Diamond may be grown at low pressures where it is the metastable form of carbon. Recent advances in a wide variety of plasma and electrical discharge methods have led to dramatic increases in growth rates. All of these methods have certain aspects in common, namely, the presence of atomic hydrogen and the production of energetic carbon-containing fragments under conditions that support high mobilities on the diamond surface. Some understanding of the processes taking place during nucleation and growth of diamond has been achieved, but detailed molecular mechanisms are not yet known. Related research has led to the discovery of a new class of materials, the "diamondlike" phases. Vapor-grown diamond and diamondlike materials may have eventual applications in abrasives, tool coatings, bearing surfaces, electronics, optics, tribological surfaces, and corrosion protection.

https://science.sciencemag.org/content/sci/241/4868/913.full.pdf

Low-Pressure, Metastable Growth of Diamond and "Diamondlike" Phases

The controlled synthesis of materials by methods that permit their assembly into functional nanoscale structures lies at the crux of the emerging field of nanotechnology. Although only one of several materials families is of interest, carbon-based nanostructured materials continue to attract a disproportionate share of research effort, in part because of their wide-ranging properties. Additionally, developments of the past decade in the controlled synthesis of carbon nanotubes and nanofibers have opened additional possibilities for their use as functional elements in numerous applications. Vertically aligned carbon nanofibers (VACNFs) are a subclass of carbon nanostructured materials that can be produced with a high degree of control using catalytic plasma-enhanced chemical-vapor deposition (C-PECVD). Using C-PECVD the location, diameter, length, shape, chemical composition, and orientation can be controlled during VACNF synthesis. Here we review the CVD and PECVD systems, growth control mechanisms, catalyst preparation, resultant carbon nanostructures, and VACNF properties. This is followed by a review of many of the application areas for carbon nanotubes and nanofibers including electron field-emission sources, electrochemical probes, functionalized sensor elements, scanning probe microscopy tips, nanoelectromechanical systems (NEMS), hydrogen and charge storage, and catalyst support. We end by noting gaps in the understanding of VACNF growth mechanisms and the challenges remaining in the development of methods for an even more comprehensive control of the carbon nanofiber synthesis process.

Vertically Aligned Carbon Nanofibers and Related Structures: Controlled Synthesis and Directed Assembly

The unique plasma-specific features and physical phenomena in the organization of nanoscale soild-state systems in a broad range of elemental composition, structure, and dimensionality are critically reviewed. These effects lead to the possibility to localize and control energy and matter at nanoscales and to produce self-organized nano-solids with highly unusual and superior properties. A unifying conceptual framework based on the control of production, transport, and self-organization of precursor species is introduced and a variety of plasma-specific non-equilibrium and kinetics-driven phenomena across the many temporal and spatial scales is explained. When the plasma is localized to micrometer and nanometer dimensions, new emergent phenomena arise. The examples range from semiconducting quantum dots and nanowires, chirality control of single-walled carbon nanotubes, ultra-fine manipulation of graphenes, nano-diamond, and organic matter to nano-plasma effects and nano-plasmas of different states of matter.

Plasma nanoscience: from nano-solids in plasmas to nano-plasmas in solids

The unique plasma-specific features and physical phenomena in the organization of nanoscale solid-state systems in a broad range of elemental composition, structure, and dimensionality are critically reviewed. These effects lead to the possibility to localize and control energy and matter at nanoscales and to produce self-organized nano-solids with highly unusual and superior properties. A unifying conceptual framework based on the control of production, transport, and self-organization of precursor species is introduced and a variety of plasma-specific non-equilibrium and kinetics-driven phenomena across the many temporal and spatial scales is explained. When the plasma is localized to micrometer and nanometer dimensions, new emergent phenomena arise. The examples range from semiconducting quantum dots and nanowires, chirality control of single-walled carbon nanotubes, ultra-fine manipulation of graphenes, nano-diamond, and organic matter, to nano-plasma effects and nano-plasmas of different states of matter.

Plasma Nanoscience: from Nano-Solids in Plasmas to Nano-Plasmas in Solids

A mechanism for carbon nanosheet formation

To examine the mechanism of film deposition in a planar rf CH4 discharge plasma, measurements were made of the spatial distributions of the deposition rate and optical emission intensity along the discharge axis between parallel electrodes. Optical‐absorption properties of the deposited carbon films were also measured over both the infrared and visible regions. To measure the spatial deposition rates, the substrate surface was elevated from the cathode electrode with the use of quartz glass plates. It was found that the spatial properties of films, which were deposited in both the ion‐sheath and bulk‐plasma regions, differ markedly from each other. The carbon films obtained from within the ion‐sheath region were found to be extremely hard, while those obtained in the bulk‐plasma region were polymerlike soft films. This disparity was thought to be due to the difference in the kinetic energy of the ions bombarding the substrate surface; that is, the substrate surface potential could be changed by elevating the substrate surface. These results were incorporated in the discussion of the deposition mechanism, with emphasis on the contribution of ion bombardment to the film‐deposition process. It was tentatively concluded that the film‐deposition rate was predominantly dependent on the product of the ion kinetic energy and ion flux density that reached the substrate surface.

Deposition mechanism of hydrogenated hard‐carbon films in a CH4 rf discharge plasma

The radio‐frequency glow discharges of several kinds of gases were examined to measure the ion sheath thickness at the cathode electrode. At intermediate gas pressures around 0.05– 0.5 Torr, the sheath thickness d depends on the pressure P in the expression P1/2d=K0 for almost all of the discharges examined. It was also pointed out that the constant K0 value decreased linearly against a mass of the predominant ion in the plasma. The discrepancy between the sheath thickness measured in this work and by theoretical solution was discussed for the argon discharge.

Plasma sheath thickness in radio-frequency discharges

Hard carbon films were produced by the rf glow discharge plasma decomposition of hydrocarbon gases diluted with H2 gas at relatively low temperature and low pressure. Optical emission spectra were measured during the deposition of the carbon films, and the correlation between the carbon film’s properties and the plasma’s condition was examined. From these results, H radicals were considered to contribute to the formation of hard carbon films. These were examined by transmission electron microscopy, and were found to be predominantly amorphous. The existence of diamond grains was confirmed in the amorphous phase. A relatively large diamond grain in the order of 10 μm was obtained under the conditions of a gas mixture ratio of 70% and a gas pressure of 5×10−3 Torr.

Deposition of hard carbon films by rf glow discharge method

The optical emission profile (OEP) between anode and cathode electrodes is examined for monitoring planar rf discharges of H2, N2, and Ar gases. Form the OEP we can easily find the boundary of the ion sheath and the plasma region and accordingly the ion sheath width at the cathode electrode. The plasma parameters are measured by the Langmuir probe method, and the relative mean electron energy Te in H2 and N2 plasmas is found to be evaluated by the ion sheath width ls with a relationship Te∝ls. The plasma density can also be estimated by both the cathode negative potential Vc and ls. A charge‐coupled device (CCD) image sensor with a 105‐mm macro lens is employed to measure the OEP within a very short time. Although the CCD measurement shows a somewhat deviated OEP, this is very useful to measure the precise ion sheath width.

Yoshio Machi

Papers

Deposition mechanism of hydrogenated hard‐carbon films in a CH4 rf discharge plasma

Plasma sheath thickness in radio-frequency discharges

Deposition of hard carbon films by rf glow discharge method

Monitoring of radio‐frequency glow‐discharge plasma

The analyses of an SiF4 plasma in an R.F. glow discharge for preparing fluorinated amorphous silicon thin films