Klemens Rumpf

Porous Silicon—A Versatile Host Material

This work reviews the use of porous silicon (PS) as a nanomaterial which is extensively investigated and utilized for various applications, e.g., in the fields of optics, sensor technology and biomedicine. Furthermore the combination of PS with one or more materials which are also nanostructured due to their deposition within the porous matrix is discussed. Such nanocompounds offer a broad avenue of new and interesting properties depending on the kind of involved materials as well as on their morphology. The filling of the pores performed by electroless or electrochemical deposition is described, whereas different morphologies, reaching from micro- to macro pores are utilized as host material which can be self-organized or fabricated by prestructuring. For metal-deposition within the porous structures, both ferromagnetic and non-magnetic metals are used. Emphasis will be put on self-arranged mesoporous silicon, offering a quasi-regular pore arrangement, employed as template for filling with ferromagnetic metals. By varying the deposition parameters the precipitation of the metal structures within the pores can be tuned in geometry and spatial distribution leading to samples with desired magnetic properties. The correlation between morphology and magnetic behaviour of such semiconducting/magnetic systems will be determined. Porous silicon and its combination with a variety of filling materials leads to nanocomposites with specific physical properties caused by the nanometric size and give rise to a multiplicity of potential applications in spintronics, magnetic and magneto-optic devices, nutritional food additives as well as drug delivery.

Magnetic Study of Fe3O4 Nanoparticles Incorporated within Mesoporous Silicon

The fabrication of low dimensional systems as ultrathin layers, nanowires, nanoparticles, and nanodots is a key trend in today’s nanotechnology. Not only the change in the physical properties of low dimensional materials compared to their bulk materials is of interest; their applicability is also a developing subject. Magnetic materials in the nanoscale range are utilized in magnetic data storage, giant magnetoresistance devices; magnetic particles are also employed in biological and medical applications. The fabrication of isolated magnetic nanoparticles is difficult to reach because the large surface areas compared to the volume oxidize easily when using metals and due to the tendency of nanoparticles to agglomerate. A controlled passivation of the particles can be carried out, but this can also lead to interactions between the metal core and the passivating materials. Magnetic iron oxide such as magnetite has the advantage of being more stable. Recently, a new preparation method has been reported based on the decomposition at high temperatures of an organic precursor in the presence of oleic acid, which leads to a monodisperse size distribution of nanopar

Magnetic Nanoparticles Embedded in a Silicon Matrix

This paper represents a short overview of nanocomposites consisting of magnetic nanoparticles incorporated into the pores of a porous silicon matrix by two different methods. On the one hand, nickel is electrochemically deposited whereas the nanoparticles are precipitated on the pore walls. The size of these particles is between 2 and 6 nm. These particles cover the pore walls and form a tube-like arrangement. On the other hand, rather well monodispersed iron oxide nanoparticles, of 5 and 8 nm respectively, are infiltrated into the pores. From their size the particles would be superparamagnetic if isolated but due to magnetic interactions between them, ordering of magnetic moments occurs below a blocking temperature and thus the composite system displays a ferromagnetic behavior. This transition temperature of the nanocomposite can be varied by changing the filling factor of the particles within the pores. Thus samples with magnetic properties which are variable in a broad range can be achieved, which renders this composite system interesting not only for basic research but also for applications, especially because of the silicon base material which makes it possible for today’s process technology.

Variable blocking temperature of a porous silicon/Fe3O4 composite due to different interactions of the magnetic nanoparticles

In the frame of this work, the aim was to create a superparamagnetic nanocomposite system with a maximized magnetic moment when magnetized by an external field and a blocking temperature far below room temperature. For this purpose, iron oxide nanoparticles of 3.8-, 5- and 8-nm size have been infiltrated into the pores of porous silicon. To fabricate tailored magnetic properties of the system, the particle size and the magnetic interactions among the particles play a crucial role. Different concentrations of the particles dispersed in hexane have been used for the infiltration to vary the blocking temperature TB, which indicates the transition between the superparamagnetic behavior and blocked state. TB is not only dependent on the particle size but also on the magnetic interactions between them, which can be varied by the particle-particle distance. Thus, a modification of the pore loading on the one hand and of the porous silicon morphology on the other hand results in a composite material with a desired blocking temperature. Because both materials, the mesoporous silicon matrices as well as the Fe3O4 nanoparticles, offer low toxicity, the system is a promising candidate for biomedical applications.

Fe3O4-nanoparticles within porous silicon: Magnetic and cytotoxicity characterization

The magnetic properties of porous silicon/Fe3O4 composites are investigated with respect to the adjustability of the blocking temperature along with an evaluation of any size-dependent changes in cytocompatibility. Fe3O4-nanoparticles have been infiltrated within mesoporous silicon, resulting in a system with tunable magnetic properties due to the matrix-morphology, the loading of the nanoparticles, and their size. In order to provide basic information regarding its suitability as a therapeutic platform, the cytotoxicity of these composites have been investigated by a trypan blue exclusion assay with respect to human embryonic kidney 293 cells, and the results compared with cell-only and known cytotoxic controls.

Investigation of a Mesoporous Silicon Based Ferromagnetic Nanocomposite

A semiconductor/metal nanocomposite is composed of a porosified silicon wafer and embedded ferromagnetic nanostructures. The obtained hybrid system possesses the electronic properties of silicon together with the magnetic properties of the incorporated ferromagnetic metal. On the one hand, a transition metal is electrochemically deposited from a metal salt solution into the nanostructured silicon skeleton, on the other hand magnetic particles of a few nanometres in size, fabricated in solution, are incorporated by immersion. The electrochemically deposited nanostructures can be tuned in size, shape and their spatial distribution by the process parameters, and thus specimens with desired ferromagnetic properties can be fabricated. Using magnetite nanoparticles for infiltration into porous silicon is of interest not only because of the magnetic properties of the composite material due to the possible modification of the ferromagnetic/superparamagnetic transition but also because of the biocompatibility of the system caused by the low toxicity of both materials. Thus, it is a promising candidate for biomedical applications as drug delivery or biomedical targeting.

Enhanced magnetic anisotropy of Ni nanowire arrays fabricated on nano-structured silicon templates

The magnetic function of a Ni-nanowire/silicon-template system has been explored in corporation with an advanced process. Arrays of nanopores with a mean diameter of 35 nm have been fabricated by anodization of silicon wafers under an external magnetic field (8 T) perpendicular to the substrate. Due to a guided supply of holes from the substrate during the anodization, well controlled straight nanopores have been formed with a high aspect ratio, and then isolated Ni nanowires have been grown along these nanopores by electrodeposition. The fabricated samples show a significantly enhanced magnetic anisotropy with little crosstalk between adjacent pores.

Porous silicon/Ni composites of high coercivity due to magnetic field-assisted etching

Ferromagnetic nanostructures have been electrodeposited within the pores of porous silicon templates with average pore diameters between 25 and 60 nm. In this diameter regime, the pore formation in general is accompanied by dendritic growth resulting in rough pore walls, which involves metal deposits also offering a branched structure. These side branches influence the magnetic properties of the composite system not only due to modified and peculiar stray fields but also because of a reduced interpore spacing by the approaching of adjacent side pores. To improve the morphology of the porous silicon structures, a magnetic field up to 8 T has been applied during the formation process. The magnetic field etching results in smaller pore diameters with less dendritic side pores. Deposition of a ferromagnetic metal within these templates leads to less branched nanostructures and, thus, to an enhancement of the coercivity of the system and also to a significantly increased magnetic anisotropy. So magnetic field-assisted etching is an appropriate tool to improve the structure of the template concerning the decrease of the dendritic pore growth and to advance the magnetic properties of the composite material.

Beyond spin-magnetism of magnetic nanowires in porous silicon

A ferromagnet–semiconductor (Si–Ni-) nanowire composite, electrochemically prepared in an n-doped (001) silicon wafer, is studied using SQUID magnetometry at magnetic fields up to 7 T parallel and perpendicular to the Ni-wires (diameter 50 nm, length 1 µm, 4 × 108 wires mm−2). Apart from the conventional spin-magnetism of Ni which is saturated at low field, an additional giant paramagnetism is observed at fields >1 T, which is nearly temperature independent, shows strict linear field dependence, strong anisotropy and a lack of saturation. Taking the difference of the orthogonal magnetizations this unconventional paramagnetism becomes obvious. It is based on mesoscopic currents driven by the symmetry breaking at the wire–silicon interface due to the Rashba field (spin-galvanic effect). Spin-polarized carriers from the Ni-wires are captured in low angular momentum quantum states . An attempt has been made to estimate the observed giant magnetic moment under simplifying assumptions.

Electrochemically Fabricated Silicon/Metal Hybrid Nanosystem with Tailored Magnetic Properties

Deposition of magnetic nanostructures into porous silicon (PS) matrices performed electrochemically or by infiltration merges the electronic properties of a semiconductor and the magnetic ones of the incorporated material. The PS templates used exhibit unconnected mesopores with a high aspect ratio of about 1000. Magnetization measurements of such hybrid materials show two characteristic field regions, one at magnetic fields below the saturation magnetization M s of the deposited metals due to the spin magnetism and another one at higher fields above M s , which is assigned to an intensively investigated magnetic contribution likely caused by a spin―orbit interaction.