Digital light processing 3D printing of dynamic magneto-responsive thiol-acrylate composites.

Additive manufacturing is one of the most promising processing techniques for fabricating customized 3D objects. For the 3D printing of functional and stimuli-triggered devices, interest is steadily growing in processing materials with magnetic properties. Synthesis routes for magneto-responsive soft materials typically involve the dispersion of (nano)particles into a non-magnetic polymer matrix. Above their glass transition temperature, the shape of such composites can be conveniently adjusted by applying an external magnetic field. With their rapid response time, facile controllability, and reversible actuation, magnetically responsive soft materials can be used in the biomedical field (e.g. drug delivery, minimally invasive surgery), soft robotics or in electronic applications. Herein, we combine the magnetic response with thermo-activated healability by introducing magnetic Fe3O4 nanoparticles into a dynamic photopolymer network, which undergoes thermo-activated bond exchange reactions. The resin is based on a radically curable thiol-acrylate system, whose composition is optimized towards processability via digital light processing 3D printing. A mono-functional methacrylate phosphate is applied as a stabilizer to increase the resins' shelf life by preventing thiol-Michael reactions. Once photocured, the organic phosphate further acts as a transesterification catalyst and activates bond exchange reactions at elevated temperature, which render the magneto-active composites mendable and malleable. The healing performance is demonstrated by recovering magnetic and mechanical properties after the thermally triggered mending of 3D-printed structures. We further demonstrate the magnetically driven movement of 3D-printed samples, which gives rise to the potential use of these materials in healable soft devices activated by external magnetic fields.

Magnetic Characteristics of Ni-Filled Luminescent Porous Silicon.

The aim of the presented work is to combine luminescent porous silicon (PSi) with a ferromagnetic metal (Ni) to modify on the one hand the photoluminescence by the presence of metal deposits and on the other hand to influence the optical properties by an external magnetic field. The optical properties are investigated especially with respect to the wavelength-shift of the photoluminescence due to the metal filling. With increasing metal deposits within PSi the photoluminescence peak is blue-shifted and furthermore an increase of the intensity is observed. Photoluminescence spectra of bare PSi show a maximum around 620 nm whereas in the case of Ni filled samples the peak is blue-shifted to around 580 nm for a deposition time of 15 min. Field dependent magnetic measurements performed with an applied field parallel and perpendicular to the surface, respectively, show a magnetic anisotropy which is in agreement with a thin film. This film-like behavior is caused by the interconnected Ni structures due to the branched porous silicon morphology. The coercivity increases with increasing metal deposition from about 150 Oe to about 450 Oe and also the magnetic anisotropy is enhanced with the growth of metal deposits. Within this work the influence of the magnetic metal filling on the optical properties and the magnetic characterization of the nanocomposites are discussed. The presented systems give not only rise to optoelectronics applications but also to magneto optical integrated devices.

New MRI contrast agents based on silicon nanotubes loaded with superparamagnetic iron oxide nanoparticles.

This article describes the preparation and fundamental properties of a new possible material as a magnetic resonance imaging contrast agent based on the incorporation of preformed iron oxide (Fe3O4) nanocrystals into hollow silicon nanotubes (Si NTs). Specifically, superparamagnetic Fe3O4 nanoparticles of two different average sizes (5 nm and 8 nm) were loaded into Si NTs of two different shell thicknesses (40 nm and 70 nm). To achieve proper aqueous solubility, the NTs were functionalized with an outer polyethylene glycol-diacid (600) moiety via an aminopropyl linkage. Relaxometry parameters r1 and r2 were measured, with the corresponding r2/r1 ratios in phosphate buffered saline confirming the expected negative contrast agent behaviour for these materials. For a given nanocrystal size, the observed r2 values are found to be inversely proportional to NT wall thickness, thereby demonstrating the role of nanostructured silicon template on associated relaxometry properties.

Porous Silicon Nanocomposites with Combined Hard and Soft Magnetic Properties.

Magnetic nanostructures of two ferromagnetic metals have been combined within porous silicon, and the magnetic switching behavior of the resulting porous silicon/metal nanocomposite has been modified by varying the arrangement. The two magnetic materials are Ni and Co, whereas Co is the magnetic harder one. These "hard/soft" magnetic nanocomposites have been achieved by two different routes. On the one hand, double-sided porous silicon has been used whereas one side has been filled with Ni nanostructures and the other one with Co nanostructures. On the other hand, Ni and Co have been deposited within one porous layer alternatingly. The filling of the pores has been carried out by electrodeposition with varying the deposition parameters. In systems which offer two distinct slopes of the hysteresis curves due to the different saturation behavior of the two types of deposited metal, magnetic exchange coupling is not present. For samples which show smooth hysteresis curves exchange, coupling between the Ni and Co nanostructures seems to be present. The aim is to control especially the structure size of the soft and the hard magnetic materials and the distance between them at the nanoscale to optimize exchange coupling resulting in a maximum energy product.

Transient surface photovoltage studies of bare and Ni-filled porous silicon performed in different ambients.

Mesoporous silicon and porous silicon/Ni nanocomposites have been investigated in this work employing light-dark surface photovoltage (SPV) transients to monitor the response of surface charge dynamics to illumination changes. The samples were prepared by anodization of a highly n-doped silicon wafer and a subsequent electrodepositing of Ni into the pores. The resulting pores were oriented towards the surface with an average pore diameter of 60 nm and the thickness of the porous layer of approximately 40 µm. SPV was performed on a bare porous silicon as well as on a Ni-filled porous silicon in vacuum and in different gaseous environments (O2, N2, Ar). A significant difference was observed between the 'light-on' and 'light-off' SPV transients obtained in vacuum and those observed in gaseous ambiences. Such behavior could be explained by the contribution to the charge exchange in gas environments from chemisorbed and physisorbed species at the semiconductor surface. PACS: 81.05.Rm; 73.20.-r; 75.50.-y; 82.45.Yz.

Magnetic interactions between metal nanostructures within porous silicon.

Electrochemically deposited magnetic nanostructures arranged in a three-dimensional system are investigated with respect to their cross-talk between each other. The nanostructures are embedded in porous silicon templates with different morphologies which means pores offering dendritic growth of different strengths. An increase of the uniformity of the pores is concomitant with an increase of the smoothness of the metal deposits which strongly influences the magnetic behavior of the system. Less dendritic structures lead to an increase of the coercivity of the nanocomposite which reveals less cross-talk between the metal deposits due to a modification of the stray fields. The system allows in a cheap and simple way to tune the magnetic interactions of magnetic nanostructures in a three-dimensional arrangement. PACS: 81.05.Rm; 81.07.Gf; 75.75.-c.

Magnetic properties of superparamagnetic nanoparticles loaded into silicon nanotubes.

In this work, the magnetic properties of silicon nanotubes (SiNTs) filled with Fe3O4 nanoparticles (NPs) are investigated. SiNTs with different wall thicknesses of 10 and 70 nm and an inner diameter of approximately 50 nm are prepared and filled with superparamagnetic iron oxide nanoparticles of 4 and 10 nm in diameter. The infiltration process of the NPs into the tubes and dependence on the wall-thickness is described. Furthermore, data from magnetization measurements of the nanocomposite systems are analyzed in terms of iron oxide nanoparticle size dependence. Such biocompatible nanocomposites have potential merit in the field of magnetically guided drug delivery vehicles. PACS: 61.46.Fg; 62.23.Pq; 75.75.-c; 75.20.-g.

Three dimensional quantitative characterization of magnetite nanoparticles embedded in mesoporous silicon: local curvature, demagnetizing factors and magnetic Monte Carlo simulations.

Magnetite nanoparticles embedded within the pores of a mesoporous silicon template have been characterized using electron tomography. Linear least squares optimization was used to fit an arbitrary ellipsoid to each segmented particle from the three dimensional reconstruction. It was then possible to calculate the demagnetizing factors and the direction of the shape anisotropy easy axis for every particle. The demagnetizing factors, along with the knowledge of spatial and volume distribution of the superparamagnetic nanoparticles, were used as a model for magnetic Monte Carlo simulations, yielding zero field cooling/field cooling and magnetic hysteresis curves, which were compared to the measured ones. Additionally, the local curvature of the magnetite particles' docking site within the mesoporous silicon's surface was obtained in two different ways and a comparison will be given. A new iterative semi-automatic image alignment program was written and the importance of image segmentation for a truly objective analysis is also addressed.

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.

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.

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.