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The structural and optical properties of technologically interesting pyrolytic carbons formed from cured novolac resin and cured novolac/biomass composites were studied by X-Ray Diffraction Analysis (XRD), and Fourier Transform Infrared (FTIR), Raman and Photoluminescence (PL) spectroscopy. Pyrolysis of the cured materials took place at temperatures in the range 400-1000 degrees C. The most important weight loss, shrinkage and structural changes of pyrolyzed composites are observed at temperatures up to 600 degrees C due to the olive stone component. In the same temperature range, the changes in pyrolyzed novolac are smaller. The spectroscopic analysis shows that novolac pyrolyzed up to 900 ( degrees )C has less defects and disorder than the composites. However, above 900 ( degrees )C, pyrolyzed novolac becomes more disordered compared to the pyrolyzed composites. It is concluded that partial replacement of novolac by olive stone in the composite materials leads to the formation of a low cost, good quality product.

We report on the application of a static (dc) electric field in the plume region during the pulsed Nd doped yttrium aluminum garnet laser deposition (PLD) of tetrahedral amorphous carbon (ta-C) films in vacuum ambient (pressure=10-4–10-3 Pa), where the working pressure is exclusively due to ablation vapor. This approach is strikingly different from the plasma- or ion-beam-assisted PLD because the mean free path at this pressure is by far longer than the target to substrate distance. Thus, the electric field interacts with individual ionized species invoking ion acceleration and gas-phase reactions among different ionized species. These phenomena are clearly dependent on the laser wavelength (first, second, or third harmonic, λ=1064, 532, and 355 nm, respectively) used for the ablation. We found that the application of the electric field causes surface smoothing (the roughness decreases from about 1 to 0.4 nm) and faster deposition rate (from about 2 to 7 nm/min) for the second and third harmonics. In addition, the phenomena are less intense in the case of the first harmonic due to the low concentration of ionized species in the plume. In addition, in the case of PLD using λ=532 nm, the electric field improves the film’s density (from 2.60 to 2.95 g/cm3). The correlations found are discussed in terms of the ablated species and the deposition mechanisms of the ta-C. [ABSTRACT FROM AUTHOR]

The structural and optical properties of nitrogenated amorphous carbon films, grown by rf-magnetron sputtering on silicon substrates, were studied by Raman and photoluminescence (PL) spectroscopy as a function of the nitrogen concentration and the substrate bias voltage V[sub b]. For films deposited with V[sub b]=10 V, the photoluminescence emission was most intense at nitrogen concentrations in the carrier gas of 25% (75% Ar), while the intensity ratio I(D)/I(G) of the Raman bands of disordered graphite (D band) and graphite (G band) partially substituted by nitrogen exhibited a minimum simultaneously observed with a minimum of G-band frequency and a maximum of G-band width. Changes in spectral characteristics of Raman scattering at a concentration of 25% (≅30 at %) are indicative of an increase of sp³-bonded fraction and disorder. PL-enhancement coincides, in this case, with structural changes and is probably correlated to the substitution of nitrogen in the tetrahedraly bonded amorphous matrix. In the case of films deposited in a pure nitrogen atmosphere, N[sub 2] = 100%, no significant PL-intensity changes appeared to exist between films deposited at low positive (10 V) and highly negative (-200 V) substrate bias. After several months of sample storage in air, samples grown at negative V[sub b] were found to preserve their structural and optical properties, while films grown at positive bias (V[sub b]=10) and nitrogen concentrations in the carrier gas above 70% (≅40 at %) delaminated. [ABSTRACT FROM AUTHOR]

The structural and optical properties of amorphous carbon films, grown by r.f.-magnetron sputtering on silicon substrates, were studied by Raman and photoluminescence spectroscopy in dependence of the substrate bias voltage Vb. The intensity ratio I(D)/I(G) of the Raman bands of disordered graphite (D-band) and graphite (G-band) decreased significantly (a) by reversing bias from positive (+10 V) to negative (−20 V) and (b) by reducing the negative bias from −200 to −120 V. The intensity ratio I(D)/I(G) exhibited an almost flat minimum in the bias-region from −120 to −20 V which is indicative of an increase of the fraction of sp3-bonded material. In the same bias-range, photoluminescence emission at 2.2–2.5 eV appeared blue-shifted and more efficient. Changes in photoluminescence energy are attributed to increase of sp3 content of the films becoming more transparent when deposited at substrate-bias between −20 and −120 V. Increase of photoluminescence intensity is probably related to increased number of defects due to increasing structural disorder. [Copyright &y& Elsevier]

The electrical properties of thick amorphous carbon (a-C) films with sequential sp[sup 3]-rich/sp[sup 2]-rich layered structure, grown by magnetron sputtering on Si substrates at room temperature, were investigated. At low electric fields, the conduction is due to the variable range hopping mechanism. At high electric fields, thermally assisted band-to-band indirect tunneling is the dominant conduction mechanism, while the Arrhenius plots of the current show a deviation from straight lines in the form of continuous bending satisfying the Meyer–Nelder rule. Comparative studies of low-frequency noise in sp[sup 2]-rich single layer and sp[sup 3]-rich/sp[sup 2]-rich layered a-C films indicate that the noise in the a-C layered originates from traps located mainly at the interfaces of the sp[sup 3]-rich/sp[sup 2]-rich bilayers. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: Metal/amorphous carbon (a-C:M) composite films are emerging as a category of very important engineering materials for surface protection. We implement pulsed laser deposition (PLD) to grow pure a-C and a-C:Ag nanocomposites. Our PLD process is assisted by a static electric field. We investigate the structural features of the a-C:Ag nanocomposites and the bonding configuration of the a-C matrix with respect to the electric field and the composition of the PLD target. For this study we use Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and X-ray diffraction (XRD). We show that the Ag mean grain size and the sp2 content of the a-C matrix are increasing with increasing Ag content in the films. [Copyright &y& Elsevier]