CVD-grown Carbon Nanotubes Research Articles

Direct Application of Carbon Nanotubes (CNTs) Grown by Chemical Vapor Deposition (CVD) for Integrated Circuits (ICs) Interconnection: Challenges and Developments.

With the continuous shrinkage of integrated circuit (IC) dimensions, traditional copper interconnect technology is gradually unable to meet the requirements for performance improvement. Carbon nanotubes have gained widespread attention and research as a potential alternative to copper, due to their excellent electrical and mechanical properties. Among various methods for producing carbon nanotubes, chemical vapor deposition (CVD) has the advantages of mild reaction conditions, low cost, and simple reaction operations, making it the most promising approach to achieve compatibility with integrated circuit manufacturing processes. Combined with through silicon via (TSV), direct application of CVD-grown carbon nanotubes in IC interconnects can be achieved. In this article, based on the above background, we focus on discussing some of the main challenges and developments in the application of CVD-grown carbon nanotubes in IC interconnects, including low-temperature CVD, metallicity enrichment, and contact resistance.

CVD-Grown Carbon Nanotube Branches on Black Silicon Stems for Ultrahigh Absorbance in Wide Wavelength Range

We report a black silicon-carbon nanotube (bSi-CNT) hybrid structure for ultrahigh absorbance at wide spectral range of wavelength (300–1200 nm). CNTs are densely grown on entire bSi stems by chemical vapor deposition (CVD) through uniformly coating Fe catalyst. The bSi-CNT not only increases the surface roughness for enhancing the light suppression, but also allows the absorption of light in a wide wavelength range over the Si band gap (>1000 nm owing to 1.1 eV) due to the small band gap of CNT (0.6 eV). At short wavelength below Si band gap (<1000 nm), the absorbance of bSi-CNT shows average of 98.1%, while bSi shows 89.4%, which is because of high surface roughness of bSi-CNT that enhancing the light trapping. At long wavelength over Si band gap, the absorbance of bSi-CNT was maintained to 96.3% because of the absorption in CNT, while absorbance of bSi abruptly reduces with increase wavelength. Especially, the absorbance of bSi-CNT was showed 93.5% at 1200 nm, which is about 30~90% higher than previously reported bSi. Simple growth of CNTs on bSi can dramatically enhances the absorbance without using any antireflection coating layer. Thus, this study can be employed for realizing high efficiency photovoltaic, photocatalytic applications.

Hierarchal growth of MoS2@CNT heterostructure for all solid state symmetric supercapacitor: Insights into the surface science and storage mechanism

Direct growth of atomically thin hierarchal MoS2 nanoflakes on CVD grown carbon nanotubes has been achieved using magnetron sputtering for electrochemical storage application. This novel technique of preparing MoS2@CNT 1D heterostructure using a combination of CVD and PVD proved to be very efficient in structurally engineering the microstructure of electrode material. The super capacitive measurements of the MoS2@CNT electrode revealed capacity of 337 mF/cm2 at the sweep rate of 5 mV/s. The storage mechanism involves both electrostatic ionic adsorption as well as Na+ intercalation into the MoS2@CNT matrix. Further, using Trassati and Dunn's methods, we found that ionic diffusion and the inner electrode material contributes majorly to the total stored charge. In order to study the phase evolution of electrode material upon several charge-discharge cycles, we employed XPS technique which indicated lower shift in the binding energies of Mo as well as S. This shift indicates partial transition of 2H→1T phase due to the transverse gliding of S plane upon the intercalation of electrolytic ions. The state supercapacitor built using symmetric MoS2@CNT electrodes was found to have high areal and volumetric capacitance of 131 mF/cm2 and 2.9 F/cm3 respectively, with high cyclic stability of 97.6% after 2500 cycles.

Scalable Preparation of High-Density Semiconducting Carbon Nanotube Arrays for High-Performance Field-Effect Transistors.

Although chemical vapor deposition (CVD)-grown carbon nanotube (CNT) arrays are considered ideal materials for constructing high-performance field-effect transistors (FETs) and integrated circuits (ICs), a significant gap remains between the required and achieved densities and purities of CNT arrays. Here, we develop a directional shrinking transfer method to realize up to 10-fold density amplification of CNT array films without introducing detectable damage or defects. In addition, the method improves the film uniformity while retaining the perfect alignment and high carrier mobility of 1600 cm2 V-1 s-1 of CVD-grown CNT arrays. By combining the density amplification method with the thermocapillary flow method developed by Rogers et al., semiconducting CNT arrays with high densities and high qualities are obtained. High-performance FETs with a channel length of 200 nm are demonstrated using these high-density semiconducting CNT arrays, yielding a record-high on-state current density of 150 μA/μm, a peak transconductance of 80 μS/μm, and a current on/off ratio of more than 104 among the CVD-grown CNT-based FETs.

Design of Electrically Conductive Structural Composites by Modulating Aligned CVD-Grown Carbon Nanotube Length on Glass Fibers.

Function-integration in glass fiber (GF) reinforced polymer composites is highly desired for developing lightweight structures and devices with improved performance and structural health monitoring. In this study, homogeneously aligned carbon nanotube (CNT) shell was in situ grafted on GF by chemical vapor deposition (CVD). It was demonstrated that the CNT shell thickness and weight fraction can be modulated by controlling the CVD conditions. The obtained hierarchical CNTs-GF/epoxy composites show highly improved electrical conductivity and thermo-mechanical and flexural properties. The composite through-plane and in-plane electrical conductivities increase from a quasi-isolator value to ∼3.5 and 100 S/m, respectively, when the weight fraction of CNTs grafted on GF fabric varies from 0% to 7%, respectively. Meanwhile, the composite storage modulus and flexural modulus and strength improve as high as 12%, 21%, and 26%, respectively, with 100% retention of the glass transition temperature. The reinforcing mechanisms are investigated by analyzing the composite microstructure and the interfacial adhesion and wetting properties of CNTs-GF hybrids. Moreover, the specific damage-related resistance variation characteristics could be employed to in situ monitor the structural health state of the composites. The outstanding electrical and structural properties of the CNTs-GF composites were due to the specific interfacial and interphase structures created by homogeneously grafting aligned CNTs on each GF of the fabric.

Drastic increase in catalyst productivity of nanoclay-supported CVD-grown carbon nanotubes by organo-modification

The organo-modification of montmorillonite (Mt) prior to catalyst insertion was proposed as a promising approach to increase carbon nanotube (CNT) yield on nanoclay (NC) supports in catalytic chemical deposition (CVD) reaction. The behavior of organo-modified montmorillonite (OMt) as the support in CVD reaction was compared with that of Mt. According to morphological images and Raman spectra, the carbon atoms form CNT in case of both Fe-loaded pristine Mt and OMt. Decreasing gas flow-rate and diminishing hydrogen fraction in growth atmosphere were found to favor improvement of CNT yield, whereas generally tended to deteriorate its quality. TGA results showed that the CNT content of OMt-supported hybrids were drastically increased compared to that of non-modified Mt. There was about two- to fifteen-fold increase in CNT yield depending on the CVD conditions. This observation was found correlated to the larger specific surface area (SSA) and d001-value of Fe-loaded OC. The evident advantages of OMt as catalyst support include efficient and scalable synthesis of high thermally stable NC/CNT hybrid at low cost.

Origins of Height Distribution within Carbon Nanotube Arrays

We investigated the phenomenon of non-uniform height distribution within CVD-grown carbon nanotube (CNT) arrays. This phenomenon is related to the activity of the catalyst which is affected by the deposition of the carbon source and the subsequent formation of pyrolysis products on the catalyst. We developed a model that considers the effects of deposition of the carbon source as well as the fact that the pyrolysis products also accumulate at the edges of CNT arrays. This model also illustrates that carbon source deposition results in moderate growth over large areas, whereas the pyrolysis products hinder growth significantly over small areas. Together, the two have a competitive effect on CNT growth and bring about the height distribution feature.

Atomic oxygen exposure behaviors of CVD-grown carbon nanotube film and its polymer composite film

Materials used in low earth orbit are exposed to damaging atomic oxygen (AO). AO exposure behaviors of carbon nanotube (CNT) film and CNT/epoxy composite film were evaluated when exposed to a nominal AO fluence of about 3.0×1020atoms/cm2. Properties of the films before and after AO exposure were evaluated by mass loss, Raman spectroscopy, X-ray photoelectron spectroscopy, tensile and electrical conductivity tests. Results show similar mass loss and different surface erosion for two kinds of films. CNT bundles collapse with the emergence of a tight arrangement of particles, and porous CNT film has more serious interior CNT erosion than dense CNT/epoxy composite film. Decreases of tensile strength and electrical conductivities are seen.

Morphological and Microstructural Property Comparison of Bulk and Aligned Cvd-Grown Carbon Nanotubes

Carbon nanotubes are one-dimensional materials found in various forms, the most important of which are bulk unordered carbon nanotubes of lengths of few microns and aligned tubes regularly a few millimeters long. Differences in length, orientation, alignment, nanotube graphitization and purity will influence the eventual performance of the material. Herein four characterization methods are used to characterize and compare bulk and aligned carbon nanotubes. Results show that the diameters of the millimeter-long aligned carbon nanotubes are approximately 60 ∼ 80 nm with thick nanotube walls of about 25–30 nm, while the diameters of the bulk carbon nanotubes are about 15–20 nm with much thinner walls. The aligned carbon nanotubes have significantly larger graphitization degrees, higher purity and greater orientation than the bulk carbon nanotubes that tend to self-agglomerate under no external stimulus. Silicon carbide matrix nanocomposites reinforced by the aligned carbon nanotubes were found to be denser that those reinforced by the bulk carbon nanotubes and also exhibit extensive, uniform, and long pullout of carbon nanotubes.

Influence of the alignment degree of CVD-grown carbon nanotubes on their functionalization and adsorption capacity

Oxygen functionalized multiwalled carbon nanotubes (CNTs) were prepared from aligned and bundle-like CVD-grown CNTs by chemical treatment with different reagents of increased oxidation degree. As the severity in the oxidation degree increases elimination of the amorphous carbon is observed. However, contrary to what was previously reported, aligned CNTs treated in less severe oxidation conditions they exhibit an increased number of oxygen containing functional groups (COH/CO groups). Meanwhile, when treated in the more severe oxidation conditions, they have the highest number of COOH groups due to the appearance of defects and shortening of the tubes. In the benzene adsorption in water, aligned CNTs display faster kinetics in all cases. The best adsorption capacity is achieved with aligned CNTs treated in very severe conditions possibly due to their enhanced solubility in water, which favors electrostatic interactions and π–π interactions between the benzene rings and the CNT walls.

A comparative study of the growth, microstructural and electrical properties of multiwall CNTs grown by thermal and microwave plasma enhanced CVD methods

Multiwalled carbon nanotubes (CNTs) were grown on silicon substrates by thermal chemical vapour deposition (CVD) and microwave plasma CVD (MPCVD). In this manuscript, an attempt has been made to compare the growth mechanism and electrical and micro-structural properties of CNTs grown by two different methods. CNTs in MPCVD is grown by tip-growth mechanism whereas in TCVD it is grown by base-growth mechanism. To compare these techniques, the CNT samples were examined using scanning and transmission electron microscopy, Raman spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy. Growth rates were 10 and 0.6 μm/min, for thermal CVD and MPCVD, respectively. Diameters ranged from ∼40–60 nm for thermal CVD grown CNTs, and 20–50 nm for MPCVD-grown CNTs. Deposition over 2″ diameter silicon wafers was achieved for the MPCVD growth technique. XRD and Raman studies revealed good crystallinity for both growth methods. However, thermal CVD grown CNTs showed more defects, with a crystallite size of ∼20 nm, compared to ∼37 nm for MPCVD. Static water contact angles of thermal CVD and MPCVD samples were 155° and 136°, respectively, indicating greater hydrophobicity for thermal CVD-grown CNTs. The electrical resistance was 75.40 Ω for MPCVD-grown CNTs, but 782.10 Ω for thermal CVD samples. Field emission studies revealed β values for thermal CVD and MPCVD samples, of 8010 and 13,570, with turn-on fields of 0.96 and 0.60 V/μm, respectively.

Mechanical properties of nanostructured carbon layers grown by CVD and PLD techniques

Pulsed laser deposition (PLD) and chemical vapour deposition (CVD) have been proven to be among the most successful techniques for growing the entire spectrum of carbon films, which can be used in a wide range of technical applications. Here an investigation has been performed to explore the effect of different growing techniques (PLD and CVD) and process parameters (such as deposition time and substrate type) on the films’ morphology and mechanical properties. The mechanical properties of the grown thin films were characterised by means of nano/micro indentation and scratch test techniques. It was observed that the thickness of the Al2O3 interlayer (between the Fe catalyst nanoparticles and the silicon substrate) is a critical parameter that can be used to significantly enhance the adhesion strength of PLD-grown carbon films. PLD-grown carbon films were in fact found to have higher adhesion to the substrate than CVD-grown carbon nanotubes (CNT), and the adhesion strength was found to increase with increasing thickness of the Al2O3 interlayer. On the other hand, CVD-grown carbon films (made of aligned CNTs) seem to offer a greater response in terms of elastic modulus. A thorough scanning electron microscopy characterisation suggested that the observed mechanical responses might be correlated to the films’ morphology at the nano/microscale. It was in fact observed that, in PLD-grown samples, an increasing deposition time and Al2O3 content leads to a grain size increase and to a clustering effect, thus to a loss in film uniformity.

Comparative study on thermal and plasma enhanced CVD grown carbon nanotubes from gas phase prepared elemental and binary catalyst particles

AbstractThe preparation of nanoparticles in the gas phase through inert gas condensation is a very versatile method to fabricate catalyst particles that act as catalytic templates for carbon nanotube synthesis since the method allows for easy control of the catalysts' elemental composition and their morphology. In this contribution we report on the synthesis of carbon nanotubes via two distinct chemical vapour deposition routes, thermal and plasma enhanced chemical vapour deposition, using such gas phase prepared elemental Fe catalyst particles as well as binary FePt nanoparticles. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Combined Raman Spectroscopy and SEM Analysis of CVD Grown Carbon Nanotubes

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Device characterization of carbon nanotubes field emitters in diode and triode configurations

Device characteristics of thermal CVD grown carbon nanotubes (CNTs) field emitters in diode and triode configurations are reported. The CNT cathode configured as field emission diode exhibited a low turn-on field of ∼3.0 V/μm, high emission current of ∼35 mA at ∼5.0 V/μm, and excellent current stability. Large signal model (dc) and small signal model (ac) of the field emission diode were also investigated. The CNT triode was achieved by integrating a TEM grid (transmission electron microscope specimen holder) as the gate electrode to the CNT cathode, with no further micro-fabrication processes needed. The CNT triode displayed gate-controlled current modulation behavior with distinct cutoff, linear and saturation regions. The triode has a gate turn-on field of ∼5.4 V/μm despite a large cathode-gate spacing of ∼120 μm. The field emission data established the basic transistor characteristics of CNT emitters in a triode amplifier configuration with good gain. The detailed dc characteristics and transistor ac parameters such as transconductance, amplification factor, and anode resistance for the CNT triode were investigated and modeled.

Atomically Resolved STM Images of CVD Grown Carbon Nanotubes

ABSTRACTAtomically resolved images of single-wall carbon nanotubes (CNT) grown in a chemical vapor deposition (CVD) chamber were obtained with the scanning tunneling microscope (STM) under ambient conditions. We found that the average diametersdof the CVD-grown CNTs appear to fall into a bimodal distribution of 1.0 and 0.6 nm, and the chiral angle Ø was observed to be close to zero degree. The summation of the lattice indices(n+m)was determined to be 14 and 9 ford=1.0nm andd=0.6nm, respectively. The most possible lattice index pairs(n, m)with a chiral angle close to zero degree are(7, 7)and(5, 4), which indicates that the larger nanotubes are metallic and the smaller nanotubes are semi-conductive.

Propertied of Field Emission Electrons for CVD-grown Carbon Nanotubes

The microstructure and field emission properties of carbon nanotubes(CNT) grown by Ni-catalytic chemical vapor deposition(CVD) were investigated. CVD-grown CNT had a high density of curved shape with randomly oriented. It was found that an increase in electric field caused an increase in field emission current and field emission sites of CNT. The maximum field emission current density was measured to be 3.6 ㎃/<TEX>$\textrm{cm}^2$</TEX> at 2.5 V/<TEX>$\mu\textrm{m}$</TEX>, while the brightness of 56 cd/<TEX>$\textrm{cm}^2$</TEX> was observed for the CNT-grown area of 0.8 <TEX>$\textrm{cm}^2$</TEX> from a phosphor screen. Field emission current at constant electric field gradually decreased initially and then stabilized with time.

Raman spectroscopy and field-emission properties of CVD-grown carbon-nanotube films

Carbon-nanotube films are very efficient cathodes for field-emission devices. This study presents a comprehensive comparison between structural, spectroscopic and field-emission properties of films of aligned and non-aligned multi-wall nanotubes (MWNTs) which are grown by thermal chemical vapour deposition. Three types of films are investigated: vertically aligned MWNTs with clean and coated nanotube side walls as well as non-aligned MWNT films. Raman spectra taken on the aligned MWNT films consist of many lines of first-, second- and third-order signals. Several lines are reported here for the first time for MWNTs. The presence of the surface coating leads to a decrease and broadening of the higher-order signals as well as an increase in the disorder-induced contributions in the first-order regime. The aligned MWNT films have excellent field-emission properties with very high emission current densities and low turn-on and threshold fields. The presence of a surface coating has no impact on the efficiency of the field-emission process. Films of non-aligned MWNTs show considerably reduced electron-emission current densities and larger critical fields.

Transport property investigation of pure and Au-doped carbon nanotubes

We report here on the investigations of the electrical transport property for single pure or Au-doped multi-walled carbon nanotubes and super-long (millimetric scale) carbon nanotubes, which were grown via CVD method. The results revealed that the structures of carbon nanotubes strongly dominate their electrical properties. It was found that doping can modulate the electrical transport property of carbon nanotubes, and the doping effect on the transport of the carbon nanotubes is discussed. The transport measurements on mat of pure or doped carbon nanotubes revealed that the resistance of the CVD-grown carbon nanotubes has negative temperature dependence.

Synthesis, Structure, and Superconducting Properties of Tantalum Carbide Nanorods and Nanoparticles

Tantalum carbide nanorods and nanoparticles have been synthesized using a vapor-solid reaction path starting with CVD grown carbon nanotube precursors. Their structures were studied using XRD, TEM, and HRSEM. Superconducting properties were characterized using a SQUID magnetometer. For reactions at lower temperatures, carbide nanorods, which replicate the ∼14 nm diameter of the precursor carbon nanotubes, are observed. For higher temperature reactions, coarsened carbide nanoparticles (100–250 nm) are observed which have spherical or cubic-faceted morphologies. A morphological Rayleigh instability is postulated as initiating the transition from nanorod to nanoparticle morphologies. Stoichiometric bulk TaC crystallizes in the rock salt structure and has a superconducting transition temperature of 9.7 K. In TaC nanorods and nanoparticles, the superconducting properties correlate with the lattice parameter. Nanoparticles with a little higher lattice parameter than the ideal one show higher Tc and higher fields at which the superconductivity disappears than stoichiometric bulk TaC.