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Nanoparticle formation by dopant exsolution (migration) from bulk host lattices is a promising approach to generate highly stable nanoparticles with tunable size, shape, and distribution. We investigated Ni dopant migration from strontium titanate (STO) lattices, forming metallic Ni nanoparticles at STO surfaces. Ex situ scanning probe measurements confirmed the presence of nanoparticles at the H2 treated surface. In situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) revealed reduction from Ni2+ to Ni0 as Ni dopants migrated to the surface during heating treatments in H2. During Ni migration and reduction, the Sr and Ti chemical states were mostly unchanged, indicating the selective reduction of Ni during treatment. At the same time, we used in situ ambient pressure grazing incidence X-ray scattering (GIXS) to monitor the particle morphology. As Ni migrated to the surface, it nucleated and grew into compressed spheroidal nanoparticles partially embedded in the STO perovskite surface. These findings provide a detailed picture of the evolution of the nanoparticle surface and subsurface chemical state and morphology as the nanoparticles grow beyond the initial nucleation and growth stages.

An ultra-high-vacuum compatible X-ray chopper system has been designed, constructed and integrated into the XTIP beamline at the Advanced Photon Source at Argonne National Laboratory. The XTIP beamline can operate at soft X-ray energies from 400 eV to 1900 eV while providing a focused beam down to about 10 µm × 10 µm into the synchrotron X-ray scanning tunneling microscopy (SX-STM) endstation instrument. The X-ray chopper is a critical component for separating topographic information from chemical information in SX-STM through phase-sensitive current detection. Depending on the experimental needs, the modulation frequency can be controlled from 100 Hz to 10 kHz. In addition, the chopper system is fully bakeable and can achieve a base pressure of 10−10 mbar. Facilities for active water cooling have been designed, but passive cooling through copper braids has been shown to be sufficient at standard chopping frequencies. Using an Fe/Al2O3/CoAl(111) sample, the separation of the SX-STM current into a chemical component and a stable feedback signal is demonstrated.

Side reactions involving surface reduction play a critical role in the failure of LiNi0.8Co0.15Al0.05O2 to reach its theoretical capacity as a cathode material for Li-ion batteries. While macroscop...

A new beamline, XTIP, has been constructed at the Advanced Photon Source to deliver monochromatic soft X-rays of between 400 and 1900 eV for the emerging synchrotron X-ray scanning tunneling microscopy technique., In recent years, there have been numerous efforts worldwide to develop the synchrotron X-ray scanning tunneling microscopy (SX-STM) technique. Here, the inauguration of XTIP, the world’s first beamline fully dedicated to SX-STM, is reported. The XTIP beamline is located at Sector 4 of the Advanced Photon Source at Argonne National Laboratory. It features an insertion device that can provide left- or right-circular as well as horizontal- and vertical-linear polarization. XTIP delivers monochromatic soft X-rays of between 400 and 1900 eV focused into an environmental enclosure that houses the endstation instrument. This article discusses the beamline system design and its performance.

Impurity contamination in thin-film solar cells remains an uncertain risk due to the little-known impact of impurities on recombination. Building upon previous work, in which we intentionally contaminated lead halide perovskite (LHP) solar cells with iron, we further examine the distribution and charge state of iron-induced defects in LHP films using synchrotron-based X-ray techniques. X-ray absorption measurements suggest that iron-rich regions, which form among iron feedstock concentrations that exceed 100 ppm, most closely resemble the chemistry of Fe2O3. Iron distributed within the bulk may form a mix of Fe2+and Fe3+, the latter of which is not expected to be recombination active, potentially allowing LHPs to incorporate more iron than traditional semiconductors. X-ray beam induced current measurements show little correlation between the presence of iron-rich regions and charge collection, which further suggests low recombination activity at these sites. These results further elucidate the recombination behavior caused by iron incorporation in LHP films, revealing insight into how inhomogeneous incorporation of impurities may mitigate photovoltaic performance degradation., National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (award number DMR-1419807), United States. Department of Energy. Office of Science User Facility (Contract No. DE-AC02-06CH11357), National Science Foundation (U.S.) (NSF EECS Award No. 1541959), Martin Family Society of Fellows for Sustainability, National Science Foundation (U.S.). Graduate Research Fellowship (Grant No. 1122374), National Science Foundation (U.S.). (CA No. EEC-1041895)

Light- and elevated temperature-induced degradation (LeTID) is a detrimental effect observed under operating conditions in p-type multicrystalline silicon (mc-Si) solar cells. In this contribution, we employ synchrotron-based techniques to study the dissolution of precipitates due to different firing processes at grain boundaries in LeTID-affected mc-Si. The synchrotron measurements show clear dissolution of collocated metal precipitates during firing. We compare our observations with degradation behavior in the same wafers. The experimental results are complemented with process simulations to provide insight into the change in bulk point defect concentration due to firing. Several studies have proposed that LeTID is caused by metal-rich precipitate dissolution during contact firing, and we find that the solubility and diffusivity are promising screening metrics to identify metals that are compatible with this hypothesis. While slower and less soluble elements (e.g., Fe and Cr) are not compatible according to our simulations, the point defect concentrations of faster and more soluble elements (e.g., Cu and Ni) increase after a high-temperature firing process, primarily due to emitter segregation rather than precipitate dissolution. These results are a useful complement to lifetime spectroscopy techniques, and can be used to evaluate additional candidates in the search for the root cause of LeTID.

There is no doubt about the potential offered by the low-cost fabrication and high efficiency of hybrid organic–inorganic perovskite solar cells. However, the service lifetimes of these devices must be increased from months to years to capitalize on their potential. The archetypal hybrid perovskite for solar cells, methylammonium lead iodide (CH3NH3PbI3, abbreviated MAPI), readily degrades in ambient atmosphere under standard operating conditions. Understanding the origin and effects of this degradation can pave the way to better engineer photovoltaic devices and the perovskite material itself. Herein we present the effects of varying pressure on the electrical performance of MAPI solar cells. Solar cell parameters, especially open circuit voltage, are significantly affected by the total ambient pressure and present an unexpected reversible behavior upon pressure cycling. We complement photoluminescence studies as a function of ambient atmosphere and temperature with first-principles density functional t...

In situ and operando measurement techniques combined with nanoscale resolution have proven invaluable in multiple fields of study. We argue that evaluating device performance as well as material behavior by correlative X-ray microscopy with

Statistical and correlative analysis are increasingly important in the design and study of new materials, from semiconductors to metals. Non-destructive measurement techniques, with high spatial resolution, capable of correlating composition and/or structure with device properties, are few and far between. For the case of polycrystalline and inhomogeneous materials, the added challenge is that nanoscale resolution is in general not compatible with the large sampling areas necessary to have a statistical representation of the specimen under study. For the study of grain cores and grain boundaries in polycrystalline solar absorbers this is of particular importance since their dissimilar behavior and variability throughout the samples makes it difficult to draw conclusions and ultimately optimize the material. In this study, we present a nanoscale in-operando approach based on the multimodal utilization of synchrotron nano x-ray fluorescence and x-ray beam induced current collected for grain core and grain boundary areas and correlated pixel-by-pixel in fully operational Cu ( In ( 1 − x ) Ga x ) Se 2 solar cells. We observe that low gallium cells have grain boundaries that over perform compared to the grain cores and high gallium cells have boundaries that under perform. These results demonstrate how nanoscale correlative X-ray microscopy can guide research pathways towards grain engineering low cost, high efficiency solar cells.

Synchrotron X-ray Scanning Tunneling Microscopy (SX-STM) is a novel imaging technique capable of providing real space chemically specific mapping with a potential of reaching atomic resolution. Determination of chemical composition along with ultra-high resolution imaging by SX-STM can be realized through excitation of core electrons by incident X-rays when their energy is tuned to an absorption edge of a particular atom during raster scanning, as is done in the conventional STM experiments. In this work, we provide a brief summary and the current status of SX-STM and discuss its applications for material science. In particular, we discuss instrumentation challenges associated with the SX-STM technique and present early experiments on Cu doped ZrTe3 single crystals.

The reasons for the start and end of the induction period of cement hydration remain topic of controversy. One long-standing hypothesis is that a thin metastable hydrate forming on the surface of cement grains significantly reduces the particle dissolution rate; the eventual disappearance of this layer re-establishes higher dissolution rates at the beginning of the acceleration period. However, the importance, or even the existence, of this metastable layer has been questioned because it cannot be directly detected in most experiments. In this work, a combined analysis using nano-tomography and nano-X-ray fluorescence makes the direct imaging of early hydration products possible. These novel X-ray imaging techniques provide quantitative measurements of 3D structure, chemical composition, and mass density of the hydration products during the induction period. This work does not observe a low density product on the surface of the particle, but does provide insights into the formation of etch pits and the subsequent hydration products that fill them.

Spatial heterogeneities in the chemical makeup of thin film photovoltaic devices are pivotal in determining device efficiency. We report the in-plane spatial distribution and degree of chlorine incorporation in organic–inorganic lead halide perovskite absorbers by means of nondestructive synchrotron-based nanoprobe X-ray fluorescence. The presence of chlorine is positively identified in CH3NH3PbI3 films synthesized with Cl-containing precursors and as an impurity in some films synthesized with nominally Cl-free precursors. The impurity may be introduced from precursors or as contaminants during film synthesis. The films formed from Cl-containing precursors contain roughly an order of magnitude higher amount of chlorine, with Cl:I values greater than 0.02 found whether Cl is present in either the organic or the inorganic precursor for both one- and two-step fabrication processes. A spatial variation in the Cl incorporation is observed within single particles and as well as between particles within a given ...

Lead halide perovskites have emerged as promising absorber materials over the last decade to increase the efficiency of photovoltaics beyond its current limits. However, to further optimize the performance of perovskites more detailed studies need to be performed, which allow for the correlation of film morphology and local electronic properties at the nanoscale. Here, we present a scanning tunneling microscopy (STM) approach to probe the effect of an applied electric field of a methylammonium formamidinium lead triiodide perovskite thin film on the film response by current–voltage spectroscopy, current imaging tunneling spectroscopy, differential conductance mapping, and x-ray absorption spectroscopy by means of synchrotron x-ray STM. We find a strong correlation between the measurement conditions and the obtained current–voltage characteristics when imaging under opposite bias polarities. In particular, we find similarities to already observed poling effects for lead halide perovskites, which result in either a positively or negatively charged interface due to ion and vacancy migration. Our results provide insight into the influence of measurement conditions such as bias polarity on the performance assessment of perovskite thin films by STM.

Disagreements about the mechanisms of cement hydration remain despite the fact that portland cement has been studied extensively for over 100 years. One reason for this is that direct observation of the change in microstructure and chemistry are challenging for many experimental techniques. This paper presents results from synchrotron nano X-ray tomography and fluorescence imaging. The data show unprecedented direct observations of small collections of C3S particles before and after different periods of hydration in 15 mmol/L lime solution. X-ray absorption contrast is used to make three dimensional maps of the changes of these materials with time. The chemical compositions of hydration products are then identified with X-ray fluorescence mapping and scanning electron microscopy. These experiments are used to provide insight into the rate and morphology of the microstructure formation.

X-Ray Microscopy 2018, Saskatoon, Canada, 19 Aug 2018 - 24 Aug 2018; Microscopy and microanalysis 24(S2), 434 - 435 (2018). doi:10.1017/S1431927618014423, Published by Cambridge University Press

X-ray beam-induced damage in nanoscale metal islands was investigated. Monolayer-high Ni islands were prepared on a Cu(111) substrate. High brilliance X-rays with photon energies between 8.45 and 8.85 keV illuminated the sample for about 11 hours. In order to track changes in the morphology of the islands, the synchrotron X-ray scanning tunneling microscopy (SX-STM) technique was utilized. The result shows that X-ray illumination onto Ni islands does not induce noticeable damage. The study demonstrates that local beam-induced changes can be studied using SX-STM.

The efficiency of solar cells with a polycrystalline absorber is typically limited by grain boundaries. Many questions need to be answered such as: which roles do elemental and structural inhomogeneities play with respect to electrical performance? By which mechanisms does degradation occur and how can it be mitigated? The answers to these questions lie at the nanoscale. Ideally, we would measure performance, composition, and structure in-situ and operando all at once. Correlative, synchrotron-based X-ray microscopy offers a step in this direction. Here, we present a novel technique, X-ray beam induced voltage (XBIV) measurements, which complements the set of X-ray microscopy techniques. Combining the penetration depth of visible-light microscopy with the spatial resolution of electron-beam methods, XBIV measurements shine light on recombination and absorber-layer bandgap variations at nanoscale resolution. We give experimental details and discuss the use of lock-in amplification based on first applications of XBIV measurements to $\text{CuIn}_{x} \text{Ga}_{1}-x\text{Se}_{2}$ solar cells.

The root cause of light- and elevated temperature-induced degradation (LeTID) in multicrystalline silicon p-type passivated emitter and rear cell (PERC) devices is still unknown. Some researchers hypothesize that high temperature firing processes dissolve metal-rich precipitates which can then participate in LeTID. To address this hypothesis, synchrotron-based X-ray techniques, including fluorescence and absorption near-edge spectroscopy, are employed. In as-grown industrial material, we observe collocated copper- and nickel-rich precipitates, which persist after firing and are below the detection limit after phosphorous diffusion. We conclude that precipitates decrease in size due to the firing process and that this may result in an increase in bulk interstitial metal concentration. We further employ microphotoluminescence at a grain versus grain boundary to highlight similarities and possible differences in degradation and regeneration behavior.

The real-space observation of magnetic structure using scanning probe microscopy (SPM) methods or synchrotron-based microscopy continues to have a tremendous impact on our understanding of functional magnetic materials. However, although SPM methods provide high spatial resolution, they lack direct chemical contrast and the ability to quantify magnetic moments. X-ray microscopy, on the other hand, can provide chemical as well as magnetic sensitivity, but the spatial resolution is limited.

It is well known that the addition of alkali elements such as Na and K during and after growth of Cu(In, Ga)Se 2 (CIGS) has beneficial effects on the electronic properties of bulk material, improving device performance significantly. While the device level effects have been measured and reported, a direct observations of the localization of Na including its chemical nature are missing, and the impact of Na on elemental and phase segregation during CIGS growth is not fully understood. We investigate these aspects to shine light on the role of Na in CIGS solar cells with the ultimate goal of increasing their conversion efficiency. Utilizing a suite of synchrotron based x-ray characterization techniques, we discuss the challenges and advantages of these techniques for investigating segregation of main constituents of CIGS, Na distribution, chemical bonding of Na, and collection efficiency in CIGS as well as their correlations.

Spatial heterogeneities in the chemical makeup of thin film photovoltaic devices are pivotal in determining device efficiency. In this study, the presence of chlorine is identified in perovskite films synthesized with Cl-containing precursors by means of nanoprobe X-ray fluorescence (Nano-XRF). Additionally, a spatial variation in the Cl incorporation is observed within a given film, and the standard deviation of Cl: I ratio across the film is large. Using Nano-XRF, the Cl incorporation in methylammonium lead iodide perovskite films can be manipulated by precursor stoichiometry ratio.

Unveiling the correlation between elemental composition, fermi-level splitting, and charge collection in perovskite solar cells (PSCs) exposed to different environments is crucial to understanding the origin of defects. This will enable defect engineering to achieve high performing and long lasting perovskite solar cells. In this contribution we measured for the first time the spatial distribution and charge collection efficiency at the nano-scale by synchrotron-based x-ray fluorescence (XRF) and x-ray beam induced current (XBIC) with sub-grain resolution, and we observe a correlation between Pb/I ratio and charge collection efficiency. In contrast to other thin-film solar cells, perovskite solar cells are highly sensitive to ambient conditions (atmosphere and illumination). As the XRF and XBIC measurements were conducted in vacuum under an x-ray source illumination, the impact of measurement conditions on the measurements need to be taken into account. Furthermore, necessary conditions for quantification of XRF/XBIC measurements are not fulfilled for perovskite solar cells. Therefore, we will discuss fundamentals of XRF/XBIC measurements of perovskite solar cells that will enable reliable quantitative, high-resolution measurements of elemental distribution and charge collection.

A tunneling smart tip of a synchrotron X-ray scanning tunneling microscope provides simultaneously localized topographic, elemental and magnetic information., Localized spectroscopy with simultaneous topographic, elemental and magnetic information is presented. A synchrotron X-ray scanning tunneling microscope has been employed for the local study of the X-ray magnetic circular dichroism at the Fe L 2,3-edges of a thin iron film grown on Cu(111). Polarization-dependent X-ray absorption spectra have been obtained through a tunneling smart tip that serves as a photoelectron detector. In contrast to conventional spin-polarized scanning tunneling microscopy, X-ray excitations provide magnetic contrast even with a non-magnetic tip. Intensity variations in the photoexcited tip current point to chemical variations within a single magnetic Fe domain.

High-speed beam choppers are a crucial part of time-resolved x-ray studies as well as a necessary component to enable elemental contrast in synchrotron x-ray scanning tunneling microscopy (SX-STM). However, many chopper systems are not capable of operation in vacuum, which restricts their application to x-ray studies with high photon energies, where air absorption does not present a significant problem. To overcome this limitation, we present a fully ultra-high vacuum (UHV) compatible chopper system capable of operating at variable chopping frequencies up to 4 kHz. The lightweight aluminum chopper disk is coated with Ti and Au films to provide the required beam attenuation for soft and hard x-rays with photon energies up to about 12 keV. The chopper is used for lock-in detection of x-ray enhanced signals in SX-STM.

The seventh edition of the international conference on Synchrotron Radiation and Neutrons in Art and Archaeology (SR2A 2016) was held September 6–8, 2016, at the Stock Exchange Room of The Art Institute of Chicago, USA. The conference was jointly organized by seven research laboratories and museums; more precisely, the Center for Scientific Studies in the Arts (NU-ACCESS) of Northwestern University, the Art Institute of Chicago, the Field Museum Chicago, the Advanced Photon Source (APS), the Oriental Institute Chicago, the Detroit Institute of Arts, and the Indianapolis Museum of Art, in close interaction with the SR2A International Committee. Nine years after the organization of the first SR2A conference in Grenoble, the Art Institute hosted the second biennial interdisciplinary meeting in the US.

Magnetism at buried interfaces plays a crucial role in many emerging phenomena, but detection of interfacial magnetism in close proximity to a surface with elemental and chemical sensitivity is a challenging task. Here, we use low temperature synchrotron x-ray scanning tunneling microscopy to investigate x-ray magnetic circular dichroism and the near edge x-ray absorption fine structure of La0.67Sr0.33MnO3-LaNiO3 superlattices. In stark contrast to the weak magnetic signal of Mn when the La0.67Sr0.33MnO3 layers are located on top, a robust x-ray magnetic circular dichroism signal is detected when they are buried underneath the LaNiO3 layers. The near edge x-ray absorption fine structure reveals the valence states of manganese, while the oxygen K-edge x-ray absorption spectra show an increase in hole formation, indicating a cogent charge transfer at the LaNiO3/La0.67Sr0.33MnO3 interface. This work demonstrates that scanning tunneling microscopy can be extended to the synchrotron X-ray study of buried interfaces by controlling the tip-sample separation in the nanometer regime.

The study of a multilayered and multicomponent system by spatially resolved X-ray fluorescence microscopy poses unique challenges in achieving accurate quantification of elemental distributions. This is particularly true for the quantification of materials with high X-ray attenuation coefficients, depth-dependent composition variations and thickness variations. A widely applicable procedure for use after spectrum fitting and quantification is described. This procedure corrects the elemental distribution from the measured fluorescence signal, taking into account attenuation of the incident beam and generated fluorescence from multiple layers, and accounts for sample thickness variations. Deriving from Beer–Lambert's law, formulae are presented in a general integral form and numerically applicable framework. The procedure is applied using experimental data from a solar cell with a Cu(In,Ga)Se2 absorber layer, measured at two separate synchrotron beamlines with varied measurement geometries. This example shows the importance of these corrections in real material systems, which can change the interpretation of the measured distributions dramatically.

We present the latest developments in the characterization of thin-film solar cells based on the combination of elemental mapping from fluorescence measurements using synchrotron x-rays, with beam induced current from electron and x-ray beams. This is a powerful method to directly correlate compositional variations with charge collection efficiency. We compare different approaches for mapping solar cells both in cross-section and in plan view on CIGS and CdTe solar cells. Based on examples from our latest research, we discuss the experimental approaches and highlight the advantages and limitations of each technique. Finally, we present an outlook to experiments that will allow x-ray based characterization to enter new fields of research that were not accessible before.

We use a nanofabricated scanning tunneling microscope tip as a detector to investigate local X-ray induced tunneling and electron emission from a single cobalt nanocluster on a Au(111) surface. The tip-detector is positioned a few angstroms above the nanocluster, and ramping the incident X-ray energy across the Co photoabsorption K-edge enables the detection of element specific electrons. Atomic-scale spatial dependent changes in the X-ray absorption cross section are directly measured by taking the X-ray induced current as a function of X-ray energy. From the measured sample and tip currents, element specific X-ray induced current components can be separated and thereby the corresponding yields for the X-ray induced processes of the single cobalt nanocluster can be determined. The detection of element specific synchrotron X-ray induced electrons of a single nanocluster opens an avenue for materials characterization on a one particle at-a-time basis.

The effect of a local external electric field on the barrier potential of a tunneling gap is studied utilizing an emerging technique, synchrotron x-ray scanning tunneling microscopy. Here, we demonstrate that the shape of the potential barrier in the tunneling gap can be altered by a localized external electric field, generated by voltages placed on the metallic outer shield of a nanofabricated coaxial metal-insulator-metal tip, resulting in a controlled linear modulation of the tunneling current. Experiments at hard and soft x-ray synchrotron beamlines reveal that both the chemical contrast and magnetic contrast signals measured by the tip can be drastically enhanced, resulting in improved local detection of chemistry and magnetization at the surface.