Collective behaviour in two-dimensional cobalt nanoparticle assemblies observed by magnetic force microscopy

Magnetic dipolar interaction in NiFe nanodot arrays formed on vertical carbon nanotubes

A new and simple method for the fabrication of densely packed magnetic nanodot arrays was developed using conventional sputtering deposition at room temperature. An anodized alumina template was employed for the formation of nanodot assemblies, consisting of carbon nanotubes (CNTs) and magnetic nanodot arrays. Each nanodot was formed exactly on top of a CNT and was arranged with a well-ordered structure in a wide range of area. It was also found that the size of dots and the distance between dots can be tailored by changing the length of CNTs, inducing a change of strength of dipolar interaction between nanodots.

General assembly method for linear metal nanoparticle chains embedded in nanotubes

We demonstrate a flexible assembly method for producing linear metal nanoparticle chains embedded in nanotubes. The chain formation is based on the Rayleigh instability after annealing metal nanowires confined in nanotubes. Beginning with metal nanowires from arbitrary synthesis methods, atomic layer deposition (ALD) was applied to coat the wires first with a sacrificial layer then with a shell layer. Subsequently, the sacrificial layer was removed leading to confined wires in nanotubes with a free volume. Finally, embedded nanoparticle chains were produced inducing the Rayleigh instability by annealing the confined nanowires. This method is quite general not only for different metals but also for different shell materials. We are able to tune the particle spacing and diameter, the shape of the nanochains, the tube diameter and the shell thickness by ALD significantly.

Synthesis and optical characterization of submicrometer gold nanotubes grown on goethite rods

Goethite (FeOOH) rods were used as templates for growing gold nanotubes with a length of a few hundred nanometers and an aspect ratio between 3 and 4. Successful uniform growth required surface modification, followed by the attachment of small Au seeds and one-step seeded growth using formaldehyde as a reducing agent, as previously reported for the growth of Au shells on silica spheres and hematite spindles. The thickness and surface roughness of the obtained shells could be adjusted by simply varying the concentration ratio between seeds (modified goethite rods) and growth reagents (HAuCl 4 and formaldehyde). The morphology of the synthesized gold nanotubes was thoroughly characterized by TEM, SEM, and AFM/MFM. The resulting gold nanotubes display well-defined plasmon resonances, with a strong longitudinal mode centered around ca. 1400 nm and a broad band in the visible resulting from the overlap of a transverse mode and a multipolar mode, as was found from theoretical modeling using the boundary element method, which provides reasonable agreement with the experimental results.

Synthesis and self-assembly of polymer-coated ferromagnetic nanoparticles

We describe the synthesis and characterization of polymer-coated ferromagnetic cobalt nanoparticles (CoNPs). The synthesis of end-functionalized polystyrene surfactants possessing amine, carboxylic acid, or phosphine oxide end-groups was accomplished using atom-transfer radical polymerization. This versatile synthetic method enabled the production of multigram quantities of these polymeric surfactants that stabilized ferromagnetic CoNPs when dispersed in organic media. An in-depth investigation into the synthesis of polystyrene-coated ferromagnetic CoNPs was also conducted using various combinations of these polymeric surfactants in the thermolysis of dicobaltoctacarbonyl (Co(2)(CO)(8)). Moreover, the application of a dual-stage thermolysis with Co(2)(CO)(8) allowed for the preparation of large samples (200-820 mg) per batch of well-defined and dispersable ferromagnetic nanoparticles. Characterization of these functionalized nanoparticle materials was then done using transmission electron microscopy, X-ray diffraction, vibrating sample magnetometry, and thermogravimetric analysis. Self-assembly of these dipolar nanoparticles was investigated in solutions cast onto supporting substrates, where local nematic-like ordering of nanoparticle chains was observed along with a tendency of adjacent chains to form "zippering" configurations, both phenomena having been predicted by recent simulations of dipolar fluids in conjunction with van der Waals interactions.

Conditional random surveying for particle deposition on a mica surface

In using microscopic imaging techniques, unbiased selection of sampling areas is often critical when judgment has to be used to find regions of interest. A conditional random sampling was designed to survey hematite particles on a mica surface using tapping-mode atomic force microscopy, based on three adapted-systematic-sampling methods designed to exclude subjective bias by limiting the freedom of arbitrarily selecting sampling areas. The results of these surveying methods were compared with the average particle surface density modeled by Poisson distribution. It was found that the conditional random sampling could survey particles effectively and improve the data reliability significantly. Ten population-known images from the same mica sheet were used to evaluate these methods, and an average relative error of 12% (maximum 21%) was obtained using the conditional random method with six sampling areas. It was used to investigate the effects of common organic pollutants, benzene, toluene, ethylbenzene, and xylenes on the transport of soil colloids.

Synthesis of magnetic microspheres with controllable structure via polymerization-triggered self-positioning of nanocrystals

A facile and novel method for the synthesis of magnetite-polymer hybrid microspheres with controllable structure is reported. On the basis of the interactions between polymers and magnetic nanocrytals, which are coated with a primary oleic acid layer, a variety of magnetic microspheres with well-tailored structure are fabricated by carefully modulating polymerization and selecting components of composite microspheres; the evolvement of hierarchical structure, and the magnetic properties of microspheres were systematically studied. Importantly, the mechanism of polymerization-triggered self-positioning of magnetic nanocrystals is investigated.

Controlling the interparticle spacing of Au-salt loaded micelles and Au nanoparticles on flat surfaces

The self-organization of diblock copolymers into micellar structures in an appropriate solvent allows the deposition of well ordered arrays of pure metal and alloy nanoparticles on flat surfaces with narrow distributions in particle size and interparticle spacing. Here we investigated the influence of the materials (substrate and polymer) and deposition parameters (temperature and emersion velocity) on the deposition of metal salt loaded micelles by dip-coating from solution and on the order and inter-particle spacing of the micellar deposits and thus of the metal nanoparticle arrays resulting after plasma removal of the polymer shell. For identical substrate and polymer, variation of the process parameters temperature and emersion velocity enables the controlled modification of the interparticle distance within a certain length regime. Moreover, also the degree of hexagonal order of the final array depends sensitively on these parameters.

Self-assembled virus-like particles with magnetic cores

Efficient encapsulation of functionalized spherical nanoparticles by viral protein cages was found to occur even if the nanoparticle is larger than the inner cavity of the native capsid. This result raises the intriguing possibility of reprogramming the self-assembly of viral structural proteins. The iron oxide nanotemplates used in this work are superparamagnetic, with a blocking temperature of about 250 K, making these virus-like particles interesting for applications such as magnetic resonance imaging and biomagnetic materials. Another novel feature of the virus-like particle assembly described in this work is the use of an anionic lipid micelle coat instead of a molecular layer covalently bound to the inorganic nanotemplate. Differences between the two functionalization strategies are discussed.

DNA-templated magnetic nanowires with different compositions: fabrication and analysis

The structure and magnetic properties of different types of templated wires are compared in this study. A long DNA molecule was used to guide the assembly of pyrrolidinone-capped Fe2O3 and CoFe2O3 particles as well as polylysine-coated gold nanoparticles. The resulting DNA-templated wires were stretched onto silicon oxide surfaces using a receding meniscus procedure. The coated, stretched, and surface-bound wires were characterized using atomic force microscopy (AFM), magnetic force microscopy (MFM), and spectroscopic methods. The results with respect to the wire properties were correlated with those determined from the bulk properties of the nanoparticles and with the properties of the bulk DNA. The MFM measurements allowed us to visualize the formation of domains along the wires as well as qualitatively compare the magnetic properties of each templated structure.

Shell-driven magnetic stability in core-shell nanoparticles

The magnetic properties of ferromagnetic-antiferromagnetic Co-CoO core-shell nanoparticles are investigated as a function of the in-plane coverage density from 3.5% to 15%. The superparamagnetic blocking temperature, the coercivity, and the bias field radically increase with increasing coverage. This behavior cannot be attributed to the overall interactions between cores. Rather, it can be semiquantitatively understood by assuming that the shells of isolated core-shell nanoparticles have strongly degraded magnetic properties, which are rapidly recovered as nanoparticles come into contact.

Synthesis, Characterization, and Self-Assembly of Pencil-Shaped CoO Nanorods

We synthesized uniformly sized, pencil-shaped CoO nanorods by the thermal decomposition of a cobalt-oleate complex, which was prepared from the reaction of cobalt chloride and sodium oleate. The diameters and lengths of the CoO nanorods were easily controlled by varying the experimental conditions, such as the heating rate and the amount of Co-oleate complex. The X-ray diffraction pattern revealed that the CoO nanorods have an extraordinary wurtzite ZnO crystal structure. These uniformly sized nanorods self-assembled to form both horizontal parallel arrangements and perpendicular hexagonal honeycomb superlattice structures. Reduction of the nanorods by heating under a hydrogen atmosphere generated either hcp Co or Co(2)C nanorods. Characterization of the CoO nanorods using X-ray absorption spectroscopy, X-ray magnetic circular dichroism spectroscopy, and magnetic measurements showed that they contain a small fraction of ferromagnetic Co impurities.

Nanoparticle-mediated local and remote manipulation of protein aggregation

The local heat delivered by metallic nanoparticles selectively attached to their target can be used as a molecular surgery to safely remove toxic and clogging aggregates. We apply this principle to protein aggregates, in particular to the amyloid beta protein (Abeta) involved in Alzheimer's disease (AD), a neurodegenerative disease where unnaturally folded Abeta proteins self-assemble and deposit forming amyloid fibrils and plaques. We show the possibility to remotely redissolve these deposits and to interfere with their growth, using the local heat dissipated by gold nanoparticles (AuNP) selectively attached to the aggregates and irradiated with low gigahertz electromagnetic fields. Simultaneous tagging and manipulation by AuNP of Abeta at different stages of aggregation allow both, noninvasive exploration and dissolution of molecular aggregates.

Seven-Nanometer Hexagonal Close Packed Cobalt Nanocrystals for High-Temperature Magnetic Applications through a Novel Annealing Process

Seven-nanometer cobalt nanocrystals are synthesized by colloidal chemistry. Gentle annealing induces a direct structural transition from a low crystalline state to the hexagonal close packed (hcp) phase without changing the size, size distribution, and the lauric acid passivating layer. The hcp structured nanocrystals can be easily redispersed in solvent for further application and processing. We found that the magnetization at saturation and the magnetic anisotropy are strongly modified through the annealing process. Monolayer self-assembly of the hcp cobalt nanocrystals is obtained, and due to the dipolar interaction, ferromagnetic behavior close to room temperature has been observed. This work demonstrates a novel approach for obtaining small size hcp structured cobalt magnetic nanocrystals for many technological applications.

Super-Stable, High-Quality Fe3 O4 Dendron-Nanocrystals Dispersible in Both Organic and Aqueous Solutions

High-quality Fe₃ O₄ nanocrystals coated with stearate groups are successfully converted to dendron-coated nanocrystals (dendron-nanocrystals). Poly(ethylene glycol) oligomers are used as major terminal groups for the dendron ligands, which afford excellent dispersibility of the dendron-nanocrystals in a broad spectrum of solvents, ranging from dichloromethane to water.

Block Copolymer Nanocomposites: Perspectives for Tailored Functional Materials

Heterogeneous materials in which the characteristic length scale of the filler material is in the nanometer range-i.e., nanocomposites-is currently one of the fastest growing areas of materials research. Polymer nanocomposites have expanded beyond the original scope of polymer-nanocrystal dispersions for refractive-index tuning or clay-filled homopolymers primarily pursued for mechanical reinforcement, to include a wide range of applications. This article highlights recent research efforts in the field of structure formation in block copolymer-based nanocomposite materials, and points out opportunities for novel materials based on inclusion of different types of nanoparticles. The use of block copolymers instead of homopolymers as the matrix is shown to afford opportunities for controlling the spatial and orientational distribution of the nanoelements. This, in turn, allows much more sophisticated tailoring of the overall properties of the composite material.