Wall-to-wall stress induced in (6,5) semiconducting nanotubes by encapsulation in metallic outer tubes of different diameters: a resonance Raman study of individual C60-derived double-wall carbon nanotubes

Intrinsic Temperature Dependence of Raman-Active Modes in Individual Isolated Single- and Double-Walled Carbon Nanotubes

The intrinsic temperature dependence of Raman-active modes in carbon nanotubes (CNTs), particularly the radial breathing mode (RBM), has been a topic of a long-standing controversy. In this study, we prepared suspended individual CNTs to investigate how their Raman spectra depend on temperature and to understand the effects of environmental conditions on this dependency. We analyzed the intrinsic temperature dependence of the main Raman-active modes, including the RBM, the moiré-activated R feature, and the G-band in double-walled carbon nanotubes (DWCNT) and single-walled carbon nanotubes (SWCNTs) after complete desorption of air. The inner tube of the DWCNT, like the desorbed SWCNTs, was free from environmental influences, resulting in minimal temperature-induced RBM frequency shifts. We show that the larger RBM shift of SWCNTs upon initial heating is not intrinsic but is due to air desorption. The R feature, attributed to moiré-activated phonon scattering and nondispersive in nature, demonstrated a quasi-linear temperature dependence, akin to the G-band but with a lower temperature coefficient. The G-band, which was largely unaffected by environmental conditions, exhibited a consistent temperature coefficient across SWCNTs, DWCNTs, and small SWCNT bundles.

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.

Moiré-Induced Vibrational Coupling in Double-Walled Carbon Nanotubes

Moiré patterns are additional, long-range periodicities in twisted crystalline bilayers. They are known to fundamentally change the electronic states of the layers, but similar effects on their mechanical and vibrational properties have not been discussed so far. Here we show that the moiré potential shifts the radial breathing mode in double-walled carbon nanotubes (DWCNTs). The change in frequency is expected to be proportional to the shift in optical transition energies, which are induced by the moiré patterns. To verify our model, we performed resonance Raman scattering on purified and sorted semiconducting DWCNTs. We find that the radial breathing mode shifts up to 14 cm^-1 higher in energy followed by displacement of optical transition energies of up to 200 meV to lower energies, in comparison to the single-walled tubes. We show how to identify the strong coupling condition in DWCNTs from their phonon frequencies and construct a Kataura plot to aid their future experimental assignment.

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.

Characterization of bundled and individual triple-walled carbon nanotubes by resonant Raman spectroscopy

The optical characterization of bundled and individual triple-walled carbon nanotubes was studied for the first time in detail by using resonant Raman spectroscopy. In our approach, the outer tube of a triple-walled carbon nanotube system protects the two inner tubes (or equivalently the inner double-walled carbon nanotube) from external environment interactions making them a partially isolated system. Following the spectral changes and line-widths of the radial breathing modes and G-band by performing laser energy dependent Raman spectroscopy, it is possible to extract important information as regards to the electronic and vibrational properties, tube diameters, wall-to-wall distances, radial breathing mode, and G-band resonance evolutions as well as high-curvature intertube interactions in isolated double- and triple-walled carbon nanotube systems.

Multiple intra-tube junctions in the inner tube of peapod-derived double walled carbon nanotubes: theoretical study and experimental evidence

The coalescence process of fullerenes in the hollow core of single walled carbon nanotubes is systematically explored by the kinetic Monte Carlo method. Two elongation (or growth) modes via the coalescence (i) between an inner tube and fullerenes and (ii) between neighboring inner tubes are identified. It is found that the coalescence of two inner tubes mostly creates a very stable intra-tube junction which is composed of multiple pentagon-heptagon pairs. As a consequence, the study predicts that the inner tube of peapod derived double walled carbon nanotubes (DWNTs) must contain many intra-tube junctions. Careful high resolution transmission electron microscopy observation on peapod-grown DWNT sample provides experimental evidence of the presence of the junctions.

Study of Carbon Nanotube-Substrate Interaction

Environmental effects are very important in nanoscience and nanotechnology. This work reviews the importance of the substrate in single-wall carbon nanotube properties. Contact with a substrate can modify the nanotube properties, and such interactions have been broadly studied as either a negative aspect or a solution for developing carbon nanotube-based nanotechnologies. This paper discusses both theoretical and experimental studies where the interaction between the carbon nanotubes and the substrate affects the structural, electronic, and vibrational properties of the tubes.

Chirality-dependent transport in double-walled carbon nanotube assemblies: the role of inner tubes

A fundamental understanding of the electrical properties of carbon nanotubes is vital when fabricating high-performance polymeric composites as well as transparent conductive films. Herein, the chirality-dependent transport mechanisms in peapod- and chemical vapor deposition-grown double-walled carbon nanotubes (DWNTs) films are discussed by identifying the chiralities of the inner and the outer tubes using fast Fourier transform image processing, as well as optical studies (e.g., Raman, UV, and photoluminescence spectroscopies). The observed conduction mechanisms are strongly dependent on the total fraction of the metallic inner and outer tubes within the DWNT samples. Furthermore, the contribution of the inner tubes to the electronic transport properties of DWNT films is confirmed by photochemically deactivating the outer tubes in both types of DWNT samples.

Double-walled carbon nanotubes: challenges and opportunities

Double-walled carbon nanotubes are coaxial nanostructures composed of exactly two single-walled carbon nanotubes, one nested in another. This unique structure offers advantages and opportunities for extending our knowledge and application of the carbon nanomaterials family. This review seeks to comprehensively discuss the synthesis, purification and characterization methods of this novel class of carbon nanomaterials. An emphasis is placed on the double wall physics that contributes to these structures' complex inter-wall coupling of electronic and optical properties. The debate over the inner-tube photoluminescence provides an interesting illustration of the rich photophysics and challenges associated with the myriad combinations of the inner and outerwall chiralities. Outerwall selective covalent chemistry will be discussed as a potential solution to the unattractive tradeoff between solubility and functionality that has limited some applications of single-walled carbon nanotubes. Finally, we will review the many different uses of double-walled carbon nanotubes and provide an overview of several promising research directions in this new and emerging field.

Made-to-order nanocarbons through deterministic plasma nanotechnology

Through a combinatorial approach involving experimental measurement and plasma modelling, it is shown that a high degree of control over diamond-like nanocarbon film sp3/sp2 ratio (and hence film properties) may be exercised, starting at the level of electrons (through modification of the plasma electron energy distribution function). Hydrogenated amorphous carbon nanoparticle films with high percentages of diamond-like bonds are grown using a middle-frequency (2 MHz) inductively coupled Ar+CH4 plasma. The sp3 fractions measured by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy in the thin films are explained qualitatively using sp3/sp2 ratios 1) derived from calculated sp3 and sp2 hybridized precursor species densities in a global plasma discharge model and 2) measured experimentally. It is shown that at high discharge power and lower CH4 concentrations, the sp3/sp2 fraction is higher. Our results suggest that a combination of predictive modeling and experimental studies is instrumental to achieve deterministically grown made-to-order diamond-like nanocarbons suitable for a variety of applications spanning from nano-magnetic resonance imaging to spin-flip quantum information devices. This deterministic approach can be extended to graphene, carbon nanotips, nanodiamond and other nanocarbon materials for a variety of applications.

NT10: recent advances in carbon nanotube science and applications

A review of recent advances in carbon nanotube science and applications is presented in terms of what was learned at the NT10 11th International Conference on the Science and Application of Nanotubes held in Montreal, Canada, June 29-July 2, 2010.