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In the frame of this work microporous silicon (mPSi) is filled with magnetic metals (Ni, Co) resulting in a composite system with specific magnetic properties corresponding to its branched morphology. The results are compared with mesoporous silicon (PSi) loaded with Ni nanoparticles (NPs) of comparable size. The two systems offer drastic differences in their magnetic response. Furthermore mPSi offers luminescence in the visible range, whereat this behavior is due to the small structure size in the range of a few nanometers (2 - 5 nm). The morphology of mesoporous silicon shows straight pores with a diameter up to 50 nm. Due to the different template morphology the responding magnetic behavior shows distinct properties related to the two systems, although the size of the metal deposits is in a similar size range. The Ni loaded mPSi system merges optical and magnetic properties leading to a nanocomposite appropriate for possible magneto-optical devices.

This work aims to combine luminescent microporous silicon with a ferromagnetic metal to create a nanocomposite system with modifiable photoluminescence due to the presence of metal deposits as well as to influence the optical behavior by an external magnetic field. Due to the metal filling of the porous silicon (PSi) the peak of the photoluminescence is blue-shifted and furthermore an increase of the intensity is observed. Magnetic characterization of the samples shows a film-like behavior with the magnetic easy axis parallel to the surface which is due to interconnections of the metal deposits. The influence of the magnetic metal filling on the optical properties is discussed and the magnetic characterization of the samples is presented.

The purpose of this work is to fabricate nanostructured silicon with two different materials of embedded magnetic nanostructures to exploit the magnetic properties of both metals and gain control of the exchange coupling between the two metals especially with respect to their volume ratio. Generally both metals are deposited alternatingly out of one solution by varying the applied potential but in a further approach the two metals are also deposited by using different metal salt solutions alternately. Furthermore a variation of the structure size and the proximity of the metal deposits modifies the exchange coupling and thus the energy product. Finally nanocomposits with an energy product as high as possible are achieved to give rise to on-chip applications using permanent nanomagnets, especially arranged in arrays.

Bi-metal magnetic nanostructures have been deposited within porous silicon (PSi) to influence the magnetic switching behavior of the PSi/metal nanocomposite and further to enhance the energy product of the system compared to single metal nanocomposites. These nanocomposites consisting of two ferromagnetic metals have been achieved by two different routes. On the one hand both metals have been deposited alternatingly out of one solution and on the other hand the two metals have been deposited by using different metal salt solutions alternately. The latter system shows two distinct slopes of the hysteresis due to the different saturation behavior of the two types of deposited metal, whereas the system produced from one electrolyte shows no kink which indicates exchange coupling between the two metals. The comparison of these two types of samples shows that the one offering exchange coupling exhibits nearly a doubled coercivity and remanence.

The purpose of this work is to combine magnetic nanostructures of two different metals within porous silicon to influence the magnetic switching behavior of this porous silicon/metal nanocomposite. These "hard/soft" magnetic nanocomposites have been achieved by two different routes. Porous silicon templates in the mesoporous regime have been employed for the electrochemical incorporation of different types of magnetic nanostructures. Two kinds of porous silicon templates have been used, on the one hand double sided porous silicon which has been achieved by etching of an ultrathin wafer in a double tank cell and on the other hand a silicon wafer has been porosified on one side only. The double-sided porous silicon has been filled with Ni on one side and with Co on the other side, also using a double tank cell. In the case of the one-sided sample both metals have been electrochemically deposited alternatingly inside the pores.

Within this work a biocompatible magnetic nanovehicle which is movable in aqueous solutions by a magnetic field is presented. The nanovehicle consists of a porous silicon template and encapsulated iron oxide nanostructures. Both materials employed, porous silicon (PSi) as well as iron oxide are known to be biocompatible. Two routes are employed to produce various iron oxide phases of the embedded nanostructures within PSi. On the one hand, readily synthesized magnetite nanoparticles (NPs) which are superparamagnetic (SPM) due to their size, are infiltrated into the pores. On the other hand chemical deposition of iron oxide inside the PSi templates is used. Mainly the synthesis of the nanocomposite as well as the magnetic properties of the systems will be discussed.

In the frame of this work porous silicon templates with different morphologies, especially with respect to the roughness of the pore-walls have been fabricated and subsequently filled with diverse ferromagnetic metals to fabricate self-assembled three dimensional arrays of nanostructures (wires, particles) with distinct magnetic properties. The fabricated templates offer a quasi-regular self-assembled pore-arrangement with oriented pores. Also the pore-filling is self-assembled by electrochemical deposition and there are multiple possibilities to tailor the magnetic behavior of these systems such as material, morphology, inter particle distance or interfaces. One critical parameter for this purpose is the roughness of the pore walls and the surface of the metal structures, respectively. Within the pores of these templates ferromagnetic metals have been deposited and the modification of the magnetic properties correlated to the morphology of the different templates has been figured out.

The aim of this work is to create a biocompatible superparamagnetic nanocomposite applicable as vehicle for magnetically guided drug delivery. Both materials, the nanostructured silicon as well as the iron oxide nanoparticles offer low toxicity. Therefore Fe3O4-nanoparticles have been incorporated within nanostructured silicon. On the one hand the iron oxide nanoparticles have been synthesized in solution and then infiltrated and on the other hand they have been chemically grown within the pores. The magnetic properties of the system have been optimized concerning the blocking temperature TB and the magnetic moment M, which are conflicting parameters. A particle-size/distance and template dependent assessment of the magnetic properties of the nanocomposite will be presented. Although the growth of iron oxide particles within the porous template is not easy to control the blocking temperature of the composite could be achieved far below room temperature.

Due to the fact that both materials porous silicon as well as iron oxide nanoparticles are biocompatible and porous silicon can be functionalized easily to attach therapeutic nucleotides the aim is to create a cytocompatible arrangement to promote magnetically guided delivery into targeted cells. Furthermore porous silicon dissolves in body fluid dependent on the porosity which increases the possible applications. Within the frame of this work the field and temperature dependent magnetic properties of the nanocomposite have been investigated as well as its cytocompatibility according to the porous silicon morphology and the iron oxide particle size.

Porous silicon in the mesoporous regime is used as template for electrodeposition of ferromagnetic metals within the pores to fabricate self-assembled three dimensional arrays of nanotubes, nanowires or nanoparticles. Depending on the electrochemical deposition parameters the geometry of the metal deposits can be tuned and thus desired magnetic properties can be achieved. The morphology of the templates is modified due to different anodization processes. Additional to a conventional etching process magnetic field assisted etching is applied. Furthermore the magnetic properties in dependence on the morphological characteristics, especially the pore-growth of the porous silicon are investigated. An important point is the concomitant modification of the magnetic coupling of metal nanostructures of adjacent pores.

Porous silicon fabricated by different formation techniques has been utilized as template for the electrodeposition of ferromagnetic metal nanowires within the pores. The porous silicon templates have been prepared by conventional and magnetic field assisted anodization, respectively whereat different morphologies have been obtained. The influence of the morphology of the employed template on the deposited metal structures and as a result on the magnetic properties of the nanocomposites is figured out. Due to a decrease of the pore-diameter, an increase of the smoothness of the deposited nanowires and also because of an increase of the distance between neighbouring ones magnetic crosstalk between them is decreased and results in a modification of the magnetic properties.

Iron oxide nanoparticles of different sizes (8, 5 and 3.8 nm) have been infiltrated into the pores of porous silicon matrices, offering a hybrid system with tunable magnetic properties due to the morphology of the matrix, the loading of the nanoparticles and their size. For biomedical applications a superparamagnetic behaviour at room temperature is required, which has been verified for the three particle sizes investigated here. Temperature dependent magnetization measurements show that the blocking temperature is far below room temperature for all three composite systems. Furthermore the toxicity of these composite samples with regard to cell-viability has been investigated. A trypan blue-based dye exclusion test has been used to determine the number of viable cells present in suspensions of HEK 293 cells and resulted in a minimum viability above 90% (by the 5th day) in the case of all three particle sizes infiltrated into the porous Si matrix.

Magnetite nanoparticles of 8 nm in size have been fabricated by high temperature decomposition with an organic coating of about 2 nm in thickness. These particles are infiltrated into the pores of porous silicon resulting in a hybrid system. For this infiltration process porous silicon with different morphologies is used and the concentration of the iron oxide particle solution has been modified to vary the distance between the particles. These modifications of the particle filling lead to a change of the strength of the magnetic inter-particle coupling within in the pores. In this way the magnetic properties of the system can be controlled, especially the position of the blocking temperature can be shifted.

The combination of porous silicon (PS) with iron oxide nanoparticles results in a nanocomposite system offering a magnetic behavior which is influenced by the properties of the nanosized particles but also by the morphology of the matrix. Mesoporous silicon, fabricated by anodization of a highly n-doped silicon wafer offers oriented pores with average diameters of 50 nm and 90 nm, respectively. Fe3O4 nanoparticles of different size (3.8, 5 and 8 nm) have been used to deposit into the pores of these matrices. The resulting composite exhibits a desired transition temperature between superparamagnetic (SPM) behaviour and blocked state which can be modified by the size of the deposited particles, the distance between the particles and by the variation of the used porous silicon morphology, respectively. Beside scanning electron microscopy (SEM) a sophisticated method for TEM-investigations has been employed to figure out the various characteristics of the system.

Metal-nanostructures are electrochemically deposited within the pores of porous silicon to achieve a hybrid material with specific magnetic properties. For this purpose first the porous silicon template is prepared by wet etching of an n+-type silicon wafer offering pore-diameters between 30 and 100 nm, depending on the anodization parameters. Secondly a ferromagnetic metal is electrodeposited into the pores, whereas the metal structures can be precipitated with various geometries and different spatial distributions depending on an accurate control of the deposition conditions. This method allows to deposit structures as spheres, ellipsoids or wires with a size up to a few micrometers in length whereas the diameter corresponds to the pore-diameter. In the case of Ni the deposition of small particles (2 - 5 nm) succeeded. These nanoparticles are covering the pore-walls and form depending on the packing density tube-like arrangements.

Ni-nanostructures are electrochemically deposited within the pores of mesoporous silicon. The porous silicon template is fabricated with straight and separated pores. The geometry of the metal deposits can be tuned in a wide size range between 50 nm and a few micrometers and thus specimens with specific magnetic properties can be achieved. Furthermore small Ni-particles between 3 and 6 nm in size can be precipitated on the pore walls with various distances. In the case of a closely packed distribution the particle arrangement results in metal tubes which cover the pore walls. The superparamagnetic Ni-particles forming such a tube are magnetically interacting which leads to a ferromagnetic behavior of the system.

The infiltration of iron oxide nanoparticles into mesoporous silicon is performed to investigate the influence of different arrangements of the particles on the magnetic behaviour of the achieved nanocomposite consisting of a semiconductor and a magnetic material. The porous silicon is fabricated from a highly doped n-type silicon wafer by anodization. The Fe3O4 particles fabricated by high temperature decomposition offer a size of 5 and 8 nm, respectively. In the present work the influence of the porous silicon matrix on the magnetic behaviour of such hybrid materials is figured out. In general iron oxide particles of a few nanometer in size are superparamagnetic but due to dipolar coupling between them the blocking temperature can be shifted, which is demonstrated by modification of the particle size and the distance between the particles.

A silicon/metal nanocomposite is fabricated electrochemically by anodization of a silicon wafer leading to a porous structure and subsequent metal deposition into the pores. The pores are oriented and separated from each other, offering a morphology in the mesoporous regime. The precipitation of the metal within the pores is performed by pulsed electrodeposition whereas the pulses are used as a tool to influence the structural characteristics of the embedded metal structures. By varying the frequency of the pulsed current the geometry of the metal structures can be modified between spherical particles and needle-like structures of a few microns in length. Magnetic characteristics of the nanocomposite samples depend on the shape of the incorporated structures but also on their magnetic interactions.

Porous silicon matrices fabricated by wet etching are used as template for the deposition of metal nanostructures. The anodization of the silicon wafer is performed on one or on both sides in employing a double-tank electrolytic cell. The resulting porous layers are subsequently filled with a ferromagnetic metal by electrodeposition. In case of double-sided samples an alternating pulsed current is applied. These two processes are extended to ultrathin samples with an entire thickness of about 60 um. The morphology of the porous structure exhibits average pore-diameters of 60 nm, a mean pore-spacing of 45 nm and a pore-length of 40 um. Ferromagnetic metals are electrochemically deposited as nanostructures with a specific distribution within the pores.

Porous silicon PS templates in the mesomacro porous regime with oriented pores clearly separated from each other and filled in a galvanic deposition process with various metals, especially ferromagnetic ones are magnetically investigated. The employment of different metals e.g. Ni, Co, NiCo together with the variation of the electrochemical deposition parameters modifies the structural characteristics of the PSmetal nanocomposite and thus leads to distinct magnetic properties of the hybrid system. Furthermore the use of different PStemplates which means a change of the porediameter and interpore spacing results also in various magnetic characteristics, especially influences on the magnetic interactions among the deposited metal nanostructures. Therefore the specimens show tailored magnetic properties like coercivity, squareness and magnetic anisotropy. The achieved nanocomposite merges electronic properties of a semiconductor with nanomagnetism and therefore opens the possibility of integrated spinbased electronic devices. © 2009 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim

Electrodeposition of ferromagnetic metal-nanostructures into a porous silicon (PS) matrix merges the electronic properties of a semiconductor and the ferromagnetic ones of the precipitated metal on one material level. The deposition is performed using a metal-salt solution as electrolyte and an applied pulsed current under cathodic conditions. The metal precipitation within a PS-template exhibiting unconnected mesopores with aspect ratios of about 1000 leads to nanostructures with a maximum elongation of a few microns (aspect ratio 100). The achieved hybrid system exhibits magnetic characteristics which drastically differ from the corresponding bulk materials. Magnetization measurements of this material show two characteristics, one at magnetic fields below the saturation magnetization MS of the deposited metals due to the spin-magnetism (e.g. MS (Ni)=0.62 T, MS (Co)=1.82 T) and another one at higher fields above MS which is assigned to a novel, recently intensively investigated magnetic contribution likely caused by spin-orbit interaction.

Porous silicon (PS) matrices with oriented pores grown perpendicular to the surface are used as template for the incorporation of magnetite nanoparticles. The Fe3O4-particles used for infiltration into the PS-template are coated with oleic acid in a hexane solution and exhibit an average diameter of 9 nm {plus minus}1 nm. The narrow size-distribution and the superparamagnetic behaviour at room temperatre are interesting features of these nanoparticles. Magnetite nanoparticles play a key role in medical applications but also the magnetic properties of such a nanoparticle/PS system are of technical interest due to the transition between superparamagnetic and ferromagnetic behaviour. Temperature dependent magnetization measurements are used to gain information about the magnetic interaction of the particles.

Nanostructuring of a silicon wafer by etching in an HF-solution leads to a porous semiconductor which offers physical properties (e.g. optical, magnetic) drastically different from the ones of the according bulk material. A change of the magnetic behaviour from diamagnetic in case of bulk silicon to ferromagnetic has been observed in mesoporous samples. This ferromagnetism depends on the morphology (porosity) of the investigated samples, which can be modified by the electrochemical process parameters and/or the doping level of the used Si-substrate. The occurrence of this novel ferromagnetic behaviour is not completely clarified yet but has to be caused by surface-effects resulting in a new kind of interface-magnetism.

Am Beispiel von Boltzmanns Experimenten zur Bestätigung der Maxwell-Theorie wird dessen hervorragende Begabung auch als Experimentalphysiker deutlich. Die dargestellten Arbeiten fanden etwa 15 Jahre vor den Hertzschen Experimenten statt, zu einem Zeitpunkt also, als die noch junge Maxwell-Theorie dringend experimenteller Bestätigung bedurfte. Boltzmann konnte mit seinen experimentellen Untersuchungen eine direkte Konsequenz der Maxwellschen elektromagnetischen Theorie, die so genannte Maxwell-Relation, bestätigen. Seine Experimente stellten nicht nur lange Zeit den stärksten Beleg für die Richtigkeit der Maxwellschen Lichttheorie dar, sondern waren eine experimentelle Spitzenleistung an der Grenze des damals Möglichen. Besonders Boltzmanns Bestimmung der Dielektrizitätskonstanten von Gasen fand noch Jahrzehnte später Erwähnung in namhaften Lehrbüchern.

Magnetische Halbleiter, in denen die Konzentration und der Spin von Ladungsträgern gemeinsam gesteuert werden können, versprechen neue elektronische Anwendungen: Eine durch elektrische Ströme gesteuerte Spin-Elektronik (Spintronik)ist das Ziel. Weil sie voll integrationsfähig in Chips sind, eignen sich magnetische Halbleiter besser als andere Materialien. Noch fehlen solche Halbleiter, aber verdünntmagnetische II-VI-Halbleiter und ferromagnetische Europium-Chalkogenide sind gute Kandidaten. Mangan-substituierte Halbleiter könnten sogar einen stabilen Ferromagnetismus bei Raumtemperatur erreichen. Und magneto-optische Halbleiterbauelemente könnten zur Steuerung schneller optischer Computer eingesetzt werden.

Herein, nanostructured silicon is fabricated with two different embedded magnetic nanostructures (Ni/Co) to exploit the magnetic properties of both metals and discern the behavior of the exchange coupling between the two—especially with respect to their volume ratio. A variation of the size as well as the volume ratio of the two metals modifies the exchange coupling and thus the energy product. Two templates are utilized: one involves the use of porous silicon (PSi) and the other entails porous silicon nanotubes (SiNTs). In the case of Psi, the bimetallic structures (Ni/Co) are electrodeposited, whereas in the case of SiNTs, Co is chemically grown within the tubes followed by the deposition of Ni on the outside. The magnetic properties of the systems strongly depend on the geometry of the deposits, on the volume ratio of the two metals, as well as on the proximity of the deposits. If the distance is small enough magnetic exchange coupling is present. Due to their size and the coupling of the bimetallic structures, the switching behavior of the hysteresis can be varied. Ultimately, nanocomposites with an energy product as high as possible is achieved to give rise to applications as permanent nanomagnets. Nanostructured silicon (porous silicon and silicon nanotubes) is used as host material for magnetic metal deposition, using two kinds of metals with a magnetic harder and softer phase, respectively. Variation of the size, volume ratio, and the proximity of the two phases are tools to adjust the magnetic properties of the composite systems.

Within this work the deposition of ferromagnetic metals in high aspect ratio mesopores will be elucidated regarding the specific nucleation and growth of the deposited nanostructures under various conditions. Since mesoporous morphologies offer dendritic pore walls these structural inhomogeneities of the matrices are critical for the deposition process. Experiments show that the nucleation preferentially starts at the “tops” of the rough surface. A further key point of the presentation is the magnetic characterization of the various nanocomposites. Magnetic characterization of the systems shows that not only the morphology of the deposited metal structures (size, shape, distribution) determines the magnetic properties but also the morphology of the template (side-pore length). In this sense magnetic coupling between the nanostructures plays a crucial role and is strongly influenced by morphological parameters. From all these findings it is possible to prepare samples with specific magnetic properties which could be of interest for on-the-chip devices.

In the frame of this work the structural and magnetic characterization of encapsulated iron oxide nanoparticles within a nanostructured silicon host is discussed. Two approaches are depicted, one to infiltrate readily synthesized Fe3O4particles into the pores and second to grow iron oxide inside the pores out of an ethanolic iron salt solution. Beside the particle size dependent investigations, a key point of the presentation is the optimization of the system with respect to the desired magnetic properties (magnetic moment as high as possible, blocking temperature below room temperature) which are controlled by the filling parameters as well as by the morphology of the host. All investigated composite systems exhibit a blocking temperature far below room temperature and due to the fact that all utilized materials are biocompatible the presented systems are promising for using as magnetic vehicle in biomedicine.