Hydrogenation of silicon surfaces alters remarkably their structural and electronic properties. The chemical passivation and thus increased surface diffusion length of adsorbate atoms changes the growth mechanism of metal thin films... more
Hydrogenation of silicon surfaces alters remarkably their structural and electronic properties. The chemical passivation and thus increased surface diffusion length of adsorbate atoms changes the growth mechanism of metal thin films leading to island nucleation and suppression of 2D growth even for low ( 0.1 ML) coverages [1]. We applied the x-ray standing wave technique to study the structure of
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Lithium-ion batteries have advanced enough to be a widespread technology, but future applications will demand higher-performing materials. Electrode materials that undergo conversion reactions are a class of high-performance materials... more
Lithium-ion batteries have advanced enough to be a widespread technology, but future applications will demand higher-performing materials. Electrode materials that undergo conversion reactions are a class of high-performance materials that could meet this challenge, as they have the capability of storing larger amounts of energy compared to intercalation-based materials. However, conversion materials suffer from energy inefficiencies during electrochemical cycling due to issues with first-cycle reversibility and charge overpotentials.1 These inefficiencies are especially hindering in conversion reaction anodes made from 3d transition metal oxides, because the anodes undergo significant structural changes during cycling.2 Further structural characterization is necessary to understand the link between cycling behavior and the energy inefficiencies of the active material. In this work, we study structure and composition changes in conversion anodes with the goal of understanding the lithiation kinetics, ionic diffusion, and nucleation and growth phenomena of such materials. Anodes with architectures of alternating Ni and NiO layers are used, as multilayer structures have been shown to control volume expansion and improve cycling lifetimes in related Li-alloy materials.3 However, the Ni/NiO multilayer architecture also enables more direct control over initial anode microstructure by tuning structural parameters, e.g. layer thickness and organization. Analysis of the layers and their interfaces with Scanning Transmission and Transmission Electron Microscopy (S/TEM) techniques and X-Ray reflectivity (XR) measurements help reveal structure-property relationships in the multilayer electrode. In particular, we elucidate the lithiation mechanism in the multilayer system in addition to changes in the layer nanostructure that have implications for understanding the effects of structural transformations on kinetics and overpotentials in the system. Complementary in-situ S/TEM lithiation experiments of model bilayer systems also present further insight into the reaction dynamics of this fascinating system. (1) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nature 2000, 407, 496. (2) Luo, L.; Wu, J.; Luo, J.; Huang, J.; Dravid, V. P. Sci. Rep. 2014, 4. (3) Fister, T. T.; Esbenshade, J.; Chen, X.; Long, B. R.; Shi, B.; Schlepütz, C. M.; Gewirth, A. A.; Bedzyk, M. J.; Fenter, P. Advanced Energy Materials 2014, 4. Acknowledgment: This work was supported as part of the Center for Electrochemical Energy Science (CEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. This work was also supported by the NUANCE Center new initiatives, and made use of the EPIC facility (NUANCE Center-Northwestern University), which has received support from the NSF MRSEC program (NSF DMR-1121262) at the Materials Research Center, The International Institute for Nanotechnology (IIN); the State of Illinois; and Northwestern University.
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The capacity of most battery insertion reactions is limited by the number of open lithium sites in electrode’s crystal structure, usually reaching one per metal atom. Conversion reactions, where the metal oxygen bond is broken to form... more
The capacity of most battery insertion reactions is limited by the number of open lithium sites in electrode’s crystal structure, usually reaching one per metal atom. Conversion reactions, where the metal oxygen bond is broken to form clusters of Li2O and metal particles, can achieve far higher lithium stoichiometry. However, this process is much less reversible than intercalation and occurs at significantly lower potentials than theoretically expected, which has limited their commercial appeal. To pinpoint the underlying connection between structural changes and the observed overpotentials for lithiation, we have studied conversion reactions in MOx (M = Ni, Fe, and Cr) thin films using the sub-nm interfacial sensitivity of in situ x-ray reflectivity. These studies show that conversion begins at elevated potentials (near 2 V) at the surface before propagating into the bulk. Heterostructures incorporating metal interlayers can also help define the direction and extent of the phase separation. We will discuss the possible origins of this effect and strategies to improve the kinetics and scalability of these model electrodes. This work was supported as part of the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. Figure 1
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Li-ion batteries (LIBs) have been widely used in the past 2 decades, due to their higher energy density and cycling ability, typically using carbon as an anode and layered LiCoO2 as a cathode. The spinel, LiMn2O4 (LMO) is cheaper, safer,... more
Li-ion batteries (LIBs) have been widely used in the past 2 decades, due to their higher energy density and cycling ability, typically using carbon as an anode and layered LiCoO2 as a cathode. The spinel, LiMn2O4 (LMO) is cheaper, safer, and more environmental friendly compared with widely-used LiCoO2 cathodes. However, LMO is known to lose capacity during cycling, ultimately due to disproportionation reactions of Mn3+ transforming to Mn2+ and Mn4+, and the subsequent dissolution of Mn2+ into the electrolyte. Yet our understanding of this process remains rudimentary, and we expect that it can be effectively addressed through interfacial modification (e.g., coatings). This work specifically focuses on developing and using a well-defined model system to observe these interfacial instabilities, and to develop suitable approaches to prevent it. Recently, we have successfully grown high quality epitaxial LMO film on multiple substrates by pulsed laser deposition (PLD). Since LMO film itself is a poor electrical conductor, we have designed systems with built-in current collectors that maintains the LMO-substrate epitaxy. Multiple approaches have been investigated to make the whole system conductive and suitable for cycling: using conductive substrates, a conductive buffer layer, or a conductive surface coating. Conductive substrate is the most straightforward method. High quality epitaxial LMO film has been grown on Nb:STO. For the conductive buffer layer approach, we have tried many possible material including TiN, Al:ZnO, ITO, SrRuO3. It turns out that ITO and SrRuO3 are robust and enable oriented LMO deposition. Graphene was adopted as conductive surface coating. We have successfully transferred graphene onto the surface of LMO film using an anhydrous method. We focus on the in situ real-time surface X-ray studies of well orientated LMO films in the standard lithium ion battery electrolytes (e.g., LiPF6 in EC/DMC). Using the high quality model films grown by PLD, we have probed the mobility of ions and interfacial atomic- and molecular- structures across the electrolyte/electrode interface, including the surface/interface structure changes and reconstruction (using XRR and CTR), individual processes of cation adsorption (resonant XRR/CTR), insertion/extraction.
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X-ray standing waves generated above a mirror surface during total external reflection have been used to locate a heavy atom layer which was embedded hundreds of angstroms above the mirror surface in a Langmuir-Blodgett multilayer. This... more
X-ray standing waves generated above a mirror surface during total external reflection have been used to locate a heavy atom layer which was embedded hundreds of angstroms above the mirror surface in a Langmuir-Blodgett multilayer. This same method has also been used to map out the ion distribution in the diffuse double layer that forms at the electrolyte / charged surface interface.
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How molecular chirality manifests at the nano- to macroscale has been a scientific puzzle since Louis Pasteur discovered biochirality. Chiral molecules assemble into meso-shapes such as twisted and helical ribbons, helicoidal scrolls... more
How molecular chirality manifests at the nano- to macroscale has been a scientific puzzle since Louis Pasteur discovered biochirality. Chiral molecules assemble into meso-shapes such as twisted and helical ribbons, helicoidal scrolls (cochleates), or möbius strips (closed twisted ribbons). Here we analyze self-assembly for a series of amphiphiles, Cn-K, consisting of an ionizable amino acid [lysine (K)] coupled to alkyl tails with n = 12, 14, or 16 carbons. This simple system allows us to probe the effects of electrostatic and van der Waals interactions in chiral assemblies. Small/wide-angle X-ray scattering (SAXS/WAXS) reveals that at low pH, where the headgroups are ionized (+1), C16-K forms high aspect ratio, planar crystalline bilayers. Molecular dynamics (MD) simulations reveal that tilted tails of the bilayer leaflets are interdigitated. SAXS shows that, with increasing salt concentration, C16-K molecules assemble into cochleates, whereas at elevated pH (reduced degree of ionization), helices are observed for all Cn-K assemblies. The shape selection between helices and scrolls is explained by a membrane energetics model. The nano- to meso-scale structure of the chiral assemblies can be continuously controlled by solution ionic conditions. Overall, our study represents a step toward an electrostatics-based approach for shape selection and nanoscale structure control in chiral assemblies.
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Chalcogenide-based phase change memory (PCM) is a key enabling technology for optical data storage and electrical nonvolatile memory. Here, we report a new phase change chalcogenide consisting of a 3D network of ionic (K···Se) and... more
Chalcogenide-based phase change memory (PCM) is a key enabling technology for optical data storage and electrical nonvolatile memory. Here, we report a new phase change chalcogenide consisting of a 3D network of ionic (K···Se) and covalent bonds (Bi-Se), K2Bi8Se13 (KBS). Thin films of amorphous KBS deposited by DC sputtering are structurally and chemically homogeneous and exhibit a surface roughness of 5 nm. The KBS film crystallizes upon heating at ∼483 K. The optical bandgap of the amorphous film is about 1.25 eV, while its crystalline phase has a bandgap of ∼0.65 eV shows 2-fold difference between the two states. The bulk electrical conductivity of the amorphous and crystalline film is ∼7.5 × 10-4 and ∼2.7 × 10-2 S/cm, respectively. We have demonstrated a phase change memory effect in KBS by Joule heating in a technologically relevant vertical memory cell architecture. Upon Joule heating, the vertical device undergoes switching from its amorphous to crystalline state of KBS at 1-1.5 V (∼50 kV/cm), increasing conductivity by a factor of ∼40. Besides the large electrical and optical contrast in the crystalline and amorphous KBS, its elemental cost-effectiveness, stoichiometry, fast crystallization kinetics, as determined by the ratio of the glass transition and melting temperature, Tg/Tm ∼ 0.5, as well as the scalable synthesis of the thin film determine that KBS is a promising PC material for next general phase change memory.
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The heat transfer capabilities of beam stops in CHESS wiggler and undulator beamlines is described. The thermal analysis for the design of these crucial in-vacuum beamline components is based on the use of a finite element analysis... more
The heat transfer capabilities of beam stops in CHESS wiggler and undulator beamlines is described. The thermal analysis for the design of these crucial in-vacuum beamline components is based on the use of a finite element analysis computer calculation and experimental heat loading tests.
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During redox reactions, oxide-supported catalytic systems undergo structural and chemical changes. Improving subsequent catalytic properties requires an understanding of the atomic-scale structure with chemical state specificity under... more
During redox reactions, oxide-supported catalytic systems undergo structural and chemical changes. Improving subsequent catalytic properties requires an understanding of the atomic-scale structure with chemical state specificity under reaction conditions. For the case of 1/2 monolayer vanadia on α-TiO2(110), we use X-ray standing wave (XSW) excited X-ray photoelectron spectroscopy to follow the redox induced atomic positional and chemical state changes of this interface. While the resulting XSW 3D composite atomic maps include the Ti and O substrate atoms and V surface atoms, our focus in this report is on the previously unseen surface oxygen species with comparison to density functional theory predictions.
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One of the main goals in surface science is the determination of the geometric structure of the surface, including the local registry of adsorbed atoms and molecules on surfaces. The X-ray Standing Wave (XSW) technique offers a powerful... more
One of the main goals in surface science is the determination of the geometric structure of the surface, including the local registry of adsorbed atoms and molecules on surfaces. The X-ray Standing Wave (XSW) technique offers a powerful method of locating the positions of atoms and molecules in and on single crystals1. This technique is now being employed with increasing frequency with the availability of x-ray radiation synchrotron sources, but is still underutilized with regards to structural determinations of adsorbates on metal surfaces.When x rays are Bragg-reflected from a crystal, the incident and diffracted waves interfere to set up a standing wave field parallel to and having the same spatial periodicity as the reflecting planes. The exact location of the peaks of this standing wave field shift relative to the atomic scattering planes as one scans through the region of total reflectivity associated with the Bragg condition. By measuring a yield characteristic of a bulk atom...
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The XSW method, due to its element specificity and phase sensitivity, opens up new interesting application areas for studies of the structure of epitaxial thin films. The purpose of this article is to demonstrate the capabilities of the... more
The XSW method, due to its element specificity and phase sensitivity, opens up new interesting application areas for studies of the structure of epitaxial thin films. The purpose of this article is to demonstrate the capabilities of the technique based on the experiments the authors have conducted at different SR sources over the past several years. We will limit the scope of this review to only the few applications that include precise lattice mismatch measurements, polarity determination, and impurity localization. We will show that the properties and the behavior of the standing wave in the epitaxial structure and attainable structural information depend on a combination of experimental parameters. The most important parameters are: the film thickness tjam, extinction length Lex, lattice mismatch kd/d = —A8 • ctgQB, determining the angular separation of the substrate and film diffraction peaks A0, and the yield depth Lyt of the secondary radiation excited by the XSW field, usually fluorescence or photoelectrons. Based on the above parameters we can describe thin film XSW experiments under the following four situations:
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The crystallographic site-specific valence band (VB) electronic structure of alpha-Fe2O3 was studied by x-ray standing wave (XSW) induced photoemission. By fine tuning the incident photon energy with respect to the 0.5 eV wide (10-14)... more
The crystallographic site-specific valence band (VB) electronic structure of alpha-Fe2O3 was studied by x-ray standing wave (XSW) induced photoemission. By fine tuning the incident photon energy with respect to the 0.5 eV wide (10-14) Bragg reflectivity peak in the back reflection geometry, we were able to controllably position the XSW antinodes with respect to the (10-14) diffraction planes. By choosing two photon energies that gave the maximum contrast for the Fe and O photoemission signals, we were able to separate out the Fe and O photoemission spectra. In the VB region of the spectra, the O component has two peaks with binding energies at 4.0 and 7.2 eV. The Fe component consists of four peaks extending 18 eV below the Fermi level with maxima at 2.7, 5.3, 8.0, and 12.4 eV. Our results show good agreement with a theoretical calculation for the Fe 3d derived final states which includes d4 and d5L photoemission final states [1]. This work was supported by the EMSI program of NSF a...
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Research Interests: Environmental Science, Geology, Geochemistry, Mineralogy, Materials Science, and 15 moreAtomic Force Microscopy, Adsorption, X Rays, Synchrotron, Ground Water, Calcite, High Resolution, Three Dimensional, MINERAL WATER, Monolayer, Aqueous Solution, Theoretical Model, Structure Determination, Standing Wave, and new combination
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Colloidal crystals of DNA-coated nanoparticles transition from face-centered cubic to body-centered cubic structures with increasing salinity; this transition is concurrent with salt-induced dehydration of the grafted DNA.
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The control of morphology in bioinspired chromophore assemblies is key to the rational design of functional materials for light harvesting. We investigate here morphological changes in perylene monoimide chromophore assemblies during... more
The control of morphology in bioinspired chromophore assemblies is key to the rational design of functional materials for light harvesting. We investigate here morphological changes in perylene monoimide chromophore assemblies during thermal annealing in aqueous environments of high ionic strength to screen electrostatic repulsion. We found that annealing under these conditions leads to the growth of extra-large ribbon-shaped crystalline supramolecular polymers of widths from about 100 nm to several micrometers and lengths from 1 to 10 μm while still maintaining a unimolecular thickness. This growth process was monitored by variable-temperature absorbance spectroscopy, synchrotron X-ray scattering, and confocal microscopy. The extra-large single-crystal-like supramolecular polymers are highly porogenic, thus creating loosely packed hydrogel scaffolds that showed greatly enhanced photocatalytic hydrogen production with turnover numbers as high as 13 500 over ∼110 h compared to 7500 when smaller polymers are used. Our results indicate great functional opportunities in thermally and pathway-controlled supramolecular polymerization.
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LiNiO2 (LNO) is a promising cathode material for next‐generation Li‐ion batteries due to its exceptionally high capacity and cobalt‐free composition that enables more sustainable and ethical large‐scale manufacturing. However, its poor... more
LiNiO2 (LNO) is a promising cathode material for next‐generation Li‐ion batteries due to its exceptionally high capacity and cobalt‐free composition that enables more sustainable and ethical large‐scale manufacturing. However, its poor cycle life at high operating voltages over 4.1 V impedes its practical use, thus motivating efforts to elucidate and mitigate LiNiO2 degradation mechanisms at high states of charge. Here, a multiscale exploration of high‐voltage degradation cascades associated with oxygen stacking chemistry in cobalt‐free LiNiO2, is presented. Lattice oxygen loss is found to play a critical role in the local O3–O1 stacking transition at high states of charge, which subsequently leads to Ni‐ion migration and irreversible stacking faults during cycling. This undesirable atomic‐scale structural evolution accelerates microscale electrochemical creep, cracking, and even bending of layers, ultimately resulting in macroscopic mechanical degradation of LNO particles. By emplo...
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Zirconium doping has a dramatically different influence on Ce reduction in the bulk than on the surface of ceria–zirconia.
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The successful integration of graphene with dielectric materials is an important step in the development of high-performance graphene-based nanoelectronics. For optimal performance, dielectric layers on graphene need to be ultrathin,... more
The successful integration of graphene with dielectric materials is an important step in the development of high-performance graphene-based nanoelectronics. For optimal performance, dielectric layers on graphene need to be ultrathin, conformal, and the growth process needs to be well-controlled and reproducible. In practice, atomic layer deposition (ALD) – a thin film growth process that utilizes self-limiting surface reactions for digital thin film thickness control – is an industrially relevant route to grow dielectric films and nanostructures for electronic applications. However, the chemical inertness and hydrophobicity of graphene typically cause direct ALD processes to yield films of poor quality (defected, non-uniform, and poorly adhered), thereby limiting the ultimate device performance. The key to successful integration of graphene and ALD film growth resides, therefore, in the ability to tailor the graphene surface in such a way that it is amenable to ALD chemistries, but ...
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A high voltage LiNi0.5Mn0.3Co0.2O2/graphite cell with a fluorinated electrolyte formulation 1.0 m LiPF6 fluoroethylene carbonate/bis(2,2,2‐trifluoroethyl) carbonate is reported and its electrochemical performance is evaluated at cell... more
A high voltage LiNi0.5Mn0.3Co0.2O2/graphite cell with a fluorinated electrolyte formulation 1.0 m LiPF6 fluoroethylene carbonate/bis(2,2,2‐trifluoroethyl) carbonate is reported and its electrochemical performance is evaluated at cell voltage of 4.6 V. Comparing with its nonfluorinated electrolyte counterpart, the reported fluorinated one shows much improved Coulombic efficiency and capacity retention when a higher cut‐off voltage (4.6 V) is applied. Scanning electron microscopy/energy dispersive X‐ray spectroscopy and X‐ray photoelectron spectroscopy data clearly demonstrate the superior oxidative stability of the new electrolyte. The structural stability of the bulk cathode materials cycled with different electrolytes is extensively studied by X‐ray absorption near edge structure and X‐ray diffraction.