Ulrich Schürmann

Tunable multiple plasmon resonance wavelengths response from multicomponent polymer-metal nanocomposite systems

A method of vapor phase codeposition has been used at elevated target temperature coupled with selective masking technique to produce a multicomponent optical material with seven different ultraviolet-visible optical active planes. Such a system consists of different nanocomposites of Teflon AF/Ag, Teflon AF/Au, and Teflon AF/Cu with sharp interfaces. The fabricated system has regions of single nanocomposite plane together with multilayer structures facilitating tunable multiple particle plasmon resonance wavelength response from a single system. Distinct double and triple plasmon wavelength resonance absorptions could be obtained from such interfaces forming multilayers of either two or three nanocomposite structures.

Physico-chemical and antimicrobial properties of co-sputtered Ag?Au/PTFE nanocomposite coatings

In this work, we used co-sputtering of noble metals together with polytetrafluorethylene (PTFE) as a method for producing antibacterial metal/polymer nanocomposite coatings, where the precious metals are only incorporated in a thin surface layer. Moreover, they are finely dispersed as nanoparticles, thus saving additional material and providing a very large effective surface for metal ion release. Nanocomposite films with thickness between 100 and 300 nm were prepared with a wide range of metal filling between 10 and 40%. The antimicrobial effect of the nanocomposite coatings was evaluated by means of two different assays. The bactericidal activity due to silver release from the surface was determined by a modification of conventional disc diffusion methods. Inhibition of bacterial growth on the coated surface was investigated through a modified proliferation assay. Staphylococcus aureus and S. epidermidis were used as test bacteria, as these species commonly cause infections associated with medical polymer devices. The antibacterial efficiency of the coatings against different bacteria was demonstrated at extremely small noble metal consumption: Au: ~1 mg m−2 and Ag: ~0.1 g m−2. The maximum ability for having an antibacterial effect was shown by the Ag–Au/PTFE nanocomposite, followed by the Ag/PTFE nanocomposite.

Controlled syntheses of Ag?polytetrafluoroethylene nanocomposite thin films by co-sputtering from two magnetron sources

Co-sputtering from two independent magnetron sources was used to prepare polymer?metal nanocomposite films. Both gradient films with increasing metal fraction and homogeneous composite films were produced from polytetrafluoroethylene?(PTFE) and silver targets using a rotatable sample holder. The structure of the pure sputtered polymer as well as the composite structure was studied. Electrical properties of the composite material near the percolation threshold show the expected, sharp change in the resistivity from 107???cm atsmall silver content to 10?3???cm after percolation. The optical absorption in the visible region due to surface plasmon resonances also has a strong dependence on the metal content, showing a red shift of the absorption peak from 405?nm to more than 500?nm at higher silver content.

Functional Polymer Nanocomposites

While extensive research has been carried out in the field of structural polymer-based nanocomposites much less investigations have been concerned with polymer nanocomposites for functional applications. Among the functional nanomaterials, nanocomposites consisting of metal nanoparticles dispersed in a dielectric matrix are of particular interest due to their novel functional properties offering hosts of new applications. Here, polymers are attractive as matrix, and several approaches have been reported to incorporate metal nanoparticles into polymers. The present review is concerned with the preparation of polymer-based nanocomposites by vapor phase co-and tandem deposition and the resulting functional properties. The techniques involve evaporation and sputtering, respectively, of metallic and organic components and inter alia allow the preparation of composites which contain alloy clusters of well defined composition. Emphasis is placed on soft-magnetic high frequency materials with cut-off frequencies well above 1 GHz and on optical composites with tuned plasmon resonances suitable for ultra thin color filters, Bragg reflectors, and other devices. In addition, antibacterial coatings and sensors for organic vapors are addressed. The latter take advantage of the steep drop of the electrical resistivity at the percolation threshold. First results are also reported on the incorporation of photo-switchable molecules into nanocomposites near the percolation threshold. Moreover, a novel approach to produce magnetic nanorods is presented.

Superposition twinning supported by texture in ZnO nanospikes

The morphology and real structure of wurtzite-type ZnO nanospikes grown by the recently introduced flame transport synthesis have been examined by means of advanced transmission electron microscopy (TEM). The rapid synthesis produces nanospikes showing a well defined texture which restricts TEM experiments to a preferred viewing direction of [2 {\overline 1}{\overline 1}3]. Forced by the specific morphology, all of the observed nanospikes show a complicated superposition of twinned domains as an intrinsic real structural feature. The high-resolution contrasts are characterized by lamellar fringes parallel to the (1 {\overline 1} 0 {\overline 1}) planes, and the quasi-kinematic diffraction patterns contain satellite peaks based on multiple scattering. All these phenomena can be interpreted by comparison of experimental and simulated data relying on a supercell approach.

Electrical and structural properties of Bi2Te3 and Sb2Te3 thin films grown by the nanoalloying method with different deposition patterns and compositions

Thin films of Bi2Te3 and Sb2Te3 were synthesized by the nanoalloying approach, which has recently been proven to yield V–VI compounds with good thermoelectric properties and has several advantages over “conventional” growth on hot substrates. Firstly, repeating layers of the elements Bi, Sb and Te with a thickness in the range between 0.2 nm and 2.4 nm were deposited on BaF2 (111) substrates in an MBE system at room temperature with different deposition patterns for different samples, i.e. in bilayer and quintuple stacks, with different starting layer thicknesses and different Te contents. Subsequently, the element layer stacks were annealed in order to induce crystallization and compound formation of Bi2Te3 and Sb2Te3 thin films. The annealed thin films were characterized using X-ray diffractometry (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The transport properties, i.e. electrical conductivities, carrier concentrations, carrier mobilities, Seebeck coefficients and thermal conductivities, were determined at room temperature for several sets of starting layer thicknesses and deposition patterns depending on the Te content. The texture was found to be strongly influenced by the starting thicknesses of the elemental layers in the deposition pattern. Results of temperature dependent measurements of the Seebeck coefficient and electrical conductivity on one sample of nanoalloyed Bi2Te3 and Sb2Te3 together with results from temperature dependent in situ XRD investigations are presented.

Inorganic Double Helices in Semiconducting SnIP

SnIP is the first atomic-scale double helical semiconductor featuring a 1.86 eV bandgap, high structural and mechanical flexibility, and reasonable thermal stability up to 600 K. It is accessible on a gram scale and consists of a racemic mixture of right- and left-handed double helices composed by [SnI] and [P] helices. SnIP nanorods <20 nm in diameter can be accessed mechanically and chemically within minutes.

Electrochemical insertion of Li into nanocrystalline MnFe2O4: a study of the reaction mechanism

The study of the mechanism of Li insertion into nanosized partially inverse spinel MnFe2O4 applying X-ray diffraction, in situ quick X-ray absorption spectroscopy, Mossbauer spectroscopy, high resolution transmission electron microscopy, 7Li MAS NMR, and electrochemical measurements yields a comprehensive picture of the individual steps occurring during Li uptake. At the very early beginning of the reaction Fe3+ on the tetrahedral site is reduced and moves to empty octahedral sites. Increasing the amount of Li to 0.7 per MnFe2O4, further Fe3+ is reduced and Mn2+ residing on the tetrahedral site moves to empty octahedral sites thus forming a defect NaCl-type structure. At least for 2 Li per MnFe2O4 reflections of the spinel disappeared in the X-ray powder pattern and only those of a monoxide are observed. No indications were found for a phase separation and Fe and Mn are homogeneously distributed over the sample. Further Li uptake leads to a stepwise conversion of the material and after insertion of 8 Li/MnFe2O4 only nanosized Mn, Fe, and Li2O are detected. After a capacity loss at the beginning of Li insertion, a constant capacity of about 266 mA h g−1 is reached after 100 cycles discharging–charging the material.

Self-organization of ultrahigh-density Fe–Ni–Co nanocolumns in Teflon® AF

We demonstrate a single-step and powerful method based on vapor-phase codeposition to fabricate self-organized, ultrahigh-density Fe–Ni–Co nanocolumnar structures in a Teflon® AF matrix. It is shown that at certain deposition parameters the structure of the metal-polymer nanocomposite changes from a cluster to a nanocolumnar morphology. These mostly monodisperse nanocolumns have an average diameter of around 6nm separated by ∼4nm Teflon® AF and were fabricated so far with aspect ratios of up to 30.

Stable production of TiOx nanoparticles with narrow size distribution by reactive pulsed dc magnetron sputtering

The pulsed dc magnetron technique was used for generating TiOx nanoparticles by sputtering from a titanium target in a gas aggregation source. It was observed that the deposition rate (DR) of nanoparticles shows a peak followed by a broad tail, even for constant operation conditions. As a key finding of the present investigation, we show that nanoparticle deposition can be stabilized at nonzero DR for the pulsed power regime. Monitoring the oxygen concentration by mass spectrometry provides insight into nanoparticle generation in different processes. Characterization of the nanoparticle film morphology based on transmission electron microscopy reveals a very narrow size distribution. Furthermore, the oxygen admixture has a significant influence on the size distribution and also on the mean size of the formed nanoparticles. In situ analysis of the chemical composition of the deposited films directly after preparation by x-ray photoelectron spectroscopy shows a major contribution by sub-oxide titanium compounds to nanoparticle production.