Enhancement of fluorine doped amorphous carbon thin films from microwave surface wave plasma activated above room temperature

Fluorinated amorphous carbon (a-C:F) thin films were synthesized above room temperature by microwave surface wave plasma chemical vapour deposition (MW SWP CVD). The effect of deposition temperature on optical, electrical, chemical and bonding properties of the a-C:F films were studied by ultraviolet–visible spectroscopy (UV–VIS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectrometry (XPS), Raman spectrometry and TEM measurements. The film exhibits high transparency and decrease in optical band gap with increasing deposition temperature. FTIR study shows the increase in CC and decrease in C–Fx bonds of the films with increasing deposition temperature. Raman study shows some important structural changes in the films due to fluorine incorporation. XPS result shows the shift of carbon peak to higher binding energy due to carbon fluorine link to the films. TEM shows the increasing graphitic layer in the films with increasing deposition temperature.     

A high resolution XPS study of sidewall functionalized MWCNTs by fluorination

Carbon nanotubes must be modified to magnify their application. Understanding the chemistry of carbon nanotubes is a crucial step towards their ultimate practical use. In this study, multi-walled carbon nanotubes (MWCNTs) were fluorinated at several different conditions. The change in the atomic structures of fluorinated MWCNTs was investigated using X-ray photoelectron spectroscopy (XPS). All core level spectra of MWCNTs were deconvoluted to several Pseudo-Voigt functions (sum of Gaussian–Lorentzian functions). The amount of doped fluorine increased with increasing doping fluorine partial pressure, and the fluorine atoms were covalently attached to the side-wall of the MWCNTs. However, the increasing rate of F1s component ratio became dull at 70%, as compared with ratios below 70%. This suggests that fluorine contents during the fluorination of carbon materials can be managed by controlling the fluorine mixing ratio.     

Surface modification of amorphous carbon thin films by 1,3-dipolar cycloaddition

Amorphous carbon thin film surfaces were successfully modified by 1,3-dipolar cycloaddition of nitrones, generated by the condensation of 4-(trifluoromethyl)benzaldehyde and N-methylhydroxylamine. Amorphous carbon thin films were deposited by electron cyclotron resonance sputtering and consisted of mainly sp²-hybridized carbon. The modification of amorphous carbon thin film surfaces with organic molecules was confirmed by X-ray photoelectron spectroscopy (XPS), Raman, and atomic force microscopy (AFM). F 1s, N 1s, and C 1s electron spectra revealed the existence of organic molecules on the surface of modified amorphous carbon thin films. The surface coverage increased with reaction temperature, reactant concentration, and reaction time.     

Surface properties of fluorinated polyimides exposed to VUV and atomic oxygen

The effect of atomic oxygen flux and VUV radiation alone and in combination on the surface of fluorinated polyimide films was studied using XPS spectroscopy. Exposure of fluorinated polyimides to VUV radiation alone caused no observable damage to the polymer surface, while an atomic oxygen flux resulted in substantial oxidation of the surface. On the other hand, exposure to VUV radiation and atomic oxygen in combination caused extensive oxidation of the polymer surface after only 2 min of exposure. The amount of oxidized carbon on the polymer surface indicated that there is aromatic ring-opening oxidation. The changes in the O1s/C1s, N1s/C1s, and F1s/C1s ratios suggested that an ablative degradation process is highly favorable. A synergistic effect of VUV radiation in the presence of atomic oxygen is clearly evidenced from the XPS study. The atomic oxygen could be considered as the main factor in the degradation process of fluorinated polyimide films exposed to a low earth orbit environment. © 1995 John Wiley & Sons, Inc.     

A new approach for selective surface modification of fluoropolymers by remote plasmas

Ethylene‐co‐tetrafluoroethylene (ETFE) and poly (vinylidene fluoride) (PVDF) films were exposed to the remote Ar, H₂, and O₂ plasmas. The modified polymer surfaces were characterized by X‐ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and contact angle measurement. The plasma exposure led to weight loss and changes in the chemical composition on the polymer surface. Selective surface modification of fluoropolymers introduces various functional groups without altering the bulk properties. The results may be summarized as follows: the remote hydrogen plasma was the most effective in alternation from C? F to C? H (abstraction of fluorine). On the other hand, the remote oxygen plasma was unfavorable to abstract fluorine atoms, but effective in dehydrogenation (abstraction of hydrogen). © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1012–1020, 2004     

Comparative study of fluorinated single- and few-wall carbon nanotubes by X-ray photoelectron and X-ray absorption spectroscopy

Pristine and ball-milled samples containing single-wall carbon nanotubes (SWCNTs) and few-wall carbon nanotubes (FWCNTs) have been fluorinated at room temperature using gaseous BrF₃ as a fluorinating agent. X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy were used to estimate the chemical composition and to probe the electronic structure of the fluorinated CNTs. Analysis of the XPS C 1s spectra revealed that fluorinated carbon atoms in SWCNTs are bounded with one CF-group at least while most of the fluorinated carbon atoms in FWCNTs are surrounded by bare carbon atoms only. The ball-milling of the samples during 1 hour has insignificant effect on CNT length and more likely produces defects in CNT surface layers. These defects increase fluorination ability of CNTs and provide access for fluorine atoms to the subsurface layers of FWCNTs. NEXAFS investigation revealed that some of CNTs, which probably constitute interior of FWCNTs or CNT ropes, are not fluorinated during the conditions used and the fluorine atoms interact more strongly with CNT surfaces having a larger curvature.     

The surface treatment and adhesive bonding of polyetheretherketone. Part II. Surface characterization

A range of surface treatments including chromate and plasma etching, which are capable of producing both chemical modification and surface topography modification of polymer surfaces, were applied to polyetheretherketone. The resulting surfaces were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and contact angle studies. An enhanced oxygen concentration measured by XPS corresponded to a high polar surface energy contribution and formed the main requirement for a high adhesive joint strength.     

Preparation and electrochemical characterization of platinum and ruthenium catalysts deposited on fluorinated carbon supports

The effect of fluorination on carbon blacks (CBs) was investigated by analyzing the CB surface functional groups. Also, fluorinated CB supported platinum–ruthenium electrocatalysts were studied. The surface characteristics of the fluorinated CB-supported catalysts were determined by Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS), and their crystallinity was evaluated using X-ray diffraction (XRD). The electrochemical properties of the fluorinated CB-supported platinum and ruthenium (Pt/Ru CBs) catalysts were analyzed by cyclic voltammetry (CV). From the FT-IR results, the introduction of a fluorine atom onto CBs was confirmed by the existence of a carbon–fluorine (C–F) characteristic peak. The XPS result also showed that the fluorine content increased with increasing heat treatment temperature. The XRD analysis revealed that the catalysts are composed of mostly Pt and Ru. This result agreed with the fact that the electrochemical activity of the Pt/Ru CB catalysts was enhanced due to the small size and a high loading of catalysts by surface modification of the CBs.     

Quantitative characterization of catalyst layer degradation in PEM fuel cells by X-ray photoelectron spectroscopy

A quantitative analysis of catalyst layer degradation was performed with X-ray photoelectron spectroscopy (XPS). XPS is quantitative, surface-sensitive, and is able to distinguish different bonding environments or chemical states of fuel cell catalyst layers and polymer electrolyte membrane. These capabilities have allowed us to explore the complex mechanisms of degradation during fuel cell operation. The elemental surface concentrations of carbon, fluorine, oxygen, sulfur, and platinum on the catalyst layer surface were measured before and after fuel cell operation, and the different chemical states of carbon and platinum were identified. Both XPS analysis and scanning electron microscopy revealed that the ionomer on the catalyst layer degraded or decreased in concentration after fuel cell operation. Ionomer degradation was characterized by a decrease of CF₃ and CF₂ species and an increase in oxidized forms of carbon (e.g. CO and CO), and an increase in less- and non-fluorinated forms of carbon (e.g. CF and graphitic), consistent with overall reduction of fluorine by about 22%. The surface concentration of fluorine and platinum also reduced from 50.1% to 38.9% and from 0.4% to 0.3%, respectively. The concentration of oxidized forms of carbon and platinum increased after fuel cell operation. The surface-sensitive XPS technique should prove useful for the quantitative monitoring of catalyst layer degradation mechanisms over the lifetime of fuel cells.     

Conducting polymer coated carbon surfaces and biosensor applications

This review article focuses on several approaches in the characterization and modification of carbon surfaces with electrocoated thin films which has been realized by recent progress in experimental methods. Electropolymerization and electrocopolymerization of π-conjugated polymers (pyrrole, carbazole, N-vinylcarbazole and aniline) onto carbon surfaces are reviewed with 348 references. Particular emphasis is placed on the recent nanoscale surface characterization techniques applied to the resulting electrocoated polymers onto carbon fibers (i.e., scanning electron microscopy (SEM), cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), focused ion beam-secondary ion mass spectroscopy (FIB-SIMS), Fourier transformed infrared spectroscopy (reflectance-FTIR), and Raman spectroscopic measurements).     

Molecular structure and hydrophobicity of polymeric fluorocarbon films deposited on PET substrates

Sputter‐coated polymeric fluorocarbon films (PFCF) on poly(ethylene terephthalate) (PET) are prepared under conditions of various power and pressure levels. By analysis of X‐ray photoelectron spectroscopy (XPS) and infrared spectroscopy (IR), low fluorine‐to‐carbon (F/C) ratios, 1.02–0.61, indicate that the deposited films are highly crosslinked‐unsaturated and the F/C ratio is found to be remarkably influenced by the power and slightly affected by the pressure due to the change in various structure components in the film molecules. The nonwettable properties of the PFCF are consistent with the results obtained by XPS‐IR analysis. The cleavage of the sharp peak assigned to C?O stretch in the FTIR spectra of the PFCF‐PET film can be thought to be an evidence related to the newly formed F? C bond on a certain site of ester group next to benzene ring in PET macromolecules. Therefore, the chemical bond at the interface between FCPF and PET can be expected to enhance the joint strength of the interface. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1843–1850, 2002     

The properties of fluorine-containing diamond-like carbon films prepared by pulsed DC plasma-activated chemical vapour deposition

Diamond-like carbon films containing up to 23.1 at. % of fluorine (F-DLC), were deposited onto silicon substrates by low-frequency, pulsed DC, plasma-activated, chemical vapour deposition (PACVD). The influence of fluorine on plasma current density, deposition rate, composition, bonding structure, surface energy, hardness, stress and biocompatibility was investigated and correlated with the fluorine content. X-ray photoelectron spectroscopy (XPS) analysis revealed the presence C–C, C–CF and C–F for F-DLC films with a low fluorine concentration (1.5–12.1 at. %), however for films with a higher fluorine content (23.0 at. %) an additional peak due to CF₂ bonding was detected. The addition of fluorine into the DLC film resulted in lower stress and hardness values. The reduction in these values was attributed to the substitution of strong C=C by weaker C–F bonds which induces a decrease in hardness. Ion scattering spectrometery (ISS) measurements revealed the presence of fluorine atoms in the outmost layer of the F-DLC films and there was no evidence of surface oxygen contamination. The water contact angle was found to increase with increasing fluorine content and has been attributed to the change of the bonding nature in the films, in particularly increasing CF and CF₂ bonds. Biocompatibility tests performed using MG-63 osteoblast-like cell cultures indicated homogeneous and optimal tissue integration for both the DLC and the F-DLC surfaces. This pulsed-PACVD technique has been shown to produce biocompatible DLC and F-DLC coatings with a potential for large area applications.     

Aging effects of repeatedly glow-discharged polyethylene: influence on contact angle,infrared absorption,elemental surface composition,and surface topography

Aging effects of repeatedly oxygen glow-discharged polyethylene surfaces were determined by water contact angle measurements, infrared (IR) spectroscopy, X-ray photoelectron (XPS) spectroscopy, and surface topography determination. Glow-discharged surfaces were stored at room temperature and in liquid nitrogen for 8 days prior to the next glow-discharge (Gld) treatment. This cycle was repeated up to 13 times. Hydrophobic recovery of the polyethylene surface, as determined by contact angle measurements, became less when the number of glow-discharge treatments increased. Hydrophobic recovery was suppressed when the samples were stored in liquid nitrogen. After five glow-discharge treatments it was possible to detect the incorporation of hydroxy groups by IR spectroscopy, while XPS spectroscopy showed that repeated glow-discharge gives rise to a higher oxygen to carbon ratio and a broadening of the C_Is peak, suggesting the incorporation of C-O and C=O bonds in the surface layer of polyethylene. The surface roughness of repeatedly glow-discharge-treated polyethylene remained almost unaltered in the submicrometer range (0.30-0.35 μm), although scanning electron microscopy revealed fine-grained structures for samples after a large number of Gld treatments. It can be concluded from this study that repeated oxygen glow-discharge treatments of polyethylene create more stable, hydrophilic surfaces than can be obtained with only one treatment. However, even after repeated glow-discharge treatments, polymer surface dynamics cause a small hydrophobic recovery.     

Preparation and fluorine enrichment behavior of fluorinated polyester

Poly[1,2-propylene glycol-3-(2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptoxy)]phthalate (PDFP) was prepared by the reaction of phthalic anhydride, epichlorohydrin, and 2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptyl glycidyl ether (DFGE). The structure of PDFP was characterized with ¹H-NMR, ¹⁹F-NMR, FTIR and the wettability of water and oil on cured PDFP coatings was investigated by contact angle meter. The results showed that the contact angle of water and diiodomethane on cured coatings increased with the increase of the fluorine content and could maximally attain 108° and 69°, respectively. At a fluorine content of 0.75%, the surface energy was 21.1 mJ/m². X-ray photoelectron spectroscopic (XPS) analysis showed that the surface F/C atomic ratio was much higher than the overall F/C molar ratio, indicating that the fluoroalkyl groups in PDFP had enriched on the coating surface. It was also found that the fluorinated polyester with longer fluoroalkyl groups had the higher tendency to enrich on the coating surface.     

Highly Hydrophobic Wood Surfaces Prepared by Treatment With Atmospheric Pressure Dielectric Barrier Discharges

Atmospheric dielectric barrier discharge (DBD) treatments of wood were done to attain water repellency on wood surfaces. A specially designed frequency controlled parallel-plate DBD reactor was utilized to produce the discharges. Ethylene, methane, chlorotrifluoroethylene and hexafluoropropylene were used as DBD reagents. Contact angle, water absorption, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements on the modified surfaces were performed. For methane and ethylene, XPS data showed an increased surface atomic concentration of carbon from 72.7% on untreated samples up to 80.7 and 96%, respectively, whereas nearly 50% fluorine concentration was observed with fluorinated reagents. The C_1s spectrum of hexafluoropropylene-DBD-treated wood sample showed that the CF₃ group was introduced in a relative amount of 19%. AFM images showed distinct features for each of the DBD treatments, such as a deposit of a thin uniform film in the case of ethylene-DBD treatment, whereas the hexafluoropropylene-DBD treatment resulted in the nucleation of plasma-derived entities at the fiber surface and the subsequent growth of a film. Under optimized conditions the water contact angle was in the range of 139°–145°. The combination of depositing a low surface energy polymer on an already rough surface gave the surface-treated wood a highly hydrophobic character.     

Multifunctional composites containing molybdenum carbides as potential electrocatalysts

The aim of the current study is to determine the feasibility of introducing fuel cell functionality on the surfaces of carbon-based composite materials. This can potentially be achieved by the synthesis of molybdenum carbides on the surfaces of carbon foam, which is a light and rigid material that can be used as structural components in aircrafts and vehicles. The current study employed physical vapor deposition (PVD) to deposit molybdenum on the carbon foam substrate. The ratio of surface molybdenum and surface carbon was determined using X-ray photoelectron spectroscopy (XPS). The combination of PVD and in situ XPS allowed for the synthesis of molybdenum-coated carbon foam samples with desirable and reproducible Mo/C ratios. The coated films were then heated in vacuum to promote the reaction between molybdenum and carbon foam to produce surface molybdenum carbides. The carbide-coated samples were further characterized using XPS, near-edge X-ray absorption fine structure (NEXAFS), and scanning electron microscopy (SEM). Platinum metal was also deposited via PVD on carbon foam, both with and without the presence of molybdenum carbide on the foam surface. The electrochemical stability of Pt-coated foams was evaluated using cyclic voltammetry (CV).     

Characterization and electrochemical properties of CF4 plasma-treated boron-doped diamond surfaces

The effect of CF₄ plasma etching on diamond surfaces, with respect to treatment time, was investigated using scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. SEM observations and Raman spectra indicated an increase in surface roughening on a scale of 10–20 nm, and an increase in crystal defect density was apparent with treatment time in the range of 10 s to 30 min. In contrast, alteration of the diamond surface terminations from oxygen to fluorine was found to be rather rapid, with saturation of the F/C atomic ratio estimated from XPS analysis after treatment durations of 1 min and more. The redox kinetics of Fe(CN)₆^3−/4− was also found to be significantly modified after 10 s of CF₄ plasma treatment. This behavior shows that C–F terminations predominantly affect the redox kinetics compared to the effect on the surface roughness and crystal defects. The double-layer capacitance (C_dl) of the electrolyte/CF₄ plasma-treated boron-doped diamond interface was found to show a minimum value at 1 min of treatment. These results indicate that a short-duration CF₄ plasma treatment is effective for the fabrication of fluorine-terminated diamond surfaces without undesirable surface damage.     

The effect of fluorination on the surface characteristics and adhesive properties of polyethylene and polypropylene

Polyethylene and polypropylene have been treated with fluorine/nitrogen or fluorine/oxygen/nitrogen mixtures at atmospheric pressure. Changes in surface chemistry and topography as well as depth of fluorination have been examined using Fourier transform infra-red analysis, X-ray photoelectron spectroscopy (X.p.s.), atomic force microscopy (AFM) and ellipsometry. Even very short exposure times caused a large substitution of the hydrogen atoms by fluorine. No change in surface topography was noticed at magnifications of up to 5000 times with the scanning electron microscope (SEM), but AFM showed that fluorination led to an increase of microroughness. The influence of fluorine or fluorine/oxygen concentration, as well as time of treatment and time of storage before adhesive bonding, on adhesion of polypropylene to steel was investigated with a bending peel test. Significant improvement in peel strength was already achieved with minor fluorination intensity. Increase of fluorination intensity did not lead to further improvement in peel strength. Analysis of the fracture surfaces was carried out with the SEM and by energy dispersive X-ray spectroscopy and X.p.s. The findings showed that the samples failed cohesively in the polymer or directly beneath the fluorinated layer. A model to describe the formation of specific interactions between substrate and adhesive is suggested.     

X-ray Photoelectron Spectroscopy Characterization of Ion-Beam Sputter-Deposited Calcium Phosphate Coatings

X-ray photoelectron spectroscopy (XPS) analysis of sputter-deposited calcium phosphate coatings on titanium substrates revealed four distinct zones. The ambient-exposed surface exhibited elevated concentrations of carbon because of atmospheric contamination. The bulk zone contained relatively constant concentrations of calcium, oxygen, phosphorus, and fluorine indicating the chemistry for calcium fluoride (CaF₂) and fluorapatite (Ca₅ (PO₄),F) formation. The transitional zone exhibited elevated titanium and oxygen photoelectron peaks suggesting the coexistence of calcium phosphate compounds within the titanium oxide. The substrate was shown to be identical to the passivated titanium surface prior to deposition.