Evaluation of protein adsorption on hydrogenated amorphous carbon films by surface plasmon resonance phenomenon

To develop an anti-thrombogenic coating, hydrogenated amorphous carbon (a-C:H) and related films were studied in terms of their protein adsorption during the initial process in thrombogenesis by surface plasmon resonance (SPR) phenomenon using a multilayer device consisting of an a-C:H layer on Au. Two a-C:H films with different hydrogen contents, a nitrogenated a-C:H (a-C:N:H) and a fluorinated amorphous carbon (a-C:F) film were prepared on the Au layer in the multilayer device. Human serum albumin (HSA) in a phosphor buffer (PB) was used as a protein. Na₂HPO₄·12H₂O, NaH₂PO₄·2H₂O and deionized water were mixed to coordinate PB. From the attenuation of reflected light, the SPR angle was determined to the angle at minimum reflection intensity. The observed behavior of the SPR angle indicated that HSA was adsorbed on all films. The SPR angle was analyzed to estimate the multilayer index of the HSA-adsorbed layer on each film. The HSA adsorption ability of both a-C:H and a-C:N:H films was similar, and the absorption ability of the a-C:F film was lower than that of the other films. Hence, the surface polarization dominates the adsorption ability of HSA on a-C:H films and related film.     

Electric properties of a–C:H/a–C:N:H/aluminum structure

To obtain photovoltaic properties using carbon films without dependence on a Si substrate by drift of carriers, a photovoltaic cell consisted of aluminum (Al) and amorphous carbon films was fabricated. Two types of amorphous carbon were deposited on smooth Al substrates and on n-type Si by radio-frequency chemical vapor deposition. The first layer is nitrogen-doped hydrogenated amorphous carbon (a–C:N:H) film, and the second layer is hydrogenated amorphous carbon (a–C:H) film. Nitrogen atoms in a–C:N:H film were introduced as N–H structure. Both type of films were confirmed to be semiconductors on the basis of the temperature dependence of electroconductivity. The a–C:N:H/n–Si structure exhibited photovoltaic characteristics Furthermore, the a–C:H/a–C:N:H/Al structure cell also exhibited photovoltaic characteristics with an open-circuit voltage and short-circuit current of 5.5 mV and 0.83 μA/cm^− 2, respectively.     

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.     

Bias enhanced diamond nucleation onto 3C–SiC(100) surfaces studied by high resolution X-ray photoelectron and high resolution electron energy loss spectroscopies

The nature of hydrogen and carbon bonding configuration formed onto 3C–SiC(100) surfaces by the diamond bias enhanced nucleation process consisting of stabilization and biasing stages were investigated by high resolution electron energy loss spectroscopy and high resolution X-ray photoelectron spectroscopy. During the stabilization stage a sp³-CH_x bonded carbonaceous mono-layer is formed onto a hydrogenated 3C–SiC(100)–C–H terminated surfaces. After the biasing stage a hydrogenated nano-diamond film is formed. It was determined that hydrogen is strongly bonded to these nano-diamond surfaces and boundaries in sp³-C–H and sp²-C–H mono-hydride configuration. In addition, CH_x (x > 1) weakly bonded surface or sub-surface species were detected. Regions which are not fully covered by the nano-diamond film expose the SiC surface covered with a very thin carbonaceous layer.     

X-ray photoemission spectroscopy of nitrogen-doped UNCD/a-C:H films prepared by pulsed laser deposition

Nitrogen-doped ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) films were deposited by pulsed laser deposition (PLD). Nitrogen contents in the films were controlled by varying a ratio in the inflow amount between nitrogen and hydrogen gases. The film doped with a nitrogen content of 7.9 at.% possessed n-type conduction with an electrical conductivity of 18 Ω^? 1 cm^? 1 at 300 K. X-ray photoemission spectra, which were measured using synchrotron radiation, were decomposed into four component spectra due to sp², sp³ hybridized carbons, C=N and C–N. A full-width at half-maximum of the sp³ peak was 0.91 eV. This small value is specific to UNCD/a-C:H films. The sp²/(sp³ + sp²) value was enhanced from 32 to 40% with an increase in the nitrogen content from 0 to 7.9 at.%. This increment probably originates from the nitrogen incorporation into an a-C:H matrix and grain boundaries of UNCD crystallites. Since an electrical conductivity of a-C:H does not dramatically enhance for this doping amount according to previous reports, we believe that the electrical conductivity enhancement is predominantly due to the nitrogen incorporation into grain boundaries.     

Structure and characteristics of amorphous (Ti,Si)–C:H films deposited by reactive magnetron sputtering

The hydrogenated amorphous carbon films doped with Ti and Si ((Ti,Si)–C:H) were deposited on silicon substrates using reactive magnetron sputtering Ti₈₀Si₂₀ composite target in an argon and methane gas mixture. The structures of the films were analyzed by X-ray photoelectron spectroscopy and Visible Raman spectroscopy. The morphologies were observed by atomic force microscope. The friction coefficients of the films were tested on the ball-on-disc tribometer. The results indicate that the sp³/sp² ratios in the films can be varied from 0.18 to 0.63 by changing Ti and Si contents at various CH₄ flow rates. The surface of the films becomes smoother and more compact as the CH₄ flow rate increases. The lowest friction coefficient is as low as 0.0139 for the film with Ti of 4.5 at.% and Si of 1.0 at.%. Especially, the film exhibits a superlow value (μ < 0.01) under ambient air with 40% relative humidity in friction process. The superlow friction coefficient in ambient air may be, attributable to synergistic effects of a combination of Ti and Si in the film.     

Fibrinogen and lysozyme adsorption on amorphous carbon film surface detected by multilayer device from the back side of the film

To develop hydrogenated amorphous carbon (a-C:H) as a biocompatible coating, a-C:H was studied in terms of its protein adsorption during the initial process of cell adsorption. A multilayer surface plasmon resonance (SPR) device consisting of an a-C:H layer on Au was built in the Kretschmann configuration to detect protein adsorption on an a-C:H film surface. From the dependence of reflectivity on the laser incident angle, SPR angle was determined to the incident angle in which the light intensity was reduced drastically. The proteins considered were lysozyme (Lyz) and fibrinogen (Fib). The SPR angle increased from 58.09 to 58.69° upon the adsorption of Lyz when the nonadsorbed Lyz was removed after introduction of 20 μM Lyz-containing solution. Upon the adsorption of Fib, the SPR angle increased from 60.95 to 61.76° when the nonadsorbed Fib were removed after the introduction of 0.4 μM Fib-containing solution. The shift in the SPR angle was small for both cases. Obtained results suggested that the number of adsorbed Lyz was higher than that of adsorbed Fib.     

Effect of structure of carbon films on their tribological properties

A comparative study of the tribological properties of a library of different carbon forms is presented. The library includes hydrogen free and hydrogenated carbon films with different bonding (CC, CH, different sp³ fractions) and structure configurations (amorphous, graphitic) leading to a wide range of densities and hardness. Reference samples (Si substrates, thermally evaporated amorphous carbon, graphitic foil) were studied as well. The tribological properties were measured using a reciprocal sliding tribometer under humid (50% RH) and dry (5% RH) air conditions. Friction coefficients were measured versus the number of sliding cycles and the wear was studied using optical profilometry and imaging as well as SEM.The friction and wear performance of the carbon films were found to depend on both the structure and the ambient conditions. Hydrogen free films have friction coefficients < 0.1 for 80% sp³ bonded films and > 0.1 for 100% sp² bonded films. The wear resistance of the hydrogen free films (much larger for sp³ bonded films) significantly decreases under dry conditions. In contrast, hydrogenated films show reduction in friction with decreasing humidity (from 0.2 under 50% RH to < 0.1 under 5% RH). The wear resistance of hydrogenated films is larger for dry and smaller for humid conditions.     

Long-term H-release of hard and intermediate between hard and soft amorphous carbon evidenced by in situ Raman microscopy under isothermal heating

We study the kinetics of the H release from plasma-deposited hydrogenated amorphous carbon films under isothermal heating at 450, 500 and 600 °C for long times up to several days using in situ Raman microscopy. Four Raman parameters are analyzed. They allow the identification of different processes such as the carbon network reorganization and the H release from sp³ or sp² carbon atoms and the corresponding timescales. Carbon reorganization with aromatization and loss of sp³ hybridization occurs first in 100 min at 500 °C. The final organization is similar at all investigated temperatures. Full H release from sp³ carbon occurs on a longer timescale of about 10 h while H release from sp² carbon atoms is only partial, even after several days. All these processes occur more rapidly with higher initial H content, in agreement with what is known about the stability of these types of films. A quantitative analysis of these kinetics studies gives valuable information about the microscopic processes at the origin of the H release through the determination of activation energies.     

Disorder and optical properties of amorphous carbon

Tight-binding calculations shed light onto the link between disorder and optical properties in pure amorphous carbon films. It is shown that the Urbach energy E_U is an excellent probe of disorder in such films. Our striking finding is that E_U varies non-monotonically with sp³ fraction and optical gap, contrary to what is seen experimentally in hydrogenated samples. Firm evidence is provided, supported by experimental measurements, that in dense films the higher the sp³ fraction the lower the disorder. The Urbach energy has a value of ∼ 0.09 eV for the 100% sp³-bonded network, it reaches a maximum of ∼ 0.3 eV for 65% sp³ content, and it then declines to low values for low-sp³ content films. Analysis of cluster distributions and bond-angle and -length distortions reveals that the Urbach edge is associated to both topological and structural disorder. Another notable finding is that the edge is rather insensitive to the spin density due to unpaired sp² sites.     

High-density plasma chemical vapor deposition of amorphous carbon films

This work investigated the influence of the plasma parameters pressure and RF power on the characteristics of amorphous carbon films deposited by high-density plasma chemical vapor deposition, using inductively coupled methane plasmas. These films show several good electrical characteristics: high resistivity, low dielectric constant and high breakdown field. After deposition, the films were characterized as follows: the thickness was measured with a step height meter and an ellipsometer; Fourier transform infrared spectroscopy was used to identify the sp² and sp³ hybridization of C and CH bonds and other possible bonds that can appear because of the hydrogen presence; atomic force microscopy was used to measure the film roughness and I–V and C–V measurements to determine the dielectric constant, the electric resistivity and the breakdown electric field. The films deposited with high-density plasmas showed good characteristics for several applications, when compared to deposition with conventional RF plasmas. These films show a better structural quality with a high sp³ to sp² ratio. Even with this high sp³ to sp² ratio, the RMS surface roughness of an approximately 300 nm thick film was only 0.24 nm. For microelectronic applications, a very low dielectric constant of only 1.68 and a high resistivity of 1.5×10¹⁴ Ω cm were obtained.     

Thermal grafting of organic monolayers on amorphous carbon and silicon (111) surfaces: A comparative study

Thermally-assisted (160 °C) liquid phase grafting of linear alkene molecules has been performed simultaneously on amorphous carbon (a-C) and hydrogen passivated crystalline silicon Si(111):H surfaces. Atomically flat a-C films with a high sp³ average surface hybridization, sp³ / (sp² + sp³) = 0.62, were grown using pulsed laser deposition (PLD). Quantitative analysis of X-ray photoelectron spectroscopy, X-ray reflectometry and spectroscopic ellipsometry data show the immobilization of a densely packed (> 3 × 10¹⁴ cm^? 2) single layer of organic molecules. In contrast with crystalline Si(111):H and other forms of carbon films, no surface preparation is required for the thermal grafting of alkene molecules on PLD amorphous carbon. The molecular grafted a-C surface is stable against ambient oxidation, in contrast with the grafted crystalline silicon surface.     

Polymeric amorphous carbon films with an extended range of optical gaps

A new type of hydrogenated amorphous carbon (a-C:H) film is prepared under low bias voltage and an extended range of plasma density in a radio-frequency plasma enhanced chemical vapor deposition system (RF-PECVD). The obtained a-C:H samples are grown on electrically floating substrates instead of substrates mounted on the powered or the grounded electrode of RF-PECVD, and have structure and properties that are significantly different from regular a-C:H films. The samples have an optical gap ranging from 1 to 4 eV, while maintaining low intrinsic stress between 0.1 and 0.2 GPa. Fractions of all types of CH_x carbon–hydrogen groups of the obtained samples are measured, analyzed, and used to locate these samples in the carbon-hydrogen ternary phase diagram. The obtained samples are located in the same general area as the regular a-C:H films, indicating configurational rather than compositional structural differences. The hydrogenation ratio of sp³ carbons in the obtained samples is found to remain at a very high level, and is used to explain their unique properties.     

Enhanced cycling stability of Ni-MH battery by depositing amorphous carbon film on the Ti1.4V0.6Ni negative electrode

Amorphous carbon (a-C) films with various thicknesses depending on the reaction time are deposited on the surface of Ti_1.4V_0.6Ni alloy electrodes for Ni-MH (nickel-metal hydride) battery by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). With the increasing deposition time, the Raman spectra show a gradually disordered sp²-bonding change of the films and the changing trend of sp²/sp³ is obtained by X-ray photoelectron spectroscopy. The a-C film of depositing for 30 min with the thickness of 400 nm shows a favorable stability in alkaline electrolyte, the capacity is enhanced by 36.2% after 50 cycles than the bare electrode, and the charge voltage is 80 mV lower than the bare one. The a-C film with high sp²-bonded carbon content effectively reduces the charge transfer resistance, and as a coating layer, the dissolution of V of the alloy is also inhibited. In particular, to get a proper discharge voltage and a stable capacity simultaneously, covering completely and an appropriate thickness of the a-C film are crucial for an expected performance.     

Structural analysis of a-C:H and a-C:H:Si films under high-pressure and high-temperature by synchrotron X-ray diffraction

Amorphous carbon films have several outstanding tribology characteristics, including high hardness, surface smoothness, and low friction. Under tribological conditions, their surface is generally exposed to high-temperature and pressure. Although the structure of amorphous carbon films is likely changed by high temperature and pressure, there have been no reports on such structural changes of the films. To obtain information about their structural changes, synchrotron X-ray diffraction was used to analyze two kinds of amorphous carbon films, a-C:H and a-C:H:Si, under high-temperature and high-hydrostatic pressure conditions. Synchrotron X-ray diffraction was applied to films pressurized by a multi-anvil press installed in the PF-AR NE5C beamline at KEK at room temperature and at a high-temperature around 200 °C. The pair distribution functions derived by Fourier transformation of the obtained scattering intensity profiles showed that the sp²/sp³ ratios for both films decreased as the pressure increased and that the sp²/sp³ ratio for the a-C:H film increased as the temperature increased. This indicates that high-pressure creates sp³ stabilization in a-C:H and a-C:H:Si films while high-temperature creates sp² transition in a-C:H film. The sp²/sp³ ratio for the a-C:H:Si film did not change much even at high-temperature due to the high thermal-oxidative stability of a-C:H:Si.     

Photoluminescence of Amorphous Multilayer Structures <Emphasis Type="Italic">a</Emphasis>-C:H/<Emphasis Type="Italic">a</Emphasis>-Si:H

The optical absorption edge and the photoluminescence spectra of amorphous hydrogenated diamond-like carbon (a-C:H) films are investigated in the temperature range 4.2–900 K. Low-dimensional multilayer structures a-C:H/a-Si:H, in which a-C:H with an optical band gap E _g = 4.5 eV is used as a barrier, are prepared for the first time. It is found that amorphous hydrogenated silicon (a-Si:H) films with a thickness d < 100 Å exhibit quantum-confinement effects. Analysis of the photoluminescence spectra and the optical absorption edge of the a-Si:H films shows that the structures a-C:H/a-Si:H have a sharp interface and that charge carriers in the a-Si:H film undergo quantization.Original Russian Text Copyright © 2005 by Fizika i Khimiya Stekla, Babaev, Kamilov, Sultanov, Askhabov.     

The infrared characteristics investigation of carbon nitride films

Two series of a-C(N):H films, with diamond-like character and graphite-like character respectively, are prepared. Without N incorporation, the two kinds of films have very close IR bands in the range of 1000–1800 cm^− 1. However, the difference in IR activity of the two series films became dramatical as N was introduced into both kinds of carbon films, which is attributed to purely electronic effect, the electronegativity of N. The N incorporated in carbon films is able to induce bond dipole of CC bonds in sp² graphite cluster, leading to a degree of dipoles for all the aromatic sp² CC bonds, a permanent electric dipole effect arises and this could lead to the increase in IR activity of the sp² clusters. As the N content exceeds 20 at.% in the carbon films, strong conjugation of C≡N bonds with aromatic graphite rings can induce conjugated π bond dipole too.     

Hard and sp2-rich amorphous carbon structure formed by ion beam irradiation of fullerene,a-C and polymeric a-C:H films

In this work, we present a comparative study of the ion irradiation effect on the mechanical and optical properties of fullerene, amorphous carbon (a-C) and polymeric hydrogenated amorphous carbon (a-C:H) films, irradiated with N ions at 400 keV in the fluence range from 10¹³ to 3×10¹⁶ N cm⁻². Modifications in the carbon structure, as function of the irradiation fluence, were investigated using the Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), Fourier transform infrared (FTIR), Raman spectroscopy, UV–Vis–near infrared (NIR) spectrophotometry and nanoindentation techniques. After high fluence, the three carbon samples are transformed into very similar hard (≈14 GPa) and non-hydrogenated amorphous carbon layers with very low optical gap (≈0.2 eV) and an unusual sp²-rich bonded atomic network. The mechanical properties of the irradiated films correlated with the bonding topologies of this new sp² carbon phase are investigated through the constraint-counting model. The results show that the structural modifications and the unusual rigidity were achieved by the distortion of the sp² carbon bond angles, giving origin to a constrained three-dimensional sp² carbon bonded network.     

Hydrogen incorporation,bonding and stability in nanocrystalline diamond films

The hydrogen concentration in hot filament and microwave plasma CVD nanocrystalline diamond films is analysed by secondary ion mass spectrometry and compared to the film grain size. The surface and bulk film carbon bonds are analysed respectively by X-ray photoelectron spectroscopy (XPS) and ultra-violet Raman spectroscopy. XPS results show the presence of the hydrogenated p-type surface conductive layer. The respective intensities of the 1332 cm^− 1 diamond peak, of the G and D bands related to sp² phases, and of the 3000 cm^− 1 CH_x stretching mode band, are compared on Raman spectra. The samples are submitted to thermal annealing under ultra-high vacuum in order to get hydrogen out-diffusion. XPS analysis shows the surface desorption of hydrogen. Thermal annealing modifies the sp² phase structure as hydrogen out diffuses.     

Deposition of hard amorphous hydrogenated carbon films by radiofrequency parallel-plate hollow-cathode plasmas

Hard amorphous hydrogenated carbon (a-C:H) films were deposited by plasma decomposition of CH₄ gas in a RF parallel-plate hollow-cathode system. The deposition system was built by placing a metallic plate in parallel to and in electrical contact with an usual RF-PECVD planar cathode. Self-bias versus RF power curves were used to make an initial characterization of plasma discharges in nitrogen gas atmospheres, for pressures between 10 and 100 mTorr. The strongly increased power consumption to obtain the same self-bias in the hollow-cathode system evidenced an increase in plasma density. The a-C:H films were deposited onto Si single crystalline substrates, in the − 50 to − 500 V self-bias range, at 5, 10 and 50 mTorr deposition pressures. The film deposition rate was found to be about four times than that usually observed for single-cathode RF-PECVD-deposited films, under methane atmosphere, at similar pressure and self-bias conditions. Characterization of film structure was carried out by Raman spectroscopy on films deposited at 10 and 50 mTorr pressures. Gaussian deconvolution of the Raman spectra in its D and G bands shows a continuous increase in the I_D/I_G integrated band intensity ratio upon self-bias increase, obeying the expected increasing behavior of the sp² carbon atom fraction. The peak position of the G band was found to increase up to − 300 V self-bias, showing a nearly constant behavior for higher self-bias absolute values. On the other hand, the G band width showed a nearly constant behavior within the entire self-bias range. Nanohardness measurements have shown that films deposited with self-bias greater than 300 V are as hard as films obtained by the usual PECVD techniques, showing a maximum hardness of about 18 GPa. Films were also found to develop high internal compressive stress. The stress dependence on self-bias showed a strong maximum at about − 200 V self-bias, with a maximum stress value of about 5 GPa.