Ca Rego

Raman characterization of boron-doped multiwalled carbon nanotubes

We present first- and second-order Raman spectra of boron-doped multiwalled carbon nanotubes. The Raman intensities are analyzed as a function of the nominal boron concentration. The intensities of both the D mode and the high-energy mode in the first-order spectra increase with increasing boron concentration, if normalized with respect to a second-order mode. We interpret this result as an indication that the high-energy mode in carbon nanotubes is defect-induced in a similar way as the D mode. Based on this result, we provide a preliminary quantitative relation between the boron concentration and the Raman intensity ratios.

Examination of the effects of nitrogen on the CVD diamond growth mechanism using in situ molecular beam mass spectrometry

Abstract Molecular beam mass spectrometry (MBMS) has been used to obtain quantitative measurements of the composition of the gas-phase species prevalent during diamond hot filament CVD using a variety of nitrogen-containing source gas mixtures. The ratio of C:N in the feedstock was maintained at 1:1, and the gas mixtures used were 0.5% each of CH 3 NH 2 and HCN in H 2 , and 0.5% CH 4 in H 2 with added NH 3 and N 2 at 0.5 and 0.25% respectively. The deposition rate and resulting film quality at optimum growth temperatures depend critically on the origin of carbon-containing species. At the relevant process temperatures, most of the gas-phase carbon exists in the form of unreactive HCN (∼70–90%) for all precursor gas mixtures (except CH 4 /N 2 ), with very little C 2 H 2 detected. As a result, poor quality diamond films were deposited at rates of less than 0.1 μm h −1 . For CH 4 /N 2 mixtures, however, equal amounts of HCN and C 2 H 2 were detected, and well-faceted diamond films were produced at higher deposition rates (∼0.45 μm h −1 ). These observations are explained in terms of the effects that nitrogen, and its resulting reaction products (NH 3 , HCN, CH 3 NH 2 , etc.), have on the gas-phase chemistry occurring during the CVD process. In particular, we suggest that N 2 can act as a catalyst for the destruction of H atoms, which in turn leads to significant changes in the gas-phase chemistry.

Gas-phase composition measurements during chlorine assisted chemical vapor deposition of diamond: A molecular beam mass spectrometric study

We have constructed a molecular beam mass spectrometer designed specifically to sample gases from a diamond chemical vapor deposition ~CVD! process chamber thereby enabling characterization of the gas-phase CVD environment. With this in situ diagnostic technique we have obtained quantitative measurements of the composition of the gas-phase species as a function of filament temperature for a variety of C/H/Cl gas mixtures. The precursor mixtures used were 1% of a chloromethane (CH 42n Cl n , n51-4! in hydrogen and 1% CH 4 in hydrogen with added Cl 2 varying from 1% to 4%. At filament temperatures optimum for diamond growth (2300°C! the relative CH4/C2H2/C2H4 product distribution measured in the gas mixture is remarkably similar to that established when CH4 is the carbon precursor species. At these filament temperatures almost all the chlorine is reduced to HCl, its concentration being proportional to the Cl fraction in the source gas, regardless of the form of the chlorine in the source gas mixture. Compositional analysis of the as-grown diamond films indicated that no chlorine was present in the bulk of the films, though trace amounts of chlorine were detected on the film surface. From these observations we surmise that chlorine atoms are involved in the gas-surface reactions which produce active growth sites on the diamond surface.

Application of diamond coatings onto small dental tools

Abstract Small dental tools such as burs and drills are commonly used in dental practice and laboratory. Conventional diamond burs used for grinding operations have a number of problems associated with heterogeneity of the crystallites, decreased cutting efficiency, need for repeated sterilisation and short life. These burs are manufactured by imbedding diamond particles into the burs using a suitable binder matrix material. The use of a diamond coating may offer an improvement in dental bur technology. Chemical vapour deposition (CVD) of diamond coatings onto the cemented tungsten carbide WC–Co substrate is problematic. Generally the adhesion of diamond coating to cemented carbide substrate is poor. It is obvious that the binder materials such as cobalt can suppress diamond growth and enhance graphitic deposits, which cause poor adhesion and low diamond nucleation density. The effects of key process parameters such as filament position, filament and substrate temperature and pre-treated substrate material on the coating properties have been investigated using a variety of analytical techniques. Characterisations of the substrates and polycrystalline diamond film morphology were analysed by scanning electron microscopy (SEM). The chemical composition was evaluated by energy dispersive spectroscopy (EDS). Raman spectroscopy was used to assess the carbon-phase purity and give an indication of the stress levels in the as-grown polycrystalline diamond films.

HFCVD diamond grown with added nitrogen: film characterization and gas-phase composition studies

Abstract Recently there has been considerable interest in the addition of nitrogen to precursor gas mixtures and its influence on the properties of the resulting diamond films. Careful control of the nitrogen concentration in the CVD process results in changes to both the film growth rate and morphology. In addition to the well-defined and precisely controlled deposition conditions, thorough characterization of the vapour-grown material is indispensable in order to tailor diamond film properties according to specific applications. High quality diamond films have been produced via hot filament CVD (HFCVD) using a precursor gas mixture of 1% methane in hydrogen with N 2 additions varying from 50 to 5000 ppm (N/C 0.01–1.0). We report the results from both the structural and compositional characterization of the as-deposited HFCVD diamond films by scanning electron microscopy, Raman spectroscopy and X-ray diffraction. Both the diamond phase purity and growth rate are maximized with 200 ppm N 2 in the gas mixture with further additions resulting in reduced growth rates and poorer film quality. We explain these findings in terms of the concentrations of gas phase species obtained from chemical equilibrium calculations and in particular the importance of the CN radical to the diamond growth process.

CVD diamond wires and tubes

Abstract Diamond has been uniformly deposited onto the surface of thin metal wires using hot filament CVD. The diamond-coated wires are stronger and stiffer than the uncoated wires. Subsequent etching of the metal core in a suitable chemical reagent allows free-standing diamond tubes to be made, the typical dimensions being 1 cm long with an internal diameter of 10–150 μm. The formation of a thick, chemical-resistant carbide layer at the metal-diamond interface when using Ti and W wires is investigated.

Investigating the fracture resistance and adhesion of DLC films with micro-impact testing

Micro-impact testing, a new repetitive contact testing technique has been developed to simulate the fatigue conditions which coatings experience in service more closely than existing testing methods. Diamond-like carbon (DLC) films deposited on silicon under different RF powers have been investigated with the new technique. The technique provides a new approach to the investigation of the fracture properties of thin films. Single-point impact data for brittle materials, such as DLC films, show an initial period of damage generation followed by catastrophic failure. The time-to-failure and the overall change in probe depth during a test are measures of the resistance of the coating to fracture. Differences in impact resistance between the DLC films are explained in terms of variations in film toughness and adhesion strength, which, in turn, are a function of the DLC deposition conditions.

Investigation of the addition of nitrogen-containing gases to a hot filament diamond chemical vapour deposition reactor

Abstract We have investigated the effect that addition of various nitrogen-containing gases, such as ammonia, methylamine and HCN, has on the composition of the films deposited in a standard hot filament diamond chemical vapour deposition reactor. Total process gas concentration (C + N) was maintained at 1% in H 2 throughout the experiments. When using a feedstock of methane and ammonia, we generally find that for methane-rich mixtures diamond films are formed; for ammonia-rich mixtures, silicon nitride is formed by reaction of the ammonia with the Si substrate; for approximately equal ratios of CH 4 to NH 3 , little film deposition occurs. Other gases, such as CH 3 NH 2 and HCN, behave similarly to 1:1 mixtures of CH 4 and NH 3 . Results are explained in terms of a simple chemical model of the gas phase chemistry and surface interactions.

Chromium interlayers as a tool for enhancing diamond adhesion on copper

Abstract Diamond films on copper suffer from poor adhesion, mainly due to the lack of affinity between carbon and copper. In this study a chromium (Cr) interlayer has been employed to improve the adhesion between the diamond films and the copper substrates. The Cr interlayer, a carbide-forming material, plays the role of an adhesive between the diamond film and the copper substrate. A hot filament chemical vapour deposition (HFCVD) system, modified to enable the substrate to be negatively biased has been used to deposit diamond. It was observed that diamond nucleation together with the deposition rate increased with bias time. A bias time of 5 min was sufficient to increase diamond nucleation and enhance the growth rate. The diamond films deposited exhibited predominantly 〈111〉 orientation, as evident from the SEM micrographs. Diamond films grown on the Cr-coated Cu substrates gave reasonable adhesion values. Pull-off tests showed that the adhesion was better than the strength of the adhesive employed, ∼14 MPa. Scratch tests revealed a critical load of ∼7 N with unbiased samples, while the critical load increased significantly to ∼38 N for samples pre-treated at –250 V for 15 min. Furthermore, it was found from Raman analysis that films with better adhesion exhibited greater Raman shift of the 1332 cm −1 diamond peak. The overall results suggest that Cr is an effective interlayer for producing adherent diamond coatings onto Cu substrates.

In-situ mass spectrometric study of the gas phase species involved in CVD of diamond as a function of filament temperature

Abstract We have used a molecular beam mass spectrometer to obtain quantitative measurements of the composition of the gas-phase species during the chemical vapour deposition (CVD) of diamond films as a function of filament temperature for a variety of different hydrocarbon precursor gases. For filament temperatures near to, and above, the optimum for diamond growth (ca. 2400 K), we find that the relative concentrations of the various stable hydrocarbon species (CH 4 , C 2 H 2 and C 2 H 4 ) present in the gas mixture, and the way that these concentrations vary with temperature, are remarkably insensitive to the particular choice of hydrocarbon feedstock gas.