Magnet-controlled plasma bombardment produces uniform nanocubes

While nanoparticles sound like a recent discovery, these tiny structures have been used for centuries. The famous Lycurgus cup, made by 4th century Roman artisans, features dichroic glass with gold and silver nanoparticles sprinkled throughout. These nanoparticles give the cup a green appearance when illuminated from the front and a red appearance when illuminated from behind.

In the centuries since the time of the ancient artisans, researchers have come a long way in understanding nanoparticles, with nanocubes proving of particular interest due to their potential applications as biosensors and gas sensors. Nanocubes and other nanoparticles can be produced using either physical or chemical methods, but physical methods tend to be preferable because they are less likely to generate organic contaminants than chemical methods. Unfortunately, uniformly-sized nanocubes are difficult to produce in sufficient quantities by physical methods.

Now, researchers from the Nanoparticles by Design Unit at the Okinawa Institute of Science and Technology (OIST) Graduate University in Japan, together with colleagues in Finland and France, have discovered a new approach to overcoming this problem. The researchers describe this approach in a paper in Advanced Functional Materials.

“The cube shape is not the lowest energy structure for iron nanoparticles,” explains Jerome Vernieres from OIST, first author of the paper, “thus, we couldn’t rely on equilibrium thermodynamics considerations to self-assemble these nanocubes”. Instead, the OIST scientists, under the guidance of Mukhles Sowwan, exploited the possibilities offered by a technique called magnetron-sputtering inert-gas condensation to create their iron nanocubes.

In this method, argon gas is first heated to convert it into an ionized plasma. The researchers then use a magnet located behind a target made of the material that will form the basis for the nanoparticles – in this case, iron – to control the shape of the plasma and direct the argon ions towards the target, hence the name ‘magnetron’. This bombardment causes iron atoms to be sputtered away from the target and collide with argon atoms and each other to form nanocubes. The challenge, however, is to make these nanocubes as uniform as possible.

“Uniformity is key in sensing applications,” says Stephan Steinhauer, also from OIST “We needed a way to control the size, shape and number of the nanocubes during their production.”

To control the size and shape of these cubes, the researchers made a simple but significant observation: iron is magnetic in its own right! In other words, the researchers discovered they could exploit the intrinsic magnetism of the target itself as an innovative way to modify the magnetic field of the magnetron. In this way, they were able to manipulate the plasma where the particles are grown, and thus control the nanocube sizes during formation.

“This is the first time uniform iron nanocubes have been made using a physical method that can be scaled for mass production,” claims Vernieres. To better understand the mechanics of this process, the OIST team collaborated with researchers from the University of Helsinki in Finland to make theoretical calculations. “The work relied heavily on both experimental methods and theoretical calculations. The simulations were important for us to explain the phenomena we were observing,” says Panagiotis Grammatikopoulos from OIST.

Once the researchers had come up with a way to produce these uniform iron cubes, the next step was to build an electronic device that could utilize the nanocubes for sensing applications. “We noticed that these cubes were extremely sensitive to the levels of gaseous nitrogen dioxide (NO₂),” says Steinhauer. “NO₂ sensing is used for a variety of different purposes, from diagnosis of asthma patients to detecting environmental pollution, so we immediately saw an application for our work.”

In collaboration with researchers from the University of Toulouse in France, the researchers from the Nanoparticles by Design Unit built a prototype NO₂ sensor that measured the change in electrical resistance of the iron nanocubes on exposure to NO₂ gas. Because even a very tiny amount of NO₂ can produce a measurable change in electrical resistance that is considerably larger than produced by other atmospheric pollutants, the iron nanocube-based sensor is both extremely sensitive and specific. “These nanocubes have many potential uses. The fact that we can produce a relatively large quantity of uniform nanocubes using an increasingly common synthesis method makes this research highly promising for industrial applications,” says Vernieres.

This story is adapted from material from the Okinawa Institute of Science and Technology Graduate University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.