In recent years, near-nano (submicron) and nanostructured materials have attracted increasingly more attention from the materials community. Nanocrystalline materials are characterized by a microstructural length or grain size of up to about 100 nm. Materials having grain size of ∼0.1 to 0.3 μm are classified as submicron materials. Nanocrystalline materials exhibit various shapes or forms, and possess unique chemical, physical or mechanical properties. When the grain size is below a critical value (∼10–20 nm), more than 50 vol.% of atoms is associated with grain boundaries or interfacial boundaries. In this respect, dislocation pile-ups cannot form, and the Hall–Petch relationship for conventional coarse-grained materials is no longer valid. Therefore, grain boundaries play a major role in the deformation of nanocrystalline materials. Nanocrystalline materials exhibit creep and super plasticity at lower temperatures than conventional micro-grained counterparts. Similarly, plastic deformation of nanocrystalline coatings is considered to be associated with grain boundary sliding assisted by grain boundary diffusion or rotation. In this review paper, current developments in fabrication, microstructure, physical and mechanical properties of nanocrystalline materials and coatings will be addressed. Particular attention is paid to the properties of transition metal nitride nanocrystalline films formed by ion beam assisted deposition process.

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Nanocrystalline materials and coatings

Field emission (FE) is based on the physical phenomenon of quantum tunneling, in which electrons are injected from the surface of materials into vacuum under the influence of an applied electric field. A variety of field emission cold cathode materials have been developed to date. In this review, we shall focus on several kinds of novel cold cathode materials that have been developed in the past decade. These include materials for microfabricated field-emitter arrays, diamond and related films, carbon nanotubes, other quasi one-dimensional nanomaterials and printable composite materials. In addition, cold cathode materials have a wide range of applications such as in flat panel displays, high-power vacuum electronic devices, microwave-generation devices, vacuum microelectronic devices and vacuum nanoelectronic devices. Applications are in consumer goods, military industries and also space technology. A comprehensive overview of the various applications is presented. Recently, recognizing the strong possibility that vacuum nanoelectronic devices using quasi one-dimensional nanomaterials, such as carbon nanotubes may emit electrons with driving voltages comparable to that of a solid-state device, there is a growing interest in novel applications of such devices. With such exciting opportunities, there is now a flurry of activities to explore applications far beyond those considered for the conventional hot cathodes that operate on thermionic emission. We shall discuss the details of a number of fascinating potential applications.

Novel cold cathode materials and applications

Recent progress in synthesis, properties and potential applications of SiC nanomaterials

Low-dimensional SiC nanostructures: Fabrication, luminescence, and electrical properties

There are a variety of methods for synthesizing or fabricating one-dimensional (1D) nanostructures containing heterojunctions between different materials. Here we review recent developments in the synthesis and fabrication of heterojunctions formed between different materials within the same 1D nanostructure or between different 1D nanostructures composed of different materials. Structures containing 1D nanoscale heterojunctions exhibit interesting chemistry as well as size, shape, and material-dependent properties that are unique when compared to single-component materials. This leads to new or enhanced properties or multifunctionality useful for a variety of applications in electronics, photonics, catalysis, and sensing, for example. This review separates the methods into vapor-phase synthesis, solution-phase synthesis, template-based synthesis, and other approaches, such as lithography, electrospinning, and assembly. These methods are used to form a variety of heterojunctions, including segmented, core/shell, branched, or crossed, from different combinations of semiconductor, metal, carbon, and polymeric materials.

The synthesis and fabrication of one-dimensional nanoscale heterojunctions.

Silicon carbide (SiC) nanowires on a silicon substrate were prepared using hot-filament-assisted chemical-vapor deposition with a solid silicon and carbon source. The SiC nanowires show good field-emitting properties as revealed by the current–voltage characteristics. Together with its ease of preparation, these SiC nanowires are shown to have great potential in the area of electron field-emitting devices.

Field-emission characteristics of SiC nanowires prepared by chemical-vapor deposition

Thin β-SiC nanorods and their field emission properties

Straight beta-silicon carbide nanorods have been grown on silicon wafers using hot filament chemical vapor deposition with iron particles as catalyst. A plate made of a C–Si–SiO2 powder mixture was used as carbon and silicon sources. Hydrogen, which was the only gas fed into the deposition system, acts both as a reactant and as a mass transporting medium. The diameter of the β-SiC nanorod ranged from 20 to 70 nm, while its length was approximately 1 μm. A growth mechanism of beta-silicon carbide nanorods was proposed. The field emission properties of the beta-silicon carbide nanorods grown on the silicon substrate are also reported.

Straight β-SiC nanorods synthesized by using C–Si–SiO2

A very low-field emission was achieved from aligned and opened carbon nanotube arrays. Field emission current densities of 10 microamperes per square centimeter were observed at applied fields of 0.6-1 V/mum, and current densities of 10 mA/cm(2) have been realized at applied fields as low as 2-2.7 V/mum. These fields are more than 2 times lower than those previously obtained for carbon nanotubes, and also represent the lowest field ever reported for any field emitting arrays at the same current densities, indicating that carbon nanotubes are superior field emitters.

Very low-field emission from aligned and opened carbon nanotube arrays

A one step procedure has been developed to grow beta-silicon carbide (β-SiC) nanorods from a solid carbon and silicon source on silicon substrates using a hot filament chemical vapor deposition. The growth process was catalyzed by the impurities of metallic particles confined in the solid source plate made of a mixture of graphite and silicon powders pressed at 150°C. Hydrogen was introduced into a reaction chamber to react with the solid source. The resulting process produced, hydrocarbon and hydrosilicon radicals, which subsequently reacted on the Si substrate surface and presumably formed SiC nanorods. The nanorods consisted of a crystalline β-SiC core with an amorphous silicon oxide shell layer. The nanorods were 10–30 nm in diameter and less than 1 μm in length. Field emission characteristics of the β-SiC nanorods were investigated using current-voltage measurements and the Fowler–Nordheim equation. The silicon carbide nanorods exhibited high electron field emission with high stability. Along with the ease of preparation, these silicon carbide nanorods are believed to have potential application in electron field emitting devices.

H. L. Lai

Papers

Field-emission characteristics of SiC nanowires prepared by chemical-vapor deposition

Thin β-SiC nanorods and their field emission properties

Straight β-SiC nanorods synthesized by using C–Si–SiO2

Very low-field emission from aligned and opened carbon nanotube arrays

Growth and emission properties of β-SiC nanorods