Angle-resolved X-ray Photoelectron Spectroscopy Measurements Research Articles

Structure-Related Electronic and Magnetic Properties in Ultrathin Epitaxial NixFe3-xO4 Films on MgO(001).

Off-stoichiometric NixFe3-xO4 ultrathin films (x < 2.1) with varying Ni content x and thickness 16 (±2) nm were grown on MgO(001) by reactive molecular beam epitaxy. Synchrotron-based high-resolution X-ray diffraction measurements reveal vertical compressive strain for all films, resulting from a lateral pseudomorphic adaption of the film to the substrate lattice without any strain relaxation. Complete crystallinity with smooth interfaces and surfaces is obtained independent of the Ni content x. For x < 1 an expected successive conversion from Fe3O4 to NiFe2O4 is observed, whereas local transformation into NiO structures is observed for films with Ni content x > 1. However, angle-resolved hard X-ray photoelectron spectroscopy measurements indicate homogeneous cationic distributions without strictly separated phases independent of the Ni content, while X-ray absorption spectroscopy shows that also for x > 1, not all Fe2+ cations are substituted by Ni2+ cations. The ferrimagnetic behavior, as observed by superconducting quantum interference device magnetometry, is characterized by decreasing saturation magnetization due to the formation of antiferromagnetic NiO parts.

Atomistic Understanding of the Formation, Structure, and Decomposition of an Fe4C Iron Carbide Phase on a Copper Substrate

Having an atomistic understanding of the formation and structural characteristics of bulk and surface iron carbide phases plays a crucial role in comprehending the mechanisms of phase transformation, microstructure formation, and surface reconstructions in the development of advanced materials with improved magnetic, mechanical, and catalytic properties. This study uses synchrotron (SR) and angle-resolved (AR) XPS, STM, LEED, and theoretical calculations (DFT) with the aim to provide a detailed experimental and theoretical discussion on the formation, stabilization, structure, and decomposition of Fe4C iron carbide. FCC(100) Fe4C iron carbide multilayer films were prepared on Cu(100) by evaporating iron in the presence of an ethylene atmosphere. Angle-resolved XPS measurements reveal that surface and interior carbon can be distinguished due to their appearance at different binding energies and reveal that the surface consists of Fe2C while the bulk has Fe4C stoichiometry. LEED and STM measurements show that the surface exhibits the p4g(2 × 2) clock reconstruction. Theory simulations show that the bulk Fe4C iron carbide has a constrained crystal lattice with alternating Fe and Fe4C2 layers, which grow in a pseudomorphic way on the copper. The carbide phase is stable up to 700 K. Above this temperature, iron diffuses into the copper while carbon remains on the surface, where it forms graphite.

Band energy diagrams of n-GaInP/n-AlInP(100) surfaces and heterointerfaces studied by X-ray photoelectron spectroscopy

Lattice matched n-type AlInP(100) charge selective contacts are commonly grown on n-p GaInP(100) top absorbers in high-efficiency III–V multijunction solar or photoelectrochemical cells. The cell performance can be greatly limited by the electron selectivity and valance band offset at this heterointerface. Understanding of the atomic and electronic properties of the GaInP/AlInP heterointerface is crucial for the reduction of photocurrent losses in III–V multijunction devices. In our paper, we investigated chemical composition and electronic properties of n-GaInP/n-AlInP heterostructures by X-ray photoelectron spectroscopy (XPS). To mimic an in-situ interface experiment with in-situ stepwise deposition of the contact material, 1 nm –50 nm thick n-AlInP(100) epitaxial layers were grown on n-GaInP(100) buffer layer on n-GaAs(100) substrates by metal organic vapor phase epitaxy. We observed (2 × 2)/c(4 × 2) low-energy electron diffraction patterns with characteristic diffuse streaks along the [011¯] direction due to PP dimers on both AlInP(100) and GaInP(100) as-prepared surfaces. Atomic composition analysis confirmed P-rich termination on both surfaces. Angle-resolved XPS measurements revealed a surface core level shift of 0.9 eV in P 2p peaks and the absence of interface core level shifts. We assigned the surface chemical shift in the P 2p spectrum to PP bonds on a surface. We found an upward surface band bending on the (2 × 2)/c(4 × 2) surfaces most probably caused by localized mid-gap electronic states. Pinning of the Fermi level by localized electronic states remained in n-GaInP/n-AlInP heterostructures. A valence band offset of 0.2 eV was derived by XPS and band alignment diagram models for the n-n junctions were suggested.

Combining advanced photoelectron spectroscopy approaches to analyse deeply buried GaP(As)/Si(1 0 0) interfaces: Interfacial chemical states and complete band energy diagrams

The epitaxial growth of the polar GaP(100) on the nonpolar Si(100) substrate suffers from inevitable defects at the antiphase domain boundaries (APDs), resulting from mono-atomic steps on the Si(100) surface. Stabilization of Si(100) substrate surfaces with As is a promising technological step enabling the preparation of Si substrates with double atomic steps and reduced density of the APDs. In this paper, 4–50-nm-thick GaP epitaxial films were grown on As-terminated Si(100) substrates with different types of doping, miscuts, and As-surface termination by metalorganic vapor phase epitaxy (MOVPE). The GaP(As)/Si(100) heterostructures were investigated by X-ray photoelectron spectroscopy (XPS) combined with gas cluster ion beam (GCIB) sputtering and by hard X-ray photoelectron spectroscopy (HAXPES). We found residuals of As atoms in the GaP lattice (∼0.2–0.3 at.%) and a localization of As atoms at the GaP(As)/Si(100) interface (∼1 at.%). Deconvolution of core level peaks revealed interface core level shifts. In As core levels, chemical shifts between 0.5 and 0.8 eV were measured and identified by angle-resolved XPS measurements. Similar valence band offset (VBO) values of 0.6 eV were obtained, regardless of the doping type of Si substrate, Si substrate miscut or type of As-terminated Si substrate surface. The band alignment diagram of the GaP(As)/Si(1 0 0) heterostructure was deduced.

From high temperature phase formation to transition metal substitution in the Fe/Al9Co2(001) system

We report the formation of several complex intermetallics as surface alloys upon the adsorption of Fe on Al9Co2(001) for different dosing conditions. Up to 4 monolayers equivalent (MLE) of Fe deposited on the substrate held at 593 K, the low energy electron diffraction pattern consists of a (2×2)R45° phase with two additional domain types rotated by ±8∘ from it. The scanning tunneling microscopy (STM) measurements show that these three types of domains have the same crystallographic structure. The lattice parameters and structural motifs point towards the formation of the high-temperature Al8Fe5 phase (γ-brass of Zn8Cu5-type structure). For Fe deposition between 593 K and 873 K, we have identified the formation of two phases, tentatively assigned to a ternary disordered Al9(Co,Fe)2 overlayer and to the monoclinic Al13Fe4(100). The stoichiometric evolution of the grown structures have been characterized by angle-resolved x-ray photoelectron spectroscopy measurements. Density functional theory based calculations have been performed to model the Al8Fe5(100)/Al9Co2(001) interface, its structural stability and to simulate the corresponding STM images.

Effects of polishing pressure and sliding speed on the material removal mechanism of single crystal diamond in plasma-assisted polishing

Plasma-assisted polishing (PAP) was confirmed to be high-efficiency and high-quality when applied to single crystal diamond (SCD) substrates. The effects of polishing pressure applied to the SCD substrate and sliding speed between the polishing plate and SCD substrate in PAP were systematically investigated in this study. Higher polishing pressures or sliding speeds resulted in a higher material removal rate (MRR) of SCD substrate, and the highest MRR achieved 5.3 μm/h. In the case of PAP conducted at low polishing pressures such as 62.5, 81.3 kPa, scanning white light interferometer (SWLI) (84-μm square) and atomic force microscope (AFM) (5-μm square) measurements revealed an atomic-scale smooth surface without the surface texture depending on the crystal direction and the lowest Sq roughness of 0.3 nm (84-μm square) was achieved. Correspondingly, at low polishing pressure of 62.5 kPa, MRRs in PAP along the <100> and <110> directions were nearly identical, suggesting that isotropic removals occurred. In the case of PAP performed at high polishing pressures such as 143.8, 246.9, and 350.0 kPa, SWLI and AFM measurements revealed a rough surface with bands of grooves along <100> direction. Correspondingly, at high polishing pressures of 246.9 and 350.0 kPa, MRRs in PAP along <100> direction were ~11 and ~12 times faster than that in <110> direction, respectively, suggesting that anisotropic removals occurred. Moreover, scanning transmission electron microscopy and angle-resolved X-ray photoelectron spectroscopy measurements confirmed that no damage or non-diamond layer was present on the PAP-processed SCD substrate. However, as the sliding speed varied, both SWLI and AFM measurements demonstrated that Sq roughness remained nearly constant with the same polishing pressure applied to the SCD substrate.

Combining Advanced Photoelectron Spectroscopy Approaches to Analyse Deeply Buried Gap(As)/Si(100) Interfaces: Interfacial Chemical States and Complete Band Energy Diagrams

The epitaxial growth of the polar GaP(100) on the nonpolar Si(100) substrate suffers from inevitable defects at the antiphase domain boundaries, resulting from mono-atomic steps on the Si(100) surface. Stabilization of Si(100) substrate surfaces with arsenic is a promising technological step enabling the preparation of Si substrates with double atomic steps and reduced density of the APDs. In this paper, 4-50 nm thick GaP epitaxial films were grown on As-terminated Si(100) substrates with different types of doping, miscuts, and As-surface termination by metalorganic vapor phase epitaxy. The GaP(As)/Si(100) heterostructures were investigated by X-ray photoelectron spectroscopy (XPS) combined with gas cluster ion beam (GCIB) sputtering and by hard X-ray photoelectron spectroscopy (HAXPES). We found residuals of arsenic atoms in the GaP lattice (0.2-0.3 at.%) and a localization of As atoms at the GaP(As)/Si(100) interface (1 at.%). Deconvolution of core level peaks revealed interface core level shifts. In As core levels, chemical shifts between 0.5-0.8 eV were measured and identified by angle-resolved XPS measurements. Similar valence band offset (VBO) values of 0.6 eV were obtained, regardless of the doping type of Si substrate, Si substrate miscut or type of As-terminated Si substrate surface. The band alignment diagram of the heterostructure was deduced.

The (110) and (320) surfaces of a Cantor alloy

The (110) and (320) surfaces of the single-phase FeCrMnNiCo solid solution have been studied on two adjacent millimeter size grains using surface science and transmission electron microscopy (TEM) techniques. The structural and chemical evolutions of the high entropy alloy (HEA) surfaces have been determined for various sputtering conditions, annealing temperatures and durations. Up to 873 K, angle-resolved X-ray photoelectron spectroscopy measurements indicate a clear Mn and Ni surface co-segregation. We propose that the surface segregation of Mn is driven by its low surface energy. The attractive interaction between Mn and Ni promotes Ni segregation which accompanied the Mn diffusion to the surface. Regarding the structures investigated by low energy electron diffraction and scanning tunneling microscopy, the (320) surface presents a terraced morphology with an ordered structure consistent with a (1×1) termination. On the contrary, the (110) surface reveals an important degree of structural disorder and local reconstructions. Its highly anisotropic morphology resembles rows propagating along the [001] direction. Above 873 K, Mn desorption occurs while the Ni content keeps increasing linearly with the temperature. TEM analysis show no evidence for HEA decomposition into metallic or intermetallic phases even after repeated annealing and sputtering cycles. The above results set the upper temperature limit above which the surface stoichiometry departs from the quinary HEA concept. It also defines the temperature range for the use of FeCrMnNiCo based coating under high vacuum conditions and for aerospace applications.

Surface Propensity of Anions in a Binary Ionic-Liquid Mixture Assessed by Full-Range Angle-Resolved X-ray Photoelectron Spectroscopy and Surface-Tension Measurements.

Angle-resolved X-ray photoelectron spectroscopy and contact-angle measurements guided by a signal attenuation model are utilized to extract molar composition and anion enrichment in the vacuum interface of a binary ionic liquid mixture, having a common quaternary ammonium cation and two different anions. By using the intensity ratio of the F1s peaks belonging to the two different anions recorded at the full electron take-off angle range, from 0° to 80°, we have determined that only a fractionally covered and anion enriched surface layer can predict the AR-XPS data, which is also consistent with surface tension measurements. Moreover, the more bulky and non-spherical anion enrichment is evident even at the conventional and the so assumed bulk sensitive take-off angle of 0°. This methodology provides a surface enrichment factor of the molecular ions and clearly serves as an experimental evidence for recently debated surface layering and/or island structure in ionic liquid systems.

Evaluation of Silicon Nitride Film Formed by Atomic Layer Deposition on the Silicon Substrate with Trench Structure Using Angle-Resolved Hard X-ray Photoelectron Spectroscopy

We evaluated the silicon nitride (SiN) films fabricated by plasma enhanced atomic layer deposition (PEALD) on the trench Si substrate. The transmission electron microscopy (TEM) observation showed that the SiN film was conformally deposited along the trench. However, from the angle-resolved hard X-ray photoelectron spectroscopy (HAXPES) measurements, the chemical bonding states in the SiN film on the trench sidewall from top to the bottom were different. On the sidewall of the trench, we found that N deficiency was generated in the SiN film, resulting in the non-stoichiometric SiN film formation. The precursor energy might not be sufficient and deactivated in the deep trench. The stoichiometric SiN film was deposited on the sidewall of the deep trench by increasing the deposition temperature.

Fluorinated vs. Zwitterionic-Polymer Grafted Surfaces for Adhesion Prevention of the Fungal Pathogen Candida albicans.

Fluorinated (F6) and zwitterionic, as well as phosphorylcholine (MPC) and sulfobetaine (MSA), copolymers containing a low amount (1 and 5 mol%) of 3-(trimethoxysilyl)propyl methacrylate (PTMSi) were prepared and covalently grafted to glass slides by using the trimethoxysilyl groups as anchorage points. Glass-surface functionalization and polymer-film stability upon immersion in water were proven by contact angle and angle-resolved X-ray photoelectron spectroscopy (AR-XPS) measurements. Antifouling performance of the grafted films was assayed against the yeast Candida albicans, the most common Candida species, which causes over 80% of candidiasis. Results revealed that the F6 fluorinated, hydrophobic copolymers performed much better in reducing the adhesion of C. albicans, with respect to both corresponding zwitterionic, hydrophilic MPC and MSA counterparts, and were similar to the glass negative control, which is well-known to inhibit the adhesion of C. albicans. A composition-dependent activity was also found, with the films of copolymer with 99 mol% F6 fluorinated co-units performing best.

Plasma Surface Preparation of III-V Materials: Quasi In-Situ XPS Measurements and Integration Guidelines

Obtaining a competitive III-V laser made with a CMOS-compatible process is a major issue in the framework of Silicon photonics. The choice of the material used to elaborate an ohmic contact for III-V substrate is quite limited and fabrication processes need to be developed [1, 2]. In this way, the surface preparation of the substrate before any deposition is the first step necessary towards success [3]. The native oxides need to be removed and the surface should not be damaged. A wet cleaning is used to remove the oxides and contaminants on the substrate surface, but the oxide regrowth occurs during the vacuum break undergone between the wet cleaning and the deposition reactor. Therefore, in-situ plasma cleaning is a mandatory complement in order to remove the substrate oxides before the deposition [3]. Helium and Argon plasma treatments have been used in this study on InGaAs and InP materials. Quasi in-situ analysis is ensured with the use of a vacuum wafer carrier [4]. Inbulk, InOx, InIn-In, Pbulk and POx concentrations have been extracted from In 3d5/2 and P 2p3/2 core level energy regions [5, 6]. Figure 1 shows the parallel angle-resolved XPS (pAR-XPS) measurements of the P 2p energy region of an InP substrate after an He plasma. Transport to the XPS have been completed with and without the use of the vacuum wafer carrier. Measurements have been performed at a collection angle of 55 ± 5° corresponding to the middle range (roughly 2-3 nm). Pbulk and Pox proportions correspond to the sum of the two doublets 3/2 and 1/2 of the phosphorus core level energy region respectively for the P-In and P-O bindings. Pox proportion is higher when an air vacuum break of 30 minutes occurs (Fig.1) whereas this concentration is almost zero using the vacuum wafer carrier. Thanks to the use of quasi in-situ XPS analysis, the removal of Pox (Fig.1) on InP is clearly demonstrated which was not the case in a previous study where an air vacuum break was realized between the surface treatment and the XPS analysis [3]. Thus, quasi in-situ characterization improves the accuracy of the results notably by limiting the oxides regrowth occurring during the wafer transfer (Fig. 1). Figure 2 and 3 show a summary of pAR-XPS measurements realized on InP samples after various plasma treatments. XPS spectra have been collected at angles of 25 ± 5° and 75 ± 5°, corresponding to photoelectrons escaping from the bulk surface (roughly 10 nm) (Fig.2) and the surface (roughly 1 nm) (Fig.3) . Two Argon plasma conditions were used. “Ar plasma soft” corresponds to the less powerfull recipe. InP substrate “as-received” was measured to determine oxide proportions of both elements, 21% for POx and 68.1% for InOx. Looking at the deep surface measurements (Fig.2), all plasma exhibit interesting results for the oxide removal. In-In bonds are created during the cleaning and may be assimilated to indium agglomerates at the InP surface [3]. Comparison with results in the near surface (Fig.3) shows that “Ar plasma” is the most efficient for the oxide removal with Pox and Inox proportions at 11.2% and 25.7%. “He plasma” is also effective but shows an higher Pox proportion close to the surface (23.7%). “Ar plasma soft” clearly shows the highest oxide proportions near the surface with Pox and Inox at 16.3% and 34.3%. From this study, it can be concluded that Ar and He plasmas are suitable candidates for the surface preparation of III-V materials prior to metallization. Moreover, the benefit to work with a quasi in-situ characterisation has been established. Measurements obtained give more accuracy to monitor the plasma surface preparation performance (Fig. 1). This work was supported by the French National Research Agency (ANR) under the ”Investissements d’avenir” programs: ANR 10-AIRT-0005 (IRT NANO-ELEC), ANR 10-EQPX-0030 (EQUIPEX FDSOI 11) and ANR-10-EQPX-0033 (EQUIPEX IMPACT). [1] L. Grenouillet, and al., 6th IEEE Int. Conf. on Gr. IV Photonics, 2009. [2] E. Ghegin, and al., EEE Trans. on Elect. Dev., vol. 64, n° 111, pp. 4408, 2017. [3] Ph. Rodriguez and al., ECS Trans., vol. 69, n° 18, pp. 251, 2015. [4] B. Pelissier and al., Microelec. eng., vol. 85, n° 11, pp. 151, 2008. [5] G. Hollinger and al., J. of Vac. Sci. & Tech. A: Vac., Surf., and Films, vol. 3, n° 16, pp. 2082, 1985. [6] G. Bruno and al., J. de Physique IV, vol. 5, n° C5, pp. 663, 1995. Figure 1

Investigation of band alignment between InAlAs and atomic-layer-deposited HfO2/Al2O3 by X-ray photoelectron spectroscopy

Band alignments between atomic layer deposition (ALD) HfO2/Al2O3 double layers and InAlAs were investigated by angle-resolved X-ray photoelectron spectroscopy (AR-XPS). The relationship of As and Hf 4f peaks reducing with the decreasing take-off angle θ indicated upward band bending on the InAlAs surface and a potential gradient in the HfO2/Al2O3 layer. AR-XPS measurements were combined with numerical calculations. The valence band and conduction band discontinuity between HfO2/Al2O3 and InAlAs were 2.49 ± 0.2 eV and 1.86 ± 0.2 eV, respectively. A low leakage current density of 7.01 × 10−7 A cm−2 at 1 V and oxide trapped charge densities of 9.925 × 1011 cm−2 were obtained.

Structures of Pt Ultra-Thin Films on Ni Substrate Prepared By Galvanic Replacement

Introduction Polymer electrolyte fuel cell (PEFC) has been expected as a high-power and clean electric source to solve the social problems such as environmental pollution and energy crisis [1]. In order to operate the PEFC at room temperature, it is necessary to promote the reaction rate of the oxygen reduction reaction (ORR), which is the rate determining step of the cathode reaction, by Pt as a cathode electro-catalyst. However, there are some problems with Pt catalyst for actual operation and wide general spread of PEFC; Pt is the expensive and precious metal and electro-catalytic activity of Pt for ORR is insufficient. In order to widely spread PEFC, therefore, it is indispensable to reduce the amount of Pt catalyst and to increase its ORR activity. It is well known, on the other hand, that ultra-thin noble metal layers formed on foreign metal surface often have higher electro-catalytic activity, because they have different surface energy from their bulk crystal [2]. Pt ultra-thin film covered with a low cost metal surface is one of the candidate as a cheaper electro-catalyst with higher electro-catalytic activity for ORR in PEFC. We have succeeded to electrochemically prepare Ni core Pt shell (Ni@Pt) nanoparticles using Ni as a cheaper metal core and demonstrated that its electro-catalytic activity for ORR is higher than Pt itself [3,4]. In these studies, such Pt thin layer coated Ni nanoparticles were prepared by a galvanic replacement technique. There are several methods to form the Pt ultra-thin film on the foreign metal surface. Among them, the galvanic replacement technique, in which such Pt ultra-thin film can be prepared by a simple operation, just dipping a substrate into a solution containing Pt ion, is one of the best candidates. However, there are few reports to structurally investigate the galvanic replacement process with an atomic dimension. In this report, the galvanic replacement process of Ni substrate surface with Pt was structurally investigated by the electrochemical and angle-resolved X-ray photoelectron spectroscopy (ARXPS) measurements. Experiments After several pre-treatments of Ni substrate, several droplets of the solution containing Pt2+ ion were added to the 0.05 M Na2SO4 solutions with pHs of 3.0, 4.0, and 5.0 as the Ni substrate was immersed into each solution during monitoring the open circuit potential (OCP). After the OCP was stabilized to a constant value, the substrate was taken out from the solution and rinsed with ultrapure water by a sonication for ca. 5 min. After drying the substrate, cyclic voltammogram (CV) and ARXPS were measured to obtain the Pt surface area and Pt film thickness, respectively. Results and Discussion In all the solutions, OCP was positively shifted and Pt existed onto the Ni substrate, although Ni oxide also existed of all the substrates. Thickness value of Pt thin film was dependent on pH value of solution. Pt thin film prepared from the solution with pH 3.0 was thinnest but its surface was quite rough. In addition, Pt black was also formed on this substrate surface and it was easy to wash it off with ultrapure water from the surface. Details of the Galvanic replacement are now under investigation.

Non-destructive determination of ultra-thin GaN cap layer thickness in AlGaN/GaN HEMT structure by angle resolved x-ray photoelectron spectroscopy (ARXPS)

Angle resolved X-ray photoelectron spectroscopy (ARXPS) and secondary ion mass spectrometry (SIMS) investigations have been carried out to characterize the GaN cap layer in AlGaN/GaN HEMT structure. The paper discusses the qualitative (presence or absence of a cap layer) and quantitative (cap layer thickness) characterization of cap layer in HEMT structure non-destructively using ARXPS measurements in conjunction with the theoretical modeling. Further the relative sensitive factor (RSF=σGaσAl) for Ga to Al ratio was estimated to be 0.963 and was used in the quantification of GaN cap layer thickness. Our results show that Al/Ga intensity ratio varies with the emission angle in the presence of GaN cap layer and otherwise remains constant. Also, the modeling of this intensity ratio gives its thickness. The finding of ARXPS was also substantiated by SIMS depth profiling studies.

Surface Analysis of 4-Methyl-2-Phenyl-Imidazole on the Cu Surface

Corrosion problems represent a large portion of the total costs in diverse industrial fields for companies every year worldwide. Moreover, appropriate corrosion control can help avoid many potential disasters that can cause serious issues including loss of life, negative social impacts, and water resource and environmental pollution. It is almost impossible to prevent corrosion, however it is possible to control it. Commonly, organic compounds are used as corrosion inhibitors. The studied compound herein, MePhI, is presented for the first time to protect any metallic material against corrosion. This compound effectively protects copper from corrosion in a highly corrosive chloride solution even after 0.5 year of immersion. To study this system X-ray photoelectron spectroscopy was employed. Using the Tougaard method a surface layer thickness of the MePhI was determined to be 0.3-0.5 nm by analyzing the background of the XPS survey spectrum. Moreover, orientation and the manner of bonding of MePhI on the Cu surface is explained especially by examining the XPS-excited Auger Cu L3M4,5M4,5 spectra, which is a “fingerprint” of a Cu(I)-MePhI connection. Angle-resolved XPS measurements suggest that both the C and N atoms of the MePhI molecule are involved in the inhibitor surface bonding. Finally, it is also suggested that MePhI molecules are lying flat on the surface.

Graphene-Coated ZnO and SiO2 as Supports for CoO Nanoparticles with Enhanced Reducibility.

The insertion of a graphene layer between cobalt and a substrate modifies the morphology and the oxidation/reduction properties of supported cobalt particles. Co forms a relatively flat structure on ZnO and SiO2 , whereas individual Co nanoparticles are formed after graphene coating of these substrates. The graphene layer moderates the formation of cobalt oxide in 5×10-7 mbar O2 and promotes the reduction of oxidized Co in H2 at lower temperature. Angle-resolved XPS measurements indicate that this is mainly a consequence of the restricted interaction of cobalt with the oxide supports. After the low-pressure redox treatments, the graphene layer maintains a relatively high quality with a small number of defect sites.

Metal nanoparticles designed PET: Preparation, characterization and biological response

In this manuscript, Ar plasma treated polyethyeterephthalate grafted with biphenyl-4,4′-dithiol interlayer and subsequently with green synthesized Pt and Pd nanoparticles is studied, focusing on the cytocompatibility of those composites. Changes in surface properties of the plasma treated and nanoparticle-grafted PET surface were studied in relation to in vitro adhesion and proliferation of mouse fibroblasts (L929) and human osteoblast (U-2 OS). Prepared samples were studied by several experimental techniques: goniometry, angle resolved X-ray photoelectron spectroscopy (ARXPS), ultraviolet–visible spectroscopy (UV–vis), Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray spectroscopy (EDS) and electrokinetic analysis. Ageing of the modified PET was accompanied by an increase of the contact angle which was due to a reorientation of the molecular polar segments produced during the plasma treatment. FTIR, UV–vis, ARXPS measurements and zeta potential indicate that the thiols were chemically bonded to the surface of the plasma treated polymers and that the thiols mediate subsequent grafting of Pt and Pd nanoparticles. Generally, EDS revealed that Pt nanoparticles are homogeneously distributed over the whole surface and Pd nanoparticles tend to aggregate slightly. In case of L929 cells, the chemical anchoring of Pt nanoparticles to PET surface significantly enhanced cell adhesion and proliferation compared to pristine PET.

Band alignment of HfO2/AlN heterojunction investigated by X-ray photoelectron spectroscopy

The band alignment between AlN and Atomic-Layer-Deposited (ALD) HfO2 was determined by X-ray photoelectron spectroscopy (XPS). The shift of Al 2p core-levels to lower binding energies with the decrease of take-off angles θ indicated upward band bending occurred at the AlN surface. Based on the angle-resolved XPS measurements combined with numerical calculations, valence band discontinuity ΔEV of 0.4 ± 0.2 eV at HfO2/AlN interface was determined by taking AlN surface band bending into account. By taking the band gap of HfO2 and AlN as 5.8 eV and 6.2 eV, respectively, a type-II band line-up was found between HfO2 and AlN.

P-depleted, Ca-rich �-TCP surfaces after calcination at 500 �C

Event Abstract Back to Event P-depleted, Ca-rich β-TCP surfaces after calcination at 500 °C Marc Bohner1, Roman Heuberger1, Jean-Christophe Hornez2 and Nicola Döbelin1 1 RMS Foundation, Switzerland 2 Université de Valenciennes, France Introduction: Thermally treating various calcium phosphates at 500°C for 24 hours reduce their chemical reactivity[1] and modify their osteoclastic response[2]. So far, these effects have been attributed to the removal of surface defects1. The aim of this study was to revisit this topic by performing a thorough XPS analysis of dense β-tricalcium phosphate (β-TCP, β-Ca3(PO4)2) samples. Materials and Methods: 99% dense β-TCP samples were produced by a combination of slip-casting and sintering (1100 °C for 3 h). Crystalline purity was assessed by applying a Rietveld refinement analysis of X-ray powder diffraction patterns (Philips PW1800 θ/2θ diffractometer, Panalytical, Almelo, The Netherlands). XPS measurements were performed using an Axis Nova instrument (Kratos Analytical Ltd, UK). The system was calibrated according to ISO 15472 and the accuracy was better than ± 0.05 eV. The spectra were analyzed using CasaXPS software (V2.3.14, Casa Software Ltd., UK). Peak shifting was corrected by referencing aliphatic carbon to 285 eV. The areas of the peaks were determined after subtraction of an iterated Shirley background, corrected by the sensitivity factors given by Kratos. Results and Discussion: The XPS survey spectra show prominent signals of O, Ca, P and C. After calcination, smaller signals of Mo and K as well as traces of Si and Na close to the detection limit were found. Before calcination, the Ca/P molar ratio was 1.53±0.02, which is close to the theoretical ratio of 1.5. After calcination, the ratio increased to 1.78±0.11 (Student’s t-test: p < 0.01). The various compounds detected after calcination (Si, Na, K, Mo) are most likely caused by a contamination during calcination. The increase of the Ca/P molar ratio of the platelets during calcination was attributed to the evaporation of phosphor pentoxide[3],[4]. Since neither more binding oxygen (P-O-P) was present nor the binding energy of phosphorus did change, the increase of the O/P molar ratio from 4.1 ± 0.1 to 5.2 ± 0.2 (Student’s t-test: p < 0.001) must be due to the formation of a P-depleted, Ca-rich phase such as hydroxyapatite (HA; Ca5(PO4)3OH), or tetracalcium phosphate (Ca4(PO4)2O; TetCP). In fact, as the O/P molar ratio can only increase from 4 to 4.3 or 4.5 by replacing β-TCP with HA or TetCP, it appears likely that a P-free, Ca-rich phase was formed, such as Ca oxide or Ca hydroxide. Angle-resolved XPS measurements confirmed the assumption that the Ca-rich phase was built on top of the calcium phosphate. Assuming the formation of an approximately 1 nm thick Ca oxide or Ca hydroxide layer during calcination is compatible with a 20% increase of Ca/P and O/P ratio as measured by XPS since XPS retrieves compositional information from the top 10 nm of the surface. Conclusions: This work presents data suggesting the formation of a P-depleted Ca-rich surface layer during the calcination of β-TCP at 500 °C, possibly Ca oxide or hydroxide. The financial support of Mathys Ltd (Bettlach, Switzerland) is kindly acknowledged