R. C. Hyer

Composites of polypyrrole and carbon black: Part III. Chemical synthesis and characterization

A new class of molecular composites of carbon black and an electronically conducting polymer, namely polypyrrole, has been synthesized by chemically polymerizing pyrrole in an aqueous dispersion of carbon black. The carbon black content of these composites can be varied from ∼5% to ∼85% (by weight). The surface area and density of these composites were compared to corresponding mixtures of carbon black and polypyrrole. The influence of carbon black on the efficiency of polymerization of pyrrole is described. The effect of carbon black content on the electronic conductivity of the composite has been mapped, and compared with the corresponding behavior of a mixture of carbon black and polyvinylchloride. The influence of the parent black characteristics (porosity, void volume, surface area) on the electronic conductivity of the resultant composite has been probed by comparing the behavior of composites derived from six commercial and experimental blacks. The temperature dependence of the composites has been studied as a function of the carbon black content. Finally, the application of these new materials is an environmental remediation scenario is demonstrated for Cr(vi) as a model pollutant.

Deposition of diamond films at low pressures and their characterization by positron annihilation, Raman, scanning electron microscopy, and x-ray photoelectron spectroscopy

We have deposited diamond films with micron‐size crystals on Si〈111〉 using low‐pressure hot‐filament‐assisted chemical vapor deposition. These films have been characterized by positron annihilation, Raman spectroscopy, scanning electron microscopy, and x‐ray photoelectron spectroscopy. In addition to the results for the electronic structure and morphology, we also present new results for the lattice defects present in these films.

Characterization of low pressure deposited diamond films by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy (XPS) has been utilized to study the electronic structure of carbon films grown by a low pressure hot filament assisted chemical vapor deposition technique with a view to optimizing the growth parameters for the diamond films. A constant flow of a mixture of high purity methane and hydrogen was maintained at a pressure of 25 Torr in the reaction chamber. The films were prepared at three different concentrations of methane: 0.25%, 0.5%, and 1.0% by density. The XPS C 1s core level and valence band spectra of these films are compared with those of graphite. Diamond has covalent sp 3 bonding while graphite has sp 2 bonding. XPS spectra exhibit features related to the difference in bonding. The plasmon loss shoulder (characteristic of graphite) associated with the main C 1s peak is found to be absent in the spectrum of films grown when the gas composition contains 0.25% methane, while it is found to build up with increasing concentration of methane. The binding energy position of the C 1s peak shows that appreciable charging occurs for the 0.25% methane concentration film. These results show that the films grown with 0.25% methane concentration closely resembled diamond. The valence band region for films grown using a gas composition consisting of 0.25% methane concentration shows considerable s-p mixing as compared with graphite. The core level and valence band results show that the film grown with a gas composition consisting of 0.5% methane concentration represents a composite of diamond and graphite. Scanning electron micrographs and Raman spectra support the conclusions drawn on the basis of XPS investigation.

Scanning tunneling microscopy of the electronic structure of chemical vapor deposited diamond films

This article discusses scanning tunneling microscopy of the electronic structure of chemical vapor deposited diamond films.

Scanning Tunneling Microscopy of the Structural and Electronic Properties of Chemical-Vapor Deposited Diamond Films

Scanning tunneling microscopy (STM) and spectroscopy have been used to characterize the structural and electronic properties of diamond films grown using hot tungsten filament and microwave plasma chemical vapor deposition. The hot-filament-grown films contained microcrystallites measuring 50 nm, while the microwave-plasma-grown films contained larger crystallites measuring 500 nm. STM tunneling current versus voltage (I-V) curves for the hot-filament-grown films exhibit a zero-current region about the Fermi level corresponding to a surface band gap of 4.1 eV, to be compared with the bulk band gap of diamond of 5.45 eV. The surface electronic density of states computed from these I-V curves is in good agreement with x-ray photoelectron and appearance potential spectroscopies. The I-V curves for the microwave plasma grown films exhibit rectifying behavior in good agreement with a Schottky model for surface band bending.

Depth and radial profiles of defects in Czochralski-grown silicon

We have studied the depth and spatial profiles of vacancies in Czochralski‐grown silicon wafers by positron annihilation spectroscopy. By using a variable energy positron beam and γ‐ray spectroscopy, we have obtained depth profiles of defects in as‐grown, annealed, and 〈100〉 epitaxial Si wafers. We discuss these results in terms of vacancies and oxygen precipitates. The bulk position lifetime measurements, made as a function of axial displacement of a positron source, resolve vacancies, and divacancies in the wafer.