Modification of carbon surfaces in cold plasmas

Various forms of carbon, including carbon black, carbon fibre and pyrolytic graphite, have been surface-modified in a cold-plasma reactor. Plasma fluorination of the carbon surfaces was performed using variable flow rates, treatment times, and types of gas. The plasma-modified carbon surfaces were characterized using electron spectroscopy (ESCA), contact angle measurements, and scanning electron microscopy (SEM). The surface chemical structure of the plasma-fluorinated carbon blacks appeared to be similar to that of commercial Fluorographite. Nitrogen-containing groups were introduced into the surfaces of carbon blacks and carbon fibres by exposure to glow discharges in mixtures of ammonia and N₂-H₂. The surface concentration of functional groups containing nitrogen decreased with time. Several mechanisms for this degradation have been proposed. Plasma-modified carbon blacks and carbon fibres have potential for improving the properties of composites by achieving appropriate levels of adhesion between filler and matrix through physical compatibility and/or chemical bonding.     

Adsorptive properties of albumin, fibrinogen, and γ-globulin on fluorinated diamond-like carbon films coated on PTFE

Fluorinated diamond-like carbon (F-DLC) films were deposited on polytetrafluoroethylene (PTFE) using radio frequency (RF) plasma-enhanced chemical vapor deposition (CVD) by changing the ratio of tetrafluoromethane (CF₄) and methane (CH₄). To enhance the adhesion strength of the F-DLC film to the PTFE substrate, the PTFE surface was modified with a N₂ plasma pre-treatment. XPS analysis of the films showed that the C–C bond decreased with increases in the CF₄ ratio, whereas the C–F bond increased with the CF₄ ratio. The F/C ratio of the film also increased with the CF₄ ratio. The pull-out test showed that the adhesion strengths of the films (CF₄-0–60%) were improved with the plasma pre-treatment. In the film without the plasma pre-treatment, adhesion strength increased with the CF₄ ratio. In contrast, in the case with the plasma pre-treatment, the adhesion strength of the F-DLC film decreased with the increased CF₄ ratio. Regarding the adsorption of albumin, fibrinogen, and γ-globulin, the amount of adsorbed albumin on the film decreased with an increasing CF₄ ratio, and the amount of adsorbed fibrinogen and γ-globulin increased with the CF₄ ratio. The CF₄-0% DLC film showed the most adsorbed albumin and the least adsorbed fibrinogen and γ-globulin. This indicates that the CF₄-0% DLC film has higher anti-thrombogenicity than the F-DLC film.     

Influence of the Chemical Composition and Topography of the Ti6Al7Nb Surface Etched in Fluorine Plasma on Its Selected Properties

Herein, a new way of the surface modification of Ti6Al7Nb alloy in fluorine plasma is presented using the mask made of stainless steel and its influence on the selected properties, including chemical composition, topography, and tribological ones. Depending on the etching process parameters, different characteristics of the surfaces are obtained. The higher is the value of the negative bias, the less fluorine concentration on the etched surface and higher etching rates. Etching using the SF₆ gas shows bigger etching rates in comparison to the etching using the CF₄ gas. Chemical composition of the modified surfaces shows greater impact on tribological characteristics than topography parameters. The lowest wear rate is observed for the sample modified using the CF₄ gas.     

The characteristics of fluorinated polycrystalline silicon oxides and thin film transistors by CF4 plasma treatment

This study describes a simple fluorinating technique by the tetrafluoromethane (CF₄) plasma treatment to form fluorinated polyoxides and polycrystalline silicon thin film transistors (TFTs). In comparison with the non-fluorinated device, the fluorinated polyoxides and devices exhibit a higher breakdown field (>8 MV/cm), low charge trapping rates, low off-state current, and low trap states. Furthermore, the performance and reliability of the fluorinated devices are also improved by the CF₄ plasma treatment. This is due to the fact that the incorporated fluorine can break strain bonds to form stronger silicon-fluorine (Si-F) bonds to passivate the generation of interface and trap states existing near the polyoxide/polysilicon interface and grain boundaries.     

A study of the surface reaction on the etched ITO thin films by using inductively coupled plasma

In this study, we investigated the etching characteristics of indium tin oxide (ITO) thin films at CF₄/Ar plasma. The maximum etch rate of 29.8 nm/min for the ITO thin films was obtained at CF₄/Ar (=80/20) gas mixing ratio. The standard conditions were the RF power of 800 W, the DC-bias voltage of −150 V, the process pressure of 2 Pa, and the substrate temperature of 40 °C. Corresponding to these etching conditions, chemical reaction of the etched ITO surface has been studied by X-ray photoelectron spectroscopy measurement to investigate the chemical reactions between the surfaces of ITO thin film and etch species. The preferential losses on the etched surfaces were investigated using atomic force microscopy.     

Plasma cleaning and the removal of carbon from metal surfaces

In an investigation of the plasma cleaning of metals and the plasma etching of carbon, a mass spectrometer was used as a sensitive process monitor. CO₂ produced by the plasma oxidation of carbon films or of organic contamination and occluded carbon at the surfaces of metals proved to be the most suitable gas to monitor. A good correlation was obtained between the measured etch rate of carbon and the resulting CO₂ partial pressure monitored continuously with the mass spectrometer.The rate of etching of carbon in an oxygen-argon plasma at 0.1 Torr was high when the carbon was at cathode potential and low when it was electrically isolated in the plasma, thus confirming the findings of previous workers and indicating the importance of ion bombardment in the etching process. Superficial organic contamination on the surfaces of the metals aluminium and copper and of the alloy Inconel 625 was quickly removed by the oxygen-argon plasma when the metal was electrically isolated and also when it was at cathode potential. Occluded carbon (or carbides) at or near the surfaces of the metals was removed slowly and only when the metal was at cathode potential, thus illustrating again the importance of ion bombardment.     

Efficient surface functionalization of vertically-aligned carbon nanotube arrays using an atmospheric pressure plasma jet system

We demonstrate the facile and efficient surface functionalization of vertically-aligned carbon nanotube (VCNT) arrays using an atmospheric pressure plasma jet (APPJ) system. The VCNT arrays were synthesized on Fe-deposited SiO₂ wafers using an acetylene carbon source by thermal chemical vapor deposition method. To functionalize the surface of the VCNT arrays, the APPJ system was ignited using nitrogen gas at high voltage of 15 kV and frequency of 25 kHz. We varied the treatment time of the APPJ and the inter-distance between plasma jet and top surface of the VCNT in order to systematically investigate the optimal conditions of the APPJ system. The hydrophobic nature of the as-grown VCNT arrays was drastically changed to hydrophilic character via the facile APPJ treatment. X-ray photoelectron spectroscopy confirmed the formation of hydrophilic functional groups such as hydroxyl and carboxyl groups, and nitrogen-doping-related functionalities such as amines, in addition to pyrrolic- and pyridinic-bonding. The results prove that the APPJ treatment is a facile and efficient method for the surface modification of nanomaterials.     

Annealing effects on structural and electrical properties of fluorinated amorphous carbon films deposited by plasma enhanced chemical vapor deposition

The annealing effects on the structural and electrical properties of fluorinated amorphous carbon (a-C:F) thin films prepared from C₆F₆ and Ar plasma are investigated in a N₂ environment at 200 mTorr. The a-C:F films deposited at room temperature are thermally stable up to 250 °C, but as the annealing temperature is increased beyond 300 °C, the fluorine incorporation in the film is reduced, and the degree of crosslinking and graphitization in the film appears to be enhanced. At the annealing temperature of 250 °C, the chemical bond structures of the film are unchanged noticeably, but the interface trapped charges between the film and the silicon substrate are reduced significantly. The increased annealing temperature contributes the decrease of both the interface charges and the effective charge density in the a-C:F film. Higher self-bias voltage is shown to reduce the charge density in the film.     

Mechanical properties evaluation of fluor-doped diamond-like carbon coatings by nanoindentation

Fluor-doped diamond-like carbon (DLC) films were grown by RF sputtering, varying the CF₄ partial pressure and the total gas pressure. Nanoindentation is required to evaluate the mechanical properties of such very thin films. The Vickers nanohardness and the Young's modulus values fall in the 55-70 GPa and 235-326 GPa ranges, respectively, the lowest values being observed for the highest degree of fluorine content in the film (80% CF₄ in the gas mixture) and for the highest processing pressure (1.5 kPa).     

Effect of gas mixing ratio on gas-phase composition and etch rate in an inductively coupled CF4/Ar plasma

In this work, we carried out investigations aimed at understanding the effect of gas mixing ratio on plasma parameters, gas phase composition and etch rate in CF₄/Ar inductively coupled plasma. For this purpose, a combination of experimental methods and modelling was used. Experiments showed that electron temperature and electron density are not very sensitive to variations of Ar content in CF₄/Ar plasma. From a zero-dimensional plasma model, the densities of both neutral and charged particles change monotonically. The analysis of surface kinetics based on an ion-assisted etching mechanism showed the possibility of non-monotonic etch rate behaviour due to a concurrence of chemical and physical etching pathways.     

Surface and bioproperties of nanocrystalline diamond/amorphous carbon nanocomposite films

Nanocrystalline diamond/amorphous carbon (NCD/a-C) nanocomposite films have been deposited by microwave plasma chemical vapour deposition from CH₄/N₂ mixtures. In order to investigate their suitability as templates for the immobilization of biomolecules, e.g. for applications in biosensors, four differently prepared surfaces, namely as-grown, hydrogen plasma treated, oxygen plasma treated, and chemically treated with aqua regia, have been thoroughly characterized by methods such as XPS, TOF-SIMS, AFM, and contact angle measurements. In addition, in order to investigate the affinity of these surface to non-specific bonding of biomolecules, they have been exposed to bovine serum albumin (BSA). It turned out that already the as-grown surface is hydrogen terminated; the degree of the termination is even slightly improved by the hydrogen plasma treatment. Reaction with aqua regia, on the other hand, led to a partial destruction of the H-termination. The oxygen plasma treatment, finally, causes a termination by O and OH, rather than by carboxylic acid groups. In addition, an increase of sp² bonded carbon is observed. All surfaces were found to be susceptible to attachment of BSA proteins, but the coverage of the hydrogen terminated was lower than that of the O-terminated film. The highest BSA concentrations were found for the aqua regia sample where the H-termination has been removed partially. Finally, our results show that even minor surface contaminations have a great influence on the BSA coverage.     

Plasma treatment effects on surface morphology and field emission characteristics of carbon nanotubes

In this study, electron field-emission properties of carbon nanotube films (CNTs) grown on silicon substrate before and after tetrafluoromethane (CF₄), hydrogen (H₂) and argon (Ar) plasma etchings were investigated. The CNTs were synthesized by thermal decomposition of methane in the presence of nickel catalyst. Our research results reveal that plasma treatment can modify the surface morphology and enhance the field-emission characteristics of CNTs regardless of the plasma used. The CNTs treated by both non-reactive and reactive plasmas have a higher density of defect and a smaller average diameter reflecting the etching effects of plasma treatments. In addition, higher emission currents and lower turn-on electric fields are also obtained for the CNTs after plasma treatment. As expected, reactive plasma treatment has a more pronounced effect on the surface morphology and field-emission characteristics of the synthesized CNTs than non-reactive plasma treatment. In particular, a huge increase in emission current (more than three orders of magnitude at high electric fields) and a substantial lower turn-on electric field are found for the CNTs after H₂ plasma treatment. This huge increase in the emitted current is primarily caused by the increase in the density of field-emission sites resulting from the change of surface morphology and the –CH _x nanoparticles redeposited on the CNTs.     

Structure of polycrystalline silicon films deposited at low temperature by plasma CVD on substrates exposed to different plasma

Polycrystalline silicon (poly-Si) films were deposited on glass substrates (corning 7059) at 300°C by a plasma enhanced chemical vapor deposition (PECVD) from a SiH₄/SiF₄ mixture. All poly-Si films were prepared under the same deposition conditions on the substrates subjected to nitrogen, hydrogen and/or CF₄ plasma with different gas pressures, just before deposition of the poly-Si films. Effects of such pretreatments for substrates on the structural properties of the resultant poly-Si films have been investigated. The Si film deposited on the substrates without any pretreatments was amorphous. However, formation of a strong 〈110〉 preferentially oriented poly-Si with improved crystallinity was obtained for the films deposited on the glass substrate after plasma pretreatments, which exhibit smoother surfaces. This result was interpreted in terms of a removal of weak Si–Si bonds during nucleation and the subsequent grain growth.     

Nanocrystallized steel surface by micro-shot peening for catalyst-free carbon nanotube growth

Nanocrystallized steel surface by micro-shot peening (MSP) were applied to carbon nanotube growth in this study. Micro-shot peening treatment severely deformed steel surface and nanocrystallized surface layer was formed by the plastic deformation. The grain sizes of the nanocrystallized layer were 10-30 nm after 300 s of MSP treatment. On the nanocrystallized surface, carbon nanotubes were formed with thermal chemical vapour deposition without catalysts. Before carbon nanotube growth, the nanocrystallized steel surface was reduced with H₂/N₂ gas at 600 °C. The carbon nanotube growth was performed at 600 °C with C₂H₂ gas carried by H₂/N₂ gas. The carbon nanotubes formed on the nano-structured surface was multi-walled carbon nanotube and the diameter was 10-20 nm. The reduction process before carbon nanotube growth was essential to form carbon nanotubes on the nanocrystallized surface with MSP.     

Characterizing the surface properties of carbon nanotubes by inverse gas chromatography

Inverse gas chromatography (IGC) was used to characterize the surface properties of pristine multi-walled carbon nanotubes (MWNTs), as well as the poly(acrylic acid) sidewall covalently functionalized MWNTs (PAA-g-MWNTs) and hydroxyl group directly grafted MWNTs (MWNTols). The dispersive component of the surface energy ( ) and the acid/base character of these samples’ surfaces were estimated by the retention time with different non-polar and polar probes at infinite dilution region. The specific free energy (ΔG ^AB) and the enthalpy (ΔH ^AB) of adsorption corresponding to acid–base surface interactions were determined. By correlating ΔH ^AB with the donor and acceptor numbers of the probes, the acidic (K _A) and the basic K _D parameters of the samples’ were calculated. The results show that chemical modification successfully reduces the dispersive component of the surface energy of MWNTs. Furthermore, MWNTs grafted with hydroxyl groups exhibit a more basic character, while MWNTs grafted with poly(acrylic acid) show a more acidic character. Overall, IGC provides useful complementary information on the changes resulted from the chemical modifications of the surface.     

Distribution of the dimensions of micropores in thin layers using the SAXS method

Investigations of the distribution of the dimensions of micropores (D _n /R) were carried out for three thin films, obtained by the pulse plasma method. The films consisted of carbon, borazone and titanium nitride respectively. The investigations were carried out using the SAXS method, i.e. averaging results for different depths and surfaces of the films. The obtained (D _n /R) functions show a fuzzy maximum in the region of 3–190nm. These films, different in the character of chemical bond, the condensation mechanism and the structure show the same micropore size distribution. Therefore such a micropore size distribution seems to be a general property of thin layers condensed from a gaseous phase.     

Water and toluene barrier properties of a polyamide 12 modified by a surface treatment using cold plasma

CF₄ and CO₂ plasma treatments have been used to modify the barrier properties of a polyamide 12 (PA12) towards permeant molecules, which present opposing characteristics: water and toluene. The surface modifications were observed by atomic force microscopy, X-ray photoelectron spectroscopy and surface Gibbs function measurements. Both treatments lead to different surfaces; one is rather hydrophobic (with CF₄) whereas the other is more hydrophilic (with CO₂. The effect of this modification on permeametric properties has been investigated by liquid water and liquid toluene permeation measurements. Our results show opposite effects of the two treatments. CF₄ plasma treatment leads to a reduction of water and toluene permeability. With CO₂ plasma treatment, in terms of permeation, two different behaviours were observed, an increase and a decrease of permeancy for water and toluene respectively. These results are in full agreement with those obtained for the surface characterization, and confirms change in the polymeric surface affinity for the permeant leading to a variation of the materials permeancy.     

Role of carbon atoms in plasma-enhanced chemical vapor deposition for carbon nanotubes synthesis

The role of carbon atoms in a dc plasma-enhanced chemical vapor deposition for carbon nanotubes (CNTs) synthesis was investigated. It was observed that at 1.33 kPa pressure of CH₄ gas in plasma, a high value of the ratio between the intensities of the graphite peak (G peak) and the disorder peak (D peak) in the Raman spectrum corresponds to the maximum value of the excited C number density in the vicinity of the Si substrate. It was found that a CH₄ gas pressure higher than 1.33 kPa leads to an increase of the relative density of the C₂, C₃ molecules and the clusters, and to a decrease of the C excited atom number density in plasma. The presence of a high amount of sp²-graphite in the composition of CNTs observed in Raman spectrum was also confirmed by the measurement of the IR-active G peak at 1584 cm^- 1 in the transmission spectrum.     

Proliferation of osteoblasts and fibroblasts on model surfaces of varying roughness and surface chemistry

Physical and chemical properties of the surfaces of implants are of considerable interest for dental and orthopedic applications. We used self-assembled monolayers (SAMs) terminated by various functional chemical groups to study the effect of surface chemistry on cell behavior. Cell morphology and proliferation on silicon wafers of various roughnesses and topographies created by chemical etching in caustic solution and by corundum sandblasting were analyzed as well. Water contact angle data indicated that oxidized wafer surfaces displayed high hydrophilicity, modification with poly(ethylene glycol) (PEG) created a hydrophilic surface, and an amino group (NH₂) led to a moderately wettable surface. A hydrophobic surface was formed by hydrocarbon chains terminated by CH₃, but this hydrophobicity was even further increased by a fluorocarbon (CF₃) group. Cell proliferation on these surfaces was different depending primarily on the chemistry of the terminating groups rather than on wettability. Cell proliferation on CH₃ was as high as on NH₂ and hydrophilic oxidized surfaces, but significantly lower on CF₃. Precoating of silicon wafers with cell culture serum had no significant influence on cell proliferation. Scanning electron microscopy indicated a very weak initial cell-surface contact on CF₃. The cell number of osteoblasts was significantly lower on sandblasted surfaces compared with other rough surfaces but no differences were detected with 3T3 mouse fibroblasts. The different surface roughnesses and topographies were recognized by MG-63 osteoblasts. The cells spread well on smooth surfaces but appeared smaller on a rough and unique pyramid-shaped surface and on a rough sandblasted surface.     

Admittance spectroscopic analysis of organic light emitting diodes with the CFX plasma treatment on the surface of indium tin oxide anodes

Admittance spectroscopic analysis was used to examine the effect of a CF_X plasma surface treatment on indium tin oxide (ITO) anodes using CF₄ gas and model the equivalent circuit for organic light emitting diodes (OLEDs) with the of ITO anode surface treated with CF_X plasma. This device with the ITO/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine/tris-(8-hydroxyquinoline) aluminum/lithium fluoride/Al structure was modeled as a simple combination of two resistors and a capacitor. The ITO anode surface treated with the CF_X plasma showed a shift in the vacuum level of the ITO, which resulted in a decrease in the barrier height for hole injection at the ITO/organic interface. Admittance spectroscopy measurements of the devices with the CF_X plasma treatment on the surface of the ITO anodes showed a change in the contact resistance, bulk resistance and bulk capacitance.