Printing Nano Building Blocks

Researchers from IBM’s Zurich Research Lab have devised a way to print particles as small as 60 nanometers in diameter with single-particle resolution. The technique lets researchers arrange tiny particles of various materials into well-defined structures on a surface–a step necessary for the mass production of devices such as nanowire transistors, biomedical sensors, and flexible, ultrasmall lenses capable of bending light.

“This is a very precise and efficient technique for taking nanoparticles with interesting properties and arranging them in an orderly fashion onto a surface,” says Tobias Kraus, a researcher on the IBM nano-patterning team. The group details its findings in a paper published in the journal Nature Nanotechnology.

To create an imprint, the IBM team first makes a template with grooves or holes only tens of nanometers deep and shaped in a desired pattern. Then the researchers move a liquid suspension containing nanoparticles over the template; the particles fill the shallow grooves or holes.

After the liquid dries, the team takes the template and presses it onto a substrate that has been prepared with a strong adhesive on its surface. The key step in the process is to ensure a difference in the strength of adhesives on the two surfaces: since the particles adhere better to a polymer layer on the substrate, they don’t stick to the original template once it’s removed. The result is a printed structure composed of single nanoparticles on the substrate.

Other researchers have previously shown that nanoparticles suspended in a liquid can be guided into patterns on a substrate. But, says Stephen Chou, head of the NanoStructure Laboratory at Princeton University, this is the first time he’s seen someone devise a way to print the structures onto the surface of another substrate with great precision. “The novelty here is that they’re able to print these particles onto another substrate,” Chou says.

“The authors describe a very clever way to use nanoparticles as ‘inks’ in a soft lithographic printing process,” says John Rogers, a professor of engineering at the University of Illinois, Urbana Champaign. Rogers’s own team has also recently devised a novel nanoscale printing technique. (See “Nanoscale Inkjet Printing.”) Along with other recent research in the area, he says, these techniques represent a new and powerful direction in nano printing.

The IBM team demonstrated the precision and versatility of the method by printing a nanoscale version of Robert Fludd’s 17th-century image of the sun. The image is composed of 20,000 gold particles, each of them only 60 nanometers in diameter.

Before any useful applications result from the research, Kraus says that his team will have to devise a way of increasing the long-range accuracy of the method–that is, a way to make sure that each particle along a long line is exactly where the researchers want it to be. They will also have to develop strategies for particles smaller than the 60-nanometer size. For IBM’s method to get below that size, the researchers will have to find the right balance between the depth of the grooves on their templates and the size of the particle. If the particle is too small and the depth too great, the particle will sit in the well and never adhere in the transfer.

Still, Kraus says, the IBM fabrication process applies to a wide range of materials–they include metals, polymers, and semiconductors–and is very reliable.