New technique enables nano self-assembly

Washington, October 24 : Researchers with the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have found a simple and yet powerfully robust way to induce nanoparticles to assemble themselves into complex arrays.

Small as they are, nanoparticles are essentially all surface; so any process that modifies the surface of a nanoparticle can profoundly change the properties of that particle.

Precisely arranging these nanoparticles is critical to tailoring the macroscopic properties during nanoparticle assembly.

Although DNA has been used to induce self-assembly of nanoparticles with a high degree of precision, this approach only works well for organized arrays that are limited in size; it is impractical for large-scale fabrication.

Ting Xu, a polymer scientist who led this project, believes that a better approach is to use block copolymers - long sequences or "blocks" of one type of monomer molecule bound to blocks of another type of monomer molecule.

"Block copolymers readily self-assemble into well-defined arrays of nanostructures over macroscopic distances," she said.

"They would be an ideal platform for directing the assembly of nanoparticles except that block copolymers and nanoparticles are not particularly compatible with one another from a chemistry standpoint. A mediator is required to bring them together," she added.

Xu and her group found such a "mediator" in the form of small molecules that will join with nanoparticles and then able attach themselves and their nanoparticle partners to the surface of a block copolymer.

For this study, Xu and her group used two different types of small molecules, surfactants (wetting agents) dubbed "PDP" and "OPAP."

These small molecules can be stimulated by light (PDP) or heat (OPAP) to sever their connection to the surface of a block copolymer and be repositioned to another location along the polymeric chain.

In this manner, the spatial distribution of the small molecule mediators and their nanoparticle partners can be precisely directed with no need to modify either the nanoparticles or the polymers.

"We've demonstrated a simple yet versatile approach to precisely controlling the spatial distribution of readily available nanoparticles over multiple length scales, ranging from the nano to the macro," said Ting Xu, a polymer scientist who led this project.

"Our technique can be used on a wide variety of nanoparticles and should open new routes to the fabrication of nanoparticle-based devices including highly efficient systems for the generation and storage of solar energy," he added.

--ANI