Aligned Multiwalled Carbon Nanotube Arrays Research Articles

Modulation of Luttinger liquid behavior by multi-channel electron transport in aligned multi-walled carbon nanotube arrays

In the case of one-dimensional systems, Luttinger liquid states are typically anticipated, which have previously been identified primarily in nanoscopic devices fabricated from isolated bundles of single- or multi-walled carbon nanotubes. In this study, the electrical transport properties of aligned multi-walled carbon nanotube arrays were investigated. The resistance of both aligned arrays and those covered with metal Mo exhibits a strong dependence on temperature, demonstrating a power-law relationship of R∼T–α below 100 K. The Luttinger liquid behavior was modulated by tuning the electron-transport channels via alerting the number of walls that contribute to the conductance within the multi-walled carbon nanotubes. The exponent α, derived theoretically as a parameter that depends on the number of conducting channels, exhibits minimal variation across the arrays and declines rapidly with the tubular structure opening due to the solid-solid reaction between the magnetron-sputtered molybdenum and the carbon atoms.

Electrically activated chemical bath deposition of CdS on carbon nanotube arrays

Electrically stimulated depositions of CdS nanoparticles on the surface of aligned multi-walled carbon nanotube (CNT) arrays were investigated by applying a voltage of 3 V to the electrodes placed into a chemical bath with an aqueous ammonium solution of CdCl2 and thiourea. The CNT arrays were fixed on positive, negative, and neutral electrodes. Morphology, composition and structure of CdS-CNT hybrids were studied using scanning electron microscopy, Raman scattering and X-ray photoelectron spectroscopy. CdS nanocrystallites in the hybrids were found to exhibit synthesis-dependent luminescence properties. S-vacancy enriched CdS grown on the positively charged CNT array possessed red photoluminescence (PL). Purple emission was attributed to small-size CdS nanocrystals synthesized on the negatively charged CNTs. The CNT array located on a neutral electrode was uniformly covered by CdS nanoparticles with a highly intense blue PL. Our results indicate a promising effect of the electrical field for enhancement of the growth rate of CdS nanocrystals.

Polarized Raman Study of Aligned Multiwalled Carbon Nanotubes Arrays under High Pressure

Tuning the intertube interaction and the topological structure of carbon nanotubes by the application of pressure may obviously affect their properties such as optical and electronic properties. However, characterizing such changes is still challenging. Here, we performed polarized Raman scattering studies on aligned multiwalled carbon nanotube arrays (MWNTAs). Unlike researchers from the previous literature, we found that the MWNTAs exhibit a polarization dependence similar to that of isolated single-walled carbon nanotubes at ambient conditions. Upon compression, the polarization dependence weakens gradually with increasing pressure up to ∼20 GPa, which has been discussed in terms of pressure-induced enhancement of intertube interactions. At around 20 GPa, the depolarization effect vanishes, which can be explained by the formation of interlinked sp3 bonding in the MWNTAs. Our results show that polarized Raman spectroscopy is an efficient method to explore not only intertube interaction but also structur...

Characterization of Vertically Aligned Multiwall Carbon Nanotube Arrays Based on Multiconductor Transmission Line Model

In this letter, a technique to extract 2-port <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$S$</tex></formula> -parameters of an arbitrary vertically aligned carbon nanotube (CNT) array is proposed based on the multiconductor transmission line (MTL) model. A closed-form analytical solution has been developed to model multiwall CNT (MWCNT) arrays under a common mode excitation. Simulated <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$S$</tex></formula> -parameters from the MTL model are compared to those from the effective single-conductor (ESC) model. The absorptions of CNT arrays are calculated, and a good matching with measurement results has been obtained. Finally, some key parameters of CNT arrays such as wave propagation velocities for different dimensions and array sizes are investigated numerically.

Shear-induced crystallisation of molten isotactic polypropylene within the intertube channels of aligned multi-wall carbon nanotube arrays towards structurally controlled composites

A melt processing route was successfully applied in the manufacturing of aligned multi-wall carbon nanotube/isotactic polypropylene (MWCNT-iPP) composites. As high as 25wt% content of the catalytic Chemical Vapour Deposition (c-CVD) derived nanotube filler could be introduced into the iPP matrix. The composite was characterised by a practically retained nanotube alignment as well as an enhanced crystallisation of iPP. Analysis of the phase iPP composition in the composite using XRD and DSC revealed a dominating α-phase with polymer molecules uniaxially oriented in the same direction as the nanotube alignment. Furthermore, kinetic in situ Synchrotron XRD studies verified that the enhanced crystallisation was induced mainly by shear stress occurring in the initial stage of infiltration MWCNT arrays by flowing molten polymer and, to a lesser extent, by the template-based growth of iPP crystallites.

Pulse plating of platinum on aligned multiwalled carbon nanotubes

In the present study, the electrochemical deposition (ECD) of platinum particles on aligned multiwalled carbon nanotube (MWCNT) arrays as novel membrane electrode assembly (MEA) in polymer electrolyte membrane fuel cells was investigated. The ECD was performed using the pulse plating technique and an electrolyte based on potassium tetranitroplatinate(II) salt. The MWCNTs were produced on thin nickel foils by chemical vapour deposition. The MWCNT arrays were electrochemically characterised by cyclic voltammetry in sulphuric acid. The platinum functionalisation of the MWCNT arrays was also investigated by cyclic voltammetry and by scanning electron microscopy. The final preparation of the MEA functionalised MWCNT on Nafion was performed by a hot pressing process.

Vertically aligned multi-walled CNT arrays coated by gold nanoparticles for surface-enhanced Raman scattering

A unique substrate for surface-enhanced Raman scattering on vertical multi-walled carbon nanotube (MWCNT) arrays coated by Au nanoparticles was reported. The vertically aligned MWCNT arrays were prepared by thermal chemical vapor deposition at temperature of 720 °C, and then coated by gold nanoparticles by sputtering. The possible mechanisms for the SERS sensitivity were discussed. Raman spectroscopy experiments for detecting Rhodamine6G were carried on and some obvious Raman peaks were observed and analyzed.

Enhancement Of Commercially-available Thermal Grease By Multi-walled Carbon nanotubes For Electronic Device Applications

Thermal grease is generally used as a thermal interface material for improved conduction between a heat source and heat sink. Here, we report the enhancement of thermal conductivity of commercially available off-the-shelf thermal grease (thermal compound LS6006) used in cooling of electronic devices, by the addition of multiwalled carbon nanotubes (MWCNTs). The thermal conductivity of the MWCNT mats and MWCNT modified thermal grease was measured relative to the thermal conductivity of the grease, which was taken as the benchmark. The thermal conductivity improves and the optimum thermal management is observed for aligned MWCNT arrays glued through the thermal grease.

One-Step Liquid-Phase Synthesis of Carbon Nanotubes: Effects of Substrate Materials on Morphology of Carbon Nanotubes

Effects of substrate materials and shapes have been investigated in the one-step liquid-phase (OS-LP) synthesis of carbon nanotubes (CNTs). Similar highly aligned multi-walled CNT (MWCNT) arrays (HACNTAs) with a height (ha) of ∼30 µm were synthesized on the substrates of commercially available stainless steel and Inconel® alloy by a resistance heating method in methanol containing cobaltocene, whereas aligned MWCNT arrays (ha≈10 µm) with slightly poor alignment were formed on the titanium substrate, compared with those of the former two. On the other hand, very short, non-aligned MWCNTs with a length of ∼1 µm were deposited on a silicon substrate. The different morphologies depending on the substrate materials are related with interaction between the pyrolytically-generated Co atoms and the surface oxide layers of the substrates. In addition, we have for the first time demonstrated that the OS-LP synthesis is suitable for CNT-coating onto intricately-shaped substrates like stainless steel mesh.

One-Step Liquid-Phase Synthesis of Carbon Nanotubes: Effects of Substrate Materials on Morphology of Carbon Nanotubes

Effects of substrate materials and shapes have been investigated in the one-step liquid-phase (OS-LP) synthesis of carbon nanotubes (CNTs). Similar highly aligned multi-walled CNT (MWCNT) arrays (HACNTAs) with a height (ha) of ∼30 µm were synthesized on the substrates of commercially available stainless steel and Inconel® alloy by a resistance heating method in methanol containing cobaltocene, whereas aligned MWCNT arrays (ha≈10 µm) with slightly poor alignment were formed on the titanium substrate, compared with those of the former two. On the other hand, very short, non-aligned MWCNTs with a length of ∼1 µm were deposited on a silicon substrate. The different morphologies depending on the substrate materials are related with interaction between the pyrolytically-generated Co atoms and the surface oxide layers of the substrates. In addition, we have for the first time demonstrated that the OS-LP synthesis is suitable for CNT-coating onto intricately-shaped substrates like stainless steel mesh.

Feedstock Diffusion and Decomposition in Aligned Carbon Nanotube Arrays

Feedstock diffusion and decomposition in the root growth of aligned carbon nanotube (CNT) arrays is discussed. A nondimensional modulus is proposed to differentiate catalyst poisoning controlled growth deceleration from one which is diffusion controlled. It is found that, at present, aligned multiwalled carbon nanotube (MWNT) arrays are usually free of feedstock diffusion resistance. However, for single-walled carbon nanotube (SWNT) arrays, since the intertube distance is much smaller than the mean free path of carbon source (ethanol here), high diffusion resistance in some currently available samples is significantly limiting the growth rate. The method presented here is also able to predict the critical lengths in different chemical vapor deposition (CVD) processes from which CNT arrays begin to meet this diffusion limit, as well as the possible solutions to this diffusion caused growth deceleration. The diffusion of carbon source inside of an array becomes more important when we found ethanol undergoes severe thermal decomposition at the reaction temperature. This means, in a typical alcohol CVD, hydrocarbons and radicals decomposed from ethanol may collide and react with the outer walls of SWNTs before reaching catalyst particles. When flow rate is low and ethanol is thoroughly decomposed, the produced SWNTs contain more soot structures than the SWNTs obtained at higher ethanol flow rate. Understanding the mass transport and reaction inside a SWNT array is helpful to synthesize longer and cleaner SWNTs.

Aligned carbon nanotube-polystyrene composites prepared by in situ polymerisation of stacked layers

In situ polymerisation provides a route to polystyrene (PS) matrix composites reinforced with aligned multi-walled carbon nanotubes (MWNTs). As shown, fully densified composites can be prepared; by varying the number of layers of aligned MWNT arrays, desired thickness of the composite can be manufactured. These aligned composites have characteristic anisotropic electrical and thermal properties.

Front Cover: Nanocoax solar cells based on aligned multiwalled carbon nanotube arrays (Phys. Status Solidi A 4/2011)

Front Cover: Nanocoax solar cells based on aligned multiwalled carbon nanotube arrays (Phys. Status Solidi A 4/2011)

Nanocoax solar cells based on aligned multiwalled carbon nanotube arrays

AbstractWe fabricated and studied solar cells based on a distributed nanocoax architecture by depositing amorphous silicon as photovoltaic medium on arrays of aligned multiwalled carbon nanotubes. These inexpensive cells demonstrate an initial efficiency of 6.1% that can be further enhanced by increasing the nanocoax density per unit area and improving the amorphous silicon quality.

Growth of Aligned MWNT Arrays Using a Micrometer Scale Local-Heater at Low Ambient Temperature

Ambient room temperature growth of aligned multi-walled carbon nanotube arrays on micrometer scale local heaters is demonstrated. High growth rates of up to 8.8 microm per second have been achieved and the growth has been monitored in situ using optical microscopy. The growth starts and ends abruptly over the length of the local heater. The terminal length of the nanotubes shows a clear dependence on growth temperature and small inhomogeneities in temperature across the heater are seen to lead to interesting microstructure of the arrays. The activation energy for growth was seen to be consistent with earlier reports for acetylene growth of nanotubes on iron catalysts.

Carbon nanotube arrays for optical design of amorphous silicon solar cells

Effective light trapping is essential for the conversion efficiency increase in thin film solar cells. Vertically aligned multiwalled carbon nanotubes (MWNTs) arrays with proper spacing form an ideal light trapping structure. In this work, we have demonstrated feasibility of the incorporation of MWNTs as back contact into amorphous silicon solar cells. Intrinsic amorphous silicon films were uniformly deposited onto vertically aligned MWNTs arrays. Scanning Electron Microscopy (SEM) was used to investigate the surface morphology of our films. The film surface area exposed to light was found to be increased dramatically due to the high-aspect ratio of MWNTs. Our findings open up a new way of managing light in thin film silicon solar cells by controlling the nano-geometry of MWNTs on substrates.

Horizontally aligned single-walled carbon nanotube field emitters fabricated on vertically aligned multi-walled carbon nanotube electrode arrays

Vertically aligned multi-walled carbon nanotube (MWCNT) arrays were fabricated on an anodic aluminum oxide membrane bonded to a Si wafer. After obtaining a protruding tip for the MWCNTs by etching away some oxide, they were used as electrodes in the fabrication of carbon nanotube field emitters. Long single-walled carbon nanotubes (SWCNTs) were spin coated on the MWCNT arrays of uniform height. Clean SWCNTs were suspended by attaching them to the tips of the vertically aligned MWCNT arrays. The spin coated SWCNTs function as emitters, while the MWCNT arrays function as electrodes. The field emission was greatly improved by coating gold on the MWCNT arrays and annealing at 400 °C. Our field emitter exhibits good field emission properties such as a low turn-on field (1.4 V/μm), high current density (122 mA/cm 2), and good stability (55 h for 10% degradation of current density from 400 μA/cm 2).

A one-step technique to prepare aligned arrays of carbon nanotubes

A simple effective pyrolysis technique has been developed to synthesize aligned arrays ofmulti-walled carbon nanotubes (MWCNTs) without using any carrier gas in a single-stage furnaceat 700 °C. This technique eliminates nearly the entire complex and expensive machinery associatedwith other extensively used methods for preparation of CNTs such as chemicalvapour deposition (CVD) and pyrolysis. Carbon source materials such as xylene,cyclohexane, camphor, hexane, toluene, pyridine and benzene have been pyrolyzedseparately with the catalyst source material ferrocene to obtain aligned arrays ofMWCNTs. The synthesized CNTs have been characterized by scanning electronmicroscopy (SEM), x-ray diffraction (XRD), transmission electron microscopy (TEM),thermogravimetric analysis (TGA) and Raman spectroscopy. In this technique,the need for the tedious and time-consuming preparation of metal catalysts andcontinuously fed carbon source material containing carrier gas can be avoided.This method is a single-step process where not many parameters are requiredto be monitored in order to prepare aligned MWCNTs. For the production ofCNTs, the technique has great advantages such as low cost and easy operation.

Photo-Thermoelectric Technique for Anisotropic Thermal Diffusivity Measurements

A photo-thermoelectric technique was employed to perform anisotropic thermal diffusivity measurements in nanocomposites and multilayer structures. In this technique a thermal wave is produced into the specimen by absorption of modulated laser radiation while the thermal wave is detected using a fast thermoelectric effect at the junction between a sharp wire probe and the sample surface. To allow effective heating and signal detection, additional thin-film layers may need to be deposited onto the samples. However, heat transport through the layers and into the thermoelectric probe may affect the thermal transport measurements. This work presents a heat conduction model that takes these effects into account and can be used to guide the selection of the probe and thin-film materials and thicknesses such that the accuracy of the thermal measurements is minimally affected. The discussion of the experimental technique is illustrated with simulation results, calibration experiments, and an example of anisotropic thermal diffusivity characterization in aligned multiwall carbon nanotube arrays.

The effect of process variables on the characteristics of carbon nanotubes obtained by spray pyrolysis

A simple spray pyrolysis setup is used to grow multi-walled carbon nanotubes (MWCNTs), from a ferrocene solution in benzene as precursor. The effects of process variables such as growth temperature, position of the aerosol generator and position in the reactor where the sample was formed were investigated. These variables have a strong influence on the graphitization degree, homogeneity, diameter and alignment of the nanotubes, as observed by TEM, SEM, XRD and Raman spectroscopy. Vertically aligned MWCNT arrays with high density were obtained in large areas (10 × 10 mm2), with high yield (2.1 mg cm−2) and at a growth rate at 1.43 μm min−1, by a suitable choice of the experimental conditions.