Fabrication of efficient field emitters with thin multiwalled carbon nanotubes using spray method

Thin multiwalled carbon nanotube (t-MWCNTs)-based field emitters are made by use of a spray method. The number of tube walls is between 2 and 6, with the corresponding outer diameters between 3 and 6 nm. They were dispersed in dichloroethane and sprayed onto metal-deposited indium tin oxide glass. After heat treatment, they were found to be tightly adhered to metal electrode. Excellent field emission characteristics were exhibited, with a large field enhancement factor and low turn-on voltage, comparable to those of singlewalled CNTs. However, the t-MWCNTs demonstrated a significantly lower degradation rate than SWCNTs in the emission current. This high emission stability was attributed to their stable edge structures, similar to conventional large-diameter MWCNTs. Therefore, t-MWCNTs could be utilized as an alternative material for field emitters.     

Quantum-mechanical simulations of field emission from carbon nanotubes

We present simulations of field emission from 2-nm long open (5,5), closed (5,5) and open (10,0) carbon nanotubes. Besides usual effects associated with the field-emission process, the total-energy distributions of the field-emitted electrons present peaks that are shifted by the electric field. Their sharpness and the evolution of their amplitude when changing the electric field depend on the semiconducting or metallic character of the nanotube.     

In-situ studies of electron field emission of single carbon nanotubes inside the TEM

Electron field emission characteristics of individual multi-walled carbon nanotubes (MWCNTs) were investigated in situ inside the transmission electron microscope (TEM). For a single MWCNT it was found that while field-emission can hardly occur from the side of the nanotube, a curved nanotube may result in finite side emission and the best emission geometry is the top emission geometry. Current-voltage (I-V) measurements made at different vacuum conditions and voltage sweeps emphasize the importance of the adsorbates on the electron field emission of MWCNTs. For a contaminated MWCNT, although the field emission current was reduced, the stability of its emission was improved. A current of up to several tens of μA was observed for a single MWCNT, but it was found that long time emission usually results in drastic structure damage that may lead to sudden emission failure.     

Influence of electric field and emission current on the configuration of nanotubes in carbon nanotube layers

The influence of applied electric field (E_av) and emission current (I_FE) on the configuration of conical layers carbon nanotubes (CLNTs) grown by CVD on the edge of Ni foil has been investigated. TEM profile imaging revealed a high concentration of nanotubes near the foil edge surface, whereas on the nanotube layers’ outer surfaces single, non-oriented nanotubes with open ends free of catalytic particles, were observed. After sufficient electric field application many nanotubes became oriented towards the anode, but one or two of them were found to be always a few microns more extended. In situ SEM investigation showed that below E_av = 3.2–3.9 V/μm, emission was achieved at the expense of originally existing free nanotube ends. Configuration changes began at larger electric fields. On the observed foil edge length (14.6–17.8 μm, with an edge thickness of 200 μm) one or two nanotubes extended towards the anode and probably became the main emitters. Upon further increasing the field to E_av = 5.7–8 V/μm and at an emission current I_FE = 2 × 10⁻⁵ A these tubes disappeared (or essentially shortened). At E_av = 8 V/μm and higher and at an exposure time up to 40 min, several tens of extended nanotubes appeared, with one or two extended well beyond the others. This nanotube configuration pattern is connected with electrostatic screening between the nanotubes. Our interpretation of the data suggests that in the investigated range of E_av and I_FE, a limited number of nanotubes are emitting and these nanotubes are constantly changing as E_av, I_FE and exposure time increase.     

Separation and purification of functionalised water-soluble multi-walled carbon nanotubes by flow field-flow fractionation

Water-soluble, functionalised multi-walled carbon nanotubes (MWNT) have been separated and purified from amorphous material through direct flow field-flow fractionation. MWNT subpopulations of relatively homogeneous, different length were obtained by collecting fractions of the raw, highly polydispersed (200-5000 nm) functionalised MWNT sample.     

Functionalization of carbon nanotubes using a silane coupling agent

A new method is developed to chemically functionalize multi-walled carbon nanotubes (MWCNTs) based on silanization reaction for use as the reinforcement for polymer matrix composites. To oxidize and create active moieties on the MWCNTs, the samples were exposed to UV light within the ozone chamber, followed by silanization using 3-glycidoxypropyltrimethoxy silane after the oxidized MWCNTs were reduced by lithium aluminum hydride. FT-IR, TEM and XPS were employed to characterize the changes in carbon nanotubes surface morphology, chemistry and physical conditions at different processing stages. The results indicate improved dispersion and attachment of silane molecules on the surface of the MWCNTs.     

Electrophoretic deposition of carbon nanotubes films as counter electrodes of dye-sensitized solar cells

Carbon nanotubes (CNTs) films have been successfully fabricated by electrophoretic deposition (EPD) technique and used as counter electrodes of dye-sensitized solar cells (DSSCs). The CNTs counter electrodes consisting of a large number of bamboo-like structures with defect-rich edge planes exhibit a highly interconnected network structure with high electrical conductivity and good catalytic activity. A high photovoltaic conversion efficiency of 7.03% is achieved for DSSCs based on the CNTs counter electrodes, which is comparable to the cell based on conventional Pt counter electrode at one sun (AM 1.5G, 100 mW cm⁻²). The results suggest that the present synthetic strategy provides a potential feasibility for the fabrication of low-cost flexible counter electrodes of DSSCs using a facile deposition technique from an environmentally “friendly” solution at low temperature.     

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.     

The preparation of multi-walled carbon nanotubes with a Ni-P coating by an electroless deposition process

The multi-walled carbon nanotubes (MWCNTs, mean diameter: 100-200 nm) with nickel-phosphorous (Ni-P) coatings were obtained by an electroless deposition process. To prepare the MWCNTs covered with continual Ni-P layers, a pre-treatment procedure comprised of acid-cleaning, sensitization and activation has been developed. The resulting MWCNTs have a uniform distribution of the Ni-P layers coated on the MWCNTs with the fibrous appearance maintained. X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations revealed that the as-coated MWCNTs were comprised of the amorphous Ni-P layers and inner carbon nanotubes covered with the Ni-P layers. These amorphous Ni-P-coated MWCNTs were used as precursors for preparing MWCNTs with nanocrystalline Ni-P(crystalline Ni and Ni₃P intermetallic compound) layers by the heat-treatment above 400 °C, which were determined by differential scanning calorimeter (DSC) and XRD studies. The results of this work provide an effective electrochemical method for preparing powdery MWCNTs with Ni-P layer as new composite materials from the aqueous solution.     

Flame-synthesis of carbon nanotubes on silicon substrates and their field emission properties

We reported the flame-synthesis of patterned multiwalled carbon nanotubes (CNTs) on silicon substrate by a shadow mask and their field emission properties. It was found that CNTs with tangled and curved morphology were preferentially grown around the cracked edges of Ni dot pattern. A crack-induced catalyst-activation growth mechanism was proposed. The patterned CNTs fabricated by such a simple flame-synthesis method exhibited good field emission characteristics with uniform emission patterns and reproducible and stable emission behaviors, although the CNTs possessed many defective graphite layers and showed relatively higher turn-on and threshold field than other reported CNTs grown by chemical vapor deposition. Our results demonstrated that such a low-cost and scaleable CNT pattern fabrication process can be expected to have favorable applications in field emission devices.     

Comparisons on properties and growth mechanisms of carbon nanotubes fabricated by high-pressure and low-pressure plasma-enhanced chemical vapor deposition

Effects of plasma pressure and the presence of nitrogen on growth of carbon nanotubes (CNTs) and their properties were studied by using microwave plasma chemical vapor deposition (MPCVD) (pressure=600–3300 Pa) and electron cyclotron resonance chemical vapor deposition (ECR-CVD) (pressure=0.3–0.6 Pa) systems. CH₄/H₂ and CH₄/N₂ were used as source gases, and Co as the catalyst. The structures and properties of CNTs were characterized by using field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Raman spectra, and field emission I–V measurements. The results show that CNTs made by higher plasma pressure system have a higher growth rate (typically 1–3 μm/min), smaller tube diameter, better field emission properties, and better tube quality. The growth rate is related to the availability of carbon source. The morphology change from spaghetti-like to well-aligned CNTs is discussed in terms of directed ions. The change in field emission properties is reasoned in terms of geometric enhancement factor and screening effect for different tube morphologies. The presence of nitrogen plasma can have the following effects: increasing tube diameter, increasing straightness of CNTs, forming of bamboo-like CNTs, deterioration of field emission properties, and shifting of Raman peak toward lower-frequency side (or increasing residual tensile stress).     

Aligned carbon nanotubes fabricated by thermal CVD at atmospheric pressure using Co as catalyst with NH3 as reactive gas

Co is used as a catalyst for chemical vapor deposition (CVD) of vertically aligned multi-walled carbon nanotubes (CNTs) in a tube furnace at atmospheric pressure. C₂H₂ and NH₃ were used for the carbon feedstock and reaction control, respectively. The CVD process parameters determine the chemical properties of the Co particles and subsequently the morphologies and field emission behavior of CNTs as they strongly depend upon the catalyst condition. The flow rate ratio of NH₃ to C₂H₂ is shown to be central to the synthesis of vertically aligned CNTs. Repeatable synthesis of vertically aligned CNTs at atmospheric pressure in a tube furnace is cost effective for large area deposition of such structures which may be used, for example, in vacuum field emission devices.     

Array geometry, size and spacing effects on field emission characteristics of aligned carbon nanotubes

Microwave plasma-enhanced chemical vapor deposition (MPECVD) has been shown capable of producing vertically aligned mutli-walled CNTs as a result of self-bias of the microwave plasma. These CNTs are relevant to field emission applications. However, it is also known that closely packed or mat-like CNTs are not effective field emitters due to field screening effects among neighboring tubes. In this study, an approach whereby “micro-” patterning of CNT arrays, adjusting their geometry, size and array spacing by conventional photolithography, rather than “nano-” patterning a single CNT by electron-beam lithography, is employed to fabricate efficient emitters with enhanced field emission characteristics. MPECVD with catalysts are used on Si substrate to fabricate micropatterned vertically aligned CNT arrays with various geometries, sizes and spacing. The field emission results show that a circular array with 20 μm spacing has the lowest turn-on field of 2 V/μm at 1 μA/cm² and achieves the highest current density of 100 μA/cm² at 3 V/μm. Investigation on the array spacing effect shows that 10 × 10 μm CNT square array with an array spacing of 20 μm displays the lowest turn-on field of 9 V/μm and achieved a very high current density of 100 mA/cm² at 20 V/μm. Furthermore, the results suggest that the array spacing of the 10 × 10 μm CNT square array can be reduced to at least 20 μm without affecting the field enhancement factor of the emitter. The results clearly indicate further optimization of spacing in the arrays of CNT emitters could result in lower turn-on field and higher current density.     

TEM image simulation study of small carbon nanotubes and carbon nanowire

Recent findings of extremely small diameter carbon nanotube and nanowire in the core of a multi-walled carbon nanotube (MWCNT) have attracted interests from broad range of researchers. Direct observation of carbon nanotube is usually done using a transmission electron microscope (TEM). When nanotubes become smaller, it becomes harder to correctly understand the TEM images, not only because of the weak scattering, but also due to the artifact that starts to appear because of the interference effect and the inappropriate defocus condition.In this study, we have shown that the artifact such as ghost fringes due to inappropriate defocus conditions of the TEM appear in the core of an MWCNT, and can be misinterpreted as either carbon nanowire or small carbon nanotube. It is also shown that, in the TEM image, it is hard to distinguish a single-walled nanotube bundle from a double-walled carbon nanotube bundle. Finally, we propose that the cross-sectional observation is necessary for the correct characterization of single- and double-walled carbon nanotube bundles.     

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.     

Controllable fabrication of SnO2-coated multiwalled carbon nanotubes by chemical vapor deposition

A simple and efficient approach for coating multiwalled carbon nanotubes (MWCNTs) with size-controllable SnO₂ nanoparticles by chemical vapor deposition has been developed using tin hydride (SnH₄) gas as the source of SnO₂ at 550 °C. The size and coverage of SnO₂ nanoparticles can be adjusted by simply controlling the deposition time and the flow rate of the SnH₄/N₂ mixture gas during the CVD procedure. In addition, by using the MWCNTs as a sacrificial template, a kind of one-dimensional chain-like SnO₂ nanostructure has been synthesized by increasing the deposition temperature to 730 °C. This technique may provide a good way to produce tunable SnO₂-MWCNT composites.