M. Arndt

Organic-organic heteroepitaxy of red-, green-, and blue-emitting nanofibers.

Self-assembly processes and organic-organic heteroepitaxy are powerful techniques to obtain highly ordered molecular aggregates. Here we demonstrate that combining both methods allows not only to fabricate highly crystalline and uniaxially oriented self-assembled nanofibers but also to tune their polarized emission. We show that submonolayer coverage of sexithiophene on top of para-sexiphenyl nanofibers is sufficient to change their emission color from blue to green. Triband emission in the red, green, and blue is generated in nanofibers with thicker sexithiophene coverage, where layers of co-oriented crystals are separated by green-emitting molecular sheets.

X-ray photoelectron spectroscopy of the effects of Ar+ ion sputtering on the nature of some standard compounds of Zn, Cr, and Fe

X-ray photoelectron spectroscopy combined with Ar+ ion etching is a powerful concept to identify different chemical states of compounds in depth profiles, important for obtaining information underneath surfaces or at layer interfaces. The possibility of occurring sputter damage is known but insufficiently investigated for corrosion products of Zn-based steel coatings like ZnCr. Hence, in this work reference materials are studied according to stability against ion sputtering. Indeed some investigated compounds reveal a very unstable chemical nature. On the basis of these findings the reliability of depth profiles of real samples can be rated to avoid misinterpretations of observed chemical species.

XPS investigation on the surface chemistry of corrosion products on ZnMgAl-coated steel

In this work, a novel evaluation strategy for the X-ray photoelectron spectroscopy (XPS) chemical assessment is proposed to identify the corrosion products formed on the surface of hot-dip galvanized ZnMgAl coatings after exposure to standardized salt spray tests. The experiments demonstrate that the investigated system exhibits a problematic differential charging behavior between the different compounds, an effect which cannot be fully compensated for by the flood gun of the XPS system, making a reliable evaluation of the individual spectra impossible by using standard evaluation and fitting methods. For that reason, a new effective approach—taking the different charge shifts into account—was implemented and its reliability experimentally verified on model mixtures of assumed corrosion products with known composition. With this new approach, the chemical surface composition of an industrial sample of a corroded ZnMgAl coating was revealed and discussed in order to clearly demonstrate the potential of the proposed method for future corrosion studies.

Alternately deposited heterostructures of α-sexithiophene–para-hexaphenyl on muscovite mica(001) surfaces: crystallographic structure and morphology

Multi-component systems of para-hexaphenyl (p-6P) and α-sexithiophene (α-6T) molecules show great promise for tuning the fluorescence colour of optically active films. As the opto-electronic properties of rod-like molecules in thin films strongly rely on their anisotropic orientation, a technique for preparation of well-defined, anisotropic multicomponent systems is required. We demonstrate that a p-6P film of less than two nanometer thickness grown on muscovite mica(001) substrates acts as an efficient alignment layer for epitaxial growth of α-6T crystallites. On top of such a p-6P alignment layer, multilayer heterostructures of alternately deposited p-6P and α-6T molecules were grown. Combined X-ray diffraction and transmission electron microscopy studies show that molecules forming α-6T crystallites align parallel to those in the p-6P crystallites leading to the perfect adoption of their herring-bone structures. This alignment is desirable for optical applications and we show that it is preserved for heterostructures composed of up to 120 alternately deposited p-6P (0.8 nm) and α-6T (3.4 nm) nominal layers (120 cycles). Although for co-evaporated α-6T–p-6P molecules formation of a mixed crystal polymorph is reported, we show that in periodically deposited α-6T–p-6P heterostructures phase separation occurs and both molecules crystallize in their well-known equilibrium structures.

Nanoscale surface analysis on second generation advanced high strength steel after hot dip galvanizing

AbstractSecond generation advanced high strength steel is one promising material of choice for modern automotive structural parts because of its outstanding maximal elongation and tensile strength. Nonetheless there is still a lack of corrosion protection for this material due to the fact that cost efficient hot dip galvanizing cannot be applied. The reason for the insufficient coatability with zinc is found in the segregation of manganese to the surface during annealing and the formation of manganese oxides prior coating. This work analyses the structure and chemical composition of the surface oxides on so called nano-TWIP (twinning induced plasticity) steel on the nanoscopic scale after hot dip galvanizing in a simulator with employed analytical methods comprising scanning Auger electron spectroscopy (SAES), energy dispersive X-ray spectroscopy (EDX), and focused ion beam (FIB) for cross section preparation. By the combination of these methods, it was possible to obtain detailed chemical images serving a better understanding which processes exactly occur on the surface of this novel kind of steel and how to promote in the future for this material system galvanic protection. FigureThe image shows an Auger mapping of a hot dip galvanised TWIP steel after removing the surface oxides by Ar sputtering. The shown signal contains intensity information based on topography and work function and elemental distribution of Mn, Fe and Zn shown in blue, green and red

Ultrathin MgO diffusion barriers for ferromagnetic electrodes on GaAs(001).

Ultrathin MgO(100) films serving as a diffusion barrier between ferromagnetic electrodes and GaAs(001) semiconductor templates have been investigated. Using Fe as an exemplary ferromagnetic material, heterostructures of Fe/MgO/GaAs(001) were prepared at 200 °C with the MgO thickness ranging from 1.5 to 3 nm. Structural characterization reveals very good crystalline ordering in all layers of the heterostructure. Auger electron spectroscopy depth-profiling and cross-sectional high-resolution transmission electron microscopy evidence diffusion of Fe into MgO and-for too thin MgO barriers-further into GaAs(001). Our results recommend a MgO barrier thickness larger than or equal to 2.6 nm for its application as a reliable diffusion barrier on GaAs(001) in spintronics devices.