Unusual high degree of unperturbed environment in the interior of single-wall carbon nanotubes

Formation of nanoribbons by carbon atoms confined in a single-walled carbon nanotube-A molecular dynamics study

Carbon nanotubes can serve as one-dimensional nanoreactors for the in-tube synthesis of various nanostructures. Experimental observations have shown that chains, inner tubes, or nanoribbons can grow by the thermal decomposition of organic/organometallic molecules encapsulated in carbon nanotubes. The result of the process depends on the temperature, the diameter of the nanotube, and the type and amount of material introduced inside the tube. Nanoribbons are particularly promising materials for nanoelectronics. Motivated by recent experimental results observing the formation of carbon nanoribbons inside carbon nanotubes, molecular dynamics calculations were performed with the open source LAMMPS code to investigate the reactions between carbon atoms confined within a single-walled carbon nanotube. Our results show that the interatomic potentials behave differently in quasi-one-dimensional simulations of nanotube-confined space than in three-dimensional simulations. In particular, the Tersoff potential performs better than the widely used Reactive Force Field potential in describing the formation of carbon nanoribbons inside nanotubes. We also found a temperature window where the nanoribbons were formed with the fewest defects, i.e., with the largest flatness and the most hexagons, which is in agreement with the experimental temperature range.

Metallocene-Filled Single-Walled Carbon Nanotube Hybrids

In this paper, the growth mechanism, structure, growth processes, growth kinetics, and optical, vibronic and electronic properties of metallocene-filled single-walled carbon nanotubes (SWCNTs) are considered. A description of the procedures used to fill the nanotubes is provided. An investigation of doping effects on metallicity-mixed SWCNTs filled with metallocenes by Raman spectroscopy, near edge X-ray absorption fine structure spectroscopy, photoemission spectroscopy, and optical absorption spectroscopy is described. The studies of doping effects on metallicity-sorted SWCNTs filled with metallocenes are discussed. Doping effects in metallicity-mixed and sorted SWCNTs upon the chemical transformation of encapsulated molecules are analyzed. A discussion of the modification of the electronic properties of filled SWCNTs is presented. Applications of metallocene-filled SWCNTs in electrochemistry, thermoelectric power generation, chemical sensors, and magnetic recording are discussed.

Efficient Inner-to-Outer Wall Energy Transfer in Highly Pure Double-Wall Carbon Nanotubes Revealed by Detailed Spectroscopy

The coaxial stacking of two single-wall carbon nanotubes (SWCNTs) into a double-wall carbon nanotube (DWCNT), forming a so-called one-dimensional van der Waals structure, leads to synergetic effects that dramatically affect the optical and electronic properties of both layers. In this work, we explore these effects in purified DWCNT samples by combining absorption, wavelength-dependent infrared fluorescence-excitation (PLE), and wavelength-dependent resonant Raman scattering (RRS) spectroscopy. Purified DWCNTs are obtained by careful solubilization that strictly avoids ultrasonication or by electronic-type sorting, both followed by a density gradient ultracentrifugation to remove unwanted SWCNTs that could obscure the DWCNT characterization. Chirality-dependent shifts of the radial breathing mode vibrational frequencies and transition energies of the inner and outer DWCNT walls with respect to their SWCNT analogues are determined by advanced two-dimensional fitting of RRS and PLE data of DWCNT and their reference SWCNT samples. This exhaustive data set verifies that fluorescence from the inner DWCNT walls of well-purified samples is severely quenched through efficient energy transfer from the inner to the outer DWCNT walls. Combined analysis of the PLE and RRS results further reveals that this transfer is dependent on the inner and outer wall chirality, and we identify the specific combinations dominant in our DWCNT samples. These obtained results demonstrate the necessity and value of a combined structural characterization approach including PLE and RRS spectroscopy for bulk DWCNT samples.

Nanotube-Based 1D Heterostructures Coupled by van der Waals Forces

1D van der Waals heterostructures based on carbon nanotube templates are raising a lot of excitement due to the possibility of creating new optical and electronic properties, by either confining molecules inside their hollow core or by adding layers on the outside of the nanotube. In contrast to their 2D analogs, where the number of layers, atomic type and relative orientation of the constituting layers are the main parameters defining physical properties, 1D heterostructures provide an additional degree of freedom, i.e., their specific diameter and chiral structure, for engineering their characteristics. The current state-of-the-art in synthesizing 1D heterostructures are discussed here, in particular focusing on their resulting optical properties, and details the vast parameter space that can be used to design heterostructures with custom-built properties that can be integrated into a large variety of applications. First, the effects of van der Waals coupling on the properties of the simplest and best-studied 1D heterostructure, namely a double-walled carbon nanotube, are described, and then heterostructures built from the inside and the outside are considered, which all use a nanotube as a template, and, finally, an outlook is provided for the future of this research field.

Constraint spaces in carbon materials

Nano-sized pores in carbon materials are recently known to give certain constraints to the encapsulated materials by keeping them inside, accompanied with some changes in their structure, morphology, stability, etc. Consequently, nano-sized pores endow the constrained materials with improved performances in comparison with those prepared by conventional processes. These pores may be called "constraint spaces" in carbon materials. Here, we review the experimental results related to these constraint spaces by classifying as nanochannels in carbon nanotubes, nanopores and nanochannels in various porous carbons, and the spaces created by carbon coating.

Carbon Nanotube Chirality Determines Properties of Encapsulated Linear Carbon Chain

Long linear carbon chains (LLCCs) encapsulated inside double-walled carbon nanotubes (DWCNTs) are regarded as a promising realization of carbyne, the truly one-dimensional allotrope of carbon. While the electronic and vibronic properties of the encapsulated LLCC are expected to be influenced by its nanotube host, this dependence has not been investigated experimentally so far. Here we bridge this gap by studying individual LLCCs encapsulated in DWCNTs with tip-enhanced Raman scattering (TERS). We reveal that the nanotube host, characterized by its chirality, determines the vibronic and electronic properties of the encapsulated LLCC. By choice of chirality, the fundamental Raman mode (C-mode) of the chain is tunable by ∼95 cm^-1 and its band gap by ∼0.6 eV, suggesting this one-dimensional hybrid system to be a promising building block for nanoscale optoelectronics. No length dependence of the chain's C-mode frequency is evident, making LLCCs a close to perfect representation of carbyne.

Extraction of Linear Carbon Chains Unravels the Role of the Carbon Nanotube Host

Linear carbon chains (LCCs) have been shown to grow inside double-walled carbon nanotubes (DWCNTs), but isolating them from this hosting material represents one of the most challenging tasks toward applications. Herein we report the extraction and separation of LCCs inside single-walled carbon nanotubes (LCCs@SWCNTs) extracted from a double-walled host LCCs@DWCNTs by applying a combined tip-ultrasonic and density gradient ultracentrifugation (DGU) process. High-resolution transmission electron microscopy, optical absorption, and Raman spectroscopy show that not only short LCCs but clearly long LCCs (LLCCs) can be extracted and separated from the host. Moreover, the LLCCs can even be condensed by DGU. The Raman spectral frequency of LCCs remains almost unchanged regardless of the presence of the outer tube of the DWCNTs. This suggests that the major importance of the outer tubes is making the whole synthesis viable. We have also been able to observe the interaction between the LCCs and the inner tubes of DWCNTs, playing a major role in modifying the optical properties of LCCs. Our extraction method suggests the possibility toward the complete isolation of LCCs from CNTs.

Investigation of growth dynamics of carbon nanotubes

The synthesis of single-walled carbon nanotubes (SWCNTs) with defined properties is required for both fundamental investigations and practical applications. The revealing and thorough understanding of the growth mechanism of SWCNTs is the key to the synthesis of nanotubes with required properties. This paper reviews the current status of the research on the investigation of growth dynamics of carbon nanotubes. The review starts with the consideration of the peculiarities of the growth mechanism of carbon nanotubes. The physical and chemical states of the catalyst during the nanotube growth are discussed. The chirality selective growth of nanotubes is described. The main part of the review is dedicated to the analysis and systematization of the reported results on the investigation of growth dynamics of nanotubes. The studies on the revealing of the dependence of the growth rate of nanotubes on the synthesis parameters are reviewed. The correlation between the lifetime of catalyst and growth rate of nanotubes is discussed. The reports on the calculation of the activation energy of the nanotube growth are summarized. Finally, the growth properties of inner tubes inside SWCNTs are considered.

Carbon : nickel nanocomposite templates - predefined stable catalysts for diameter-controlled growth of single-walled carbon nanotubes

Carbon : nickel (C : Ni) nanocomposite templates (NCTs) were used as catalyst precursors for diameter-controlled growth of single-walled carbon nanotubes (SWCNTs) by chemical vapor deposition (CVD). Two NCT types of 2 nm thickness were prepared by ion beam co-sputtering without (type I) or with assisting Ar(+) ion irradiation (type II). NCT type I comprised Ni-rich nanoparticles (NPs) with defined diameter in an amorphous carbon matrix, while NCT type II was a homogenous C : Ni film. Based on the Raman spectra of more than 600 individual SWCNTs, the diameter distribution obtained from both types of NCT was determined. SWCNTs with a selective, monomodal diameter distribution are obtained from NCT type I. About 50% of the SWCNTs have a diameter of (1.36 ± 0.10) nm. In contrast to NCT type I, SWCNTs with a non-selective, relatively homogeneous diameter distribution from 0.80 to 1.40 nm covering 88% of all SWCNTs are obtained from NCT type II. From both catalyst templates predominantly separated as-grown SWCNTs are obtained. They are free of solvents or surfactants, exhibit a low degree of bundling and contain negligible amounts of MWCNTs. The study demonstrates the advantage of predefined catalysts for diameter-controlled SWCNT synthesis in comparison to in situ formed catalysts.

Investigating the Effect of Carbon Nanotube Diameter and Wall Number in Carbon Nanotube/Silicon Heterojunction Solar Cells

Suspensions of single-walled, double-walled and multi-walled carbon nanotubes (CNTs) were generated in the same solvent at similar concentrations. Films were fabricated from these suspensions and used in carbon nanotube/silicon heterojunction solar cells and their properties were compared with reference to the number of walls in the nanotube samples. It was found that single-walled nanotubes generally produced more favorable results; however, the double and multi-walled nanotube films used in this study yielded cells with higher open circuit voltages. It was also determined that post fabrication treatments applied to the nanotube films have a lesser effect on multi-walled nanotubes than on the other two types.

Theoretical infrared phonon modes in double-walled carbon nanotubes

In this theoretical work, we study the polarized infrared spectra of double-walled carbon nanotubes (DCNTs) as a function of their diameters, chiralities and lengths.

Double-walled carbon nanotube processing

Single-walled carbon nanotubes (SWCNTs) have been the focus of intense research, and the body of literature continues to grow exponentially, despite more than two decades having passed since the first reports. As well as extensive studies of the fundamental properties, this has seen SWCNTs used in a plethora of applications as far ranging as microelectronics, energy storage, solar cells, and sensors, to cancer treatment, drug delivery, and neuronal interfaces. On the other hand, the properties and applications of double-walled carbon nanotubes (DWCNTs) have remained relatively under-explored. This is despite DWCNTs not only sharing many of the same unique characteristics of their single-walled counterparts, but also possessing an additional suite of potentially advantageous properties arising due to the presence of the second wall and the often complex inter-wall interactions that arise. For example, it is envisaged that the outer wall can be selectively functionalized whilst still leaving the inner wall in its pristine state and available for signal transduction. A similar situation arises in DWCNT field effect transistors (FETs), where the outer wall can provide a convenient degree of chemical shielding of the inner wall from the external environment, allowing the excellent transconductance properties of the pristine nanotubes to be more fully exploited. Additionally, DWCNTs should also offer unique opportunities to further the fundamental understanding of the inter-wall interactions within and between carbon nanotubes. However, the realization of these goals has so far been limited by the same challenge experienced by the SWCNT field until recent years, namely, the inherent heterogeneity of raw, as-produced DWCNT material. As such, there is now an emerging field of research regarding DWCNT processing that focuses on the preparation of material of defined length, diameter and electronic type, and which is rapidly building upon the experience gained by the broader SWCNT community. This review describes the background of the field, summarizing some relevant theory and the available synthesis and purification routes; then provides a thorough synopsis of the current state-of-the-art in DWCNT sorting methodologies, outlines contemporary challenges in the field, and discusses the outlook for various potential applications of the resulting material.

Double-wall carbon nanotubes for wide-band, ultrafast pulse generation

We demonstrate wide-band ultrafast optical pulse generation at 1, 1.5, and 2 μm using a single-polymer composite saturable absorber based on double-wall carbon nanotubes (DWNTs). The freestanding optical quality polymer composite is prepared from nanotubes dispersed in water with poly(vinyl alcohol) as the host matrix. The composite is then integrated into ytterbium-, erbium-, and thulium-doped fiber laser cavities. Using this single DWNT-polymer composite, we achieve 4.85 ps, 532 fs, and 1.6 ps mode-locked pulses at 1066, 1559, and 1883 nm, respectively, highlighting the potential of DWNTs for wide-band ultrafast photonics.

Role of intertube interactions in double- and triple-walled carbon nanotubes

Resonant Raman spectroscopy studies are performed to access information about the intertube interactions and wall-to-wall distances in double- and triple-walled carbon nanotubes. Here, we explain how the surroundings of the nanotubes in a multiwalled system influence their radial breathing modes. Of particular interest, the innermost tubes in double- and triple-walled carbon nanotube systems are shown to be significantly shielded from environmental interactions, except for those coming from the intertube interaction with their own respective host tubes. From a comparison of the Raman results for bundled as well as individual fullerene-peapod-derived double- and triple-walled carbon nanotubes, we observe that metallic innermost tubes, when compared to their semiconducting counterparts, clearly show weaker intertube interactions. Additionally, we discuss a correlation between the wall-to-wall distances and the frequency upshifts of the radial breathing modes observed for the innermost tubes in individual double- and triple-walled carbon nanotubes. All results allow us to contemplate fundamental properties related to DWNTs and TWNTs, as for example diameter- and chirality-dependent intertube interactions. We also discuss differences in fullerene-peapod-derived and chemical vapor deposition grown double- and triple-walled systems with the focus on mechanical coupling and interference effects.

Wall-selective probing of double-walled carbon nanotubes using covalent functionalization

Double-walled carbon nanotubes (DWNTs) present an original coaxial geometry in which the inner wall is naturally protected from the environment by the outer wall. Covalent functionalization is introduced here as an effective approach to investigate DWNT devices. Performed using an aryldiazonium salt, the functionalization is reversible upon thermal annealing and occurs strictly at the surface of the outer wall, leaving the inner wall essentially unaltered by the chemical bonding. Measurements on functionalized DWNT transistors show that the electrical current is carried by the inner wall and provide unambiguous identification of the metallic or semiconducting character of both walls. New insights about current saturation at high bias in DWNTs are also presented as an illustration of new experiments unlocked by the method. The wall-selectivity of the functionalization not only enables selective optical and electrical probing of the DWNTs, but it also paves the way to designing novel electronic devices in which the inner wall is used for electrical transport while the outer wall chemically interacts with the environment.

Raman and fluorescence spectroscopic studies of a DNA-dispersed double-walled carbon nanotube solution

We performed resonant Raman/fluorescence spectroscopic studies on double-walled carbon nanotubes (DWNTs) that were dispersed in an aqueous single stranded DNA solution. The luminescence signals from the inner tubes of DWNTs are intensified in the isolated state of each individual DWNT. The completely depressed radial breathing modes (RBMs) associated with the outer tubes (whether semiconducting or metallic) via the mechanical wrapping and the strong charge transfer between DNA and the outer tubes support our interpretation that the bright luminescence and sharp absorption spectra come from only the inner tubes, and not from isolated SWNTs. The circumferentially wrapped DNA on the outer tubes of individually isolated DWNTs in an aqueous solution gives rise to strong charge transfer to the semiconducting and metallic outer tubes as well as to generating physical strain in the outer tubes.

In situ Raman spectroelectrochemical study of 13C-labeled fullerene peapods and carbon nanotubes

C60 fullerene peapods and double-walled carbon nanotubes (DWCNTs) containing highly 13C enriched C60 and inner tubes, respectively, are studied using Raman spectroscopy and in situ Raman spectroelectrochemistry in order to follow the influence of 13C enrichment on the vibrational pattern of these carbon nanostructures. The Raman response of 13C60 after encapsulation in fullerene peapods differs from that of isotope-natural species, (Nat)C60. The Raman A(g)(2) mode of encapsulated 13C60 is upshifted in frequency compared to that of the (Nat)C60 peapods with the same filling factor. The chemical doping of 13C60 peapods (peapod = C(60)@SWCNT) with K-vapor leads to the downshift of the A(g)(2) mode, similar to the case of (Nat)C60 peapods. The 13C60 peapods were successfully transformed into DWCNTs, which confirms high filling of single-walled (SW) CNTs with 13C60. The DWCNTs exhibited distinctly downshifted G and D Raman modes for inner tubes, which proves that only inner tubes were enriched by 13C. The in situ Raman spectroelectrochemistry of (Nat)C60 exhibits strong anodic enhancement, while for 13C60 peapods the enhancement is only weak. On the other hand, the electrochemical charging of the inner-tube-labeled DWCNTs (13C(i)-DWCNTs) followed the behavior of ordinary (Nat)C(i)-DWCNTs as indicated by in situ Raman spectroelectrochemistry. In addition, the spectroelectrochemical behavior of the G mode of inner tubes in 13C(i)-DWCNTs is followed from the start of the electrochemical doping, which was not feasible for (Nat)C(i)-DWCNTs.

Spectroelectrochemistry of carbon nanostructures

This review is focused on charge-transfer reactions at carbon nanotubes and fullerenes. The spectroelectrochemistry of fullerenes deals with the spin states of fullerenes, the role of mono-anions and the reactivity of higher charged states in C60. The optical (Vis-NIR) spectroelectrochemistry of single-walled carbon nanotubes (SWNTs) follows changes in the allowed optical transitions among the Van Hove singularities. The Raman spectroelectrochemistry of SWNT benefits from strong resonance enhancement of the Raman scattering. Here, both semiconducting and metallic SWNTs are analyzed using the radial breathing mode (RBM) and G-modes as well as the second order (D, G') and intermediate frequency modes. Raman spectroelectrochemistry of SWNT allows the addressing of index-identified tubes and even single isolated nanotubes. Optical and Raman spectroelectrochemistry of fullerene peapods, C60@SWNT and C70@SWNT indicates effective shielding of the intratubular fullerene (peas). The most striking effect in the spectroelectrochemistry of peapods is the so-called "anodic Raman enhancement" of intratubular C60. Double-walled carbon nanotubes (DWNTs) give a specific spectroscopic response in Vis-NIR spectroelectrochemistry for the inner and the outer tube. They are better distinguishable by Raman spectroelectrochemistry which allows a precise tracing of the specific doping response of outer/inner tubes.

Selective tuning of the electronic properties of coaxial nanocables through exohedral doping

The electronic properties of exohedrally doped double-walled carbon nanotubes (DWNTs) have been investigated using density functional theory and resonance Raman spectroscopy (RRS) measurements. First-principles calculations elucidate the effects of exohedral doping on the M@S and S@M systems, where a metallic (M) tube is either inside or outside a semiconducting (S) one. The results demonstrate that metallic nanotubes are extremely sensitive to doping even when they are inner tubes, in sharp contrast to semiconducting nanotubes, which are not affected by doping when the outer shell is a metallic nanotube (screening effects). The theoretical predictions are in agreement with RRS data on Br2- and H2SO4-doped DWNTs. These results pave the way to novel nanoscale electronics via exohedral doping.

Dynamics of carbon nanotube growth from fullerenes

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.

Anharmonic phonon lifetimes in carbon nanotubes: evidence for a one-dimensional phonon decay bottleneck

High-resolution Raman spectroscopy is applied to suspended single-walled carbon nanotubes (SWNTs) to elucidate the puzzling differences in the lifetime of the radial breathing mode (RBM) obtained from different experimental techniques. Whereas recent tunneling experiments suggest a room temperature RBM lifetime as long as 10 ns, previous Raman experiments yield lifetimes shorter than 2 ps. The lifetimes obtained in this study are longer than 5 ps-a significant step in the direction of the tunneling results. We argue that the remaining discrepancy is due to the existence of phonon decay bottlenecks caused by the one-dimensional nature of nanotubes. Numerical simulations of the RBM decay show that it is possible to reconcile the short lifetimes measured spectroscopically with the long lifetimes obtained in tunneling experiments.

The Intermediate Frequency Modes of Single- and Double-Walled Carbon Nanotubes: A Raman Spectroscopic and In Situ Raman Spectroelectrochemical Study

The intermediate frequency modes (IFM) of single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs) were analyzed by Raman spectroscopy and in situ Raman spectroelectrochemistry. The inner and outer tubes of DWCNTs manifested themselves as distinct bands in the IFM region. This confirmed the diameter dependence of IFM frequencies. Furthermore, the analysis of inner tubes of DWCNTs allowed a more-precise assignment of the bands in the IFM region to features intrinsic for carbon nanotubes. Although the inner tubes in DWCNTs are assumed to be structurally perfect, the role of defects on IFM was discussed. The dependence of IFM on electrochemical charging was also studied. In situ spectroelectrochemical data provide a means to distinguish the bands of the outer and inner tubes.

In-Depth Study into the Interaction of Single Walled carbon Nanotubes with Anthracene and p-Terphenyl

Solubilization of single walled carbon nanotubes (SWNT) in the presence of polycyclic aromatic hydrocarbons (PAHs) such as p-terphenyl and anthracene has been shown. The suspensions formed are stable for periods greater than 48 months but to date experimental research is scarce regarding the interactions that are taking place. Spectroscopic analysis such as Raman and fluorescence are used to probe the interactions occurring between the PAHs and the SWNT over a wide concentration range. Previous studies show the fluorescence of the PAHs is quenched on interaction with SWNT and in the case of p-terphenyl, the spectrum is red shifted. This result prompted a study of a large range of concentrations to quantify the degree of interaction between the SWNT and PAHs. It was found at high concentrations that both the PAHs and SWNT formed aggregates and at lower concentrations it was found that free PAHs and isolated SWNT were interacting. The radial breathing modes (RBMs) in Raman spectroscopy gave detail as to how diameter selective the PAH samples are when compared to the pristine SWNT modes. An increase in the wavenumber of the RBMs for both composite spectra was observed and it is believed that such a result is due to the debundling of the SWNT on interaction with the PAHs. It was also found that anthracene and p-terphenyl selectively interact with SWNT and the selected SWNT were found to be within a distinct diameter range and possessed unique physical properties.

NMR evidence for gapped spin excitations in metallic carbon nanotubes

We report on the spin dynamics of 13C isotope enriched inner walls in double-wall carbon nanotubes using 13C nuclear magnetic resonance. Contrary to expectations, we find that our data set implies that the spin-lattice relaxation time (T1) has the same temperature (T) and magnetic field (H) dependence for most of the inner-wall nanotubes detected by NMR. In the high-temperature regime (T approximately > or = 150 K), we find that the T and H dependence of 1/T1T is consistent with a 1D metallic chain. For T approximately < or = 150 K we find a significant increase in 1/T1T with decreasing T, followed by a sharp drop below approximately = 20 K. The data clearly indicate the formation of a gap in the spin excitation spectrum, where the gap value 2delta approximately = 40 K (congruent to 3.7 meV) is H independent.

Electrochemical Doping of Chirality-Resolved Carbon Nanotubes

Raman spectra of electrochemically charged single-wall carbon nanotubes (HiPco) were studied by five different laser photon energies between 1.56 and 1.92 eV. The bands of radial breathing modes (RBM) were assigned to defined chiralities by using the experimental Kataura plot. The particular (n,m) tubes exhibit different sensitivity to electrochemical doping, monitored as the attenuation of the RBM intensities. Tubes which are in good resonance with the exciting laser exhibit strong doping-induced drop of the RBM intensity. On the other hand, tubes whose optical transition energy is larger than the energy of an exciting photon show only small changes of their RBM intensities upon doping. This rule presents a tool for analysis of mixtures of single-walled carbon tubes of unknown chiralities. It also asks for a re-interpretation of some earlier results which were reported on the diameter-selectivity of doping. The radial breathing mode in strongly n- or p-doped nanotubes exhibited a blue-shift. A suggested interpretation follows from the charging-induced structural changes of SWCNTs bundles, which also includes a partial de-bundling of tube ropes.

Isotope engineering of carbon nanotube systems

The synthesis of a unique isotope engineered system, double-wall carbon nanotubes with natural carbon outer and highly 13C 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.

Atomic correlation between adjacent graphene layers in double-wall carbon nanotubes

Atomic correlation between adjacent graphene layers was elucidated for double-wall carbon nanotubes (DWNTs) through a chiral index assignment of two nested nanotubes by high-resolution transmission electron microscopy. Our analysis provides a rather constant diameter difference close to 0.75 nm but no chiral angle correlation between the constituent nanotubes in the concentric DWNTs. The local atomic correlation as a commensurate graphene stacking was repeatedly found in eccentric DWNTs and circumscribed nanotubes, which should lead to elastic deformation and bundling of nanotubes.

A Sonochemical Route to Single-Walled Carbon Nanotubes under Ambient Conditions

A chemical route to single-walled carbon nanotubes (SWCNTs) under ambient conditions has been developed. Silica powder was immersed in a mixture solution of ferrocene and p-xylene. After sonication at atmospheric pressure and room temperature, we obtained high-purity SWCNTs. Sonochemical effects may lead to producing high-purity SWCNTs. The process could be readily generalized to synthesize other forms of carbon-based materials, such as fullerenes, multiwalled nanotubes, carbon onions, and diamond, in liquid solution under ambient conditions.

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 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.