Carbon Nanotubes Growth Process Research Articles

Investigation of Carbon Nanotube Growth on Multimetal Layers for Advanced Interconnect Applications in Microelectronic Devices

In this paper we present the microstructural study behind the growth of carbon nanotubes (CNTs) on the multimetal buffer layers due to its importance in microelectronics and microelectromechanical systems applications. Two different buffer layers, i.e., aluminum (Al) and titanium nitride (TiN), were deposited on the conductive layers of tantalum/copper/tantalum. A thick iron film was used as a catalyst layer to grow the CNTs. The fundamental mechanism behind the formation of catalyst nanoparticles on these two buffer layers, i.e., Al and TiN, was studied and analyzed by various characterization tools, such as atomic force microscopy, X-ray photoelectron spectroscopy, and scanning electron microscope. The formation of aluminum oxide nanoparticles during the CNT growth process was observed in the case of aluminum buffer layer. From the experimental results, it was concluded that TiN can be used as a stable buffer layer on the conductive metal lines. The CNTs growth on both buffer layers was found to be in random directions, which is due to the formation of bigger and less dense catalyst nanoparticles in comparison with the CNTs grown on the conventional buffer layer of thermally grown silicon dioxide on the silicon substrate, on which vertically aligned CNTs are grown.

Direct evidence for lip-lip interactions in multi-walled carbon nanotubes

The stability of open edged multi-walled carbon nanotubes has been investigated by using in situ high resolution transmission electron microscopy at elevated temperatures. Formation of inter-shell structures was experimentally observed for the first time and attributed to a robust interaction between adjacent concentric shells (so-called lip-lip interaction). The fl uctuating behavior of the inter-shell structures suggests a mechanism by which the carbon atoms can pass in or out through the inter-shell edges during carbon nanotube growth or shrinkage processes.

Photolithographic fabrication of gated self-aligned parallel electron beam emitters with asingle-stranded carbon nanotube

In this paper we report on the development of a photolithographic process to fabricate agated-emitter array with single-stranded carbon nanotubes (CNTs) self-aligned tothe center of the emitter gate using plasma-enhanced chemical vapor deposition(PECVD). Si tips are formed on a silicon wafer by anisotropic etching of Si usingSiO2 as a mask.Deposition of a SiO2 insulating layer and Cr–W electrode layers creates protrusions above the Si tips.This wafer is polished, and the Cr–W on the tips is removed. Etching of theSiO2 using hydrofluoric acid is performed to expose the gated Si tip. Incorporation ofa novel diffusion process produces single-stranded CNTs by depositing a thinNi layer on the Si tips and thermally diffusing the Ni layer to yield a catalystparticle for single-stranded CNT growth. The large surface to volume ratio at theapex of the Si tip allows a Ni particle to remain to act as a catalyst to grow asingle-stranded CNT for fabricating the CNT based emitter structure. Diffusion of the Ni iscarried out in situ during the heating phase of the PECVD CNT growth process at600 °C. The diameters of the observed CNTs are on the order of 20 nm. The field emissioncharacteristics of the gated field emitters are evaluated. The measured turn-on voltage ofthe gated emitter is 5 V.

Long Carbon Nanotubes Grown on the Surface of Fibers for Hybrid Composites

Hybrid composite architectures employing traditional advanced composites and carbon nanotubes offer significant potential mechanical and multifunctional performance benefits. The architecture investigated here is composed of aligned fibers with carbon nanotubes grown radially on their surface. A novel process for rapidly growing dense, long, high-quality, aligned carbon-nanotube forests is employed. Two fundamental issues related to realizing hybrid composite architectures are investigated experimentally: wetting of the carbon nanotubes by thermoset polymers and retention of mechanical (stiffness and strength) properties of the fibers after the carbon-nanotube growth process. Wetting of carbon-nanotube forests by two commercial polymers (including a highly viscous epoxy) is demonstrated at rates conducive to creating a fully dispersed carbon-nanotube/matrix region around the fibers in a typical composite. Single-fiber tension tests indicate no mechanical degradation for alumina fibers undergoing the carbon-nanotube growth process. Results indicate that hybrid carbon-nanotube/composite architectures are feasible, and future work focuses on mechanical and multifunctional property characterization of other hybrid architectures and scaling to a continuous carbon-nanotube growth process.

Atomic-Scale In-situ Observation of Carbon Nanotube Growth from Solid State Iron Carbide Nanoparticles

We have first observed the nucleation and growth process of carbon nanotubes (CNTs) from iron carbide (Fe 3C) nanoparticles in chemical vapor deposition with C 2H 2 by in situ environmental transmission electron microscopy. Graphitic networks are formed on the fluctuating iron carbide nanoparticles, and subsequently CNTs are expelled from them. Our atomic scale observations suggest that carbon atoms diffuse through the bulk of iron carbide nanoparticles during the growth of CNTs.

Growth control of carbon nanotubes by plasma enhanced chemical vapor deposition

Plasma enhanced chemical vapor deposition (PECVD), which enables growth of vertically aligned carbon nanotubes (CNTs) directly onto a solid substrate, is considered to be a suitable method for preparing CNTs for nanoelectronics applications such as electron sources for field emission displays (FEDs). For these purposes, establishment of an efficient CNT growth process has been required. We have examined growth characteristics of CNTs using a radio frequency PECVD (RF-PECVD) method with the intention to develop a high efficiency process for CNT growth at a low enough temperature suitable for nanoelectronics applications. Here we report an effect of pretreatment of the catalyst thin film that plays an important role in CNT growth using RF-PECVD. Results of this study show that uniform formation of fine catalyst nanoparticles on the substrate is important for the efficient CNT growth.

Spectroscopic investigation on carbon nanotubes coated with ZnO nanoparticles

ZnO nanoparticles were successfully coated on the surface of carbon nanotubes (CNTs) during the final stage of the growth process of CNTs using radio-frequency plasma enhanced chemical vapour deposition. During the coating process, Zn or ZnO particles with oxygen vacancies can be selectively anchored to the oxygen atom of the oxygen-containing groups on the surface of CNTs and can be perfectly oxidized by active oxygen atoms in the annealing process. The appropriate amount of coated nanoparticles effectively reduces the formation of various structural defects induced by oxygen or hydrogen atoms on the surface of the CNT wall, which can be evaluated through a decrease in the intensity ratio of disorder graphitic band (D peak) over graphitic C–C stretching band (G peak) in the Raman spectrum as well as a distinct decrease in the intensity peak, corresponding to C–H stretching vibration, in the infrared spectrum.

Environmental transmission electron microscopy observations of the growth of carbon nanotubes under nanotube–nanotube and nanotube–substrate interactions

The growth process of carbon nanotubes (CNTs) under CNT–CNT and CNT–substrate interactions has been observed directly by environmental transmission electron microscopy. Even a free standing CNT occasionally swings during the growth until it touches the substrate. In addition, we show that the growth direction of CNTs changes due to the interaction between CNTs.

A Study on Reaction Mechanism of the Catalytic CVD Synthesis of SWNTs

The catalytic reaction mechanism in the growth process of single-walled carbon nanotubes (SWNTs) is considered for carbon monoxide catalytic CVD (COCCVD) and alcohol catalytic CVD (ACCVD) methods. Combinatory CVD experiments were carried out employing a wide variety of catalyst metals, ranging from commonly used transition metals such as Co and Mo to noble metals such as Au and Pt. As a result, we propose that difference in eletronegativity is a key factor to choose a pair of binary catalysts. In the efficient catalyst reaction for a combination of catalyst metals with large electronegativity difference, the metal atom with larger electronegativity functions as a carbon bonding site, and other metal atoms help the dissociation of H2. For the combination of similar electronegativities, the metals have no work-share. Furthermore, catalytic activity of Au and Pt are demonstrated for COCCVD in addition to the previously reported ACCVD with adequate heat-treatment condition.

A Computational Study of Vertical Partial-Gate Carbon-Nanotube FETs

A vertical partial gate carbon nanotube (CNT) field-effect transistor (FET), which is amenable to the vertical CNT growth process and offers the potential for a parallel CNT array channel, is simulated using a self-consistent atomistic approach. We show that the underlap between the gate and the bottom electrode (required for isolation between electrodes) is advantageous for transistor operation because it suppresses ambipolar conduction. A vertical CNTFET with a gate length that covers only 1/6 of the channel length has a much smaller minimum leakage current than one without underlap, while maintaining comparable on current. Both n-type and p-type transistor operations with balanced performance metrics can be achieved on a single partial gate FET by using proper bias schemes. Even with a gate underlap, it is demonstrated that increasing the CNT diameter still leads to a simultaneous increase of on current and minimum leakage current. Along with a partial gate, the simulated transistor features a significant amount of air between the surface of the channel CNT and the gate insulator, as is caused by the vertical CNT growth process. Filing this pore with a high-k insulator is shown to have the potential to decrease the on current, due to electrostatic phenomena at the source-channel contact.

Matchstick-like carbon nanotube synthesis and structure

Matchstick-like carbon nanotubes (CNTs) have been synthesized by catalytic pyrolysis of thiophene, using a mixture of cobalt nitrate and calcium nitrate as a catalyst. The nanotubes thus grown are aligned, straight, and short, and have spherical heads at their tips. High resolution transmission electron microscopy, selected area electron diffraction, and energy dispersive X-ray spectroscopy results indicate that the spherical heads of the matchstick-like CNTs are crystalline Co9S8 nanoparticles and the CNTs have herringbone-type graphite structure. The effect of temperature on the formation of the matchstick-like CNTs has been investigated. A simple growth model is proposed to describe the growth process of the matchstick-like CNTs.

Direct observation of single-walled carbon nanotube growth processes on SiO 2 substrate by in situ scanning electron microscopy

We have observed the extension process of individual single-walled carbon nanotubes (SWCNTs) on SiO 2 substrate during chemical vapor deposition using in situ scanning electron microscopy. Cobalt-filled apo-ferritin was used for dispersion of Co catalysts on the substrate, minimizing overlapping of SWCNTs grown. The growth of a SWCNT exhibited an incubation stage before nucleation, a growth stage with an average growth rate of 3–5 nm/s, and termination of extension after the growth stage. It is likely that catalyst migration at the SWCNT tip occurred for continued extension of SWCNTs on the substrate.

Synthesis and characterization of larger diameter carbon nanotubes from catalytic pyrolysis of polypropylene

Carbon nanotubes with larger diameter of about 160 nm have been synthesized through catalytic decomposition of polypropylene and maleated polypropylene using Ni as catalysts in autoclave at 700 °C. Field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HRTEM) images reveal the morphologies and structures of carbon nanotubes. The possible growth process of carbon nanotubes was also discussed. The results indicate that reaction temperature, maleated polypropylene and Ni catalysts play key roles in the synthesis of large diameters carbon nanotubes. Also, using waste polypropylene as starting material for preparing carbon nanotubes is feasible.

A Vapor-Liquid-Solid Model for Chemical Vapor Deposition Growth of Carbon Nanotubes

Although carbon nanotubes (CNTs) with a variety of morphologies have been successfully synthesized, there is no clear physical picture of the growth process. Correspondingly, the growth mechanism is still not clear up to now. Here we suggest a VLS model for the growth process of CNTs, which involves a liquid or liquid-like state catalyst. The basic idea is that, due to the high thermal conductivity and nanometer size of the catalyst and the fast diffusion of carbon atoms in it, both the temperature and the carbon atom distribution across it are uniform. The supersaturation level can be expressed as a function of the carbon concentration and temperature, which determines the nucleation dynamics and growth kinetics. Based on this model, the growth rate equation was obtained to describe the growth kinetics of carbon nanotubes, which shows good accordance with the experimental results.

Quantitative Characterization of the Morphology of Multiwall Carbon Nanotube Films by Small-Angle X-ray Scattering

Films of multiwall carbon nanotubes (MWCNTs) grown by thermal chemical vapor deposition were studied using small-angle X-ray scattering (SAXS). We assessed the extent of alignment of carbon nanotubes (CNTs) by examining relative SAXS intensities as a function of azimuthal angle. We also identified features in the SAXS patterns that correspond well to CNT diameters measured through high-resolution transmission electron microscopy. For the case of thick films, corresponding to CNTs with lengths on the order of a millimeter, we were able to study the morphology of the films as a function of distance from the catalyst substrate. We examined two different films in which the morphologies of CNTs range from vertically aligned to entangled and tortuous. We determined that the alignment of CNTs as well as their average diameter can vary significantly throughout the film, demonstrating the utility of SAXS for quantitative structural analysis of CNT films, indicating the potential to reveal new information about the CNT growth process, and relating variations in morphology to evolution of the catalyst and reaction conditions.

A molecular dynamics study of the carbon–catalyst interaction energy for multi-scale modelling of single wall carbon nanotube growth

The interaction energy between a graphene sheet and a catalytic substrate-supported metal cluster has been studied by classical molecular dynamics simulation, in the wider context of a multi-scale modelling investigation of the growth process of single wall carbon nanotubes (SWNTs). The contact angle of the metal cluster on the substrate is proportional to the binding energy of the potential function representing the interaction between the metal cluster and the substrate, and the total interaction energy between the metal cluster and the graphene sheet depends on contact angle. However, the interaction energy per unit area was found to be 1.0 eV Å −2 (16.03 J m −2), independent of the contact angle, using Ni as catalyst.

A Tight-Binding Molecular Dynamics Simulation Analysis of Carbon Nanotube Growth Process Parameters

This paper employs tight-binding molecular dynamics simulations to investigate the effects upon the final nanotube morphology of three principal carbon nanotube growth process parameters, namely the radii of the initial carbon nanotube patterns, the temperatures of the carbon nanotube patterns, and the energies of the incident carbon atoms. The simulation results provide an understanding of the roles played by these parameters in the fabrication of carbon nanotubes from an atomic perspective. It is shown that carbon nanotube patterns with small radii tend to form close-capped ends, while patterns with larger radii tend to promote lifted-up growth. Furthermore, the effect of pattern temperature is more obvious in the case of larger radii carbon nanotube patterns, in which disordered structures are evident at low pattern temperatures. Finally, it is shown that incident atoms with higher energies not only form unstable bonds with other carbon atoms, but may also destroy the covalent bonds of previously deposited carbon atoms. The present results provide a valuable insight into the promotion of close-capped nanotubes or lifted-up growth during the fabrication of carbon nanotubes.

Hydrogen termination for the growth of carbon nanotubes on silicon

Vertically aligned carbon nanotubes (CNTs) are grown on Fe-covered nano-structured Si tip apexes and planar Si substrates. Under the similar condition of a hot wire filament chemical vapor deposition technique, the growth and adhesion behaviors of the CNTs strongly depend on the chemical treatment of the Si surfaces, which determines the predominant chemical terminations. The Si substrates are terminated by hydrogen, or covered by a native oxide layer, or treated with tetramethyl ammonium hydroxide. The former provides an extremely strong adhesion, presumably due to the formation of Si–C direct bonds created in the initial stage of the CNT growth process, whereas the latter adheres poorly, and the third not at all, as revealed by images of scanning and transmission electron microscopy. A related concept of the CNT growth is suggested.

Growth process of close-packed aligned carbon nanotubes on SiC

Densely aligned carbon nanotubes (CNTs) by surface decomposition of SiC (0001¯) C-face in vacuum were observed by plan-view transmission electron microscopy. It was found that the wall number of CNTs was directly proportional to the diameter of CNTs. Comparing with the theoretical calculation, it was revealed that all of the carbon atoms remained on the surface after the selective evaporation of Si atoms by decomposition of each monolayer of SiC (0001¯), and then constructed the CNT walls with the minimum diffusion distance at the interface.

Fabrication and growth mechanism of carbon nanotubes by catalytic chemical vapor deposition

The synthesis of carbon nanotubes (CNTs) by chemical vapor deposition (CVD) of methane on catalyst of Ni–Cu–Al powder was reported. The influence factors on the growth morphology of CNTs, such as reaction time and different carrier gases concerning hydrogen, nitrogen as well as no carrier gas were investigated using SEM and TEM. The results showed that the reaction time brought no significant influence to the diameter of CNTs, but the average length of the CNTs was obviously increased with prolonging the reaction time. Relatively pure CNTs could be obtained with hydrogen as the carrier gas but with poor product rate compared with the CNTs with no carrier gas. Proper amount of CNTs with pure characteristic could be obtained with nitrogen as the carrier gas. Finally, a growth mechanism of section-seal is proposed for analyzing the growth process of general CNTs.