Nanostructured materials: basic concepts and microstructure☆

Laser ablation in liquids : Applications in the synthesis of nanocrystals

▪ Abstract The synthesis of nanocrystalline diamond films from carbon-containing noble gas plasmas is described. The nanocrystallinity is the result of new growth and nucleation mechanisms, which involve the insertion of C2, carbon dimer, into carbon-carbon and carbon-hydrogen bonds, resulting in hetereogeneous nucleation rates on the order 1010 cm−2 s−1. Extensive characterization studies led to the conclusion that phase-pure diamond is produced with a microstructure consisting of randomly oriented 3–15-nm crystallites. By adjusting the noble gas/hydrogen ratio in the gas mixture, a continuous transition from micro- to nanocrystallinity is achieved. Up to 10% of the total carbon in the nanocrystalline films is located at 2 to 4 atom-wide grain boundaries. Because the grain boundary carbon is π-bonded, the mechanical, electrical, and optical properties of nanocrystalline diamond are profoundly altered. Nanocrystalline diamond films are unique new materials with applications in fields as diverse as tribolo...

Nanocrystalline diamond films1

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respect, dislocation pile-ups cannot form, and the Hall–Petch relationship for conventional coarse-grained materials is no longer valid.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

Ultrananocrystalline diamond (UNCD) films, grown using microwave plasma-enhanced chemical vapor deposition with gas mixtures of Ar–1%CH4 or Ar–1%CH4–5%H2, have been examined with transmission electron microscopy (TEM). The films consist of equiaxed nanograins (2–10 nm in diameter) and elongated twinned dendritic grains. The area occupied by dendritic grains increases with the addition of H2. High resolution electron microscopy shows no evidence of an amorphous phase at grain boundaries, which are typically one or two atomic layer thick (0.2–0.4 nm). Cross-section TEM reveals a noncolumnar structure of the films. The initial nucleation of diamond occurs directly on the Si substrate when H2 is present in the plasma. For the case of UNCD growth from a plasma without addition of H2, the initial nucleation occurs on an amorphous carbon layer about 10–15 nm thick directly grown on the Si substrate. This result indicates that hydrogen plays a critical role in determining the nucleation interface between the UNCD...

Microstructure of ultrananocrystalline diamond films grown by microwave Ar–CH4 plasma chemical vapor deposition with or without added H2

Core‐level photoabsorption has been used to determine the sp2 and sp3 bonding content of nanocrystalline diamond thin films grown using C60 or CH4 precursors. The C(1s) absorption spectra show clear bulk diamond excitonic and sp3 features with little evidence of sp2 bonding, while the Raman spectra measured from these same films are ambiguous and indeterminate. This result can be attributed to the local structure (near‐neighbor bonding) sensitivity of core‐level photoabsorption that is insensitive to domain size, unlike Raman spectroscopy.

Characterization of nanocrystalline diamond films by core‐level photoabsorption

https://digital.library.unt.edu/ark:/67531/metadc706864/m2/1/high_res_d/71703.pdf

Physical and tribological properties of diamond films grown in argoncarbon plasmas

Friction and wear properties of smooth diamond films grown in fullerene + argon plasmas☆

In this study, we explored and compared the friction and wear performance of smooth (30 nm, root mean square) and nanocrystalline diamond films (average grain size ≈ 15 nm) grown in an Ar-C60 microwave, plasma with the friction and wear performance of laserpolished microcrystalline diamond films (grain size ≈ 20–50 μm) grown in a conventional CH4-H2 microwave plasma. Tribological tests were run under 2-N load in open air (30–50% relative humidity) and in dry N2. The frictional performances of both the as-grown smooth and laser polished diamond films were comparable. They both afforded very low friction coefficients (0.06–0.15) and low wear rates (2–6 × 10−7 mm3/N.m) to counterface Si3N4 balls. We used Raman spectroscopy, electron microscopy, and atomic force microscopy to ascertain the structural, chemical, and surface topographical characteristics of the films and correlated the results with the friction and wear mechanisms of diamond films in open air and dry N2.

D. M. Gruen

Papers

Microstructure of ultrananocrystalline diamond films grown by microwave Ar–CH4 plasma chemical vapor deposition with or without added H2

Characterization of nanocrystalline diamond films by core‐level photoabsorption

Physical and tribological properties of diamond films grown in argoncarbon plasmas

Friction and wear properties of smooth diamond films grown in fullerene + argon plasmas☆

Durability and tribological performance of smooth diamond films produced by Ar-C60 microwave plasmas and by laser polishing