Catálogo de publicaciones - libros

An overview of the relative stability of various carbon structures with characteristic sizes in the nanoscale region is presented with major emphasis on ultrananocrystalline diamond (UNCD), which has very diverse structures at the nanoscale. Heats of formation of nanodiamond particles of different morphologies are reported.

Nanometer sized diamond has been found in meteorites, proto-planetary nebulae and interstellar dusts, as well as in residues of detonation and in diamond films. Remarkably, the size distribution of diamond nanoparticles appears to be peaked around 2–5 nm, and to be largely independent of preparation conditions. Using calculations, we have shown that in this size range nanodiamond has a fullerene-like surface and, unlike silicon and germanium, exhibits very weak quantum confinement effects. We called these carbon nanoparticles their atomic structure, predicted by simulations, is consistent with many experimental findings. In addition, we carried out calculations of the stability of nanodiamond which provided a unifying explanation of its size distribution in extra-terrestrial samples, and in ultra-crystalline diamond films. Here we present a summary of our theoretical results and we briefly outline work in progress on doping of nanodiamond with nitrogen.

Recent advances in the fabrication and characterization of semiconductor and metallic nanowires are proving very successful in meeting the high expectations of nanotechnologists. Although the nanoscience surrounding bonded carbon nanotubes has continued to flourish over recent years the successful synthesis of the analogue, diamond nanowires, has been limited. This prompts questions as to whether diamond nanowires are fundamentally unstable. By applying knowledge obtained from examining the structural transformations in nanodiamond, a framework for analyzing the structure and stability of diamond nanowires may be established. One possible framework will be discussed here, supported by results of density functional theory calculations used to study the structural relaxation of nanodiamond and diamond nanowires. The results show that the structural stability and electronic properties of diamond nanowires are dependent on the surface morphology, crystallographic direction of the principal axis, and the degree of surface hydrogenation.

Computational studies of growth mechanisms on diamond surfaces based on C precursor have been reviewed. The investigations have postulated reaction mechanisms with diamond growth occurring by insertion of C into the C-H bonds of the hydrogen-terminated diamond surface or into π- bonded carbon dimers on dehydrogenated diamond surfaces. Reaction barriers for both growth and renucleation at (011) and (100) diamond surfaces had been calculated using quantum chemistry approaches. Preliminary results on growth mechanism involving CN precursors are also reported.

Interstellar molecular clouds exhibit infrared emission features characteristic of C-H stretching bands that are attributed to dust grains of hydrogen terminated diamond. Within our solar system, primitive chondritic meteorites (among the most well preserved, least altered, and least metamorphosed material that initially formed in our solar system) have been found to contain pristine dust grains that predate the formation of the solar system. Nanometer-sized diamonds are ubiquitous in primitive chondrites at 1–1400 ppm, often representing a significant component of carbon in the matrices of these meteorites. The astronomical sources, which produced the nanodiamonds, and the relative contribution of the sources to the nanodiamond population have not been fully established. The presence of trapped Xe with specific isotopic anomalies (characteristic of explosive nucleosynthesis) in nanodiamond isolates from meteorites suggests at least a subpopulation of the nanodiamonds formed in association with supernovae. However, all attempts to isolate that subpopulation have failed and the exact nature of the Xe carriers is not known. Asymptotic giant branch (AGB) carbon stars are a likely source of presolar nanodiamonds based on their pervasive dust production, however the available isotopic evidence can neither support nor eliminate AGB stars as sources. Similarly, the solar nebula should not be neglected as a possible source since the available isotopic evidence cannot exclude it. Regardless of source, comparative microstructural studies indicate the majority of the nanodiamonds formed by low-pressure vapor condensation. Microstructural and trapped element isotopic data on meteoritic nanodiamonds are discussed in terms of the origin and formation mechanisms of the nanodiamonds.

We recently reported [1,2] the discovery and isolation of new members of the hydrogen-terminated diamond series, ∼1 to ∼2 nm sized higher diamondoids from petroleum. Crystallographic studies [1,2] revealed a wealth of diamond molecules that are nanometer-sized rods, helices, discs, pyramids, etc. Highly rigid, well-defined, readily derivatizable structures make them valuable molecular building blocks for nanotechnology. We now produce certain higher diamondoids in gram quantities. Although more stable than graphite particles of comparable size, higher diamondoids are extraordinarily difficult to synthesize. Attempts to synthesize them were abandoned in the 1980’s. We examined extracts of diamond-containing materials synthesized by CO laser-induced gas-phase synthesis [3] and commercial CVD in an attempt to detect diamantane to undecamantane. However, high-sensitivity GCMS detected no diamondoids in these materials.

This work deals with the investigation of the microwave plasma assisted CVD process employed for nanocrystalline diamond (NCD) deposition, using an Ar/H/CH feed gas. The different steps required in order to insure the process control and optimization are considered. The stable process parameters providing the discharge stability and reproducibility are first determined. Then, the influence of the growth parameters on the film characteristics is examined. In particular, the effect of the surface temperature is probed combining both and measurements. Finally, the potentiality of NCD films for the achievement of high frequency surface acoustic wave devices is illustrated.

This work deals with the modelling of Ar/H/CH microwave discharges used for nanocrystalline diamond film deposition. Two thermochemical models are developed and used in order to estimate the discharge composition, the gas temperature and the average electron energy in the frame of a quasi-homogeneous plasma assumption. The first one takes into account the formation of hydrocarbons containing up to two carbon atoms, while in the second heavier aliphatic molecules and poly-aromatic hydrocarbons are considered along with soot particle nucleation.

Detonation nanodiamonds are shown to be effective seeds for growth of CVD diamond films as they provide high nucleation density on a substrate and can be placed on shaped surfaces and even into porous materials. XPS, AES and TEM analyses give useful information on the early stage of diamond growth. The transfer molding technique for manufacturing various diamond shapes is described. As the nucleation side of free-standing films produced by molding is the “working” surface, emphasis is placed on the study of its properties, such as topography, impurity contamination, thermal conductivity, and wettability. Diamond items grown directly on patterned Si templates are illustrated.

Nanocrystalline carbon film of a prevailing diamond character can be deposited by direct current glow discharge (DC GD) chemical vapor deposition (CVD) from a methane-hydrogen mixture. The growth of a nanodiamond film by the DC GD CVD process occurs on top of a hydrogenated -coordinated carbon precursor (graphitic-like) as confirmed by near-edge x-ray adsorption fine structure (NEXAFS) and transmission electron microscopy (TEM). No surface pretreatment is necessary in order to induce film growth and the thickness of the precursor layer is ∼ 200 nm. By high resolution TEM and x-ray diffraction (XRD) it has been established the nanocrystalline nature of the films and that the average size of the diamond crystallites is about 5 nm. Preferential vertical alignment of the basal planes in the precursor was determined by cross- section HR TEM as well as by angle-resolved NEXAFS. Electron energy loss spectroscopy (EELS) demonstrated the -coordinated character of the nanodiamond films surface. Secondary ion mass spectroscopy (SIMS) established a drastic increase in the adsorbed hydrogen content accompanying the nanodiamond formation. Whereas the hydrogen concentration in the precursor layer is only a few percent it increases to ∼15–20 at.% in the nanodiamond film. Density measurements X-ray reflection (XRR) of the films increases with thickness as expected from the phase composition of the films determined from the spectroscopic methods. From a microscopic perspective nanodiamond film and growth is explained as a sub-surface process in terms of a four step cyclic process: (i) Formation of a dense, hydrogenated carbon coordinated oriented layer; (ii) precipitation of clusters in this graphitic phase, (iii) growth of nanodiamond particles up to ∼5 nm in size by energetic species bombardment of the diamond/hydrogenated carbon — interface. It involves preferential displacement of carbon coordinated carbon atoms leaving coordinated carbon atoms intact, leading to expansion of the diamond phase, (iv) frustration of growth of the diamond particles and (v) growth of the film formed of an agglomerate of nanodiamond particles by a cyclic process which involves process 2–4. From a macroscopic perspective nanodiamond formation is suggested to be associated with a sub-surface nano-graphite — nanodiamond phase transition triggered by stress relaxation.