CNT Growth Research Articles

Synthesis of Vertically Aligned Carbon Nanotubes by dc PECVD

Chemical vapor deposition (CVD) is one of the various synthesis methods that have been employed for CNT growth. In particular, Ren et al reported that large areas of vertically aligned multi-wall carbon nanotubes could be grown using plasma enhanced chemical vapor deposition (PECVD). In the present study, we synthesized aligned CNT arrays using a direct current (dc) PECVD system. The synthesis of CNTs requires a metal catalyst layer, etchant gas, and a carbon source. In this study, the substrate consisted of Si wafers with 10, 30, and 50 nm Ni-sputtered film. Ammonia (NH3) and acetylene (C2H2) were used as the etchant gases and carbon source, respectively. NH3 pretreatment was processed using a flow rate of 180 sccm for 10 min. CNTs were grown on pretreated substrates at 30% C2H2:NH3 flow ratios for 10 min. Carbon nanotubes with diameters ranging from 60 to 80 nanometers and lengths of about 2.7 μm were obtained. Vertical alignment of the carbon nanotubes was observed by FE-SEM.

The growth of carbon nanotube stacks in the kinetics-controlled regime

A CNT stack formation technique is used for the pseudo in-situ monitoring of CNT growth in the kinetics-controlled regime. CNT stacks are fabricated by water-assisted selective etching and the cyclic introduction of ethylene into the chemical vapor deposition (CVD) reactor. By varying the growth temperature and the ethylene flow rate within growth cycles, the reaction was shown to be first order with an activation energy of 201.2 kJ/mol. This technique can be implemented to monitor the initial stage of CNT growth by deceasing the ethylene flow time in growth cycles. Within one minute, the CNT growth reaches steady state. The formation of CNT stacks could be tailored to investigate the effects of growth parameters, such as pressure, temperature, and carbon source flow rate, on the CNT growth.

Growth Mechanism of Long Aligned Multiwall Carbon Nanotube Arrays by Water-Assisted Chemical Vapor Deposition

Highly aligned arrays of multiwalled carbon nanotube (MWCNT) on layered Si substrates have been synthesized by chemical vapor deposition (CVD). The effect of the substrate design and the process parameters on the growth mechanism were studied. Adding water vapor to the reaction gas mixture of hydrogen and ethylene enhanced the growth which led to synthesis of longer CNT arrays with high density. Environmental scanning electron microscopy (ESEM), energy-dispersive spectroscopy (EDS), and atomic force microscopy (AFM) were used to analyze the CNT morphology and composition. Quadrupole mass spectroscopy (QMS) provided in-situ information on the gas spices within the reaction zone. On the basis of results, we verified the top growth mechanism and evaluated the reason of decline and stoppage of the CNT growth after extended period of deposition. Multilayered Si substrates with a top film of Al2O3, having appropriate roughness, provide favorable conditions to form catalyst islands with uniform distribution and size. Using water-assisted CVD process and optimized substrate design, our group succeeded to grow vertically aligned, patterned MWCNT up to 4-mm long. The arrays were of high purity and weak adhesion which allowed to be peeled off easily from the substrate.

Deposition and characteristics of iron–silicon thin film catalyst for CNT growth

Fe–Si catalyst thin films for the growth of carbon nanotubes were prepared using co-sputter deposition. As-deposited Fe–Si films consist of different amounts of α-Fe and amorphous Si. The amount depends on the Si concentration in the film. Hydrogen plasma etched Fe–Si films become particles having different sizes. The particle size is also dependent on the Si concentration. Correlation among the Si concentration, the particle size, and the growth rate of carbon nanotube was made. Optimal growth of carbon nanotubes at 370 °C was obtained at an average particle size of 45 nm or a Si concentration of 21%.

A detailed model for the flame synthesis of carbon nanotubes and nanofibers

Many experimental investigations of CNT/CNF flame synthesis have been recently reported. However, there are as yet no comprehensive models regarding their formation, growth or structure. Herein, a CNT/CNF growth rate model is proposed that is applicable for any method of CNT/CNF production (although our particular interest lies in flame synthesis in ethylene/air flames). While it is usual for most existing models to consider only a single carbon-carrying gas that contributes towards carbon deposition, our extended model can consider a complex hydrocarbon mixture that can mimic a flame environment. The model shows that before carbon nucleation is initiated, the surface density of carbon atoms increases as they are added through diffusion from the leading face of the catalyst nanoparticle. Over time, the diffusion potential decreases due to a reduction in the carbon atom concentration gradient. However, with the onset of nucleation and growth, diffusion is reinstated as the major driving potential for carbon atom transport through the nanoparticle. Steady carbon deposition and filament growth occurs once there is a stable carbon cluster size due to nucleation. The results support our hypothesis that flame synthesis can be a much faster and higher throughput process than CVD. The CNT/CNF growth rate decreases with increasing height above the burner. Decreasing temperature at higher locations leads to slower catalyst deactivation, but the CO concentration, which is the major contributor to carbon deposition, also decreases with increasing height above the burner. One of the major findings in this work is the contribution of CO to CNT formation by flame synthesis. The concentration of hydrocarbons in the vicinity of the toroidal zone near, which most of the CNT growth is observed, is negligible compared to CO concentration. Our results provide a basis to conduct future multiscale simulations of CNT/CNF growth.

Growth of mechanically fixed and isolated vertically aligned carbon nanotubes and nanofibers by DC plasma-enhanced hot filament chemical vapor deposition

Vertically aligned, mechanically isolated, multiwalled carbon nanotubes (MWCNTs) and nanofibers (MWCNFs) were grown using an array of catalyst nickel nanowires embedded in an anodic aluminum oxide (AAO) nanopore template using DC plasma-enhanced hot filament chemical vapor deposition (HFCVD). The nickel nanowire array, prepared by electrodeposition of nickel into the pores of a commercially available AAO membrane, acts as a template for CNT and CNF growth. It also provides both a mechanical “fixed support” boundary condition and enforces sufficient spatial separation of the CNT/CNFs from each other to enable reliable and well-controlled mechanical testing of individual vertically aligned CNT/CNFs. In contrast with other AAO-templated growth methods, no post-growth etching of the AAO is required, since the CNTs/CNFs grow out of the pores and remain vertically aligned. A mixture of hydrogen and methane was used for the growth, with hydrogen acting as a dilution and source gas for the DC plasma, and methane as the carbon source. A negative bias was applied to the sample mount to generate the DC plasma. The filaments provided the necessary heat for dissociation of molecular species, and also heat the sample itself significantly. Both of these effects assist the CNT/CNF growth. Minimal heating came from the low-power plasma. However, the associated DC field was essential for the vertical alignment of the CNTs and CNFs. Scanning electron, transmission electron, and atomic force microscopy confirm that the CNT/CNFs are composed of graphitic layers, and form a vertically aligned, relatively uniform, and dense array across the AAO template. A significant number of the structures grown are indeed high quality nanotubes, as opposed to more defective nanofibers that are often predominant in other growth methods. This method has the advantage of being scalable and consuming less power than other techniques that grow vertically aligned CNTs/CNFs.

Synthesis of carbon nanotubes on alumina-based supports with different gas flow rates by CCVD method

Several methods for the synthesis of carbon nanotubes have been developed in the last decade. The CVD process and their associated parameters affect the structure of the resulting nanotubes. In this work CNT growth has been studied on different supported catalysts with different rates of gas flow as one of the critical points. Different supports were prepared by mixing nanosized alumina with tetraethyl orthosilicate (TEOS) by a chemical method at low temperature and iron as the metal catalyst was impregnated by 5%, 10% and 20% weight of the supports. Methane was used as a carbon source for the synthesis of CNTs at 800°C-1000°C. Aluminium-based support, supported catalysts and CNT samples have been characterised by TEM, SEM, BET and XRD.

In-Flight Kinetic Measurements of the Aerosol Growth of Carbon Nanotubes by Electrical Mobility Classification

We describe an on-the-fly kinetic study of gas-phase growth of multiwalled carbon nanotubes. The methodology employs electrical mobility classification of the CNT, which enables a direct measure of CNT length distribution in an aerosol reactor. The specific experiment employs two mobility classification steps. In the first step we mobility classify the catalyst particle, in this case Ni, created by pulsed laser ablation, to generate a stream of monodisperse particles. This then determined the diameter of the CNT, when a hydrocarbon/H2 mix is added in a heated aerosol reactor. A second electrical mobility classification step allows us to determine the length distribution of the CNTs. We found that CNT growth from ethylene required the addition of small amounts of water vapor, whereas growth from acetylene did not. We show that acetylene, which always has small amounts of acetone present when purchased, can provide the oxygen source to prevent catalyst coking. By varying the temperature of the growth, we were able to extract Arrhenius growth parameters. We found an activation energy for growth approximately 80 kJ mol(-1) from both acetylene and ethylene, which is considerably lower than previous works for substrate-grown CNTs (E(a) = 110-150 kJ mol(-1)). Furthermore, we observed that our aerosol CNT growth rates were about 2 orders of magnitude higher than those for substrate-grown CNTs. The dominant growth mechanism of CNT previously proposed is based upon bulk diffusion of carbon through nickel particles. However, on the basis of the lower activation energy found in this work, we proposed that the possible mechanism of gas-phase growth of CNT is correlated with both surface (E(a) = 29 kJ mol(-1)) and bulk diffusion (E(a) = 145 kJ mol(-1)) of carbon on nickel aerosol particles. Finally, the experimental approach described in this work should be amenable to other nanowire systems grown in the aerosol phase.

Nano-Scale MOSFET Devices Fabricated Using a Novel Carbon-Nanotube-based Lithography

AbstractPECVD-grown carbon nanotubes on (100) silicon substrates have been studied and exploited for electron emission applications. The growth of CNT's is achieved by a mixture of hydrogen and acetylene gases in a CVD reactor and a 2-5nm thick nickel is used as the seed for the growth. The presence of DC-plasma yields a vertical growth and allows deposition at temperatures below 650°C. The grown nano-tubes are encapsulated by means of an insulating TiO2 layer, leading to beam-shape emission of electrons from the cathode towards the opposite anode electrode. The electron emission occurs using an anode-cathode voltage of 100 V with ability of direct writing on a photo-resist coated substrates. Straight lines with widths between 50 and 200nm have been successfully drawn. Scanning electron and transmission electron microscopy have been used to investigate the quality and fineness of the results. This technique has been applied on P-type (100) silicon substrates for the formation of the gate region of N-MOSFET devices, showing a drive current of 310μA/μm and Cox of 0.7μF/cm2.

Fabrication of 100 nm gate length MOSFET's using a novel carbon nanotube-based nano-lithography

PECVD-grown carbon nanotubes on (1 0 0)silicon substrates have been studied and exploited for electron emission applications. After the growth of vertical CNT's [Y. Abdi, J. Koohsorkhi, J. Derakhshandeh, S. Mohajerzadeh, H. Hosseinzadegan, M.D. Robertson, C. Benet, EMRS Spring Meeting, Strasbourg, France, May 2005] the grown nanotubes are encapsulated by means of an insulating TiO 2 layer, leading to beam-shape emission of electrons from the cathode towards the opposite anode electrode. The electron emission occurs using an anode–cathode voltage of 100 V with ability of direct writing on a photo-resist-coated substrates. Straight lines with widths between 50 and 200 nm have been successfully drawn. This technique has been applied on P-type (1 0 0)silicon substrates for the formation of the gate of N-MOSFET devices. The successful realization of MOSFET devices indicates its usefulness for applications in nano-electronic devices. This device has inversion C OX exceeding 0.7 μF/cm 2, drive current equal to 310 μA/μm.

Effects of NH 3 plasma pretreatment on the growth of carbon nanotubes

In CVD, it is believed that the catalyst is a crucial parameter for the diameter and the density of the CNTs. Typical thin catalyst film thickness should be less than 20 nm. While the catalyst film thickness is more than 30 nm, it is very difficult to grow thinner CNTs. Pretreatment further to break the film into desired particles should be taken. In this paper, the pretreatment effect of NH 3 plasma on CNT growth was studied. Multi-wall carbon nanotubes have been grown with NH 3 plasma pretreatment to the thick catalyst films by CVD. The diameter and density of CNTs changed with the plasma power and pretreatment time. The diameter of grown CNTs decreased from 10 to 4 nm as plasma intensity increased. NH 3 plasma power and pretreatment time have to be optimized in order to grow uniform and density controlled nanotubes.

ENHANCED GROWTH OF CARBON NANOTUBES ON SELECTED AREA USING AN AQUEOUS CATALYST

This paper describes the work carried out to produce patterned arrays of multi-wall carbon nanotubes (MWCNTs) using a liquid catalyst and a microwave plasma technique. By this method we can control the CNT growth on selective sites by engineering the substrate surface and yet maintain a very high CNT growth rate (~10 μm per minute). The catalyst is made by hydrochloric acid solution which contains high concentration of ferromagnetic metal ions. Upon dispersing the liquid catalyst on a silicon nitride substrate containing exposed silicon patterns, the catalyst only precipitates on the hydrophilic sites (i.e. silicon surface). After performing the CNT growth in a microwave reactor with CH 4/ N 2 mixture plasma, nanotubes can only be found to grow on exposed silicon surface instead of the main area covered by silicon nitride. This method allows us to produce multi-wall carbon nanotubes on the selective area as in the silicon trenches, concavities and thin film edges.

Development of triode type RF plasma enhanced CVD equipment for low temperature growth of carbon nanotube

Triode-type radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) equipment has been developed in order to grow well-aligned carbon nanotubes on Si and glass substrates at 550 °C. The CVD equipment employs a grid electrode in addition to the cathode and anode electrodes. The grid electrode allows the growth of a well-aligned carbon nanotube with an inside and an outside diameter of 7 and 17 nm, respectively. Moreover, the patterning growth of the well-aligned CNT on a glass substrate was also demonstrated.

Wafer-level ordered arrays of aligned carbon nanotubes with controlled size and spacing onsilicon

Aligned carbon nanotubes with uniform outer diameters, ordered over wafer-scale areaswere fabricated using porous alumina templates on silicon. Anodization of aluminium filmsdeposited on corrugated silicon surfaces, patterned using interference lithography, led toperiodic pore structures over wafer-scale areas with controlled spacing and symmetry, andwith pore sizes independently controlled through the anodization conditions.Nickel deposited at the base of the pores catalysed CNT growth during PECVD.These results demonstrate a wafer-scale approach to the control of the size, pitch,ordering symmetry, and position of nanotubes and nanowires in a rigid insulatingscaffold.

Effect of catalyst preparation on the carbon nanotube growth rate

A series of silica supported Fe catalysts were prepared by different methods in order to obtain varying Fe particle sizes. The catalysts were characterized by XRD, TPR, BET, and TEM. The CNT growth from CO disproportionation was studied in order to establish a relationship between the CNT growth rate and the particle size. We found that there is an optimum catalyst particle size at around 13–15 nm which will lead to the maximum growth rate. The influence of the metal loading on the growth rate was also investigated. A CNT growth model has been formulated to explain the experimental results.

Effect of plasma conditions on fabrication of multi-walled carbon nanotubes grown perpendicularly on Hastelloy C276®

We prepared multi-walled carbon nanotubes (MWCNTs) on Hastelloy C276® by a direct current (dc) plasma-assisted hot filament chemical vapor deposition in CH 4/N 2 atmosphere. Two-step deposition method, consisting of nucleation step and the subsequent CNT growth step, was employed and MWCNTs were grown by the catalytic growth with metal particles, which were precipitated from Hastelloy C276® and mainly consisted of Ni. The plasma generated at the current density of 30 mA/cm 2 was necessary to form nuclei for CNT growth, while the reducing of the plasma current density to the range between 10 and 20 mA/cm 2 was needed to grow MWCNTs. Based on these results, we proposed the possible mechanisms of the nucleus formation and MWCNT growth.

The effects of ammonia on the growth of large-scale patterned aligned carbon nanotubes using thermal chemical vapor deposition method

A systematic study on the effects of NH 3 on the growth of carbon nanotubes in atmospheric thermal-CVD was undertaken. Large-scale patterned aligned CNTs were prepared by thermal-CVD method in temperature of 900 °C. The effects of NH 3 on the growth stage were investigated. Patterned Ni-catalyst of thickness 10 nm was prepared by optical lithography and e-gun evaporation on p-Si(100) substrates lift-off process. Following a heating process, the CNTs were then grown at 900 °C by thermal-CVD method using argon (Ar), NH 3 and methane (CH 4) as the diluting, reactive and carbon source gases, respectively. In the experimental results, aligned CNTs could be synthesized with mixing ratio of CH 4/NH 3 larger or less than 1 varying the fraction of Ar dilution. The role of NH 3 was considered to enhance the catalyst activity by preventing it from being passivated. Furthermore, vertically aligned free-standing CNTs were observed at low CNT density when dealing the introduction of NH 3; meanwhile, a transition of growth characteristics from spaghetti-like to vertically-aligned was observed when the growth condition was switched from low NH 3 to high NH 3 ratio. Without Ar dilution, high NH 3 flow relative to CH 4 seemed to hamper the CNT growth as well as the degree of straightness and vertical alignment. A kinetics-based growth mechanism, either on carbon supply limited kinetics or on carbon diffusion limited kinetics, in combination with the proposed role of NH 3, can successfully explain the effect of NH 3 in different experimental conditions. Furthermore, the base growth mode was observed on all process conditions of thermal CVD in this work, and a model based on the effect of surface radius of curvature of catalyst particles on the surface energy based on broken bond model was adopted to explain the formation of bamboo-like structure and the correlation between the spacing of cone-shaped graphite and its straightness.

The growth mode change in carbon nanotube synthesis in plasma-enhanced chemical vapor deposition

We carried out experiments to investigate the origin of the growth modes in CVD synthesis of multi-wall carbon nanotubes. The results obtained show that the mode of CNT growth depends on the adhesion of the catalyst particle to the substrates. While many growths of CNTs via thermal CVD revealed base growth modes, the thermal synthesis of the CNTs on anodic aluminum oxide showed a tip growth mode indicating that the growth method itself does not determine the growth mode. In the syntheses via plasma-enhanced CVD with varying the plasma power, a change between the base- and tip-growth modes could be observed. In the growth process driven thermally, CNTs grew on partially interconnected particles, involving large adhesion force between the grouped particles and substrate, and, therefore the CNTs synthesis in a tip growth mode was difficult. However, in plasma the bombardment effectively isolated nanoparticles and promoted the tip growth mode.

Synthesis of Carbon Nanotubes from Catalytic Decomposition of C2H2through Pd/Al2O3Catalysts

CNTs have been synthesized by catalytic <TEX>$C_2H_2$</TEX> decomposition through <TEX>$Pd/Al_2O_3$</TEX> at low temperature. The CNTs were grown to a length of about 10 <TEX>${\mu}$</TEX>m and diameter 150-200 nm with multiwalled structure. Pd catalysts have two major roles; one is the active catalyst for <TEX>$C_2H_2$</TEX> decomposition, the other is a nucleation site of CNT's growth.

Production of carbon nanotubes in a packed bed and a fluidized bed

AbstractCarbon nanotubes (CNTs) prepared from ethylene decomposition over the Fe/Al2O3 catalyst are studied in a packed bed (PB) reactor and a nanoagglomerate fluidized bed reactor (NABR), respectively. CNTs sampled at different reaction times are characterized by TEM, Raman spectroscopy, and particle size analysis. The bulk density and agglomerate size of CNTs increase significantly with reaction time in a PB, while it remains at a stable level in NABR. Also, CNTs with good morphology, narrow diameter distribution, and fewer lattice defects are obtained in an NABR, rather than in a PB. In contrast to the unavoidable jamming due to volume increase of CNTs observed in a PB, a continuous CNT growth process is attained in an NABR, even though the amount of CNTs in an NABR is 6–7 times that in a PB. The flow dynamics, available space for growing, and mass and heat transfer can be controlled in an NABR, which favors the large‐scale production of CNTs with uniform properties.