High‐transmission dielectric metasurface with 2π phase control at visible wavelengths

Recently, metasurfaces have received increasing attention due to their ability to locally manipulate the amplitude, phase and polarization of light with high spatial resolution. Transmissive metasurfaces based on high‐index dielectric materials are particularly interesting due to the low intrinsic losses and compatibility with standard industrial processes. Here, it is demonstrated numerically and experimentally that a uniform array of silicon nanodisks can exhibit close‐to‐unity transmission at resonance in the visible spectrum. A single‐layer gradient metasurface utilizing this concept is shown to achieve around 45% transmission into the desired order. These values represent an improvement over existing state‐of‐the‐art, and are the result of simultaneous excitation and mutual interference of magnetic and electric‐dipole resonances in the nanodisks, which enables directional forward scattering with a broad bandwidth. Due to CMOS compatibility and the relative ease of fabrication, this approach is promising for creation of novel flat optical devices.

     

Continuous flow synthesis and characterization of tailor-made bare gold nanoparticles for use in SERS

We describe a method for the synthesis of gold nanoparticles in a stainless steel continuous flow tubular reactor using tetrachloroauric acid as a precursor but without using a classical reducing agent. Gold(III) ion is reduced by stainless steel to form gold nanoparticles which are collected at the end of the coil. A single-phase system is introduced that generates dispersed nanoparticles in the absence of reducing agents on their surface. By controlling flow rates and temperature, the size of the nanoparticles can be tuned in the range from 24 nm to 36 nm. The reproducibility of the preparation was investigated, relative standard deviation of both the wavelength of the peak and the intensity of the plasmonic absorption band were determined and found to vary by 0.15 % and 6.5 %, respectively. Flow synthesis is found to be an excellent alternative to chemical methods to produce stable gold nanoparticles of varying size in an efficiently way. The particles obtained also perform very well when used as a substrate in surface enhanced Raman scattering as shown by the characterization of carboxylated single walled carbon nanotubes.

Three overlooked chemical approaches toward 3-naphthalimide amonafide N-derivatives

Three efficient strategies for derivatization of the anticancer drug candidate amonafide (Quinamed, originally AS1413) are described. Unprecedented reductive amination of aryl aldehydes, S_NAr, and addition–elimination reactions, while using readily available starting materials, give quick entry to potential libraries of novel 3-aryl, 3-benzyl N-derivatives of amonafide. The selective anticancer activity of this important DNA intercalation agent is expected to be enhanced by expanding the diversity of amonafide N-derivatives. The synthetic routes reported in this work are general and readily applicable.     

Fluorescent 2-(2′-hydroxybenzofuran)benzoxazole (HBBO) borate complexes: synthesis,optical properties,and theoretical calculations

The multi-step synthesis, structural and optical properties of original luminescent borate complexes derived from 2-(2′-hydroxybenzofuran)benzoxazole (HBBO) are reported. Functionalization at position 3 of the benzofuran ring was readily achieved through an electrophilic cyclization key step followed by a Sonogashira cross-coupling reaction. The optical properties of the resulting boron difluoride dyes highlight different photophysical behaviors depending on the nature of the substitution at position 3 of the benzofuran core (^tBu-phenylacetylene or NⁿBu₂-phenylacetylene). The NⁿBu₂-phenylacetylene moiety favors a sizeable intramolecular charge transfer as evidenced by a strong solvatochromism; a feature further confirmed by ab initio calculations.     

Applicability of density functional theory in reproducing accurate vibrational spectra of surface bound species

The structural equilibrium parameters, the adsorption energies, and the vibrational frequencies of the nitrogen molecule and the hydrogen atom adsorbed on the (111) surface of rhodium have been investigated using different generalized‐gradient approximation (GGA), nonlocal correlation, meta‐GGA, and hybrid functionals, namely, Perdew, Burke, and Ernzerhof (PBE), Revised‐RPBE, vdW‐DF, Tao, Perdew, Staroverov, and Scuseria functional (TPSS), and Heyd, Scuseria, and Ernzerhof (HSE06) functional in the plane wave formalism. Among the five tested functionals, nonlocal vdW‐DF and meta‐GGA TPSS functionals are most successful in describing energetics of dinitrogen physisorption to the Rh(111) surface, while the PBE functional provides the correct chemisorption energy for the hydrogen atom. It was also found that TPSS functional produces the best vibrational spectra of the nitrogen molecule and the hydrogen atom on rhodium within the harmonic formalism with the error of ?2.62 and ?1.1% for the N? N stretching and Rh? H stretching frequency. Thus, TPSS functional was proposed as a method of choice for obtaining vibrational spectra of low weight adsorbates on metallic surfaces within the harmonic approximation. At the anharmonic level, by decoupling the Rh? H and N? N stretching modes from the bulk phonons and by solving one‐ and two‐dimensional Schrödinger equation associated with the Rh? H, Rh? N, and N? N potential energy we calculated the anharmonic correction for N? N and Rh? H stretching modes as ?31 cm^?1 and ?77 cm^?1 at PBE level. Anharmonic vibrational frequencies calculated with the use of the hybrid HSE06 function are in best agreement with available experiments. © 2014 Wiley Periodicals, Inc.     

The Effect of the Complexation of <Emphasis Type="Italic">p-N,N</Emphasis>-Dimethylaminobenzoic Acid and <Emphasis Type="Italic">p-N,N</Emphasis>-Dimethylaminobenzonitrile with LaCl<Subscript>3</Subscript> on Spectral-luminescent Parameters of Fluorophores

The LE band fluorescence enhancement of p-N,N-dimethylaminobenzoic acid (DMABA) and p-N,N-dimethylaminobenzonitrile (DMABN) was found in aprotic acetonitrile and butyronitrile at the addition of LaCl₃. The corresponding ICT fluorescence band remains unchanged. This enhancement is explained by the decrease of the internal conversion rate constant in a coordination complex with LaCl₃. The formation of the coordination complex between DMABA and LaCl₃ in ethanol is accompanied by the efficient fluorescence quenching in LE and ICT bands, in parallel with the enhancement of ICT/LE emission ratio. The experimental data are well described by the proposed kinetic schemes.     

Simultaneous acquisition of pulse EPR orientation selective spectra

High resolution pulse EPR methods are usually applied to resolve weak magnetic electron-nuclear or electron-electron interactions that are otherwise unresolved in the EPR spectrum. Complete information regarding different magnetic interactions, namely, principal components and orientation of principal axis system with respect to the molecular frame, can be derived from orientation selective pulsed EPR measurements that are performed at different magnetic field positions within the inhomogeneously broadened EPR spectrum. These experiments are usually carried out consecutively, namely a particular field position is chosen, data are accumulated until the signal to noise ratio is satisfactory, and then the next field position is chosen and data are accumulated. Here we present a new approach for data acquisition of pulsed EPR experiments referred to as parallel acquisition. It is applicable when the spectral width is much broader than the excitation bandwidth of the applied pulse sequence and it is particularly useful for orientation selective pulse EPR experiments. In this approach several pulse EPR measurements are performed within the waiting (repetition) time between consecutive pulse sequences during which spin lattice relaxation takes place. This is achieved by rapidly changing the main magnetic field, B(0), to different values within the EPR spectrum, performing the same experiment on the otherwise idle spins. This scheme represents an efficient utilization of the spectrometer and provides the same spectral information in a shorter time. This approach is demonstrated on W-band orientation selective electron-nuclear double resonance (ENDOR), electron spin echo envelope modulation (ESEEM), electron-electron double resonance (ELDOR)--detected NMR and double electron-electron resonance (DEER) measurements on frozen solutions of nitroxides. We show that a factors of 3-6 reduction in total acquisition time can be obtained, depending on the experiment applied.     

Gelfand�CTsetlin algebras and cohomology rings of Laumon spaces

Laumon moduli spaces are certain smooth closures of the moduli spaces of maps from the projective line to the flag variety of GL _n . We calculate the equivariant cohomology rings of the Laumon moduli spaces in terms of Gelfand–Tsetlin subalgebra of U(gl _n ) and formulate a conjectural answer for the small quantum cohomology rings in terms of certain commutative shift of argument subalgebras of U(gl _n ).     

Analysis of the piezoelectric properties of tangentially polarized, stripe-electroded cylinders

The electromechanical properties of thin tangentially polarized, stripe-electroded piezoceramic elements are considered. Theoretical analysis is made based on a piecewise linear approximation of the actual nonuniform electric field in the volume of the stripe-electroded cylinder design. Analytical expressions are derived for the electromechanical properties (including the effective coupling coefficient) of the tangentially polarized elements as a function of the geometry and electrode configuration (i.e., electrode width and spacing). The properties of stripe-electroded cylinders are calculated and compared with the ideal case of a uniform circumferential electric field, which may be realized in practice with segmented cylinders comprised of 33-polarized bar elements. The reasons for the reduction in the effective coupling coefficient of tangentially polarized stripe-electroded cylinders compared to the ideal coupling coefficient k(33) achievable with the segmented cylinder designs are discussed. The results of calculations of the electromechanical properties are in good agreement with experimentally obtained data on a variety of samples.     

Normal state resistivity of Ba<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>: evidence for multiband strong-coupling behavior

The normal state resistivity in multiband superconductors has been analyzed in the framework of Eliashberg theory. The results are compared with measurements of the temperature dependence of normal state resistivity of high-purity Ba_0.68K_0.32Fe₂As₂ single crystals with the highest reported transition temperature T _c = 38.5 K. The experimental data demonstrate strong deviations from the Bloch-Grüneisen behavior, namely the tendency to saturation of the resistivity at high temperatures. The observed behavior of the resistivity is explained within the two band scenario when the first band is strongly coupled and relatively clean, while the second band is weakly coupled and is characterized by much stronger impurity scattering.