Nanostructure Creation Takes Bottom Up Approach

A bottom-up approach to the self-assembly of microscopic particles into larger crystalline structures — by altering the concentrations of tiny particles and their magnetic fields — could produce the basic components for advanced optics, data storage and bioengineering.

Until recently, it was difficult and time-consuming to produce nanostructures using current technologies. The traditional method for creating the man-made crystals was more of a “top-down” approach using lithography or molding techniques. These structures cannot easily be created in three dimensions.

Now, scientists at Duke University have developed a bottom-up approach to assembling microscopic particles into larger crystalline structures that could make it possible to fabricate complex materials in many different structures.

A nanostructure. (Image: Benjamin Yellen)

“Not only did we develop the theoretical underpinning for this new technique, but we demonstrated in the lab that we could create more than 20 different programmed structures,” said Benjamin Yellen, assistant professor of mechanical engineering and materials science at Duke’s Pratt School of Engineering. “Our approach is much more ‘bottom up’ in that we’re starting at the level of a model ‘atom’ and working our way up.”

The scientists manipulated the magnetization within a liquid solution to coax magnetic and nonmagnetic particles to form intricate nanostructures such as chains, lattices and rings. These nanostructures were formed inside a ferrofluid, a solution consisting of suspended nanoparticles composed of iron-containing compounds.

A unique property of these fluids is that they become highly magnetized in the presence of external magnetic fields. The particles less magnetic than the ferrofluid behave similarly to negative charges, while particles that are more magnetic than the ferrofluid act like positive charges. The opposite particles thus attract one another to form structures resembling salt crystals.

The research team controlled the patterns and shapes of the nanostructures by varying the concentrations of the particles and the magnetization of the ferrofluid. The structure of the nanostructures decides their application, Yellen said.

Yellen foresees that these nanostructures can be used in advanced optical devices such as sensors, where different nanostructures could be designed to possess custom-made optical properties. He also predicts that rings made of metal particles could be used to design antennas as well as materials that demonstrate artificial “optical magnetism” and negative magnetic permeability.

The work appeared in Nature Communications.

For more information, visit: www.duke.edu