Size-dependent phase transition from amorphous to crystalline in antimony clusters on crystalline surfaces

This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Peer Reviewe

Cross-sectional TEM preparation of hybrid inorganic/organic materials systems by ultramicrotomy

Preparation of hybrid inorganic-organic systems (HIOS) for transmission electron microscopy (TEM) in cross sectional view is the key for understanding the interfacial structure. Strikingly different materials properties like hardness, cleavability and heat sensitivity limit the number of applicable preparation strategies. Successful preparation of a HIOS system combining ZnO and para-sexiphenyl (6P) is realized by ultramicrotomy. It is shown that the alignment of the cutting plane with respect to the (0001) cleavage plane of ZnO plays a decisive role for successful preparation of extended TEM lamellae and the preservation of the HIOS structure. In particular, for (0001) oriented ZnO substrates the optimum cut direction is parallel to the HIOS interface. In cross-sectional high-resolution TEM images (100) lattice planes of 6P are observed proving the appropriate preparation strategy.Peer Reviewe

Random matrix description of decaying quantum systems

This contribution describes a statistical model for decaying quantum systems (e.g. photo-dissociation or -ionization). It takes the interference between direct and indirect decay processes explicitely into account. The resulting expressions for the partial decay amplitudes and the corresponding cross sections may be considered a many-channel many-resonance generalization of Fano's original work on resonance lineshapes [Phys. Rev 124, 1866 (1961)]. A statistical (random matrix) model is then introduced. It allows to describe chaotic scattering systems with tunable couplings to the decay channels. We focus on the autocorrelation function of the total (photo) cross section, and we find that it depends on the same combination of parameters, as the Fano-parameter distribution. These combinations are statistical variants of the one-channel Fano parameter. It is thus possible to study Fano interference (i.e. the interference between direct and indirect decay paths) on the basis of the autocorrelation function, and thereby in the regime of overlapping resonances. It allows us, to study the Fano interference in the limit of strongly overlapping resonances, where we find a persisting effect on the level of the weak localization correction.Comment: 16 pages, 2 figure

Origin of a-plane (Al,Ga)N formation on patterned c-plane AIN/sapphire templates

a-plane (Al,Ga)N layers can be grown on patterned c-plane AlN/sapphire templates with a ridge direction along [1bar 100]Al2O3. Scanning nanobeam diffraction reveals that the formation of a-plane layers can be explained by nucleation of c-plane (Al,Ga)N with [11bar 20](Al,Ga)N[0001]Al2O3 at the ridge sidewalls. Faster growth of the top (11bar 20)(Al,Ga)N facet in the vertical direction leads to the overgrowth of c-plane (Al,Ga)N nucleated on the horizontal ridge and trench surfaces. Phase separation into binary GaN and AlN takes place during the first growth stages. However, this fades out and does not influence the composition of the final thick a-plane (Al,Ga)N layer

A General Synthetic Route Towards Highly Dispersed Metal Clusters Enabled by Poly ionic liquid s

The ability to synthesize a broad spectrum of metal clusters MCs with their size controllable on a subnanometer scale presents an enticing prospect for exploring nanosize dependent properties. Here we report an innovative design of a capping agent from a polytriazolium poly ionic liquid PIL in a vesicular form in solution that allows for crafting a variety of MCs including transition metals, noble metals, and their bimetallic alloy with precisely controlled sizes amp; 8764;1 nm and record high catalytic performance. The ultrastrong stabilization power is a result of an unusual synergy between the conventional binding sites in the heterocyclic cations in PIL and an in situ generated polycarbene structure induced simultaneously to the reduction reactio

TEM investigations of Ga(Sb,As) quantum dots grown on a seed layer of (In,Ga)As quantum dots

This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Peer Reviewe

Structural properties of Co2TiSi films on GaAs(001)

Co2TiSi films were grown by molecular beam epitaxy on GaAs(001) and analyzed using reflection high-energy electron diffraction, and electron microscopy. In addition, X-ray diffraction was combined with lattice parameter calculations by density functional theory comparing the L21 and B2 structures and considering the influence of non-stoichiometry. Columnar growth is found and attributed to inhomogeneous epitaxial strain from non-random alloying. In films with thicknesses up to 13 nm, these columns may be the origin of perpendicular magnetization with the easy axis perpendicular to the sample surface. We found L21 and B2 ordered regions, however the [Co]/[Ti]-ratio is changing in dependence of the position in the film. The resulting columnar structure is leading to anisotropic B2-ordering with the best order parallel to the axes of the columns

Ordered structure of FeGe₂ formed during solid-phase epitaxy

Fe3Si/Ge(Fe,Si)/Fe3Si thin-film stacks were grown by a combination of molecular beam epitaxy and solid-phase epitaxy (Ge on Fe3Si). The stacks were analyzed using electron microscopy, electron diffraction, and synchrotron x-ray diffraction. The Ge(Fe,Si) films crystallize in the well-oriented, layered tetragonal structure FeGe2 with space group P4mm. This kind of structure does not exist as a bulk material and is stabilized by the solid-phase epitaxy of Ge on Fe3Si. We interpret this as an ordering phenomenon induced by minimization of the elastic energy of the epitaxial film

Response of caatinga vegetation to decreasing levels of canopy cover

Muito embora o desmatamento da caatinga seja conduzido visando principalmente a abertura de novas áreas para produção de alimentos e para utilização da madeira, sua manipulação está sendo indicada como uma maneira de aumentar a disponibilidade de forragem. A resposta da vegetação da caatinga a vários níveis de desmatamento está sendo estudada em dois experimentos no Centro Nacional de Pesquisa de Caprinos, em Sobral, Ceará. A remoção total da copa das árvores produziu um aumento de seis a oito vezes na produção do estrato herbáceo durante o primeiro ano. Esta mesma intensidade de resposta foi obtida com a manutenção dos níveis da copa das árvores entre 20 e 30% e entre 50 e 60%. A regeneração das espécies cortadas foi rápida, e o nível de cobertura da copa para o tratamento com desmatamento total foi praticamente igual (aproximadamente 95%) ao das áreas não desmatadas (controle) quatro anos após o desmatamento. Como resultado dessa regeneração, a produção de biomassa do estrato herbáceo e a produção de folhas das árvores também foram similares ente os dois tratamentos. O desmatamento não somente favoreceu o estabelecimento de novas plantas, mas também a produção de biomassa originária da regeneração das áreas desmatadas foi cerca de dez vezes superior à produção das árvores da área-controle. A alta produção do estrato herbáceo durante o primeiro ano e a resposta de espécies desejáveis como sabiá (Mimosa caesalpiniaefolia) ao desmatamento indicam que a utilização de um sistema de manipulação seletivo da caatinga pode ser desenvolvido.Although clearing of the caatinga forest is conducted primarily to open areas for crop production and to harvest wood, vegetation manipulation is being promoted as a means of increasing forage availability to livestock. Response of caatinga vegetation to various levels of canopy cover removal is being studied in two experiments at the CNPC, Sobral, CE, Brazil. Complete removal of the tree canopy resulted in a six to eightfold increase in first year production of herbaceous vegetation. This magnitude of increase was also true for canopy covers of 20% - 30% and 50% - 60%. Caatinga species coppice readily following cutting and, therefore, canopy cover for the cleared treatment (complete removal) was equal to that of the control (approximately 95%) by the end of the fourth year post treatment. As a result, herbaceous vegetation and leaf litter yields for the two treatments were similar, too. Sabiá (Mimosa caesalpiniaefolia), a tree species which produces palatable browse and valuable wood, responded very favorably to clearing. Not only did many new plants establish, but by the fourth year post treatment sabiá coppice on the cleared areas produced as much as ten times more foliar biomass than did the intact trees on the control area. The high herbaceous vegetation yields under a thinned canopy and the favorable responses to cutting of tree species such as sabiá indicate that a multiple use approach involving selectively thinned caatinga could be developed whereby production of the naturally renewable wood and forage resources would be optimize