Nanoporous silicon nitride-based membranes of controlled pore size, shape and areal density: Fabrication as well as electrophoretic and molecular filtering characterization.

A new route will be presented for an all-parallel fabrication of highly flexible, freestanding membranes with well-defined porosity. This fabrication is based on arrays of well-defined Au nanoparticles (NPs) exhibiting a high degree of hexagonal order as obtained in a first step by a proven micellar approach. These NP arrays serve as masks in a second reactive ion etching (RIE) step optimized for etching Si and some important Si compounds (silicon oxide, silicon nitride) on the nanoscale. Application to commercially available silicon nitride membranes of well-defined thickness, delivers a diaphragm with millions of nanopores of intended and controlled size, shape, and areal density with narrow distributions of these parameters. Electrophoretic transport measurements indicated a very low flow resistance of these porous membranes in ionic solutions as expected theoretically. Size-selective separation of protein molecules was demonstrated by real-time fluorescence microscopy.

Electrochemically-Driven Insertion of Biological Nanodiscs into Solid State Membrane Pores as a Basis for "Pore-In-Pore" Membranes.

Nanoporous membranes are of increasing interest for many applications, such as molecular filters, biosensors, nanofluidic logic and energy conversion devices. To meet high-quality standards, e.g., in molecular separation processes, membranes with well-defined pores in terms of pore diameter and chemical properties are required. However, the preparation of membranes with narrow pore diameter distributions is still challenging. In the work presented here, we demonstrate a strategy, a "pore-in-pore" approach, where the conical pores of a solid state membrane produced by a multi-step top-down lithography procedure are used as a template to insert precisely-formed biomolecular nanodiscs with exactly defined inner and outer diameters. These nanodiscs, which are the building blocks of tobacco mosaic virus-deduced particles, consist of coat proteins, which self-assemble under defined experimental conditions with a stabilizing short RNA. We demonstrate that the insertion of the nanodiscs can be driven either by diffusion due to a concentration gradient or by applying an electric field along the cross-section of the solid state membrane. It is found that the electrophoresis-driven insertion is significantly more effective than the insertion via the concentration gradient.

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

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.

Arrays of quasi-hexagonally ordered silica nanopillars with independently controlled areal density, diameter and height gradients.

A consecutive fabrication approach of independently tailored gradients of the topographical parameters distance, diameter and height in arrays of well-ordered nanopillars on smooth SiO2-Si-wafers is presented. For this purpose, previously reported preparation techniques are further developed and combined. First, self-assembly of Au-salt loaded micelles by dip-coating with computer-controlled pulling-out velocities and subsequent hydrogen plasma treatment produce quasi-hexagonally ordered, 2-dimensional arrays of Au nanoparticles (NPs) with unidirectional variations of the interparticle distances along the pulling direction between 50-120 nm. Second, the distance (or areal density) gradient profile received in this way is superimposed with a diameter-controlled gradient profile of the NPs applying a selective photochemical growth technique. For demonstration, a 1D shutter is used for locally defined UV exposure times to prepare Au NP size gradients varying between 12 and 30 nm. Third, these double-gradient NP arrangements serve as etching masks in a following reactive ion etching step delivering arrays of nanopillars. For height gradient generation, the etching time is locally controlled by applying a shutter made from Si wafer piece. Due to the high flexibility of the etching process, the preparation route works on various materials such as cover slips, silicon, silicon oxide, silicon nitride and silicon carbide.

Cyclic photochemical re-growth of gold nanoparticles: Overcoming the mask-erosion limit during reactive ion etching on the nanoscale.

THE BASIC IDEA OF USING HEXAGONALLY ORDERED ARRAYS OF AU NANOPARTICLES (NP) ON TOP OF A GIVEN SUBSTRATE AS A MASK FOR THE SUBSEQUENT ANISOTROPIC ETCHING IN ORDER TO FABRICATE CORRESPONDINGLY ORDERED ARRAYS OF NANOPILLARS MEETS TWO SERIOUS OBSTACLES: The position of the NP may change during the etching process and, thus, the primary pattern of the mask deteriorates or is completely lost. Furthermore, the NP are significantly eroded during etching and, consequently, the achievable pillar height is strongly restricted. The present work presents approaches on how to get around both problems. For this purpose, arrays of Au NPs (starting diameter 12 nm) are deposited on top of silica substrates by applying diblock copolymer micelle nanolithography (BCML). It is demonstrated that evaporated octadecyltrimethoxysilane (OTMS) layers act as stabilizer on the NP position, which allows for an increase of their size up to 50 nm by an electroless photochemical process. In this way, ordered arrays of silica nanopillars are obtained with maximum heights of 270 nm and aspect ratios of 5:1. Alternatively, the NP position can be fixed by a short etching step with negligible mask erosion followed by cycles of growing and reactive ion etching (RIE). In that case, each cycle is started by photochemically re-growing the Au NP mask and thereby completely compensating for the erosion due to the previous cycle. As a result of this mask repair method, arrays of silica nanopillar with heights up to 680 nm and aspect ratios of 10:1 are fabricated. Based on the given recipes, the approach can be applied to a variety of materials like silicon, silicon oxide, and silicon nitride.

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.

Geometry-induced spin-ice structures prepared by self-organization on the nanoscale.

We use non-close packed colloidal lithography to prepare hexagonal magnetic antidot arrays with varying diameters at a period of 205 nm. Smaller antidots are attractive for applications as spin waveguides in magnonics. Larger antidots form a magnetically frustrated system, i.e. Kagome spin-ice, as we prove by magnetic force microscopy. The simple but effective approach successfully extends the limits of top-down lithography previously used to study spin-ice configurations and emergent magnetic monopoles towards smaller structures.

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.

Controlled positioning of nanoparticles on a micrometer scale.

For many applications it is desirable to have nanoparticles positioned on top of a given substrate well separated from each other and arranged in arrays of a certain geometry. For this purpose, a method is introduced combining the bottom-up self-organization of precursor-loaded micelles providing Au nanoparticles (NPs), with top-down electron-beam lithography. As an example, 13 nm Au NPs are arranged in a square array with interparticle distances >1 µm on top of Si substrates. By using these NPs as masks for a subsequent reactive ion etching, the square pattern is transferred into Si as a corresponding array of nanopillars.

Tuning the properties of magnetic thin films by interaction with periodic nanostructures.

The most important limitation for a significant increase of the areal storage density in magnetic recording is the superparamagnetic effect. Below a critical grain size of the used CoCrPt exchange-decoupled granular films the information cannot be stored for a reasonable time (typically ten years) due to thermal fluctuations arbitrary flipping of the magnetization direction. An alternative approach that may provide higher storage densities is the use of so-called percolated media, in which defect structures are imprinted in an exchange-coupled magnetic film. Such percolated magnetic films are investigated in the present work. We employ preparation routes that are based on (i) self-assembly of Au nanoparticles and (ii) homogeneous size-reduction of self-assembled polystyrene particles. On such non-close-packed nanostructures thin Fe films or Co/Pt multilayers are grown with in-plane and out-of-plane easy axis of magnetization. The impact of the particles on the magnetic switching behavior is measured by both integral magnetometry and magnetic microscopy techniques. We observe enhanced coercive fields while the switching field distribution is broadened compared to thin-film reference samples. It appears possible to tailor the magnetic domain sizes down to the width of an unperturbed domain wall in a continuous film, and moreover, we observe pinning and nucleation at or close to the imprinted defect structures.

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

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.

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

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.

Arrays of size and distance controlled platinum nanoparticles fabricated by a colloidal method.

Based on emulsion polymerization in the presence of a Pt complex, polystyrene (PS) particles were prepared exhibiting a well defined average diameter with narrow size-distribution. Furthermore, the colloids contain a controlled concentration of the Pt precursor complex. Optimized coating of Si substrates with such colloids leads to extended areas of hexagonally ordered close-packed PS particles. Subsequent application of plasma etching and annealing steps allows complete removal of the PS carriers and in parallel nucleation and growth of Pt nanoparticles (NPs) which are located at the original center of the PS colloids. In this way, hexagonally arranged spherical Pt NPs are obtained with controlled size and interparticle distances demonstrating variability and precision with so far unknown parameter scalability. This control is demonstrated by the fabrication of Pt NP arrays at a fixed particle distance of 185 nm while systematically varying the diameters between 8 and 15 nm. Further progress could be achieved by seeded emulsion polymerization. Here, Pt loaded PS colloids of 130 nm were used as seeds for a subsequent additional emulsion polymerization, systematically enlarging the diameter of the PS particles. Applying the plasma and annealing steps as above, in this way hexagonally ordered arrays of 9 nm Pt NPs could be obtained at distances up to 260 nm. To demonstrate their stability, such Pt particles were used as etching masks during reactive ion etching thereby transferring their hexagonal pattern into the Si substrate resulting in corresponding arrays of nanopillars.

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.

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.