Angle-resolved X-ray Photoelectron Spectroscopy Data Research Articles

Investigation of Oxide Thickness on Technical Aluminium Alloys—A Comparison of Characterization Methods

In this study the oxide layer of technical 6xxx aluminium surfaces, pickled as well as passivated, were comparatively investigated by means of transmission electron microscopy (TEM), Auger electron and X-ray photoelectron spectroscopy (AES, XPS), the latter in two different operating modes, standard and angle resolved mode. In addition, confocal microscopy and focused ion beam cutting were used for structural studies of the surfaces and for specimen preparation. The results illustrate in detail the strengths and weaknesses of each measurement technique. TEM offers a direct way to reliably quantify the thickness of the oxide layer, which is in the range of 5 nm, however, on a laterally restricted area of the surface. In comparison, for AES, the destructiveness of the electron beam did not allow to achieve comparable results for the thickness determination. XPS was proven to be the most reliable method to reproducibly quantify the average oxide thickness. By evaluating the angle resolved XPS data, additional information on the average depth distribution of the individual elements on the surface could be obtained. The findings obtained in this study were then successfully used for the investigation of the increase in the aluminium oxide thickness on technical samples during an aging test of 12 months under standard storage conditions.

Surface analysis insight note: Straightforward concentration depth profiling by angle‐resolved X‐ray photoelectron spectroscopy using a Tikhonov regularization algorithm

Angle‐resolved X‐ray photoelectron spectroscopy (ARXPS) is a technique used for depth‐dependent analysis in the near‐surface region of samples. Calculation of a concentration depth profile using ARXPS requires an inverse Laplace transform, which adds considerable complexity to the analysis. In this insight note, the Tikhonov regularization algorithm for depth profile reconstruction from ARXPS data is examined. The steps required to produce a concentration depth profile are provided. The discussion includes strategies that deal with elastic scattering, electron attenuation, choice of regularization terms and optimization of the regularization parameters. The method is implemented in a Microsoft Excel spreadsheet that allows users to calculate a depth profile for data collected at up to five angles for five different peak components.

Non-destructive depth profile evaluation of multi-layer thin film stack using simultaneous analysis of data from multiple X-ray photoelectron spectroscopy instruments

Non-destructive depth profile evaluation of multi-layer thin film stacks using simultaneous analysis of angle-resolved X-ray photoelectron spectroscopy data from multiple instruments is demonstrated. The data analysis algorithm, called the maximum smoothness method, was originally designed to handle data from a single XPS instrument with a single X-ray energy; in this work, the algorithm is extended to provide a simultaneous analysis tool which can handle data from multiple instruments with multiple X-ray energies. The analysis provides depth profiles for multilayer stacks that cannot be obtained by conventional data analysis methods. In this paper, metal multi-layer stack samples with total thickness greater than 10 nm are analyzed with the maximum smoothness method to non-destructively obtain depth profiles, with precise information on the chemical states of atoms in the surface layer (<2 nm) and the overall layer stack structure, which can only be obtained by analyzing the data from multiple instruments.

Non-destructive initial-profile-free depth profile evaluation of thin-film sample using angle-resolved X-ray photoelectron spectroscopy and profile smoothing regularization

A new data analysis method that offers a non-destructive evaluation of the depth profile of the sample from the angle-resolved X-ray photoelectron spectroscopy data is demonstrated. The analysis method is based on the smoothing of the depth profile and is constructed by a simple equation compared to conventional methods. It does not require pre-existing information about the profile. The unnecessity is convenient in many scenes in material science. Details of the theory, including the techniques to omit the necessity of the initial value in the analysis process, are explained. The new method has been applied to SiN thin film samples to show that the reasonable depth profile can be actually obtained without pre-existing information about the depth profile.

Dry adhesion study of polyester/melamine clear coats on galvanized steel

The dry adhesion strength of polyester/melamine clear coats varying in their branching degrees were applied on galvanized steel panels and investigated by pull-off and T-bend testing. It was found that pull-off tensile adhesion is mainly dominated by the type of coating and less influenced by the pretreatment of the substrate. In contrast, the critical T-bend strain is mainly affected by the surface treatment of the galvanized layer with higher stiffness but lower ductility compared to the clear coat. Pull-off tensile strength at room temperature is correlating with the glass transition temperature Tg and a strong viscoelastic contribution is ascribed to the work of adhesion. Samples with adhesive as well as cohesive failure modes were further investigated by X-ray photoelectron spectroscopy (XPS) to determine the interfacial chemistry. Angle-resolved XPS data suggest that the nitrogen from the melamine crosslinker plays a decisive role for the adhesion of the coatings. Regarding the melamine distribution a micro- and a nanostructural effect has to be considered: while maximum melamine concentrations over coating depth are usually found in the bulk region of these clear coats, this work proposes that the remaining melamine at the interface segregates towards the metallic substrate.

Platinum atomic layer deposition on metal substrates: A surface chemistry study

The surface chemistry associated with the atomic layer deposition (ALD) of platinum films on metallic nickel substrates using trimethyl(methylcyclopentadienyl)platinum(IV) (MeCpPtMe3) and oxygen was characterized by using X-ray photoelectron spectroscopy (XPS). The uptake of the MeCpPtMe3 ALD precursor was found to be self-limiting between 525 and 625 K, but to lead to multilayer deposition at 675 K. The adsorbed species display a C:Pt ratio of approximately 5:1, suggesting that all methyl moieties are lost upon activated bonding to the surface but that the methylcyclopentadienyl group remains coordinated to the adsorbed Pt atoms. Oxygen treatment of that surface leads to the complete removal of the carbon-containing species from the surface and to the formation of a thin NiO film. Further dosing with MeCpPtMe3, in the next ALD cycle, fully reduces that NiO film to metallic Ni(0) and adds more Pt to the surface. However, no Pt film buildup was seen after several ALD cycles. Angle-resolved XPS data suggested that the deposited Pt migrates below the NiO that forms during the O2 exposures and possibly alloys with the metallic Ni substrate at that stage.

Nanostructure of Surface Films on Ni18P Alloy in Sulfate Solutions by the Maximum Entropy Method.

NiP alloys are very often used in industry, due to their outstanding performance in corrosion and wear. Alloys with high phosphorus content (≥16 atom % P) are amorphous and show high corrosion resistance in both neutral and acidic solutions irrespective of the presence of chloride ions. The reason for this behavior is attributed to the formation of a “P-enriched surface layer” whose exact nature is still under debate. In this work, an iterative algorithm based on the application of maximum entropy method on nondestructive angle-resolved X-ray photoelectron spectroscopy data has been applied to the investigation of the surface layer grown on Ni18P alloys following mechanical polishing and anodic polarization in sulfate solutions. The results show that the outermost region of the examined alloy has a complex layered structure: (1) an uppermost hydrocarbon contamination layer about 1 nm thick, which includes also adsorbed water; (2) a nickel (poly)phosphate layer of about 1 nm; (3) a highly phosphorus-enriched interface being about 2 nm thick with a marked phosphorus concentration gradient, from 70 to 20 atom %; and (4) bulk alloy with the stoichiometric composition. These findings, together with the chemical state of the different phosphorus compounds, allow us to conclude that the high corrosion and wear resistance of NiP alloys might be ascribed to the presence of a thin, self-repairing nickel (poly)phosphate layer grown on a strongly P-enriched interface. Because the Auger parameter of P at the interface is similar to that of elemental P, it might be also concluded that the interface is enriched in elemental phosphorus.

Redox chemistry, solubility, and surface distribution of Pt(II) and Pt(IV) complexes dissolved in ionic liquids

An X-ray photoelectron spectroscopy (XPS) study of Pt(II) and Pt(IV) complexes dissolved in ionic liquids (ILs) revealed the occurrence of a rich redox chemistry, which is partly induced by X-ray radiation. Depending on the IL anion either a reduction of Pt(IV) to Pt(II) in the case of ethylsulfate ([EtOSO3]−)-based ILs, or an oxidation of Pt(II) to Pt(IV) in the case of bis[(trifluoromethyl)sulfonyl]imide ([Tf2N]−)-based ILs was observed upon X-ray irradiation. Moreover, surface enrichment or depletion effects of the Pt complexes were deduced from angle-resolved XPS (ARXPS) data; while the Pt complexes dissolved in imidazolium-based ILs tended to deplete from the surface, in pyrrolidinium-based solutions a surface enrichment of the Pt complexes was observed.

SiON metrology using angular and energy distributions of photoelectrons

Angle-resolved X-ray photoelectron spectroscopy (ARXPS) is a useful tool for non-destructive in-depth analysis of near surface regions. However, the reconstruction of depth profile from ARXPS data is an ill-posed mathematical problem. Thus, the main goal of this work was to develop a new, iterative algorithm based on the least square fitting which allows to solve this problem. The depth profiles were restored by dividing sample in thin virtual box shaped layers each with a different concentration. To extract information on the depth distribution, this algorithm is based on the analysis of the angular peak intensities along with the inelastic background. In addition, the physically trivial constraint of atomic fractions adding up to unity was imposed. The model takes into account the effect of elastic scattering and anisotropy of the photoelectric cross section. To test the algorithm, experimental spectrum for SiON samples on Si substrate were measured with a Thermo Theta Probe electron spectrometer for off-normal emission angles in the range between 25° and 75°. A very good agreement was found between the measured spectra and obtained spectra from the algorithm.

Temperature dependence study of Pd–Cu supported bimetallic films by photoelectron spectroscopy and cyclic voltammetry

Vapor deposition method is used to prepare layered Pd–Cu bimetallic films, with different thickness of Pd over a Cu film supported on glassy carbon. X-ray photoelectron spectroscopy (XPS) measurements of core level binding energy (BE) and valence band region are used to investigate the contribution of charge transfer resulting from the bonding of these two dissimilar metals in a layered structure. As this layered bimetallic film is annealed at increasing temperature from 298 to 650K, atomic inter-diffusion occurs to form an alloy. We differentiate this interfacial charge transfer effect between these two dissimilar metals from that of surface alloying resulting from the annealing effect. Cyclic voltammetry (CV) tests combined with XPS confirm that (1) the surface is free of Cu atoms for Pd films with thicknesses between 2.0 and 7.0±0.3nm at room temperature and (2) Pd–Cu inter-diffusion propagates to the surface and the formation of surface alloys take place at temperature greater than 350K. For these Pd thicknesses, the difference in BE shifts of Cu 2p and Pd 3d peaks between layered and alloyed structures are +0.16eV and −0.17eV, respectively. This difference in BE shifts allows for a clear distinction between these two bimetallic structures and the tailoring of the optimum configuration that enhances catalytic activity toward formic acid. Additionally, angle resolved XPS data as a function of temperature and XPS depth profiling at room temperature provide insight about the inter-diffusion length. Furthermore, CV measurements in H2SO4 and formic acid confirm that for a Pd–Cu bimetallic surface with a 3.8±0.3nm Pd film over Cu shows significantly improved activity toward formic acid oxidation compared to bulk Pd at 298K and after being annealed at temperature of 350K.

Instrument-related geometrical factors affecting the intensity in XPS and ARXPS experiments

The angular dependence of the X-ray photoelectron spectroscopy (XPS) signal is influenced not only by the electron take-off angle, but also by instrument-related geometrical factors. The XPS signal is, in fact, integrated over the overlap between the X-ray beam, the spectrometer analysis volume, and the sample surface. This overlap depends on the size and shape of the spectrometer analysis volume and X-ray beam, as well as on their relative orientation. In this paper it is described the models and protocols for the characterization of the parameters defining the geometry of an XPS instrument. The protocols include practical methods for assessing the spectrometer analysis area and the X-ray beam spot dimension. Simple systems consisting of flat and “thick” gold films on silicon wafers were employed. The parameters found with those samples are transferable to other more complex systems since they are geometrical in nature. The method allows for the prediction of the actual intensity of XPS peaks, hence removing the need of normalizing the peak areas to the area of a determined substrate peak. The associated reduction of the uncertainty in half is of special importance since the quantitative analysis of angle-resolved XPS data could be very sensitive to noise. Two rotating and one non-rotating XPS instruments are described. Some examples of the applications of the method are also provided.

Liquid phase formation of alkyl- and perfluoro-phosphonic acid derived monolayers on magnesium alloy AZ31 and their chemical properties

Alkyl- and perfluoro-phosphonic acid derived SAMs were successfully formed on Mg alloy by liquid phase method for the first time. The chemical and anticorrosive properties of the prepared SAMs on magnesium alloys were characterized using contact angle measurements, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electrochemical measurements. Water contact angle measurements revealed that the maximum advancing/receding water contact angles of n-octyl (OP: CH3(CH2)7PO(OH)2), n-dodecyl (DP: CH3(CH2)11PO(OH)2), n-octadecyl (ODP: CH3(CH2)17PO(OH)2) phosphonic acid, and 2-(perfluorohexyl)ethyl (PFEP: CF3(CF2)5CH2CH2PO(OH)2) phosphonic acid were 105.1/64.7°, 108.3/69.6°, 111.9/75.2°, and 115.2/67.4° respectively. In the case of alkylphosphonic acid SAMs (OP, DP, and ODP), the advancing and receding water contact angles increased with an increase in the preparation time. The angle-resolved XPS (AR-XPS) data revealed that the film thicknesses of the OP, DP, ODP, PFEP on Mg alloy were estimated to be 0.8, 1.2, 1.7, and 1.1nm, respectively. The XPS O 1s data support that the phosphonic acid derived SAM is covalently bound to the oxide or hydroxide surface of the Mg alloy in a monodenate or bidenate manner. Chemical stability of the alkyl- and perfluoro-phosphonic acid modified Mg alloy surfaces was investigated using aqueous solutions at pH=4.0, 7.0, and 10.0. The contact angles of OP, DP, and PFEP modified Mg surface decreased rapidly within the first 5min after immersion in all the aqueous solutions and were less than 20°. On the other hand, the contact angles of the ODP modified Mg alloy after immersion in aqueous solutions at pH 4, 7 and 10 for 5min were 45.1°, 89.3,° and 85.5°, respectively. The ODP modified Mg alloy had highest chemical stability in four types of the phosphonic acid derived SAMs used in this study, indicating that the molecular density of ODP on Mg alloy would be higher than those of OP, DP, PFEP on Mg alloy. The corrosion resistance of ODP modified Mg alloy was investigated by potentiodynamic polarization curve measurements. The ODP modified Mg alloy exhibits protective properties in a solution containing Cl− ions compared to unmodified Mg alloy.

Encoding of stoichiometric constraints in the composition depth profile reconstruction from angle resolved X‐ray photoelectron spectroscopy data

The inversion of angle resolved XPS data is a difficult problem, due to mathematical complexity of accurate model of electron transport, sensitivity to noise and small number of measured points limiting the depth resolution and the accuracy of the calculated depth profiles. Extended maximum entropy model for the calculation of apparent compositions (measured intensity ratios) is presented which allows to encode linear relationship between element concentrations. These can be used to calculate a solution with additional stoichiometric relationships and analyze, simultaneously, signals from several synthetic peak components. The synthetic peak components used in the model can represent various core level energy shifts, which are otherwise difficult to determine unambiguously from the measured signal. Element concentrations linked with stoichiometric relationships are used to identify compound materials with known density and inelastic scattering properties. The new model, thus allows self‐consistent recovery of depth profiles using maximum entropy method without the assumption of homogeneous density. The improved accuracy of the recovered depth profiles is demonstrated on the analysis of HfON/SiON overlayers. Various artefacts in the depth profiles obtained with standard maximum entropy model, which assumes constants density are identified. Copyright © 2011 John Wiley &amp; Sons, Ltd.

A promising concept for using near-surface measuring angles in angle-resolved x-ray photoelectron spectroscopy considering elastic scattering effects

The increasing number of applications of very thin films requires both reliable thin-layer and interface characterization. A powerful method for characterization in the nanometer thickness range is the angle-resolved x-ray photoelectron spectroscopy (ARXPS). This is a nondestructive depth-profiling method, which can provide elemental content as well as chemical information. Two of the drawbacks of ARXPS are, that it requires dedicated mathematical modeling and that, at least up until now, its use has been restricted away from near-surface angles. In this paper we present a method for the mathematical description of a few, hitherto unaccounted, measurement effects in order to improve the simulations of ARXPS data for complex surface structures. As an immediate application, we propose a simple algorithm to consider the effects of elastic scattering in the standard ARXPS data interpretation, which in principle would allow the use of the whole angular range for the analysis; thus leading to a significant increase in the usable information content from the measurements. The potential of this approach is demonstrated with model calculations for a few thin film examples.

Self-assembled monolayers of flufenaminate anions on mild steel surface formed in aqueous solution

Adsorption of derivative of phenylanthranilic acid – flufenamic acid (FFA) on the “oxide-free” and oxidized surface of mild steel in neutral borate buffer solution was studied by ellipsometry and XPS. Anodic polarization curves reveal that complete suppression of the anodic dissolution of iron is achieved at FFA concentration C in = 3.8 mM. Besides, adding FFA substantially shifts the pitting potential from 0.06 V to 0.67 V. Ellipsometric studies have shown that at the applied potential −0.65 V, when the surface is free from the oxide layer, FFA forms monomolecular layer. To characterize the surface layers formed after exposing the sample in 5 mM FFA solution the XPS was used to assess the composition and the thickness of the layers. Using the intensities of the Fe 2p, Fe 3p, N 1s, F 1s, O 1s and C 1s and analyzing the angle resolved XPS data the FFA molecules have been shown to form monomolecular layer in which FFA is (vertically or slightly inclined) anchored by iron cations through oxygen atoms of carboxyl group to the surface and the fluorine atoms of CF 3 groups form the utmost layer. Similar orientation is also assumed for FFA molecules adsorbed on the oxidized iron surface. It seems that the layer formed by FFA or similar molecules may serve a robust interface for grafting other substances on such a functionalized surface.

Surface-directed spinodal decomposition in the pseudobinary alloy (HfO2)x(SiO2)1−x

Hf silicate films (HfO2)0.25(SiO2)0.75 with thicknesses in the range 4–20 nm were grown on silicon substrate by atomic layer deposition at 350 °C. Hf distributions in as-grown and 800 °C annealed films were investigated by high resolution transmission electron microscopy (HRTEM), angle-resolved x-ray photoelectron spectroscopy (ARXPS), and medium energy ion scattering (MEIS). HRTEM images show a layered structure in films thinner than 8 nm. The ARXPS data also reveal a nonuniform distribution of Hf throughout the film depth. Diffusion of SiO2 to the film surface after a longer time anneal was observed by MEIS. All these observations provide evidence for surface-directed spinodal decomposition in the pseudobinary (HfO2)x(SiO2)1−x alloy system.

Structure of Ultra-Thin Diamond-Like Carbon Films Grown with Filtered Cathodic Arc on Si(001)

The structure of 3 nm and 15 nm diamond-like carbon films, grown on Si(001) by filtered cathodic arc, was studied by angle-resolved X-ray photoelectron spectroscopy (ARXPS) and transmission electron microscopy (TEM). The ARXPS data was deconvolved by employing simultaneous-fitting, which allowed for a clear deconvolution of the Si 2p and C 1s spectra into their different chemical contributions. An analysis of the take-off angle dependence of the peak intensities allowed for an independent identification of the physical origin of the chemical species. It was shown that the C 1s peak at 283.3 eV and the Si 2p peak at 99.6 eV correspond to SiC, and that the C/Si interface of the 3 nm film consists of a stoichiometric approximately 1 nm SiC layer. To quantify the sp(3)-sp(2) ratio it was necessary to take into account not only their associated C 1s XPS-peak intensities, but also their take-off angle dependence. The thickness of the films obtained through ARXPS closely agrees with cross-sectional TEM images.

Influence of the aliphatic chain length of imidazolium based ionic liquids on the surface structure

The molecular surface structure of four imidazolium based ionic liquids was studied with two surface sensitive techniques. Angle resolved neutral impact collision ion scattering spectroscopy (ARNICISS) allows us to determine elemental concentration depth profiles and to obtain information about the topography of the surface. Angle resolved X-ray photoelectron spectroscopy (ARXPS) can be used to study the chemical composition of the surface. The room temperature ionic liquids (RTILs) 1-ethyl-3-methylimidazolium [EMIM], 1-butyl-3-methylimidazolium [BMIM], 1-hexyl-3-methylimidazolium [HMIM], and 3-methyl-1-octylimidazolium [OMIM] bis(trifluoromethylsulfonyl)imide [Tf(2)N] were investigated at 293 K. No evidence of surface active impurities was observed. The majority of previous studies about these RTILs with ARXPS or other surface sensitive techniques dealt only with single examples of these substances or with different combination of the anion and cation. In this present study a homologous series of the four RTILs mentioned above was investigated. This means that only the number of carbon atoms in the aliphatic chain of the cation is varied. Due to this procedure it is possible to study the influence of the chain length, which is a part of the imidazolic ring, on the composition of the surface and the surface near region. In this paper we demonstrate the potential of ARNICISS as a surface sensitive technique to study the surface structure of the RTILs. Furthermore, we combine our NICISS data with ARXPS data, to get a better comprehension of the influence of the aliphatic chain length. After the presentation of the results we develop a model of the surface structure of different RTILs. We have discovered two different surface structures that depend on the number of carbon atoms inside the aliphatic chain.

Application of angle-resolved X-ray photon electron spectroscopy for interface and layer growth studies demonstrated on Ti/Ta-based films deposited on SiO2

X-ray photoelectron spectroscopy (XPS) is one of the main methods for elemental and bonding-state characterization in the near-surface region. Applying it as angle-resolved XPS (ARXPS), it is promising for non-destructive depth profiling in the nanometre-thickness range. The challenges for the application of ARXPS are discussed for the early film growth of Ti and Ta on SiO(2) produced by in situ magnetron sputtering. The measurements showed several phase-formation processes at the interfaces, which were used as the basis for the ARXPS data analysis and resulted in different interlayer growth information for Ti and Ta. From a systematic ARXPS data interpretation in combination with simulations, limits of the method are critically discussed.

Structure of Hafnium Silicate Films Formed by Atomic Layer Deposition

Hf distributions in as-grown and annealed (HfO2)0.25(SiO2)0.75 films with thicknesses in the range 4-13 nm were investigated by high resolution transmission electron microscopy (HRTEM), angle resolved X-ray photoelectron spectroscopy (ARXPS) and medium energy ion scattering (MEIS). HRTEM images show a layered structure in films thinner than 8 nm. ARXPS data also show a non-uniform distribution of Hf throughout the film depth. A diffusion of SiO2 to the film surface after a longer time anneal was observed by MEIS. All these observations provide evidence for surface-directed spinodal decomposition in the pseudobinary (HfO2)x(SiO2)1-x alloy system.