A drop full of nanoparticles is dragged across a substrate covered with nanometric barriers and holes. When the nanoparticles encounter these obstacles, they detach from the liquid and are captured by the holes. Image: Valentin Flauraud.
A drop full of nanoparticles is dragged across a substrate covered with nanometric barriers and holes. When the nanoparticles encounter these obstacles, they detach from the liquid and are captured by the holes. Image: Valentin Flauraud.

Whether making pens or building space shuttles, the manufacturing process consists of creating components and then carefully assembling them. But when it comes to infinitely small structures, manipulating and assembling high-performance nanoparticles on a substrate is no mean feat.

Researchers in the Laboratory of Microsystems at the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have now come up with a way to position hundreds of thousands of nanoparticles very precisely on a 1cm2 surface. They were able to place each nanoparticle within 1nm – compared with 10–20nm using conventional methods – and oriented within 1°.

Their work, which is reported in a paper in Nature Nanotechnology, sets the stage for the development of nanometric devices such as optical detection equipment and biological sensors. "If we manage to place gold nanoparticles 1nm apart, we could, for example, confine light to an extraordinary degree and detect or interact with individual molecules," said Valentin Flauraud, the lead author.

For their study, the researchers used gold nanoparticles that were grown chemically in a liquid. "These nanoparticles exhibit better properties than those produced through evaporation or etching, but it is more difficult to manipulate them, because they are suspended in a liquid," said Flauraud.

Their novel positioning technique involves taking a drop of liquid full of nanoparticles and heating it so that the nanoparticles cluster in a given spot. This drop is then dragged across a substrate covered in nanometric barriers and holes.

When the nanoparticles encounter these obstacles, they detach from the liquid and are captured by the holes. "It's a little like playing miniature golf," Flauraud explained. Each trap is designed to orient a nanoparticle in a specific way.

"The challenge was to figure out how the liquid, the particles and the substrate interact at the nanometric scale so we could trap the nanoparticles effectively," said Massimo Mastrangeli, the second author on the paper and now a researcher at the Max Planck Institute for Intelligent Systems in Stuttgart, Germany.

To show how well their method works, the researchers took on several challenges. First, they tested the optical properties of their system with a powerful transmission electron microscope in EPFL's Interdisciplinary Center for Electron Microscopy (CIME).

Next, they showed that their technique could be used to produce geometrically complex structures by writing out the alphabet with nanoparticles, producing the smallest segment display in the world. "All of this work was conducted at EPFL and is the result of strong synergies between the various technical platforms and the labs," said Jürgen Brugger, head of EPFL’s Laboratory of Microsystems. "It's an excellent example of how top-down and bottom-up methods can be combined, opening the door to numerous unexplored fields of nanotechnology."

This story is adapted from material from EPFL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.