Height Of Carbon Nanotubes Research Articles

Effect of different carrier gases and their flow rates on the growth of carbon nanotubes

The present paper examines the effect of different carrier gases and their flow rates on the growth of carbon nanotubes (CNTs). A theoretical model is developed incorporating the charging rate of the carbon nanotube, kinetics of all the plasma species, and the growth rate of the CNTs because of diffusion and accretion of ions on the catalyst nanoparticle. The three different carrier gases, i.e., argon (Ar), ammonia, and nitrogen, are considered in the present investigation, and flow rates of all the three carrier gases are varied individually (keeping the flow rates of hydrocarbon and hydrogen gas constant) to investigate the variations in the number densities of hydrocarbon and hydrogen ions in the plasma and their consequent effects on the height and radius of CNT. Based on the results obtained, it is concluded that Ar favors the formation of CNTs with larger height and radius whereas ammonia contributes to better height of CNT but decreases the radius of CNT, and nitrogen impedes both the height and radius of CNT. The present work can serve to the better understanding of process parameters during growth of CNTs by a plasma enhanced chemical vapor deposition process.

Modeling carbon nanotube growth on the catalyst-substrate surface subjected to reactive plasma

The paper presents a theoretical model to study the growth of the carbon nanotube (CNT) on the catalyst substrate surface subjected to reactive plasma. The charging rate of the CNT, kinetics of electron, ions and neutral atoms, the growth rate of the CNT because of diffusion and accretion of ions on the catalyst nanoparticle inclusion of the issue of the plasma sheath is undertaken in the present model. Numerical calculations on the effect of ion density and temperature and the substrate bias on the growth of the CNT have been carried out for typical glow discharge plasma parameters. It is found that the height of CNT increases with the ion density of carbon ions and radius of CNT decreases with hydrogen ion density. The substrate bias also affects the growth rate of the CNT. The field emission characteristics from the CNTs can be analyzed from the results obtained.

Growth mechanism and field emission behavior of carbon nanotubes grown over 300 nm thick aluminium interlayer

The influence of thick aluminium (Al) ~300nm interlayer on the growth and field emission (FE) properties of carbon nanotubes (CNTs) deposited on silicon coated with a 2nm iron (Fe) catalyst was studied. The CNTs were grown over silicon substrate with and without Al-interlayer via CVD. It was observed that the presence of such high thickness of the interlayer on the substrate resulted in higher growth rate, narrower diameters and longer height of CNTs compared to CNTs grown on silicon (Si) substrate coated only with Fe. Al-interlayer hinders the diffusion of Fe into silicon, hence promotes the growth rate. Literature reports that a thick layer of Al causes Fe to diffuse into it, negatively affecting the growth. However, in our experiments, no evidence of depletion of Fe from the substrate was observed. Unique patterns of grown CNTs could be attributed to anisotropic Al-melting over the silicon substrate resulting in Al/Fe rich and deficient regions. The drastic improvement of current density from 0.41mA/cm2 to 20mA/cm2 at a field of 3.5V/μm was found with Al-interlayer CNT grown samples. These mechanisms of improvements in field emission characteristics have been discussed in detail.

Numerical calculations on the field emission of carbon nanotubes

Carbon nanotubes (CNTs) have been proven experimentally to be well suited for field emission applications. However, the exact process of electron emission from CNTs is still not fully understood. In this paper, we numerically investigate the effects of electrostatic field screening at the CNT emitter tips by finite element method and show the impact on device performance. We further observe that to achieve maximum emission efficiency of CNTs in an infinite line, one need to have an intertube distance of three times the CNT height. These results can be used to develop an optimized CNT field emission array that fits within the constraints of currently available fabrication capabilities.

Field emission properties from a carbon nanotube array with parallel grid

One of the models for the carbon nanotube array with parallel grids is proposed. The actual electric field at the top of the carbon nanotubes and the field enhancement factor are calculated analytically with the image charge method and floated sphere model. The effects of the geometrical parameters of the device and the contact resistance on actual electric field, field enhancement factor at the top of carbon nanotubes, and the field emission current from the gated carbon nanotubes are investigated. The calculation results show that the carbon nanotube array has the best density for field emission when the intertube distance is twice the height of carbon nanotube. The actual electric field and the field emission current from gated carbon nanotube are greatly reduced by the contact resistance. When the contact resistance is larger than 800 kΩ, the emission current from carbon nanotube tends to be zero and the field emission properties are improved via modulating gate voltage.

Highly efficient carbon nanotube growth on plasma pretreated stainless steel substrates

We present here the direct growth of carbon nanotubes (CNT) on austenitic stainless steel (SUS316L) sheets containing catalytic elements that enable repeated growth without extra deposition of buffer and catalytic layers. We compared the effects of substrate pretreatment methods consisting of a combination of air-annealing and Ar-plasma treatment. The air-annealing and plasma-treatments were performed using a thermal furnace and cylindrical plasma chamber to induce morphological changes in the substrate surface. The roughness of the substrates was found to be considerably altered by annealing temperature, plasma pretreatment temperature, and growth temperature. The highest CNT height of 23.5μm was obtained using SUS316L samples that were plasma-treated and air-annealed at 725°C. Finally, the CNT growth efficiency was found to be enhanced considerably by the substrate pretreatments.

Open-Ended Aligned Carbon Nanotube Arrays Produced Using CO2-Assisted Floating-Ferrocene Chemical Vapor Deposition

The influence of CO2, in concentrations of up to 7600 ppm, on the preparation of aligned carbon nanotube (CNT) arrays from ethylene using floating-ferrocene chemical vapor deposition (CVD) at 750 °C was investigated. The CO2-assisted floating-ferrocene CVD method facilitates the well-controlled growth of aligned CNT arrays; the quality of the aligned CNT arrays was significantly improved in the presence of CO2, as demonstrated by improved alignment and crystallinity. Under the assistance of CO2, CNT arrays were linear and vertical. CNTs grown with CO2 were also higher than those grown in the absence of CO2, with the optimum CO2 concentration of 760 ppm producing a 50% enhancement in CNT height. Varying the concentration of CO2 also controlled the diameter and wall numbers of the aligned CNTs. CNTs synthesized in the presence of 7600 ppm CO2 had a diameter of 8.0 ± 1.6 nm, and an average wall number of 4 ± 1. In particular, both open-ended and triple-walled CNTs were clearly observed.

Evolution of directly-spinnable carbon nanotube growth by recycling analysis

Carbon nanotubes (CNTs) grown on substrate-bound catalysts by CVD are influenced by the catalyst, which changes over the course of the process. The evolution of the CNT growth is revealed by breaking the process into recycling increments and using the phenomenon of ‘direct spinnability’ as a target characteristic. Using acetylene alone, it was found that the first four cycles gave 100% regrowth in height and mass yield of CNTs, with both properties falling to around 20% on the 5th cycle. A decrease in nanotube diameter was observed whilst the areal density increased. With the addition of hydrogen a 100% regrowth for the second cycle was observed, followed by a decrease to around 55%, 18% and 11% in both height and yield for subsequent cycles. The diameter increased, whilst the areal density decreased in subsequent cycles. In the absence of hydrogen the CNTs have around seven walls, decreasing to about three by the seventh cycle. With hydrogen, CNTs have five or six walls for all cycles. Raman spectroscopy indicates an increase in disorder in later cycles. Spinnability is high for initial cycles but drops sharply on the fourth cycle, or third cycle with hydrogen, as the nanotube forest tortuosity markedly increases.

Tailoring of three-dimensional carbon nanotube architectures by coupling capillarity-induced assembly with multiple CVD growth

We present a general method for the construction of 3D carbon nanotube (CNT) architectures with structural integrity and stability by the combination of capillary action and catalytic vapor-phase deposition (CVD). Using this method, patterned CNTs undergo the transformation from a vertically aligned structure to a hierarchically dual porosity material in a controllable way, which can be tuned by sequential modulation of the water-wetting and the CVD re-growth. By controlling the predesign of the substrate patterns, the CNT height, and the sequence of water wetting-CVD runs, diverse shapes and hybrid structures have been fabricated. This simple and versatile method might be extendable to the organization of other filamentary nanostructures for the construction of complex architectures made of various 1D building blocks.

Synthesis of Ultra-Long Super-Aligned Double-Walled Carbon Nanotube Forests

The pre-treatment (catalyst reduction with H2) time effect on the carbon nanotube (CNT) growth is reported. The total CNT height, the initial growth rate, the diameter, the number of walls, and the alignment in the CNT forests change with the catalyst reduction time. Densely packed, vertically super-aligned, double-walled CNT (DWCNT) forests with 9 mm height were synthesized in 10 hrs. We find that the density and the size of catalysts plays an important role in the alignment of the DWCNT forests, which is evidenced by atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy.

Investigation on the Plasma-Induced Emission Properties of Large Area Carbon Nanotube Array Cathodes with Different Morphologies.

Large area well-aligned carbon nanotube (CNT) arrays with different morphologies were synthesized by using a chemical vapor deposition. The plasma-induced emission properties of CNT array cathodes with different morphologies were investigated. The ratio of CNT height to CNT-to-CNT distance has considerable effects on their plasma-induced emission properties. As the ratio increases, emission currents of CNT array cathodes decrease due to screening effects. Under the pulse electric field of about 6 V/μm, high-intensity electron beams of 170–180 A/cm2 were emitted from the surface plasma. The production mechanism of the high-intensity electron beams emitted from the CNT arrays was plasma-induced emission. Moreover, the distribution of the electron beams was in situ characterized by the light emission from the surface plasma.

Field Emission Stability of Anodic Aluminum Oxide Carbon Nanotube Field Emitter in the Triode Structure

In this work, fabricated field emission (FE) triode arrays with the anodic aluminum oxide (AAO) template carbon nanotube (CNTs) as the field emitters are numerically analyzed. To obtain physically sound self-consistent solution, a set of Maxwell's equations coupling with Lorentz equation are solved simultaneously using a finite difference time domain particle-in-cell method. The FE current is then computed with the Fowler-Nordheim equation. To validate the simulation model, we firstly calibrate the collected electron current density between the measured AAO-CNTs and calculated result. The FE current is dominated by, such as density, height, diameter, and tilt angle of CNTs, and applied bias, respectively. A high density of CNTs will result in strong screening effect among adjacent CNTs and reduce the magnitude of electric field. Consequently, it significantly affects the emitted and collected electron current densities. However, the structure with much higher density of CNTs obviously emits more stable current than that of a low density of CNTs. There is an optimal setting on the height and diameter of the CNTs within the explored structure which exhibits the highest current density. When we vary the density of CNTs, the structure with high density (say the number of CNTs is greater than 30) shows the most stable and smallest fluctuation on the current density against the randomly generated samples of the height and tilt angle of CNTs.

Optimization of Vertically Aligned Nature of CNTs for Improved Field Emission Behavior

For improved field emission behavior, we demonstrate the PECVD technique to grow highly ordered, selectively grown, vertically aligned carbon nanotubes (CNTs) with optimum density. We report the optimization of vertically aligned CNTs by optimizing catalyst thickness, dot size of catalyst and the spacing between them. We made an attempt to optimize the catalyst thickness to understand the effect over density, diameter and height of CNTs. SEM images reveal that the catalyst thickness as well as the dot size and the spacing is helpful in optimizing the vertical alignment of CNTs. The vertically aligned CNTs provide a large number of tips as compared to horizontally aligned CNTs over an area. Also, the high value of the enhancement factor required to achieve a better FE behavior is also plausible with vertical CNTs. The field emission results confirm that the vertically aligned CNTs bring a large current density at relatively low threshold field. We were able to achieve a current density of 100 mA/cm2 at a field of 6 V/µm. Density and therefore screening effect also becomes important factors that hinder the performance of CNT‐based emitters. However, in our typical samples we observed less discrepancy in the density, and the density so calculated was in the order of 109/cm2.

Investigation on Growth Behavior of CNTs Synthesized by Atmospheric Pressure Plasma Enhanced Chemical Vapor Deposition System on Fe Catalyzed Substrate

We have studied growth behavior of carbon nanotubes (CNTs) on iron (Fe) catalyzed substrate using newly developed atmospheric pressure plasma enhanced chemical vapor deposition (AP-PECVD) system. To investigate the improved growth performance with simple equipment and process on large scale, a new AP-PECVD system containing different concept on downstream gas was designed and manufactured. As a catalyst, either sputtered or evaporated Fe thin film on SiO2/Si substrate was used and acetylene gas was used as a carbon source. We observed growth behavior of CNTs such as height, rate and density were strongly affected by plasma power. The maximum height of 427 microm and 267 microm was synthesized under RF plasma power of 30 W for 30 min and 40 W for 3 min, respectively. The growth rate dramatically increased to 6.27 times as plasma power increased from 30 to 40 W which opens the possibility the mass production of CNTs. By SEM and TEM observation, it was verified the grown CNTs was consists of mixture of single-wall and multi-wall CNTs. The graphitization ratio was measured to be 0.93, indicating that the graphitized CNTs forest was formed and relatively high purity of CNTs was synthesized, being useful for nano-composite materials to reinforce the strength. From our experiments, we can observe that the height and growth rate of CNTs is strong function of plasma power.

Geometric Characterization of Carbon Nanotubes by Atomic Force Microscopy in Conjunction with a Tip Characterizer

An atomic force microscopy (AFM) probe tip characterizer with 14 line and space structures and two knife edges was fabricated by means of a superlattice technique. The shape of a probe tip both before and after AFM imaging was acquired by this tip characterizer with general variations <1.5 nm; depending on imaging conditions. The geometric structures of carbon nanotubes (CNTs) on a SiO2 substrate were studied by dynamic mode AFM in conjunction with this tip characterizer. Contact points between the tip and the CNTs were detected by observing changes in the AFM phase images. A modified CNT width correction model was established to calculate the estimated and upper-limit widths of two CNTs. The experimental results showed that imaging under a weak attractive force was suitable for obtaining accurate CNT height measurements, whereas a weak repulsive force provided the most accurate widths. Differing heights and widths between the two CNTs suggested that one CNT was double-walled, whereas the other had more than two walls; these results agree with transmission electron microscopy (TEM) measurements of the CNTs.

7.3: Morphology Effect on Field Emission Property of Carbon Nanotube Emitters in Triode Structure

AbstractIn this work, process variation on the field emission property of the fabricated field emission (FE) triode arrays with carbon nanotubes (CNTs) as the field emitters are explored. To fabricate the FE triode structure, nanoporous anodic aluminum oxide (AAO) thin layer was first prepared on the Si substrate, followed by the growth of vertically aligned carbon nanotubes in AAO pore channels by electron cyclotron resonance chemical vapor deposition. The SiO2 dielectric and Al gate electrode layers required for the triode structure were directly deposited on the CNTs. Reactive ion and wet etches were then used to open the FE area in the triode. We employ the finite difference time domain particle‐in‐cell method to calculate the structure FE property, where the results are calibrated with the collected FE current of the fabricated samples. The FE current of structure is then examined with respect to different shape, random height and angle of the CNT emitters. for symmetric shapes, like circle and square, the emission beam has been more focusing than triangle and trapezoid. The electric field of random height of CNTs is stronger than that of random angle. It is implied that the vertical CNTs has better FE property. Having the symmetric shape and vertical CNTs in triode structure benefits the FE sources for advanced flat display technologies.

Numerical simulation of field emission efficiency of anodic aluminum oxide carbon nanotube field emitter in the triode structure

In this work, we study the field emission (FE) properties of carbon nanotubes (CNTs) field emitter in the triode structure fabricated with anodic aluminum oxide (AAO) template. To obtain the self-consistent solution, a set of two-dimensional Maxwell's equations coupling with Lorentz equation are solved with a finite difference time domain (FDTD) particle-in-cell (PIC) method. The FE current is then computed with the Fowler–Nordheim equation. Calculated result of the collected current density shows good agreement with the measured data for the fabricated sample. Effect of the height and number of CNTs on the collected electron current density is thus investigated. We find that the explored structure with few number of CNTs exhibit high current density due to insignificant screening effect. The current density is low and approaches to stable current level when the number of CNTs increases. The simulation further predicts that the structure with the height of CNTs = 0.42 μm has a maximum current density. The FE efficiency (i.e., the collected electron current density/the emission current density) versus the number of CNTs is estimated. The explored AAO-CNTs triode structure maintains high FE efficiency (∼95%) with a nonzero gate voltage.

Properties of Gated Single Free-Standing Carbon Nanotube with Multiple Growth Sequence

In this paper, we discuss an approach to fabricating a carbon nanotubes (CNTs) gate electrode structure for cold cathode emitter applications. The field emission behavior of the structure was studied by using single vertically aligned free-standing CNTs of various lengths and gate electrodes of various sizes to determine the optimal emission structure. A single CNT electrode source with a gate electrode was fabricated by combining optical lithography, electron-beam lithography (EBL), and inductively coupled plasma chemical vapor deposition (ICP-CVD). A single mask was adopted to define the gated structure and nano sized catalyst for CNT growth. A vertically aligned CNT was then grown within the gate hole by ICP-CVD. The CNT height inside the gate could be lengthened by multiple growth processes without affecting the field emission capability. The best turn-on fields, defined as the field required to generate an emission current of 1 nA, were approximately 1.66 and 1.38 V/µm, respectively. The turn-on field emission current saturated when the CNT length was about the same as the gate height.

Multi-barrier layer-mediated growth of carbon nanotubes

Variation in the height of carbon nanotubes (CNTs) grown has been co-related to the type of multi-barrier-layer used. Initially, various types of barrier-layers such as Al, Al 2O 3, Al/SiO 2, Al 2O 3/SiO 2 were prepared onto a n-type Si (100) substrate. The thickness of SiO 2 was ∼ 550 nm, where as, Al 2O 3 and Al were ∼ 15 nm thick. These samples were covered with ∼ 1 nm thick Fe catalyst layer. The coated samples were subjected to the thermal chemical vapor deposition (T-CVD) process. SEM analysis showed that, for Al 2O 3/SiO 2 barrier layers, the average height of the CNTs was ∼ 10 μm, where as, for other types of samples it was less than ∼ 1 μm. To investigate this, multi-barrier layers were characterized by dynamic secondary ion mass spectrometry (D-SIMS). The observed variation in height of CNTs is attributed to the variation in diffusivity of Fe atoms into multi-barriers-layers. The results showed that, diffusion of Fe catalyst atoms could severally affect height of CNTs.

Density Control of Carbon Nanotubes through the Thickness of Fe/Al Multilayer Catalyst

The density control of vertically aligned carbon nanotubes (CNTs) has been studied using a thermal chemical vapor deposition (CVD) method in correlation with the thicknesses of the Fe/Al multilayer catalyst on a Si substrate. The density of CNTs could be controlled through the thicknesses of both the Al layer and Fe catalyst layer. The low density of the Fe catalyst grains formed by thermal treatment, caused the formation of CNTs with random orientations, and resulted in a decrease in the CNT height. The optimum film thickness of the Fe/Al multilayer catalyst for obtaining long CNTs (220 µm) was determined for the case of a substrate temperature of 700 °C.