Alfred Plettl

The effect of substrate surface nanotopography on the behavior of multipotnent mesenchymal stromal cells and osteoblasts.

Hexagonally arranged Gold nanoparticles with controllable diameters and inter-particle distances were deposited on thick SiO2 layers on top of Si wafers and used as masks during subsequent reactive ion etching. In this way, arrays of nanopillars are obtained with well-defined diameters (10/30 nm), inter-pillar distances (50-120 nm) and heights (20-35 nm), all on the nanoscale. Such nanotopographies served as substrate for multipotent mesenchymal stromal cells (MSC) and human osteoblasts (OB) allowing to study cellular responses to purely topographically patterned interfaces. Focus was put on adhesion, proliferation and differentiation of the cells. It turned out experimentally that adhesion is comparable for both cell types practically independent of topographical details at the substrate surface. Topography induced proliferation enhancement, however, is again independent of geometrical details in case of MSC, but significantly sensitive to pillar height in case of OB with a clear preference towards short nanopillars (20 nm). A high sensitivity to topographic details is also observed for osteogenic differentiation of MSC, in that case with a preference towards higher nanopillars (50 nm). The present experimental data also allow the important conclusion that cell proliferation and differentiation can be optimized simultaneously by fine-tuning nanoscaled topographical parameters.

Growth of thin, flat, epitaxial (111) oriented gold films on c-cut sapphire

AbstractAnewgrowthprocedurehasbeendevelopedtopreparethin(<50nm),flat(roughness 0.3–1nm rms ),epitaxialAufilmsontopofaninsulatingsubstratewithoutusingamagneticseedlayer.ThiscouldbeobtainedbydepositingfirstaNbseedlayer(61nm)atroomtemperatureontopofc-cutsapphirefollowedbyevaporatingAuatarateof0.05–0.1nm/sat300 C.Thisprocedureresultedin(111)orientedflatAufilmsevenforthicknesswellbelow50nm.Theflatnessofthesefilmswasconsistentlyconfirmedbyatomicforceandscanningtunnelingmicroscopy,theirexcellentepitaxialquality(rocking width 0.1 –0.3 ) byX-raydiffractometryand reflected high energyelectron diffraction. 2001ElsevierScienceB.V.Allrightsreserved. Keywords:Gold;Niobium;Aluminumoxide;Epitaxy;Growth;Metallicfilms;X-rayscattering,diffraction,andreflection;Reflectionhigh-energyelectrondiffraction(RHEED) 1. IntroductionThere is immense interest in flat epitaxial andthin, yet electrically conducting, metallic films,which then can be used for experiments likee.g. attenuated total reflection (ATR) [1], electro-depositionofnanoclusters[2,3]orscanningtunnel-ing microscopy(STM) on organic monolayers [4].In this context, Au films are especiallyattractivedue to their chemical stability, which is essential,if, after film preparation under ultrahigh vacuum(UHV),experimentsdemandfurtherexsitusteps.In our case, aiming at studying surface sensitiveelectrical transport properties of such thin films,the application of magnetic seedlayers had to beexcluded, since otherwise additional spin-depen-dent scattering mechanisms would contributeto the resistance. Furthermore, to make surfacescatteringthedominantprocess,competingdefectscatteringprocesseshavetobeminimizedleadingtothedemandofthin,flat,highqualityepitaxialAu films. As a consequence of these combinedqualityrequirements, the well known substratesusedtogrowepitaxialAufilms,i.e.mica[5–7]andMgO [8] turn out to be unsuitable. On mica, thepercolation thickness of epitaxial Au films neces-saryto observe electrical conductance appears tobe close to 30 nm. Thus, for epitaxial Au filmsbelow30nmelectricalconductancecannolongerbeguaranteed.ForMgO,ontheotherhand,Feor

Peptide-equipped tobacco mosaic virus templates for selective and controllable biomineral deposition

Summary The coating of regular-shaped, readily available nanorod biotemplates with inorganic compounds has attracted increasing interest during recent years. The goal is an effective, bioinspired fabrication of fiber-reinforced composites and robust, miniaturized technical devices. Major challenges in the synthesis of applicable mineralized nanorods lie in selectivity and adjustability of the inorganic material deposited on the biological, rod-shaped backbones, with respect to thickness and surface profile of the resulting coating, as well as the avoidance of aggregation into extended superstructures. Nanotubular tobacco mosaic virus (TMV) templates have proved particularly suitable towards this goal: Their multivalent protein coating can be modified by high-surface-density conjugation of peptides, inducing and governing silica deposition from precursor solutions in vitro. In this study, TMV has been equipped with mineralization-directing peptides designed to yield silica coatings in a reliable and predictable manner via precipitation from tetraethoxysilane (TEOS) precursors. Three peptide groups were compared regarding their influence on silica polymerization: (i) two peptide variants with alternating basic and acidic residues, i.e. lysine–aspartic acid (KD)x motifs expected to act as charge-relay systems promoting TEOS hydrolysis and silica polymerization; (ii) a tetrahistidine-exposing polypeptide (CA4H4) known to induce silicification due to the positive charge of its clustered imidazole side chains; and (iii) two peptides with high ZnO binding affinity. Differential effects on the mineralization of the TMV surface were demonstrated, where a (KD)x charge-relay peptide (designed in this study) led to the most reproducible and selective silica deposition. A homogenous coating of the biotemplate and tight control of shell thickness were achieved.

Tailoring particle arrays by isotropic plasma etching: an approach towards percolated perpendicular media

Plasma etching of densely packed arrays of polystyrene particles leads to arrays of spherical nanostructures with adjustable diameters while keeping the periodicity fixed. A linear dependence between diameter of the particles and etching time was observed for particles down to sizes of sub-50 nm. Subsequent deposition of Co/Pt multilayers with perpendicular magnetic anisotropy onto these patterns leads to an exchange-decoupled, single-domain magnetic nanostructure array surrounded by a continuous magnetic film. The magnetic reversal characteristic of the film-particle system is dominated by domain nucleation and domain wall pinning at the particle locations, creating a percolated perpendicular media system.

Fabrication of two-dimensional Au@FePt core-shell nanoparticle arrays by photochemical metal deposition

In this report, we experimentally demonstrate that single platinum nanoparticles exhibit the necessary catalytic activity for the optically induced reduction of H[AuCl4] complexes to elemental gold. This finding is exploited for the parallel Au encapsulation of FePt nanoparticles arranged in a self-assembled two-dimensional array. Magnetic force microscopy reveals that the thin gold layer formed on the FePt particles leads to a strongly increased long-term stability of their magnetization under ambient conditions.

Controlled photochemical particle growth in two-dimensional ordered metal nanoparticle arrays

We report on a universal technique which allows us to precisely manipulate the diameter of metal nanoparticles in two-dimensional particle arrays. The approach is demonstrated here for hexagonally ordered gold nanoparticle arrays fabricated by means of diblock copolymer micelle lithography (BCML). The particles are used as nucleation centers in seed-mediated photochemical metal deposition, whereby the particle diameter increases. Repeatedly combining photochemical growth with thermal annealing steps additionally facilitates controlling the shape of the particles.

Functional nanostructures by organized macromolecular-metallic hybrid systems

Stabilization of nanoparticles by block copolymers does allow accurate control of particle size and inter particle distance, and offers the possibility of producing thin optically transparent films and a new technique for lithography in the nanometer range. The approach is based on micelles of diblock copolymers with a polar core, which are formed in organic solution and whose core is able to bind a transition-metal compound. Chemical conversion of the inorganic species within the nanocompartment is employed to prepare sterically stabilized inorganic crystallites or clusters. The particle size and interparticle distance of these crystallites or clusters can be controlled exclusively by the film forming block copolymer. Controlled coagulation of spherical block copolymer micelles allows to agglomerate several clusters of equal size in one compartment and to prepare strings of the clusters. After film formation the polymer shell can be removed entirely by using an oxygen plasma technique resulting in the deposition of the naked clusters on different substrates without destroying the former particle organization.

Freeze Fracture Approach to Directly Visualize Wetting Transitions on Nanopatterned Superhydrophobic Silicon Surfaces: More than a Proof of Principle

Freeze fracturing is applied to make the wetting behavior of artificially nanopatterned Si surfaces directly visible. For this purpose, almost hexagonally arranged nanopillars of fixed areal density (127 μm(-2)) and diameters (35 nm) but varying heights (40-150 nm) were fabricated on silicon. Measurement of contact angles (CAs) including hysteresis allowed to distinguish between the Wenzel (W) and the Cassie-Baxter (CB) states with droplets completely wetting the pillars or residing on top of them, respectively. Providing additional depth contrast by evaporating the ice replica with thin carbon and (typically 3 nm) platinum layers under 45° allowed resolving 3D features of 5 nm within the ice replica. In this way, laterally sharp transitions from CB- to W-states could be revealed, indicating the formation of zero-curvature water surfaces even on the nanoscale.

Platinum nanoparticles from size adjusted functional colloidal particles generated by a seeded emulsion polymerization process

Summary The benefits of miniemulsion and emulsion polymerization are combined in a seeded emulsion polymerization process with functional seed particles synthesized by miniemulsion polymerization. A systematic study on the influence of different reaction parameters on the reaction pathway is conducted, including variations of the amount of monomer fed, the ratio of initiator to monomer and the choice of surfactant and composition of the continuous phase. Critical parameters affecting the control of the reaction are determined. If carefully controlled, the seeded emulsion polymerization with functional seed particles yields monodisperse particles with adjustable size and functionalities. Size-adjusted platinum-acetylacetonate containing latex particles with identical seed particles and varied shell thicknesses are used to produce arrays of highly ordered platinum nanoparticles with different interparticle distances but identical particle sizes. For that, a self-assembled monolayer of functional colloids is prepared on a solid substrate and subsequently treated by oxygen plasma processing in order to remove the organic constituents. This step, however, leads to a saturated state of a residual mix of materials. In order to determine parameters influencing this saturation state, the type of surfactant, the amount of precursor loading and the size of the colloids are varied. By short annealing at high temperatures platinum nanoparticles are generated from the saturated state particles. Typically, the present fabrication method delivers a maximum interparticle distance of about 260 nm for well-defined crystalline platinum nanoparticles limited by deformation processes due to softening of the organic material during the plasma applications.

Plasmonic nanostructures fabricated using nanosphere-lithography, soft-lithography and plasma etching

Summary We present two routes for the fabrication of plasmonic structures based on nanosphere lithography templates. One route makes use of soft-lithography to obtain arrays of epoxy resin hemispheres, which, in a second step, can be coated by metal films. The second uses the hexagonal array of triangular structures, obtained by evaporation of a metal film on top of colloidal crystals, as a mask for reactive ion etching (RIE) of the substrate. In this way, the triangular patterns of the mask are transferred to the substrate through etched triangular pillars. Making an epoxy resin cast of the pillars, coated with metal films, allows us to invert the structure and obtain arrays of triangular holes within the metal. Both fabrication methods illustrate the preparation of large arrays of nanocavities within metal films at low cost. Gold films of different thicknesses were evaporated on top of hemispherical structures of epoxy resin with different radii, and the reflectance and transmittance were measured for optical wavelengths. Experimental results show that the reflectivity of coated hemispheres is lower than that of coated polystyrene spheres of the same size, for certain wavelength bands. The spectral position of these bands correlates with the size of the hemispheres. In contrast, etched structures on quartz coated with gold films exhibit low reflectance and transmittance values for all wavelengths measured. Low transmittance and reflectance indicate high absorbance, which can be utilized in experiments requiring light confinement.