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Journal of synchrotron radiation 27(5), 1200 - 1208 (2020). doi:10.1107/S1600577520009789, Nanoprobe X-ray diffraction (nXRD) using focused synchrotron radiation is a powerful technique to study the structural properties of individual semiconductor nanowires. However, when performing the experiment under ambient conditions, the required high X-ray dose and prolonged exposure times can lead to radiation damage. To unveil the origin of radiation damage, a comparison is made of nXRD experiments carried out on individual semiconductor nanowires in their as-grown geometry both under ambient conditions and under He atmosphere at the microfocus station of the P08 beamline at the third-generation source PETRA III. Using an incident X-ray beam energy of 9 keV and photon flux of 10$^{10}$ s$^{−1}$, the axial lattice parameter and tilt of individual GaAs/In$_{0.2}$Ga$_{0.8}$As/GaAs core–shell nanowires were monitored by continuously recording reciprocal-space maps of the 111 Bragg reflection at a fixed spatial position over several hours. In addition, the emission properties of the (In,Ga)As quantum well, the atomic composition of the exposed nanowires and the nanowire morphology were studied by cathodoluminescence spectroscopy, energy-dispersive X-ray spectroscopy and scanning electron microscopy, respectively, both prior to and after nXRD exposure. Nanowires exposed under ambient conditions show severe optical and morphological damage, which was reduced for nanowires exposed under He atmosphere. The observed damage can be largely attributed to an oxidation process from X-ray-induced ozone reactions in air. Due to the lower heat-transfer coefficient compared with GaAs, this oxide shell limits the heat transfer through the nanowire side facets, which is considered as the main channel of heat dissipation for nanowires in the as-grown geometry., Published by Wiley-Blackwell, [S.l.]

It is demonstrated by a detailed structural analysis that the crystallinity and the efficiency of small molecule based organic photovoltaics can be tuned by solvent vapor annealing (SVA). Blends made of the small molecule donor 2,2′-[(3,3′″,3″″,4′-tetraoctyl[2,2′:5′,2″:5″,2′″:5′″,2″″-quinquethiophene]-5,5″″-diyl)bis[(Z)-methylidyne(3-ethyl-4-oxo-5,2-thiazolidinediylidene)]]bis-propanedinitrile (DRCN5T) and the acceptor [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) were annealed using solvent vapors with either a high solubility for the donor (tetrahydrofuran), the acceptor (carbon disulfide) or both (chloroform). The samples were analyzed by grazing-incidence wide-angle X-ray scattering (GIWAXS), electron diffraction, X-ray pole figures, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). A phase separation of DRCN5T and PC71BM is induced by SVA leading to a crystallization of DRCN5T and the formation of a DRCN5T enriched layer. The DRCN5T crystallites possess the two dimensional oblique crystal system with the lattice parameters a = 19.2 Å, c = 27.1 Å, and β = 111.1° for the chloroform case. No major differences in the crystal structure for the other solvent vapors were observed. However, the solvent choice strongly influences the size of the DRCN5T enriched layer. Missing periodicity in the [010]-direction leads to the extinction of all Bragg reflections with k ≠ 0. The annealed samples are randomly orientated with respect to the normal of the substrate (fiber texture).

We report on the influence of oxygen partial pressure for the development of surface oxides covering the industrial aluminum alloy standard 6063 at temperatures ranging from room temperature to 500° C. Using an array of synchrotron-based techniques, we followed the change in oxide thickness, chemical composition, and the lateral distribution of alloying elements. The impact of the oxygen chemical potential is most visible at high temperatures where the oxide composition changes from mostly Al based to mostly Mg based. This is in stark contrast to the ultra-high vacuum (UHV) conditions where only a partial compositional transition is observed. The microscopy data demonstrate that in the UHV case, Mg segregation onto the surface occurs firstly at grain boundaries at 300° C and secondly at sites over the entire surface at 400° C. Further, the initial oxide thickness is 45 A, as determined by XPS and XRR, decreases in all observed cases after heating to 300° C. At higher temperatures, however, the oxygen partial pressure highly influences the resulting oxide thickness as evident from our X-ray reflectivity data. (Less)

Nanotechnology 31(21), 1-7 (2020). doi:10.1088/1361-6528/ab7590, We report on the direct correlation between the structural and optical properties of single, as-grown core-multi-shell GaAs/In$_{0.15}$Ga$_{0.85}$As/GaAs/AlAs/GaAs nanowires. Fabricated by molecular beam epitaxy on a pre-patterned Si(111) substrate, on a row of well separated nucleation sites, it was possible to access individual nanowires in the as-grown geometry. The polytype distribution along the growth axis of the nanowires was revealed by synchrotron-based nanoprobe x-ray diffraction techniques monitoring the axial 111 Bragg reflection. For the same nanowires, the spatially-resolved emission properties were obtained by cathodoluminescence hyperspectral linescans in a scanning electron microscope. Correlating both measurements, we reveal a blueshift of the shell quantum well emission energy combined with an increased emission intensity for segments exhibiting a mixed structure of alternating wurtzite and zincblende stacking compared with the pure crystal polytypes. The presence of this mixed structure was independently confirmed by cross-sectional transmission electron microscopy., Published by IOP Publ., Bristol

Applied physics letters 117(1), 011601 - (2020). doi:10.1063/5.0013925, Ni$_x$Fe$_{3���x}$O$_4$ thin films with varying Ni amount (0 ��� x ��� 1.5) were deposited on MgO(001) via reactive molecular beam epitaxy. The growth process was monitored during film deposition by means of X-ray diffraction. All prepared films exhibit a well-ordered structure with complete vertical crystallinity throughout the whole film growth and flat surfaces of the final films independent of the Ni amount. An enhancement of the vertical compression in the initial growth continuously decreases up to a film thickness of 8 nm. During further growth, all films exhibit residual and constant vertical compression with lateral adaption of the final films to the substrate lattice, as observed by high energy surface X-ray diffraction experiments. Hard X-ray photoelectron spectroscopy measurements of the final films reveal increasing Fe$^{3+}$:Fe$^{2+}$ ratios for higher Ni content and point to additional NiO agglomerations within the films exceeding the stoichiometric Ni amount of x = 1., Published by American Inst. of Physics, Melville, NY

Typically, core–shell–shell semiconductor nanowires (NWs) made from III–V materials with low lattice mismatch grow pseudomorphically along the growth axis, i.e. the axial lattice parameters of the core and shell materials are the same. Therefore, both the structural composition and interface strain of the NWs are encoded along directions perpendicular to the growth axis. Owing to fluctuations in the supplied growth species during molecular beam epitaxy (MBE) growth, structural parameters such as local shell thickness, composition and strain may differ between NWs grown onto the same substrate. This requires structural analysis of single NWs instead of measuring NW ensembles. In this work, the complete structure of single GaAs/(In,Ga)As/GaAs core–shell–shell NW heterostructures is determined by means of X-ray nanodiffraction using synchrotron radiation. The NWs were grown by MBE on a prepatterned silicon (111) substrate with a core diameter of 50 nm and an (In,Ga)As shell thickness of 20 nm with a nominal indium concentration of 15%, capped by a 30 nm GaAs outer shell. In order to access single NWs with the X-ray nanobeam being incident parallel to the surface of the substrate, a single row of holes with a separation of 10 µm was defined by electron-beam lithography to act as nucleation centres for MBE NW growth. These well separated NWs were probed sequentially by X-ray nanodiffraction, recording three-dimensional reciprocal-space maps of Bragg reflections with scattering vectors parallel (out-of-plane) and perpendicular (in-plane) to the NW growth axis. From the out-of-plane 111 Bragg reflection, deviations from hexagonal symmetry were derived, together with the diameters of probed NWs grown under the same conditions. The radial NW composition and interface strain became accessible when measuring the two-dimensional scattering intensity distributions of the in-plane 2{\overline 2}0 and 22{\overline 4} reflections, exhibiting well pronounced thickness fringes perpendicular to the NW side planes (truncation rods, TRs). Quantitative values of thickness, composition and strain acting on the (In,Ga)As and GaAs shells were obtained via finite-element modelling of the core–shell–shell NWs and subsequent Fourier transform, simulating the TRs measured along the three different directions of the hexagonally shaped NWs simultaneously. Considering the experimental constraints of the current experiment, thicknesses and In content have been evaluated with uncertainties of ±2 nm and ±0.01, respectively. Comparing data taken from different single NWs, the shell thicknesses differ from one to another.

We present results from the anodization of single crystal Al(100) and Al(111) surfaces and the aluminum alloy AA 6063 studied in situ by X-ray reflectivity and electrochemical impedance spectroscopy. We observe that the anodic oxide layer grows linearly with the anodization potential and that the thicknesses are similar for all samples. However, the thicknesses obtained from X-ray reflectivity are higher than that obtained from electrochemical impedance spectroscopy. We attribute the higher thicknesses to an outer porous oxide layer, which is not detected by electrochemical impedance spectroscopy. Both, electrochemical impedance spectroscopy and X-ray reflectivity suggests that a more heterogeneous and rough oxide is formed on AA 6063 due to the influence of the alloying elements and intermetallic particles during the growth.

Anodization of aluminum alloys AA 6082 and AA 7075 was investigated in-situ with integrated electrochemical and synchrotron-based X-ray reflectivity (XRR) methods providing complementary information about the anodic processes taking place on the alloys. The stepwise potentiostatic polarization measurements reveal dynamic processes of the anodic oxide formation and dissolution, and the following electrochemical impedance spectroscopy measurements detect the break of the native oxide and the growth of typical two-layer anodic oxide film, while the XRR measurements show the growth of entire anodic oxide film whose thickness increases linearly with the increasing applied potential. The results indicate that while a stable anodic oxide can be formed on the both alloys with a similar growth factor, AA 7075 shows a thinner thickness of the barrier layer and a lower resistance of the oxide film. The electrochemical results suggest both localized and uniform anodic dissolution processes, which are more pronounced on AA 7075, demonstrating the effect of alloying elements on the growth of anodic oxides.

Understanding how specific atom sites on metal surfaces lower the energy barrier for chemical reactions is vital in catalysis. Studies on simplified model systems have shown that atoms arranged as steps on the surface play an important role in catalytic reactions, but a direct comparison of how the light-off temperature is affected by the atom orientation on the step has not yet been possible due to methodological constraints. Here we report in situ spatially resolved measurements of the CO production over a cylindrical-shaped Pd catalyst and show that the light-off temperature at different parts of the crystal depends on the step orientation of the two types of steps (named A and B). Our finding is supported by density functional theory calculations, revealing that the steps, in contrast to what has been previously reported in the literature, are not directly involved in the reaction onset but have the role of releasing stress., The authors thank the Knut and Alice Wallenberg Foundation, the Swedish Research Council, the Swedish Foundation for Strategic Research, and the Crafoord Foundation. Support by the MAX IV staff is gratefully acknowledged. The calculations were performed at C3SE through a SNIC grant. J.E.O. acknowledges support from the Spanish Ministry of Economy (MAT2013-46593-C6-4-P) and the Basque Government (IT621-13).

Journal of synchrotron radiation 25, 1664 - 1672 (2018). doi:10.1107/S1600577518013218, Since the properties of functional materials are highly dependent on their specific structure, and since the structural changes, for example during crystallization, induced by coating and annealing processes are significant, the study of structure and its formation is of interest for fundamental and applied science. However, structure analysis is often limited to ex situ determination of final states due to the lack of specialized sample cells that enable real-time investigations. The lack of such cells is mainly due to their fairly complex design and geometrical restrictions defined by the beamline setups. To overcome this obstacle, an advanced sample cell has been designed and constructed; it combines automated doctor blading, solvent vapor annealing and sample hydration with real-time grazing-incidence wide- and small-angle scattering (GIWAXS/GISAXS) and X-ray reflectivity (XRR). The sample cell has limited spatial requirements and is therefore widely usable at beamlines and laboratory-scale instruments. The cell is fully automatized and remains portable, including the necessary electronics. In addition, the cell can be used by interested scientists in cooperation with the Institute for Crystallography and Structural Physics and is expandable with regard to optical secondary probes. Exemplary research studies are presented, in the form of coating of P3HT:PC$_{61}$PM thin films, solvent vapor annealing of DRCN5T:PC$_{71}$BM thin films, and hydration of supported phospholipid multilayers, to demonstrate the capabilities of the in situ cell., Published by IUCr, [S.l.]

The journal of physical chemistry / A 122(49), 28075 - 28084 (2018). doi:10.1021/acs.jpcc.8b08205, Doping with halide ions is a popular method toalter the properties of metal phthalocyanines (MPcs),particularly magnetic and electrical nature of organic semiconductorsfor applications in spintronic or electronic devices.Doping can cause a structural rearrangement in MPc packing,and the physical properties may be correlated with molecularpacking. Films of a planar and magnetic MPc, manganese-(II)phthalocyanine (MnPc), are chosen for iodine dopingstudy. The optical, magnetic, and electrical properties ofpristine- and iodine-doped MnPc thin films are investigatedand can be directly associated with their molecular structure.Two-dimensional grazing incidence synchrotron X-ray diffractionreveals structural disorder in MnPc films upon iodineinfusion induced by the reorientation of ordered, edge-on molecular configuration to tilted and face-on configurations in arandom fashion. The film morphology changes accordingly, where in the uniform crystallites reorganize in a disordered manner.The ferromagnetic nature of the pristine film gets weakened because of iodine species and favors antiferromagnetic coupling.The study of electrical properties at room temperature by conducting atomic force microscopy reveals that the conductance isenhanced independently of the film thickness because of the disorder induced by iodine inclusion., Published by ACS, Washington, DC

Journal of synchrotron radiation 25(5), 1389 - 1394 (2018). doi:10.1107/S1600577518010597, High-energy surface X-ray diffraction (HESXRD) provides surface structural information with high temporal resolution, facilitating the understanding of the surface dynamics and structure of the active phase of catalytic surfaces. The surface structure detected during the reaction is sensitive to the composition of the gas phase close to the catalyst surface, and the catalytic activity of the sample itself may affect the surface structure, which in turn may complicate the assignment of the active phase. For this reason, planar laser-induced fluorescence (PLIF) and HESXRD have been combined during the oxidation of CO over a Pd(100) crystal. PLIF complements the structural studies with an instantaneous two-dimensional image of the CO$_2$ gas phase in the vicinity of the active model catalyst. Here the combined HESXRD and PLIF operando measurements of CO oxidation over Pd(100) are presented, allowing for an improved assignment of the correlation between sample structure and the CO$_2$ distribution above the sample surface with sub-second time resolution., Published by Wiley-Blackwell, [S.l.]

Catalysis letters 148(9), 2901 - 2910 (2018). doi:10.1007/s10562-018-2487-0, Liquid organic hydrogen carriers (LOHCs) have great potential as a hydrogen storage medium needed for a future sustainable energy system. Dehydrogenation of LOHCs requires a catalyst, such as supported Pd nanoparticles. Under reaction conditions, hydrogen and carbon may diffuse into the bulk of supported Pd catalyst particles and affect their activity and selectivity. The detailed understanding of this process is critical for the use of LOHCs in future hydrogen storage technologies. In this work, we studied these processes in-situ on a Pd model catalyst using high-energy grazing incidence X-ray diffraction. Pd nanoparticles were evaporated in ultra-high vacuum on a polished $α$-Al$_2$O$_3$(0001) substrate. The particles, with an initial average size of ~ 3.4 nm, were investigated at elevated temperature during their interaction with H$_2$ and methylcyclohexane (MCH) representing a model LOHC. The interaction with H$_2$ was studied in-situ at partial pressures up to 1 bar and temperatures between 300 and 500 K. At 300 K, the Pd nanoparticles (NPs) show a transition from $α$-PdH to $β$-PdH as a function of the H$_2$ pressure. The transition occurs gradually, which is attributed to the heterogeneity of the NP system. The hydrogen uptake in $β$-PdH$_x$ at 300 K and 1 bar is estimated to be XH ~ 0.37 ± 0.03 indicating that the miscibility gap is narrowed for the nanoparticular system. With increasing temperature, X$_H$ decreases until no $β$-PdH phase is formed anymore at 500 K. At the same temperature, we studied the interaction of the Pd/sapphire model catalyst with MCH, both in the presence and in the absence of H$_2$. In the absence of H$_2$, carbon is formed and diffuses into the bulk yielding PdC$_x$ with a C concentration of around x ~ 0.05 ± 0.01. In the presence of H$_2$ in the gas phase, bulk carbon formation in the Pd/sapphire model catalyst is completely suppressed. These results show that Pd nanoparticles act as an adequate catalyst for the dehydrogenation of MCH., Published by Springer Science + Business Media B.V, Dordrecht [u.a.]

Understanding nanoparticle catalysis requires novel approaches in which adjoining crystal orientations can be studied under the same reactive conditions. Here we use a curved palladium crystal and near-ambient pressure X-ray photoemission spectroscopy to characterize chemical species during the catalytic oxidation of CO in a whole set of surfaces vicinal to the (111) direction simultaneously. By stabilizing the reaction at fixed temperatures around the ignition point, we observe a strong variation of the catalytic activity across the curved surface. Such spatial modulation of the reaction stage is straightforwardly mapped through the photoemission signal from active oxygen species and poisoning CO, which are shown to coexist in a transient regime that depends on the vicinal angle. Line-shape analysis and direct comparison with ultrahigh vacuum experiments help identifying and quantifying all such surface species, allowing us to reveal the presence of surface oxides during reaction ignition and cooling-off., We acknowledge financial support from the Spanish Ministry of Economy (Grants MAT-2012-38567-C02-02, MAT-2016-78293-C6, MAT-2017-88374-P) and Basque Government (Grant IT-1255-19).

Topics in catalysis 60(6-7), 492 - 502 (2017). doi:10.1007/s11244-016-0714-8, One layer thick iron oxide films are attractive from both applied and fundamental science perspectives. The structural and chemical properties of these systems can be tuned by changing the substrate, making them promising materials for heterogeneous catalysis. In the present work, we investigate the structure of FeO(111) monolayer films grown on Ag(100) and Ag(111) substrates by means of microscopy and diffraction techniques and compare it with the structure of FeO(111) grown on other substrates reported in literature. We also study the NO adsorption properties of FeO(111)/Ag(100) and FeO(111)/Ag(111) systems utilizing different spectroscopic techniques. We discuss similarities and differences in the data obtained from adsorption experiments and compare it with previous results for FeO(111)/Pt(111)., Published by Baltzer, Bussum

We study the structure–function relationship of alumina supported platinum during the formation of ammonia from nitrogen oxide and dihydrogen by employing in situ X-ray absorption and Fourier transform infrared spectroscopy. Particular focus has been directed towards the effect of oxygen on the reaction as a model system for emerging technologies for passive selective catalytic reduction of nitrogen oxides. The suppressed formation of ammonia observed as the feed becomes net-oxidizing is accompanied by a considerable increase in the oxidation state of platinum as well as the formation of surface nitrates and the loss of NH-containing surface species. In the presence of (excess) oxygen, the ammonia formation is proposed to be limited by weak interaction between nitrogen oxide and the oxidized platinum surface. This leads to a slow dissociation rate of nitrogen oxide and thus low abundance of the atomic nitrogen surface species that can react with the adsorbed hydrogen species. In this case the consumption of hydrogen through the competing water formation reaction and decomposition/oxidation of ammonia are of less importance for the net ammonia formation.

The spin polarization of Pt in Pt/NiFe2O4 and Pt/Fe bilayers is studied by interface-sensitive x-ray resonant magnetic reflectivity to investigate static magnetic proximity effects. The asymmetry ratio of the reflectivity is measured at the Pt L3 absorption edge using circular polarized x-rays for opposite directions of the magnetization at room temperature. The results of the 2% asymmetry ratio for Pt/Fe bilayers are independent of the Pt thickness between 1.8 and 20 nm. By comparison with ab initio calculations, the maximum magnetic moment per spin polarized Pt atom at the interface is determined to be (0.6±0.1) μB for Pt/Fe. For Pt/NiFe2O4 the asymmetry ratio drops below the sensitivity limit of 0.02 μB per Pt atom. Therefore, we conclude, that the longitudinal spin Seebeck effect recently observed in Pt/NiFe2O4 is not influenced by a proximity induced anomalous Nernst effect.

International audience; We present results from the anodization of an aluminum single crystal [Al(111)] and an aluminum alloy [Al 6060] studied by in situ x-ray reflectivity, in situ electrochemical impedance spectroscopy and ex situ scanning electron microscopy. For both samples, a linear increase of oxide film thickness with increasing anodization voltage was found. However, the slope is much higher in the single crystal case, and the break-up of the oxide film grown on the alloy occurs at a lower anodization potential than on the single crystal. The reasons for these observations are discussed as are the measured differences observed for x-ray reflectivity and electrochemical impedance spectroscopy. (C) 2014 Author(s)

X-ray photoelectron emission microscopy (XPEEM) was used in combination with other microscopic and spectroscopic techniques to follow the surface development of an aluminum brazing sheet during heating. The studied aluminum alloy sheet is a composite material designed for vacuum brazing. Its surface is covered with a native aluminum oxide film. Changes in the chemical state of the alloying elements and the composition of the surface layer were detected during heating to the melting temperature. It was found that Mg segregates to the surface upon heating, and the measurements indicate the formation of magnesium aluminate. During the heating the aluminum oxide as well as the silicon is observed to disappear from the surface. Our measurements is in agreement with previous studies observing a break-up of the oxide and the outflow of the braze cladding onto the surface, a process assisted by the Mg segregation and reaction with surface oxygen. This study also demonstrates how XPEEM can be utilized to study complex industrial materials.