CNT Arrays Research Articles

Self-heating Schottky emission from a ballasted carbon nanotube array

A ballast resistor is utilized in a low density vertically aligned carbon nanotube array. Based on the nature of the ballast resistor, the uniformity of the emission improves remarkably. A highly stable field emission current is obtained under a constant voltage and a current density of 300mA/cm2 is achieved. Joule heat generated by this field emission current increases the temperature of the CNT array significantly. The high temperature changes the emission to Schottky emission regime. The Schottky emission achieves 900mA/cm2, which is three times the field emission current density. Simulation result shows the corresponding temperature is about 1700K. A color change of the emission area is observed after the experiment. When compared to the conventional Schottky cathode, the emitter is self-heating and no extra heater is needed. This is the first report of a successful utilization of a ballast resistor in a CNT based emission array and the first observation of Schottky emission from a vertically aligned CNT array used as an electron emitter.

Study of Growth Enhancement of Multiwalled Carbon Nanotubes by Chlorine-Assisted Chemical Vapor Deposition

In this paper, we investigate the effect of chlorine as a growth promoter in the synthesis of multiwalled carbon nanotubes (MWCNTs). We find that the addition of chlorine gas to the chemical vapor deposition process reduces the amount of amorphous carbon deposited on the MWCNT surfaces and results in a longer catalyst lifetime. By using optimum growth parameters, the catalyst remains active for 75 min. Moreover, the growth rate is also relatively high resulting in the maximum CNT array height of 3.5 mm. The MWCNT arrays grown with and without chlorine are compared.

Evaluation of the shear force of single cancer cells by vertically aligned carbon nanotubes suitable for metastasis diagnosis

Vertically aligned carbon nanotube (VACNT) arrays have been demonstrated as probes for rapid quantifying of cancer cell deformability with high resolution. Through entrapment of various cancer cells on CNT arrays, the deflections of the nanotubes during cell deformation were used to derive the lateral cell shear force using a large deflection mode method. It is observed that VACNT beams act as sensitive and flexible agents, which transfer the shear force of cells trapped on them by an observable deflection. The metastatic cancer cells have significant deformable structures leading to a further cell traction force (CTF) than primary cancerous one on CNT arrays. The elasticity of different cells could be compared by their CTF measurement on CNT arrays. This study presents a nanotube-based methodology for quantifying the single cell mechanical behavior, which could be useful for understanding the metastatic behavior of cells.

The role of density in the mechanical response of CNT turfs

Solvent capillary and axial compression can be used to increase the density of as-grown vertically aligned carbon nanotube arrays, herein referred to as turfs. During solvent evaporation, carbon nanotubes cluster together along their vertical growth axis, and axial mechanical compression densifies through compressing tubes into tangled positions and decreasing the empty space between tubes in directions primarily perpendicular to the vertical growth axis. The elastic modulus of the turfs, measured by nanoindentation, is between four to fifteen times higher after densification than that of pristine turfs and scales directly with densification ratio, while the adhesive properties remain unchanged. The electrical conductivity of the densified structure also scales with density. This suggests that multiple methods of post-growth modification of CNT arrays can achieve densified turfs with tailored mechanical properties without corresponding degradation of the electrical contact behavior.

Natural gas storage on silicon, carbon, and silicon carbide nanotubes: a combined quantum mechanics and grand canonical Monte Carlo simulation study

Grand canonical Monte Carlo (GCMC) simulation combined with ab initio quantum mechanics calculations were employed to study methane storage in homogeneous armchair open-ended single-walled silicon nanotubes (SWSiNTs), single-walled carbon nanotubes (SWCNTs), and single-walled silicon carbide nanotubes (SWSiCNTs) in triangular arrays. Two different groups of nanotubes were studied: the first were (12,12) SiNTs, (19,19) CNTs, and (15,15) SiCNTs and the second were (7,7) SiNTs, (11,11) CNTs, and (9,9) SiCNTs with the diameters of 26 and 15 A for the first and second groups, respectively. The simulations were carried out at different thermodynamic states. The potential energy functions were calculated using ab initio quantum mechanics and then fitted with (12,6) Lennard–Jones potential model as a bridge between first-principles calculations and GCMC simulations. The absolute, excess, and delivery adsorption isotherms of methane were calculated for two groups of nanotubes. The specific surface area and the isosteric heat of adsorption were computed. The radial distribution functions for the adsorbed molecules on different nanotubes were also calculated. Different isotherm models were fitted with the simulation adsorption data. According to the results, the excess uptake value of methane adsorption in (11,11) CNT array exceeded the US Department of Energy target (180 V/V at 298 K and 35 bar). The results also indicate that SiNTs and SiCNTs are not desirable materials compared with corresponding CNTs for natural gas storage.

Controlled Growth of Well-Aligned Carbon Nanotubes, Electrochemical Modification and Electrodeposition of Multiple Shapes of Gold Nanostructures

An efficient method has been developed to synthesize well-aligned multi-walled carbon nanotubes (MWCNTs) on a conductive Ta substrate by chemical vapour deposition (CVD). Free-standing MWCNTs arrays were functionalized through electrochemical oxidation with the formation of hydroxyl and carboxyl functional groups. Using a new oven drying technique, we demonstrate that the unidirectionally aligned and laterally spaced geometry of the CNT arrays can be retained after being subjected to each step of electrochemical modification. Samples were analyzed by using a field emission scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transformed infrared (FTIR) and Raman spectroscopy. Useful electrochemical methods for the synthesis of various gold nanostructures onto the aligned MWCNTs were then presented for the first time. The results demonstrated that flowerlike nanoparticle arrays, nanosheets and nanoflowers were obtained on the aligned CNTs under different experimental conditions. These kinds of aligned-CNT/Au nanostructures hybrid materials introduced by these efficient and simple electrochemical methods could lead to development of a new generation device for ultrasensitive catalytic and biological application.

Digital Image Correlation: Visualizing Strain Evolution and Coordinated Buckling within CNT Arrays by In Situ Digital Image Correlation (Adv. Funct. Mater. 22/2012)

Digital Image Correlation: Visualizing Strain Evolution and Coordinated Buckling within CNT Arrays by In Situ Digital Image Correlation (Adv. Funct. Mater. 22/2012)

Super‐Aligned Carbon Nanotube Films as Current Collectors for Lightweight and Flexible Lithium Ion Batteries

AbstractCarbon nanotube (CNT) current collectors with excellent flexibility, extremely low density (0.04 mg cm−2), and tunable thickness are fabricated by cross‐stacking continuous CNT films drawn from super‐aligned CNT arrays. Compared with metal current collectors, better wetting, stronger adhesion, greater mechanical durability, and lower contact resistance are demonstrated at the electrode/CNT interface. Electrodes with CNT current collectors show improvements in cycling stability, rate capability, and gravimetric energy density over those with metal current collectors. These results suggest that CNT films can function as a promising type of current collector for lightweight and flexible lithium ion batteries with high energy density.

Physical properties of carbon nanotube sheets drawn from nanotube arrays

We report mechanical, thermal, and electrical properties of novel sheet materials composed of multiwalled carbon nanotubes, drawn from a CNT array. At low loading there is some slippage of CNTs but at higher loading tensile strength σ0=7.9MPa and Young’s modulus E=310MPa. The room-temperature thermal conductivity of the CNT sheet was 2.5±0.5Wm−1K−1, giving a thermal conductivity to density ratio of κ/ρ=65Wm−1K−1g−1cm3. The heat capacity shows 1D behavior for T>40K, and 2D or 3D behavior at lower temperatures. The room-temperature specific heat was 0.83Jg−1K−1. The iV curves above 10K have Ohmic behavior while the iV curve at T=2K is non-Ohmic, and a model to explain both ranges is presented. Negative magnetoresistance was found, increasing in magnitude with decreasing temperature (−15% at T=2K and B=9T). The tensile strength, Young’s modulus and electrical conductivity of the CNT sheet are low, in comparison with other CNT materials, likely due to defects. Thermal conductivity is dominantly phononic but interfacial resistance between MWCNTs prevents the thermal conductivity from being higher.

Large-Displacement Indentation Testing of Vertically Aligned Carbon Nanotube Arrays

Mechanical properties and deformation of the vertically aligned carbon nanotube arrays (VA-CNTs) has been examined with large-displacement indentation tests inside a scanning electronic microscope (SEM). The in-situ indentation allows for establishment of the load-depth responses and real-time observations of the deformation processes. Under a cylindrical, flat indenter, the VA-CNTs exhibit elastic deformation at small displacement and then plastic deformation at large displacement. The critical indentation pressure (Pm), a measure of collapsing stress of the CNT arrays, is obtained. The magnitude of Pm is approximately equal to the collapsing stress of the carbon nanotube arrays obtained under uniaxial compression.

Vertically aligned multiwalled carbon nanotubes for pressure, tactile and vibration sensing

We report a simple method for the micro–nano integration of flexible, vertically aligned multiwalled CNT arrays sandwiched between a top and bottom carbon layer via a porous alumina (Al2O3) template approach. The electromechanical properties of the flexible CNT arrays have been investigated under mechanical stress conditions. First experiments show highly sensitive piezoresistive sensors with a resistance decrease of up to ∼35% and a spatial resolution of <1 mm. The results indicate that these CNT structures can be utilized for tactile sensing components. They also confirm the feasibility of accessing and utilizing nanoscopic CNT bundles via lithographic processing. The method involves room-temperature processing steps and standard microfabrication techniques.

Dyes in Vertically Aligned Carbon Nanotube Arrays for Solar Cell Applications

ABSTRACTThe infiltration of dissolved dyes into vertically aligned carbon nanotube arrays (va-CNT) is reported. The ultra hydrophobic surface of the CNT forest can be wetted and hence infiltrated for an appropriate choice of solvent. The dye-infiltrated CNT array forms a well ordered bulk-heterojunction structure for organic solar cells in which the CNT can act as a large electrode or, for appropriate energy levels, as an acceptor material. Derivatives of the small molecule copper phthalocyanine or the polymer poly(3-hexylthiophene) were used as dyes. Drop coating was chosen as the infiltration technique resulting in a completely embedded CNT forest. Field emission secondary electron microscopy analysis illustrates the final layer quality. Common electrical characterization under AM1.5 illumination proves photosensitivity and implies photovoltaic behavior of the composite.

Nitrogen-doped carbon nanotube arrays grown on graphene substrate

Nitrogen-doped carbon nanotube (N-doped CNT) arrays have been synthesized on graphene substrate by chemical vapor deposition process, in which iron nanoparticles (NPs) assembled on the graphene sheet were generated in situ from the reduction of Fe 3O 4 NPs/reduced graphene oxide (RGO) and were used as catalyst. The morphology and structure of the N-doped CNT arrays were investigated by field emission scanning electron microscope and high-resolution transmission electron microscope. The N-doped CNTs were bamboo-shaped and the density can be controlled by modulating the density of catalyst NPs on RGO sheets. The concentration and incorporation of nitrogen were studied by elemental analysis, X-ray photoelectron spectroscope and Raman analysis, and the results showed that the nitrogen content was around 3 wt.%. Because of the good conductivity of graphene structure, N-doped CNT arrays grown on graphene substrate may be promising candidates as noble metal-free electrodes for oxygen reduction reaction in the future.

Quantitative Characterization of Vertically Aligned Multi-Walled Carbon Nanotube Arrays Using Small Angle X-Ray Scattering

We have used small angle X-ray scattering (SAXS) to quantitatively characterize the morphology of vertically aligned (VA) multiwall carbon nanotube (MWCNT) arrays. We examined the extent of alignment of MWCNTs in terms of order parameter by analyzing SAXS intensity as a function of azimuthal angle. The SAXS measurements at different heights of CNT arrays from the substrate reveal two distinct morphologies and increasing alignment. We are able to quantitatively characterize a real variation in CNT diameters of the VA-MWCNTs through model fitting of the SAXS spectra. It found that the average CNT diameter increases with increasing distance from the substrate.

Determination of Trace Metals by Anodic Stripping Voltammetry Using a Carbon Nanotube Tower Electrode

AbstractAnodic stripping voltammetry (ASV) trace detection of the heavy metals ions Pb2+, Cd2+, Cu2+ and Zn2+ was done at highly aligned multi‐wall carbon nanotube tower electrodes (CNT tower electrodes). The CNT tower electrode is one type of CNT array that consists of millions of ordered super long CNTs. Osteryoung square‐wave stripping voltammetry was used for the stripping step. This electrode material shows good resolution for individual detection of these four metal ions with calculated detection limits of 12 nM, 25 nM, 44 nM and 67 nM for Pb2+, Cd2+, Cu2+ and Zn2+, respectively, for a deposition time of 120 s. The detectable limit was as low as 0.5 nM for a 10 min deposition.

The effects of catalyst treatment on fast growth of millimeter-long multi-walled carbon nanotube arrays

An optimized strategy was developed for fast growth of millimeter-long CNT arrays using chemical vapor deposition (CVD). Growth temperature of 800 °C was firstly determined, and catalyst heat treatment conditions were then optimized to probe the full potential of growth rate. 1.5 mm long CNT arrays were obtained in 10 min under optimized growth and catalyst heat treatment conditions. The growth rate of CNT arrays strongly depends on the growth temperature and catalyst heat treatment. Insufficient reduction could not reduce iron oxide into metallic state or/and crack down catalyst film into particles, but excessive treatment may result in large particles due to Ostwald ripening process. This method would offer more freedoms in designing the fast growth of high-purity, long CNT arrays.

HIERARCHICAL CARBON NANOTUBE-INORGANIC HYBRID STRUCTURES INVOLVING CNT ARRAYS AND CNT FIBERS

Carbon nanotube-inorganic hybrid materials have stimulated a new boost in CNT research as a new class of multifunctional materials with properties distinct from the well-known CNT composites. Synergistic effects based on interfacial charge and heat transfer processes commend these hybrids for use in photocatalysis, gas sensors and in electrochemical devices. A major challenge is the synthesis of hybrids with hierarchical architectures and controlled interfaces. Common wet-chemical techniques have the major drawback that the CNT array typically collapses upon drying due to stresses imposed by the surface tension of the solvent. In this work, we synthesized hierarchical hybrids with coherent 3D architectures using unique CNT fibers, based on the "Cambridge process" as well as vertical arrays of CNTs ("carpets"). We demonstrate that separating the liquid reactants from the CNT array is key to preserving the hybrid's architecture. In addition, we show that the presence of benzyl alcohol as a linking agent is beneficial to maximizing the interfacial area in TiO 2-coated CNT fibers.

Model Calculation for the Field Enhancement Factor of Carbon Nanowall Array

To estimate the field emission current associated with an array of carbon nanowalls (CNWs), the model of the floating rods between anode and cathode plates was proposed. An approximate formula for the enhancement factor was derived, showing that the interwall distance of the CNW array critically affects the field emission. The field enhancement factor was almost one order of magnitude less than that of vertically aligned CNTs. Considering the field emission current density, the field emission can be optimized when the interwall distance is comparable with the wall height. For same separation distance, the macroscopic field strength of the CNW array is almost one order of magnitude higher than that of vertical CNT array to obtain the emission current of 1mA from the cathode surface of 1cm2.

Tunable MEMS capacitors using vertical carbon nanotube arrays grown on metallines

In this work, tunable MEMS capacitors are realized using a vertically grown carbonnanotube array. The vertical CNT array forms an effective CNT membrane, which canbe electrostatically actuated like the conventional metal plates used in MEMScapacitors. The CNT membrane is grown on titanium nitride metal lines, with aAl/Fe bi-layer as buffer layer and catalyst material respectively, using chemicalvapor deposition process. Two different anchor configurations are investigated. Amaximum capacitance of 400 fF and maximum tunability of 5.8% is extracted from theS-parameter measurements. Using the tunable MEMS vertical array capacitor a voltagecontrolled oscillator (VCO) is demonstrated showing promise for integrating CNTs forcommunications applications.

A novel approach to fabricate high volume fraction nanocomposites with long aligned carbon nanotubes

Conventional micro-fiber-reinforced composites provide insight into critical structural features needed for obtaining maximum composite strength and stiffness: the reinforcements should be long, well aligned in a unidirectional orientation, and should have a high reinforcement volume fraction. It has long been a challenge for researchers to process CNT composites with such structural features. Here we report a method to quickly produce macroscopic CNT composites with a high volume fraction of millimeter long, well aligned CNTs. Specifically, we use the novel method, shear pressing, to process tall, vertically aligned CNT arrays into dense aligned CNT preforms, which are subsequently processed into composites. Alignment was confirmed through SEM analysis while a CNT volume fraction in the composites was calculated to be 27%, based on thermogravimetric analysis data. Tensile testing of the preforms and composites showed promising mechanical properties with tensile strengths reaching 400MPa.