John C. Angus

Charge Transfer Equilibria Between Diamond and an Aqueous Oxygen Electrochemical Redox Couple

Undoped, high-quality diamond is, under almost all circumstances, one of the best insulators known. However, diamond covered with chemically bound hydrogen shows a pronounced conductivity when exposed to air. This conductivity arises from positive-charge carriers (holes) and is confined to a narrow near-surface region. Although several explanations have been proposed, none has received wide acceptance, and the mechanism remains controversial. Here, we report the interactions of hydrogen-terminated, macroscopic diamonds and diamond powders with aqueous solutions of controlled pH and oxygen concentration. We show that electrons transfer between the diamond and an electrochemical reduction/oxidation couple involving oxygen. This charge transfer is responsible for the surface conductivity and also influences contact angles and zeta potentials. The effect is not confined to diamond and may play a previously unrecognized role in other disparate systems.

Growth of Diamond Seed Crystals by Vapor Deposition

Carbon was deposited on virgin, natural diamond powder from methane gas at 1050°C and 0.3 Torr. The deposits were identified as new diamond by chemical analysis, chemical etching, density measurements, x‐ray and electron diffraction, microwave absorption, electron spin resonance, and visual observations. The crystalline quality of the new diamond layers has not been established; it cound range from polycrystalline material with a large number of defects to true epitaxial layers.

Hydrogen and Oxygen Evolution on Boron‐Doped Diamond Electrodes

The evolution of hydrogen and oxygen was studied on diamond electrodes containing approximately 1021 boron atom/cm3. Voltammetry showed a wide potential window [−1.25 to +2.3 V vs. standard hydrogen electrode (SHE)] without significant water decomposition. This window was much narrower for poor quality diamond films with appreciable sp2 content. A redox couple observed at +1.7 V indicates oxidation of the diamond surface prior to oxygen evolution. The extent of surface oxidation increased with sp2 content. Anodic polarization made the diamond surface hydrophilic; x‐ray photoelectron spectroscopy showed an increase in oxygen coverage and the presence of carbon‐oxygen bonds. The estimated capacitance of the interface ranged from 0.05 μF/cm2 for high quality diamond to 5 μF/cm2 for low quality diamond. Preliminary measurements of the exchange current densities for oxygen and hydrogen evolution indicated slow kinetics compared to metals or highly oriented pyrolytic graphite.

Dense ‘‘diamondlike’’ hydrocarbons as random covalent networks

Dense ‘‘diamondlike’’ hydrocarbon films (a‐C:H) are a class of amorphous solids which can be condensed as thin films from nonequilibrium environments by several plasma deposition techniques. Films with hydrogen atom fractions from 0.5 to 0.6 select a structure with an average coordination number close to the theoretical value at which stabilization by bonding and destabilization by strain energy are balanced. This coordination number is achieved by the incorporation of the hydrogen and by the presence of trigonally (sp2) bonded carbon in the carbon skeletal network. Theory predicts a range of hydrogen atom fraction, outside of which fully constrained, random hydrocarbon networks cannot exist. The dense hydrocarbon films show extreme values of physical properties, including hardness and argon diffusivity, which can be related to their unusual structure.

Applications of Diamond Thin Films in Electrochemistry

Electrochemical reactions typically involve electron transfer between an electrode and a dissolved chemical species at a solid-electrode/liquid-electrolyte interface. Three broad classes of electrochemical applications may be identified: (1) synthesis (or destruction), in which an applied potential is used to bring about a desired chemical oxidation or reduction reaction; (2) analysis, in which the current/potential characteristics of an electrode are used to determine the type and concentration of a species; and (3) power generation. These broad types of applications require stable, conductive, chemically robust, and economical electrodes. Diamond electrodes, fabricated by chemical vapor deposition, provide electrochemists with an entirely new type of carbon electrode that meets these requirements for a wide range of applications. The first reports of electrochemical studies using diamond were in the mid-1980s. During the past several years, the field has attracted increasing attention. This review summarizes the electrochemical properties of diamond that make it a unique electrode material and that distinguish it from conventional carbon electrodes.

Diamond and diamond-like films

Abstract The ability to grow thin films of polycrystalline diamond at atmospheric pressure and below is a relatively new development. This technology, while still immature, has potential electronic applications in thermal management, packaging, lithography and active electronic devices. Non-electronic applications include abrasives, wear resistance surfaces, tool coatings, optical coatings and IR optics. The current status of low pressure diamond growth, and the related field of diamond-like films, will be reviewed and possible future directions indicated.

Orientation relationship between chemical vapor deposited diamond and graphite substrates

Diamond was deposited on synthetic graphite, highly oriented pyrolytic graphite and on substrates covered with graphite powder. Scanning electron microscopy and transmission electron microscopy were used to examine the samples. A strong preference for nucleation of diamond on the edges of the graphite sheets was observed. The graphite and the diamond have a preferential orientation relationship in which the diamond (111) plane is parallel to the graphite (0001) plane, and the diamond [110] direction is parallel to the graphite [1120] direction. This orientation means that the puckered hexagons in the diamond (111) plane retain the same orientation as the flat hexagons in the original graphite sheet. We conclude that the diamond can nucleate with an epitaxial relationship to the graphite. Some of the edges of the graphite sheets may have been converted to diamond by the atomic hydrogen.

Growth of boron‐doped diamond seed crystals by vapor deposition

p‐type semiconducting diamond was grown by vapor deposition from a 0.83% diborane in methane gas mixture at 1050°C and 0.2 Torr on 0 to 1‐μ nominal size natural type‐I diamond powder. Total mass increases of about 9% were achieved which correspond to average linear growth rates of less than 10−3 μ/day. Evidence showing the growth was boron‐doped diamond included chemical etching, x‐ray and electron diffraction, density measurements, Seebeck and resistivity measurements, chemical analysis, optical measurements, induced electron emission spectroscopy, and scanning electron microscopy. The crystalline quality of the new diamond has not been established; it may be highly defective. A distinct change in color of the diamond seed crystals from an off‐white or gray for virgin crystals to light blue after growth was observed. The results are further confirmation that diamond may be grown at low pressures where it is thermodynamically metastable with respect to graphite. It is also further evidence that boron is t...

Voltammetry Studies of Single‐Crystal and Polycrystalline Diamond Electrodes

Boron-doped polycrystalline and near-single-crystal quality diamond electrodes were studied by voltammetry. A redox couple with E 1/2 = + 1.83 V vs. standard hydrogen electrode was detected on the polycrystalline electrodes but was absent on the single-crystal electrodes. The results strongly suggest that the couple is associated with reactivity at the grain boundaries. Plasma fluorination of polycrystalline diamond electrodes using CF 4 in a radio frequency plasma eliminated the redox couple at + 1.83 V but did not alter the potential range of water stability. Cathodic polarization of as-grown, polycrystalline diamond electrodes caused an irreversible addition of oxygen to the surface. Subsequent anodic polarization added additional oxygen and made the surface hydrophilic. Single-crystal electrodes also displayed an increase in oxygen coverage upon both cathodic and anodic polarization. Voltammetry studies of electrodes covered with a thin sp 2 carbon surface layer indicate that the redox couple at + 1.83 V corresponds to multiple processes including the etching of sp carbon in the grain boundaries.

Diamond Growth at Low Pressures

Diamond synthesis has attracted attention ever since it was established in 1797 that diamond is a crystalline form of carbon. Initially, synthesis was attempted at high pressures because diamond is the densest carbon phase. As understanding of chemical thermodynamics developed through the 19th and 20th centuries, the pressure-temperature range of diamond stability was explored. These efforts culminated in the announcement in 1955 of a process for diamond synthesis with a molten transition metal solvent-catalyst at pressures where diamond is thermo-dynamically stable. Worldwide sales of synthetic diamond now approach 330 million carats (73 tons) with a market price of between