X-ray Photoelectron Spectroscopy Sampling Depth Research Articles

The Substrate and Composition Dependence of Plasma Polymer Stability

AbstractFilm stability of functionalised plasma polymer (PP) films in aqueous environments is an issue. Whilst methods are routinely employed to enhance aqueous stability, the issue itself has not previously been systematically studied. Herein, we describe a method using gradient PP surfaces whereby aqueous stability can be explored as a function of PP composition and substratum. X‐ray photoelectron spectroscopy (XPS) is used to examine PP films on glass and Thermanox “as‐prepared” and post‐immersion (24 h) in distilled H2O. Our data indicate stability becomes an issue with increase in functionalisation, but that functionalised PPs are much more stable when deposited on polymeric Thermanox than when deposited onto glass. Substratum influence in PP films thicker than the XPS sampling depth is a surprise.magnified image

Improvements in the Reliability of X-ray Photoelectron Spectroscopy for Surface Analysis

An overview is given of improvements in the reliability of X-ray photoelectron spectroscopy (XPS) measurements over the past three decades. Early inter-laboratory comparisons showed wide variations in measurements of peak energies and relative peak intensities for the same material. Procedures now exist for calibration of instrumental energy and intensity scales. It was generally necessary in early work to assume that the surface was homogeneous over the XPS sampling depth. It is now possible to assess the extent of compositional inhomogeneities in depth with angle-resolved XPS measurements or from analyses of the photoelectron energy distributions in the vicinity of major peaks. The surface sensitivity of XPS was assumed in early work to be given by the so-called "universal curve" for effective attenuation lengths (EALs) as a function of electron energy. Calculations of electron inelastic mean free paths (IMFPs), however, show appreciable material dependencies. In addition, elastic scattering of the signal electrons is often significant. As a result, the EAL can be appreciably different from the corresponding IMFP. Brief mention is also made of the development of NIST data resources for XPS: an XPS database, IMFP and EAL databases, and standard test data for XPS. The standard test data for XPS are a useful teaching tool for estimating uncertainties of peak parameters for overlapping peaks in an XPS or other spectrum.

Direct Verification of the Core−Shell Structure of Shell Cross-Linked Micelles in the Solid State Using X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) is used to probe the nanostructure of shell cross-linked micelles in the solid state. A novel ABC triblock copolymer was synthesized via atom transfer radical polymerization (ATRP) comprising quaternized nitrogen atoms in the relatively hydrophilic “A” block, cross-linkable hydroxy groups in the middle “B” block, and neutral nitrogen atoms in the relatively hydrophobic “C” block. Two types of micelles were prepared: conventional micelles with the C block in the core in aqueous media and inverted micelles with the A block in the core in nonaqueous media. Efficient shell cross-linking of both types of micelles was achieved using divinyl sulfone (DVS), which reacts with the hydroxy groups in the central B block at room temperature. Although the XPS sampling depth of 2−10 nm is comparable to the dimensions of the micelle layers, and despite the probability of some micelle deformation in the absence of solvent, analysis of the high-resolution N 1s spectra provided the first direct evidence for the expected core−shell nature of SCL micelles in the solid state.

Investigation of the Water-Induced Reorganization of Polycaprolactone−Poly(fluoroalkylene oxide)−Polycaprolactone Triblock Copolymer Films by Angle-Dependent X-ray Photoelectron Spectroscopy

The water-induced reorganization of a series of poly(caprolactone)−poly(fluoroalkylene oxide)−poly(caprolactone) (PCL−PFPE−PCL) thin film surfaces was studied by angle-dependent X-ray photoelectron spectroscopy (XPS). The reorganization was studied as a function of polymer chain length, water exposure time, and XPS sampling depth. The prepared films were exposed to water and then frozen in liquid nitrogen to preserve the surface composition during XPS analysis. The XPS results showed the PFPE block segment length influenced the extent and rate of segment reorganization at the surface. The detected changes were shown to be reversible upon re-exposure to air and the effect of liquid-nitrogen freezing was determined to be negligible.

Vapour-phase chemical derivatisation for the determination of surface functional groups by X-ray photoelectron spectroscopy

A vacuum adsorption apparatus has been used for vapour-phase chemical derivatisation of standard polymers, to identify specific reactions for carboxylic acid and carbonyl functional groups. The reactivity and selectivity of trifluoroethanol (TFE)–pyridine-di-tert-butylcarbodiimide and 3-(trifluoromethyl)phenylhydrazine for these respective groups were studied. Depths of reaction were also investigated using F 1s/F 2s peak intensity ratios determined by X-ray photoelectron spectroscopy (XPS). The TFE–/pyridine-/di-tert-butylcarbodiimide system was found to be suitable for the selective derivatisation of carboxylic acid groups, achieving ca. 75% conversion of carboxylic acid groups. The depths of fluorine incorporation for both the TFE and TFAA reactions were found to be invariant with reaction time over the XPS sampling depth for the range of conditions studied. Derivatisation of carbonyl groups using 3-(trifluoromethyl)phenylhydrazine was not sufficiently selective, reactions being observed with hydroxy and carboxylic acid groups. Flame treatment of polyethylene produced oxidised surfaces with a variety of functional groups. Using the methods described above and previously reported the hydroxy and carboxylic acid group concentrations were found to be invariant with flame intensity, with the latter having the consistently lower concentration.

Composition depth profiles of plasma‐fluorinated poly(ethylene terephthalate) fibers

AbstractComposition depth profiles of the outer 50 Å of plasma‐fluorinated poly(ethylene terephthalate) fibers were obtained by angle‐dependent X‐ray photoelectron spectroscopy (XPS). The effect of sample geometry on XPS sampling depth and the depth distribution function (DDF) was determined theoretically for cylindrical and hemispherical surfaces. The theoretical DDFs are nonexponential. For cylindrical surfaces, the effect is small, a 22% increase in surface sensitivity. The average XPS sampling depth for smooth, properly oriented fibers is shown to vary, as it does for a planar surface, as the sine of the nominal takeoff angle. The DDF appropriate for cylindrical surfaces was incorporated into a computer program for inversion of angle‐dependent XPS data to obtain composition depth profiles of the fibers. Plasma‐fluorinated PET fibers were used to demonstrate the use of angle‐dependent XPS on fibers. XPS results indicate that most fluorination occurs within the top few “monolayers,” attack is preferentially at the phenyl ring, both CHF and  CF2  moieties are formed, and fluorination causes partial loss of aromaticity. © 1994 John Wiley & Sons, Inc.

Surface studies of polyethylene modified by flame treatment

Effects of flame treatment on polyethylene (PE) surfaces have been studied using contact-angle measurements, attenuated total reflection infrared spectroscopy (ATR-IR), X-ray photoelectron spectroscopy (XPS), and an adhesion test. A significantly higher level of surface oxidation was induced by flame treatment on PE than on polypropylene (PP) under a given set of conditions, and unlike the case of PP the oxidation of PE is not confined to the XPS sampling depth on the polymer surface even for a mild flame treatment. High-energy resolution XPS has revealed several different functionalities on the surface and ATR-FTIR has clearly detected the peak at 1715 cm–1 which can be attributed to CO groups. Large contact-angle hysteresis was obtained and was attributed to chemical heterogeneity on the surface. Good adhesion with an epoxy adhesive has been obtained after flame treatment.

Additional in-depth information obtainable from the energy loss features of photoelectron peaks: the oxidation and reduction of an Fe/Cr alloy in oxygen at low partial pressures and ultra high vacuum

The oxidation and subsequent vacuum annealing of an Fe/Cr alloy has been studied by X-ray photo-electron spectroscopy (XPS). In addition to the conventional ways of extracting compositional depth information with this spectroscopy (i.e. by ion sputtering and angle resolved XPS), a special study has been made of the use of energy loss features within the XPS spectra to provide such information. There are three classes of energy losses that may prove useful: (i) the height of the inelastically scattered electron background immediately adjacent to the peak of interest which is defined by the term electron energy loss tail height (ELTH), (ii) the slope of the background some distance (e.g. 25 eV) from the photo-electron peak, the post-peak slope (P-PS), and (iii) the slope of the base line associated with the spectrum itself. The former categories provide definitive information concerning the hierarchy of discrete layers on the surface of a material, whilst the latter provides a means of identifying subtle concentration gradients that exist within the XPS sampling depth. The use of these methods in addition to the more conventional depth profiling studies have enabled a series of comprehensive models to be proposed for the formation of passivating layers on the Fe/Cr alloy in a variety of oxidation and reduction conditions at modest temperatures (200–300°C). The methods described have the added advantage that they provide the electron spectroscopist with the possibility of carrying out a retrospective depth profile long after the analysis has been completed.

SIMS and XPS studies of polyurethane surfaces 2. Polyurethanes with fluorinated chain extenders

AbstractA series of fluorine‐containing segmented poly(ether urethanes) and poly(ether urethane ureas), previously characterized by angular dependent x‐ray photoelectron spectroscopy (XPS) have been re‐examined using static secondary ion mass spectrometry (SIMS). Comparison of the data from the two techniques has shown that the intensity of secondary ion signals which characterize the hard and soft segments can be used quantitatively for direct analyis of the molecular composition at the surface. Moreover, the SIMS sampling depth for the conditions used has been shown to be ∼ 10 Å, significantly less than the XPS sampling depth under normal conditions, and similar to the XPS sampling depth at high glancing take‐off angle (with respect to the surface normal).

Surface analysis of gold containing polyimide films

Polyimide films with metallic surfaces were produced by thermal treatment of solutions containing HAuCl4⋅3H2O, hydrogen tetrachloroaurate trihydrate, and either Ciba–Geigy XU 218, poly[5(6)-amino-1-(4′ phenyl)-1,3,3 trimethylindane-3,3′,4,4′-benzophenonetetracarboxylimide] or BTDA-ODA, poly [ p, p′-oxybis (phenylene)-3,3′,4,4′-benzophenonetetracarboxylimide]. Scanning electron microscopy indicated that 0.1-μ particles covered a large portion of the air side surface; whereas, 10-μ particles covered the glass side surface of these films. Auger electron spectroscopy and x-ray photoelectron spectroscopy (XPS) indicated that the particles were gold in the metallic state. Model imide compounds were synthesized and used to evaluate the XPS spectra of both unmodified and gold containing films. Surface analysis data suggested that only a small amount of gold was located within the XPS sampling depth and that the observed gold particles resided underneath a polyimide overlayer. The discrete metal particles had an unexpected influence on the photoelectron spectra of the polyimide and the influence was dependent on particle size. A schematic model of the multilayered modified film surface was developed and the model further probed by electron flood gun and metal vapor deposition studies.

An XPS study of the UV photoreduction of transition and noble metal oxides

The photoreduction of polycrystalline samples of MoO 3, WO 3, Cu 2O, CuO, PdO and PtO by UV irradiation was investigated by X-ray photoelectron spectroscopy (XPS). Photoreduction of these metal oxides was found upon irradiation with the emission lines from a mercury vapor lamp (2537 Å) at room temperature. Other oxides, like ZnO and V 2O 5, where not photoreduced. Mo 6+ and W 6+ were observed to partially photoreduce to Mo 5+ and W 5+. In the case of the copper oxides, both Cu 2+ and Cu 1+ were found to completely photoreduce to the metallic state. The photoreduction of PdO and PtO was observed to proceed slowly through the first few monolayers and was 75% completed within the XPS sampling depth ( ∼ 40 Å) after long irradiation times. All photoreduced species were observed to reoxidize upon exposure to the atmosphere. The oxidation of photoreduced metallic Pt and Pd at room temperature is unique since those metals usually do not oxidize in air. A model has been developed to explain and predict the extent of reduction of metal oxides surfaces by UV irradiation. It shows that depending on band gap and chemical stability, complete or partial reduction of many oxide surfaces can be achieved.

Studies of the composition, ion-induced reduction and preferential sputtering of anodic oxide films on Hg 0.8Cd 0.2Te by XPS

Angle-resolved studies of anodic oxide films on Hg 0.8 Cd 0.2 Te, and on cadmium and tellurium bulk oxides, by X-ray photoelectron spectroscopy (XPS) have revealed the formation of a discrete altered overlayer, containing reduced tellurium (Te 0) species, following inert gas ion bombardment. In the presence of Cd 2+ species, recombination is shown to occur, to yield a Te 2− (telluride) species, in the form of CdTe. Only Te 2− is observed on ion-induced reduction of CdTeO 3, whereas there is no evidence for significant Te 2− formation on either TeO 2 or anodic oxides on CMT, reflecting the low cadmium content of these latter two oxides. In addition, preferential cation sputtering leads to a homogeneous (within the XPS sampling depth) Te 4+-depleted layer in Cd-Te mixed oxides. The phenomenon is characterised by an effective sputter yield ratio ( S Te 4+ / S Cd 2+ ) of 1.83 (±0.06). The anodic oxide on CMT, when corrected for preferential Te 4+ sputtering, yields a (Te 4+):(Cd 2+) ratio of 4.6 (±0.5), in excellent agreement with the bulk Te:Cd ratio in Hg 0.8Cd 0.2Te. An anodic oxide film composition of approximately < 25 mol% CdTe 2O 5 and > 75 mol% TeO 2 and/or mixed Hg-Te oxides is proposed.