Well-aligned Carbon Nanotubes Research Articles

Field Emission Current Uniformity and Stability of Well-Aligned Carbon Nanotubes Synthesized in Methanol

Field emission current uniformity and the stability of well-aligned carbon nanotubes (CNTs) synthesized in methanol have been evaluated. For the growth of CNTs, a cobalt-loaded silicon substrate was heated by applying a direct current through the substrate in methanol. Well-aligned CNTs were grown vertically on the substrates at temperatures of 600 and 900 °C. The uniformity and the stability of field electron emission from the CNTs were measured with a scanning probe field emission current (SPFEC) system. The CNTs grown at 900 °C, which had higher crystallinity according to laser Raman spectroscopic analysis than those grown at 600 °C, showed excellent uniformity and high stability of the emission current. Over all measured areas of 2 ×2 mm2 on the well-aligned CNTs grown at 900 °C, a uniform emission current larger than 100 nA was obtained at an anode voltage of 280 V, when the gap from the CNTs to the anode was 30 µm. Emission current stability with a range of 5 to 6 µA at an anode voltage of 300 V was observed in the CNTs grown at 900 °C.

Self-Assembled Monolayer-Assisted Chemical Transfer of In Situ Functionalized Carbon Nanotubes

A low-temperature flexible process, named "chemical transfer", was developed to assemble well-aligned carbon nanotube (ACNT) structures onto various substrates. The technology was featured by (1) in situ functionalization of ACNTs with reactive functional groups during the CVD process and (2) covalently bonded interface with a self-assembled monolayer (SAM) of conjugated thiol molecules as the bridging ligand and conduction path at the ACNT/gold interface. The effectiveness of the in situ functionalization was characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). I-V response and the interfacial strength of the chemically transferred structure were studied. Results showed an Ohmic contact, low electrical resistivity, and improved CNT-substrate adhesion. This novel technique shows promising applications for positioning ACNTs as electrical interconnects or thermal interface materials on temperature-sensitive substrates.

High Molecular Weight Polyethylene Nanospheres: Synthesis, Physical and Mechanical Properties—Second Harmonic Generation

The weak second harmonic light generating from carbon nanotubes are detected. The signal intensity closely related to the density of pi-bonds attributed to the defects in the rolled graphene sheets, which is stimulated to have anharmonic oscillation as strongly affected by the environment. The intensities of SHG are diminished in order of well-aligned multi-wall carbon nanotubes (MWCNTs), randomly-aligned MWCNTs, and then to single-wall CNTs.

High dispersion of γ-MnO 2 on well-aligned carbon nanotube arrays and its application in supercapacitors

The well-aligned carbon nanotube arrays (ACNTs) were used as supporting material and the γ-MnO 2/ACNT electrode with high dispersion of γ-MnO 2 has been prepared by electrochemically induced deposition method. The crystal structure and morphology of the γ-MnO 2/ACNT electrode were investigated by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The capacitive properties of γ-MnO 2/ACNT electrode were characterized by cyclic voltammetry and galvanostatic charge–discharge method. The specific capacitance of the γ-MnO 2/ACNT electrode is as high as 784 F g − 1 based on γ-MnO 2 and 234 F g − 1 based on γ-MnO 2/ACNT composites in 0.1 M Na 2SO 4 aqueous solution from 0 to 1 V when the charge–discharge current density is 1 mA cm − 2 . Additionally, the electrode shows excellent power characteristics, high electrochemical reversibility and excellent long-term charge–discharge cycle stability.

Optical polarizer made of uniaxially aligned short single-wall carbon nanotubes embedded in a polymer film

An intrinsic characteristic of carbon nanotubes (CNTs), which are allotropes of carbon, is one-dimensional anisotropy. It is derived from the peculiar geometrical shape of the molecules and is manifested in their electrical, thermal, mechanical, and optical properties. Here, we report an optical polarizer that makes effective use of such anisotropy of CNTs. The polarizer we fabricated is made of uniaxially aligned CNTs embedded in a polymer film. Thanks to the $\ensuremath{\pi}$ plasmon-originated broad absorption spectrum and strong optical anisotropy of single-wall CNTs (SWCNTs), the film exhibits a degree of polarization of $\ensuremath{\sim}90%$ with keeping flat transmittance through the spectral region from 350 to 800 nm. In order to efficiently obtain well-aligned CNTs, we used SWCNTs shortened into a length of less than 200 nm. We also observed enhancement of the degree of polarization at the wavelengths of Van Hove singularities.

Synthesis of well-aligned bamboo-like carbon nanotube arrays from ethanol and acetone

We report on a novel and facile thermal chemical vapour deposition method for fabricating dense and vertically well-aligned bamboo-like carbon nanotube (CNT) arrays on a copper substrate from ethanol and acetone for the first time. The effect of growth time, temperature and catalysts has been systematically studied. Using ethanol as the carbon source, well-aligned CNTs were produced at 800 °C, and random, long and large carbon fibres/tubes were formed at 900 °C. When acetone was used, mushroom-like carbon nanostructures were formed at 800 °C, and well-aligned CNTs were produced at 850 °C. In contrast, large amounts of random carbon micro-fibres were formed at 800 °C using ethanol as the carbon source when Fe or Ni was employed as the substrate. The electrochemical properties of the novel mushroom-like carbon nanostructures are presented; the growth mechanisms for the formation of the bamboo-like CNTs and the mushroom-like carbon nanostructures are discussed.

Selective growth of well-aligned carbon nanotubes by APCVD

Well-aligned good-quality carbon nanotube (CNT) array was grown on silicon substrate by atmospheric pressure chemical vapor deposition (APCVD) through SiO2 masking. First, the patterned substrate was pretreated with NH3 and then CNTs were synthesized at 800 °C using Ni as the catalyst, acetylene (C2H2) as the carbon source material and N2 as the carrier gas. Effects of the NH3-pretreatment time, the flow ratio of [C2H2]/[NH3] and the CNT growth time on the qualities of CNT array were analyzed in detail. It was found that good-quality CNTs with an average length of around 15 μm could be grown by pretreating the Si substrate with NH3 for 10 min and then conducting the CNT growth with a flow ratio of [C2H2]/[NH3] = 30/100. Furthermore, the field emission property of CNT array was investigated using a diode structure. It was found that the turn-on electric field decreased with increasing CNT length. The turn-on electric field as low as about 2 V/μm with an emission current density of 10 μA/cm2 was achieved for a CNT-array diode with the tube length near 18 μm. For the same device, the emission current density could be elevated to 10 mA/cm2 with the applied voltage of 3.26 V/μm.

Fabrication of single-walled carbon nanotube flexible strain sensors with high sensitivity

This work demonstrates a fabrication technique of high sensitivity flexible strain sensors at room temperature. The grown well-aligned millimeter-long single-walled carbon nanotube (SWCNT) was transferred from the silicon substrate to the pretrenched flexible substrate. The sensor design allows effective adhesion between SWCNT and flexible substrate for SWCNT lengthwise strain and piezoresistivity change. Experimental results show that the sensor achieves a high strain resolution of 0.004%. The measured piezoresistive gauge factor of the flexible sensor is 269. The demonstrated fabrication technique of flexible sensors shows advantage of high sensitivity, high quality, and is suitable for mass production.

Electrocatalytic oxidation of methanol on a platinum modified carbon nanotube electrode

A thin film of poly(o-aminophenol) (POAP) was formed by electropolymerization on well-aligned multi-walled carbon nanotubes (MWNTs). The electrochemical properties of POAP/MWNTs nanocomposites have been investigated by cyclic voltammetry in 1.0 M H2SO4 and 5 mM K3[Fe(CN)6]/1 M KCl solutions. Platinum nanoparticles (PtNPs) were electrochemically deposited onto the POAP/MWNTs nanocomposites by potential cycling between +0.50 and −0.70 V at a scan rate of 50 mV sec−1 in 0.1 M NaCl solution containing 5 mM Na2PtCl6. The electrocatalytic oxidation of methanol at the nanocomposites of PtNPs/POAP/MWNTs was studied in a 0.2 M H2SO4 solution. Compared to bare MWNTs and POAP modified glassy carbon electrodes, POAP/MWNTs electrode significantly enhances the catalytic efficiency of Pt nanoparticles for methanol oxidation. This improvement in performance is due not only to the high surface area and the fast electron transfer rate of nanotubes, but also to a decrease of the average particle size of Pt nanoparticles and an increase of the specific surface of electrodes in the presence of the POAP film.

A multiscale experiment on the tribological behavior of aligned carbon nanotube/ceramic composites

The tribological behavior of well-aligned carbon nanotube (CNT)/alumina nanocomposites was studied at multiple length scales. Pin-on-disk, microscratch and atomic force microscopy were used to test three nanomaterials: alumina matrix, thin-walled and thick-walled CNT/alumina composites. This work demonstrates that the frictional coefficient of the composites tested at all the length scales depends on the buckling behavior of the nanotubes and applied loading. Analysis is presented to assess the role of CNT buckling in determining the friction.

Structure‐Dependent Electrical Properties of Carbon Nanotube Fibers

Spun carbon nanotube (CNT) fibers have great potential for conducting and sensing applications owing to their unique, tunable electrical properties. Here we report the electron transport properties of neat, well-aligned CNT fibers spun from arrays of millimeter-long CNTs. The conductivity of asspun CNT fibers is around 595.2 S cm at room temperature, and its variation with temperature shows a semiconductive behavior from 300 to 75.4 K. The electron transport was found to follow a three-dimensional (3D) hopping mechanism. Importantly, it was found that chemical treatments may significantly affect the conductivities of as-spun fibers. Oxidizing the CNT fibers in air or HNO3 increased the conductivities, while covalent bonding of Au nanoparticles to the CNT fibers remarkably improved conductivity and changed conduction behavior. Conversely, annealing CNT fibers in Ar+ 6% H2 at 800 °C or under the CNT array growth conditions at 750 °C led to a dramatic decrease in conductivity. Owing to their conjugated and highly anisotropic 1D structures, carbon nanotubes (CNTs) are a fascinating new class of electronic materials from both theoretical and applied standpoints. The excellent conductivities of CNTs and their ability to carry very high current density, along with their high thermal conductivity, chemical stability, and mechanical strength, make CNTs uniquely promising for a broad range of applications, including building blocks for nanoscale electronic devices, microsensors for bio-agents and chemicals, and power cables for space shuttles. The electrical resistivity q of individual CNTs has been measured under ballistic conductions to be as low as 10 X cm for single-walled and 3× 10 X cm for multiwalled CNTs, respectively, indicating that CNTs may be better conductors than metals such as copper at room temperature. However, in most cases, due to the presence of various defects or impurities formed during the CNT growth, the conductivities of individual CNTs are often much lower than those under ballistic conduction with nanotubes free of defects. The electron transport in CNT assemblies is different from that in individual nanotubes. It has been reported that singlewalled carbon nanotube (SWNT) fibers, either synthesized directly by vertical floating chemical vapor deposition (CVD) methods or extruded from a super-acid suspension, exhibit room-temperature resistivities in the range of 1 × 10 to 7 × 10 X cm, which is nearly 100 times higher than the resistivities of single nanotubes. The resistivities of multiwalled carbon nanotube (MWNT) fibers are typically one or two orders of magnitude higher than that of SWNT fibers. Such large differences between single nanotubes and fiber assemblies may arise from a high impurity content (such as amorphous carbon and catalytic particles) in the fibers, which may profoundly affect electron transport by causing significant scattering, and contact resistances between nanotubes. Therefore, two approaches can be used to improve the electrical conductivity of CNT fibers: 1) minimize the contact resistances between nanotubes by improving the alignment of CNTs and by increasing the lengths of individual tubes; 2) improve the conductivity of individual CNTs by post-synthesis treatments. Itwas the objective of the study reported here to use these two approaches to produce CNT fibers with high conductivity and to study the fundamental conduction mechanisms of the CNT fibers. Thin and clean CNT fibers (typically 3 lm in diameter) were spun from arrays of well-aligned, millimeter-long CNTs, which were synthesized using ethylene CVD on a Fe catalyst film. By measuring the resistance of CNT fibers at temperatures from 300 K to 75.4 K, we investigated the electronic properties of as-spun fibers and their possible conducting mechanisms. It was also found that the conductivity of CNT fibers could be tuned through mild post-treatments. The spun CNT fibers were post-treated with five different procedures: 1) Annealing in air at 480 °C for half an hour in an attempt to clean off the amorphous carbon, whose oxidation temperature is often around 400 °C. 2) Oxidizing in dilute 5 M HNO3 solution at 40 °C to cause a weak chemical C O M M U N IC A TI O N

Field Emission Characteristics of Well-Aligned Carbon Nanotubes Synthesized in Organic Liquids

We have characterized the field electron emission from well-aligned carbon nanotubes (CNTs) synthesized in methanol using cobalt (Co) as a catalyst on silicon (Si) substrates. In order to prepare CNTs aligned vertically to the substrate, the physicochemical state of the Co catalyst was controlled by annealing in air at a temperature of 900 °C as a thermal preoxidation prior to the liquid-phase growth. For the CNT growth, the Co-loaded Si substrate was heated by applying a direct current through the substrate in methanol. Vertically well-aligned CNTs of different crystallinities were synthesized at temperatures of 600 and 900 °C. Field electron emission characteristics of the synthesized well-aligned CNTs were measured using diode-type electrodes in a high-vacuum chamber. A low-threshold electric field of approximately 0.9 V·µm-1 and a very stable emission were observed in the CNTs with high crystallinity grown at a temperature of 900 °C.

High-density and well-aligned carbon nanotubes formed by surface decomposition of SiC

The structure and diameter of the densely aligned carbon nanotubes (CNTs) by surface decomposition of SiC ( 0 0 0 1 ¯ ) C-face in vacuum were investigated by cross-sectional, plan-view transmission electron microscopy and electron diffraction methods. Zigzag-type CNTs were confirmed to be selectively formed and the formation mechanism was proposed from crystallographic analysis. Furthermore, the wall number of CNTs was found to be directly proportional to the diameter of CNTs. Comparing with the theoretical calculation, it was revealed that all of the carbon atoms remained on the surface after the selective evaporation of Si atoms by decomposition of each monolayer of SiC ( 0 0 0 1 ¯ ) , and then constructed the CNT walls with the minimum diffusion distance at the interface.

Quantitative limitation of active site and characteristics of chemical oxidized well-aligned carbon nanotubes

For many years, acid treatment processes have been used to modify substances for their applications on energy storage devices, such as electrochemical capacitors, fuel cells, etc. It is obvious that the increase of surface area and functional groups has beneficial influence on the amount of reactant adsorption. However, the quantitative limitation of active site has been poorly discussed until today. In this experiment, nitric acid (2 M and 14 M) was introduced at 90 °C over different periods of time to oxidate the well-aligned multi-wall carbon nanotubes (MWNTs), which were synthesized directly using carbon cloth (CC) as the substance. Quinoidal functional groups on active CNTs, such as -COOH and -OH, were found to be the result of chemical reaction. Pt catalyst, which was extracted from H 2PtCl 6 · 6H 2O, was deposited onto the carbon nanotubes using electroless plating in chloroplatinic acid solution. The extent of Pt adsorption was measured and was substantially larger than the as-prepared CNTs. It was found that the state of quantitative limitation existed in both 2 M and 14 M systems. A model was also developed to illustrate the limitation in the active site due to the chemical oxidation.

Preparation of parallel arrayed carbon nanotube film and its structure research

In an atmosphere of hydrogen and methane mixture, we have grown well-aligned carbon nanotube film on a Si slice by microwave plasma enhanced chemical vapor deposition. Observing and analyzing through SEM, TEM, and Raman spectra, we found that carbon nanotubes were vertically arrayed in the film. Their diameters are approximately 40–60 nm, and the lengths are about 4.0 μm. This well-aligned carbon nanotube film will be a very promising material for electron emitters in field emission displays.

Distortion of carbon nanotube array and its influence on carbon nanotube growth and termination

Well-aligned carbon nanotubes (CNTs) were produced by thermal chemical vapor deposition (CVD) of C2H4/H2/Ar on Fe/Al catalyst films. CNT growth kinetics and the array microstructure were investigated for the densely grown CNT films. A new CNT curling-controlled growth model is presented for the CNT growth kinetics. In this curling-controlled base-growth model, the initial CNT growth rate is relatively stable but the rate decreases dramatically after the initial period. It is found that, instead of the CNT film height, the curling of freshly grown CNTs is the essential cause of the CNT growth rate decrease and termination and this is in consistent with the Knudsen-diffusion theory.

The use of camphor-grown carbon nanotube array as an efficient field emitter

Three-dimensional growth of well-aligned high-purity multiwall carbon nanotubes (CNTs) is achieved on silicon, nickel-coated silicon and cobalt-coated silicon substrates by thermal decomposition of a botanical carbon source, camphor, with different catalyst concentrations. Field emission study of as-grown nanotubes in a parallel-plate diode configuration suggests them to be an efficient emitter with a turn-on field of ∼1 V/μm (for 10 μA/cm 2) and a threshold field of ∼4 V/μm (for 10 mA/cm 2). Maximum current density lies in a range of 20–30 mA/cm 2 at 5.6 V/μm with significant reversibility. Prolonged stability test of camphor-grown CNT emitters suggests a life time of ∼5 months under continuous operation. A new feature, metal-assisted electron emission from CNTs, has been addressed. Isolated nanotubes used as a cold cathode in a field emission microscope reveal the pentagonal emission sites and hence the atomic structure of the nanotube tips.

A generic process of growing aligned carbon nanotube arrays on metals and metalalloys

Aligned carbon nanotube (CNT) arrays are integral towards the development of severalapplications such as field emission, interconnects in silicon technology, and chemicaland biological sensing. Even though the synthesis of CNTs has been describedextensively in the literature, there has not been significant success in growing uniform,well-aligned CNT arrays on pure metal surfaces other than metals that catalyseCNT growth themselves. In this paper, we describe a method of growing alignedCNT arrays on a variety of pure metals, metal alloys, and conductive ceramicsusing a bimetallic iron/alumina composite catalyst at low temperatures (550 to700 °C). We believe that the addition of alumina to the iron catalyst significantly reducescatalyst–metal underlayer interactions that have traditionally proven to be a barrier for thegrowth of CNTs on metals. The alumina also minimizes surface diffusion of iron and allowsthe formation of a high density of uniformly dispersed catalyst nanoparticles to act asnucleation sites for well-aligned CNT arrays. Despite the presence of non-conductingalumina from the catalyst, the contact resistance between the CNTs and the metalunderlayer was observed to be quite low, emphasizing the usefulness of this approach topractical applications. Our process was successful in growing aligned CNTs even oncommercial steel plates and may be applicable for substrates of any shape orsize.

Laser direct writing carbon nanotube arrays on transparent substrates

The authors report the synthesis of multiwalled carbon nanotubes on transparent substrates by utilizing a diode laser to locally heat the catalysts to high temperature. Well-aligned bamboo-shaped multiwalled carbon nanotubes can be synthesized on glass substrate covered with a carbon black layer. If the carbon black was substituted by the commercial graphite inner coating, randomly oriented but high quality multiwalled carbon nanotubes were obtained with excellent field emission performance. This approach facilitates the fabrication of multiwalled carbon nanotube field emitters for field emission flat panel displays without limitations on the geometry size and temperature requirement of the substrate.

Preparation of well-aligned carbon nanotubes by pyrolysis of phenolic resin in anodic alumina pores

Well-aligned carbon nanotubes (CNTs) of high quality were synthesized by pyrolysis of phenolic resin at 800 °C in anodic alumina oxide (AAO) pores under argon protection. The innocuous source materials and safe operational conditions permit this method to synthesize well-aligned CNTs in large-scale and low cost. The formation mechanism of the synthesized CNTs is also proposed in this work by a series of visual sketches and is proved with obvious evidence. Firstly, phenolic resin nanotubes form in the template pores through the evaporation of solvent. Heat treatment then transfers these tubes into CNTs.