Dendrimer-assisted low-temperature growth of carbon nanotubes by plasma-enhanced chemical vapor deposition

Using a shielded growth approach and N2-annealed, nearly monodispersed Fe2O3 nanoparticles synthesized by interdendritic stabilization of Fe3+ species within fourth-generation poly(amidoamine) dendrimers, carbon nanotubes and nanofibers were successfully grown at low substrate temperatures (200-400 degrees C) by microwave plasma-enhanced chemical vapor deposition.     

Low-temperature growth of single-walled carbon nanotubes by water plasma chemical vapor deposition

Preferential growth of pure single-walled carbon nanotubes (SWNTs) over multi-walled carbon nanotubes (MWNTs) was demonstrated at low temperature by water plasma chemical vapor deposition. Water plasma lowered the growth temperature down to 450 degrees C, and the grown nanotubes were single-walled without carbonaceous impurities and MWNTs. The preferential growth of pure SWNTs over MWNTs was proven with micro-Raman spectroscopy, high-resolution transmission electron microscopy, and electrical characterization of the grown nanotube networks.     

Preferential growth of single-walled carbon nanotubes on silica spheres by chemical vapor deposition

The preferential growth of single-walled carbon nanotubes (SWNTs) on silica spheres with various diameters was realized for the first time by chemical vapor deposition (CVD) of methane. SWNTs tend to wrap the silica spheres to form a new superstructure of uniform SWNT nanoclaws when the diameters of the silica spheres are larger than 400 nm. The SWNTs obtained on silica spheres have highly graphitic tubular walls as characterized by Raman spectroscopy and HRTEM. This is a new method to obtain tunable uniform elastic deformation of SWNTs, which may act as the model for the study about the effect of delocalized bending on the properties of SWNTs. In addition, the combination of SWNTs with monodispersed silica spheres could conveniently integrate SWNTs into photonic crystals.     

Study of the quality of carbon nanotubes produced by chemical vapor deposition

Method for obtaining carbon nanotubes by the method of chemical vapor deposition with varied amount of catalyst, reaction duration, and temperature is considered. The synthesis of carbon nanotubes with various mode parameters was experimentally studied. The quality of the carbon nanotubes was examined by Raman spectroscopy. It was found that the defectiveness of the carbon nanotubes depends on the synthesis parameters.     

Combinatorial synthesis of carbon nanotubes by the catalytic chemical vapor deposition method

Summary Bimetallic (Fe-Co) catalyst samples prepared from different precursors over various supports were tested in carbon nanotube (CNT) production. In order to quicken the evaluation of the performance of the catalysts a combinatorial arrangement was used.</o:p>     

Controlling the diameter of carbon nanotubes in chemical vapor deposition method by carbon feeding

It was found that the diameter distribution of single-walled carbon nanotubes (SWNTs) grown by the chemical vapor deposition (CVD) method could be controlled by the carbon feeding rate at the growth stage. A unified hypothesis on the relationship between nanoparticle size, growth condition, growth temperature, and diameter of the resulting nanotubes was developed and used to explain the relationship. It was shown that the diameters of SWNTs can be controlled even when highly polydisperse nanoparticles were used as catalyst. Such control enabled us to synthesize uniform small-diameter SWNTs at low carbon feeding rates. Additionally, understanding of the important role of the carbon feeding rate can be used to explain the cause of low growth efficiency in most CVD processes. It would also help us to design methods to improve the growth efficiency of CVD growth of nanotubes.     

Formation and growth mechanism of dissimilar coiled carbon nanotubes by reduced-pressure catalytic chemical vapor deposition

Dissimilar coiled carbon nanotubes were prepared by catalytic chemical vapor deposition (CCVD) on finely divided Co nanoparticles supported on silica gel under reduced pressure and at lower gas flow rates. The morphology of the regular coiled carbon nanotubes were examined by TEM, while the polygonization characteristics of the helix were examined by SAED. Observations were made on other forms of irregular coils with various shapes by TEM. On the basis of the heptagon-pentagon construction theory, we proposed a helix formation mechanism, which involves a carbon core formation centering on a catalytic particle followed by carbon helices growth controlled by kinetics.     

Influence of the catalyst type on the growth of carbon nanotubes via methane chemical vapor deposition

The preparation of the catalyst is one of the key parameters which governs the quality of carbon nanotubes (CNTs) grown by catalyzed chemical vapor deposition (CVD). We investigated the influence of three different procedures of catalyst preparation on the type and diameter of CNTs formed under identical growth conditions via methane CVD. In the first one, chemically synthesized colloidal iron oxide or iron molybdenum alloy nanoparticles were used, which were homogeneously deposited on silicon substrates by spin coating to prevent them from coalescence under CVD growth conditions. The obtained multiwall CNTs (MWNTs) exhibited diameters corresponding to the catalyst particle size, whereas no formation of single-wall CNTs (SWNTs) was observed. In the second method, commercial porous alumina nanoparticles were used in association with iron and molybdenum salts and the Fe/Mo catalyst was formed in situ. We determined that the alumina concentration significantly influenced the morphology of the catalyst and that below a critical value of the range of 1 g/L no CNTs were formed. While yielding nearly defect-free SWNTs, their diameter could not be controlled using this procedure, resulting in a large distribution of tube sizes. In a third, new preparation method, associating alumina and iron-based nanoparticles, SWNTs of a different size and narrower diameter distribution as compared to the second method were obtained. Our results are evidence of the essential role of alumina particles in the formation of SWNTs, and the newly developed method opens up a way to the synthesis of diameter-controlled SWNTs via catalyzed CVD.     

Selected-area growth of carbon nanotubes by the combination of focused ion beam and chemical vapor deposition techniques.

In this article, we report a technique for growing carbon nanotubes in a more controllable fashion, which enables us to synthesize nanotubes directly in various forms of designed patterns. This nanofabrication process is based on a combination of focused ion beam (FIB) and chemical vapor deposition (CVD) techniques. In this process, arrays of conductive patterns were first deposited on silicon substrates by directing a gaseous compound (C(9)H(16)Pt) via the capillary needle-sized nozzles within a FIB system. The substrates were then coated with catalyst and further modified by the FIB to localize the position of the catalyst. Finally, the growth of carbon nanotubes on the designed substrates was carried out by CVD of hydrocarbon gases. This fabrication technique has the advantage of positioning carbon nanotubes in selected locations. This may open up opportunities for the direct synthesis of carbon nanotubes onto almost any substrate material, thus allowing fabrication of carbon nanotube-based devices.     

Effect of processing temperature on growing bamboo-like carbon nanotubes by chemical vapor deposition

The present article demonstrates a simple, eco-friendly route for the fabrication of carbon nanotubes (CNTs) with different morphologies, including the fascinating bamboo-like structures without complex catalyst/support preparation procedures. A thermal chemical vapor deposition (CVD) technique that utilized natural pozzolan supports and a solid carbon source, that is, a mixture of camphor and ferrocene in a weight ratio of 20:1, was carried out at different temperatures where the ferrocene played also the role of catalyst. The pozzolan chemical composition and mineral identification were determined by energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. The morphology of the fabricated CNTs was studied via scanning and transmission electron microscopies (SEM and TEM). It was revealed that both conventional tubular and bamboo-like nanotubes grow at 750 °C while the bamboo-like morphology prevails at 850 °C. The better nanostructure uniformity at higher deposition temperature was accompanied by an improved nanotube graphitization degree that was verified by Raman spectroscopy. Yet, the reduction of the CNTs production yield was recorded by thermogravimetric analysis (TGA). The experimental data are interpreted and discussed as an interplay between the CNTs processing temperature, morphology and growth mechanism. Thus, the growth of either tubular or bamboo-like nanostructures is suggested to be ruled by the competitive surface and bulk diffusions of carbon onto and into the catalyst surface. The growth depends on the size of catalyst nanoparticles sintered at different temperatures. The favorable role of the pozzolan supporting materials in the formation of bamboo-like tubes is emphasized.     

Direct evidence for root growth of vertically aligned single-walled carbon nanotubes by microwave plasma chemical vapor deposition

The root growth mode of extremely dense and vertically aligned single-walled carbon nanotubes (SWNTs) synthesized by microwave plasma chemical vapor deposition was clarified by a new method, marker growth, which does not require transmission electron microscopy. SWNT layers were grown intermittently on a substrate, and a line between the layers was used as a marker to identify the growth mode. Micro-Raman spectroscopy revealed that the SWNT layers have the same diameter distribution.     

Model of carbon nanotube growth through chemical vapor deposition

This Letter outlines a model to account for the catalyzed growth of nanotubes by chemical vapor deposition. It proposes that their formation and growth is an extension of other known processes in which graphitic structures form over metal surfaces at moderate temperatures through the decomposition of organic precursors. Importantly, the model also states that the form of carbon produced depends on the physical dimensions of the catalyzed reactions. Experimental data are presented that correlate nanotube diameters to the size of the catalyst particles. Nanotube stability as a function of nanotube type, length and diameter are also investigated through theoretical calculations.     

Selective chemical vapor deposition synthesis of double-wall carbon nanotubes on mesoporous silica

Double-wall carbon nanotubes (DWNTs) have been selectively synthesized over Fe/Co loaded mesoporous silica by catalytic chemical vapor deposition of alcohol. Several silica materials with desired pore diameter and morphology have been investigated for the DWNT growth. The diameter distribution and selectivity of the DWNT are found to depend on the reaction temperature, pore size, and thermal stability of the support material. A high-yield synthesis of DWNTs has been achieved at 900 degrees C over high-temperature stable mesoporous silica. The outer diameter of DWNTs is found to be in the range of 1.5-5.4 nm with a "d" spacing of 0.38 +/- 0.02 nm between inner and outer layers, which is much larger than those of multiwall carbon nanotubes.     

Cobalt ultrathin film catalyzed ethanol chemical vapor deposition of single-walled carbon nanotubes

We report a simple and efficient chemical vapor deposition (CVD) process that can grow oriented and long single-walled carbon nanotubes (SWNTs) using a cobalt ultrathin film ( approximately 1 nm) as the catalyst and ethanol as carbon feedstock. In the process, millimeter- to centimeter-long, oriented and high-quality SWNTs can grow horizontally on various flat substrate surfaces, traverse slits as large as hundreds of micrometers wide, or grow over vertical barriers as high as 20 microm. Such observations demonstrate that the carbon nanotubes are suspended in the gas flow during the growth. The trace amount of self-contained water (0.2-5 wt %) in ethanol may act as a mild oxidizer to clean the nanotubes and to elongate the lifetime of the catalysts, but no yield improvement was observed at the CVD temperature of 850 degrees C. We found that tilting the substrates supporting the Co ultrathin film catalysts can grow more, longer carbon nanotubes. A mechanism is discussed for the growth of long SWNTs.     

Magic carbon clusters in the chemical vapor deposition growth of graphene

Ground-state structures of supported C clusters, C(N) (N = 16, ..., 26), on four selected transition metal surfaces [Rh(111), Ru(0001), Ni(111), and Cu(111)] are systematically explored by ab initio calculations. It is found that the core-shell structured C(21), which is a fraction of C(60) possessing three isolated pentagons and C(3v) symmetry, is a very stable magic cluster on all these metal surfaces. Comparison with experimental scanning tunneling microscopy images, dI/dV curves, and cluster heights proves that C(21) is the experimentally observed dominating C precursor in graphene chemical vapor deposition (CVD) growth. The exceptional stability of the C(21) cluster is attributed to its high symmetry, core-shell geometry, and strong binding between edge C atoms and the metal surfaces. Besides, the high barrier of two C(21) clusters' dimerization explains its temperature-dependent behavior in graphene CVD growth.     

Synthesis of double-walled carbon nanotubes by catalytic chemical vapor deposition and their field emission properties

Double-walled carbon nanotubes (DWCNTs) were synthesized by catalytic chemical vapor deposition using Fe-Mo/MgO as a catalyst at 1000 degrees C under the mixture of methane and hydrogen gas. The nanotubes were purified by acid but were not damaged. Thermogravimetric analysis revealed the purity of the tubes to be about 90%. The high-resolution transmission electron microscopy image showed that DWCNTs have inner tube diameters of 1.4-2.6 nm and outer tube diameters of 2.3-3.4 nm. We observed radial breathing modes in Raman spectra, which are related to the diameter of inner nanotubes. The purified DWCNTs were mixed with organic vehicles and glass frit, and then they were screen-printed on glass substrate coated with indium tin oxide. The field emission properties of the screen-printed DWCNT films were examined by varying the amount of glass frit ingredient within the DWCNT paste. The results showed that DWCNT emitters had good emission properties such as turn-on field of 1.33-1.78 V/microm and high brightness. When the applied anode voltage was gradually increased, current density and brightness became saturated. We also observed DWCNTs adsorbed on the anode plate; they were DWCNTs peeled off from the cathode plate for field emission measurement.     

A magnetism-assisted chemical vapor deposition method to produce branched or iron-encapsulated carbon nanotubes

A magnetism-assisted chemical vapor deposition method was developed to synthesize branched or iron-encapsulated carbon nanotubes. In the process, the external magnetic field can promote the coalescence or division of the catalyst particles, causing the formation of branched or encapsulated nanostructures. This finding will extend the understanding of the chemical vapor deposition method in a magnetic field and promote the applications of branched or encapsulated nanostructures.     

Bulk synthesis of large diameter semiconducting single-walled carbon nanotubes by oxygen-assisted floating catalyst chemical vapor deposition

Semiconducting single-walled carbon nanotubes (s-SWCNTs) with a mean diameter of 1.6 nm were synthesized on a large scale by using oxygen-assisted floating catalyst chemical vapor deposition. The oxygen introduced can selectively etch metallic SWCNTs in situ, while the sulfur growth promoter functions in promoting the growth of SWCNTs with a large diameter. The electronic properties of the SWCNTs were characterized by laser Raman spectroscopy, absorption spectroscopy, and field effect transistor measurements. It was found that the content of s-SWCNTs in the samples was highly sensitive to the amount of oxygen introduced. Under optimum synthesis conditions, enriched s-SWCNTs can be obtained in milligram quantities per batch.     

Enhancing the pyrolysis of scrap rubber for carbon nanotubes/graphene production via chemical vapor deposition

Carbon nanostructures are considered nowadays as very important materials for both fundamental research and industrial applications because of their well-defined morphologies, which leads to excellent performance in various fields. This study presents the preparation of carbon nanostructures starting from cheap source represented by scrap rubber after pursuing optimized pyrolysis of scrap rubber at 500^oC as deduced from thermal gravimetric analysis (TGA). The resulting cracked hydrocarbons from pyrolysis were collected over a well-designed Fe-Ni-Cu/MgO as catalyst via chemical vapor deposition (CVD), in which a growth temperature of 750^oC was undertaken for 60 min. A further attempt was elaborated where the scrap rubber was exposed to thermal aging at 90^oC for 14 days prior to CVD of its pyrolysis products in order to enhance the cracking process and increase the yield of the lighter hydrocarbons produced which leads to formation of well-defined carbon nanostructures. Characterizations on the produced carbon nanostructures were achieved using transmission electron microscopy (TEM) and Raman spectroscopy. The adsorption of methylene blue on the carbon nanostructures was also studied. The characterizations confirmed that the morphology of the resulting carbon nanostructures derived from scrap rubber without prior thermal aging composed of graphene sheets wrapping carbon nanotubes (CNTs-A). After thermal aging of scrap rubber prior to pyrolysis and CVD, the produced carbon nanostructures composed principally of CNTs (CNTs-B) in a well-defined form in higher yield. The Langmuir model appeared to be best-fitting the adsorption of MB on both samples. High monolayer adsorption capacity of 95 mg MB/g was accomplished in case of CNTs-A versus 60 mg MB/g in case of CNTs-B, respectively. Ultraviolet-Visible (UV-Vis.) spectroscopic study revealed that the presence of MB molecules on the surface of CNTs may enhance the electronic properties of the prepared samples.     

Polymer decoration on carbon nanotubes via physical vapor deposition

The polymer decoration technique has been widely used to study the chain folding behavior of polymer single crystals. In this article, we demonstrate that this method can be successfully adopted to pattern a variety of polymers on carbon nanotubes (CNTs). The resulting structure is a two-dimensional nanohybrid shish kebab (2D NHSK), wherein the CNT forms the shish and the polymer crystals form the kebabs. 2D NHSKs consisting of CNTs and polymers such as polyethylene, nylon 66, polyvinylidene fluoride and poly(L-lysine) have been achieved. Transmission electron microscopy and atomic force microscopy were used to study the nanoscale morphology of these hybrid materials. Relatively periodic decoration of polymers on both single-walled and multi-walled CNTs was observed. It is envisaged that this unique method offers a facile means to achieve patterned CNTs for nanodevice applications.