Vertically Aligned Multi-walled Carbon Nanotubes Research Articles

Growth of vertically-aligned carbon nanotube forests on conductive cobalt disilicide support

We report the thermal chemical vapor deposition of vertically-aligned multiwalled carbon nanotube forests directly onto electrically conductive cobalt disilicide (CoSi2) support using Fe as catalyst. We find that CoSi2 support layer is able to prevent the agglomeration of the catalyst and favor vertically-aligned growth better than a SiO2 support and comparable to an Al2O3 support. This is an unusual behavior for a conductive support. This is because CoSi2 has a lower surface energy than most metals or metallic compounds. This has great benefits in the application of CoSi2 as support for CNTs as horizontal and vertical interconnects.

Monolayer formation of human osteoblastic cells on vertically aligned multiwalled carbon nanotube scaffolds

Monolayer formation of SaOS-2 (human osteoblast-like cells) was observed on VACNT (vertically aligned multiwalled carbon nanotubes) scaffolds without purification or functionalization. The VACNT were produced by a microwave plasma chemical vapour deposition on titanium surfaces with nickel or iron as catalyst. Cell viability and morphology studies were evaluated by LDH (lactate dehydrogenase) release assay and SEM (scanning electron microscopy), respectively. The non-toxicity and the flat spreading with monolayer formation of the SaOs-2 on VACNT scaffolds surface indicate that they can be used for biomedical applications.

Total re-establishment of superhydrophobicity of vertically-aligned carbon nanotubes by Co2 laser treatment

Total restitution of original superhydrophobicity of vertically-aligned multi-walled carbon nanotubes (VACNT) was achieved by CO2 laser treatment. Superhydrophobic VACNT films obtained by microwave plasma chemical vapour deposition (MWCVD) were pre-treated with oxygen plasma to convert them in superhydrophilic films. Treatments with CO2 laser at different powers were carried out on the superhydrophilic films for controlling of wettability. Changing in both surface energy and roughness of the VACNT films, was evaluated by contact angle measurement and atomic force microscopy images, respectively.

Vertically-aligned carbon nano-tube membrane filters with superhydrophobicity and superoleophilicity

A membrane filter possessing both superhydrophobicity and superoleophilicity is of great interest for the possible separation of oil and water. Such a filter was realized in this study by synthesizing vertically-aligned multi-walled carbon nano-tubes on a stainless steel mesh. The dual-scale structure, nano-scale needle-like tubes on the mesh with micro-scale pores, combined with the low surface energy of carbon amplified both hydrophobicity and oleophilicity. For the tests, diesel was selected as a representative of low viscosity oils. The contact angles for diesel and water were 0° and 163±4°. The nano-tube filter could separate diesel and water layers, and even surfactant-stabilized emulsions. The successful phase separation of the high viscosity lubricating oil and water emulsions was also carried out. The separation mechanism can be readily expanded to a variety of different hydrophobic and oleophilic liquids. The simple nano-tube filter might be practically employed in environmental and chemical separation processes including oil spill cleanup.

Wettability control on vertically-aligned multi-walled carbon nanotube surfaces with oxygen pulsed DC plasma and CO 2 laser treatments

This work presents a method to obtain control on wettability of vertically-aligned multi-walled carbon nanotube surfaces, using oxygen pulsed directly current plasma and CO 2 laser irradiation. Superhydrophilic surfaces were obtained by the oxygen pulsed DC plasma treatment to incorporate polar groups on VACNT. The CO 2 laser was used to irradiate the functionalized samples, at different power levels, in order to restore the superhydrophobic characteristic. Evaluation of polar and dispersive components of superficial energy was performed by Owens and Wendt method.

Regular and reverse nanoscale stick-slip behavior: Modeling and experiments

We recently proposed a new nanoscale friction model based on the bristle interpretation of single asperity contacts. The model is mathematically continuous and dynamic which makes it suitable for implementation in nanomanipulation and nanorobotic modeling. In the present paper, friction force microscope (FFM) scans of muscovite mica samples and vertically aligned multi-wall carbon nanotubes (VAMWCNTs) arrays are conducted. The choice of these materials is motivated by the fact that they exibit different stick-slip behaviors. The corresponding experimental and simulation results are compared. Our nanoscale friction model is shown to represent both the regular and reverse frictional sawtooth characteristics of the muscovite mica and the VAMWCNTs, respectively.

Cytotoxicity analysis of vertically aligned multi-walled carbon nanotubes by colorimetric assays

A new methodology to evaluate the acute cytotoxicity of vertically aligned multi-walled carbon nanotube (VACNT) scaffolds was presented. A comparison between three different colorimetric assays was performed: (i) lactate dehydrogenase, (ii) 2-(4,5-dimethyl-2-thiazolyl)-3,5-diphenyl-2H-tetrazolium bromide, and (iii) neutral red. The results confirmed that a simple removing of VACNT scaffolds from the cell culture before the colorimetric assays, is necessary to obtain a high level of cell viability.

An evaluation of cell proliferation and adhesion on vertically-aligned multi-walled carbon nanotube films

Biocompatibility tests were performed on vertically-aligned multi-walled carbon nanotube (MWCNT) films produced by microwave plasma chemical vapor deposition on titanium substrates with iron (Fe) and nickel (Ni) as catalysts. The cell adhesion and morphology of L-929 mouse fibroblast cells were studied by high resolution scanning electron microscopy, after up to 7 days incubation periods. Cell viability and proliferation were evaluated by two “in vitro” tests: (1) 2-(4,5-dimethyl-2-thiazolyl)-3,5-diphenyl-2H-tetrazolium bromide (MTT), and (2) lactate dehydrogenase (LDH) assays. Low level of bioavailable Fe and Ni was determined by inductively coupled plasma optical emission spectrometry. Neither functionalization nor purification of MWCNT films was necessary to obtain good response to the biocompatibility tests. Efficient cell growth and non-toxicity suggest the use MWCNTs in tissue regeneration. The MWCNT films stimulated the cell growth, showing a proliferation 20% higher than on Ti.

Enhanced field emission stability and density produced by conical bundles of catalyst-free carbon nanotubes

Self-assembled bundling and catalyst removal can enhance the field emission stability and density of vertically-aligned multiwalled carbon nanotubes (VA-MWCNTs). These catalyst-free, opened tip, VA-MWCNTs offered better emission stability than the as grown samples. Both the emission stability and density were further enhanced as the opened-tip MWCNTs self-assembled into arrays of conical bundles. Theoretical simulation suggests that higher emission density was due to the reduced screening effects. The simulated local fields at the tips of the bundles suggest for a two-order of magnitude lower electric field loading on MWCNTs and contribute to prolong emission stability needed for practical applications.

Glucose Biosensors Based on Vertically-Aligned Multi-Walled Carbon Nanotubes

AbstractVertically-aligned multiwalled carbon nanotubes (VA-MWCNTs) were grown using plasma enhanced chemical vapor deposition (PECVD) technique. These VA-MWCNTs were then dip coated by Poly methyl methacrylate (PMMA) followed by annealing. Samples were then polished to expose the tips of CNTs. Biological molecules Glucose Oxidase (GOx) were then immobilized on the exposed tips of these nanoelectrode ensembles. Here we present further characterization of these devices, with results on the detection limits and measurement stability. We found that these sensors can be reused for longer than eight months when kept in proper storage conditions.

Comparing Field Emission Stability of Lithography-free, Modified Multi-walled Carbon Nanotubes

AbstractField emission from carbon nanotubes (CNTs) has been known for more than a decade but there is no commercialized product available in the market. Apparently, we need to improve our basics understanding on stable field emission from CNTs. Here we compared the field emission properties of as grown vertically-aligned multi-walled carbon nanotubes (MWCNTs) to two types of modified MWCNTs: 1) Conical bundles of opened-tip MWCNTs, and 2) Opened-tip MWCNTs embedded in poly-methyl methacrylate (PMMA). We found that both types of modified MWCNTs have lower emission thresholds and better emission stability than the as grown samples. Among these modified samples, MCNTs embedded in PMMA has lower emission thresholds and better emission stability. We attributed these improvements to the filling of spacing between MWCNTs with PMMA that has higher dielectric constant than vacuum.

Fabrication of Nano Particles on Vertically Aligned Multi-Wall Carbon Nanotubes by E-Beam Evaporation

A novel composite electrode-containing gold/platinum nanoparticles on the vertically aligned multi-wall nanotubes (MWCNTs) by electron beam evaporation (e-beam evaporation) is reported herein. The gold/platinum nanoparticles were less than 10 nm, and distributed uniformly on the surface of vertically aligned MWCNTs under the control of e-beam evaporation. The composite electrodes could be applied in the field of biomedical sensor and energy storage devices. This process was straightforward and contained high specific surface areas of gold/platinum nanoparticles on the MWCNTs. It was helpful to enhance the efficiency of organic fuel decomposition as well as advance the sensor precision. In addition, the nanoparticles on the MWCNTs could also facilitate the high electron mobility and chemical stability.

Friction and adhesion properties of vertically aligned multi-walled carbon nanotube arrays and fluoro-nanodiamond films

Sliding friction and adhesion properties of vertically aligned multi-walled carbon nanotube (VAMWCNT) arrays and fluoro-nanodiamond (F-ND) films on glass substrate have been quantitatively investigated in current study using atomic force microscopy. It was found that VAMWCNT arrays result in lower friction compared to F-ND films. Friction forces were also found to be consistently higher in nitrogen environment than in ambient environment for both samples and a surface chemistry based hypothesis was proposed. However, no apparent dependence of relative humidity was found on adhesion forces for both F-ND and VAMWCNT samples, indicating lack of correlation between nanoscale adhesion and friction. The implications from current study for designing movable components in micro- and nanoelectromechanical system devices were also discussed.

Effects of interfaces on nano-friction of vertically aligned multi-walled carbon nanotube arrays

Sliding friction properties of vertically aligned multi-walled carbon nanotube (VAMWNT) arrays have been investigated in current study in a quantitative manner. The VAMWNT arrays have been fabricated on an anodic aluminum oxide template by chemical vapor deposition at 650 °C. Friction force was measured in air by a modified atomic force microscopy (AFM) cantilever-bead assembly with 15 μm diameter borosilicate sphere attached to the end of the regular AFM cantilever. Quantitative measurements were achieved by using a novel in situ calibration methods recently developed based on diamagnetic levitation [Q. Li, K.-S. Kim, A. Rydberg, Rev. Sci. Instrum. 77 (2006) 065105-1-13]. The effects of different interfaces were studied using both cantilever-bead assembly coated with and without Al thin layer coatings. A reverse stick–slip behavior was observed in the current system as compared to the normal stick–slip behavior found in the literature.

Vertically Aligned Carbon Nanotubes as Cytocompatible Material for Enhanced Adhesion and Proliferation of Osteoblast-Like Cells

In this study, we describe the spatial organization of CAL-72 osteoblast-like cells on arrays of vertically aligned multi-walled carbon nanotubes (VACNTs). It was observed that, unlike cell growth on non-patterned surfaces, the cell attachment and spreading process on VACNTs was significantly enhanced. Additionally, since carbon nanotubes are known to possess resilient mechanical properties and are chemically stable, the effect of periodic arrays of VACNTs on CAL-72 osteoblast-like cells was also studied. The periodicity and alignment of VACNTs considerably influenced growth, shape and orientation of the cells by steering toward the nanopattern. This situation is of great interest for the potential application of VACNTs in bone bioenginnering. This data provides evidence that CAL-72 osteoblast-like cells can sense physical features at the nanoscale. These results give a fascinating insight into the ways in which cell growth can be influenced by man-made nanostructures and could provide a framework for achieving controlled cell guidance with controlled organization and special physical properties.

Direct growth of three-dimensional multicomponent micropatterns of vertically aligned single-walled carbon nanotubes interposed with their multi-walled counterparts on Al-activated iron substrates

Vertically aligned single-walled carbon nanotubes (VA-SWNTs) have been synthesized by an Al-activated plasma-enhanced chemical vapor deposition (PECVD) method. On this basis, we have developed a facile but effective method for direct growth of multicomponent micropatterns of VA-SWNTs interposed within the patterned areas surrounded by vertically aligned multi-walled carbon nanotubes (VA-MWNTs) onto an Al-activated iron substrate. The Al-activated iron substrate was prepared by region-selectively depositing an Al thin layer on either an Fe-coated substrate or a commercially available iron foil. This simple approach allows for an effective integration of various aligned carbon nanotubes with different electronic characteristics into multifunctional materials and devices.

Concept of nonvolatile memory based on multiwall carbon nanotubes

In this study, a novel concept is proposed for molecular electronics that uses verticallyaligned multiwall carbon nanotubes as nonvolatile memory elements. Nanotubes grown ona patterned substrate may be opened by partially deleting the outer molecularlayer or layers, so that the inner core is able to move along the vertical tube axis.Mounting another dielectric plate above the nanotube forest at a specific distancefrom the tube caps can provide two stable van der Waals states of the inner core,providing for nonvolatile data storage. A device built using this architecture canfunction as a two-dimensional memory array. At each cross point in the array,a multiwall carbon nanotube exists in either the separated off-state or in thecontact on-state, and can be switched between these states by applying voltagepulses at the corresponding electrodes. A theoretical memory density as high as1013 memory elements per square centimetre is possible, with an operation frequency exceeding100 GHz. Significant physical characteristics of such a device are bi-stability andreversibility. Such a device can function both as nonvolatile random access memory and asterabit solid-state storage.

Capacitive measurements for a novel ECIS (electrolyte–carbon nanotubes–insulator–semiconductor) structure

A novel capacitor, named an ECIS (electrolyte–carbon nanotubes–insulator–semiconductor), is proposed in this paper. By using a bonding technique, the characteristics of both the field-effect-based EIS (electrolyte–insulator–semiconductor) cell and vertically-aligned MWNT (multi-walled carbon nanotube) array are combined together to design the ECIS capacitor. Under a few hypotheses, the frequency dependence of the ECIS sample is also observed and analysed in this work. The new capacitor structure can be employed to integrate CNTs (carbon nanotubes) into CMOS chips. The prospect of future research is briefly outlined for the application of this novel capacitor in the field of potential-metric sensors.

Structural control of vertically aligned multiwalled carbon nanotubes by radio-frequency plasmas

Plasma-enhanced chemical vapor deposition is the only technique for growing individual vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) at desired locations. Inferior graphitic order has been a long-standing issue that has prevented realistic applications of these VA-MWCNTs. Previously, these VA-MWCNTs were grown by a one-plasma approach. Here, we demonstrate the capability of controlling graphitic order and diameters of VA-MWCNTs by decoupling the functions of the conventional single plasma into a dual-plasma configuration. Our results indicate that the ionic flux and kinetic energy of the growth species are important for improving graphitic order of VA-MWCMTs.

High-density vertically aligned multiwalled carbon nanotubes with tubular structures

Ammonia (NH3) gas was thought to be essential for the growth of vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) and led to the formation of bamboo-like structures. Here, we show that VA-MWCNTs with ideal tubular structures can be grown on substrates by various mixed gases with or without NH3 gas. The growth of these VA-MWCNTs is guided by a growth model that combined the dissociative adsorption of acetylene molecules (C2H2) and the successive vapor-liquid-solid growth mechanism. Results indicate that the key factor for growing these VA-MWCNTs is a balance between the decomposition rate of the C2H2 molecules on the iron catalyst and the subsequent diffusion and segregation rates of carbon.