A nucleation site and mechanism leading to epitaxial growth of diamond films

A diamond nucleation site responsible for epitaxial growth of diamond on silicon by chemical vapor deposition (CVD) is identified in high-resolution transmission electron microscopic images. Other sites in the same sample leading to polycrystalline growth, but deleterious to epitaxial CVD growth, are also described. A mechanism for the heteroepitaxial growth of diamond is suggested, in which etching of the nondiamond carbon binder exposes and removes nonadherent nanodiamond nuclei, leaving intact only those directly nucleated on the silicon substrate. This work enhances our understanding of diamond nucleation and heteroepitaxial growth and its potential applications.     

Polycrystalline CVD Diamond Films with High Electrical Mobility

Advances in the deposition process have led to dramatic improvements in the electronic properties of polycrystalline diamond films produced by chemical vapor deposition (CVD). It is now possible to produce CVD diamond with properties approaching those of IIa natural diamonds. The combined electron-hole mobility, as measured by transient photoconductivity at low carrier density, is 4000 square centimeters per volt per second at an electric field of 200 volts per centimeter and is comparable to that of the best single-crystal IIa natural diamonds. Carrier lifetimes measured under the same conditions are 150 picoseconds for the CVD diamond and 300 picoseconds for single-crystal diamond. The collection distance at a field of 10 kilovolts per centimeter is 15 micrometers for the CVD diamond as compared to 30 micrometers for natural diamonds. The electrical qualities appear to correlate with the width of the diamond Raman peak. Also, although the collection distance at the highest fields in the films nearly equals the average grain size, there is no evidence of deleterious grain boundary effects.     

Modified phases of diamond formed under shock compression and rapid quenching

Two modified forms of carbon were quenched by a rapid-cooling technique from graphite sheets shock-compressed to 65 gigapascals and 3700 K. One form, ;;n-diamond," which was obtained from the most rapidly cooled part, has a crystal structure close to that of cubic diamond. The other form, found in the relatively slow-cooled part, was comparable to an i-carbon prepared by an ion-beam technique. The n-diamond is interpreted as a metastable form, the same as hexagonal diamond, converted from graphite through a martensitic transition, for which either the region or the path may be different from that of hexagonal diamond. The second form was found to be produced through reconstruction.     

Eclogitic diamond formation at jwaneng: No room for a recycled component

Eclogitic diamonds have a large range of delta13C values, whereas peridotitic diamonds do not. Paired delta15N-delta13C-N variations in 40 eclogitic diamonds from the Jwaneng kimberlite in Botswana show that neither the influence of recycled biogenic carbon nor the global and primordial heterogeneity of mantle carbon are likely for the origin of the large delta13C range; the data instead support a fractionation process. It is proposed that carbonatitic mantle melts from which diamonds crystallize undergo different evolutions before diamond precipitation, when percolating through either a peridotite or an eclogite. These different evolutions, reflecting the presence or absence of olivine, can account for their respective delta13C distributions.     

Carbon structures with three-dimensional periodicity at optical wavelengths

Porous carbons that are three-dimensionally periodic on the scale of optical wavelengths were made by a synthesis route resembling the geological formation of natural opal. Porous silica opal crystals were sintered to form an intersphere interface through which the silica was removed after infiltration with carbon or a carbon precursor. The resulting porous carbons had different structures depending on synthesis conditions. Both diamond and glassy carbon inverse opals resulted from volume filling. Graphite inverse opals, comprising 40-angstrom-thick layers of graphite sheets tiled on spherical surfaces, were produced by surface templating. The carbon inverse opals provide examples of both dielectric and metallic optical photonic crystals. They strongly diffract light and may provide a route toward photonic band-gap materials.     

Direct spinning of carbon nanotube fibers from chemical vapor deposition synthesis

Many routes have been developed for the synthesis of carbon nanotubes, but their assembly into continuous fibers has been achieved only through postprocessing methods. We spun fibers and ribbons of carbon nanotubes directly from the chemical vapor deposition (CVD) synthesis zone of a furnace using a liquid source of carbon and an iron nanocatalyst. This process was realized through the appropriate choice of reactants, control of the reaction conditions, and continuous withdrawal of the product with a rotating spindle used in various geometries. This direct spinning from a CVD reaction zone is extendable to other types of fiber and to the spin coating of rotating objects in general.     

Long-range ordered carbon clusters: a crystalline material with amorphous building blocks

Solid-state materials can be categorized by their structures into crystalline (having periodic translation symmetry), amorphous (no periodic and orientational symmetry), and quasi-crystalline (having orientational but not periodic translation symmetry) phases. Hybridization of crystalline and amorphous structures at the atomic level has not been experimentally observed. We report the discovery of a long-range ordered material constructed from units of amorphous carbon clusters that was synthesized by compressing solvated fullerenes. Using x-ray diffraction, Raman spectroscopy, and quantum molecular dynamics simulation, we observed that, although carbon-60 cages were crushed and became amorphous, the solvent molecules remained intact, playing a crucial role in maintaining the long-range periodicity. Once formed, the high-pressure phase is quenchable back to ambient conditions and is ultra-incompressible, with the ability to indent diamond.     

Magnetic susceptibility of molecular carbon: nanotubes and fullerite

Elemental carbon can be synthesized in a variety of geometrical forms, from three-dimensional extended structures (diamond) to finite molecules (C(60) fullerite). Results are presented here on the magnetic susceptibility of the least well-understood members of this family, nanotubes and C(60) fullerite. (i) Nanotubes represent the cylindrical form of carbon, intermediate between graphite and fullerite. They are found to have significantly larger orientation-averaged susceptibility, on a per carbon basis, than any other form of elemental carbon. This susceptibility implies an average band structure among nanotubes similar to that of graphite. (ii) High-resolution magnetic susceptibility data on C(60) fullerite near the molecular orientational-ordering transition at 259 K show a sharp jump corresponding to 2.5 centimeter-gram-second parts per million per mole of C(60). This jump directly demonstrates the effect of an intermolecular cooperative transition on an intramolecular electronic property, where the susceptibility jump may be ascribed to a change in the shape of the molecule due to lattice forces.     

Low-pressure, metastable growth of diamond and "diamondlike" phases

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.     

Poly(phenylcarbyne): A Polymer Precursor to Diamond-Like Carbon

The synthesis of poly(phenylcarbyne), one of a class of carbon-based random network polymers, is reported. The network backbone of this polymer is composed of tetrahedrally hybridized carbon atoms, each bearing one phenyl substituent and linking, by means of three carbon-carbon single bonds, into a three-dimensional random network of fused rings. This atomic-level carbon network backbone confers unusual properties on the polymer, including facile thermal decomposition, which yields diamond or diamond-like carbon phases at atmospheric pressure.     

Carbon-free fullerenes: condensed and stuffed anionic examples in indium systems

Condensed, well-ordered analogs of the fullerenes occur in the hexagonal phases Na(96)In(97)Z(2) (Z = nickel, palladium, or platinum). Large cages of In(74) (D3h) and M(60) (= In(48)Na(12), D3d) share pentagonal faces to generate a double-hexagonal close-packed analog of NiAs. All these polyhedra are centered by partially disordered In(10)Z clusters within deltahedra of sodium atoms that cap all inner faces of the cages, namely, Z@In(10)@Na(39)@In(74) and Z@In(10)@Na(32)@Na(12)In(48) "onions" The highest filled bands in these compounds apparently involve localized electron pairs on surface features on In(74)-based layers. Structural and electronic relations between diamond and Naln (stuffed diamond structure) parallel those between certain fullerenes and Na(96)In(97)Z(2) and, presumably, other valence-driven intermetallic phases.     

中国滨海盐沼湿地碳收支与碳循环过程研究进展

滨海盐沼湿地由于其较高的初级生产力和较缓慢的有机质降解速率而成为缓解全球变暖的有效蓝色碳汇,近年来引起全球范围内的热切关注。我国滨海盐沼湿地分布较广,国内学者对滨海盐沼湿地碳循环及碳收支研究取得了一定进展,深入研究滨海盐沼湿地碳循环有助于对全球碳循环及全球变化的理解,并为利用滨海湿地进行碳的增汇减排提供科学依据。主要从我国滨海盐沼湿地碳循环主要观测方法、碳收支与碳循环过程及特点、碳库的组成与影响因素、气态碳的输入输出、潮汐作用对其碳收支的影响这5个方面出发,对国内的滨海盐沼湿地碳循环与碳收支的研究进展进行了归纳总结,并对今后的研究方向给出如下建议:(1)加强滨海盐沼湿地土壤碳库在深度上和广度上的研究;(2)标准化滨海盐沼湿地碳储量、碳通量的量化方法和观测技术;(3)在研究尺度上要宏观、微观并重,同时加强长期原位监测湿地碳通量的变化与室内模拟研究;(4)量化在潮汐影响下滨海盐沼湿地碳与邻近生态系统之间的横向交换通量。只有对我国滨海盐沼湿地碳库收支进行更准确的评估和长期的碳库动态变化监测,方可进一步认识我国盐沼湿地对全球气候变化的影响及其反馈作用,这对于预测全球变化及制定湿地碳储备功能的提升策略具有重要的意义。     

A new type of meteoritic diamond in the enstatite chondrite abee

Diamonds with delta(13)C values of -2 per mil and less than 50 parts per million (by mass) nitrogen have been isolated from the Abee enstatite chondrite by the same procedure used for concentrating Cdelta, the putative interstellar diamond found ubiquitously in primitive meteorites and characterized by delta(13)C values of -32 to -38 per mil, nitrogen concentrations of 2,000 to 12,500 parts per million, and delta(15)N values of -340 per mil. Because the Abee diamonds have typical solar system isotopic compositions for carbon, nitrogen, and xenon, they are presumably nebular in origin rather than presolar. Their discovery in an unshocked meteorite eliminates the possibility of origins normally invoked to account for diamonds in ureilites and iron meteorites and suggests a low-pressure synthesis. The diamond crystals are approximately 100 nanometers in size, are of an unusual lath shape, and represent approximately 100 parts per million of Abee by mass.     

The mechanism of diamond nucleation from energetic species

A model for diamond nucleation by energetic species (for example, bias-enhanced nucleation) is proposed. It involves spontaneous bulk nucleation of a diamond embryo cluster in a dense, amorphous carbon hydrogenated matrix; stabilization of the cluster by favorable boundary conditions of nucleation sites and hydrogen termination; and ion bombardment-induced growth through a preferential displacement mechanism. The model is substantiated by density functional tight-binding molecular dynamics simulations and an experimental study of the structure of bias-enhanced and ion beam-nucleated films. The model is also applicable to the nucleation of other materials by energetic species, such as cubic boron nitride.     

汽相生长金刚石微粒及金刚石薄膜分析

用化学汽相沉积法已长出金刚石微粒及金刚石薄膜,并对它进行了扫描电镜观查,X-射线衍射分析,喇曼光谱分析,硬度测试,证明它确是金刚石并具有金刚石的性能.     

Hexagonal diamonds in meteorites: implications

A new polymorph of carbon, hexagonal diamond, has been discovered in the Canyon Diablo and Goalpara meteorites. This phase had been synthesized recently under specific high-pressure conditions in the laboratory. Our results: provide strong evidence that diamonds found in these meteorites were produced by intense shock pressures acting on crystalline graphite inclusions present within the meteorite before impact, rather than by disintegration of larger, statically grown diamonds, as some theories propose.     

PHYSIOLOGY OF THE AMINO ACIDS

We have followed the amino acids from their entrance into the alimentary tract in the form of food proteins through the successive steps of digestion, absorption into the blood stream and passage from the blood stream into the tissues, where they are concentrated by some unknown mechanism to many times their concentration in the blood plasma. We have seen something of the way in which certain of the amino acids can be transformed into one another in the body or synthesized from ammonia and keto acids. However, we have had to admit that our bodies can form in such ways only about half of the different amino acids that are required, and that the other half must be made for us by plants, bacteria or other organisms which have greater synthetic powers than we. And finally we have seen something of the manifold fates of the amino acids after they have entered our tissues; how they may be destroyed and their nitrogenous parts turned into urea in the liver before it is possible to put them to their more specialized uses, how their carbon fractions can be used to form glucose, how they may sacrifice themselves to protect us from toxic products, how they can serve as source material for certain vitamins, hormones and other compounds with physiological functions still to be identified, and how finally those amino acids which are not deflected to these various fates may enter into the proteins of the tissues and become for a time parts of our living structures.     

UV-B辐射对植物光合器官和光合作用过程的影响*

 植物光合作用是植物受UV-B辐射影响的主要一个方面。UV-B辐射诱导的活性氧的产生进一步破坏类囊体膜;叶绿素在UV-B辐射下降解或其合成过程受阻,但不同植物间叶绿素对UV-B辐射的抗性存在一定差异;UV-B辐射下,PSⅡ的两个主要组分D1蛋白和LHCⅡ合成减少,但D1蛋白可通过D1蛋白的周转维持PSⅡ的活性;UV-B辐射诱导主要光合酶RuBP羧化酶活性的降低,且不同植物对UV-B辐射强高度的响应程度不同,而C4植物中烟酰胺腺嘌呤二核苷酸-苹果酸酶（NADP-ME）却起到保护植物免受UV-B损伤的作用。在光合作用的主要过程中,UV-B辐射通过影响PSⅡ上各反应活性而降低电子传递速率,对碳同化过程的影响除降低酶活性外,另一个重要方面就是调节气孔的运动。在UV-B辐射对植物光合作用影响的研究中应进一步揭示其作用机理,并注重当前UV-B辐射水平及其与其它环境因子综合作用对植物光合作用的影响研究。     

Epitaxial growth of diamond films on si(111) at room temperature by mass-selected low-energy c+ beams

Diamond films ( approximately 0.7 micrometer thick) have been epitaxially grown on Si(111) substrates at room temperature with mass-selected 120-electronvolt C(+) ions. The diamond reflections observed in x-ray diffraction are well localized at their predicted positions, indicating that (i) the diamond(111) and (220) planes are parallel to the Si(111) and (220), respectively; (ii) the diamond rotational spread around its (111) normal is approximately 1.7 degrees ; and (iii) the mosaic block size is approximately 150 A. The film growth is discussed in terms of subplantation-a shallow subsurface implantation model. This discovery is an important step toward diamond semiconductor devices.     

Prediction of new low compressibility solids

An empirical model and an ab initio calculation of the bulk moduli for covalent solids are used to suggest possible new hard materials. The empirical model indicates that hypothetical covalent solids formed between carbon and nitrogen are good candidates for extreme hardness. A prototype system is chosen and a first principles pseudopotential total energy calculation on the system is performed. The results are consistent with the empirical model and show that materials like the prototype can have bulk moduli comparable to or greater than diamond. It may be possible to synthesize such materials in the laboratory.