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.     

Preparation of diamond like carbon thin films above room temperature and their properties

Diamond like carbon (DLC) thin films were deposited on p-type silicon (p-Si), quartz and ITO substrates by microwave (MW) surface-wave plasma (SWP) chemical vapor deposition (CVD) at different substrate temperatures (RT ∼ 300 °C). Argon (Ar: 200 sccm) was used as carrier gas while acetylene (C₂H₂: 20 sccm) and nitrogen (N: 5 sccm) were used as plasma source. Analytical methods such as X-ray photoelectron spectroscopy (XPS), FT-IR and UV–visible spectroscopy were employed to investigate the structural and optical properties of the DLC thin films respectively. FT-IR spectra show the structural modification of the DLC thin films with substrate temperatures showing the distinct peak around 3350 cm^− 1 wave number; which may corresponds to the sp² C–H bond. Tauc optical gap and film thickness both decreased with increasing substrate temperature. The peaks of XPS core level C 1 s spectra of the DLC thin films shifted towards lower binding energy with substrate temperature. We also got the small photoconductivity action of the film deposited at 300 °C on ITO substrate.     

Characterisation of the secondary glow region of a biased microwave plasma by optical emission spectroscopy

Despite considerable study, the mechanisms underlying the process of bias enhanced nucleation (BEN) as applied to CVD diamond remain unclear. However, early in the bias process a bright disc-like secondary plasma appears just above the substrate surface and recent reports implicate this region in the generation of oriented nuclei.Spatially resolved optical emission spectroscopy (OES), photography and electrical measurements have been used to study this region in detail for H₂/CH₄/Ar, H₂/Ar and pure Ar microwave plasmas. For a clean tungsten substrate the secondary plasma appears once the bias exceeds a value of ∼100 V; interestingly this coincides with threshold bias for significant enhanced nucleation. OES actinometry shows that axial profiles of relative atomic hydrogen concentration and the electron temperature are peaked 1.8 mm above the substrate. The emission intensity profiles show strong similarities with the conditions existing close to the cathode of a DC glow plasma; electric field strengths are of similar magnitude. It is suggested that the secondary glow seen in biased microwave plasmas can be understood as a DC glow plasma which is generated by secondary electrons arising from ion bombardment of the substrate. These are then strongly accelerated in the sheath fields, leading to high inelastic process rates directly above the growth substrate.     

Plasma enhanced growth of single walled carbon nanotubes at low temperature: A reactive molecular dynamics simulation

Low-temperature growth of carbon nanotubes (CNTs) has been claimed to provide a route towards chiral-selective growth, enabling a host of applications. In this contribution, we employ reactive molecular dynamics simulations to demonstrate how plasma-based deposition allows such low-temperature growth. We first show how ion bombardment during the growth affects the carbon dissolution and precipitation process. We then continue to demonstrate how a narrow ion energy window allows CNT growth at 500 K. Finally, we also show how CNTs in contrast cannot be grown in thermal CVD at this low temperature, but only at high temperature, in agreement with experimental data.     

Effect of Fe catalyst thickness and C2H2/H2 flow rate ratio on the vertical alignment of carbon nanotubes grown by chemical vapour deposition

Carbon nanotubes (CNTs) were fabricated by Chemical Vapour Depositon using a C₂H₂/H₂ mixture. They were grown on Si/SiO₂ substrate with Fe film as catalyst, deposited using thermal evaporation technique. The aim of this work is to emphasize the role of the Fe catalyst thickness and the C₂H₂/H₂ flow rate ratio to grow vertically aligned CNTs by thermal CVD. In order to investigate these aspects, four Fe metal films with thickness of 2.5, 3.5, 7.5 and 16 nm were deposited on Si/SiO₂ substrate and CNTs were grown with different C₂H₂/H₂ flow rate ratios, from 5/95 to 30/70 by thermal CVD at 750 °C. Results showed that CNTs were not vertically aligned with 16 nm catalyst thickness for all flow rate ratios, while CNTs were always vertically aligned for iron thickness less than 3.5 nm and vertically aligned only for a C₂H₂/H₂ flow rate ratio greater than 20/70 for the 7.5 nm catalyst thickness. Morphology and structural information about CNTs and Fe metal clusters were provided by field emission gun-scanning electron microscopy (FEG-SEM), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). Our results also indicate that for each flow rate ratio exists a critical thickness of iron catalyst under which vertically aligned CNTs are obtained.     

Experiments on C nanotubes synthesis by Fe-assisted ethane decomposition

We performed experiments on the synthesis of carbon nanotubes (CNTs) by iron-catalyzed chemical vapor deposition (CVD) in C₂H₆ + H₂ atmosphere. We varied flow-rates of reactant gases (ethane: 30–120 sccm, hydrogen: 0–120 sccm), as well as their ratio, in order to study the evolution of the growth kinetics. We used scanning electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy to investigate the morphologies, dimensions and crystalline structure of the samples. Our results demonstrate the crucial role played by H₂ in the enhancement of C diffusion-rate and in the consequent development of ordered and smooth graphene layers. A faster growth-rate is achieved by the increase of C₂H₆ flow-rate. However, if H₂ flow-rate is not adequately enhanced, the improvement is only apparent. The excess of C supplied gives rise to deposition of amorphous carbon onto the CNT walls, and to the co-production of different nanostructures. A substantial agreement is found with results reported for CVD growth of CNTs by the use of different catalysts, reactants and gas-flowing setups.     

Hot filament enhanced CVD synthesis of carbon nanotubes by using a carbon filament

The low temperature synthesis of single-walled carbon nanotubes (CNTs) on a Si substrate has been reported. Single-walled CNTs were grown from a C₂H₂ and H₂ mixture by a hot-filament enhanced CVD method using a carbon filament. The catalyst was silica-supported iron–cobalt prepared by sol–gel method. We observed the influence of the catalyst material, C₂H₂ concentration, growth pressure and substrate temperature on the formation of the CNTs. By optimizing the catalyst, both single-walled CNTs and multiwalled CNTs could be synthesized, depending on the reaction conditions. The formation of single-walled CNTs occurred when the carbon supply was kept low, i.e. low C₂H₂ concentration and low reaction pressure. The diameter of tubes decreased with increasing the substrate temperature. By optimizing the growth conditions, a small diameter of 0.65 nm single-walled CNTs, estimated from Raman scattering spectrum, was achieved even the low substrate temperature as 660 °C.     

Growth mechanism and field emission behavior of carbon nanotubes grown over 300 nm thick aluminium interlayer

The influence of thick aluminium (Al) ~ 300 nm interlayer on the growth and field emission (FE) properties of carbon nanotubes (CNTs) deposited on silicon coated with a 2 nm iron (Fe) catalyst was studied. The CNTs were grown over silicon substrate with and without Al-interlayer via CVD. It was observed that the presence of such high thickness of the interlayer on the substrate resulted in higher growth rate, narrower diameters and longer height of CNTs compared to CNTs grown on silicon (Si) substrate coated only with Fe. Al-interlayer hinders the diffusion of Fe into silicon, hence promotes the growth rate. Literature reports that a thick layer of Al causes Fe to diffuse into it, negatively affecting the growth. However, in our experiments, no evidence of depletion of Fe from the substrate was observed. Unique patterns of grown CNTs could be attributed to anisotropic Al-melting over the silicon substrate resulting in Al/Fe rich and deficient regions. The drastic improvement of current density from 0.41 mA/cm² to 20 mA/cm² at a field of 3.5 V/μm was found with Al-interlayer CNT grown samples. These mechanisms of improvements in field emission characteristics have been discussed in detail.     

Low temperature growth of carbon nanotubes using Ni nanopowder mixed with Ag-paste as catalyst

Growth of carbon nanotubes (CNTs) at low temperature is a critical issue in the development of CNTs for diversified applications. The screen printing method, which used silver paste mixed with CNTs and glass powder as the raw material, was usually adopted for fabricating cathode of field emission device. In this work, Ni nanopowder mixed with commercial Ag-paste was prepared and screen-printed on the glass substrate, and then subjected to sintering in air at 500 °C. Growth of CNTs was performed in a self-designed cold-walled chemical vapour deposition (CVD) chamber at 500 °C under a pressure between 1 and 10 Torr. Field emission property was tested and the turn-on field of 2.4 V/μm was obtained for the specimen containing 2.7 wt.% Ni nanopowder. CNT density can be controlled by adjusting the concentration of Ni nanopowder in silver paste. This method proposed a simple route for synthesizing CNTs at relatively low temperature in a conventional CVD system, which is capable of fabricating large area cathode with proper CNT density.     

Role of iron catalyst particles density in the growth of forest-like carbon nanotubes

Carbon nanotubes (CNTs) were fabricated by Chemical Vapour Depositon using a C₂H₂/H₂ mixture. They were grown on Si/SiO₂ substrate with Fe film as catalyst, deposited using thermal evaporation technique. The aim of this work is to emphasize the role of the Fe catalyst and the C₂H₂/H₂ flow rate ratio to grow vertically aligned CNTs. Fe metal samples with the deposition times ranging from 1 min to 16 min were deposited and CNTs were grown with different C₂H₂/H₂ flow rate ratio, from 5/95 to 30/70 by thermal CVD at 750 ^oC. Results show that CNTs were not vertically aligned with the longest catalyst deposition time for all flow rate ratios, while CNTs were always vertically aligned for deposition time less than 4 min and vertically aligned only for a C₂H₂ flow rate greater than 20% for the 7 min catalyst deposition time. Morphological and structural information about CNTs and Fe metal clusters were provided by field emission gun-scanning electron microscopy (FEG-SEM), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). An accurate balance between the Fe metal clusters density and the C₂H₂/H₂ flow rate ratio favours to achieve of a good vertical alignment     

Microcrystalline diamond growth in presence of argon in millimeter-wave plasma-assisted CVD reactor

The additions of argon and oxygen to H₂–CH₄ feed gas and high-electron-density plasma generated by the millimeter-wave power were used to deposit microcrystalline diamond films having high quality and high growth rate simultaneously. Microcrystalline diamond films were grown on silicon substrates with 60–90 mm diameter in the millimeter-wave plasma-assisted CVD reactor based on 10 kW gyrotron operating at a frequency of 30 GHz. The growth process and morphology of diamond films at wide variation of parameters (gas pressure, substrate temperature, microwave power, argon and oxygen concentrations in gas mixtures Ar–H₂–CH₄ and Ar–H₂–CH₄–O₂) are investigated. For understanding of growth conditions the investigations of the plasma parameters (electron density and gas temperature) in novel CVD reactor are presented.     

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.     

Localized fabrication of carbon nanotubes forest at a needle electrode by atmospheric pressure corona discharge

Characteristics of the growth of multi-walled carbon nanotubes (CNTs) at a localized surface of a needle-shaped graphite cathode by corona discharge were investigated with varied temperature, 400–1000 °C. For the CNTs' growth, C₂H₄ in H₂ stream was used as carbon source. The CNTs were observed in aligned form on the tip of a needle-shaped cathode under the present condition. It was observed that the temperature range for the CNTs' growth with corona discharge, 600–800 °C, was lower than that that without corona discharge, 700–1000 °C. In the relevant reaction system, strong electric field at the cathode tip enforced the CNTs to grow straightly to form the free-standing aligned CNT forest at a specifically narrow tip zone of the needle cathode.     

RF-PECVD growth and nitrogen plasma functionalization of CNTs on copper foil for electrochemical applications

This work describes an efficient way to improve the adhesion, growth rate and density of CNTs on copper substrate using radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD). The adhesion of an alumina buffer layer to the copper substrate is critical for the successful growth of CNTs. Hydrogen plasma was performed on the copper substrate to reduce copper oxide from the surface. The effect of two intermediate layers (Ti, Ni), as individual or in combination, between alumina and copper substrate on the CNT growth has been investigated. Furthermore, a nitrogen plasma treatment was carried out to functionalize the obtained CNTs. Electrochemical measurements were performed using CNTs grown on a copper substrate as electrodes and LiClO₄ as electrolyte. The specific capacitance of the obtained electrodes increases from 49 up to 227 Fg^− 1 for untreated and nitrogen-plasma treated CNTs at a scan rate of 10 mVs^− 1, respectively.     

A time‐dependent multiphysics,multiphase modeling framework for carbon nanotube synthesis using chemical vapor deposition

A time‐dependent multiphysics, multiphase model is proposed and fully developed here to describe carbon nanotubes (CNTs) fabrication using chemical vapor deposition (CVD). The fully integrated model accounts for chemical reaction as well as fluid, heat, and mass transport phenomena. The feed components for the CVD process are methane (CH₄), as the primary carbon source, and hydrogen (H₂). Numerous simulations are performed for a wide range of fabrication temperatures (973.15–1273.15 K) as well as different CH₄ (500–1000 sccm) and H₂ (250–750 sccm) flow rates. The effect of temperature, total flow rate, and feed mixture ratio on CNTs growth rate as well as the effect of amorphous carbon formation on the final product are calculated and compared with experimental results. The outcomes from this study provide a fundamental understanding and basis for the design of an efficient CNT fabrication process that is capable of producing a high yield of CNTs, with a minimum amount of amorphous carbon. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Three-terminal CNTs gas sensor for N2 detection

A novel three-terminal gas sensor was fulfilled by utilizing the vertically aligned carbon nanotubes (CNTs) mat. Carbon nanotubes were synthesized by thermal chemical vapor deposition (thermal CVD) at 700 °C under C₂H₂ gas flow rate of 30 sccm. Upon exposure to a with and without N₂ environment at the room temperature of 25 °C, the electrical resistance of as-made devices was found to increase and to return back, respectively. Compared to a low bias one, the sensitivity increased when applying a high source drain bias voltage. Furthermore, the device became more sensitive for N₂ detection by applying a negative gate voltage. It was concluded that the alteration of free holes concentration in the CNTs mat played the major mechanism for the N₂ gas detection.     

Low temperature growth mechanisms of vertically aligned carbon nanofibers and nanotubes by radio frequency-plasma enhanced chemical vapor deposition

Using radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD), carbon nanofibers (CNFs) and carbon nanotubes (CNTs) were synthesized at low temperature. Base growth vertical turbostratic CNFs were grown using a sputtered 8 nm Ni thin film catalyst on Si substrates at 140 °C. Tip growth vertical platelet nanofibers were grown using a Ni nanocatalyst in 8 nm Ni films on TiN/Si at 180 °C. Using a Ni catalyst on glass substrate at 180 °C a transformation of the structure from CNFs to CNTs was observed. By adding hydrogen, tip growth vertical multi-walled carbon nanotubes were produced at 180 °C using FeNi nanocatalyst in 8 nm FeNi films on glass substrates. Compared to the most widely used thermal CVD method, in which the synthesis temperature was 400–850 °C, RF-PECVD had a huge advantage in low temperature growth and control of other deposition parameters. Despite significant progress in CNT synthesis by PECVD, the low temperature growth mechanisms are not clearly understood. Here, low temperature growth mechanisms of CNFs and CNTs in RF-PECVD are discussed based on plasma physics and chemistry, catalyst, substrate characteristics, temperature, and type of gas.     

The use of aliphatic alcohol chain length to control the nitrogen type and content in nitrogen doped carbon nanotubes

Nitrogen doped carbon nanotubes (N-CNTs) were synthesized using acetonitrile/alcohol mixtures as the nitrogen, carbon and oxygen sources using the chemical vapor deposition (CVD) method. XPS analysis of the CNTs produced from an acetonitrile/ethanol mixture using different CVD temperatures (700–1000 °C), revealed that nitrogen incorporation in N-CNTs decreased with an increase in CVD temperature and that the type of nitrogen species incorporated also varied. Molecular nitrogen and a low content of pyridinic nitrogen was obtained in N-CNTs grown at 700 and 800 °C, while quaternary nitrogen was noted in all N-CNTs grown. Use of 20% acetonitrile/ROH (R = CH₃, C₂H₅, C₄H₉, C₅H₁₁, C₇H₁₅ and C₈H₁₇) mixtures allowed the C/O ratio to be changed whilst the N content in the precursor mixture was kept constant. The N content in the N-CNTs grown at 850 °C increased with the alcohol chain length and also controlled the nitrogen species incorporated, an effect related to the oxygen content of the reactant mixtures.     

Growth of high-quality carbon nanotubes on free-standing diamond substrates

Carbon nanotubes (CNTs) were grown on diamond-coated Si substrates and free-standing diamond wafers to develop efficient thermal interface materials for thermal management applications. High-quality, translucent, free-standing diamond substrates were processed in a 5 kW microwave plasma chemical vapor deposition (CVD) system using CH₄ as precursor. Ni and Ni-9%W-1.5%Fe catalyst islands were deposited to nucleate CNTs directly onto the diamond substrates. Randomly-oriented multi-walled CNTs forming a mat of ∼5 μm thickness and consisting of ∼20 nm diameter tubes were observed to grow in a thermal CVD system using C₂H₂ as precursor. Transmission electron microscopy and Raman analyses confirmed the presence of high-quality CNTs on diamond showing a D/G peak ratio of 0.2-0.3 in Raman spectra.     

Anodic titanium oxide: A new template for the synthesis of larger diameter multi-walled carbon nanotubes

Carbon nanotubes (CNTs) with larger diameter were synthesized over anodic titanium oxide (ATO) template by CVD method using acetylene as carbon source. The porous titanium oxide was obtained by anodization of titanium metal in a mixture of 1 M H₂SO₄ + 0.5% HF electrolyte at a constant applied potential of 40 V. The XRD analysis of anodized titanium revealed that rutile and anatase forms of TiO₂ are formed due to anodization. Further, SEM analysis was used to follow the development of pores on titanium surface. The TEM analysis revealed that the formed CNTs are straight and hollow with uniform wall thickness as well as larger diameter (70–80 nm). HRTEM study showed that the formed CNTs are multi-walled and their wall thickness is around 2–3 nm. Further, the structural features of the formed CNTs were studied by XRD. Raman spectroscopy was used to study the degree of graphitization of CNTs. The Lewis acid sites of TiO₂ present in the internal surface of the pores play an important role in the catalytic decomposition of acetylene and hence the formation of CNTs. When increasing the carbon deposition time, the wall thickness of CNTs is not increased significantly, indicating that the decomposition of acetylene is due to Lewis acid sites of TiO₂ and not due to thermal decomposition. Further, the morphology of CNTs formed over ATO template was compared with that of CNTs formed on Co electrodeposited ATO. There is no significant difference in morphology as well as wall thickness was observed between the CNTs grown over ATO with and without Co catalyst. But, still further investigations are necessary to study the structural differences between the CNTs grown over ATO with and without Co catalyst.