Prevention of Si-contaminated nanocone formation during plasma enhanced CVD growth of carbon nanotubes

We investigated the growth behavior and morphology of vertically aligned carbon nanotubes (CNTs) on silicon (Si) substrates by direct current (DC) plasma enhanced chemical vapor deposition (PECVD). We found that plasma etching and precipitation of the Si substrate material significantly modified the morphology and chemistry of the synthesized CNTs, often resulting in the formation of tapered-diameter nanocones containing Si. Either low bias voltage (∼500 V) or deposition of a protective layer (tungsten or titanium film with 10-200 nm thickness) on the Si surface suppressed the unwanted Si etching during growth and enabled us to obtain cylindrical CNTs with minimal Si-related defects. We also demonstrated that a gate electrode, surrounding a CNT in a traditional field emitter structure, could be utilized as a protection layer to allow growth of a CNT with desirable high aspect ratio by preventing the nanocone formation.     

Effect of tantalum nitride supporting layer on growth and morphology of carbon nanotubes by thermal chemical vapor deposition

The role of tantalum nitride (TaNx) thin films as buffer layers on the control of nucleation and growth of aligned carpet-like carbon nanotubes (CNTs) has been proved. TaNx thin films have been deposited on Si by controlled magnetron sputtering process. Multiwall CNTs have been synthesized at 850 °C using an aerosol of ferrocene diluted in toluene. Electron microscopy images show a strong correlation between the growth rate and morphology of the CNTs and the initial composition of the TaNx thin films. Multi-scale investigations reveal that both morphology and structure of the CNTs are determined by the properties of the TaNx films. Raman and X-ray photoelectron spectroscopy, high resolution TEM imaging at the submicrometric and atomic scales have been used to confirm these hypotheses.     

Synthesis of carbon nanocapsules and carbon nanotubes by an acetylene flame method

The fabrication of carbon nanocapsules and carbon nanotubes (CNTs) using an acetylene flame method was investigated. Carbon nanocapsules, a graphitic structure of nanoparticles with a hollow core, were synthesized using catalyst-free acetylene flames while CNTs were formed with the presence of cobalt-based catalysts in addition to acetylene flames. When the synthesis of these materials was carried out, the results showed that a massive amount of high-purity carbon nanocapsules with a particle size in the range of 15-30 nm can be produced with the acetylene flame method. The CNTs produced were multi-walled carbon nanotubes measuring a few micrometers in length and 20-30 nm in diameter. The acetylene flame method holds great potential for the cost-effective production of CNTs as well as carbon nanocapsules.     

Diameter control of carbon nanotubes by changing the concentration of catalytic metal ion solutions

We have developed a simple new method to control the diameter of carbon nanotubes (CNTs) using catalytic nanoparticle arrays fabricated by filling the pores of well-ordered porous anodic aluminum oxide (AAO) templates with a metal ion solution. Fe ion solution was used to fill the pores in which Co had been deposited electrochemically, and then the template was dried naturally on a magnet. After this process, the pores were widened in NaOH solution. Well-graphitized multi-walled CNTs were grown from almost all the pores and were very long in length and homogeneous in diameter. We were able to control the diameter of CNTs, simply, by changing the concentration of iron ion solution. For example, the average outer diameters of the CNTs are 7 ± 1.5, 13 ± 1, and 17 ± 1 nm when the concentrations of Fe ion in their mother solutions were 1.0 × 10⁻³, 3.0 × 10⁻³, and 6.0 × 10⁻³ M, respectively. The inner diameters of these CNTs corresponded to the calculated diameters of Fe nanoparticles by assuming that all Fe ions contained in each pore are reduced to a single nanoparticle. This means that homogeneous nanoparticles are made in each pore. Our new method could be used to fabricate homogeneous nanoparticles from most metal ion solutions.     

Synthesis of vertically aligned, double-walled carbon nanotubes from highly active Fe-V-O nanoparticles

Monodispersed Fe-V-O nanoparticles were prepared by a liquid-phase synthesis to be used as catalysts for carbon nanotube (CNT) growth. Vertically aligned, dense CNTs have been grown from the highly active Fe-V-O nanoparticles by chemical vapor deposition. Diameter distribution of CNTs (3.7 ± 0.6 nm) was consistent with that of the original nanoparticles (3.1 ± 0.5 nm), and the value was smaller than those of other reported vertically aligned CNTs from as-prepared nanoparticles. TEM study showed that the CNTs consisted mainly of double-walled CNTs (single: 14%, double: 74%, and triple: 12%). The CNT diameter increased to 4.4 ± 0.8 nm as the growth temperature was increased from 810 to 870 °C. Energy dispersive X-ray spectroscopy of nanoparticles before and after the CNT growth revealed that the V content decreased from 7.2 to 2.7 at.%, suggesting that the segregation of Fe and V played an important role for the high activity of the Fe-V-O nanoparticles.     

Synthesis of vertically aligned carbon nanotubes on metal deposited quartz plates

Without plasma aid, we have successfully synthesized vertically aligned carbon nanotubes (CNTs) on iron-, cobalt- or nickel-deposited quartz plates by chemical vapor deposition with ethylenediamine as a precursor. The amine serves as both etching reagent for the formation of metal nanoparticles and carbon source for the growth of aligned carbon nanotubes. The carbon nanotubes were vertically aligned in high density on a large area of the plain silica substrates. The density and diameter of CNTs is determined by the thickness of the deposited metal film and the length of the tubes can be controlled by varying the reaction time. High-resolution transmission electron microscopy analysis reveals that the synthesized CNTs are multiwalled with a bamboo-like structure. Energy dispersive X-ray spectra demonstrate that the CNTs are formed as tip growths. Raman spectrum provides definite evidence that the prepared CNTs are multiwalled graphitic structure.     

Pulsed laser deposition-assisted patterning of aligned carbon nanotubes modified by focused laser beam for efficient field emission

Patterned carbon nanotube (CNT) arrays have been synthesized on patterned substrates created via pulsed laser deposition (PLD) of the precursor catalyst films with a mask. Arrays of CNTs in square and hexagonal patterns with tube lengths of 8 μm and 16 μm were created on silicon or quartz substrates, respectively. Using the method of laser cutting, as-grown CNT patterns were pruned by focused He-Ne laser beam. It is found that after pruning, CNTs tend to cluster together and form welded junctions. The comparison of field emission properties of CNTs before and after pruning shows that laser modification of CNT morphologies effectively enhanced the emission currents.     

Synthesis of carbon nanotubes over gold nanoparticle supported catalysts

The synthesis of carbon nanotubes (CNTs) through the catalytic decomposition of acetylene was carried out over gold nanoparticles supported on SiO₂-Al₂O₃. Monodispersed gold nanoparticles with 1.3-1.8 nm in diameter were prepared by the liquid-phase reduction method with dodecanethiol as protective agent. The carbon products formed after acetylene decomposition consist of multi-walled carbon nanotubes with layered graphene sheets, carbon nanofilaments (CNFs), and carbon nanoparticles encapsulating gold particles. The observed CNTs have outer diameters of 13-25 nm under 850 °C. The influence of several reaction parameters, such as kind of carriers, reaction temperature, gas flow rate, was investigated to search for optimum reaction conditions. The CNTs were observed at a relatively low temperature (550 °C). The silica-alumina carrier showed higher activity for the formation of CNTs than others used in the screening test. With increasing temperature, the CNTs showed cured structures having thick diameters and inside compartments. When Au content on the support was over 5 wt.%, the gold nanoparticles coagulated to form large ones >20 nm in diameter and became encapsulated with graphene layers after decomposition of acetylene.     

Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature

Carbon nanotubes (CNTs) were grown using a modified atmospheric pressure plasma with NH₃(210 sccm)/N₂(100 sccm)/C₂H₂(150 sccm)/He(8 slm) at low substrate temperatures (?500 °C) and their physical and electrical characteristics were investigated as the application to field emission devices. The grown CNTs were multi-wall CNTs (at 450 °C, 15-25 layers of carbon sheets, inner diameter: 10-15 nm, outer diameter: 30-50 nm) and the increase of substrate temperature increased the CNT length and decreased the CNT diameter. The length and diameter of the CNTs grown for 8 min at 500 °C were 8 μm and 40 ± 5 nm, respectively. Also, the defects in the grown CNTs were also decreased with increasing the substrate temperature (The ratio of defect to graphite (I_D/I_G) measured by FT-Raman at 500 °C was 0.882). The turn-on electric field of the CNTs grown at 450 °C was 2.6 V/μm and the electric field at 1 mA/cm² was 3.5 V/μm.     

Growth and electrical characterization of high-aspect-ratio carbon nanotube arrays

The remarkable properties of carbon nanotubes (CNTs) make them attractive for microelectronic applications, especially for interconnects and nanoscale devices. In this paper, we describe a microelectronics compatible process for growing high-aspect-ratio CNT arrays with application to vertical electrical interconnects. A lift-off process was used to pattern catalyst (Al₂O₃/Fe) islands to diameters of 13 or 20 μm. After patterning, chemical vapor deposition (CVD) was involved to deposit highly aligned CNT arrays using ethylene as the carbon source, and argon and hydrogen as carrier gases. The as-grow CNTs were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results demonstrated that the CNTs have high purity, and form densely-aligned arrays with controllable array size and height. Two-probe electrical measurements of the CNT arrays indicate a resistivity of ∼0.01 Ω cm, suggesting possible use of these CNTs as interconnect materials.     

Ionic peapods from carbon nanotubes and phosphotungstic acid

Using a mild wet-chemical route, cage-like phosphotungstic acid (HPW) molecules of 1.2 nm diameter were successfully filled into opened carbon nanotubes (CNTs) with cavity diameter of 2 nm. High resolution transmission electron microscope revealed that HPW molecules arrayed in a chain in the tube cavity, forming a new kind of peapod structure. In aqueous solution these unusual peapods showed higher ionic property than the opened nanotubes, with a ζ-potential of −47.5 ± 1.3 mV, and dissolved finely as a stable solution. Raman spectra confirmed the strong interaction between HPW molecules and CNT graphite wall. It could substantially tune the electronic properties of CNTs and thus promote their applications in novel electronic devices.     

The use of camphor-grown carbon nanotube array as an efficient field emitter

Three-dimensional growth of well-aligned high-purity multiwall carbon nanotubes (CNTs) is achieved on silicon, nickel-coated silicon and cobalt-coated silicon substrates by thermal decomposition of a botanical carbon source, camphor, with different catalyst concentrations. Field emission study of as-grown nanotubes in a parallel-plate diode configuration suggests them to be an efficient emitter with a turn-on field of ∼1 V/μm (for 10 μA/cm²) and a threshold field of ∼4 V/μm (for 10 mA/cm²). Maximum current density lies in a range of 20-30 mA/cm² at 5.6 V/μm with significant reversibility. Prolonged stability test of camphor-grown CNT emitters suggests a life time of ∼5 months under continuous operation. A new feature, metal-assisted electron emission from CNTs, has been addressed. Isolated nanotubes used as a cold cathode in a field emission microscope reveal the pentagonal emission sites and hence the atomic structure of the nanotube tips.     

Carbon nanotubes prepared from three-layered copolymer microspheres of acrylonitrile and methylmethacrylate

Carbon nanotubes (CNTs) were synthesized from fine three-layered copolymer microspheres using the polymer blend technique. Diameter of PMMA core/Poly(AN-co-MMA) shell-1/PMMA shell-2 microspheres, prepared by a radical soap-free emulsion polymerization of methylmethacrylate (MMA) and acrylonitrile (AN), was between 400 nm and 500 nm. Microspheres were subjected to melt-spinning at 305 °C, stabilizing in oxygen at 220 °C for 4 h, and finally carbonizing at 1000 °C for 30 min. FE-SEM study of carbonized sample revealed the presence of CNTs arrays on carbon blocks. Similar arrays were observed in a comparative CNTs sample prepared from three-layered microspheres with the pure PAN shells-1 layers. HRTEM showed that the CNTs derived from copolymer microspheres had different structure when compared to the control sample, i.e. CNTs often adhered to each other and contained the internal compartments. The insufficient PMMA shell-2 coating of copolymer microspheres is believed to be a reason for CNTs adhesion. The possible mechanisms of the carbon block formation and the adhesion of CNTs are introduced.     

The cutting of multi-walled carbon nanotubes and their strong interfacial interaction with polyamide 6 in the solid state

The cutting of multi-walled carbon nanotubes (MWCNTs) using solid state shear milling (S³M) method and their strong interfacial interaction with polyamide 6 (PA6) in the solid state were studied. Transmission electron microscopy showed that after milling, the CNTs were greatly reduced in length, and disentangled, being straighter with open ends. Fourier transform infrared spectra and differential scanning calorimeter analysis indicated the existence of strong interfacial interactions between MWCNTs and PA6 of the pan-milled PA6/CNTs powder. It was further quantified by thermogravimetric analysis that about 30 wt.% of PA6 formed a strong combining force with CNTs after pan-milling. The mechanism of cutting CNTs and the reason for their strong interfacial interactions with PA6 in the solid state were discussed. A fine and homogeneous dispersion of CNTs throughout PA6 matrix was observed by scanning electron microscopy. The tensile properties of the composites prepared by the S³M method were significantly improved compared to those of pure PA6 and composites prepared by conventional melt mixing. Upon incorporation of only 1.5 wt.% MWCNTs, the tensile modulus of PA6 was enhanced from 2448 MPa to 4439 MPa, by about 80%, and the tensile strength was increased by about 23%.     

Optimization of water assisted chemical vapor deposition parameters for super growth of carbon nanotubes

The optimization of water assisted chemical vapor deposition (WA-CVD) was carried out to synthesize ultra long, vertically aligned, densely packed carbon nanotube (CNT) forests. The effect of various WA-CVD parameters (viz. the flow rate of the reactant gas mixture and its injection temperature, growth kinetics, ramp rate and growth temperature) on the height of the CNTs was studied. A hypothesis for catalytic activity is proposed on the basis of the X-ray photoelectron spectroscopic analysis of the CNT grown substrates and further verified at the optimum condition. The effect of temperature on the growth of the CNTs is studied. The gas flow rate and injection temperature influence the onset of oxidation of the substrates, which in turn affects the CNT growth rate. A growth kinetics study is performed in order to monitor the growth temperature. The role of the onset of oxidation of the iron catalyst in the growth of the CNTs is studied by varying the ramp rate. The precise CNT growth temperature for WA-CVD is determined by growth temperature studies. The optimum condition allows ∼2.2 ± 0.002 mm long CNTs to be obtained.     

Synthesis of diamond films and nanotips through graphite etching

A hot filament CVD process based on hydrogen etching of graphite has been developed to synthesize diamond films and nanotips. The graphite sheet was placed close to the substrate and only hydrogen was supplied during deposition. No hydrocarbon feed gases are required for this process. High quality diamond films were synthesized with high growth rate on P-type (1 0 0)-oriented silicon wafers without discharge or bias. The diamond growth rate is approximately five times higher than that through conventional hot filament chemical vapor deposition using a gas mixture of methane and hydrogen (1 vol.% methane) under similar deposition conditions. The diamond films synthesized in this process exhibit smaller crystallites and contain smaller amount of non-diamond carbon phases. Synthesis of well-aligned diamond nanotips with various orientation angles was achieved on the CVD diamond-coated Si substrate when the substrate holder was negatively biased in a DC glow discharge. The nanotips grown at locations far enough from the sample edges are aligned vertically, while those around the sample edges are tilted and point away from the sample center. The alignment orientation of the nanotips appears to be determined by the direction of the local electric field lines on the sample surfaces.     

Mass production of aligned carbon nanotube arrays by fluidized bed catalytic chemical vapor deposition

A parametric study investigating the impacts of loading amount of active phase, growth temperature, H₂ reduction, space velocity, and apparent gas velocity on the intercalated growth of vertically aligned carbon nanotube (CNT) arrays among lamellar catalyst was performed. A series of Fe/Mo/vermiculite catalysts with Fe/vermiculite ratio of 0.0075-0.300 were tested. Metal particles were dispersed among the layers of vermiculite after H₂ reduction. Uniform catalyst particles, with a size of 10-20 nm and a density of 8.5 × 10¹⁴ m⁻², were formed among the vermiculite layers at 650 °C. CNTs with high density synchronously grew into arrays among the vermiculites. With the increasing growth temperature, the alignment of CNTs intercalated among vermiculites became worse. Moreover, intercalated CNTs were synthesized among vermiculite layers in various flow regimes. The as-grown particles were with a size of 1-2 mm when the fluidized bed reactor was operated in particulate fluidization and bubbling fluidization, while the size of the as-grown products decreased obviously when they grown in the turbulent fluidized bed. Based on the understanding of the various parameters investigated, 3.0 kg/h of CNT arrays were mass produced in a pilot plant fluidized bed reactor.     

Co-Mo catalyzed growth of multi-wall carbon nanotubes from CO decomposition

We investigated the growth of multi-wall carbon nanotubes (CNTs) catalyzed by SiO₂-supported Co-Mo bi-metallic catalyst in flowing CO at 700 °C. We found that both Co and Mo are present in catalytic particles at the tips of CNTs, but their compositions vary from one catalytic particle to another and significantly deviate from the initial mixing composition. The Co concentration and distribution in the catalytic particle of a CNT largely determines the length of the CNT. The CNT growth process is carbon adsorption on exposed area of a catalytic particle and subsequent precipitation at the CNT-catalyst interface or open CNT wall edges. The encapsulation of a catalytic particle was found to occur by the growth of the open-edged graphene walls around the particle. Two types of long CNTs were observed: one with their CNT walls ended at the CNT-particle interface, and the other with their CNT walls open to the environment. The former have diameters similar to their catalytic particle size while the latter have larger diameters.     

Effects of hydrogen on carbon nanotube formation in CH4/H2 plasmas

Carbon nanotubes (CNTs) were synthesized using CH₄/H₂ plasmas and plasmas simulated using a one-dimensional fluid model. The thinnest and longest CNTs with the highest number density were obtained using CH₄/H₂ = 27/3 sccm at 10 Torr. These conditions allowed CNTs to grow for 90 min without any meaningful loss of catalyst activity. However, an excess H₂ supply to the CH₄/H₂ mixture plasma made the diameter distribution of the CNTs wider and the yield lower. Hydrogen concentration is considered to affect catalyst particle size and activity during the time interval before starting CNT growth (=incubation period). With CH₄/H₂ = 27/3 sccm for a growth time of 10 min efficient CNT growth was achieved because the amount of carbon atoms in the CNTs and that calculated from simulation showed good agreement. The effect of hydrogen etching on CNTs was analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy by observing CNTs treated by H₂ plasma after CNT growth. It was confirmed that (a) multi-walled CNTs were not etched by the H₂ plasma, (b) the C 1s XPS spectra of the CNTs showed no chemical shift after the treatment, and (c) C-H bonds were produced in CNTs during their growth.     

The effect of feedstock and process conditions on the synthesis of high purity CNTs from aromatic hydrocarbons

High purity multi-walled carbon nanotubes were synthesized from aromatic hydrocarbons (benzene, toluene, xylene and trimethyl benzene) using ferrocene as the source of Fe catalyst. Screening studies of aromatic feeds at 675 °C, residence time of 14 s and Fe/C atom ratio of 1.07%, resulted in feedstock carbon conversion of 20-31%, CNT yield of 19.8-30.5%, and catalyst yield of 5.3-8.3 (g CNT/g catalyst). While the quality of the CNTs as determined by TGA, SEM, TEM and Raman spectroscopy, were high and comparable for different feedstocks; their carbon conversion, CNT yield and catalyst yield differed noticeably. A process optimization study for toluene feed showed that carbon conversion of more than 39%, CNT yield of 38.7% and catalyst yield of 18.3 can be achieved at temperature of 800 °C, Fe/C atom ratio of 0.47%, and residence time of 10-20 s.