Iris Hölken

Sacrificial Template Synthesis and Properties of 3D Hollow-Silicon Nano- and Microstructures

Novel three-dimensional (3D) hollow aero-silicon nano- and microstructures, namely, Si-tetrapods (Si-T) and Si-spheres (Si-S) were synthesized by a sacrificial template approach for the first time. The new Si-T and Si-S architectures were found as most temperature-stable hollow nanomaterials, up to 1000 °C, ever reported. The synthesized aero-silicon or aerogel was integrated into sensor structures based on 3D networks. A single microstructure Si-T was employed to investigate electrical and gas sensing properties. The elaborated hollow microstructures open new possibilities and a wide area of perspectives in the field of nano- and microstructure synthesis by sacrificial template approaches. The enormous flexibility and variety of the hollow Si structures are provided by the special geometry of the sacrificial template material, ZnO-tetrapods (ZnO-T). A Si layer was deposited onto the surface of ZnO-T networks by plasma-enhanced chemical vapor deposition. All samples demonstrated p-type conductivity; hence, the resistance of the sensor structure increased after introducing the reducing gases in the test chamber. These hollow structures and their unique and superior properties can be advantageous in different fields, such as NEMS/MEMS, batteries, dye-sensitized solar cells, gas sensing in harsh environment, and biomedical applications. This method can be extended for synthesis of other types of hollow nanostructures.

Study of tetrapodal ZnO-PDMS composites: a comparison of fillers shapes in stiffness and hydrophobicity improvements.

ZnO particles of different size and structures were used as fillers to modify the silicone rubber, in order to reveal the effect of the filler shape in the polymer composites. Tetrapodal shaped microparticles, short microfibers/whiskers, and nanosized spherical particles from ZnO have been used as fillers to fabricate the different ZnO-Silicone composites. The detailed microstructures of the fillers as well as synthesized composites using scanning electron microscopy have been presented here. The tensile elastic modulus and water contact angle, which are important parameters for bio-mimetic applications, of fabricated composites with different fillers have been measured and compared. Among all three types of fillers, tetrapodal shaped ZnO microparticles showed the best performance in terms of increase in hydrophobicity of material cross-section as well as the stiffness of the composites. It has been demonstrated that the tetrapodal shaped microparticles gain their advantage due to the special shape, which avoids agglomeration problems as in the case for nanoparticles, and the difficulty of achieving truly random distribution for whisker fillers.

Biaxial flexural strength of new Bis-GMA/TEGDMA based composites with different fillers for dental applications.

OBJECTIVE The aim of this study was to evaluate whether the mechanical properties of the modern dental composites can be improved by using tetrapodal ZnO particles as fillers in a Bis-GMA/TEGDMA matrix. Another aim was to test whether the mechanical properties of the composite are influenced by functionalization of the ZnO particles with lauric acid to achieve antibacterial activity. METHODS Different filler materials and particle shapes (spherical zirconia, spherical zinc oxide, tetrapodal zinc oxide) were used to produce Bis-GMA/TEGDMA based composites with a filler content of 40wt.-% and 60wt.-%, respectively. In addition, functionalization with lauric acid was investigated. For the biaxial flexural strength testing 104 test disks (N=8) with a diameter of 15mm and a thickness of 1.5mm were produced. RESULTS Functionalization with lauric acid resulted in a decrease in biaxial flexural strength for all filler materials. The biaxial flexural strength decreased when using a higher filler content with spherical particles but increased when using tetrapodal zinc oxide particles. SIGNIFICANCE A higher durability of the composites using tetrapodal zinc oxide particles. An antibacterial functionalization with lauric acid cannot be recommended as the mechanical stability of the composite will be reduced.

Functional Ecofriendly Coatings for Marine Applications

The worldwide increasing number of off-shore wind turbines brings along the need for innovative polymer coatings which on the one hand can withstand the extreme forces acting on the rotor blades and on the other hand can provide adequate antifouling properties to counter biocorrosion of the basement. The development of environmentally friendly and commercially realizable coating systems is most important since the prohibition of tributyltin (TBT)-based antifouling coatings in 2008. In this study an alternative polymer/particle composite coating based on polythiurethan (PTU) with tetrapodal shaped ZnO (t-ZnO) as additive is investigated with respect to chemical, mechanical and antifouling properties. Overall, four different composites with filler amounts of 0 wt% t-ZnO, 1 wt% t-ZnO, 5 wt% t-ZnO, 10 wt% t-ZnO were tested. Preliminary antifouling experiments were carried out under artificial habitat conditions at an aquarium, imitating Pacific Sea water. For mechanical and chemical characterization tensile tests, adhesion tests and contact angle measurements were performed. An optimum of mechanical and antifouling properties were found for a filler amount of 5 wt% t-ZnO in PTU.

On the physical robustness of RF on-chip nanostructured security

The main objective of this paper is to investigate the physical robustness of a newly-proposed RF-based fingerprinting and security technique. This technique aims at securing electronic chips utilizing the fabrication technology as well as chip packaging. It is based on using micro- and nanostructured composite materials mixed with a dielectric fixing matrix. Several experimental investigations on the repeatability, temperature-robustness and response to physical attacks on different mixtures and particle sizes are herewith discussed.

Flame based growth of ZnO nano- and microstructures for advanced optical, multifunctional devices, and biomedical applications (Conference Presentation)

The recent flame based growth strategy offers a simple and versatile fabrication of various (one, two, and three-dimensional) nano- and microstructures from different metal oxides (ZnO, SnO2, Fe2O3, etc.) in a desired manner.[1] ZnO structures ranging from nanoscales wires to macroscopic and highly porous 3D interconnected tetrapod networks have been successfully synthesized, characterized and utilized for various applications. The ZnO micro- and nanoneedles grown at walls in silicon trenches showed excellent whispering gallery mode resonances and photocatalytic properties.[2] Using the same strategy, large polycrystalline micro- and nanostructured ZnO platelets can be grown with grains interconnected together via grain boundaries and these grain boundaries exhibit a higher conductivity as compared to individual grains.[3] This flame transport synthesis (FTS) approach offers the growth of a large amount of ZnO tetrapods which have shown interesting applications because of their 3D spatial shape and micro-and nanoscale size, for example, interconnected tetrapods based devices for UV-detection and gas sensing.[4-5] Because of their complex 3D shape, ZnO tetrapods can be used as efficient filler particles for designing self-reporting,[6] and other interesting composites. The nanostructured materials exhibit an important role with respect to advanced biomedical applications as grown ZnO structures have shown strong potentials for antiviral applications.[7] Being mechanically strong and micro-and nanoscale in dimensions, these ZnO tetrapods can be easily doped with other elements or hybridized with various nanoparticles in form of hybrid ZnO tetrapods which are suitable for various multifunctional applications, for example, these hybrid tetrapods showed improved gas sensing properties.[8] The sacrificial nature of ZnO allows the for growth of new tetrapods and 3D network materials for various advanced applications, for example, highly porous and ultra light carbon based Aerographite materials[9] and hollow silicon tetrapods.[10] These carbon based highly porous network can be further utilized for growth of new hybrid 3D nanomaterials, for example, Aerographite- GaN[11] and Aerographite-ZnO[12] for advanced optical and other applications.

RF Nanostructured Security

This chapter gives an intensive overview of some recent micro‐ and nanostructured Radio Frequency (RF) security issues. It identifies the challenges of tomorrow’s security problems and why this has been a big relevance not only to nano-communications but also to other applications. A short overview on the traditional Physical Unclonable Functions (PUFs) introduces the reader into the concept of applied electromagnetic waves interacting with nanomaterials. Major security and fingerprinting contributions, which are newly‐proposed and implemented by the authors, are concluded in this chapter. These security techniques are based on artificially‐synthesized disordered micro and nano materials. A potential on‐chip realization and integration scenario of such approach is also discussed. Novel material synthesis technologies and functional prototype production processes are illustrated. Extraction process of RF fingerprints, based on near‐field scattering measurements, is included as well. Finally, statistical analysis and distance measures of similarity, uniqueness and orthogonality of the extracted fingerprints are carefully investigated at the end of this chapter.

Tetrapodal ZnO Particles for Substrate Mode Scattering in Flexible Organic Light-Emitting Diodes

A thin polymer foil containing tetrapodal zinc oxide microparticles for volume scattering is produced. This foil is used as bendable substrate for an organic light-emitting diode.