Study of CuO Nanowire Growth on Different Copper Surfaces.

Cupric oxide (CuO) nanowires were produced by thermal oxidation of copper surfaces at temperatures up to 450 °C. Three different surfaces, namely a copper foil as well as evaporation deposited copper and an application relevant sputtered copper film on Si(100) substrates were characterized ex-situ before and after the experiment. The development of oxide layers and nanowires were monitored in-situ using grazing incidence small angle X-ray scattering. The number density of nanowires is highest for the sputtered surface and lowest for the surface prepared by evaporation deposition. This can be linked to different oxide grain sizes and copper grain boundary diffusions on the different surfaces. Small grains of the copper substrate and high surface roughness thereby lead to promoted growth of the nanowires.

A new device for high-temperature in situ GISAXS measurements.

A heating stage originally designed for diffraction experiments is implemented into a Bruker NANOSTAR instrument for in situ grazing incidence small-angle x-ray scattering experiments. A controlled atmosphere is provided by a dome separating the sample environment from the evacuated scattering instrument. This dome is double shelled in order to enable cooling water to flow through it. A mesoporous silica film templated by a self-assembled block copolymer system is investigated in situ during step-wise heating in air. The GISAXS pattern shows the structural development of the ordered lattice of parallel cylindrical pores. The deformation of the elliptical pore-cross section perpendicular to the film surface was studied with increasing temperature. Moreover, the performance of the setup was tested by controlled in situ heating of a copper surface under controlled oxygen containing atmosphere.

Synergy of multi-scale toughening and protective mechanisms at hierarchical branch-stem interfaces.

Biological materials possess a variety of artful interfaces whose size and properties are adapted to their hierarchical levels and functional requirements. Bone, nacre, and wood exhibit an impressive fracture resistance based mainly on small crystallite size, interface organic adhesives and hierarchical microstructure. Currently, little is known about mechanical concepts in macroscopic biological interfaces like the branch-stem junction with estimated 10(14) instances on earth and sizes up to few meters. Here we demonstrate that the crack growth in the upper region of the branch-stem interface of conifer trees proceeds along a narrow predefined region of transversally loaded tracheids, denoted as sacrificial tissue, which fail upon critical bending moments on the branch. The specific arrangement of the tracheids allows disconnecting the overloaded branch from the stem in a controlled way by maintaining the stem integrity. The interface microstructure based on the sharply adjusted cell orientation and cell helical angle secures a zig-zag crack propagation path, mechanical interlock closing after the bending moment is removed, crack gap bridging and self-repairing by resin deposition. The multi-scale synergetic concepts allows for a controllable crack growth between stiff stem and flexible branch, as well as mechanical tree integrity, intact physiological functions and recovery after the cracking.

Cellulose in never-dried gel oriented by an AC electric field.

Never-dried cellulose gel obtained by slow coagulation from LiCl/N,N-dimethylacetamide (DMAc) solution was exposed to an alternating current electric field. Making use of the birefringence of oriented cellulose and by means of wide-angle X-ray scattering, it was demonstrated that preferred orientation of cellulose molecules parallel to the electric field lines is induced in the cellulose gel. The preferred orientation remained unchanged for several days after storage in water and persisted after drying of the cellulose gel.

MP2 and DFT calculations on circulenes and an attempt to prepare the second lowest benzolog, [4]circulene.

MP2 and DFT calculations have been carried out for [n]circulenes for n=3 to 20 in order to predict the strain energy and topology of these cyclically condensed aromatic systems. To synthesise [4]circulene (2), 1,5,7,8-tetrakis(bromomethyl)biphenylene (14) was prepared from the corresponding tetramethyl derivative (8) and subjected to various dehalogenation reactions; all attempts to obtain [2.2]biphenylenophane (7) as a precursor for 2 by this route failed. Treatment of 14 with sodium sulfide furnished the thiaphanes 16 and 17, thermal and photochemical desulfurization of which also failed to provide 7. In a second approach [2.2]paracyclophane was converted to the pseudo-geminal dithiol 23, which was subsequently bridged to the thiaphanes 22 and 24. On flash vacuum pyrolysis at 800 degrees C these were converted exclusively into phenanthrene (30). An approach to dehydrochlorinate the commercial product PARYLENE C to the tetrahydro[4]circulene 7 led only to polymerisation. The X-ray structures of the intermediates 8, 14, 17, 23, 24, 26, and 35 are reported.

Complex of silylene with nitrogen: a combined matrix-spectroscopic and density functional theory study.

The quest for a new and productive route to matrix-isolated silylene 1 has led to the observation that the presence of nitrogen is favorable, as under such conditions 1 is stabilized by formation of a donor-acceptor complex 1 x N2. Either matrix photolysis of diazidosilane or pulsed high-vacuum flash pyrolysis of 1,1,1-trimethyldisilane, followed by condensation of the products with nitrogen at 10 K, were applied as synthetic procedures. In a similar manner, complex 1 x CO can be generated. The results are supported by isotopic-labeling studies. The structural elucidation of the new species is based on the comparison of experimental IR and UV/Vis spectra with data from density functional theory calculations.

Tetrakis(trimethylsilyl)tetrahedrane.

Tetrakis(trimethylsilyl)tetrahedrane 3 has been synthesized upon irradiation of tetrakis(trimethylsilyl)cyclobutadiene 8, which can be prepared either by thermal nitrogen elimination from trimethylsilyl[1,2,3-tris(trimethylsilyl)-2-cycloprop-1-enyl]diazomethane 7 or by mild oxidation of cyclobutadiene dianion 9 with 1,2-dibromoethane. The structural characterization of tetrahedrane 3 has been achieved by X-ray crystallography. The surprising thermal stability of 3 - which is stable up to 300 degrees C - is discussed.

Reactions of silicon atoms with methane and silane in solid argon: a matrix-spectroscopic study.

The reaction of silicon atoms with methane (1) and silane (7) in argon at 10 K has been studied. Methane (1) is found to be inert to silicon atoms, but reaction occurs upon photochemical excitation under formation of methylsilylene (2). On the contrary, silane (7) is spontaneously converted into a mixture of silylsilylene (10) and disilene (12). Subsequent irradiation generates the butterfly-type disilyne Si(2)H(2) (14), together with a third photoproduct, which we assume to be the doubly bridged Si(2)H(4) isomer 13. The structural elucidation of the new species is based on the comparison of the experimental IR and UV/Vis spectra with data from density functional theory calculations. The results are supported by isotopic labeling studies.

Thioformaldehyde S-sulfide (Thiosulfine).

Matrix isolation spectroscopy allows the direct identification of ylide 1 and its cyclic isomer 2. They were obtained by pyrolysis of 1,2,4-trithiolane under high vacuum; the cyclic compound forms from 1 by thermal ring closure in a kinetically controlled reaction.

Solid Xenon: A Medium for Unusual Photoreactions.

Can propane be fragmented photolytically with light of wavelength greater than 170 nm [Eq. (1)]? One might think: No! By photolysis in a xenon matrix, in fact, three variants are available to reach this goal.