Fluoride Chemistry in Tin Halide Perovskites.

Tin is the frontrunner for substituting toxic lead in perovskite solar cells. However, tin suffers the detrimental oxidation of SnII to SnIV . Most of reported strategies employ SnF2 in the perovskite precursor solution to prevent SnIV formation. Nevertheless, the working mechanism of this additive remains debated. To further elucidate it, we investigate the fluoride chemistry in tin halide perovskites by complementary analytical tools. NMR analysis of the precursor solution discloses a strong preferential affinity of fluoride anions for SnIV over SnII , selectively complexing it as SnF4 . Hard X-ray photoelectron spectroscopy on films shows the lower tendency of SnF4 than SnI4 to get included in the perovskite structure, hence preventing the inclusion of SnIV in the film. Finally, small-angle X-ray scattering reveals the strong influence of fluoride on the colloidal chemistry of precursor dispersions, directly affecting perovskite crystallization.

Quantification of Nanoscale Density Fluctuations in Hydrogenated Amorphous Silicon.

The nanostructure of hydrogenated amorphous silicon (a-Si∶H) is studied by a combination of small-angle x-ray scattering (SAXS) and small-angle neutron scattering (SANS) with a spatial resolution of 0.8 nm. The a-Si∶H materials were deposited using a range of widely varied conditions and are representative for this class of materials. We identify two different phases that are embedded in the a-Si∶H matrix and quantified both according to their scattering cross sections. First, 1.2 nm sized voids (multivacancies with more than 10 missing atoms) which form a superlattice with 1.6 nm void-to-void distance are detected. The voids are found in concentrations as high as 6×10^{19} cm^{-3} in a-Si∶H material that is deposited at a high rate. Second, dense ordered domains (DOD) that are depleted of hydrogen with 1 nm average diameter are found. The DOD tend to form 10-15 nm sized aggregates and are largely found in all a-Si∶H materials considered here. These quantitative findings make it possible to understand the complex correlation between structure and electronic properties of a-Si∶H and directly link them to the light-induced formation of defects. Finally, a structural model is derived, which verifies theoretical predictions about the nanostructure of a-Si∶H.

Mechanical stability and fibrinolytic resistance of clots containing fibrin, DNA, and histones.

Neutrophil extracellular traps are networks of DNA and associated proteins produced by nucleosome release from activated neutrophils in response to infection stimuli and have recently been identified as key mediators between innate immunity, inflammation, and hemostasis. The interaction of DNA and histones with a number of hemostatic factors has been shown to promote clotting and is associated with increased thrombosis, but little is known about the effects of DNA and histones on the regulation of fibrin stability and fibrinolysis. Here we demonstrate that the addition of histone-DNA complexes to fibrin results in thicker fibers (increase in median diameter from 84 to 123 nm according to scanning electron microscopy data) accompanied by improved stability and rigidity (the critical shear stress causing loss of fibrin viscosity increases from 150 to 376 Pa whereas the storage modulus of the gel increases from 62 to 82 pascals according to oscillation rheometric data). The effects of DNA and histones alone are subtle and suggest that histones affect clot structure whereas DNA changes the way clots are lysed. The combination of histones + DNA significantly prolongs clot lysis. Isothermal titration and confocal microscopy studies suggest that histones and DNA bind large fibrin degradation products with 191 and 136 nM dissociation constants, respectively, interactions that inhibit clot lysis. Heparin, which is known to interfere with the formation of neutrophil extracellular traps, appears to prolong lysis time at a concentration favoring ternary histone-DNA-heparin complex formation, and DNase effectively promotes clot lysis in combination with tissue plasminogen activator.

Deep eutectic solvents for the self-assembly of gold nanoparticles: a SAXS, UV-Vis, and TEM investigation.

In this work, we report the formation and growth mechanisms of gold nanoparticles (AuNPs) in eco-friendly deep eutectic solvents (DES; choline chloride and urea). AuNPs are synthesized on the DES surface via a low-energy sputter deposition method. Detailed small angle X-ray scattering (SAXS), UV-Vis, and cryogenic transmission electron microscopy (cryo-TEM) investigations show the formation of AuNPs of 5 nm diameter. Data analysis reveals that for a prolonged gold-sputtering time there is no change in the size of the particles. Only the concentration of AuNPs increases linearly in time. More surprisingly, the self-assembly of AuNPs into a first and second shell ordered system is observed directly by in situ SAXS for prolonged gold-sputtering times. The self-assembly mechanism is explained by the templating nature of DES combined with the equilibrium between specific physical interaction forces between the AuNPs. A disulfide-based stabilizer, bis((2-mercaptoethyl)trimethylammonium) disulfide dichloride, was applied to suppress the self-assembly. Moreover, the stabilizer even reverses the self-assembled or agglomerated AuNPs back to stable 5 nm individual particles as directly evidenced by UV-Vis. The template behavior of DES is compared to that of nontemplating solvent castor oil. Our results will also pave the way to understand and control the self-assembly of metallic and bimetallic nanoparticles.

Nonaqueous microemulsions based on n,n'-alkylimidazolium alkylsulfate ionic liquids.

The ternary system composed of the ionic liquid surfactant (IL-S) 1-butyl-3-methylimidazolium dodecylsulfate ([Bmim][DodSO4]), the room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium ethylsulfate ([Emim][EtSO4]), and toluene has been investigated. Three major mechanisms guiding the structure of the isotropic phase were identified by means of conductometric experiments, which have been correlated to the presence of oil-in-IL, bicontinuous, and IL-in-oil microemulsions. IL-S forms micelles in toluene, which swell by adding RTIL as to be shown by dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) experiments. Therefore, it is possible to form water-free IL-in-oil reverse microemulsions ≤10 nm in size as a new type of nanoreactor.

Development of nanoparticle bulk morphology analysis: a multidomain XRD approach.

The appearance of twin/stacking faults in nanoparticles creates strains affecting the catalytic, optical, and electrical properties of nanomaterials. Currently, there is a lack of experimental tools for a numeric characterization of these defects in samples. Therefore, many structure-property correlations are poorly understood. Here, we report the exploration of the twinning effect on the XRD pattern and its practical application. We developed a new approach focused on the special mutual orientation of periodic fcc segments, domains. Using computational simulations, we found that the more domains, the smaller the height ratio of 220 to 111 diffraction peaks. Knowing this correlation, we performed the XRD bulk morphology and size analysis of Au and AuPt samples. The obtained results were compared with the results of TEM and SAXS analyses. In a broader context, our multidomain XRD method is a simple alternative to TEM which enables unraveling the structure-property correlations in NP studies.

Solvent and A-Site Cation Control Preferred Crystallographic Orientation in Bromine-Based Perovskite Thin Films.

Preferred crystallographic orientation in polycrystalline films is desirable for efficient charge carrier transport in metal halide perovskites and semiconductors. However, the mechanisms that determine the preferred orientation of halide perovskites are still not well understood. In this work, we investigate crystallographic orientation in lead bromide perovskites. We show that the solvent of the precursor solution and organic A-site cation strongly affect the preferred orientation of the deposited perovskite thin films. Specifically, we show that the solvent, dimethylsulfoxide, influences the early stages of crystallization and induces preferred orientation in the deposited films by preventing colloidal particle interactions. Additionally, the methylammonium A-site cation induces a higher degree of preferred orientation than the formamidinium counterpart. We use density functional theory to show that the lower surface energy of the (100) plane facets in methylammonium-based perovskites, compared to the (110) planes, is the reason for the higher degree of preferred orientation. In contrast, the surface energy of the (100) and (110) facets is similar for formamidinium-based perovskites, leading to lower degree of preferred orientation. Furthermore, we show that different A-site cations do not significantly affect ion diffusion in bromine-based perovskite solar cells but impact ion density and accumulation, leading to increased hysteresis. Our work highlights the interplay between the solvent and organic A-site cation which determine crystallographic orientation and plays a critical role in the electronic properties and ionic migration of solar cells.

Distribution of counterions around lignosulfonate macromolecules in different polar solvent mixtures.

Lignosulfonate is a colloidal polyelectrolyte that is obtained as a side product in sulfite pulping. In this work we wanted to study the noncovalent association of the colloids in different solvents, as well as to find out how the charged sulfonate groups are organized on the colloid surface. We studied sodium and rubidium lignosulfonate in water-methanol mixtures and in dimethyl formamide. The number average molecular weights of the Na- and Rb-lignosulfonate fractions were 7600 g/mol and 9100 g/mol, respectively, and the polydispersity index for both was 2. Anomalous small-angle X-ray scattering (ASAXS) was used for determining the distribution of counterions around the Rb-lignosulfonate macromolecules. The scattering curves were fitted with a model constructed from ellipsoids of revolution of different sizes. Counterions were taken into account by deriving an approximative formula for the scattering intensity of the Poisson-Boltzmann diffuse double layer model. The interaction term between the spheroidal particles was estimated using the local monodisperse approximation and the improved Hayter-Penfold structure factor given by the rescaled mean spherical approximation. Effective charge of the polyelectrolyte and the local dielectric constant of the solvent close to the globular polyelectrolyte were followed as a function of the methanol content in the solvent and lignosulfonate concentration. The lignosulfonate macromolecules were found to aggregate noncovalently in water-methanol mixtures with increasing methanol or lignosulfonate content in a specific directional manner. The flat macromolecule aggregates had a nearly constant thickness of 1-1.4 nm, while their diameter grew when counterion association onto the polyelectrolyte increased. These results indicate that the charged groups in lignosulfonate are mostly at the flat surfaces of the colloid, allowing the associated lignosulfonate complexes to grow further at the edges of the complex.

Pyridine Controlled Tin Perovskite Crystallization.

Controlling the crystallization of perovskite in a thin film is essential in making solar cells. Processing tin-based perovskite films from solution is challenging because of the uncontrollable faster crystallization of tin than the most used lead perovskite. The best performing devices are prepared by depositing perovskite from dimethyl sulfoxide because it slows down the assembly of the tin-iodine network that forms perovskite. However, while dimethyl sulfoxide seems the best solution to control the crystallization, it oxidizes tin during processing. This work demonstrates that 4-(tert-butyl) pyridine can replace dimethyl sulfoxide to control the crystallization without oxidizing tin. We show that tin perovskite films deposited from pyridine have a 1 order of magnitude lower defect density, which promotes charge mobility and photovoltaic performance.

Direct Observation of the Xenon Physisorption Process in Mesopores by Combining In Situ Anomalous Small-Angle X-ray Scattering and X-ray Absorption Spectroscopy.

The morphology and structural changes of confined matter are still far from being understood. This report deals with the development of a novel in situ method based on the combination of anomalous small-angle X-ray scattering (ASAXS) and X-ray absorption near edge structure (XANES) spectroscopy to directly probe the evolution of the xenon adsorbate phase in mesoporous silicon during gas adsorption at 165 K. The interface area and size evolution of the confined xenon phase were determined via ASAXS demonstrating that filling and emptying the pores follow two distinct mechanisms. The mass density of the confined xenon was found to decrease prior to pore emptying. XANES analyses showed that Xe exists in two different states when confined in mesopores. This combination of methods provides a smart new tool for the study of nanoconfined matter for catalysis, gas, and energy storage applications.

Silver ion incorporation and nanoparticle formation inside the cavity of Pyrococcus furiosus ferritin: structural and size-distribution analyses.

Highly symmetrical protein cage architectures from three different iron storage proteins, heavy and light human ferritin chains (HuHFt and HuLFt) and ferritin from the hyperthemophilic bacterium Pyrococcus furiosus (PfFt), have been used as models for understanding the molecular basis of silver ion deposition and metal core formation inside the protein cavity. Biomineralization using protein cavities is an important issue for the fabrication of biometamaterials under mild synthetic conditions. Silver nanoparticles (AgNPs) were produced with high yields within PfFt but not within HuHFt and HuLFt. To explain the molecular basis of silver incorporation, the X-ray crystal structure of Ag-containing PfFt has been solved. This is the first structure of a silver containing ferritin reported to date, and it revealed the presence of specific binding and nucleation sites of Ag(I) that are not conserved in other ferritin templates. The AgNP encapsulated by PfFt were further characterized by the combined use of different physical-chemical techniques. These showed that the AgNPs are endowed with a narrow size distribution (2.1 +/- 0.4 nm), high stability in water solution at millimolar concentration, and high thermal stability. These properties make the AgNP obtained within PftFt exploitable for a range of applications, in fields as diverse as catalysis in water, preparation of metamaterials, and in vivo diagnosis and antibacterial or tumor therapy.

Yttria-zirconia coatings studied by grazing-incidence small-angle X-ray scattering during in situ heating.

The morphology of sol-gel derived dip-coated yttria-doped zirconia films containing variable amounts of yttria has been studied using in situ grazing-incidence small-angle X-ray scattering (GISAXS) whilst heated incrementally to 1000 °C. A procedure to analyse in situ GISAXS data has been devised which allows a quantitative analysis of time-dependent GISAXS data tracing processes such as chemical reactions or manufacturing procedures. To achieve this, the relative positions of the Yoneda peak and the through beam are used to fix the vertical q scale when the sample thickness is subject to fluctuations due to chemical reactions or deposition processes. A version of Beaucage's unified model with a structure factor from Hosemann's model for paracrystals describes the yttria-zirconia film data best. It is interpreted in terms of particles forming from a polymeric gel network and subsequently agglomerating into larger units subject to Ostwald ripening as both size and average separation distance of the scattering objects increase. The sample with the highest yttria content shows progressive surface roughening from 850 °C which may indicate the onset of chemical segregation.

Identifying the location of Cu ions in nanostructured SAPO-5 molecular sieves and its impact on the redox properties.

Combining X-ray Absorption Fine Spectroscopy (XAFS) with Anomalous Small-Angle X-ray Scattering (ASAXS) determines the location of Cu2+ ions in silicoaluminophosphate (SAPO-5) frameworks prepared by hydrothermal crystallization or impregnation. As expected, for the hydrothermally prepared sample, incorporation in the SAPO-5 framework was observed. For the first time preferential location of Cu2+ ions at the inner and outer surfaces of the framework is determined. Temperature-Programmed Reduction (TPR) and X-ray Photoelectron Spectroscopy (XPS) investigations demonstrated that such Cu2+ is stable in an argon (Ar) atmosphere up to 550 °C and can only be reduced under a hydrogen atmosphere. In contrast, Cu2+ deposited by impregnation on the pure SAPO-5 framework can be easily reduced to Cu+ in an Ar atmosphere. At lower Cu amounts, mononuclear tetrahedrally coordinated Cu species were formed which are relatively stable in the monovalent form. In contrast, at higher Cu amounts, CuO particles were found which change easily between the mono- and bivalent species.

Photoluminescence of atomic gold and silver particles in soda-lime silicate glasses.

We report the chemistry and photophysics of atomic gold and silver particles in inorganic glasses. By synchrotron irradiation of gold-doped soda-lime silicate glasses we could create and identify unambiguously the gold dimer as a stable and bright luminescing particle embedded in the glassy matrix. The gold dimer spectra coincide perfectly with rare gas matrix spectra of Au(2). The glass matrix is, however, stable for years, and is hence perfectly suited for various applications. If the irradiated gold-doped sample is annealed at 550 degrees C a bright green luminescence can be recognized. Intense 337 nm excitation induces a decrease of the green luminescence and the reappearance of the 753 nm Au(2) emission, indicating a strong interrelationship between both luminescence centers. Time-dependent density functional theory (TD-DFT) calculations indicate that the green luminescence can be assigned to noble metal dimers bound to silanolate centers. These complexes are recognized as the first stages in the further cluster growth process, which has been investigated with small-angle x-ray scattering (SAXS). In silver-doped glasses, Ag(0) atoms can be identified with electron paramagnetic resonance (EPR) spectroscopy after synchrotron activation. Annealing at 300 degrees C decreases the concentration of Ag(1), but induces an intense white light emission with 337 nm excitation. The white luminescence can be decomposed into bands that are attributed to small silver clusters such as Ag(2), Ag(3) and Ag(4), and an additional band matching the green emission of gold-doped glasses.

Cationic polyacrylamide induced nanoparticles assembly in a cellulose nanofiber network.

Polyacrylamides of different molecular weight, charges and dosages allow to control the retention and distribution of nanoparticles (NPs) in composites, and optimise composite properties and functionality. Our aim is to evaluate the effect of high molecular weight (13 MDa) cationic polyacrylamide (CPAM) charge and dosage on SiO2 (74 nm) NP's assembly in cellulose nanofibers composites. Engineered cellulose/SiO2 composites were investigated by SEM, SAXS and DLS. SEM images show the local area retention of NPs into the cellulose matrix. SAXS provides an average NPs distribution and inter-NPs distance over complete volume of composite. DLS gives the hydrodynamic radius of CPAM adsorbed onto SiO2 NPs in a suspension. SAXS analysis reveals a structure conformation made of spherical SiO2 NPs core of diameter 74 nm surrounded by a CPAM polyelectrolyte shell 2.5 nm thick. Surprisingly, CPAM induced assembly of SiO2 NPs with constant inter-nanoparticle distance, which is irrelevant of polymer charge density. However, NPs retention in the cellulose fibre network increases with CPAM dosage. The assembly mechanism is governed by the balance of electrostatic and steric forces following CPAM coverage onto NPs and the inter-nanoparticle CPAM bridging conformation. This maintains the constant inter-nanoparticle distance and the assembly of NPs in the cellulose network.

Crystal Phase Transitions in the Shell of PbS/CdS Core/Shell Nanocrystals Influences Photoluminescence Intensity.

We reveal the existence of two different crystalline phases, i.e., the metastable rock salt and the equilibrium zinc blende phase within the CdS-shell of PbS/CdS core/shell nanocrystals formed by cationic exchange. The chemical composition profile of the core/shell nanocrystals with different dimensions is determined by means of anomalous small-angle X-ray scattering with subnanometer resolution and is compared to X-ray diffraction analysis. We demonstrate that the photoluminescence emission of PbS nanocrystals can be drastically enhanced by the formation of a CdS shell. Especially, the ratio of the two crystalline phases in the shell significantly influences the photoluminescence enhancement. The highest emission was achieved for chemically pure CdS shells below 1 nm thickness with a dominant metastable rock salt phase fraction matching the crystal structure of the PbS core. The metastable phase fraction decreases with increasing shell thickness and increasing exchange times. The photoluminescence intensity depicts a constant decrease with decreasing metastable rock salt phase fraction but shows an abrupt drop for shells above 1.3 nm thickness. We relate this effect to two different transition mechanisms for changing from the metastable rock salt phase to the equilibrium zinc blende phase depending on the shell thickness.

In situ study of atomic structure transformations of Pt-Ni nanoparticle catalysts during electrochemical potential cycling.

When exposed to corrosive anodic electrochemical environments, Pt alloy nanoparticles (NPs) undergo selective dissolution of the less noble component, resulting in catalytically active bimetallic Pt-rich core-shell structures. Structural evolution of PtNi6 and PtNi3 NP catalysts during their electrochemical activation and catalysis was studied by in situ anomalous small-angle X-ray scattering to obtain insight in element-specific particle size evolution and time-resolved insight in the intraparticle structure evolution. Ex situ high-energy X-ray diffraction coupled with pair distribution function analysis was employed to obtain detailed information on the atomic-scale ordering, particle phases, structural coherence lengths, and particle segregation. Our studies reveal a spontaneous electrochemically induced formation of PtNi particles of ordered Au3Cu-type alloy structures from disordered alloy phases (solid solutions) concomitant with surface Ni dissolution, which is coupled to spontaneous residual Ni metal segregation during the activation of PtNi6. Pt-enriched core-shell structures were not formed using the studied Ni-rich nanoparticle precursors. In contrast, disordered PtNi3 alloy nanoparticles lose Ni more rapidly, forming Pt-enriched core-shell structures with superior catalytic activity. Our X-ray scattering results are confirmed by STEM/EELS results on similar nanoparticles.

Supramolecular Structures of Enzyme Clusters.

The structural characterization of subtilisin mesoscale clusters, which were previously shown to induce supramolecular order in biocatalytic self-assembly of Fmoc-dipeptides, was carried out by synchrotron small-angle X-ray, dynamic, and static light scattering measurements. Subtilisin molecules self-assemble to form supramolecular structures in phosphate buffer solutions. Structural arrangement of subtilisin clusters at 55 °C was found to vary systematically with increasing enzyme concentration. Static light scattering measurements showed the cluster structure to be consistent with a fractal-like arrangement, with fractal dimension varying from 1.8 to 2.6 with increasing concentration for low to moderate enzyme concentrations. This was followed by a structural transition around the enzyme concentration of 0.5 mg mL-1 to more compact structures with significantly slower relaxation dynamics, as evidenced by dynamic light scattering measurements. These concentration-dependent supramolecular enzyme clusters provide tunable templates for biocatalytic self-assembly.

A closer look at the structure of sterically stabilized liposomes: a small-angle X-ray scattering study.

The evaluation of the radial electron density profile of a drug containing a sterically stabilized liposomal system is described. Using synchrotron small-angle X-ray scattering, we were able to characterize the hydrophilic shell of the polyethylene glycol chains. Using a Gaussian model for describing the electron density profile along the normal of the bilayer, we got an asymmetric distribution of PEGylated lipids in accordance with theoretical considerations. Moreover, we used anomalous X-ray scattering to study the localization of a hydrophobic drug (a kinase inhibitor), which revealed that these molecules are mainly located in the hydrocarbon chain region of the phospholipid bilayer.