Influence of Fe nanoparticles diameters on the structure and electron emission studies of carbon nanotubes and multilayer graphene

In this paper we report the effect of Fe film thickness on the growth, structure and electron emission characteristics of carbon nanotubes (CNTs) and multilayer graphene deposited on Si substrate. It is observed that the number of graphitic shells in carbon nanostructures (CNs) varies with the thickness of the catalyst depending on the average size of nanoparticles. Further, the Fe nanoparticles do not catalyze beyond a particular size of nanoclusters leading to the formation of multilayer graphene structure, instead of carbon nanotubes (CNTs). It is observed that the crystallinity of CNs enhances upon increasing the catalyst thickness. Multilayer graphene structures show improved crystallinity in comparison to CNTs as graphitic to defect mode intensity ratio (I_D/I_G) decreases from 1.2 to 0.8. However, I_2D/I_G value for multilayer graphene is found to be 1.1 confirming the presence of at least 10 layers of graphene in these samples. CNTs with smaller diameter show better electron emission properties with enhancement factor (γ_C = 2.8 × 10³) in comparison to multilayer graphene structure (γ_C = 1.5 × 10³). The better emission characteristics in CNTs are explained due to combination of electrons from edges as well as centers in comparison to the multilayer graphene.     

Enhanced electron emission from titanium coated multiwalled carbon nanotubes

Enhanced field emission properties and improved crystallinity of titanium (Ti) coated multiwalled carbon nanotubes (MWCNTs), prepared by microwave plasma enhanced chemical vapour deposition have been observed. Ti films of extremely low thicknesses (0.5 nm, 1.0 nm and 1.5 nm) were coated over carbon nanotubes (CNTs) and their field emission behaviour was investigated. The turn on field of Ti coated CNTs was found to be low (~ 0.8 V/μm) as compared to pristine CNTs (~ 1.8 V/μm). The field enhancement factor for Ti coated CNTs was quite large (~ 1.14 × 10⁴) as compared to pristine CNTs (~ 6 × 10³). This enhancement in electron emission is attributed to the passivation of defects and improved crystallinity of CNTs. Surface morphological and microstructural studies were carried out to investigate the growth of pristine and Ti coated CNTs. It was observed that Ti nanoclusters adsorb on the edges of MWCNTs and increase their crystallinity. This increase is directly correlated with the thickness of Ti film deposited. Micro Raman spectroscopy confirmed the improved crystallanity of Ti coated CNTs.     

Field emission of carbon nanotubes grown on nickel substrate

Carbon nanotubes (CNTs) have been synthesized directly on the electrically conducting nickel substrate without additional catalyst. Field emission properties of the as-prepared sample were characterized using parallel plate diode configurations. It was observed that the field emission qualitatively follows the conventional Fowler–Nordheim (F–N) theory from the straight line of ln(I/V²) versus 1/V plot at the high applied field region. The uniformity and stability of the electron emission have also been examined. The low electron turn-on field (E_to) and high emission current density indicates the potential applications of this new CNT-based emitter.     

Structural modifications and enhanced Raman scattering from multiwalled carbon nanotubes grown on titanium coated silicon single crystals

Multiwalled carbon nanotubes (MWCNTs) were grown on 10 nm iron (Fe) film by microwave plasma enhanced chemical vapor deposition using titanium (Ti) film as an interlayer. The Ti interlayer of thickness 5 nm-20 nm was sandwiched between Fe and silicon (Si) using thermal evaporation. Enhanced Raman response was observed in MWCNTs with increasing Ti interlayer thickness. This was related with the plasmonic effects occurring at the interface of the CNTs and the metallic support in a three layer system (Ti-Fe-CNTs). The increase in the G mode optical strength is attributed to surface enhanced resonance Raman scattering. Moreover, the increase in the D-mode and 2D-mode intensity is explained on the basis of double resonance effects. The crystallinity in the samples was calculated using I_D/I_G ratio. It was found that that I_D/I_G ratio decreases in three layer system with increasing Ti interlayer thickness as compared to a two layer (Fe-CNTs) system as reported earlier.     

Dispersion and field emission properties of multi-walled carbon nanotubes by high-energy milling

Multi-walled carbon nanotubes (MWNTs) were high-energy milled for 2 h in texanol after a polycarboxylic acid polymeric dispersant had been added in order to enhance the dispersion. The degree of MWNT dispersion was significantly enhanced by high-energy milling compared to the intact sample, which increased the density of surface-exposed MWNTs with a screen-printed paste. However, the emission properties of high-energy milled MWNTs did not show such a high emission current density relative to their increased surface-exposed density. Further investigation using Raman spectroscopy and X-ray diffraction evidenced the milling-induced MWNT damage, which explained the relatively lower emission current density of high-energy milled MWNTs.     

Optimization of field emission properties of carbon nanotubes by Taguchi method

It is the purpose of this study to evaluate the field emission property of carbon nanotubes (CNTs) prepared by microwave plasma-enhanced chemical vapor deposition (MPCVD) method. Nickel layer of 5 nm in thickness on 20-nm thickness titanium nitride film was transformed into discrete islands after hydrogen plasma pretreatment. CNTs were then grown up on Ni-coated areas by MPCVD. Through the practice of Taguchi method, superior CNT films with very low emission onset electric field, about 0.7 V/μm (at J = 10 μA/cm²), are attained without post-deposition treatment. It is found that microwave power has the most important influence on the field emission characteristics of CNT films. The increase of methane flow ratio will downgrade the degree of graphitization of CNT and thus its field emission characteristics. Scanning electron microscope and transmission electron microscopy (TEM) observation and energy dispersive X-ray spectrometer analysis reveal that CNT growth by MPCVD is based on tip-growth mechanism. TEM micrographs validate the hollow, bamboo-like structure of the multi-walled CNTs.     

Influence of structural and preparation parameters of Fe2O3/Al2O3 catalysts on rate of production and quality of carbon nanotubes

The influence of catalytic and operational parameters on the rate of growth and quality of carbon nanotubes has been investigated. A series of Fe₂O₃/Al₂O₃ catalysts prepared by different methods were investigated under conditions of synthesis of CNTs via the process of CVD of ethylene. Deposition experiments were carried out in a thermogravimetric hot-wall reactor, which enables continuous monitoring of the evolution of carbon mass with time. Controlled explosive burning (CEB) of precursor compounds was found to be the most effective method of preparation of the catalyst with respect to rate of deposition and yield of CNTs. This result has been attributed to the presence of hematite particles of small diameter on the catalyst. The presence of hydrogen in the gas feed mixture, even at small concentration, proved to be beneficial for the rate of production of MWCNTs and to result in the synthesis of CNTs of narrower diameter distribution. Yield and quality of MWCNTs depend on the concentration of the carbon source (ethylene) in the feed mixture and on temperature of deposition. Under the present experimental conditions, the optimal reaction temperature was found to be 650 °C. The products of the deposition were characterized using scanning electron microscopy and Raman spectroscopy.     

Field emission properties of RuO2 thin film coated on carbon nanotubes

Carbon nanotubes (CNTs) have been widely applied in field emission (FE) due to their high geometric aspect ratio and low work function. More recently, researchers have introduced ruthenium dioxide (RuO₂) as a field emitter because of its excellent chemical and thermal stability due to its oxide nature. This study used the surface morphology of CNTs and the field emission (FE) stability of RuO₂ to improve FE characteristics. Since the work functions of CNTs and RuO₂ are very close, this study combined these two elements by applying a thin film of RuO₂ on the CNT surface. In the process of covering the tips of CNTs with a thin film of RuO₂ eventually obtained the best matching of these two elements. The study not only enhanced the FE performance of CNTs but also extended FE lifetime by applying a thin film of RuO₂ on the CNT tips.     

Field emission characteristics from tapered ZnO nanostructures grown onto vertically aligned carbon nanotubes

Zinc oxide (ZnO) nanostructures were grown on vertically aligned carbon nanotubes (CNTs) using thermal chemical vapor deposition (CVD) to enhance the field emission characteristics. The shape of ZnO nanostructure was tapered. Scanning electron microscopy (SEM) image showed the ZnO nanostructures were grown onto CNT surface uniformly. The field electron emission of pristine CNTs and ZnO-coated CNTs were measured. The results showed that ZnO nanostructures grown onto CNTs could improve the field emission characteristics. The ZnO-coated CNTs had a threshold electric field at about 3.1 V/μm at 1.0 mA/cm². The results demonstrated that the ZnO-coated CNT is an ideal field emitter candidate material. The stability of the field emission current was also tested.     

Synthesis of large-area single-walled carbon nanotube films on glass substrate and their field electron emission properties

Large-area and homogeneous single-walled carbon nanotube (SWCNT) films have been deposited via arc discharge directly on glass substrate coated with a layer of indium tin oxide film. The characterization, by means of electron microscopy and Raman spectroscopy, shows that the as-grown films are uniformly woven and consist of SWCNT with diameters ranging from 0.82 to 1.15 nm. As a cathode material, the field emission test indicates the films have low turn-on field of ∼1.2 V/μm at 10 μA/cm² emission current, and high emission intensity causing luminance of about 7000 cd/cm² with fine uniformity. The best performing sample exhibits a constant degradation of less than 3% per hour at an emission current of around 1 mA. Measuring with the high voltage (2000 V) on the films for 2.0 h increased the field enhancement factor from 4500 to 5400 at the high field region. The results are of significance to the development of field emission display using nanoemitters.     

Tunable field emission properties of carbon nanotube arrays by engineering Fe catalysts

High-quality carbon nanotube (CNT) arrays composed of nanotubes with different diameters and wall numbers were synthesized by water-assisted chemical vapor deposition (CVD) from engineered Fe catalysts. Interestingly, the distribution of nanotube diameter and wall number broadened over 2.5 times as the catalytic Fe thickness increased. The mean diameter and wall number of nanotubes increased monotonically with the Fe thickness, while the calculated CNT area density within an array dropped about 32 times. Field emission measurements revealed that the turn-on voltage for CNT arrays decreased from 3.5 to 2.5 V/µm with the increased catalytic Fe thickness. It was believed that the screening effect from the proximity of neighboring nanotubes has a dominant influence than the CNT diameter on the resulting turn-on voltage.     

Characterization and enhanced field emission properties of carbon nanotube bundle arrays coated with N-doped nanocrystalline anatase TiO2

Anatase TiO₂ nanocrystals (NCs) were deposited onto patterned carbon nanotube (CNT) bundle arrays to form a TiO₂/CNT composite using metal organic chemical vapor deposition (MOCVD) using titanium-tetraisopropoxide (Ti(OC₃H₇)₄) as a source reagent. The N-doped TiO₂/CNT composite was then fabricated using nitrogen plasma treatment. The structural and spectroscopic properties of TiO₂/CNT composites were characterized by field-emission scanning electron microscopy, micro-Raman spectroscopy and X-ray photoelectron spectroscopy. The combined geometrical structure and low electron affinity effects of N-doped TiO₂ led to a low turn-on field of 1.0 V μm⁻¹ at a current density of 10 μA cm⁻², a low threshold field of 1.9 V μm⁻¹ at a current density of 1 mA cm⁻², a high field enhancement factor of 3.0 × 10³, and long-term stability for the N-doped TiO₂/CNT composite. The results revealed that the N-doped TiO₂/CNT composite can be a potential candidate for field emission devices.     

Preparation and visible-light-driven photoelectrocatalytic properties of boron-doped TiO2 nanotubes

In the present study, chemical vapour deposition (CVD) was applied to dope boron into TiO₂ nanotubes anodized Ti in C₂H₂O₄·2H₂O + NH₄F electrolyte with the goal of improving the photocatalytic (PC) activity under visible light. The undoped TiO₂ nanotubes had a highly self-organized structure. However, after doping through CVD, TiO₂ nanotubes suffered from an observable disintegration of morphological integrity. X-ray diffraction (XRD) results confirmed that annealing temperature had an influence on the phase structure and boron impurities could retard anatase–rutile phase transition. Diffuse reflectance absorption spectra (DRS) analysis indicated that B-doped samples displayed stronger absorption in both UV and visible range. B-doped TiO₂ nanotubes electrode annealed at 700 °C through CVD showed higher photoelectrocatalytic (PEC) efficiency in methyl orange (MO) degradation than that annealed at 400 °C and 550 °C. MO degradation was substantially enhanced with the increasing applied bias potential. Moreover, there was a synergetic effect between the electrochemical and photocatalytic processes, and the synergetic factor R reached 1.45. B-doped TiO₂ nanotubes electrode showed good stability after 10 times by repeating photoelectrocatalysis of MO.     

Direct synthesis of Y-junction carbon nanotubes by microwave-assisted pyrolysis of methane

Both Y-junction carbon nanotubes and individual carbon nanotubes were synthesized without any additive catalyst by microwave decomposition of methane. Detailed microstructures of as-synthesized products have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. The results show that these Y-junction CNTs possess an internal bamboo-shaped structure, and some three dimensional multi-terminal junctions are also observed on CNTs. As gas flow rate decreased to 15 sccm, only individual nanotubes could be obtained. A possible mechanism is proposed for the synthesis of the Y-junction carbon nanotubes on these observations. This technique may also have great potential in making other nano-structured carbon materials on a large scale and at low cost.     

Effect of silane flow rate on structural,electrical and optical properties of silicon thin films grown by VHF PECVD technique

Hydrogenated silicon thin films deposited by VHF PECVD process for various silane flow rates have been investigated. The silane flow rate was varied from 5 sccm to 30 sccm, maintaining all other parameters constant. The electrical, structural and optical properties of these films were systematically studied as a function of silane flow rate. These films were characterized by Raman spectroscopy, Scanning Electron Microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy and UV–visible (UV–Vis) spectroscopy. Different crystalline volume fraction (22%–60%) and band gap (∼1.58 eV–∼1.96 eV) were achieved for silicon thin films by varying the silane concentration. A transition from amorphous to nanocrystalline silicon has been confirmed by Raman and FTIR analysis. The film grown at this transition region shows the high conductivity in the order of 10⁻⁴ Ω⁻¹ cm⁻¹.     

Effects of hydrogen dilution on CNx film properties deposited using rf PECVD from a mixture of ethane,nitrogen and hydrogen

Carbon nitride (CN_x) thin films were deposited using radio frequency plasma enhanced chemical vapor deposition (rf PECVD) from a mixture of ethane (C₂H₆), nitrogen (N₂) and hydrogen (H₂) gases. The C₂H₆ and N₂ flow rates were kept constant, while the H₂ flow rate was varied. The effects of hydrogen dilution on the growth rate and structural properties of the films were studied. It was found that a significant increase in the films growth rate was observed with the introduction of H₂ at as low as 25 standard cubic centimeters per minute (sccm). A set of CN_x films deposited from C₂H₆:N₂ mixture without any inclusions of H₂ were also presented in this work as a reference to compare the differences between those two sets and to understand the roles of H₂ to the films properties. At highest H₂ flow rate, the structure of the films changed from polymeric to graphitic and the quenching of PL was observed. Furthermore, higher N incorporation with lower E_g was obtained for these films compared to those of C₂H₆:N₂ films. The change in the structure of the films corresponds to changes in their chemical bonding. As N incorporation increased, the porosity of the films increases and thus affects the disorder in the film structures.     

Investigation of the cytotoxicity of carbon nanotubes using hydroxyapatite as a nano-matrix towards mouse fibroblast cells

The cytotoxicity of carbon nanotubes adhered on hydroxyapatite matrix synthesized by catalytic chemical vapor deposition was investigated towards mouse fibroblast cells, using MTT assay method. The results demonstrate that carbon nanotubes adhered on hydroxyapatite matrix possess no evident short-term toxicity and can be considered biocompatible with L929 mouse fibroblast cells in culture, while the long-term negative effects, that are evidenced after reseeding, are probably due to physical rather than chemical interactions.     

Electrochemical characterization of carbon nanotube forests grown on copper foil using transition metal catalysts

Growth of vertical, multiwalled carbon nanotubes (CNTs) on bulk copper foil substrates can be achieved by sputtering either Ni or Inconel thin films on Cu substrates followed by thermal chemical vapor deposition using a xylene and ferrocene mixture. During CVD growth, Fe nanoparticles from the ferrocene act as a vapor phase delivered catalyst in addition to the transition metal thin film, which breaks up into islands. Both the thin film and iron are needed for dense and uniform growth of CNTs on the copper substrates. The benefits of this relatively simple and cost effective method of directly integrating CNTs with highly conductive copper substrates are the resulting high density of nanotubes that do not require the use of additional binders and the potential for low contact resistance between the nanotubes and the substrate. This method is therefore of interest for charge storage applications such as double layer capacitors. Inconel thin films in conjunction with Fe from ferrocene appear to work better in comparison to Ni thin films in terms of CNT density and charge storage capability. We report here the power density and specific capacitance values of the double layer capacitors developed from the CNTs grown directly on copper substrates.     

Oxygen plasma assisted end-opening and field emission enhancement in vertically aligned multiwall carbon nanotubes

This paper highlights the changes in micro-structural and field emission properties of vertically aligned carbon nanotubes (VACNTs) via oxygen plasma treatment. We find that exposure of very low power oxygen plasma (6 W) at 13.56 MHz for 15–20 min, opens the tip of vertically aligned CNTs. Scanning electron microscopy and transmission electron microscopy images were used to identify the quality and micro-structural changes of the nanotube morphology and surfaces. Raman spectra showed that the numbers of defects were increased throughout the oxygen plasma treatment process. In addition, the hydrophobic nature of the VACNTs is altered significantly and the contact angle decreases drastically from 110° to 40°. It was observed that the electron field emission (EFE) characteristics are significantly enhanced. The turn-on electric field (ETOE) of CNTs decreased from ∼0.80 V μm⁻¹ (untreated) to ∼0.60 V μm⁻¹ (oxygen treated). We believe that the open ended VACNTs would be immensely valuable for applications such as micro/nanofluidic based filtering elements and display devices.     

Field emission characteristics of fast grown nanocrystalline diamond/amorphous carbon composite films by microwave plasma-enhanced chemical deposition method

This study synthesized the nanocrystalline diamond/amorphous carbon (NCD/a-C) composite films by the microwave plasma-enhanced chemical vapor deposition (MPCVD) system with Ar/CH₄/N₂ mixtures. A localized rectangular-type jet-electrode with high density plasma was used to enhance the formation of NCD/a-C films, and a maximum growth rate of 105.6 µm/h was achieved. The content variations of sp² and sp³ phases via varying nitrogen gas flow rates were investigated by using Raman spectroscopy. The NCD/a-C film which synthesized with 6% nitrogen concentration and no hydrogen plasma etching treatment possessed a low turn-on electric field of 3.1 V/µm at the emission current of 0.01 µA.