Growth of carbon nanotubes by open-air laser-induced chemical vapor deposition

Carbon nanotubes have remarkable mechanical, electronic and electrochemical properties, but the full potential for application will be realized only if the growth of high quantity and quality carbon nanotubes can be optimized and well controlled. In this study, carbon nanotubes have been successfully grown on fused quartz rods by a novel open-air laser-induced chemical vapor deposition (LCVD) technique with gold palladium nanoparticles as catalyst material. In this LCVD technique, a curtain of inert nitrogen gas was used to shield the deposition zone from the surrounding environment and allows the growth of the nanotubes to occur under open-air conditions. A 35-W continuous CO₂ laser was used as a heat source to induce a local temperature rise on the substrate surface covered with metal nanoparticles, subsequently resulting in deposition of multi-wall carbon nanotubes. The carbon nanotubes deposited in this study are derived from a precursor mixture that consists of propane and hydrogen, and are in tangled form with different diameters (10-250 nm) and structures. Raman spectroscopy, transmission and scanning electron microscopy are used to investigate the microstructure and composition of the carbon nanotubes.     

Growth kinetics and microstructure of carbon nanotubes using open air laser chemical vapor deposition

Open air laser-induced chemical vapor deposition (LCVD) was used to rapidly deposit a carbon nanotube forest on a moving substrate prepared with gold–palladium nanoparticles. Butane and propane were used as the carbon source, which was combined with hydrogen gas. Nanotube growth kinetics and alignment characteristics were strongly dependent on laser power density, substrate velocity, catalyst particle size, and the type of precursor mixture used. An increase in the hydrogen concentration improved the purity of the nanotubes by reducing the formation rate of amorphous carbon on the catalytic surface. A multi-stage growth model was proposed.     

Laser-induced carbon CVD on a moving fused quartz substrate: morphological and oscillatory deposition characteristics

CO₂ Laser-induced chemical vapor deposition (LCVD) is being investigated as a possible technique of depositing uniform carbon coatings on moving fused quartz substrates. A CO₂ laser is used to locally heat the substrate surface and create a hot spot where pyrolysis of hydrocarbon species occurs and subsequently deposits a layer of carbon film. The results indicate that uniform carbon film stripes can be deposited on moving fused quartz rods using pyrolytic LCVD only under certain deposition conditions, otherwise oscillation of the stripe width or substrate damage by laser radiation will occur. The transition region for oscillation, as well as the period of oscillation, is identified. Substrate damage can be induced by reducing the laser spot size, even when the substrate receives a lower irradiation intensity. Raman spectra of the carbon film stripes indicate that the film consists of disordered graphitic material. The ratio of the D-peak to the G-peak intensity found in the Raman spectrum is used to characterize the degree of disorder for the carbon film. The range of the Raman peak ratios is 0.795-0.935.     

Selective growth of diamond and carbon nanostructures by hot filament chemical vapor deposition

Diamond and carbon nanostructures have been synthesized selectively on differently pretreated silicon substrates by hot filament chemical vapor deposition in a CH₄/H₂ gas mixture. Under typical conditions for CVD diamond deposition, carbon nanotube and diamond films have been selectively grown on nickel coated and diamond powder scratched silicon surface, respectively. By initiating a DC glow discharge between the filament and the substrate holder (cathode), well aligned carbon nanotube and nanocone films have been selectively synthesized on nickel coated and uncoated silicon substrates, respectively. By patterning the nickel film on silicon substrate, pattern growth of diamond and nanotubes has been successfully achieved.     

Deposition rate and temperature of carbon films during laser-induced CVD on a moving substrate

The feasibility of using pyrolytic laser-induced chemical vapor deposition (LCVD) to deposit carbon coatings on moving fused quartz substrates is investigated. This LCVD system uses a CO₂ laser to locally heat a substrate in open air to create a hot spot. Pyrolysis of hydrocarbon species occurs and subsequently deposits a layer of carbon film onto the substrate surface. The results of this study indicate that growth kinetics and the geometry of uniform carbon stripes deposited by pyrolytic LCVD strongly related to the laser power, the traverse velocity of the substrate, the type of hydrocarbon species used in deposition, and the diameter of the substrate. The deposition rate of carbon film increases exponentially with the laser power, while an increase in traverse velocity of the substrate will also increase the deposition rate until a maximum deposition rate is reached; further increases in the traverse velocity will decrease the deposition rate. We suspect that this optimal deposition rate is caused by substrate motion, which affects the substrate surface temperature, and consequently the effective surface area available for film deposition. The substrate temperature is observed to behave linearly with the deposition parameters considered in this study.     

Nucleation of diamond over nanotube coated Si substrate using hot filament chemical vapor deposition (CVD) system

We studied the use of carbon nanotubes as a seeding layer for the nucleation of diamond on Si (100) substrate by using a hot filament chemical vapor deposition (HFCVD) system. Prior to deposition, substrates were seeded with multi-wall carbon nanotube (MWCNT) powder which was prepared separately. MWCNTs were used as nucleation precursors. The diamond grains grew essentially over the nanotubes with a higher growth density in comparison with the un-seeded substrates. The scanning electron microscopy (SEM) image of surface morphology shows crystallites of cauliflower shaped grains. The micro Raman spectroscopic results show a sharp peak at 1,332 cm^-1 corresponding to diamond phase. X-ray photoelectron spectroscopic study show the presence of carbon (C_1s) phase. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Field emission from patterned carbon nanotube emitters produced by microwave plasma chemical vapor deposition

Large area carbon nanotube patterns were fabricated by microwave plasma chemical vapor deposition. The carbon nanotubes were grown on pre-patterned catalyst films. Scanning electron microscopy and Raman spectroscopy were used to characterize the structure of the carbon nanotubes. The carbon nanotubes were very uniform and approximately 100 nm in diameter. The Raman spectrum shows a good graphitization for the carbon nanotubes. Aligned growth was found on the pattern line area. Field emission characteristics of the patterns were characterized. A threshold field of 2.0 V/μm and emission current density of 1.1 mA/cm² at 3.6 V/μm were achieved. A clear and stable image showing the patterns were obtained.     

Optimizing substrate surface and catalyst conditions for high yield chemical vapor deposition grown epitaxially aligned single-walled carbon nanotubes

Single-crystal stable-temperature (ST)-cut quartz substrates, which have a (0 1 1 1) crystallographic plane with their surface normal lying close to 38° from the y axis ([0 1 0]), were annealed in air prior to use as a support for aligned carbon nanotube growth by chemical vapor deposition. Very smooth substrate surfaces were obtained with annealing times in the vicinity of 15 h at a temperature of 750 °C. These smooth surfaces are ideal for the growth of horizontally aligned SWCNTs with high spatial density, while less dense SWCNTs were obtained with less smooth surfaces. Under optimized growth conditions, only SWCNT are observed and they can grow to lengths in excess of 100 μm. Our findings suggest structural defects interfere with the growth process. A binary Fe/Co catalyst was employed to grow the nanotubes. No obvious dependence on the Fe:Co ratio is observed.     

Synthesis of carbon nanotubes grown by hot filament plasma-enhanced chemical vapor deposition method

We prepared two kinds of catalytic layers onto n-typed silicon substrate—nickel by r.f.-magnetron sputtering and iron (III) nitrate metal oxide by spin coating. For iron (III) nitrate metal oxide 0.5 mol of ferric nitrate nonahydrate [Fe₂(NO₃)₃·9H₂O] ethanol solution was coated onto silicon by spin coater at different rotation speeds (rev./min). Carbon nanotubes were synthesized on both Ni and iron (III) nitrate metal oxide layers by the HFPECVD (hot filament plasma-enhanced chemical vapor deposition) method. We used ammonia (NH₃) and acetylene (C₂H₂) for the dilution gas and a carbon precursor for the growth of the carbon nanotubes, respectively. We could observe the relationship between the catalytic cluster density and the nanotube density with scanning electron microscopy (SEM) images. The density of carbon nanotubes on iron (III) nitrate metal oxide was controlled by the rev./min of the spin coater. Transmission electron microscopy (TEM) image shows multi-walled carbon nanotube where the catalyst was found in the tip of the carbon nanotube. Electron dispersive X-ray spectrometry (EDS) peaks for CNT's tip show that it was constituted with nickel and iron, respectively. Raman spectroscopy of nanotubes shows D-band and G-band peaks approximately 1370 and 1590 cm⁻¹.     

Carbon nanotube junctions obtained by pulsed liquid injection chemical vapour deposition

The effect of substrate surface roughness on the synthesis of carbon nanotube (CNT) junctions is studied. CNTs were obtained by a pulsed liquid injection chemical vapour deposition system (PLICVD) and grown on quartz substrates with different roughnesses. Nickel particles were used as catalyst and acetone as the carbon precursor. Results shown that CNTs growth depend strongly on the substrate irregularity. When roughness is present, the presence of CNT junctions are increased. On the quartz surface, without any modification of roughness, CNTs are not obtained. Thus, a growth mechanism for CNT junctions, based on the substrate roughness is suggested. This method represents an important alternative to produce CNTs for applying them in nanoelectronic devices.     

Comparison of carbon nanotube modified electrodes for photovoltaic devices

Substrates with four different nanotube modifications have been prepared and their electron transport properties measured. Two modification techniques were compared; covalent chemical attachment of both single and multi-walled carbon nanotubes to transparent conductive (fluorine doped tin oxide) glass surfaces and chemical vapour deposition (CVD) growth of both single and multi-walled carbon nanotubes on highly doped conductive silicon wafers. These carbon nanotube modified substrates were investigated using scanning electron microscopy and substrates with nanotubes grown via CVD have a much higher density of nanotubes than substrates prepared using chemical attachment. Raman spectroscopy was used to verify that nanotube growth or attachment was successful. The covalent chemical attachment of nanotubes was found to increase substrate electron transfer substantially compared to that observed for the bare substrate. Nanotube growth also enhanced substrate conductivity but the effect is smaller than that observed for covalent attachment, despite a lower nanotube density in the attachment case. In both modification techniques, attachment and growth, single-walled carbon nanotubes were found to have superior electron transfer properties. Finally, solar cells were constructed from the nanotube modified substrates and the photoresponse from the different substrates was compared showing that chemically attached single-walled nanotubes led to the highest power generation.     

Synthesis of multi-walled carbon nanotubes by catalytic chemical vapor deposition using Cr2 − xFexO3 as catalyst

Chromium oxide and iron oxide solid solution was used as a catalyst for multi-walled carbon nanotubes synthesis by the catalytic chemical vapor deposition technique. The catalyst was prepared by the solution combustion synthesis method. Natural gas (NG) was employed as a carbon source for the carbon nanotube growth instead of methane, which is typically used. The carbon nanotube synthesis was carried out under H₂/NG and Ar/NG atmospheres at 950 °C. The Cr_2 − xFe_xO₃ catalyst was capable to produce carbon nanotubes only in H₂/NG atmospheres. Partial elimination of the catalyst after the synthesis was possible using a concentrated solution of HNO₃.     

Re-growth of single-walled carbon nanotube by hot-wall and cold-wall chemical vapor deposition

Single-walled carbon nanotube (SWCNT) was synthesized from short nanotubes using chemical vapor deposition (CVD) and the associated factors affecting the re-growth of the SWCNT were both investigated and optimized. Long, dense nanotubes were prepared from a mixture of acetylene and ethanol on air-annealed ST-cut quartz substrates by hot-wall CVD. Raman and photoluminescence analyses of the resulting material demonstrated that SWCNT was generated from the initial seeds since the chiralities of the seeds were maintained in the re-grown SWCNT. The re-growth of SWCNT was also achieved by cold-wall CVD. In both CVD systems, the efficiency of SWCNT re-growth was largely determined by the pretreatment conditions and growth parameters. By varying these factors, the growth of SWCNT from seeds was controlled. The re-growth mechanism is discussed based on experimental observations.     

Optimization of methane cold wall chemical vapor deposition for the production of single walled carbon nanotubes and devices

Carbon nanotubes are synthesized by cold wall chemical vapor deposition (CVD) using methane as the carbon source and iron thin film catalyst. The yield of thin nanotubes as determined by scanning electron microscopy (SEM) is strongly dependent on the precise CVD process and the preparation of the substrate. The effects of pressure (5–80 kPa), temperature (700–950 °C), substrate conditioning (air preheat) and metallization (Fe, Al, Mo) on thin nanotube yield are reported. High yields of thin nanotubes are obtained under optimum conditions. These thin nanotubes are candidates to be single walled carbon nanotubes (SWNTs) and Raman spectroscopy, photoluminescence spectroscopy and electrical transport provide evidence that, at least at optimum conditions, many, and perhaps all of the thin nanotubes are single walled. Single nanotube field effect transistors are fabricated and factors affecting device yield are reported. Optimum single nanotube device yield does not necessarily coincide with the optimum nanotube yield.     

Selective growth of CNT by using triode-type radio frequency plasma chemical vapor deposition method

Triode-type radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) equipment has been used in order to grow well-aligned carbon nanotubes on Si and glass substrates below 600 °C. This CVD equipment allows the growth of a well-aligned carbon nanotube with an inside and an outside diameter of 7 and 17 nm, respectively. The selective growth of the CNT was demonstrated.     

Sol–gel preparation of catalyst particles on substrates for hot-filament CVD nanotube deposition

Carbon nanotubes (CNTs) were grown by hot-filament chemical vapour deposition and iron was the catalyst. The aim of this research was to produce aligned CNTs attached to a substrate.For a homogeneous distribution of the catalyst particles on substrate plates various sol–gel mixtures were used containing iron nitrate and tetrabutyltitanate forming an inorganic network. The produced gels were spin-coated on the substrates and after drying a temperature treatment between 650 and 800 °C was applied. For comparison iron nitrate was dissolved in ethanol and spin-casted on the substrate.Parameter variation during hot-filament CVD growth resulted in the formation of different types of nanotubes and also sectors of aligned CNTs were observed. The nanotube types were: double-wall CNTs, multiwall CNTs, bamboo-type CNTs and fishbone carbon nanofibers.     

Synthesis of oriented nanotube films by chemical vapor deposition

Oriented nanotube films (20-35 μm thick) were synthesised on flat silicon substrates by chemical vapor deposition (CVD) of a gas mixture of acetylene and nitrogen. For the CVD we used metal oxide clusters formed by spin coating an iron(III) nitrate ethanol solution onto a silicon substrate and subsequent heating. The cluster density and its effects on the nanotube density were investigated as a function of the iron(III) nitrate concentration and the synthesis temperature. A high nanotube density was achieved with a high density of iron oxide clusters as nucleation centres for the growth of nanotubes. The cluster density was controlled by the iron(III) concentration of the ethanolic coating solution and by the synthesis temperature. The perpendicular orientation of the nanotubes with respect to the substrate surface is attributed to a high density of nanotubes.     

Carbon nanoflake growth from carbon nanotubes by hot filament chemical vapor deposition

We report the growth of carbon nanoflakes (CNFs) on Si substrate by the hot filament chemical vapor deposition without the substrate bias or the catalyst. CNFs were grown using the single wall carbon nanotubes and the multiwall carbon nanotubes as the nucleation center, in the Ar-rich CH₄–H₂–Ar precursor gas mixture with 1% CH₄, at the chamber pressure and the substrate temperature of 7.5 Torr and 840 °C, respectively. In the H₂-rich condition, CNF synthesis failed due to severe etch-removal of carbon nanotubes (CNTs) while it was successful at the optimized Ar-rich condition. Other forms of carbon such as nano-diamond or mesoporous carbon failed to serve as the nucleation centers for the CNF growth. We proposed a mechanism of the CNF synthesis from the CNTs, which involved the initial unzipping of CNTs by atomic hydrogen and subsequent nucleation and growth of CNFs from the unzipped portion of the graphene layers.     

Self-assembling of multi-walled carbon nanotubes on a porous carbon surface by catalyst-free chemical vapor deposition

Synthesis of carbon nanotubes (CNTs) by catalyst-free chemical vapor deposition (CFCVD) is one of the most important challenges in nanotube science. Self-assembling multi-walled CNTs (MWCNTs) were produced on a porous carbon surface using carbon black (CB) as a substrate, at 800 °C by the decomposition of diluted ethylene. MWCNTs with an outer-diameter distribution of 20–80 nm, examined by scanning and transmission electron microscopy, could be self-assembled on pore structures of CB surface by CFCVD.     

Prospective growth region for chemical vapor deposition synthesis of carbon nanotube on C–H–O ternary diagram

Chemical vapor deposition has become a standard process for synthesizing carbon nanotubes. Since the successful use of chemical vapor deposition for the first time, much effort has been expended into exploring various carbon sources that can be used to synthesize carbon nanotubes, such as methane, ethane, and ethanol. However, whole perspectives for suitable carbon sources have not been clear. In this study, we performed experiments in order to determine that the appropriate C–H–O components ratio in raw materials can be used to synthesize carbon nanotubes. We also examined a variety of raw materials in our newly developed round-trip-type vacuum furnace in order to determine whether they could be used to synthesize a carbon nanotube. We used Raman spectroscopy to identify the developed carbon nanotube, and we plotted the component ratios of effective and ineffective materials on a C–H–O ternary diagram; in this diagram, the growth region became highly apparent. It should be noted that for the growth of the carbon nanotube, this region should satisfy the equation O < C < (H + O) in molar ratio. Furthermore, it was observed that adjusting the component ratios by mixing raw materials did not cause an inconsistency in the growth region.