Laser-induced unzipping of carbon nanotubes to yield graphene nanoribbons

Irradiation of undoped and doped multi-walled carbon nanotubes by an excimer laser (energy ～200-350 mJ) yields graphene nanoribbons (GNRs). The GNRs so obtained have been characterized by transmission electron microscopy, atomic force microscopy and Raman spectroscopy.     

Graphene formation by unzipping carbon nanotubes using a sequential plasma assisted processing

We report the realization of graphene nanosheets by means of unzipping carbon nanotubes grown on silicon substrates. The formation of carbon nanotubes is possible with a gas mixture of methane and hydrogen in a direct-current plasma enhanced chemical vapor deposition reactor at a temperature of 700 °C. To avoid the undesired agglomeration of nickel islands as the catalyst layer, a hydrogen-assisted pre-treatment has been used. Vertically aligned CNTs are placed horizontally on a silicon substrate and unzipped using a sequential passivation and etching process in a reactive ion etching unit. A mixture of hydrogen, oxygen and SF₆ gases are used to result in proper unzipping of horizontal CNTs. Scanning electron microscopy, transmission electron microscopy, atomic force microscopy and Raman spectroscopy have been exploited to investigate the physical properties of the grown nano-structures. In addition, the composition of the passivation layer has been examined using energy dispersive spectroscopy. Multilayered graphene sheets with a height of 3 nm have been obtained.     

Graphene layers produced from carbon nanotubes by friction

Graphene layers have been produced from multi-walled carbon nanotube (MWCNT) bulk materials by friction when polished on ground-glass, offering a novel and effective method to produce graphene layers, which, more importantly, could be transferred to other substrates by rubbing. Field emission scanning electron microscopy, Raman spectroscopy, atomic force microscopy, transmission electron microscopy and selected area electron diffraction confirmed the formation of graphene layers. They were thought to be peeled away from the MWCNT walls due to friction. The reflection spectra showed that absorption of as-produced graphene layers decreased with wavelength in the range of 250–400 nm, compared to the MWCNT bulk material having strong absorption at 350 nm. Nanoscratch test was used to determine the mechanical properties of graphene films, suggesting the tolerance of as-produced graphene film to flaws introduced by scratch.     

Purification of single-wall carbon nanotubes produced by arc plasma jet method

In the arc plasma jet method, a large amount of soot including single-wall carbon nanotubes (SWNTs) can be produced in a short time (1–2 g/min). However, a lot of impurities, such as amorphous carbon and catalyst metals, are included in the produced soot besides SWNT. Purification is indispensable to apply SWNTs industrially, but it was difficult until recently. Here, we report that SWNTs can be purified easily in large quantities by reflux in the hydrogen peroxide solution using catalyst of iron particle, which can activate the oxidation reaction of hydrogen peroxide solution. Higher than 90 wt.% purity of SWNTs are obtained by this technique.     

Chemical-free graphene by unzipping carbon nanotubes using cryo-milling

Current paper reports synthesis of chemical free graphene by unzipping of the carbon nanotubes (CNTs) using high strain rate deformation at 150 K. A specially designed cryomill operating at 150 K was used for the experiments. The mechanism of unzipping was further explored using molecular dynamics (MD) simulations. Both experimental and simulation results reveal two modes of unzipping through radial and shear loading.     

Graphene nanoribbons (GNRs) by unzipping MWCNTs for the improvement of PMMA microcellular foams

This work presents the cellular microstructures and properties of PMMA/graphene nanoribbons (GNRs) microcellular foams. GNRs were obtained by oxidative unzipping multiwalled carbon nanotubes and solvent thermal reduction in dimethylformamide (DMF), then they were mixed with PMMA to fabricate PMMA/GNRs nanocomposites by solution blending. Subsequently, supercritical carbon dioxide (scCO₂) as a friendly foaming agent was applied to fabricate PMMA/GNRs microcellular foam by a batch foaming in a special mold. The morphology of cell structure was analyzed by scanning electron microscopy and image software, showing that the addition of a smaller content of GNRs caused a fine cellular structure with a higher cell density (～3 × 10¹¹ cells/cm³) and smaller cell sizes (～1 μm) due to their remarkable heterogeneous nucleation effect. The mechanical testing of PMMA/GNRs microcellular foams demonstrated that the obtained GNRs also could be used as a reinforcing filler to increase the mechanical properties of PMMA foams. An improvement in the compressive strength of ～80% (about 39% increase standardized by specific compressive strength) was achieved by 1.5 wt % GNRs addition, and the thermal stability of PMMA/GNRs foams was enhanced too. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45182.     

Filling single-wall carbon nanotubes

Single-wall nanotube (SWNT)-based hybrid materials represent a quite recent research field. On the basis of previous works performed on multi-wall carbon nanotubes (MWNTs), the goal is to fill the inner SWNT cavity with various compounds, whose the combination with the surrounding carbon tube is expected to allow peculiar physical phenomena to occur and/or peculiar properties to be obtained. As compared to MWNTs, regular SWNT inner cavity is really nanometric, i.e., in the dimension range where quantum phenomena could occur. However, a major drawback is that filling ∼1-nm wide tubes is less easy (i.e., compared to MWNTs), and the related driving forces not fully understood. Who is working in the field, what and how are the SWNT-based hybrid nano-materials prepared so far, what could possibly be the filling mechanisms, are questions that are discussed in this paper. A review of the existing literature is made, with a focus on C₆₀@SWNTs (peapods), which appear to be the most amazing—and the most promising—SWNT-based hybrid nano-materials to date.     

Radial followed by longitudinal unzipping of multiwalled carbon nanotubes

A hybrid structure of graphene nanoleaves attached on a carbon nanotube and rectangular graphene nanosheets (GNSs) were fabricated by means of radial followed by longitudinal unzipping of crystalline multiwalled carbon nanotubes (MWCNTs) grown by catalytic chemical vapor deposition, using a chemical method containing sulfuric acid and nitric acid. Consecutive unzipping in the radial direction and then in the longitudinal direction of a nanotube has never been reported. Our simple chemical approach will open the path to fabrication for rectangular GNSs in the hundred nanometers by few hundred nanometers scale and a broad application of MWCNTs to the novel carbon nanostructure.     

Graphene nanoribbons formed by a sonochemical graphene unzipping using flavin mononucleotide as a template

When the width of a graphene nanoribbon (GNR) is only a few nanometers, it possesses semiconducting properties that enable various high-end electronic applications. In this study, we report that the dense and stable dispersion of a natural graphite formed using flavin mononucleotide (FMN) as a surfactant produces GNRs as small as 10 nm in width. High-resolution transmission electron microscopy reveals GNRs with various widths, along with a graphene flake containing straight-edged GNRs and cuts, depending on substrate treatments. Such nanoribbon formation originates from sonochemical graphene unzipping with a one-dimensional FMN supramolecular ribbon as a template. Raman spectroscopy demonstrates the universal intensity ratio of D over D′ bands near 4, supporting formation of continuous edge defect. Thermal annealing enhances the optical contrast and van der Waals interactions of the graphene film, resulting in increased conductivity compared to the as-prepared graphene film, which is also better than that of reduced graphene oxide.     

Structure changes of single-wall carbon nanotubes and single-wall carbon nanohorns caused by heat treatment

Raman spectra and transmission electron microscope images showed that diameter enlargement of HiPco, a kind of single-wall carbon nanotube, accompanied by tube-wall corrugation was caused by heat treatment (HT) at 1000 to 1700 °C. Further enlargement accompanied by straightening of the tube walls and incorporation of carbon fragments within the tubes became obvious after HT at 1800 to 1900 °C. The transformation of some single-wall carbon nanotubes into multi-wall nanotubes was observed after HT at 2000 °C, and most single-wall tubes were transformed into multi-wall ones by HT at 2400 °C. What influence the Fe contained in the HiPco tubes had on these structure changes was unclear; similar changes were observed in single-wall carbon nanohorns that did not contain any metal. This indicates that thermally induced changes in the structure of single-wall carbon nanotubes can occur without a metal catalyst. Heat treatment increased the integrity of the nanotube-papers, and this increase may have been due to tube-tube interconnections created by HT.     

Photoluminescence intensity of single-wall carbon nanotubes

The photoluminescence (PL) intensity of a single-wall carbon nanotube (SWNT) is calculated for each (n, m) by multiplying the photon-absorption, relaxation and photon-emission matrix elements. The intensity depends on chirality and “type I vs type II” for smaller diameter semiconducting SWNTs (less than 1 nm). By comparing the calculated results with the experimental PL intensity of SWNTs prepared by chemical vapor deposition at different temperatures, we find that the abundance of (n, m) nanotubes with smaller diameters should exhibit a strong chirality dependence, which may be related to the stability of their caps.     

Chemical doping of single-wall carbon nanotubes

Single-wall carbon nanotubes can be doped, or intercalated, with electron donors or acceptors, similar to graphite and some conjugated polymers. The resulting materials show many of the same features: enhanced electrical conductivity, conduction electron paramagnetism, partial or complete reversibility, and cyclability. Reactions may be carried out in vapor or liquid phase, or electrochemically. Structural information is sketchy at best, due to the limited quality of currently available materials and solvent-related effects. Recent developments in coagulation-based fiber extrusion and partially aligned materials offer new opportunities for novel material modifications by chemical doping.     

Structure modification of single-wall carbon nanotubes

Various purification processes were applied to single-wall carbon nanotubes synthesized by metal catalyzed laser ablation. Structure modifications introduced by these processes were investigated by high-resolution transmission electron microscopy and Raman spectroscopy. An apparent structure modification after purification was the increase of bundle size although breaking of nanotubes and a change of nanotube diameter distribution were also observed. More vigorous attacking of single-wall carbon nanotube structure was identified by a strong mixed-acid treatment.     

Synthesis of Y-junction single-wall carbon nanotubes

Y-junction single-wall carbon nanotubes (SWNTs) are synthesized using controlled catalysts by chemical vapor deposition. Mo-doped Fe nanoparticles supported by aluminum oxide particles are used as catalysts for growing Y-junction single-wall carbon nanotubes. Distribution of Mo-doped Fe particles plays an important role in Y-junction formation. Transmission electron microscopy confirmed the formation of single-walled structures of Y-junctions with diameters of 2-5 nm. Radial breathing mode peaks in Raman spectra show that our sample has both metallic and semiconducting nanotubes, indicating the possible formation of Y-branching with different electrical properties. The different electrical properties of branch and stem can be utilized in nanoscale three terminal electronic devices. The growth mechanism of Y-junction SWNTs is proposed.     

Characterization of doped single-wall carbon nanotubes by Raman spectroscopy

Single-wall carbon nanotubes were grown by thermal chemical vapor deposition using either boron- or nitrogen-containing feedstocks or both. Carrier doping was evidenced by hardenings of the G band in Raman spectra, and the estimated carrier concentration reached ∼0.4%. In the G′ and D band spectra, a doping-induced component was observed at the high- or low-energy side of the original one. However, the appearance of the new component did not always coincide with the carrier doping. The doped SWCNTs often show radial breathing mode peaks in the off-resonance region, indicating a defect-induced modification of absorption spectrum.     

Characterization of single-wall carbon nanotubes by N2 adsorption

N₂ adsorption isotherms at 77 K of single-wall carbon nanotubes (SWNTs), multi-wall carbon nanotubes (MWNTs), and mixtures of these carbon nanotubes (CNTs) were analyzed for differences in their pore size distributions (PSDs). The PSDs, calculated in the microporous region by the Horvath–Kawazoe method and in the mesoporous region by the BJH method, are in agreement with the structures of both types of CNTs deduced from high-resolution transmission electron microscopy. A characteristic peak in the microporous region in the PSD of SWNTs is not present in the PSDs of MWNTs and impurities such as amorphous carbon, metal residues of catalysts, etc. The evaluation of this peak is proposed as a convenient tool for the quantitative characterization of SWNT purity in carbon nanotube-containing samples.     

Purification process for single-wall carbon nanotubes

Single-wall carbon nanotubes (SWNTs) have exceptional strength and stiffness and high thermal and electrical conductivity, making them excellent candidates for aerospace structural materials. However, one of the most fundamental challenges is purifying the SWNTs. The purpose of this study was to develop a simple purification process for SWNTs, along with an understanding of the purification process. In addition, uncomplicated analytical methods were sought to screen and compare various purification methods. In this study, we demonstrate an easy method of cleaning SWNTs and evaluating their purity. The cleaning method, which employed oxidative heat treatment followed by acid reflux, was straightforward, inexpensive, and fairly effective. The purification mechanism was determined to be, first, that much of the non-nanotube carbon and iron catalyst was oxidized and, second, that the acid washing removed the iron oxide, leaving relatively pure SWNTs. Also, it was shown that a combination of thermal gravimetric analysis and Raman spectroscopy, both of which take only a few minutes and require little sample preparation, are sufficient as qualitative screening tools to determine the relative purity of SWNTs. Other analytical techniques were used to verify the validity of the screening techniques.     

Solid-liquid-solid growth mechanism of single-wall carbon nanotubes

Reasons are presented which suggest that the liquefaction of the catalytic particles is a decisive condition for formation of single wall carbon nanotubes (SWNTs) by physical synthesis techniques. It is argued that the SWNT growth mechanism is a kind of solid-liquid-solid graphitization of amorphous carbon or other imperfect carbon forms catalyzed by molten supersaturated carbon-metal nanoparticles. The assumption of low temperature melting of these nanoparticles in contact with amorphous carbon followed by its precipitation in the form of SWNTs allows to explain qualitatively the experimentally observed SWNT growth rates and temperature dependence of the SWNT yield. Guidelines for increasing SWNT yield are proposed.