Multiwalled Carbon Nanotube Forests Research Articles

Effects of electrode shape in micro-electro-discharge patterning of carbon nanotube forests

This paper reports the performance of different tool electrode structures in micropatterning of vertically aligned carbon nanotube (CNT) forests through dry micro-electro-discharge machining (μEDM). Micro cylindrical electrodes of tungsten with flat and hemispherically rounded tips were employed in this study. The rounded-tip electrode was shaped using a custom wet μEDM process. Both types of electrodes were used to pattern arrays of micro test slots at different depths and discharge voltages on a multi-walled CNT forest from which the resultant discharge gaps were experimentally characterized. The rounded-tip electrode showed a 2.1–2.4× reduction in the discharge gap with applied voltages of 15–20 V, suggesting its potential to improve the lateral resolution of the patterning process when compared with the flat-tip case. Finite element analysis of the process configuration with these two electrodes was conducted and showed good agreement with the improvement factors of the discharge gap experimentally observed. However, the process using the rounded-tip electrode resulted in larger variations on the bottom surfaces of the created patterns, while the use of the flat-tip electrode provided higher uniformity on these surfaces, revealing an apparent trade-off relationship between the discharge gap and the patterning uniformity for the two electrode types under the tested conditions. Elemental analysis of the processed CNT forest surfaces showed no detectable contamination from the electrodes onto the forest structures patterned using either electrode.

Optimizing platinum-free counter electrodes for dye-sensitized solar cells: Multiwalled carbon nanotube forests covered with ruthenium layers

Multiwalled carbon nanotube (MWCNT) forests have extremely large surface areas and high catalytic activity. To use them as alternative materials for Pt/fluorine-doped tin oxide (FTO) conventional counter electrodes (CEs) in dye-sensitized solar cells (DSSCs), a unique Ru metal layer, prepared with 600-cycle atomic layer deposition, was employed. The best Ru-coated MWCNT forest (Ru/MWCNT) CE performance was achieved by examining the experimental conditions for the MWCNT forests, including deposition processes, catalyst thicknesses and synthesis time. The results revealed that 20μm-thick MWCNT forests with numerous defects/disordered sites exhibit excellent CE performance. In practice, Ru/MWCNT CEs prepared under optimal synthesis conditions show low charge transfer resistance (Rct of approximately 2.5 ohm) and series resistance (Rs of approximately 14 ohm) that are even better than those for Pt/FTO CEs (Rct=6.36 ohm and Rs=16.4 ohm). Owing to these excellent Rct and Rs values, dye-sensitized solar cells with optimized Ru/MWCNT CEs showed better performance than those containing Pt/FTO CEs. Finally, post-heat treatment of Ru/MWCNT CEs increased the cell efficiency of the DSSCs.

Carbon nanotube substrates enhance SARS-CoV-2 spike protein ion yields in matrix-assisted laser desorption–ionization mass spectrometry

Nanostructured surfaces enhance ion yields in matrix-assisted laser desorption–ionization mass spectrometry (MALDI-MS). The spike protein complex, S1, is one fingerprint signature of Sars-CoV-2 with a mass of 75 kDa. Here, we show that MALDI-MS yields of Sars-CoV-2 spike protein ions in the 100 kDa range are enhanced 50-fold when the matrix–analyte solution is placed on substrates that are coated with a dense forest of multi-walled carbon nanotubes, compared to yields from uncoated substrates. Nanostructured substrates can support the development of mass spectrometry techniques for sensitive pathogen detection and environmental monitoring.

Dry Synthesis of Binder-Free Ruthenium Nitride-Coated Carbon Nanotubes as a Flexible Supercapacitor Electrode.

Ruthenium nitride was successfully deposited on a multiwalled carbon nanotube (MWCNT) forest grown on a stainless-steel mesh substrate by radiofrequency plasma-assisted pulsed laser deposition. This novel dry fabrication method for flexible supercapacitor electrodes eliminates toxic byproducts and the need for any binder component. Experimental results show a successful thin film coating of the individual MWCNTs with RuNx under various synthesis conditions. The electrochemical characterization demonstrates a significant improvement in capacitance of the synthesized RuNx-MWCNT electrode compared to the bare MWCNT forest, with a large potential window of 1.2 V. Capacitance values as high as 818.2 F g-1 (37.9 mF cm-2) have been achieved.

Tailoring the surface morphology of carbon nanotube forests by plasma etching: A parametric study

The top surface of carbon nanotube (CNT) “forests” produced by thermal chemical vapor deposition (CVD) often has a tangled morphology, owing to the self-organization of the CNTs at the initial stage of the CVD process. Removal of this top “crust” layer, without damaging the intrinsic microstructure of the CNT forest is often a key step for further applications. This paper studies the tailored use of Ar/O2 plasma etching to modify the surface morphology of multi-walled CNT forests. First, we investigate the effects of process parameters including plasma power, flow rate, and gas composition on the etching of CNT forests. As a result, we identify the plasma conditions that successfully remove the top crust yet maintain the structural shape of a CNT forest. Second, we prepare CNT forests having different packing densities and heights to study the influence of the initial characteristics on the etching result. We experimentally confirm that the etching rate depends strongly on the density and morphology of the crust layer. We also compare the material removal rate in vertical and lateral directions. Finally, we explore the enhancement of alignment and chemical uniformity of the top surface of CNT forests by the Ar/O2 plasma

Towards 3D self-assembled rolled multiwall carbon nanotube structures by spontaneous peel off.

Controlling the 3D assembly of individual nanomaterials can be a challenging task. However, it opens up opportunities for the production of increasingly complex nanostructures. Unusual rolled multiwall carbon nanotube structures are synthesized here by simply inducing a change of precursor composition during the growth of multiwall carbon nanotube forests. The multiwall carbon nanotube structures are comprised of nitrogen-doped and undoped sections, and are obtained via a detailed peel off and roll mechanism. These results open new doors for the development of increasingly complex nanostructures.

Janus Structured Multiwalled Carbon Nanotube Forests for Simple Asymmetric Surface Functionalization and Patterning at the Nanoscale

The asymmetric functionalization of nanomaterials is desired to develop nanostructures with increasingly exotic properties. Nevertheless, it remains a general challenge and a difficult task. Janus ...

Infrared absorbance of vertically-aligned multi-walled CNT forest as a function of synthesis temperature and time

In this paper, the growth of optimized vertically aligned multi-walled carbon nanotube (VA-MWCNT) forests by LPCVD method for use in a large-area absorber in infrared detectors is presented. The effect of synthesis temperature (500−700 °C) and time (1−10 min) on the optical absorption coefficient in the infrared (2−20 μm) is investigated by FT-IR measurement at various incident angles (15-80°). The structural properties of VA-MWCNT are characterized by SEM, TEM and Raman spectroscopy. Spectral measurements show an increasing absorption with the height of the forest that results at increased synthesis time and temperature. However, the absorption coefficient decreases with increasing synthesize time and temperature, while it is also affected by other properties, such as diameter, density, alignment, and uniformity. Moreover, the reduction in absorption at oblique incident angles demonstrates the relevance of surface properties. Finally, a circular graphite waveguide system is used to model the absorption characteristics of an MWCNT forest.

Flame-Synthesis of Carbon Nanotube Forests on Metal Mesh Structure: Dependence, Morphology, and Application.

Multi-walled carbon nanotubes (MWCNTs) in the form of “forests” were synthesized directly on the surface of stainless steel (SS) mesh from ethanol flame volume. The growth dependence of the MWCNT forests on the porosity of SS mesh substrate and the morphologies and growth mechanism of the MWCNT forests were investigated in detail by a combination of turbulent flow simulation, scanning electron microscopy (SEM), transmission electron microscope (TEM), and Raman and X-ray diffraction (XRD) spectroscopy. The growth height of the MWCNT forests exhibited a strong dependence on the flame gas flow rate controlled by the porosity of SS mesh substrate, and the maximum averaged height of the MWCNT forests reached 34 μm. Most MWCNTs grew perpendicularly on the surface of SS wires, and some branch, welded, and spiral structures were observed by SEM and TEM. The MWCNT-decorated mesh was used as a novel heating element to weld glass-fabric-reinforced polyetherimide (GF/PEI) thermoplastics. We found that the maximum tensile lap-shear strength (LSS) of the welded joints could reach 39.21 MPa, an increase of 41% in comparison with that of conventional SS mesh-based joints.

Penetration depth variation in atomic layer deposition on multiwalled carbon nanotube forests

Atomic layer deposition (ALD) of Al2O3 on tall multiwalled carbon nanotube forests shows concentration variation with depth in discrete steps. While ALD is capable of extremely conformal deposition in high aspect ratio structures, decreasing penetration depth has been observed over multiple thermal ALD cycles on 1.3 mm tall multiwalled carbon nanotube forests. Scanning electron microscopy imaging with energy dispersive x-ray spectroscopy elemental analysis shows steps of decreasing intensity corresponding to decreasing concentrations of Al2O3. A study of these steps suggests that they are produced by a combination of diffusion limited precursor delivery and the increase in precursor adsorption site density due to nuclei growing during the ALD process. This conceptual model has been applied to modify literature models for ALD penetration on high aspect ratio structures, allowing two parameters to be extracted from the experimental data. The Knudsen diffusion constant for trimethylaluminum (TMA) in these carbon nanotube forests has been found to be 0.3 cm2 s−1. From the profile of the Al2O3 concentration, the sticking coefficient of TMA in the TMA/water thermal ALD process was found to be 0.003.

Multi-walled carbon nanotube forests covered with atomic-layer-deposited ruthenium layers for high-performance counter electrodes of dye-sensitized solar cells

Abstract On account of the high surface area and good catalytic activity of multi-walled carbon nanotube (MWNT) forests, they have great potential for use as alternative materials to conventional Pt/fluorine-doped tin oxide (FTO) counter electrodes (CEs). Nevertheless, the inferior charge collection efficiency of MWNT forest CEs is a drawback that needs to be overcome. Here, we uniquely introduce a Ru layer on MWNT forests by a typical atomic layer deposition (ALD) process. Under an optimal ALD cycle number of 600, Ru layers directly connect vertically grown MWNTs by acting as their bridges while also retaining the microporous structure of the MWNTs. As a result, deposition of the Ru layer under the optimal conditions causes a decrease in the resistances of as-synthesized MWNT forests (charge transfer resistance Rct: from 155.3 Ω to 4.12 Ω; series resistance Rs: from 205.1 Ω to 12 Ω); these decreased resistances are better than even those of Pt/FTO CEs (Rct = 14.2 Ω and Rs = 17.6 Ω). Dye-sensitized solar cells with the optimally deposited Ru/MWNT forest CEs show performances equivalent to those of devices with Pt/FTO CEs on account of the excellent Rct and Rs values of the former CEs.

Interlaminar fracture toughness of hybrid carbon fiber‐carbon nanotubes‐reinforced polymer composites

In this study, a relatively low temperature synthesis technique; graphitic structure by design (GSD), was employed to grow multi‐walled carbon nanotubes (MWCNTs) over carbon fiber fabrics. The MWCNTs forests were grown into two different morphologies, uniform and patterned. Hybrid carbon fiber‐reinforced polymer composites (CFRPs) were fabricated based on the hybrid reinforcement. Double cantilever beam tests were performed to investigate the effect of the surface grown nano‐reinforcements on the Mode I interlaminar fracture toughness (GIc) of the hybrid CFRPs. Results revealed that the surface grown MWCNTs enhanced the GIc of the CFRPs by 22 and 32%, via uniform and checkerboard‐patterned growth morphologies, respectively. Fractography was also employed to reveal the MWCNTs’ role in interlaminar crack stoppage and deflection resulting in improving GIc of the hybrid CFRPs. POLYM. COMPOS., 40:E1470–E1478, 2019. © 2018 Society of Plastics Engineers

Multiwalled carbon nanotube forests grown on the surface of synthetic diamond crystals

Abstract Forests of multiwalled carbon nanotubes (MWCNTs) were grown on the facets of synthetic diamond crystals by treating them in a rotary chemical vapor deposition reactor. Nickelocene was used as a source of both nickel catalyst and carbon for the formation of MWCNTs. The diamond crystals were not subjected to any surface treatment prior to the MWCNT growth. The MWCNTs were anchored in nickel particles adhered to the diamond facets and were partially filled with nickel. These morphological features of the diamond-MWCNT structures together with nanotube branching suggest the formation of MWCNTs by the combined growth mechanism.

Nitrogen-doped multiwalled carbon nanotubes decorated with copper(I) oxide nanoparticles with enhanced capacitive properties

We report on the enhanced capacitive properties of a copper(I) oxide nanoparticle (Cu2O NP)-decorated multiwalled carbon nanotube (MWCNT) forest with nitrogen (N) doping. A careful in situ solid-state dewetting and plasma doping method was developed that ensured homogeneous decoration and contamination-free Cu2O NPs with N doping on the nanotube sidewalls. The morphology and structure of the hybrid materials were characterised by scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, Raman spectroscopy and X-ray photoemission spectroscopy. The electrochemical performance of the hybrid materials was investigated by cyclic voltammetry and galvanostatic charge/discharge tests in a 0.1 M Na2SO4 electrolyte. The electrochemical tests demonstrated that the Cu2O NP/N-MWCNT electrode exhibits a specific capacitance up to 132.2 F g−1 at a current density of 2.5 A g−1, which is 30% higher than that of the pure MWCNT electrode. Furthermore, the electrode could retain the specific capacitance at 85% stability over 1000 cycles. These observations along with the simple assembly method for the hybrid materials suggest that the Cu2O NP/N-MWCNT could be a promising electrode for supercapacitor applications.

Growth of vertically aligned multiwalled carbon nanotubes forests on metal alloy Ni-Nb-N with low content of catalyst

This research shows the possibility of carbon nanotubes (CNTs) formation on the surface of low nickel (∼ 10 at.%) Ni-Nb-N amorphous metal alloy film by CVD method at 550 °C of the gas mixture based on acetylene. The structure of CNT were studied by transmission and scanning-electron microscopy, energy-dispersive X-ray and the Raman spectroscopy.

Fabrication and characterization of a CNT forest integrated micromechanical resonator for a rarefied gas analyzer in a medium vacuum atmosphere

This study focuses on the development of a multi-walled carbon nanotube (MWCNT) forest integrated micromechanical resonator working as a rarefied gas analyzer for nitrogen (N2) and hydrogen (H2) gases in a medium vacuum atmosphere. The resonant response is detected in the form of changes in the resonant frequency or damping effects, depending on the rarefied gas species. The carbon nanotube (CNT) forest on the resonator enhances the effective specific surface area of the resonator, such that the variation of the resonant frequency and the damping effect based on the gas species increase significantly. We developed the fabrication process for the proposed resonator, which consists of standard micro-electro-mechanical systems (MEMS) processes and high-density CNT synthesis on the resonator mass. The high-density CNT synthesis was realized using multistep alternate coating of two types of ferritin proteins that act as catalytic iron particles. Two devices with different CNT densities were fabricated and characterized to evaluate the effect of the surface area of the CNT forest on the resonant response as a function of gas pressures ranging from 0.011 to 1 Pa for N2 and H2. Considering the damping effect, we found that the device with higher density was able to distinguish N2 and H2 clearly, whereas the device with lower density showed no difference between N2 and H2. We confirmed that a larger surface area showed a higher damping effect. These results were explained based on the kinetic theory of gases. In the case of resonant frequency, the relative resonant frequency shift increased with gas pressure and surface area because of the adsorption of gas molecules on the resonator surfaces. Higher density CNT forest adsorbed more gas molecules on the surfaces. The developed CNT forest integrated micromechanical resonator could successfully detect N2 and H2 gases and distinguish between them under pressures of 1 Pa.

Investigation on thermal and cryogenic behaviour of carbon nanotube spun yarns using a dynamic mechanical analyser

Pure carbon nanotube (CNT) yarns were spun from highly aligned multiwall CNT forests, and their thermal and cryogenic behaviour under cyclic heating–cooling treatment was studied using a Dynamic Mechanical Analyser over a temperature range from −100 to 200 °C. The effect of static tension force applied on the CNT yarn during the treatment was also examined. The study primarily focused on the dimensional changes of the CNT spun yarns in response to thermal and cryogenic treatment, but the changes in morphology and mechanical properties were also discussed. The results revealed a special thermal/cryogenic behaviour of the CNT spun yarns, and provided technical information and engineering data for the design and application of CNT spun yarns.

Sheet resistance measurements of carbon nanotube forests for extended electrodes

Owing to their outstanding electrical properties, carbon nanotubes (CNTs) are currently being considered as interconnects in microelectronic circuits and large area electrodes. For large area electrodes, CNT must grow as dense and vertically aligned forests, displaying sheet resistances as low as possible. Here we present two straightforward methods for the assessment of the sheet resistance of CNT forests. The first one is based on the DC electrical characterization of a CNT forest bar with distributed Mo contacts. The second consists in RF measurements, at different frequencies, of capacitors containing CNT forests as top electrodes. The two methods enable to assess the sheet resistance of multi-walled CNT forests grown by chemical vapor deposition on SiO2 substrates using thin Fe catalyst layers grown on thin Ti stabilizer films. DC characterization points out to a clear dependency of the CNT sheet resistance on the thickness of Fe catalyst, which can be related to the CNT area density. Additionally, the variations of the capacitor effective area stressed by RF measurements confirm this dependency and provide us with a method for the estimation of nanotube area density.

Multiwalled carbon nanotube forest grown via chemical vapor deposition from iron catalyst nanoparticles by ToF-SIMS

The authors report the time-of-flight secondary ion mass spectrometry of carbon nanotubes grown catalytically from iron nanoparticles. Both positive and negative ion spectra were obtained using a cluster ion source (Bi32+ primary ions at 50 keV). The positive mode contains an intense C+ signal. The Cn− peaks in the negative ion spectrum show an odd–even effect in their intensities.

Optical reflection and absorption of carbon nanotube forest films on substrates

The correlation between the height and density of multi-walled carbon nanotube forests and their optical properties in the mid-infrared region was investigated using nanotube forests grown on Al/Si, Al/Nb/Si, and fused silica substrates. Measurements of the hemispherical reflectance and transmittance of carbon nanotube forests are presented. Analyses by an effective medium approximation and a circular waveguide model are compared. It is found that circular waveguides with graphite walls of reduced conductivity can generate similar spectra of the absorption coefficients as carbon nanotube forests do. Parameters from the waveguide model can describe qualitatively the density and alignment of carbon nanotubes in the forest. With a proper density, a randomly modulated forest of less than 20 μm in height can generate a hemispherical reflectance of less than 0.002 in the mid-infrared region.