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Evidence for Reduced Hydrogen-Bond Cooperativity in Ionic Solvation Shells from Isotope-Dependent Dielectric Relaxation
Get PDFWe find that the reduction in dielectric response (depolarization) of water caused by solvated ions is different for H2O and D2O. This isotope dependence allows us to reliably determine the kinetic contribution to the depolarization, which is found to be significantly smaller than predicted by existing theory. The discrepancy can be explained from a reduced hydrogen-bond cooperativity in the solvation shell: we obtain quantitative agreement between theory and experiment by reducing the Kirkwood correlation factor of the solvating water from 2.7 (the bulk value) to ∼1.6 for NaCl and ∼1 (corresponding
to completely uncorrelated motion of water molecules) for CsCl
Effect of ionic strength on the structure and elongational kinetics of vimentin filaments
Get PDFIntermediate filaments are a major structural element in the cytoskeleton of animal cells that mechanically integrate other cytoskeletal components and absorb externally applied stress. Their role is likely to be linked to their complex molecular architecture which is the product of a multi-step assembly pathway. Intermediate filaments form tetrameric subunits which assemble in the presence of monovalent salts to form unit length filaments that subsequently elongate by end-to-end annealing. The present work characterizes this complex assembly process using reconstituted vimentin intermediate filaments with monovalent salts as an assembly trigger. A multi-scale approach is used, comprising static light scattering, dynamic light scattering and quantitative scanning transmission electron microscopy (STEM) mass measurements. Light scattering reveals the radius of gyration (Rg), molecular weight (Mw) and diffusion coefficient (D) of the assembling filaments as a function of time and salt concentration (cS) for the given protein concentration of 0.07 g L−1. At low cS (10 mM KCl) no lateral or elongational growth is observed, whereas at cS = 50–200 mM, the hydrodynamic cross-sectional radius and the elongation rate increases with cS. Rgversus Mw plots suggest that the mass per unit length increases with increasing salt content, which is confirmed by STEM mass measurements. A kinetic model based on rate equations for a two step process is able to accurately describe the variation of mass, length and diffusion coefficient of the filaments with time and provides a consistent description of the elongation accelerated by increasing cS
Quantifying single plasmonic nanostructure far -fields with interferometric and polarimetric k-space microscopy
Get PDFOptically resonant nanoantennae are key building blocks for metasurfaces, nanosensors and nanophotonic light sources due to their ability to control the amplitude, phase,
directivity and polarization of scattered light. Here, we report an experimental technique
for the full recovery of all degrees of freedom encoded in the far-field radiated by a
single nanostructure using a high-NA Fourier microscope equipped with digital off-axis
holography. This method enables full decomposition of antenna-physics in its multipole
contributions and gives full access to the orbital and spin angular momentum properties
of light scattered by single nano-objects. Our results demonstrate these capabilities through a quantitative assessment of the purity of the ‘selection rules’ for orbital angular momentum transfer by plasmonic spiral nanostructures
Epitaxy of Anthraquinone on (100) NaCl: A Quantitative Approach
Get PDFA growth cell suitable for microscopic in situ observation of well-controlled crystal growth from the vapor phase is used to study the heteroepitaxial growth of anthraquinone crystals on a (100) NaCl substrate. In this, the morphology, orientation, nucleation, and growth rate of the crystals is studied as a function of driving force, Δμ/kT. At the lowest Δμ/kT, the crystals are block-shaped and show no preferential orientation with respect to the substrate. Increasing the driving force leads to the growth of oriented block- and needle-shaped crystals, which nucleate from macrosteps on the substrate. At the highest Δμ/kT, crystals nucleate on the flat surface areas or at monatomic steps on the substrate, resulting in a dramatic increase in epitaxial needle density. Growth rate measurements show an exponential behavior as a function of Δμ/kT. In all cases, the supply of growth units proceeds via surface diffusion over the NaCl substrate surface toward the anthraquinone crystals. At the lowest Δμ/kT, growth is partly limited by integration of the growth units at the crystal surfaces. At intermediate driving force, kinetic roughening sets in, leading to rounded needle tips. At the highest supersaturation, growth is completely governed by the supply of growth units via surface diffusion, leading to tip splitting as a consequence of morphological instability
Efficient Green Emission from Wurtzite AlxIn1–xP Nanowires
Get PDFDirect band gap III−V semiconductors, emitting efficiently in the amber −green region of the visible spectrum, are still missing, causing loss in efficiency in light emitting diodes operating in this region, a phenomenon known as the “green gap”. Novel geometries and crystal symmetries however show strong promise in overcoming this limit. Here we develop a novel material system, consisting of wurtzite Alx In1−xP nanowires, which is predicted to have a direct band gap in the green region. The nanowires are grown with selective area metalorganic vapor phase epitaxy and show wurtzite crystal purity from transmission electron microscopy. We show strong light emission at room temperature between the near-infrared 875 nm (1.42 eV) and the “pure green” 555 nm (2.23 eV). We investigate the band structure of wurtzite AlxIn1−xP using time-resolved and temperature-dependent photoluminescence measurements and compare the experimental results with density functional theory simulations, obtaining excellent agreement. Our work paves the way for high-efficiency green light emitting diodes based on wurtzite III-phosphide nanowires
Molecular Insight into the Slipperiness of Ice
Get PDFABSTRACT: Measurements of the friction coefficient of steel-on-ice over a large temperature range reveal very high friction at low temperatures (−100 °C) and a steep decrease in the friction coefficient with increasing temperature. Very low friction is only found over the limited temperature range typical for ice skating. The strong decrease in the friction coefficient with increasing temperature exhibits Arrhenius behavior with an activation energy of Ea ≈ 11.5 kJ mol−1. Remarkably, molecular dynamics simulations of the ice−air interface reveal a very similar activation energy for the mobility of surface molecules. Weakly hydrogen-bonded surface molecules diffuse over the surface in a rolling motion, their number and mobility increasing with increasing temperature.
This correlation between macroscopic friction and microscopic molecular mobility indicates that slippery ice arises from the high mobility of its surface molecules, making the ice surface smooth and the shearing of the weakly bonded surface molecules easy
Broadband highly directive 3D nanophotonic lenses
Get PDFControlling the directivity of emission and absorption at the nanoscale holds great promise for improving the performance of optoelectronic devices. Previously, directive structures have largely been centered in two categories—nanoscale antennas, and classical lenses. Herein, we utilize an evolutionary algorithm to design 3D dielectric nanophotonic lens structures leveraging both the interference-based control of antennas and the broadband operation of lenses. By sculpting the dielectric environment around an emitter, these nanolenses achieve directivities of 101 for point-sources, and 67 for finite-source nanowire emitters; 3× greater than that of a traditional spherical lens with nearly constant performance over a 200 nm wavelength range. The nanolenses are experimentally fabricated on GaAs nanowires, and characterized via photoluminescence Fourier microscopy, with an observed beaming half-angle of 3.5° and a measured directivity of 22. Simulations attribute the main limitation in the obtained directivity to imperfect alignment of the nanolens to the nanowire beneath
Observation of Ultrafast Vibrational Energy Transfer in Fibrinogen and Fibrin Fibers
Get PDFWe study the secondary structure of the blood protein fibrinogen using two-dimensional infrared spectroscopy. With this technique, we identify the amide I′ vibrational modes of the antiparallel β-sheets and turns of fibrinogen. We observe ultrafast energy flow among these amide I′ vibrational modes with a time constant of ∼7 ps. This energy transfer time constant does not change significantly upon fibrin fiber formation, indicating that the secondary structure of the fibrinogen monomers remains largely unchanged in the polymerization process
