Monte‐Carlo simulation of x‐ray photoelectron emission from silver

Silver 3d x‐ray photoelectron spectroscopy (XPS) spectra were simulated with the Monte‐Carlo method using an effective energy‐loss function that was derived from a reflected electron energy‐loss spectroscopy (REELS) analysis based on an extended Landau approach. After confirming that Monte‐Carlo simulation based on the use of the effective energy‐loss function can successfully describe the experimental REELS spectrum and Ag 3d XPS spectrum, we applied Monte‐Carlo simulation to predict the angular distribution of Ag 3d x‐ray photoelectrons for different x‐ray incidence angles and different photoelectron take‐off angles. The experimental photoelectron emission microscope that we are constructing was confirmed as being close to the optimum configuration, in which the x‐ray incident angle as measured from the surface normal direction is 74° and the photoelectron take‐off angle is set normal to the surface. The depth distribution functions of the Ag 3d X‐ray photoelectrons for different energy windows suggest that the photoelectron emission microscope will exhibit greater surface sensitivity for narrower photoelectron energy windows. Copyright © 2003 John Wiley & Sons, Ltd.     

Mathematical topographical correction of XPS images using multivariate statistical methods

For rough heterogeneous samples, the contrast observed in XPS images may result from both changes in elemental or chemical composition and sample topography. Background image acquisition and subtraction are frequently utilized to minimize topographical effects so that images represent concentration variations in the sample. This procedure may significantly increase the data acquisition time. Multivariate statistical methods can assist in resolving topographical and chemical information from multispectral XPS images. Principal component analysis (PCA) is one method for identification of the highest correlation/variation between the images. Topography, which is common to all of the images, will be resolved in the first most significant component. The score of this component contains spatial information about the topography of the surface, whereas the loading is a quantitative representation of the topography contribution to each elemental/chemical image. The simple‐to‐use self‐modelling mixture analysis (Simplisma) method is a pure variable method that searches for the source of most differences in the data and therefore has the potential to distinguish between chemical and topographical phases in images. The mathematical background correction scheme is developed and validated by comparing results to the experimental background correction for samples with differing degrees of topography. Copyright © 2004 John Wiley & Sons, Ltd.     

Study of the electron standing wave states in scanning tunneling spectroscopy of Si(111) surface

Electron standing wave (ESW) states excited in the vacuum gap near sample surface by operating the scanning tunneling microscopy in field emission regime have been widely used to probe the local electronic properties of novel materials and structures. For accurate interpretation of the ESW states spectra, a simple numerical approach is developed based on a one‐dimensional model, and the transmission coefficient is accurately calculated using the transfer matrix method. Effects correlated with the potential distribution in the tunneling gap are discussed. By this method, main features of the experimental spectra obtained on Si(111)7 × 7 surfaces are successfully simulated. Factors affecting the spectra are discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Leaching of PVP from polyacrylonitrile/PVP blending membranes: A comparative study of asymmetric and dense membranes

Poly(N‐vinyl‐2‐pyrrolidone) (PVP) has been often used as an additive to improve the structure and performances of asymmetric membrane. In this work, we examined the leaching of PVP from polyacrylonitrile/PVP asymmetric membranes regarding the effect of leaching time, PVP content, and the molecular weight of PVP. Also, comparative studies of dense membranes were performed. It was found that the water contact angle on the dense membrane surface is very low compared with that on the asymmetric membrane surface. Results from X‐ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy‐attenuated total reflection (FTIR‐ATR) showed that more amount of PVP exists at the surface layer of the dense membrane than at the asymmetric one. If the dense membrane was immersed into water for several hours and then dried in the air, the water contact angle increases and closes to that on the asymmetric membrane surface. Although leaching time was extended from 2 h to 15 days, PVP leaches out little from the asymmetric membrane. The leaching of PVP mainly occurs during the phase‐inversion process. Furthermore, the surface features were examined by atomic force microscopy and field emission scanning electron microscopy, respectively. In comparison with PVP K30, more PVP K90 remains in the asymmetric membrane based on the FTIR‐ATR spectra. However, it can be concluded from the results of XPS that at the most outer surface of the asymmetric membrane (e.g., in depth about 1～2 nm), the residual PVP K90 is almost the same with PVP K30. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1490–1498, 2006     

Annealing of silver nanolayers sputtered on polytetrafluoroethylene

Silver nanolayers sputtered on polytetrafluoroethylene and their changes induced by post‐deposition annealing at 100–300 °C are studied. Changes in surface morphology and roughness are examined by atomic force microscopy and by measurement of electrical sheet resistance by two‐point technique. Chemical composition was determined by X‐ray photoelectron spectroscopy (XPS) and electrokinetic analysis in dependence on the gold layer thickness. The annealing at 300 °C leads to significant rearrangement of the silver layer, and the transition threshold increases to 35 nm. The presence of oxidized structures on silver‐coated samples is observed in XPS spectra and by electrokinetic analysis, too. Annealing of pristine and silver‐coated poly(tetrafluoroethylene) at 300 °C results in significant change of the sample surface morphology and chemistry. There is observed formation of isolated silver islands on the surface, which could be connected with silver melting. Later, the silver agglomeration takes place. Copyright © 2013 John Wiley & Sons, Ltd.     

Molecular dynamics simulation of surface morphology and thermodynamic properties of polyaniline nanostructured film

To develop predictive models in nanostructured films, there is an ongoing research to validate molecular dynamics (MD) simulation results with experimental data. The morphology and surface topography of polyaniline (PANI) nanostructured film coated on a TiO₂ nanocrystalline surface were investigated by scanning electron microscopy and atomic force microscopy, respectively. The atomistic model of the simulated PANI was generated using energy minimization with a condensed‐phase optimized molecular potential for atomistic studies force field function to reach a thermodynamic equilibrium state. Various parameters of PANI such as density, energy, cavity size, and free volume distributions are calculated. MD simulation has also been used to obtain specific volume (V) as a function of temperature (T). It is demonstrated that this V–T curve can be used to determinate glass transition temperature T_g, reliably. Although experimental data available for the PANI film are very limited, simulation results such as density and T_g are in good agreement with the experimental values reported in the literature. Comparison of the surface topography of PANI demonstrates a reasonable trend between atomic force microscopy image analysis and the MD simulation results at various temperatures. Copyright © 2014 John Wiley & Sons, Ltd.     

Photoelectron core levels for enargite,Cu3AsS4

The mineral chemistry of enargite surfaces and near‐surfaces prepared by fracture within an ultrahigh vacuum are investigated using XPS and synchrotron radiation XPS (SRXPS). The purpose of the study is to identify surface core‐level line positions in high‐resolution photoelectron spectra. The XPS spectra obtained using monochromatic Al Kα radiation show that there are near‐identical monosulphide line positions in S 2p spectra for the As–S and Cu–S bonds in enargite. The SRXPS spectra for sulphur are remarkably similar to the XPS spectra for sulphur, showing that there is minimal difference in the chemical environment between sulphur atoms at the surface and in the mineral matrix. The Cu 2p XPS data have only cuprous contributions and a minor surface contribution. The surface Cu contribution is observed as a high‐binding‐energy tail in the Cu 2p_3/2 spectrum. The As 3d data for both XPS and SRXPS show contributions from arsenic atoms at the surface and in the bulk mineral matrix. The surface contribution is distinct and is found 1 eV below the bulk contribution. The results of the study suggest that, following fracture, the enargite surface is reorganized in such a manner that the surface is characterized by protrusions of individual arsenic atoms. Copyright © 2004 John Wiley & Sons, Ltd.     

Quantification of surface roughness effect on elastically backscattered electrons

Electron inelastic mean free path can be obtained from a measured elastic peak electron spectroscopy spectrum combined with a Monte Carlo simulation. It is thus necessary to know the influence of various experimental factors to the measured and calculated results. This work investigates the effect of the surface roughness or the surface topography on the intensity of the elastic peak. A Monte Carlo simulation, by taking into account of realistic surface roughness for both Gaussian and non‐Gaussian type rough surfaces experimentally prepared, has been employed to study the surface topography effect. The simulations of elastic peak electron spectroscopy were performed for both planar and rough Al and Cu surfaces and for varied primary energies ranging from 200 to 2000 eV. To quantify the surface roughness effect, the surface roughness parameter is introduced according to the ratio of elastic peak intensities between a rough surface and an ideal planar surface. Simulation results have shown that surface roughness parameter is important in a certain range of emission angle and particularly for large emission angles. For grazing emission, the elastic peak intensity can be largely enhanced by roughness even at nanometer scale. Copyright © 2014 John Wiley & Sons, Ltd.     

Surface characterization and radical decay studies of oxygen plasma‐treated PMMA films

Polymethylmethacrylate (PMMA) films were modified by RF oxygen plasma with various powers applied for different periods, and the effects of these parameters on the surface properties such as hydrophilicity, surface free energy (SFE), chemistry, and topography were investigated by water contact angle, goniometer, X‐ray photoelectron spectroscopy (XPS), and atomic force microscopy, and the types of the created free radicals and their decay were detected by electron spin resonance spectroscopy (ESR). SFE and contact angle results varied depending on the plasma parameters. Oxygen plasma treatment (100 W–30 min) enhanced the hydrophilicity of PMMA surface as shown by decreasing the water contact angle from 70° to 26°. XPS analysis showed the change in the amounts of the present functionalities as well as formation of new groups as free carbonyl and carbonate groups. The roughness of the surface increased considerably from ~2 nm to ~75 nm after 100 W–30 min oxygen plasma treatment. ESR analysis indicated the introduction of peroxy radicals by oxygen plasma treatment, and the intensity of the radicals increased with increasing the applied power. Significant decrease in radical concentration was observed especially for the samples treated with higher powers when the samples were kept under the atmospheric conditions. As a conclusion, RF plasma, causes changes in the chemical and physical properties of the materials depending on the applied parameters, and can be used for the creation of specific groups or radicals to link or immobilize active molecules onto the surface of a material. Copyright © 2012 John Wiley & Sons, Ltd.     

Topography and surface energy dependent calcium phosphate formation on Sol-Gel derived TiO2 coatings

Heterogeneous nucleation and growth of calcium phosphate (CaP) on sol-gel derived TiO(2) coatings was investigated in terms of surface topography and surface energy. The topography of the coatings was derived from AFM measurements, while the surface energy was determined with contact angle measurements. The degree of precipitation was examined with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The precipitation of CaP was found to be dependent on both topography and surface energy. A high roughness value when combining the RMS roughness parameter S(q) with the number of local maxima per unit area parameter S(ds) enhances CaP formation. The hydrophilicity of the coating was also found to be of importance for CaP formation. We suggest that the water contact angle, which is a direct measure of the hydrophilicity of the surface, may be used to evaluate the surface energy dependent precipitation kinetics rather than using the often applied Lewis base parameter.     

Investigation into the surface modification of aragonite whiskers

Aragonite whiskers (AWs) were treated with several fatty acid surfactants and silane coupling agent in order to determine the optimal modifier by using contact angle measurements. The results revealed that the AWs modified by fatty acids showed more remarkable increase in the contact angle than by silane, suggesting the former were preferentially applied in modifying AWs. While the samples coated with fatty acids exhibited hydrophobicity with contact angles ranging from 104.08° to 137.87° with increasing of carbon chain length. Therefore, the highest contact angle of AWs treated by oleic acid was discussed in detail as an example, which was characterized by field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), thermo‐gravimetry analyses (TGA), X‐ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). FESEM and TEM results showed a thin layer coated on the modified sample surface. Both the results of TGA and XPS confirmed organic groups existed in the sample of AWs treated by oleic acid. FTIR demonstrated that calcium dioleate was formed in the modification process. Further, modification mechanism was proposed based on the obtained results. Copyright © 2016 John Wiley & Sons, Ltd.     

EC STM investigations of corrosion due to chloride solutions on thin CrN coatings

The focus of the investigations presented is to evaluate local alterations caused by chloride ions affecting thin, magnetron-sputtered CrN layers. Scanning-probe microscopy and analysis techniques are used for this estimation. Thin CrN layers were deposited by reactive magnetron sputtering. They were investigated in electrochemical scanning tunnelling microscopy (EC STM) by cyclic voltammetry in 1 mol L(-1) NaCl. Simultaneously, the surface topography changes were recorded with STM.Above 100 mV the anodic oxidation leads to formation of chromium(III) hydroxide and at sample potentials above 350 mV oxidation of Cr(OH)(2) and Cr(OH)(3) towards chromium(VI) as a soluble chromate starts. Transpassive dissolution of the coating takes place above 900 mV. Yellow colour of the electrolyte is a visible sign for the formation of chromium(VI). Changes of the surface topography indicate the formation of surface layers at anodic potentials. At cathodic potentials increase in current is measured due to the reduction of chromium(III) hydroxide to divalent chromium and metallic chromium. Roughness of surface topography increases.Follow-up explorations with scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), atomic-force microscopy (AFM), scanning tunnelling microscopy/scanning tunnelling spectroscopy (STM/STS) and X-ray photoelectron spectroscopy (XPS) not only evidence the formation of various chromium oxides, but also indicate the existence of chromium hydroxide.     

A novel route for synthesis of NiCoP/SiO2 hydrodesulfurization catalysts with active S species

We report a new route for synthesis of NiCoP/SiO₂ with active S species using NiS and CoS as precursors. The as‐prepared catalysts were characterized using X‐ray powder diffraction, N₂‐adsorption specific surface area measurements, inductively coupled plasma atomic emission spectroscopy, transmission electron microscopy, and X‐ray photoelectron spectroscopy (XPS). The catalyst was highly active during the dibenzothiophene hydrodesulfurization (HDS) of NiCoP/SiO₂. The XPS spectra of the as‐obtained samples show that the surface of NiCoP contains a small amount of S species, which may be responsible for the higher HDS activity.     

ToF-SIMS measurements with topographic information in combined images

In 2D and 3D time-of-flight secondary ion mass spectrometric (ToF-SIMS) analysis, accentuated structures on the sample surface induce distorted element distributions in the measurement. The origin of this effect is the 45° incidence angle of the analysis beam, recording planar images with distortion of the sample surface. For the generation of correct element distributions, these artifacts associated with the sample surface need to be eliminated by measuring the sample surface topography and applying suitable algorithms. For this purpose, the next generation of ToF-SIMS instruments will feature a scanning probe microscope directly implemented in the sample chamber which allows the performance of topography measurements in situ. This work presents the combination of 2D and 3D ToF-SIMS analysis with topographic measurements by ex situ techniques such as atomic force microscopy (AFM), confocal microscopy (CM), and digital holographic microscopy (DHM). The concept of the combination of topographic and ToF-SIMS measurements in a single representation was applied to organic and inorganic samples featuring surface structures in the nanometer and micrometer ranges. The correct representation of planar and distorted ToF-SIMS images was achieved by the combination of topographic data with images of 2D as well as 3D ToF-SIMS measurements, using either AFM, CM, or DHM for the recording of topographic data.     

Topography and Field Effects in Secondary Ion Mass Spectrometry – Part I: Conducting Samples

Quantitative chemical characterization of surfaces with topography by secondary ion mass spectrometry (SIMS) remains a significant challenge due to the lack of systematic and validated measurement methods. In this study, we combine an experimental approach using a simple model system with computer simulation using SIMION, to understand and quantify the key factors that give rise to unwanted topographic artefacts in SIMS images of conducting samples with microscale topography. Experimental data are acquired for gold wires (diameters 33 to 125 μm) mounted onto silicon wafers. Significant loss of ion intensities and shadowing arise from the distortion of the extraction field, and the chemical analysis over the whole of the sample surface is difficult. For large primary ion incidence angles of ≥55° to the surface normal, a fraction of the primary ions are scattered from the target and impact the substrate, emitting secondary ions that may be mistaken as originating from the wire. For conducting samples, topographic field effects may be reduced by the use of a smaller extraction voltage and an extraction delay. The effects of an extraction delay on ion intensities, mass resolution and time-of-flight are studied, and its application is demonstrated on an anisotropically etched silicon sample. The use of a simple sample holder with a V-shaped groove to reduce topographic field effects for wires is also presented. Using these results, we provide clear guidance to analysts for the diagnosis and identification of topography effects in SIMS, and present key recommendations to minimize them in practical analysis.     

Surface characterization and barrier properties of plasma‐modified polyethersulfone/layered silicate nanocomposites

This study describes the preparation of polyethersulfone (PES)/layered silicate nanocomposites (PLSNs) by mixing PES polymer chain into organically‐modified layered silicate in 1‐methyl‐2‐pyrrolidinone (NMP) solution. Both X‐ray diffraction data and transmission electron microscopy images of PLSNs indicate that the silicate layers were almost exfoliated and randomly distributed into the PES matrix. The mechanical and barrier properties of PLSNs show remarkable enhancement in the storage modulus and water/oxygen permeability when compared with that of neat PES matrix. Surfaces modification of PES and PLSN films with various treated times, system pressures, and radio frequency (RF) powers were performed using a mixture of oxygen (O₂) and nitrogen (N₂) plasmas. The topographical and physical properties of plasma‐modified PES and PLSN surfaces were investigated using scanning probe microscopy (SPM), contact‐angle measurements, and X‐ray photoelectron spectroscopy (XPS). These results indicate that the surface roughness of PLSNs with the same condition of plasma modification is lower than that of neat PES matrix and is probably due to the increase of stiffness with the presence of inorganic layered silicates in PES matrix. The surface properties of the PES and PLSNs are also changed from hydrophobic to hydrophilic. The XPS spectra suggest that the exposure of the PES and PLSNs to the plasmas led to the combination of etching reactions of polymer surface initiated by plasma and the following addition reactions of new oxygen‐ and nitrogen‐containing functional groups onto polymer surfaces to change their surface properties. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3185–3194, 2006     

Local surface charge dissipation studied using force spectroscopy method of atomic force microscopy

We propose herein a method to study local surface charge dissipation in dielectric films using force spectroscopy technique of atomic force microscopy. By using a normalization procedure and considering an analytical expression of the tip‐sample interaction force, we could estimate the characteristic time decay of the dissipation process. This approach is completely independent of the atomic force microscopy tip geometry and considerably reduces the amount of experimental data needed for the calculation compared with other techniques. The feasibility of the method was demonstrated in a freshly cleaved mica surface, in which the local charge dissipation after cleavage followed approximately a first‐order exponential law with the characteristic time decay of approximately 7–8 min at 30% relative humidity (RH) and 2–3.5 min at 48% RH. Copyright © 2015 John Wiley & Sons, Ltd.     

Nondestructive compositional depth profiling using variable‐kinetic energy hard X‐ray photoelectron spectroscopy and maximum entropy regularization

We discuss the calculation of nondestructive compositional depth profiles from regularization of variable kinetic energy hard X‐ray photoelectron spectroscopy (VKE‐XPS) data, adapting techniques developed for angle‐resolved XPS. Simulated TiO₂/Si film structures are analyzed to demonstrate the applicability of regularization techniques to the VKE‐XPS data and to determine the optimum choice of regularization function and the number of data points. We find that using a maximum entropy‐like method, when the initial model/prior thickness is similar to the simulated film thickness, provides the best results for cases where prior knowledge of the sample exists. For the simple structures analyzed, we find that only five kinetic energy spectra are necessary to provide a good fit to the data, although in general, the number of spectra will depend on the sample structure and noisiness of the data. The maximum entropy‐like algorithm is then applied to two physical films of TiO₂ deposited on Si. Results suggest interfacial intermixing. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.     

Effect of hydration of polyamide membranes on the surface electrokinetic parameters: surface characterization by x-ray photoelectronic spectroscopy and atomic force microscopy

The surface and the solid/liquid interface of two polyamide membranes, one experimental (B0) and one commercial (NF45), have been characterized by X-ray photoelectronic spectroscopy (XPS), atomic force microscopy (AFM), and zeta potential, respectively. The surface roughness, determined by AFM data analysis, is different for the two membranes, and results show that the commercial NF45 membrane presents a much lower roughness than the experimental B0 membrane. XPS data indicate that the surface of membrane NF45 is similar to that of pure polyamide, while membrane B0 contains a considerable amount of impurities. The homogeneity in depth of both membranes was also studied by determining the composition profile at different analysis angles. Streaming potential along the membrane surface or tangential streaming potential (TSP) measurements with NaCl solutions at different concentrations were carried out with both membranes to determine the zeta potential and the electrokinetic surface charge density, and a correlation between membrane surface and interface parameters is made. Some differences in atomic concentrations of membrane surface elements and X-ray photoelectronic spectra of the samples used in TSP measurements and after a drying process at 90 degrees C for 24 h can be observed when they are compared with those for fresh membranes. Electrokinetic parameters for membrane NF45 (TSP, zeta potential, and surface electrokinetic charge density) obtained from three different series of measurements strongly decrease as a result of membrane use, but for membrane B0 they are practically independent of the number of measurements. This difference in the electrokinetic behavior of the two membranes has been related to the hydration process of the surface for each sample studied by XPS and AFM.     

Site-selective EXAFS in mixed-valence compounds using high-resolution fluorescence detection: a study of iron in Prussian Blue

A quantitative analysis is presented for the site-selective Fe K-edge absorption spectra of Prussian Blue: Fe(4)[Fe(CN)(6)](3) x xH(2)O (x = 14-16). The site-selective spectra were recorded using high-resolution fluorescence detection of the K beta emission from a polycrystalline sample. The K beta fluorescence lines arising from the high-spin and low-spin sites are shifted in energy. Since the emission features partially overlap, fluorescence-detected absorption spectra using different emission energies represent different linear combinations of the pure high-spin and low-spin EXAFS. A numerical method was used to extract the individual site EXAFS spectra from the experimental data. The analysis yields a range of solutions. A unique solution can be obtained if homovalent model compounds are used to simulate the K beta fluorescence emission from the two Fe sites in Prussian Blue. EXAFS analysis of the range of spectra obtained in the numerical method yields almost identical interatomic distances for the different spectra while the Debye-Waller factors vary considerably. The distances obtained in the EXAFS fit correspond to the crystallographic distances.