Dawit Gedamu

Rapid Fabrication Technique for Interpenetrated ZnO Nanotetrapod Networks for Fast UV Sensors

Two flame-based synthesis methods are presented for fabricating ZnO-nanostructure-based UV photodetectors: burner flame transport synthesis (B-FTS)and crucible flame transport synthesis (C-FTS). B-FTS allows rapid growth of ZnO nanotetrapods and in situ bridging of them into electrical contacts. The photo detector made from interconnected ZnO nanotetrapod networks exhibits fast response/recovery times and a high current ratio under UV illumination.

Solvent Free Fabrication of Micro and Nanostructured Drug Coatings by Thermal Evaporation for Controlled Release and Increased Effects

Nanostructuring of drug delivery systems offers many promising applications like precise control of dissolution and release kinetics, enhanced activities, flexibility in terms of surface coatings, integration into implants, designing the appropriate scaffolds or even integrating into microelectronic chips etc. for different desired applications. In general such kind of structuring is difficult due to unintentional mixing of chemical solvents used during drug formulations. We demonstrate here the successful solvent-free fabrication of micro-nanostructured pharmaceutical molecules by simple thermal evaporation (TE). The evaporation of drug molecules and their emission to a specific surface under vacuum led to controlled assembling of the molecules from vapour phase to solid phase. The most important aspects of thermal evaporation technique are: solvent-free, precise control of size, possibility of fabricating multilayer/hybrid, and free choice of substrates. This could be shown for twenty eight pharmaceutical substances of different chemical structures which were evaporated on surfaces of titanium and glass discs. Structural investigations of different TE fabricated drugs were performed by atomic force microscopy, scanning electron microscopy and Raman spectroscopy which revealed that these drug substances preserve their structurality after evaporation. Titanium discs coated with antimicrobial substances by thermal evaporation were subjected to tests for antibacterial or antifungal activities, respectively. A significant increase in their antimicrobial activity was observed in zones of inhibition tests compared to controls of the diluted substances on the discs made of paper for filtration. With thermal evaporation, we have successfully synthesized solvent-free nanostructured drug delivery systems in form of multilayer structures and in hybrid drug complexes respectively. Analyses of these substances consolidated that thermal evaporation opens up the possibility to convert dissoluble drug substances into the active forms by their transfer onto a specific surface without the need of their prior dissolution.

Procedures and Properties for a Direct Nano-Micro Integration of Metal and Semiconductor Nanowires on Si Chips

1-dimensional metal and semiconductor nanostructures exhibit interesting physical properties, but their integration into modern electronic devices is often a very challenging task. Finding the appropriate supports for nanostructures and nanoscale contacts are highly desired aspects in this regard. In present work we demonstrate the fabrication of 1D nano- and mesostructures between microstructured contacts formed directly on a silicon chip either by a thin film fracture (TFF) approach or by a modified vapor-liquid-solid (MVLS) approach. In principle, both approaches offer the possibilities to integrate these nano-meso structures in wafer-level fabrications. Electrical properties of these nano-micro structures integrated on Si chips and their preliminary applications in the direction of sensors and field effect transistors are also presented.

Effect of Al Sn — Doping on properties of zinc oxide nanostructured films grown by magnetron sputtering

Metal doping in nanostructured zinc oxide is important for device applications. To obtain improved performances for practical applications, Aluminum (Al) and Tin (Sn)-doping in zinc oxide nanostructured layers were investigated. Samples were grown by magnetron sputtering and studied by X-ray diffraction (XRD), micro-Raman, scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) techniques. It was observed that nanoparticles are interconnected and form porous network of individual nanoparticles. It is found clear evidence of changes of different properties after doping with aluminum or tin in zinc oxide nanostructured films grown by magnetron sputtering.

Zinc oxide micro- and nanostructures as multifunctional materials

Zinc oxide (ZnO) is emerging as a useful multifunctional material due to its toughness, but it has the drawback that it is also very brittle. This brittleness can be overcome by incorporating ZnO nanostructures into polymer matrices to produce materials such as heavy-duty lacquers. However, ZnO also has special sensing, photocatalytic, and antiviral behavior, all of which can be enhanced by forming nanoand microstructures to increase the ratio of surface area to volume. One main problem for these applications is how to control the synthesis parameters—temperature and pressure—which affect not only the quantity of particles or assemblies produced but also their shape. The shape in turn influences the physical and chemical properties of the material. Depending on the parameters, different crystal-growth directions are favored during ZnO particle synthesis, facilitating formation of one of these structural variations.1 Previously, precursors such as ZnO micropowder or dimethyl zinc were used as starting materials. Now, through the use of vacuum systems, plasma sources, and optimization of carrier and reactive gases, synthesis has moved toward higher throughput. However, for many commercial applications these methods are still too complex or too slow. For our investigation, we made ZnO tetrapod structures by two methods: sputter deposition, and an alternative, high-throughput method we have developed based on oxidation of zinc powder by heating in a furnace in air.2 The high-throughput method gives us the possibility to synthesize a large quantity (up to the kilogram scale) of nanostructured particles at low cost. Figure 1. Methylene blue solution under the photocatalytic effect of zinc oxide with UV light at (a) 0, (b) 30, (c) 60, and (d) 90min.

ZnO core spike particles and nano-networks and their wide range of applications

In our approach we are producing a polymer composite material with ZnO core spike particles as concave fillers. The core spike particles are synthesized by a high throughput method. Using PDMS (Polydimethylsiloxane) as a matrix material the core spike particles achieve not only a high mechanical reinforcement but also influence other material properties in a very interesting way, making such a composite very interesting for a wide range of applications. In a very similar synthesis route a nanoscopic ZnO-network is produced. As a ceramic this network can withstand high temperatures like 1300 K. In addition this material is quite elastic. To find a material with these two properties is a really difficult task, as polymers tend to decompose already at lower temperatures and metals melt. Especially under ambient conditions, often oxygen creates a problem for metals at these temperatures. If this material is at the same time a semiconductor, it has a high potential as a multifunctional material. Ceramic or classical semiconductors like III-V or IIVI type are high temperature stable, but typically brittle. This is different on the nanoscale. Even semiconductor wires like silicon with a very small diameter do not easily built up enough stress that leads to a failure while being bent, because in a first order approximation the maximum stress of a fiber scales with its diameter.