Synthesis and characterization of nanocapsules with shells made up of Al13 tridecamers

The synthesis of samples by the sol-gel method with aluminum tri-sec-butoxide as cation precursor, 2-propanol as solvent, and sulfuric acid as hydrolysis catalyst gave rise to nanocapsules with an average diameter of 20 nm and a shell thickness of 3.5 nm. The analysis of the X-ray diffraction patterns and the 27Al MAS NMR spectra showed that the shell of the nanocapsules was made up of Al13 tridecamers ordered in a noncrystalline symmetry. The interaction between the capsule's shells opened the capsule structure, producing curved fibers, but maintaining the atomic local order. This opening of the capsules favored the reordering of the atomic local order of Al13 tridecamers into the one of crystalline boehmite, when the sample was aged at room temperature for several days; it also increased the pore volume and the specific surface area of the sample. The crystallization transformed the curved fibers into rods made of small crystalline boehmite bars. The capsule morphology was preserved after calcining the nonaged sample at 700 degrees C, indicating that the transformation of the phase made up of ordered Al13 tridecamers into a noncrystalline alumina was pseudomorphic. We describe and partially explain one of the possible atomic ordering evolutions from the one of an isolated Al13 tridecamer, to the phase forming the nanocapsules shell, until eventually coming to the ordering corresponding to boehmite crystalline rods.     

Photochemical release of aldehydes from alpha-acetoxy nitroveratryl ethers

Photolabile aldehyde-releasing precursors (alpha-acetoxy ethers) were prepared by reduction of the corresponding esters with DIBAL and quenching the intermediate aluminum hemiacetal with acetic anhydride. These species smoothly released aldehydes upon irradiation with UV light at 350 nm. Using this method, not only simple model aliphatic aldehydes were liberated but also specimens relevant for the flavor and fragrance industry (methional, phenylacetaldehyde, and (R)-citronellal). [reaction: see text]     

Polarity effects on ZnO films grown along the nonpolar [1120] direction

The surface electrical properties of ZnO thin films grown along the nonpolar [1120] direction have been investigated by Kelvin probe microscopy on a nanometer scale. Two different charge domains, with a 75 meV work function difference, coexist within the ZnO surface, which is covered by rhombohedral pyramids whose sidewalls are shown to be {1011}-type planes. The presence and relative orientation of the two kinds of charge domains are explained in terms of the atomic arrangement at the {1011} polar surfaces.     

Holographic phase-shift measurement during development of a fixed grating in lithium niobate crystals

We report what is believed to be the first direct measurement of the grating phase-shift evolution during white-light illumination for the development of a fixed grating in an Fe-doped lithium niobate crystal. Stabilized holographic recording is shown to be essential for such measurements. Experimental data are in good agreement with theory and allow computation of the relevant material parameters for the sample under analysis. The results are of the utmost relevance for understanding the advantageous behavior of oxidized samples in hologram fixing.     

Reentrant hidden order at a metamagnetic quantum critical end point

Magnetization measurements of URu2Si2 in pulsed magnetic fields of 44 T reveal that the hidden order phase is destroyed before appearing in the form of a reentrant phase between approximately 36 and 39 T. Evidence for conventional itinerant electron metamagnetism at higher temperatures suggests that the reentrant phase is created in the vicinity of a quantum critical end point.     

Computational studies on pseudorotaxanes by molecular dynamics and free energy perturbation simulations

A computational scheme that comprises the utilization of the AMBER force field with RESP charges and an explicit solvent model for acetonitrile proved to be useful for studying the structures and energetics of pseudorotaxanes of benzidine and 4,4'-biphenol with cyclobis(paraquat-p-phenylene). The scheme can be further utilized for modeling [2]rotaxanes.     

Some existence and nonexistence theorems for compact graphs of constant mean curvature with boundary in parallel planes

Given H≥0 and bounded convex curves α₁, ...,⇌_n, α in the plane z=0 bounding domains D₁, …, D_n, D, respectively, with if i ∈ j and with D_i ⊂ D, we obtain several results proving the existence of a constanth depending only on H and on the geometry of the curves α_i, α such that the Dirichlet problem for the constant mean curvature H equation: where may accept or not a solution.