Controllable preparation and properties of single-/double-walled carbon nanotubes

In this paper, we discussed recent studies done in our laboratories with a floating catalyst chemical vapor deposition (CVD) method. We can grow single- or double-walled carbon nanotubes (SWNTs/DWNTs) with different kinds of catalysts. Single-walled carbon nanotubes without amorphous carbon coating were prepared by thermally decomposing acetylene (C₂H₂) at the temperature range of 750–1200 °C with ferrocene as catalyst. While with sulfur promoted ferrocene catalyst, double-walled carbon nanotubes were mass-produced by pyrolizing C₂H₂ at the temperature range of 900–1100 °C. Furthermore, tunable growth of DWNTs with different diameter was achieved in our experiment. It is found that DWNTs produced at lower carbon partial pressure have much smaller inner tubes, even DWNTs with the smallest inner diameter of 0.4 nm was found in here. As convenient and effective tool, radial breathing mode (RBM) of Raman scattering technique can be used to distinguish SWNTs from DWNTs. In further studies of Raman scattering with DWNTs, the possible match of the inner tubes and the outer tubes according to the RBM bands was assigned, and different chirality types were discussed according to the diameter and chirality dependence of resonant Raman vibration. We also investigated the temperature-dependent frequency shift of resonant Raman spectra of DWNTs in the range of 78–650 K. We found that different RBM peaks, which are relative to different tube diameters, have different temperature coefficient of frequency shift, and the larger diameter carbon nanotubes have more RBM frequency downshift with increasing temperature. It is ascribed to the RBM frequency variation to the temperature dependence of the stretching force constant of C–C bond. Besides, Polarized Raman spectra were preformed on well-aligned SWNTs structure fabricated through post-growth method and found that the angular dependence of Raman intensity is consistent well with the predictions of the resonance Raman theory.     

Spectroscopy of single- and double-wall carbon nanotubes in different environments

Individual single-wall carbon nanotubes (SWNTs) and double-wall carbon nanotubes (DWNTs) were suspended in water for optical studies using sodium-cholate and other surfactants. We used time-resolved photoluminescence (PL) spectroscopy to study the influence of tube chirality and diameter as well as of the environment on nonradiative decay in small diameter tubes. The studies provide evidence for PL from small diameter core tubes in DWNTs and for a correlation of nonradiative decay with tube diameter and exciton red shift as induced by interaction with the environment.     

Analysis of wave propagation in carbon nanotubes via elastic shell theories

This paper investigates wave propagation in both single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs) via two developed elastic shell theories: Love’s thin cylindrical shell theory and the Cooper–Naghdi thick cylindrical shell theory. In studying DWNTs, the van der Waals effect is accounted for and modeled with the two theories. The elastic thick shell theory, in which the shear and inertia effects are taken into account, is developed first to investigate the wave propagations of CNTs to provide more accurate wave dispersions for higher modes. The material properties of the CNTs that are used in the two shell theories are proposed, and the expression of the inertia moment of the cross area in the thick shell theory is recommended. The dispersion results that are derived via the two theories are compared to show the feasibility of those theories in studying CNTs. Radius-dependent wave propagation results in SWNTs and DWNTs are also studied via the two theories.     

Selective growth of vertically aligned double- and single-walled carbon nanotubes on a substrate at 590 °C

Vertically aligned double-?and single-walled carbon nanotubes (DWNTs and SWNTs) were synthesized on a substrate at 590?°C by hot-filament chemical vapor deposition. An optimized combination of iron and aluminum layers as the catalyst resulted in iron particles ranging from 1-5?nm floating in an aluminum matrix after annealing. Selective synthesis of DWNTs and SWNTs from such particles was achieved by adjusting the dilution ratio of acetylene that was used as the source gas. The yield of DWNTs among all CNTs was as high as 81%, while that of SWNTs was almost 100%. The diameter distribution of DWNTs was narrow, with a standard deviation of about 12%.     

The effect of sulfur on the structure of carbon nanotubes produced by a floating catalyst method

The effect of sulfur on the structures (shell number and diameter distribution) of carbon nanotubes (CNTs) was investigated in detail using high resolution transmission electronic microscopy (HRTEM) and Raman spectroscopy. Single-walled (SWNTs), double-walled (DWNTs), and multi-walled carbon nanotubes (MWNTs) with different diameter distributions were obtained only by increasing the sulfur addition amount with methane as carbon source. A similar structure change was found for ethanol as carbon source with changing the sulfur addition amount. These results indicate that addition of sulfur is necessary to enhance the growth of SWNTs and DWNTs, independent of the carbon source for our method. Based on the growth parameter study, HRTEM observations and kinetic considerations, the role of sulfur in the nucleation and growth of CNTs was discussed.     

Effect of omitting terms involving tube radii difference in shell models on buckling solutions of DWNTs

Continuum cylindrical shell models have been widely applied in the buckling analysis of carbon nanotubes. An explicit expression for the critical buckling strain of double-walled carbon nanotubes (DWNTs) may be obtained based on cylindrical shell models. The expression is usually simplified by neglecting the terms involving outer and inner tube radii difference. In this brief note, we present the critical buckling strains of DWNTs with the inclusion of these terms and investigate the quantitative effect of neglecting these terms on the critical strain. It was found that the omission of the terms related to outer and inner tube radii difference leads to an overprediction of the critical buckling strain as well as a possible change in the buckling mode shape. It is also observed that the effect of the terms is especially significant for DWNTs with small inner radius but is negligible when the inner radius is relatively large.     

In situ Raman study on single- and double-walled carbon nanotubes as a function of lithium insertion

We investigated the electrochemical lithium ion (Li(+)) insertion/desertion behavior on highly pure and bundled single- and double-walled carbon nanotubes (SWNTs and DWNTs) using an in situ Raman technique. In general, two storage sites could host Li(+) in SWNT and DWNT bundles when varying an external potential: a) the outer surface sites, and b) the interstitial spaces within the bundles. The most sensitive changes in the tangential mode (TM) of the Raman spectra upon doping with Li(+) can be divided into two regions. The first region was found from 2.8 to 1.0 V (the coverage of Li(+) on the outer surface of a bundled nanotube) and was characterized by the loss of resonant conditions via partial charge transfer, where the G(+) line of the SWNT and the TM of the outer tube of DWNTs experienced a highly depressed intensity, but remained almost constant in frequency. The appearance of a Breit-Wigner-Fano (BWF) profile provided strong evidence of metallic inner tubes within DWNTs. The second region was observed when the applied potentials ranged from 0.9 to 0 V and was characterized by Li(+) diffusion into the interstitial sites of the bundled nanotube material. This phenomenon invoked a large downshift of the G(-) band in SWNTs, and a small downshift of the TM of the inner tube of DWNTs caused by expansion of the C--C bonds due to the charge transferred to the nanotubes, and the disappearance of the BWF profile through the screening effect of the interstitial Li(+) layers.     

Opening and reversible filling of single-walled carbon nanotubes with various materials

Single-walled carbon nanotubes have been advocated as perfect candidates for the sustainable miniaturisation of electronic and mechanical nanoscale devices. The encapsulation of selected compounds within the inner hollow cavity of SWNTs allows controlled preparation of nano-meter size "nanowires" and "nanocables" with purpose-tailored physical properties. Therefore is crucial to have control of opening and closing their tips. In a previous study we showed that molten metal hydroxide [MOH (M==Cs, Na)] is filled into the carbon nanotubes and can be easily washed out with water leaving opened nanotubes. Following this approach we have explored the use of milder ways to open SWNTs that can be easily scalable for the production of large amount of opened SWNTs. The opened tubes have then successfully been filled in solution with various inorganic and organic materials.     

Growth of double-walled carbon nanotubes using a conditioning catalyst

Here we describe the effect of different synthetic conditions on the quality and purity of double-walled carbon nanotubes (DWNTs) with the aid of a conditioning catalyst. By lowering the reaction temperature down to 875 degrees C and utilizing a conditioning catalyst, increased purity and a decreased inner diameter of the DWNTs was achieved, while adverse results were observed with increasing reaction temperature. Based on detailed high-resolution transmission electron microscopy studies on the diameter distribution of the tubes, preferential growth conditions for DWNTs over single-wall carbon nanotubes are identified solely from increased carbon solubility considerations (caused by an increased portion of active carbon species by use of Mo) for the same distribution of metal particles.     

Nanoscale optical imaging of excitons in single-walled carbon nanotubes

We present simultaneous near-field photoluminescence (PL) and Raman imaging of single-walled carbon nanotubes (SWNTs) with a spatial resolution better than 15 nm. Highly localized excitation is used to visualize the spatial extent of the contributing excited states. For SWNTs on glass, we typically observe highly confined PL from short segments of about 20 nm in length. The PL from micelle-encapsulated SWNTs on mica is extended along the tube up to several hundreds of nanometers. We find that near-field enhancement is much stronger for photoluminescence than for Raman scattering, an observation that is explained by the low intrinsic quantum yield of SWNTs.     

Synthesis of uniform double-walled carbon nanotubes using iron disilicide as catalyst

The preparation of carbon nanotube (CNT) materials with high purity is critical for many potential applications. These materials not only need to be free of carbonaceous impurities but also have uniform diameters. Within the CNT family, double-walled carbon nanotubes (DWNTs), as the simplest member of multiwalled carbon nanotubes, have demonstrated good potential in many bulk applications. However, the synthesis of DWNTs with uniform diameter and high purity is still a challenge. Here, a method to prepare high-purity DWNTs using iron disilicide (FeSi2) as catalyst is demonstrated. Over 90% of CNTs in the sample were DWNTs with a narrow diameter distribution in the range of 4-5 nm. An additional advantage of using FeSi2 as catalyst is to simplify the process to prepare suitable catalyst because commercially available FeSi2 can be used directly without any further treatment.     

Chirality‐Selective Photoluminescence Enhancement of ssDNA‐Wrapped Single‐Walled Carbon Nanotubes Modified with Gold Nanoparticles

In this work, a convenient method to enhance the photoluminescence (PL) of single‐walled carbon nanotubes (SWNTs) in aqueous solutions is provided. Dispersing by single‐stranded DNA (ssDNA) and modifying with gold nanoparticles (AuNPs), about tenfold PL enhancement of the SWNTs is observed. More importantly, the selective PL enhancement is achieved for some particular chiralities of interest over all other chiralities, by using certain specific ssDNA sequences that are reported to recognize these particular chiralities. By forming AuNP–DNA–SWNT nanohybrids, ssDNA serves as superior molecular spacers that on one hand protect SWNT from direct contacting with AuNP and causing PL quench, and on the other hand attract the AuNP in close proximity to the SWNT to enhance its PL. This PL enhancement method can be utilized for the PL analysis of SWNTs in aqueous solutions, for biomedical imaging, and may serve as a prescreening method for the recognition and separation of single chirality SWNTs by ssDNA.     

Studies of Size Effects on Carbon Nanotubes' Mechanical Properties by Using Different Potential Functions

We use molecular mechanics calculations to study size effects on mechanical properties of carbon nanotubes. Both single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs) are considered. The size-dependent Young's modulus decreases with the increasing tube diameter for a reactive empirical bond order (REBO) potential function. However, we observe a contrary trend if we use other potential functions such as the modified Morse potential function and the universal force field (UFF). Such confliction is only obtained for small tubes within cutoff diameters (3 nm for REBO and 1.5 nm for others). In light of these predictions, Young's moduli of large nanotubes concur with experimental results for all the potential functions. No matter which potential function is used, the Poisson's ratio decreases with the increasing tube diameter. We also study the chirality effects on mechanical properties of SWNTs. We find that the Young's moduli are insensitive to the chirality of nanotubes. The chirality effect on the Poisson's ratio is significant for the UFF but not the REBO or modified Morse potential functions.     

Symmetry analysis of relative motions in a double-wall carbon nanotube

The binding energy of a double-wall carbon nanotube (DWNT) is theoretically studied as a function of the relative longitudinal shift and relative rotation of the component single-wall carbon nanotubes (SWNTs). It is shown that the binding energy is an oscillating function of the relative shift and rotation, with the oscillation period depending on the relations between symmetry elements of the SWNTs. The results of numerical calculations of the binding energy of DWNTs, performed in the approximation of weak van der Waals interlayer interaction, are presented.     

Atomic nanotube welders: boron interstitials triggering connections in double-walled carbon nanotubes

Here we demonstrate that the incorporation of boron (B) atoms between double-walled carbon nanotubes (DWNTs) during thermal annealing (1400-1600 degrees C) results in covalent nanotube "Y" junctions, DWNT coalescence, and the formation of flattened multiwalled carbon nanotubes (MWNTs). These processes occur via the merging of adjacent tubes, which is triggered by B interstitial atoms. We observe that B atom interstitials between DWNTs are responsible for the rapid establishment of covalent connections between neighboring tubes (polymerization), thereby resulting in the fast annealing of the carbon cylinders with B atoms embedded in the newly created carbon nanotube network. Once B is in the lattice, tube faceting (polygonization) starts to occur, and the electronic properties are expected to change dramatically. Therefore, B atoms indeed act as atomic nanotube fusers (or welders), and this process could now be used in assembling novel electronic nanotube devices, nanotube networks, carbon nanofoams and heterojunctions exhibiting p-type electronic properties.     

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.     

Synthesis and electronic properties of ferrocene-filled double-walled carbon nanotubes

Double-walled carbon nanotubes (DWNTs) are filled with ferrocene molecules by a vapour diffusion method for the first time. The as-synthesized ferrocene-filled DWNTs are characterized by transmission electron microscopy (TEM), energy-dispersive x-ray spectrometry (EDX) and Raman spectroscopy. Electronic properties of double-walled carbon nanotubes (DWNTs) filled with ferrocene molecules are studied by fabricating them as the channels of field-effect transistor (FET) devices. Our results reveal that electronic properties of ferrocene-filled DWNTs are greatly modified due to the charge transfer between ferrocene molecules and DWNTs. In addition, after ferrocene molecules are decomposed inside DWNTs, electronic properties of DWNTs exhibit a further change due to Fe encapsulation, and unipolar n-type semiconducting DWNTs are consequently obtained.     

Highly selective sorting of semiconducting single wall carbon nanotubes exhibiting light emission at telecom wavelengths

Single wall carbon nanotubes (SWNTs) are known for their exceptional electronic properties.However,most of the synthesis methods lead to the production of a mixture of carbon nanotubes having different chiralities associated with metallic (m-SWNTs) and semiconducting (s-SWNTs) characteristics.For application purposes,effective methods for separating these species are highly desired.Here,we report a protocol for achieving a highly selective separation of s-SWNTs that exhibit a fundamental optical transition centered at 1,550 nm.We employ a polymer assisted sorting approach,and the influence of preparation methods on the optical and transport performances of the separated nanotubes is analyzed.As even traces of m-SWNTs can critically affect performances,we aim to produce samples that do not contain any detectable fraction of residual m-SWNTs.     

Raman spectroscopy study of isolated double-walled carbon nanotubes with different metallic and semiconducting configurations

A double-walled carbon nanotube (DWNT) provides the simplest system to study the interaction between concentric layers in carbon nanotubes. The inner and outer walls of a DWNT can be metallic (M) or semiconducting (S), and each of the four possible configurations (M@M, M@S, S@S, S@M) has different electronic properties. Here we report, for the first time, detailed Raman spectroscopy experiments carried out on individual DWNTs, where both concentric tubes are measured under resonance conditions, in order to understand the dependence of their electronic and optical properties according to their configuration. Interestingly, for the three DWNTs that were studied, the inner-outer tube distance (e.g., 0.31-0.33 nm) was less than the interlayer spacing in graphite. We believe these results have important implications in the fabrication of electronic devices using different types of S and M tubular interconnects.     

Photoluminescence mapping of "as-grown" single-walled carbon nanotubes: a comparison with micelle-encapsulated nanotube solutions

Band gap photoluminescence (PL) is observed from "as-grown" single-walled carbon nanotubes (SWNTs) in solid form. The relative PL intensities for six specific semiconducting SWNTs are compared directly to those of the micelle-encapsulated SWNTs' solutions to investigate the influence of the micelle dispersion process on PL measurements. The results indicate that sodium dodecyl sulfate (SDS) and sodium cholate (SC) selectively solubilize smaller-diameter nanotubes, whereas sodium dodecylbenzene sulfonate (SDBS) solution does not exhibit significant diameter selectivity within the diameter range studied here (d(t) = 0.829-0.966 nm).