High-density Carbon Nanotubes Research Articles

Floating Bimetallic Catalysts for Growing 30cm-Long Carbon Nanotube Arrays with High Yields and Uniformity.

Ultralong carbon nanotubes (CNTs) are considered as promising candidates for many cutting-edge applications. However, restricted by the extremely low yields of ultralong CNTs, their practical applications can hardly be realized. Therefore, new methodologies should be developed to boost the growth efficiency of ultralong CNTs and alleviate their areal density decay at the macroscale level. Herein, a facile, universal, and controllable method for the in-situ synthesis of floating bimetallic catalysts (FBCs) is proposed to grow ultralong CNT arrays with high yields and uniformity. Ferrocene and metal acetylacetonates serve as catalyst precursors, affording the successful synthesis of a series of FBCs with controllable compositions. Among these FBCs, the optimized FeCu catalyst increases the areal density of ultralong CNT arrays to a record-breaking value of ∼8100 CNTs mm-1, and exhibits a lifetime 3.40 times longer than that of Fe, thus achieving both high yields and uniformity. A 30-centimeters-long and high-density ultralong CNT array is also successfully grown with the assistance of FeCu catalysts. As evidenced by our kinetic model and molecular dynamics simulations, the introduction of Cu into Fe can simultaneously improve the catalyst fluidity and decrease carbon solubility, and an optimal catalytic performance will be achieved by balancing this tradeoff. This article is protected by copyright. All rights reserved.

Assembly and Alignment of High Packing Density Carbon Nanotube Arrays Using Lithographically Defined Microscopic Water Features.

High packing density aligned arrays of semiconducting carbon nanotubes (CNTs) are required for many electronics applications. Past work has shown that the accumulation of CNTs at a water-solvent interface can drive array self-assembly. Previously, the confining interface was a large-area, macroscopic feature. Here, we report on the CNT assembly on microscopic water features. Water microdroplets are formed on 10-100 μm wide hydrophilic stripes patterned on a substrate. Exposure to CNTs dispersed in solvent accumulates CNTs at the microdroplet-solvent interface, driving their alignment and deposition at the microdroplet-solvent-substrate contact line. Compared with macroscopic methods in which the contact line uncontrollably moves across the substrate as it is pulled out of the liquids, the hydrophilic patterns and microdroplets allow pinning of the contact line. As CNTs deposit, the contact line self-translates, allowing for dense CNT packing. We realize monolayer CNT arrays aligned within ±3.9° at density of 250 μm-1 and field effect transistors with a high current density of 1.9 mA μm-1 and transconductance of 1.2 mS μm-1 at -0.6 V drain bias and 60 nm channel length.

Impact of dynamic density decay of growing carbon nanotube forests on electrical resistivity

Vertically aligned carbon nanotube (CNT) forests densely formed on diverse metallic substrates are desirable as large-area electrodes and thermal interface materials. In this study, we achieved high-density CNT forests on stainless steel and Cu substrates by preventing the interaction between the metallic elements and Fe catalysts at high temperatures. We found that coating the substrate surface with a combination of Ta and SiO2 effectively suppressed the diffusion of metallic elements of the substrate to the catalytic layer. The macroscopic electrical resistivity of longer forests was found to increase as the number density of CNTs decreased during the growth process. This increase in resistivity was reproduced by the dynamic density decay model in which CNT density decreases with forest growth. We point out the importance of the relationship between the length and density of CNT forests for applications.

High-Speed Carbon Nanotube Photodetectors for 2 μm Communications.

In the era of big data, the growing demand for data transmission capacity requires the communication band to expand from the traditional optical communication windows (∼1.3-1.6 μm) to the 2 μm band (1.8-2.1 μm). However, the largest bandwidth (∼30 GHz) of the current high-speed photodetectors for the 2 μm window is considerably less than the developed 1.55 μm band photodetectors based on III-V materials or germanium (>100 GHz). Here, we demonstrate a high-performance carbon nanotube (CNT) photodetector that can operate in both the 2 and 1.55 μm wavelength bands based on high-density CNT arrays on a quartz substrate. The CNT photodetector exhibits a high responsivity of 0.62 A/W and a large 3 dB bandwidth of 40 GHz (setup-limited) at 2 μm. The bandwidth is larger than that of existing photodetectors working in this wavelength range. Moreover, the CNT photodetector operating at 1.55 μm exhibits a setup-limited 3 dB bandwidth over 67 GHz at zero bias. Our work indicates that CNT photodetectors with high performance and low cost have great potential for future high-speed optical communication at both the 2 and 1.55 μm bands.

Imaging mechanism and contrast separation in low-voltage scanning electron microscopy imaging of carbon nanotube arrays on SiO2/Si substrate

Polymer-sorted high-density carbon nanotube (CNT) arrays have shown great potential to extend the silicon-based Moore's law. Imaging the CNT arrays on insulators like SiO2/Si using low-voltage scanning electron microscopy (LVSEM) to acquire array information like the alignment, density, and distribution of residual polymers is necessary. Such a task remains challenging due to the nanoscale CNT body (1–2 nm in diameter), nanoscale tube-to-tube separation (1–10 nm), the broadening of the apparent diameter, and the complex image contrast caused by the insulating substrate and polymer residues. In this study, the imaging mechanism for this system is investigated. Two methods are developed to separate the three dominant contrasts, i.e. topographic contrast, charge contrast, and material contrast, by selecting the take-off angle and energy of the emitted electrons as enabled by changing the working distance or the deceleration voltage. The contrast formation and separation mechanism is further confirmed by the dynamic contrast evolution due to the electron-beam-induced deposition of amorphous carbon. The contrast separation method is further applied to an individual CNT, reducing its apparent diameter from 36 nm to 6 nm. This result hints at the potential for LVSEM to count the density exceeding 150 CNTs/μm of CNT arrays. Finally, a comparative study of LVSEM and transmission electron microscopy confirms the failure of LVSEM to resolve CNTs in a bundle. The results suggest that the density of CNT arrays may be underestimated in reported SEM data. The proposed method can serve as a useful tool for further study and application of arrayed CNTs.

Bifunctional lithiophilic carbon fibers with hierarchical structure for high-energy lithium metal batteries

Lithium (Li) metal is an impeccable candidate anode for satisfying the energy density requirements of next-generation Li batteries. However, Li dendritic growth and fragile solid-elecrolyte interphase (SEI) caused by the high reactivity between Li and electrolyte are the primary challenges for its large-scale applications. Herein, a bifunctional lithiophilic hierarchical substrate composed of high-density nitrogen-doped carbon nanotubes and Co nanoparticles encapsulated in graphene (Co@G) decorated carbon fibers (Co-N-CNT-CF) can modulate the structural dimensions and hierarchy of Li nucleation/growth and alleviate Li volume expansion, achieving the homogeneous Li plating/stripping behavior. Density functional theory (DFT) calculations and experimental results confirm the highly lithiophilicity of the substrate, which exhibits a low Li nucleation overpotential, enhanced Coulombic efficiency (CE), small voltage hysteresis, and ultrastable lifespan without dendritic formation. As coupled with the thick LiFePO4 and LiCoO2 (∼2 mA h cm−2) cathodes, the Co-N-CNT-CF@Li composite anode (N/P = 3) enables a high reversible capacity, high Li utilization, and improved cycle stability. This work engineers a hierarchical structure of the three-dimensional (3D) lithiophilic carbon substrate for realizing highly reversible, dendritic-free Li metal anodes.

Growth of high-density single-wall carbon nanotubes with a uniform structure using a CoRu catalyst

The inefficient production of structurally uniform single-wall carbon nanotubes (SWCNTs) is an obstacle to their practical use in high-performance electronic devices. We have synthesized SWCNTs with a narrow diameter distribution (1.35 ± 0.25 nm) using a CoRu catalyst. Monodispersed nanoparticles with a narrow size distribution (2.4 ± 0.6 nm) and different compositions were prepared and used as catalysts for SWCNT growth. A furnace with an 80 cm-long uniform temperature zone (±10 °C) was designed and used to study the effect of catalyst composition on the growth of SWCNTs under the same conditions. By optimizing the composition of the bimetallic CoRu catalyst, SWCNTs with a uniform structure were efficiently synthesized. In addition, the effect of the growth conditions of temperature and carbon feed rate was investigated, and it was found that with an increase in yield, the structural uniformity of SWCNTs usually became worse. Both catalysts with elements in the suitable proportions and appropriate growth conditions are critical to achieving the high-efficiency structure-controlled growth of SWCNTs.

Hierarchical Nanocomposites Electrospun Carbon NanoFibers/Carbon Nanotubes as a Structural Element of Potentiometric Sensors.

This work proposes new carbon materials for intermediate layers in solid-contact electrodes sensitive for potassium ions. The group of tested materials includes electrospun carbon nanofibers, electrospun carbon nanofibers with incorporated cobalt nanoparticles and hierarchical nanocomposites composed of carbon nanotubes deposited on nanofibers with different metal nanoparticles (cobalt or nickel) and nanotube density (high or low). Materials were characterized using scanning electron microscopy and contact angle microscopy. Electrical parameters of ready-to-use electrodes were characterized using chronopotentiometry and electrochemical impedance spectroscopy. The best results were obtained for potassium electrodes with carbon nanofibers with nickel-cobalt nanoparticles and high density of nanotubes layer: the highest capacity value (330 µF), the lowest detection limit (10−6.3 M), the widest linear range (10−6–10−1) and the best reproducibility of normal potential (0.9 mV). On the other hand the best potential reversibility, the lowest potential drift (20 μV·h−1) in the long-term test and the best hydrophobicity (contact angle 168°) were obtained for electrode with carbon nanofibers with cobalt nanoparticles and high density of carbon nanotubes. The proposed electrodes can be used successfully in potassium analysis of real samples as shown in the example of tomato juices.

Fabrication of Inkjet-Printed Carbon Nanotube for Enhanced Mechanical and Strain-Sensing Performance

This paper presents fabrication of inkjet-printed carbon nanotube film on flexible substrate for wearable electronics applications. The density of CNT films is optimized by droplet spacing (DS) and multiple passes to provide the best strain behavior. It is found that low-density carbon nanotubes have fewer conductive pathways resulting in less change and low GF under applied strain. Conversely, high-density carbon nanotubes have more conductive paths, and they are not easily broken under strain, resulting in poor strain-sensing ability. The inkjet printing process can adjust uniformity and density of CNT film through DS and multiple passes to optimize its strain characteristics. The highest GF of 3.36 was obtained under strain ranging from 71 to 3128 με when CNT printed by DS of 23 μm and 20 passes. The relative change in resistance under various strains, ranging from 71 to 3128 με, had a stable peak value for each 20 strain/release cycle which proved its repeatability and stability. Furthermore, inkjet-printed CNT sensors monitored human movement of various joints and distinguished bending angle demonstrating its potentially practical application in wearable electronics.

Microstructure and mechanical properties of Mg-14Li-1Al/MWCNTs composites prepared by electrophoretic deposition and accumulative roll bonding

Mg-14Li-1Al/MWCNTs composites were prepared by electrophoretic deposition and accumulative roll bonding (ARB). The mechanical properties and microstructure was characterized by tensile test, microhardness, optical microscope and transmission electron microscope. After 6 cycles of ARB, the yield strength, ultimate tensile strength and elongation of the composite are 213 MPa, 266 MPa and 21.6%, respectively. Compared with the as-cast alloy and as-rolled alloy, the yield strength and ultimate tensile strength of the composite were increased by 142.04% and 82.05%, 79.72% and 60.24%, respectively. A highest specific strength of 189 kNm/kg was obtained. The highest microhardness value of the composite is 84.6 HV, which is 66.86% and 49.73% higher than that of the as-cast (50.7 HV) and as-rolled (56.5 HV) alloys. High density dislocations, ultrafine grains, and carbon nanotubes with uniform dispersion were observed in the Mg-14Li-1Al/MWCNTs composites. The strengthening of the composites is mainly contributed to fine grain strengthening and dislocation strengthening.

Synergistic Flame Retardant Effects of Carbon Nanotube‐Based Multilayer Nanocoatings

AbstractIn an effort to develop highly functionalized flame retardant materials, hybrid nanocoatings are prepared by alternately depositing a positively charged polyaniline (PANi) and negatively charged montmorillonite (MMT) using the layer‐by‐layer (LbL) assembly technique. Carbon nanotubes (CNTs) are employed in polymer nanocomposites as effective reinforcement, where nanotubes are stabilized in MMT aqueous solution. The 3D structure and high density of CNTs deposited in the PANi/CNTs‐MMT multilayers produce thicker and heavier coatings in comparison to the LbL assemblies without CNTs. Vertical and horizontal flame testing show that the incorporation of CNTs improves fire resistance. Additionally, cone calorimetry reveals that stacking two nanomaterials (MMT and CNTs) in a single coating shows a significant reduction in peak heat release rate (up to 51%), total smoke release (up to 47%), and total heat release (up to 37%) for the polyurethane foam. The enhancement of flame retardancy is attributed to a synergistic effect; MMT serves as a physical barrier that retards the diffusion of heat and gas. The addition of CNTs strengthens the thermal stability and high char yield. These results, coupled with the simplicity with which the LbL deposition is applied, present a viable alternative to halogen‐free flame retardant nanocoatings to natural and synthetic fibers.

Thermal and Structural Properties of High Density Polyethylene/Carbon Nanotube Nanocomposites: A Comparison Study

The effects of functionalization of carbon nanotubes on the properties of nanocomposite sheets prepared from high-density polyethylene (HDPE) and carbon nanotubes (CNTs) were investigated. Carbon nanotubes were first oxidized, followed by amine group functionalization. The Fourier transform-infrared (FTIR) spectroscopy results confirm the presence of oxygenated and amide groups at the surface of the CNTs after each treatment. The HDPE/CNT nanocomposites sheets were prepared using a melt compounding method. Six types of CNTs were used; pristine Single-walled Carbon nanotubes (SWCNT) and pristine Multi-walled Carbon nanotubes (MWCNT), oxidized (O-SWCNT and O-MWCNT) and amide (Amide-SWCNT and Amide-MWCNT). All prepared nanocomposite sheets were characterized using Thermal gravimetric analysis (TGA), Differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electronic microscope (SEM). TGA results measured increased thermal stability of the polymer with the addition of CNTs, O-MWCNT showed the best enhancement. XRD measurements confirmed that the addition of CNTs did not change the crystal structure of the polymer, although the crystallinity was decreased. The maximum crystallinity decrease resulted from O-SWNTs addition to the polymer matrix. SEM imaging showed that oxidized and functionalized CNTs have more even dispersion in the polymer matrix compared with pristine CNTs.

A study of catalyst particles encapsulated inside multiwalled carbon nanotubes on zeolite and montmorillonite

Synthesis of carbon nanotubes on zeolite and montmorillonite is presented. Prior to synthesis, minerals were enriched by particles of iron. Synthesis of carbon nanotubes was performed by hot filament chemical vapour deposition using methane as a source of carbon. A high density of carbon nanotubes was observed in the whole volume of zeolite and montmorillonite. Iron containing particles catalysing the formation and growth of carbon nanotubes were visualized at the ends of nanotubes by field-emission scanning electron microscopy in a combined mode of secondary and backscattered electrons. 57Fe Mössbauer spectroscopy was employed to find the oxidation state of iron in Fe-montmorillonite before the synthesis of nanotubes and in the structures of nanocomposites after the synthesis. The metallic particles in the structures of nanocomposites at the ends of carbon nanotubes are characterized by Mössbauer spectroscopy mainly as ferromagnetic cementite (Fe3C).

Growth of Homogeneous High-Density Horizontal SWNT Arrays on Sapphire through a Magnesium-Assisted Catalyst Anchoring Strategy.

In-situ growth of high-density single-walled carbon nanotube (SWNT) arrays with homogeneity is highly desirable for integrated circuits. However, disastrous migration and aggregation of catalyst nanoparticles on substrate has greatly limited the area of as-grown SWNT arrays. Herein, we develop a magnesium-assisted catalyst anchoring strategy to restrain catalyst nanoparticles sintering on substrate. Magnesium modification ameliorates sapphire surface by high temperature solid reaction and thus provides a stronger metal-support interaction (SMSI). Hereby, we realize the direct growth of high-density SWNT arrays that fully cover an entire 10×10 mm2 substrate with the local highest density of ≈110 tubes μm-1 using iron as catalyst. This strategy was also proven universal when employing solid carbide catalysts.

Deposition of High-Density Carbon Nanotube-Containing Nickel-Based Composite Films by Low-Pressure Cold Spray

Carbon nanotubes (CNTs) have a large specific surface area and, thus, are expected to have applications as electrodes, e.g., in fuel cells. A CNT-containing composite metal film with high electrode performance could be realized by implementing a large number of CNTs on the film. However, a composite film with a high density of CNTs has not yet been obtained. In the thermal spray field, including cold spray, metal–CNT composite films with more than 20 wt% of carbon nanotubes has not yet been deposited. In this study, high-density CNT-containing metal-based composite films were deposited onto an aluminum substrate by low-pressure cold spray from electroless nickel-plated CNT particles. The electroless nickel plating process caused slight structural damage to the CNTs, whereas the cold spray process did not cause any significant structural damage to the CNTs. Two phases were observed in the films: a CNT-rich layer and a nickel-rich layer. However, the CNTs maintained sufficient adhesive strength to remain attached to the bottom nickel phase. Composite films with at least 65 wt% CNT were produced with our method.

Synthesis of Vertical Carbon Nanotube Interconnect Structures Using CMOS-Compatible Catalysts.

Synthesis of the vertically aligned carbon nanotubes (CNTs) using complementary metal-oxide-semiconductor (CMOS)-compatible methods is essential to integrate the CNT contact and interconnect to nanoscale devices and ultra-dense integrated nanoelectronics. However, the synthesis of high-density CNT array at low-temperature remains a challenging task. The advances in the low-temperature synthesis of high-density vertical CNT structures using CMOS-compatible methods are reviewed. Primarily, recent works on theoretical simulations and experimental characterizations of CNT growth emphasized the critical roles of catalyst design in reducing synthesis temperature and increasing CNT density. In particular, the approach of using multilayer catalyst film to generate the alloyed catalyst nanoparticle was found competent to improve the active catalyst nanoparticle formation and reduce the CNT growth temperature. With the multilayer catalyst, CNT arrays were directly grown on metals, oxides, and 2D materials. Moreover, the relations among the catalyst film thickness, CNT diameter, and wall number were surveyed, which provided potential strategies to control the tube density and the wall density of synthesized CNT array.

Ultralong Electrospun Copper–Carbon Nanotube Composite Fibers for Transparent Conductive Electrodes with High Operational Stability

Recent advances in nanotechnology have provided new materials which have the potential to surpass copper and aluminum alloys in electrical conductivity, weight and ampacity [2-6]. Among these carbon nanotubes (CNTs) stand out due to their remarkable thermal and electrical conductivity and ampacity (103 times of copper), low density, abundance of precursor materials and supreme mechanical properties. However, making these materials into a continuous fiber or macrostructures has remained the main obstacle. A promising approach to tackle this issue is employing CNTs as nanofillers in copper matrixes. Subramaniam et al. [5] combined several simple fabrication to produce high density CNT (45 vol%)-Cu composite films with specific conductivity 26% greater than copper. These results show that by improving the interfacial interactions between the CNTs and the copper matrix strong and highly conductive nanocomposites can be made. In this research continuous CNT-Cu nanocomposite is produced using electrospinning method. For improving the interfacial interaction the surface of the CNTs were first coated with a thin layer of copper using electroless deposition methods (the process is explained in details in [7]).

Study of self-organized structure in carbon nanotube forest by fractal dimension and lacunarity analysis

In this work, a fractal analysis of a morphology of carbon nanotube (CNT) forest was conducted. A self-similarity between the formation of catalyst nanoparticles, low and high-density CNT forest growth, and Raman mapping was investigated, for a catalyst thickness of 0.8, 1.0, and 1.2 nm. To explain reasons behind fluctuations of the CNT forest structure, a fractal dimension (Df) and lacunarity (Λ) were calculated, using a box-counting method. During the fractality analysis of catalyst particles, the highest Df was obtained for the 0.8-nm-thick film (Df = 1.91), while the lowest one was noted for the Fe catalyst thickness of 1.2 nm (Df = 1.72). The progressive increase of the catalyst thickness affected the dewetting, Ostwald ripening and diffusion processes, affecting the catalyst size distribution and arrangement. The same thickness variation led to a similar decreasing trend of Df changes, which was observed in the planar and linear analysis of the distribution of CNTs and in the IG/ID ratio distribution in Raman mapping, confirming the complexity and the self-organizing properties of CNT forest. The self-similarity and scaling of properties of self-organized CNT forest structure were proved by the analysis of fractal dimension and lacunarity analysis and revealed different stages of the catalyst annealing.

Experimental and numerical approach to study mechanical and fracture properties of high-density polyethylene carbon nanotubes composite

This work investigates the elasto-plastic and fracture properties of high-density polyethylene (HDPE) carbon nanotubes (CNTs) composite using a multi-scale finite element (MSFE) approach. The composites consist of CNTs, which are randomly embedded within the HDPE matrix throughout the volume. CNT reinforcement is numerically modelled by considering elastic properties while the elasto-plastic constitutive law is used for the matrix (HDPE) composition. Elastic properties of composites are estimated at meso-scale by numerical modelling of a representative volume element (RVE). Hollomon’s (power-hardening law) model has been employed to get the plastic properties of the polymer composite. Equivalent consecutive properties are predicted at meso-scale, further, these properties have been used at macro-scale simulation for fracture analysis. A detailed study of stress intensity factors has been presented for the different volume fraction of CNTs in the composite composition. The presented numerical results are supported by experimental investigations.

In-situ polymerization and covalent modification on aramid fiber surface via direct fluorination for interfacial enhancement

Poor adhesive performance is considered to be a major disadvantage of aramid fibers in the fabrication of aramid fiber reinforced polymer composites, and it's a main drawback to restrict its wide application. In this paper, fluorine-related free radicals induced by direct fluorination were detected by Electron Paramagnetic Spectrum (EPR), which were considered as active spots to initiate copolymerization between acrylic acid (AA) and divinylbenzene (DVB), and the copolymer was chemically grafted onto AF surface, thus aramid fiber was covalently decorated by plenty of –COOH groups. Furthermore, amine-functionalized carbon nanotubes were devised to undergo the condensation reaction with the –COOH groups on fiber surface so that high density of carbon nanotubes could be chemically grafted on aramid fiber surface. Testing of mechanical properties showed that there was almost no damage to the mechanical property of aramid fibers, and interfacial shear strength of modified fiber increased by 53.1% and 69.1% after grafting polyacrylic acid-divinylbenzene copolymer and carbon nanotubes in sequence, respectively. Mathematical analysis for the changes and relationships between surface energy, surface roughness of fibers and their IFSS revealed that specific surface energy made a greater contribution to the enhancement of IFSS than surface roughness, which proved that surface energy was more helpful to the interfacial properties of the composites. Additionally, electric conductivity of organic aramid fiber improved obviously with a conductivity of 0.48 S/m.