Unhydrogenated Amorphous Carbon Research Articles

In vitro cytocompatibility evaluation of hydrogenated and unhydrogenated carbon films

Hydrogenated amorphous carbon (a-C:H) films and unhydrogenated amorphous carbon (a-C) films are promising as artificial joint coatings due to their excellent wear resistance. In this paper, the cytocompatibility of a-C:H film and a-C film was evaluated to determine the more appropriate type for prosthesis modification. The a-C:H film was deposited by electron convolute resonance plasma enhanced chemical vapor deposition (ECR-PECVD) and a-C film was fabricated by magnetic filtered cathodic vacuum arc (MFCVA). The microstructure and physical characteristics of the films were investigated by Raman spectroscopy, contact angle measurement, atom force microscopy (AFM), electrokinetic analysis, electrical resistivity measurement and Hall effect measurement. Bovine serum albumin (BSA) and serum protein adsorption on a-C and a-C:H films was determined by micro BCA assay. RAW264.7 macrophages were cultured on the a-C:H and a-C films for 24h to evaluate the cell death, cell activation and inflammatory cytokine release. Primary mouse osteoblasts were cultured on the a-C:H and a-C films for 2h, 1day, 3days and 6days with biological tests performed to evaluate the cell adhesion, cell viability and cell morphology. The results show that a-C film is N type semiconductor with unpaired electrons. Protein adsorption assay shows that the a-C film can covalently bind more serum proteins than a-C:H film. The unpaired electrons of a-C film contribute to its better ability to covalently bind bioactive proteins than a-C:H film, and the superior adsorption and bioactivity of the adhesion proteins on a-C film further induce the better biological performance of a-C film. In a word, a-C film induces lower inflammatory reaction and higher ostoblasts viability than a-C:H film.

Deposition of unhydrogenated amorphous carbon films by sublimation of C 60 fullerene in electron beam excited plasma

C 60 fullerene clusters are used as a carbon source to deposit unhydrogenated amorphous carbon films. C 60 clusters are sublimated by heating up to 850 °C. The sublimated fullerene powders are injected to an electron beam excited argon plasma and dissociated to be active species. Consequently, the carbon species condense as a thin film on the negatively biased substrates that are immersed in the plasma. Deposition rates of approximately 1.0 µm/h and the average surface roughness of 0.2 nm are achieved. X-ray diffraction pattern reveals that the microstructure of the film is amorphous while fullerene film deposited without the plasma shows crystal structure. Raman spectroscopic analysis shows that the films are one of the types of diamond-like carbon films. The nano-indentation technique is used for hardness measurement of the films and results in hardness up to about 20 GPa.

Bonding structure and haemocompatibility of silicon-incorporated amorphous carbon

Although silicon doping of amorphous carbon films can lower the intrinsic compressive stress, doping alone cannot ensure a stress free state. An additional thermal processing is needed for complete stress relaxation. This additional process can, however, alter the surface properties of the film and its relative haemocompatibility. In this study, unhydrogenated amorphous carbon film incorporated with silicon was synthesized by magnetron sputtering and annealed to relieve stress. The evolution of the bonding structure and compatibility with blood were investigated. The results showed that the clustering of the sp 2-bonded carbon was retarded, and the amorphous network disordering was increased when annealing temperature increased to 600 °C. Surface thrombogenesis of the films were evaluated using in-vitro platelet adhesion assay. The static evaluation of the films incubated in human platelet-rich-plasma showed an increase in the number of adherent platelets, but with lower activation percentage.

Temperature dependence of the electron paramagnetic resonance of unhydrogenated carbon nitride

Temperature-dependent electron paramagnetic resonance (EPR) and conductivity measurements were performed on unhydrogenated amorphous carbon nitride (a-CN x ) thin films in an attempt to investigate electronic transport mechanisms in the layers. The films were synthesized by pulsed laser ablation of graphite in a nitrogen remote plasma. As nitrogen incorporation decreased from 25 to 16 at.% with increasing laser intensity, the electrical conductivity at room temperature, σ, increased from 0.33 to 17 (Ω·cm) −1. This behavior correlates well with increasing spin densities (from 4 to 9×10 20 cm −3) and a broadening of the Lorentzian EPR line ( g=2.0028), from 2.6 to 3.8 G. The EPR linewidth can be decomposed into temperature independent and temperature dependent parts. At 4 K, the EPR signal is 1.9 G wide, independent of nitrogen content. However, the temperature-dependent part of the EPR linewidth broadens with temperature, with a T −1/4 dependence, thus indicating that spin relaxation also occurs through a hopping mechanism. Between 100 and 270 K, a hopping conduction mechanism is evidenced by a linear dependence of ln( σ) vs. T −1/4. Values of T 0, as deduced from the EPR measurements, are close to those obtained from conductivity measurements for films with the lowest nitrogen, thus suggesting a correspondence between spins and charge carriers. However, such a correspondence is not observed at higher nitrogen content, as the EPR-deduced T 0 values become much smaller than the ones obtained from conductivity measurements. These results are explained in terms of the different arrangement of the carbon sp 2 bonded domains in the films.

Spectroscopic studies of nitrogenated amorphous carbon films prepared by ion beam sputtering

Spectroscopic analysis of the unhydrogenated amorphous carbon nitride (a-CNx) films, prepared by ion beam sputtering, was done by ellipsometry, Raman scattering and x-ray photoelectron spectroscopic (XPS) studies. The optical gap of the films was estimated from the Tauc’s analysis of the (n,k) data obtained from spectroscopic ellipsometry. In addition to the commonly observed D and G bands at ∼1350 cm−1 and 1550 cm−1, respectively, we detected a separate band at ∼1450 cm−1 in the Raman spectrum of a-CNx films. This intermediate peak was unambiguously identified as the N band arising out of the nitrogen–nitrogen bonding, due to the incorporation of nitrogen in the a-C network. High resolution XPS C 1s and N 1s peaks were used to estimate the nitrogen content in the films and various bonding configurations were identified from their Gaussian deconvolution. An increase in the intensity ratio of CN and CC components, I(CN)/I(CC), in the C 1s spectra signified increasing nitrogenation of the carbon network. Deconvolution of the N 1s spectra revealed that the sp3-hybridized C–N component remained unchanged at ∼40%, however, the sp1-hybridized C≡N component increased in intensity when the nitrogen content in the network increased systematically. Increasing nitrogenation led to the graphitization of the network and resulted in a gradual decrease in the optical gap, along with simultaneous increase in the ID/IG and IN/IG intensity ratios in the Raman band.

Direct quantitative detection of the sp3 bonding in diamond-like carbon films using ultraviolet and visible Raman spectroscopy

The bonding in a series of unhydrogenated amorphous carbon films has been analyzed quantitatively using Raman spectroscopy excited by laser light in both the visible and ultraviolet regions of the spectrum. The asymmetry of the peak near 1550 cm−1 in the visible Raman spectra is correlated with the percentage of sp3 bonding in the films. The ultraviolet Raman spectra exhibit two broad Raman peaks at 1650 and 1100 cm−1, due to sp2 and sp3 vibrational modes, respectively. The former is a resonance feature associated with a large proportion of paired sp2 sites, while the latter is a weighted phonon density-of-states for the distorted random network of sp3 sites. The position and relative intensity of the two peaks are shown to be strongly correlated with the percentage of sp3 sites in the films, providing a reliable measure of sp3 bonding which is both semiquantitative and nondestructive.

Amorphous carbon films rich in diamond deposited by magnetron sputtering

In this work evidence is presented that unhydrogenated amorphous carbon films approaching diamond can be deposited by rf magnetron sputtering at room temperature. In situ spectroscopic ellipsometry (SE) studies, performed in the energy range 1.5–5.5 eV, have been used to monitor and characterize the firms during deposition, as well as to define the range of deposition conditions which produce films rich in sp 3 C-C bonds. The internal stress developed in these films during deposition was also measured by the cantilever laser technique. A strong dependence was found between the sp 3 volume fraction, calculated by analyzing the SE data, and the internal stress in the films. The effects of bias voltage and thickness on the film stress were also examined and are discussed. It was finally observed that films beyond a critical thickness and with an internal stress level above 6–7 GPa became metastable when exposed to air at atmospheric pressure.

Quantitative Studies of Tetrahedral Bonding in Amorphous Carbon Films Using Ultraviolet Raman Spectroscopy

ABSTRACTThe bonding in a series of unhydrogenated amorphous carbon films has been analysed quantitatively using Raman spectroscopy with ultraviolet excitation. The Raman spectra exhibit two broad Raman peaks at 1650 cm−1 and 1100 cm−1, due to sp2 and sp3 vibrational modes respectively. The former is a resonance feature associated with a large proportion of paired sp2 sites, while the latter is a weighted vibrational density-of-states for the distorted random network of sp3 sites. The position and relative intensity of the two peaks are shown to be strongly correlated with the percentage of sp3 sites in the films, providing a reliable measure of sp3 bonding which is both quantitative and non-destructive.

Influence of water vapor and oxygen on the tribology of carbon materials with sp 2 valence configuration

The tribological behaviors of graphite, non-graphitic carbon, carbon-carbon composite, and unhydrogenated amorphous carbon in nitrogen and air are investigated. The friction and wear of all carbon materials can be similarly influenced by water vapor and oxygen. Carbon materials generally exhibit a high friction and wear behavior when there is a lack of lubricant vapors in the environment. Water vapor, even in small quantities, is effective in lubricating carbon materials-a concentration of 100 ppm in nitrogen can lubricate most of the materials examined in this study. Depending on the water vapor content in the environment and the friction regime involved, oxygen can act either as a lubricant or an agent for tribochemical wear. The tribological properties of carbon materials depend on gases and vapors in the environment that can interact with carbon atoms. In spite of having a wide range of physical properties, the family of sp 2 carbon materials can exhibit similar friction and wear behaviors in various environments owing to their common valence structure.

Optical constants of hydrogenated and unhydrogenated amorphous carbon in the 05–12-eV range

The real and imaginary parts of the refractive index have been obtained in a wide energy range (0.5-12 eV) for films (0.2-0.3 µm thick) of amorphous carbon with different hydrogen contents (0-40 at. %) to obtain more insight into the electronic structure of these materials. Reflectance and transmittance measurements have been processed with two different approaches in the low- and high-energy parts of the spectra. The results are compared with the known optical properties of bulk diamond and graphite to obtain the ratio between trigonal and tetrahedral carbon atoms. It was found that the number of trigonal carbon sites increases from 20% to 80%, decreasing the hydrogen concentration from 40 to 15 at.%.

Photoluminescence fatigue in amorphous carbon ( a-C) films prepared by d.c. magnetron sputtering

We have studied the fatigue effect in unhydrogenated amorphous carbon ( a-C) films prepared by d.c. magnetron sputtering of graphite. At 12 K the fatifued photoluminescence spectra show neither a spectral shape change nor peak shift during illumination. The fatigued photoluminescence intensity at a fixed energy shows a t − b time process during illumination. The self-recovery of the fatigued photoluminescence in the dark requires much more time than hydrogenated amorphous silicon ( a-Si : H) and less than amorphous phosphorus ( a-P). The light induced defects in a-C films are in metastable states and explained to be π defects rather than dangling bonds (σ defects) as in a-Si : H or hydrogenated amorphous carbon ( a-C : H).

Optical absorption and photoluminescence of unhydrogenated amorphous carbon films

Unhydrogenated amorphous carbon (a-C) films were prepared by a laser beam technique. Optical absorption, photoluminescence (PL) and the dependence of PL spectra on the incident laser intensity were studied. The optical gap, peak energy of the PL spectrum and band tail width change with preparation bias voltage. At room temperature PL spectra exhibit a superlinear behaviour at lower incident intensity, linear behaviour at higher intensities, and a large blue shift of peak energy with increasing excitation intensity. These results can be qualitatively explained in the light of the distribution of gap states and tail state population.

Structural properties and energetics of amorphous forms of carbon.

We have made a comparative theoretical study of the most common forms of unhydrogenated amorphous carbon (\ensuremath{\alpha}-C), namely, of the dense, diamondlike phase and the low-density evaporated \ensuremath{\alpha}-C (e-C). Emphasis is given to the connection among the structure, energetics, and stability of these phases. To make the simulations of the amorphous structures (formed by quenching the liquid) tractable, we used the Monte Carlo method, combined with the empirical-potential approach. Our analysis employs a powerful total-energy-partitioning scheme, which is proved very useful in treating the energetics of disordered systems. It is found that threefold ${\mathit{sp}}^{2}$ sites are the energetically favorable geometries in e-C, and thus they are by far more numerous. The nonplanar character of ${\mathit{sp}}^{2}$ sites and the absence of sixfold rings indicate that medium-range order is rather not significant in e-C. The increasing graphitic character of e-C, as the temperature is raised, is explained by resorting to the effective temperatures ${\mathit{T}}^{\mathrm{*}}$, at which the atoms freeze in their lattice positions. For diamondlike \ensuremath{\alpha}-C, the simulations show that there exist two distinctly different dense structures. The ``as-quenched'' one (i-C) is mostly ${\mathit{sp}}^{3}$ bonded, but it is metastable. Upon annealing, it converts into a second phase (i-${\mathrm{C}}^{\mathrm{*}}$), mostly ${\mathit{sp}}^{2}$ bonded, with a significant energy gain. A specific mechanism is proposed for this transition. The insensitivity of density to annealing is explained if we use the concept of the ``glass transition temperature'' ${\mathit{T}}^{\mathrm{*}}$. Finally, by introducing an isotropic bulk modulus for the amorphous phase, it is found that e-C has a much lower compressibility than i-${\mathrm{C}}^{\mathrm{*}}$, enhancing the distinguishability among the two low-coordinated forms of \ensuremath{\alpha}-C.