R. Pfeiffer

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.

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.

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.

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.

Double-Wall Carbon Nanotubes

Double-wall carbon nanotubes (DWNTs) are the simplest archetypicalmanifestation of MWNTs and as such combine the outstanding properties of SWNTswith the possibility to study concentric intertube interactions with highprecision. Two complementary routes for the efficient growth of DWNTs arediscussed. Firstly, SWNTs filled with various carbon sources, such as fullerenes oracenes can form inner-shell tubes by a high-temperature treatment underclean-room conditions. Inner–outer tube pairs can be identified with a well-definedmutual chirality. Isotope labeling and full isotope substitution is possible.Using different carbon sources, DWNTs with intrinsic functionality andspecial electronic and magnetic properties can be grown. Alternatively, adirect growth using chemical vapor deposition and subsequent purification isdescribed. Large-scale growth of very long continuous DWNTs for application incomposites and as advanced field emission sources are straightforward forthis technique. The two techniques for growth of DWNTs are evaluatedwith respect to their scientific novelty and application potential. Stability,electronic structure, transport and mechanical properties are reviewed.

Catalyst and Chirality Dependent Growth of Carbon Nanotubes Determined Through Nano‐Test Tube Chemistry

[*] Dr. Hidetsugu Shiozawa, Prof. S. Ravi P. Silva Advanced Technology Institute, University of Surrey, Guildford, GU2 7XH (UK) Email: h.shiozawa@surrey.ac.uk; s.silva@surrey.ac.uk Dr. Christian Kramberger, Dr. Rudolf Pfeiffer, Prof. Hans Kuzmany, Prof. Thomas Pichler Faculty of Physics, University of Vienna, Strudlhofgasse 4, 1090 Vienna (Austria) Dr. Zheng Liu, Dr. Kazu Suenaga Research Center for Advanced Carbon Materials, AIST, Tsukuba 305-8565 (Japan) Dr. Hiromichi Kataura Nanotechnology Research Institute, AIST, Tsukuba 305-8562 and JST, CREST (Japan)

Diameter selective reaction processes of single-wall carbon nanotubes

A method is presented which allows the study of diameter selective reactions in single-wall carbon nanotubes with an unprecedented accuracy. It is based on the transformation of fullerene peapods into double-wall carbon nanotubes and the study of the resulting diameter distribution of the inner nanotubes with Raman spectroscopy. This yields a spectral resolution increase of about 40 for the modes of different tubes. The method is demonstrated for the diameter selective healing of nanotube defects and yield from C70 peapod samples. The growth of very small diameter inner tubes from C70 peapods is demonstrated, which challenges the models of inner nanotube formation. An anomalous absence of middiameter inner tubes is explained by the suppressed amount of C70 peapods in the transition region between standing and lying C70 configurations.

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.

Temperature dependence of the optical excitation lifetime and band gap in chirality assigned semiconducting single-wall carbon nanotubes

The temperature dependence of optical excitation lifetime 1 / and transition energies Eii were measured for bucky papers of single-wall carbon nanotubes SWCNTs and inner tubes in double-wall carbon nanotubes DWCNTs using resonant Raman scattering on the radial breathing mode. The temperature dependence of and Eii is the same for both types of samples and is independent of tube chirality. The data suggest that electron-phonon interaction is responsible for the temperature dependence of Eii and . The temperature independent contribution to is much larger in SWCNT than in DWCNT samples. This is explained by the different nanotube environment in the two types of samples. for the inner tubes of the DWCNTs is only 30 meV below 150 K, which is comparable to that found for individual SWCNTs and is considered as intrinsic to the tubes.