Showing papers on "Field electron emission published in 2000"

Scanning field emission from patterned carbon nanotube films

Abstract: The investigation of the field emission (FE) properties of carbon nanotube (CNT) films by a scanning anode FE apparatus, reveals a strong dependence on the density and morphology of the CNT deposit. Large differences between the microscopic and macroscopic current and emission site densities are observed, and explained in terms of a variation of the field enhancement factor β. As a consequence, the emitted current density can be optimized by tuning the density of CNTs. Films with medium densities (on the order of 107 emitters/cm2, according to electrostatic calculations) show the highest emitted current densities.

Current saturation mechanisms in carbon nanotube field emitters

Abstract: Recent studies have shown a current limiting effect in the field emission behavior of carbon nanotubes. In this letter, we demonstrate that an individual nanotube exhibits current saturation above 100 nA of emission current, and we show that this current saturation is a direct result of an adsorbate-enhanced field emission mechanism. Current saturation results from the displacement of adsorbates from configurations of tunneling enhancement as electric field and current are increased. Saturation is concurrent with rapid fluctuations in emission current and distinctive changes in the field emission patterns. At high fields, the adsorbate states are completely removed from the nanotube. A single, clean single-walled nanotube shows no evidence of current saturation for emission currents reaching 2 μA.

Field emission from well-aligned, patterned, carbon nanotube emitters

Abstract: We have developed a process to fabricate well-aligned, patterned, carbon nanotube field emitters on glass substrates. The process consists of depositing and patterning a nickel-based metal line on the glass substrate followed by a bias-enhanced microwave plasma chemical vapor deposition to grow carbon nanotube emitters. A turn-on field of 1.2 V/μm, and emission currents of 1 mA/cm2 at 3 V/μm were achieved on well-aligned carbon nanotube emitters. A test of cathode-ray tube lighting elements now underway suggests a lifetime of exceeding 10 000 h.

Characterizing the Structure and Properties of Individual Wire-Like Nanoentities

Abstract: A new approach to the characterization of the mechanical and electrical properties of individual nanowires and nanotubes is demonstrated by in-situ transmission electron microscopy (TEM). The technique allows a one-to-one correlation between the structure and properties of the nanowires. Recent developments include the determination of the Young’s modulii of carbon nanotubes and semiconductor nanowires, femtogram nanobalance of a single fine particle, field emission of carbon nanotubes, and quantum ballistic conductance in carbon nanotubes.

Field emission properties of carbon nanotubes

Abstract: We have investigated the field emission properties of nanotube thin films deposited by a plasma enhanced chemical vapor deposition process from 2% CH4 in H2 atmosphere Depending on the deposition of the metallic catalyst [Fe(NO3)3 in an ethanol solution or sputtered Ni] the nanotube films showed a nested or continuous dense distribution of tubes The films consisted of multiwalled nanotubes (MWNTs) with diameters ranging from 40 down to 5 nm, with a large fraction of the tubes having open ends The nanotube thin film emitters showed a turn-on field of less than 2 V μm−1 for an emission current of 1 nA An emission site density of 10 000 emitters per cm−2 is achieved at fields around 4 V μm−1 The emission spots, observed on a phosphorous screen, show various irregular structures, which we attribute to open ended tubes A combined measurement of the field emitted electron energy distribution (FEED) and the current-voltage characteristic allowed us to determine the work function at the field emission site

In situ-grown carbon nanotube array with excellent field emission characteristics

Abstract: In situ catalytic thermal decomposition method was used for producing aligned multiwalled carbon nanotubes (MWNTs) in bulk quantities on stable and electrically conducting substrates. Very low turn-on electric fields of 0.75 V/μm and low threshold fields of ∼1.6 V/μm (for current density of 10 mA/cm2) were obtained from the MWNT arrays grown on TiN substrate. Furthermore, large emission current densities of 1–3 A/cm2 were obtained at reasonably low fields of less than ∼8 V/μm. These enhanced emission properties are tentatively attributed to the oriented and high-density nature of the emitting carbon nanotube structure and the high-conductivity, stable nature of the TiN substrate onto which the nanotubes are attached.

Field emission of different oriented carbon nanotubes

Abstract: Field emission data from aligned high-density carbon nanotubes (CNTs) with orientations parallel, 45°, and perpendicular to the substrate have been obtained The large-area uniformly distributed CNTs were synthesized on smooth nickel substrates via dc plasma-assisted hot filament chemical vapor deposition CNTs with diameters in the range of 100–200 nm were employed in this study The different orientations were obtained by changing the angle between the substrate and the electrical field direction The growth mechanism for the alignment and orientation control of CNTs has been discussed The CNTs oriented parallel to the substrate have a lower onset applied field than those oriented perpendicular to the substrate This result indicates that electrons can emit from the body of the CNT, which means that the CNT can be used as a linear emitter The small radius of the tube wall and the existence of defects are suggested as the reasons for the emission of electrons from the body of the tubes

Structures with high number density of carbon nanotubes and 3-dimensional distribution

Abstract: A composite is described having a three dimensional distribution of carbon nanotubes. The critical aspect of such composites is a nonwoven network of randomly oriented fibers connected at their junctions to afford macropores in the spaces between the fibers. A variety of fibers may be employed, including metallic fibers, and especially nickel fibers. The composite has quite desirable properties for cold field electron emission applications, such as a relatively low turn-on electric field, high electric field enhancement factors, and high current densities. The composites of this invention also show favorable properties for other an electrode applications. Several methods, which also have general application in carbon nanotube production, of preparing these composites are described and employ a liquid feedstock of oxyhydrocarbons as carbon nanotube precursors.

Effect of sp2-phase nanostructure on field emission from amorphous carbons

Abstract: Electron field emission from amorphous carbon is found to depend on the clustering of the sp2 phase. The size of the sp2 phase is varied by thermal annealing and it dominates the effect of other parameters, such as chemical composition, surface termination, sp3 content, or conductivity. The optimum size of the sp2 phase is determined by Raman spectroscopy and is of the order of 1 nm. The field emission originates from the sp2 regions and is facilitated by the large field enhancement from more conductive sp2 clusters in an insulating sp3 matrix.

Insights into the structure of BN nanotubes

Abstract: The following features of multiwalled BN nanotubes were discovered using a field emission high-resolution analytical transmission electron microscope: (i) coexistence of hexagonal and rhombohedral stacking in nanotube shell assembly; (ii) flattening of nanotube cross section, which makes possible clear atomic resolution of the core structure in a three-shelled nanotube; and (iii) change in chirality of tubular layers from armchair to zigzag arrangement in a 30° double-walled nanotube kink, as revealed by atomically resolved images of tube wall segments.

Semiconductor nanowires: synthesis, structure and properties

Abstract: Highly pure, ultra long and uniform-sized semiconductor nanowires in bulk-quantity have been synthesized by novel methods of laser ablation and thermal evaporation of semiconductor powders mixed with metal or oxide catalysts. Transmission electron microscopic study shows that decomposition of semiconductor sub-oxides and the defect structure play an important role in enhancing the formation and growth of high-quality semiconductor nanowires. The morphology, microstructure, optical and electrical properties of the nanowires have been characterized systematically by Raman scattering, photoluminescence and field emission. A new growth mechanism, namely oxide-assisted growth, is proposed based on the microstructure and different morphologies of the nanowires observed.

Application of carbon nanotubes as electrodes in gas discharge tubes

Abstract: turn-on fields. Recent experiments have reported turn-on electric fields in the range of 1.5‐3 V/mm. 3‐5 The nanotubes emitters, especially the SWNTs, are capable of producing stable electron emission with a current density exceeding 4 A/cm 2 ~Ref. 5! which make them attractive cold-cathode materials for microvacuum electronic applications. Assynthesized SWNTs are in the form of either free-standing mat or powder, unsuitable for device applications. We have processed the raw materials to uniform films by a spray method. 6 Adhesion between the substrates and the films is enhanced by introducing either a carbon-dissolving or a carbide-forming metal interlayer. In this letter, we report the effects of carbon nanotube coating on the performance of the gas discharge tubes. The direct current ~dc! breakdown voltages of GDTs fabricated using SWNT-coated electrodes were measured and compared with commercial GDTs. A significant reduction in the breakdown voltage and voltage fluctuation ~over 1000 surges! was observed for the nanotubebased GDTs as compared to typical commercial devices.

Role of the localized states in field emission of carbon nanotubes

Abstract: We have performedab initio pseudopotential electronic structure calculations for various edge geometries of the (n,n) single-wall nanotube with or without applied fields. Among the systems studied, the one with a zigzag edge exposed by a slant cut is found to be the most favorable for emission due to the existence of unpaired dangling bond states around the Fermi level. The next most favorable geometry is the capped nanotube wherep-bonding states localized at the cap and pointing in the tube axis direction occur at the Fermi level. A scaling rule for the induced field linear in the aspect ratio of the tube is also obtained.

New mechanism for electron emission from planar cold cathodes: the solid-state field-controlled electron emitter

Abstract: A new mechanism for electron emission from planar cathodes is described The theoretical analysis shows that, with an ultrathin wide band-gap semiconductor layer (UTSC) on a metal, the surface barrier is lowered to approximately 01 eV due to the creation of a space charge induced by the electrons injected from the metal The barrier height depends mostly on the UTSC thickness and not on the state of the surface, as in thermionic and field emissions This mechanism explains the measured stable emission at 300 K and 10(-7) Torr, with a threshold field of only approximately 50 V/mm, from these solid-state field-controlled emitters

Effect of work function and surface microstructure on field emission of tetrahedral amorphous carbon

Abstract: The work function of tetrahedral amorphous carbon (ta-C) has been measured by Kelvin probe to lie in the range 4–5 eV, irrespective of its sp3 content or nitrogen addition. This implies that the surface barrier to emission is dominant and that emission changes caused by sp3 bonding or nitrogen addition are not directly due to changes in work function. Hydrogen, oxygen, and argon plasma treatments are all found to increase the emission of a-C, but hydrogen and argon treatments are found to reduce the work function while oxygen treatment increases it. Detailed studies of the surface with varying plasma treatment conditions suggest that the changes in emission arise mainly from changes in the surface microstructure, such as the formation of sp2 regions within the sp3 bulk. The need for local field enhancement mechanisms to account for emission over the sizeable barrier is emphasized, which may arise from local chemical nonhomogeneity, or formation of nanometer-size sp2 clusters embedded in an sp3 matrix.

The role of outgassing in surface flashover under vacuum

Abstract: Results of high-speed electrical and optical diagnostics are used as a basis to discuss a new surface flashover model. Outgassing, caused by electron stimulated desorption, is found to play a crucial role in the temporal flashover development. Dielectric unipolar surface flashover under vacuum is experimentally characterized by a three-phase development, which covers a current range from 10/sup -4/ A to 100 A. Phase one comprises a fast (several nanoseconds) buildup of a saturated secondary electron avalanche reaching current levels of 10 to 100 mA. Phase two is associated with a slow current amplification reaching currents in the Ampere level within typically 100 ns. The final phase is characterized by a fast current rise up to the impedance-limited current on the order of 100 A. The development during phase two and three is described by a zero-dimensional model, where electron-induced outgassing leads to a Townsend-like gas discharge above the surface. This is supported by time-resolved spectroscopy that reveals the existence of excited atomic hydrogen and ionic carbon before the final phase. The feedback mechanism toward a self-sustained discharge is due to space charge leading to an enhanced field emission from the cathode. A priori unknown model parameters, such as outgassing rate and gas density buildup above the surface, are determined by fitting calculated results to experimental data. The significance of outgassing is also discussed with a view to microwave surface flashover.

Carbon nanotube field emission array and method for fabricating the same

Abstract: A field emission array (FEA) using carbon nanotubes having characteristics of low work function, durability and thermal stability, and a method for fabricating the same are provided. The field emission array uses carbon nanotubes as electron emission sources, thereby lowering a work function and dropping driving voltage. Accordingly, a device can be driven at low voltage. In addition, resistance to gases, which are generated during the operation of a device, is improved, thereby increasing the life span of an emitter. The method prints a mixed paste using extrusion or screen printing and performs sintering, thereby fusing carbon nanotubes such that the carbon nanotubes are aligned in a single direction.

Field Emission Patterns Originating from Pentagons at the Tip of a Carbon Nanotube

Abstract: Field emission of electrons from multiwall carbon nanotubes (MWCNTs) has been investigated in ultra-high vacuum by field emission microscopy (FEM). MWCNTs whose tips are capped by curved graphite layers give FEM patterns consisting of 6 bright pentagons, each of which has a dark region in its center. Interference fringes are also observed between adjacent pentagonal rings. The present result shows a direct evidence of the presence of 6 pentagons on a tip of a closed carbon nanotube.

Effect of surface morphology of Ni thin film on the growth of aligned carbon nanotubes by microwave plasma-enhanced chemical vapor deposition

Abstract: Aligned carbon nanotubes were synthesized on Ni-coated Si substrates using microwave plasma-enhanced chemical vapor deposition. The surface morphology of Ni thin films was varied with the rf power density during the rf magnetron sputtering process. It was found that the growth of carbon nanotubes was strongly influenced by the surface morphology of Ni thin film. Pure carbon nanotubes were synthesized on Ni thin film with uniformly distributed grain sizes, whereas large amounts of carbonaceous particles were produced in addition to the nanotubes, when the nanotubes were grown on Ni thin film with widely distributed grain sizes. With decreasing Ni-grain size, the diameter of nanotubes decreased and the length increased. High-resolution transmission electron microscope images clearly demonstrated the nanotubes to be multiwalled, and the graphitized structures were confirmed from the Raman spectra. Efficient field emission was observed from the diode structure with the nanotube tips.

Nanomeasurements of individual carbon nanotubes by in situ TEM

Abstract: Property characterization of nanomaterials is challenged by the small size of the structure because of the difficulties in manipulation. Here we demonstrate a novel approach that allows a direct measurement of the mechanical and electrical properties of individual nanotube-like structures by in situ transmission electron microscopy (TEM). The technique is powerful in a way that it can be directly correlated to the atomic-scale microstructure of the carbon nanotube with its physical properties, thus providing a complete characterization of the nanotube. Applications of the technique will be demonstrated in measurements of the mechanical properties, the electron field emission, and the ballistic quantum conductance of individual carbon nanotubes. A nanobalance technique is demonstrated that can be applied to measure the mass of a single tiny particle as light as 22 fg (1 f = 10 -15 ).

Synthesis and field-emission behavior of highly oriented boron carbonitride nanofibers

Abstract: Large-area highly oriented boron carbonitride (BCN) nanofibers with various compositions were synthesized directly on polished polycrystalline nickel substrates from a gas mixture of N2, H2, CH4, and B2H6 by bias-assisted hot-filament chemical-vapor deposition. The morphology of BCN nanofibers was examined by scanning electron microscopy, the nanofiber structure was studied by high-resolution transmission electron microscopy, and the chemical composition of individual nanofibers was determined by electron energy-loss spectroscopy. Field-emission behavior of the BCN nanofibers was characterized and a high emission current density of about 20–80 mA/cm2 at a low electric field of 5–6 V/μm implies a promising application as field-emission sources.

Synthesis and field-emission behavior of highly oriented boron carbonitride nanofibers

Abstract: Large-area highly oriented boron carbonitride (BCN) nanofibers with various compositions were synthesized directly on polished polycrystalline nickel substrates from a gas mixture of N-2, H-2, CH4, and B2H6 by bias-assisted hot-filament chemical-vapor deposition. The morphology of BCN nanofibers was examined by scanning electron microscopy, the nanofiber structure was studied by high-resolution transmission electron microscopy, and the chemical composition of individual nanofibers was determined by electron energy-loss spectroscopy. Field-emission behavior of the BCN nanofibers was characterized and a high emission current density of about 20-80 mA/cm(2) at a low electric field of 5-6 V/mu m implies a promising application as field-emission sources. (C) 2000 American Institute of Physics. [S0003-6951(00)04018-3].

Microstructure and its effect on field electron emission of grain-size-controlled nanocrystalline diamond films

Abstract: Nanocrystalline diamond films were grown by microwave plasma assisted chemical vapor deposition using N-2 and CH4 as precursors. The microstructure of the films such as the diamond grain size, graphite content, and N incorporation, was controlled by introducing a small amount of hydrogen gas (0-10 sccm) in the growth. Effects of the growth parameters on the film microstructure were investigated using transmission electron microscopy, x-ray diffraction, Raman spectroscopy, and secondary ion mass spectroscopy. A surface stabilizing model is suggested to explain the formation mechanism of the uniformly grain size-controlled nanocrystalline diamond. A systematic investigation on the film microstructure and their field electron emission (FEE) property is presented for various films of different diamond grain sizes and graphite contents. It was found that the FEE property highly depended on the diamond/graphite mixed phase structure. Novel field emission properties (1 V/mum emission threshold and 10 mA/cm(2) emission current) are obtained by optimizing the growth parameters. A transport-tunneling mechanism is applied to explain the experimental observations. Our results showed that nanocrystalline diamond film can be a very promising cold cathode material for field emission applications. (C) 2000 American Institute of Physics. [S0021-8979(00)07117-6].

Field emission from quasi-aligned SiCN nanorods

Abstract: We report on the preparation and field emission properties of quasi-aligned silicon carbon nitride (SiCN) nanorods. The SiCN nanorods are formed by using a two-stage growth method wherein the first stage involves formation of a buffer layer containing high density of nanocrystals by electron cyclotron resonance plasma enhanced chemical vapor deposition and the second stage involves using microwave plasma enhanced chemical vapor deposition for high growth rate along a preferred orientation. It should be noted that growth of the SiCN nanorods is self-mediated without the addition of any metal catalyst. Scanning electron microscopy shows that the SiCN nanorods are six-side-rod-shaped single crystals of about 1–1.5 μm in length and about 20–50 nm in diameter. Energy dispersive x-ray spectrometry shows that the nanorod contains about 26 at. % of Si, 50 at. % of C, and 24 at. % of N. Characteristic current–voltage measurements indicate a low turn-on field of 10 V/μm. Field emission current density in excess of ...