Assembly of magnetic nanoparticles into higher structures on patterned magnetic beads under the influence of magnetic field

Manipulation of dipolar magnetism in low-dimensional iron oxide nanoparticle assemblies

Trench-patterned iron oxide nanoparticles are switched between a superspin glass and a superferromagnetic state.

Formation and Self-assembly of Gold Nanoplates through an Interfacial Reaction for Surface-Enhanced Raman Scattering

3D hierarchical architectures assembled from individual particles have attracted great interest because they displayed novel properties from the individual building blocks as well as their complex structures. Here we present a new strategy to form 3D hierarchical gold (Au) nanostructures via an interfacial reduction reaction. An aniline (ANI) derivative, N-(3-amidino)-aniline (NAAN), and HAuCl4 were separately dissolved in toluene and water to form an organic/water interface. Au nanoplates formed at the interface and subsequently moved to the aqueous phase. As a capping agent for the nanoplate formation, the oxidized NAAN, i.e., poly(N-(3-amidino)-aniline) (PNAAN), also facilitated the self-assembly of Au nanoplates into 3D hierarchical Au nanoflowers (AuNFs) through π-π stacking. The individual AuNF exhibited good surface-enhanced Raman scattering (SERS) response both in enhancement factor and reproducibility because it integrates the SERS enhancement effects of individual Au nanoplates and their hierarchical structures. This is the first report depicting the one-pot formation and self-assembly of Au nanoplates into 3D organized hierarchical nanostructures through the molecular interaction of conducting polymer.

Magnetic nanoparticle assembly arrays prepared by hierarchical self-assembly on a patterned surface

Inverted pyramid hole arrays were fabricated by photolithography and used as templates to direct the growth of colloidal nanoparticle assemblies. Cobalt ferrite nanoparticles deposit in the holes to yield high quality pyramid magnetic nanoparticle assembly arrays by carefully controlling the evaporation of the carrier fluid. Magnetic measurements indicate that the pyramid magnetic nanoparticle assembly arrays preferentially magnetize perpendicular to the substrate.

Synthesis and assembly of nanomaterials under magnetic fields

Traditionally, magnetic field has long been regarded as an important means for studying the magnetic properties of materials. With the development of synthesis and assembly methods, magnetic field, similar to conventional reaction conditions such as temperature, pressure, and surfactant, has been developed as a new parameter for synthesizing and assembling special structures. To date, magnetic fields have been widely employed for materials synthesis and assembly of one-dimensional (1D), two-dimensional (2D) or three-dimensional (3D) aggregates. In this review, we aim to provide a summary on the applications of magnetic fields in this area. Overall, the objectives of this review are: (1) to theoretically discuss several factors that refer to magnetic field effects (MFEs); (2) to review the magnetic-field-induced synthesis of nanomaterials; the 1D structure of various nanomaterials, such as metal oxides/sulfide, metals, alloys, and carbon, will be described in detail. Moreover, the MFEs on spin states of ions, magnetic domain and product phase distribution will be also involved; (3) to review the alignment of carbon nanotubes, assembly of magnetic nanomaterials and photonic crystals with the help of magnetic fields; and (4) to sketch the future opportunities that magnetic fields can face in the area of materials synthesis and assembly.

Ring stains in the presence of electromagnetohydrodynamic interactions

In a recent paper [Das et al., Phys. Rev. E 85, 046311 (2012)], we delineated the role of electrokinetic transport in modifying the classical "coffee stain" effect. In this study, we extend this calculation to incorporate the consequences of a generalized electromagnetohydrodynamic transport in the coffee stain phenomenon. The magnetohydrodynamic (MHD) effect enhances the velocities at the beginning of the drop life, whereas the electrokinetic effect increases the "disordering" effect in particle deposition at the end of the drop, triggered by a velocity divergence. For a suitable combination of the strength of the MHD and electrokinetic transport, however, this disordering effect is substantially enhanced, and, most nonintuitively, such velocity divergence and the disordering effect may occur at a time that is much earlier than the end of the drop life, or may occur even instantaneously after the start of the drop evaporation. This work will provide useful insight in the understanding of the dynamics of mesoscopic patterns formed as the magnetic nanocrystals deposit in the presence of a combined transport driven by evaporation and magnetic field effects.

Pattern transfer nanomanufacturing using magnetic recording for programmed nanoparticle assembly

We report a novel nanomanufacturing technique that incorporates patterned arrays built entirely from Fe₃O₄ nanoparticles into a flexible and transparent polymer film. First, the nanoparticles are patterned using the enormous magnetic field gradients at the surface of commercial disk drive media, and then the resulting architecture is transferred to the surface of a polymer film by spin-coating and peeling. Since the particles are immobilized by the field gradients during the spin-coating process, the patterned array is preserved after peeling. To demonstrate the potential of this technology, we fabricate a 5 mm diameter all-nanoparticle diffraction grating capable of producing a white-light optical spectrum. We also demonstrate several extensions to this technology, where, by adding an external magnetic field during assembly, we create both periodic variations in topography, as well as a nanocomposite with two vertically and horizontally separated nanoparticle layers. As this technique leverages the nanometer resolution inherent in current magnetic recording technology, strong potential exists for low-cost nanomanufacturing of optical and electronic devices from a variety of nanomaterials with ∼10 nm resolution.