The growth and assembly behavior of cobalt magnetic nanocrystallites under an external magnetic field were studied. Co polycrystalline wires with an average length of 2 mm and diameter of 13 mm were formed by the self-assembly of Co nanocrystallites (15 nm on average) under the induction of a 0.25 T external magnetic field. The wires were nearly parallel because their axes were all parallel to the magnetic line of force. The M s and H c values of the sample, 111 emu g 21 and 389 Oe, are higher than those of the sample prepared without an external magnetic field applied (91 emu g 21 and 375 Oe), which might be associated with the special nanostructure in which Co nanocrystallites were arranged in polycrystalline wires acting as permanent magnetic dipoles. The process could be used to fabricate large arrays of uniform wires of some magnetic materials and improve the magnetic properties of nanoscale magnetic materials.

Polycrystalline nickel wires with an average length of 10 µm and diameter of about 200 nm were prepared at 70°C by a hydrothermal process with a 0.25 T magnetic field applied. Studies show that the magnetic field induced one-dimensional assembly of acicular Ni nanocrystallites with dimensions of 200 nm in length and 10-30 nm in diameter, leading to the formation of the polycrystalline nickel wires. Magnetic measurements show saturation magnetization (Ms) of the sample prepared in a 0.25 T external magnetic field is higher than that of the sample synthesized without an external magnetic field applied. It is suggested that the one-dimensional self-assembly of acicular nickel nanocrystallites with their magnetic easy axes [111] aligned along the magnetic line of force results in the improvement of the magnetic properties.

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Rational assembly of carbon nanostructures into large‐area films is a key step to realize their applications in ubiquitous electronics and energy devices. Here, a self‐assembly methodology is devised to organize diverse carbon nanostructures (nanotubes, dots, microspheres, etc.) into homogeneous films with potentially infinite lateral dimensions. On the basis of studies of the redox reactions in the systems and the structures of films, the spontaneous deposition of carbon nanostructures onto the surface of the copper substrate is found to be driven by the electrical double layer between copper and solution. As a notable example, the as‐assembled multiwalled carbon nanotube (MWCNT) films display exceptional properties. They are a promising material for flexible electronics with superior electrical and mechanical compliance characteristics. Finally, two kinds of all‐solid‐state supercapacitors based on the self‐assembled MWCNT films are fabricated. The supercapacitor using carbon cloth as the current collector delivers an energy density of 3.5 Wh kg−1 and a power density of 28.1 kW kg−1, which are comparable with the state‐of‐the‐art supercapacitors fabricated by the costly single‐walled carbon nanotubes and arrays. The supercapacitor free of foreign current collector is ultrathin and shows impressive volumetric energy density (0.58 mWh cm−3) and power density (0.39 W cm−3) too.

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