Curved polymer and polyelectrolyte brushes beyond the Daoud-Cotton model

We revise the classical Daoud-Cotton (DC) model to describe conformations of polymer and polyelectrolyte chains end-grafted to convex spherical and cylindrical surfaces. In the framework of the DC model, local stretching of chains in the brush does not depend on the degree of polymerization of grafted chains, and the polymer density profile follows a single-exponent power law. This model, however, does not correspond to a minimum in free energy of the curved brush. The nonlocal (NL) approximation exploited in the present paper implies the minimization of the overall free energy of the brush and predicts that the polymer density profile does not follow a single-exponent power law. In the limit of large surface curvature the NL approximation provides the same scaling laws for brush thickness and free energy as the local DC model. Numerical prefactors are however different. Extra extension of chains in the brush interior region leads to larger equilibrium brush thickness and lower free energy per chain. A significant difference between outcomes of the two models is found for brushes formed by ionic polymers, particularly for weakly dissociating (p H-sensitive) polyelectrolytes at low solution salinity.     

Binary group 15 polyazides. structural characterization of [Bi(N3)4]-, [Bi(N3)5]2-, [bipy·Bi(N3)5]2-, [Bi(N3)6]3-, bipy·As(N3)3, bipy·Sb(N3)3, and [(bipy)2·Bi(N3)3]2 and on the lone pair activation of valence electrons

The binary group 15 polyazides As(N(3))(3), Sb(N(3))(3), and Bi(N(3))(3) were stabilized by either anion or donor-acceptor adduct formation. Crystal structures are reported for [Bi(N(3))(4)](-), [Bi(N(3))(5)](2-), [bipy·Bi(N(3))(5)](2-), [Bi(N(3))(6)](3-), bipy·As(N(3))(3), bipy·Sb(N(3))(3), and [(bipy)(2)·Bi(N(3))(3)](2). The lone valence electron pair on the central atom of these pnictogen(+III) compounds can be either sterically active or inactive. The [Bi(N(3))(5)](2-) anion possesses a sterically active lone pair and a monomeric pseudo-octahedral structure with a coordination number of 6, whereas its 2,2'-bipyridine adduct exhibits a pseudo-monocapped trigonal prismatic structure with CN 7 and a sterically inactive lone pair. Because of the high oxidizing power of Bi(+V), reactions aimed at Bi(N(3))(5) and [Bi(N(3))(6)](-) resulted in the reduction to bismuth(+III) compounds by [N(3)](-). The powder X-ray diffraction pattern of Bi(N(3))(3) was recorded at 298 K and is distinct from that calculated for Sb(N(3))(3) from its single-crystal data at 223 K. The [(bipy)(2)·Bi(N(3))(3)](2) adduct is dimeric and derived from two BiN(8) square antiprisms sharing an edge consisting of two μ(1,1)-bridging N(3) ligands and with bismuth having CN 8 and a sterically inactive lone pair. The novel bipy·As(N(3))(3) and bipy·Sb(N(3))(3) adducts are monomeric and isostructural and contain a sterically active lone pair on their central atom and a CN of 6. A systematic quantum chemical analysis of the structures of these polyazides suggests that the M06-2X density functional is well suited for the prediction of the steric activity of lone pairs in main-group chemistry. Furthermore, it was found that the solid-state structures can strongly differ from those of the free gas-phase species or those in solutions and that lone pairs that are sterically inactive in a chemical surrounding can become activated in the free isolated species.     

Self-assembly and photocatalytic properties of polyoxoniobates: {nb(24)o(72)}, {nb(32)o(96)}, and {k(12)nb(96)o(288)} clusters

Three novel polyoxoniobates, KNa(2)[Nb(24)O(72)H(21)]·38H(2)O (1), K(2)Na(2)[Nb(32)O(96)H(28)]·80H(2)O (2), and K(12)[Nb(24)O(72)H(21)](4)·107H(2)O (3) with molecular triangle, molecular square, and cuboctahedral molecular cage geometries, respectively, have been successfully synthesized by conventional aqueous methods. All the compounds are built from [Nb(7)O(22)](9-) fundamental building units. Compound 1 is the first isolated {Nb(24)O(72)} cluster, featuring three heptaniobate clusters linked in a ring by three additional NbO(6) octahedra, while compound 2 is the largest isopolyoxoniobate cluster reported to date, consisting of four heptaniobate clusters linked by four additional NbO(6) octahedra. Compound 3 is the largest solid aggregation of polyoxoniobates, assembled by four {KNb(24)O(72)} clusters joined by four K ions. To our knowledge, it is the first time these polyoxoniobate clusters have been crystallized with only alkali-metal counterions, thereby giving them the possibility of being redissolved in water. ESI-MS spectra indicate that compounds 1 and 2 remain structural integrity when the pure, crystalline polyanion salts are dissolved in water, while compound 3 is partially assembled into Nb(24) fragments. The UV-vis diffuse reflectance spectra of these powder samples indicate that the corresponding well-defined optical absorption associated with E(g) can be assessed at 3.35, 3.17, and 3.34 eV, respectively, revealing the presence of an optical band gap and the nature of semiconductivity with a wide band gap. UV-light photocatalytic H(2) evolution activities were observed for these compounds with Co(III)(dmgH)(2)pyCl as a cocatalyst and TEA as a sacrificial electron donor.     

Water Deficit and Salt Stress Diagnosis Through LED Induced Chlorophyll Fluorescence Analysis in Jatropha curcas L.

LED induced chlorophyll fluorescence analysis is employed to investigate the effect of water deficit and salt stress upon the growth process of Jatropha curcas L.. Red(Fr) and far-red(FFr) chlorophyll fluorescence around 685 nm and 735 nm, respectively, were observed and examined as a function of the stress intensity(salt concentration and water deficit). The fluorescence ratio Fr/FFr which is a valuable nondestructive and nonintrusive indicator of the chlorophyll content of leaves was exploited to monitor the jatropha plants under stress. The data indicated that salinity plays a minor role in the chlorophyll concentration of leaves for NaCl concentrations in the 25 to 200 mM range. The fluorescence ratio also permitted the detection of damage caused by water deficit in the early stages of the plants growing process. A significant variation of the Fr/FFr ratio was observed in the first 10 days of the experiment, and before signs of visual stress became apparent. The results suggest that the Fr/FFr ratio is an early-warning indicator of water deficit stress     

Plasmonic properties of aluminum nanorings generated by double patterning

In this Letter we evaluate a technique for the efficient and flexible generation of aluminum nanorings based on double patterning and variable shaped electron beam lithography. The process is demonstrated by realizing nanorings with diameters down to 90 nm and feature sizes of 30 nm utilizing a writing speed of one ring per microsecond. Because of redepositions caused by involved etching processes, the material of the rings and, therefore, the impact on the plasmonic properties, are unknown. This issue, which is commonly encountered when metals are nanostructured, is solved by adapting a realistic simulation model that accounts for geometry details and effective material properties. Based on this model, the redepositions are quantified, the plasmonic properties are investigated, and a design tool for the very general class of nanofabrication techniques involving the etching of metals is provided.     

Quantitative pupil analysis in stimulated emission depletion microscopy using phase retrieval

The resolution attainable with stimulated emission depletion (STED) microscopy greatly depends on the quality of the STED laser focus. So far, visual inspection of a measured STED focus has been the only convenient means of gauging the source of aberrations. Here we describe a method, requiring no instrument modifications, for obtaining an equivalent to the complex pupil function at the back aperture of the objective and show that it provides quantitative information about aberration sources (including aberrations induced by the objective or sample). We show the accuracy of this field representation to be sufficient for reconstructing the STED focus in three dimensions and determining corrective steps.     

电子衍射

廿世纪的二十年代是物理学发展中最令人向往的时期之一。普朗克在热辐射理论中提出的量子假说,经过爱因斯坦和密立根从理论上的发展和光电效应实验的验证已经被人们普遍接受。X射线衍射经劳厄和布拉格父子等人的工作取得了丰硕成果。特别是康普顿对X射线散射的研究,使电磁辐射的波粒两象性得到了     

里德伯常量的测量

里德伯常量提出已经１００多年了，随着理论的不断完善和测量技术的不断提高，里德伯常量的测量不断登上新台阶，本文详细讨论了里德伯常量测量中的各种误差来源及提高其测量精度的方法，并给出了里德伯常量的最新进展情况。