Metal nanostructure arrays in quasi-regular porous silicon

One crucial point of this project is to merge semiconducting and ferromagnetic materials on one level exhibiting desired physical properties at room temperature which can be reached by nanostructured semiconductor/metal hybrid systems. As a substrate material silicon is preferentially promoted due to its dominant position in today`s micro- and nano-technology but also because of its particularly suitable nanomachined properties. By anodization of a silicon wafer, employing adequate process-parameters, porous silicon (PS) offering a quasi- regular pore-arrangement with channels grown perpendicular to the wafer surface is achieved. These high-aspect- ratio (~ 1000) pores of adjustable diameter between 30 nm and 100 nm are obtained by a proper choice of current density and carefully selected electrolyte parameters. To produce a nanocomposite system, in a second cathodic deposition process transition metals (Ni, Co, Fe, Cu) and some of their alloys are precipitated in the porous silicon templates. Magnetic properties like coercivity and squareness of hysteresis of (Ni-, Co-, NiCo-, Fe-)-PS metal/semiconductor hybrids are tuneable via the morphology of the PS-membrane, the electrochemical deposition parameters and the choice of the deposited metal. Investigations on the specimens are carried out by SQUID- magnetometry, FTIR-spectroscopy, magneto-optical methods as well as by scanning electron microscopy. Initially, in the frame of this project the mesoporous structures fabricated at present, will be extended to achieve a broad morphological range of nanostructured matrices providing a basis for a variety of resulting tailored magnetic properties after metal deposition. The upgrading will imply not only macropores in the micrometer range but also multilayered PS-structures with varying adjacent porosities within one sample as well as double-sided etched wafers. Subsequently within the produced structures either a transition metal will be precipitated by electrodeposition or magnetic nanoparticles will be deposited electroless by immersion at monitored conditions. The deposition process will especially include the precipitation of Fe and magnetite which is not only interesting due to the magnetic behaviour but also to its applicability in nanobiology and nanomedicine. As observed so far from Ni/PS and Co/PS nanocomposites the magnetic characteristics of the hybrid material always exhibit a temperature dependent novel non-saturating behaviour at high magnetic fields far above the saturation magnetization of the embedded metals whose occurrence can not yet be sufficiently interpreted and has to be clarified. Furthermore the different morphologies of the PS-matrices as well as the distinct structures of the metal precipitations (mainly carried out by SEM and TEM) will be correlated with the associated magnetic characteristics together with transport and magnetoresistance measurements, as well as with optical, magneto-optical and MFM- investigations. In addition the link to the process parameters will be figured out. ESR and FMR measurements are going to be useful to gain information about the Si/SiOx /metal-interface. Moreover an experiment to demonstrate the injection of spin-polarized electrons from a precipitated ferromagnetic metal into silicon will be arranged. To this purpose a defined oxide layer within the pores as a tunnel barrier will be grown and the sample-surface smoothened for metallization as well.

The project Metal nanostructure arrays in quasi-regular porous silicon dealt with the fabrication of porous silicon with oriented pores and the subsequent filling with ferromagnetic materials. The filling of the pores with various metals could be controlled to achieve nanocomposites with desired magnetic properties. The key topic of the project was to combine a nanostructured semiconductor with magnetic nanostructures on one material level resulting in a hybrid system offering ferromagnetic properties at room temperature whereat the system is compatible to processes of todays microtechnology. Such systems could be employed e.g. as magneto-optical devices, sensors or in biomedical applications. One of the most important aims was the development of the processes to fabricate quasi-regular pore- arrangements with different morphology (dimensions 50 100 nm) and the deposition of tailored magnetic nanostructures, tunable in their size and geometry. During these experiments the metal deposition process within meso/macropores has been elucidated and also the magnetic properties of the achieved nanoparticle/wire arrays have been investigated in detail. Special attention has been laid on the magnetic coupling between the deposited nanostructures. Furthermore the deposition process could be sophisticated to deposit Ni-nanoparticles of only a few nanometers in size at the pore-walls to form quasi metal tubes. Due to their close arrangement the particles magnetically interact and thus the system offers a ferromagnetic behaviour. Considering the nanocomposite system, the dendritic pore-growth of the porous silicon templates influences both, the deposition process and also the resulting magnetic properties. Thus a novel anodization technique by applying a magnetic field during the pore formation has been employed to decrease the pore-wall roughness. As a result the magnetic properties of the samples after metal-filling have been significantly modified so that the samples become comparatively hard magnetic. A further key topic was the infiltration of superparamagnetic iron oxide nanoparticles (different sizes between 4 and 10 nm) into the pores showing interesting results with regard to magnetic inter-particle interactions. By tuning the particle filling within the pores and by varying the morphology of the template the blocking temperature (transition between superparamagnetic behaviour and blocked state) could be influenced and a value far below room temperature could be attained. Since both employed materials (porous silicon and iron oxide) are biocompatible these results are of interest for using the nanocomposite for biomedical applications such as magnetically guided drug delivery. Also cytotoxicity evaluations of these porous silicon/iron oxide systems showed encouraging results.