Chiral angle-dependent defect evolution in CVD-grown single-walled carbon nanotubes

Defects are ubiquitous in nanomaterials and it is critical to understand and control defect densities in these materials for electronic, chemical, and mechanical applications. Until now the relationship between nanomaterial structure and defect density during synthesis was limited to theoretical studies with no experimental confirmation of the predictions. Here we study defect evolution during the synthesis of individual single-walled carbon nanotubes (SWCNTs) using in situ Raman spectroscopy. SWCNTs are an important class of nanomaterials, and offer the unique ability to study the effect of their chiral angle on defect evolution during growth – a widely explored theoretical area that still lacks experimental confirmation. Our data reveals the first experimental evidence of chiral angle dependence on the defect density in SWCNTs, with lower defect density for higher chiral angle SWCNTs despite their faster growth rate. Modeling of the kinetics of defect generation reveals formation energy as the critical factor driving steady-state defect densities, with higher formation energies for topological defects in higher chiral angle SWCNTs and lower energies for low chiral angle SWCNTs.     

Effect of catalyst combination on growth of single-walled carbon nanotubes

A new effective catalysts combination of iron — nickel for alcohol CVD technique was found. This catalyst catalyzed well as well as the typical catalyst of iron — cobalt catalysts, but gave a different diameter distribution. Calculating their electrical density of states under the assumption of their solid lattice structures, the result was fairly consistent with experimental results. The number of electrical states near Fermi level that is considered to be important for catalytic reaction is enough and the DOS of iron – nickel catalyst was quite similar to that of cobalt unlike manganese – copper catalyst. Consequently, a blend of catalysts that has a similar DOS to cobalt and has enough states near the Fermi level can be a good catalyst for alcohol CVD.     

Chiral-selective growth of single-walled carbon nanotubes on stainless steel wires

Single-walled carbon nanotubes (SWCNTs) were successfully grown on calcined stainless steel wires at 700 °C using CO as the carbon source. By contrast, the raw stainless steel wires produced only necklace-like multi-walled carbon nanotubes. Photoluminescence spectroscopy studies showed that SWCNTs grown from calcined stainless steel have a narrow diameter distribution and a high chiral selectivity of (6,5) nanotubes. The pre-growth heat treatment of the stainless steel leads to formation of iron and chromium oxides. The reduction of iron oxide results in formation of Fe nanoparticles which, anchored by chromium oxide, account for the chiral-selective growth of SWCNTs.     

Gadolinium and europium catalyzed growth of single-walled carbon nanotubes

The inner transition metals, gadolinium (Gd) and europium (Eu) have been shown to catalyze the growth of single-walled carbon nanotubes (SWCNTs) using chemical vapor deposition. The Gd and Eu nanocatalysts, prepared using a diblock copolymer templating method and characterized by atomic force microscopy, were uniformly spaced over a large deposition area with an average diameter of 1.9 nm and narrow size distribution. Characterization by transmission electron microscopy and Raman spectroscopy confirms the presence of SWCNTs catalyzed by Gd and Eu with an average diameter of 2.05 nm.     

Ferrocene-filled single-walled carbon nanotubes

Ferrocene molecules are successfully introduced into the inner hollow space of Single-walled carbon nanotubes (SWNTs) to get ferrocene-filled SWNTs (Fc@SWNTs). This nanohybrid material was carefully characterized by high resolution microscopy, FTIR spectrum, and Cyclic voltammetry (CV). This new material may not only act as air stable n-type field-effect transistors based on nanotubes, but it may also be employed as building blocks for various devices based on the redox activity of ferrocene. What’s more, upon high temperature annealing, the encapsulated ferrocene molecules will decompose and change into interior tubes, forming double-walled carbon nanotubes (DWNTs). This provides convincing evidence that ferrocene molecules are inserted into the hollow cavities SWNTs. This result also presented a controllable way to synthesize DWNTs.     

Lattice-directed growth of single-walled carbon nanotubes with controlled geometries on surface

How to control the orientations of single-walled carbon nanotubes (SWCNTs) on surface is the key point to controlling their geometries. In this work, we chose quartz (0 0 1), MgO (0 0 1) and layered mica with 3-, 4- and 6-fold symmetry, respectively as substrates to grow SWCNTs using gas-flow and lattice-directed modes. The produced SWCNTs were aligned along the symmetrical directions and displayed the homologous angles of 120°, 90° and 60° during growth on quartz (0 0 1), MgO (0 0 1) and mica surfaces, respectively. The obtained SWCNTs with controlled geometries would have wide applications in nanoelectronic devices in the future.     

Chemistry of single-walled carbon nanotubes

In this Account we highlight the experimental evidence in favor of our view that carbon nanotubes should be considered as a new macromolecular form of carbon with unique properties and with great potential for practical applications. We show that carbon nanotubes may take on properties that are normally associated with molecular species, such as solubility in organic solvents, solution-based chemical transformations, chromatography, and spectroscopy. It is already clear that the nascent field of nanotube chemistry will rival that of the fullerenes.     

Formylation of single-walled carbon nanotubes

Formyl or aldehyde groups are transferred to the surface of single-walled carbon nanotubes (SWCNTs) by reaction of reduced carbon nanotubes with N-formylpiperidine. This could open the way for more versatile chemical modification reactions of carbon nanotubes than is currently possible using functionalization methods reported to date. The formylated SWCNTs were characterized by thermogravimetric analysis-mass spectrometry and Raman, UV-vis-NIR and FTIR spectroscopy. The location and distribution of the functional groups was determined by AFM using electrostatic interactions with gold nanoparticles. The formylated SWCNTs were further derivatized with a fluorescent dye and studied using fluorescence spectroscopy.     

Promotion of cell growth with magnetically enhanced single-walled carbon nanotubes

We demonstrate that purified and functionalized single walled carbon nanotubes (SWNTs) promote the growth of NIH3T3 mouse fibroblast cells under a magnetic field. The SWNTs are functionalized in acidic solutions by attaching carboxyl groups (–COOH) on their surfaces. Functionalized SWNTs (fSWNTs) exhibit a ferromagnetic property when dispersed in water. Cytotoxicity after the delivery of the fSWNTs into the cells is significantly reduced due to the complete removal of toxic metallic impurities during the functionalization process. The efficient uptake of the fSWNTs by the cells is confirmed through transmission electron microscopy (TEM). It is discovered that the growth of the NIH3T3 cells treated with the fSWNTs is enhanced by up to 25% than control cells when an external magnetic field is applied. Our findings may lead to the non-invasive and non-toxic drug delivery as well as targeted cell therapy with fSWNTs.     

Influence of carbon structure of the anode on the synthesis of single-walled carbon nanotubes in a carbon arc plasma

Arc plasma evaporation of carbon electrodes doped with various catalysts is one of the most effective methods of single-walled carbon nanotube fabrication. It was found that the reaction yield is strongly influenced not only by the appropriate choice of the catalyst(s), but also by the type of carbon material used for electrode fabrication. Several different carbon powders i.e. graphite powders, glassy carbon and coke, have been tested in order to establish which parameters (primary particle size, granulation, density or conductivity of the electrode) affected the outcome of the reaction the most. The highest yield of single-walled nanotubes was found for anodes fabricated from graphite powders, whilst the electrodes made from glassy carbon or coke yielded significantly smaller amounts of nanotubes. The reaction zone where carbon radicals nucleate (close to the arc gap) was probed by optical absorption spectroscopy. The estimated temperature distributions and contents of C₂ radicals did not depend on the anode characteristics.     

Controlling the growth of single-walled carbon nanotubes on surfaces using metal and non-metal catalysts

Thanks to the development of controlled synthesis techniques, carbon nanotubes, a 20-year-old material, are doing better at finding practical applications. The history of carbon nanotube growth with controlled structure is reviewed. There have been two main categories of catalysts used for carbon nanotube growth, metal and non-metal. For the metal catalysts, the growth process and the mechanism involved have been adequately discussed, with a widely accepted vapor–liquid–solid growth mechanism. The strategies for preparing single-walled carbon nanotube samples with well-defined structures such as geometry, length and diameter, electronic property, and chirality have been well developed based on the proposed mechanism. However, a clear mechanism is still being explored for non-metal catalysts with a hypothesis of a vapor–solid growth mechanism. Accordingly, the controlled growth of carbon nanotubes with a non-metal catalyst is still in its infancy. This review highlights the structure-control growth approach for carbon nanotubes using both metal and non-metal catalysts, and tries to give a full understanding of the possible growth mechanisms.     

Effects of catalyst pre-treatment on the growth of single-walled carbon nanotubes by microwave CVD

Layers of carbon nanotubes were deposited by microwave CVD on oxidized silicon substrates coated with Al-Fe-Mo catalyst films. To achieve a tube growth at about 973 K, the ion bombardment of the catalyst surface has to be avoided. The appropriate pre-treatment of the substrates is essential for the deposition of single-walled carbon nanotubes. Annealing in air is preferable to the frequently used reducing pre-treatment prior to the deposition as a higher area density of the tubes and a better reproducibility of deposition can be obtained. To figure out this finding, selected samples were investigated by analytical transmission electron microscopy and Raman spectroscopy. It is shown that the pre-treatment has a strong effect on the size and distribution of the catalyst particles.     

Hydrogen storage on Li-doped single-walled carbon nanotubes: Computer simulation using the density functional theory

The first principal calculation based on the density functional theory was performed to investigate the hydrogen storage behavior of Li-doped single-walled carbon nanotubes (SWCNTs). It was found that, through Li-doping, two new adsorption sites for hydrogen molecules are created in addition to the inherent three adsorptive sites which are exterior, interior and interstitial regions of pristine SWCNTs: the first site (denoted ‘region 1’) is the nanotube's sidewall whose electronic distribution status is influenced by the doped Li atoms. The second site (denoted ‘region 2’) exists on the positively charged Li atoms which result from the transfer of electrons from the Li atoms to the SWCNTs. The calculations show that although the adsorption energy in region 1 increases somewhat, the adsorption behavior of hydrogen is marginally different from that of pristine SWCNTs. However, in region 2, at least three hydrogen molecules can be adsorbed by each charged Li-atom, and based on the maximum Langmuir coverage (of 0.55), 1.1 hydrogen molecules can be adsorbed onto each charged Li-atom. When this result is considered together with the effective specific surface area, the hydrogen storage capacities of Li-doped SWCNTs with the doping ratio of LiC₁₅ are approximately 0.1 wt% in region 1 and 1.17 wt% in region 2 at 10 MPa and 300 K so that the total H₂ storage capability is 1.27 wt%, which agrees well with previously reported results.     

Torsional behavior of single-walled carbon nanotubes

Atomistic simulations were performed to investigate the deformation behavior of single-walled carbon nano-tubes (SWCNTs) under torsional loading. The evolutions of the potential energy and stresses were presented. Radial distribution functions (RDFs) were calculated to analyze structural evolution during torsional deformation. The results show that during torsion, the tensile stress component along the tube axis is most significant and other stress components are almost negligible. The tensile stress stretched the C–C bonds until they reached the bond length of 0.18 nm. The torsional strength of the SWCNTs is about 30% of the tensile strength. Buckling took place at a few degrees of torsional angle and propagated along the tube as the torsional angle increased, and collapse of the tube wall followed buckling. These structural evolutions can be well described with the RDFs. Two new peaks appeared at 0.21 nm and 0.18 nm in the RDFs, corresponding to the minimum spacing between the atoms in the collapsed layers, and the maximum bond length that can be reached in stretching before rupture.     

Electron-induced cutting of single-walled carbon nanotubes

Electron beam irradiation with moderate fluences of approximately 10¹⁶-10¹⁷ electrons per cm² is used for controllable, bulk-scale cutting of single-walled carbon nanotubes (SWCNTs). The effectiveness of high energy electron irradiation in cutting SWCNTs is dependent on the nature of the sidewall. While pristine nanotubes are very stable under irradiation conditions, ozonated SWCNTs combined with a moderate fluence of electrons resulted in bulk-scale cutting of nanotubes. The length distribution of the cut SWCNTs could be controlled by adjusting the irradiation fluence. The average length of the cut nanotubes was 65 nm with 85% of the nanotubes shorter than 100 nm.     

Photophysics of individual single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are cylindrical graphitic molecules that have remained at the forefront of nanomaterials research since 1991, largely due to their exceptional and unusual mechanical, electrical, and optical properties. The motivation for understanding how nanotubes interact with light (i.e., SWNT photophysics) is both fundamental and applied. Individual nanotubes may someday be used as superior near-infrared fluorophores, biological tags and sensors, and components for ultrahigh-speed optical communications systems. Establishing an understanding of basic nanotube photophysics is intrinsically significant and should enable the rapid development of such innovations. Unlike conventional molecules, carbon nanotubes are synthesized as heterogeneous samples, composed of molecules with different diameters, chiralities, and lengths. Because a nanotube can be either metallic or semiconducting depending on its particular molecular structure, SWNT samples are also mixtures of conductors and semiconductors. Early progress in understanding the optical characteristics of SWNTs was limited because nanotubes aggregate when synthesized, causing a mixing of the energy states of different nanotube structures. Recently, significant improvements in sample preparation have made it possible to isolate individual nanotubes, enabling many advances in characterizing their optical properties. In this Account, single-molecule confocal microscopy and spectroscopy were implemented to study the fluorescence from individual nanotubes. Single-molecule measurements naturally circumvent the difficulties associated with SWNT sample inhomogeneities. Intrinsic SWNT photoluminescence has a simple narrow Lorentzian line shape and a polarization dependence, as expected for a one-dimensional system. Although the local environment heavily influences the optical transition wavelength and intensity, single nanotubes are exceptionally photostable. In fact, they have the unique characteristic that their single molecule fluorescence intensity remains constant over time; SWNTs do not "blink" or photobleach under ambient conditions. In addition, transient absorption spectroscopy was used to examine the relaxation dynamics of photoexcited nanotubes and to elucidate the nature of the SWNT excited state. For metallic SWNTs, very fast initial recovery times (300-500 fs) corresponded to excited-state relaxation. For semiconducting SWNTs, an additional slower decay component was observed (50-100 ps) that corresponded to electron-hole recombination. As the excitation intensity was increased, multiple electron-hole pairs were generated in the SWNT; however, these e-h pairs annihilated each other completely in under 3 ps. Studying the dynamics of this annihilation process revealed the lifetimes for one, two, and three e-h pairs, which further confirmed that the photoexcitation of SWNTs produces not free electrons but rather one-dimensional bound electron-hole pairs (i.e., excitons). In summary, nanotube photophysics is a rapidly developing area of nanomaterials research. Individual SWNTs exhibit robust and unexpectedly unwavering single-molecule fluorescence in the near-infrared, show fast relaxation dynamics, and generate excitons as their optical excited states. These fundamental discoveries should enable the development of novel devices based on the impressive photophysical properties of carbon nanotubes, especially in areas like biological imaging. Many facets of nanotube photophysics still need to be better understood, but SWNTs have already proven to be an excellent starting material for future nanophotonics applications.     

单壁碳纳米管分离研究

根据导电性能不同,单壁碳纳米管可以分为金属型和半导体型。目前所有方法制备的单壁碳纳米管,其产物为金属型和半导体型单壁碳纳米管的混合物,且很难将它们分开,这极大地阻碍了单壁碳纳米管在很多领域的应用。本文介绍了单壁碳纳米管的结构与导电性能的关系,着重综述了最新金属型和半导体型单壁碳纳米管的分离方法。