Carbon Nanotube Carpets Research Articles

Growth of a cup-stacked carbon nanotube carpet with a superhydrophobic surface

Growth of a cup-stacked carbon nanotube carpet with a superhydrophobic surface

Vertically aligned carbon nanotube‐based composite: Elaboration and monitoring of the nanotubes alignment

ABSTRACTWe present the different elaboration steps of a composite formed of carbon nanotubes (CNT) carpet embedded in an epoxy polymer. Detailed characterization at each step of the elaboration process is performed. The good alignment of CNT in as‐grown carpets is kept all along the elaboration process of the composite, as it is measured at both macro and microscopic scales by X‐ray scattering. We also ensured by X‐ray fluorescence measurements that the iron‐based catalyst particles used for the synthesis were removed from the carpet after a high temperature post‐annealing treatment. These measurements give valuable information for further applications involving unidirectional nanotube composites and membranes, where CNT alignment is a key parameter. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39730.

Growth of a cup-stacked carbon nanotube carpet with a superhydrophobic surface

A carpet structure composed of high purity cup-stacked carbon nanotubes (CSCNTs) was synthesized by a catalytic chemical vapor deposition method. In a 15-min growth time, the CSCNT carpet with a height up to ∼300 μm and carbon purity above 99.9 wt% was prepared. Transmission electron microscopy observations indicated that the CSCNTs with diameters ranging from 80 nm to 230 nm consist of truncated conical graphene layers. Based on the experimental results, a possible “base growth” mechanism was proposed for the CSCNTs. The CSCNT carpet is highly flexible and superhydrophobic, showing a contact angle of ∼155o. It can be easily harvested from the original growth substrate and be transferred to target substrates. It may therefore find applications as a water-proof and self-cleaning coating layer.

Investigating the mechanism of collective bidirectional growth of carbon nanofiber carpets on metallic substrates

Bidirectional-growth of carbon nanofibers is a rare phenomenon found on free-standing catalyst particles, in contrast to the most commonly studied tip- and base-growth mechanisms for carbon nanostructures synthesized through thermal chemical vapor deposition. We reveal the underlying mechanisms of collective bidirectional growth in NixPd1−x-catalyzed carbon nanofiber carpets grown on a palladium substrate with varying nickel film thickness by monitoring the fiber growth evolution. The results show that the collective bidirectional growth is promoted and controlled by the chemical and physical restructuring of the sub-surface portion of the metal stack which undergoes micro-fragmentation as a result of the incorporation, diffusion, and precipitation of carbon. Carbon nanofiber growth can be controlled by engineering the catalyst-underlayer materials properties such as grain size, chemical composition and alloying. Since the determining factor whether carbon nanofibers or nanotubes are obtained is a strong function of catalyst size, the understanding of this growth mechanism can be transferred to the field of carbon nanotube synthesis. By keeping the grain size small enough to ensure carbon nanotube instead of carbon nanofiber growth, achieving dense, vertically aligned carbon nanotube carpets on metallic substrates might be possible, which is a prerequisite for carbon nanotube integration in integrated circuits.

Plasma Fluorination of Vertically Aligned Carbon Nanotubes

Functionalization of vertically aligned multiwalled carbon nanotube carpets was performed via exposure to CF4 or Ar:F2 RF plasmas. Rapid fluorination was observed via X-ray photoelectron spectroscopy (XPS) with surface fluorine concentration, bonding type, and patterning dependent on gas mixture and exposure time. Surface properties of the v-MWCNTs forests have been changed by the introduction of fluorine-containing groups, as demonstrated via surface wettability studies, while scanning electron microscopy shows that overall nanotube alignment and separation is conserved. Scanning X-ray photoelectron spectromicroscopy (SPEM) shows that the plasma treatment results in selective functionalization of the surface tips of the nanotubes. This opens the way to nanotube carpet structures with activated surfaces, which maintain the desirable conductive properties of the pristine nanotubes near to the substrate.

Interaction of Discharges in Heptane with Silicon Covered by a Carpet of Carbon Nanotubes

Discharges in heptane in pin‐to‐plate configuration are produced between a platinum wire and a (100)‐oriented silicon wafer coated by a carbon nanotube (CNT) carpet. This carpet is used to simulate the behavior of a nanostructured surface in electro‐discharge machining (EDM) where small protrusions on the surface could play a similar role. CNTs behave like simple electrical conductors between the discharge and the silicon wafer. They act as if they would focus the current on smaller areas. The average diameter of impacts is about five times smaller if the silicon wafer is coated by a CNT carpet. The underlying silicon surface is heated by the plasma and melts, forming a central spot surrounded by a serrated trailing edge. The current density being about one order of magnitude larger when a CNT carpet is present, the induced magnetic field stirs the molten silicon, creating serrations all around the impact. Hot nanoparticles of carbon coming from the plasma fall and roll randomly on the silicon surface where they create wavy micro‐channels. Nanowires that are detached from the surface are covered by nanoparticles of platinum in the plasma and embedded within an amorphous carbon layer deposited on the nanotube. However, these effects can only be observed if the current density is high enough (>≈10 A µm−2 depending on the material) like in micro‐EDM but not in nano‐EDM.

Splitting of a Vertical Multiwalled Carbon Nanotube Carpet to a Graphene Nanoribbon Carpet and Its Use in Supercapacitors

Potassium vapor was used to longitudinally split vertically aligned multiwalled carbon nanotubes carpets (VA-CNTs). The resulting structures have a carpet of partially split MWCNTs and graphene nanoribbons (GNRs). The split structures were characterized by scanning electron microscopy, transmission electron microscopy, atomic force microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. When compared to the original VA-CNTs carpet, the split VA-CNTs carpet has enhanced electrochemical performance with better specific capacitance in a supercapacitor. Furthermore, the split VA-CNTs carpet has excellent cyclability as a supercapacitor electrode material. There is a measured maximum power density of 103 kW/kg at an energy density of 5.2 Wh/kg and a maximum energy density of 9.4 Wh/kg. The superior electrochemical performances of the split VA-CNTs can be attributed to the increased surface area for ion accessibility after splitting, and the lasting conductivity of the structure with their vertical conductive paths based on the preserved GNR alignment.

The role of band structure in electron transfer kinetics in low‐dimensional carbon

AbstractThe role of band structure in heterogeneous electron transfer kinetics at graphene and carbon nanotube electrodes is discussed within Gerischer‐Marcus model. Raman spectroscopy was used to estimate the averaged density of defects in anodized epitaxial graphene, single‐walled carbon nanotube mats, and multi‐walled carbon nanotube carpets. Comparison of the density of defects with cyclic voltammetry data reveals correlation between the defect induced electronic states near the Fermi level and electrochemical efficiency. Electron transfer at low dimensional carbon electrodes is additionally enhanced by layer curvature and doping. Results show that defect induced states are responsible for large dispersion of heterogeneous electron transfer rate constant at different graphitic nanocarbon electrodes.

Mimicking water striders’ legs superhydrophobicity and buoyancy with cabbage leaves and nanotube carpets

Abstract

Wetting behavior and activity of catalyst supports in carbon nanotube carpet growth

A simple, reliable, and non-destructive approach based on contact angle measurements is described for predicting the activity of catalyst supports in carbon nanotube (CNT) carpet growth. The basic component of the surface free energy of different alumina supports - determined from the van Oss-Good-Chaudhury model and the Young-Dupré equation - was found to correlate with the activity of Fe catalyst during water-assisted CVD growth of CNT carpets.

Improvement of the identification of multiwall carbon nanotubes carpet thermal conductivity by pulsed photothermal method

Thermal properties in multiwall carbon nanotubes carpets and micro-devices are investigated using a nanosecond photothermal method. Gradually, the identification model and experimental protocol are performed to increase the method accuracy for the thermal conductivity determination. In the experimental protocol, a nanosecond UV monopulse laser beam is used to heat the surface of a multilayer (600 nm of Ti/20 μm of carbon nanotube carpet) sample. In the 1D identification model with two layers and a thermal contact resistance, the effect of the laser excitation temporal shape is taken into account. In this study, this first approach allows to improve the accuracy of apparent thermal conductivity measurements of multiwall carbon nanotubes carpet. The carbon nanotubes carpet apparent thermal conductivity value went from being to 180 ± 5 W×m−1×K−1. In the second approach, two laser beams are coupled in order to increase the interaction time duration from 27 ns to 60 ns. It becomes possible to probe different depths in the carpet. The obtained value (180 W×m−1×K−1) confirms the pulsed photothermal method consistency for porous samples. Finally, assuming that the carbon nanotubes are parallel and without any defects, the equivalent intrinsic thermal conductivity of a single carbon nanotube is estimated to be around 3600 W×m−1×K−1.

Local current mapping of single vertically aligned multi-walled carbon nanotube in a polymer matrix

This paper reports our work on the development of thin composite films based on aligned multi-walled carbon nanotubes (CNTs) forest embedded in epoxy polymer matrix dedicated for 3D-interconnection in flexible substrate. To reach this goal, information about local CNT properties is strongly needed. In this study, aligned multi-walled carbon nanotube carpets were prepared by aerosol Assisted catalytical chemical vapor deposition (AA-CCVD) process. Such carpets were then embedded in a polymer matrix, subsequently thinned by chemical mechanical polishing (CMP) process and finally treated by plasma process. Then, to determine local conducting properties of CNT, the surface of the composite film is studied by conductive AFM (C-AFM) and scanning spreading resistance microscopy. These methods are discussed and compared. C-AFM appears to be the most efficient method to obtain high resolution current maps. For the first time, a local electrical property mapping and current versus applied voltage characteristics over single carbon nanotube in vertical arrangement are obtained. These results show metallic behavior for nanotubes and emphasize that the electrical conduction is mainly driven by the external walls.

Control over the Diameter, Length, and Structure of Carbon Nanotube Carpets Using Aluminum Ferrite and Iron Oxide Nanocrystals as Catalyst Precursors

Both colloidal iron oxide and aluminum ferrite nanocrystals can initiate the growth of vertically aligned carbon nanotube (CNT) carpets through water-assisted chemical vapor deposition (CVD). The outer diameters of the CNTs ranged from 3 to 18 nm, and this dimension correlated well with the diameters of the starting nanocrystals. The smallest particles (4 nm aluminum ferrite nanocrystals) yielded CNT carpets with high percentages (60%) of single-walled CNT species, whereas larger particles (iron oxide nanocrystals over 25 nm) formed carpets with more double, triple, and larger multiwalled structures. CNTs grown by aluminum ferrite nanocrystals were uniformly of higher quality (IG/ID = 11.4) than comparable materials formed from pure iron oxides (IG/ID = 9.8). We speculate that the presence of aluminum in the nanocatalyst may slow the acetylene decomposition at the catalyst surface, leading to less production of amorphous carbon material.

High-performance field emission from a carbon nanotube carpet

We have created a field emitter composed of a carbon nanotube (CNT) yarn, which was prepared by direct spinning through chemical vapor deposition and then formed into a carpet structure by tying the yarn to a conductive substrate before cutting it. The structure of the carpet is arranged to induce the tips of the CNT yarn to protrude toward the anode for maximum electron emission. The turn-on field, threshold field, and field enhancement factor of the device are 0.33, 0.48V/μm, and 19,141, respectively. Extremely low operating electric fields and a high field enhancement factor result from the high density of CNT emitters with high crystallinity, the electrically good contact between the emitters and the substrate, and the effects of the multistage structure. The emission is stable even at a high current density of 2.13mA/cm2, attributed to the strong adhesion between the emitters and the substrate. The emission performance is found to be customizable by adjusting the structure, for example, the CNT pile density. These results are relevant for practical applications, such as large-area flat-panel displays, large-area low-voltage lamps, and X-ray sources.

Growth of long and aligned multi-walled carbon nanotubes on carbon and metal substrates

Well aligned, long and dense multi-walled carbon nanotubes (CNT) can be grown on both carbon fibres and any metal substrates compatible with the CNT synthesis temperature. The injection-CVD process developed involves two stages, including fibre pretreatment by depositing a SiO2-based sub-layer from an organometallic precursor followed by CNT growth from toluene/ferrocene precursor mixture. Carbon substrates, as well as metals, can easily be treated with this process, which takes place in the same reactor and does not need any handling in between the two stages. The aligned CNT carpets obtained are similar to the ones grown on reference quartz substrates. The CNT growth rate is fairly high (ca. 30 μm min−1) and it is possible to control CNT length by varying the CNT synthesis duration. The thickness of the SiO2-based sub-layer can be varied and is shown to have an influence on the CNT growth. This layer is assumed to play a diffusion barrier layer role between the substrate and the iron based catalyst nanoparticles producing CNT. The CNT anchorage to the carbon fibres has been checked and good overall adhesion proved, which is in favour of a good transfer of electrical charge and heat between the nanotubes and fibre.

A combination of capillary and dielectrophoresis-driven assembly methods for wafer scale integration of carbon-nanotube-based nanocarpets

The wafer scale integration of carbon nanotubes (CNT) remains a challenge for electronic and electromechanical applications. We propose a novel CNT integration process relying on the combination of controlled capillary assembly and buried electrode dielectrophoresis (DEP). This process enables us to monitor the precise spatial localization of a high density of CNTs and their alignment in a pre-defined direction. Large arrays of independent and low resistivity (4.4 × 10−5 Ω m) interconnections were achieved using this hybrid assembly with double-walled carbon nanotubes (DWNT). Finally, arrays of suspended individual CNT carpets are realized and we demonstrate their potential use as functional devices by monitoring their resonance frequencies (ranging between 1.7 and 10.5 MHz) using a Fabry–Perot interferometer.

Catalyst–support interactions and their influence in water-assisted carbon nanotube carpet growth

We report results from characterization studies focused on a diverse selection of catalyst support materials in order to understand what makes a good catalyst support during carbon nanotube (CNT) carpet growth via water-assisted chemical vapor deposition. The growth and catalyst morphological changes occurring for thin Fe layers deposited on Al2O3, MgO, TiN, and ZrO2 are compared. The growth behaviors of the catalyst substrates were evidently different, with Al2O3/Fe supporting CNT carpet growth and showing the highest activity and longest lifetime. The TiN/Fe catalyst also supported CNT carpet growth, albeit with much lower activity, shorter lifetime, and lower CNT quality while MgO/Fe and ZrO2/Fe did not support CNT carpet growth under standard growth conditions. Studies using a combination of atomic force microscopy and X-ray photoelectron spectroscopy revealed a general correlation between the catalyst behavior (activity and lifetime) and the 3D evolution of the catalyst for active catalysts (Al2O3/Fe and TiN/Fe). Analysis of inactive catalysts under standard conditions (MgO/Fe and ZrO2/Fe) raise interesting questions related to additional chemical interactions between the substrate and catalyst that could influence nucleation and CNT growth. This work provides a step toward understanding the challenges that arise in engineering efficient CNT growth processes on a desired substrate.

Carbon nanotube growth from metallic nanoparticles deposited by pulsed-laser deposition on different substrates

Carbon nanotubes carpets were grown by RF plasma enhanced chemical vapor deposition on various substrates coated by Fe and Ni transition metals that act as catalyst. C2H2 gas was used for the carbon source. The results show that carbon nanotubes CNT can be grown on Si3N4/Si and SiO2/Si substrates only with an Fe catalyst. They are typically formed by multi-walled graphene layers, and can be obtained for a temperature as low as 550°C. Nanotubes grown on TiN/SiO2/Si substrate from Fe or Ni catalysts present bamboo-like nanostructures and are obtained for particular experimental conditions. This study demonstrates substrate-to-catalyst effect on the CNT growth and their microstructures indicating that the adhesion force of nanoparticles on substrates is a main parameter. Catalyst particles are spherical and several tens of nm in diameter (weak adhesion strength) when deposited onto SiO2/Si or Si3N4/Si, the tip growth mode of nanotube is favored. On TiN/SiO2/Si substrate, particles are larger (large adhesion strength) and CNT growth is no more in tip mode, bamboo-like structures are obtained. When an Fe-Ni catalyst multilayer has been deposited onto the different substrates, carbon nanotube microstructures show multi-walled graphene parallel layers on Si3N4/Si and SiO2/Si insulating substrates, and bamboo-like microstructures on TiN/SiO2/Si conductor substrate.

Diffusion-bonded CNT carpets for fundamental CDI studies

ABSTRACTUncertainty about future energy and water supplies suggests a pressing need to develop efficient technologies for water desalination. Capacitive deionization (CDI), a method that captures ions in the electrical double layer (EDL) of an electrochemical capacitor, is a promising technology that can potentially fulfill those requirements. Similar to supercapacitors, ideal CDI electrodes should have a large electrolyte-accessible specific surface area available for ion adsorption with rapid charging/discharging characteristics. Unlike supercapacitors, CDI electrodes are required to operate in aqueous electrolytes with low ionic concentrations in a non-linear charging regime. To explore this practically and theoretically important regime, we developed robust, electrochemically-compatible carbon nanotube (CNT) carpet electrodes that posses a well-defined and uniform pore structure that is more readily analyzed in comparison to the random and multi-scale pore structure of typical carbon electrodes. The fabricated electrodes were characterized using cyclic voltammetry and potentiostatic charging in aqueous NaCl solutions (no = 20 - 90 mM) using a three electrode setup. Examination of the CV and potentiostatically-measured capacitances were consistent with EDL behavior dictated by the Stern layer. However, some deviations from the expected behavior were observed with increasing salt concentration during potentiostatic testing.

Comparison of the efficiency of various substrates in growing vertically aligned carbon nanotube carpets

Vertically aligned multi-walled carbon nanotube (VACNT) carpets have been synthesized on metal and isolating thin films without the use of a pre-deposited catalyst. We used ferrocene as precursor powder catalyst and acetylene as a carbon source to grow VACNT carpets using three-temperature-zone chemical vapor deposition (TTZ-CVD). Tuning the carrier gas Ar flow rate and the sublimation time of ferrocene in the TTZ-CVD extended the lifetime of the catalyst, increasing the efficiency of growth. Increasing the number of ferrocene loads increased the growth height of VACNT carpets on metal and isolating thin films. The effective growth rates of VACNT carpets were 10.4, 7.13, 4.8, 3, 2, 9.55, 7.6, 13.3, and 32.32 μm/min, respectively, on Cu, Ti, Au, Pt, Mo, Cr, Ag, Ni, and Si. VACNT carpets such as SiO 2 and Al 2O 3 also grew on the isolating thin films with growth rates of 37.28 and 23.8 μm/min, respectively. By controlling the growth parameters, the growth of VACNT carpets on selective substrates can be controlled.