High-density Carbon Nanotubes Research Articles

Bundling dynamics regulates the active mechanics and transport in carbon nanotube networks and their nanocomposites

High-density carbon nanotube networks (CNNs) continue to attract interest as active elements in nanoelectronic devices, nanoelectromechanical systems (NEMS) and multifunctional nanocomposites. The interplay between the network nanostructure and its properties is crucial, yet current understanding remains limited to the passive response. Here, we employ a novel superstructure consisting of millimeter-long vertically aligned single walled carbon nanotubes (SWCNTs) sandwiched between polydimethylsiloxane (PDMS) layers to quantify the effect of two classes of mechanical stimuli, film densification and stretching, on the electronic and thermal transport across the network. The network deforms easily with an increase in the electrical and thermal conductivities, suggestive of a floppy yet highly reconfigurable network. Insight from atomistically informed coarse-grained simulations uncover an interplay between the extent of lateral assembly of the bundles, modulated by surface zipping/unzipping, and the elastic energy associated with the bent conformations of the nanotubes/bundles. During densification, the network becomes highly interconnected yet we observe a modest increase in bundling primarily due to the reduced spacing between the SWCNTs. The stretching, on the other hand, is characterized by an initial debundling regime as the strain accommodation occurs via unzipping of the branched interconnects, followed by rapid rebundling as the strain transfers to the increasingly aligned bundles. In both cases, the increase in the electrical and thermal conductivity is primarily due to the increase in bundle size; the changes in network connectivity have a minor effect on the transport. Our results have broad implications for filamentous networks of inorganic nanoassemblies composed of interacting tubes, wires and ribbons/belts.

Synthesis of nanocomposites based on nanotubes and silicates

In situ synthesis of nanocomposites based on carbon nanotubes and zeolite/montmorillonite was carried out in a hot filament CVD reactor where the precursors (methane and hydrogen) are activated by carbonized tungsten filaments heated up to 2200°C. In nanocomposites formed both on zeolite and montmorillonite we observed cross-linking of the catalytic particles by nanotubes and creation of carbon nanotube bridges and three-dimensional networks. The length of nanotube bridges was in a range from several nm to nearly 10μm. A high density of carbon nanotubes was observed in the whole volume of zeolite. The high catalytic efficiency of zeolite is most likely caused by its structure that allows anchoring of Fe3+ catalytic particles in the pores and prevents their migration from the sample. At the ends of the nanotubes grown on zeolite we observed particles of the catalyst. In montmorillonite, the particles catalyzing the growth of carbon nanotubes may be present not only on the external surface but also in the interlayer voids of the mineral. Its catalytic efficiency is enhanced as proved by the higher amount of CNTs and their bundles. In the course of CNTs synthesis probably also clumps of Fe3+ catalytic particles arise, which may be the reason for formation of bundles of nanotubes.

Electron field emission properties of highly dense carbon nanotube arrays

In this paper, we have studied field emission properties of highly dense arrays of multi-walled carbon nanotubes (CNTs) used as cathodes in diode-type field emission devices with a phosphor screen. For the high-density CNT emitters it is demonstrated that the emission sites are located on the CNT-cathode edges, which is direct experimental evidence of the ‘edge effect’. The results of computer simulations (using ‘ANSYS Electromagnetic’ software) are presented to confirm the experimental data and to analyze the effect of patterning on the electric field distribution for high-density CNT arrays. It is shown that selective-area removal of nanotubes in the arrays leads to the formation of additional edges characterized by the high field enhancement factor and enhanced emission from the CNT cathodes. In addition, scanning probe microscopy techniques are employed to examine surface properties of the high-density CNT arrays. For CNT arrays of ‘short’ nanotubes, the work function distribution over the sample surface is detected using a scanning Kelvin microscopy method.

Non-destructive characterization of structural hierarchy within aligned carbon nanotube assemblies.

Understanding and controlling the hierarchical self-assembly of carbon nanotubes (CNTs) is vital for designing materials such as transparent conductors, chemical sensors, high-performance composites, and microelectronic interconnects. In particular, many applications require high-density CNT assemblies that cannot currently be made directly by low-density CNT growth, and therefore require post-processing by methods such as elastocapillary densification. We characterize the hierarchical structure of pristine and densified vertically aligned multi-wall CNT forests, by combining small-angle and ultra-small-angle x-ray scattering (USAXS) techniques. This enables the nondestructive measurement of both the individual CNT diameter and CNT bundle diameter within CNT forests, which are otherwise quantified only by delicate and often destructive microscopy techniques. Our measurements show that multi-wall CNT forests grown by chemical vapor deposition consist of isolated and bundled CNTs, with an average bundle diameter of 16 nm. After capillary densification of the CNT forest, USAXS reveals bundles with a diameter >4 μm, in addition to the small bundles observed in the as-grown forests. Combining these characterization methods with new CNT processing methods could enable the engineering of macro-scale CNT assemblies that exhibit significantly improved bulk properties.

High-performance local back gate thin-film field-effect transistors using sorted carbon nanotubes on an amino-silane treated hafnium oxide surface

Wafer-scale fabrication and characterization of local back gate semiconducting nanotube thin-film transistors (SN-TFTs) are reported in this paper. The local back gate voltage of the corresponding SN-TFT controls the individual transistor switching. In order to achieve high performance, a high-k dielectric material is employed as a gate oxide and this helped to achieve low-voltage operations, much steeper sub-threshold voltage swings and higher transconductance values. A simple procedure to deposit a high-density single-walled carbon nanotube thin film on an amino-silane-treated hafnium oxide (HfOX) surface is suggested such that a good density of nanotubes is realized without degrading the device on–off current ratio and mobility values. The density of the nanotubes achieved on the silanized HfOX surface is about 40–45 nanotubes µm−2. SN-TFTs exhibit an excellent p-type output characteristic with distinct linear and saturation regions. Local back gate SN-TFTs exhibit an on–off current ratio exceeding 104 and a steep sub-threshold slope of 400 mV/decade. SN-TFTs achieve a maximum current density of 13 μA µm−1, an average threshold voltage of −0.5 V, a maximum normalized transconductance of 18.5 μS µm−1 and exhibit a maximum carrier mobility of 60.6 cm2(Vs)−1.

Effect of substrate temperature on aligned high-density carbon nanotubes deposited by RF-PECVD

The high-density carbon nanotubes (CNTs) are synthesized on Fe/Si substrate in the mixture of acetylene and hydrogen gas by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) method. The effects of substrate temperature on the growth of CNTs are studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The quality of CNTs is improved considerably by increasing the substrate temperature while the beam density is increased and graphitization degree (I D/I G) is enhanced. The best aligned CNTs are prepared at 750 °C, the beam D density is about 1.6 × 103/μm2, and I D/I G is about 0.93. Temperature influence is also discussed.

A CMOS Compatible Carbon Nanotube Growth Approach

ABSTRACTIn future technology nodes, 22nm and below, carbon nanotubes (CNTs) may provide a viable alternative to Cu as an interconnect material. CNTs exhibit a current carrying capacity (up to 109 A/cm2), whilst also providing a significantly higher thermal conductivity (SWCNT ~ 5000 WmK) over Copper (106 A/cm2 and ~400WmK). However, exploiting such properties of CNTs in small vias is a challenging endeavor. In reality, to outperform Cu in terms of a reduction in via resistance alone, densities in the order of 1013 CNTs/cm2 are required. At present, conventional thermal CVD of carbon nanotubes is carried out at temperatures far in excess of CMOS temperature limits (400 C). Furthermore, high density CNT bundles are most commonly grown on insulating supports such as Al2O3 and SiO2 as they can effectively stabilize metallic nanoparticles at elevated temperatures but this limits their application in electronic devices. To circumvent these obstacles we employ a remote microwave plasma to grow high density CNTs at a temperature of 400 C on conductive underlayers such as TiN. We identify some critical factors important for high-quality CNTs at low temperatures such as control over the catalyst to underlayer interaction and plasma growth environment while presenting a fully CMOS compatible carbon nanotube synthesis approach

Synthesis and device applications of high-density aligned carbon nanotubes using low-pressure chemical vapor deposition and stacked multiple transfer

For nanotube-based electronics to become a viable alternative to silicon technology, high-density aligned carbon nanotubes are essential. In this paper, we report the combined use of low-pressure chemical vapor deposition and stacked multiple transfer to achieve high-density aligned nanotubes. By using an optimized nanotube synthesis recipe, we have achieved high-density aligned carbon nanotubes with density as high as 30 tubes/μm. In addition, a facile stacked multiple transfer technique has been developed to further increase the nanotube density to 55 tubes/μm. Furthermore, high-performance submicron carbon nanotube field-effect transistors have been fabricated on the high-density aligned nanotubes. Before removing the metallic nanotubes by electrical breakdown, the devices exhibit on-current density of 92.4 μA/μm and normalized transconductance of 13.3 μS/μm. Moreover, benchmarking with the aligned carbon nanotube transistors in the literature indicates that our devices exhibit the best performance so far, which is attributed to both the increased nanotube density and scaling down of channel length. This study shows the great potential of using such high-density aligned nanotubes for high performance nanoelectronics and analog/RF applications.

Synthesis of Copper Nanocatalysts with Tunable Size Using Diblock Copolymer Solution Micelles

Self-assembled solution micelles prepared from polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) and polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP), have been employed as templates to synthesize copper nanocatalysts which are regarded as an excellent catalyst system for 1D nanomaterial synthesis. We have demonstrated that uniform-sized nanoparticles with diameters ranging from 1 to 15 nm have been generated. We have revealed that nanocatalyst size can be rationally tailored by adjusting the interaction between copper precursors and ligands and metal sequestration time. Ordered arrays of copper nanocatalysts derived from depositing a monolayer of solution micelles exhibit excellent thermal stability and do not agglomerate during the thermal treatment at 850 °C, typical growth temperature for 1D nanomaterial using the chemical vapor deposition technique. High-density and aligned single-walled carbon nanotubes with uniform diameter have been synthesized using the chemical vapor deposition technique. The average diameter is 1.4 nm, which is on the same order of catalyst size, around 2.0 nm. The combination of tunable size and spacing with superb thermal stability and outstanding catalytic activity offered by this new copper nanocatalyst system will enable growth of high-yield 1D nanomaterials with controllable diameter and spacing consistently and reproducible properties. It also paves a new path to study the effect of nanocatalyst size on 1D nanomaterial synthesis and their properties.

Growth of High-Density Self-Aligned Carbon Nanotubes and Nanofibers Using Palladium Catalyst

In this paper we demonstrate vertical self-aligned growth of carbon nanotubes (CNT) and carbon nanofibers (CNF) using 1 nm of Pd as the catalyst material. Results were compared with those obtained using traditional catalysts (Co, Fe, and Ni). Pd is of interest as it has been demonstrated to be an excellent material for electrical contact to nanotubes. CNT were grown using plasma-enhanced chemical vapor deposition (PECVD) at 450°C to 500°C and using atmospheric-pressure chemical vapor deposition (APCVD) between 450°C and 640°C. The results were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. High-density (1011 cm−2 to 1012 cm−2) self-aligned CNT growth was obtained using APCVD and Pd as the catalyst, while Co and Fe resulted in random growth. TEM revealed that the CNT grown by Pd with PECVD form large bundles of tubes, while Ni forms large-diameter CNF. It was found that the CNT grown using Pd or Ni are of low quality compared with those grown by Co and Fe.

Growth of carbon nanotubes through selective deposition of nanoparticles

This paper describes the selective growth of a high density of Carbon NanoTubes (CNTs) (up to 2.5 1011 CNT cm−2) in nanometric (140 to 300 nm) holes etched in silicon dioxide (vias). The method involves (i) the electrografting of a thin layer of poly(hydroxyethyl methacrylate) (polyHEMA) at the Si bottom of the SiO2 vias; (ii) independent synthesis of nanometric iron oxide nanoparticles (NP) by the polyol method, the NPs being capped by diethylene glycol (DEG); (iii) HF cleaning of the SiO2 structure; (iv) selective deposition of the NPs on the polyHEMA layer at the bottom of the vias; (v) burn-off of the polymer; (vi) Chemical Vapour Deposition (CVD) growth of the CNTs. The success of the experiment is based on the fine tuning of the selective interactions between the capping of the NPs and the polymer deposited exclusively at the bottom of the vias.

High-Density Short-Height Directly Grown CNT Patterned Emitter on Glass

Vertically aligned, high-density (4× 109 /cm2) carbon nanotubes (CNTs) with an average short height of 1.5 μm were grown directly on a Cr electrode patterned on a glass substrate at a substrate temperature below 550 °C by water-assisted chemical vapor deposition (CVD). Threshold field of the emission of the patterned grown CNT could be tuned with the pattern shape, and it was 1.8 V/μm at 1 μA/cm2 for the emitter with a hole spacing parameter of S = 20 μm. The stability of field emission of the patterned CNT emitter on glass was confirmed with continuously working more than 24 hours at a pulse current density of 70 μA/cm2 with the duty ratio of 7.5 %. [DOI: 10.1380/ejssnt.2010.336]

Modification of Working Electrode Surface with Carbon Nanotubes as an Electrochemical Sensor for Estimation of Melting Points of DNA

Screen-printed with three electrode system was used in this study. A working electrode has been printed from carbon nanotubes based paste on silver layer modified with nano-patterned structures for the first case. In the second case, vertically aligned carbonnanotubes were grown on the Au working electrode. The process of the nanotubes growing was tested to create homogenous and high density carbon nanotubes layer directly on the thick-film silver layer. Based on the characterization of electrodes, we used Au based for detection of nucleic acids. Moreover, we were able to estimated melting points of DNA.

Synthesis of Carbon Nanotubes and Nanofibers by Thermal CVD on SiO<SUB>2</SUB> and Al<SUB>2</SUB>O<SUB>3</SUB> Support Layers

In this work, catalytic thermal chemical vapor deposition method, using a mixture of methane and hydrogen at atmospheric pressure in a horizontal tubular quartz furnace, was used to grow carbon nanostructured materials. Silicon wafers with SiO2 or Al2O3 layers were used as support for thin nickel film deposition used as catalyst. It has been shown that the interaction between catalysts and substrates is of critical importance for carbon nanotube growth. However, this mechanism is not completely understood. Here, the interaction between catalyst nickel film and two different oxide layers supported on silicon wafers was studied as well as the influence of both support systems (SiO2/Si and Al2O3/Si) on the carbon nanostructures growth at different temperatures and process running times. The substrates were characterized by atomic force microscopy and the carbon nanostructured materials were studied by Raman spectroscopy, high resolution scanning and transmission electron microscopy. At higher temperatures it was observed a high density of carbon nanotubes grown over Al2O3 support layer when compared to SiO2 support layer showing a different behavior for Ni catalyst on each of the substrates. A quite different Ni catalyst behavior was observed at lower temperatures due to the formation of carbon nanofibers instead of carbon nanotubes on both substrates.

Carbon Nanotube Growth from Diamond

We demonstrate that nanosize diamond particles in a stable solid phase can act as nuclei for carbon nanotube (CNT) growth by chemical vapor deposition. This growth process is explained by a new growth mechanism, where the surface diffusion of carbon adatoms on the solid surface of nanoparticles plays an important role, contrary to the bulk diffusion in conventional metal catalysts. The use of diamond nanoparticles overcomes the issues of deactivation due to fusion with each other or reaction with substrate materials in the CNT growth using conventional metal catalysts and promotes high-density CNT growth from highly dense nanoparticles on any substrate.

Fabrication and Electrical Characterization of Densified Carbon Nanotube Micropillars for IC Interconnection

Closely packed carbon nanotube (CNT) bundles are expected to have higher conductivity than copper and could potentially replace copper for electrical and thermal conductors in IC chips. However, it is extremely difficult, if not impossible, to controllably grow closely packed CNT bundles. We report on a novel postgrowth capillary densification method, which results in dramatic increase of CNT site density. Bare CNT pillars are densified approximately 15 times. High-density CNT micropillars and micrologs with round cross sections were fabricated from CNTs coated with parylene prior to densification. These CNT micropillars and micrologs could be used as basic building blocks for future IC interconnection. Electrical characterization results show that the densification process does not mitigate the electrical conducting performance of CNTs.

A novel spherical carbon

We developed a novel spherical carbon material. The spherical carbon is composed of a high density of carbon nanotubes or nanofilaments, and includes an oxidized diamond particle as a core. Syntheses of this carbon in high volume with high selectivity may be possible. It is expected that this carbon will be useful as a catalyst material for fuel cells, electric double-layer capacitors, etc.

Synthesis of high-purity carbon nanotubes over alumina and silica supported bimetallic catalysts

Carbon nanotubes (CNTs) were synthesized by a catalytic chemical vapor deposition method (CCVD) of ethylene over alumina and silica supported bimetallic catalysts based on Fe, Co and Ni. The catalysts were prepared by a precipitation method, calcined at 600 ?C and in situ reduced in hydrogen flow at 700?C. The CNTs growth was carried out by a flow the mixture of C2H4 and nitrogen over the catalyst powder in a horizontal oven. The structure and morphology of as-synthesized CNTs were characterized using SEM. The as-synthesized nanotubes were purified by acid and basic treatments in order to remove impurities such as amorphous carbon, graphite nanoparticles and metal catalysts. XRD and DTA/TG analyses showed that the amounts of by-products in the purified CNTs samples were reduced significantly. According to the observed results, ethylene is an active carbon source for growing high-density CNTs with high yield but more on alumina-supported catalysts than on their silica-supported counterparts. The last might be explained by SMSI formed in the case of alumina-supported catalysts, resulting in higher active phase dispersion.

All ink-jet-printed carbon nanotube thin-film transistor on a polyimide substrate with an ultrahigh operating frequency of over 5 GHz

We report a flexible carbon nanotube (CNT) thin-film transistor (TFT) fabricated solely by ink-jet printing technology. The TFT is top gate configured, consisting of source and drain electrodes, a carrier transport layer based on an ultrapure, high-density (>1000 CNTs/μm2) CNT thin film, an ion-gel gate dielectric layer, and a poly(3,4-ethylenedioxythiophene) top gate electrode. All the TFT elements are ink-jet printed at room temperature on a polyimide substrate without involving any photolithography patterning or surface pretreatment steps. This CNT-TFT exhibits a high operating frequency of over 5 GHz and an on-off ratio of over 100. Such an all-ink-jet-printed process eliminates the need for lithography, vacuum processing, and metallization procedures and thus provides a promising technology for low-cost, high-throughput fabrication of large-area high-speed flexible electronic circuits on virtually any desired flexible substrate.

High-yield synthesis of carbon nano-onions in counterflow diffusion flames

High-yield synthesis of carbon nanotubes (CNTs) and nano-onions (CNOs) on a catalytic nickel substrate using counterflow diffusion flames was investigated. With ethylene fixed at 5%, methane varied from 15% to 45% in the upper flow, and air supplied in the lower flow, only a moderate CH 4 concentration (25%) could yield high density CNTs. However, when oxygen was increased to 50% in the lower flow, only CNOs were synthesized. An increase in methane concentration from 15% to 45% led to a higher yield and a greater diameter (ranging from 5 to 60 nm) of CNOs. To examine the role of mixed fuel, it was observed that as ethylene was removed and only 45% methane and nitrogen were supplied in the upper flow, no CNOs could be generated. While, as methane was increased to 50% or 55%, high-yield CNOs were synthesized and the yield increased with methane concentration. Note that the key parameters influencing the formation and yield of CNOs are both the oxygen and fuel concentrations. There was a critical threshold value of oxygen concentration, 30%, for onset of CNOs synthesis. Also, the critical threshold value of methane concentration for onset of CNOs formation decreased with increasing oxygen concentration or ethylene concentration.