Metal nanostructures sculpted in mid-air by magnetic fields

Metal nanomaterials could be key to the development of transformative new electronic and energy devices. However, ensuring that these nanomaterials have the appropriate mechanical and electrical properties requires them to be formed with consistent shapes and surfaces. Reaping the benefits of their properties requires them to be produced using scalable techniques.

Nanomaterials – which are made from nanoscale particles of between one and 100nm – tend to be created within a liquid matrix; this is not only extremely expensive to scale, but it also cannot be used to make pure metals such as aluminium or magnesium. Alternative techniques, which tend to involve creating a cloud of particles from a condensing vapour, are very difficult to control.

Engineers from UC-Riverside approached this challenge by vaporising metals within a magnetic field to direct the reassembly of atoms into predictable shapes.

The researchers created nanomaterials from iron, copper, and nickel in a gas phase. They placed solid metal within a powerful electromagnetic levitation coil to heat the metal beyond its melting point, causing it to vaporise. The droplets of metal vapour levitated in the gas, and moved under the influence of the forces applied by the magnetic field.

This allowed the droplets to bond in an orderly manner that could be predicted based on the type of metal and how the magnetic fields were applied. While nanoparticles of iron and nickel (which are ferromagnetic) form string-like structures, copper nanoparticles form clusters of globules. Aggregates of iron and nickel produced a porous surface when deposited on a carbon film, while the copper produced a more compact and solid surface. The properties of these surfaces mirrored at a larger scale the properties of the nanoparticle types.

According to the researchers, the magnetic field can be considered an “add-on”, meaning that this technique could be applied to create predictable structures from any vapour-phase nanoparticles. This could allow for greater control over the electrical and mechanical properties of nanomaterials.

“This “field directed” approach enabled one to manipulate the assembly process and change the architecture of the resulting particles from high fractal dimension objects to lower dimension string-like structures,” said Professor Michael Zachariah, a chemical engineering expert at UC-Riverside. “The field strength can be used to manipulate the extent of this arrangement.”