman spectroscopy was used to investigate the resulting tube systems. The spectral range of the radial breathing mode of the inner shell nanotubes was found to consist of well separated narrow lines. From the 41 geometrically allowed tubes between 250 and 470 cm−1 36 were identified. The observed number of lines was more than two times the number of allowed tubes. This effect is found to be due to a splitting of all Raman lines into doublets or triplets.

Resonance Raman Properties of Pristine and Intercalated HiPCO SWNTs

The Raman spectrum of single wall carbon nanotubes (SWNTs) prepared by high pressure CO decomposition (HiPCO process) has been measured using different lasers. The G band, D band and radial breathing mode (RBM) were analysed. A strong Breit‐Wigner‐Fano type (metallic) contribution to the G band was found in the spectra measured with green lasers, while spectra measured with red lasers indicate resonances of semiconducting SWNTs. Potassium intercalation (n‐doping) is readily achieved and it shifts the Fermi level so that resonances from the first van Hove singularity of metallic SWNTs as well as from the first three van Hove singularities of semiconducting tubes are lost. On the other hand, only the peaks associated with large diameter tubes are affected upon iron(III)‐chloride intercalation (p‐doping).

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