Homogeneous nucleation of diamond powder is reported. The experiments were performed in a low-pressure microwave-plasma reactor. The deposits were collected downstream of the reaction zone and subjected to wet oxidation to remove nondiamond carbons. The residues were analyzed by optical and electron microscopy, electron diffraction, and Raman spectroscopy. A variety of hydrocarbons diluted in argon, hydrogen, or oxygen gas mixtures were tested. In most cases only nondiamond materials, like graphite and carbyne, were obtained. Homogeneous nucleation of diamond was clearly observed in dichloromethane- and trichloroethylene-oxygen mixtures. The particles formed had crystalline shapes, mostly hexagonal. The largest particles were about 0.2 μm, although most of the particles were on the order of 50 nm in diameter. The powder was identified to be a mixture of polytypes of diamond.

The effects of heteroatom addition on the nucleation of solid carbon in a low-pressure plasma reactor were investigated. Silane or diborane were added to acetylene mixed in hydrogen or argon. Oxygen was added to some of the diborane containing gas mixtures. Silane containing mixtures resulted in powder comprised of weakly bonded amorphous hydrogenated carbon-silicon material. The addition of diborane resulted in substantial production of diamond particles, 5 to 450 nm in diameter, under the conditions that show no diamond formation without diborane present. The observed yield of the oxidation-resistant powder produced in boron-containing mixtures reached 1.3 mg/h with the linear growth rates of diamond particles on the order of 102–104 μm/h. Implication of these results to interstellar dust formation is discussed.

Diamond particles 10–500 nm in diameter were produced by microwave-assisted combustion of acetylene in oxygen. Both premixed and diffusion flame configurations were investigated. A mixture of cubic and hexagonal polytypes of diamond were identified. Larger particle sizes were observed at lower reactor pressure and higher C to O atomic ratios. C to O atomic ratios between 0.83 and 1.0 produced crystalline diamond powder while other ratios produced graphite, soot, and amorphous carbon phases. Diamond formation was not observed when reaction pressures were above 150 Torr.

Space groups and atomic coordinates for the 4H, 6H, 8H, 10H, 15R, and 21R polytypes of diamond are presented. The systematic method used to determine the highest symmetry space group for diamond polytypes is described.

Diamond films were deposited by a cyclic growth-etch process for up to 72 h. Initial growth rates are typical for the deposition of quality diamond films by continuous process chemical vapor deposition, however, they show a distinct decline as growth progresses. The films show a crystalline faceting characteristic of good quality diamond, but the intensity of the 1332 cm−1 diamond Raman band decreases after 10 h of growth, with a loss of all characteristic carbon Raman bands at 72 h of growth. The present cycling experiments differ from typical continuous diamond deposition processes in that the gas phase composition during the etching cycle is significantly richer in OH, O, and H. Oxygen is proposed to poison the growing surface by forming strongly chemisorbed sites which are trapped in the growing film. Defective carbon deposited above the trapped oxygen etches rapidly in subsequent cycles, and the buildup of such trapped oxygen defects may account for the observed decline in growth rate and quality.