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Measurements of XPS survey, core levels (N 1s, O 1s, Pb 4f, I 3d), and valence band (VB) spectra of CH3NH3PbI3 (MAPbI3) hybrid perovskite prepared on different substrates (glass, indium tin oxide (ITO), and TiO2) aged under different light-soaking conditions at room temperature are presented. The results reveal that the photochemical stability of MAPbI3 depends on the type of substrate and gradually decreases when glass is replaced by ITO and TiO2. Also, the degradation upon exposure to visible light is accompanied by the formation of MAI, PbI2, and Pb0 products as shown by XPS core levels spectra. According to XPS O 1s and VB spectra measurements, this degradation process is superimposed on the partial oxidation of lead atoms in ITO/MAPbI3 and TiO2/MAPbI3, for which Pb–O bonds are formed due to the diffusion of the oxygen ions from the substrates. This unexpected interaction leads to additional photochemical degradation. © 2021 by the author. Licensee MDPI, Basel, Switzerland.

The design of epitaxial semiconductor-superconductor and semiconductor-metal quantum devices requires a detailed understanding of the interfacial electronic band structure. However, the band alignment of buried interfaces is difficult to predict theoretically and to measure experimentally. This work presents a procedure that allows to reliably determine critical parameters for engineering quantum devices; band offset, band bending profile, and number of occupied quantum well subbands of interfacial accumulation layers at semiconductor-metal interfaces. Soft X-ray angle-resolved photoemission is used to directly measure the quantum well states as well as valence bands and core levels for the InAs(100)/Al interface, an important platform for Majorana-zero-mode based topological qubits, and demonstrate that the fabrication process strongly influences the band offset, which in turn controls the topological phase diagrams. Since the method is transferable to other narrow gap semiconductors, it can be used more generally for engineering semiconductor-metal and semiconductor-superconductor interfaces in gate-tunable superconducting devices.

The design of epitaxial semiconductor-superconductor and semiconductor-metal quantum devices requires a detailed understanding of the interfacial electronic band structure. However, the band alignment of buried interfaces is difficult to predict theoretically and to measure experimentally. This work presents a procedure that allows to reliably determine critical parameters for engineering quantum devices; band offset, band bending profile, and number of occupied quantum well subbands of interfacial accumulation layers at semiconductor-metal interfaces. Soft X-ray angle-resolved photoemission is used to directly measure the quantum well states as well as valence bands and core levels for the InAs(100)/Al interface, an important platform for Majorana-zero-mode based topological qubits, and demonstrate that the fabrication process strongly influences the band offset, which in turn controls the topological phase diagrams. Since the method is transferable to other narrow gap semiconductors, it can be used more generally for engineering semiconductor-metal and semiconductor-superconductor interfaces in gate-tunable superconducting devices.

Currently, platinum is the most widely used catalyst for low temperature proton exchange membrane fuel cells (PEMFC). However, the kinetics at the cathode are slow, and the price of platinum is high. To improve oxygen reduction reaction (ORR) kinetics at the cathode, platinum can be alloyed with rare earth elements, such as yttrium. We report that Pt3Y has the potential to be over 2 times more active for the ORR compared with Pt inside a real fuel cell. We present detailed photoemission analysis into the nature of the sputtered catalyst surface, using synchrotron radiation photoelectron spectroscopy (SRPES) to examine if surface adsorbates or impurities are present and can be removed. Pretreatment removes most of the yttrium oxide in the surface leaving behind a Pt overlayer which is only a few monolayers thick. Evidence of a substochiometric oxide peak in the Y 3d core level is presented, this oxide extends into the surface even after Ar+ sputter cleaning in-situ. This information will aid the development of new highly active nanocatalysts for employment in real fuel cell electrodes., QC 20220615

We present studies on the structure and chemical states of a catalyst developed by immobilizing palladium on S-terminated GaAs(001). Hard x-ray photoelectron spectroscopy (HX-PES) of core-level and valence band photoemission consistently indicates that the organopalladium molecules are reduced on the surface yielding Pd nanoparticles with a metallic nature. This finding is supported by high-resolution observations using scanning electron microscopy and backscattered electron image. HX-PES results also reveal that a portion of S atoms forming the S-termination is oxidized during the formation of Pd nanoparticles.

type:Journal Article, A vapor-phase-grown epitaxial ZnS:Ag layer simultaneously codoped with In and N on GaAs substrate exhibited a 436-nm light emission and p-type conduction with a low resistivity. X-ray photoemission spectroscopy revealed that the In 3d5/2 electron binding energy of the codoped ZnS:In,N layer was smaller by 0.5 eV than that of the ZnS:In independently doped layer, although the 2p3/2 electron binding energies of Zn and S of the codoped layer agreed well with those of the independently doped layer, respectively. The reduction of binding energy was ascribed to an increase in the electronic relaxation energy for core-hole states in photoemission and reflects a large charge transfer between the In and N atoms at the first neighbor sites through covalent sp3 bonding orbitals. An increase of the spectral intensity at around 4 eV relative to the valence band maximum observed for the codoped layer corresponds to a new state at –3.67 eV from the valence band maximum due to a strong coupling between the In 5s and N 2p orbitals at the first neighbor sites, derived from a first-principle band structure calculation for ZnS:(In,2N)., source:http://www.aip.org

type:Journal Article, Electronic structure of Nd2−xCexCuO4−y (x=0 and 0.15) thin films was examined by x-ray photoelectron spectroscopy. The films were prepared by rf magnetron sputtering and successive annealing under reducing condition (reduction). The reduced films showed semiconducting and superconducting behaviors depending on the value x. The Cu, O, and Nd core-level spectra revealed that the doped electrons were predominantly in CuO2 plane of the Nd2CuO4 crystal. The Cu core-level spectra from x=0.15 films before and after the reduction suggested that the reduction added electrons to Cu4s-O2p extended conduction band, and strengthened Cu O bond covalency to screen the core hole state by mobile itinerant electrons. Journal of Applied Physics is copyrighted by The American Institute of Physics., source:http://www.aip.org, source:https://doi.org/10.1063/1.346722

type:Journal Article, For Na-free and Cu-deficient CuInS2 thin films with chalcopyrite structure, electron spectra of core levels and valence band were similar to each other in spite of a difference in Cu deficiency. Incorporation of Na with the CuInS2 films caused peak shifts by -0.5 eV for the core levels without changes in both linewidth and shape. The electron density of states, deeper by 1-5 eV than that from Cu d-S p hybridization centered at around 2.5 eV, increased by the Na incorporation, and increments in the electron density of states, were larger for the films with larger the Cu deficiency. The Na incorporation yielded a surface layer expressed as (Na,Cu)InS2 on the Cu deficient CuInS2 films. (C) 1998 American Institute of Physics., source:http://www.aip.org, source:https://doi.org/10.1063/1.122168

type:Journal Article, By using x-ray photoelectron spectroscopy we have examined changes in the electronic structure of CuInS2 thin films with co-incorporation of O and Na that raised up the photovoltaic performance of CdS/CuInS2 solar cells. The electron binding energies of both the core levels and valence band for the film with the co-incorporation were smaller by 0.9 eV than those for the film without the co-incorporation. For the co-incorporated film an increase in the spectral intensity of the valence band was observed below 1-6 eV from the Cu 3d(10) nonbonding states, and it was due to both the Na s, p-S p hybridization and the In s-Op hybridization. The surface of the co-incorporated film can be expressed as (Cu, Na)In(S, O)(2). The (Cu, Na)In(S, O)(2) phase is related to the enlarged open-circuit voltage (0.75 eV) of n-CdS/p-CuInS2 solar cells. (C) 2000 American Institute of Physics., source:http://www.aip.org, source:https://doi.org/10.1063/1.1320014

In this paper, the interface between Al2O3 and p-type 4H-SiC is evaluated using x-ray photoelectron spectroscopy (XPS) measurements. These studies are made on dielectricsemiconductor test structures with Al2O3 as dielectric with different pre- and post-deposition treatments. XPS measurements on the as-deposited samples with two different pre-surface cleaning methods have shown no formation of a SiO2 interlayer. However, after the post deposition rapid thermal annealing (RTA) at 1100 °C in N2O for 60 s, a SiO2 interlayer is formed. The surface band bending was determined from Si 2p core level peak shifts measured using XPS. These results suggest that Al2O3 deposited on the p-type 4H-SiC have a net positive oxide charge which is complementary to that of Al2O3 on n-type 4H-SiC. From these shifts it was found that the asdeposited RCA cleaned sample had an oxide charge of 5.6×1013 cm-2, as compared to standard cleaned samples, having 4.6×1013 cm-2. A further reduction in oxide charge was observed after annealing at 1100 °C in N2O, down to a value of 4×1013 cm-2., QC 20161125

The results of XPS measurements (core levels and valence bands) of P+, Ca+, P+Ca+ and Ca+P+ ion implanted (E=30 keV, D=1×1017cm-2) commercially pure titanium (cp-Ti) and first-principles density functional theory (DFT) calculations demonstrates formation of various structural defects in titanium dioxide films formed on the surface of implanted materials. We have found that for double implantation (Ti:P+,Ca+ and Ti:Ca+,P+) the outermost surface layer is formed mainly by Ca and P, respectively, i.e. the implantation sequence is very important. The DFT calculations show that under P+ and Ca+P+ ion implantation the formation energies for both cation (P-Ti) and anion (P-O) substitutions are comparable, which can induce the creation of [PO4]3- and Ti-P species. For Ca+ and P+Ca+ ion implantation the calculated formation energies correspond to Ca2+-Ti4+ cation substitution. This conclusion is in agreement with XPS Ca 2p and Ti 2p core levels and valence-band measurements and DFT calculations of the electronic structure of related compounds. The conversion of implanted ions to Ca2+ and [PO4]3- species provides a good biocompatibility of cp-Ti for further formation of hydroxyapatite. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Several GPU-based algorithms have been developed to ac-celerate biomolecular simulations, but although they pro-vide benefits over single-core implementations, they have not been able to surpass the performance of state-of-the art SIMD CPU implementations (e.g. GROMACS), not to mention efficient scaling. Here, we present a heteroge-nous parallelization that utilizes both CPU and GPU re-sources efficiently. A novel fixed-particle-number sub-cell algorithm for non-bonded force calculation was developed. The algorithm uses the SIMD width as algorithmic work unit, it is intrinsically future-proof since it can be adapted to future hardware. The CUDA non-bonded kernel imple-mentation achieves up to 60% work-efficiency, 1.5 IPC, and 95% L1 cache utilization. On the CPU OpenMP-parallelized SSE-accelerated code runs overlapping with GPU execution. Fully automated dynamic inter-process as well as CPU-GPU load balancing is employed. We achieve threefold speedup compared to equivalent GROMACS CPU code and show good strong and weak scaling. To the best of our knowledge this the fastest GPU molecular dynamics implementation presented to date., QC 20140901

Disorder broadening of core-level binding energies is a general effect observed in random alloys, and identifies an opportunity for studying specific local environments experimentally. Here we study it in an archetypical system: face-centered-cubic Cu50Au50. While the disorder broadening is clearly detectable at Au, at Cu it is below the detection limit. We supplement experiments by a theoretical study where we model the alloy by a large supercell constructed as a special quasirandom structure and calculate binding-energy shifts at all sites in the supercell. Theory shows that the suppression of the disorder broadening at Cu results from a delicate balance between the influence of local chemical environment and inhomogeneous lattice distortions on the site-resolved core-level shifts. Surprisingly, even larger relaxation-induced shifts are observed at Au sites.

The electronic structure of phenylalanine, tyrosine, tryptophan, and 3-methylindole in the gas phase was investigated by x-ray photoemission spectroscopy (XPS) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the C, N, and O K-edges. The XPS spectra have been calculated for the four principal conformers of each amino acid, and the spectra weighted by the Boltzmann population ratios calculated from published free energies. Instead of the single peaks expected from the stoichiometry of the compounds, the N 1s core level spectra of phenylalanine and tryptophan show features indicating that more than one conformer is present. The calculations reproduce the experimental features. The C and O 1s spectra do not show evident effects due to conformational isomerism. The calculations predict that such effects are small for carbon, and for oxygen it appears that only broadening occurs. The carbon K-edge NEXAFS spectra of these aromatic amino acids are similar to the published data of the corresponding molecules in the solid state, but show more structure due to the higher resolution in the present study. The N K-edge spectra of tryptophan and 3-methylindole differ from phenylalanine and tyrosine, as the first two both contain a nitrogen atom located in a pyrrole ring. The nitrogen K-edge NEXAFS spectra of aromatic amino acids do not show any measurable effects due to conformational isomerism, in contrast to the photoemission results. Calculations support this result and show that variations of the vertical excitation energies of different conformers are small, and cannot be resolved in the present experiment. The O NEXAFS spectra of these three aromatic compounds are very similar to other, simpler amino acids, which have been studied previously., QC 20110429

We present studies on the structure and chemical states of a catalyst developed by immobilizing palladium on S-terminated GaAs(001). Hard x-ray photoelectron spectroscopy (HX-PES) of core-level and valence band photoemission consistently indicates that the organopalladium molecules are reduced on the surface yielding Pd nanoparticles with a metallic nature. This finding is supported by high-resolution observations using scanning electron microscopy and backscattered electron image. HX-PES results also reveal that a portion of S atoms forming the S-termination is oxidized during the formation of Pd nanoparticles., journal article

Disorder broadening of core-level binding energies is a general effect observed in random alloys, and identifies an opportunity for studying specific local environments experimentally. Here we study it in an archetypical system: face-centered-cubic Cu50Au50. While the disorder broadening is clearly detectable at Au, at Cu it is below the detection limit. We supplement experiments by a theoretical study where we model the alloy by a large supercell constructed as a special quasirandom structure and calculate binding-energy shifts at all sites in the supercell. Theory shows that the suppression of the disorder broadening at Cu results from a delicate balance between the influence of local chemical environment and inhomogeneous lattice distortions on the site-resolved core-level shifts. Surprisingly, even larger relaxation-induced shifts are observed at Au sites.

Free neutral potassium clusters of two different mean sizes were produced using a gas aggregation source. The K 3p core level was probed by means of synchrotron radiation based photoelectron spectroscopy. The absolute binding energy and the intensity ratio of the surface and bulk spectral features were used to obtain independent estimates of the cluster size. Plasmon satellites in core level spectra of free metallic clusters were observed. Surface and bulk plasmon energies were determined. The interplay between the core line and plasmon integrated intensities is discussed.

Electron correlations are responsible for many profound phenomena in solid-state physics. A classical example is the 6-eV satellite in the photoelectron spectrum of Ni. Until now the satellite structure has only been investigated at the L shell and more shallow levels. Here we report a high-kinetic-energy photoemission spectroscopy (HIKE) investigation of Ni metal. We present 1s and 2p photoelectron spectra, obtained using excitation energies up to 12.6 keV. Our investigation demonstrates that the energy position of the satellite relative to the main line is different for the 1s and the 2p levels. In combination with electronic structure calculations, we show that this energy shift is attributed to unique differences in the core-valence coupling for the K and L2,3 shells in 3d transition metals, resulting in different screening of the core holes.

By means of synchrotron-radiation-based core-level spectroscopies we demonstrate that the degree of corrugation in graphene nanomesh on lattice-mismatched transition-metal substrates critically depends on the strength of chemical bonding at the interface. The degree of interfacial orbital hybridization between graphene and metal states is rising in the series Pt(111)-Ir(111)-Rh(111)-Ru(0001). This growing strength of hybridization is accompanied by a gradual change in graphene morphology from nearly flat to strongly corrugated. We provide a comparison of the pore size and period for the cases of graphene and h-BN nanomesh on Rh(111).

Bismuth adsorbate-stabilized (2x1) and (2x4) reconstructions of the GaAs(100) surfaces have been studied by first-principles calculations, valence-band and core-level photoelectron spectroscopies, and scanning tunneling microscopy. It is demonstrated that large Bi atom size leads to the formation of the pseudogap at the Fermi energy and to the lower energy of an adsorbate-derived surface band, which contributes to the stabilization of the exceptional Bi/GaAs(100)(2x1) reconstruction. It is proposed that the Bi/GaAs(100)(2x4) reconstructions include asymmetric mixed Bi-As dimers, in addition to the Bi-Bi dimers. Based on the calculations, we solve the atomic origins of the surface core-level shifts (SCLSs) of the Bi 5d photoemission spectra from the Bi/GaAs(100)(2x4) surfaces. This allows for resolving the puzzle related to the identification of two SCLS components often found in the measurements of the Bi 5d and Sb 4d core-level emissions of the Bi/III-V and Sb/III-V(100)(2x4) surfaces. Finally, the reason for the absence of the common (2x4)-beta 2 structure and additional support for the stability of the (2x1) structure on the Bi/III-V(100) surfaces are discussed in terms of Bi atom size and subsurface stress.

Bismuth adsorbate-stabilized (2x1) and (2x4) reconstructions of the GaAs(100) surfaces have been studied by first-principles calculations, valence-band and core-level photoelectron spectroscopies, and scanning tunneling microscopy. It is demonstrated that large Bi atom size leads to the formation of the pseudogap at the Fermi energy and to the lower energy of an adsorbate-derived surface band, which contributes to the stabilization of the exceptional Bi/GaAs(100)(2x1) reconstruction. It is proposed that the Bi/GaAs(100)(2x4) reconstructions include asymmetric mixed Bi-As dimers, in addition to the Bi-Bi dimers. Based on the calculations, we solve the atomic origins of the surface core-level shifts (SCLSs) of the Bi 5d photoemission spectra from the Bi/GaAs(100)(2x4) surfaces. This allows for resolving the puzzle related to the identification of two SCLS components often found in the measurements of the Bi 5d and Sb 4d core-level emissions of the Bi/III-V and Sb/III-V(100)(2x4) surfaces. Finally, the reason for the absence of the common (2x4)-beta 2 structure and additional support for the stability of the (2x1) structure on the Bi/III-V(100) surfaces are discussed in terms of Bi atom size and subsurface stress., QC 20100525

Decay spectra of N2O following excitation to the N terminal (Nt) → π*, N central (Nc) → π* and O is → π* intermediate states are reported. The final states reached after participator decay show resonant enhancement consistent with a local-density-of-states analysis based on the Mulliken population of the valence molecular orbitals. In particular, the X-state is resonantly enhanced mostly after excitation from the Nt 1s and the O Is core levels to the π*, while the B-state is mostly enhanced following the excitation of the Nc Is → π* intermediate state. Below the Nt Is threshold, the lowest lying peak related to spectator decay falls at lower binding energy than the highest lying participator peak. This can be attributed to a particularly strong screening effect exerted by the excited electron in the LUMO.

A method is presented to determine the photoelectron surface core-level shift (SCLS) of 3d transition metals using x-ray photoelectron spectroscopy. The experimental difficulties arising from the relatively large broadening of photoemission lines in the 3d transition series can be overcome by the analysis of the angular dependence of photoemission spectra. The proposed method has been demonstrated using well-defined single-crystal surfaces of copper. The observed values of the SCLS for copper are compared with those predicted by both ab initio calculations and a macroscopic atom model. The experimental determination of SCLSs opens alternative routes for collecting thermochemical data for surfaces/interfaces., Materials Science and Engineering, Mechanical, Maritime and Materials Engineering

A method is presented to determine the photoelectron surface core-level shift (SCLS) of 3d transition metals using x-ray photoelectron spectroscopy. The experimental difficulties arising from the relatively large broadening of photoemission lines in the 3d transition series can be overcome by the analysis of the angular dependence of photoemission spectra. The proposed method has been demonstrated using well-defined single-crystal surfaces of copper. The observed values of the SCLS for copper are compared with those predicted by both ab initio calculations and a macroscopic atom model. The experimental determination of SCLSs opens alternative routes for collecting thermochemical data for surfaces/interfaces., Materials Science and Engineering, Mechanical, Maritime and Materials Engineering

We describe the implementation of a simple technique to simulate core-level spectra within the Car-Parrinello plane-waves molecular dynamics framework. The x-ray absorption (XA) spectra are generated using the transition potential technique with the effect of the core hole included through a specifically developed pseudopotential for the core-excited atom. Despite the lack of 1s core orbitals in the pseudopotential treatment, the required transition moments are accurately calculated without reconstruction of the all-electron orbitals. The method is applied to the oxygen XA spectra of water in its various aggregation states, but it is transferable to any first-row element. The computed spectra are compared favorably with the results from all-electron cluster calculations, as well as with experimental data. The periodicity of the plane-wave technique improves the description of condensed phases. The molecular dynamics simulation enables in principle a proper treatment of thermal effects and dynamical averaging in complex systems., Part of urn:nbn:se:su:diva-286

A technique based on the use of x-ray photoelectron spectroscopy was developed to measure heterojunction band discontinuities with an uncertainty of + or - 0.04 eV and changes in band discontinuities for a specific heterojunction interface with an uncertainty of + or - 0.01 eV. This technique was used to investigate Ge-GaAs, GaAs-A1As, ZnSe-GaAs, and ZnSe-Ge heterojunctions. It was discovered that microscopic dipoles present at abrupt heterojunction interfaces can substantially affect observed band discontinuities. Variations in heterojunction band discontinuities as functions of crystallographic orientation, growth sequence, and growth conditions were observed. It was established that heterojunction band discontinuities depend on microscopic properties of the interface and cannot be predicted from individual semiconductor properties alone. Based on electrostatic considerations, it was shown that polar heterojunction interfaces cannot be atomically abrupt but must require at least two interfacial transition planes to be consistent with experimental observations.