MAX IV is Ready to Make the Invisible Visible

In a ceremony held on June 21, the brightest day of the year in the northern hemisphere, Swedish Prime Minister Stefan Löfven and H.M. King Carl XVI Gustaf, together with Director Christoph Quitmann, inaugurated MAX IV Laboratory in the presence of about 500 staff, funders, stakeholders, and guests from all over the world.     

Stacking in sediments of colloidal hard spheres

We use computer simulations to investigate the crystallization dynamics of sedimenting hard spheres in large systems (hundreds of thousands of particles). We show that slow sedimentation results primarily in face-centered cubic (fcc) stacked crystals, instead of random hexagonal close packed or hexagonal close packed (hcp) crystals. We also find slanted stacking faults, in the fcc regions. However, we attribute the formation of fcc to the free energy difference between fcc and hcp and not to the presence of these slanted stacking faults. Although the free energy difference between hcp and fcc per particle is small (only 10(-3) times the thermal energy), it can become considerable, when multiplied by the number of particles in each domain. The ratio of fcc to hcp obtained from dynamic simulations is in excellent agreement with well-equilibrated Monte Carlo simulations, in which no slanted stacking faults were found. Our results explain a range of experiments on colloids, in which the amount of fcc increases upon lowering the sedimentation rate or decreasing the initial volume fraction.     

Dense regular packings of irregular nonconvex particles

We present a new numerical scheme to study systems of nonconvex, irregular, and punctured particles in an efficient manner. We employ this method to analyze regular packings of odd-shaped bodies, both from a nanoparticle and from a computational geometry perspective. Besides determining close-packed structures for 17 irregular shapes, we confirm several conjectures for the packings of a large set of 142 convex polyhedra and extend upon these. We also prove that we have obtained the densest packing for both rhombicuboctahedra and rhombic enneacontrahedra and we have improved upon the packing of enneagons and truncated tetrahedra.     

Micromechanically tuned ring resonator in silicon on insulator

We demonstrate a monolithically integrated micromechano-optical device where the resonance wavelength of a silicon ring resonator is tuned by perturbing the evanescent field with an electrostatically actuated silicon nitride microcantilever. The resonance wavelength can be tuned over 125 pm.     

Integrated mechano-optical hydrogen gas sensor using cantilever bending readout with a Si3N4 grated waveguide

We demonstrate a proof of concept of a novel and compact integrated mechano-optical sensor for H(2) detection based on a microcantilever suspended above a Si(3)N(4) grated waveguide. The fabricated devices are mechanically and optically modeled and characterized. Sensing operation of the sensor is demonstrated with 1% H(2) in N(2). The error in detection of the cantilever bending induced by absorption of H(2) is estimated to be approximately 10 nm. Significantly improved sensitivity (down to ～33 pm) is expected for reduced initial bending of the microcantilever. The simulation and experimental results are in good agreement and provide a good guideline for further optimization of the sensor.     

Nanoparticle Growth in Supported Nickel Catalysts during Methanation Reaction—Larger is Better

A major cause of supported metal catalyst deactivation is particle growth by Ostwald ripening. Nickel catalysts, used in the methanation reaction, may suffer greatly from this through the formation of [Ni(CO)₄]. By analyzing catalysts with various particle sizes and spatial distributions, the interparticle distance was found to have little effect on the stability, because formation and decomposition of nickel carbonyl rather than diffusion was rate limiting. Small particles (3–4 nm) were found to grow very large (20–200 nm), involving local destruction of the support, which was detrimental to the catalyst stability. However, medium sized particles (8 nm) remained confined by the pores of the support displaying enhanced stability, and an activity 3 times higher than initially small particles after 150 h. Physical modeling suggests that the higher [Ni(CO)₄] supersaturation in catalysts with smaller particles enabled them to overcome the mechanical resistance of the support. Understanding the interplay of particle size and support properties related to the stability of nanoparticles offers the prospect of novel strategies to develop more stable nanostructured materials, also for applications beyond catalysis.     

On the structure 0f nitroformazans

A syn,s-cis structure, best described as mesomeric, is proposed for a series of symmetrical nitroformazans, based on the X-ray crystal structure of 3-nitro-1,5-di(2,6-dimethylphenyl)formazan. Spectroscopic measurements, however, do not exclude the possibility of rapid tautomerism in solution.     

Photo-oxidative degradation of bisphenol A polycarbonate and its possible initiation processes

During UV degradation of bisphenol A polycarbonate (BPA-PC), photo-Fries rearrangements and photo-oxidation reactions take place, however, in outdoor exposure conditions the photo-oxidation reaction is the most dominant one. To initiate this autocatalytic oxidation process, an initiating radical is required. In this research two possible sources for photo-initiation are explored, viz. hydroperoxides and peroxides formed by thermo-oxidation and charge transfer complexes between the polymer and oxygen. A comparison of the photodegradation rate of thermo-oxidized and undegraded polycarbonate samples was made. It was shown that the formed thermo-oxidation products in BPA-PC did not affect the photo-oxidation rate. Charge transfer complexes (CTCs) between oxygen and polycarbonate were studied by UV absorption spectroscopy at different air pressures. The concentration of CTCs increased with oxygen pressures. These CTCs absorb wavelengths in the region of terrestrial sunlight and could cause the initiation of the photo-oxidation. The influence of oxygen pressure on the photodegradation of BPA-PC was studied by irradiating the samples with a by the polymer absorbing wavelength, viz. 250 nm. The absorption of this wavelength leads to photo-Fries products. It was shown that at higher oxygen pressures the rate of the photo-Fries reaction is reduced, which was explained by quenching of the photo-Fries reaction by oxygen.     

Streptococcus mutans and Streptococcus intermedius adhesion to fibronectin films are oppositely influenced by ionic strength

Bacterial adhesion to protein-coated surfaces is mediated by an interplay of specific and nonspecific interactions. Although nonspecific interactions are ubiquitously present, little is known about the physicochemical mechanisms of specific interactions. The aim of this paper is to determine the influence of ionic strength on the adhesion of two streptococcal strains to fibronectin films. Streptococcus mutans LT11 and Streptococcus intermedius NCTC11324 both possess antigen I/II with the ability to bind fibronectin from solution, but S. intermedius binds approximately 20x less fibronectin than does the S. mutans strain under identical conditions. Both strains as well as fibronectin films are negatively charged in low ionic strength phosphate buffered saline (PBS, 10x diluted), but bacteria appear uncharged in high ionic strength PBS. Physicochemical modeling on the basis of overall cell surface properties (cell surface hydrophobicity and zeta potentials) demonstrates that both strains should favor adhesion to fibronectin films in a high ionic strength environment as compared to in a low ionic strength environment, where electrostatic repulsion between equally charged surfaces is dominant. Adhesion of S. intermedius to fibronectin films in a parallel plate flow chamber was completely in line with this modeling, while in addition atomic force microscopy (AFM) indicated stronger adhesion forces upon retraction between fibronectin-coated tips and the cell surfaces in high ionic strength PBS than in low ionic strength PBS. Thus, the dependence of the interaction on ionic strength is dominated by the overall negative charge on the interacting surfaces. Adhesion of S. mutans to fibronectin films, however, was completely at odds with theoretical modeling, and the strain adhered best in low ionic strength PBS. Moreover, AFM indicated weaker repulsive forces upon approach between fibronectin-coated tips and the cell surfaces in low ionic strength PBS than in high ionic strength PBS. This indicated that the dependence of the interaction on ionic strength is dominated by electrostatic attraction between oppositely charged, localized domains on the interacting surfaces, despite their overall negative charge. In summary, this study shows that physicochemical modeling of bacterial adhesion to protein-coated surfaces is only valid provided the number of specific interaction sites on the cell surfaces is low, such as on S. intermedius NCTC11324. Nonspecific interactions are dominated by specific interactions if the number of specific interaction sites is large, such as on S. mutans LT11. Its ionic strength dependence indicates that the specific interaction is electrostatic in nature and operative between oppositely charged domains on the interacting surfaces, despite the generally overall negatively charged character of the surfaces.