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The article reviews recent progresses in functionalization of single-walled carbon nanotubes (SWCNTs). We describe detailed studies of electronic, chemical, and structural properties of intercalated and filled carbon nanotubes using state-of-the-art spectroscopic and microscopic techniques. Intercalation with alkali metal allows one to finely tune the electron doping level of SWCNTs. A photoemission study of potassium intercalation compounds of SWCNT bundles unveils a transition of the metallic ground state of an SWCNT bundle from a Tomonaga–Luttinger liquid state to a Fermi liquid state. In turn, the filling of SWCNTs with fullerenes and organic molecules facilitates a reality of nanochemistry using carbon nanotubes as a nanoreactor. Spectroscopic and microscopic studies of the interconversion of encapsulated fullerenes and organic molecules show catalytic and noncatalytic processes for the inner tube growth from different filling precursors. Unique atomic wires and clusters can be created inside SWCNTs. We present that such chemical reactions of the encapsulated molecules result in the evolution of doping and bonding properties of the carbon nanotube host. Keywords: carbon nanotube; functionalization; intercalation; nanochemistry; Raman scattering; electron energy loss spectroscopy; photoemission spectroscopy; transmission electron microscopy

From nano-test tube chemistry we determine details of low temperature growth of carbon nanotubes. Single-walled carbon nanotubes of various diameters are grown at different temperatures from a Pt organometallic precursor encapsulated inside template single-walled tubes. At the low temperature limits where the reactions are slower and milder, the inner-tube diameters are optimally determined by the outer-tube diameter. The growth temperature is determined for a given tube chirality and is found to be dependent on the diameter. It is lower for thinner tubes and can be as low as 600 °C for the (7.3) tube. In situ nano-test tube chemistry within a state-of-the-art microscope provides an idealistic nano-clean room experiment, which allows monitoring of a snapshot in the catalytic growth of a single-walled tube out of a Pt nanocrystal.

Ultra-high curvature single-walled carbon nanotubes (SWCNTs) with diameters down to 0.37 nm were prepared by transformation of FeCp2 peapods to double-walled CNTs (DWCNTs). Results from resonance Raman scattering and high resolution transmission electron microscopy (TEM) were compared to calculations on the molecular dynamical, many electron corrected extended tight binding, and density functional theory (DFT) level. The growth process was found to be catalytic from Fe3C particles inside the tubes with dimensions of a crystallographic unit cell. The electronic structure showed dramatic deviations from tight binding results. The family behavior leads to level crossing already for and transitions. Experimental results fitted well into a Kataura–Popov plot and allowed assignment for the observed Raman lines of the inner tubes. Experimental and calculated radial breathing mode (RBM) frequencies showed a systematic difference indicating a radial expansion of the smallest inner tubes and a radial compression for the larger tubes.

We studied SWCNTs and derivatives separated according to their metallicity using X-ray diffractometry, optical absorption, and Raman spectroscopy. The X-ray study evidences a complete absence of graphite and metal catalysts. Optical absorption shows that the metallic sample is of high purity, however, a small amount of metallic tubes are present in the semiconducting sample. We synthesized double-wall carbon nanotubes from the separated SWCNTs by filling-in C60's and heating these samples. The Raman spectra of the inner tubes are markedly distinct in the two types of samples and also allows to characterize the efficiency of the separation.

[*] 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)

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.

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.

Double-wall carbon nanotubes (DWCNTs) grown directly via a CVD method and DWCNTs produced by annealing C 60 and ferrocene filled single-wall carbon nanotubes were studied with X-ray, diffraction (XRD) and Raman spectroscopy. The samples showed the typical XRD patterns of hexagonally bundled tubes. The frequencies and linewidths of the inner tube radial breathing (RBMS) were (within experimental precision) equal in all three samples only the intensities differed. Finally, all samples showed the typical clustering of the inner tube RBMs due to the distribution of inner-outer tube pairs. The only difference was the population of specific inner-outer tube pairs where the CVD and ferrocene based DWCNTs were rather similar and both diffared from the C 60 based DWCNTs.

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.

The radial breathing mode in the Raman spectrum of 13 C isotope enriched single walled carbon nanotubes is inhomogeneously broadened due to the random distribution of isotopes. We study this effect theoretically using density functional theory within the local density approximation and compare the result with experiments on isotope engineered double walled carbon nanotubes in which the inner tubes were grown from a mixture of 13 C enriched fullerenes and natural fullerenes. As explained by the calculations, this synthesis procedure leads to an increased inhomogeneity compared to a case when only enriched fullerenes are used. The good agreement between the measurements and calculations shows the absence of carbon diffusion along the tube axis during inner tube growth, and presents a strong support of the theory that inner tube growth is governed by Stone-Wales transformations following the interconnection of fullerenes.

The energy dispersion of the D* (G') band shows a strong diameter dependence according to Raman measurements on double walled carbon nanotubes. The Raman shift of the small diameter inner tubes shows an average softening. There is an oscillation around an average linear behavior with slopes of 108 cm -1 /eV and 61 cm -1 /eV for the outer and inner tubes, respectively. We show that the experimental observation can be reproduced by simple model calculations if the curvature effects are taken into account. The phonon softening with increasing curvature was proven by first principles calculations.

The growth process of inner-shell tubes from C 60 peapods and ferrocene (FeCp 2 ) filled single-wall carbon nanotubes was studied with Raman spectroscopy and X-ray diffraction (XRD). In the peapod case, it is shown that the Raman signal of the fullerenes decays about a factor five faster than the XRD peak of the 1D fullerene chain. The growth of the inner-shell tubes coincides well with the 1D chain decay. In the FeCp 2 case, the inner tubes grow already at 600 °C in about one hour. Additionally, even for the same starting material, the inner-outer tube pair distribution is different than for C 60 based inner tubes.

We demonstrate filling of single-wall carbon nanotubes with ferrocene. Encapsulated ferrocenes are utilized as a precursor to secondary inner tubes and iron dopant. The reaction pathway is traced by Raman and photoemission spectroscopy. From a comparison to the inner-tube growth from C 60 fillers we propose different mechanisms for the inner-tube growth from fullerene and non-fullerene precursors. This work pioneers a new chemistry using carbon nanotubes as a nano reaction furnace.

The filling and depletion of carbon nanotubes with C60 fullerenes is reported using solvents. We use X-ray diffractometry and Raman spectroscopy to show that large diameter carbon nanotubes can be filled with fullerenes using methanol with a high efficiency and can be also removed from the inside of the tubes using dichlorobenzene. The latter solvent effectively removes weakly bound C60s from inside the large diameter tubes, which is explained by the high solubility of C60 in dichlorobenzene. � 2007 Elsevier B.V. All rights reserved.

The growth of double-walled carbon nanotubes from peapods was studied. The transformation was monitored by the decrease of fullerene Raman lines, the growth of inner tube Raman lines, and the development of X-ray diffraction patterns. A visual check of the growth process by HRTEM provided additional information. From the difference in time constants for the bleaching of fullerene Raman lines and for the growth of nanotube Raman lines, the existence of an intermediate phase was concluded that was eventually observed in X-ray diffraction and HRTEM. Time constants for the growth of large diameter inner tubes were up to a factor two larger than for small diameter inner tubes. The results fully support the fullerene coalescence growth model triggered by Stone-Wales transformations.

The past decade has witnessed a substantial increase in research on synthesis techniques for pure and doped carbon nanotubes and their involved parameters. However, there is a missing link between the ideal synthesis conditions for high quality functionalized structures with heteroatoms such as nitrogen and the formation of different bonding environments through the actual process. We describe here an approach to prepare high quality and substitutionally N-doped single and double walled carbon nanotubes with a defined diameter range from a nondiluted carbon/nitrogen feedstock. This not only illustrates the dependence on the combined synthesis parameters but also provides a fundamental insight into the formation of nitrogen volatile species during the feedstock pyrolysis versus the actual N-incorporation profile within the walls.

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.

We have synthesized double wall carbon nanotubes (DWNTs) with few defects and little amorphous carbon by hot wall chemical vapor deposition (CVD) of alcohol. Catalysts for the DWNT growth were made from cobalt and molybdenum acetates. Scanning electron microscopy, transmission electron microscopy, multi frequency resonance Raman spectroscopy and optical absorption spectroscopy were used for characterization of the product with regard to DWNT yield, the nanotube diameter distribution, defect concentration and amorphous carbon content. Base pressures lower than 1 × 10 −5 mbar in the CVD reactor considerably suppress defects in the DWNTs. Optimized growth conditions for DWNT formation are presented.

In a Raman map of double-wall carbon nanotubes, the radial breathing modes of the inner tubes are grouped into clusters with similar resonance behavior. These clusters are located close to the resonances of SDS wrapped HiPco tubes and can spread over 30 cm -1 . Compared to the HiPco tubes the resonances of the inner tubes are shifted about 50 meV to the red. Each cluster represents one fixed inner tube type in several different outer tube types and each component of a cluster originates from a well defined inner/outer tube pair. Elaborate inter-shell interaction models showed an increasing charge transfer from the outer to inner tube with decreasing diameter difference. Additionally, many DWCNTs systems seem to be metallic, i.e. they have a finite density of states at the Fermi level.

Nanotube growth by chemical vapour deposition carbon is carried out for two different catalyst preparation methods: a wet chemistry catalyst is compared to a thin metal film multilayer catalyst. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs are recorded to investigate the nanotube alignment and the number of walls. Resonance Raman spectroscopy is used for analysis of diameters and the defect concentration.

High quality single wall carbon nanotubes (SWCNT) were grown using n-heptane as the carbon feedstock and Ni/Co acetylacetonate catalyst precursors. They were characterized with regards to morphology, diameter distributions and yield using microscopic and spectroscopic techniques. n-Heptane provides a cheap and effective carbon source for the selective synthesis of clean SWCNTs at 680 °C with improved yield in comparison to other carbon sources. In addition we show the applicability of selective growth of individual SWCNTs rooting from lithographically patterned catalyst islands at this low temperature.

The interior of single-wall carbon nanotubes is studied by filling fullerenes into the one-dimensional nanospace inside the tubes using different filling methods. Filling from solution is demonstrated as efficient as filling from the vapor phase. Evidence for successful filling comes from a transformation of the encapsulated carbon atoms to a secondary tube centered inside the primary tube. Isotope-labeled fullerenes are used to disentangle the Raman response from the two shells in double-wall carbon nanotubes.

The synthesis of double-wall carbon nanotubes (DWCNTs) with highly unperturbed inner shells is reported using the catalytic vapor deposition method. Temperature dependent and high resolution Raman measurements show an enhanced phonon life-time of the inner tubes with respect to the outer ones and similar diameter SWCNTs. This proves that the inner tubes are unperturbed similar to the inner tubes in peapod-grown DWCNTs. The presence of the outer tube is argued to protect the inner tube from interaction with impurities and also to stabilize the growth of defect free inner tubes. The current material underlines the application potential of DWCNTs.

A range of mono- and bis-functionalised fullerenes have been synthesised and inserted into single-walled carbon nanotubes. The effect of the size and shape of the functional groups of the fullerenes on the resultant 1D arrays formed within the nanotubes was investigated by high resolution transmission electron microscopy and X-ray diffraction. The addition of non-planar, sterically bulky chains to the fullerene cage results in highly ordered 1D structures in which the fullerenes are evenly spaced along the internal nanotube cavity. Theoretical calculations reveal that the functional groups interact with neighbouring fullerene cages to space the fullerenes evenly within the confines of the nanotube. The addition of two functional groups to opposite sides of the fullerene cages results in a further increase in the separation of the fullerene cages within the nanotubes at the cost of lower nanotube filling rates., This work was financially supported by FWF project I83-N20 (ESF IMPrESS), the Royal Society, the European Research Council (ERC), "Fundacao para a Ciencia ea Tecnologia" through the program Ciencia 2008, the project SeARCH (Services and Advanced Research Computing with HTC/HPC clusters) and Nottingham Nanoscience and Nanotechnology Centre (NNNC).

Single wall carbon nanotubes filled with C60 were analyzed using resonance Raman scattering and electron energy loss spectroscopy. In order to obtain concentrations of the fullerene molecules inside the tubes, the scattering intensity from the fullerenes relative to that from the tubes was used. Since the scattering intensity from the tubes is subject to strong fluctuations, the determination of the concentrations is shown to require averaging of results from different lasers and from all observable Raman lines. The fluctuations are shown to be intrinsic and a consequence of photoselective resonance scattering. Calibration of absolute concentrations can be obtained from electron energy loss spectroscopy performed on the same samples. Samples with three different diameters were analyzed and good agreement between the fullerene concentrations measured by the two methods was obtained.

Single wall carbon nanotubes (SWCNTs) synthesized by the recently developed HiPco® method differ from those obtained by other techniques because their diameter distribution is broader and is centered at a smaller value. Therefore, HiPco® samples contain an unusual wide selection of different nanotubes. Combining this feature with different dopant molecule sizes makes it possible to study a special, diameter selective intercalation phenomenon. Here we present a comparative resonant Raman spectroscopic study on the n-type (K) and p-type (FeCl3) doping of a HiPco® and a laser ablation grown SWCNT sample. Diameter selectivity is observed in the former but not in the latter case. We suggest that the solvability of dopands is diameter dependent with a minimum in the range d = 0.9–1.2 nm.

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.

The article reviews recent progresses in functionalization of single-walled carbon nanotubes (SWCNT). Detailed studies of electronic, chemical, and structural properties of intercalated and filled carbon nanotubes using state-of-the-art spectroscopic and microscopic techniques are described. Intercalation with alkali metal allows one to fine tuned the electron-doping level of SWCNT. A photoemission study of potassium intercalation compounds of SWCNT bundles unveils a transition of the metallic ground state of a SWCNT bundle from Tomonaga–Luttinger liquid state to Fermi liquid state. In turn, the filling of single-walled carbon nanotubes with fullerenes and organic molecules facilitates a reality of nanochemistry using carbon nanotubes as a nanoreactor. A combined spectroscopic and microscopic investigation of the interconversion of encapsulated fullerenes and organic molecules unveils either a catalytic or a noncatalytic process for the inner-tube growth from different filler precursors. Such encapsulated reactions vary the chemical status of filling material which eventually leads to a modulation of the doping level of the carbon nanotube host. Keywords: carbon nanotube; functionalization; intercalation; nanochemistry; Raman scattering; electron energy loss spectroscopy; photoemission spectroscopy; transmission electron microscopy

The defect-induced D line and its overtone are fundamental signatures in the Raman spectra of carbon nanomaterials. An analysis of these lines as a function of laser excitation energy is reported for double-walled carbon nanotubes where the inner tubes represent high-curvature nanotube species. From 13C substituted inner tubes it is demonstrated that the quasilinear relations between laser energy and line position (Raman dispersion) cross over unexpectedly for low-energy excitation for the inner and outer tube shells. The result is quantitatively explained by a curvature-induced phonon softening and first-principles calculations of the optical transition energies.

Mechanisms for controlling the assembly of molecular arrays in carbon nanotubes via alteration of the size and geometry of the functional groups attached to the molecules inserted into the nanotubes are studied. As model compounds, a series of structurally related fullerenes functionalized with polyaryl groups (C(60)X, where X is a polyaryl group) of various lengths are synthesized to explore this effect. These molecules are inserted into single-walled carbon nanotubes (SWNTs) under mild conditions to prevent their decomposition and to form C(60)X@SNWT structures. The molecular chains thus formed are studied by high-resolution transmission electron microscopy, X-ray diffraction, and Raman spectroscopy, revealing that the functional groups increase the interfullerene separation proportionally with the size of X. However, the functional groups themselves appear to adopt various orientations with respect to each other and exhibit intermolecular pi-pi interactions within the cavities of the carbon nanotubes. All these effects create a distribution of observed interfullerene separations in nanotubes, which are examined by theoretical simulations and interpreted in terms of molecular geometries and intermolecular interactions.

Vibrational modes of $^{13}\mathrm{C}$ isotope enriched single-walled carbon nanotubes are inhomogeneously broadened due to the random distribution of isotopes. We study this effect on the radial breathing mode theoretically using density-functional theory within the local-density approximation and compare the result with experiments on inner tubes in double-walled carbon nanotubes grown from $^{13}\mathrm{C}$-enriched fullerenes. Increased inhomogeneity was achieved by growing inner tubes from a mixture of enriched and natural fullerenes, which is explained by the calculations. This shows the absence of carbon diffusion along the tube axis during inner tube growth, supporting the theory of inner tube growth by Stone-Wales transformations from interconnected fullerenes.

We report on detailed studies of the $n$-type doping dependence of the Raman response of double-wall carbon nanotubes using potassium intercalation. The charge transfer is monitored by a shift of the G line. Upon doping the G line shifts to higher frequencies for the outer and to lower frequencies for the inner tubes. This is explained by different Coulomb interactions for the inner and outer tubes. The response of the radial breathing mode upon doping shows that a charge transfer from the dopant happens predominantly to the outer tubes at low doping. Charge transfer to the inner tubes occurs at higher doping levels. The previously observed cluster behavior of the inner tube RBM response allows a detailed analysis of the dependence of the inner tube doping from specific inner tube--outer tube configurations.

The temperature dependence of optical excitation lifetime $1∕\ensuremath{\Gamma}$ and transition energies ${E}_{ii}$ 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 $\ensuremath{\Gamma}$ and ${E}_{ii}$ 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 ${E}_{ii}$ and $\ensuremath{\Gamma}$. The temperature independent contribution to $\ensuremath{\Gamma}$ is much larger in SWCNT than in DWCNT samples. This is explained by the different nanotube environment in the two types of samples. $\ensuremath{\Gamma}$ for the inner tubes of the DWCNTs is only $\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ below $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is comparable to that found for individual SWCNTs and is considered as intrinsic to the tubes.

The present state of Raman scattering from carbon nanotubes is reviewed. In the first part of the presentation the basic concepts of Raman scattering are elucidated with particular emphasis on resonance scattering. The classical and the quantum-mechanical description are presented and the basic experimental instrumentation and procedures are described. Special Raman techniques are discussed. Eventually, a short review on the electronic structure of single wall carbon nanotubes (SWCNTs) is given. The second part of the presentation deals with Raman scattering from SWCNTs. A group theoretical analysis and the origin of the basic Raman lines are described. For the radial breathing mode the observed quantum oscillations and the unusual strong Raman cross section are discussed. For the G line the resonance behavior and the response to doping is demonstrated and the calculated dependence of the line frequency on the tube diameter is summarized. For the D-line and for the G'- line the dispersion is demonstrated and its origin from a triple resonance is described. Finally, the response from pristine and doped peapods is elucidated. In the third part most recent results are reported from Raman spectroscopy of double wall carbon nanotubes (DWCNTs). The unusual narrow lines with widths down to 0.4 cm -1 indicate clean room conditions for the growth process of the inner tubes. (n,m) assignment to them and high curvature effects are discussed. Results for DWCNTs where the inner tubes are highly 13 C substituted are reported with respect to Raman and NMR spectroscopy. Eventually, it is demonstrated that the RBM Raman lines of the inner tubes cluster into groups of up to 14 lines where each member of the cluster represents a pair of inner-outer tubes.

The nanospace inside single wall carbon nanotubes was studied by Raman spectroscopy and high resolution transmission electron microscopy. To explore this space, C60 molecules were filled into the tubes to create a peapod system. Upon doping with electron donors the cage of the tubes as well as the cages of the fullerenes received charge and eventually turned into C60 −6 molecules. These molecules react to a linear and single bonded polymeric phase. Alternatively, upon annealing at high temperatures the C60 molecules serve as a carbon source to grow a second tube inside the primary tube. This tube exhibits extremely narrow Raman lines for the radial breathing mode indicating a highly defect free material. They thus apostrophe the inside of the tube cage as a nano clean room. The small size of the inner-shell tubes allows for a full assignment of the observed radial breathing modes to chiral vectors. The deviation of electronic and vibrational properties of the tubes from a tight binding behavior is demonstrated.

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.

Raman studies on double‐wall carbon nanotubes (DWCNTs) obtained from peapods showed that the radial breathing mode (RBM) response of the inner tubes is split into more components than there are geometrically possible inner tubes. This was attributed to the possibility that one inner tube type may form in more than one outer tube type. Recent resonance Raman investigations revealed an even more complex and interesting behavior. The split components appear to be grouped into species belonging to the same inner tube chirality and the width of the splitting increases with decreasing tube diameter.

The physical properties of double‐wall carbon nanotubes (DWCNT) with highly 13C enriched inner walls were studied with Raman spectroscopy and nuclear magnetic resonance (NMR). An inhomogeneous broadening of the vibrational modes is explained by the random distribution of 12C and 13C nuclei based on ab‐initio calculations. The growth of DWCNTs from natural and 13C enriched fullerene mixtures indicates that carbon does not diffuse freely along the tube axis during the inner tube growth. The high curvature of the small diameter inner tubes manifests in an increased distribution of the NMR chemical shift tensor components.

The analysis of the Raman scattering cross section of the radial breathing modes of double-wall carbon nanotubes allowed to determine the optical transitions of the inner tubes. The Raman lines are found to cluster into species with similar resonance behavior. The lowest components of the clusters correspond well to SDS wrapped HiPco tubes. Each cluster represents one particular inner tube inside different outer tubes and each member of the clusters represents one well defined pair of inner and outer tubes. The number of components in one cluster increases with decreasing of the inner tube diameter and can be as high as 14., Comment: 5 pages, 3 figures

Raman spectroscopy on carbon nanotubes (CNT) yields a rich variety of information owing to the close interplay between electronic and vibrational properties. In this paper, we review the properties of double wall carbon nanotubes (DWCNTs). In particular, it is shown that SWCNT encapsulating C$_{60}$, so-called peapods, are transformed into DWCNTs when subject to a high temperature treatment. The inner tubes are grown in a catalyst free environment and do not suffer from impurities or defects that are usually encountered for as-grown SWCNTs or DWCNTs. As a consequence, the inner tubes are grown with a high degree of perfection as deduced from the unusually narrow radial breathing mode (RBM) lines. This apostrophizes the interior of the SWCNTs as a nano-clean room. The mechanism of the inner nanotube production from C$_{60}$ is discussed. We also report recent studies aimed at the simplification and industrial scaling up of the DWCNT production process utilizing a low temperature peapod synthesis method. A splitting of the RBMs of inner tubes is observed. This is related to the interaction between the two shells of the DWCNTs as the same inner tube type can be encapsulated in different outer ones. The sharp appearance of the inner tube RBMs allows a reliable assignment of the tube modes to (n,m) indexes and thus provides a precise determination of the relation between the tube diameter and the RBM frequencies.

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 C(60), we discuss intercalated C(60) compounds and polymerized C(60), 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 C(60) encapsulated into SWCNTs (C(60) peapods) is discussed.

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 is shown to originate from the possibility that one type of inner tube may form in different types of outer tubes and the fact that the inner tube RBM frequency depends on the diameter of the enclosing tube. Finally, a comparison of the inner tube RBMs and the RBMs of tubes in bundles gave clear evidence that the interaction in a bundle is stronger than the interaction between inner and outer tubes., 6 pages, 7 figures, submitted to Eur. Phys. J. B

The synthesis of a unique isotope engineered system, double-wall carbon nanotubes with natural carbon outer and highly $^{13}\mathrm{C}$ enriched inner walls, is reported from isotope enriched fullerenes encapsulated in single-wall carbon nanotubes (SWCNTs). The material allows the observation of the $D$ line of the highly defect-free inner tubes that can be related to a curvature induced enhancement of the electron-phonon coupling. Ab initio calculations explain the inhomogeneous broadening of inner tube Raman modes due to the distribution of different isotopes. Nuclear magnetic resonance shows a significant contrast of the isotope enriched inner SWCNTs compared to other carbon phases and provides a macroscopic measure of the inner tube mass content. The high curvature of the small diameter inner tubes manifests in an increased distribution of the chemical shift tensor components.

Pristine single-wall carbon nanotubes (SWCNT's) filled with C 6 0 are investigated with resonance Raman spectroscopy. Both totally symmetric modes of C 6 0 exhibit a surprising splitting into two components with a slightly different resonance behavior and an apparent loss of polarization. The latter can be understood from a symmetry reduction of the fullerene molecules in the center of the tubes and/or by an anisotropy of the electric field seen by the C 6 0 peas inside the SWCNT pods due to depolarization effects. The splitting is explained from molecular-dynamics simulations which show that the two bands emerge from a coupling of the C 6 0 totally symmetric modes to the fullerene translational mobility inside the tube. The doublet is thus the spectroscopic fingerprint of different mobilities of C 6 0 fullerenes in the tubes.

We report on the single wall carbon nanotube specific 13C isotope enrichment. The high temperature annealing of isotope enriched fullerenes encapsulated in SWCNTs yields double wall carbon nanotubes with a high isotope enrichment of the inner wall. The methods helps to discriminate the Raman signal of the outer and inner tubes. The vibrational spectra evidences that no carbon exchange occurs between the two walls.

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.

We performed vapor phase filling of SWCNTs with various kinds of fullerenes. C60, C70, 13C enriched C60 and mixtures of these were used. The resulting peapods as well as the DWCNTs obtained by annealing were characterized with multi frequency Raman spectroscopy. Simultaneous filling with C60 and C70 was found to take place with the same efficiency. When using the 13C enriched C60 for DWCNT synthesis, mode softening for all inner shell lines was observed, while the outer shell response remained unchanged. The yield of inner shell CNTs is almost the same for C60 and C70 except that C70 is very inefficient in forming mid diameter inner CNTs with RBM frequencies close to 365 cm−1. If inner CNTs are grown from mixed peapods of C70 and 13C enriched C60 these particular CNTs are mainly built from C60. This is confirmed by the observation of distinct line shifts in the inner shell RBM.

Double wall carbon nanotubes were prepared by vacuum annealing of single wall carbon nanotubes filled with ${\mathrm{C}}_{60}$. Strong evidence is provided for a highly defect free and unperturbed environment in the interior of the tubes. This is concluded from unusual narrow Raman lines for the radial breathing mode of the inner tubes. Lorentzian linewidths scale down to $0.35\text{ }{\mathrm{c}\mathrm{m}}^{\ensuremath{-}1}$ which is almost $10$ times smaller than linewidths reported so far for this mode. A splitting is observed for the majority of the Raman lines. It is considered to originate from tube-tube interaction between one inner tube and several different outer tubes. The highest RBM frequency detected is $484\text{ }\mathrm{c}{\mathrm{m}}^{\mathrm{\ensuremath{-}}\mathrm{1}}$ corresponding to a tube diameter of only $0.50\text{ }\mathrm{n}\mathrm{m}$. Labeling of the Raman lines with the folding vector is provided for all inner tubes. This labeling is supported by density functional calculations.

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.