Sp3 Fraction Research Articles

Tribological properties of diamond-like carbon films deposited by negative carbon ion beam

Excellent antiwear diamond-like carbon (DLC) films were prepared on silicon wafers by a novel negative carbon ion beam deposition system. This system does not need any gas in the chamber; deposition occurs under high vacuum. The ion source has good control of the C− beam energy (from 50 to 150 eV) and produces a 0.5-in.-diam ion beam for DLC coating. All coatings showed ultrasmooth surfaces (Ra∼1 nm) and good adhesion. The results of infrared and Raman spectroscopy show that the degree of diamond-like character in the films, the sp3 fraction, increases as a function of deposition energy in this particular energy range. The nanoindentation hardness of the films also increases from 15 to 50 GPa as deposition energy increases. The wear rates and friction coefficients against a M-50 steel bearing ball were measured in a pin-on-disk tribometer with a normal load of 9.8 N, calculated nominal Hertzian stress of 0.8–1.8 GPa. The measured wear rates of the DLC films were of the order of 10−7 and 10−8 mm3/N m depending on the deposition energy of the carbon beam. The friction coefficients in the steady state were found to be around 0.1. These results indicate excellent applications for abrasion resistant DLC coatings. We are developing a rectilinear negative carbon ion beam deposition system (up to 8 in.) for large area application, and many of the process scale-up issues have been solved.

Modulation of the antigen presentation activity in foot and mouth disease virus (FMDV) vaccines by two adjuvants: avridine and a water soluble fraction of Mycobacterium sp.

We have previously demonstrated that the presence of the antigen presenting cells (APC) is critical in the induction and maintenance of the immune response in animals infected or immunized with inactivated FMDV. The use of immunological adjuvants has been repeatedly shown to be essential for the improvement of the immunogenicity in FMDV vaccines. Specifically, we have previously shown that the addition of the synthetic lipoamide Avridine (AVR) or a water soluble fraction of Mycobacterium sp. (WSF) significantly increased the immune response and protection against FMDV challenge. Here, we study the effect of these adjuvants on the induction of APC activity in mice immunized with inactivated FMDV. Both adjuvants were able to induce a long lasting antibody response which correlates with an efficient APC activity. Experiments using sequential cell transfers showed that the presence of the APC activity is not related with the efficiency of keeping free antigen in the vaccinated host. Interestingly, APC from animals immunized with AVR as adjuvant elicited virus neutralizing antibodies, while those APC obtained from donors vaccinated using WSF as adjuvant (or just an oil emulsion) induced anti-FMDV detectable only by ELISA. The analysis of the antibody response to a well studied synthetic peptide raised evidences that indicate that this difference could be explained by a differential presentation of viral B epitopes when different adjuvants were used. These results suggest that the induction of APC should be considered as one of the critical factors in the process of improving the immunogenicity of experimental FMDV vaccines.

Correlation between gap density of states and recombination processes in high electronic quality a-C:H

The investigation on electronic density of states (DOS) and photoluminescence processes in a-C:H films has attracted, in the last years, interest, due to the foreseen technological application of carbon based materials. In this paper we present a new model for electronic DOS of a-C:H having sp2 fraction less than a percolation threshold. This model is able to explain the optical absorption coefficient for photon energies in the range 0.7 to 3.5 eV. Moreover, with this model we correlate, as it happens in a-Si:H, the photoluminescence quantum efficiency and the density of states close to the Fermi energy. Such states, however, can have either diamagnetic or paramagnetic nature so that some of them can be detected by electron spin resonance and others by optical transitions.

Analysis of amorphous carbon thin films by spectroscopic ellipsometry

Using spectroscopic ellipsometry (SE), we have measured the optical properties of amorphous carbon (a-C) films ∼10–30 nm thick prepared using a filtered beam of C+ ions from a cathodic arc. Such films exhibit a wide range of sp3-bonded carbon contents from 20 to 76 at.% as measured by electron energy loss spectroscopy (EELS), and a range of optical gaps from 0.65 eV (20 at.% sp3 C) to 2.25 eV (76 at.% sp3 C) as measured by SE. SE data from 1.5 to 5 eV have been analyzed by applying the most widely used effective medium theory (EMT) namely that of Bruggeman with isotropic screening, assuming a model of the material as a composite with sp2 C and sp3 C components. Although the atomic fractions of sp3 C deduced by SE with the Bruggeman EMT correlate monotonically with those obtained by EELS, the SE results range from 10 to 25 at.% higher. The possible origins of this discrepancy are discussed within the framework of an optical composite. Improved agreement between SE and EELS is obtained by employing a simple form for the EMT, in which the effective dielectric function is determined as a volume-fraction-weighted average of the dielectric functions of the two components.

Improvement of wear resistance of pulsed laser deposited diamond-like carbon films through incorporation of metals

We have investigated the characteristics of diamond-like carbon (DLC), DLC doped with Cu, and DLC doped with Ti deposited by a sequential pulsed laser ablation of two targets. The composition of these films was determined by Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy and transmission electron microscopy studies showed typical features of DLC with a high fraction of sp 3 bonded carbon in the doped films as well as in the undoped films. Wear resistance measurements made on the samples by means of the `crater grinding method' showed that DLC+2.75% Ti has the highest wear resistance, while that of pure DLC has the lowest amongst the samples. Careful analysis of the Raman data indicates a significant shift to shorter wavelength with the addition of metal, which means that the compressive stress in the DLC films has been reduced. We envisaged that the reduction in the compressive stress promotes the wear resistance of the coatings. The XPS studies showed evidence for the formation of Ti–C bonding in the Ti doped sample. Thus metal-doped DLC coatings are expected to improve the tribological properties and enhance the performance of components coated with metal-doped DLC.

X-ray diffraction studies of the effects of N incorporation in amorphous CNx materials

The effects of nitrogen incorporation on the atomic-scale structure of amorphous CNx samples have been studied for 0, 5, 20, and 30 at. % N concentration, by x-ray diffraction. Significant differences in the structure are observed on the incorporation of only 5 at. % N, and the changes in structure continue as further N is added. From the experimental data, we are able to obtain directly the average bond distances and then calculate the average bond angles for each of the samples. The average first neighbor distance shows a gradual decrease from 1.55 Å for 0 at. % N, to 1.44 Å for 30 at. % N, and a similar trend is observed in the position of the second neighbor peak. This gives a corresponding increase in the average bond angle from 108° to 114°. The results show an increase in the fraction of sp2 bonded carbon atoms with increasing N concentration, and there is evidence for the presence of significant numbers of C≡N and C=N bonds. These results are also consistent with stress, hardness, and optical gap measurements for these samples.

Deposition of tetrahedral hydrogenated amorphous carbon using a novel electron cyclotron wave resonance reactor

Highly tetrahedral hydrogenated amorphous carbon (ta-C:H) is deposited with a novel, 13.6 MHz excited electron cyclotron wave resonance (ECWR) plasma source. The ion flux of an acetylene and a nitrogen plasma was investigated by mass spectrometry and retarding field measurements. The ECWR gives a dissociation degree between 15% and 80% depending on gas flow rate. Ion current densities up to 2 mA/cm2 can be achieved, corresponding to ta-C:H deposition rates of 2 nm/s. The fraction of sp3 bonded carbon atoms and mass density are strongly related to the amount of hydrogen in the ion flux. For low hydrogen ion fluxes (10%), a sp3 fraction of 70% and a mass density of 2.85 g/cm3 can be achieved. At higher hydrogen ion fluxes (40%), the sp3 fraction and the mass density fall to 55% and 2.55 gm/cm3, respectively.

Structure of plasma-deposited amorphous hydrogenated boron-carbon thin films

Amorphous hydrogenated boron-carbon (a-B1xCx:H) thin films were prepared by radio-frequency plasma deposition using CH4 and B2H6 diluted in H2 as precursor gases. The composition and density were determined by ion-beam analysis. The densities of the a-B:H and a-C:H films are similar with ϱ ≡ 1.72 g cm3. a value typical of hard and dense a-B:H and a-C:H films. Ternary a-B1,xCx:H films are less dense, having a minimum density of ϱ ≡ 1.48 g cm3 at x = 0.71. Fourier transform infrared spectroscopy was used for qualitative and quantitative analysis of the bonding structures. The properties of films up to x = 0.2 are comparable to those of pure a-B:H films. However, small carbon contents influence the film structure, mainly reducing the number of B-H-B bridge bonds, which are abundant in pure a-B:H films. Low film densities at x = 0.5 to 0.8 correlate with the maximum intensity of the extrinsic B-C vibrational mode around 1200 cm★ due to terminal -CHv groups, indicating a less cross-linked network structure. Modifications of the network are indicated for x> 0.7. Below, carbon is predominantly sp★ hybridised and bonded between boron neighbours. Above x = 0.7 the network becomes carbon dominated with an increasing number of carbon-carbon bonds and an increasing fraction of sp★ hybridised carbon was observed. The extrinsic B-C vibrational mode at 1200 cm★ and the intrinsic B-C network vibration at 1500 cm★ were quantified over the whole range of a-B1xCx:H film compositions. The absorption constant of the extrinsic B-C vibrational mode at 1200 cm★ was estimated.

Amorphous Diamond Films Deposited by Pulsed-Laser Ablation: the Optimum Carbon-Ion Kinetic Energy and Effects of Laser Wavelength

AbstractA systematic study has been made of changes in the bonding and optical properties of hydrogen-free tetrahedral amorphous carbon (ta-C) films, as a function of the kinetic energy of the incident carbon ions measured under film-deposition conditions. Ion probe measurements of the carbon ion kinetic energies produced by ArF and KrF laser ablation of graphite are compared under identical beam-focusing conditions. Much higher C+ kinetic energies are produced by ArF-laser ablation than by KrF for any given fluence and spot size. Electron energy loss spectroscopy and scanning ellipsometry measurements of the sp3 bonding fraction, plasmon energy, and optical properties reveal a well-defined optimum kinetic energy of 90 eV to deposit ta-C films having the largest sp3 fraction and the widest optical (Tauc) energy gap (equivalent to minimum near-gap optical absorption). Tapping-mode atomic force microscope measurements show that films deposited at near-optimum kinetic energy are extremely smooth, with rms roughness of only ~ 1 Å over distances of several hundred nm, and are relatively free of particulates.

Investigations of diamond-graphite hybrids and fullerenes with seven-membered rings

The nature of diamond-graphite hybrids has been studied by molecular mechanics, by examining the structures of species such as C 84H x wherein the sp 3 to sp 2 carbon ratio is varied progressively. The dependence of the average coordination number on the atom fraction of hydrogen has been examined in the light of the random covalent network model. The HOMO-LUMO gap has been estimated in a graphite-like C 110H x and in a diamond-like C 120H x as a function of the sp 3 sp 2 carbon atom ratio. The gap increases exponentially with the fraction of sp 3 carbon. Shapes of fullerene-like structures with 7-membered rings, in addition to 6- and 5-membered rings, have been investigated along with structures of bent nanotubes having similar ring systems.

Mechanical properties and Raman spectra of tetrahedral amorphous carbon films with high sp3 fraction deposited using a filtered cathodic arc

Abstract The mechanical and structural properties of the tetrahedral amorphous carbon (ta-C) films deposited by the filtered cathodic vacuum arc technique on silicon at room temperature have been studied over the carbon ion energy range 15–200 eV. High (about 80% or higher) sp3 bond fractions were obtained for almost all ion energies investigated (E > 50 eV), with a maximum of about 87% at about 95 eV. The variations in the mechanical properties have been correlated to the sp3 fraction and show an almost linear dependence on the small variation in sp3 content. The maximum hardness, Young's modulus, stress critical load and minimum friction coefficient all coincide with the highest sp3 fraction as determined by electron-energy-loss spectroscopy. All films are atomically smooth. The Raman spectra of these films when fitted with a skewed Lorentzian, a parameter Q, which measures the degree of skewness, is noticeably dependent on films with sp2 content below 30% and can also be used to verify the ion energy corresponding to the maximum sp3 content. The change observed in the mechanical properties as well as the skewness parameter Q is attributed to a change in the local ordering of the sp2 bonded atoms in the ta-C sp3 matrix.

Electron emission and structure properties of cesiated carbon films prepared by negative carbon ion beam

The work function, field emission property (turn-on field), and Auger electron spectroscopy of cesiated carbon films on Si (100) have been investigated for codeposition of Cs neutral and C− ion beams at different energies (25–150 eV). The higher energy (150 eV) C− ion beam produces the lower work function surface (1.1 eV) as well as sp3 rich carbon film. The work function depends both on cesium concentration as well as on the sp3 fraction in the carbon films. The turn-on field of the film can be as low as 7 V/μm. The thermal stability of the low work function surface has been investigated for postdeposition annealing up to 600 °C. An extremely high stability cold cathode has been made by forming cesium carbide.

Optical properties of amorphous diamond prepared by a mass-separated ion beam: Correlation with the Raman spectra

The optical properties of amorphous-diamond films prepared by a mass-separated ion beam are studied and compared with the shape of the corresponding Raman spectra. The Raman measurements with moderate spatial resolution indicate that the films possess the sp3 fraction distributed along the film surface, the thinner edges being less diamondlike than the thicker center. The optical parameters of the film material, such as the refractive index and absorption coefficient, follow the spatial changes of the sp3 fraction, and the amorphous-diamond center is characterized by a relatively high refractive index (n∼2.50) and quite low absorption (α∼0.2×104 cm−1 at 633 nm). Numerical correlation between the Raman spectra and optical parameters proposes a common origin for their spatial distribution, namely, impurities perturbing the three-dimensional diamondlike network and causing sp2-coordinated carbon areas.

Tetrahedral amorphous carbon (ta-C) coatings for processing tools to reduce contamination in the analysis of trace elements

The possibility of reducing contamination in trace element analysis by coating processing tools with tetrahedral amorphous carbon (ta-C) coating has been studied. The 0.5 μm thick ta-C coating was deposited on pressing tool pellets made of Mo, W and WC-Co hard metal using the plasma accelerator method. The method was optimised to get high adhesion and high sp3 fraction (approx. 80%) to ensure minimal wear. The contamination of sugar test samples pressed with uncoated pellets was clearly detected with particle induced X-ray emission method (PIXE) for all the pellet materials used. In the samples pressed with ta-C coated pellets contamination from the pellets was below the detection limit, i.e. < 0.1–1 ppm. Furthermore, ta-C coatings withstood the pressing and normal cleaning procedures without any delamination or other kind of damage. The results obtained should be applicable to other processing tools as well.

Electron field emission from amorphous carbon-cesium alloys

Hard carbon films can be prepared by the condensation of energetic carbon species at and below room temperature. These hydrogen-free films are primarily tetrahedrally coordinated and contain high fractions of sp3 bonding. Field emission from these and other forms of carbon has been considered previously, but it was generally unstable or based on surface treatments that limit their operating conditions. We report electron emission from amorphous carbon-cesium (a-C:Cs) thin films at applied fields as low as 7 V/μm. This emission characteristic is relatively insensitive to surface treatment; films left under ambient laboratory environment for more than six months show these favorable characteristics with no pretreatment. We describe the fabrication process and emission properties of these films.

Raman diagnostics of amorphous diamond-like carbon films produced with a mass-separated ion beam

Abstract Raman spectra of the amorphous hydrogen-free diamond-like films prepared in well-controlled conditions by using a mass-separated carbon ion beam are investigated. The Raman spectra of a series of samples deposited by the same mass-separator set-up but with different energies of carbon ions and characterized by different sp3 fractions are compared. The correlation between the sp3 fraction and the numerical parameters of Raman spectra recorded in the 100–4100 cm−1 range, such as (i) the intensity ratios R (1) = I 500 I 1550 , R (2) = I 2100 I 3000 , R (3) = I 2100 I 1550 and (ii) the average slope S = (I 1300 −I 1100 ) I 1100 , is established for each of the five wavelengths of Ar+ laser excitation. Possible interference distortion of the shape of Raman spectra is taken into account. The presented results agree with the theoretically predicted properties of tetrahedral amorphous carbon and confirm the possibility of the effective quantitative characterization of this material by using Raman spectroscopy. The correlations obtained are used for practical studies of spatial properties of the film structure and a significant decrease of the film quality towards the edges of diamond-like film is observed.

Electronic and atomic structure of undoped and doped ta-C films

Undoped and doped tetrahedrally bonded amorphous carbon films (ta-C) were grown in UHV by alternating deposition of low energy mass separated 12 C + ions and dopant ions. For undoped ta-C the fraction of sp 3 -bonded carbon atoms was determined by EELS as a function of ion energy. The observed energy dependence is compared to data from vacuum arc-deposited ta-C and to existing nucleation models. With XPS and UPS measurements, the valence band structure and the chemical bonding of B- and N-doped as well as undoped ta-C films was characterized.

Boronated tetrahedral amorphous carbon (ta-C:B)

We have deposited boronated highly tetrahedral amorphous carbon (ta-C:B) films with low stress using a filtered cathodic vacuum arc (FCVA). The sp3 fraction, hardness and resistivity were measured as a function of the ion energy and were found to reach a maximum above 50 eV for B concentrations of 2 and 4%. The most significant result we found was that highly tetrahedral a-C:B film (sp3≈80%) with low stress (1–3 GPa) with B concentrations up to 4% could be obtained. The B in the films was found to be predominantly (∼75%) sp2 bonded The bond length and angle of ta-C:B found using the radial distribution function were similar to ta-C, confirming its tetrahedral nature. Additionally, the stress in the films did not vary with the ion energy or sp3 fraction unlike in undoped ta-C films. The ta-C:B films also exhibited higher resistivity than ta-C. This is believed to be related to the reduction of defect density measured by electron spin resonance, although the optical band gap was similar to ta-C (2.0–2.4 eV).

Selective chemical etching of hexagonal boron nitride compared to cubic boron nitride

A BN film containing comparable amounts of sp2 and sp3 phases was subjected to a gas-phase chemical etch in a hot-filament environment containing 1% CH4 in H2. After a partial etch, examination by FTIR shows that the sp2 was preferentially etched, leaving a larger sp3 fraction than in the unetched film. The possibility that preferential etching could be used to increase the purity of cBN films is discussed.

Defects, doping, and conduction mechanisms in nitrogen-doped tetrahedral amorphous carbon

First principles methods are used to study N doping of diamondlike amorphous carbon. A structural model containing 216 atoms is introduced, whose properties are in agreement with the available experimental data. The topological and electronic properties for different N doping concentrations are investigated. We find that N occurring in tetrahedral sites or chains of an even number of π bonded sites results in an increase of the Fermi energy, while N incorporation in strained network sites induces structural changes that lead to an increase in the sp2 fraction of the material. The prevalent conduction mechanisms are identified and discussed. While the Fermi energy increases upon N doping, the localization of the conduction-band-tail states limits extended state conduction. These results are compared to the recent experimental reports on N doping of ta-C and we find that the nondoping threefold N incorporation (N30) is energetically most likely, which explains the low doping efficiency seen in experiments.