The possible way to evaluate the purity of double-walled carbon nanotubes over single wall carbon nanotubes by chemical doping

Synthesis of outer tube-selectively nitrogen-doped double-walled carbon nanotubes by nitrogen plasma treatment

We demonstrate selective nitrogen doping on the outer tubes of double walled carbon nanotubes by nitrogen plasma treatment. Such selectivity improves the chemical activity of the outer tubes, including their wettability and oxygen reduction reaction properties, while retaining the structural, optical, and electrical properties of the inner tubes.

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-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.

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

We measure resonant Raman scattering from 11 individual C(60)-derived double-wall carbon nanotubes all having inner semiconducting (6,5) tubes and various outer metallic tubes. The Raman spectra show the radial breathing modes (RBM) of the inner and the outer tubes to be simultaneously in resonance with the same laser energy. We observe that an increase in the RBM frequency of the inner tubes is related to an increase in the RBM frequency of the outer tubes. The Raman spectra also contain a sharp G(-) feature that increases in frequency as the nominal diameter of the outer metallic tubes decreases. Finally, the one-phonon second-order D-band mode shows a two-way frequency splitting that decreases with decreasing nominal wall-to-wall distance. We suggest that the stress which increases with decreasing nominal wall-to-wall distance is responsible for the hardening that is observed in the frequencies of the RBM, D and G(-) modes of the inner (6,5) semiconducting tubes.

Processing and properties of highly enriched double-wall carbon nanotubes

Carbon nanotubes consist of one or more concentric graphene cylinders and are under investigation for a variety of applications that make use of their excellent thermal, mechanical, electronic and optical properties. Double-wall nanotubes are ideal systems for studying the interwall interactions influencing the properties of nanotubes with two or more walls. However, current techniques to synthesize double-wall nanotubes produce unwanted single- and multiwall nanotubes. Here, we show how density gradient ultracentrifugation can be used to separate double-wall nanotubes from mixtures of single- and multiwall nanotubes through differences in their buoyant density. This technique results in samples that are highly enriched in either single- or double-wall nanotubes of similar outer wall diameter, with the double-wall nanotubes being, on average, approximately 44% longer than the single-wall nanotubes. The longer average length of the double-wall nanotubes provides distinct advantages when they are used in transparent conductors.

Amino acid functionalization of double-wall carbon nanotubes studied by Raman spectroscopy

Double-wall carbon nanotubes (DWNT) have been functionalized with lysine after a strong oxidation with MnO4- in acid solution which, as suggested by the Raman spectra, attacked the external nanotube of the DWNT.

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.