Multimodal X-ray imaging of grain-level properties and performance in a polycrystalline solar cell.

The factors limiting the performance of alternative polycrystalline solar cells as compared with their single-crystal counterparts are not fully understood, but are thought to originate from structural and chemical heterogeneities at various length scales. Here, it is demonstrated that multimodal focused nanobeam X-ray microscopy can be used to reveal multiple aspects of the problem in a single measurement by mapping chemical makeup, lattice structure and charge collection efficiency simultaneously in a working solar cell. This approach was applied to micrometre-sized individual grains in a Cu(In,Ga)Se2 polycrystalline film packaged in a working device. It was found that, near grain boundaries, collection efficiency is increased, and that in these regions the lattice parameter of the material is expanded. These observations are discussed in terms of possible physical models and future experiments.

The self-healing of defects induced by the hydriding phase transformation in palladium nanoparticles.

Nanosizing can dramatically alter material properties by enhancing surface thermodynamic contributions, shortening diffusion lengths, and increasing the number of catalytically active sites per unit volume. These mechanisms have been used to explain the improved properties of catalysts, battery materials, plasmonic materials, etc. Here we show that Pd nanoparticles also have the ability to self-heal defects in their crystal structures. Using Bragg coherent diffractive imaging, we image dislocations nucleated deep in a Pd nanoparticle during the forward hydriding phase transformation that heal during the reverse transformation, despite the region surrounding the dislocations remaining in the hydrogen-poor phase. We show that defective Pd nanoparticles exhibit sloped isotherms, indicating that defects act as additional barriers to the phase transformation. Our results resolve the formation and healing of structural defects during phase transformations at the single nanoparticle level and offer an additional perspective as to how and why nanoparticles differ from their bulk counterparts.

Identifying Defects with Guided Algorithms in Bragg Coherent Diffractive Imaging.

Crystallographic defects such as dislocations can significantly alter material properties and functionality. However, imaging these imperfections during operation remains challenging due to the short length scales involved and the reactive environments of interest. Bragg coherent diffractive imaging (BCDI) has emerged as a powerful tool capable of identifying dislocations, twin domains, and other defects in 3D detail with nanometer spatial resolution within nanocrystals and grains in reactive environments. However, BCDI relies on phase retrieval algorithms that can fail to accurately reconstruct the defect network. Here, we use numerical simulations to explore different guided phase retrieval algorithms for imaging defective crystals using BCDI. We explore different defect types, defect densities, Bragg peaks, and guided algorithm fitness metrics as a function of signal-to-noise ratio. Based on these results, we offer a general prescription for phasing of defective crystals with no a priori knowledge.

Bragg Coherent Diffractive Imaging of Zinc Oxide Acoustic Phonons at Picosecond Timescales.

Mesoscale thermal transport is of fundamental interest and practical importance in materials such as thermoelectrics. Coherent lattice vibrations (acoustic phonons) govern thermal transport in crystalline solids and are affected by the shape, size, and defect density in nanoscale materials. The advent of hard x-ray free electron lasers (XFELs) capable of producing ultrafast x-ray pulses has significantly impacted the understanding of acoustic phonons by enabling their direct study with x-rays. However, previous studies have reported ensemble-averaged results that cannot distinguish the impact of mesoscale heterogeneity on the phonon dynamics. Here we use Bragg coherent diffractive imaging (BCDI) to resolve the 4D evolution of the acoustic phonons in a single zinc oxide rod with a spatial resolution of 50 nm and a temporal resolution of 25 picoseconds. We observe homogeneous (lattice breathing/rotation) and inhomogeneous (shear) acoustic phonon modes, which are compared to finite element simulations. We investigate the possibility of changing phonon dynamics by altering the crystal through acid etching. We find that the acid heterogeneously dissolves the crystal volume, which will significantly impact the phonon dynamics. In general, our results represent the first step towards understanding the effect of structural properties at the individual crystal level on phonon dynamics.

Stability Limits and Defect Dynamics in Ag Nanoparticles Probed by Bragg Coherent Diffractive Imaging.

Dissolution is critical to nanomaterial stability, especially for partially dealloyed nanoparticle catalysts. Unfortunately, highly active catalysts are often not stable in their reactive environments, preventing widespread application. Thus, focusing on the structure-stability relationship at the nanoscale is crucial and will likely play an important role in meeting grand challenges. Recent advances in imaging capability have come from electron, X-ray, and other techniques but tend to be limited to specific sample environments and/or two-dimensional images. Here, we report investigations into the defect-stability relationship of silver nanoparticles to voltage-induced electrochemical dissolution imaged in situ in three-dimensional detail by Bragg coherent diffractive imaging. We first determine the average dissolution kinetics by stationary probe rotating disk electrode in combination with inductively coupled plasma mass spectrometry, which allows in situ measurement of Ag+ ion formation. We then observe the dissolution and redeposition processes in single nanocrystals, providing unique insight about the role of surface strain, defects, and their coupling to the dissolution chemistry. The methods developed and the knowledge gained go well beyond a "simple" silver electrochemistry and are applicable to all electrocatalytic reactions where functional links between activity and stability are controlled by structure and defect dynamics.

Three-dimensional imaging of dislocation dynamics during the hydriding phase transformation.

Crystallographic imperfections significantly alter material properties and their response to external stimuli, including solute-induced phase transformations. Despite recent progress in imaging defects using electron and X-ray techniques, in situ three-dimensional imaging of defect dynamics remains challenging. Here, we use Bragg coherent diffractive imaging to image defects during the hydriding phase transformation of palladium nanocrystals. During constant-pressure experiments we observe that the phase transformation begins after dislocation nucleation close to the phase boundary in particles larger than 300 nm. The three-dimensional phase morphology suggests that the hydrogen-rich phase is more similar to a spherical cap on the hydrogen-poor phase than to the core-shell model commonly assumed. We substantiate this using three-dimensional phase field modelling, demonstrating how phase morphology affects the critical size for dislocation nucleation. Our results reveal how particle size and phase morphology affects transformations in the PdH system.

Three Dimensional Variable-Wavelength X-Ray Bragg Coherent Diffraction Imaging.

We present and demonstrate a formalism by which three-dimensional (3D) Bragg x-ray coherent diffraction imaging (BCDI) can be implemented without moving the sample by scanning the energy of the incident x-ray beam. This capability is made possible by introducing a 3D Fourier transform that accounts for x-ray wavelength variability. We demonstrate the approach by inverting coherent Bragg diffraction patterns from a gold nanocrystal measured with an x-ray energy scan. Variable-wavelength BCDI will expand the breadth of feasible in situ 3D strain imaging experiments towards more diverse materials environments, especially where sample manipulation is difficult.

Avalanching strain dynamics during the hydriding phase transformation in individual palladium nanoparticles.

Phase transitions in reactive environments are crucially important in energy and information storage, catalysis and sensors. Nanostructuring active particles can yield faster charging/discharging kinetics, increased lifespan and record catalytic activities. However, establishing the causal link between structure and function is challenging for nanoparticles, as ensemble measurements convolve intrinsic single-particle properties with sample diversity. Here we study the hydriding phase transformation in individual palladium nanocubes in situ using coherent X-ray diffractive imaging. The phase transformation dynamics, which involve the nucleation and propagation of a hydrogen-rich region, are dependent on absolute time (aging) and involve intermittent dynamics (avalanching). A hydrogen-rich surface layer dominates the crystal strain in the hydrogen-poor phase, while strain inversion occurs at the cube corners in the hydrogen-rich phase. A three-dimensional phase-field model is used to interpret the experimental results. Our experimental and theoretical approach provides a general framework for designing and optimizing phase transformations for single nanocrystals in reactive environments.

BATTERIES. Topological defect dynamics in operando battery nanoparticles.

Topological defects can markedly alter nanomaterial properties. This presents opportunities for "defect engineering," where desired functionalities are generated through defect manipulation. However, imaging defects in working devices with nanoscale resolution remains elusive. We report three-dimensional imaging of dislocation dynamics in individual battery cathode nanoparticles under operando conditions using Bragg coherent diffractive imaging. Dislocations are static at room temperature and mobile during charge transport. During the structural phase transformation, the lithium-rich phase nucleates near the dislocation and spreads inhomogeneously. The dislocation field is a local probe of elastic properties, and we find that a region of the material exhibits a negative Poisson's ratio at high voltage. Operando dislocation imaging thus opens a powerful avenue for facilitating improvement and rational design of nanostructured materials.

Vildagliptin therapy reduces postprandial intestinal triglyceride-rich lipoprotein particles in patients with type 2 diabetes.

AIMS/HYPOTHESIS: We assessed the effects of vildagliptin, a novel dipeptidyl peptidase IV inhibitor, on postprandial lipid and lipoprotein metabolism in patients with type 2 diabetes. SUBJECTS, MATERIALS AND METHODS: This was a single-centre, randomised, double-blind study in drug-naive patients with type 2 diabetes. Patients received vildagliptin (50 mg twice daily, n=15) or placebo (n=16) for 4 weeks. Triglyceride, cholesterol, lipoprotein, glucose, insulin, glucagon and glucagon-like peptide-1 (GLP-1) responses to a fat-rich mixed meal were determined for 8 h postprandially before and after 4 weeks of treatment. RESULTS: Relative to placebo, 4 weeks of treatment with vildagliptin decreased the AUC(0-8h) for total trigyceride by 22+/-11% (p=0.037), the incremental AUC(0-8h) (IAUC(0-8h)) for total triglyceride by 85+/-47% (p=0.065), the AUC(0-8h) for chylomicron triglyceride by 65+/-19% (p=0.001) and the IAUC(0-8h) for chylomicron triglyceride by 91+/-28% (p=0.002). This was associated with a decrease in chylomicron apolipoprotein B-48 (AUC(0-8h), -1.0+/-0.5 mg l(-1) h, p=0.037) and chylomicron cholesterol (AUC(0-8h), -0.14+/-0.07 mmol l(-1) h, p=0.046). Consistent with previous studies, 4 weeks of treatment with vildagliptin also increased intact GLP-1, suppressed inappropriate glucagon secretion, decreased fasting and postprandial glucose, and decreased HbA(1c) from a baseline of 6.7% (change, -0.4+/-0.1%, p<0.001), all relative to placebo. CONCLUSIONS/INTERPRETATION: Treatment with vildagliptin for 4 weeks improves postprandial plasma triglyceride and apolipoprotein B-48-containing triglyceride-rich lipoprotein particle metabolism after a fat-rich meal. The mechanisms underlying the effects of this dipeptidyl peptidase IV inhibitor on postprandial lipid metabolism remain to be explored.

Subcellular organization of bile acid amidation in human liver: a key issue in regulating the biosynthesis of bile salts.

To extend our knowledge of how the synthesis of free bile acids and bile salts is regulated within the hepatocyte, bile acid-CoA:amino acid N-acyltransferase and bile acid-CoA thioesterase activities were measured in subcellular fractions of human liver homogenates. Some bile acids, both conjugated and unconjugated, have been reported to be natural ligands for the farnesoid X receptor (FXR), an orphan nuclear receptor. The conversion of [(14)C]choloyl-CoA and [(14)C]chenodeoxycholoyl-CoA into the corresponding tauro- and glyco-bile acids or the free bile acids was measured after high-pressure liquid radiochromatography. There was an enrichment of the N-acyltransferase in the cytosolic and the peroxisomal fraction. Bile acid-CoA thioesterase activities were enriched in the cytosolic, peroxisomal, and mitochondrial fractions. The highest amidation activities of both choloyl-CoA and chenodeoxycholoyl-CoA were found in the peroxisomal fraction (15-58 nmol/mg protein/min). The K(m) was higher for glycine than taurine both in cytosol and the peroxisomal fraction.These results show that the peroxisomal de novo synthesis of bile acids is rate limiting for peroxisomal amidation, and the microsomal bile acid-CoA synthetase is rate limiting for the cytosolic amidation. The peroxisomal location may explain the predominance of glyco-bile acids in human bile. Both a cytosolic and a peroxisomal bile acid-CoA thioesterase may influence the intracellular levels of free and conjugated bile acids.

Inhibitory effects of dopamine and isoprenaline on antral motor activity stimulated by acetylcholine or physostigmine in vitro.

Continuous pressure recordings were made in guinea-pig antral pouches in vitro. The antral phasic activity was stimulated by acetylcholine and by the cholinesterase inhibitory agent physostigmine. Dopamine and isoprenaline both significantly impaired antral pressure responses to physostigmine, whereas the acetylcholine-induced response was not significantly reduced. This suggests that both dopamine and isoprenaline to some extent act by reducing the spontaneously released acetylcholine, suggesting that both drugs act on intramural cholinergic neural pathways.

Effects of alpha- and beta-adrenergic agents and dopamine on gastric motor activity stimulated by acetylcholine or physostigmine.

The inhibitory actions of phenylephrine, adrenaline, isoprenaline and dopamine on the antral and fundic pressure responses to exogenous acetylcholine or physostigmine were tested on isolated whole guinea-pig stomachs. Phenylephrine, adrenaline and dopamine markedly inhibited pressure responses to physostigmine, while the responses to acetylcholine were impaired to a less extent. The antral inhibition was more pronounced than the fundic. Isoprenaline was significantly less potent, but like the former inhibited physostigmine more than acetylcholine in the antrum. It may be concluded that adrenergic agents and dopamine exert their inhibition of gastric motor activity partly on intramural cholinergic neural pathways. Dopaminergic as well as alpha-adrenergic influence appear to be more effective than beta-adrenergic. The pronounced effect on phasic antral activity indicates principal inhibitory actions on propulsive motility and gastric emptying.