Characteristics and surface energy of silicon-doped diamond-like carbon films fabricated by plasma immersion ion implantation and deposition

Diamond-like carbon (DLC) films doped with different silicon contents up to 11.48 at.% were fabricated by plasma immersion ion implantation and deposition (PIII-D) using a silicon cathodic arc plasma source. The surface chemical compositions and bonding configurations were determined by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results reveal that the sp³ configuration including Si–C bonds increases with higher silicon content, and oxygen incorporates more readily into the silicon and carbon interlinks on the surface of the more heavily silicon-doped DLC films. Contact angle measurements and calculations show that the Si-DLC films with higher silicon contents tend to be more hydrophilic and possess higher surface energy. The surface states obtained by silicon alloying and oxygen incorporation indicate increased silicon oxycarbide bonding states and sp³ bonding states on the surface, and it can be accounted for by the increased surface energy particularly the polar contribution.     

Effect of deposition voltage on the microstructure of electrochemically deposited hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films were deposited on Si substrates by electrolysis in a methanol solution at ambient pressure and a low temperature (50 °C), using various deposition voltages. The influence of deposition voltage on the microstructure of the resulting films was analyzed by visible Raman spectroscopy at 514.5 nm and X-ray photoelectron spectroscopy (XPS). The contents of sp³ bonded carbon in the various films were obtained by the curve fitting technique to the C1s peak in the XPS spectra. The hardness and Young’s modulus of the a-C:H films were determined using a nanoindenter. The Raman characteristics suggest an increase of the ratio of sp³/sp² bonded carbon with increasing deposition voltage. The percentage of sp³-bonded carbon is determined as 33–55% obtained from XPS. Corresponding to the increase of sp³/sp², the hardness and Young’s modulus of the films both increase as the deposition voltage increases from 800 V to 1600 V.     

Substrate temperature effects on the microhardness and adhesion of diamond-like thin films

Diamond-like films were deposited on silicon substrates by r.f. plasma-enhanced chemical vapor deposition from gas methane. In this study, the substrate temperature, T_S, was varied in a wide range from 20 to 370°C while maintaining fixed other important process parameters such as r.f. power (70 W) or pressure (2.5 Pa). The increase of T_S causes an increase of the sp²/(sp²+sp³) bonded carbon ratio and a decrease of the hydrogen content. These changes produce a great modification of the mechanical properties: microhardness, friction coefficient and adhesion. The variations of mechanical properties with T_S correlate well with the sp²/(sp²+sp³) bonded carbon ratio and the hydrogen content in the films showing a gradual transformation of the diamond-like structure into a more sp²-rich one.     

Characterization of carbonaceous films deposited on metal substrates by liquid-phase electrodeposition in methanol

In this study, we report the characterization of carbonaceous films deposited on metal substrates by liquid-phase electrodeposition in methanol. The characterization of carbonaceous films by electrodeposition was examined by means of Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (EELS), secondary ion mass spectrometry (SIMS), atom probe (AP) and high resolution-elastic recoil detection analysis (HR-ERDA). From these results, it was found that the films deposited on the metal substrates were composed of the sp² and sp³ carbon contents, of which the ratio was about 7:3. Furthermore, the films by electrodeposition contained much hydrogen. The hydrogen contents in the surface were about 60 at.% and those in the subsurface were a few 10 at.%.     

Structural dependence of corrosion resistance of amorphous carbon films against nitric acid

To investigate the structural dependence of the corrosion resistance of amorphous carbon (a-C:H) films, three different types of a-C:H films etched by nitric acid were evaluated using a surface plasmon resonance (SPR) device with a multilayer structure consisting of an a-C:H layer on Ag. Two non-hydrogenated amorphous carbon (a-C) films and one hydrogenated a-C:H film were synthesized to estimate the effects of the sp²/sp³ ratio and hydrogenation, respectively. A flow cell for the introduction of nitric acid solution was placed on the amorphous carbon layer of the multilayer structure. A 0.3 mM nitric acid solution was used in the etching tests. The Kretschmann configuration was used for SPR measurement, and the SPR angle was determined as the angle with minimum reflectivity. The SPR angle decreased with increasing duration of nitric acid injection into the flow cell, indicating that the film was corroded by the nitric acid. The thickness of the films was calculated from the SPR angle. The rates of decrease in the thickness were 2.2, 0.8, and 1.6 nm/h for the a-C films with lower and higher sp² contents and the 17 at.% hydrogenated a-C:H film, respectively. Although the hydrogen content had little effect on the rate of change in the film thickness, the film thickness clearly decreased with decreasing sp²/sp³ ratio. These results indicate that the sp²/sp³ ratio is an important factor determining the chemical resistance to nitric acid solution.     

The effect of CNT content on the surface and mechanical properties of CNTs doped diamond like carbon films

The carbon nanotubes (CNTs) doped diamond like carbon films were carried out by spinning coating multi-walled carbon nanotubes (CNTs) on silicon covered with diamond like carbon films via PECVD with C₂H₂ and H₂. The results show that the I_D/I_G and sp²/sp³ ratios are proportional to the CNT contents. For wettability and hydrogen content, the increase of CNT content results in more hydrophobic and less hydrogen for CNT doped DLC films. As for mechanical properties, the hardness and elastic modulus increases linearly with increasing CNT content. The residual stress is reduced for increasing CNT content. As for the surface property, the friction coefficient is reduced for higher CNT content. For CNT doped DLC films, the inclusion of horizontal CNT into DLC films increases the hardness, elastic modulus and reduces the hydrogen content, friction coefficient and residual stress. Like the light element and metal doping, the CNT doping has effects on the surface and mechanical properties on DLC which might be useful to specific application.     

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.     

Antibacterial properties of polycrystalline diamond films

Electronic and mechanical properties, and their biocompatibility, make diamond-based materials promising biomedical applications. The cost required to produce high quality single crystalline diamond films is still a hurdle to prevent them from commercial applications, but the emergence of polycrystalline diamond (PCD) films grown by chemical vapour deposition (CVD) method has provided an affordable strategy. PCD films grown on silicon wafer have been used throughout and were fully characterised by SEM, XPS, Raman spectroscopy and FTIR. The samples contain nearly pure carbon, with impurities originated from the CVD growth and the silicon etching process. Raman spectroscopy revealed it contained tetrahedral amorphous carbon with small tensile stress. The sp² carbon content, comprised between 16.1 and 18.8%, is attributed to the diamond grain boundaries and iron-catalysed graphitisation. Antibacterial properties of PCD films were performed with two model bacteria, i.e. Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive) using direct contact and shaking flask methods. The samples showed strong bacteriostatic properties against S. aureus and E. coli with the direct contact method and no influence on planktonic bacterial growth. These results suggest that the bacteriostatic mechanism of PCD films is linked to their surface functional groups (carbon radicals and –NH₂ and –COOH groups) and that no diffusible molecules or components were involved.     

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.     

Effect of substrate bias voltage on the properties of diamond-like carbon thin films deposited by microwave surface wave plasma CVD

Diamond-like carbon (DLC) thin films were deposited on silicon and ITO substrates with applying different negative bias voltage by microwave surface wave plasma chemical vapor deposition (MW SWP-CVD) system. The influence of negative bias voltage on optical and structural properties of the DLC film were investigated using X-ray photoelectron spectroscopy, UV/VIS/NIR spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. Optical band gap of the films decreased from 2.4 to 1.7 with increasing negative bias voltage (0 to − 200 V). The absorption peaks of sp³ CH and sp² CH bonding structure were observed in FT-IR spectra, showing that the sp²/sp³ ration increases with increasing negative bias voltage. The analysis of Raman spectra corresponds that the films were DLC in nature.     

Blood compatibility of La2O3 doped diamond-like carbon films

La₂O₃ doped diamond-like carbon films (DLC) with different concentration were deposited by using Radio-Frequency magnetron sputtering. The microstructure and surface properties of DLC films were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact angle test. The blood compatibility of the samples was evaluated by tests of platelet adhesion. Results show the sp²-bonded C content increases with increasing of La₂O₃ concentration doped. A remarkable decrease of platelet adhered on the surface of the La₂O₃ doped DLC films was observed comparing to the Chrono flex used in clinical application, suggesting that La₂O₃ doped DLC is able to enhance its blood compatibility. The mechanism of hemocompatibility of doped films was discussed. Our results demonstrate that La₂O₃ doped DLC films are potentially useful biomaterials with good blood compatibility.     

Dependence of the composition and bonding structure of carbon nitride films deposited by direct current plasma assisted pulsed laser ablation on the deposition temperature

Carbon nitride films were deposited by direct current plasma assisted pulsed laser ablation of a graphite target under nitrogen atmosphere. Atomic force microscopy (AFM), Fourier transform infrared (FTIR), Raman, and X-ray photoelectron spectroscopy (XPS) were used to characterize the surface morphology, bonding structure, and composition of the deposited films. The influence of deposition temperature in the range 25–400 °C on the composition and bonding structure of carbon nitride films was systematically studied. AFM images show that surface roughness and cluster size increase monotonically with deposition temperature. XPS, FTIR, and Raman spectra indicate directly the existence of CN, CN, and CN bonds in the deposited films. The increase of deposition temperature results in a drastic decrease in the N/C ratio, the content of CN bond and N atoms bonded to sp³ C atoms, in addition to the increase in the content of disorder sp² C atoms and N atoms bonded to sp² C atoms in the deposited films. Raman spectra show that the intensity ratio of D peak over G peak increases with increasing deposition temperature to 200 °C, then decreases with the further increase of deposition temperature, which results from the continuous growth of sp² cluster in the films.     

Effect of surface treatments on the electronic properties of ultra-nanocrystalline diamond films

We present the soft x-ray spectroscopic study of the ultra-nanocrystalline diamond (UNCD) films with different surface treatments. The samples were prepared by means of microwave plasma enhanced chemical vapor deposition (MPECVD) and the different surface treatments are applied to alter their field emission properties. The electronic properties were subsequently examined by the soft x-ray absorption and x-ray emission spectroscopy at carbon 1s threshold. From the experimental results, there is no significant variation in electronic structure of oxygen- and hydrogen-plasma treated UNCD films. On the other hand, the biased treated UNCD film shows more remarkable change on the sp² and sp³ states. The formation of sp² bonding and the reduction of sp³ bonding are the consequence of the improved electron field emission properties.     

Spectroscopic studies of nanocrystalline diamond materials

The structural and electronic properties of nanocrystalline diamond films synthesized by a modified hot-filament chemical vapour deposition process were investigated by both bulk- and surface-sensitive techniques. Diffraction and microscopy data show the films to consist of diamond grains with an average crystallite size of about 10–15 nm and a root-mean-square roughness of similar size. Carbon core-level excitations in transmission electron energy-loss spectroscopy reveal an sp² content below 5%. The low energy loss spectra are quite similar to that of diamond crystal. The high sp³ content in the films was also confirmed by C 1s photoelectron plasmon energy loss features in X-ray photoemission experiments and by X-ray excited Auger-electron spectroscopy. We find that the hydrogen covered diamond surface gets contaminated after storage for several months under ambient conditions. Heating up to 500°C in vacuo is required to desorb the adsorbate layer.     

Diamond nanoseeding on silicon: Stability under H2 MPCVD exposures and early stages of growth

Detonation nanodiamond dispersed on silicon surfaces underwent different H₂ MPCVD exposures. The induced changes at the surface have been characterized in situ by XPS and XEELS. Then, a short CH₄/H₂ growth step was applied. This sequential study revealed an excellent stability of detonation nanodiamond. The sp³ etching rate is insufficient to remove nanodiamond even under intense H₂ plasma. The H₂ exposure could be successfully used to remove C–C sp² carbon without altering sp³ seeds. Moreover, the formation of silicon carbide observed after the hydrogen treatment is thought to be helpful to enhance the adhesion of nanodiamond particles on the substrate.     

Thickness dependency of field emission in amorphous and nanostructured carbon thin films

Thickness dependency of the field emission of amorphous and nanostructured carbon thin films has been studied. It is found that in amorphous and carbon films with nanometer-sized sp² clusters, the emission does not depend on the film thickness. This further proves that the emission happens from the surface sp² sites due to large enhancement of electric field on these sites. However, in the case of carbon films with nanocrystals of preferred orientation, the emission strongly depends on the film thickness. sp²-bonded nanocrystals have higher aspect ratio in thicker films which in turn results in higher field enhancement and hence easier electron emission.     

Ultraviolet and visible Raman analysis of thin a-C films grown by filtered cathodic arc deposition

Amorphous carbon thin films with a wide range of sp² fraction from 20 to 90% grown by filtered cathodic arc deposition have been examined by ultraviolet (UV) at 325 nm and visible Raman spectroscopy at 457 nm excitation wavelength. The comprehensive study of behaviour of G, D and T band with sp²/sp³ content has been carried out. The upwards shift of the G peak with sp³ content was observed for both excitation wavelengths. It was also found that the I(D)/I(G) ratio decreases with sp³ content for UV and visible excitations, and for high sp³ content I(D)/I(G) tends to zero. The dispersion of the G peak is also investigated in this work as a function of sp² content.     

Preparation and blood compatibility of carbon/TiO2 nanocomposite

A carbon/TiO₂ nanocomposite, which consists of carbon film with various sp³C content and TiO₂ nanowire arrays, has been synthesized, in which the top surface of TiO₂ nanowire arrays prepared using hydrothermal method on fluorine-doped tin oxide glass were coated with carbon thin films. The carbon thin films with a higher, medium and lower sp³C content were deposited by pulsed magnetic filtered cathodic vacuum arc deposition, plasma-enhanced chemical vapor deposition and magnetron sputtering deposition, respectively. The surface morphology and structure of TiO₂ nanowire arrays were investigated by scanning electron microscopy, transmission electron microscope and X-ray diffraction. The sp³C content in carbon films was characterized using Raman spectroscopy. The blood compatibility of the samples including the TiO₂ nanowire arrays, carbon films and carbon/TiO₂ nanocomposite was assessed by tests of platelet adhesion in vitro. Results showed that the carbon/TiO₂ composite can effectively improve the anticoagulant function compared to the single materials. It is believed that the excellent blood compatibility of the carbon/TiO₂ nanocomposite is attributed to a joint function of surface properties adjusted by nanowire arrays and electronic structure of carbon thin films.     

Structure and mechanical properties of oxygen doped diamond-like carbon thin films

In this study, structure and mechanical properties of doped diamond-like carbon (DLC) films with oxygen were investigated. A mixture of methane (CH₄), argon (Ar) and oxygen (O₂) was used as feeding gas, and the RF-PECVD technique was used as a deposition method. The thin films were characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (RS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and a combination of elastic recoil detection analysis and Rutherford backscattering (ERDA-RBS). Nano-indentation tests were performed to measure hardness. Also, the residual stress of the films was calculated by Stoney equation. The XPS and ERDA-RBS results indicated that by increasing the oxygen in the feeding gas up to 5.6 vol.%, the incorporation of oxygen into the films' structure was increased. The ratio of sp² to sp³ sites was changed by the variation of oxygen content in the film structure. The sp²/sp³ ratios are 0.43 and 1.04 for un-doped and doped DLC films with 5.6 vol.% oxygen in the feeding gas, respectively. The Raman spectroscopy (RS) results showed that by increasing the oxygen content in doped DLC films, the amount of sp² CC aromatic bonds was raised and the hydrogen content reduced in the structure. The attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) confirmed the decrease of hydrogen content and the increase the ratio of CC aromatic to olefinic bonds. Hardness and residual stress of the films were raised by increasing the oxygen content within the films' structure. The maximum hardness (19.6 GPa) and residual stress (0.29 GPa) were obtained for doped DLC films, which had the maximum content of oxygen in structure, while the minimum hardness (7.1 GPa) and residual stress (0.16 GPa) were obtained for un-doped DLC films. The increase of sp³ CC bonds between clusters and the decrease of the hydrogen content, with a simultaneous increase of oxygen in the films' structure is the reason for increase of hardness and residual stress.     

The carbon nitride films prepared at various substrate temperatures by vacuum cathodic arc method

Carbon nitride films have been grown by vacuum cathodic arc method in the substrate temperature range of 100–500 °C. The bonding structure of the films was investigated by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and infrared (IR) spectroscopy. With increasing substrate temperature, the films indicate various characteristics. At 100 °C, it can be described as a network similar to DLC in which aromatic sp²C phase is cross-linked by sp³C phase. Between 200 and 400 °C, with increasing substrate temperature the films become graphitized and the sp²CN phase increases, meanwhile the non-aromatic sp²CN phase appears at the edges of aromatic clusters in planar position as well as in out-of-planar regions. While at 500 °C the non-aromatic sp²CN phase almost comes to the same level as the aromatic sp²CN phase. So in the network of the film the aromatic sp²C phase is cross-linked by the non-aromatic sp²C phase. Based on the variation of the microstructure of the films, a comprehensive assignment pattern for the XPS C1s and N1s at different substrate temperature is proposed. In addition, the interpretation of p electron band in valence band spectra at various substrate temperatures is also discussed.