Photoconductivity of DLC film deposited by pulsed discharge plasma CVD

DLC films were deposited by a new pulsed DC discharge plasma chemical vapour deposition (CVD) using hydrogen and methane gas mixture. When methane concentration (Cm) i.e. CH₄/(H₂ + CH₄) was increased from 3 to 40%, the graphitization of the carbon film increases as evident from Raman study. When Cm was increased to 30%, DLC film shows photoconducting property. The white light photoconductivity (S = I_l/I_d, where I_l is light current and I_d is dark current) measured with solar simulator under AM 1.5 condition was approximately 20 at room temperature. The photoconductivity was not clear when Cm was lower than 20%. ESR measurements also show that the electron spin density was slightly decreased with decreasing concentration of methane. Thus we can conclude here that at higher concentrations of methane at 30%, Sp² content of the film increases and the DLC film becomes photoconducting.     

Effect of oxygen plasma treatment on non-thrombogenicity of diamond-like carbon films

The non-thrombogenicity of oxygen-plasma-treated DLC films was investigated as surface coatings for medical devices. DLC films were deposited on polycarbonate substrates by a radio frequency plasma enhanced chemical vapor deposition method using acetylene gas. The deposited DLC films were then treated with plasma of oxygen gas at powers of 15 W, 50 W, and 200 W. Wettability was evaluated by water contact angle measurements and the changes in surface chemistry and roughness were examined by X-ray photoelectron spectroscopy and atomic force microscope analysis, respectively. Each oxygen-plasma-treated DLC film exhibited a hydrophilic nature with water contact angles of 11.1°, 17.7° and 36.8°. The non-thrombogenicity of the samples was evaluated through the incubation with platelet-rich plasma isolated from human whole blood. Non-thrombogenic properties dramatically improved for both 15 W- and 50 W-oxygen-plasma-treated DLC films. These results demonstrate that the oxygen plasma treatment at lower powers promotes the non-thrombogenicity of DLC films with highly hydrophilic surfaces.     

Effect of O2 plasma pretreatment on structural and optical properties of ZnO films on PES substrate by atomic layer deposition

ZnO films were deposited on the O₂ plasma treated polyethersulfone (PES) substrates by atomic layer deposition. X-ray diffraction (XRD) measurements reveals that the grains in ZnO films show strongly (0 0 2) preferential orientation, when the duration of plasma pretreatment increases. The decreased grain size and improved crystallinity results in the decreased surface roughness of ZnO films. In contrast, when the duration of plasma pretreatment increases to 60 min, the surface roughness increases again due to the increased grain size and worse crystallinity. In photoluminescence measurement, slight blue shift of near-band-edge emission occurs with increasing duration of plasma pretreatment up to 30 min.     

Improved microwave plasma cavity reactor for diamond synthesis at high-pressure and high power density

Microwave plasma assisted synthesis of diamond is experimentally investigated using high purity, 2–5% CH₄/H₂ input gas chemistries and operating at high pressures of 180–240 Torr. A microwave cavity plasma reactor (MCPR) was specifically modified to be experimentally adjustable and to enable operation with high input microwave plasma absorbed power densities within the high-pressure regime. The modified reactor produced intense microwave discharges with variable absorbed power densities of 150–475 W/cm³ and allowed the control of the discharge position, size, and shape thereby enabling process optimization. Uniform polycrystalline diamond films were synthesized on 2.54 cm diameter silicon substrates at substrate temperatures of 950–1150 °C. Thick, freestanding diamond films were synthesized and optical measurements indicated that high, optical-quality diamond films were produced. The deposition rates varied between 3 and 21 μm/h and increased as the operating pressure and the methane concentrations increased and were two to three times higher than deposition rates achieved with the MCPR operating with equivalent input methane concentrations and at lower pressures (≤ 140 Torr) and power densities.     

Characterization of physical and biomedical properties of nitrogenated diamond-like carbon films coated on polytetrafluoroethylene substrates

A technique to coat hydrogen-free diamond-like carbon (DLC) films on polytetrafluoroethylene (PTFE) substrates has been developed by sputtering of a negatively biased graphite target in a mixture of argon and nitrogen plasma. The coated films were characterized by various methods to investigate their chemical, electronic features, and particularly their biomedical properties. DLC films produced by this method have up to 20% sp³ carbon bonds depending on the nitrogen concentration in the plasma. Raman spectroscopy revealed that, bond-disorder increases with nitrogen doping. The average grain size of DLC decreases in the nitrogen doped samples by almost 30%. The roughness of the uncoated PTFE substrate surfaces decreased dramatically from 660 nm to 170 nm after DLC coating. However, the nitrogen contents in the plasma have little effects on the roughness, the cluster size, and shapes. Electronic band gap of the samples decreases with adding nitrogen from ~ 2 eV in nitrogen-free samples to ~ 1 eV in nitrogenated samples. Lower adhesion and aggregation of platelets on PTFE surfaces coated with DLC-10% nitrogen and DLC-20% nitrogen have been observed while there is greater adhesion of platelets on DLC-30% nitrogen and DLC-40% nitrogen.     

Duplex DLC coatings fabricated on the inner surface of a tube using plasma immersion ion implantation and deposition

A duplex plasma immersion ion implantation and deposition (PIIID) process, involving carbon ion implantation and diamond-like carbon (DLC) deposition, is proposed to modify the inner surface of a tube. In the research, samples of GCr15 bearing steel were placed inside a tube in the vacuum chamber. After the vacuum chamber was evacuated to a base pressure of 6 × 10^− 3 Pa, C₂H₂ gas was introduced into the chamber, and the tube was biased by a negative pulsed bias. Since a pulsed glow discharge (PGD) plasma can be formed by the bias, carbon ion implantation and DLC film deposition process can be obtained by biasing the tube with a high and low bias, respectively. To synthesize different DLC films, single PIIID processes employing a low voltage (several kV) PGD method and duplex PIIID processes combining the high (several tens kV) and low voltage PGD techniques were carried out. The as-synthesized films were characterized by Raman spectrum, nano-indentation, scratch, tribological and electrochemical tests. Raman results show that duplex DLC films were synthesized by this duplex PIIID process. In addition, compared with the single DLC film synthesized by the low voltage PGD process, the duplex DLC films can obtain a high wear and corrosion resistances. Furthermore, using this duplex PIIID method, batch treatment of outer-rings of the bearing was realized.     

Diamond-like carbon films synthesized under atmospheric pressure synthesized on PET substrates

Diamond-like carbon films were synthesized under atmospheric pressure (AP-DLC) and their gas barrier properties and hardness were measured. The AP-DLC films were uniformly obtained by RF-plasma CVD method at room temperature with a size of 450 mm². The growth rate increased as a function of C₂H₂ concentration and the average growth rate was around 12 μm/min. The maximum deposition rate was ~ 1 μm/s, which is approximately 2000 times larger than that by low-pressure plasma CVD of 1–2 μm/h. The gas barrier properties of AP-DLC films, ~ 1 μm thick, were 5–10 times larger than those of uncoated PET substrates. The microhardness of AP-DLC films was around 3 GPa, measured by the nano-indentation method. The issue lies in the removal of macro-particles of the films to improve the microhardness and the surface roughness.In this paper, we report the physical properties of DLC films synthesized under atmospheric pressure by the radio-frequency CVD method. We also summarize a brief history of PET bottle coating by vacuum-DLC films, as well as that of the development of atmospheric pressure technology and related DLC films, focused on gas barrier properties and micro-hardness.     

Electrical and optical properties of diamond-like carbon films deposited by pulsed laser ablation

Pulsed laser ablation of a graphite target was carried out by ArF excimer laser deposition at a laser wavelength of 193 nm and fluences of 10 and 20 J/cm² to produce diamond-like carbon (DLC) films. DLC films were deposited on silicon and quartz substrates under 1 × 10^? 6 Torr pressure at different temperatures from room temperature to 250 °C. The effect of temperature on the electrical and optical properties of the DLC films was studied. Laser Raman Spectroscopy (LRS) showed that the DLC band showed a slight increase to higher frequency with increasing film deposition temperature. Spectroscopic ellipsometry (SE) and ultraviolet–visible absorption spectroscopy showed that the optical band gap of the DLC films was 0.8–2 eV and decreased with increasing substrate temperature. These results were consistent with the electrical resistivity results, which gave values for the films in the range 1.0 × 10⁴–2.8 × 10⁵ Ω cm and which also decreased with deposition temperature. We conclude that at higher substrate deposition temperatures, DLC films show increasing graphitic characteristics yielding lower electrical resistivity and a smaller optical band gap.     

Preparation of diamond like carbon thin films above room temperature and their properties

Diamond like carbon (DLC) thin films were deposited on p-type silicon (p-Si), quartz and ITO substrates by microwave (MW) surface-wave plasma (SWP) chemical vapor deposition (CVD) at different substrate temperatures (RT ∼ 300 °C). Argon (Ar: 200 sccm) was used as carrier gas while acetylene (C₂H₂: 20 sccm) and nitrogen (N: 5 sccm) were used as plasma source. Analytical methods such as X-ray photoelectron spectroscopy (XPS), FT-IR and UV–visible spectroscopy were employed to investigate the structural and optical properties of the DLC thin films respectively. FT-IR spectra show the structural modification of the DLC thin films with substrate temperatures showing the distinct peak around 3350 cm^− 1 wave number; which may corresponds to the sp² C–H bond. Tauc optical gap and film thickness both decreased with increasing substrate temperature. The peaks of XPS core level C 1 s spectra of the DLC thin films shifted towards lower binding energy with substrate temperature. We also got the small photoconductivity action of the film deposited at 300 °C on ITO substrate.     

Electron field emission from filtered arc deposited diamond-like carbon films using Au and Ti layers

In this work, tetrahedral diamond-like carbon (DLC) films are deposited on Si, Ti/Si and Au/Si substrates by a new plasma deposition technique — filtered arc deposition (FAD). Their electron field emission characteristics and fluorescent displays of the films are tested using a diode structure. It is shown that the substrate can markedly influence the emission behavior of DLC films. An emission current of 0.1 μA is detected at electric field E_DLC/Si=5.6 V/μm, E_DLC/Au/Si=14.3 V/μm, and E_DLC/Ti/Si=5.2 V/μm, respectively. At 14.3 V/μm, an emission current density J_DLC/Si=15.2 μA/cm², J_DLC/Au/Si=0.4 μA/cm², and J_DLC/Ti/Si=175 μA/cm² is achieved, respectively. It is believed that a thin TiC transition layer exists in the interface between the DLC film and Ti/Si substrate.     

Localized DLC etching by a non-thermal atmospheric-pressure helium plasma jet in ambient air

Using a versatile atmospheric-pressure helium plasma jet, diamond-like carbon (DLC) films were etched in ambient air. We observed that the DLC films are etched at a nominal rate of around 60 nm/min in the treated area (230 μm in diameter) during a 20-min exposure. The etching rate increased after the initial 10-min exposure. During this period, the flat DLC surface was structurally modified to produce carbon nanostructures with a density of ~ 2.4 × 10¹¹ cm^− 2. With this increase in surface area, the etching rate increased. After 20 min, the DLC film had a circular pattern etched into it down to the substrate where silicon nanostructures were observed with sizes varying from 10 nm to 1 μm. The initial carbon nanostructure formation is believed to involve selective removal of the sp²-bonded carbon domains. The carbon etching results from the formation of reactive oxygen species in the plasma.     

Experimentally defining the safe and efficient,high pressure microwave plasma assisted CVD operating regime for single crystal diamond synthesis

The detailed experimental behavior of a microwave plasma assisted chemical vapor deposition (MPACVD) reactor operating within the high, 180–300 torr, pressure regime is presented. An experimental methodology is described that first defines the reactor operating field map and then enables, while operating at these high pressures, the determination of the efficient, safe and discharge stable diamond synthesis process window. Within this operating window discharge absorbed power densities of 300–1000 W/cm³ are achieved and high quality, single crystal diamond (SCD) synthesis rates of 20–75 μm/h are demonstrated. The influence of several input experimental variables including pressure, N₂ concentration, CH₄ percentage and substrate temperature on SCD deposition is explored. At a constant pressure of 240 torr, a high quality, high growth rate SCD synthesis window versus substrate temperature is experimentally identified between 1030 and 1250 °C. When the input nitrogen impurity level is reduced below 10 ppm in the gas phase the quality of the synthesized diamond is of type IIa or better.     

Thin-film nanocomposites of diamond-like carbon and titanium oxide; Osteoblast adhesion and surface properties

Thin films of a novel, nanocomposite material consisting of diamond-like carbon and polycrystalline/amorphous TiO_x (DLC-TiO_x, x ≤ 2) were prepared using pulsed direct-current plasma enhanced chemical vapour deposition (PECVD). Results from Raman spectroscopy indicate that the DLC and TiO_x deposit primarily as segregated phases. Amorphous TiO₂ is found to be present on the surface region of the film and there is evidence for the presence of crystalline TiO in the bulk of the film. The hydrophilicity of the DLC-TiO_x films increased with increasing titanium content. Culture studies with human osteoblasts revealed that the differences in three-day cell adhesion properties (count, morphology and area) between DLC and DLC-TiO_x films containing up to 13 at.% Ti were not statistically significant. However, the cell count was significantly greater for the films containing 3 at.% of Ti in comparison to those containing 13 at.% of Ti. A post-plasma treatment with Ar/O₂ was used to reduce the water contact angle, θ, by nearly 40° on the DLC-TiO_x films containing 3 at.% of Ti. A cell culture study found that the osteoblast count and morphology after three days on these more hydrophilic films did not differ significantly from those of the original DLC-TiO_x films. We compare these results with those for SiO_x-incorporated DLC films and evaluate the long-term osteoblast-like cell viability and proliferation on modified DLC surfaces with water contact angles ranging from 22° to 95°.     

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.     

Growth and properties of hydrogen-free DLC films deposited by surface-wave-sustained plasma

Hydrogen-free diamond-like carbon (DLC) films were deposited by a new surface-wave-sustained plasma physical vapor deposition (SWP-PVD) system in various conditions. Electron density was measured by a Langmuir probe; the film thickness and hardness were characterized using a surface profilometer and a nanoindenter, respectively. Surface morphology was investigated using an atomic force microscope (AFM). It is found that the electron density and deposition rate increase following the increase in microwave power, target voltage, or gas pressure. The typical electron density and deposition rate are about 1.87 × 10¹¹–2.04 × 10¹² cm^− 3 and 1.61–14.32 nm/min respectively. AFM images indicate that the grain sizes of the films change as the experimental parameters vary. The optical constants, refractive index n and extinction coefficient k, were obtained using an optical ellipsometry. With the increase in microwave power from 150 to 270 W, the extinction coefficient of DLC films increases from 0.05 to 0.27 while the refractive index decreases from 2.31 to 2.11.     

Diamond-like carbon thin film with controlled zeta potential for medical material application

With the purpose of applying diamond-like carbon (DLC) thin films as a biocompatible material, we experimented with introducing functional groups such as amino and carboxyl groups to the surface of DLC thin films by plasma surface treatment. From the results, it was found that the contact angle values of the DLC thin films surface were decreased with increasing in the OCO bonded network on the surface. Measurement of the zeta potential when the amounts of the functional groups were varied showed that this successfully varied the zeta potential over the wide range of − 48 mV to + 12 mV. It was found that when carboxyl groups are introduced by O₂ plasma modification of the DLC thin films surface, the zeta potential was lower than that of untreated DLC thin films sample. It was also found that amino groups can be introduced to the DLC thin films surface by NH₃ plasma treatment, the zeta potential was higher than that of untreated DLC thin films sample. This means that zeta potential of the DLC thin films can also be controlled by controlling these two functional groups. Therefore, we have succeeded in developing a multifunctional DLC thin films that does not use polymers and is suitable as a biocompatible material.     

Microstructure and optical band gap control of DLC film deposited by pulsed discharge plasma CVD

DLC films were deposited on silicon and quartz glass substrates by pulsed discharge plasma chemical vapor deposition (CVD), where the plasma was generated by pulsed DC discharge in H₂–CH₄ gas mixture at about 90 Torr in pressure, and the substrates were located near the plasma. The repetition frequency and duty ratio of the pulse were 800 Hz and 20%, respectively. When CH₄ / (CH₄ + H₂) ratio, i.e. methane concentration (Cm), increased from 3 to 40%, C₂ species in the plasma was increased, and corresponding to the increase of C₂, deposition rate of the film was increased from about 0.2 to 2.4 μm/h. The absorption peaks of sp³C–H and sp²C–H structures were observed in the FT-IR spectra, and the peak of sp²C–H structure was increased with increasing Cm, showing that sp² to sp³ bonding ratio was increased when Cm was increased. Corresponding to these structural changes due to the increase of Cm, optical band gap (Eg) was decreased from 3 to 0.5 eV continuously when Cm was increased from 3 to 40%.     

Deposition and characterization of diamond-like carbon films by microwave resonator microplasma at one atmosphere

Diamond-like carbon (DLC) films have been deposited at atmospheric pressure by microwave-induced microplasma for the first time. Typical precursor gas mixtures are 250 ppm of C₂H₂ in atmospheric pressure He. Chemically resistant DLC films result if the Si (100) or glass substrate is in close contact with the microplasma, typically at a standoff distance of 0.26 mm. The films deposited under this condition have been characterized by various spectroscopic techniques. The presence of sp³ CH bonds and ‘D’ and ‘G’ bands were observed from FTIR and Raman spectroscopy, respectively. The surface morphology has been derived from SEM and AFM and shows columnar growth with column diameters of approximately 100 nm. Likely due to the low energy of ions striking the surface, the hardness and Young's modulus for the films were found to be 1.5 ± 0.3 GPa and 60 ± 15 GPa respectively with a film thickness of 2 μm. The hypothesis that a high flux of low energy ions can replace energetic ion bombardment is examined by probing the plasma. Rapid deposition rates of 4–7 μm per minute suggest that the method may be scalable to continuous coating systems.     

Atmospheric plasma deposition of diamond-like carbon coatings

The atmospheric pressure plasma-enhanced chemical vapor deposition of diamond-like carbon (DLC) has been investigated. The DLC coatings were grown with a mixture of acetylene, hydrogen and helium that was fed through a linear plasma source. The plasma was driven with radio frequency power at 27.12 MHz. Deposition rates exceeded 0.10 µm/min at substrate temperatures between 155 and 200 °C. Solid-state carbon-13 nuclear magnetic resonance revealed that the coatings contained approximately 43% sp²-bonded carbon and 57% sp³-bonded carbon. Coefficient of friction values for the coatings were found to be 0.24 ± 0.02, which is within the range observed for vacuum deposited DLC.     

Cross-sectional hydrogen content and mass density profiles of diamond-like carbon film by neutron and X-ray reflectivity

The cross-sectional profiles of hydrogen content and mass density of diamond-like carbon (DLC) film were investigated using X-ray and neutron reflectivity. DLC films were prepared using a plasma CVD technique by varying the H₂/(H₂ + CH₄) ratio gas source from 0 to 0.9. The cross-sectional hydrogen content and mass density profiles of the films were calculated by neutron and X-ray reflectivity, and the results were compared with those from elastic recoil detection analysis (ERDA).The fitted simulation showed that the mass density gradually decreased with increasing depth, whereas the hydrogen content increased with depth. In both ERDA and reflectivity measurements, the average hydrogen content was more than 30% in all films and tended to increase with the H₂/(CH₄ + H₂) ratio. However, there was a difference in hydrogen content values between the two analyses ranging from 5.1 % to 8.5%.