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The fabrication and characterization of ‘self-supporting’ diamond tubes grown by chemical vapor deposition (CVD) are reported. Diamond layers were deposited onto tungsten wires with diameters of 238 μm; the tungsten cores were subsequently completely removed by etching to leave ‘self-supporting’ diamond tubes with a diameter of approximately 400 μm and length of 20 mm. A hot-filament CVD system fed with ethanol highly diluted in hydrogen was employed. Growth rates of 7.8 μm h −1 have been measured and incubation times >3 h have been estimated. Scanning electron microscopy of cross-sections revealed columnar structures, which terminate on sharp (111) facets on the tube's external surface. Raman spectroscopy showed that the tube structure is predominantly composed of CC sp 3 bonds, with intrinsic tensile stresses.

Porous diamond and porous carbon structures were deposited by hot-filament chemical vapor deposition (HFCVD) onto natural fibers derived from pyrolized bamboo substrates. 'Seeding' of the substrate with diamond dust prior the HFCVD process, allowed the fabrication of porous diamond, replicating the porous surface geometry of the bamboo fibers. As indicated by scanning electron microscopy and Raman spectroscopy, respectively, the films deposited on 'seeded' substrates are poly-faceted and are composed predominantly of C-C sp bonds. 2001 Elsevier Science B.V. All rights reserved. 3

Tungsten wires were used as templates for the deposition of tubes of micro-crystalline diamond or of 'spongy' carbon nano- structures. A hot-filament chemical vapor deposition system fed with highly diluted ethanol in hydrogen and argon was used. Argon was intentionally added to the feed at different concentrations to affect the crystalline growth. As the argon concentration was increased beyond 65 vol.% in the reactor feed, the morphology drastically changed from micro-crystalline diamond to complex carbon nano-structures. While the samples produced at argon concentrations of 65 vol.% or less resulted in a radial columnar structure of micro-crystalline diamond, the samples produced at higher argon concentrations showed radial flaws and 'spongy' structures. Each type of morphology is of potential interest for the use of these tubes as molecular filters or in electron emission devices. 2001 Elsevier Science B.V. All rights reserved.

Diamond-coated stainless-steel blades promise smoother cutting and greater durability. In this work the feasibility of coating stainless-steel blades with diamond and diamond-like carbon (DLC) by hot-filament chemical vapor deposition was investigated. Two pre-deposition seeding procedures using either diamond dust or oil droplets were tested. As revealed by scanning electron microscopy and Raman spectroscopy, both seeding methods allow the production of diamond films also containing some DLC. The oil-seeding method, however, is the most successful in promoting diamond nucleation. Film deposition along the blade edge is also enhanced by its curvature.

We present experimental results of chemical vapor deposition (CVD) diamond growth from ethanol vapor with the substitution of hydrogen by helium in concentrations varying from 0 to 95 vol.% He. Scanning electron microscopy (SEM) analysis revealed that the addition of helium to the gas phase influenced the growth rate and film structure. The deposition rate of diamond increases up to a helium concentration of 40 vol.% and then decreases. We attribute this behavior to an increase in the density of vacancies in the film or to a change in the reaction kinetics provoked by the chemical inertness of helium. Diamond films with well-defined crystalline facets and of good quality (measured by Raman spectroscopy) were obtained with the addition of up to 70 vol.% He. Use of helium also provokes an increase in film porosity, with the appearance of a strong room temperature photoluminescence with a Gaussian peak at approximately 635 nm. The intensity of the luminescence increases with the concentration of helium and is probably related to nanoporous defects introduced into the diamond structure.

Carbon nitride films were prepared by combined hot filament (HF) and radiofrequency (RF) plasma enhanced chemical vapor deposition techniques from three mixtures, C2H6–N2–Ar, C2H4–N2–Ar and C2H2–N2–Ar, denominated M1, M2 and M3, respectively. Depositions were carried out in a vacuum chamber fitted with two parallel-plate electrodes driven by a 40-MHz generator and a Ta filament resistively heated by an AC power supply. The deposition rates, composition and structural properties were compared for films deposited using the same flow rates of the hydrocarbon, nitrogen and argon mixture fed to the vacuum chamber, and the same radiofrequency and filament power. The deposition rate, R, was determined by dividing the film height, obtained using a profilemeter, by the deposition time. Reflection infrared and Raman spectroscopies were used to investigate the molecular structure while Rutherford backscattering spectroscopy was used to determine the elemental composition. Using combined RF and HF excitation: (i) R increased from mixture M1 to mixture M3; (ii) the concentration of CC bonds was higher in the films deposited in the M3 mixture; and (iii) the incorporation of nitrogen, determined by the nitrogen to carbon atomic ratio, increased from mixture M1 to M3. For the mixture M3, a comparison of the film structure and deposition rate of films produced using HF, RF and combined HF–RF excitation, was made. Both R and the concentration of C–H bonds were higher when HF excitation alone was used. An interpretation of the role of the plasma in the decrease in R is suggested. The influence of the degree of saturation of the hydrocarbon molecules in each mixture on R and on the film structure is highlighted. The Raman spectroscopy results revealed that diamond-like carbon is formed under HF excitation while the use of RF, alone or combined with HF, eliminates the diamond phases.

We studied the effects on the growth kinetics and properties of diamond and carbon-like diamond films obtained by the introduction of argon at low to high concentrations (0–85 vol.% Ar) into the feed mixture (ethanol and hydrogen) of a hot-filament chemical vapor deposition (CVD) reactor. Scanning electron microscopy (SEM) analysis revealed that the addition of argon induces an increase in the diamond grain size, and increases the flaws between the grains and the density of vacancy defects. Well-faceted diamond films of good quality (measured by Raman spectroscopy) have been obtained using up to 65 vol.% of argon in the gaseous mixture, while higher concentrations (85 vol.% Ar) produce diamond-like carbon or other complex carbon structures. We also observed an increase in the diamond growth rate with argon addition that was associated with an increase in carbon free radicals (up to ∼40 vol.% Ar) or to the increase in the filament temperature necessary to keep the substrate temperature constant at higher Ar concentrations (above ∼40 vol.% Ar). Room temperature photoluminescence spectroscopy also confirmed that argon addition increases the density of vacancy defects in the diamond structure.

In this work, we studied the properties of thin nanoporous carbon coatings on very thick porous silicon (PS) layers of very high porosity. The PS was obtained by etching pores into crystalline Si wafers in a process controlled to prevent crack formation during etching and drying. The nanoporous carbon was made by dipping the PS samples in diluted polyvinyl alcohol and submitting the coating to pyrolysis at different temperatures (in the 513–843 K range). The porous carbon coatings obtained on porous silicon using this method presented good adherence, uniform thickness and good optical transparency, the greatest luminescence intensity being obtained at a pyrolysis temperature of 513 K. Such a composite nanoporous structure of two type IV materials may find a large number of future applications, ranging from luminescent devices to molecular sieves.

Diamond growth on non-diamond substrates initially requires a heterogeneous nucleation step on the substrate surface and, depending on the substrate material, the nucleation time may be very long and the nuclei density may also be very low, compromising the film quality. Seeding of substrates prior to chemical vapor deposition (CVD), typically using diamond grains, is commonly employed to increase nucleation of the growing film. Alternative nucleation methods are of great interest. In this work, we demonstrate the feasibility of using natural hematite to increase nucleation on silicon. This mineral in lamellae form is very soft and minimizes damage to the substrate surface. Hematite grains of a silica-rich phase improved the crystalline faceting and the coalescence of the growing diamond, indicating that complex oxides, such as iron silicates, may favor diamond nucleation. The nucleation density was approximately 0.8 × 10 6 nuclei cm -2 , which is a thousand times greater than that obtained on the 'unseeded' silicon surface. Samples 'seeded' with hematite grains of the silica-rich phase presented sharper peaks at 1332 cm -1 (higher aspect-ratio), indicating a higher diamond quality.

The effect of nitrogen incorporation on the electrical properties of diamond is currently under intense study. In this work, diamond grown by hot filament chemical vapor deposition was exposed to a radiofrequency (40 MHz) glow discharge of pure nitrogen for times of between 5 min and 1 h. The effect of this exposure on the chemical and elemental composition of the samples was assessed by Raman and X-ray photoelectron spectroscopy. Changes in the electrical resistance with increasing film nitrogenation and temperature were measured. The scope and limitations of this approach to nitrogenation are evaluated.

Abeles (in E. Wolf (Ed.), Progress in Optics, North-Holland, 1968, p. 251) has shown that the processes of determination of the refractive index and thickness of a transparent film may be completely separated. When under varying incidence angle the relative reflectances of p-polarized light at a film-covered and uncovered substrate's surfaces become equal, this means that the angle of incidence coincides with the Brewster angle, θB1, of the ambient–film interface. From a complementary point of view, for a film with non-uniform thickness it is an angle where the interference fringes of all the interference patterns, corresponding to the different film thicknesses, disappear. Moreover, there is yet one angle, θB2, where a sample's reflection also ceases to depend on the film's thickness and the interference fringes disappear: this is the Brewster angle of the film–substrate interface. Since the reflectances from a growing film or from different places of a tapered film have the same values at these angles θB1 and θB2 the interference patterns should intersect each other at these specific points. For a weak absorptive film the diminution, due to film's absorption, of the reflectance at the angle θB1 allows us to determine the extinction coefficient of the film.

Thick porous silicon (PS) layers, made by anodic etching of crystalline Si wafers, have been coated with diamond films using the hot-filament chemical vapor deposition (CVD) technique. Ethanol diluted in hydrogen was used as the carbon source for diamond deposition. We observed that diamond nucleation occurs predominantly on the top of the PS spikes, creating individual islands of growth. Nucleation is apparently homogeneous over the PS surface and the islands grow independently until the complete coalescence of the diamond film. Although the diamond film is polycrystalline, it does not have the usual columnar structure of diamond deposited on c-Si. A good adherence between diamond and PS was observed in diamond–PS and diamond–PS–diamond structures. The problems of the intrinsic and thermal stresses have been addressed. Samples were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), Raman and photoluminescence spectroscopy.

Earlier we proposed a new type of dielectric anisotropic protective coating for Brewster-angle windows (BAW) that retain both the position of the Brewster angle and the true zero of the substrate's reflection factor for p-polarized light [1] . The useful property of such coatings consists in the independence of their optical quality from their thickness and this makes them less sensitive to surface damage. On the other hand, by adjusting their thickness one can influence the reflection of the s-polarized light. Here we will show that for any given substrate one can choose a certain value of the horizontal tensor component of the negative anisotropic coating film and then, by adjusting the film's thickness, reduce to zero the reflection factor for s-polarized light as well. In the region of positive anisotropy the same optical thickness of a film maximizes the s-polarized light reflectance which may be much greater than that of a clean substrate. Both these variants may be useful under adoption of BAW in resonant laser systems for second and higher harmonic generation.

Nitrogenated diamond and carbon nitride films have been produced using ammnonia as the source of atomic nitrogen. A feed mixture of C 2 H 5 OH+NH 3 +H 2 was used in a hot filament CVD reactor. The deposition parameters were optimized for the growth of well-faceted crystals onto either SiO 2 or undoped diamond substrates. Film structure and morphology were studied using optical and atomic force microscopies, and Raman and photoluminescence spectroscopies. Three types of film, according to the deposition substrate used and the nitrogen doping, have been identified. All films presented semiconductor characteristics and their deep and shallow activation energies have been determined from resistance versus temperature measurements.

Conducting Langmuir-Blodgett (LB) films have been fabricated from parent polyaniline (PAni) which was doped with functionalized acids. In order to optimize experimental conditions for the formation of stable Langmuir monolayers and their subsequent transfer onto solid substrates, PAni was dissolved in ten different combinations of chloroform solutions. Use was made of camphor sulfonic acid, dodecyl benzene sulfonic acid, and toluenesulfonic acid, and of the solvents N-methyl pyrrolidine and m-cresol as processing agents. Because acidic subphases have been employed, as-deposited LB films were already doped, which was confirmed by the appearance of a polaronic band in the UV-Vis absorption spectra. The absorbance peak increases with the number of deposited layers indicating that a suitable multilayer buildup is accomplished. When analysed by atomic force microscopy, PAni LB films show a fibrillar structure with the fibril width ranging from ≈60 to 160 nm.

A new method of determination of thin film anisotropy from the angular dependence of the reflectance interference patterns for s- and p-polarized light is proposed and tested experimentally. The method is based on the different phase angle dependence of polarized light on the incident angle. As a result, the interference patterns of the reflected s- and p-polarized light beams exhibit a different number of oscillations in their angular dependence. The high sensitivity of the method is shown by its application to the interference patterns of a specially prepared multilayer structure with a calculated anisotropy.