Radio-frequency Plasma-enhanced Chemical Vapor Research Articles

An amorphous Si-O-N coating on magnesium to retard corrosion & improve biocompatibility

Fast corrosion and the resulting adverse biological responses impede the applications of magnesium-based implants. Here, an amorphous Si-O-N coating is prepared on Mg via radio frequency plasma-enhanced chemical vapor deposition (PECVD) to address those issues. This coating is compact and uniform, which can firmly adhere to the substrate. It provides efficient shielding, thus reduces corrosion rate from 36.16 μm/y to 16.14 μm/y. The coating itself is biocompatible, and coated Mg exhibits excellent biocompatibility to rat/mouse mesenchymal stem cells. PECVD-obtained amorphous coatings based on biocompatible elements could be a solution for Mg-based implants.

The native and metastable defects and their joint density of states in hydrogenated amorphous silicon obtained from the improved dual beam photoconductivity method

In this study, undoped hydrogenated amorphous silicon (a-Si:H) thin films deposited under moderate dilution ratios of silane by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) have been investigated using steady-state photoconductivity and improved dual beam photoconductivity (DBP) methods to identify changes in multiple gap states in annealed and light-soaked states. Four different gap states were identified in annealed state named as A, B, C, and X states. The peak energy positions of these Gaussian distributions are consistent with those recently identified by Fourier transform photocurrent spectroscopy (FTPS). After in situ light soaking, their density increases with different rates as peak energy positions and half-widths remain unaffected. The electron-occupied A and B states located below the dark Fermi level and their density and ratios in the annealed and light-soaked states correlate well with those defects detected by time-domain pulsed electron paramagnetic resonance (EPR) experiments. The A, B, and X states located closer to the middle of the bandgap anneal out at room temperature in dark and define the “fast” states. However, the C states show no sign of room temperature annealing such that they must define the “slow” states in undoped a-Si:H. The results found in this study indicate that the anisotropic disordered network is a more appropriate model than previously proposed defect models based on the continuous random network to define the nanostructure of undoped a-Si:H, where multiple defects, D0 and non-D0 defects, can be identified by using the improved DBP method.

Superhydrophobic and Electrochemical Performance of CF2-Modified g-C3N4/Graphene Composite Film Deposited by PECVD.

The physicochemical properties of functional graphene are regulated by compositing with other nano-carbon materials or modifying functional groups on the surface through plasma processes. The functional graphene films with g-C3N4 and F-doped groups were produced by controlling the deposition steps and plasma gases via radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD). The first principles calculation and electrochemistry characteristic of the functional graphene films were performed on Materials Studio software and an electrochemical workstation, respectively. It is found that the nanostructures of functional graphene films with g-C3N4 and F-doped groups were significantly transformed. The introduction of fluorine atoms led to severe deformation of the g-C3N4 nanostructure, which created gaps in the electrostatic potential of the graphene surface and provided channels for electron transport. The surface of the roving fabric substrate covered by pure graphene is hydrophilic with a static contact angle of 79.4°, but the surface is transformed to a hydrophobic state for the g-C3N4/graphene film with an increased static contact angle of 131.3° which is further improved to 156.2° for CF2-modified g-C3N4/graphene film exhibiting the stable superhydrophobic property. The resistance of the electron movement of CF2-modified g-C3N4/graphene film was reduced by 2% and 76.7%, respectively, compared with graphene and g-C3N4/graphene.

Improvement of Surface Properties of Aluminum Alloy-Based Composites by Multi-Layer DLC Coating

In recent years, it has been a challenge for the transportation sector to reduce the weight of vehicles for higher fuel efficiency. This expands the use of light metals such as Al alloys. However, these materials have low hardness and surface strength. In this study, Al-alloy (AC8A; Al–Si–Cu–Ni–Mg) composites reinforced with Al2O3 fibers were used as base materials and were coated with multilayer diamond-like carbon (DLC) films to improve their properties. The multilayer DLC films were deposited on the Al alloy composites by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) using methane and acetylene gases as the raw materials. As the number of DLC film layers increased, the Young’s modulus of the samples decreased and the following ratios of hardness (H) to elastic modulus (E) also increased: H/E, H3/E2, H2/E. In addition, the critical number of peeling slides increased as the composite volume fraction and the number of layers increased.

Electrochemical behavior of the double-layer diamond-like carbon/plasma electrolytic oxidation on AZ31 alloy: A comparison of different PEO interlayers

In the present study, a double-layer diamond-like carbon (DLC)/plasma electrolytic oxidation (PEO) coating was developed on the AZ31 substrate and its electrochemical behavior was studied. In this regard, after fabrication of PEO coatings using two different electrolytes (silicate (PEOSi) and phosphate (PEO-Ph) compounds), DLC thin films were synthesized on the substrates using the radio frequency plasma-enhanced chemical vapor deposition (RFPECVD) method. Results showed the successful formation of thin and crack-free DLC layer with the thickness of about 1 μm on the middle PEO layers. However, the structure and chemical stability of the final DLC layer was dependent on the PEO composition. Moreover, the electrochemical studies in buffer phosphate saline (PBS) at the normal body temperature (37 ± 0.5 °C) showed that while both PEO-Ph and PEO-Si layers significantly decreased the corrosion rate of substrate, PEO-Ph revealed better electrochemical performance. On the other hand, DLC/PEO-Si was more effective than DLC/PEO-Ph to decrease the corrosion rate of substrates owing to the greater bonding of DLC coating on PEO-Si layer. In summary, crack-free DLC/PEO coatings could be a promising candidate to prevent from the high corrosion rate of magnesium alloys, making it appropriate for biomedical applications.

Surface treatment of biopolymer films Polylactic acid and Polyhydroxybutyrat with angular changing oxygen plasma ‒ More than just gradual purification

Bio-based and biodegradable polymers such as polylactic acid (PLA) and polyhydroxybutyrat (PHB) are developing into increasingly important raw materials. Especially when it comes to replacing conventional polymers in order to save resources and reduce environmental impact. Such biopolymers often conform to mechanical requirements of industry, but inappropriate surface characteristics such as poor barrier properties and increased degradation limit their application. Appropriate surface modification, such as the deposition of thin amorphous hydrogenated carbon films (a-C:H) produced by low-temperature mediated radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) can provide a remedy here. Prior to this, the surfaces are mostly cleaned/activated with oxygen plasma to enable better deposition of the subsequent a-C:H layer. It has been noticed that the O2 treatment, strongly changes the surface properties. It is now to be determined how a change in the O2 plasma treatment geometry affects the surface properties and whether this can be used accordingly. Surface morphology was analyzed by AFM, surface free energy (SFE) was determined using a contact angle goniometer, and changed barrier properties were investigated via the water vapor transmission rate. Top surfaces were studied by synchrotron supported X-ray spectroscopy, revealing angle-dependent changes in C and O content and binding conditions. The present angle-dependent O2 treatment not only changed the surface roughness and its chemical composition, but also, contrary to expectations, improved the barrier properties of PLA up to 18% at highest value, while the transmittance of PHB increased by more than 10%. A similar change in the mobility of the contact angle hysteresis is observed for both materials.

Influence of hydrogen gas flow ratio on the properties of silicon- and nitrogen-doped diamond-like carbon films by plasma-enhanced chemical vapor deposition

We deposited silicon- and nitrogen-doped diamond-like carbon (Si–N–DLC) films by radio frequency (RF) plasma-enhanced chemical vapor deposition using H2 and Ar as dilution gases. We investigated the influence of the H2 flow ratio [H₂/(H₂ + Ar)] on the structure; chemical bonding; and mechanical, tribological, optical, and surface properties of the Si–N–DLC films. The optical bandgap increased from 1.88 eV to 2.06 eV with the H2 flow ratio, and it had a negative correlation with the sp2 CC bonding fraction in the films. When the H2 flow ratio increased from 0% to 100%, the critical load of the Si–N–DLC films increased by ~14% due to a reduction of ~26% in the internal stress of the films. As the H2 flow ratio increased, the friction coefficient and specific wear rate decreased by ~11% and ~35%, respectively, whereas the pure water contact angles of the film surfaces increased from 74.7° to 86.5°. The film surfaces had root mean square roughnesses (RMS) less than 0.2 nm. Therefore, the improvement in their tribological properties and the increase in their contact angles were probably due to the hydrogen termination of dangling bonds. The friction coefficient and wear rate of the Si–N–DLC film deposited at a H2 flow ratio of 0% (100%) changed little (increased) after annealing at 773 K.

Characterising a Custom-Built Radio Frequency PECVD Reactor to Vary the Mechanical Properties of TMDSO Films.

Plasma-polymerised tetramethyldisiloxane (TMDSO) films are frequently applied as coatings for their abrasion resistance and barrier properties. By manipulating the deposition parameters, the chemical structure and thus mechanical properties of the films can also be controlled. These mechanical properties make them attractive as energy adsorbing layers for a range of applications, including carbon fibre composites. In this study, a new radio frequency (RF) plasma-enhanced chemical vapour deposition (PECVD) plasma reactor was designed with the capability to coat fibres with an energy adsorbing film. A key characterisation step for the system was establishing how the properties of the TMDSO films could be modified and compared with those deposited using a well-characterized microwave (MW) PECVD reactor. Film thickness and chemistry were determined with ellipsometry and X-ray photoelectron spectroscopy, respectively. The mechanical properties were investigated by nanoindentation and atomic force microscopy with peak-force quantitative nanomechanical mapping. The RF PECVD films had a greater range of Young’s modulus and hardness values than the MW PECVD films, with values as high as 56.4 GPa and 7.5 GPa, respectively. These results demonstrated the varied properties of TMDSO films that could in turn be deposited onto carbon fibres using a custom-built RF PECVD reactor.

Optical Constant and Conformality Analysis of SiO2 Thin Films Deposited on Linear Array Microstructure Substrate by PECVD

SiO2 thin films are deposited by radio frequency (RF) plasma-enhanced chemical vapor deposition (PECVD) technique using SiH4 and N2O as precursor gases. The stoichiometry of SiO2 thin films is determined by the X-ray photoelectron spectroscopy (XPS), and the optical constant n and k are obtained by using variable angle spectroscopic ellipsometer (VASE) in the spectral range 380–1600 nm. The refractive index and extinction coefficient of the deposited SiO2 thin films at 500 nm are 1.464 and 0.0069, respectively. The deposition rate of SiO2 thin films is controlled by changing the reaction pressure. The effects of deposition rate, film thickness, and microstructure size on the conformality of SiO2 thin films are studied. The conformality of SiO2 thin films increases from 0.68 to 0.91, with the increase of deposition rate of the SiO2 thin film from 20.84 to 41.92 nm/min. The conformality of SiO2 thin films decreases with the increase of film thickness, and the higher the step height, the smaller the conformality of SiO2 thin films.

A critical study on different hydrogen plasma treatment methods of a-Si: H/c-Si interface for enhanced defect passivation

Defect engineering of amorphous-crystalline silicon interface is a critical aspect for high efficiency silicon heterojunction solar cells. In this work, we have done a comparative study of different hydrogen plasma treatment (HPT) methods namely Pre, Post and Intermediate (Inter) HPT using radio frequency plasma-enhanced chemical vapor deposition (rf-PECVD) to understand the passivation mechanism to obtain state of the art passivation. It was observed that Inter and Post HPT can give high minority carrier lifetime on both p and n type wafers and has lesser interface defect states. The analysis from Fourier Transform Infrared Spectroscopy and high resolution Transmission Electron Microscopy shows that these HPT methods results in higher bulk disorder yet atomically sharp and superior interface due to hydrogen diffusion which saturates the interface dangling bonds. While a Pre-HPT results in epitaxial growth leading to poorer performance inspite of lowest dihydrides in the bulk which further highlights the importance of the interface. We have demonstrated Inter and Post HPT of ultrathin (5 nm) a-Si:H films which could give surface recombination velocity as low as 13 cm/s. The degradation due to hydrogen effusion in these HPT a-Si:H films has also been studied.

Nucleation and growth dynamics of graphene grown by radio frequency plasma-enhanced chemical vapor deposition

We investigated the nucleation and grain growth of graphene grown on Cu through radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) at different temperatures. A reasonable shielding method for the placement of copper was employed to achieve graphene by RF-PECVD. The nucleation and growth of graphene grains during PECVD were strongly temperature dependent. A high growth temperature facilitated the growth of polycrystalline graphene grains with a large size (~ 2 μm), whereas low temperature induced the formation of nanocrystalline grains. At a moderate temperature (790 to 850 °C), both nanocrystalline and micron-scale polycrystalline graphene grew simultaneously on Cu within 60 s with 50 W RF plasma power. As the growth time increased, the large graphene grains preferentially nucleated and grew rapidly, followed by the nucleation and growth of nanograins. There was competition between the growth of the two grain sizes. In addition, a model of graphene nucleation and grain growth during PECVD at different temperatures was established.

Silica Layer Used in Sensor Fabrication from a Low-Temperature Silane-Free Procedure

Silica (SiO2, silicon dioxide—a dielectric layer commonly used in electronic devices) is widely used in many types of sensors, such as gas, molecular, and biogenic polyamines. To form silica films, core shell or an encapsulated layer, silane has been used as a precursor in recent decades. However, there are many hazards caused by using silane, such as its being extremely flammable, the explosive air, and skin and eye pain. To avoid these hazards, it is necessary to spend many resources on industrial safety design. Thus, the silica synthesized without silane gas which can be determined as a silane-free procedure presents a clean and safe solution to manufactures. In this report, we used the radio frequency (rf = 13.56 MHz) plasma-enhanced chemical vapor deposition technique (PECVD) to form a silica layer at room temperature. The silica layer is formed in hydrogen-based plasma at room temperature and silane gas is not used in this process. The substrate temperature dominates the silica formation, but the distance between the substrate and electrode (DSTE) and the methane additive can enhance the formation of a silica layer on the Si wafer. This silane-free procedure, at room temperature, is not only safer and friendlier to the environment but is also useful in the fabrication of many types of sensors.

Hydrophobic Anti-Reflective Coating of Plasma-Enhanced Chemical Vapor Deposited Diamond-Like Carbon Thin Films with Various Thicknesses for Dye-Sensitized Solar Cells

Diamond-like carbon (DLC) thin films, prepared by a radio frequency plasma-enhanced chemical vapor deposition (PECVD) system, were investigated for application as an anti-reflective coating (ARC) for dye-sensitized solar cells (DSSCs) with a change in film thickness. The strength of the Raman spectrum, G-peak position, and ID/IG ratio, related to sp3 bonds in the DLC thin films, is directly attributed to some tribological properties including surface roughness, hardness, elastic modulus, friction coefficient, and contact angle. Some optical properties, such as transmittance, refractive index, and absorption coefficient, were examined after changing the thickness of DLC thin films. The optimal short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) values were obtained for the significantly improved conversion efficiency (CE) from 4.92% to 5.35% in the 60 nm thick PECVD DLC ARC for DSSCs with hard and hydrophobic surfaces.

Multifractal investigation of Ag/DLC nanocomposite thin films

The objective of this study is the experimental investigation of the silver in diamond-like carbon (Ag/DLC) nanocomposite prepared by the co-deposition of radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) and RF-sputtering. Atomic force microscopy (AFM), X-ray diffraction analyses, ultraviolet–visible (UV–visible) spectroscopy measurements were applied to describe the three-dimensional surface texture data in connection with the statistical, and multifractal analyses. Additional information about structure–property relationships in prepared Ag/DLC nanocomposite was studied in detail to allow a better understanding of the surface micromorphology. The performed analysis revealed the studied samples have multifractal properties and can be included in novel algorithms for graphical representation of complex geometrical shapes and implemented in computer simulation algorithms.

Characterization of nano-bio silicon carbide

Plasma-enhanced chemical vapor deposition, reactive magnetron sputtering, hot-wire chemical vapor deposition and radio frequency plasma-enhanced chemical vapor deposition were used to develop technology for preparation of nano-bio silicon carbide coating of ceramic materials for dental applications. The effect of the bias voltage applied to the ceramic prostheses and dental crowns on the crystallization processes have been recognized. The optimal bias voltage applied to conductive substrate was –200 V, whereas for dielectric substrate the bias voltage Vbias did not affect the properties of SiC coating. The analysis of CVCs and spectroscopic diagnostics as the methods for studying the mechanism of interfacial rearrangements to investigate SiC phase transition in nano silicon carbide coatings were used. The conductivity of the SiC coating coincided with the conductivity on the dielectric (µn0 = 1012…1013 сm–1·s–1·V–1). The conductive substrate had a significant effect on the properties of the coating and thus depended on the bias voltage Vbias. The conductivity increased by three-four orders of magnitude (µn0 = 3·1017 сm–1·s–1·V–1), if the bias voltage Vbias = –200 V. The increase of the bias voltage (Vbias = –600 V) led to a decrease in the conductivity (µn0 = 1011…1012 сm–1·s–1·V–1). It was found that there was the double injection regime with bimolecular recombination in this structure with the dependence I = V3/2 for CVCs of SiC. The luminescence spectrum of SiC coating on non-dielectric ceramics (if Vbias = – 200 V during deposition) was significantly different from the luminescence spectrum of SiC coating on dielectric ceramics. Increasing the applied voltage to the substrate Vbias during deposition led to increasing the fraction of hexagonal polytypes. Directions in the crystal lattice according to the photoluminescence spectra were identified from the comparing the values of the width of the non-phonon parts of stacking faults and deep level spectra in the low-temperature photoluminescence with arrangements of atoms in the SiC lattice structure. The displacement of each atom participating in photoluminescence allowed to find the correlation with technology of SiC deposition and to develop technology of SiC coating on the dental materials.

Surface modification of SiOx film anodes by laser annealing and improvement of cyclability for lithium-ion batteries

SiOx (x = 0.66, 0.76 and 1.04) film anodes with the different oxygen content were prepared by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) through adjusting SiH4/N2O (R) gas flow ration. The introduction of oxygen can alleviate volume expansion in the discharge/charge process at the expense of cyclic specific capacity. To obtain both higher capacity and cyclically stable SiOx film anodes, KrF excimer laser system operated at λ = 248 nm can induce to form nanoparticles on the surface of SiO0.76 film, which is related to the formation of solid electrolyte interphase (SEI) area during charge/discharge process. The SEI area becomes larger and mechanically robust with increase of laser energy density, which suppresses structural degeneration for giving lithium-ion battery longer cycle life. At the laser energy density of 266 mJ/cm2, SiO0.76 annealed film anode delivers capacity of 1731 mAh/g and capacity retention of 94.8% after 300 cycles, which are much higher than that of as-deposited anode. Increasing laser energy density to 433 mJ/cm2, SiO0.76 film anode shows higher capacity retention of 97.4% after 300 cycles.

Fabrication of Nano-Onion-Structured Graphene Films from Citrus sinensis Extract and Their Wetting and Sensing Characteristics.

Graphene and its derivatives have acquired substantial research attention in recent years because of their wide range of potential applications. Implementing sustainable technologies for fabricating these functional nanomaterials is becoming increasingly apparent, and therefore, a wide spectrum of naturally derived precursors has been identified and reformed through various established techniques for the purpose. Nevertheless, most of these methods could only be considered partially sustainable because of their complexity as well as high energy, time, and resource requirements. Here, we report the fabrication of carbon nano-onion-interspersed vertically oriented multilayer graphene nanosheets through a single-step, environmentally benign radio frequency plasma-enhanced chemical vapor deposition process from a low-cost carbon feedstock, the oil from the peel of Citrus sinensis orange fruits. C. sinensis essential oil is a volatile aroma liquid principally composed of nonsynthetic hydrocarbon limonene. Transmission electron microscopy studies on the structure unveiled the presence of hollow quasi-spherical carbon nano-onion-like structures incorporated within graphene layers. The as-fabricated nano-onion-incorporated graphene films exhibited a highly hydrophobic nature showing a water contact angle of up to 1290. The surface energies of these films were in the range of 41 to 35 mJ·m-2. Moreover, a chemiresistive sensor directly fabricated using C. sinensis-derived onion-structured graphene showed a p-type semiconductor nature and a promising response to acetone at room temperature. With its unique morphology, surface properties, and electrical characteristics, this material is expected to be useful for a wide range of applications.

3-D Morphological Analysis of Carbon-Nickel Nanocomposite Thin Films

The study's aim was to identify the 3-D surface spatial parameters that describe the 3-D surface microtexture of the nickel–carbon (Ni–C) nanocomposite thin films composed of Ni nanoparticles with different average sizes embedded in amorphous hydrogenated carbon, prepared by the combining radio frequency magnetron sputtering technique and plasma-enhanced chemical vapor deposition (RF-PECVD). The deposition time was varied at 7, 10 and 13 min, respectively. The sample investigation was performed using an atomic force microscope, and the obtained data were analyzed and visualized using MountainsMap® Premium software to determine their stereometric surface engineering characteristics. The results from this study provide not only fundamental insights into the texture characteristics, but also directions toward their implementation in nanotribological models.

Diamond-like carbon coating for effective electrical insulation of Cu and Al wires

The physical and mechanical properties, electrical insulation, chemical resistance, and heat resistance of diamond-like carbon (DLC) films fabricated using the radio frequency plasma-enhanced chemical vapor deposition method (RF-PECVD) on Cu (DLC/Cu) and Al (DLC/Al) substrates were evaluated in this study. The electrical insulation testing results showed that the dielectric breakdown voltage of both DLC/Cu and DLC/Al samples was approximately 145 kV/mm. For assessing the heat resistance, the samples were stored at a high temperature of 200 °C for a fixed time duration, and the changes in the insulation voltage resistance were evaluated. Based on the voltage resistance measurement results of the electrical insulation after heat resistance testing, the study confirmed that the DLC film is heat stable for 7 days of heating. In addition, testing confirmed that the DLC coating remained stable with no deterioration when immersed in both acidic and alkaline solutions, thus indicating high chemical resistance. These findings suggest that DLC is suitable for application as a thin and effective electrical insulation coating, and its properties support the realization of small coil lengths with square wires.

Comparison of the properties of a-C:H films deposited from methane and heptane precursors: study of the mechanical, chemical and structural properties

In this work, thin films of hydrogenated amorphous carbon (a-C:H) were deposited on silicon substrates from heptane and methane precursors by the radio frequency plasma-enhanced chemical vapor deposition (PECVD) technique, varying the self-bias voltage from −100 V to −900 V. As a liquid, heptane is safer, easier to handle, to store and stock, and presented advantages over methane as a precursor in the deposition of a-C:H films by PECVD as it allows deposition rates more than 4 times higher than of methane. Diamond-like carbon films deposited from heptane were also harder (22.5 GPa) and presented smaller intrinsic stress (3.5 GPa) than those obtained from methane (18.8 GPa and 4.1 GPa, respectively). In addition, these films have a smaller hydrogen content and a reduced sp2 character. The observed behavior is consistent with the subplantation model, caused by the ion bombardment energy during deposition. The differences observed in the deposition from both precursors are related to the different number of carbon atoms of the parent molecules and lead to a change of structure and hardness that could be closely related to the structural changes observed by Raman spectroscopy. Films deposited from heptane may be promising for applications due to a higher hardness to elastic modulus ratio H/E of approximately 0.13 when compared to films deposited from methane.