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The NO2 adsorption properties of macroscopic multiwalled carbon nanotube (MWCNT) ropes and acid treated MWCNT ropes, obtained by the floating catalyst chemical vapour deposition process, have been examined. The structural characterisation shows that these ropes constitute bundles of MWCNTs. This bundled structure is found to control the electrical and gas adsorption properties of the material. The electrical resistance of these ropes decreases upon exposure to NO2 with a high sensitivity even at room temperature. The adsorption of the NO2 onto MWCNT bundles is found to be more stable with temperature in comparison to isolated MWCNTs revealing the complex nature of the adsorption process. These adsorption sites, which are created within the bundles of carbon nanotubes, are more stable requiring higher desorption energy. The surface of the MWCNT ropes is also modified with acid treatment, which increases the response to NO2 by a factor 100% due to increased polar interactions between the gas molecules and the existing functionalised surface. These results suggest the possibility of using these macroscopic MWCNT ropes as low cost gas sensing materials.

Mechanical properties of long multi-walled CNT ropes prepared using the floating catalyst chemical vapour deposition method were tested, obtaining an average tensile strength and Young’s modulus of 210 MPa and 2.2 GPa, respectively. Furthermore, the ropes showed excellent NH 3 detecting sensitivity at both high and low NH 3 concentrations. Increasing the temperature, NH 3 desorbed from the ropes, indicating an exothermal absorption reaction.

This paper reviews the process of growing large area arrays of carbon nanotubes (CNT) to be used as sensing platforms. First the work explores different methods to prepare Ni catalyst particles. More precisely, the work shows how the nanostructured arrays of catalyst are obtained by thermal annealing of Ni thin films, laser nanostructuring of Ni thin films and formation by organic dendrimers in aqueous solution. The growth process by chemical vapour deposition (CVD) is detailed and the influence of the catalyst particles on the final morphology of the tubes is discussed. A discussion of the advantages and disadvantages of each Ni nanoparticle technique to grow CNT arrays as a sensing platform is presented.

Carbon-based materials have long been identified as a suitable thin film cathode for use in electron emitting structures. This is in part due to the relative ease in the extraction of electrons at low applied electric fields, but also due to their enviable material properties that include chemical and high thermal stability, excellent electrical integrity and mechanical robustness. These thin film cathodes can also be deposited over large areas at low temperatures. It has been reported for a number of years that thin carbon films with low threshold fields can give emission current densities sufficient for flat panel displays, but further commercial development has been hindered due to poor uniformity of emission. A suitable solution to this problem has failed to be found, partly due there being no single accepted model which explains the electron emission process from these films. We have first attempted to categorise the amorphous carbon films found in the literature and two generic models emerge which can be used to explain the emission process. One is based on space charge-induced band bending for films with low defect densities and the second model for films with high defect densities is based on nanoscale dielectric inhomogeneities. Backed by experimental data, we will show computer simulation results that confirm our findings from a qualitative point of view and propose a model to explain the low threshold electron field emission from a majority of defective diamond-like carbon (DLC) thin films. Integrated structures that can be fabricated with gates suitable for a planar device structure will also be introduced.

from 0 s to 18 s) exhibits a stable current with a fluctuation of only 0.08 lA. Hence, the ripple measured during the extraction was not a simple noise but an oscillation that reflected a change in the interlayer structure, such as atomic configuration of carbons in neighboring layers. The observed MWCNT is in the high temperature condition, so that the electron transport in the MWCNT may be diffusive. Although further investigations are required on the influence of a thermal effect on the electrical conductivity of MWCNTs, in order to clarify the mechanism of the electrical conductivity, the measured fluctuation of the current might be a ripple corresponding to the numerical variation of p-bonding atoms in neighboring layers. Recently, it is reported that a small ripple on the sliding force during the extraction of the inner shell is obtained by the molecular mechanics calculations based on the van der Waals attraction between the interlayers, and the ripple period corresponds to that of the CNT honeycomb structure [10]. Our experimental result suggests that the ripple of the current reflects the electronic structure that orig- inated in the atomic structure of the MWCNT. In conclusion, we observed a controlled interlayer sliding deformation of MWCNTs with measuring electrical conductivity in TEM. The in situ measurement demonstrated that the electrical conductivity reversibly changed corresponding to the displacement of the inner shell. The ripple of the current might be caused by relative variation of atomic configuration in the neighboring layers. The electromechanical characteristic of MWCNTs is an attractive, as it can be utilized in developing elemental applications of nanoelectromechanical system.

Low threshold fields of 1.6 V/μm and 2.6 V/μm were obtained for field emission from multiwalled carbon nanotubes and boron-doped multiwalled carbon nanotubes embedded in polystyrene, respectively. A Fowler–Nordheim analysis of the results together with sheet resistivity data illustrate that the higher carbon nanotube concentrations in a polystyrene matrix result in larger effective emission areas, at the expense of higher operating threshold fields.

This paper investigates the introduction of nano-sized Ag cluster into a SiO/sub 2/ matrix by Ag/sup +/ implantation on thermally oxidised SiO/sub 2/ layers. The embedded Ag nanoclusters create conducting paths for the emissive electrons from substrate to emission surface and give rise to a local electric field enhancement due to an electrical inhomogeneity effect.

Summary form only given. The ubiquitous CRT is finally being overtaken in terms of volume production by flat panel technologies, predominantly based on liquid crystals. It has taken over 30 years for the first LCD based displays to be established, and, at present, to dominate the market. Field emission (FE) based displays are currently at a turning point in establishing a toehold in the hugely competitive market. Carbon nanotubes (CNT) have played a major role in attracting research and development into the FED sector. There has been sustained research into the electron field emission properties of carbon nanotubes (CNT), usually performed on non-aligned random mats, vertically aligned mats, and ordered arrays of CNT on various substrates. In this paper we will introduce some of the key parameters that need be optimised in the design of the cathodes, the definitions used for enhancement factors, and how the location of the anode electrode plays a major role for the correct interpretation of results between different groups. We will also show the importance of nanoscale design over large area for producing FE displays, and how the low temperature growth of CNT structures allows for a step change in the development of these devices. Other routes to producing large area mats of cathodes will also be discussed together with characterisation methodologies for the cathodes. Within the paper we will also present a review of our recent research in the study of the geometric enhancement factor of individual CNT by means of computer simulations. This also leads to an alternative definition of the applied electric field, and enhancement factor. The conclusions drawn from the simulations are backed by experimental field emission from individual CNT, in both two and three terminal configurations to offer further understanding of the mechanism in which these structures operate

We present a novel approach, which will potentially allow for low-temperature-substrate synthesis of carbon nanotubes using direct-current plasma-enhanced chemical vapour deposition. The approach utilizes top-down plasma heating rather than conventional heating from a conventional substrate heater under the electrode. In this work, a relatively thick titanium layer is used as a thermal barrier to create a temperature gradient between the Ni catalyst surface and the substrate. We describe the growth properties as a function of the bias voltage and the hydrocarbon concentrations. The heating during growth is provided solely by the plasma, which is dependent only on the process conditions, which dictate the power density and the cooling of the substrate, plus now the thermal properties of the "barrier layer". This novel approach of using plasma heating and thermal barrier allows for the synthesis of carbon nanotubes at low substrate temperature conditions to be attained with suitable cooling schemes.

The intercalation of metals within carbon nanotube structures has extended the potential applications of these materials to possible quantum memory elements as well as high density magnetic storage media. In our study we use methodologies based on wet chemistry and solid state physical (excimer laser) processes to incorporate silver nanoparticles in single and multiwall carbon nanotubes. We show high resolution TEM as evidence for the formation of very long ( ∼ 100–150 nm) silver quantum wires within SWCNT, and their properties are probed using various analytical techniques. The variation of the silver intercalated nanotube percentage and yield are compared for the SW‐ and MW‐CNTs, when using wet chemistry versus physical processes.

The observation of field induced electron emission from room temperature grown carbon nanofibers at low (5 V/mum) macroscopic electric fields is reported. The nanofibers were deposited using methane as a source gas in a conventional rf plasma enhanced chemical vapor deposition reactor using a Ni metal catalyst previously subjected to an Ar plasma treatment. Analysis of the scanning electron microscopy images of the nanofibers show them to possess an average diameter of 300 nm and that the nanofibers are observed to be radially dispersed over an area of 50 mum in diameter. No evidence of hysteresis in the current-voltage characteristic or conditioning of the emitters is observed. The mechanism for emission at low fields is attributed to field enhancement at the tips rather than from the surrounding amorphous carbon film which is shown to have a higher threshold field (20 V/mum) for emission.

Pulse laser ablation and subsequent laser nanostructuring at room temperature has been employed to produce nanostructured Ni on SiO2/Si substrates for catalytic growth of carbon nanotubes. The resultant nanostructured surface is seen to consist of nanometer sized hemispherical droplets whose mean diameter is controlled by the initial metal thickness, which in turn is readily controlled by the number of laser pulses. Vertically aligned multiwall carbon nanotube mats were then grown using conventional plasma enhanced chemical vapor deposition. We show that within a single processing technique it is possible to produce the initial metal-on-oxide thin film to a chosen thickness but also to be able to alter the morphology of the film to desired specifications at low macroscopic temperatures using the laser parameters. The influence of the underlying oxide is also explored to explain the mechanism of nanostructuring of the Ni catalyst. (C) 2004 American Institute of Physics.

Traditionally, the emission of electrons from materials have been explained using either the Fowler–Nordheim emission mechanism where high electric fields are used to extract electrons from surfaces or using conventional thermal emission where high currents are used to “boil” off electrons to vacuum. In this letter, we propose an alternative mechanism for electron emission from highly compressive thin films based on stress-induced “band structure” modification of nano-ordered sp2 regions in the thin films. Experimental results are recorded which show that the localized compressive stress governs electron emission in the amorphous carbon thin films studied here rather than the surface nanostructures/features or the diamond-like sp3 hybridized bond component. This analysis is in agreement with the concept of an internal or nongeometric field enhancement from sp2 nanostructures giving rise to high dielectric inhomogeneity within the carbon thin film. The results presented could be extended to explain the ano...

In this work, Ag–SiO2 nanocomposite layers were synthesized by introducing Ag nanoclusters into thermally oxidized SiO2 layers, using ion implantation. The field-emission (FE) properties of these layers were studied and correlated with the results from atomic force microscopy and transmission electron microscopy measurements. These nanocomposites exhibit good FE properties and give an emission current of 1nA at electric fields as low as 13V∕μm, for a dose of 5×1016Ag+∕cm2, compared with 204V∕μm for “bare” SiO2 layers. It is clearly demonstrated that the good FE properties of these nanocomposites are attributed to two types of local-field enhancement: one due to the surface morphology and the other due to electrical inhomogeneity. The isolated conductive Ag nanoclusters embedded in the electrically insulating SiO2 matrix provide a field enhancement due to the electrical inhomogeneity effect. Moreover, the implanted Ag ions diffuse to the surface, during the implantation process, and create dense surface-pr...

Chemical vapor-synthesized carbon nanotubes are typically grown at temperatures around 600 °C. We report on the deployment of a titanium layer to help elevate the constraints on the substrate temperature during plasma-assisted growth. The growth is possible through the lowering of the hydrocarbon content used in the deposition, with the only source of heat provided by the plasma. The nanotubes synthesized have a small diameter distribution, which deviates from the usual trend that the diameter is determined by the thickness of the catalyst film. Simple thermodynamic simulations also show that the quantity of heat, that can be distributed, is determined by the thickness of the titanium layer. Despite the lower synthesis temperature, it is shown that this technique allows for high growth rates as well as better quality nanotubes. © 2005 American Institute of Physics.

Diamond-like carbon (DLC) films have always had high intrinsic stress due to their metastable structure and the fine balance between film density and bond stability. We show the effects of high intrinsic stress on the electron field emission performance, where a lower electric field for emission is recorded with increasing stress in the DLC films. In addition to examining “as deposited” films with different magnitudes of intrinsic stress, we subject the DLC films to external pressure by physically bending the a-C∕silicon substrates. The result is a phenomenon where electrons are “squeezed” out of the films, and can be applied to the fabrication of stress sensors.

Vertically aligned carbon nanofibers (VACNFs) were synthesized on Ni-coated Si substrates using a dc plasma-enhanced chemical-vapor deposition system. The size of the Ni islands used as catalyst to grow the VACNFs was formed by both thermal annealing and laser processing on thin metal layers. It was observed that the diameter of the carbon nanofibers is strongly dependent on the initial Ni island dimension. By varying the laser power from 228 to 279mJ∕cm2, the size of these Ni islands can be controlled independent of the initial Ni film thickness. Electron field-emission results show that the emission threshold field is dependent on both the height and radius of these VACNFs and also field shielding effects. Threshold fields as low as 2V∕μm was obtained from the sample with the largest height over radius ratio.

Field emission properties of carbon nanotubes under mechanical stress have been investigated. The emission threshold fields initially decrease from 2.3to0.6V∕μm before rising back to 3.1V∕μm with increasing mechanical stress applied externally to the film. This behavior from nanotube composites has not been reported and is believed to be associated with modification to the work function of the nanotubes. This work suggests a possible application for these composite films as electromechanical high power switches.

The electron field emission properties of highly graphite like (sp2 rich) amorphous carbon films have been investigated. These films were prepared by dual ion beam-assisted deposition technique, where the assisting energies were varied from 0 to 800 eV. Threshold fields as low as 8 V/μm is observed at an assisting energy of 400 eV, which is comparable to the best threshold fields observed in high sp3 carbon films. Surface nanostructures are found on these films during growth, but are thought not to be the primary reason for the observed low threshold fields. The combination of a highly graphite-like structure with a high intrinsic compressive stress and a high local (electronic) density, obtained from x-ray photoelectron spectroscopy, is identified as the source for the field enhancement. The controllable stress is thought to modify the band structures of the graphite-like sp2 rich component in the films, which results in high dielectric inhomogeneity. This analysis is in agreement with the concept of an ...

We compare the field emission characteristics of multiwalled carbon nanotubes (MWNTs) and boron-doped MWNTs mixed in a polystyrene matrix. The concentration of the nanotubes was varied by weight ratio to that of the polystyrene (PS). Results indicate that the threshold fields are mainly controlled by the electric-field screening effect on the surface due to the high emission current densities measured. The lowest threshold field of 1.6 V/μm was observed from a MWNT-PS ratio of 11.1 wt %. Emission current densities, together with the threshold fields for these films, show that the emission properties are comparable to those of vertically aligned CNTs as well as arrays of carbon fibers. As the procedure to prepare these nonaligned CNT-PS composites is simple, it is proposed that the methodology could be a means of producing large area cathodes suitable for use in flat panel displays.

The mechanism of electron field emission from different forms of amorphous carbon (a-C) thin films is discussed. We show that it is possible to explain electron emission from a variety of amorphous carbon films by understanding the nature of electron states near the Fermi level. The films can be described as consisting of conductive sp2 C clusters lying within a more insulating sp3 C matrix. We show that the trend in the threshold field for emission can be explained in terms of improvements in the connectivity between these sp2 clusters and that cluster concentration and size can be in turn controlled by the choice of deposition conditions. The presence of the dielectric inhomogeneity between the two regions of sp2 clusters and sp3 C matrix is also shown to be very important in understanding the apparently low barrier heights that appear in the analysis of the current-voltage emission characteristics using the Fowler–Nordheim theory. This article attempts to set the framework for a unified model for elect...

The field emission properties of amorphous hydrogenated carbon (a-C:H) films and nitrogenated a-C:H (a-C:H:N) films subjected to 10 keV Co60 ion implantation are investigated as a function of ion dose. The average threshold electric field for conditioned nitrogenated and non-nitrogenated unimplanted films is found to be 27 and 29 V/μm. Implantation of C60 ions to a dose of 7.5×1013 cm−2 results in an increase in the threshold field of both the nitrogenated and non-nitrogenated films. However, implantation to a dose of 1.25×1014 cm−2 results in a reduction in the threshold fields to values close to those found in the unimplanted samples. At larger doses of 2.65×1014 cm−2, the threshold field for the non-nitrogenated samples remain largely unchanged though there is an increase in the refractive index consistent with an increase in the optical density of the film. In the nitrogenated film, the average threshold field at the highest dose again increases to values comparable to those found for C60 implantation...

Carbon nanotubes and nanofibres are typically synthesised under substrate temperatures above 600°C. Here we investigate the influence of the substrate temperature and the plasma conditions on the growth of vertically aligned carbon nanostructures using Direct Current plasma Chemical Vapour Deposition, at temperatures below 550°C. These nanostructures are produced using a C2H2 based plasma and nickel thin film as the catalyst. We found that preferential deposition of amorphous carbon takes place as the synthesis temperature is lowered below 500°C. However, lowering the carbon concentration in the gas feedstock (2. It was found that by varying the applied plasma bias during high temperature synthesis, we increased the growth rate up to 165 nm/min. Based on the observations of experimental process variations and the morphology of the synthesised structures, we propose a growth mechanism for such low temperature growth and examine the resulting morphology changes.