Angle-dependent XPS Research Articles

Tuning surface hydrophilicity/hydrophobicity of hydrocarbon proton exchange membranes (PEMs)

The effect of annealing on the surface hydrophilicity of various representative classes of hydrocarbon-based proton exchange membranes (PEMs) is investigated. In all cases, a more hydrophilic membrane surface develops after annealing at elevated temperatures. The annealing time also had some influence, but in different ways depending on the class of PEM. Longer annealing times resulted in more hydrophilic membrane surfaces for copolymerized sulfonated poly(ether ether ketone) (SPEEK-HQ), while the opposite behavior occurred in sulfonated poly(aryl ether ether ketone) (Ph-SPEEK), sulfonated poly(aryl ether ether ketone ketone) (Ph-m-SPEEKK) and sulfonated poly (aryl ether ether nitrile) (SPAEEN-B). Increased surface hydrophilicity upon annealing results from ionic cluster decomposition, according to the “Eisenberg–Hird–Moore model” (EHM). The increased surface hydrophilicity is supported by contact angle (CA) measurements, and the cluster decomposition is auxiliarily supported by probing the level of atomic sulfur (sulfonic acid) within different surface depths using angle-dependent XPS as well as ATR-FTIR. Membrane acidification leads to more hydrophilic surfaces by elimination of the hydrogen bonding that occurs between strongly-bound residual solvent (dimethylacetamide, DMAc) and PEM sulfonic acid groups. The study of physicochemical tuning of surface hydrophilicity/hydrophobicity of PEMs by annealing and acidification provides insights for improving membrane electrode assembly (MEA) fabrication in fuel cell (FC).

Towards monomaterial p-n junctions: Single-step fabrication of tin oxide films and their non-destructive characterisation by angle-dependent X-ray photoelectron spectroscopy

The application of a non-destructive method for characterization of electronic structure of an ultra-thin SnO1<x<2 layer synthesized by spin coating on Si wafers was demonstrated. Utilizing angle dependent XPS, we quantified stoichiometry changes inside the SnO1<x<2 layers of thickness comparable with the electron attenuation length. The O/Sn concentration varied from 1.25 near the SnOx surface to 1.10 near the substrate/overlayer interface. Deviations from ideal stoichiometry are caused by defects, and defect levels affect the band structure of the SnOx layers. By investigation of the valence band region, followed by main core level position tracking, changes of electronic parameters like energy levels shift were identified. The results indicated a downward energy levels shift by 0.45 eV in SnOx layers at the SiO2/SnOx interface. In combination with the detected upward energy levels shift in the substrate's electronic structure, these results suggest a negative charge displacement across the SiO2 layer. As a consequence, there is a significant carrier concentration gradient in the layer, from a nearly insulating oxide at the SnOx surface to a semiconducting one at the bottom of the SnOx film. The results showed that the application of a simple and cost-effective method allows tuning the materials' properties towards the one-step fabrication of materials with ambipolar doping.

A comparative study about electronic structures at rubrene/Ag and Ag/rubrene interfaces

The contact between the electrode and the organic semiconductor is one of the most crucial factors in determining the organic device performance. The development and production technology of different organic devices require the understanding of different types of metal/organic semiconducting thin film interfaces. Comparisons about the electronic structures at Rubrene/Ag and Ag/Rubrene interfaces have been studied using photoemission spectroscopy. The Ag on rubrene interfaces is found to show more interesting and complex natures than its counterpart. The vacuum level (VL) was shifted about 0.51 eV from push back effect for deposition of 5 Å rubrene onto Ag film whereas the electronic features of silver was only suppressed and no energy shift was resulted. While the deposition of 5 Å Ag onto rubrene film leads to the diffusion of the Ag atoms, as a cluster with quantum size effect, inside the film. Angle dependent XPS measurement indicates that diffused metal clusters were present at entire probed depth of the film. Moreover these clusters dope the uppermost surface of the rubrene film which consequences a shift of the electronic states of thick organic film towards higher binding energy. The VL was found to shift about 0.31 eV toward higher binding energy whereas the shift was around 0.21 eV for the electronic states of rubrene layer.

Oxidation of rubrene thin films: an electronic structure study.

The performances of organic semiconductor devices are crucially linked with their stability at the ambient atmosphere. The evolution of electronic structures of 20 nm thick rubrene films exposed to ambient environment with time has been studied by UV and X-ray photoemission spectroscopy (UPS and XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT). XPS, NEXAFS data, and DFT calculated values suggest the formation of rubrene-epoxide and rubrene-endoperoxide through reaction of tetracene backbone with oxygen of ambient environment. Angle dependent XPS measurement indicates that the entire probed depth of the films reacts with oxygen by spending only about 120 min in ambient environment. The HOMO peak of pristine rubrene films almost disappears by exposure of 120 min to ambient environment. The evolution of the valence band (occupied states) and NEXAFS (unoccupied states) spectra indicates that the films become more insulating with exposure as the HOMO-LUMO gap increases on oxidation. Oxygen induced chemical reaction completely destroys the delocalized nature of the electron distribution in the tetracene backbone of rubrene. The results are relevant to the performance and reliability of rubrene based devices in the environment.

Surface orientation of hydrophilic groups in sulfonated poly(ether ether ketone) membranes

Sulfonated poly(ether ether ketone) copolymers (SPEEK) with a range of sulfonate contents (SC 77–51%) were synthesized via nucleophilic substitution polycondensation from hydroquinone and sulfonated hydroquinone. Membranes obtained by solvent casting from dimethylacetamide onto glass surfaces were analyzed for surface behavior. The surfaces of a membrane were hydrophobic in air, but hydrophilic in water. This surface behavior was corroborated by water contact angle vs. time, using sessile drop measurements. Hydrophilic sulfonic group aggregates on SPEEK chain and various media contacting with the top or bottom surfaces of the membrane during the fabrication process caused differences in surface behavior. Angle-dependent XPS showed that there was a higher atomic S/C ratio of the bottom surface than on the corresponding top surface. The hydrophilic sulfonic groups were in higher concentration within the membrane, with the concentration gradually decreasing towards the surface for SPEEK-HQ-80 and SPEEK-HQ-70 membranes. Acidification with strong acid and higher temperature induced a more hydrophilic surface on a membrane than a milder acidification process. The depth profile at the membrane surface was examined by a combination of contact angle, XPS and ATR-FTIR.

π-Interactions as a tool for an easy deposition of meso-tetraferrocenylporphyrin on surfaces

A bottom-up approach was employed to prepare novel Self-Assembled Monolayers (SAMs) in which a naphthyl moiety acted as a “π-binder” for unfunctionalised meso-tetraferrocenylporphyrin (H2TFcP). Four naphthalene derivatives with an appropriate functional group were synthesized and SAMs were prepared both on gold and ITO surfaces. Mixed H2TFcP–naphthalene films were thoroughly characterized using UV-Vis, XPS and electrochemical techniques. In particular, angle-dependent XPS experiments revealed an almost perpendicular orientation of the porphyrin on surfaces, suggesting that an intercalation occurred among naphthalene units. A large amount of porphyrin was deposited on both the surfaces (in the order of 10−10 mol × cm−2), comparable to that afforded by more conventional covalent linkages. However, significant differences in homogeneity between SAMs on gold and ITO resulted in a diverse electrochemical behaviour. The electrochemical activity of the oxidised porphyrin was restored by prolonged exposure of the modified gold electrode (tens of seconds) to a negative potential, whereas no response was detected after the same treatment on ITO. This novel approach provides a general and versatile strategy to bind meso-substituted porphyrins on a pre-formed monolayer without the necessity for further functionalisations.

Investigation of surface Sr segregation in model thin film solid oxidefuel cell perovskite electrodes

While SOFC perovskite oxide cathodes have been the subject of numerous studies, the critical factors governing their kinetic behavior have remained poorly understood. This has been due to a number of factors including the morphological complexity of the electrode and the electrode- electrolyte interface as well as the evolution of the surface chemistry with varying operating conditions. In this work, the surface chemical composition of dense thin film SrTi1−xFexO3-δelectrodes, with considerably simplified and well-defined electrode geometry, was investigated by means of XPS, focusing on surface cation segregation. An appreciable degree of Sr-excess was found at the surface of STF specimens over the wide composition range studied. The detailed nature of the Sr-excess is discussed by means of depth and take-off angle dependent XPS spectra, in combination with chemical and thermal treatments. Furthermore, the degree of surface segregation was successfully controlled by etching the films, and/or preparing intentionally Sr deficient films. Electrochemical Impedance Spectroscopy studies, under circumstances where surface chemistry was controlled, were used to examine and characterize the blocking effect of Sr segregation on the surface oxygen exchange rate.

Application of differential charging in XPS for structural study of Langmuir–Blodgett films

Differential charging in XPS was recently shown to be a novel technique for studying in-depth structural information of discrete cadmium layers in Langmuir–Blodgett (LB) multilayer films. Here we report structural modification in multilayer LB films after sulfidation using differential charging in angle-dependent XPS that are not observable by the x-ray reflectivity technique. An AFM study suggests less modification in compact LB films in comparison to the non-compact ones. The differential charging in the LB multilayers changes after sulfidation due to the formation of cadmium sulfide nanostructures in the cadmium arachidate LB matrix, which was reflected prominently in the differential charging of Cd 3d5/2 XPS peaks. It was found that, even after sulfidation, the compact multilayer LB films were differentially charged in the out-of-plane (in-depth) direction, whereas this kind of differential charging was not apparent in rough and non-compact films. Our results clearly indicate that for LB films with compact structure a partial layered structure survives the impact of particle formation, whereas for non-compact films the modification is large and no specific conclusion could be drawn. While x-ray reflectivity cannot provide specific information about the internal structure of the post-sulfidation films it shows that the total thickness of the films reduces in all cases.

Study of interfaces and band offsets in TiN/amorphous LaLuO 3 gate stacks

TiN/LaLuO 3 (LLO) gate stacks formed by molecular beam deposition have been investigated by X-ray photoelectron spectroscopy, medium energy ion scattering, spectroscopic ellipsometry, scanning transmission electron microscopy, electron energy loss spectroscopy and atomic force microscopy. The results indicate an amorphous structure for deposited LLO films. The band offset between the Fermi level of TiN and valence band of LLO is estimated to be 2.65 ± 0.05 eV. A weaker La–O–Lu bond and a prominent Ti2p sub-peak which relates to Ti bond to interstitial oxygen have been identified for an ultra-thin 1.7 nm TiN/3 nm LLO gate stack. The angle-dependent XPS analysis of Si2s spectra as well as shifts of La4d, La3d and Lu4d core levels suggests a silicate-type with Si-rich SiO x LLO/Si interface. Symmetrical valence and conduction band offsets for LLO to Si of 2.2 eV and the bandgap of 5.5 ± 0.1 eV have been derived from the measurements. The band alignment for ultra-thin TiN/LLO gate stack is affected by structural changes.

Plasma Polymerization of Amines and Acetone: Functionalized Surfaces for Microarray Applications

AbstractPlasma polymerization (PP) afforded amine as well as carbonyl containing films which are suitable for use in microarray analysis. The polymerization process with acetone (AC), ethylenediamine (EDA) and diethylamine (DEA) has to be performed in the presence of argon as a carrier gas to be successful. Different activation steps of the polymer based support yield surfaces which differ significantly in their elemental composition after the PP step. Angle dependent XPS proves a take off angle dependence of the carbonyl PP film in contrast to a homogeneous PP film for amine functionalized PP layers while microarray experiments have been successful for both functional films.

Cellular response to nanoscale elastin-like polypeptide polyelectrolyte multilayers

Ionic elastin-like polypeptide (ELP) conjugates are a new class of biocompatible, self-assembling biomaterials. ELPs composed of the repeat unit (GVGVP) n are derived from the primary sequence of mammalian elastin and produced in Escherichia coli. These biopolymers exhibit an inverse transition temperature that renders them extremely useful for applications in cell-sheet engineering. Cationic and anionic conjugates were synthesized by the chemical coupling of ELP to polyethyleneimine (PEI) and polyacrylic acid (PAA). The self-assembly of ELP–PEI and ELP–PAA using the layer-by-layer deposition of alternately charged polyelectrolytes is a simple, versatile technique to generate bioactive and biomimetic surfaces with the ability to modulate cell–substratum interactions. Our studies are focused on cellular response to self-assembled multilayers of ionic (GVGVP) 40 incorporated within the polymeric sequence H 2N–MVSACRGPG–(GVGVP) 40–WP–COOH. Angle-dependent XPS studies indicated a difference in the chemical composition at the surface (∼10 Å below the surface) and subsurface regions. These studies provided additional insight into the growth of the nanoscale multilayer assembly as well as the chemical environment that the cells can sense. Overall, cellular response was enhanced on glass substrata coated with ELP conjugates compared with uncoated surfaces. We report significant differences in cell proliferation, focal adhesions and cytoskeletal organization as a function of the number of bilayers in each assembly. These multilayer assemblies have the potential to be successfully utilized in the rational design of coatings on biomaterials to elicit a desired cellular response.

Precise synthesis of dendron-like hyperbranched polymers and block copolymers by an iterative approach involving living anionic polymerization, coupling reaction, and transformation reaction

Dendritic hyperbranched poly(methyl methacrylate)s (PMMA)s, whose branched architectures resemble the “dendron” part(s) of dendrimer, were synthesized by an iterative methodology consisting of two reactions in each iteration process: (a) a coupling reaction of α-functionalized, living, anionic PMMA having twotert-butyldimethylsilyloxymethylphenyl (SMP) groups with benzyl bromide (BnBr)-chain-end-functionalized PMMA, and (b) a transformation reaction of the introduced SMP groups into BnBr functionalities. These two reactions, (a) and (b), were repeated three times to afford a series of dendron-like, hyperbranched (PMMA)s up to third generation. Three dendron-like, hyperbranched (PMMA)s different in branched architecture were also synthesized by the same iterative methodology using a low molecular weight, functionalized 1,1-diphenylalkyl anion prepared fromsec-BuLi and 1,1-bis(3-tert-butyldime-thylsilyloxymethylphenyl)ethylene in the reaction step (b) in each iterative process. Furthermore, structurally similar, dendron-like, hyperbranched block copolymers could be successfully synthesized by the iterative methodology using α-functionalized, living, anionic poly(2-(perfluorobutyl) ethyl methacrylate) (PRfMA) in addition to α-functionalized, living PMMA. Accordingly, the resulting block copolymers were comprised of both PMMA and PRfMA segments with different sequential orders. After the block copolymers were cast into films and annealed, their surface structures were characterized by angle-dependent XPS and contact angle measurements. All three samples showed significant segregation and enrichment of PRfMA segments at the surfaces.

Quantitative and high mass ToF-SIMS studies of siloxane segregation in hydrogel polymers using cryogenic sample handling techniques

This paper examines the capabilities of cryogenic sample handling to examine composition and structure of hydrogel materials where siloxane components are central to the analysis. XPS analysis of multicomponent polymers with cryogenic sample handling following exposure to aqueous environments has revealed the composition and kinetics of near surface reorganization for siloxane and fluorocarbon containing polymers. In this study we report results from a ToF-SIMS protocol for cryogenic sample handling applied to the analysis of surface changes upon hydration/dehydration of hydrogel polymers. Comparison of results from angle dependent XPS and ToF-SIMS are discussed for a range of commercial soft contact lens materials. Both methods detected changes in surface chemistry between the hydrated (frozen) and dehydrated surfaces. Analysis of the hydrated surfaces detected polymer components indicative of the commercial formulation as well as ice clusters. Analysis of the dehydrated materials detected changes in surface chemistry relative to the hydrated surface in addition to loss of water due to sample dehydration. A quantitative standard additions method for ToF-SIMS data was used to determine submonolayer amounts of PDMS impurities at the surface of the hydrogels. ToF-SIMS analysis of a series of seven poly (allyl methacrylate- g-dimethylsiloxane), AMA- g-DMS, graft copolymers in the hydrated state revealed high mass oligomeric ion distributions for systems with bulk PDMS content greater than 25 wt.%. This marks the first time that detection of high mass oligomeric ion distributions from hydrated (frozen) surfaces has been reported. Analysis of the dehydrated surface detected formation of high mass oligomeric ion distributions for systems with PDMS bulk content greater than 15 wt.%, but only detected these ion distributions in wet (frozen) samples when the bulk concentration was greater than 25 wt.%.

XPS characterization of the surface immobilization of antibacterial furanones

Brominated furanones have attracted recent interest as antibacterial compounds. To utilize them as protective coatings in biomedical device applications, they must be covalently immobilized onto solid surfaces; however, interfacial coupling protocols developed for other biomolecules are not applicable to furanones. An azide reaction scheme has enabled covalent immobilization onto fluorinated ethylene propylene copolymer but its chemistry is less predictable, requiring detailed characterization by XPS and tapping mode AFM after each step of the immobilization sequence. XPS curve fitting resolved components in the C 1s, N 1s and Br 3d regions. Angle dependent XPS was used to assess the depth distributions and layer thicknesses. The results indicated successful covalent immobilization of furanones; however, side reactions occurred. In addition to the expected C Br, a contribution from bromine ion (Br −) was detected, indicating that photo-degradation of furanones took place during UV illumination, and this reaction was found to increase with illumination time. The Br − was removed by washing with water, whereas the C Br signal from immobilized furanone remained. Spectroscopic characterization will assist in elucidating the structure of furanone coatings, understanding their mode of action when covalently immobilized on surfaces, and rationally designing and optimizing an effective antibacterial coating for biomedical applications.

Angle-dependent XPS study of functional group orientation for aminosilicone polymers adsorbed onto cellulose surfaces

Thin layers of aminosilicones were adsorbed onto cellulose surfaces and examined by angle-dependent XPS (ADXPS) to study the orientation of the amino endgroup relative to the main polymer chain. The results indicate a preferential orientation of the amino group away from the air interface. Furthermore, the degree of endgroup depletion at the air interface was dependent on the polymer chain length and the coverage. A model is proposed in which the greatest effect occurs at near-monolayer coverage, with the endgroups adsorbed at the silicone/cellulose interface and the polymer chains forming loops that extend toward the air interface.

Synthesis and surface characterization of novel perfluorooctyl-functionalized polymers with well-defined architectures

We have successfully synthesized two kinds of novel well-defined chain-end-functionalized polystyrenes with one, two, three, and four perfluorooctyl (C 8 F 17 ) groups and structurally analogous block copolymers, poly[styrene-b-4-(3-perfluoroocyl)propoxystyrene], by means of living anionic polymerization, followed by the Williamson reaction with C 8 F 17 (CH 2 ) 3 Br. The surface properties and compositions of the films prepared from both C 8 F 17 -functionalized polymers have been characterized by contact angles using water and dodecane droplets and angle-dependent XPS measurement. It was observed that C 8 F 17 groups were segregated and preferentially enriched at their topmost surfaces and the extent of the enrichment increased with the number of C 8 F 17 group. The dependence of extent of C 8 F 17 group enrichment on fluorine content however appears to be different between chain-end-functionalized polymer and block copolymer.

Surface treatment of polyethylene by fast atom beams

Ultra-high molecular-weight polyethylene (UHMWPE) was treated by fast atom beams (FAB) obtained from He, Ar, H2 and N2 with about 1 kV accelerating voltage and estimated fluence of 1017 particles cm−2. The modified surface layers were characterised by X-ray photoelectron spectroscopy (XPS or ESCA) and dynamic microhardness measurements. Each applied FAB treatment results in the increase of the bulk plasmon loss energy (EL) of the C 1s peak. This implies the formation of graphitic-type material and/or hydrogenated amorphous carbon (or carbon nitride in case of treatment by N atoms) in the modified surface layer. FAB treatment by N atoms leads to the incorporation of ≈35 at.% of N, which is about three times higher than the value obtained previously after low-kilo-electron-volt N2+ ion-beam treatment. The related C 1s peak shows that the overwhelming portion of C is bound to N, while the N 1s peak reflects that N is present in at least three chemical bonding states. Angle-dependent XPS studies of the nitrogen-FAB-treated UHMWPE reveal the presence of a N-rich subsurface layer with a topmost layer containing less N. This is in agreement with the calculated depth distribution of N atoms. Each applied FAB treatment leads to significant increase in the surface microhardness.

Surface properties of methacrylic copolymers containing a perfluoropolyether structure

New copolymers obtained by free radical copolymerization of methylmethacrylate (MMA) with a methacrylic monomer containing a perfluoropolyether structure (PFEUMA) were prepared. The products showed an excellent surface activity evidencing hydrophobic and lipophobic properties; the water contact angle of the films was always very high (>105°). Angle-dependent XPS analyses showed that the fluorinated monomer preferentially concentrates at the air surface and a fluorine gradient normal to the surface is present. A surface reorganisation was observed by changing the interface. The wettability increased when the films were dipped in water; after complete drying, the surface regained its hydrophobicity.

Various-sized stearyl poly(ethylene oxide) coupling-polymer blending poly(ether urethane) material for surface study and biomedical applications

Three types of stearyl poly(ethylene oxide) (SPEO) with different vatues of M 2 , 2300, 6000 and 12000, were systhesized by an anion polymerization from the ethylene oxide (Eo) monomer with the initiator, sodium salt stearyl alcohol. Consequently de amphiphilic coupling polyaers of 4,4'-didhenylmethane diisocyanate (MDI)-SPEO (SPBO-MDI SPEO, MSPEO 2300, 6000 and 12000) were synthesized and the blended poly(other unethane) samples of PEL-MSPEOs (PEL-MSPEO 2300, 6000 and 12000) were prepared via a solution process. The surface analysis with ATR-FTIR, angle-dependent XPS and two methods of contact angle measurements (sessile drop and captive bubble) were perfomed. The results of the analysis indicate that the dominant factor for the monoblock behavior of long-chained MSPEO 12 000 is the PEO chain. However, for short-chained MSPEO the factor is C 18 and the dominant effect is not so overwhelming compared to that of PEO for long chained MSPEO. Simultaneously, the size of the molecule will also dramatically affect MSPEOs' kinetic behavior including the monoblock mobility and the reconstructing of the surface conformation. i.e. the smaller the size, the faster the movement. The static test of clotting time, PRT, was also performed. The results indicated that the modified surface could resist the clotting much more effectively than that of the untreated surface.

Molecular orientation-photoconductivity relationship study of phthalocyanine polymer-oriented thin films

The molecular orientation–photoconductivity relationships of several kinds of phthalocyanine polymer (PPc)-oriented thin films have been studied in double-layered photoreceptor devices, where the charge-generation layers (CGLs) are phthalocyanine polymer-oriented thin films and the charge-transportation layers (CTLs) are composed of hole transporting materials of tetraphenyl benzidine or hydrazone. The oriented thin films containing PPc dispersed in polyvinyl difluoride (PVDF) were prepared by the electric field orientation. The results showed that the photosensitivities of the phthalocyanine polymer (PPcs)-oriented thin films were higher than those of the unoriented PPcs thin films, and varied with their molecular structures and the molecular stacking in the films. This was thought to be due to the molecular orientation effect, which was demonstrated by the analyses of the polarized fluorescence, DSC, FTIR reflection absorption spectroscopy (FTIR-RAS), and angle-dependent XPS. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2331–2339, 2000