S.E. Rodil

Raman and infrared modes of hydrogenated amorphous carbon nitride

Features in the Raman and infrared (IR) spectra of highly sp3 bonded hydrogenated amorphous carbon nitride films are assigned. The Raman spectra show three main features all found in a-C itself, the G and D peaks at 1550 and 1350 cm−1, respectively, and the L peak near 700 cm−1. The intensity ratio of the D and G peaks, I(D)/I(G), is found to scale as (band gap)−2, which confirms that nitrogen induces carbon to form sp2 graphitic clusters. The intensity of the L mode is found to scale with the D mode, supporting its identification as an in-plane rotational mode of sixfold rings in graphitic clusters. A small feature at 2200 cm−1 due to C≡N modes is seen, but otherwise the Raman spectra resembles that of a-C and shows no specific features due to N atoms. The hydrogen content is found to have a strong effect on the IR spectra at 1100–1600 cm−1 making this band asymmetric towards the 1600 cm−1 region.

Is stress necessary to stabilise sp3 bonding in diamond-like carbon?

Abstract The role of compressive stress in producing sp3 bonding in diamond-like carbon is of interest both technologically and scientifically. Stress limits the maximum thickness of adherent films, and it is desired to produce much thicker films for protective coatings and for making micro-electromechanical systems. Stress is important theoretically, because it is often linked to the deposition process. A strong correlation between macroscopic stress and sp3 fraction in diamond-like carbons has been noted, particularly for tetrahedral amorphous carbon (ta-C). However, a survey of data shows that a given stress produces films with sp3 contents between 20 and 85%, while for a given sp3 content, stresses between 2 and 19 GPa have been found. We propose that the main cause of stress is ion bombardment, and that a low energy of only 20 eV/ion is needed to produce films with an sp3 content over 70%. We discuss the various models linking stress and the sp3 fraction in ta-C. The role of densification vs. compressive stress in stabilising sp3 bonding is also discussed.

Properties of carbon films and their biocompatibility using in-vitro tests

In this paper we report the results of a comparative study of the biological response of amorphous carbon coated stainless steel. Films of amorphous carbon (a-C), amorphous carbon nitride (a-CN) and hydrogenated amorphous carbon (a-C:H) were deposited on stainless steel substrates (AISI 316L) using a dc magnetron sputtering system. In-vitro studies were carried out on the coated samples using human osteoblasts cell culture lines and fibroblasts. Preliminary biocompatibility was assessed by cell adhesion and proliferation, as determined by a spectroscopic technique. Comparison of the optical absorbance results between control uncoated disks and the test cultures provided a semi-quantitative analysis of the cytotoxic effect of the different carbon compounds. Osteoblasts cells were grown on uncoated steel, a-C, a-CN and Ti coated steel samples. The degree of fibroblast adhesion measured at 24 h is very similar for all the test samples, however, osteoblasts adhesion was higher for a-C films. Similarly, cellular proliferation at 7 days showed an outstanding increase of osteoblasts cells for a-C and Ti in contrast with uncoated steel. The physical film properties, such as, roughness measured by atomic force microscopy, surface composition determined by both Rutherford Backscattering and Auger Spectroscopy and the electro-optic properties of the films were also determined. The relation between film properties and cellular response is discussed.

Role of sp2 phase in field emission from nanostructured carbons

It is shown that sp2 phase organization plays an important role in the field emission from nanostructured carbons. Emission is found to depend on the cluster size, anisotropy, and mesoscale bonding of the sp2 phase, and the electronic disorder. It is found by Raman spectroscopy that increasing the size of sp2 clusters in the 1–10 nm range improves emission. Anisotropy in the sp2 phase orientation can help or inhibit the emission. sp2 clusters embedded in the sp3 matrix or electronic disorder induced by localized defects oriented in the field direction can provide a local field enhancement to facilitate the emission.

Resonant Raman spectra of amorphous carbon nitrides: the G peak dispersion

Abstract A general model is presented for the interpretation of the Raman spectra of amorphous carbon nitrides measured at any excitation energy. The Raman spectra can be explained in terms of an amorphous carbon based model, without need of extra peaks due to CN, NN or NH modes. We classify amorphous carbon nitride films in four classes, according to the corresponding N-free film: a-C:N, a-C:H:N, ta-C:H:N and ta-C:N. In all cases, a multi-wavelength Raman study allows a direct correlation of the Raman parameters with the N content, which is not generally possible for single wavelength excitation. The G peak dispersion emerges as a most informative parameter for Raman analysis. UV Raman enhances the sp1 CN peak, which is usually too faint to be seen in visible excitation.

Infrared spectra of carbon nitride films

The assignment of the vibrational modes in amorphous carbon nitride (CN) films is discussed by considering CN films deposited using a variety of methods. The infrared (IR) spectra of CN show three main absorption bands. In hydrogenated CN samples, CHx and NHx groups give rise to stretching vibrations at 3000 and 3400 cm−1, respectively. A weaker sharp band is observed approximately 2200 cm−1 due to CN–sp1 bonds. Finally, there is a broad band between 1000 and 2000 cm−1. It is usually stated that the effect of nitrogen into carbon films is to break the symmetry of the sp2 carbon bonds making the Raman ‘G’ (graphitic) and ‘D’ (disorder) modes IR active, so the broad band between 1000 and 2000 cm−1 is similar in both IR and Raman spectra. However, it is shown that nitrogen is not necessary to have significant IR activity in the 1000–2000 cm−1 region. Also, Raman spectroscopy in carbon is always a resonant process, so that the spectra depend on the excitation energy. Therefore, the similarity of the visible Raman and IR spectra of some CNs is generally a coincidence. We show that the IR broad band in the 1000–2000 cm−1 region is an electronic effect and is not due to activation of IR forbidden modes due to symmetry breaking. This explains the IR spectra not only of CN films but also of N-free amorphous carbon films and is related to the presence of the system of delocalized π bonds with increasing conjugation.

Nitrogen Incorporation into Tetrahedral Hydrogenated Amorphous Carbon

Structural changes induced by the incorporation of nitrogen into ta-C : H films have been studied by Electron Energy Loss Spectroscopy, X-Ray Photoelectron Spectroscopy, Fourier Transformed Infrared Spectroscopy and Ultraviolet-Visible Spectroscopy. ta-C:H films have been synthesised using a low pressure Electron Cyclotron Wave Resonance (ECWR) source which provides a plasma beam with a high degree of ionisation and dissociation. Nitrogen was incorporated by adding N 2 to the C 2 H 2 plasma used for the deposition of ta-C : H films. The N/C atomic ratio in the films rises rapidly until the N 2 /C 2 H 2 gas ratio reaches three, and then increases more gradually, while the deposition rate decreases steeply. Chemical sputtering of the forming films and the formation of molecular nitrogen within the films limit the maximum nitrogen content to about N/C = 0.6. For low nitrogen content the films retain their diamond-like properties, however as N/C atomic ratio increases, a polymeric-like material is formed, with > C=N- structures and terminating C=N and NH groups that decrease the connectivity of the network.

Optical gap in carbon nitride films

Abstract In this paper we study the effect of introducing nitrogen into different carbon networks. Two kinds of carbon nitride films were deposited: (a) Using a DC-magnetron sputtering system sp 2 bonded carbon nitride (a-CN) films were deposited and (b) Using a combination of filtered cathodic vacuum arc and a low-pressure N 2 plasma source, N was introduced into sp 3 carbon networks (ta-C), leading to the formation of a more dense CN film named ta-CN. For ta-CN films we found that the optical gap initially decreases as the N content and the sp 2 fraction rises, but above a certain N quantity there is a level-off of the value, and the gap then remains constant despite further increases in the fraction and clustering of the sp 2 phase. However, for a-CN films the optical gap increases with the nitrogen content. These two different trends are not easily explained using the same framework as that for carbon films, in which any decrease in the band gap is associated to an increase in the sp 2 fraction or its clustering. Here we discuss the conditions that lead to high optical gap in sp 2 -bonded carbon nitride samples, which are clearly not associated to the presence of any crystalline super-hard phase. We also compared other differences in properties observed between the two films, such as deposition rate, infrared and Raman spectra.

Effect of graphitic inclusions on the optical gap of tetrahedral amorphous carbon films

High sp3 fraction tetrahedral amorphous carbon (ta-C) films can be prepared using the filtered cathodic vacuum arc (FCVA). A by-product of the deposition process are small micrometer sized graphitic particles which are also incorporated into the film. The particle coverage of FCVA films is typically <5%, and thus the effect of these graphite inclusions have been largely ignored in earlier optical gap measurements of ta-C. By incorporating a better filter design (e.g., S-bend filter), the particle coverage can be reduced to 0.1%. In this article, we show that the effect of these graphitic inclusions is to scatter or absorb light which significantly affects the optical gap measurement and hence reduces the “apparent” optical gap of the ta-C film. By comparing two ta-C films with different particle coverage but the same sp3 content of 85%, we show that we can correct for the effect of these inclusions. Our results confirm that the E04 gap of a 85% sp3ta-C matrix is 3.6 eV. The importance of considering these...

Oral bacterial adhesion on amorphous carbon and titanium films: effect of surface roughness and culture media.

Implant infections can cause severe problems from malfunctioning to dangerous sepsis affecting the health of the patient. For many years, titanium has been the most common material used on dental implants due to their mechanical and biocompatibility properties. Recent studies suggest that amorphous carbon (a-C) films can be possible candidates for coating dental implants, improving some important features like biocompatibility and bone formation. In the oral cavity, the risk of an implant infection is high due to multiple species are capable to colonize this site and these biofilm infections can limit the use of these medical devices. The purpose of this study was to evaluate the influence of the surface chemistry, roughness, and culture media in the bacterial colonization process. To achieve this, a-C and Ti films were deposited on rough and smooth surfaces and cultured with different microorganisms belonging to the oral microbiota with mycoplasma medium (MM) or human saliva (HS). Samples were incubated for 24 h, after this, samples were sonicated and the number of attached bacteria was determined by counting the colony-forming units (CFUs) from each sample. The proportion of the species in the biofilms was determined using checkerboard DNA-DNA hybridization. Data were analyzed by Students t test using Bonferronis modification of Students t test and differences on the proportion of the bacterial species attached to each surface were determined using the Mann-Whitney test. Results show an increased number of CFUs on rough surfaces, especially on the a-C surfaces. The incubation media were an important factor on the adhesion of certain taxa, whereas other species were more sensitive to surface chemistry and others to surface roughness.