Vertically Aligned Carbon Nanotube Films Research Articles

Enhancement of mechanical properties of vertically aligned carbon nanotube arrays due to N+ ion irradiation

In this work we apply N+ ion irradiation on vertically aligned carbon nanotube (VACNT) arrays in order to increase the number of connections and joints in the CNT network. The ions energy was 50 keV and fluence 5 × 1017 ions cm−2. The film was 160 μm thick. SEM images revealed the ion irradiation altered the carbon bonding and created a sponge-like, brittle structure at the surface of the film, with the ion irradiation damage region extending ∼4 μm in depth. TEM images showed the brittle structure consists of amorphous carbon forming between nanotubes. The significant enhancement of mechanical properties of the irradiated sample studied by the cyclic nanoindentation with a flat punch indenter was observed. Irradiation on the VACNT film made the structure stiffer, resulted in a higher percentage recovery, and reduced the energy dissipation under compression. The results are encouraging for further studies which will lead to create a class of materials—ion-irradiated VACNT films—which after further research may find application in storage or harvesting energy at the micro/nanoscale.

A novel hybrid method combining ASP with PECVD for in-situ low temperature synthesis of vertically aligned carbon nanotube films

In this study, an in-situ low temperature method to cost-effectively grow vertically aligned carbon nanotube (VACNT) films has been developed by combining advanced active screen plasma (ASP) technique with plasma enhanced chemical vapour deposition (PECVD). A novel active screen consisting of 316 stainless steel cylinder with double top lids was designed for the in-situ low-cost high-efficient preparation of catalyst films within a PECVD device, and VACNT films were grown from the catalyst films subsequently at low temperatures (≤500°C). The deposited catalyst films and the VACNT films were characterised by SEM, AFM, XRD, Raman and TEM/EDS. The results show that the catalyst films consist of stainless steel nanoparticles with sizes around 50nm. The growth of the CNTs is related to the PECVD temperature and the density of the CNTs is determined by the status of the catalyst films. The CNTs are multiwalled with nanoparticles at their tips. The mechanisms behind the in-situ low temperature synthesis are discussed based on the plasma physics, growth parameters and physical status of the catalyst films.

Highly Anisotropic Adhesive Film Made from Upside-Down, Flat, and Uniform Vertically Aligned CNTs.

We have created a multifunctional dry adhesive film with transferred vertically aligned carbon nanotubes (VA-CNTs). This unique VA-CNT film was fabricated by a multistep transfer process, converting the flat and uniform bottom of VA-CNTs grown on atomically flat silicon wafer substrates into the top surface of an adhesive layer. Unlike as-grown VA-CNTs, which have a nonuniform surface, randomly entangled CNT arrays, and a weak interface between the CNTs and substrates, this transferred VA-CNT film shows an extremely high coefficient of static friction (COF) of up to 60 and a shear adhesion force 30 times higher (12 N/cm2) than that of the as-grown VA-CNTs under a very small preloading of 0.2 N/cm2. Moreover, a near-zero normal adhesion force was observed with 20 mN/cm2 preloading and a maximum 100-μm displacement in a piezo scanner, demonstrating ideal properties for an artificial gecko foot. Using this unique structural feature and anisotropic adhesion properties, we also demonstrate effective removal and assembly of nanoparticles into organized micrometer-scale circular and line patterns by a single brushing of this flat and uniform VA-CNT film.

Controlling the morphology of Vertically-aligned carbon nanotubes using Langmuir-Blodgett deposited CoFe2O4, Fe3O4, and Fe nanoparticles with palmitic acid

The synthesis of Vertically-aligned carbon nanotubes (VA-CNTs) from Langmuir-Blodgett (LB) films of laboratory made CoFe2O4 nanoparticles (NPs), Fe3O4 NPs, and commercially available Fe NPs was performed to compare the catalytic activity of these catalyst NPs. VA-CNT films synthesized of CoFe2O4 NPs were approximately 2–3 times thicker than these synthesized of Fe3O4 NPs and Fe NPs. Palmitic acid (C16) molecules were mixed with the above catalyst NPs to control the distance between catalyst NPs in LB films, and adjust the number density of VA-CNTs synthesized from these LB films. Catalyst NPs were not dispersed uniformly and composed the domain of catalyst NPs in the C16 mixed L films. From optimizing the area ratio of catalyst NPs to C16 molecules, VA-CNT films synthesized of the catalyst NPs and C16 mixture exhibited large thicknesses compared with these synthesized of only catalyst NPs. VA-CNT films synthesized of CoFe2O4 NP-C16 LB films exhibited the largest thicknesses among these synthesized of three types of catalyst NP-C16 LB films, and enable to obtain a VA-CNT film up to 136μm. We also performed the evaluation of VA-CNT diameters to investigate how the catalyst types, sizes, distributions, and area ratios affect CNT diameters. The mean inner and outer diameters were almost constant regardless of the catalyst types and area ratios. The mean coefficients of variation (CVs) of the CNT outer diameters synthesized of CoFe2O4 NPs, Fe3O4 NPs, and Fe NPs were 19.5%, 20.5%, and 28.9%, respectively.

Investigation on Carbon Nanotubes as Thermal Interface Material Bonded With Liquid Metal Alloy

Vertically aligned carbon nanotube (VACNT) films with high thermal conductance and mechanical compliance offer an attractive combination of properties for thermal interface applications. In current work, VACNT films synthesized by the chemical vapor deposition method were used as thermal interface material (TIM) and investigated experimentally. The liquid metal alloy (LMA) with melting point of 59 °C was used as bonding material to attach VACNT films onto copper plates. In order to enhance the contact area of LMA with the contact surface, the wettability of the contact surface was modified by plasma treatment. The thermal diffusivity, thermal conductivity, and thermal resistance of the synthesized samples were measured and calculated by the laser flash analysis (LFA) method. Results showed that: (1) VACNT films can be used as TIM to enhance the heat transfer performance of the contact surface; (2) the LMA can be used as bonding material, and its performance is dependent on the LMA wettability on the contact surface. (3) When applying VACNT film as the TIM, LMA is used as the bonding material. After plasma treatment, comparison of VACNT films with the dry contact between copper and silicon showed that thermal diffusivity can be increased by about 160%, the thermal conductivity can be increased by about 100%, and the thermal resistance can be decreased by about 31%. This study shows the advantages of using VACNT films as TIMs in microelectronic packaging.

Influence of Temperature and Time For Direct Hydroxyapatite Electrodeposition on Superhydrophilic Vertically Aligned Carbon Nanotube Films

Superhydrophilic vertically aligned multi-walled carbon nanotubes were used as substrate to direct hydroxyapatite electrodeposition. Conventional Nickel/Titanium alloys were used for comparison. Results show that the bath temperature and the hydrophobicity control affect the hydroxyapatite/carbon nanotube composites obtained by electrodeposition. These results were proved using X-ray diffraction, Energy-dispersive X-ray spectroscopy, field emission gun scanning electron microscopy and Raman spectroscopy.

Wafer-Scale Transfer of Vertically Aligned Carbon Nanotube Arrays

The first critical step in making vertically aligned carbon nanotube (VACNT)-based thermal interface materials is to transfer the VACNTs on a large scale. Although VACNTs have been transferred by several methods, they were only transferred inadvertently in most cases. Here we report well-controlled weak-oxidation-assisted transfer of VACNTs. Specifically, after a short time of weak oxidation, we found that VACNTs could be easily detached from the native growth substrates, and thus, a freestanding VACNT film was obtained. Then the VACNTs could be assembled onto specific substrates for its real applications. More importantly, the repeated growth-transfer synthesis of VACNT arrays can be realized in one batch by introducing an additional process of weak oxidation in chemical vapor deposition, which makes the strategy more effective. Surprisingly, no degradation in the quality was observed before and after the weak oxidation according to thermogravimetric analysis and Raman spectra of VACNTs. Enhanced thermal and mechanical properties were achieved after reactive ion etching (RIE) and subsequent metallization of the surfaces of the VACNTs, and this might be due to the removal of impurities such as amorphous carbon and entangled CNTs by RIE. These findings provide an efficient approach for transferring VACNTs, which is important for the application of VACNTs in thermal management.

Elastomer-infiltrated vertically aligned carbon nanotube film-based wavy-configured stretchable conductors.

Elastomer-infiltrated vertically aligned carbon nanotube (VACNT) forests are good candidates for use as stretchable conductors that can retain the electrical properties under relatively large stretching. The electrical performance can be further enhanced in terms of high stretchability and small change in the electrical resistance by using a wavy configuration. In this work, we present a wavy-structured high-performance stretchable conductor prepared by a simple prestraining approach based on polydimethylsiloxane (PDMS)-infiltrated VACNT films. Prior to the infiltration process, the VACNT forests can also be easily micropatterned by a PDMS stamp-assisted contact transfer printing technique. The conductive VACNT forest patterns are fully infiltrated with highly elastic PDMS, and the PDMS/VACNT film is conformally and strongly bonded to the prestrained PDMS substrate with the help of an intermediate thin PDMS layer, resulting in mechanical robustness of the whole device. The fabricated wavy VACNT conductor shows a small resistance change ratio of less than 6% with a tensile strain of up to 100% (prestrained level) and a high reversibility under multiple stretching/releasing cycles with a maximum strain of 100%.

Effect of growth pressure on the synthesis of vertically aligned carbon nanotubes and their growth termination.

When vertically aligned carbon nanotubes (VACNTs) are synthesized by thermal chemical vapor deposition (TCVD), their structural features such as height and density can be determined by TCVD growth conditions. In this study we investigated the effect of growth pressure on the structural features of VACNTs. Changes in growth pressure significantly affected the height, density, and crystalinity of synthesized VACNTs. In addition, we suggest that the growth termination of VACNTs could be due to the lack of carbon feedstock supply to the center of the VACNT film induced by the pressure-dependent adsorption of amorphous carbon at the edge of the VACNT film. In addition, the field emission characteristics of the VACNT film were carried out. The turn-on voltage of the VACNT film was 1.62 V/microm and the field enhancement factor (beta) was 2478. These results provide useful information for practical applications of VACNTs, such as field emission display and X-ray source.

Heat Capacity, Thermal Conductivity, and Interface Resistance Extraction for Single-Walled Carbon Nanotube Films Using Frequency-Domain Thermoreflectance

Thermal interface materials (TIMs) based on vertically aligned carbon nanotube (VACNT) films promise the unusual combination of high thermal conductance and low elastic modulus, properties that are critical for TIM performance and reliability. This paper reports through-plane thermal conductivity, heat capacity, and boundary resistances measured using frequency domain thermoreflectance for seven VACNT films. The films are composed of single-walled carbon nanotubes grown using chemical vapor deposition on Si substrates and have thicknesses from 5.3 to 81 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex></formula> . The top surfaces of the films are coated with 100-nm Ti and 100-nm Ag as a transducer layer for the thermoreflectance measurement. The roughness of the top surface of each sample is measured using atomic force microscopy. The average thermal conductivity of the films ranges from 2.1 to 3.5 W <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm m}^{-1}~{\rm K}^{-1}$</tex></formula> and the average heat capacity ranges from 5.5 to 9.3 J <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm cm}^{-3}~{\rm K}^{-1}$</tex></formula> . The heat capacity of the films is used to calculate the volume fraction of tubes that contributes to thermal conduction. The CNT-metal boundary resistances are below 1 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm m}^{2}~{\rm K}~{\rm MW}^{-1}$</tex></formula> and the CNT-substrate resistance ranges from 3 to 80 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm m}^{2}~{\rm K}~{\rm MW}^{-1}$</tex></formula> . Variations of the nanotube alignment and morphology throughout the film are also examined using scanning electron micrographs to provide a comprehensive analysis of these films.

Carbon nanotube cathode with capping carbon nanosheet

Here, we report a vertically aligned carbon nanotube (VACNT) film capped with a few layer of carbon nanosheet (FLCN) synthesized by chemical vapor deposition using a carbon source from iron phthalocyanine pyrolysis. The square resistance of the VACNT film is significantly reduced from 1500Ω/□ to 300Ω/□ when it is capped with carbon nanosheet. The VACNT capped with carbon nanosheet was transferred to an ITO glass substrate in an inverted configuration so that the carbon nanosheet served as a flexible transparent electrode at the bottom and the VACNT roots served as emission tips. Because all of the VACNTs start growing from a flat silicon substrate, the VACNT roots are very neat and uniform in height. A field emission test of the carbon nanosheet-capped VACNT film proved that the CNT roots show better uniformity in field emission and the carbon nanosheet cap could also potentially serve as a flexible transparent electrode, which is highly desired in photo-assisted field emission.

Effect of morphology on the strain recovery of vertically aligned carbon nanotube arrays: An in situ study

We report on the distinctly different mechanical responses of two vertically aligned carbon nanotube (VACNT) films, subjected to large displacement (up to 70μm) flat punch indentations. The VACNT films were synthesized using the same chemical vapor deposition (CVD) technique but for varying reaction times, which resulted in their different thicknesses (480 and 160μm, respectively) and morphologies. In situ tests reveal that the shorter, more aligned VACNT film deforms via an instantaneous vertical shearing of the material directly underneath the indenter tip, which is manifested as a rapid displacement burst in the load–displacement response when tested at rates of 100nm/s and above. The resultant buckles were of a more permanent nature leading to their low recoverability (22–40%). In contrast, we find the thicker, more tortuous VACNT film to show a higher (∼80%) recovery and a more compliant response. These differences in the mechanical response of the VACNTs are discussed in the framework of foam-like deformation with a particular emphasis on their different morphological features, namely density and tortuosity.

S111054 高配向カーボンナノチューブ膜の接触熱抵抗に及ぼす接触条件の影響([S111-05]第19回卒業研究コンテスト(5))

Carbon nanotubes have been reported to have high thermal conductivity. A vertically aligned carbon nanotube (VA-CNT) film is composed of densely packed and highly-aligned CNTs on a SiC substrate. This VA-CNT film is expected to have superior heat conductivity than a conventional thermal-grease heat transfer method. However, it is not clear that how much contact pressure and contact roughness affect the contact thermal conductivity of the VA-CNT films. We developed an apparatus that measures the contact thermal resistance by an one-directional steady-state heat flow method. We found out that there was suitable surface roughness for the minimum contact thermal resistance of the VA-CNT film.

An evaluation of chondrocyte morphology and gene expression on superhydrophilic vertically-aligned multi-walled carbon nanotube films

Cartilage serves as a low-friction and wear-resistant articulating surface in diarthrodial joints and is also important during early stages of bone remodeling. Recently, regenerative cartilage research has focused on combinations of cells paired with scaffolds. Superhydrophilic vertically aligned carbon nanotubes (VACNTs) are of particular interest in regenerative medicine. The aim of this study is to evaluate cell expansion of human articular chondrocytes on superhydrophilic VACNTs, as well as their morphology and gene expression. VACNT films were produced using a microwave plasma chamber on Ti substrates and submitted to an O2 plasma treatment to make them superhydrophilic. Human chondrocytes were cultivated on superhydrophilic VACNTs up to five days. Quantitative RT-PCR was performed to measure type I and type II Collagen, Sox9, and Aggrecan mRNA expression levels. The morphology was analyzed by scanning electron microscopy (SEM) and confocal microscopy. SEM images demonstrated that superhydrophilic VACNTs permit cell growth and adhesion of human chondrocytes. The chondrocytes had an elongated morphology with some prolongations. Chondrocytes cultivated on superhydrophilic VACNTs maintain the level expression of Aggrecan, Sox9, and Collagen II determined by qPCR. This study was the first to indicate that superhydrophilic VACNTs may be used as an efficient scaffold for cartilage or bone repair.

Infrared absorption coefficients of vertically aligned carbon nanotube films

Vertically aligned carbon nanotube (VACNT) films have promising applications in solar cells, radiometers, and photonic devices. We have fabricated VACNT films with thicknesses of 10, 36, 45, and 120 μm on silicon substrates. The absorption coefficient of each film is obtained from the measured regular transmittance from 1 to 19 μm wavelengths, using a ratio method. The degree of alignment of the VACNT films is estimated by comparison of experiments with an analysis based on the effective medium theory. The effect of thickness on the absorption coefficient of the VACNT films is elucidated according to their morphologies.

Electrochemical Charge Storage Properties of Vertically Aligned Carbon Nanotube Films: Effects of Thermal Oxidation

Vertically aligned carbon nanotube (VACNT) films were synthesized and investigated in vitro for their potential use as a neural stimulation electrode. Materials and electrochemical (EC) characterization (cyclic voltammetry, electrochemical impedance spectroscopy, and high-rate potential transient measurements) were performed before and after flash oxidation in O2 at various temperatures and over a wide frequency range. The results showed distinct EC behavior within three ranges of treatment temperature. Oxidative thermal treatments that did not visibly etch the VACNT film caused a significant improvement in electrode performance compared to the as-deposited electrode. Surprisingly, flash oxidation within a narrow temperature range (400 < T ≤ 450 °C) selectively increased capacitance/charge injection at high frequencies (102–104 Hz). A phenomenological model is proposed to explain the temperature-dependent behavior and indicates the importance of modifying a porous coating to increase the charging rate rather than maximizing the total charge accumulated at long times for high-rate charge storage applications.

Impact of nanotube density and alignment on the elastic modulus near the top and base surfaces of aligned multi-walled carbon nanotube films

The mechanical compliance of vertically aligned carbon nanotube (VACNT) films renders them promising as interface materials that can accommodate thermal expansion mismatch. Here we study the relationship between the detailed morphology and elastic modulus of multi-walled VACNT films with thicknesses ranging from 98 to 1300μm. A systematic analysis of scanning electron micrographs reveals variations in nanotube alignment and density among samples and within different regions of a given film. Nanoindentation of both top and bottom film surfaces using an atomic force microscope with spherical indenters with radii between 15 and 25μm provides evidence of the modulus differences. The top surface is shown to have a higher modulus than the base, with out-of-plane modulus values of 1.0–2.8MPa (top) and 0.2–1.4MPa (base). The indentation data and microstructural information obtained from electron microscopy are interpreted together using an open cell foam model to account for differences in nanotube alignment and density, which are generally lower at the base and yield predictions that are consistent with the modulus data trends. This work shows that microstructure analysis complements property measurements to improve our understanding of nanostructured materials.

Biomineralization of Superhydrophilic Vertically Aligned Carbon Nanotubes

Vertically aligned carbon nanotubes (VACNT) promise a great role for the study of tissue regeneration. In this paper, we introduce a new biomimetic mineralization routine employing superhydrophilic VACNT films as highly stable template materials. The biomineralization was obtained after VACNT soaking in simulated body fluid solution. Detailed structural analysis reveals that the polycrystalline biological apatites formed due to the -COOH terminations attached to VACNT tips after oxygen plasma etching. Our approach not only provides a novel route for nanostructured materials, but also suggests that COOH termination sites can play a significant role in biomimetic mineralization. These new nanocomposites are very promising as nanobiomaterials due to the excellent human osteoblast adhesion.

Mechanical characterization of aligned multi-walled carbon nanotube films using microfabricated resonators

Owing to their unique mechanical and thermal properties, vertically aligned carbon nanotube (VACNT) films are promising for use as advanced thermal interface materials. While there has been much research on the thermal properties of VACNT films, the mechanical modulus along the in-plane direction has received little attention. This paper reports a new technique for measuring this property using the resonant frequency shift in microfabricated resonators integrated with VACNT films. A comprehensive description of the technique includes microfabrication details, the laser Doppler methodology for extracting the resonant frequency, uncertainty analysis, and a model for data interpretation. A model accounting for the spatial variation of nanotube alignment relies on electron microscopy data and is consistent with the strong thickness dependence of the modulus data. The modulus of 0.5–100 μm-thick multi-walled nanotube films varies from 8 to 300 MPa, which is a promising value for a variety of applications including thermal interfaces.

Increasing mouse embryonic fibroblast cells adhesion on superhydrophilic vertically aligned carbon nanotube films

We have analyzed the adhesion of mouse embryonic fibroblasts (MEFs) genetically modified by green fluorescence protein (GFP) gene cultured on vertically-aligned carbon nanotubes (VACNTs) after 6 days. The VACNTs films grown on Ti were obtained by microwave plasma chemical vapor deposition process using Fe catalyst and submitted to an oxygen plasma treatment, for 2 min, at 400 V and 80 mTorr, to convert them to superhydrophilic. Cellular adhesion and morphology were analyzed by scanning electron, fluorescence microscopy, and thermodynamics analysis. Characterizations of superhydrophilic VACNTs films were evaluated by contact angle and X-Ray Photoelectron Spectroscopy. Differences of crowd adhered cells, as well as their spreading on superhydrophilic VACNTs scaffolds, were evaluated using focal adhesion analysis. This study was the first to demonstrate, in real time, that the wettability of VACNTs scaffolds might have enhanced and differential adherence patterns to the MEF-GFP on VACNTs substrates.