Electrochemical behavior of amorphous carbon obtained from camphor

Amorphous carbon films were deposited on alumina plate by pyrolysing camphor at different temperatures with thermal chemical vapor deposition (CVD) technique. Carbon coated alumina plates were used as working electrode to ascertain their electrochemical behavior in different electrolytic media. Electrochemical windows of these carbon films were found to be suitable in the potential range of 1.05 to −0.30 V versus SCE in acid medium. In the presence of redox electrolyte, cathodic-anodic peak separation was found to be the same as that obtained with diamond. Raman spectra of carbon films were studied to explain some of their electrochemical behavior.     

Nanoindentation and nanoscratching studies of amorphous carbon films

Hydrogen-free amorphous carbon (a-C) films prepared by RF magnetron sputtering were deposited on Si substrates in thin films, at various negative bias voltages V_b (i.e. Ar-ion energies), and in thick layered-structure films with alternative values of V_b. The main purposes of this work are to present preliminary results concerning the effect of Ar-ion bombardment during deposition on the elastic properties of thin a-C films with Ar⁺ energies in the range 30–200 eV, and the adhesion failure which limits their thickness and usefulness for practical applications, and the enhancement of hardness and scratch resistance of sputtered a-C films developed in a layered structure. The results show a significant improvement in the elastic properties of layered structure films and their stability. The combination of high hardness and relative low elastic modulus which the layered films exhibit make them more resistant to plastic deformation during contact, as confirmed by scratch testing.     

Resonant Raman studies of tetrahedral amorphous carbon films

Resonant Raman scattering has been used to study the tetrahedral amorphous carbon films deposited by the filtered cathodic vacuum arc technique. The excitation wavelengths were 244, 488, 514 and 633 nm, corresponding to photon energies of 5.08, 2.54, 2.41 and 1.96 eV, respectively. In the visible Raman spectra only vibrational modes of sp²-bonded carbon (G and D peaks) are observed, while a wide peak (called the T peak) can be observed at approximately 1100 cm⁻¹ by UV-Raman spectra which is associated with the vibrational mode of sp³-bonded carbon. Both the position and the width of the G peak decrease almost linearly with increasing excitation wavelength, which is interpreted in terms of the selective ππ* resonant Raman scattering of sp²-bonded carbon clusters with various sizes. The G peak position in the UV-Raman spectra, the T peak position and the intensity ratios of I_D/I_G and I_T/I_G all exhibit maximum or minimum values at the carbon ion energy of 100 eV. The changes of these spectral parameters are discussed and correlated with the sp³ fraction of carbon atoms in the films.     

Defect studies and photoelectrical properties of phosphorus doped amorphous carbon films

This paper reports on the successful deposition of n-type phosphorus doped carbon (n-C:P) thin films and fabrication of n-C:P/p-Si cells by pulsed laser deposition (PLD) using graphite target at room temperature. The cell performances have been tested in the dark for the current–voltage (I–V) rectifying curve and I–V working curve under illumination when exposed to AM 1.5 SUN illumination condition (100 mW/cm², 25 °C). The cells fabricated using 7% of phosphorus by weight percentages in the graphite target (Pwt%) show the highest energy conversion efficiency, η=1.14% and fill factor, FF=41%. The quantum efficiency of the cells is observed to improve with Pwt%. The dependence of Pwt% on the optical and physical properties of the deposited films and the photovoltaic characteristic of the n-C:P/p-Si heterojunction cells are discussed.     

Multi-walled carbon nanotubes on amorphous carbon films

Nanotubular structures composed of layered graphite sheets or other layered materials have been studied intensely by both scanning and transmission electron microscopy. In this paper, we will show how graphite structures, that are inherent to the production process of the amorphous carbon support films, used for both SEM and TEM studies can be easily mistaken for the actual sample structures. We will further report that these artifacts appear in both commercial as well as homemade holey carbon support films on copper grids, and suggest that to successfully study the “real” nanotubular structures only support films made from materials other than carbon should be used.     

Electrochemical properties of N-doped hydrogenated amorphous carbon films fabricated by plasma-enhanced chemical vapor deposition methods

Nitrogen-doped hydrogenated amorphous carbon thin films (a-C:N:H, N-doped DLC) were synthesized with microwave-assisted plasma-enhanced chemical vapor deposition widely used for DLC coating such as the inner surface of PET bottles. The electrochemical properties of N-doped DLC surfaces that can be useful in the application as an electrochemical sensor were investigated. N-doped DLC was easily fabricated using the vapor of nitrogen contained hydrocarbon as carbon and nitrogen source. A N/C ratio of resulting N-doped DLC films was 0.08 and atomic ratio of sp³/sp²-bonded carbons was 25/75. The electrical resistivity and optical gap were 0.695 Ω cm and 0.38 eV, respectively. N-doped DLC thin film was found to be an ideal polarizable electrode material with physical stability and chemical inertness. The film has a wide working potential range over 3 V, low double-layer capacitance, and high resistance to electrochemically induced corrosion in strong acid media, which were the same level as those for boron-doped diamond (BDD). The charge transfer rates for the inorganic redox species, Fe^2+/3+ and Fe(CN)₆^4−/3− at N-doped DLC were sufficiently high. The redox reaction of Ce^2+/3+ with standard potential higher than H₂O/O₂ were observed due to the wider potential window. At N-doped DLC, the change of the kinetics of Fe(CN)₆^3−/4− by surface oxidation is different from that at BDD. The rate of Fe(CN)₆^3−/4− was not varied before and after oxidative treatment on N-doped DLC includes sp² carbons, which indicates high durability of the electrochemical activity against surface oxidation.     

Electrochemical studies of kerosene-pyrolysedcarbon films

Polycrystalline thin films of conducting carbon are deposited on alumina substrates by the pyrolysis of kerosene vapour at 1000C for 2h in argon atmosphere. Preliminary structural analysis is done by XRD, laser-Raman, FTIR and SEM studies. The electrochemical behaviour of as-grown conducting carbon films was investigated in various electrolytes at different pH and the performance was compared with that of platinum and glassy electrodes. The electrochemical window of the kerosene carbon electrode in 100mm H2SO4 was found to be 2.91V which is greater than that of glassy carbon (2.79V) and platinum (2.02V). Cyclic voltammetry reveals that Pt electrode has almost an equal tendency towards hydrogen and oxygen evolution, whereas glassy carbon favours hydrogen evolution and kerosene carbon favours oxygen evolution. It is suggested that the kerosene carbon electrode can be used as an oxygen electrode more efficiently. Unlike diamond films or glassy electrodes, kerosene carbon thin films are of low cost and good stability; they are also easy to grow on various ceramic substrates of any size. Moreover, these electrodes are very economical and promising for application in chlor-alkali industry.     

Osteoblasts attachment on amorphous carbon films

In this work we studied the osteoblasts response to amorphous carbon (a-C) films deposited on stainless steel substrates with different surface textures. For osteoblasts cells, attachment to the substrate is the first step in the process of cell/surface interactions which affects subsequent cellular and tissue response. Amorphous carbon films are characterized by very smooth surfaces that imaged the surface roughness of the substrate and many of their applications rely on this property. However, in the biomedical field the cell response is strongly influenced by the topography and particularly, for osteoblasts cells it has been shown that rough surfaces enhances cellular attachment and differentiation. Therefore, in this work we modified the surface roughness of the substrate in order to obtain carbon films with different values of average surface roughness. The substrates were abraded or fine-polished to obtain four different average roughness: 0.01, 1.5, 2.1 and 3.5 μm. Surface topography before and after deposition of the a-C films was evaluated by profilometry and scanning electron microscopy (SEM), while chemical composition was determined by X-ray photoelectron spectroscopy. Human osteoblasts cells were used to evaluate the effect of the different surface finishes on the adhesion. The number of attached cells was determined by a colorimetric technique after 24 h of incubation, while morphological and cytoskeletal changes were monitored using SEM. The cellular attachment on a-C surfaces increases monotonically with the roughness attaining up to 160% more cells than the positive control.     

Crystalline structures of carbon complexes in amorphous carbon films

Amorphous carbon (a-C) films, 20 nm thick, were deposited by sputtering on (001) Si substrates. A negative bias voltage was applied to the substrate during deposition to induce Ar⁺ ion bombardment with an energy of ∼230 eV. The film microstructure was investigated by conventional transmission electron microscopy as well as high-resolution electron microscopy. The films consist of an amorphous carbon matrix and crystallites with a platelet form. The crystallites are energetically metastable and easily degrade under electron beam irradiation. In addition, they are usually oriented along their sixfold or threefold axes, exhibiting lattice parameters larger than those of graphite and diamond. X-ray reflectivity density measurements and high-resolution electron microscopy observations indicate that these crystallites consist of packed complex carbon clusters. X-ray diffraction measurements in rocking curve geometry support the existence of oriented crystallites with interplanar spacings corresponding to those observed by transmission electron microscopy.     

Iodine doping of amorphous diamond-like carbon films

Amorphous diamond-like carbon (a:DLC) films have been doped by incorporation of iodine during the films deposition. XPS and AES analysis shows the existence of iodine atoms with constant concentration of 0.9% along the iodine doped DLC film (a:I-DLC). The optical and electronic properties of the doped films were studied. Optical measurements in the visible light show that iodine affects the interband absorption of the a:DLC films. Iodine causes decreasing of the optical energy gap, from 1.07 to 0.78 eV and affects the density of states at the conducting band. Like the optical measurements, electrical measurements show that iodine also decreases the activation energy of the films from 0.34 to 0.22 eV. This shows that although both gaps decrease, the optical energy gap remains different from that of electrical gap, also after doping.     

Magnetotransport properties of undoped amorphous carbon films

Anomalous positive magnetoresistance (MR) up to 36% was observed at 2 K and 12 T in the undoped amorphous carbon (a-C) film deposited on glass substrate by pulsed laser deposition at 500 °C. There is no tendency of saturation of MR with increase of the magnetic field. The MR decreases as the measurement temperature increases from 2 to 80 K, and could hardly be observed above 80 K. As the deposition temperature grows from 300 to 600 °C, the disorder degree of the a-C film decreases, and the value of MR also decreases, indicating that the lower disorder degree results in a smaller MR. The mechanism of this MR could be ascribed to the wave function shrinkage. It could help to understand the MR phenomenon in amorphous material systems.     

Optical properties and new carbon forms of sputtered amorphous carbon films

Amorphous carbon films were deposited by r.f. magnetron sputtering at various bias voltages V_b applied on Si substrate. We studied the optical properties of the films using in situ spectroscopic ellipsometry (SE) measurements in the energy region 1.5–5.5 eV. From the SE data analysis the dielectric function ε(ω) of the a-C films was obtained, providing information about the electronic structure and the bonding configuration of a-C films. Based on the SE data the films are classified in three categories. In Category I and II belong the films developed with V_b≥0 V (rich in sp² bonds) and −100≤V_b<0 V (rich in sp³ bonds), respectively. The dielectric function of the films belonging in these two categories can be described with two Lorentz oscillators located in the energy range 2.5–5 eV (π–π^*) and 9–12 eV (σ–σ^*). A correlation was found between the oscillator strength and the sp² and sp³ contents. The latter were calculated by analyzing the ε(ω) with the Bruggeman effective medium theory. In films deposited with V_b<−100 V (Category III), the formation of a new and dense carbon phase was detected which exhibits a semi-metallic optical behavior and the ε(ω) can be described with two oscillators located at ∼1.2 and ∼5.5 eV.     

Oral bacterial adhesion on amorphous carbon films

It is now well established that all the different forms of amorphous carbon films are biocompatible and suitable for specific biomedical applications. On the other hand, bacterial adhesion on implant surfaces has also a strong influence on the healing and long-term outcome of biomedical devices and this has not been thoroughly studied for the carbon films. The purpose of this study was to evaluate the bacterial adhesion on graphite-like amorphous carbon (a-C) films in comparison to titanium (Ti) films and stainless steel (SS) substrates using different bacteria strains from the normal oral microbiota. Medical grade stainless steel discs of 15 mm in diameter were coated by either Ti or a-C films using magnetron sputtering. The bacterial adhesion of single species and a mixture of nine different microorganisms was tested on the three surfaces. The bacteria were anaerobically incubated on the surfaces for 24 h, then colony forming units (CFUs) were counted. The total amount of CFUs was found higher on the a-C and SS surfaces in comparison to Ti films when the nine strains were mixed together, suggesting that Ti surfaces are better than the a-C and SS to avoid bacterial adhesion. However, when single species were analyzed the individual strains showed different adhesion profiles. Some species like Aggregatibacter actinomycetemcomitans, Eikenella corrodens, Campylobacter rectus, and Fusobacterium nucleatum were found in higher counts on the a-C surfaces, while other species like Actinomyces israelii, Porphyromonas gingivalis, Prevotella intermedia and Streptococcus sanguinis were found in lower counts comparing to the Ti films. These results suggested that the determination of anti-bacterial properties of a surface by studying the bacterial adhesion of individual strains, as usually done, might not be representative of the in vivo response, where more than one strain are surely present.     

Studies of carbon ion self-implantation into hydrogenated amorphous carbon films

The properties of polymer-like amorphous hydrogenated carbon thin films with low defect density have been studied. These films were implanted with carbon ions with a dose range of 10¹²–10¹⁶ cm⁻². The purpose of the study is to investigate the effects of ion beam damage on this type of film. Optical absorption measurements observe a narrowing of the optical band gap, suggesting the introduction of a large number of defect states subsequent to the implantation resulting in the broadening of the band tails, only after a threshold ion dose of 10¹⁵ cm⁻². Nuclear reaction analysis suggests also a reduction in the hydrogen content of the film which coincides with film thinning.     

Surface morphology and evolution of amorphous carbon thin films

The surface morphology of disordered carbon films grown by nanosecond pulsed laser ablation of graphite is reviewed. It is shown that the presence of a background gas can have a profound effect on the plume of material ejected during ablation. At low pressures smooth films are produced but at higher pressures rough films with an evolution from a nodular morphology to a large area cluster-assembled morphology occurs. The surface morphology changes with increasing background pressure as a result of collisions, which reduce the kinetic energy of the ejected material and allow for cluster formation within the plume. It is shown that the energy of some of the carbon ablated species in vacuum can exceed 100 eV. The nature of the species present in the plume is discussed in terms of electron–ion recombination and impact ionisation/excitation. The cluster-assembled films are shown to be useful as a scaffold for supporting metal nanoparticles to produce substrates for surface enhanced Raman spectroscopy.     

Structure and friction properties of laser-patterned amorphous carbon films

In the paper we report on laser surface modification of super hard micrometer-thick tetrahedral amorphous carbon (ta-C) films in the regime of single-shot irradiation with KrF laser pulses (wavelength 248 nm, pulse duration 20 ns), aimed at investigations of the laser-induced changes of the structure and surface properties of the ta-C films during graphitization and developing ablation processes. Based on the analysis of surface relief changes in the laser-irradiated spots, characteristics of the single-shot graphitization and ablation of the 2-μm-thick ta-C film are determined. Using Raman spectroscopy, it is found that during the graphitization regime the structure transformation and growth of graphitic clusters occur according to the relationship I(D)/I(G) ∝ L_a², but after reaching the ablation threshold the Tuinstra-Koenig relationship I(D)/I(G) ∝ 1/L_a describes further growth of the graphitic cluster size (L_a) during developing ablation of the ta-C film with nanosecond pulses. The maximal size of graphitized clusters is estimated as L_a = 4–5 nm. The studies of nanomechanical properties of laser-patterned ta-C films using the lateral force microscopy and force modulation microscopy have evidenced lower friction forces (between diamond-coated tips and film surface) and lower stiffness in the laser-graphitized areas. The laser-produced graphitic layer acts as a solid lubricant during sliding of the diamond-coated tips on the ta-C film surface in ambient air (~ 50% RH); the lubricating role of adsorbed water layers is suggested to be significant at low loads on the tips. The results of this work demonstrate that the UV laser surface texturing in the regime of graphitization is a promising technique to control the friction and surface elasticity of super hard amorphous carbon films on the micro and nanoscale.     

Ion beam deposition of amorphous hydrogenated carbon films on amorphous silicon interlayer: Experiment and simulation

A thick layer of amorphous silicon (a-Si) was deposited on industrial grade crystalline n-Si < 111 > substrate by means of electron beam evaporation. On top of a-Si layer, amorphous hydrogenated carbon (a-C:H) film was grown by direct ion beam deposition from acetylene precursor gas. In order to study on atomic level the a-C:H film growth on amorphous silicon, a theoretical model was developed in a form of reaction rate (kinetic) equations. Numerical simulation using this model has revealed that the ratio of sp³/sp² content in the film is heavily influenced by relaxation rate of the carbon atoms in a sub-surface region of the film that were activated by ion irradiation. The final structure of a-C:H film does not depend much on elemental composition and structure of amorphous Si coating, provided that deposition procedure is not terminated at its initial stage but continues for more than 60 s. It became evident, therefore, that the use of a-Si interlayer with a-C:H films could be particularly beneficial when a need arises to minimize or eliminate the effect of the substrate. As one of such cases, a poor adhesion of amorphous carbon on steel and other ferrous alloys could be mentioned.     

Comparative study on the structural make-up and mechanical behavior of silicon and silver doped amorphous carbon films

Silicon - Silicon (Si) and silver (Ag) doped amorphous carbon (a-C) thin film were deposited on chrome nitrided 316 LVM stainless steel using filtered cathodic vacuum arc (FCVA) deposition...     

The vacuum-annealed undoped polycrystalline CVD diamond electrodes: the impedance-spectroscopy and photoelectrochemical studies

The electrochemical impedance, photocurrent, and photopotential are measured in the supporting electrolyte solution (2.5 M H₂SO₄) for electrodes made of the undoped polycrystalline CVD diamond films annealed in vacuum at 1500-1640 °C. Analysis of the impedance spectra allowed concluding that both the amount of a nondiamond conducting phase in the electrode bulk and its conductivity increased upon annealing. With increasing annealing temperature, the photopotential passed through a maximum at ∼1570 °C. The photosensitivity of “metal-like” samples (annealed at temperatures ≥1630 °C) is negligibly small. Judging from the sign of the photopotential (positive) and photocurrent (cathodic), the studied material formally behaves as a p-type semiconductor. It is suggested that the photoeffects are caused by the structure defects, in particular, dislocations in diamond crystallites, formed close to intercrystalline boundaries during the high-temperature annealing.     

Field emission from metal-containing amorphous carbon composite films

Metal-containing (Co, Al and Ti) amorphous carbon composite films (a-C:Me) have been prepared by the filtered cathodic arc technique using metal-containing graphite targets at room temperature. Field emission properties of the heat-treated a-C:Me films were improved and were found to be dependent on the metal content and variety of metals. After heat-treatment at 550°C in a mixture of acetylene and nitrogen gases, the field emission properties of a-C:Co films were significantly improved, in which Co acted as catalysts to enhance graphitization as well as formation of carbon nanotubes during heat-treatment. A threshold electric field of less than 2 V/μm was obtained from the heat-treated a-C:Co composite films without conditioning. The heat-treated a-C:Al and a-C:Ti films, though the conditioning step could be avoided and relatively low threshold fields could be obtained, exhibited relatively low emission site densities, however. The a-C:Me films, which can be deposited with a high rate at room temperature and require a relatively low temperature, heat-treatment process to enhance electron emission, are promising for practical applications in field emission display.