Quantitative surface analysis by X-ray photoelectron spectroscopy and X-ray excited auger electron spectroscopy

It is shown that X-ray excited KLL Auger electron spectra allow it to describe measured signal strengths similarly to X-ray photoelectron signals, thus offering valuable information on the quantitative surface composition of a solid sample. The principal equation and corresponding fundamental parameters are discussed. As a result Auger spectra of C, N, O, F, and Na can be easily used in a multiline approach for quantitative analysis. LMM and MNN spectra give rise to more problems, due to their more complicated structure, uncertainties with regard to the background and the influence of Coster-Kronig transitions. These problems are overcome by the use of empirical ratios of the strongest lines of 2p/LMM or 3d/MNN. Since these ratios are independent of sample composition, they allow it to transform the Auger signal into the corresponding photoelectron signal, provided that a standard sample has been measured. Thus a true additional information is obtained and moreover difficulties in cases of photoelectron spectra with overlapping lines from other chemical elements can be overcome.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday     

Surface composition of dairy powders observed by X-ray photoelectron spectroscopy and effects on their rehydration properties

The surface composition of three dairy powders was investigated by X-ray photoelectron spectroscopy. These spray-dried casein powders were more or less enriched in hygroscopic material (lactose and/or minerals). The principal limitation of these high protein content powders is their poor rehydration ability. Consequently, information about surface composition is required in order to get a better understanding of rehydration behaviour (i.e. wetting time and time of rehydration). The obtained results indicate that the surface of the three powders was dominated by proteins. Lactose and minerals are marginal compounds at the surface whereas the surface coverage of fat was over represented. A correlation between the lactose surface content and the wetting time of the powders was found, but no relationship with the surface fat. Moreover, as the surface is partly depleted in minerals and lactose, it is concluded that these compounds are principally located in the bulk of the particle. Therefore this observation could be related with a wetting time of the powders only slightly affected by the addition of hygroscopic material whereas the time of rehydration was strongly improved; powder wetting being more affected by the surface composition whereas powder dispersion being more influenced by the powder bulk composition.     

X-ray photoelectron and scanning Auger electron spectroscopy study of electrodeposited ZnCr coatings on steel

Zn–Cr alloyed coatings electrochemically deposited are of high interest for leading steel manufacturing companies because of their novel properties and high corrosion resistance compared with conventional Zn coatings on steel. For tuning and optimizing the properties of the electrodeposited Zn–Cr coatings, a broad range of the deposition conditions must be studied. For this reason, two different types of material were investigated in this study, one with a low electrolyte temperature and one with an elevated electrolyte pH, compared with the standard values. Because different corrosion performance and delamination behaviour of the layers were observed for the two types, advanced surface analysis was conducted to understand the origin of this behaviour and to discover differences in the formation of the coatings. The topmost surface, the shallow subsurface region, and the whole bulk down to the coating–steel interface surface were analysed in detail by X-ray photoelectron spectroscopy (XPS) and high-resolution scanning Auger electron spectroscopy to determine the elemental and the chemical composition. For better understanding of the resulting layer structure, multiple reference samples and materials were measured and their Auger and XPS spectra were fitted to the experimental data. The results showed that one coating type is composed of metallic Zn and Cr, with oxide residing only on the surface and interface, whereas the other type contains significant amounts of Zn and Cr oxides throughout the whole coating thickness.     

Morphology of melt-pressed latex films examined by scanning and freeze-fracture electron microscopy

We report freeze-fracture transmission electron microscopy (FFTEM) and scanning electron microscopy (SEM) studies of films prepared by melt-pressing micrometer-size polystyrene (PS) and poly(methyl methacrylate) (PMMA) latex samples prepared by surfactant-free emulsion polymerization. Film samples were prepared in two ways intended to influence the nature of packing in the solid: slow evaporation of water at 40°C and 1 atm, or freezedrying. These powder samples were then gently pressed into transparent films using a Carver press. Samples prepared from freeze-dried latex are much less ordered then those from slowly dried dispersions. Two kinds of close packing, face-centered cubic (fcc) and body-centered cubic (bcc), may exist in different parts of an individual sample. An excellent example of bcc ordering was observed in a sample prepared by slowly drying a PMMA latex dispersion. Under very similar conditions, small regions of both fcc and bcc ordering were observed in PS samples. However, distortion of ideal bcc or fcc packing produces the same pattern of fracture. © 1994 John Wiley & Sons, Inc.     

Quantitative characterization of DNA films by X-ray photoelectron spectroscopy

We describe the use of self-assembled films of thiolated (dT)25 single-stranded DNA (ssDNA) on gold as a model system for quantitative characterization of DNA films by X-ray photoelectron spectroscopy (XPS). We evaluate the applicability of a uniform and homogeneous overlayer-substrate model for data analysis, examine model parameters used to describe DNA films (e.g., density and electron attenuation length), and validate the results. The model is used to obtain quantitative composition and coverage information as a function of immobilization time. We find that when the electron attenuation effects are properly included in the XPS data analysis, excellent agreement is obtained with Fourier transform infrared (FTIR) measurements for relative values of the DNA coverage, and the calculated absolute coverage is consistent with a previous radiolabeling study. Based on the effectiveness of the analysis procedure for model (dT)25 ssDNA films, it should be generally valid for direct quantitative comparison of DNA films prepared under widely varying conditions.     

Polypyrrole-coated poly(vinyl chloride) powder particles: surface chemical and morphological characterisation by means of X-ray photoelectron spectroscopy and scanning electron microscopy

Polypyrrole (PPy)-coated poly(vinyl chloride) (PVC) powder particles were prepared by the in situ chemical polymerisation of pyrrole in aqueous solutions in the presence of PVC powder particles. The PVC particles in suspension served as a hydrophobic substrate for the in situ polymerisation of pyrrole using iron chloride as the oxidising agent and sodium p-toluene sulfonate. In these conditions, tosylate-doped PPy (PPyTS) was obtained and chlorides were inserted as minor codoping species. In some cases, the pyrrole was polymerised after incubating the PVC particles with poly(N-vinyl pyrrolidone). Scanning electron microscope (SEM) micrographs showed that the PVC particles retained their initial, quasispherical shape after coating by PPy. At low magnification, the coated PVC particles appeared smooth, but at high magnification, they exhibited a decoration by elementary nanoparticles of about 200-nm size due to PPy bulk powder grains. Elemental analysis indicated a mass loading of PPy in the range 1–58% w/w. Specific surface analysis by X-ray photoelectron spectroscopy (XPS) resulted in the spectra of the PPy-coated PVC particles resembling those of bulk powder PPyTS even for low PPy mass loading. The surface fraction of PPy repeat units was found to vary in the 55–91% range. This result is consistent with the SEM observation of the PPy nanoparticles at the surface of PVC powder grains. However, despite the important loading of PPy, the XPS estimation of the overlayer thickness is in favour of a patchy coating rather than continuous coatings of PPy.     

Rapid characterization of ultrafiltration membranes by scanning electron microscopy

Physicochemical properties of ultrafiltration membranes were studied by scanning electron microscopy. The membrane elemental composition (carbon, oxygen, and sulfur) was determined by energy dispersion analysis. The elements were shown to be homogeneously distributed along the membrane. A homogeneous pore distribution on the membrane surface was found after covering it with a thin gold layer. The pore sizes are 50 nm. The topographic analysis of the permeate-side of the membrane indicated its anisotropy.     

Structural and electronic characterization of self-assembled molecular nanoarchitectures by X-ray photoelectron spectroscopy

Molecular monolayers and similar nanoarchitectures are indicative of the promising future of nanotechnology. Therefore, many scientists recently devoted their efforts to the synthesis, characterization, and properties of mono- and multilayer-based systems. In this context, X-ray photoelectron spectroscopy is an important technique for the in-depth chemical and structural characterization of nanoscopic systems. In fact, it is a surface technique suitable for probing thicknesses of the same order of the photoelectron inelastic mean free paths (a few tens of ångströms) and allows one to immediately obtain qualitative and quantitative data, film thickness, surface coverage, molecule footprint, oxidation states, and presence of functional groups. Nevertheless, other techniques are important in obtaining a complete spectroscopic characterization of the investigated systems. Therefore, in the present review we report on X-ray photoelectron spectroscopy of self-assembled molecular mono- and multilayer materials including some examples on which other characterization techniques produced important results.

X-ray standing waves and scanning electron microscopy — Energy dispersive X-ray emission spectroscopy study of gold nanoparticles

A systematic characterization of mono-disperse nanoparticles with nominal diameters of 25 nm, 46 nm, 73 nm, 100 nm, 115 nm, and 250 nm was performed using X-ray standing waves (XSW). The samples were prepared on Si-wafer pieces and analyzed at DELTA synchrotron facility at beamline BL8 under grazing incidence geometry of the primary radiation. Additionally, SEM-EDX inspections of single particles as well as population-density checks were conducted. Particles with smaller diameters were able be characterized by XSW while the larger ones were not completely covered by the interference field produced by the provided 15 keV monochromatic radiation of BL8. The results of the measurements were compared with those of numerical simulations. The extension of the interference field perpendicular to the Si-wafer reflector was determined to be 83 nm ± 4 nm.     

Investigation of Ti/CuO interface by X-ray photoelectron spectroscopy and atomic force microscopy

The Ti/CuO interface has been studied by the techniques of X-ray photoelectron spectroscopy and atomic force microscopy. Thin films of titanium were deposited on a CuO substrate at room temperature by the e-beam technique. The photoelectron spectra of titanium and copper were found to exhibit significant chemical interaction at the interface. The titanium overlayer was observed to get oxidized to TiO₂, while the CuO was observed to get reduced to elemental copper. This chemical interaction was observed to occur until a thickness of 7 nm of the titanium overlayer. For thicknesses greater than this value, the presence of unreacted titanium in the sample was detected. Barrier characteristics at the Ti/CuO interface were also carried out for substrate temperatures of 300°C, 400°C, 500°C, and 600°C as a function of the titanium overlayer thickness. A linear trend in the barrier thickness of the overlayer was observed between 400°C and 600°C substrate temperatures. The atomic force microscopy micrographs of the unannealed samples depicted layer-by-layer growth of elemental titanium on copper. At the Ti/CuO interface in such samples, the micrographs exhibited island formation of TiO₂ corresponding to the Volmer-Weber growth model. This formation has been interpreted as the relaxation in the strain energy. The percentage coverage of the underlying substrate by the TiO₂ islands showed a linear trend for the thicknesses of the titanium overlayer investigated. The average size of these islands also showed a linear trend as a function of the thickness of the overlayer.     

The morphological characterization of particles by automated scanning electron microscopy

Summary A method is described for the characterization and the classification according to shape of microscopic objects by automated SEM. The classification is performed by a hierarchical cluster analysis on a set of Fourier coefficients that are calculated from a set of radii, measured between a well defined centroid point and the contour lines of the object. This method is incorporated in existing commercial software for automated X-ray and size analysis of airborne particulate matter (PRC, Tracor Northern). Two examples demonstrate the possibilities and limitations of this method.

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Morphologische Charakterisierung mikroskopischer Objekte mit Hilfe der Raster-Elektronenmikroskopie

Zusammenfassung Eine Methode für die Charakterisierung und Klassifizierung mikroskopischer Objekte nach ihrer Form durch automatische Raster-Elektronenmikroskopie wurde beschrieben. Die Klassifizierung wird mittels einer hierarchischen Clusteranalyse unter Verwendung eines Satzes von Fourier-Koeffizienten durchgeführt, die aus einem Satz von Radien — gemessen zwischen einem exakt definierten Mittelpunkt und den Konturlinien der Objekte — berechnet werden. Diese Methode wird in eine kommerziell erhältliche Software für automatische Röntgen- und Größenverteilungs-analyse von luftgetragenen Staubteilchen eingebaut (PRC, Tracor Northern). An Hand von Beispielen werden die Möglichkeiten und Limitierungen dieser Methode dargestellt.

     

Surface characterization of immunosensor conjugated with gold nanoparticles based on cyclic voltammetry and X-ray photoelectron spectroscopy

This investigation describes the surface characterization of rabbit immunoglobulin G (IgG) conjugated with gold nanoparticles. Goat anti-rabbit immunoglobulin G tagged with 5 nm gold nanoparticles was applied to detect the IgG. Then, the autocatalyzed deposition of Au³⁺ onto the surface of anti-IgGAu increased the surface area per gold nanoparticle. The immobilization chemistries and the atomic concentrations of Au_4f, P_2p, S_2p, C_1s, N_1s and O_1s of the resulting antibody-modified Au electrodes were determined by X-ray photoelectron spectroscopy (XPS). The sulfur that is involved in the cysteamine binding and the enlargement of the gold nanoparticles are identified using cyclic voltammetry. The results reveal that the surface area per gold particle, following the autocatalyzed deposition Au³⁺ on the surface of anti-IgGAu, was approximately seven times higher than that before deposition.     

Sequestered carbon on clay mineral probed by electron paramagnetic resonance and X-ray photoelectron spectroscopy

This paper describes the interaction among soil organic matter components with kaolinite, an important clay mineral present in tropical soils, especially in Brazil. XPS data show that the soil organic matter adsorbed on kaolinite has aromatic and aliphatic structures, with phenolic and/or alcoholic functions and carbonyl carbons (CO) of amide and/or carboxylic groups. The N1s spectrum of the kaolinite shows an asymmetric peak that is assigned to amide and protonated ammines probably from humin. The interaction between them is strong enough to resist chemical oxidative or reductive attack besides loose amide functionalities. EPR data show that reductive treatment reduces some Fe3+ of the kaolinite structure, loosing organic components. A schematic representation of the reduction of structural Fe3+ in the concentrated domains and consequently increased concentration of Fe3+ ions in diluted domains of the spectrum is presented. This reinforces the hypothesis that humin is a stable carbon sink in soils when adsorbed to clays.     

Structural characterization of langmuir-blodgett films of a cellulose derivative by X-ray photoelectron spectroscopy

The method of variable angle x-ray photoelectron spectroscopy has been applied to transferred monolayers of a new cellulose derivative. The results support a model for the monolayer in which the cellulose backbone lies flat on the substrate and the sidechains extend perpendicular to the surface. Furthermore, a significant portion of the substrate surface is found to be bare, indicating that the transferred monolayer contains major defects in the form of holes.     

Intact and fragmented triosmium clusters on MgO: characterization by X-ray absorption spectroscopy and high-resolution transmission electron microscopy

Oxidative fragmentation of the clusters Os(3)(CO)(12) adsorbed on MgO powder was investigated by X-ray absorption spectroscopy and scanning transmission electron microscopy (STEM). Exposure of the clusters to air leads to their fragmentation, oxidation of the osmium, and formation of ensembles consisting of three Os atoms. X-ray absorption near-edge spectra demonstrate the oxidative nature of the fragmentation process. Extended X-ray absorption fine structure (EXAFS) spectra indicate an average Os-Os distance of 3.33 Angstrom and an Os-Os coordination number of 2, consistent with the formation of ensembles of three Os atoms on the support. STEM images confirm the presence of such trinuclear ensembles, and the diameters of the observed scattering centers (6.0 Angstrom) match that indicated by the EXAFS results.     

Energy-dispersive and x-ray photoelectron spectroscopy and electron microscopy of new quininium-plastic membrane electrodes

New quininium (Qn) plastic membrane electrodes of the conventional type were constructed and characterized. They are based on incorporation of Qn-reineckate (QnRn) ion-pair, Qn-phosphotungstate (Qn3-PT), or Qn-phosphomolybdate (Qn3PM) ion associate into a poly(vinyl chloride) membrane. The electrodes are selective for Qn and have been successfully used for the determination of Qn2SO4 in pharmaceutical tablets. Nevertheless, they showed, as almost all other ion-selective electrodes, limited life times. Energy dispersive- (EDS) and X-ray photoelectron spectroscopy (XPS), as well as electron microscopy were applied to investigate the cause of this limitation in the life times of the electrodes. The results indicated that the electrodes lose their activity after prolonged soaking as a result of leaching of the ion exchanger from the membranes into the test solution in addition to deformation at the surface of the expired electrode.     

Crystal polymorphism of protein GB1 examined by solid-state NMR spectroscopy and X-ray diffraction

The study of micro- or nanocrystalline proteins by magic-angle spinning (MAS) solid-state NMR (SSNMR) gives atomic-resolution insight into structure in cases when single crystals cannot be obtained for diffraction studies. Subtle differences in the local chemical environment around the protein, including the characteristics of the cosolvent and the buffer, determine whether a protein will form single crystals. The impact of these small changes in formulation is also evident in the SSNMR spectra; however, the changes lead only to correspondingly subtle changes in the spectra. Here, we demonstrate that several formulations of GB1 microcrystals yield very high quality SSNMR spectra, although only a subset of conditions enable growth of single crystals. We have characterized these polymorphs by X-ray powder diffraction and assigned the SSNMR spectra. Assignments of the 13C and 15N SSNMR chemical shifts confirm that the backbone structure is conserved, indicative of a common protein fold, but side chain chemical shifts are changed on the surface of the protein, in a manner dependent upon crystal packing and electrostatic interactions with salt in the mother liquor. Our results demonstrate the ability of SSNMR to reveal minor structural differences among crystal polymorphs. This ability has potential practical utility for studying the formulation chemistry of industrial and therapeutic proteins, as well as for deriving fundamental insights into the phenomenon of single-crystal growth.     

Application of scanning electrochemical microscopy and scanning electron microscopy for the characterization of carbon-spray modified electrodes

Different gold surfaces modified by carbon-spray have been investigated by scanning electron microscopy (SEM) and scanning electrochemical microscopy (SECM). A transformation of the SECM image to a distance-location profile is proposed which assists the correlation of both images. The structures found in the transformed SECM images of carbon-spray layers on gold substrates can be explained by the topographic features visible in the SEM pictures. Tempering the carbon spray results in an increased density of electrochemically reactive carbon particles which could be confirmed by cyclic voltammetric investigations. Gold minigrids modified with carbon spray expose some areas of especially large currents which could not be predicted from their SEM images. This effect may result from particles located at the edge of a wire intersection having relatively large active surfaces per particle. They contribute significantly to the total current of the minigrid.